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Article No : o05_o02

Heterogeneous Catalysis and Solid Catalysts,2. Development and Types of Solid Catalysts

OLAF DEUTSCHMANN, Institut f€ur Technische Chemie und Polymerchemie, Universit€at

Karlsruhe (TH), Karlsruhe, Germany

HELMUT KNOZINGER, Department Chemie, Universit€at M€unchen, M€unchen,

Germany

KARL KOCHLOEFL, Rosenheim, Germany

THOMAS TUREK, Institut f€ur Chemische Verfahrenstechnik, TU Clausthal,

Clausthal-Zellerfeld, Germany

1. Development of Solid Catalysts . . . . . . . 483

2. Classification of Solid Catalysts . . . . . . . 484

2.1. Unsupported (Bulk) Catalysts . . . . . . . . 485

2.1.1. Metal Oxides. . . . . . . . . . . . . . . . . . . . . . 485

2.1.1.1. Simple Binary Oxides . . . . . . . . . . . . . . . 485

2.1.1.2. Complex Multicomponent Oxides . . . . . . 488

2.1.2. Metals and Metal Alloys . . . . . . . . . . . . . 493

2.1.3. Carbides and Nitrides . . . . . . . . . . . . . . . 493

2.1.4. Carbons . . . . . . . . . . . . . . . . . . . . . . . . . 494

2.1.5. Ion-Exchange Resins and Ionomers . . . . . 494

2.1.6. Molecularly Imprinted Catalysts . . . . . . . 495

2.1.7. Metal – Organic Frameworks . . . . . . . . . 495

2.1.8. Metal Salts . . . . . . . . . . . . . . . . . . . . . . . 496

2.2. Supported Catalysts . . . . . . . . . . . . . . . . 496

2.2.1. Supports . . . . . . . . . . . . . . . . . . . . . . . . . 496

2.2.2. Supported Metal Oxide Catalysts . . . . . . . 497

2.2.3. Surface-Modified Oxides . . . . . . . . . . . . . 497

2.2.4. Supported Metal Catalysts . . . . . . . . . . . . 498

2.2.5. Supported Sulfide Catalysts . . . . . . . . . . . 498

2.2.6. Hybrid Catalysts . . . . . . . . . . . . . . . . . . . 499

2.2.7. Ship-in-a-Bottle Catalysts . . . . . . . . . . . . 501

2.2.8. Polymerization Catalysts . . . . . . . . . . . . . 502

2.3. Coated Catalysts . . . . . . . . . . . . . . . . . . 502

3. Production of Heterogeneous Catalysts . 502

3.1. Unsupported Catalysts. . . . . . . . . . . . . . 503

3.2. Supported Catalysts . . . . . . . . . . . . . . . . 507

3.2.1. Supports . . . . . . . . . . . . . . . . . . . . . . . . . 507

3.2.2. Preparation of Supported Catalysts . . . . . . 507

3.3. Unit Operations in Catalyst Production. 508

4. Characterization of Solid Catalysts . . . . 511

4.1. Physical Properties . . . . . . . . . . . . . . . . 512

4.1.1. Surface Area and Porosity . . . . . . . . . . . . 512

4.1.2. Particle Size and Dispersion . . . . . . . . . . 513

4.1.3. Structure and Morphology . . . . . . . . . . . . 513

4.1.4. Local Environment of Elements . . . . . . . . 516

4.2. Chemical Properties . . . . . . . . . . . . . . . 517

4.2.1. Surface Chemical Composition . . . . . . . . 517

4.2.2. Valence States and Redox Properties . . . . 518

4.2.3. Acidity and Basicity . . . . . . . . . . . . . . . . 521

4.3. Mechanical Properties . . . . . . . . . . . . . 523

4.4. Characterization of Solid Catalysts under

Working Conditions. . . . . . . . . . . . . . . . 523

4.4.1. Temporal Analysis of Products (TAP

Reactor) . . . . . . . . . . . . . . . . . . . . . . . . . 523

4.4.2. Use of Isotopes . . . . . . . . . . . . . . . . . . . . 524

4.4.3. Use of Substituents, Selective Feeding, and

Poisoning . . . . . . . . . . . . . . . . . . . . . . . . 524

4.4.4. Spatially Resolved Analysis of the Fluid

Phase over a Catalyst. . . . . . . . . . . . . . . . 525

4.4.5. Spectroscopic Techniques . . . . . . . . . . . . 525

5. Design and Technical Operation of Solid

Catalysts . . . . . . . . . . . . . . . . . . . . . . . . 525

5.1. Design Criteria for Solid Catalysts . . . . 525

5.2. Catalytic Reactors . . . . . . . . . . . . . . . . 529

5.2.1. Classification of Reactors . . . . . . . . . . . . 529

5.2.2. Laboratory Reactors . . . . . . . . . . . . . . . . 529

5.2.3. Industrial Reactors . . . . . . . . . . . . . . . . . 530

5.2.4. Special Reactor Types and Processes . . . . 536

5.2.5. Simulation of Catalytic Reactors . . . . . . . 538

5.3. Catalyst Deactivation and Regeneration 538

5.3.1. Different Types of Deactivation . . . . . . . . 538

5.3.2. Catalyst Regeneration . . . . . . . . . . . . . . . 540

5.3.3. Catalyst Reworking and Disposal . . . . . . . 540

References . . . . . . . . . . . . . . . . . . . . . . . 541

1. Development of Solid Catalysts

The development of a catalytic process in-volves the search for the catalyst and theappropriate reactor, and typically occurs in asequence of steps at different levels. Figure 1

shows a scheme summarizing this evolutionaryprocess.

Small-scale reactors are used for screening todetermine the optimal catalyst formulation. Sincecatalyst development and sequential screening areslow and cost-intensive processes, high-through-

DOI: 10.1002/14356007.o05_o02

put experimentation (HTE) techniques [15–21]which permit parallel testing of small amounts ofcatalyst in automated systems have attracted greatinterest (see also ! Combinatorial Methods inCatalysis and Materials Science). Companies suchas Symyx Technologies, Santa Clara (www.symyx.com), hte, Heidelberg (www.hte-compa-ny.de), and Avantium Technologies, Amsterdam(www.avantium.nl) already offer HTE-baseddevelopment of catalysts or other materials. TheHTE-based search for catalysts usually starts witha first phase (stage I or discovery) in which largecatalyst libraries, often with several hundred ma-terials, are categorized into promising and lesspromising candidates by use of relatively simpleand fast analysis techniques. One example isinfrared thermography for detection of exothermicreactions with spatial resolution. To decrease thenumber of experiments, optimization methodsbased on genetic algorithms may be used to derivesubsequent catalyst generations from the perfor-mance of the members of the preceding generation[22]. In stage II, the more interesting materials,typically less than 50 candidates, are subjected totests under much more realistic process conditionswith more detailed characterization. For this pur-pose, a variety of parallel-reactor systems has beendeveloped. A crucial point in high-throughputexperimentation is the precise and fast analytical

quantification of reaction starting materials andproducts. Especially promising for obtaining fastand detailed on-line information during catalysttesting is high-throughput multiplexing gas chro-matography [21]. Instead of performing time-consuming chromatographic analyses duringparallelized catalyst testing one after the other,samples are rapidly injected into the separation bymeans of a special multiplexing injector. Theobtained chromatogram is a convolution of over-lapping time-shifted single chromatograms andmust therefore be mathematically deconvoluted.This new technique was successfully used for thestudy of palladium-catalyzed hydrogenation reac-tions [23].

High-throughput experimentation is a modernand accelerated version of classical catalyst de-velopment by trial and error. A famous earlyexample of this approach is the discovery of theiron-based ammonia synthesis catalyst, duringwhich 2500 catalysts were tested in 6500 experi-ments [1]. In recent years is has become evidentthat the empirical search for new or improvedcatalyst formulations can be successfully aidedby knowledge-based (expert) systems or molec-ular design [24–26]. State-of-the-art computa-tional tools for the effective molecular-scaledesign of catalytic materials are summarized in[27]. A striking example is the theoretical pre-diction of bimetallic ammonia synthesis catalysts[28]. As the rate-limiting step in heterogeneouslycatalyzed ammonia synthesis is the dissociativeadsorption of N2, an optimum strength of themetal – nitrogen interaction is required for highammonia synthesis activity. The resulting volca-no-shaped relationship shows, in agreement withexperimental evidence, that Ru and Os, followedby Fe, are the best pure metal catalysts (Figure 2).

First-principles DFT calculations were usedto predict that alloys of metals with high and lowadsorption energy should give rise to bindingenergies close to the optimum. Based on thesecalculations, a Co – Mo catalyst was developedthat has much higher ammonium synthesis ac-tivity than the individual metals and is even betterthan Ru and Fe at low ammonia concentrations.

2. Classification of Solid Catalysts

Solid catalysts are extremely important in large-scale processes [29–33] for the conversion of

Figure 1. Scheme for catalyst development and design (from[14], modified)

484 Heterogeneous Catalysis and Solid Catalysts, 2. Development and Types of Solid Catalysts Vol. 17

chemicals, fuels, and pollutants. Many solidmaterials (elements and compounds) includingmetals, metal oxides, and metal sulfides, arecatalysts. Only a few catalytic materials used inindustry are simple in composition, e.g., puremetals (e.g., Ni) or binary oxides (such as g-Al2O3, TiO2). Typical industrial catalysts, how-ever, consist of several components and phases.This complexity often makes it difficult to assessthe catalytic material’s structure.

In the following a variety of families of ex-isting catalysts are described, and selected ex-amples are given. These families include (1)unsupported (bulk) catalysts; (2) supported cat-alysts; (3) confined catalysts (ship-in-a-bottlecatalysts); (4) hybrid catalysts; (5) polymeriza-tion catalysts, and several others. The selectedexamples not only include materials which are inuse in industry, but also materials which are notyet mature for technological application butwhich have promising potential.

2.1. Unsupported (Bulk) Catalysts

2.1.1. Metal Oxides

Oxides are compounds of oxygen in which the Oatom is the more strongly electronegative com-ponent. Oxides of metals are usually solids. Theirbulk properties largely depend on the bondingcharacter between metal and oxygen. Metal oxi-des have widely varying electronic propertiesand include insulators (e.g., Al2O3, SiO2), semi-

conductors (e.g., TiO2, NiO, ZnO), metallic con-ductors (typically reduced transition metal oxi-des such as TiO, NbO, and tungsten bronzes),superconductors (e.g., BaPb1�xBixO3), and high-Tc superconductors (e.g., YBa2Cu3O7�x).

Metal oxides make up a large and importantclass of catalytically active materials, their sur-face properties and chemistry being determinedby their composition and structure, the bondingcharacter, and the coordination of surface atomsand hydroxyl groups in exposed terminatingcrystallographic faces. They can develop acid-base and redox properties. Metal oxides can havesimple composition, like binary oxides, but manytechnologically important oxide catalysts arecomplex multicomponent materials.

2.1.1.1. Simple Binary Oxides

Simple binary oxides of base metals may behaveas solid acids or bases or amphoteric materials[34]. These properties are closely related to theirdissolution behavior in contact with aqueoussolutions. Amphoteric oxides (e.g., Al2O3, ZnO)form cations in acidic and anions in basic milieu.Acidic oxides (e.g., SiO2) dissolve with forma-tion of acids or anions. Transition metal oxides intheir highest oxidation state (e.g., V2O5, CrO3)behave analogously. Basic oxides (e.g., MgO,lanthanide oxides) form hydroxides or dissolveby forming bases or cations. These dissolutionproperties must be considered when such oxidesare used as supports and impregnated from aque-ous solutions of the active phase precursor [35,

Figure 2. Calculated turnover frequencies (TOF) for ammonia synthesis as a function of the adsorption energy of nitrogen forvarious transition metals and alloys (reprinted with permission from [28]).

Vol. 17 Heterogeneous Catalysis and Solid Catalysts, 2. Development and Types of Solid Catalysts 485

36]. The dissolution properties also are closelyrelated to the surface properties of the oxides incontact with a gas phase, where the degree ofhydration/hydroxylation of the surface is a criti-cal parameter. Silica, alumina and magnesia arecommonly used catalysts and catalyst supportsrepresentative for a wide range of surface acid –base properties.

Aluminas are amphoteric oxides, whichform a variety of different phases depending onthe nature of the hydroxide or oxide hydroxideprecursor and the conditions of their thermaldecomposition. Bayerite, nordstrandite, boehm-ite, and gibbsite can be used as starting materials.The thermal evolution of the various poorlycrystalline transitional phases (namely h-, Q-,g-, c-, and k-Al2O3) and of the final crystalline,thermodynamically stable a-Al2O3 phase (co-rundum) is shown in Figure 3. The structures ofthese oxides can be described as close-packedlayers of oxo anions with Al3þ cations distributedbetween tetrahedral and octahedral vacancy po-sitions. Stacking variations of the oxo anionsresult in the different crystallographic forms ofalumina. The most commonly used transitionalphases are h- and g-Al2O3, which are oftendescribed as defect spinel structures [37] thatincorporate Al3þ cations in both tetrahedral andoctahedral sites. The Al sublattice is highly dis-ordered, and irregular occupation of the tetrahe-dral interstices results in a tetragonal distortion ofthe spinel structure. There is a higher occupancyof tetrahedral cation positions in g-Al2O3, and ahigher density of stacking faults in the oxygensublattice of h-Al2O3. Crystallites are preferen-tially terminated by anion layers, and these layersare occupied by hydroxyl groups for energeticreasons [38].

Acidic and basic sites and acid-base pair siteshave been identified on the surfaces of aluminas[40]. Thermal treatment of hydroxylated oxidesleads to partial dehydroxylation with formationof coordinatively unsaturated O2� ions (basicsites) and an adjacent anion vacancy which ex-poses 3- or 5-coordinate Al3þ cations (Lewis acidsites). The remaining hydroxyl groups can beterminal or doubly or triply bridging with theparticipation of Al3þ in tetrahedral and/or octa-hedral positions. The properties of the resultingOH species range from very weakly Brønstedacidic to rather strongly basic and nucleophilic[38, 40]. As a result of this complexity, aluminasurfaces develop a rich surface chemistry andspecific catalytic properties [41].

Besides their intrinsic catalytic properties andtheir use as catalysts in their own right (e.g., forelimination reactions, alkene isomerization [41],and the Claus process [42]), aluminas are fre-quently used as catalyst supports for oxides andmetals. The surface area and particle size ofaluminas can be controlled by the preparationconditions, and their redox and thermal stabilitygive the supported active phases high stabilityand ensure a long catalyst lifetime.

Silicas are weakly Brønsted acidic oxideswhich occur in a variety of structures such asquartz, tridymite and cristobalite (! Silica) [43,44]. The most commonly used silica in catalysisis amorphous silica. The building blocks of silicaare linked SiO4 tetrahedra, with each O atombridging two Si atoms. Bonding within the solidis covalent. At the fully hydrated surface, thebulk structure is terminated by hydroxyl (silanol)groups, SiOH [40, 43, 44]. Two types of thesegroups are usually distinguished: isolated groupsand hydrogen-bonded vicinal groups. Fully hy-drated samples, calcined at temperatures below473 K, may contain geminal groups Si(OH)2 [40,43, 44]. Heating in vacuum removes the vicinalgroups by dehydroxylation, i.e., condensation toform H2O and Si – O – Si linkages (siloxanebridges). Complete removal of the hydroxylgroups occurs at temperatures well above973 K in vacuo and is believed to result insignificant changes in surface morphology.

The surface hydroxyl groups are only weaklyBrønsted acidic and therefore hardly develop anycatalytic activity. They are, however, amenableto hydrogen-bonding [45] and they are usually

Figure 3. The dehydration sequences of the aluminum tri-hydroxides in air (adopted from [39])


regarded as the most reactive native surfacespecies, which are available for functionalizationof silicas. The siloxane bridges are (at least afterheating at elevated temperatures) essentially un-reactive. For this reason and because of the lowacidity of silanol groups, silicas are not used asactive catalysts, but they play an important role asoxide supports and for the synthesis of functio-nalized oxide supports (see Section 2.2.5).

Tailored silicas can be synthesized by con-trolling the preparation conditions [43, 44]. Thus,surface area, particle size and morphology, po-rosity and mechanical stability can be varied bymodification of the synthesis parameters.

In addition to amorphous silicas, the crystal-line microporous silica silicalite I can be obtainedby hydrothermal synthesis [46]. This material hasMFI structure and can be considered as the parentsiliceous extreme of zeolite ZSM-5.

Large-pore mesoporous structures, the so-called porosils, have also been reported[46–48]. The dimensions of their linear andparallel pores can be varied from 2 to 10 nm ina regular fashion. These pores can thereforeaccommodate bulky molecules and functionalgroups.

The incorporation of foreign elements such asAl3þ substituting for Si4þ induces Brønsted acid-ity and creates activity for acid catalysis.

MagnesiumOxide is a basic solid. It has thesimple rock salt structure, with octahedral coor-dination of magnesium and oxygen. Ab initiomolecular orbital calculations indicated that the

electronic structure is highly ionic, with theMg2þO2� formalism being an accurate represen-tation of both bulk and surface structures [49].The lattice is commonly envisaged to terminatein (100) planes incorporating five-coordinate(5c) Mg2þ and O2� ions [50] (see Figure 4). Thismodel appears to be physically accurate for MgOsmoke, which may be regarded as a model crys-talline metal oxide support [51]. Although the(100) plane is electrically neutral, hydroxylgroups are present on the surfaces of polycrys-talline MgO. These groups and the O2- anions areresponsible for the basic properties, coordina-tively unsaturated Mg2þ ions being only weakLewis acid sites. The hydroxyl groups are alsohighly nucleophilic.

These properties dominate the surface chem-istry of MgO. Organic Brønsted acids have beenshown to be chemisorbed dissociatively to formsurface-bound carbanions and surface hydroxylgroups [52]. Even the heterolytic dissociativeadsorption of dihydrogen on polycrystallineMgO has been reported. Mg2þO2� pairs withMg2þ and O2� in 4- or 3-coordination seem toplay a crucial role.

The presence of low-coordinate Mg2þ andO2� ions (see Figure 4) on the MgO surface afteractivation at high temperatures has been demon-strated [50, 53, 54], and the unique reactivity of3c centers has been discussed [55].

MgO has also been used as a host matrix fortransition metal ions (solid solutions) [56].These materials permit the properties of isolatedtransition metal ions to be studied.

Figure 4. Representation of a surface plane (100) of MgO showing surface imperfections such as steps, kinks, and corners wichprovide sites for ions of low coordination (adopted from [50]).


Transition Metal Oxides [57–59] can bestructurally described as more or less densepackings of oxide anions, the interstices ofwhich are occupied by cations. The bonding,however, is never purely ionic, but rather mixedionic-covalent, sometimes also developing me-tallic character (e.g., bronzes). The surface ofthese oxides is often partially occupied by hy-droxyl groups, so they possess some acidiccharacter. However, it is the variability in oxi-dation states and the possibility of formingmixed-valence and nonstoichiometric com-pounds that are responsible for their importantredox catalytic properties. The most frequentlyused transition metal oxides are those of theearly transition metals (mostly suboxides).Fields of application are particularly selectiveoxidation and dehydrogenation reactions.

Titania TiO2 exists in two major crystallo-graphic forms: anatase and rutile. Anatase is themore frequently used modification since it devel-ops a larger surface area, although it is a meta-stable phase and may undergo slow transforma-tion into the thermodynamically stable rutilephase above ca. 900 K. Vanadium impuritiesseem to accelerate the rutilization above820 K. Other impurities such as surface sulfateand phosphate seem to stabilize the anatasephase. The anatase ! rutile phase transitionmust be sensitively controlled for supportedVOx/TiO2, which plays a significant role in se-lective oxidation and NOx reduction catalysis.

Titania is a semiconductor with a wide bandgap and as such is an important material forphotocatalysis [60, 61].

Zirconia has attracted significant interest inthe recent past as a catalyst support and as a basematerial for the preparation of strong solid acidsby surface modification with sulfate or tungstategroups [62]. The most important crystallographicphases of ZrO2 for catalytic applications aretetragonal and monoclinic. The latter is thethermodynamically stable phase. Higher surfaceareas, however, are developed by the metastabletetragonal phase, which is stabilized at low tem-peratures by sulfate impurities or intentionaladdition of sulfate or tungstate.

ZrO2 is the base material for the solid-stateelectrolyte sensor for the measurement of oxygenpartial pressure in, e.g., car exhaust gases [63].

The solid electrolyte shows high bulk conductiv-ity for O2� ions.

Other Transition Metal Oxides are used assupported catalysts or as constituents of complexmulticomponent catalysts.

Only a few examples are reported on theapplication of the unsupported binary oxides ascatalysts. Iron oxide Fe2O3 and chromium oxideCr2O3 catalyze the oxidative dehydrogenation ofbutenes to butadiene. Fe2O3-based catalysts areused in the high-temperature water gas shiftreaction [64] and in the dehydrogenation ofethylbenzene [65]. Vanadium pentoxide V2O5

is active for the selective oxidation of alkenes tosaturated aldehydes [66]. Acidic transition metaloxides such as vanadium pentoxide and molyb-denum trioxide MoO3 can be used for the syn-thesis of formaldehyde by oxidative dehydroge-nation of methanol, while the more basic ironoxide Fe2O3 leads to total oxidation [67]. Zincoxide ZnO is used as a catalyst for the oxidationof cyclohexanol to cyclohexanone.

2.1.1.2. Complex Multicomponent Oxides

Complex multicomponent oxides play a majorrole as catalytic materials.

Aluminum Silicates are among the mostimportant ternary oxides. Four-valent Si atomsare isomorphously substituted by trivalent Alatoms in these materials. This substitution cre-ates a negatively charged framework of inter-connected tetrahedra. Exchangeable cations arerequired for charge compensation when protonsare incorporated as charge-compensating ca-tions, OH groups bridging Si and Al atoms arecreated which act as Brønsted acidic sites.

Amorphous Silica – Alumina can be pre-pared by precipitation from solution. This mixedoxide is a constituent of hydrocarbon crackingcatalysts.

Zeolites. Hydrothermal synthesis can beused for preparation of a large family of crystal-line aluminosilicates, known as zeolites (! Zeo-lites), which are microporous solids with poresizes ranging from ca. 3 to 7 A

�[46, 68, 69].

Characteristic properties of these structurallywell-defined solids are selective sorption of small


molecules (molecular sieves), ion exchange, andlarge surface areas. Zeolites possess a frameworkstructure of corner-linked SiO4

4� and AlO45�

tetrahedra with two-coordinate oxygen atomsthat bridge two tetrahedral centers (so-called Tatoms). Zeolite frameworks are open and containchannels (straight or sinusoidal) or cages ofspherical or other shapes. These cages are typi-cally interconnected by channels. The evolutionof several zeolite structures from the primarytetrahedra via secondary building blocks is dem-onstrated in Figure 5 [70]. The diameter of thechannels is determined by the number n of Tatoms surrounding the opening of the channels asn-membered rings. Small-pore zeolites contain6- or 8-membered rings (diameter d: 2.8< d < 4 A

�), medium-pore zeolites contain 10-

membered rings (5 < d < 6 A�) and the openings

of large-pore zeolites are constructed of 12-mem-bered rings (d > 7 A

�). Examples of small-pore

zeolites are sodalite and zeolite A, of medium-pore zeolites ZSM-type zeolites (see Figure 5),while large-pore zeolites include faujasites andzeolites X and Y (see Figure 5).

The H forms of zeolites develop strongBrønsted acidity and play a major role inlarge-scale industrial processes such as catalyticcracking, the Mobil MTG (methanol-to-gaso-line) process and several others.

Besides Si and Al as T atoms P atoms can alsobe incorporated in zeolite structures. In addition,transition metal atoms such as Ti, V, and Cr cansubstitute for Si, which leads to oxidation cata-lysts of which titanium-silicalite-1 (TS1) is themost outstanding catalyst for oxidation, hydrox-ylation, and ammoxidation with aqueous H2O2

[71].Basic properties can be created in zeolites by

ion-exchange with large alkali metal ions such asCsþ and additional loading with CsO [72].

AluminumPhosphates (AlPO) [73, 74] areanother family of materials whose structures aresimilar to those of zeolites. They can be regardedas zeolites in which the T atoms are Si and Al.More recently they have been named zeotypes,the T atoms of which are Al and P. In contrast toaluminosilicate zeolites, AlPOs typically have a

Figure 5. Structures of four representative zeolites and their micropore systems and dimensions [70]


Al/P atomic ratio of 1/1, so that the frameworkcomposition [AlPO4] is neutral. Therefore, thesesolids are nonacidic and have hardly any appli-cation as catalysts. However, acidity can beintroduced by substituting Al3þ by divalentatoms, which yields metal aluminophosphates(MAPOs), e.g., MnAPO or CoAPO, or by partialsubstitution of formally pentavalent P by Si4þ togive silicoaluminophosphates (SAPO). TheAlPO family contains members with many dif-ferent topologies which span a wider range ofpore diameters than aluminosilicate zeolites.

Mesoporous solid acids with well-definedpore structures can be obtained by replacing acertain amount of Si atoms in MCM-type oxidesby Al atoms.

Clays (! Clays) are aluminosilicate miner-als (montmorillonite, phyllosilicates (smectites),bentonites, and others). Montmorillonite is analuminohydroxysilicate and is the main constit-uent of most clay minerals. It is a 2 : 1 clay, i.e.,one octahedral AlO6 layer is sandwiched be-tween two tetrahedral SiO4 layers. Montmorillo-nites are reversibly swellable and possess ion-exchange capacity. They can be used as catalystsupports. The structural layers can be linkedtogether by introducing inorganic pillars whichprevent the layers from collapsing at highertemperatures when the swelling agent is evapo-rated (pillared clays) [75]. A bimodal micro-/mesoporous pore size distribution can thus beobtained. Pillaring can be achieved with a widevariety of reagents including hydroxy aluminumpolymers, zirconia hydroxy polymers, silica, andsilicate pillars. Catalytically active componentsmay be built in by the pillaring material, e.g.,transition metal oxide pillars.

Mixed Metal Oxides are multimetal multi-phase oxides which typically contain one or moretransition metal oxide and exhibit significant

chemical and structural complexity [76, 77].Their detailed characterization is therefore ex-tremely difficult, and structure-property relation-ships can only be established in exceptionalcases. Bulk mixed metal oxide catalysts arewidely applied in selective oxidation, oxydehy-drogenation, ammoxidation and other redox re-actions. Several examples of mixed metal oxidesand their application in industrial processes aresummarized in Table 1.

Vanadium Phosphates (e.g., VOHPO4

0.5 H2O) are precursors for the so-called VPOcatalysts, which catalyze ammoxidation reac-tions and the selective oxidation of n-butane tomaleic anhydride. It is proposed that the crystal-line vanadyl pyrophosphate phase (VO)2P2O7 isresponsible for the catalytic properties of theVPO system. The vanadium phosphate precursorundergoes transformations in reducing and oxi-dizing atmospheres, as shown in the followingscheme [78]:

As discussed by GRASSELLI [10] effectiveammoxidation (and oxidation) catalysts are mul-tifunctional and need several key properties,including active sites which are composed of atleast two vicinal oxide species of optimal metal –oxygen bond strengths. Both species must be

readily reducible and reoxidizable.The individual active sites must be spatially

isolated from each other (site-isolation concept) toachieve the desired product selectivities. Theyshould either be able to dissociate dioxygen andto incorporate the oxygen atoms into the lattice, orthey must be located close to auxiliary reoxidationsites which contain metals having a facile redox

Table 1. Examples of mixed metal oxide catalysts and their applications*

Catalyst Active phases Industrial processes

Copper chromite CuCr2O4, CuO low-temperature CO conversion, oxidations, hydrogenation

Zinc chromite ZnCr2O4, ZnO methanol synthesis (high pressure)

Copper/zinc chromite CuxZn1�xCr2O4, CuO methanol synthesis (low pressure)

Iron molybdate Fe(MoO4)3, MoO3 methanol to formaldehyde

Zinc ferrite ZnFe2O4 oxidative dehydrogenation

Chromia – alumina CrxAl2�xO3 dehydrogenation of light alkanes

* Adapted from [76]


couple. These sites are generally distinct from eachother. They must, however, be able to communi-cate with each other electronically and spatially sothat electrons, lattice oxygen, and anion vacanciescan readily move between them. The lattice mustbe able to tolerate a certain density of anionvacancies without structural collapse [10]. It isclear that these complex requirements can only beachieved by multicomponent materials.

GRASSELLI [10, 79] has listed three key cata-lytic functionalities required for effective am-moxidation/oxidation catalysts:

1. An a-H-abstracting component, which maybe Bi3þ, Sb3þ, Te4þ, or Se4þ.

2. A component that chemisorbs alkene/ammo-nia and inserts oxygen/nitrogen (Mo6þ,Sb5þ).

3. A redox couple such as Fe2þ/Fe3þ, Ce3þ/Ce4þ, or U5þ/U6þ to facilitate lattice oxygentransfer between bulk and surface of the solidcatalyst.

An empirical correlation was found betweenthe electron configurations of the various metalcations and their respective functionalities [10,

79] as shown in Table 2. This correlation can beused to design efficient catalysts.

Bismuth Molybdates are among the mostimportant catalysts for selective oxidation andammoxidation of hydrocarbons [78, 10]. Thephase diagram shown in Figure 6 demonstratesthe structural complexity of this class of ternaryoxides [80]. The catalytically most importantphases lie in the compositional range Bi/Mo atom-ic ratio between 2/3 and 2/1 and area-Bi2Mo3O12,b-Bi2Mo2O9, and g-Bi2MoO6. An industriallyused Bi molybdate catalyst was optimized in

Table 2.Electronic structure of some catalytically active elements and

their functionalities [79]

a-H abstraction Alkene

chemisorption/O

insertion

Redox

couple

Example

Bi3þ 5d106s26p0 Mo6þ 4d05s0 Ce3þ/Ce4þ Bi2O3 � nMoO3

Fe2þ/Fe3þ M2þa M3þ

b BixMovOz

Te4þ 4d105s25p0 Mo6þ 4d05s0 Ce3þ/Ce4þ Te2MoO7

(TeaCebMov)Oz

Sb3þ 4d105s25p0 Sb5þ 5d05s0 Fe2þ/Fe3þ FexSbyOz

U5þ 5f16d07s0 Sb5þ 5s05p0 U5þ/U6þ USb3O10

Se4þ 3d104s24p0 Te6þ 5s05p0 Fe2þ/Fe3þ FeaSe4þb Te6þc Ox

Figure 6. Phase compositions2/3: Bi2O3 � 3 MoO3; 1/1: Bi2O3 � 2 MoO3; 2/1 (K): Bi2O3 � MoO3 (koechlinite); 2/1 (H): Bi2O3 � MoO3 (high-temperatureform); 3/1 (L): 3 Bi2O3 � 2 MoO3 (low-temperature form); 3/1 (H): 3 Bi2O3 � 2 MoO3 (high-temperature form) [11]


several steps and has the empirical formula(K,Cs)a(Ni,Co,Mn)9.5(Fe,Cr)2.5BiMo12Ox [10].This material is supported on 50 % SiO2 and wassubsequently optimized further to give a catalystwith the empirical formula (K,Cs)a(Ni,Mg,Mn)7.5(Fe,Cr)2.3Bi0.5Mo12Ox.

Antimonites are a second important class ofammoxidation catalysts [10], the most importantof which are those containing at least one of theelements U, Fe, Sn, Mn, or Ce, which all havemultiple oxidation states. Many formulations ofcatalysts have been proposed over the years.Those of current commercial interest have ex-tremely complex compositions, e.g., Na0–3(Cu,Mg,Zn,Ni)0–4(V,W)0.05–1Mo0.1–2.5Te0.2–

5Fe10Sb13–20Ox [10, 81].

Scheelites. Numerous multicomponent oxi-des adopting the scheelite (CaWO4) structurewith the general formula ABO4 are known[82]. This structure tolerates cation replacementsirrespective of valency provided that A is a largercation than B and that there is charge balance. Anadditional property of the scheelite structure isthat it is often stable with 30 % or more vacanciesin the A cation sublattice. As an example,Pb2þ1�3xBi

3þ2x &xMo6þO2�

4 , where f indicates a va-cancy in the Bi3þ (A cation) sublattice, possessesscheelite structure. The materials are active forselective oxidation of C3 and C4 alkenes, whichinvolves formation of allyl species followed byextraction of O2� from the lattice. Replenish-ment of the created vacancies occurs by oxygenchemisorption at other sites and diffusion of O2�

ions within the solid. The introduction of Acation vacancies has a significant effect on allylformation, and the more open structure whichprevails when cation vacancies are present facil-itates O2� transport.

Perovskite is a mineral (CaTiO3) which isthe parent solid for a whole family of multicom-ponent oxides with the general formula ABO3

[57, 83]. The common feature, which also re-sembles that of the scheelite-type oxides, is thesimultaneous presence of a small, often highlycharged, B cation and a large cation A, oftenhaving a low charge. The structure also toleratesa wide variety of compositions. As an example,La3þ1�xSr

2þx Y3þO2�

3�1=2x is active for methane cou-pling. Other applications of perovskite-type oxi-

des in catalysis are in fuel cells, as catalysts forcombustion and for DeNOx reactions.

Hydrotalcites are another family of solidswhich tolerate rather flexible compositions [76,84, 85]. Hydrotalcite is a clay mineral. It is ahydroxycarbonate of Mg and Al of general for-mula [Mg6Al2(OH)16]CO3 � 4H2O. The compo-sitional flexibility of the hydrotalcite lattice per-mits the incorporation of many different metalcations and anions to yield solids with the generalformula ½M2þ

1�xM3þx ðOHÞ�xþðAn�Þx=n�nH2O (M2þ

¼ Mg2þ; Ni2þ; Zn2þ ect.; M3þ ¼ Al3þ; Fe3þ;Cr3þ, etc.; An� ¼ CO2�

3 ; SO2�4 ; NO�3 , etc.). Hy-

drotalcites develop large surface areas and basicproperties. They have consequently been appliedas solid catalysts for base-catalyzed reactionsfor fine-chemicals synthesis, polymerization ofalkene oxides, aldol condensation, etc. Hydrotal-cite-type phases (and also malachite (rosasite)-and copper zinc hydroxycarbonate (aurichalcite)-type phases) can also be used as precursors for thesynthesis of mixed oxides by thermal decompo-sition, for example, Cu – Zn and Cu – Zn – Crcatalysts [76].

Heteropolyanions are polymeric oxo an-ions (polyoxometalates) formed by condensationof more than two kinds of oxo anions [86, 87].The amphoteric metals of Groups 5 (V, Nb, Ta)and 6 (Cr, Mo, W) in the þ5 and þ6 oxidationstates, respectively, form weak acids which read-ily condense to form anions containing severalmolecules of the acid anhydride. Isopolyacidsand their salts contain only one type of acidanhydride. Condensation can also occur withother acids (e.g., phosphoric or silicic) to formheteropolyacids and salts. About 70 elements canact as central heteroatoms in heteropolyanions.The structures of heteropolyanions are classifiedinto several families according to similarities ofcomposition and structure, such as Keggin typeXM12O40

n�, Dawson type X2M18O62n�, and

Anderson type XM6O24n�, where X stands for

the heteroatom. The most common structuralfeature is the Keggin anion, for which the cata-lytic properties have been studied extensively.Typically the M atoms in catalytic applicationsare either Mo or W. Heteropoly compounds canbe applied as heterogeneous catalysts in theirsolid state. Their catalytic performance is deter-mined by the primary structure (polyanion), the


secondary structure (three-dimensional arrange-ment of polyanions, counter cations, and water ofcrystallization, etc.), and the tertiary structure(particle size, pore structure, etc.) [88, 89]. Incontrast to conventional heterogeneous catalysts,on which reactions occur at the surface, thereactants are accommodated in the bulk of thesecondary structure of heteropoly compounds.Certain heteropolyacids are flexible, and polarmolecules are easily absorbed in interstitial posi-tions of the bulk solid, where they form a pseu-doliquid phase [88, 89].

Heteropoly compounds develop acidic andoxidizing functions, so that they can be used foracid and redox catalysis. In addition, polyanionsare well-defined oxide clusters. Catalyst design istherefore possible at the molecular level. Thepseudoliquid provides a unique reactionenvironment.

Some solid heteropolyacids have high thermalstability and are therefore suitable for vapor-phase reactions at elevated temperatures. Thethermal stability of several heteropolyacids de-creases in the sequence H3PW12O40 > H4Si-W12O40 > H3PMo12O40 > H4SiMo12O40 [88,89]. It can be enhanced by formation of theappropriate salts [90, 91].

Because of their multifunctionality, hetero-polyacids catalyze a wide variety of reactionsincluding hydration and dehydration, condensa-tion, reduction, oxidation, and carbonylationchemistry with Keggin-type anions of V, Mo[89, 90, 92–94]. A commercially important pro-cess, the oxidation of methacrolein, is catalyzedby a Cs salt of H4PVMo11O40. Heteropoly saltswith extremely complex compositions have beenproposed, e.g., for the oxydehydrogenation ofethane. A Keggin-type molybdophosphoric saltwith formula K2P1.2MO10W1Sb1Fe1Cr0.5-

Ce0.75On was found to be the most efficientamong the tested solids in terms of activity,selectivity, and stability [95].

2.1.2. Metals and Metal Alloys

Metals and metal alloys are used as bulk, unsup-ported catalysts in only a few cases.Metal gauzesor grids are used as bulk catalysts in stronglyexothermic reactions which require catalyst bedsof small height. Typical examples are platinum –rhodium grids used for ammonia oxidation in

the nitric acid process [96] and silver grids for thedehydrogenation of methane to formaldehyde.

Skeletal (Raney-type) catalysts, particularlyskeletal nickel catalysts, are technologically im-portant materials [97] which are specificallyapplied in hydrogenation reactions. However,their application is limited to liquid-phase reac-tions. They are used in particular for the produc-tion of fine chemicals and pharmaceuticals.Skeletal catalysts are prepared by the selectiveremoval of aluminum from Ni – Al alloy parti-cles by leaching with aqueous sodium hydroxide[97]. Besides skeletal Ni, cobalt, copper, plati-num, ruthenium, and palladium catalysts havebeen prepared, with surface areas between 30 and100 m2 g�1. One of the advantages of skeletalmetal catalysts is that they can be stored in theform of the active metal and therefore require nopre-reduction prior to use, unlike conventionalcatalysts, the precursors of which are oxides ofthe active metal supported on a carrier.

Fused catalysts are particularly used as alloycatalysts. The synthesis from a homogeneousmelt by rapid cooling may yield metastablematerials with compositions that can otherwisenot be achieved [98]. Amorphous metal alloyshave also been prepared (metallic glasses) [98,99].

Oxide materials can also be fused for catalyticapplications [98]. Such oxides exhibit a complexand reactive internal interface structure whichmay be useful either for direct catalytic applica-tion in oxidation reactions or in predeterminingthe micromorphology of resulting catalytic ma-terials when the oxide is the catalyst precursor.The prototype of such a catalyst is the multiplypromoted iron oxide precursor of catalysts usedfor ammonia synthesis [6, 100].

2.1.3. Carbides and Nitrides [6, 101]

Monometallic carbides and nitrides of early tran-sition metals often adopt simple crystal structuresin which the metal atoms are arranged in cubicclose-packed (ccp), hexagonal close-packed(hcp), or simple hexagonal (hex) arrays. C andN atoms occupy interstitial positions betweenmetal atoms (interstitial alloys). The materialshave unique properties in terms of meltingpoint (> 3300 K), hardness (> 2000 kg mm�2),and strength (> 3 � 105 MPa). Their physical


properties resemble those of ceramic materials,although their electronic and magnetic propertiesare typical of metals. Carbon in the carbidesdonates electrons to the d band of the metal, thusmaking the electronic characteristics of, e.g.,tungsten and molybdenum resemble more close-ly those of the platinum group metals.

Bulk carbides and nitrides, e.g., of tungstenand molybdenum, can be prepared with surfaceareas between 100 and 400 m2 g�1 by advancedsynthetic procedures [101], so that they can beapplied as bulk catalysts. They catalyze a varietyof reactions for which noble metals are stillpreferentially used. Carbides and nitrides areexceptionally good hydrogenation catalysts, andthey are active in hydrazine decomposition. Car-bides of tungsten and molybdenum are alsohighly active for methane reforming, Fischer –Tropsch synthesis of hydrocarbons and alcohols,and hydrodesulfurization, and the nitrides areactive for ammonia synthesis and hydrodenitro-genation [100]. The catalytic properties of car-bides can be fine tuned by treatment with oxygen,which leads to the formation of oxycarbides[102]. While clean molybdenum carbide is anexcellent catalyst for C – N bond cleavage(cracking of hydrocarbons), molybdenum oxidecarbide is selective for skeletal isomerization[102].

In conclusion, carbides and nitrides, especial-ly those of tungsten and molybdenum, may wellbe considered as future substitutes for platinumand other metals of Groups 8 – 10 as catalysts.

2.1.4. Carbons [103]

Although carbons are frequently used as catalystsupports, they may also be used as catalysts intheir own right [104, 105]. Carbons exist in avariety of thermodynamic phases (allotropes ofcarbon) and metastable structures, which areoften ill defined (see also! Carbon, 1. General).The surface chemistry of carbons is rather com-plex [40, 103]. Carbon surfaces may contain avariety of functional groups, particularly thosecontaining oxygen, depending on the prove-nience and pretreatment of the carbon. At a singleadsorption site several chemically inequivalenttypes of heteroatom bonds may form. Stronginteractions between surface functional groupsfurther complicate the variety of surface chemi-

cal structures derived for the most importantcarbon – oxygen system. Two functions of thecarbon surface act simultaneously during a cata-lytic reaction. Firstly, the reactants are chemi-sorbed selectively on the carbon surface by ionexchange via oxygen functional groups or direct-ly by dispersion forces involving the graphitevalence-electron system. The second function isthe production of atomic oxygen occurring on thegraphene basal faces of all sp2 carbon materials[103].

Carbon can already be catalytically activeunder ambient conditions and in aqueous media.Therefore efforts have been made to applycarbons as catalysts in condensed phases. Itsapplication in the gas phase under oxidizingconditions is severely limited by its tendencyto irreversible oxidation.

Catalytic applications of carbons include theoxidation of sulfurous to sulfuric acid, the selec-tive oxidation of hydrogen sulfide to sulfur withoxygen in the gas phase at ca. 400 K, the reactionbetween phosgene and formaldehyde, and theselective oxidation of creatinine by air in physi-ological environments.

A potential technological application of car-bon catalysts involves the catalytic removal ofNO by carbon [103].

More recently, carbon nanotubes (CNT) andnanofibers (CNF) have found significant interestas catalysts and catalyst supports [103, 106].These materials, especially nanotubes, exhibitinteresting electronic, mechanical and thermalproperties that are clearly different from those ofactivated carbons. High mechanical strength andresistance to abrasion in combination with highaccessibility of active sites are advantages ofCNT-based catalysts which make them veryattractive for liquid-phase reactions, where themicroporosity of activated carbons often limitsthe catalytic performance. Due to their highelectrical conductivity and oxidation stability,CNTs are also highly interesting carrier materialsfor proton-exchange membrane fuel cell(PEMFC) and direct methanol fuel cell (DMFC)catalysts [107].

2.1.5. Ion-Exchange Resins and Ionomers

Ion-exchange resins (! Ion Exchangers) arestrongly acidic organic polymers which are pro-


duced by suspension copolymerization of styrenewith divinylbenzene and subsequent sulfonationof the cross-linked polymer matrix [108]. Thismatrix is insoluble in water and organic solvents.Suspension polymerization yields sphericalbeads which have different diameters in the range0.3 – 1.25 mm. The Gaussian size distributionof the beads can be influenced by the polymeri-zation parameters.

A network of micropores is produced duringthe copolymerization reaction. The pore size isinversely proportional to the amount of cross-linking agent. In the presence of inert solventssuch as isoalkanes during the polymerization,which dissolve the reactive monomers andprecipitate the resulting polymers, beads withan open spongelike structure and freely acces-sible inner surface are obtained. The matrix isthen a conglomerate of microspheres which areinterconnected by cavities or macropores.Macroporous resins are characterized by mi-cropores of 0.5 – 2 nm and macropores of20 – 60 nm, depending on the degree ofcross-linking.

Strongly acidic polymeric resins are thermallystable at temperatures below 390 – 400 K.Above 400 K, sulfonic acid groups are split offand a decrease in catalytic activity results.

Industrially, acidic resins are used in theproduction of methyl tert-butyl ether [109].

The ionomer Nafion is a perfluorinated poly-mer containing pendant sulfonic acid groupswhich is considered to develop superacidic prop-erties. It can be used as a solid acid catalyst forreactions such as alkylation, isomerization, andacylation [110].

2.1.6. Molecularly Imprinted Catalysts[111]

Molecular imprinting permits heterogeneous su-pramolecular catalysis to be performed on sur-faces of organic or inorganic materials withsubstrate recognition. Heterogeneous catalystswith substrate specificity based on molecularrecognition require a material having a shape-and size-selective footprint on the surface or inthe bulk. The stabilization of transition states byimprinting their features into cavities or adsorp-tion sites by using stable transition-state analo-gues as templates is of particular interest.

Imprinted materials can be prepared on thebasis of Al3þ-doped silica gel [112] and of cross-linked polymers [113, 114]. Chiral molecularfootprint cavities have also been designed andimprinted on the surface of Al3þ-doped silica gelby using chiral template molecules.

When transition-state or reaction-intermedi-ate analogues are used as templates for molecularimprinting, specific adsorption sites are created.Such molecular footprints on silica gel consist ofa Lewis site and structures complementary to thetemplate molecules. These structures can stabi-lize a reacting species in the transition state andlower the activation energy of the reaction, thusmimicking active sites of natural enzymes andcatalytic antibodies.

Although this approach seems to have a highpotential for heterogeneous catalysis, the realapplication of imprinted materials as catalystsstill remains to be demonstrated.

2.1.7. Metal – Organic Frameworks [115,116]

Metal – organic frameworks (MOFs) are highlyporous, crystalline solids consisting of a three-dimensional network of metal ions attached tomultidentate organic molecules. Similar to zeo-lites, the spatial organization of the structuralunits gives rise to a system of channels andcavities on the nanometer length scale. A mile-stone for the development of MOFs was thesynthesis of MOF-5 in 1999 [117]. This materialconsists of tetrahedral Zn4O6þ clusters linked byterephthalate groups and has a specific surfacearea of 2900 m2 g�1. MOF-177 has an evenlarger specific surface area of 4500 m2 g�1

[118]. By selection of the linker length, the sizeof the resulting pores can be tailored.

Due to their extremely high surface areas andtheir tunable pore structure with respect to size,shape, and function, MOFs are highly interestingmaterials for various applications. Examples arethe adsorption of gases such as hydrogen ormethane targeted at the replacement of com-pressed-gas storage, removal of impurities innatural gas, pressure-swing separation of noblegases (krypton, xenon), and use as catalysts[119]. Despite their higher metal content com-pared to zeolites, the use of MOFs in heteroge-neous catalysis is restricted due to their relatively


low stability at elevated temperature and in thepresence of water vapor or chemical reagents. Inaddition, the metal ions in MOFs are oftenblocked by the organic linker molecules and aretherefore not accessible for catalytic reactions.However, successful applications of especiallystable Pd MOFs in alcohol oxidation, Suzuki C–C coupling and olefin hydrogenation have beenreported [120]. It can be expected that the numberof successful catalytic studies using MOFs willgrow considerably.

2.1.8. Metal Salts

Although salts can be environmentally harmful,they are still used as catalysts in some techno-logically important processes. FeCl3 – CuCl2 isa catalyst for chlorobenzene production, andAlCl3 is still used for ethylbenzene synthesis andn-butane isomerization.

2.2. Supported Catalysts

Supported catalysts play a significant role inmany industrial processes. The support provideshigh surface area and stabilizes the dispersion ofthe active component (e.g., metals supported onoxides). Active phase – support interactions,which are dictated by the surface chemistry ofthe support for a given active phase, are respon-sible for the dispersion and the chemical state ofthe latter. Although supports are often consideredto be inert, this is not generally the case. Supportsmay actively interfere with the catalytic process.Typical examples for the active interplay be-tween support and active phase are bifunctionalcatalysts such as highly dispersed noble metalssupported on the surface of an acidic carrier.

To achieve the high surface areas and stabilizethe highly disperse active phase, supports aretypically porous materials having high thermo-stability. For application in industrial processesthey must also be stable towards the feed and theymust have a sufficient mechanical strength.

2.2.1. Supports

Many of the bulk materials described in Sec-tion 2.1 may also function as supports. The mostfrequently used supports are binary oxides in-cluding transitional aluminas, a-Al2O3, SiO2,MCM-41, TiO2 (anatase), ZrO2 (tetragonal),MgO etc., and ternary oxides including amor-phous SiO2 – Al2O3 and zeolites. Additionalpotential catalyst supports are aluminopho-sphates, mullite, kieselguhr, bauxite, and calci-um aluminate. Carbons in various forms (char-coal, activated carbon) can be applied as supportsunless oxygen is required in the feed at hightemperatures. Table 3 summarizes importantproperties of typical oxide and carbon supports.

Silicon carbide, SiC, can also be used as acatalyst support with high thermal stability andmechanical strength [121]. SiC can be preparedwith porous structure and high surface area bybiotemplating [122]. This procedure yields ce-ramic composite materials with biomorphic mi-crostructures. Biological carbon preforms arederived from different wood structures byhigh-temperature pyrolysis (1100 – 2100 K)and used as templates for infiltration with gas-eous or liquid Si to form SiC and SiSiC ceramics.During high-temperature processing the micro-structural details of the bioorganic preforms areretained, and cellular ceramic composites withunidirectional porous morphologies and aniso-tropic mechanical properties can be obtained.

Table 3. Properties of typical catalyst supports

Support Crystallographic phases Properties/applications

Al2O3 mostly a and g SA up to 400; thermally stable three-way cat., steam reforming and many other cats.

SiO2 amorphous SA up to 1000; thermally stable; hydrogenation and other cats.

Carbon amorphous SA up to 1000; unstable in oxid. environm., hydrogenation cats.

TiO2 anatase, rutile SA up to 150; limited thermal stability; SCR cats.

MgO fcc SA up to 200; rehydration may be problematic; steam reforming cat.

Zeolites various (faujasites, ZSM-5) Highly defined pore system; shape selective; bifunctional cats.

Silica/alumina amorphous SA up to 800; medium strong acid sites; dehydrogenation cats.; bifunctional catalysts.

SA ¼ surface area in m2/g


These materials show low density, high specificstrength, and excellent high-temperature stabili-ty. Although they have not yet found applicationin catalysis, the low-weight materials may wellbe advantageous supports for high-temperaturecatalysis processes.

Monolithic supports with unidirectionalmacrochannels are used in automotive emissioncontrol catalysts (! Automobile Exhaust Con-trol) where the pressure drop has to be minimized[123]. The channel walls are nonporous or maycontain macropores. For the above applicationthe monoliths must have high mechanicalstrength and low thermal expansion coefficientsto give sufficient thermal shock resistance.Therefore, the preferred materials of monolithstructures are ceramics (cordierite) or high-qual-ity corrosion-resistant steel. Cordierite is a natu-ral aluminosilicate (2 MgO � 2 Al2O3 � 5 SiO2).The accessible surface area of these materialscorresponds closely to the geometric surface areaof the channels. High surface area is created bydepositing a layer of a mixture of up to 20different inorganic oxides, which include transi-tional aluminas as a common constituent. Thisso-called washcoat develops internal surfaceareas of 50 to 300 m2/g [124, 125].

Silica, MCM-41, and polymers can be func-tionalized for application as supports for thepreparation of immobilized or hybrid catalysts[40, 43, 126–132]. The functional groups mayserve as anchoring sites (surface bound ligands)for complexes and organometallic compounds.Chiral groups can be introduced for the prepara-tion of enantioselective catalysts (seeSection 2.2.6).

2.2.2. Supported Metal Oxide Catalysts

Supported metal oxide catalysts consist of atleast one active metal oxide component dis-persed on the surface of an oxide support [9,133, 134]. The active oxides are often transitionmetal oxides, while the support oxides typicallyinclude transitional aluminas (preferentially g-Al2O3), SiO2, TiO2 (anatase), ZrO2 (tetragonal),and carbons.

Supported vanadia catalysts are extremelyversatile oxidation catalysts. V2O5/TiO2 is usedfor the selective oxidation of o-xylene to phthalicanhydride [135, 136] and for the ammoxidation

of alkyl aromatics to aromatic nitriles [136]. Thelatter reaction is also catalyzed by V2O5/Al2O3

[136]. The selective catalytic reduction (SCR)of NOx emissions with NH3 in tail gas fromstationary power plants is a major application ofV2O5 – MoO3 – TiO2 and V2O5 – WO3 – TiO2

[137, 138]. MoO3 – Al2O3 and WO3 – Al2O3

(promoted by oxides of cobalt or nickel) arethe oxide precursors for sulfided catalysts (seeSection 2.2.5) for hydrotreating of petroleum(hydrodesulfurization, hydrodenitrogenation,hydrocracking) [7, 139, 140]. WO3 – ZrO2 de-velops acidic and redox properties [141, 142].When promoted with Fe2O3 and Pt it can beapplied as a highly selective catalyst for thelow-temperature isomerization of n-alkanes toisoalkanes [143]. Re2O7 – Al2O3 is an efficientmetathesis catalyst [144]. Cr2O3 – Al2O3 andCr2O3 – ZrO2 are catalysts for alkane dehydro-genation and for dehydrocyclization of, e.g.,n-heptane to toluene [145].

The above-mentioned transition metal oxideshave lower surface free energies than the typicalsupport materials [9, 146]. Therefore, they tendto spread out on the support surfaces and formhighly dispersed active oxide overlayers. Thesesupported oxide catalysts are thus frequentlycalled monolayer catalysts, although the supportsurface is usually not completely covered, even atloadings equal to or exceeding the theoreticalmonolayer coverage. This is because most of theactive transition metal oxides (particularly thoseof V, Mo, and W) form three-dimensional islandson the support surface which have structuresanalogous to molecular polyoxo compounds[9, 133].

2.2.3. Surface-Modified Oxides

The surface properties, that is acidity and basici-ty, of oxides can be significantly altered bydeposition of modifiers. The acid strength ofaluminas is strongly enhanced by incorporationof Cl� into or on the surface. This may occurduring impregnation with solutions containingchloride precursors of an active component [35]or by deposition of AlCl3. Chlorinated aluminasare also obtained by surface reaction with CCl4[40]. The presence of chlorine plays an importantrole in catalytic reforming with Pt – Al2O3 cat-alysts [8].


Strongly basic materials are obtained by sup-porting alkali metal compounds on the surface ofalumina [147]. Possible catalysts include KNO3,KHCO3, K2CO3, and the hydroxides of the alkalimetals supported on alumina.

Sulfation of several oxides, particularly te-tragonal ZrO2, yields strong solid acids, whichwere originally considered to develop superaci-dic properties [148–150], because, like tung-stated ZrO2 (see Section 2.2.2), they also cata-lyze the isomerization of n-alkanes to isoalkanesat low temperature.

2.2.4. Supported Metal Catalysts

Metals typically have high surface free energies[146] and therefore a pronounced tendency toreduce their surface areas by particle growth.Therefore, for applications as catalysts they aregenerally dispersed on high surface area supports,preferentially oxides such as transitional aluminas,with the aim of stabilizing small, nanosized par-ticles under reaction conditions [35, 151]. Thiscan be achieved by some kind of interactionbetween a metal nanoparticle and the supportsurface (metal – support interaction:), which mayinfluence the electronic properties of the particlesrelative to the bulk metal. This becomes particu-larly significant for raftlike particles of monatomicthickness, for which all atoms are surface atoms.Furthermore, the small particles expose increasingnumbers of low-coordinate surface metal atoms.Both electronic and geometric effects may influ-ence the catalytic performance of a supportedmetal catalyst (particle-size effect). Aggregationof the nanoparticles leads to deactivation.

Model supported metal catalysts having uni-form particle size and structure can be preparedby anchoring molecular carbonyl clusters onsupport surfaces, followed by decarbonylation[152, 153]. Examples are Ir4 and Ir6 clusters onMgO and in zeolite cages.

Bimetallic supported catalysts contain two dif-ferent metals, which may either be miscible orimmiscible as macroscopic bulk alloys. The com-bination of an active and an inactive metal [e.g., Niand Cu (miscible) or Os and Cu (immiscible)]dilutes the active metal on the particle surface.Therefore, the catalytic performance of reactionsrequiring ensembles of several active metal atomsrather than single isolated atoms is influenced [154,

155]. Selectivities of catalytic processes can thusbe optimized. Typically, the surface compositionof binary alloys differs from that of the bulk. Thecomponent having the lower surface free energy isenriched in the surface layer. For example, Cu islargely enriched at the surface of Cu – Ni alloys,even at the lowest concentration. Also, surfacecompositions of binary alloys may be altered bythe reaction atmosphere.

In industrial application, supported metal cat-alysts are generally used as macroscopic spheresor cylindrical extrudates. By special impregna-tion procedures, metal concentration profileswithin the pellet can be created in a controlledway. Examples are schematically shown in Fig-ure 7 [35]. The choice of the appropriate con-centration profile may be crucial for the selectiv-ity of a process because of the interplay betweentransport and reaction in the porous mass of thepellet. For example for the selective hydrogena-tion of ethyne impurities to ethene in a feed ofethene, eggshell profiles are preferred.

Applications of supported metal catalysts,such as noble (Pt, Pd, Rh) or non-noble (Ru, Ni,Fe, Co) metals supported on Al2O3, SiO2, oractive carbon include hydrogenation and dehy-drogenation reactions. Ag on Al2O3 is used forethene epoxidation. Supported Au catalysts areactive for low-temperature CO oxidation. Multi-metal catalysts Pt – Rh – Pd on Al2O3 modifiedby CeO2 as oxygen storage component are usedon a large scale in three-way car exhaust catalysts[125]. Pt supported on chlorinated Al2O3 is thebifunctional catalyst used for catalytic reform-ing, isomerization of petroleum fractions, etc.

Modification of supported Pt catalysts bycinchona additives yields catalysts for the enan-tioselective hydrogenation ofa-ketoesters [156].

2.2.5. Supported Sulfide Catalysts

Sulfided catalysts of Mo and W supported on g-Al2O3 or active carbons are obtained by sulfida-

Figure 7. The four main categories of macroscopic distribu-tion of a metal within a support


tion of oxide precursors (supported MoO3 orWO3; see Section 2.2.2) in a stream of H2/H2S. They are typically promoted with Co or Niand serve (in large tonnage) for hydrotreatingprocesses of crude oil, including hydrodesulfur-ization (HDS) [7, 139, 140], hydrodenitrogena-tion HDN [140], and hydrodemetalation HDM[157]. Currently, the CoMoS and NiMoS modelsare most accepted for describing the active phase.These phases consist of a single MoS2 layer orstacks of MoS2 layers in which the promoteratoms are coordinated to edges [7, 140], as shownin Figure 8. This figure also indicates that Comay simultaneously be present as Co9S8 and as asolid solution in the Al2O3 support matrix. It isinferred that the catalytic activity of the MoS2

layers is related to the creation of sulfur vacan-cies at the edges of MoS2 platelets. These vacan-cies have recently been visualized on MoS2

crystallites by scanning tunneling microscopy(STM) [158].

2.2.6. Hybrid Catalysts [126, 128, 129, 131,132]

Hybrid catalysts combine homogeneous and het-erogeneous catalytic transformations. The goalof the approach is to combine the positive aspectsof homogeneous catalysts or enzymes in terms ofactivity, selectivity, and variability of steric andelectronic properties by, e.g., the appropriatechoice of ligands (including chiral ligands[159]) with the advantages of heterogeneouscatalysts such as ease of separation and recoveryof the catalyst. This can be achieved by immobi-lization (heterogenization) of active metal com-plexes, organometallic compounds, or enzymeson a solid support.

There are several routes for the synthesis ofimmobilized homogeneous catalysts:

1. Anchoring the catalytically active species viacovalent bonds on the surface of suitableinorganic or organic supports such as SiO2,mesoporous MCM-41, zeolites, polystyrenes,and styrene – divinylbenzene copolymers[126–128, 132]. The polymerization or copo-lymerization of suitably functionalized mo-nomeric metal complexes is also known.

2. Chemical fixation by ionic bonding using ionexchange.

3. Deposition of active species on surfaces ofporous materials by chemi- or physisorption,or chemical vapor deposition (CVD). The‘‘ship-in-bottle’’ principle belongs to this syn-thetic route, but is treated separately inSection 2.2.7.

4. Molecularly defined surface organometallicchemistry may also yield immobilized activeorganometallic species.

The reagents for covalent bonding on sili-ceous materials (SiO2, MCM-41) are often alk-oxy- or chlorosilanes which are anchored to thesurface by condensation reactions with surfacehydroxyl groups [126–128, 132]. Functionalgroups thus created on the surface can includephosphines, amines etc., which serve as an-chored ligands for active species that undergoligand-exchange reactions. Careful control ofthe density of functional groups leads to spatialseparation of active complexes (site isolation)and thus helps to avoid undesired side reactions[160].

Immobilized enzymes (! Biocatalysis, 2. Im-mobilized Biocatalysts) are frequently used inbiocatalysis and in organic synthesis. The syn-thesis and catalytic performance of this class ofheterogenized materials is discussed in severalreview articles [132, 161].

Dendrimers [162] which are functionalized atthe ends of the dendritic arms can be used forimmobilization of metal complexes. A catalyticeffect is thus generated at the periphery of thedendrimer. Dendrimers with core functionalitieshave also been synthesized. The resemblance ofthe produced structures to prosthetic groups inenzymes led to the introduction of the worddendrizymes [163]. Dendrimers have found ap-plication, e.g., in membrane reactors.

Figure 8. Three forms of Co present in sulfided Co – Mo/Al2O3 catalysts: as sites on the MoS2 edges (the so-calledCo – Mo – S phase), as segregated Co9S8, and as Co2þ ionsin the support lattice


Immobilized homogeneous catalysts are usedfor selective oxidation reactions, for hydrogena-tion, and for C – C coupling reactions. Theyhave proved very efficient in asymmetric synthe-sis [128, 131, 164].

Special processes with immobilized catalystsare supported (solid) liquid-phase catalysis(SLPC) [165] and supported (solid) aqueous-phase catalysis (SAPC) [166]. In SLPC a solu-tion of the homogeneous catalyst in a high-boiling solvent is introduced into the porevolume of a porous support by capillary forces,and the reactants pass the catalyst in the gasphase. For example, the active phase – a mixtureof vanadium pentoxide with alkali metal sulfatesor pyrosulfates – is present as a melt in the poresof the SiO2 support under the working conditionsof the oxidation of SO2 [167]. In SAPC hydro-phobic organic reactants are converted in theliquid phase. The catalyst consists of a thin filmof water on the surface of a support (e.g., porousSiO2) and contains an active hydrophilic organ-

ometallic complex [166]. The reaction takesplace at the interface between the water film andan organic liquid phase containing the hydropho-bic reactant. The nature of these catalyst systemsis schematically shown in Figure 9.

A new and improved version of SLPC usesionic liquids for immobilization of homogeneouscatalysts in supported ionic liquid phase (SILP)systems. The advantage of ionic liquids over thepreviously used solvents is their extremely lowvapor pressures that allow for long-term immo-bilization of homogeneous catalysts. A variety ofreactions have already been successfully studied[168, 169]. A related novel concept uses solidcatalysts with ionic liquid layer (SCILL) as amethod to improve the selectivity of heteroge-neous catalysts. The sequential hydrogenation ofcyclooctadiene to cyclooctene and cyclooctaneon a commercial Ni catalyst coated with an ionicliquid was tested as model system. Compared tothe original catalyst, the coating of the internalsurface with the ionic liquid strongly enhanced

Figure 9. Schematic representation of SAP catalysis


the maximum intrinsic yield of the intermediateproduct [170].

2.2.7. Ship-in-a-Bottle Catalysts [171]

Metal complexes which are physically entrappedin the confined spaces of zeolite cages (confinedcatalysts) are known as ship-in-a-bottle catalystsor tea-bag catalysts. The entrapped complexesare assumed to retain many of their solutionproperties. The catalytic performance can bemodified in a synergistic manner by shape selec-tivity, the electrostatic environment, and theacid-base properties of the zeolite host. Ligandsfor metal centers in the zeolite cages includeethylenediamine, dimethylglyoxime, variousSchiff bases, phthalocyanines, and porphyrins[171, 172]. The entrapped complexes can beobtained via three principal routers [171]:

1. Reaction of the preformed flexible ligand withtransition metal previously introduced into

the zeolite cages (flexible ligand method).The synthesis of a zeolite entrapped metalsalen complex is schematically shown inFigure 10.

2. Assembling the ligand from smaller speciesinside the zeolite cavities (ship-in-a-bottletechnique). For example, the synthesis of azeolite-entrapped metal phthalocyanine isschematically shown in Figure 11.

3. Synthesis of the zeolite structure around thepreformed transition metal complex (zeolitesynthesis technique).

The success of ship-in-a-bottle catalysts incatalytic processes has still to be demonstrated.

Zeolite-encapsulated complexes have alsobeen suggested as model compounds for mim-icking enzymes. These zeolite-based enzymemimics are called zeozymes to describe a catalyt-ic system, in which the zeolite replaces theprotein mantle of the enzyme, and the entrappedmetal complex mimics the active site of theenzyme (e.g., an iron porphyrin) [173].

Figure 11. Ship-in-a-bottle synthesis of zeolite-encaged metal phthalocyanines

Figure 10. Synthesis of zeolite-entrapped metal salen complexes by the flexible ligand method


Host – guest supramolecular compoundsmay also be mentioned in this context [111, 174].

2.2.8. Polymerization Catalysts

Ziegler – Natta (! Polyethylene, Section 3.2.2)catalysts are mixtures of solid and liquid com-pounds containing a transition metal such as Ti orV [175, 176]. TiCl4 combined with Al(C2H5)3 orother alkyl aluminum compounds were found tobe active for olefin polymerization. More activecatalysts were produced commercially by sup-porting the TiCl4 on solid MgCl2, SiO2 or Al2O3

to increase the amount of active titanium. Cur-rently, Ziegler – Natta catalysts are produced byball milling MgCl2 with about 5 % of TiCl4, andthe cocatalyst is Al(C2H5)3.

The Phillips catalyst (! Polyethylene, Sec-tion 3.2.1) consists of hexavalent surface chro-mate on high surface area silicate supports. Cr6þ

is reduced by ethylene or other hydrocarbons,probably to Cr2þ and Cr3þ, the catalyticallyactive species [175, 176].

More recently, so-called single-site catalystsusing metallocenes as active species were devel-oped (! Metallocenes, ! Polyethylene, Sec-tion 3.2.3) [176, 177]. The activity of thesematerials is dramatically enhanced by activationwith methylaluminoxane (MAO), obtained byincomplete hydrolysis of Al(CH3)3, the catalyticperformance of which is significantly more ver-satile than that of the classical Ziegler – Natta orPhillips catalysts. Activities and the nature of thepolymeric product can be tailored by the choiceof the metal and ligands.

2.3. Coated Catalysts

In addition to bulk and supported catalysts, coatedcatalysts can be considered as a third class ofcatalysts. In contrast to traditional catalyst geome-tries such as powders, tablets, spheres, and rings,coated catalysts are catalytically active layers ap-plied on inert structured surfaces. These activelayers consist of bulk or supported catalysts. Theuse of coated catalysts has recently become in-creasingly popular. Examples for such systems are:

. Egg-shell catalysts deposited on an inertcarrier

. Monolithic honeycombs for environmental ap-plications or for multiphase reactions [178,179]

. Structured packings [180]

. Foams and sponges [181]

. Fibers and cloths [182]

. Catalytic-wall reactors

. Catalytic filters for flue gas treatment anddiesel exhaust after-treatment [183]

. Membrane – electrode assemblies for fuelcells [184]

. Microstructured reactors with coated channels[185]

Advantages of coated catalysts are optimalusage of the active mass, high selectivity at lowdiffusion lengths, highly efficient mass transferfrom fluid phases to the solid catalyst layer, andlow pressure drop.

3. Production of HeterogeneousCatalysts

The development of heterogeneously catalyzedreactions for the production of chemicals initiat-ed the preparation of the required catalysts on atechnical scale. Up to the end of World War II,solid catalysts were produced predominantly inprocess companies such as IG Farben and BASFin Germany, and Standard Oil Company andUOP in the USA [186, 187]. About ten yearslater some independent catalyst producing com-panies were founded in the USA, WesternEurope, and Japan [186]. At present more than15 international companies [186, 188] are pro-ducing solid catalysts on multitonne scale; forexample:

. Synetix (ICI Catalysts and ICI Catalco)

. Davison Chemicals and Grace

. SUD-Chemie Catalyst Group (incl. UCI, Hou-dry, Prototec in the USA, NGC, CCIFE inJapan, UCIL in India and AFCAT, SYNCATin South Africa)

. UOP and Katalystiks

. BASF (incl. Engelhard Corp., Calsicat)

. Monsanto

. Shell and Criterion Catalysts

. Akzo Chemicals

. Johnson Matthey

. Haldor Topsøe


. Evonik Degussa

. Nippon Shokubai

. Nikki Chemical

In 1991 the catalyst world market achieved aturnover of about $ 6 � 109 [186, 187, 189],grew to $ (8 – 9) � 109 in 1996, and reached$ 13 � 109 in 2008.

Approximately 24 – 28 % of produced cata-lysts were sold to the chemical industry and 38 –42 % to petrochemical companies including re-fineries. 28 – 32 % of solid catalysts were used inenvironmental protection, and 3 – 5 % in theproduction of pharmaceuticals [186, 189].

The catalytic properties of solid catalystsare strongly affected by the preparation meth-od, production conditions, and quality ofsource materials. Therefore, it is necessary tocontrol each production step and the physicalor mechanical properties of all intermediates.To attain a better reproducibility of catalystproduction, batch procedures were mainly re-placed by continuous operations, such as pre-cipitation, filtration, drying, calcination, andforming.

Automation of various operations and com-puter control of different equipments wereinstalled in catalyst production lines [186]. Re-cently, SPC (statistic process control) and QA(quality assurance) were integrated into thecatalyst production process. Some companies,especially in Western Europe and in the USA,produce solid catalysts according to ISO Stan-dard which guarantees a standard catalyst qualityto the customer [186, 190].

Catalysts applied in several industrial pro-cesses can be subdivided into the followingcategories:

. Unsupported (bulk) catalysts

. Supported catalysts

3.1. Unsupported Catalysts

Unsupported catalysts represent a large categoryand are applied in numerous industrial processes.Various preparation methods were adopted in thepast decades in the commercial production ofunsupported catalyst, such as mechanical treat-ment or fusion of catalyst components, precipi-tation, coprecipitation, flame hydrolysis, andhydrothermal synthesis [98, 188, 191–195].

Mechanical Treatment, for example, mix-ing, milling, or kneading of catalytic active ma-terials or their precursors with promoters, struc-ture stabilizers, or pore-forming agents, is one ofthe simplest preparation methods [188,191–193]. In some cases, however, the requiredintimate contact of catalyst components couldnot be achieved and therefore the activity, selec-tivity, or thermal stability of catalysts prepared inthis way was lower than of those prepared byother methods. However, recent improvementsin the efficiency of various aggregates for themechanical treatment of solids resulted in activi-ty enhancement. An important advantage ofthese methods is that formation of wastewateris avoided.

Industrial catalysts produced by mechanicaltreatment are summarized in Table 4.

Fusion of Components or Precursors isused for the production a small group of unsup-ported catalysts. The fusion process [98] permitsthe synthesis of alloys consisting of elementswhich do not mix in solution or in the solid state.However, preparation of unsupported catalystsby fusion is an energy-consuming and quiteexpensive process.

The most important application of this methodis the production of ammonia synthesis catalystsbased on the fusion of magnetite (Fe3O4) with

Table 4. Unsupported catalysts prepared by mechanical treatment (MT) or by fusion (F)

Catalyst* Preparation method Application

Fe2O3(K, Cr, Ce, Mo) MT ethylbenzene dehydrogenation (styrene production)

Fe2O3(K) MT Fischer – Tropsch synthesis

ZnO – Cr2O3 MT hydrogenation of carbonyl compounds

Fe3O4(K, Al, Ca, Mg) F NH3 synthesis

V2O5 – K2S2O7 F SO2 oxidation (H2SO4 production)

Pt/Rh grid F NH3 oxidation (HNO3 production)

* Elements in parentheses are promoters.


promoters such as oxides of K, Al, Ca, and Mg[98]. Another example is the preparation of SO2

oxidation catalysts by fusion of V2O5 with Kpyrosulfate (K2S2O7) [98]. Some producers in-corporate Cs oxide as an activity promoter in thiscatalyst.

Quite recently, amorphous alloys composed,e.g., of Pd and Zr, so-called metallic glasseswerefound to be active in catalytic oxidations [98, 99].

Industrial catalysts produced by the fusionprocess are listed in Table 4.

Precipitation and Coprecipitation are themost frequently applied methods for the prepa-ration of unsupported catalysts or catalyst sup-ports [188, 191–195]. However, both methodshave the major disadvantage of forming largevolumes of salt-containing solutions in the pre-cipitation stage and in washing the precipitate.

Source materials are mainly metal salts, suchas sulfates, chlorides, and nitrates. Acetates,formates, or oxalates are used in some cases. Inindustrial practice nitrates or sulfates are pre-ferred. Basic precipitation agents on an industrialscale are hydroxides, carbonates, and hydroxo-carbonates of sodium, potassium, or ammonium.

Alkali metal nitrates or sulfates formed asprecipitation byproducts must be washed out ofthe precipitate. Thermally decomposable anions,such as carbonates and carboxylates and cationslike NH4

þ are especially favored in catalystproduction.

Coprecipitation of two or more metal cationsis a suitable operation for the homogeneousdispersion of the corresponding oxides, especial-ly if the catalyst precursors have a defined crys-talline structure, for example, Cu(OH)NH4CrO4

or Ni6Al2(OH)16CO3. After thermal treatment,binary oxides such as CuO – Cr2O3 and NiO –Al2O3 are formed [188, 191–195].

Precipitation and coprecipitation can be car-ried out in batch or continuous operations.

If the metal salt solution is placed in theprecipitation vessel and the precipitating agentis added, then the pH changes continuouslyduring the precipitation. Coprecipitation shouldbe carried out in the reverse manner (addition ofthe metal salt solution to the precipitation agent)to avoid sequential precipitation of two or threemetal species.

If the metal salt solution and the precipitatingagent are simultaneously introduced into the

precipitation vessel, then it is possible to keepthe pH constant. However, the residence timeof the precipitate in the vessel changescontinuously.

Finally, if the metal salt solution and theprecipitating agent are continuously introducedin the precipitation vessel, and the reaction pro-ducts are removed continuously, then pH andresidence time can be kept constant [188,191–195].

Besides pH and residence time, other precipi-tation parameters, such as temperature, agitation,and concentration of starting solutions, affect theproperties of the precipitate. The choice of an-ions, the purity of raw material, and the use ofvarious additives also play an important role[188, 191–195].

In general, highly concentrated solutions, lowtemperatures and short ageing times result infinely crystalline or amorphous materials whichare difficult to wash and filter. Lower concentra-tions of the solutions, higher temperatures, andextended ageing provide coarse crystalline pre-cipitates which are easier to purify and separate[188, 191–195].

The industrial production of precipitated cat-alysts usually involves the following steps:

. Preparation of metal salt solution and of pre-cipitating agent (dissolution, filtration)

. Precipitation

. Ageing of the precipitate

. Washing of the precipitate by decantation

. Filtration

. Washing of the filter cake (spray drying)

. Drying

. Calcining

. Shaping

. Activation

Operations such as filtration, drying, calcina-tion etc. are discussed in Section 3.3.

Typical unsupported industrial catalysts pro-duced by precipitation or coprecipitation arecompiled in Table 5.

The Sol – Gel Process [196] involves theformation of a sol, followed by the creation of agel. A sol (liquid suspension of solid particlessmaller than 1 mm) is obtained by the hydrolysisand partial condensation of an inorganic salt ora metal alkoxide. Further condensation of sol


particles into a three-dimensional network re-sults in the formation of a gel. The porosity andthe strength of the gel are strongly affected byconditions of its formation. For example, slowcoagulation, elevated temperature, or hydrother-mal posttreatment increase the crystalline frac-tion of the gel.

Alumina and silica can be produced fromsodium aluminate or sodium silicate by treatmentwith nitric, hydrochloric, or sulfuric acid. In thisprocess, first sols and then gels are formed.Washing the sodium from the gels is essential[193, 194, 196].

Spherical silica or silica-alumina gels areproduced directly by injecting drops of a gellingmixture into oil at a proper rate to allow setting ofthe gel. To avoid bursting during drying, the gelbeads are washed to reduce the salt content(NaNO3, NaCl, or Na2SO4). Finally, the beadsare dried and calcined [193, 194].

Based on the sol – gel process, high-puritymaterials such as alumina, TiO2, ZrO2 are pro-duced on an industrial scale. Raw materials arethe corresponding metal alkoxides, e.g., Al(C12 – C18 alkoxide)3 and Ti(n-C4H9O)4 [196].

Flame Hydrolysis. In flame hydrolysis[197] a mixture of the catalyst or support precur-sor, hydrogen, and air is fed into the flame of acontinuously operating reactor. Precursors(mainly chlorides such as AlCl3, SiCl4, TiCl4 orSnCl4) are hydrolyzed by steam (formed by H2

oxidation). The products of flame hydro1ysis areoxides. More than 100 000 t/a of so-calledfumed silica, alumina, or titania are producedby Degussa, Wacker (both Germany), and Cabot(USA).

Thermal Decomposition of metal – inor-ganic or metal – organic catalyst precursors is

sometimes used in industrial catalyst production.For example, mixtures of Cu- and Zn(NH3)4(HCO3)2 decompose at 370 K to formbinary Cu – Zn carbonates, which are trans-formed during calcination into the correspondingbinary oxides, used as low temperature water gasshift catalysts [193].

Industrial production of Cu – Cr oxides (cop-per chromites), used in the hydrogenation ofcarbonyl compounds, is based on the thermaldecomposition of a basic copper ammoniumchromate [CuNH4(OH)CrO4] at 620 – 670 K[193, 194].

Highly active Ni catalysts for the hydrogena-tion of fats and oils are obtained by the thermaldecomposition of Ni formate [(HCOO)2Ni] at390 – 420 K. The decomposition is usually car-ried out in hard fat, which protects Ni againstoxidation [193].

Catalysts or supports produced by flame hy-drolysis or thermal decomposition of inorganiccomplexes are summarized in Table 6.

Hydrothermal Synthesis [46, 82] is a veryimportant preparation method for zeolites andother molecular sieves.

Currently, the importance of zeolites in indus-trial catalysis is still increasing. They are used ascatalysts or supports not only in petrochemicaloperations but also in the production of finechemicals.

In hydrothermal synthesis (see also ! Zeo-lites, Section 6.1) a mixture of silicon andaluminum compounds containing alkali metalcations, water, and in some cases organic com-pounds (so-called templates) is converted intomicroporous, crystalline aluminosilicates [194,199].

Common sources of silicon are colloidalsilica, water glass, fumed silica, and silicon

Table 5. Catalysts (their precursors) or supports prepared by precipitation or coprecipitation

Catalysts (precursors) or supports Source materials Application

Alumina Na aluminate, HNO3 support, dehydration, Claus process

Silica Na silicate (water glass), H2SO4 support

Fe2O3 Fe(NO3)3, NH4OH ethylbenzene dehydrogenation (styrene production)

TiO2 Fe titanate, titanyl sulfate, NaOH support, Claus process, NOx reduction

CuO – ZnO – (Al2O3) Cu, Zn, (Al) nitrates, Na2CO3 LTS, methanol synthesis

Fe molybdate Fe(NO3)3, (NH4)2MoO4, NH4OH methanol oxidation to formaldehyde

Vanadyl phosphate vanadyl sulfate, NaHPO4 butane oxidation to maleic acid anhydride

NiO – Al2O3 Ni, Al nitrates, Na2CO3 hydrogenation of aromatics

NiO – SiO2 Ni nitrate, Na silicate, Na2CO3 hydrogenation of aromatics


alkoxides. Aluminum can be introduced as alumi-num hydroxide, metahydroxide, or aluminate salts.Common templates are tetrapropyl- or tetraethy-lammonium bromides or hydroxides [194, 199].

Hydrothermal synthesis is a complex processconsisting of three basic steps: achievement ofsupersaturation, nucleation, and crystal growth.It is affected by the hydrogel molar composition,a1kalinity, temperature, and time [194, 199].

In general, synthesis is carried out at 360 –450 K under atmospheric or autogenous waterpressure (0.5 – 1 MPa) with residence times of1 – 6 d. After synthesis, the crystalline productis separated by filtration or centrifugation,washed, dried, and calcined. Sodium-containingzeolites, which are the products of the hydrother-mal synthesis, are converted into acidic forms byexchange of sodium ions with the ammonium,followed by thermal treatment [194, 199].

Zeolites used in the industrial catalysis areabove all Y zeolite, mordenite, ZSM-5, ZSM-11,and zeolite b [46, 81].

To improve the thermal stability of zeolites,especially of Y zeolite, Al ions are extracted fromthe lattice by steaming or acid treatment. Forexample, fluid-cracking catalysts (amorphousaluminosilicates) contain 10 – 50 % of ultra-stable Y zeolites [194, 199].

Related to zeolites are other molecular sievessuch as aluminum phosphates (AlPO) and silico-aluminum phosphates (SAPO), the importanceof which in industrial catalysis is growing.SAPO-11 was applied recently in the isomeriza-tion of cyclohexanone oxime to -caprolactam,instead of sulfuric acid in a demonstration unit[46].

Other Preparation Methods include con-densation of more than two kinds of oxo anions,such as MoO4

2�;WO42�; HPO4

2�, etc. to give

heteropolyacids such as H3PW12O40 orH3PMo12O40 [88].

In industrial practice, the source materials areNa2HPO4, Na2WO4, or Na2MoO4 solutions. Hy-drolysis and subsequent condensation are carriedout with HCl. The heteropolyacids are extractedwith organic so1vents. Heteropolyacids are verystrong acids with Hammett acidity functionHo < � 8. They have found industrial applica-tion in acid-catalyzed reactions conducted in theliquid phase, such as hydration, esterification,and alkylation. Their activity is evidently higherthan that of inorganic acids [88]. Their K or Cssalts are used as catalysts in the selective vaporphase oxidation of propene to acrolein or iso-butene to methacrolein [88].

Another preparation method is based on thetreatment of alumina or aluminosilicate withgases such as HF, HCl, BF3, AlCl3 to createhighly acidic centers. Such catalysts are activein the skeletal isomerization of hydrocarbons,e.g., n-C4 or n-C5 [194].

Skeletal Catalysts, also called porous me-tals, consist of the metal skeleton remaining afterthe less noble component of an alloy was re-moved by leaching with alkali, preferentiallyNaOH. Skeletal catalysts were discovered in1925 and introduced into chemistry by Raney,and therefore some bear his name, e.g., Raney Nior Co.

The group of skeletal catalysts [97] includesNi, Co, Cu, Fe, Pt, Ru, and Pd. The second alloycomponent can be Al, Si, Zn, or Mg, with Albeing used preferentially.

Small amounts of a third metal such as Cr orMo have been added to the binary alloy asactivity promoter.

Ni – Al, Cu – Al, and Co – Al alloys withdifferent grain sizes are commercially available

Table 6. Catalysts (their precursors) or supports prepared by sol – gel process (SG), flame hydrolysis (FH), or thermal decomposition (TD) of

inorganic metal complexes

Catalyst (precursor) or support Source material Preparation method Application

Alumina (high-purity) Al alkoxides hydrolysis, SG support for noble metals

Alumina (acidic, low bulk density) AlCl3 FH support or additive

Silica (low bulk density) SiCl4 FH support or additive

TiO2 Ti(n-C4H9O)4 hydrolysis, SG support

TiO2 (low bulk density) TiCl4 FH support or additive

CuO – ZnO Cu, Zn (NH3)4HCO3 TD low-temperature shift, methanol

CuO – Cr2O3 Cu(NH4)OHCrO4 TD hydrogenation of carbonyl compounds

Ni (kieselguhr) Ni formate TD hydrogenation of fats and oils


and can be leached out before use. Davison-Grace and Degussa provide finished extremelypyrophoric Ni or Co skeletal catalysts protectedby water in commercial quantities. Skeletal Nifound technical application in the hydrogenationof aliphatic or aromatic nitro compounds andnitriles.

3.2. Supported Catalysts

The main feature of supported catalysts is that theactive material forms only a minor part and isdeposited on the surface of the support [192–194,198].

In some cases, the support is more or less inert,e.g., a-alumina, kieselguhr, porous glass, cera-mics. In other cases the support takes part in thecatalytic reaction, as in the case of bifunctionalcatalytic systems, e.g., alumina, aluminosilicate,zeolites, etc. [192–194, 198].

Additionally, some supports can alter thecatalytic properties of the active phase. Thisso-called strong metal – support interaction(SMSI) can decrease, for example, the chemi-sorption capacity of supported metals (Pt –TiO2) or can hinder the reduction of supportedmetal oxides (Ni silicate, Ni and Cu aluminates,etc.) [192–194].

3.2.1. Supports

Currently, various industrial supports are avail-able in multitonne quantities possessing a widerange of surface areas, porosities, shapes, andsizes.

Widely used supports include alumina, silica,kieselguhr, porous glass, aluminosilicates, mo-lecular sieves, activated carbon, titania, zincoxide, silicates such as cordierite (2 MgO �Al2O3 � 5 SiO2) and mullite (3 Al2O3 � 2 SiO2),and Zn and Mg aluminates [192–194].

The supports are produced by specializedproducers or directly by catalyst producers. Wellknown support manufacturers are:

. Grace Davison (USA, UK, Germany)

. Alcoa (USA)

. Sasol (former CONDEA) (Germany)

. BASF (former Engelhard) (USA)

. Saint Gobain Norpro (USA)

. Evonik Degussa (Germany)

. Cabot Corp. (USA)

. Corning (USA)

In the past, mostly natural supports, e.g.,bauxite, pumice and kieselguhr, were used incatalyst production. At present (with the excep-tion of kieselguhr) mainly synthetic supportswith ‘‘tailored’’ physical properties are preferredin industrial catalysis.

Because the majority of supports also havecatalytic properties (e.g., alumina, aluminosili-cates, zeolites) their production methods aredescribed in Section 3.1.

3.2.2. Preparation of Supported Catalysts

The broad application of supported catalysts inindustrial catalysis led to the development ofnumerous preparation methods applicable on atechnical scale. Some of these methods areidentical with those used in the production ofunsupported catalysts, e.g., mechanical treat-ment, precipitation, thermal decomposition ofmetal – inorganic or metal – organic com-plexes and therefore they will be discussed herevery briefly.

Mechanical Treatment, e.g., kneading of acatalyst precursor with a support is applied, e.g.,in the production of kieselguhr-supported Ni(precursor NiCO3) [191, 193, 194]. Also, MoO3

supported on Al2O3 is sometimes produced bythis process. However, the distribution of theactive phase on the support is in some cases notsufficient [191].

Better results are obtained by the combinationof mechanical and thermal treatment which,results in spreading [9] of, e.g., MoO3 on Al2O3

or of V2O5 on Al2O3 or TiO2.

Impregnation by pore filling of a carrierwith an active phase is a frequently used produc-tion method for supported catalysts [35, 191].The object of this method is to fill pores of thesupport with a solution of the catalyst precursor,e.g., a metal salt of sufficient concentration toachieve the desired loading. If higher loadingswith active phases are required, it is mostlynecessary to repeat the impregnation after dryingor calcination of the intermediate. Examples of


catalysts prepared by the pore filling method areNi or Co on Al2O3 – MoO3, MoO3 on alumino-silicates including zeolites, Ni or Ag on a-alu-mina, noble metals on active carbon, etc. [188,191–194].

Adsorption is a very good method toachieve uniform deposition of small amountsof active component on a support. Powdersor particles exposed to metal salt solutionsadsorb equilibrium quantities of salt ions, inaccordance with adsorption isotherms. Adsorp-tion may be either cationic or anionic, dependingon the properties of the carrier surface. Forexample, alumina (depending on the adsorptionconditions, mainly on the pH of the solution)adsorbs both cations and anions. Silica weaklyadsorbs cations, while magnesia strongly adsorbsanions [188].

Adsorption of PdCl2 from aqueous solution ondifferent aluminas is very fast, and a high equi-librium concentration (ca. 2 wt %) can be ob-tained. The Pd deposition takes place mainlyin an outer shell (egg shell profile) of shapedparticles [188, 194].

With H2[PtCl4] only 1 wt % Pt loading onalumina is possible owing to from the flat ad-sorption isotherm [188, 194].

The addition of oxalic, tartaric, and citric acidto the metal salt solution changes the profiles ofactive component on the carrier. In general, withincreasing acid strength the metal ions are forceddeeper into the support particles [188].

Ion Exchange [35, 188] is very similar toionic adsorption but involves exchange of ionsother than protons. Lower valence ions, such asNaþ or NHþ4 can be exchanged with highervalence ions, for example, Ni2þ or Pt4þ. Thismethod is used mainly in the preparation ofmetal-containing zeolites, e.g., Ni- or Pd-con-taining Y zeolites or mordenites used in petro-leum-refining processes.

Thermal Decomposition of inorganic ororganic complexes in the presence of a support.This method is identical with that used in thepreparation of unsupported catalysts by thermaldecomposition of precursors. Supports can beused either as powder or preshaped. For example,Ni or Co deposited on kieselguhr or silica isproduced by this method [193].

Precipitation Onto the Support is carriedout in a similar way as in the case of unsupportedcatalysts [188, 191, 198]. Supports, mainly aspowders, are slurried in the salt solution, andalkali is added. Rapid mixing is essential to avoidprecipitation in the bulk.

Uniform precipitation can be achieved byusing urea rather than conventional alkalis[198]. An appropriate amount of urea is addedto the metal salt – support slurry and the mixtureis heated while stirring. At 360 K urea decom-poses slowly to NH3 and CO2, and precipitationtakes place homogeneously over the surface andin pores of the support. This method is calleddeposition – precipitation [198] and is used es-pecially in the production of highly active Ni –SiO2 or Ni – Al2O3 catalysts.

Reductive Deposition is a preparationmethod in which especially precious metals aredeposited on the carrier surface by reduction ofaqueous metal salts, mainly chlorides or nitrates,with agents such as H2, Na formate, formalde-hyde, and hydrazine. Examples of commercialcatalysts produced by this method are preciousmetals on active carbon, SiO2 or a-Al2O3. Re-ductive deposition is preferred especially in thecase of bimetallic supported catalysts such asPt – Re or Pd – Rh [200].

Heterogenization of Homogeneous Cata-lysts is based on the binding of metal com-plexes to the surface or entrapment in pores ofthe inorganic or organic support [132]. Suchcatalysts are used mainly in stereospecific hy-drogenations in the production of fine chemicalsor pharmaceuticals.

Enzymes [132] can also be heterogenized.They found industrial application in biochemicalprocesses. A prominent example is the isomeri-zation of glucose to fructose in the production ofsoft drinks.

3.3. Unit Operations in CatalystProduction

As in other branches of the chemical industry,unit operations have also been established incatalyst production in the past few decades, forexample, in operations such as filtration, drying,calcination, reduction and catalyst forming [188,


190, 194]. Continuous operations are favoredbecause of larger throughputs, lower operatingcosts, and better quality control. Additionally,factors such as environmental pollution and ha-zards to human health can be minimized moreeasily.

Filtration, Washing. The main purpose ofthese operations is to separate precipitates and toremove byproducts and possible impurities. Inbatch operations mainly plate-and-frame filterpresses are used [188, 194]. The washing of thefilter cake proceeds in countercurrent to thedirection of the filtration.

Continuous vacuum rotary filters are widelyused. By changing the speed of the filter drum(covered with filter cloth), the quantity of slurryfiltered and the thickness of the filter cake can bevaried over a broad range. As the drum rotates,there is a washing phase in which water issprayed against the moving filter cake. Finally,the filter cake is scraped or blown to remove itfrom the drum [194].

Another filtration equipment is the centrifuge.However, its application is possible only whenthe filtered material is grainy or crystalline, e.g.,zeolites. Washing can be carried out by introduc-ing washing water into the centrifuge. Centri-fuges can operate either in a discontinuous orcontinuous manner [194].

Drying. Because drying conditions such asrate, temperature, duration, or gas flow rate canchange the physical properties of the resultingmaterial, it is important to measure and keepthese parameters constant. For example the po-rosity of precipitated catalysts depends on thedrying procedure. The drying of impregnatedsupports can change the distribution of activecomponents. Their uniform distribution can beobtained only if all the liquid is evaporatedspontaneously [188, 191, 194]. Usually, dryingproceeds up to 400 K.

For the drying of filter cake, various tools areused, e.g., box furnaces with trays, drum dryers,rotary kilns, and spray dryers [188, 191, 194].

The main problems with drums and rotarydryers are the feeding of the wet filter cake andremoval of adhering material from the walls.Because lumps are usually formed in the dryingprocess, the resulting material must pass a gran-ulator equipped with a sieve [194].

Spray dryers provide microspherical materi-als with a narrow particle-size range. Spraydryers are equipped with a nozz1e or a rotatingdisk to disperse the watery slurry of the filter cakein a stream of hot air [188, 194, 201].

All the above drying equipment operates incontinuous mode.

Small batches of catalyst precursors are driedin box furnaces with trays [191].

For the drying of extrudates, continuouslyoperating belt dryers have found technical appli-cation [194].

Calcination. The main object of calcination(thermal treatment in oxidizing atmosphere) is tostabilize physical and chemical properties of thecatalyst or its precursor. During calcination,thermally unstable compounds (carbonates, hy-droxides, or organic compounds) decompose andare usually converted to oxides. During calcina-tion new compounds may be formed, especiallyif the thermal treatment is carried out at highertemperatures [134]. For example, in the thermaldecomposition of Cu or Ni nitrate deposited onalumina, not only CuO or NiO but also Cu or Nialuminate is additionally formed [134].

Furthermore amorphous material can becomecrystalline. Various crystalline modifications canundergo reversible or irreversible changes.

Physical and mechanical properties and porestructures can also change. The calcination temper-ature is usually slightly higher than that of thecatalyst operating temperature [188, 191, 194, 201].

For the calcination of powder or granulate,rotary kilns are preferably used [191, 194]. Smal-ler batches of powdered catalysts are calcined inbox or muffle furnaces with trays, as in the case ofdrying. The gases that are mainly used for heatingare in direct contact with the material beingcalcined [188, 191, 194].

Pellets or extrudates are calcined in belt ortunnel furnaces. The tunnel-type calciner canoperate at substantially higher temperatures(close to 1270 – 1470 K) than the belt type(870 – 1070 K). The tunnel calciner is especial-ly suitable for firing ceramic carriers. The mate-rial being calcined is taken in boxes or cartswhich are recycled to the entrance via a continu-ous chain or belt [194].

Reduction, Activation, and Protection.Reduction, activation, or passivation, is in


several cases the last step in catalyst production.These operations are performed by the catalystproducer or in the plant of the client.

For example, in the production of Ni catalystsfor the liquid-phase hydrogenation of fats andoils, the reduction of NiO deposited on kiesel-guhr is carried out exclusively by the catalystproducer. The reduction of powders (50 –500 mm particle size) is performed on an indus-trial scale in fluid-bed reactors. The reducedmaterial is pyrophoric and must be protectedwith a hard fat such as tallow, to make itshandling easy and safe. The finished catalyst issupplied in the form of flakes or pastilles [188,194, 202].

The reduction of metal oxides such as NiO,CuO, CoO, or Fe2O3 is carried out with H2

at elevated temperature (> 470 K) and has twosteps. In the first step metal nuclei are formed.In the second, nuclei accumulate to formmetal crystallites. The rates of both processesdepend on temperature and on the nature of thesubstrate [188]. Reduction at lower temperatures(<570 K) provides a narrow distribution of smallmetal crystallites. Reduction at higher tempera-tures (> 670 K) gives a broader distribution andlarger metal crystallites [188].

Reduction of some oxides, such as those of Cuand Fe, is exothermic and needs to be carried outcarefully with H2 diluted with N2.

Water, the reduction product, has negativeeffects on the rate and on the extent of reduction[188]. In industrial practice, where H2 is re-cycled, the removal of water by freezing out(below 270 K) and by adsorption on molecularsieve is essential.

To achieve optimal activity, partial reductionof oxidic catalysts is common [188, 203]. Forexample, Ni catalysts for fat and oil hydrogena-tion contain about 50 – 60 wt % of metallic Ni,45 – 35 wt % NiO, and about 5 wt % Ni silicate.

When the reduction of shaped oxidic catalystsis conducted by the catalyst producer, then theactive material is protected either with a high-boiling liquid such as higher aliphatic alcohols orC14 – C18 paraffins [203] or it is passivated. Inthis procedure, chemisorbed hydrogen is removedin a gas stream composed of N2 and about 0.1 –1.0 vol % of O2 at ambient temperature. After thistreatment catalysts can be handled in air withoutany precautions [202]. The activity is restored inthe client’s plant by treatment with H2 [188].

The activation of hydrotreating catalysts com-posed of Ni- or Co-promoted MoO3 – Al2O3 iscarried out with H2 containing 10 vol % of H2S[134]. In the past, this activation was performedexclusively in the hydrodesulfurization plants.However, presulfiding at catalyst producers isbecoming more common [188].

Mainly electrical or gas-heated shaft reactorsare used for the reduction of extrudates, spheres,or pellets in plants of catalyst producers.

Catalyst Forming. The size and shape ofcatalyst particles depend on the nature of thereaction and on the type of applied reactor.

Reactions in the liquid phase require smallparticles or even powders (50 – 200 mm). Suchmaterials are made by grinding of a dried orcalcined precursor, e.g., filter cake, using gran-ulators equipped with sieves to give uniformparticle size [188, 194, 201, 203].

Catalysts for fluidized bed reactors (0.05 –0.25 mm) are usually made by spray drying or bycooling molten material droplets (V2O5) in an airstream [188, 194, 201].

Spheres consisting of Al2O3, SiO2, or alumi-nosilicate with 3 – 9 mm diameter are used pref-erentially as a support for catalysts inmoving-bedor ebullating-bed reactors. They are produced bythe so-called oil-drop method (see Section 3.1).Spheres prepared in this way possess sufficientabrasion resistance [188, 194, 201].

Another method for producing spheres isbased on agglomeration of powder by moisteningon a rotating disk (spherudizer) [188, 194, 201].As the spheres reach the desired diameter theyare removed automatically and transported to thedryer and calciner. Such spheres are suitable forfixed bed reactors.

Other methods for forming spherical particlesinclude tumbling short, freshly extruded cylin-ders in a rotating drum [194].

In the briquetting technique, the plastic mix-ture of catalyst powder with a binder is fedbetween two rotating rolls provided with hol-lowed-out hemispheres [194].

Extrusion of pastes containing catalyst pow-der, binders and lubricants is a frequently usedindustrial shaping method [188, 194, 201]. De-pending on the properties of the paste, press orscrew extruders are applied. Press extruders areprincipally suitable for viscous pastes. Screwextruders are preferred for thixotropic masses.


In both cases, pastes are forced through a die, andthe extruded material is cut with a special device toa desired length and falls onto a moving belt thattransports it through a drier or calciner [188, 194,201]. Poly(vinyl alcohol), powdered stearine, andAl stearate are used as lubricants. If the mass beingextruded contains alumina, then peptizing agentssuch as nitric acid are added mainly to improve themechanical strength [188, 193, 194].

Another type of binder is calcium aluminatecement, which sets up by treatment with steam[188, 194].

The extruded material can have differentshapes, such as cylinders (noodles), hollow cy-linders (macaroni), or ribbed cylinders. The sizesdepend on the shape and are in the range of 1.5 –15 mm diameter [194, 203].

Added organic lubricants and pore-formingagents can be removed by calcination in a streamof air.

Special extrusion techniques and equipmentare necessary to produce honeycombs.

Extruding is less expensive than pelletizing,but extrudates have less resistance to abrasionthan pellets. Extrudates are suitable for differenttypes of fixed-bed reactors operating in the gas ortrickle phase.

Pelletizing is a very common method for cata-lyst forming. It is based on compression of a certainvolume of powder in a die between two movingpunchers, one of which also serves to eject theformed pellet [188, 194, 201]. Depending onthe size and the shape of the prepared pellets, thematerial being pelletized must be crushed andforced through a corresponding sieve [194]. Fur-thermore, lubricants such as graphite, Al stearate,pol(vinyl alcohol), kaolin, and bentonite are addedbefore the material enters the tabletting machine.The fluidity of the material is required to assurehomogeneous filling of the die [188, 194, 201]. Asin the case of extrudates, organic lubricants can beremoved by calcination of the pellets.

Industrial pelleting machines are equippedwith around thirty dies and produce about 10 literof pellets per hour or more, depending on theirshape and size. Pressures in the range 10 –100 MPa in the pelleting machine are common[188, 194, 201].

Commercially, cylindrical pellets with sizessuch as 3 � 3, 4.5 � 4.5, 5 � 5, or 6 � 6 mmare offered [188, 194, 201]. Production of 3 � 3mm pellets is more expensive than that of larger

sizes. Besides cylindrical pellets, various com-panies provide rings, cogwheels, spoked wheels,multihole pellets, etc. [202, 203].

Pellets of different shapes and sizes are suit-able for various types of fixed-bed reactors.

Coating of Inert Supports [204] with a thinlayer of catalytically active material is requiredfor manufacture of coated catalysts. A variety ofmethods for coating with catalysts are available.One can distinguish between material-dependentmethods for the preparation of thin catalyticallyactive layers on supports and material-indepen-dent coating methods [185]. Material-dependentmethods are anodic oxidation of aluminum oraluminum alloys, which gives rise to a layer witha one-dimensional and unidirectional pore sys-tem with adjustable properties [205], and forma-tion of porous layers on FeCrAl alloys by heattreatment. Material-independent coating tech-nologies can be grouped according to the stateof aggregation of the catalyst precursor [204].

Gaseous catalyst precursors can be trans-formed into coatings by chemical vapor deposi-tion (CVD) or physical vapor deposition (PVD).Coating methods based on a liquid phase com-prise sol – gel methods, deposition of catalystsuspensions, and combinations of both techni-ques. Depending on the adjusted viscosity of thesols or suspension, the liquid precursors may beapplied on surfaces by dip coating, spraying,printing, or rolling. Solid catalyst powders canbe applied, e.g., by flame spray deposition orpowder plasma spraying.

A coating procedure that has been intensivelystudied is the manufacture of monoliths, e.g., ascatalysts for pollution control [206]. Oxides suchas Al2O3, CeO2, and ZrO2 (washcoat) are depos-ited on monoliths with a honeycomblike struc-ture by dipping into an aqueous slurry containingprimary particles (about 20 nm in diameter) ofthese materials [125]. The excess slurry is blownout, and after drying and calcination a thin cata-lyst layer is obtained, the thickness of which canbe tailored by adjusting the slurry properties andrepetition of the dip-coating step.

4. Characterization of Solid Catalysts

Catalytic activity and selectivity critically de-pend on the morphology and texture, surface


chemical composition, phase composition, andstructure of solid catalysts. Therefore, manyphysical and chemical methods are used in catal-ysis research to characterize solid catalysts and tosearch for correlations between structure andperformance of catalysts. These methods includeclassical procedures [207] as well as techniquesdeveloped more recently for the study of thechemistry and physics of surfaces [207].

4.1. Physical Properties

4.1.1. Surface Area and Porosity [208, 209]

The specific surface area of a catalyst or support(in m2/g) is determined by measuring the volumeof gas, usually N2, needed to provide a mono-molecular layer according to the Brunauer –Emmett – Teller (BET) method.

In this approach, the determination of themonolayer capacity is based on the physisorptionof the test gas. The volume adsorbed at a givenequilibrium pressure can be measured by staticmethods, namely, volumetric or gravimetricmeasurements. Flow or dynamic techniques arealso applied.

The total surface area of a porous material isgiven by the sum of the internal and externalsurface areas. Pores are classified as micropores(pore width < 2 nm), mesopores (pore width2 – 50 nm), and macropores (pore width>50 nm) according to IUPAC definitions [210].

The specific pore volume, pore widths, andpore-size distributions for micro- and mesoporesare determined by gas adsorption. For meso-pores, the method is based on the dependenceof the pressure of capillary condensation on theradius of a pore in which condensation takesplace, which is given by the Kelvin equation:

lnðp=poÞ ¼ V

RT

2s cosqrk

� �

ð1Þ

where p/po ¼ pressure/saturation pressure, V isthe molar volume, s the surface tension of theliquid adsorbate, Q the contact angle betweenadsorbate and adsorption layer on pore walls(hence, Q ¼ 0 is a good approximation), and rK

is Kelvin radius of a pore assuming cylindricalshape. Since an adsorption layer is typicallyformed before capillary condensation occurs, thegeometric radius rp of a pore is given by the sum

of the Kelvin radius rK and the thickness of theadsorption layer t: rp ¼ rK þ t. Mesopore sizedistributions can be calculated when adsorptionand desorption isotherms are available in the fullpressure range up to p/po ¼ 0.95. The mesoporevolume Vp is assumed to be completely filled atthis relative pressure, which corresponds torp � 20 nm.

In the micropore range (pore width < 2 nm),where the pore dimensions are comparable tomolecular dimensions, pore filling occurs ratherthan condensation [209]. The Dubinin – Radush-kevich and the Dubinin – Stoeckli theories thenpermit the estimation of pore dimensions fromphysisorption data. In addition, several empiricalmethods exist, such as the t-method [211] and theas-method [212]. In the original t-method theamount of nitrogen adsorbed at 77 K was plottedagainst t, the corresponding multilayer thicknesscalculated from a universal N2 isotherm, while intheas method the multilayer thickness t is replacedby the reduced adsorptionas. Here,as is defined asthe dimensionless adsorption na=nax such thatas ¼1 at p/po ¼ 0.4, and na is the adsorbed amount inmoles of the adsorbate (e.g., N2) at a given relativepressure and nax is the amount adsorbed (in moles)at a relative pressure of 0.4.

For meso- and macroporous materials (porewidth > 2 nm), the pore size distribution isdetermined by measuring the volume of mercury(or another nonwetting liquid) forced into thepores under pressure [188]. The measurement,carried out with a mercury pressure porosimeter,depends on the following relation:

P ¼ 2ps cosarp

ð2Þ

where P is pressure, s is surface tension ofmercury, and a is the contact angle of mercurywith solid. At pressures of 0.1 – 200 MPa, poresize distributions in the range of 3.75 – 7500 nmcan be measured. Because the actual shape of thepores is not exactly cylindrical as assumed in thederivation of the above equation, the calculatedpore sizes and distributions can deviate appreci-ably from the actual values shown by electronmicroscopy.

For pore systems with narrow pore size dis-tributions, the average pore radius can be ap-proximated by using

rp ¼ 2 Vp=Sp ð3Þ


where Vp is pore volume, and Sp is the surfacearea. The pore volume of a catalyst or support isgiven by

Vp ¼ 1=rp�1=r ð4Þ

where rp and r are the particle and true densi-ties, respectively. The former is determined bya pycnometer using a nonpenetrating liquid,such as mercury, whereas the true density isobtained by measuring the volume of the solidpart of a weighed sample by helium displace-ment.

In certain instances, pore dimensions can bedetermined by high-resolution electron micros-copy (HREM) [213].

4.1.2. Particle Size and Dispersion [214]

The surface area of active metals dispersed on asupport deserves particular consideration sincethe metal surface area and particle size (which areinterrelated quantities) determine the catalyticproperties of supported metal catalysts. The met-al dispersion D is given byD ¼ NS/NT, whereNS

is the number of metal atoms exposed at thesurface andNT is the total number of metal atomsin a given amount of catalyst. The fraction ofsurface atoms D can be determined if NS isexperimentally available. It can be determinedby chemisorption measurements with adsorp-tives that strongly bind to the metal but whichinteract negligibly with the support at the chosentemperatures and pressures. H2, CO, NO, andN2O have been used for this purpose at or aboveroom temperature [214], and static, dynamic, anddesorption methods have been applied. Satura-tion values of the chemisorbed amounts permitNS to be calculated if the chemisorption stoichio-metries are known.

Dispersion is directly related to particle sizeand particle size distribution. Assuming reason-able model shapes for the metal particles, aver-age particle sizes can be calculated from thechemisorption data.

Average crystallite size distributions can bedetermined independently from X-ray diffrac-tion line broadening [214–216], and small-angleX-ray scattering (SAXS) permits the determina-tion of particle sizes and particle size distribu-tions, but also of the specific surface area of themetal and of the support [214–216].

Electron microscopy offers the unique oppor-tunity to observe catalyst morphologies over theentire range of relevant particle sizes [213, 214,217–219]. Particle shapes and sizes of the sup-port or active phase and their size distributionscan be extracted from micrographs, but structuralinformation can be also obtained by electron-diffraction and lattice-imaging techniques [213].

4.1.3. Structure and Morphology

X-ray Powder Diffraction (see also !Structure Analysis by Diffraction, Section 2.6)(XRD) is a routine technique for the identifica-tion of phases present in a catalyst [215, 220]. It isbased on the comparison of the observed set ofreflections of the catalyst sample with those ofpure reference phases, or with a database (Pow-der Diffraction File (PDF) distributed by ICDD,the International Centre for Diffraction Data).XRD studies can now be carried out in situ on theworking catalyst [220], and the use of synchro-tron radiation permits dynamic experiments inreal time [221]. Time-resolved studies on a time-scale of seconds are now becoming possible.Quantification of phase compositions can alsobe performed.

More sophisticated analysis of the diffractionpatterns of crystalline materials provides detailedinformation on their atomic structure. The Riet-veld method is used for structure refinements.Perhaps more importantly for catalytic materials,the local atomic arrangement of amorphous cat-alysts is based on the Debye equation, whichgives the intensity scattered by a collection ofrandomly distributed atoms. The Fourier trans-form of the Debye equation gives the radialdistribution function (RDF) of electrons, fromwhich the number of atoms (electrons) located inthe volume between two spheres of radius r andr þ dr around a central atom, i.e., the radialdensity of atoms, can be obtained [220]. Thisapproach has been applied for the structuralanalysis of amorphous or poorly crystalline cat-alyst materials and of small metal particles.

X-ray Absorption Spectroscopy (XAS)[222–224], is the method of choice where theapplicability of XRD for structure analysesceases to be possible. Because of their highphoton flux, synchrotron facilities are thepreferred sources for XAS experiments. The


physical principle of XAS is the ejection of aphotoelectron from a core level of an atom byabsorption of an X-ray photon. The position ofthe absorption edge gives the binding energy ofthe electron in the particular core level and is thuscharacteristic of the respective element and itschemical state (see also ! Surface and Thin-Film Analysis, 2. Electron Detection) and theshape of the absorption edge provides informa-tion on the distribution of the local density ofstates (LDOS).

The ejected photoelectron wave is backscat-tered at neighboring atoms, and the scatteredwave interferes with the outgoing primary wave.This interference results in a modulation of theabsorption coefficient at energies between 50 and1000 eV beyond the absorption edge (extendedX-ray absorption fine structure, EXFAS). Analy-sis of these oscillations provides information onthe chemical nature of atoms at well-defineddistances from the central (ionized) atom andgives coordination numbers. Qualitative informa-tion on coordination of the central atom may alsobe obtained from the observation of pre-edgepeaks. Information on dynamic and static disordercan also be extracted from the EXAFS. Hence, adetailed microscopic picture of the structure of acatalyst can be derived. XAS is particularly at-tractive for studies of catalysts under workingconditions, although there are limitations regard-ing temperature [222]. The combined applicationof XAS and XRD on the same sample usingsynchrotron radiation for in situ studies is an idealtool in catalysis research [225].

Electron Microscopy and Diffraction[213, 217–219], (see also! Microscopy, Chap.2). When electrons penetrate through matter in anelectron microscope, contrast is formed by dif-ferential absorption (amplitude contrast) or bydiffraction phenomena (phase contrast). Electronmicrographs of catalyst materials can provide foridentification of phases, images of surfaces andtheir morphologies, and elemental compositionsand distributions. Image interpretations are oftennot straightforward and need expert analysis.Several variants of electron microscopy use dif-ferent electron optics and working principles andtherefore have to be chosen according to theproblem to be solved.

Conventional transmission electron micros-copy (CTEM) operates in the 100 – 200 kV

range of electron energies, and imaging is basedon amplitude contrast in the bright-field mode.Point resolutions of 0.2 – 0.3 nm can beachieved in favorable cases. A typical applica-tion of CTEM in catalysis research is the exami-nation of metal particle sizes and their distribu-tions in supported catalysts.

Dark-field images are produced when thedirectly transmitted electron beam is excludedby the objective aperture, and only diffractedelectrons are used for imaging. This mode ofoperation selectively detects crystallites withcrystallographic spacings within a narrow range.

High-resolution electron microscopy(HREM) can be performed in CTEM instrumentsby modifying the mode of imaging, or in dedi-cated instruments operating at electron energiesof 0.5 – 1.0 MeV. HREM images can be directlyrelated to the atomic structure of the material[226]. Lattice fringes can be resolved, and thedetermination of the spacings of atomic planes isenabled. Support particles can thus be identified,and the crystal structure of heavy metal particleshaving sizes in the range down to 1 nm can beinvestigated.

In dedicated scanning transmission electronmicroscopes (STEM) an annular detector pro-vides the image formed from diffracted beams,while the central transmitted beam can be furtheranalyzed by using an electron spectrometer tosimultaneously provide elemental analysis. Theintensity distribution of electrons scattered athigh angles (40 – 150 mrad) depends on thesquare of the atomic number Z according to theRutherford scattering cross section. The STEMimages are therefore also called Z-contrastimages, and they are particularly useful for thestudy of catalysts containing small metal parti-cles [213].

In scanning electron microscopy (SEM) theimage is produced by scanning a finely focusedprobe beam in a raster pattern across the specimensurface. Emitted signals such as backscatteredand secondary electrons are detected and usedfor image formation. Secondary electrons aremost commonly used. The best resolutions thatcan be achieved with current generation SEMinstruments are approximately 1 nm. SEM ismost useful for studying sample topographies,and it can be applied with a significant back-ground pressure of a reactive gas while the sampleis observed (environmental SEM or ESEM).


Selected area electron diffraction (SAED)provides information on phase compositions andstructures at a microscopic level. The combina-tion of microdiffraction patterns and bright-fieldimages enables the determination of shapes andexposed facets in dispersed phases in solidcatalysts.

Analytical electron microscopy (AEM) per-mits the determination of the elemental compo-sition of a solid catalyst at the microscopic levelby energy dispersive detection of the electron-induced X-ray emission. Energy dispersive spec-troscopy (EDS) is sensitive for elements withatomic numbers Z > 11. For lighter elements(Z < 11), electron energy loss spectroscopyEELS is applied.

Controlled atmosphere electron microscopy(CAEM) [213, 227] is arousing considerableinterest as it will permit the observation ofchanges in the catalyst structure and morphologyunder reaction conditions.

Vibrational Spectroscopy [228] (! Infra-red and Raman Spectroscopy). Vibrational spec-troscopy is one of the most promising and mostwidely used methods for catalyst characteriza-tion, since it provides detailed structural infor-mation on the solid catalyst material and onsurface groups and adsorbates. Several vibration-al spectroscopic methods can be applied in situ,and they can be successfully used for studies onill-defined high surface area porous materials. Insituations where X-ray diffraction techniques arenot applicable, vibrational spectroscopy can of-ten provide information on phase transitions andchanges in compositions of bulk catalyst materi-als, on their crystallinity, and on the nature ofsurface functional groups. Most vibrational spec-troscopic methods are not surface-sensitive, butthey become surface-sensitive when vibrationalspectra are recorded for groups or adsorbates thatare present exclusively at the material’s surface.Representative examples for the structural char-acterization of solid catalysts by vibrational spec-troscopy are bulk oxides (including simple bina-ry oxides, multicomponent materials such asoxidation catalysts, and zeolites and molecularsieves), and supported oxides (e.g., monolayer-type catalysts), and sulfides. The vibrationalanalysis of surface groups, particularly of hy-droxyl groups, can also be addressed. In manycases surface hydroxyl groups (e.g., on oxides)

are simply formed by dissociative chemisorptionof water molecules, which reduces the surfacefree energy. Hydroxyl groups can also be con-stituents of the solid-state structure, for exampleas in zeolites.

There are several methods and techniques ofvibrational spectroscopy which are particularlysuitable in catalysis research. Infrared transmis-sion – absorption spectroscopy is the mostcommonly used technique. The KBr disk tech-nique is routine for transmission spectroscopyof powder samples. However, for in situ inves-tigations pressed self-supporting wafers have tobe used. Samples which exhibit only weak bulkabsorption, and the average particle size d ofwhich is smaller than the wavelength of theinfrared radiation (d < l) are optimally suitedfor the transmission mode. Transmission –absorption infrared spectroscopy has been par-ticularly successful in elucidating the structureof hydroxyl groups [40, 228]. More stronglyabsorbing materials, and particularly those hav-ing average particle sizes greater than the wave-length of the infrared radiation, which thereforecause significant scattering losses in transmis-sion, may preferentially be studied by diffusereflectance spectroscopy (DRS, DRIFT). Apowerful technique for structural studies oncatalytic materials under extreme temperatureand pressure conditions is laser Raman spec-troscopy (LRS), although laser heating and la-ser-induced fluorescence may cause serious pro-blems. One way, among others [228], to avoidfluorescence is to use of UV light instead ofvisible radiation for spectral excitation [229].LRS has been successfully applied for the struc-tural characterization of complex oxides, zeo-lites, and supported oxides and sulfides [228].Surface-enhanced Raman spectroscopy (SERS)has found some application in studies of finelydivided metal catalysts, particularly silver[230]. Second harmonic generation (SHG) andsum frequency generation (SFG) [231, 232] arenonlinear optical techniques with high surfacesensitivity which will probably find increasingapplication in studies relevant to catalysis.

Neutron Techniques [233] include neutrondiffraction and inelastic neutron scattering(INS). Both techniques are particularly sensitiveto light elements (such as H or D) and providecomplimentary structural information to XRD.


4.1.4. Local Environment of Elements

Nuclear spectroscopic methods provide informa-tion on the local environment of several selectedelements.

M€ossbauer spectroscopy and time differ-ential perturbed angular correlation(TDPAC) belong to the class of techniques whichdetect solid-state properties mediated by hyper-fine interactions via nuclear spectroscopy [234].Both techniques are g spectroscopies; they arebulk techniques and can be applied under in situconditions, although M€ossbauer spectroscopyrequires low temperatures.

M€ossbauer spectroscopy (M€ossbauer Spec-troscopy) [234, 235] provides information onoxidation states, phases, lattice symmetry, andlattice vibrations. Its application is limited toelements which exhibit the M€ossbauer effect,such as iron, cobalt, tin, iridium, ruthenium,antimony, and platinum. Particularly valuableinformation on catalyst structures has been ob-tained for iron catalysts for Fischer-Tropsch andammonia synthesis, and for cobalt-molybdenumhydrodesulfurization catalysts.

The time differential observation of the per-turbed angular correlation of g rays emitted fromradioactive nuclei (TDPAC) [234, 236, 237] is ag-spectroscopic technique which also allows thedetermination of hyperfine interactions such asnuclear electric quadrupole interactions (NQI).The NQI parameters enable local structural in-formation around the g emitter to be extracted.The technique has been successfully applied instudies on molybdenum-containing catalysts,and its application to tungsten seems promising.

Solid State Nuclear Magnetic Reso-nance [238–240] (see also ! Nuclear Mag-netic Resonance and Electron Spin ResonanceSpectroscopy, Chap. 4). NMR spectroscopy inheterogeneous catalysis principally allows thecharacterization of the chemical and structuralenvironment of atoms in the catalysts (or inspecies adsorbed on catalyst surfaces). NMRstudies on catalysts can be carried out over awide range of temperatures and pressures, aswell as in the presence of gases and liquids.Information can therefore be derived about thestructures of catalysts and their thermal orchemical transformations. In addition, specific

adsorbent – adsorbate interactions, the natureof chemically bonded surface species, andchemical reactions occurring at the catalystsurface can be studied. Most elements of interestin catalysis have isotopes that can be studiedwith modern NMR spectrometers. Isotope en-richments may be desirable or even necessaryfor certain elements, for example, 17O.

NMR spectra of solids are often complex sincestructure-dependent interactions such as dipolarinteractions, chemical shift interactions, quadru-polar interactions (for nuclei with spin I > 1/2)contribute strongly to the shape and position ofNMR lines. Because of their structure-dependencethese interactions are the main source of informa-tion on the structural environment of the nucleus inquestion. The selectivedetermination of the relatedinteraction parameters of structurally inequivalentnuclei is the major goal of an NMR experiment. Inwell-crystallized samples, the interaction para-meters adopt unique values, while in poorlycrystallized or amorphous powders they must bedescribed by distribution functions.

The anisotropy of the above-mentioned inter-actions results in line broadening, and the spectraof polycrystalline samples consist of a broadsuperposition of signals arising from differentorientations of the crystallites relative to thedirection of the external magnetic field Bo,weighted by the statistical probability with whicheach orientation occurs (powder patterns). Spe-cial techniques have been developed which re-move or at least reduce substantially these line-broadening effects and permit highly resolvedNMR spectra of powders with individual lines forinequivalent nuclei to be recorded. The mostimportant of these techniques are dipolar decou-pling, magic-angle spinning (MAS), and doubleoriented rotation (DOR). Cross-polarization(CP) improves the sensitivity for nuclei with lownatural abundance and allows the spatial prox-imity of nuclei to be monitored.

Typical examples for structural characteriza-tions by solid-state NMR [238] are studies onzeolites using 27Al and 29Si NMR. Informationon the distribution of Al in the environment of Siatoms and on the possible presence of nonframe-work Al species has been obtained. The locationof exchangeable alkali metal ions has been stud-ied by 23Na and 133Cs NMR. Vanadium- andmolybdenum-based catalysts have successfullybeen characterized by 51V and 95Mo NMR.


4.2. Chemical Properties

4.2.1. Surface Chemical Composition

The atomic composition of a catalyst surfaceplays a decisive role for the catalytic properties.Electron and ion spectroscopies [241] are sur-face-sensitive analytical tools which provide in-formation on the atomic composition within thetopmost atomic layers. The information depth,i.e., the number of atomic layers contributing tothe measured signal, depends on the method.Concentration profiles can be obtained by sputteretching of the surface by ion bombardment. Theapplication of these particle spectroscopies re-quires ultrahigh-vacuum (UHV) conditions.

The basis for the identification of atoms onsurfaces of solid materials by electron spectro-scopies, such as Auger electron spectroscopy(AES) and X-ray photoelectron spectroscopy(XPS) are the electronic binding energies. Withion spectroscopies, such as low-energy ion scat-tering (LEIS) and Rutherford backscattering(RDS), surface atoms are identified by theirnuclear masses. Ion bombardment of surfaces isaccompanied by sputtering processes (surfaceetching) which lead to the removal of secondaryionic and neutral particles. These are analyzed bymass spectroscopic techniques, such as second-ary ion mass spectroscopy (SIMS), and second-ary neutral mass spectroscopy (SNMS). Lessfrequently used is laser microprobe mass analy-sis (LAMMA). Relevant information on theproperties of the various surface analytical tech-niques is summarized in Table 7.

The physical principles of the various techni-ques have been discussed in several articles andmonographs [241, 242].

Electron Spectroscopy (AES, XPS) (seealso! Surface and Thin-Film Analysis, 2. Elec-

tron Detection, Chap. 3). These techniques useelectrons as information carriers. The electronscan be produced by the absorption of photonsresulting in photoemission. In XPS, X-rayphotons are used to ionize core levels, and thekinetic energyEk of the emitted photoelectrons ismeasured. The energy balance is given by:

Ek ¼ hn�Eb�F ð5Þ

This equation permits the electron binding ener-gy Eb (relative to the Fermi level) to be measuredwhen the photon energy hn and the work functionF of the spectrometer are known. The bindingenergies are characteristic for a particularelement.

As a result of the photoionization a singlyionized atom is formed, which can also be pro-duced by electron impact. The core hole (e.g., inthe K shell) can be filled by an electron from ahigher shell (e.g., the L1 shell) and the energy ofthis de-excitation process can be released byemission of an X-ray photon (X-ray fluorescence,XRF) or can be transferred to another electron(e.g., in the L2 shell) which is then emitted with awell-defined kinetic energy (Auger process).This kinetic energy is determined by the orbitalenergies EK, EL1, and EL2 of the three orbitalsinvolved. The Auger energy EKLL is then givenby:

EKLL ¼ EK�EL1�EL3�dE�F ð6Þwhere dE is a relaxation energy, and F thespectrometer work function. Clearly, EKLL ischaracteristic for an element and independent ofthe initial ionization process. Thus, both techni-ques permit the elemental constituents of a sur-face to be identified.

The information depth of both electron spec-troscopies is determined by the mean free path ofthe emitted electrons, which depends on thekinetic energy of the electron in the solid matrix.

Table 7. Characteristics of surface analytical techniques in standard applications (adapted from ref. [241])

Information Technique

AES XPS LEIS RBS SIMS SNMS

Surface sensitivity (monolayers) 2 – 5 5 – 10 1 – 2 20 – 50 2 – 4 2 – 4

Detection limits (monolayers) 10�2 – 10�3 10�2 10�3 10�3 10�6 10�6

Quantification* þ þþ þ þþþ – þChemical information* (þ) þ – – þ –

Structural information* – (þ) (þ) þ (þ) –

* Increasing number of positive signs indicates better capabilities; parentheses indication of restriction to special conditions.


This dependence is known [241–243]. The elec-tron mean free path is typically larger in oxidesthan in metals at equal energy, and it is particu-larly large for zeolites because of their lowdensity. Together with reported ionization crosssections and, in the case of AES, Auger decayprobabilities, quantitative surface analysis is pos-sible. The ratios of integral peak areas are pro-portional to concentration ratios. These can beanalyzed as a function of preparation and treat-ment conditions of a given catalyst system (e.g.,supported metal, oxide, or sulfide catalysts) andcompared with model calculations [242]. Infor-mation on the elemental distributions and ondispersions of active components thus becomesavailable.

Ion-scattering Spectroscopies [241] (seealso ! Surface and Thin-Film Analysis, 3. IonDetection, Chap. 3). In ion-scattering spectro-scopies solid surfaces are bombarded with mono-energetic ions, which are scattered on the topatomic layer (ion energies of about 0.5 – 5 keV,low-energy ion scattering (LEIS) [244, 245]) orwithin near-surface regions (ion energies ofabout 0.1 – 23 MeV, Rutherford backscattering(RBS) [246, 247]). In both cases the collisionkinematics can be described as simple binarycollisions, so that the kinetic energy of the back-scattered ion is directly dependent on the ratio ofthe masses of the projectile and the scatteringtarget atom and on the scattering angle. The massof the projectile is known and the scatteringangle is fixed and determined by the geometryof the spectrometer. Thus, the mass, and hencethe identity, of the scattering target atoms can bedetermined unequivocally.

The LEIS technique provides information onthe nature of the atomic constituents of thetopmost atomic layers. Quantitative analysis,however, is difficult since neutralization proba-bility, which makes the technique surface sensi-tive, is not easily available. Only a few percent ofthe primary ions are backscattered as ions in thecase of noble gas ion (e.g., Heþ). The techniquecan be applied for the characterization of realcatalyst surfaces, although surface roughnessreduces the signal intensity.

In contrast, in the energy regime of RBS thescattering cross sections can be calculated exact-ly. As a consequence, quantitative analysis ispossible by RBS, but the surface sensitivity is

lower than for LEIS. In optimal cases an infor-mation depth of 1 – 5 nm can be achieved. Acombined application of LEIS, RBS, and perhapsXPS is often most informative [241].

Secondary Particles. Ion bombardment ofa surface leads to ion etching with the release ofatoms and molecular fragments with varyingcharges (anions, cations, and neutrals) and exci-tation states. The mass analysis of secondary ionsby mass spectrometry [secondary ion mass spec-troscopy (SIMS)] has been developed as a highlysensitive and powerful surface analytical method(! Surface and Thin-Film Analysis, 3. Ion De-tection, Chap. 1) [248, 249]. Although destruc-tive because of the need for sputtering, the sput-tering rate can be kept low in the so-called staticmode (low primary-ion current density) so thatthe surface remains essentially unchanged. Sincethe sputtered particles are preferentially releasedfrom the first two atomic layers, the SIMS tech-nique is surface-sensitive. In contrast to the ion-scattering techniques, not only atomic constitu-ents of a surface can be detected but informationon the local environment of an atom in the surfacecan be obtained by analysis of molecular frag-ments. The detection of light elements, particu-larly hydrogen, is also possible. Quantification ofthe method is difficult, although not entirelyimpossible.

A high percentage of the sputtered secondaryparticles are neutral and must be postionized formass spectroscopic analysis [secondary neutralmass spectroscopy (SNMS), ! Surface andThin-Film Analysis, 3. Ion Detection, Chap. 2][250, 39]. Post-ionization can be achieved byelectron impact in a plasma or by an electronbeam. Alternatively, resonant and nonresonantlaser ionization can be applied. Applications ofSNMS for catalyst characterization have still notbeen reported.

4.2.2. Valence States andRedoxProperties

Electron Spectroscopies. X-ray photoelec-tron spectroscopy (XPS) and Auger electronspectroscopy (AES), in addition to elementalanalysis, permit information to be obtained onthe valence and bonding states of a given ele-ment. This is due to the fact that the core-levelbinding energies and Auger kinetic energies are


dependent on the chemical state, which leads tocharacteristic chemical shifts. In solids, the Ma-delung potential also plays an important role[242, 251, 252]. In addition, the ionization of anatom leads to relaxation phenomena which pro-vide a relaxation energy that is carried on by theemitted photoelectron. The binding energy of acore electron in level C of an atom is given by:

EbðCÞ ¼ const:þX

j

ðqj=RjÞþkq�Rea ð7Þ

where qj are the charges of all other atoms and Rj

their distances from the core-ionized atom. Theconstant is the Hartree – Fock energy of the coreelectron in the atomic level C for the free atom, kis the change of the core potential resulting fromthe removal of an electron, andRea represents theextra-atomic polarization energy.

For an Auger transition involving the atomiclevels C, C0, and C00, the kinetic energy Ek(C, C0,C00) of the Auger electron is related to the photo-electron binding energy Eb(C) in good approxi-mation by:

a0 ¼ Eb ðCÞþEk ðC;C0 :C0 0Þ ¼ const:þRea ð8Þ

where a0 is the so-called Auger parameter.Chemical shifts and the Auger parameter

provide detailed information on the chemicalstate of an element as regards its oxidation stateand its local environment. The latter is reflectedin the Auger parameter, which is dependenton the extra-atomic polarization energy andhence, on the structural and bonding character-istics of the atom under consideration. So-called Wagner plots [252, 253] in which theXP binding energy, the Auger energy, and theAuger parameter are correlated for families ofcompounds often permit the analysis of a com-pound of unknown structural and bondingcharacteristics.

Optical Spectroscopy and Electron Para-magnetic Resonance. Optical excitations inthe UV, VIS, and NIR regions and electronparamagnetic resonance (EPR) are classicaltechniques which provide information on theelectron configuration (oxidation state) of a metalcenter and on the symmetry of the ligand sphere[254–257]. While optical spectroscopy is appli-cable to practically all systems, EPR is limited toparamagnetic species, i.e. those which containone or more unpaired electrons.

UV – VIS – NIR spectroscopy covers a widerange of energies (typically 0.5 – 6 eV or 4000to 50 000 cm�1 or wavelengths (2500 to200 nm) as shown in Figure 12. Several typesof transitions occur in this range, namely, charge-transfer (CT) and d – d transitions, as also indi-cated in Figure 12. The first class of excitationsinvolves two adjacent atoms, one of which istypically a metal center and the other a ligand oranother metal atom. Electromagnetic radiationcan promote charge transfer from the ligand (L)to the metal (M), from the metal (M) to the ligand(L) or from one metal center to another. Thesetransitions are therefore called ligand-to-metalCT (LMCT), metal-to-ligand CT (MLCT), andmetal-to-metal CT (MMCT), respectively. Suchtransitions occur in molecular complexes and innonmolecular solids, such as metal oxides. Theenergy of CT transitions depends on the symme-try and oxidation state of the metal center and onthe nature of the ligand or of the second metalatom [258]. Hence, information on these proper-ties can be extracted from CT spectra. Thesespectra are relatively intense since they are di-pole-allowed. In contrast, metal-centered or in-tra-atomic transitions in transition metal atoms orions (ligand-field or d – d transitions) are ofmoderate or weak intensity because they areforbidden by the Laporte (orbital) selection rule(and also by the spin selection rule) unless theselection rules are relaxed by vibronic or spin –orbit coupling. The d – d transitions also provideinformation on the electron configuration and onthe symmetry of a complex (or local environment

Figure 12. Energy ranges of different types of electronictransitions (adopted from [254])


in a solid). Typical systems that have been stud-ied by optical spectroscopy are transition metaland base metal oxides, transition metal ions orcomplexes grafted on support surfaces, and tran-sition metal ion-exchanged zeolites.

While spectra of liquid samples can be re-corded in the transmission mode, catalyst pow-ders must be studied by the diffuse reflectancetechnique (DRS) [255, 256, 259]. The measureddiffuse reflectance can be converted into the so-called Schuster – Kubelka – Munk (SKM)function, which is directly proportional to theabsorption coefficient. The wavelength depen-dence of the SKM function is thus equivalent toan absorption spectrum, provided the scatteringcoefficient is independent of the wavelength.This condition is often fulfilled in the UV andVIS spectral regions. Simple quartz cells for insitu treatments can be designed, to which an EPRtube can also be connected for simultaneousoptical spectroscopy and EPR on the same sam-ple [254].

Luminescence spectroscopy [260, 261] hasproved to be a valuable addition to the spectros-copy techniques for characterization of solidcatalysts under well-defined conditions.

As mentioned above, electron paramagneticresonance (EPR) is used to study paramagneticspecies in catalytic materials. Besides the simplequalitative (and quantitative) detection of thepresence of paramagnetic sites, the spin densitydistribution at the paramagnetic center and on theneighboring atoms can be deduced from thespectra. Simulations of EPR spectra are oftenuseful for full interpretation. The extremelyhigh sensitivity of the EPR technique can be anadvantage but also a drawback because the im-portance of minority radical species may beoveremphasized. EPR is not surface-sensitive.However, radical species in the surface can easilybe identified by exposing the sample to paramag-netic O2, which leads to significant broadening ordisappearance of signals of surface species be-cause of dipole – dipole interactions.

Several reviews on the use of EPR in catalystcharacterization have been published [254, 262,263]. Typical applications of EPR are the detec-tion of paramagnetic states of transition metalions and analysis of the symmetry of their ligandsphere and/or their coordination, redox proper-ties of catalytic materials and their surfaces, andsurface anion or cation radicals deliberately pro-

duced by organic molecules as probes for theredox properties of the solid catalyst. Radicalspecies (e.g., in connection with coke formation)formed during catalytic reactions have also beendetected.

Thermal Analysis. Thermoanalytical tech-niques such as differential thermal analysis(DTA), thermogravimetry (TG), and differentialscanning calorimetry (DSC) are well-establishedmethods (! Thermal Analysis and Calorimetry)in solid-state chemistry [264, 265] which havesuccessfully been applied to investigating thegenesis of solid catalytic materials. They canalso be used to follow reduction and oxidationprocesses by measuring either thermal effectsand/or weight changes. When combined with anon-line mass spectrometer, changes in the gas-phase composition occurring during chemicaltransformations of the solid sample can be moni-tored simultaneously.

In temperature-programmed reduction (TPR),as first described by ROBERTSON et al. [266], astream of inert gas (N2 or Ar) containing ca.5 vol % H2 is passed through the catalyst bed of aflow reactor containing a reducible solid catalyst[267]. By monitoring continuously the H2 con-centration in the gas stream and its eventualconsumption with a thermal conductivity detec-tor while heating the sample with a linear tem-perature ramp of ca. 10 K/min, the rates ofreduction are obtained as a function of time (ortemperature). The total amount of H2 consumeddetermines the reduction equivalents present inthe catalyst, and detailed analysis of the experi-ment permits the kinetic parameters of the reduc-tion process to be determined and provides in-formation on reduction mechanisms. Character-istic numbers which depend on the experimentalparameters (amount of reducible species present,H2 concentration, flow rate, and temperatureramp) have been defined [268, 269]. These num-bers must be kept in certain ranges for optimalperformance of the experiment.

TPR experiments have been used to investi-gate the reduction behavior of bulk and supportedreducible species, solid solutions, promoted met-al catalysts, metals in zeolites, and of supportedsulfides and of nitrides [267].

Temperature-programmed oxidation (TPO)is an equally valuable technique for investigatingthe oxidation kinetics and mechanisms of reduced


materials [267]. Cyclic application of TPR andTPO provides information on the redox behaviorof catalytic materials, e.g., of catalysts for selec-tive catalytic oxidations.

4.2.3. Acidity and Basicity

Acid-catalyzed reactions are among the industri-ally most important hydrocarbon conversions.Acid sites can be classified as Lewis acidic sites,such as coordinatively unsaturated cations (e.g.,Al3þ on the surface of partially dehydroxylatedalumina, and Brønsted acidic sites, which aretypically surface OH groups as, e.g., in H formsof zeolites. Carbenium and carbonium ions arethought to be formed by protonation of hydro-carbons on these groups. Surface oxygen ionsmay function as Lewis basic centers, and if strongenough they may abstract protons from hydro-carbon molecules to form carbanion intermedi-ates. A typical solid base is MgO.

For characterization of acid and base proper-ties, the nature (Lewis or Brønsted) of the sites,the acid or base strength, and the number of sitesper unit surface area of a solid catalyst must bedetermined. Brønsted acidity is almost certainlyrequired for all acid-catalyzed reactions. How-ever, the mechanistic details on surfaces aresignificantly different from the well-known car-benium and carbonium ion chemistry in solution,because of the lack of the stabilizing effect ofsolvation in heterogeneously catalyzed gas-phase reactions. As shown by KAZANSKY [270],the electronic ground state of surface acidic OHgroups of oxides and in H forms of zeolites isessentially covalent. The main differences intheir acid strength are thought to be due to theenergetic positions of their electronically excitedheterolytic terms. Similarly, the interaction ofacid groups with alkenes does not result in theformation of adsorbed carbenium ions but ratherin the formation of more stable covalent alkox-ides. Basic sites (surface O2� ions) in the vicinityof OH groups could be involved in this process.Carbenium ions (and even more so carboniumions) are therefore not considered to be reactionintermediates in solid acid catalysis, but ratherexcited unstable ion pairs or transition statesresulting from electronic excitation of covalentsurface alkoxy species. Because of the proposedbifunctional nature of active acid sites in hetero-

geneous acid catalysis, it is necessary to charac-terize both the acidic and basic properties of solidcatalysts. Many different methods have beendeveloped for the characterization of acidity, butonly little is known about the basic character,particularly of materials that are typically con-sidered to be acid catalysts.

Chemical Characterization [271]. Titra-tion methods in aqueous medium are not veryinformative, because H2O tends to strongly mod-ify surface properties by molecular or dissociativechemisorption. Therefore, nonaqueous methodshave been proposed, in which the solvent (e.g.,benzene or isooctane) does not or only weaklyinteract with the catalyst surface. Hammett in-dicators were used to determine the acid strengthin terms of the Hammett – Deyrup H0 function:

H0 ¼ �log aHþ ðfB=fBHþ Þ ð9Þwhere aþH is the proton activity and fB and fBH

þ arethe activity coefficients of the basic probe and itsprotonated form, respectively. A series of Ham-mett indicators covers the range of �18 < H0

4, where H0 ¼ �12 corresponds to 100 %H2SO4.

Site densities and acid strength distributionswere determined by the n-butylamine titrationmethod [272]. As these titration and indicatormethods can yield erroneous results [273], theyare not frequently used today.

Isosteric heats of adsorption of strong bases(e.g., pyridine) may be considered as measures ofacid strength. However, a discrimination be-tween Lewis and Brønsted sites is not possible.

Temperature-programmed desorption [alsocalled thermal desorption spectroscopy (TDS)]of basic probe molecules has been developed as apowerful tool for the characterization of solidacids [274, 275]. In this method, a strong base isisothermally pre-adsorbed on an acidic catalystand then exposed to a stream of inert gas (e.g.,He). Heating by a temperature ramp (ca. 10 K/min) leads to desorption of the base. The integralarea of the desorption peak gives the total acidsite density, and the position of the peak maxi-mum provides the activation energy of desorp-tion (which may be close or identical to the heatof adsorption), which can be considered to be ameasure of the acid strength. This approach hasbeen applied for investigations of H forms ofzeolites using ammonia as the probe [276, 277].


However, discrimination between Lewis andBrønsted acid sites is again only possible withthe assistance of, e.g., vibrational spectroscopy.

Microcalorimetry [274]. Differential heatsof adsorption of probe molecules can be mea-sured with high accuracy by heat-flow calorime-try and differential scanning calorimetry. Thesedata provide information on the acid (or base)strength distribution. Ammonia and other amineshave been used as probes for acid sites on oxides[278] and in H forms of zeolites [279, 280], andcarbon dioxide and sulfur dioxide were adsorbedas acidic probes on several oxides [278].

Vibrational Spectroscopy [281–286]. Trans-mission infrared spectroscopy is the most frequen-tly applied technique for investigations into acidicand basic properties of solid catalysts. Surfacehydroxyl groups can easily be detected since theyfunction as dipolar oscillators. However, thestretching frequency of unperturbed OH groupscan not be taken as a measure of the acid strength.Lewis acidic and basic centers can only be detectedby vibrational frequencies with the adsorption ofsuitable probe molecules. Criteria for the selectionof optimal probe molecules have been defined byKNOEZINGER et al. [281, 286].

The use of basic probe molecules permits adiscrimination between Brønsted and Lewis acidsites. When a base B is adsorbed on an acidic OHgroup, hydrogen bonding followed eventually byprotonation of the base may occur (Eq. 3):

OHþB�OH��B�O��HþB ð10Þ

The strength of the hydrogen bond and the abilityof the OH group to protonate the base is deter-mined by the acid strength of the surface OHgroup and by the base strength (or proton affinity)of B. When hydrogen bonding occurs, the in-duced frequency shift of the O – H stretchingmode Dn�OH is a measure of the strength of thehydrogen bond DHB (Eq. 11) [287] and hence, ofthe acidity of the OH group.

jDn~OHj1=2 � DHB ð11Þ

Simultaneously, internal molecular modes of thebase B are modified, particularly when proton-ation occurs. These changes can also be used forthe interpretation of the bonding type of the

probe. Strong bases such as the traditional probemolecules ammonia and pyridine are protonatedby even very weak Brønsted sites which may notbe at all relevant in acid catalysis. Weaker basessuch as nitriles, carbon monoxide, and evendinitrogen and dihydrogen only undergo hydro-gen bonding (hydrogen bonding method [282]),but due to their weak interactions they are veryspecific and can provide very detailed informa-tion on the properties of acidic surfaces.

The same bases can be used for the detectionof Lewis acid sites L, with which they formsurface coordination compounds:

LþB�L B ð12Þ

The frequency shifts of the internal B modes are ameasure of the nature and strength of the coordi-native bond. For example, carbon monoxidewhen coordinated to L sites undergoes verytypical shifts of the C – O stretching frequencywhich provide information on the nature of theelement and of the coordinative bond, on theoxidation state, and on the coordination of the Lsite [281, 286].

The investigation of basic sites O2� by acidicprobe molecules AH with analysis of the vibra-tional spectra is much less advanced than that ofacid sites. The hydrogen-bond method can inprinciple be applied:

O2�þH�A�O2��H�A ð13Þ

Here the shift of the H–A stretching mode is ameasure of the hydrogen-bond strength andhence, the basic character (proton affinity) of thesurface O2� site. Recently, CH compounds suchas trichloromethane [282], acetylene and substi-tuted acetylenes [286, 288] and even methane[289] were proposed and successfully tested asacidic probe molecules. Pyrrole [285] and seve-ral Lewis acids [281] have also been used.

Surface chemical transformations of, e.g.,CO2, alcohols, ketones, acetonitrile, and pyridinegave detailed information on the bifunctionalacid – base pair character of several oxides,particularly of alumina [281, 290].

Nuclear Magnetic Resonance [291–293].Solid-state 1H magic-angle spinning (MAS)NMR spectroscopy measures proton chemicalshifts, which were thought to reflect the depro-


tonation energies of surface OH groups. How-ever, proton chemical shifts are also verysensitive to hydrogen bonding. Therefore,changes in proton chemical shifts induced byhydrogen bonding of probe molecules can alsobe used for the characterization of protic acidity.

Dissociative chemisorption of CH3I was pro-posed for characterization of surface basicity by13C NMR spectroscopy [294].

4.3. Mechanical Properties [295]

Catalyst particles are exposed to diverse mechan-ical strains during transportation, charging to thereactor, and operation. In fixed bed reactorscatalyst particles must withstand pressure causedby the mass of the catalyst charge and erosion byhigh-velocity gas streams. In fluidized- and mov-ing-bed reactors, the particles must resist attritionfrom rubbing against each other and from collid-ing with the walls of the reactor system. Thetechnical performance of catalysts depends ontheir mechanical strength to maintain integrityfor a reasonable time in spite of these strains.

There are three types of methods for determin-ing the strength of catalysts used under staticconditions [296]. For pellets and rings with noareas of distortion (preferably sized 1 cm or larg-er), the crushed (or crush) strength is determinedby exerting pressure on the specimen placed be-tween two horizontal plates of a hydraulic press.The upper plate moves down until the specimen iscrushed, at which point the pressure is recorded.The test is repeated for several particles, and thevalues are averaged. In the knife-edge hardnesstest, the upper plate of the press is replaced by aknife with a 0.3 mm edge. A mass of 1 kg isapplied to the knife and the percentage of brokensamples is recorded. The mass is then raised inincrements of 1 kg, and the test is repeated until100 % of the particles are broken or until a mass of10 kg is reached. Catalyst particles of irregularform are tested in a cylinder provided with a ram.After a definite pressure is applied, the sample isdischarged, and the weight percent of fines formedduring the test is determined by screening.

Tests for impact strength and resistanceagainst abrasion or attrition are carried out underdynamic conditions. For impact testing of verystrong catalysts (e.g., ammonia synthesis cata-lysts), a mass of 500 – 1000 g is dropped on the

particle from a standard height and the percent-age of unbroken, split, and broken samples out of20 or more is recorded.Abrasion tests on tabletedand extruded catalysts are carried out in a rotatinghorizontal steel cylinder provided with one baf-fle. The percent of fines (based on the mass of thecatalyst tested) formed after 1 h is reported asattrition loss. The attrition loss of fluid catalystsis measured by exposing the catalyst particles to ahigh-velocity air stream in a glass pipe. The finesformed during the test (prevented from escapingby a filter) are reported as attrition loss expressedas the percentage of the sample charged [297].

4.4. Characterization of SolidCatalysts under Working Conditions

The best description of a catalytic mechanism isthe corresponding catalytic cycle. As a first step,a detailed product analysis is required to differ-entiate between a single clean reaction and sys-tems undergoing parallel and/or consecutive re-actions. The microkinetic approach, as outlinedin ! Heterogeneous Catalysis and Solid Cata-lysts, 1. Fundamentals, Section 2.2, for the pre-diction of the overall rate of a catalytic reactiontaking into account the surface chemistry of thecatalyst and the elementary reactions involved, isthe most promising procedure to predict a mech-anism. However, the results of microkineticanalyses may not always be unequivocal, anddiscrimination between different kinetic modelsmay not be straightforward. Therefore, addition-al information is necessary to prove or disprovethe sequence of elementary steps (catalytic cy-cle) that represents the mechanism of a catalyzedreaction at the molecular level.

Quantum chemical calculations at variouslevels of sophistication and computer modelingprocedures now permit structures of adsorbedintermediates and transition states to be elucidat-ed and reaction energy diagrams to be computed(see! Heterogeneous Catalysis and Solid Cat-alysts, 1. Fundamentals, Section 2.3).

4.4.1. Temporal Analysis of Products(TAP Reactor)

Transient kinetics measurements can also pro-vide quantitative values of kinetic parameters


and elucidate individual reaction steps [298,299]. Pulse reactors are one type of transientreactors. A valuable laboratory pulse reactor(transient operation) is the TAP reactor (TAP ¼temporal analysis of products). Pulses containingsmall amounts of reactants (1013 – 1017 mole-cules per pulse) are injected into the evacuatedreactor containing the catalyst bed. The reactant/product molecules leaving the reactor (responsesignal) are analyzed by mass spectroscopy with atime resolution of less than 100 ms. This ap-proach permits surface processes on solid cata-lysts such as adsorption, reaction, and desorptionto be studied, and reaction mechanisms andkinetic models to be established [300–302].

In another kind of transient experiment, stepchanges in concentrations are effected, and theresponse of product concentration is measured asa function of time. The analysis of this responseprovides details of the course of reaction andpermits kinetic parameters to be determined.

4.4.2. Use of Isotopes

A powerful technique for the kinetic and mecha-nistic study of heterogeneous catalytic reactionsis steady-state isotopic-transient kinetic analysis(SSITKA) [299, 303]. The technique is based onthe detection of isotopic labels in the reactoreffluent species versus time following a stepchange in the isotopic labeling of one of thereactants in the reactor feed. Reactant and prod-uct concentrations and flow rates remain undis-turbed during the step change and – in the ab-sence of isotopic mass effects – steady-state con-ditions are maintained under isotopic-transientoperation. In contrast to other transient experi-ments, the steady-state kinetic behavior of thecatalyst surface can be studied. Steady-state ki-netic and mechanistic information which can beobtained from SSITKA includes concentrationsof different types of adsorbed reaction intermedi-ates, coverages, surface lifetimes, site heteroge-neity, activity distributions, and identification ofpossible mechanisms [299].

The use of isotopes can greatly aids the eluci-dation of catalytic mechanisms [304]. The mostfrequently used isotopes are 2H, 13C, 14C, and18O. Deuterium-exchange reactions with organicreactants yield isotopic distribution patternswhich are often specific enough to eliminate a

number of conceivable mechanisms. When car-ried out in conjunction with structure variations,isotopic distribution patterns may be effective innarrowing the range of possible mechanisms,even though such studies cannot give ‘‘the mech-anism’’ [5]. Deuterium labeling is also used todetermine which carbon atoms end up where orwhether a reaction is inter- or intramolecular [5].13C labeling can be used for the same purpose.Although nonradioactive labels are preferred,radioactive tracers such as 14C have also beenused [305]. 18O labeling has been applied toelucidate the relative rates of CO and CO2 inmethanol synthesis [306].

Kinetic isotope effects [307, 308] are due tothe different masses of a given element and itscorresponding isotope. The resulting differencein zero-point energy may lead to an increase inactivation energy of the labeled molecule andtherefore a reduction of the rate. Whether akinetic isotope effect occurs or not when an atomin a certain position or group is isotopicallylabeled (i.e., an X �� H bond is replaced byX �� D) for a catalytic reaction of interest pro-vides information on whether weakening or rup-ture of the X �� H bond is involved in a kineti-cally significant elementary step.

4.4.3. Use of Substituents, SelectiveFeeding, and Poisoning

Modification of organic molecules with suitablesubstituent groups may provide valuable infor-mation on reaction mechanisms from the stereo-chemistry of the reaction of interest [5, 309, 2].Substituents generally also have electronic ef-fects on the reactivity of a parent reactant (sub-stituent effects). Resulting linear free-energyrelationships for a series of substituents alsoassist the determination of kinetically significantreaction steps of a conceivable reaction mecha-nism [310, 311, 3, 4], since the substituentsdirectly affect the relative energy of the transitionstate and hence the activation barrier of a kineti-cally significant step.

Modification of molecules by substituentsmay also cause intra- or intermolecular stericeffects [5], and steric interactions between ad-sorbate and catalyst surface can be studied. Thelatter studies provide almost the only way todirectly probe the steric nature of active catalytic


sites without confusion with adsorption sites thatare not catalytic sites [5].

Selective feeding and scavenging have beenproposed for the characterization of reactionintermediates [5]. Suppose Q is a suspectedintermediate for a particular reaction. This hy-pothesis can be tested by adding (feeding) acompound to the reaction feed which is suppos-edly adsorbed to form the suspected intermediateQ, and by testing whether the added compound isindeed converted to the expected product. Inscavenging, a compound is added which shouldreact with the intermediate Q to form anothercompound which is not normally a product.

The nature of catalytic sites can be tested andtheir density estimated by selective poisoning[312].

4.4.4. Spatially Resolved Analysis of theFluid Phase over a Catalyst

Analysis of the temperature and concentrationprofiles in the fluid phase over a working catalystoften provides valuable information on the func-tional behavior of the catalyst. Since, at hightemperatures, conversion of reactants can alsoproceed via homogeneous reactions in the fluidphase aside from catalytic conversion, interac-tion of homogeneous and heterogeneous chemi-cal reactions and mass and heat transfer in thecatalyst-containing reactor becomes important tofully understand the function of the catalyst. Inparticular in these cases, temporally and spatiallyresolved profiles provide a more stringent test formodel development and evaluation.

Useful data arise from the experimentalresolution of local velocity profiles by laserDoppler anemometry/velocimetry (LDA, LDV)[313–315] and of spatial and temporal speciesprofiles by in situ, noninvasive methods such asRaman and laser-induced fluorescence (LIF)spectroscopy. For instance, an optically acces-sible catalytic channel reactor can be used toevaluate models for heterogeneous and homo-geneous chemistry as well as transport by thesimultaneous detection of stable species by Ra-man measurements and OH radicals by planarlaser-induced fluorescence (PLIF) [316, 317].For example, the onset of homogeneous ignitionof methane oxidation in a platinum-coated cat-alytic channel can be monitored by means of the

distribution of OH radicals. While catalyticoxidation of methane along the channel wallsreleases some OH radicals, at a certain point inthe reactor a transition to homogeneous oxida-tion occurs accompanied by high concentrationsof OH radicals in the flame region. Since tran-sient phenomena such as ignition, extinction,and oscillations of reactions are very sensitive totransport and kinetics, they can serve as mea-sures for critical evaluation of theoretical mod-els. For instance, the reliability of differentheterogeneous and homogeneous reactionschemes proposed in the literature was investi-gated by comparison of the experimentally de-rived ignition distances with numerical elliptictwo-dimensional simulations of the flow field byusing combinations of a variety of schemes[318, 319].

4.4.5. Spectroscopic Techniques

Any spectroscopic technique which is surface-sensitive, has sufficiently high sensitivity, andcan be applied under catalytic working condi-tions can provide valuable information on thenature of sufficiently long lived intermediates.However, spectroscopy often detects spectatorspecies rather than reaction intermediates. There-fore, it is mandatory to demonstrate that trueintermediates are in fact seen. This can be doneby varying critical reaction parameters and mon-itoring the response of spectroscopic signal in-tensities as a function of time. Sufficiently hightemporal resolution of the applied spectroscopictechnique is therefore required.

5. Design and Technical Operation ofSolid Catalysts

5.1. Design Criteria for Solid Catalysts[320]

Solid catalysts are used in a surprising variety ofshapes including powders and irregularly shapedparticles, regular particles such as spheres andcylinders, and more complex geometries likemonolithic honeycombs, gauzes, and fibers. Themost suitable geometry must be carefully select-ed and adjusted according to the properties ofthe catalytically active material and specific


requirements of the chemical reaction and cata-lytic reactor. Only rarely is the catalytic reactionso fast that the outer geometric surface area of anonporous catalytic body is sufficiently large.Hence, porous catalysts are mainly used in whichthe catalytically active surface area inside thestructure often exceeds the geometric surfacearea by several orders of magnitude. In thesecases, the pore structure must be accessible tothe reactants while products are allowed toleave. Design criteria for solid catalysts comprisethe choice of appropriate geometries with respectto highest possible catalyst utilization and prod-uct selectivity in a given reactor. These goalsshould be achieved at the lowest possible pres-sure drop over the reactor.

Diffusion, Mass- and Heat-Transfer Ef-fects [12, 321, 322]. Heterogeneous catalyticreactions take place on the external and internalsurfaces (pores) of the catalyst. External andinternal concentration and temperature gradi-ents can build up in the fluid (gas or liquid)boundary layer around the catalyst particles andinside the pores if mass and heat transfer be-tween the bulk of the fluid and the active sur-faces are not sufficiently fast. Such gradientstend to exist (1) in fixed-bed reactors chargedwith large, porous catalysts; (2) during opera-tion at low mass flow velocities (mass flow rateper unit cross-sectional area); or (3) in the caseof highly exothermic reactions. On the otherhand, gradient effects usually are absent whensmall catalyst particles are used in fluidized-bedreactors.

Because such internal and external gradientscan substantially reduce the activity and selec-tivity of the catalyst, conditions have beendelineated under which their adverse effectscan be minimized [12, 321, 322]. By carefullymatching operating conditions, catalyst, andreactor, optimum catalyst performance can beensured.

Mass and heat transfer in heterogeneous cata-lytic reactions occur in two ways.External trans-port to the external surface involves diffusionthrough the more or less stationary hydrodynam-ic boundary layer that surrounds the catalystparticle. The thickness of this layer depends onthe characteristics of the fluid and its flow ratepast the particle, and affects the rate of mass andheat transfer. Internal transport to the stationary

fluid in the pores of catalyst particle is controlledby diffusion alone.

Depending on the relative rates of the trans-port processes and the catalytic reaction, three orfour types of regimes can be distinguished. In thekinetic regime the rates of external and internalmass transport are much higher than the rate ofthe chemical reaction. Therefore, concentrationand temperature gradients between the fluid andthe center of a catalyst particle are negligible, andthe catalyst is fully utilized.

In the internal diffusion regime, mass trans-port in the catalyst pores is about as fast as, orslower than, the chemical reaction. In this casethere are considerable concentration gradientsalong the length of the pores, the effectivenessof the catalyst is impaired, and the apparentenergy of activation is lower than that observedin the kinetic regime.

At an even lower ratio of rates of transport andconversion, there is an intermediate regime inwhich the reaction takes place only on the exter-nal surface of the catalyst particles while theinternal surface area in the pores is inactive.Because of the limited heat transfer in this re-gime, exothermic reactions can overheat thecatalyst, and this results in a higher activity thanthat corresponding to the temperature of the fluid.

Finally, on further decreasing the ratio of therates of transport and conversion (e.g., by raisingthe temperature of the fluid), the external masstransfer regime is reached in which the reactionrate is controlled by mass transfer, and the con-centration of the reactants at the surface of thecatalyst particles drops. Raising the reactor tem-perature in this regime has little effect on thereaction rate, and the apparent activation energydrops.

Effectiveness Factor [12, 321, 322]. Theeffectiveness factor h is the ratio of the actualreaction rate observed on a porous catalyst parti-cle to the rate that would be obtained if the insideof the particle were exposed to the temperatureand reactant concentrations of the fluid. Mathe-matical analysis [12, 323–332] of mass transferin porous particles of different shapes has shownthat the effectiveness factor is a function of adimensionless quantity, called the Thiele modu-lusj [323]: for a sphere of radiusR and for a platesealed on one side and on the edges the thicknessof which is L, j is defined by the following


equations:

sphere : js ¼ Rkvc

m�1s

Deff

� �1/2

ð14Þ

plate : jL ¼ Lkvc

m�1s

Deff

� �1/2

where kv is the reaction rate constant per unit ofgross catalyst volume, cs is the concentration onthe surface, m is the reaction order, and Deff theeffective diffusion coefficient, given by

Deff ¼ DQt

ð15Þ

where D is the diffusion coefficient for a pair offluids taking into account binary and Knudsendiffusion, Q the void fraction of the porous mass,and t a factor allowing for tortuosity and varyingcross sections of the pores.

Equation (14) for the plate can also be used forarbitrary catalyst geometry if L is interpreted ascharacteristic diffusion length, i.e. the ratio ofcatalyst particle volume and its external surfacearea.

For first-order reactions (m ¼ 1), the effec-tiveness factors are as follows (tan is hyperbolictangent):

Sphere : h ¼ 3

js

1

tanh js

� 1

js

� �

ð16Þ

Plate : h ¼ tanh jL

jL

Correlation between the effectiveness factor andthe Thiele modulus for nonexothermic reactionsis shown in Figure 13 [328]. The effectiveness

factor is about unity for j < 1 and inverselyproportional to j for j > 3.

If the intrinsic velocity rate constant kv (Eq.15) cannot be determined directly, another di-mensionless modulus Q has been derived[326–328]. For first-order reactions occurring ina sphere, it is defined by

Q � j2h ¼ R2

DeffVcCs

� �

dn

dt

� �

ð17Þ

where dn/dt is the conversion rate in moles persecond of the reactant in the catalyst volume Vc.The effectiveness factor as a function of Q isshown in Figure 14 for a moderate energy ofactivation (E ¼ 10 RT; first-order reaction in aspherical particle) and variable enthalpy changeDH (l is the thermal conductivity of the catalyst).For exothermic reactions (b > 0), the effective-ness factor goes through a maximum value ex-ceeding unity because of the interaction of twoopposing effects. Poor mass transfer lowers theefficiency of the catalyst, whereas insufficientheat transfer raises catalyst temperature andreaction rate.

Effects on Selectivity [12, 321, 322]. Theeffect of mass- and heat-transport processes onthe selectivity of reactions yielding more thanone product depends on the selectivity type. In atype I reaction at low effectiveness factors, theobserved selectivity factor changes from k1/k2 toðk1DAeffk

�12 D�1BeffÞ1/2

, provided the order of thetwo reactions (A!X and B!Y) is the same. If,as usual, the ratio of the effective diffusivities is

Figure 13. Effectiveness factor w as a function of the Thielemodulus ws or sL

Flat plate sealed on one side and on edges, first-order reaction;D Same, second-order reaction; þ Spherical particle, first-order reaction* Reproduced with permission [328]

Figure 14. Effectiveness factor as a function of modulus(Eq. 17)--- Unstable region* Reproduced with permission [333]


smaller than k1/k2, the selectivity will drop at loweffectiveness factors. However, if the diffusivityratio is high, the selectivity can increase in aporous catalyst. This is the situation in the so-called shape-selective zeolites [12]. If the reac-tion orders are different, the reaction with thelower order is favored in the porous catalyst atlow h values.

In reaction type II, the effectiveness factor hasno influence on the selectivity if the orders of thetwo reactions are equal. Otherwise, the effect ofdifferent orders is the same as in type II reactions[322, 333].

For an isothermal first-order reaction of typeIII occurring at low effectiveness factor on aporous plate, the observed selectivity is approxi-mately the square root of the intrinsic rate con-stant ratio k1/k2 [334]. Figure 15 shows compar-ative conversions of A to X for type III ateffectiveness factors h ¼ 1 and h < 0.3, fork1/k2 ¼ 4; the maximum yield and selectivityboth drop by ca. 50 % at the low effectivenessfactor [334].

Temperature gradients caused by exothermicreactions favor reactions with higher apparentenergies of activation. Because such undesiredside reactions as decomposition and oxidativedegradation generally have high energies of ac-tivation, large catalysts having narrow porestructure can have an unfavorable effect on theselectivity and product yield.

In the regime of external mass transfer, resis-tance of the boundary layer to diffusion hassimilar effects on the selectivity of parallel andconsecutive reactions as does diffusion in pores.

Catalyst Geometries and Transfer Pro-cesses. Industrial processes that occur in thekinetic regime include reactions conducted influidized-bed reactors using catalysts 0.05 –0.25 mm in size [12, 322]. Because diffusioncoefficients in liquids are smaller by severalorders of magnitude than those in the gaseousphase, liquid-phase operation in the kineticregime requires a finely powdered catalyst.

Because the resistance to flow of catalystsincreases steeply with decreasing size, use ofcatalysts smaller than 2 – 3 mm in fixed-bedreactors is restricted to radial-flow reactors withsmall bed length. Ring-shaped and tablet cata-lysts show relatively favorable pressure drop anddiffusion characteristics.

In various partial oxidation processes (e.g., o-xylene to phthalic anhydride), good results havebeen obtained with so-called eggshell catalysts inwhich the active catalyst mass is applied in a thinlayer of a few tenths of a millimeter to theexternal surface of an inert, nonporous support.

If the reaction rate is higher and the regime ofinternal diffusion cannot be avoided, it is oftenadvantageous to use catalysts with a bimodalpore system in which micropores (< 2 nm) areconnected by macropores (> 50 nm) to theexternal surface area of the catalyst particle. Themicropores provide the needed high active sur-face area, whereas the macropores facilitate masstransport to and from the micropores. Such cat-alysts are especially suitable for operation atprocess pressures below 3 MPa. At these pres-sures mass transport into pores smaller than100 – 1000 nm occurs increasingly throughKnudsen diffusion involving molecular colli-sions with the pore wall rather than betweenmolecules [12, 322].

In fast processes occurring in the regime ofexternal mass transfer, the overall conversionrate is independent of the porosity of the catalystand is essentially proportional to its externalsurface area [329]. Examples of such reactionsare oxidation of ammonia to nitric acid andammoxidation of methane to hydrogen cyanidecarried out at 1070 – 1270 K on stacks of fine-mesh Pt – Rh gauze.

Figure 15. Effect of the effectiveness factor on catalystselectivity* Reproduced with permission [334]


5.2. Catalytic Reactors [335–339]

5.2.1. Classification of Reactors [33, 335,337]

Catalytic reactors can be classified by their modeof operation under steady-state or transient con-ditions or on their mode of contacting/mixing ofreactants and solid catalyst.

Typical steady-state reactors are packed-bedtubular reactors under continuous flow condi-tions, either plug flow or mixed flow. In an idealtubular reactor ideal mixing takes place in theradial direction, but there is no mixing in the axialdirection. Plug flow is attained in this case. Plugflow reactors can be operated either in integral ordifferential mode. In the latter case, single-passexperiments in small-scale reactors provide thedata for differential conditions required for anal-ysis of the reaction kinetics. As an alternative, theeffluent from the differentially operating reactorcan also be recycled externally or internally, thusapproaching a well-mixed reactor system, thecontinuous-flow stirred tank reactor (CSTR).Without inlet and outlet feed a continuous recy-cle flow results, characteristic of a batch reactorin which the feed composition changes with time(transient conditions as opposed to steady-stateconditions). The catalyst must not necessarilybe kept in a packed bed but can be suspended inthe liquid or gaseous fluid reactant mixture. In thefluidized-bed mode, the solid catalyst consistingof fine powder (particle diameter 10 – 200 mm)is kept in motion by an upward gas flow (fluid-ized-bed reactor, see Section 5.2.3). If the fluid isa liquid the catalyst can be suspended easily in aCSTR by efficient stirring (slurry reactor, seeSection 5.2.3). In so-called riser reactors, cata-lyst material is continuously introduced into andremoved from the reactor with the reactant andproduct feeds.

Batch and semi-batch reactors operate undertransient conditions. Discontinuous step or pulseoperation necessarily results in transientconditions.

Several catalytic reactor types are schemat-ically shown in Figure 16. Tubular fixed bedreactors (A) have an inlet flow n_0 and an outletflow n_ of the reactant-product mixture. Theadiabatic fixed-bed reactor is shown in Fig-ure 16 B. Multitube fixed-bed reactors (C) areused for highly exothermic reactions such as

the oxidation of o-xylene to phthalic anhydride.The principle of a CSTR is demonstrated as D.A fluidized-bed reactor with catalyst recircula-tion is sketched in E. A slurry CSTR reactor isschematically shown in F. An alternative forthree-phase reactions (gas – liquid – solid) isthe packed bubble column or slurry reactor(G). Discontinuous batch reactors with internaland external recycling operating under tran-sient conditions are depicted as H and I,respectively.

5.2.2. Laboratory Reactors [335, 336]

A compilation of laboratory reactors and theiroperation mode is given in Figure 17.

Laboratory reactors are small-scale reactors.Steady-state fixed-bed tubular flow reactors aremost frequently used for catalyst testing anddetermination of the reaction kinetics on thelaboratory scale. These reactors are favoredbecause of the small amounts of catalyst re-quired, the ease of operation, and the low cost.Stirred tank reactors (e.g., with fixed or rotatingbasket) and batch suspension reactors are lessfrequently used. Provided that heat- and mass-transfer limitations can be neglected under theoperating conditions in a tubular flow reactor,the catalyst bed is isothermal, and the pressuredrop across the catalyst bed is negligible, thereaction rate can be determined from the massbalance for a component i. This is usually veri-fied in microreactors operating under differen-tial conditions. The reaction rate per unit massrW (mol s�1 kg�1) is then given by Equation(18):

dxi

d WF0i

� � ¼ �nirw; ð18Þ

where Xi is the conversion of component i, ni itsstoichiometric coefficient, W the catalyst mass(kg), and Fi

o the molar rate of component i at thereactor inlet (mol s�1). The ratio W/Fi

o is thespace – time.

Ancillary techniques in laboratory units forcatalyst testing such as generation of feedstreams and product sampling are discussed in[340].

The TAP reactor, a laboratory pulse reactor, isdescribed in Section 4.4.1.


5.2.3. Industrial Reactors [33, 337]

Various types of industrial reactors and theiroperation mode are listed in Figure 18.

Catalytic Fixed-Bed Reactors [341–343].In the chemical industry fixed-bed reactors arethe standard type of reactors for heterogeneously

catalyzed gas-phase reactions (two-phase reac-tors). Fixed catalyst beds can be realized invarious ways. Randomly packed beds (deepbeds) require catalyst particles having differentshapes such as spheres, cylinders, rings, flat diskpellets, or crushed material of a defined sievefraction. The geometries and dimensions of thecatalyst particles are dictated by pressure drop

Figure 16. Schematic representation of several types of catalytic reactorsA) Tubular fixed bed reactor; B) Adiabatic fixed-bed reactor; C) Multitube fixed-bed reactor; D) CSTR; E) Fluidized-bedreactor with catalyst recirculation; F) Slurry CSTR reactor; G) Packed bubble column or slurry reactor; H), I) Discontinuousbatch reactors


and heat- and mass-transfer considerations. Theuse of monolithic catalysts significantly reducesthe pressure drop across the catalyst bed. Thistype of catalyst is employed for example forautomobile exhaust gas purification and for theremoval of nitrogen oxides from tail gases ofpower stations.

Fixed-bed reactors can be operated underadiabatic or nonadiabatic conditions. Adiabaticreactors can be applied for reactions with lowheats of reaction such as gas purification. Theyconsist of a cylindrical tube in which the cata-lyst is packed on a screen and is traversed inaxial direction (Fig. 16 B). This design is par-ticularly suitable when short residence timesand high temperatures are required. In this casea fixed bed of large diameter and small height(5 – 20 mm) is used (shallow bed). As anexample, for ammonia oxidation in nitric acidplants the fixed bed consists of several layers ofplatinum wire gauze with bed diameters up to

several meters. This type of reactor is limited tosmall catalyst volumes. The radial flow conceptis preferred when large amounts of catalyst arerequired [341]. In this reactor type, the catalystis charged in the annular space around anaxially located tube. The reactants are travers-ing radially, either from the inside or from theoutside of perforated plate rings. Because of thelow pressure drop, smaller catalyst pellets(4 � 4 or 3 � 3 mm) can be used in this reac-tor type.

Only limited conversions can be achieved byadiabatic reactors because of the necessary con-trol of the adiabatic temperature change. Multi-stage reactors consisting of several sequentialadiabatic stages which are separated by inter-stage heat exchangers have therefore beenintroduced.

Nonadiabatic operation can be achievedwith fixed-bed reactors which are cooled orheated through the reactor walls. Efficient heat

Figure 17. Classification of laboratory reactors


exchange results in so-called isothermal reactors.A typical example is the multitubular reactorschematically shown in Fig. 16 C, which isused for highly exothermic and temperaturesensitive reactions (e.g., o-xylene oxidation)with, e.g., salt melts as heat-transfer media. Ina multitube reactor, the pressure drop must bethe same in each tube so that the gas flow isdistributed uniformly over the tubes. Smallchanges in the packing density in the tubescan cause uneven heat transfer and, in the caseof highly exothermic reactions, hot spots andselectivity loss. For these reasons multitubereactors are filled by special equipment thatcharges each tube with the same amount ofcatalyst at a definite rate. After filling, the tubesare checked for pressure drop, and if necessary,the charge is adjusted.

Autothermal reactors can favorably be ap-plied for exothermic and temperature sensitivereaction systems. The conventional reactor de-sign consists of an adiabatic packed-bed reactorcoupled with a countercurrent heat exchanger in

which the cold reactant feed is brought to reac-tion temperature.

Multifunctional reactors are being developed[344] with the goal of improving operationconditions which are not necessarily optimallydetermined in standard fixed-bed reactorconfigurations.

The following industrial processes are per-formed in various types of fixed-bed reactors:

i. ‘‘deep-bed’’ adiabatic system:. Isomerization of C4 – C6 alkanes or of light

gasoline at 620 – 770 K, 20 – 40 bar on Pt-alumina (HCI activated) or on Pt-H-mordenite.

. Catalytic reforming of heavy gasoline usinga cascade of single bed reactors at 700 –820 K, 20 – 25 bar on K-promoted Cr2O3-Al2O3 catalyst.

. Hydrocracking of vacuum gas oil at 670 –770 K, 20 – 40 bar, using single or two stageprocesses on Ni-MoO3-Y-zeolite-aluminaand Pt-mordenite-alumina, respectively.

Figure 18. Classification of industrial reactors


ii. ‘‘multibed’’ adiabatic system:. Ammonia synthesis at 670 – 770 K, 200 –

300 bar on K-, Mg-, Al-promoted ironcatalysts.

. Oxidation of SO2 in the sulfuric acid pro-duction at 720 – 770 K, atmospheric pres-sure on K2SO4-V2O5 catalysts. Reactorwith externally located heat exchangers arein operation.

iii. ‘‘radial flow’’ system:. Water gas shift (HTS) at 620 – 670 K,

25 – 50 bar on Cr2O3-Fe2O3 catalysts.

iv. ‘‘shallow-bed’’ system:. Methanol dehydrogenation to formalde-

hyde at 870 K, atmospheric pressure onmetallic Ag (granulate).

. Ammonia oxidation to NOx at 1170 K at-mospheric pressure on Pt/Rh-grids.

v. ‘‘quench’’ system:. Methanol synthesis at 490 – 520 K, 50 –

100 bar on Cu-ZnO-Al2O3 catalysts.

vi. ‘‘multitube’’ system:. Methanol synthesis at 490 – 520 K, 50 –

100 bar on Cu-ZnO-Al2O3 catalysts.. Oxidation of ethylene to ethylene oxide at

470 – 520 K, atmospheric pressure on Ag-a-alumina.

. Oxidation of o-xylene to phthalic anhydrideat 640 – 680 K, at atmospheric pressure onV2O5-TiO2 catalysts.

. Hydrogenation of benzene to cyclohexane at470 – 520 K, 35 bar on Ni-SiO2 catalysts.

. Dehydrogenation of ethylbenzene to styreneat 770 – 870 K, atmospheric or reducedpressure on promoted (K, Ce, Mo) Fe-oxide.

Fluidized Bed Reactors [345–348]. Fluid-ized-bed reactors are preferred over fixed-bedreactors if rapid catalyst deactivation occurs oroperation in the explosive regime is required. Inthis type of reactor, an initially stationary bed ofcatalyst is brought to a fluidized-state by anupward stream of gas or liquid when the volumeflow rate of the fluid exceeds a certain limitingvalue, the minimum fluidization volume flowrate. The catalyst particles are held suspendedin the fluid stream at this or higher flow rates. Thepressure drop of fluid passing through the fluid-ized bed is equal to the difference between theweight of the solid catalyst particles and the

buoyancy divided by the cross-sectional area ofthe bed. Major advantages of fluidized-bed re-actors are excellent gas – solid contact, goodgas – solid heat and mass transfer, and highbed-wall heat transfer coefficients.

The gas distribution in the fluidized bed isperformed industrially by, for example, perforat-ed plates, nozzles, or bubble caps which aremounted at the reactor bottom.

Depending on the volume flow rate of the fluiddifferent types of fluidized beds form. Fluidiza-tion with a liquid feed leads to a uniform expan-sion of the bed. In contrast, solid-free bubblesform when fluidization is carried out in a gasstream. These bubbles move upwards and tend tocoalesce to larger bubbles as they reach increas-ing heights in the bed. At high gas volume flowrates, solid particles are carried out of the bed. Tomaintain steady-state operation of such a turbu-lent fluidized bed, the solid catalyst particlesentrained in the fluidizing gas must be collectedand transported back to the reactor bed. This canbe achieved most easily with an integrated cy-clone, as schematically shown in Figure 19 A. Acirculating fluidized bed is finally formed at stillhigher gas volume rates. An efficient externalrecycle system, as shown in Figure 19 B, isrequired for such operating conditions becauseof the high solids entrainment.

Catalytic cracking is carried out in fluidized-bed reactors because the solid acid catalysts arerapidly deactivated by coke deposition. The cat-alyst must therefore continuously be dischargedfrom the reactor and regenerated in an air-fluid-ized regenerator bed where the coke is burnedoff. The regenerated catalyst is then returned to

Figure 19. Schematic representation of two forms of gas –solid fluidized beds under turbulent flow conditions


the fluidized-bed reactor. The heat of combustionof the coke can be used for preheating of thereactant feed.

The main advantages of fluidized-bed reactorsare:

. Uniform temperature distribution (due to in-tensive solid mixing)

. Large solid-gas exchange area

. High heat-transfer coefficient between bed andimmersed heating or cooling surfaces.

However, these reactors have also some dis-advantages, e.g.:

. Expensive catalyst separation and purificationof reaction products (installation of cyclonesand filters)

. Undesired bypass of reactants due to bubbledevelopment

. Catalyst attrition

. Erosion of internals resulting from high solidsvelocities

Fluidized-bed reactors found very broad ap-plication in the petroleum refining and produc-tion of chemicals, for example:

. Catalytic cracking of vacuum gas oil to gaso-line at 720 – 820 K on aluminosilicates con-taining ultrastable Y-zeolites.

. Fischer – Tropsch synthesis (Synthol process)from CO und H2 at 620 – 670 K and 15 –30 bar on promoted Fe-oxide catalysts.

. Ammoxidation of propylene to acrylonitrile(SOHIO-process) at 670 – 770 K, 1 – 2 baron promoted Bi-MoOx catalysts.

. Oxidation of naphthalene or o-xylene to phtha-lic anhydride at 620 – 650 K, at atmosphericpressure on V2O5–SiO2 catalysts.

A special type of the fluidized-bed reactor isthe so-called riser reactor. This reactor consists ofa vertical tube in which the reaction takes place inthe presence of the entrained catalyst. Catalystcoming from the riser tube is collected in thevessel, before passing through the stripper to theregenerator (fluidized-bed type). The riser reac-tor is mainly used in the catalytic cracking ofheavy oils on highly active zeolitic catalysts.

The moving-bed reactor [345] operates withspherical catalyst particles larger (2 – 6 mm)

than those used in the fluid-bed system. In thestandard arrangement, catalyst particles are mov-ing slowly through the agitated bed. Catalystreaching the reactor top is transported into theregenerator. Using mechanical or pneumaticconveyer the regenerated catalyst is returning tothe bottom of the reactor.

The main advantage of the moving-bed reac-tor is lower catalyst attrition than in the fluidized-bed system. The disadvantage is a poor heattransfer, and therefore this reactor is not suitablefor exothermic reactions.

The moving-bed reactor found applicationmainly in petroleum cracking.

Slurry Reactors [349]. The aim of slurryreactors is intimate contact between a gas phasecomponent (which is to be dissolved in a liquid-phase component) and a finely dispersed solidcatalyst (three-phase reactors). The particle sizeof the solid catalyst is kept sufficiently small(< 200 mm) that it remains suspended by theturbulence of the liquid in the slurry reactor. Thisis in contrast to three-phase fluidized-bed reac-tors, in which an upward liquid flow is required tosuspend the larger catalyst particles.

On the basis of the contacting pattern of thephases and the mechanical devices that achievecontact and the mass transfer, nine types of slurryreactors can be distinguished [349]:

1. Slurry bubble column2. Countercurrent column3. Cocurrent upflow4. Cocurrent downflow5. Stirred vessel6. Draft-tube reactor7. Tray column8. Rotating-disk or multiagitator column9. Three-phase spray column

Slurry reactors are industrially applied for amultitude of processes [350], many of which areheterogeneously catalyzed processes for hydro-genation of edible oils. A new development is thecontinuous Fischer – Tropsch slurry synthesisprocess of SASOL in South Africa [351].

Three-phase reactors are classified as fixed-bed or suspension reactors depending on thecatalyst arrangement and shape:

Fixed-bed reactors operate either in the trick-le-bed or in the bubble-flow mode [349]. In the


first case, liquid reactants or reactants dissolvedin a solvent are flowing downward through thecatalyst bed and the gaseous reactants are con-ducted in the countercurrent or concurrentdirection.

In the bubble-flow reactors, liquid and gas-eous reactants are fed into the bottom of thecolumn and are flowing upward through thecatalyst fixed-bed.

The trickle-bed arrangement has some advan-tages such as:

. Fast diffusion of gases through the liquid filmto the catalyst surface

. Lower back-mixing

. No problems with catalyst separation

. Selective removal of catalytic poison in theentrance zone of the bed

. Simple catalyst regeneration

Drawbacks of trickle-bed reactors are:

Poor heat transferPartial utilization of the catalyst in case of the

incomplete wettingPossibility of ‘‘brooks’’ formation

The successful performance of the trickle-phase reactors depends on the suitable diame-ter/length ratio, catalyst shape and size, and theliquid flow distribution through the catalyst bed.The catalyst particle size is limited by the al-lowed pressure drop. Larger sizes (6 – 10 mmdiameter) are therefore preferred, which, howev-er, can bring diffusion problems.

The ‘‘bubble-flow’’ version is favored in par-ticular for reactions with a low space velocity. Agood heat transfer and no problems with anincomplete catalyst wetting are the main advan-tages of the ‘‘bubble-flow’’ reactors.

Both types of the three-phase reactors havefound numerous industrial applications, e.g.:

. Hydrotreating of petroleum fractions at 570 –620 K, 30 – 60 bar on Ni–MoO3–Al2O3

catalysts.. Hydrocracking of high boiling distillates at

570 – 670 K, 200 – 220 bar on Ni–MoO3–Y–zeolite–alumina catalysts.

. Selective hydogenation of C4-fractions (re-moval of dienes and alkines) at 300 –325 K, 5 – 20 bar on Pd-Al2O3 catalysts.

. Hydrogenation of aliphatic carbonyl com-pounds to alcohols at 370 – 420 K, 30 bar onCuO–Cr2O3 catalysts.

Suspension reactors [349, 350] are operatedsuccessfully in the chemical industry because oftheir good heat transfer, temperature control,catalyst utilization, and simple design. Becauseof the small catalyst particle size, there are noproblems with internal diffusion. Suspensionreactors operate either in the discontinuous or inthe continuous mode. One serious disadvantageis the difficult catalyst separation, especially iffine particles have to be removed from the vis-cous liquid.

Currently two types of suspension reactors arein use: stirred vessels and three-phase bubblecolumns.

In the case of stirred vessels the catalyst parti-cles (mainly smaller than 200 mm) are suspendedin the liquid reactant or solutions of reactants,whereas gaseous reactants are introduced at thebottom of the vessel through perforated tubes,plates or nozzles. The vessels are equipped withdifferent types of stirrers or turbines keeping thecatalyst suspended. Cooling and heating coils aswell as the gas recycle system belong to thestandard equipment. Stirred vessels operate mostlydiscontinuously. However, if continuous operationis favored, then stirred vessels are arranged in acascade to complete the required conversion.

Bubble columns are mainly continuously op-erating three-phase reactors. The gas is intro-duced at the bottom of the column throughnozzles, perforated plates or tubes. In the stan-dard arrangement the liquid reactant flows in thecocurrent direction with the gas. In some casesstirrers are installed to keep powdered catalystsin the suspension.

The gaseous reactants can be recycled by anexternal loop or by an internal system, such as aVenturi jet tube. This equipment is driven by arecycle of the slurry using a simple pump. TheVenturi tube is sucking the gas from the free boardabove the reactor back into the slurry. Heat ex-changers can be installed in the loop in both cases.

The main advantages of bubble columns aresimple and low-priced construction, good heattransfer, and good temperature control.

Suspension reactors are used predominantlyfor fat and oil hydrogenation 420 – 470 K, at 5 –15 bar using various Ni–kieselguhr catalysts.


Also, hydrogenolysis of fatty acid methylesters to fatty alcohols is performed in suspensionreactors at 450 – 490 K, 200 – 300 bar on pro-moted copper chromites (CuO–Cr2O3).

Further applications are: syntheses of acetal-dehyde and of vinyl acetate according to theWacker process.

5.2.4. Special Reactor Types and Processes

Microstructured Reactors [352–357]. Amicrostructured reactor can be defined as a seriesof interconnecting channels having diametersbetween 10 and 1000 mm that are formed in aplanar surface in which small quantities of re-agents are manipulated (see also ! Microreac-tors – Modeling and Simulation). Among theadvantages of microstructured reactors over con-ventional catalytic reactors are high heat-transfercoefficients, increased surface-to-volume ratiosof up to 50 000 m2 m�3, as opposed to 1000 m2

m�3 for conventional catalytic laboratory reac-tors, shorter mixing times, and localized controlof concentration gradients. The small scales re-duce exposure to toxic or hazardous materials,and the enclosed nature of the microstructuredreactors permits greater ease of containment inthe event of a runaway reaction. Furthermore, thehighly efficient heat transfer as well as highsurface areas available for adsorption of radicalsallow reactions to be carried out beyond theexplosion limit [354, 358, 359].

Despite their small size, microstructured re-actors can be used for synthetic chemistry [360],since as few as 1000 microchannels operatingcontinuously could produce 1 kg of product perday. While several types of laboratory-scalemicrostructured reactors are already commer-cially available, further developments are stillrequired to make microstructured reactors ma-ture for industrial application. One of the mostadvanced designs targeted at large-scale appli-cations for exothermic gas-phase reactions is theDEMiS project of Degussa and Uhde [361]. Thestate of development of microstructured reactorsfor heterogeneously catalyzed gas-phase andliquid-phase reactions has been summarized[185]. Coating of wall materials with catalysts,strategies for replacement of spent catalysts, andthe reduction of overall apparatus size appear tobe the most challenging obstacles to be over-

come. It is also evident that in many cases moreactive catalysts are required for full utilization ofmicrostructured reactors.

Unsteady-State Reactor Operation [362].Forced unsteady-state reactor operation has beenapplied to continuous catalytic processes in fixedand f1uidized-bed reactors. This operating modecan lead to improved reactor performance. Non-linearity of chemical reaction rates and complex-ity of reaction systems are responsible for con-version or selectivity improvements under forcedunsteady-state conditions [363].

An unsteady-state in a fixed-bed reactor canbe created by oscillations in the inlet compositionor temperature (control function) such as sche-matically shown in Figure 20. The most widelyapplied technique in a fixed-bed reactor is peri-odic flow reversal. Examples of this operationmode in industrial applications are SO2 oxida-tion, NOx reduction by NH3, and oxidation ofvolatile organic compounds (VOC). In fluidized-beds for exothermic reactions, favorable un-steady-state conditions of the catalyst can beachieved by catalyst circulation inside the reac-tor. The unsteady-state operation in the fluidized-bed is applied for example for the partial oxida-tion of n-butane to maleic anhydride on vanadylpyrophosphate catalysts developed by DuPont.In the first step, n-butane diluted with an inert gasis contacted in the riser reactor (residence time10 – 30 s) with spherical catalyst particles(100 mm). The partial oxidation of n-butane pro-ceeds on account of the lattice oxygen in thesurface layer of vanadyl pyrophosphate. In thesecond step, the partially reduced catalyst istransported into the fluidized-bed reactor wherethe catalyst reoxidation takes place. The obtainedselectivity was about 10 % higher than that in themultitubular fixed-bed reactor, operating under

Figure 20. Stepwise variation of inlet parameters to createunsteady state conditions


steady-state conditions. A further group of forcedunsteady-state processes uses the combination ofa chemical reaction with the separation of pro-ducts (chromatographic reactor). Systems ap-plied till today operating on the principle ofchromatographic columns are filled with a cata-lyst possessing suitable adsorption properties,such as Pt - Al2O3. Pulses of reactant are peri-odically injected into this reactor which is purgedby carrier gas during periods between the pulses.This operation can provide a higher conversionfor reversible reactions if one of the reactionproducts is adsorbed on the catalyst more strong-ly than the other one. The feasibility of thisprinciple was tested in the dehydrogenation ofcyclohexane to benzene on pilot scale.

Membrane Reactors [364–366]. In mem-brane reactors (! Membrane Reactors) cata-lytic conversion is coupled with a separationeffect provided by the integrated membrane.High conversions can be achieved for equilibri-um-restricted reactions when one of the reactionproducts can be removed from the reactionmixture by diffusion through a permselectivemembrane. Hydrocarbon dehydrogenation re-actions are the best example for this application.In addition, catalytic membrane reactors havebeen proposed for improved control of the se-lectivity of some catalytic reactions. For exam-ple, controlled introduction of a reactant into thereactor by selective or preferential permeationmay limit possible secondary reactions of thetarget product.

Two types of membranes can be distin-guished: dense membranes and porous mem-branes. Dense metallic membranes consist ofthin metal foils. Palladium and palladium al-loys (PdAg, PdRu) are specific for hydrogenpermeation. Dense oxide membranes are usu-ally solid electrolytes such as ZrO2 and CeO2,which are permeable for O2 [366]. Porousmembranes are typically made of oxides, al-though carbon membranes have also been used[367]. Ceramic membranes consist of severallayers of material with progressively decreas-ing pore size. The top layer with the smallestpore size controls the separation. Most of thesemembranes are produced by sol – gel techni-ques. Intrinsic catalytic properties can be in-troduced into these membranes, which can beproduced as cylindrical tubes forming the basis

for tubular reactors. Zeolite membranes canalso be prepared, and the structure of the zeoliteshould permit high selectivity in separationprocesses.

Despite the potential of membrane reactors,their development is still not mature for industrialapplication.

Reactive Distillation [368–370]. In reac-tive distillation (! Reactive Distillation) frac-tional distillation and chemical reaction are per-formed simultaneously, e.g., for a reaction of thetype

AþB!CþDin which at least one of the products has avolatility which is different form those of theother compounds. The most attractive features ofreactive distillation are:

1. The separation of at least one of the productsby distillation drives reactions to completionwhich are otherwise equilibrium-limited.

2. Reactions in which high concentrations of aproduct or one of the reactants lead to unde-sired side reaction, can be carried out.

Despite these advantages, several chemicaland physical limitations exist in practice forits use in chemical processes, so that reactivedistillation has found application for only a fewimportant reactions. Industrially important pro-cesses are various etherification, esterification,alkylation, and isomerization reactions.

IFP, Mobil, Neste Oy, Snamprogetti, Texaco,and UOP are providing licenses for plants toproduce tert-butyl and tert-amyl ethers from thecorresponding olefins and methanol or ethanolusing strong acidic resins as solid catalysts.

Reactions under Supercritical Conditions[371–374]. The major advantage of supercrit-ical fluids as solvents in catalytic reactions isthe fact that carbon dioxide and water can beused as environmentally benign solvents. Whenin their supercritical state, these nontoxic com-pounds are good solvents for many organiccompounds. A multicomponent system undersupercritical conditions may behave like a sin-gle gaslike phase with advantageous physicalproperties. Under reaction conditions this leadsto [371]:


1. Higher reactant concentrations2. Elimination of contact problems and diffusion

limitations in multiphase reaction systems3. Easier separations and downstream proces-

sing4. In situ extraction of coke precursors5. Strongly pressure and temperature dependent

solvent properties near the critical point6. Higher diffusivities than in liquid solvents7. Better heat transfer than in gases8. Use of clustering to alter selectivities

Hence, supercritical fluids offer strategies formore economical and environmentally benignprocess design, mainly because of enhancedreaction rates, prolongation of catalyst lifetime,and simplification of downstream processing.

5.2.5. Simulation of Catalytic Reactors[13]

Catalytic reactors are generally characterized bya complex interaction of various physical andchemical processes. Therefore, the challenge incatalysis is not only the development of newcatalysts to synthesize a desired product, but alsothe understanding of the interaction of the cata-lyst with the surrounding reactive flow field.Sometimes, the exploitation of these interactionscan lead to the desired product selectivity andyield. This challenge calls for the development ofreliable simulation tools that integrate detailedmodels of reaction chemistry and computationalfluid dynamics (CFD) modeling of macroscaleflow structures. The consideration of detailedmodels for chemical reactions, in particular forheterogeneous reactions, however, is still verychallenging due to the large number of speciesmass-conservation equations, their highly non-linear coupling, and the wide range of timescalesintroduced by the complex reaction networks.

Currently available multipurpose commercialCFD codes can simulate very complex flow con-figurations including turbulence and multicom-ponent species transport. However, CFD codesstill have difficulties in implementing complexmodels for chemical processes, in which an areaof recent development is the implementationof detailed models for heterogeneous reactions.Several software packages have been developedfor modeling complex reaction kinetics in CFD

such as CHEMKIN [375], CANTERA [376],DETCHEM [377], which also offer CFD codesfor special reactor configurations such as channelflows and monolithic reactors. These kineticpackages and also a variety of user written sub-routines for modeling complex reaction kineticshave meanwhile been coupled to several com-mercial CFD codes. Aside from the commerciallywidespread multipurpose CFD software packagessuch as FLUENT [378], STAR-CD [379], FIRE[380], CFD-ACEþ [381], and CFX [382], avariety of multipurpose and specialized CFDcodes have been developed in academia and atresearch facilities such as MP-SALSA [383].

From a reaction-engineering perspective,computational fluid dynamics simulations havematured to a powerful tool for understandingmass and heat transport in catalytic reactors.Initially, CFD calculations focused on a betterunderstanding of mixing, mass transfer to en-hance reaction rates, diffusion in porous media,and heat transfer. Later, the flow field and heattransport models were also coupled with modelsfor heterogeneous chemical reactions. So far,most of these models are based on the mean-field approximation as discussed in ! Hetero-geneous Catalysis and Solid Catalysts, 1. Funda-mentals, Section 2.3.3, in which the local state ofthe surface is described by its coverage withadsorbed species averaged on a microscopicscale.

Detailed CFD simulations of catalytic reac-tors, often including multistep reaction mechan-isms, have been carried out for catalytic channelreactors with laminar [384] and turbulent [385]flow fields, monolithic reactors [386–390], fixed-bed reactors [391, 392], wire-gauze reactors[393, 394], reactors with multiphase flow[378], and others. CFD simulations are becominga powerful tool for understanding the behavior ofcatalytic reactors and in supporting the designand optimization of reactors and processes.

5.3. Catalyst Deactivation andRegeneration

5.3.1. Different Types of Deactivation

As has been observed in the laboratory and inindustrial application, heterogeneous catalystsare deactivated during time on stream. For


example, in fluid-bed catalytic cracking and pro-pene ammoxidation the catalyst life is limited to afew seconds or minutes, while in other reactions,such as NH3 and CO oxidation the catalystremains active for several years. Not only lossof activity but also a decrease in selectivity isusually caused by catalyst deactivation[395–397].

The activity loss can be compensated withincertain limits by increasing the reaction temper-ature. However, if such compensation is notefficient enough, the catalyst must be regeneratedor replaced.

The main reasons for catalyst deactivationare:

1. Poisoning2. Fouling3. Thermal degradation4. Volatilization of active components

Some types of catalyst deactivation are re-versible, e.g., catalyst fouling and some specialtypes of poisoning. Other types of deactivationare mostly irreversible [395, 396, 398].

Catalyst Poisoning. The blocking of activesites by certain elements or compounds accom-panied by chemisorption or formation of surfacecomplexes are the main causes of catalyst de-activation [395, 396, 398]. If the chemisorptionis weak, reactivation may occur; if it is strong,deactivation results. Chemical species oftenconsidered as poisons can be divided in fiveclasses:

1. Group 15 and 16 elements such as As, P, S, Se,and Te

2. Metals and ions, e.g., Pb, Hg, Sb, Cd3. Molecules with free electron pairs that are

strongly chemisorbed, e.g., CO, HCN, NO4. NH3, H2O and organic bases, e.g., aliphatic or

aromatic amines, pyridine, and quinoline5. Various compounds which can react with

different active sites, e.g., NO, SO2, SO3,CO2.

Elements of class 1 and their compounds, forexample, H2S, mercaptans, PH3, and AsH3, arevery strong poisons for metallic catalysts, espe-cially for those containing Ni, Co, Cu, Fe, andnoble metals [395, 396, 398].

Elements of the class 2 can form alloys withactive metals and deactivate various systems inthis way [395, 399].

CO is chemisorbed strongly on Ni or Co andblocks active sites. Below 450 K and at elevatedpressure the formation of volatile metal carbo-nyls is possible [395, 396], and catalyst activity isstrongly reduced.

Ammonia, amines, alcohols, and water arewell known poisons for acidic catalysts, espe-cially for those based on zeolites [395, 400].

Catalysts or carriers containing alkali metalsare sensitive to CO2, SO2, and SO3.

Catalyst Fouling. Because most catalystsand supports are porous, blockage of pores,especially of micropores, by polymeric com-pounds is a frequent cause of catalyst deactiva-tion. At elevated temperatures (> 770 K) suchpolymers are transformed to black carbonaceousmaterials generally called coke [395, 401]. Cat-alysts possessing acidic or hydrogenating – de-hydrogenating functions are especially sensitiveto coking.

There are different types of coke, such as Ca,Cb, carbidic or graphitic coke, and whisker car-bon [395]. Ca is atomic carbon formed as a resultof hydrocarbon cracking on nickel surfacesabove 870 K. Ca carbon can be transformed athigher temperatures to polymeric carbon (Cb)which has a strongly deactivating effect. Cacarbon can also dissolve in metals and formsmetal carbides, and it may precipitate at grainboundaries. Metal-dissolved carbon may alsoinitiate the growth of carbon whiskers, whichcan bear metal particles at their tops.

Coke formation can be minimized, for exam-ple, in methane steam reforming by sufficientlyhigh steam/methane ratio or/and by the alkaliza-tion of the carrier.

ThermalDegradation. One type of thermaldegradation is the agglomeration of small metalcrystallites below the melting point, called sin-tering [395]. The rate of sintering increases withincreasing temperature. The presence of steam inthe feed can accelerate the sintering of metalcrystallites.

Another type of thermal degradation are solid-solid reactions occurring especially at highertemperatures (above 970 K). Examples are reac-tions between metals, such as Cu, Ni, Co, and


alumina carriers which result in the formation ofinactive metal aluminates [395, 402].

Also, phase changes belong to the category ofthermal degradations. A prominent example isthe reduction of the surface area of alumina from250 m2 g�1 (g phase) to 1 – 2 m2 g�1 (a phase)by thermal treatment between 870 and 1270 K.

Alternating oxidation and reduction of thesystem, as well as temperature fluctuations, areoften accompanied by activity losses and cancause mechanical strain in catalyst pellets.Therefore, mechanical strength of catalyst par-ticles has major industrial importance.

Volatization of Active Components. Somecatalytic systems containing P2O5, MoO3, Bi2O3,etc. lose their activity on heating close to thesublimation point. Cu, Ni, Fe and noble metalscan escape from catalysts after conversion tovolatile chlorides if traces of chlorine are presentin the feed.

5.3.2. Catalyst Regeneration

The regeneration of metallic catalysts poisonedby Group 16 elements is generally rather diffi-cult. For example, oxidation of sulfur-poisonedmetallic catalysts converts metal sulfides to SO3,which desorbs from metals. However, if thecatalyst or carrier contains Al2O3, ZnO, MgOthen SO3 forms the corresponding sulfates. Whenthe catalyst is subsequently brought on-line un-der reducing conditions, then H2S is formed fromsulfates and the catalyst will be repoisoned [395,403].

Therefore it is necessary to remove poisonsfrom the feed as completely as possible. Furtherprevention is the installation of a guard-bedcontaining effective poison adsorbents in frontof the reactor.

Ni catalysts poisoned with CO or HCN canbe regenerated by H2 treatment at temperaturesthat allow formation of methane and NH3,respectively.

The original activity of acidic catalysts poi-soned partially by H2O, alcohols, NH3, andamines can be restored by thermal treatment atsufficiently high temperatures.

From the industrial point of view, the regen-eration of coked catalysts is very important. Theremoval of coke depends on its structure and on

the catalyst composition. Alkali metals, especial-ly potassium, accelerate coke gasification. Oxi-dation is the fastest gasification reaction, but it ishighly exothermic [395, 404]. To maintain thetemperature within allowed limits, mixtures ofO2, steam, and N2 are mainly used to remove thecoke.

Catalysts deactivated by thermal degradationare very difficult to regenerate. Certain Pt –Al2O3 catalysts, deactivated as a result of thermalPt sintering, can be partly regenerated by chlorinetreatment at elevated temperatures, which makesPt redistribution possible.

5.3.3. Catalyst Reworking and Disposal

The leaching of precious metals from spentcatalysts is widely practiced. Producers of noblemetal catalysts, such as Engelhard Corp./BASF,Johnson Matthey, and Umicore have plants forthis purpose. Yields of recovered precious metalsare 90 – 98 % depending on the original metalcontent and on the nature of the support used.

Besides precious metals, Ni from spent Ni –kieselguhr catalysts used in fat and oil hydro-genations are reworked to a large extent. BeforeNi leaching, fats must be removed.

Hydrotreating catalysts containing Mo, Ni orCo are also reworked. Before leaching of themetals, coke and sulfur are removed by roasting[395, 396].

In various cases, however, problems arisewhen the price of recovered components doesnot cover the costs of catalyst reworking. Thefinal alternative is the disposal of the spent cata-lyst. In general, catalysts containing Al, Si, Fecan be disposed of without any special precau-tions or can be used in construction materials.However, if such catalysts contain Ni or V accu-mulated during their use, then removal of theseelements below legislative limits is necessary[395].

Spent Cu- and Cr-containing catalysts aresometimes accepted by metallurgical plants.

Before disposal, spent catalysts containingvarious contaminants need to be encapsulated toavoid their release into water. Materials used forencapsulation are, e.g., bitumen, cement, wax,and polyethylene [395]. Nevertheless, the dis-posal of encapsulated catalysts is not only ex-pensive but is becoming increasingly difficult.


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Further Reading

D. K. Chakrabarty, B. Viswanathan: Heterogeneous Cataly-

sis, New Age Science, Tunbridge Wells 2009.

K. Ding, Y. Uozumi (eds.): Handbook of Asymmetric Het-

erogeneous Catalysis, Wiley-VCH, Weinheim 2008.


E. M. Gaigneaux, M. Devillers, D. E. de Vos, S. Hermans, P.

Jacobs, J. Martens, R. Ruiz (eds.): Scientific Bases for the

Preparation of Heterogeneous Catalysts, Elsevier, Am-

sterdam 2006.

D. L. Marmaduke (ed.): Progress in Heterogeneous Cataly-

sis, Nova Science Publ., New York, NY 2008.

N. Mizuno (ed.): Modern Heterogeneous Oxidation Cataly-

sis, Wiley-VCH, Weinheim 2009.

G. F. Swiegers: Mechanical Catalysis, Wiley, Hoboken, NJ

2008.

R. A. van Santen, M. Neurock: Molecular Heterogeneous

Catalysis, Wiley-VCH, Weinheim 2006.


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