Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Catalytic Properties of Ceria and CeO2-Containing Materials

The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts. © 2001 Elsevier Science B.V. All rights reserved.

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Mechanisms of catalyst deactivation

Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing). The key features and considerations for each of these deactivation types is reviewed in detail with reference to the latest literature reports in these areas. Two case studies on the deactivation mechanisms of catalysts used for cobalt Fischer-Tropsch and selective catalytic reduction are considered to provide additional depth in the topics of sintering, coking, poisoning, and fouling. Regeneration considerations and options are also briefly discussed for each deactivation mechanism.

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Heterogeneous Catalyst Deactivation and Regeneration: A Review

The use of structured catalysts in the chemical industry has been considered for years. Conventional fixed-bed reactors have some obvious disadvantages such as maldistributions of various kinds (including a nonuniform access of reactants to the catalytic surface), high pressure drop in the bed, etc. Structured catalysts are promising as far as elimination of these setbacks is concerned. Two basic kinds of structured catalysts can be distinguished: Structural packings covered with catalytically active material, similar in design to those used in distillation and absorption columns and/or static mixers. Good examples of catalysts of this kind are those offered by Sulzer, clearly developed by Sulzer column packings and static mixers. As in packed beds, there is an intensive radial convective mass transport over the entire cross-section of these packings. Structural packing catalysts and the reactors containing them are, however, not within the scope of this review. Monolithic catalysts are continuou...

Monoliths in Heterogeneous Catalysis

Catalysis and combustion have long been linked. In fact, the science of catalysis stems from Davy's discovery [1] that platinum wires could promote the flameless combustion of flammable fuel-air mixtures. Today, catalysis is a mainstay of our modern chemical industry. Oxidation catalysts are used not only for the complete oxidation of fuels to carbon dioxide and water, as in radiant catalytic tent heaters and fume abatement devices, but also for the selective partial oxidation of hydrocarbons or other “fuels” to produce basic chemicals such as ethylene oxide (from ethylene), terephthalic acid (from p-xylene), and nitric acid (from ammonia). However, despite the long-known capability of catalysts to oxidize hydrocarbons without significant production of carbon monoxide, soot, or thermal NOx, there seemed little possibility that catalytic oxidation reactors could ever displace conventional flame combustors as primary fuel combustors. This is because the volumetric heat release rates of conventional...

Catalysis in Combustion

Poison-resistant catalysts for the simultaneous control of hydrocarbon, carbon monoxide, and nitrogen oxide emissions

Experiments designed to answer certain questions about the mechanism of operation and of deactivation of catalysts near stoichiometric conditions were carried out. This involved special catalysts preparations tested in laboratory equipment and in vehicles. We concluded that the poisoning process at exhaust stoichiometry appears to be very similar to the poisoning process under net oxidizing conditions; that is, only a sharply defined outer shell of the pellets is poisoned, this shell progressing inward as time elapses. Based on these experiments, the following catalyst design concept was developed: To utilize the different properties of Pt, Pd and Rh, all three will have to be employed in a special design corresponding to their ratio in ores. To avoid interferences, they should not be coimpregnated over the same support surface; one way to avoid coimpregnation is to impregnate them in separate layers in the catalyst pellets. Due to its relative poison insensitivity Pt should be the outer layer, while Pd and Rh should be protected from the poison front. Consequenty, the Pt impregnation depth (from the outer surface) is given by the depth to which the poisons penetrate. Due to the importance of Rh in catalyzing the NO reduction reaction at the (diffusionmore » controlled) rich end of the A/F scale, Rh should be impregnated as close to the surface of the pellet as possible, while still being protected from poisoning. Consequently, Rh should be impregnated right below the Pt layer and Pd underneath the Rh layer. Due to the transient nature of the feedstream's A/F, it appears to be beneficial to include an agent in catalyst formulations which, by storing and slowly releasing some of the components from the surface, tends to smooth out the effects of A/F oscillations. The results are encouraging; real-time engine dynamometer and vehicle aging tests are required, however, to prove their durability in actual automobile service.« less

Poison-resistant catalysts for the simultaneous control of hydrocarbon, carbon monoxide and nitrogen oxide emissions

Effects of platinum and palladium impregnation on the performance and durability of automobile exhaust oxidizing catalysts

Stable isothermal multiplicities were observed during carbon monoxide oxidation in an integral reactor, filled with alumina supported platinum catalysts. The multiplicities were investigated in the conversion-temperature, conversion-inlet carbon monoxide concentration and conversion-mass flow rate domains. The region of multiplicities was found to widen significantly upon catalyst aging which enhanced the pellets' diffusive resistances. Several intermediate stable steady states were found between the highest and lowest steady states, both experimentally and theoretically. All the above phenomena could be well interpreted by the interactions of the kinetics of carbon monoxide oxidation with intrapellet diffusion resistances.

Multiple steady states in an isothermal, integral reactor: the catalytic oxidation of carbon monoxide over platinum--alumina

The application of monolithic catalysts to automotive emission control has frequently been complicated by unexplained thermal degradation phenomena. This paper reports experimental and theoretical studies aimed at the understanding of how heat is generated and distributed in catalytic monoliths during an exothermal, mass transfer limited reaction.

Theoretical considerations indicate that under certain operating conditions the steady state solid catalyst temperature can exceed the adiabatic reaction temperature. It is also shown that the catalyst overtemperature is influenced by the nature of the reactive species and by the geometry of the catalyst. These predictions have been verified by experiments in a novel tubular reactor.

L. Louis Hegedus

Papers

Poison-resistant catalysts for the simultaneous control of hydrocarbon, carbon monoxide, and nitrogen oxide emissions

Poison-resistant catalysts for the simultaneous control of hydrocarbon, carbon monoxide and nitrogen oxide emissions

Effects of platinum and palladium impregnation on the performance and durability of automobile exhaust oxidizing catalysts

Multiple steady states in an isothermal, integral reactor: the catalytic oxidation of carbon monoxide over platinum--alumina

Temperature excursions in catalytic monoliths