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

Focuses on the surface chemistry and spectroscopy of chromium in inorganic oxides. Characterization of the molecular structures of chromium; Mechanics of hydrogenation-dehydrogenation reactions; Mobility and reactivity on oxidic surfaces.

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Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides.

The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPR H2 and TPR CO ), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “ in situ ” XRD methods Five different compounds of iron oxides were characterized: hematite α-Fe 2 O 3 , goethite α-FeOOH, ferrihydrite Fe 5 HO 8 ·4H 2 O, magnetite Fe 3 O 4 and wustite FeO In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300 °C Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe 2 O 3  → 2 Fe 3 O 4  → 6 Fe, three-stage mechanism 3Fe 2 O 3  → 2Fe 3 O 4  → 6FeO → 6Fe is postulated especially when temperature of reduction overlaps 570 °C Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe 2+  ⇌ 2Fe 3+  + Fe, occurring at both the atomic scale on two-dimensional interface border Fe 3 O 4 /Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wustite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe 3 O 4  + Fe Above 570 °C, the appearance of wustite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “ in situ ” XRD method In the case of FeO–H 2 system, instead of one-step simple reduction FeO → Fe, a much more complex two-step pathway FeO → Fe 3 O 4  → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe 3 O 4  → FeO → Fe up to 880 °C should be reconsidered as a result of the accompanying FeO disproportionation wustite ⇌ magnetite + iron manifesting its role above 150 °C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide The disproportionation reaction of FeO does not consume hydrogen and occurs above 200 °C much easier than FeO reduction in hydrogen above 350 °C The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions The disproportionation reaction wustite ⇌ magnetite + iron makes simple wustite reduction FeO → Fe a much more complicated process In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wustite → magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe 3 O 4  Depending on TPR H2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380 °C Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction

Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

Iron oxide surfaces

Temperature-programmed-reduction studies of nickel oxide/alumina catalysts: effects of the preparation method

The mechanism of reduction of iron oxide by hydrogen

A precipitation technique at constant pH value was used to prepare a series of alumina-aluminum borates (AABs) with various Al/B atomic ratios. These materials were used as the supports of Co-Mo catalysts. Hydrodesulfurization (HDS) of Kuwait atmospheric gas (AGO) oil was carried out over these presulfided catalysts in a bench-scale trickle bed reactor at 400 psi and 340 C. All CoMo/AAB catalysts are much more active than the conventional CoMo/Al[sub 2]O[sub 3] catalyst on HDS reactions. A correlation exists between the acidity and the HDS activity of the catalysts. The high activities of the CoMo/AAB catalysts can be rationalized on the presence of boron. On one hand, it can increase the metal dispersions and hydrogenation capabilities. On the other hand, it can enhance the acidities and cracking abilities of the catalysts. The desulfurization data can be fitted with a pseudo-second-order rate equation. The activation energy for desulfurization is found to be 26 kcal/mol.

Hydrodesulfurization Reactions of Atmospheric Gas Oil over CoMo/Alumina-Aluminum Borate Catalysts

A series of chromium-promoted copper catalysts with various Cr to Cu molar ratios were prepared with the co-precipitation method. The promotional effects of chromium on copper catalysts were examined by X-ray powder diffraction (XRD), nitrous oxide decomposition, and the dehydrogenation reaction of ethanol. The dehydrogenation reaction was carried out in a continuous-flow microreactor between 523 and 583 K under atmospheric pressure. The results indicated that the promotional effect was dependent on the Cr/Cu molar ratio, and the predominant decay of catalysts in this study was caused by sintering. The catalyst with the Cr/Cu molar ratio of 4/40 has the highest activity and stability. The surrounded well-dispersed chromia strongly influenced the catalytic properties of copper metal. It also showed that the over-promotation of a catalyst has a disastrous effect on the total make of product. The ethanol dehydrogenation reaction follows a first-order reaction, and the kinetics for deactivation can be described by a second-order expression.

Chiuping Li

Papers

Temperature-programmed-reduction studies of nickel oxide/alumina catalysts: effects of the preparation method

The mechanism of reduction of iron oxide by hydrogen

Hydrodesulfurization Reactions of Atmospheric Gas Oil over CoMo/Alumina-Aluminum Borate Catalysts

Effect of chromium promoter on copper catalysts in ethanol dehydrogenation

Characterization of unsupported copper—chromium catalysts for ethanol dehydrogenation