Journal of Molecular Catalysis A: Chemical 173 (2001) 275286
Poisoning and deactivation of palladium catalysts
Peter Albers a,, Jrg Pietsch b, Stewart F. Parker ca Infracor GmbH, Degussa-Hls Group, Department of Physical Chemistry, P.O. Box 1345, D-63403 Hanau, Germany
b Degussa-Hls AG, Silica, Silanes & Catalysts, P.O. Box 1345, D-63403 Hanau, Germanyc ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
Received 23 August 2000; received in revised form 20 October 2000; accepted 26 October 2000
Some of the major causes for deactivation and premature degradation of palladium catalysts are briefly summarised. Theseinclude particle growth for various reasons, coke deposition and coke transformation, the influence of the support materialon long term stability and modifications at the palladium surface itself such as valency changes or the formation of simplebut stable molecular surface species. In addition, variations of physical properties as well as chemical poisoning, corrosionand leaching are discussed. In spite of the large body of literature concerning the common phenomenon catalyst deactivationspecific information accessible for the various kinds of palladium catalysts being used worldwide is still quite limited. Thereis a serious need for future studies on properties of palladium-based catalytic systems regarding, for example, the interactionsbetween carbon, hydrogen and supported palladium as a ternary system. Observations on different deactivation processes onPd/SiO2 catalysts occuring under industrial conditions are compiled. It was tried to roughly differentiate between differentdegrees of coking and coke transformation in mainly thermally or purely catalytically driven catalyst coking on the onehand and of moderate or enhanced corrosion phenomena or changes of the properties of the palladium itself on the other. 2001 Elsevier Science B.V. All rights reserved.
Keywords: Palladium catalysts; Catalyst deactivation; Palladium; Hydrogen; Carbon; Hydrogenation; Oxidation; Sintering; Agglomeration;Coking; Poisoning; Leaching
Uncontrolled and accidental poisoning or deactiva-tion of palladium catalysts by various mechanisms canbe a considerable financial burden in chemical industryand is still on focus of academic as well as technolog-ical research. Some typical large scale applications ofpalladium catalysts are their use in the hydrogenationof organic fine chemicals, aromatic hydrogenations,
petroleum refining, the selective hydrogenation ofacetylene to ethylene, the production of acetaldehydeby oxidation of ethene, the production of vinyl-acetateand, to an increasing extent, the use in different kindsof automotive exhaust gas catalysts [1,2].
As a result of the great economic relevance ofpalladium-based catalysts in these and related fieldsof chemical technology many investigations havebeen performed to study the physico-chemical prop-erties of these materials in more detail and to re-veal the various reasons for loss of activity or se-lectivity under unfavourable or irregular operationconditions. A better understanding of deactivationprocesses is essential for improving and optimising
1381-1169/01/$ see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S1 3 8 1 -1 1 69 (01 )00154 -6
276 P. Albers et al. / Journal of Molecular Catalysis A: Chemical 173 (2001) 275286
process conditions, the catalysts themselves and forcircumventing premature catalyst degradation in orderto minimise additional costs.
Conversely, controlled poisoning of palladium-basedcatalytic systems can have a beneficial impact onperformance. A prominent example is the use of leadcompounds in the production of the Lindlar catalystto improve and to fine-tune its properties such asthe selectivity in the hydrogenation of CC to C=Cbonds by blocking certain active sites [3,4].
In the present contribution, some of the most impor-tant reasons for the deactivation of palladium catalystsare sketched. A few examples and references wereselected from the large body of existing literature oncatalyst deactivation. They are focused predominantlyon some recent and typical studies and observationsincluding own work. A complete review addressingall different aspects of catalyst deactivation is beyondthe scope of this short summary. Additional detailsand related references are accessible from authori-tative surveys and proceedings (e.g. , and theliterature cited therein).
2. Typical reasons for deactivation of palladiumcatalysts
2.1. Crystallite growth/sintering/agglomeration
Morphological changes of palladium entities due toparticle growth by different processes including sin-tering and agglomeration  are a major cause fora decrease of acitivity down to uneconomic levels oreven complete deactivation of palladium catalysts. Ithas been observed that under the influence of hydro-gen particle growth of finely dispersed, unsupportedpalladium may occur even at temperatures as low as330 K [11,12]. The various parameters governing par-ticle growth phenomena have to be considered alreadyduring the first steps of the preparation of palladiumcatalysts. Unfavourable and undesired effects maybe suppressed by means of adequate impregnationagents and procedures, by controlled temperature andby the use of suitable support materials .
An impact of the properties of the metal precursoras well as the support on the final palladium dispersionobtained under identical preparation conditions wasobserved .
Conditioning treatments of the support materials bypurification and by surface modification are well es-tablished as useful procedures especially in the fieldof carbonaceous catalyst supports. Some examples arethe treatments with oxidising agents such as hydro-gen peroxide or by using acids the remove ashes andcontaminants. For activated carbons or carbon blacksthe cleaning and surface conditioning leads (e.g.) tothe generation of surface functional groups and to anincreasing accessability of other relevant sites includ-ing aromatic/graphitic structures and sites with fewvicinal hydrogen atoms down to non-conjugated dou-ble bonds or vinylic/allylic entities. This can be ofparamount influence on the precious metal dispersionto be obtained in the final Pd/C catalyst which areused (e.g.) in hydrogenation reactions . Thefine structure of the carbon support materials [19,20],their purity, their surface chemistry and, therefore, theprecious metal/support interactions have to be exam-ined and optimised [1518,20] to establish adequatepreparation conditions to achieve the best performancepossible for the given application of the palladiumcatalyst.
Regarding the stability of the palladium dispersionduring reduction treatments the selection of suitablesupport materials is of great importance as well. Undera given set of conditions variations of precious metaldispersion and the loss of active area of a palladiumcatalyst with increasing temperature may be enhancedin the case of rather inert support materials comparedto SiO2 or Al2O3 [21,22]. This is also the case underthe actual operation conditions of supported palladiumcatalysts: an influence of the support on the stability ofparticle size was observed. Pd/TiO2 catalysts showedsintering at 500C under the influence of hydrogenwhereas Pd/Al2O3 catalysts were found to be more re-sistant under the same conditions . The Pd-basedcatalysts were identified as the most efficient ones forthe catalytic combustion of methane to carbon dioxideand water. Also in these applications it was found thatthe tendency to sintering and poisoning and the ther-mal stability is dependent on the support . Con-sidering the long term stability of the Pd-dispersion,again, interdependencies between support material andthe resistance of the final Pd-catalysts against catalystdeactivation were also found for other support mate-rials such as ZrO2, MgO, SiO2/AlPO4 [25,26] or forternary systems .
P. Albers et al. / Journal of Molecular Catalysis A: Chemical 173 (2001) 275286 277
Not only modification of the support but also of theprecious metal component may be helpful: additionof (e.g.) Pt was shown to be effective in preventingcatalyst deactivation by improving the heat resistanceand the dispersion of supported oxidised palladiumparticles . Here, finely dispersed Pt crystallites,tightly attached to the support surface, were found toserve as anchoring sites for Pd.
In the selective catalytic reduction of NOx bymethane on Pd/zeolite catalysts a pronounced loss ofacitivity was observed depending on the presence ofwater vapour. Formation of palladium oxide was de-tected and it was concluded that the tendency towardsPd-agglomeration was promoted by water . Inaddition to particle growth effects, valency changesbetween PdOx and metallic Pd may simultaneouslybe of prominent relevance . This illustrates thatseveral positive or detrimental influences could acttogether and might affect the degree and the stabil-ity of the dispersion of palladium particles includingchanges of the chemical valency in oxidation of par-ticle surfaces on the one hand or the formation ofpalladium hydride on the other (Section 2.2.2).
In addition to variations of the particle size a criticalenhancement of the physical or chemical interactionsbetween palladium and organic reactants, interme-diates, by-products or degradation products, endingwith significant catalyst coking, can be of importance.The influence of feed ratios during the operation ofPd/activated carbon catalysts that are deactivated bymeans of sintering or by coking has been revea