In this review basic concepts are discussed which in last 30 years formed the background of the research on alloy catalysts. An attempt is made to describe the present state of understanding of the real and potential effects induced by alloying metals. A brief discussion is also devoted to the quantum theory of alloys, practical applications and speculations about the future developments in catalysis by alloys.

This review deals with the mechanism that is arbitrarily called below the Mars and Van Krevelen (MvK) mechanism. The characteristic feature of this mechanism is that some products of the reaction leave the solid catalysts’ surface with one or more constituents of the catalysts’ lattice. First in this review, the position of this mechanism amongst other ways of catalytic activation will be defined. Then, the experimental evidence for this mechanism will be presented. Some less common reactions running with this mechanism will be discussed in more detail, among them the deoxygenation of nitrocompounds and carboxylic acids.The participation of the lattice components (O, S, Cl, H) in the formation of products leads to a relation of the catalytic activity with the thermodynamic parameters characterizing the catalysts lattice. A proper use of such quantified relation will be also discussed.

The hydrogenation of propanal and of acetone have been studied as model for reactions of aldehydes and ketones. It has been confirmed that on all metals studied here, propanal reacts more slowly than acetone, just opposite to the homogeneous catalysts. Competitive hydrogenation experiments brought results which strongly suggest that this finding should be related to the different adsorption modes, possible with either homogeneous or heterogeneous catalysts. A solid surface offers contiguous adsorption sites which induce a higher population of more firmly adsorbed (and less reactive) species. Observations made on ether formation could be explained by means of an older model, according to which two different species react on a large ensemble of surface atoms.

In this study several metal catalysts have been applied (viz. Pt, Pd, Rh and Ga-promoted Pt) in the reactions of 1-propanol and 2-propanol (in hydrogen). Information obtained in this way is used to elucidate the effects of alcohols (n- and iso-alcohols) on the hydrogenation of carbonyl compounds over the catalysts mentioned above. Effects of gaseous additives, effects of promoters thereon and, some other effects can be explained by the model suggested by Van der Burg et al. Discussion based on the results obtained by this study and on information from the literature, leads to the conclusion that in hydrogenation of carbonyl compounds the addition of the first hydrogen atom is the most probable rate-determining step.

The role of carbonaceous deposits in the skeletal isomerisation of n-butene was studied in a pulse reactor. High activity and selectivity were observed under conditions when the catalyst was almost free of carbonaceous deposits. Carbonaceous deposits deactivate the catalyst and suppress both isobutene and by-product formation. However, since reactions leading to byproducts are suppressed more strongly, the relative concentration (yield) of isobutene might increase during the first few pulses. it is concluded that a selective monomolecular mechanism operates on the uncovered OH-groups. Anchoring hydrocarbon molecules on a surface is a more difficult step than subsequent oligomerisation. Deposits (mostly oligomers) that form early, crack non-selectively to isobutene and larger amounts of propene and pentenes. (C) 1999 Elsevier Science B.V. All rights reserved.

The hydrogenation of acetone and of propanal have been studied over Pd and Gapromoted Pd catalysts. The main effect of the promoter is to create new sites bearing the more reactive adsorption mode of propanal and acetone.

The oxidation of allyl iodide was studied on the period IV metal oxides with 18 O 2 . In this reaction the selectivity and activity of the period IV metal oxides were determined as a function of temperature. The activity pattern of the allyl iodide oxidation reaction shows the same saw-tooth like pattern as the oxidation of H 2 and CH 4 ; i.e. maxima at MnO 2 , Co 3 O 4 and CuO. However, the differences in activity of the period IV metal oxides in the oxidation of allyl iodide are within one-order of magnitude, which is much smaller than for the oxidation of H 2 and CH 4 . Although in the oxidation of allyl iodide the first step, i.e. abstraction of a hydrogen, is bypassed, the activity is still determined by the average metal–oxygen bond strength: the higher the M–O bond strength the lower the activity. The selectivity pattern looks similar to the activity pattern, only the selectivity pattern varies in anti-phase with the activity pattern: a high M–O bond strength results into a high selectivity to acrolein. The oxidation of allyl iodide with 18 O 2 and the oxidation of propylene with 18 O 2 on Co 3 O 4 revealed that the total as well as the selective oxidation products were formed with lattice oxygen from the metal oxide. Reaction of an adsorbed intermediate with gas phase oxygen can only take place if the surface of the oxide is strongly reduced.

The reactivity of lattice oxygen of vanadium oxide catalysts was studied with the oxygen isotopic exchange reaction. The reactivity of pure V2O5 is compared with the reactivity of Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, δ-VOPO4, and (VO)2P2O7. According to their behaviour in the oxygen exchange reaction, two types of vanadium oxide catalysts could be distinguished. The first type of catalysts only showed exchange activity in the R2 exchange mechanism and the second type showed activity in both the R1 and R2 exchange mechanisms (mechanisms in which, respectively, one or two oxygen atoms of the gas phase molecule are exchanged with oxygen atoms of the metal oxide). The catalysts which belong to the first group are bulk V2O5 and δ-VOPO4 and the catalysts which belong to the second group are Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, and (VO)2P2O7. If only the R2 mechanism is observed then diffusion of lattice oxygen is probably faster than when both the R1 and R2 mechanisms are observed. The activity of the supported vanadium oxide catalysts in the oxygen exchange reaction is dependent on the support. The reactivity order is V2O5/TiO2>V2O5/Al2O3∼V2O5/SiO2.

Two methods have been used to determine the individual rate constants of the isotopic exchange reaction of oxygen on metal oxides. The best way to determine the three rate constants is to use the kinetic model of Klier and co-workers and fit the equations to the experimental data. The method of Tsuchiya and co-workers can be used to check the results obtained by the method of Klier and co-workers. The three rate constants,R0,R1, andR2, of the three different exchange mechanisms are determined for the period IV metal oxides at various temperatures. The rate constants are correlated with parameters characterising the oxide, i.e. the position of the metal element in the periodic table and the average metal–oxygen bond strength.

The oxidation of allyl iodide was studied on the period IV metal oxides with O-18(2). In this reaction the selectivity and activity of the period IV metal oxides were determined as a function of temperature. The activity pattern of the allyl iodide oxidation reaction shows the same saw-tooth like pattern as the oxidation of H-2 and CH4; i.e. maxima at MnO2, Co3O4 and CuO. However, the differences in activity of the period IV metal oxides in the oxidation of allyl iodide are within one-order of magnitude, which is much smaller than for the oxidation of H-2 and CH4 Although in the oxidation of allyl iodide the first step, i.e. abstraction of a hydrogen, is bypassed, the activity is still determined by the average metal-oxygen bond strength: the higher the M-O bond strength the lower the activity. The selectivity pattern looks similar to the activity pattern, only the selectivity pattern varies in anti-phase with the activity pattern: a high M-O bond strength results into a high selectivity to acrolein. The oxidation of allyl iodide with O-18(2) and the oxidation of propylene with O-18(2) On Co3O4 revealed that the total as well as the selective oxidation products were formed with lattice oxygen from the metal oxide. Reaction of an adsorbed intermediate with gas phase oxygen can only take place if the surface of the oxide is strongly reduced. (C) 1999 Elsevier Science B.V. All rights reserved.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The role of acid strength in skeletal isomerisation of n-butene to isobutene was studied on a series of open-surface catalysts and several ZSM-5 samples. The highest selectivity was obtained on the open-surface catalysts of moderate acid strength (H0 ca. −5). When the acidity is lower, activity of the catalysts is not sufficient, when the acidity is too high, oligomerisation and by-product formation are induced. In this study, only limited modification of the acidity of zeolites had been achieved and therefore, the effects on butene isomerisation were not very pronounced and, they were observable only at high temperatures (ca. 750K).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Several classes of active and selective catalysts for butene skeletal isomerization are already known. The most promising ones are those, based on zeolites or zeotypes. These materials suppress side reactions, and they are stable on stream. This paper shows why this is so: dimerization and oligomerization (the latter being the source of deactivation) are suppressed when a proper structure of the microporous catalyst is chosen; i.e., noncrossing, 10-membered ring (10-MR) channels. It is concluded, at variance with most of the early literature that the prevailing mechanism, which has to be induced by the catalyst, is a monomolecular mechanism. The bimolecular mechanism should be suppressed as much as possible.

The present paper reviews the best anode electrocatalysts, mainly the alloys, which have been tested in direct liquid-feed fuel cells fed with methanol, ethanol or formic acid. It attempts to interpret the alloys catalytic behaviours by using the Nørskov and co-workers’ theoretical work [A. Ruban, B. Hammer, P. Stoltze, H.L. Skriver, J.K. Nørskov, J. Mol. Catal. A 115 (1997) 421; B. Hammer, J.K. Nørskov, Adv. Catal. 45 (2000) 71; J. Greeley, J.K. Nørskov, M. Maurikakis, Annu. Rev. Phys. Chem. 53 (2002) 319], who proposed surface theories and databases about the metals d-band centre shift and the segregation. It also attempts to suggest new alloys combinations. For example, for the methanol oxidation, the best catalyst is Pt-Ru and the following features make this catalyst stand out: the d-band centre of Pt shifts down what supposes weaker molecules adsorption and Pt strongly segregates. From this analysis, it is suggested that the Pd-Ni alloy may be a potentially good catalyst. Similar interpretations are given for the three fuel cell systems regarded in the present paper.

Catalytic behaviour of CeO2 in the reduction of benzoic acid has been studied in the temperature range 523–723 K. Two types of catalytic behaviour are observed in the whole temperature range. One type is observed from 523 up to 723 K. In this range, the selectivity to benzaldehyde can be higher than 95% and the reaction proceeds by the redox (Mars and van Krevelen) mechanism, with the oxygen vacancy as the active site. The activity is controlled by the steady-state concentration of oxygen vacancies under reaction conditions. The second type behaviour is observed above 648 K. Under these conditions the conversion of benzoic acid achieves nearly 100%, but the selectivity to benzaldehyde decreases, while that to toluene and benzene increases. In this range of temperatures, the reaction proceeds too far, due to a very high concentration of vacancies. Besides the redox mechanism decar☐ylation (hydrogenolysis) of the relative stable adsorbed benzoic acid takes place. The results observed with the reaction of adsorbed benzoic acid, under a flow of H2 reflects the two types of reaction. The potential promotion effects by Al, Cr, Mn, Fe, Ga, Zr, In and Pb oxides added to CeO2 were investigated. The catalytic behaviour of CeO2 changes by the addition of the metal oxides, positive effects are observed after adding Mn, Zr, In and Pb oxides.

Iron-based catalysts appeared to be very active and selective in the hydrogenation of acetic acid to acetaldehyde. The active and selective catalyst consists of a metallic and an oxidic phase. Probably, the metal is needed to activate hydrogen, and the oxide is needed to provide the reaction site for the selective hydrogenation. Catalyst pretreatment and reaction conditions must be carefully controlled: the catalyst must be prereduced and the hydrogen/acid ratio must be higher than four. Only then are both iron-containing phases formed and kept stable during the reaction. The function of hydrogen is twofold: it must keep the catalyst in its active, partly reduced, form and it acts as reactant in the hydrogenation of acetic acid.

In this review, the most relevant aspects of skeletal isomerization of n-butene to isobutene are discussed: the nature of the active sites, the prevailing mechanism of the skeletal isomerization, and the relation of this to that of n-butane. It is concluded that the prevailing mechanism of skeletal isomerization ofn-butene is monomolecular (in contrast to butane isomerization) and requires Bronsted acid (OH) active sites. The selectivity and catalytic stability can be influenced by the shape selectivity of zeolites and zeotypes. These effects are explained on the basis of the knowledge on the prevailing mechanism.

Hydrogenations of two model compounds (acetone, propanal) have been studied with Pt/SiO2 catalysts, promoter-free or promoted by Ga, Ge or Fe compounds. It appeared that hydrogenation of acetone is faster than that of propanal, but the promotion effect is more pronounced with the aldehyde than with the ketone. This is in compliance with earlier work. A tentative explanation of the observed phenomena is suggested, based on the theoretical analysis by Delbecq and Sautet.