Heterogeneous Catalysis and Solid Catalysts

7/30/2019 Heterogeneous Catalysis and Solid Catalysts

1/110

Heterogeneous Catalysisand Solid Catalysts

OLAF DEUTSCHMANN, Institut fur Technische Chemie und Polymerchemie, Universitat Karlsruhe (TH), Enges-

serstr. 20, Karlsruhe, Germany

HELMUT KN OZINGER, Department Chemie, Universitat Munchen, Butenandtstr. 5 13 (Haus E), Munchen,

Germany 81377

KARL KOCHLOEFL, Schwarzenbergstr. 15, Rosenheim, Germany 83026

THOMAS TUREK, Institut fur Chemische Verfahrenstechnik, TU Clausthal, Leibnizstr. 17, Clausthal-Zellerfeld,

Germany

1. Introduction. . . . . . . . . . . . . . . . . . . . . 2

1.1. Types of Catalysis . . . . . . . . . . . . . . 2

1.2. Catalysis as a Scientific Discipline . . 31.3. Industrial Importance of Catalysis. . 5

1.4. History of Catalysis . . . . . . . . . . . . . 5

2. Theoretical Aspects . . . . . . . . . . . . . . . 7

2.1. Principles and Concepts. . . . . . . . . . 8

2.1.1. Sabatiers Principle . . . . . . . . . . . . . . 8

2.1.2. The Principle of Active Sites . . . . . . . 8

2.1.3. Surface Coordination Chemistry . . . . . 9

2.1.4. Modifiers and Promoters . . . . . . . . . . 10

2.1.5. Active Phase Support Interactions . . 10

2.1.6. Spillover Phenomena . . . . . . . . . . . . . 12

2.1.7. Phase-Cooperation and Site-IsolationConcepts . . . . . . . . . . . . . . . . . . . . . . 12

2.1.8. Shape-Selectivity Concept . . . . . . . . . 13

2.1.9. Principles of the Catalytic Cycle. . . . . 14

2.2. Kinetics of Heterogeneous Catalytic

Reactions . . . . . . . . . . . . . . . . . . . . 14

2.2.1. Concepts of Reaction Kinetics

(Microkinetics) . . . . . . . . . . . . . . . . . 16

2.2.2. Application of Microkinetic Analysis . 17

2.2.3. Langmuir Hinshelwood Hougen

Watson Kinetics . . . . . . . . . . . . . . . . 18

2.2.4. Activity and Selectivity . . . . . . . . . . . 202.3. Molecular Modeling in Heterogeneous

Catalysis. . . . . . . . . . . . . . . . . . . . . . 20

2.3.1. Density Functional Theory . . . . . . . . . 21

2.3.2. Kinetic Monte Carlo Simulation . . . . . 22

2.3.3. Mean-Field Approximation . . . . . . . . 22

2.3.4. Development of Multistep Surface

Reaction Mechanisms . . . . . . . . . . . . 23

3. Development of Solid Catalysts . . . . . 23

4. Classification of Solid Catalysts . . . . . 25

4.1. Unsupported (Bulk) Catalysts . . . . . 25

4.1.1. Metal Oxides. . . . . . . . . . . . . . . . . . . 25

4.1.2. Metals and Metal Alloys . . . . . . . . . . 33

4.1.3. Carbides and Nitrides . . . . . . . . . . . . 34

4.1.4. Carbons . . . . . . . . . . . . . . . . . . . . . . 344.1.5. Ion-Exchange Resins and Ionomers . . 35

4.1.6. Molecularly Imprinted Catalysts . . . . 35

4.1.7. Metal Organic Frameworks . . . . . . 36

4.1.8. Metal Salts . . . . . . . . . . . . . . . . . . . . 36

4.2. Supported Catalysts. . . . . . . . . . . . . 36

4.2.1. Supports . . . . . . . . . . . . . . . . . . . . . . 37

4.2.2. Supported Metal Oxide Catalysts . . . . 37

4.2.3. Surface-Modified Oxides . . . . . . . . . . 38

4.2.4. Supported Metal Catalysts . . . . . . . . . 38

4.2.5. Supported Sulfide Catalysts . . . . . . . . 39

4.2.6. Hybrid Catalysts . . . . . . . . . . . . . . . . 404.2.7. Ship-in-a-Bottle Catalysts . . . . . . . . . 41

4.2.8. Polymerization Catalysts . . . . . . . . . . 42

4.3. Coated Catalysts . . . . . . . . . . . . . . . 43

5. Production of Heterogeneous

Catalysts . . . . . . . . . . . . . . . . . . . . . . 43

5.1. Unsupported Catalysts. . . . . . . . . . . 44

5.2. Supported Catalysts. . . . . . . . . . . . . 47

5.2.1. Supports . . . . . . . . . . . . . . . . . . . . . . 48

5.2.2. Preparation of Supported Catalysts . . . 48

5.3. Unit Operations in Catalyst

Production . . . . . . . . . . . . . . . . . . . . 496. Characterization of Solid Catalysts . . 52

6.1. Physical Properties. . . . . . . . . . . . . . 52

6.1.1. Surface Area and Porosity . . . . . . . . . 52

6.1.2. Particle Size and Dispersion . . . . . . . 54

6.1.3. Structure and Morphology . . . . . . . . . 54

6.1.4. Local Environment of Elements . . . . . 56

6.2. Chemical Properties. . . . . . . . . . . . . 57

6.2.1. Surface Chemical Composition. . . . . . 57

6.2.2. Valence States and Redox

Properties . . . . . . . . . . . . . . . . . . . . . 59

6.2.3. Acidity and Basicity . . . . . . . . . . . . . 62

2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a05_313.pub2


2/110

6.3. Mechanical Properties . . . . . . . . . . 64

6.4. Characterization of Solid Catalysts

under Working Conditions. . . . . . . . 64

6.4.1. Temporal Analysis of Products (TAP

Reactor) . . . . . . . . . . . . . . . . . . . . . . 65

6.4.2. Use of Isotopes . . . . . . . . . . . . . . . . . 65

6.4.3. Use of Substituents, Selective Feeding,and Poisoning . . . . . . . . . . . . . . . . . . 65

6.4.4. Spatially Resolved Analysis of the

Fluid Phase over a Catalyst . . . . . . . . 66

6.4.5. Spectroscopic Techniques. . . . . . . . . . 66

7. Design and Technical Operation

of Solid Catalysts. . . . . . . . . . . . . . . . 67

7.1. Design Criteria for Solid

Catalysts . . . . . . . . . . . . . . . . . . . . . 67

7.2. Catalytic Reactors . . . . . . . . . . . . . . 70

7.2.1. Classification of Reactors . . . . . . . . . 70

7.2.2. Laboratory Reactors . . . . . . . . . . . . . 707.2.3. Industrial Reactors . . . . . . . . . . . . . . 72

7.2.4. Special Reactor Types and Processes . 77

7.2.5. Simulation of Catalytic Reactors . . . . 79

7.3. Catalyst Deactivation and

Regeneration . . . . . . . . . . . . . . . . . . 80

7.3.1. Different Types of Deactivation . . . . . 80

7.3.2. Catalyst Regeneration . . . . . . . . . . . . 81

7.3.3. Catalyst Reworking and Disposal . . . . 82

8. Industrial Application and Mechanisms

of Selected Technically RelevantReactions . . . . . . . . . . . . . . . . . . . . . . 82

8.1. Synthesis Gas and Hydrogen . . . . . 82

8.2. Ammonia Synthesis . . . . . . . . . . . . . 83

8.3. Methanol and Fischer Tropsch

Synthesis . . . . . . . . . . . . . . . . . . . . . 84

8.3.1. Methanol Synthesis . . . . . . . . . . . . . . 84

8.3.2. Fischer Tropsch Synthesis . . . . . . . 86

8.4. Hydrocarbon Transformations. . . . . 87

8.4.1. Selective Hydrocarbon Oxidation

Reactions . . . . . . . . . . . . . . . . . . . . . 87

8.4.2. Hydroprocessing Reactions . . . . . . . . 918.5. Environmental Catalysis . . . . . . . . . 94

8.5.1. Catalytic Reduction of Nitrogen Oxides

from Stationary Sources . . . . . . . . . . 94

8.5.2. Automotive Exhaust Catalysis . . . . . . 95

1. Introduction

Catalysis is a phenomenon by which chemicalreactions are accelerated by small quantities of

foreign substances, called catalysts. A suitablecatalystcan enhance the rate of a thermodynam-ically feasible reaction but cannot change theposition of the thermodynamic equilibrium.Most catalysts are solids or liquids, but theymay also be gases.

The catalytic reaction is a cyclic process.According to a simplified model, the reactantor reactants form a complex with the catalyst,thereby opening a pathway for their transforma-tion into the product or products. Afterwards thecatalyst is released and the next cycle canproceed.

However, catalysts do not have infinite life.Products of side reactions or changes in thecatalyst structure lead to catalyst deactivation.In practice spent catalysts must be reactivated orreplaced (see Chapter Catalyst Deactivationand Regeneration).

1.1. Types of Catalysis

If the catalyst and reactants or their solutionform a common physical phase, then the reaction

iscalledhomogeneouslycatalyzed.Metalsaltsoforganic acids, organometallic complexes, andcarbonyls of Co, Fe, and Rh are typical homoge-neous catalysts. Examples of homogeneously

catalyzed reactions are oxidation of toluene tobenzoic acid in the presence of Co and Mnbenzoatesandhydroformylationofolefinstogivethe corresponding aldehydes. This reaction iscatalyzed by carbonyls of Co or Rh.

Heterogeneous catalysis involves systems inwhich catalyst and reactants form separatephysical phases. Typical heterogeneous cata-lysts are inorganic solids such as metals, oxides,sulfides, and metal salts, but they may also beorganic materials such as organic hydroperox-ides, ion exchangers, and enzymes.

Examples ofheterogeneously catalyzedreac-tions are ammonia synthesis from the elementsover promoted iron catalysts in the gas phase andhydrogenation of edible oils on Ni kieselguhrcatalysts in the liquid phase, which are examplesofinorganic and organic catalysis, respectively.

Electrocatalysis is a special case of hetero-geneous catalysis involving oxidation or reduc-tion by transfer of electrons. Examples are

the use of catalytically active electrodes inelectrolysis processes such as chlor-alkali elec-trolysis and in fuel cells.

2 Heterogeneous Catalysis and Solid Catalysts


3/110

In photocatalysis light is absorbed bythe catalyst or a reactant during the reaction.This can take place in a homogeneous or het-erogeneous system. One example is the utili-zation of semiconductor catalysts (titanium,zinc, and iron oxides) for photochemical deg-

radation of organic substances, e.g., on self-cleaning surfaces.

In biocatalysis, enzymes or microorganismscatalyze various biochemical reactions. Thecatalysts can be immobilized on various carrierssuch as porous glass, SiO2, and organic poly-mers. Prominent examples of biochemical re-actions are isomerization of glucose to fructose,important in the production of soft drinks, byusing enzymes such as glucoamylase immobi-

lized on SiO2, and the conversion of acryloni-trile to acrylamide by cells of corynebacteriaentrapped in a polyacrylamide gel.

The main aim of environmental catalysis isenvironmental protection. Examples are thereduction of NOx in stack gases with NH3 onV2O5 TiO2 catalysts and the removal of NOx,CO, and hydrocarbons from automobile exhaustgases by using the so-called three-way catalystconsisting of Rh Pt CeO2 Al2O3 depos-ited on ceramic honeycombs.

The term green catalytic processes has beenused frequently in recent years, implying thatchemical processes may be made environmen-tally benign by taking advantage of the possiblehigh yields and selectivities for the target pro-ducts, with little or no unwanted side productsand also often high energy efficiency.

The basic chemical principles of catalysisconsist in the coordination of reactant mole-cules to central atoms, the ligands of whichmay be molecular species (homogeneous and

biocatalysis) or neighboring atoms at the surfaceof the solid matrix (heterogeneous catalysis).Although there are differences in the details ofvarious types of catalysis (e.g., solvation effectsin the liquid phase, which do not occur insolid gas reactions), a closer and undoubted-ly fruitful collaboration between the separatecommunities representing homogeneous, het-erogeneous, and biocatalysis should be strong-ly supported. A statement by David Parker (ICI)during the 21st Irvine Lectures on 24 April1998 at the University of St. Andrews shouldbe mentioned in this connection, namely, that,

. . . at the molecular level, there is little todistinguish between homogeneous and hetero-geneous catalysis, but there are clear distinc-tions at the industrial level [1].

1.2. Catalysis as a Scientific Discipline

Catalysis is a well-established scientificdiscipline, dealing not only with fundamentalprinciples or mechanisms of catalytic reac-tions but also with preparation, properties, andapplications of various catalysts. A number ofacademic and industrial institutes or laborato-ries focus on the study of catalysis andcatalytic processes as well as on the improve-ment of existing and development of newcatalysts.

International journals specializing in cataly-sis include Journal of Catalysis, Journal of

Molecular Catalysis (Series A: Chemical; Se-ries B: Enzymatic), Applied Catalysis (SeriesA: General; Series B: Environmental),ReactionKinetics and Catalysis Letters, Catalysis Today,Catalysis Letters, Topics in Catalysis,Advancesin Organometallic Catalysis, etc.

Publications related to catalysis can also

be found in Journal of Physical Chemistry,Langmuir, and Physical Chemistry ChemicalPhysics.

Well-known serials devoted to catalysis areHandbuch der Katalyse [edited by G.-M.Schwab, Springer, Wien, Vol. 1 (1941) - Vol.7.2 (1943)], Catalysis [edited by P. H. Emmett,Reinhold Publ. Co., Vol. 1 (1954) - Vol. 7(1960)], Catalysis Science and Technology[edited by J. R. Anderson and M. Boudart,Springer, Vol. 1 (1981) - Vol. 11 (1996)],

Catalysis Reviews (edited by A. T. Bell and J. J.Carberry, Marcel Dekker), Advances in Cata-lysis (edited by B. C. Gates and H. Knozinger,Academic Press), Catalysis (edited by J. J.Spivey, The Royal Society of Chemistry),Studies in Surface Science and Catalysis (editedby B. Delmon and J. T. Yates), etc.

Numerous aspects of catalysis were thesubject of various books. Some, published since1980, are mentioned here:

C. N. Satterfield, Heterogeneous Catalysisin Practice, McGraw Hill Book Comp., NewYork, 1980.

Heterogeneous Catalysis and Solid Catalysts 3


4/110

D. L. Trimm, Design of Industrial Catalysts,Elsevier, Amsterdam, 1980.

J. M. Thomas, R. M. Lambert (eds.), Char-acterization of Heterogeneous Catalysts, Wiley,Chichester, 1980.

R. Pearce, W. R. Patterson (eds.), Catalysisand Chemical Processes, John Wiley, NewYork, 1981.

B. L. Shapiro (ed.), Heterogeneous Cataly-sis, Texas A & M Press, College Station, 1984.

B. E. Leach (ed.), Applied Industrial Catal-ysis, Vol. 1, 2, 3, Academic Press, New York,1983 1984.

M. Boudart, G. Djega-Mariadassou, Kineticsof Heterogeneous Reactions, Princeton Univer-sity Press, Princeton, 1984.

F. Delannay (ed.), Characterization ofHeterogeneous Catalysts, Marcel Dekker, NewYork, 1984.

R. Hughes, Deactivation of Catalysts, Aca-demic Press, New York, 1984.

M. Graziani, M. Giongo (eds.), FundamentalResearch in Homogeneous Catalysis, Wiley,New York, 1984.

H. Heinemann, G. A. Somorjai (eds.), Ca-talysis and Surface Science, Marcel Dekker,New York, 1985.

J. R. Jennings (ed.), Selective Developmentin Catalysis, Blackwell Scientific Publishing,London, 1985.

G. Parshall, Homogeneous Catalysis, Wiley,New York, 1985.

J. R. Anderson, K. C. Pratt, Introduction toCharacterization and Testing of Catalysts, Ac-ademic Press, New York, 1985.

Y. Yermakov, V. Likholobov (eds), Homo-geneous and Heterogeneous Catalysis, VNUScience Press, Utrecht, Netherlands, 1986.

J. F. Le Page, Applied Heterogeneous Catal-ysis Design, Manufacture, Use of Solid Cat-

alysts, Technip, Paris, 1987.G. C. Bond, Heterogeneous Catalysis, 2nd

ed., Clarendon Press, Oxford, 1987.P. N. Rylander,Hydrogenation Methods,Ac-

ademic Press, New York, 1988.A. Mortreux, F. Petit (eds.), Industrial Ap-

plication of Homogeneous Catalysis, Reidel,Dordrecht, 1988.

J. F. Liebman, A. Greenberg, MechanisticPrinciples of Enzyme Activity, VCH, New York,1988.

J. T. Richardson, Principles of Catalytic De-velopment, Plenum Publishing Corp., NewYork, 1989.

M. V. Twigg (ed.), Catalyst Handbook,Wolfe Publishing, London, 1989.

J. L. G. Fierro (ed.), Spectroscopic Charac-terization of Heterogeneous Catalysts, Elsevier,Amsterdam, 1990.

R. Ugo (ed.), Aspects of Homogeneous Ca-talysis, Vols. 1 7, Kluwer Academic Publish-ers, Dordrecht, 1990.

W. Gerhartz (ed.), Enzymes in Industry,VCH, Weinheim, 1990.

R. A. van Santen, Theoretical HeterogeneousCatalysis, World Scientific, Singapore, 1991.

J. M. Thomas, K. I. Zamarev (eds.), Per-

spectives in Catalysis, Blackwell ScientificPublications, Oxford, 1992.B. C. Gates, Catalytic Chemistry, Wiley,

New York, 1992.G. W. Parshall, S. D. Ittel, Homogeneous

Catalysis, 2nd ed., Wiley, New York, 1992.J. J. Ketta (ed.), Chemical Processing Hand-

book, Marcel Dekker, New York, 1993.J. A. Moulijn, P. W. N. M. van Leeuwen,

R. A. van Santen (eds.), Catalysis An Inte-grated Approach to Homogeneous, Heteroge-

neous and Industrial Catalysis, Elsevier,Amsterdam, 1993.

J. W. Niemantsverdriet, Spectroscopy in Ca-talysis, VCH, Weinheim, 1993.

J. Reedijk (ed.), Bioinorganic Catalysis, M.Dekker, New York, 1993.

G. A. Somorjai, Introduction to SurfaceChemistryandCatalysis,Wiley,NewYork,1994.

J. M. Thomas, W. J. Thomas, Principles andPractice of Heterogeneous Catalysis, VCH,Weinheim, 1996.

R. J. Wijngarden, A. Kronberg, K. R. Wes-terterp, Industrial Catalysis Optimizing Cat-alysts and Processes, Wiley-VCH, Weinheim,1998.

G. Ertl, H. Knozinger, J. Weitkamp (eds.),Environmental Catalysis, Wiley-VCH,Weinheim,1999.

G. Ertl, H. Knozinger, J. Weitkamp (eds.),Preparation of Solid Catalysts, Wiley-VCH,Weinheim, 1999.

B. Cornils, W. A. Herrmann, R. Schlogl, C.-H. Wong, Catalysis from A Z, Wiley-VCH,Weinheim, 2000.



5/110

B. C. Gates, H. Knozinger (eds.), Impact ofSurface Science on Catalysis, Academic, SanDiego, 2000.

A comprehensive survey of the principles andapplications: G. Ertl, H. Knozinger, F. Schuth, J.Weitkamp (eds.): Handbook of HeterogeneousCatalysis, 2nd ed. with 8 volumes and 3966pages, Wiley-VCH, Weinheim 2008.

The firstInternational Congress on Catalysis(ICC) took place in 1956 in Philadelphia and hassince been held every four years in Paris (1960),Amsterdam (1964), Moscow (1968), PalmBeach (1972), London (1976), Tokyo (1980),Berlin (1984), Calgary (1988), Budapest(1992), Baltimore (1996), Granada (2000)),Paris (2004) and Seoul (2008). The 15th Con-

gress will be held in Munich in 2012. Presentedpapers and posters have been published in theProceedings of the corresponding congresses.The International Congress on Catalysis Coun-

cil (ICC) was renamed at the Council meeting inBaltimore 1996. The international organizationis now called International Association of Ca-talysis Societies (IACS).

In 1965 the Catalysis Society of NorthAmerica was established and holds meetings inthe USA every other year.

The European Federation of Catalysis Soci-eties (EFCATS) was established in 1990. TheEUROPACATConferences are organized underthe auspices of EFCATS. The first conferencetook place in Montpellier (1993) followed byMaastricht (1995), Cracow (1997), Rimini(1999), and Limerick (2001).

Furthermore, every four years (in the evenyear between two International Congresses onCatalysis) anInternational Symposium focusingon Scientific Basis for the Preparation of Het-

erogeneous Catalysts is held in Louvain-LaNeuve (Belgium).

Other international symposia or congressesdevoted to catalysis are: International ZeoliteConferences, International Symposium of Cat-alyst Deactivation, Natural Gas ConversionSymposium, Gordon Conference on Catalysis,TOCAT (Tokyo Conference on Advanced Cata-lytic Science and Technology), InternationalSymposium of Acid-Base Catalysis, the Europe-an conference series, namely the Roermond,Sabatier- and Schwab-conference, and the Tay-lor Conference.

1.3. Industrial Importance

of Catalysis

Because most industrial chemical processes arecatalytic, the importance and economical sig-nificance of catalysis is enormous. More than80 % of the present industrial processes estab-lished since 1980 in the chemical, petrochemi-cal, and biochemical industries, as well as in theproduction of polymers and in environmentalprotection, use catalysts.

More than 15 international companies havespecialized in the production of numerous cata-lystsappliedinseveralindustrialbranches.In2008the turnover in the catalysts world market wasestimatedtobeaboutUS-$ 13 109(seeChapter

Production of Heterogeneous Catalysts).

1.4. History of Catalysis

The phenomenon of catalysis was first recog-nizedbyBERZELIUS [2,3] in 1835. However, somecatalytic reactions such as the production ofalcoholic beverages by fermentation or the man-ufacture of vinegar by ethanol oxidation werepracticed long before. Production of soap by fat

hydrolysis and diethyl ether by dehydration ofethanol belong to the catalytic reactions thatwere performed in the 16th and 17th centuries.

Besides BERZELIUS, MITSCHERLICH [3] wasalso involved at the same time in the study ofcatalytic reactions accelerated by solids. Heintroduced the term contact catalysis. This termfor heterogeneous catalysis lasted for more than100 years.

In 1895 OSTWALD [3,4] defined catalysis asthe acceleration of chemical reactions by the

presence of foreign substances which are notconsumed. His fundamental work was recog-nized with the Nobel prize for chemistry in1909.

Between 1830 and 1900 several practicalprocesses were discovered, such as flamelesscombustion of CO on a hot platinum wire, andthe oxidation of SO2 to SO3 and of NH3 to NO,both over Pt catalysts.

In 1912 SABATIER [3,5] received the Nobelprize for his work devoted mainly to the hydro-

genation of ethylene and CO over Ni and Cocatalysts.



6/110

The first major breakthrough in industrialcatalysis was the synthesis of ammonia fromthe elements, discovered by HABER [3,6,7] in1908, using osmium as catalyst. Laboratoryrecycle reactors for the testing of various am-monia catalysts which could be operated at high

pressure and temperature were designed byBOSCH [3]. The ammonia synthesis was com-mercialized at BASF (1913) as the Haber

Bosch [8] process. MITTASCH [9] at BASF devel-oped and produced iron catalysts for ammoniaproduction.

In 1938 BERGIUS [3,10] converted coal toliquid fuel by high-pressure hydrogenation inthe presence of an Fe catalyst.

Other highlights of industrial catalysis were

the synthesis of methanol from CO and H2 overZnO Cr2O3 and the cracking of heavier pe-troleum fractions to gasoline using acid-activat-ed clays, as demonstrated by HOUDRY [3,6] in1928.

The addition of isobutane to C3 C4 olefinsin the presence of AlCl3, leading to branchedC7 C8 hydrocarbons, components of high-quality aviation gasoline, was first reported byIPATIEFF et al. [3,7] in 1932. This invention led toa commercial process of UOP (USA).

Of eminent importance for Germany, whichpossesses no natural petroleum resources, was

the discovery by FISCHER andTROPSCH [11] of thesynthesis of hydrocarbons and oxygenated com-pounds from CO and H2 over an alkalized ironcatalyst. The first plants for the production ofhydrocarbons suitable as motor fuel started up inGermany 1938. After World War II, Fischer-

Tropsch synthesis saw its resurrection in SouthAfrica. Since 1955 Sasol Co. has operated twoplants with a capacity close to 3 106 t/a.

One of the highlights of German industrialcatalysis before World War II was the synthe-sis of aliphatic aldehydes by ROELEN [12] bythe addition of CO and H2 to olefins in thepresence of Co carbonyls. This homogeneous-ly catalyzed reaction was commercialized in1942 by Ruhr-Chemie and is known as Oxo

Synthesis.During and after World War II (till 1970)numerous catalytic reactions were realized onan industrial scale (see also Chapter Applica-tion of Catalysis in Industrial Chemistry).Some important processes are compiled inTable 1.

Table 2 summarizes examples of catalyticprocesses representing the current status of thechemical, petrochemical and biochemical in-dustry as well as the environmental protection

(see also Chapter Application of Catalysis inIndustrial Chemistry).

Table 1. Important catalytic processes commercialized during and after World War II (until 1970) [13,14]

Year of

commercialization

Process Catalyst Products

1939 1945 dehydrogenation Pt Al2O3 toluene from methylcyclohexane

dehydrogenation Cr2O3 Al2O3 butadiene from n-butane

alkane isomerization AlCl3 i-C7 C8 from n-alkanes

1946 1960 oxidation of aromatics V2O5 phthalic anhydride from

naphthalene and o-xylenehydrocracking Ni aluminosilicate fuels from high-boiling petroleum

fractions

polymerization

(Ziegler Natta)

TiCl4 Al(C2H5)3 polyethylene from ethylene

dehydrogenation Fe2O3 Cr2O3 KOH styrene from ethylbenzene

oxidation (Wacker process) PdCl2 CuCl2 acetaldehyde from ethylene

1961 1970 steam reforming Ni a-Al2O3 Co, (CO2), and H2 from methane

ammoxidation Bi phosphomolybdate acrylonitrile from propene

fluid catalytic cracking H zeolites aluminosilicates fuels from high boiling fractions

reforming bimetallic catalysts (Pt, Sn, Re, Ir) gasoline

low-pressure methanol

synthesis

Cu ZnO Al2O3 methanol from CO, H2, CO2

isomerization enzymes immobilized on SiO2 fructose from glucose (production

of soft drinks)

distillate dewaxing ZSM-5, mordenites removal of n-alkanes from gasoline

hydrorefining Ni , CO MoSx hydrodesulfurization, hydrodenitrification



7/110

2. Theoretical Aspects

The classical definition of a catalyst statesthat a catalyst is a substance that changes therate but not the thermodynamics of a chemicalreaction and was originally formulated byOSTWALD [4]. Hence, catalysis is a dynamicphenomenon.

As emphasized by BOUDART [19], the condi-tions under which catalytic processes occur onsolid materials vary drastically. The reactiontemperature can be as low as 78 K and as highas 1500 K, and pressures can vary between 109

and 100 MPa. The reactants can be in the gasphase or in polar or nonpolar solvents. Thereactions can occur thermally or with the assis-tance of photons, radiation, or electron transferat electrodes. Pure metals and multicomponentand multiphase inorganic compounds can act ascatalysts. Site-time yields (number of productmolecules formed per site and unit time) as lowas 105 s1 (corresponding to one turnover per

day) and as highas 109 s1 (gas kinetic collisionrate at 1 MPa) are observed.

It is plausible that it is extremely difficult, ifnot impossible, to describe the catalytic phe-nomenon by a general theory which covers theentire range of reaction conditions and observedsite-time yields (reaction rates). However, thereare several general principles which are consid-

ered to be laws or rules of thumb that are usefulin many situations. According to BOUDART [19],the value of a principle is directly related to itsgenerality. In contrast, concepts are more spe-cialized and permit an interpretation of phenom-ena observed for special classes of catalysts orreactions under given reaction conditions.

In this chapter, important principles andconcepts of heterogeneous catalysis are dis-cussed, followed by a section on kineticsof heterogeneously catalyzed reactions. Thechapter is concluded by a section on the deter-mination of reaction mechanisms in heteroge-neous catalysis.

Table 2. Important catalytic processes commercialized after 1970 [1518]

Year of

commercialization

Process Catalyst Product

1971 1980 automobile emission control Pt Rh CeO2 Al2O3(three-way catalyst)

removal of NOx, CO, CHx

carbonylation (Monsanto process) organic Rh complex acetic acid from methanol

MTG (Mobil process) zeolite (ZSM-5) gasoline from methanol

1981 1985 alkylation (Mobil Badger) modified zeoli te (ZSM-5) ethylbenzene from ethylene

selective catalytic reduction (SCR;

stationary sources)

V Ti (Mo, W) oxides

(monoliths)

reduction of NOx with NH3 to N2

esterification (MTBE synthesis) ion-exchange resin methyl-tert-butyl ether from

isobutene methanol

oxidation (Sumitomo Chem.,

2-step process)

1. Mo, Bi oxides acrylic acid from propene

2. Mo, V, PO (heteropolyacids)

oxidation (Monsanto) vanadylphosphate maleic anhydride from n-butane

fluid-bed polymerization (Unipol) Ziegler Natta type polyethylene and polypropylene

hydrocarbon synthesis (Shell) 1. Co (Zr,Ti) SiO2 middle distillate from CO H22. Pt SiO2

environmental control

(combustion process)

Pt Al2O3 (monoliths) deodoration

1986 2000 oxidation with H2O2 (Enichem) Ti silicalite hydroquinone and catechol

from phenol

hydration enzymes acrylamide from acrylonitrile

ammoxidation (Montedipe) Ti silicalite cyclohexanone oxime from

cyclohexanone, NH3, and H2O2dehydrogenation of C3, C4 alkanes

(Star and Oleflex processes)

Pt(Sn) zinc

aluminate,

Pt Al2O3

C3, C4 olefins

2000 catalytic destruction of N2O from

nitric acid tail gases (EnviNOx

process, Uhde)

Fe zeolite removal of nitrous oxide

HPPO (BASF-Dow,

Degussa-Uhde)

Ti silicalite propylene from propene



8/110

2.1. Principles and Concepts

2.1.1. Sabatiers Principle

The Sabatier principle proposes the existence ofan unstable intermediate compound formed be-

tween the catalyst surface and at least one of thereactants [5]. This intermediate must be stableenough to be formed in sufficient quantities andlabile enough to decompose to yield the finalproduct or products. The Sabatier principle isrelated to linear free energy relationships suchas a Brnsted relation [19]. These relations dealwith the heat of reaction q (thermodynamicquantity) and the activation barrier E (kineticquantity) of an elementary step in the exother-

mic direction (q > 0). With an empirical pa-rameter a (0 < a < 1) and neglecting entropyeffects, a Brnsted relation can be written as

DE a Dq;

where DE is the decrease in activation energycorresponding to an increase Dq in the heat ofreaction. Hence, an elementary step will have ahigh rate constant in the exothermic directionwhen its heat of reaction q increases. Since the

activation barrier in the endothermic direction isequal to the sum of the activation energy Eandthe heat of reaction, the rate constant will de-crease with increasing q.

The Brnsted relationship represents a bridgebetween thermodynamics and kinetics and, to-gether with the Sabatier principle, permits aninterpretation of the so-called volcano plots firstreported by BALANDIN [20]. These volcano curvesresult when a quantity correlated with the rate ofreaction under consideration is plotted against a

measure of the stability of the intermediatecompound. The latter quantity can be the heatof adsorption of one of the reactants or the heat offormation of a bulk compound relative to thesurface compound, or even the heat of formationof any bulk compound that canbe correlated withthe heat of adsorption, or simply the position ofthe catalytic material (metal) along a horizontalseries in the Periodic Table [263].

As an example, Figure 1 shows the volcanoplot for the decomposition of formic acid on

transition metals [21]. The intermediate in thisreaction was shown to be a surface formate.Therefore, the heats of formationDHfof the bulk

metal formates were chosen as the measure ofthe stability of the intermediate. At low values ofDHf, the reaction rate is low and corresponds tothe rate of adsorption, which increases withincreasing heat of formation of the bulk for-mates (representing the stability of the surfacecompound). At high values ofDHf the reactionrate is also low and corresponds to the desorp-tion rate, which increases with decreasing DHf.As a consequence, a maximum in the rate ofreaction (decomposition of formic acid) is ob-

served at intermediate DHf values which isneither a pure rate of adsorption nor a pure rateof desorption but which depends on both.

2.1.2. The Principle of Active Sites

The Sabatier principle of an unstable surfaceintermediate requires chemical bonding of re-actants to the catalyst surface, most likely be-tween atoms or functional groups of reactant

and surface atoms. This leads to the principle ofactive sites. When LANGMUIR formulated hismodel of chemisorption on metal surfaces [22],

Figure 1. Volcano plot for the decomposition of formicacid. The temperature Tat which the rate of decomposition vhas a fixed value is plotted against the heat of formationDHfof the metal formate (adopted from [31]).



9/110

he assumed an array of sites which were energet-ically identical and noninteracting, and whichwould adsorb just one molecule from the gasphaseinalocalizedmode.TheLangmuiradsorp-tion isotherm results from this model. The sitesinvolved can be considered to be active sites.

LANGMUIR was already aware that the as-sumption of identical and noninteracting siteswas an approximation which would not hold forreal surfaces, when he wrote [23]: Most finelydivided catalysts must have structures of greatcomplexity. In order to simplify our theoreticalconsideration of reactions at surfaces, let usconfine our attention to reactions on plane sur-faces. If the principles in this case are wellunderstood, it should then be possible to extend

the theory to the case of porous bodies. Ingeneral, we should look upon the surface asconsisting of a checkerboard. LANGMUIR thusformulated the surface science approach to het-erogeneous catalysis for the first time.

The heterogeneity of active sites on solidcatalyst surfaces and its consequences wereemphasized by TAYLOR [24], who recognizedthat There will be all extremes between thecase in which all atoms in the surface are activeand that in which relatively few are so active. In

other words, exposed faces of a solid catalystwill contain terraces, ledges, kinks, and vacan-cies with sites having different coordinationnumbers. Nanoscopic particles have edges andcorners which expose atoms with different co-ordination numbers [25]. The variation of coor-dination numbers of surface atoms will lead todifferent reactivities and activities of the corre-sponding sites. In this context, Schwabs adli-neation theory may be mentioned [26], whichspeculated that one-dimensional defects con-

sisting of atomic steps are of essential impor-tance. This view was later confirmed by surfacescience studies on stepped single-crystal metalsurfaces [27].

In addition to variable coordination numbersof surface atoms in one-component solids, thesurface composition may be different from thatof the bulk and different for each crystallogra-phic plane in multicomponent materials (surfacesegregation [28]). This would lead to a hetero-geneity of the local environment of a surfaceatom and thus create nonequivalent sites.

Based on accurate kinetic measurementsand on the Taylor principle of the existence of

inequivalent active sites, BOUDART et al. [29]coined the terms structure-sensitive and struc-ture-insensitive reactions. A truly structure-in-sensitive reaction is one in which all sites seemto exhibit equal activity on several planes of asingle crystal. Surprisingly, many heteroge-

neously catalyzed reactions turned out to bestructure-insensitive. Long before experimentalevidence for this phenomenon was available andbefore a reliable interpretation was known,TAYLOR predicted it by writing [24]: Theamount of surface which is catalytically activeis determined by the reaction catalyzed. Inother words, the surface of a catalyst adaptsitself to the reaction conditions for a particularreaction. The driving force for this reorganiza-

tion of a catalyst surface is the minimization ofthe surface free energy, which may be achievedby surface-reconstruction [30,31]. As a conse-quence, a meaningful characterization of activesites requires experiments under working (insitu) conditions of the catalytic system.

The principle of active sites is not limited tometals. Active sites include metal cations, an-ions, Lewis and Brnsted acids, acid basepairs (acid and base acting simultaneously inchemisorption), organometallic compounds,

and immobilized enzymes. Active sites mayinclude more than one species (or atom) toform multiplets [20] or ensembles [32]. Amandatory requirement for these sites to beactive is that they are accessible for chemisorp-tion from the fluid phase. Hence, they mustprovide free coordination sites. Therefore,BURWELL et al. [33,34] coined the term coordi-natively unsaturated sites in analogy with ho-mogeneous organometallic catalysts. Thus, ac-tive sites are to be considered as atoms or groups

of atoms which are embedded in the surface of amatrix in which the neighboring atoms (orgroups) act as ligands. Ensemble and ligandeffects are discussed in detail by SACHTLER [35]and quantum chemical treatments of geometricensemble and electronic ligand effects on metalalloy surfaces are discussed by HAMMER andNRSKOV [36].

2.1.3. Surface Coordination Chemistry

The surface complexes formed by atomsor molecules are now known to usually



10/110

resemble a local structure similar to molecularcoordination complexes. The bonding in thesesurface complexes can well be described in alocalized picture [37,38]. Thus, important phe-nomena occuring at the surface of solid catalystsmay be described in the framework of surfacecoordination chemistry or surface organometal-lic chemistry [39,40].

This is at variance with the so-called bandtheory of catalysis, which attempted to corre-late catalytic performance with bulk electronicproperties [4143]. The shortcomings of thistheory in oxide catalysis are discussed bySTONE [44].

2.1.4. Modifiers and Promoters

The performance of real industrial catalysts isoften adjusted by modifiers (additives) [45,46].A modifier is called a promoter when it in-creases the catalyst activity in terms of reactionrate per site. Modifiers may also affect a cat-alysts performance in an undesired manner. Inthis case the modifier acts as a catalyst poison.However, this simple distinction between pro-moters and poisons is less straightforward for

reactions yielding more than one product inparallel or consecutive steps, of which only oneis the desired product. In this case not only highactivity but also high selectivity is desired. Theselectivity can be improved by adding sub-stances that poison undesirable reactions. Inexothermic reactions excessively high reactionrates may lead to a significant temperatureincrease (sometimes only locally: hot spots)which can yield undesirable products (e.g., COand CO2 in selective catalytic oxidation). A

deterioration of the catalyst due to limited cata-lyst stability may also occur. Consequently, amodifier is required which decreases the reac-tion rate so that a steady-state temperature andreaction rate can be maintained. Although themodifier acts as a poison in these cases, it is infact a promoter as far as selectivity and catalyststability are concerned.

Modifierscanchangethebindingenergyofanactive site or its structure, or disrupt an ensembleof atoms, e.g., by alloying an active with aninactive metal. A molecular approach toward anunderstanding of promotion in heterogeneouscatalysis was presented by HUTCHINGS [47].

As an example, the iron-based ammoniasynthesis catalyst is promoted by Al2O3 andK2O [48]. Alumina acts as a textural promoter,as it prevents the rapid sintering of pure ironmetal. It may also stabilize more active sites onthe iron surface (structural promoter). Potassi-

um oxide appears to affect the adsorption kinet-ics and dissociation of dinitrogen and the bind-ing energy of nitrogen on adjacent iron sites(electronic promoter).

The addition of Co to MoS2-based cata-lysts supported on transitional aluminas hasa positive effect on the rate of hydrodesulfur-ization of sulfur-containing compounds at Co/(Co Mo) ratios below ca.0.3[49] (see SectionSupported Metal Catalysts). The active phase

is proposed to be the so-called CoMoS phasewhich consists of MoS2 platelets, the edges ofwhich are decorated by Co atoms. The lattermay act as structural and electronic promoterssimultaneously.

Another example concerns bifunctional cat-alysts for catalytic reforming [50], which con-sist of Pt supported on strongly acidic aluminas,the acid strength of which is enhanced by mod-ification with chloride. Since these materialslose chlorine during the catalytic process, the

feed contains CCl4 as a precursor of the surfacechloride promoter.

2.1.5. Active Phase Support

Interactions

Several concepts have proved valuable ininterpreting phenomena which are pertinent tocertain classes of catalysts. In supported cata-lysts, the active phase (metal, oxide, sulfide)

undergoes active phase-support interactions[5153]. These are largely determined by thesurface free energies of the support and activephase materials and by the interfacial free ener-gy between the two components [5153]. Activetransition metal oxides (e.g., V2O5, MoO3,WO3) have relatively low surface free energiesas compared to typical oxidic support materialssuch as g-Al2O3, TiO2 (anatase), and SiO2.Although the interfacial free energies betweenactive phase and support are not known, theinteraction between the two components ap-pears to be favorable, with the exception ofSiO2-supported transition metal oxides. As a



11/110

consequence spreading and wetting phenomenaoccur if the thermal treatment of the oxidemixtures is carried out at temperatures suffi-ciently high to induce mobility of the activeoxide. As a rule of thumb, mobility of a solidtypically occurs above the Tammann tempera-

ture, which is equal to half the melting point ofthe bulk solid. As a result, the active transitionmetal oxide tends to wet the support surface andforms a monolayer (monolayer-type catalysts).

Transition and noble metals typically havehigh surface free energies [52], and therefore,smallparticles or crystallites tend to agglomeratetoreducetheirsurfacearea.Stabilizationofnano-sizemetal particles therefore requires depositionon the surface of supports providing favorable

metal-support interactions (MSI). The smallerthe particle the more its physical properties andmorphology can be affected by these interac-tions. Therefore, the nature of the support mate-rialforagivenmetalalsocriticallyinfluencesthecatalytic properties of the metal particle.

Supported metals are in a nonequilibriumstate and therefore still tend to agglomerate atsufficientlyhigh temperatures in reducing atmo-spheres. Hence, deactivation occurs because ofthe reduced metal surface area. Regeneration

can typically be achieved by thermal treatmentin an atmosphere in which the active metal isoxidized. The surface free energies of transitionand noble metal oxides are significantly lowerthan those of the parent metals, so that theirspreading on the support surface becomesmore favorable. Subsequent reduction undersufficiently mild conditions can restore the high

degree of metal dispersion [dispersion D isdefined as the ratio of the number of metalatoms exposed at the particle surface (NS) tothe total number of metal atoms NT in theparticle (D NS/NT)].

So-called strong metal-support interactions

(SMSI) may occur, e.g., for Pt TiO2 andRh TiO2 [51,53,54]. As shown experimen-tally, the adsorption capacity for H2 and CO isdrastically decreased when the precursorfor the catalytically active metal on the supportis reduced in H2 at temperatures aboveca. 770 K [51,54]. Simultaneously, the oxidesupport is slightly reduced. Although severalexplanations have been proposed for the SMSIeffect, the most probable explanation is encap-

sulation of the metal particle by support oxidematerial. Encapsulation may occur when thesupport material becomes mobile. Althoughthe electronic properties of the metal particlemay be affected by the support oxide in theSMSI state, the decrease of the adsorptioncapacity appears to be largely due to a geomet-ric effect, namely, the resulting inaccessibilityof the metal surface.

The various possible morphologies and dis-persions of supported metals are schematically

shown in Figure 2. The metal precursor typical-ly is well dispersed after impregnation of thesupport. Low-temperature calcination may leadto well-dispersed oxide overlayers, while directlow-temperature reduction leads to highly dis-persed metal particles. This state can also bereached by low-temperature reduction of thedispersed oxide precursor, this step being

Figure 2. Schematic representation of metal-support interactions (adopted from [50])



12/110

reversible by low-temperature reoxidation. Forthe preparation of highly dispersed Ni catalysts,it is important to remove the water that is formedby hydrogen reduction of NiO. H2 diluted withN2 is used for this purpose. Surface compoundformation may also occur by a solid-state reac-

tion between the active metal precursor and thesupport at high calcination temperatures. Re-duction at high temperatures may lead to parti-cle agglomeration when cohesive forces aredominant, and to so-called pillbox morpholo-gies when adhesive forces are dominant. In bothcases, the metal must be mobile. In contrast,when the support is mobile, sintering of thesupport can occur, and the small metal particlesare stabilized on the reduced surface area

(cohesive forces). Alternatively, if adhesiveforces are dominant encapsulation (SMSIeffect) may occur.

2.1.6. Spillover Phenomena

In multiphase solid catalysts spillover may oc-cur of an active species (spillover species) ad-sorbed or formed on one phase (donor phase)onto a second phase (acceptor) which does not

form the active species under the same condi-tions [5557]. A well-known example is hydro-gen spillover from Pt, on which dihydrogenchemisorbs dissociatively, onto WO3 with for-mation of a tungsten bronze [58]. According toSOMORJAI [59] the spillover phenomenon mustbe regarded as one of the modern concepts insurface science and heterogeneous catalysis.Nevertheless, the exact physical nature of spill-over processes has only rarely been verifiedexperimentally. The term is typically used to

explain nonlinear effects (synergistic effects)of the combination of chemically differentcomponents of a catalytic material on itsperformance.

Besides hydrogen spillover, oxygen spilloverhas been postulated to play an important role inoxidation reactions catalyzed by mixed oxides.For example, the addition of antimony oxide toselective oxidation catalysts enhances the cata-lytic activity at high levels of selectivity by afactor of up to five relative to the Sb-free system,although antimony oxide itself is completelyinactive.

Observations of this kind motivated DELMONet al. [60,61] to formulate the remote-controlconcept to explain the fact that all industrialcatalysts used for the partial oxidation of hydro-carbons or in hydrotreatment are multiphasicand that particular phase compositions develop

synergy effects. The remote-control concept is,however, not undisputed.

2.1.7. Phase-Cooperation and Site-Isola-

tion Concepts

GRASSELLI [62] proposed the phase-cooperationconceptfor partial oxidation and ammoxidationreactions. It is suggested that two phases (e.g.,

a-Bi2Mo3O12 and g-Bi2MoO6) cooperate in thesense that one phase performs the actual cata-lytic function (a-phase) and the other (g-phase)the reoxidation function. The concept could beverified for many other multiphase, multicom-ponent mixed metal oxide catalysts, such asmulticomponent molybdates and multicompo-nent antimonates [62,63].

Another concept most relevant for selectiveoxidation and ammoxidation is the site-isolationconcept first formulated by CALLAHAN and

GRASSELLI [64]. Site isolation refers to the sepa-ration of active sites from each other on thesurface of a heterogeneous catalyst and is con-sidered to be the prerequisite for obtaining thedesired selective partial oxidation products. Theconcept states that reactive surface lattice oxy-gen atoms must be structurally isolated fromeach other in defined groupings on a catalystsurface to achieve selectivity. The number ofoxygen atoms in a given isolated grouping de-termines the reaction channel through the stoi-

chiometry requirements imposed on the reactionby the availability of oxygen at the reaction site.It was postulated that two and up to five adjacentsurface oxygen atoms would be required for theselective oxidation of propene to the desiredproduct acrolein. Lattice groupings with morethan five oxygen atoms would only produce totaloxidation products (CO and CO2), whilecompletely isolated single oxygen atoms wouldbe either inactive or could produce allyl radicals.The latter would couple in the vapor phase togive hexadiene and ultimately benzene. Thesescenarios are schematically shown in Figure 3.



13/110

2.1.8. Shape-Selectivity Concept

Zeolites and related materials have crystallinestructure and contain regular micropores, thediameters of which are determined by the struc-ture of the materials. The pore sizes are welldefined and have dimensions similar to those ofsmall organic molecules. This permits shape-selective catalysis to occur. The geometric con-straints may act on the sorption of reactants, onthe transition state of the catalyzed reaction, oron the desorption of products. Correspondingly,shape-selective effects have been classified asproviding reactant shape selectivity, restricted

transition state shape selectivity, and product

shape selectivity [65,66]. These scenarios areschematically illustrated in Figure 4 for thecracking ofn-heptane and 1-methylhexane (re-actant shape selectivity), for the transalkylationofm-xylene (transition state shape selectivity),and for the alkylation of toluene by methanol(product shape selectivity). In the first example,the kinetic diameter ofn-heptane is smaller thanthat of 1-methylhexane. The latter is not able toenter micropores, so that shape-selective crack-ing ofn-heptane takes place when both hydro-carbons are present in the feed. An examplefor shape-selective control of the transition

Figure 3. Site-isolation principle. Schematic of lattice oxygen arrangements on hypothetical surfaces. Anticipated reactionpaths of propene upon contact with these surfaces (NR no reaction; adopted from [62])



14/110

state is the transalkylation of m-xylene. Thereaction is bimolecular and the formation of1,2,4-trimethylbenzene has a less bulky transi-tion state than the formation of 1,3,5-trimethyl-benzene. The latter product can thus not beformed if the pore size and geometry is carefullyadapted to the transition state requirements.

Finally, p-xylene can be selectively formed bymethylation of toluene with methanol and zeo-lites whose pore openings only allow p-xyleneto be released. The o and m isomers eitheraccumulate in zeolite cages or are isomerizedto p-xylene.

2.1.9. Principles of the Catalytic Cycle

The most fundamental principle in catalysis is

thatofthe catalytic cycle,whichmaybebasedona redefinition of a catalyst by BOUDART [67]: Acatalyst is a substance that transforms reactantsinto products, through an uninterrupted andrepeatedcycle of elementary steps in which thecatalyst is changed through a sequence ofreac-tive intermediates, until the last step in the cycleregenerates the catalyst in its original form.

The catalytic substance or active sites maynot be present originally, but may be formed byactivation during the start-up phase of the cata-lytic reaction. The cycle must be uninterruptedand repeated since otherwise the reaction isstoichiometric rather than catalytic. The number

of turnovers, a measure of catalyst life, must begreater than unity, since the catalyst wouldotherwise be a reagent. The total amount ofcatalyst (active sites) is typically small relativeto the amounts of reactants and products in-volved (catalytic amounts). As a consequence,the reactive intermediates can be treated by the

kinetic quasi-steady-state approximation ofBODENSTEIN.

The activity of the catalyst is defined by thenumber of cycles per unit time or turnovers orturnover frequency (TOF; unit: s1). The life ofthe catalyst is defined by the number of cyclesbefore it dies.

2.2. Kinetics of Heterogeneous

Catalytic Reactions [6776]

The catalytic cycle is the principle of catalyticaction. The mechanism of a catalyzed reactioncan be described by the sequence of elementaryreaction steps of the cycle, including adsorption,surface diffusion, chemical transformations ofadsorbed species, and desorption, and it is thebasis for deriving the kinetics of the reaction. Itis assumed that for each individual elementarystep the transition-state theory is valid. An earlytreatise of the kinetics of heterogeneously cata-

lyzed reactions was published by SCHWAB [77].The various aspects of the dynamics of sur-

face reactions and catalysis have been classified

Figure 4. Classification of shape-selective effects



15/110

by ERTL [31] into five categories in terms of timeand length scales, as shown schematically in

Figure 5. In the macroscopic regime, the rate ofa catalytic reaction is modeled by fitting empir-ical equations, such as power laws, to experi-mental data, so as to describe its concentrationand pressure dependence and to determine rateconstants that depend exponentially on temper-ature. This approach was very useful in chemi-cal engineering for reactor and process design.Assumptions about reaction schemes (kineticmodels) provide correlations between the sur-face coverages of intermediates and the external

variables, an approach that led to the Temkinequation [78] modeling the kinetics of ammoniasynthesis.

Improved kinetic models could be developedwhen atomic processes on surfaces and theidentification and characterization of surfacespecies became available. The progress of acatalytic reaction is then described by a micro-kinetics approach by modeling the macroscopickinetics through correlating atomic processeswith macroscopic parameters within the frame-

work of a suitable continuum model. Continu-um variables for the partial surface coveragesare, to a first approximation, correlated to ex-ternal parameters (partial pressures and temper-ature) by the Langmuir lattice model of a surfaceconsisting of identical noninteracting adsorp-tion sites.

The formulation of rate laws for the fullsequence of elementary reactions will usuallylead to a set of nonlinear coupled (ordinary)differential equations for the concentrations(coverages) of the various surface species in-volved. The temporal behavior of the reactionsystem under constant continuous-flow condi-

tions may be nonstationary transient. In certainparameter ranges it may be oscillatory or even

chaotic. Also, there may be local variations insurface coverages which lead to coupling of thereaction with transport processes (e.g., particlediffusion, heat transfer). The formation of spa-tiotemporal concentration profiles on a meso-scopic scale is the consequence of these nonlin-ear dynamic phenomena.

Since the Langmuir lattice model is not validin reality, the continuum model can describe thereaction kinetics only to a first approximation.Interactions between adsorbed species occur,

and adsorbed particles occupy nonidenticalsites, so that complications arise in the descrip-tion of the reaction kinetics. Apart from theheterogeneity of adsorption sites, surfaces mayundergo structural transformations. Surface sci-ence investigations provide information onthese effects on an atomic scale.

As mentioned above, it is assumed that thetransition-state theory is valid for description ofthe rates of individual elementary steps. Thistheory is based on the assumption that at all

stages along the reaction coordinate thermalequilibrium is established. Temperature then isthe only essential external macroscopic param-eter. This assumption can only be valid if energyexchange between all degrees of motionalfreedom of the particles interacting with thesolid acting as a heat bath is faster than theelementary step which induces nuclear motions.Energy transfer processes at the quantumlevel are the basic requirements for chemicaltransformations.

Nonlinear dynamics and the phenomenaoccuring at the atomic and quantum levels werereviewed by ERTL [31].

Figure 5. Schematic classification of the various aspects of the dynamics of surface reactions (adopted from [31])



16/110

2.2.1. Concepts of Reaction Kinetics

(Microkinetics)

The important concepts of (catalytic) reactionkinetics were reviewed by BOUDART [67,68,79,80], and by CORTRIGHT and DUMESIC [74].

The term microkinetics was defined to de-note reaction kinetics analyses that attempt toincorporate into the kinetic model the basicsurface chemistry involved in the catalyticreaction at a molecular level [73,74]. An im-portant prerequisite for this approach is thatreaction rates are measured in the absence ofheat- and mass-transfer limitations. The kineticmodel is based on a description of the catalyticprocess in terms of information and/or assump-

tions about active sites and the nature of ele-mentary steps that make up the catalytic cycle.The ultimate goal of a kinetic analysis is thedetermination of preexponential factors andactivation energies (cf. Arrhenius equation) forall elementary steps in forward and reversedirection. Usually there is not sufficient infor-mation available to extract the values of allkinetic parameters. However, it has been es-tablished that in many cases the observed ki-netics are controlled by a limited number of

kinetic parameters [73,74]. Questions to beanswered in this situation are: (1) how manykinetic parameters are required to calculate theoverall rate from a reaction scheme? (2) Whatspecies are the most abundant intermediates onthe catalyst surface under reaction conditions?(3) Does the reaction scheme include a rate-determining step for the kinetic parameters ofinterest under the reaction conditions? Gener-ally, only a few parameters are kineticallysignificant, although it is difficult to predict

which parameters control the overall rate of thecatalytic process. Therefore, initial estimatesrequire a larger set of parameters than areultimately necessary for the kinetic descrip-tion of the catalytic process of interest. Be-sides experimental values of kinetic para-meters for individual elementary reactions(often resulting from surface sciencestudies on single-crystal surfaces), quantumchemical calculations permit mechanistic in-vestigations and predictions of kinetic para-meters [3638].

Assume that a kinetic model has been estab-lished which consists of n elementary steps,

each proceeding at a net rate

ri rfirri i 1; 2;:::; n 1

The subscripts f and r stand for forward andreverse, respectively. As mentioned above,the validity of the Bodenstein steady-state con-cept can be assumed. The kinetic steady state isthen defined by:

sir ri 2

where r is the net rate rf rr of the overallcatalytic reaction defined by a stoichiometricequation. si is the stoichiometric number ofthe ith step, i.e., the number of times that this

step must occur for the catalytic cycle toturnover once. If the transition-state theory isvalid for each individual elementary step, theratio of the forward rate rfi to the reverse raterri of step i is given by the De Donder rela-tion [81,82]:

rfi=rri expAi=RT 3

where Ai is the affinity of step i:

Ai @Gi=@jiT;P 4

where ji is the extent of reaction of step i.At steady state, the affinity for each step but

one may be very small as compared to theaffinity A of the overall reaction. Each step butone is then in quasi-equilibrium. The step that isnot in quasi-equilibrium (subscript d) is calledthe rate-determining step (rds) as defined byHORIUTI [83]. As a consequence of this defini-

tion, the following inequalities are valid:

rfi ) rfd; and rri ) rrd i 6 d

If there is an rds, then the affinity Ai 0 forall values ofi except for the rds (i 6 d), i.e., all(or almost all) of the affinity for the catalyticcycle is dissipated in the rds, hence

A sdAd 5

It follows that

rf=rr rfd=rrd 6



17/110

At steady state sd(rf rr) rfd rrd.Hence

sdrf rfd and sdrr rrd: 7

The stoichiometric equation for the overallreaction can always be written such that sd is

equal to unity. It is then clear that the rds isappropriately and uniquely named as the step forwhich the forward and reverse rates are equal tothe forward and reverse rates, respectively, ofthe overall reaction [67].

Clearly the rds (if there is one) is the onlykinetically significant step. A kinetically signif-icant step is one whose rate constants or equi-librium constant appear in the rate equation forthe overall reaction. In some cases there is no rdsin the Horiuti sense, but frequently only a few ofthe elementary steps in a catalytic cycle arekinetically significant. It is sometimes said thata rate-limiting step is the one having the smallestrate constant. However, rate constants can oftennot be compared because they have differentdimensions.

The relative importance of rate constants ofelementary steps in a catalytic cycle providesuseful guidelines for the development of activityand selectivity. This can be achieved by

parametric sensitivity analysis [84], which wasfirst proposed by CAMPBELL [85] for analysis ofkinetic parameters of catalytic reactions (seealso ref. [74]). CAMPBELL [85] defined a degreeof rate control for any rate constant ki in acatalytic cycle turning over at a rate r

Xi ki=r@r=@ki 8

where the equilibrium constant for step i and allother rate constants are held constant. The mainadvantage of this mathematical operation is its

simplicity. It turns out that HORIUTIs rds, as theonly kinetically significant step in a catalyticcycle, has a degree of rate control Xi 1,whereas the X values for all other steps areequal to zero. Clearly, all intermediate valuesof Xi are possible, and probable in most cases.

As a catalytic cycle turns over at the quasi-steady state, the steady-state concentrations(coverages) of the reactive intermediates maybe significantly different from the values thatthey would attain if they were at equilibrium

with fluid reactants or products. The steady-state concentrations (coverages) of reactive in-termediates may be lower or higher than the

equilibrium values. The reason for this phenom-enon is kinetic coupling between elementarysteps at the steady state, where the net rate ofeach step is equal to the net rate of the overallreaction multiplied by the stoichiometric num-ber of the step. With kinetic coupling, a reactive

intermediate can accumulate as a reactant or bedepleted as a product [68,79,81].

The principle ofmicroscopic reversibility isstrictly valid only for reactions at equilibrium.Awayfrom equilibrium, it remains valid providedthat transition-state theory is still applicable,which appears to be the case in heterogeneouscatalysis [19]. Hence, the principle remainsvalid for any elementary step in a heteroge-neous catalytic reaction. However, the princi-

ple must be applied with caution to a catalyticcycle, as opposed to a single elementary reac-tion. If valid, the principle of microscopicreversibility allows the calculation of a rateconstant if the second rate constant andthe equilibrium constant Ki of an elementaryreaction i are known: kfi/kri Ki.

2.2.2. Application of Microkinetic

Analysis

Two of the most intensively studied systems inheterogeneous catalysis are CO oxidation overnoble metals and ammonia synthesis. In bothcases, pioneering work using microkinetic anal-ysis led to a better understanding of the catalyticcycle and new fundamental insights, whichsupported design and optimization of the cata-lytic applications. In industry, CO oxidationover Pt and Pd was one of the first systems usedfor automobile emission control and is a key

intermediate step in many technical systems forhydrocarbon transformations. Ammonia syn-thesis once the driving force for a newchemical industry still is one of the mostimportant technical applications of heteroge-neous catalysis. These technical aspects of COoxidation and ammonia synthesis are discussedin Chapter Industrial Application and Mechan-isms of Selected Technically Relevant Reac-tions. Since CO oxidation on noble metals hasbeen the major working system in surface

science and has led to elucidation of manyfundamental issues of reactions on catalyticsurfaces, such as oscillatory kinetics and



18/110

spatio-temporal pattern formation [86], this sys-tem will be exemplarily used for the illustrationof microkinetic analysis.

CO oxidation on noble metals (Pt, Pd, etc.)

CO1/2O2!CO2 9

is relatively well understood, based on surfacescience studies. Molecular oxygen is chemi-sorbed dissociatively, while CO binds asso-ciatively [87,88]. Molecular CO then reacts withatomic oxygen in the adsorbed state:

O22*!O2;ads!2Oads 10

CO*!COads 11

COadsOads!CO22* 12

Here * denotes a free surface site and thesubscript ads an adsorbed species. The reac-tion steps (10)(12) suggest that CO oxidation isa Langmuir Hinshelwood process in whichboth reacting species are adsorbed on the cata-lyst surface. The reverse of reaction (10), i.e.,the recombination of two oxygen atoms is ki-netically insignificant at temperatures belowca. 600 K. Possible Eley Rideal steps suchas (13), in which a gas-phase molecule reactswith an adsorbed species

COOads!CO2* 13

were found to be unlikely.Quantitative experiments led to a schematic

one-dimensional potential-energy diagram char-

acterizing the elementary steps on the Pd(111)surface (Fig. 6). Most of the energy is liberatedupon adsorption of the reactants, and the activa-tion barrier for the combination of the adsorbedintermediates is relatively small; this step is onlyweakly exothermic, and the heat of adsorption

(activation energy for desorption) of CO2 is verylow.

The sequence of elementary steps (10) (12) is quite simple. The overall kinetics, how-ever, is not. This is due to the nonuniformity ofthe surface and segregation of the reactants intosurface domains at higher coverages. As a con-sequence, the reaction between the surface spe-cies COads and Oads can only occur at theboundaries between these domains. A simple

Langmuir Hinshelwood treatment of the ki-netics is therefore ruled out, except for thespecial case of low surface coverages by COadsand Oads, when these are randomly distributedand can be considered to a first approximation asbeing part of an ideal surface.

2.2.3. Langmuir Hinshelwood

Hougen Watson Kinetics [89,70,72,90]

The Langmuir Hinshelwood Hougen Watson (LHHW) approach is based on theLangmuir model describing the surface of acatalyst as an array of equivalent sites whichdo not interact either before or after chemisorp-tion. Further, for derivation of rate equations, itis assumed that both reactants and products areequilibrated with surface species that react on

Figure 6. Schematic one-dimensional potential-energy diagram characterizing the CO O2 reaction on Pd(111) [88]



19/110

the surface in a rate-determining step. Surfacecoverages are correlated with partial pressuresor concentrations in the fluid phase by means ofLangmuir adsorption isotherms. It was men-tioned above that the Langmuir model is unre-alistic. Moreover, it was demonstrated in Sec-

tion Concepts of Reaction Kinetics (Microki-netics) that the surface coverages of adsorbedspecies are by no means identical to the equi-librium values predicted by the Langmuir ad-sorption isotherm for reaction systems in whichkinetic coupling occurs, and rate-determiningsteps do not generally exist.

Despite these weaknesses, the LHHW kinet-ics approach has proved valuable for modelingheterogeneous catalytic reactions for reactor and

process design. The kinetic parameters whichare determined by fitting the rate equations toexperimental data, however, do not have astraightforward physical meaning. As an alter-native, simple power-law kinetics for straight-forward reactions (e.g., A ! B) can be used fortechnical application.

Often it is difficult to discriminate between twoor more kinetic models within the accuracy limitsof the experimental data. Sophisticated mathe-matical procedures have thereforebeendeveloped

for the discrimination of rival models [91].

As an example for a typical LHHW rateequation consider the reaction

AB C:

The form of rate equation is as follows [91]:

r krdsNTKi PAPBPC=Keq

1KAPAKBPBKCPCP

j KjPjn

rate factor driving force

inhibition term

14

The numerator is a product of the rate con-stant of the rds krds, the concentration of activesites NT, adsorption equilibrium constants Ki,and the driving force for the reaction. The latteris a measure of how far the overall reaction isfrom thermodynamic equilibrium. The overallequilibrium constant Keq, can be calculated fromthermodynamics. The denominator is an inhibi-tion term which takes into account the competi-tive adsorption of reactants and products.

A few examples of LHHW rate equations aresummarized in Table 3. A collection of usefulLHHW rate equations and kinetic data foralmost 100 industrially important catalytic re-

actions is available in [92].

Table 3. General structure of Langmuir type rate equations 90

Reaction Controlling step Net rate Kinetic

constant

Driving

force

Adsorption

term

1. A P a. adsorption of A kApA(1 SQ) kAQA kA pApPK

1 KApA Kppp

b. surface reaction single-site

mechanism

kSQp kSQp kSKA pApPK

1 KApA Kppp

c. desorption of P kpQp kppp(1 SQ) kpkSKA pApPK

1 KApA Kppp

2. A PQ surface reaction, A (ads) reacts

with vacant site

kSQA(1 SQ) kSQp kSKA pApPpQ

K(1 KApA Kppp KQpQ)

2

3. A2 2 P a. dissociative adsorption of A2 kApA2(1 SQ)2

kAQA kA pA2

p2p

K (1 KApA2 Kppp)2

b. surface reaction following

dissociative

adsorption of A2

kSQA kSQp kSKA pA2 ppK

1 KApA2 Kppp

4. AB P a. adsorption of A kApA(1 SQ) kAQA kA pApp

KpB1 KApA KBpB Kppp

b. surface reaction kSQAQB kSQp(1 SQ) kSKAKB pApBppK

(1 KApA KBpB Kppp)2

c. desorption of p kpQp kppp(1 SQ) kpKSKAKB pApBppK

1 KApA KBpB Kppp

5. 12

AB P dissociative adsorption of A2,

only half of which react

kApA2(1 SQ) kAQA kA pA2 pp

KpB1 KApA2 KBpB Kppp

6. A 12

B P adsorption of A, which reacts

with half of B produced from

the dissociative adsorption of B2

kApA(1 SQ) kAQA kA PApP

KpB21 KApA KBpB2 Kppp

7. A22 B 2 P a. dissociative adsorption of A2 kApA2(1 SQ)2 kAQA

2kA PA2

p2P

KpB2

1 KApA KBpB Kppp

b. surface reaction following

the dissociative

of adsorption A

kSQAQB kSp(1 SQ) kSKAKB PA2pBpPK

1 KApA2 KBpB Kppp



20/110

2.2.4. Activity and Selectivity

Catalytic activity is expressed in terms ofreaction rates, preferably normalized to thesurface area of the active phase (e.g., metalsurface area for supported metal catalysts).

These surface areas can be obtained by suitablechemisorption techniques (see Section Physi-cal Properties). As an alternative to these arealrates, specific rates are also used which arenormalized to catalyst weight. The best possi-ble measure of catalytic activity, however, isthe turnover rate or turnover frequency, since itis normalized to the number of active sites andrepresents the rate at which the catalytic cycleturns over. For comparison of rates reported by

different research groups, the methodology forthe determination of the number of active sitesmust be carefully reported. The hitherto unre-solved problem is that the site densities mea-sured prior to the catalytic reaction are notnecessarily identical to those available underreaction conditions.

A readily available measure of catalytic ac-tivity is space time yield, expressed in units ofamount of product made in the reactor per unittime and unit reactor volume.

A considerable obstacle for the comparisonof catalytic activities for a given reaction thatwere obtained in different laboratories for thesame catalyst is the use of different reactors. Fora series of catalysts, reasonable comparisons ofactivities or rates are possible when relativevalues are used.

Conversion data alone, or conversion versustime plots are not sufficient as a measure ofcatalytic activity.

Selectivity can be defined as the amount of

desired product obtained per amount of con-sumed reactant. Selectivity values are only use-ful if the conversion is also reported. A simplemeasure of selectivity is the yield (yield se-lectivity conversion). Selectivities can alsobe used to indicate the relative rates of two ormore competing reactions; competition mayoccur when several reactants form products inparallel (type I):

when one reactant transforms into several pro-ducts in parallel (type II):

or in consecutive reactions (type III):

The selectivity is defined as the ratio of therate of formation of the desired product to therate of consumption of the starting material [93].Thus, the selectivities for product X for the first-order reactions I and II is r1/(r1 r2), whereas itis (r1 r2)/r1 for type III.

In the case of type I or II reactions, selectivityfor X or Y is independent of the conversion ofthe starting material. In type III reactions, theselectivity for X is 100 % initially, decreasesgradually with increasing conversion, and dropsto zero at 100 % conversion. At an intermediateconversion, there is a maximum yield of Xwhich depends on the ratio of the rate constants

k1 and k2 of the rates r1 and r2. The integratedrate equations are:

A expk1t 15

X k1=k2k1 expk1texpk2t

16

where [A] is the concentration of unconvertedA, [X] the concentration of A converted toproduct X, and t time. The maximum yield is

reached at

t k1k21 lnk1=k2 17

2.3. Molecular Modeling in

Heterogeneous Catalysis

Modeling of catalytic reactions is applied atmany levels of complexity covering severalorders of length and time scales. It ranges from

complete description of the dynamics of areaction through adsorbate adsorbate interac-tions to the simple mean-field approximations



21/110

and macrokinetic models discussed in SectionLangmuir Hinshelwood Hougen WatsonKinetics. The different approaches can be rep-resented in a hierarchy of models (Table 4).

In this section, frequently used models arepresented that either describe the molecular be-havior of the catalytic cycle directly or are basedon the molecular picture. Often, the output of acomputation using a more sophisticated methodserves as input for a computation using a lessdetailed model; for instance activation energiescomputed by DFTareoften used as parameters inkinetic Monte Carlo and simulations.

2.3.1. Density Functional Theory

In real ab initio calculations, in which the time-dependent Schrodinger equation is solved toobtain the complex N-electron wavefunctionY, the number of atoms of the system studiedis very limited, and therefore quantum mechan-ical calculations in heterogeneous catalysis arealmost exclusively based on the DFT approach.Based on the Hohenberg Kohn theorem, theground-state energy of an atom or molecule is

completely determined by the electron density.Eventhoughtheexactfunctionaldependenceof theenergy on the electron density is not known, ap-proximate functionals can be developed (Kohn Sham formalism) that lead to the much simplercomputed electron density.

There are two major methods for DFT simu-lations of catalytic systems: In the first, thecluster algorithm, the molecules studied aremetal clusters including the adsorbed particles.The advantage of this approach is that the specialshape of catalytic clusters can be taken intoaccount, and methods developed for gas-phasechemistry can be used, so that computational

costs are relatively low. Disadvantages are thelimited number of atoms in the cluster, currently(ca. 2008) a few hundred, and the fact that metalclusters in general have different properties to

three-dimensional metals. Some prominent soft-ware tools using the cluster algorithm areGAUSSIAN [94] and TURBOMOLE [95].

The second approach, usually denoted by theterms planar waves or periodic boundaries,is much more popular in heterogeneous cataly-sis. The algorithm is based on a supercell ap-proach, i.e., structures to be calculated must beperiodic in three dimensions. This approach isespecially advantageous when considering sur-face structures, because a real solid surface is

built on expansion from a small metal cluster ormetal slab into three dimensions. In particular,the metallic properties are better described. Thethird dimension is a disadvantage, because thesolid cell must be periodic in this direction aswell. Aside from the problem of choosing ap-propriate functionals, the size of the cell and theconvergence criteria are significant for DFTcomputations to provide reliable information.Some prominent software tools using the planarwaves approach are CASTEP [96], DACAPO

[97], and VASP [98].Even though still very computer time con-

suming, DFT can be used to calculate the stabil-ity and frequencies for all reactants, intermedi-ates, and products, as well as activation barriersof the elementary reactions [99105]. Recently,complete reaction mechanisms including prop-erties of intermediates have been developedbased on DFT computations alone, for instance,for CO oxidation over RuO2(110) [105], epoxi-dation of ethylene over Ag [106], methanoldecomposition over Cu [107], ammonia synthe-sis over Ru [108], and decomposition of N2O onFe-ZSM-5 [109]. DFT simulation not only helps

Table 4. Hierarchy of methods of modeling catalytic reactions

Method of modeling Simplification Application

Ab initio calculation Most fundamental approach Not yet significant in heterogen eous catalysis

Density functional theory (DFT) Replacement of the N-electron wave

function by the electron density

Dynamics of reactions, activation barriers, adsorbed

structures, frequencies

Kinetic Monte Carlo (kMC) Details of dynamics neglected Adsorbate adsorbate interactions on catalytic surfaces

and nanoparticles

Langmuir Hinshelwood

(mean-field approximation, MF)

Detailed configuration of the adsorbate

structure neglected

Microkinetic modeling of catalytic reactions in technical

systems

Power-law kinetics All mechanistic aspects neglected Scaleup and reactor design for black-box systems



22/110

to understand the fine details of catalytic reac-tions, for instance, the effect of surface steps onstability of intermediates [108] and the impact ofcoverage on activation energies [110], but also toelucidate the broader picture, for example, byfinding a relationship between activation energy

and chemisorption energy [111].

2.3.2. Kinetic Monte Carlo Simulation

Diffusion of adsorbates on catalytic surfaces iscrucial for catalytic reactions. Furthermore, in-teractions between adsorbates can be substantialand lead to ordered structures such as islandsand influence the energetic state of the surface,

which also implies dependence of the activationbarriers for adsorption, diffusion, reaction, anddesorption on the surface coverage and theactual configuration of the adsorbates. The ad-sorbed species can be associated with a surfacesite, and thus a lattice representation of a two-dimensional surface can be constructed. In thecase of catalytic particles, a three-dimensionalstructure can be used with individual two-di-mensional facets that can differ in their catalyticactivity. In the three-dimensional case, special

care is needed for appropriate treatment ofedges and corners. Even reconstruction of sur-faces can be taken into account. At each surfacesite the local environment (presence of adsor-bates, catalyst morphology/crystal phase) willdetermine the activation energies. If the inter-actions between the adsorbates, the surface, andthe gas phase are known, such parameters couldtheoretically be derived from DFT simulations,and the kinetics can be computed by the kineticMonte Carlo method (kMC) [105,112118].

Each molecular event, i.e., adsorption, desorp-tion, reaction, diffusion, is computed and leadsto a new configuration of adsorbed species onthe surface lattice. Aside from this very detaileddescription of the process, time averaging of thetime-dependent computed reaction rates andsurface coverage can then lead to overall rateexpressions. However, the computational effortneeded is immense, not only due to the kMCsimulation but also because of the huge numberof fundamental DFT computations needed toprovide reliable activation barriers for all possi-ble individual steps. Experimental derivation ofthis information is even more exhausting. Most

of the adsorbate adsorbate interactions, suchas the formation of ordered structures, mayappear at low temperature and pressure, wherediffusion is slow and the rate of impingement ofgas-phase molecules is small, respectively. Un-der these conditions kMC may be the only

description that is accurate, while at high tem-perature and pressure, the adsorbates are ratherrandomly dispersed on the surface, and theassumptions of the mean-field approximationmay be valid.

2.3.3. Mean-Field Approximation

[119122]

In the mean-field approximation, a continuousdescription is considered instead of the detailedconfigurations of the system discussed above.Hence, the local state of the catalytic surface onthe macroscopic or mesoscopic scale can berepresented by mean values by assuming ran-domly distributed adsorbates on the surface,which is viewed as being uniform. The state ofthe catalytic surface is described by the temper-ature Tand a set of surface coverages ui, that is,the fraction of the surface covered with adsor-

bate i. The surface temperature and the cov-erages depend on time and spatial position in themacroscopic system (reactor), but are averagedover microscopic local fluctuations. Underthose assumptions a chemical reaction can bedefined as

XNg NsNbi1

n0

ikAi!XNgNsNbi1

n00

ikAi

where Ai denote gas-phase species, surface spe-cies, and bulk species. The Ns surface speciesare those that are adsorbed on the top monoa-tomic layer of the catalytic particle, while theNbbulk species are those found in the inner solidcatalyst.

Steric effects of adsorbed species and variousconfigurations, e.g., type of chemical bond be-tween adsorbate and solid, can be taken intoaccount by using the following concept: Thesurface structure is associated with a surface site

density G that describes the maximum numberof species that can be adsorbed on unit surfacearea. Each surface species is associated with a



23/110

coordination number si describing the numberof surface sites which are covered by this spe-cies. Under the assumptions made, a multistep(quasi-elementary) reaction mechanism can beset up. The molar net production rate is thengiven as

si XKsk1

nikkfk

YNgNsNbj1

cn

0

jk

j :

where Ks is the number of surface reactions, ciare the species concentrations, which are given,e.g., in mol m2 for the Ns adsorbed species andin, e.g., mol m3 for the Ng and Nb gaseous andbulk species. With Qi cisiG

1, the variationsof surface coverages follow:

@Qi@t

sisiG

:

Since the temperature and concentrations ofgaseous species depend on the local position inthe reactor, the set of surface coverages alsovaries with position. However, no lateral inter-action of the surface species between differentlocations on the catalytic surface is modeled inthis approach. This assumption is justified bythe fact that the computational cells in reactor

simulations are usually much larger than therange of lateral interactions of the surface pro-cesses. In each of these cells, the state of thesurface is characterized by mean values (mean-field approximation).

The binding states of adsorption of all speciesvary with the surface coverage, as discussed inSection Kinetic Monte Carlo Simulation. Thisadditional coverage dependence can be mod-eled in the expression for the rate coefficient byan additional function leading to:

kfk AkTbkexp

EakRT

YNsi1

Qmiki exp

eikQi

RT

:

For adsorption reactions sticking coefficientsare commonly used, which can be converted toconventional rate coefficients.

2.3.4. Development of Multistep Surface

Reaction Mechanisms [122]

The development of a reliable surface reactionmechanism is a complex process. A tentative

reaction mechanism can be proposed based onexperimental surface-science studies, on analo-gy to gas-phase kinetics and organometalliccompounds, and on theoretical studies, in-cluding DFT and kMC calculations as well assemi-empirical calculations [123,124]. This

mechanism should include all possible pathsfor formation of the chemical species underconsideration in order to be elementary-likeand thus applicable over a wide range of con-ditions. The mechanistic idea then needs to beevaluated against numerous experimentally de-rived data, which are compared with theoreticalpredictions based on the mechanism. Here,simulations of the laboratory reactors requireappropriate models for all significant processes

in order to evaluate the intrinsic kin

Date post:	14-Apr-2018
Category:	Documents
Upload:	biodiesel-pala-fredricksen
View:	251 times
Download:	0 times

Heterogeneous Catalysis and Solid Catalysts

Documents