Oxidation catalysts redox mechanism

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

It is well known also that higher alkanes suffer radical gas phase oxidation above 723 K. Therefore, their use requires catalysts active and selective for deNOx at lower temperatures. The mechanism of NOx elimination is still debated a redox mechanism involving Cu ions is probable, and isolated Cu cations exchanged into MFI [4,5] or mordenite [6] have been found to be more active than CuO clusters. It must be emphasized, however, that acid zeolites exhibit good activity at high temperature, and acid mechanisms have been proposed [7-10]. In presence of Cu this acid mechanism disappears probably due to the decrease of the acidity of mordenite upon Cu exchange [6]. According to... 

Eisenberg et ah—the redox mechanistic perspective over Rh and Pt/Sn systems. The term redox mechanism in catalysis is used to describe cases where the catalyst itself undergoes changes in oxidation state during the course of the mechanism. It does not refer to the oxidation state changes of reactants, products, or associated intermediates, although associated intermediates are often involved in the course of the mechanism. 

Fiolitakis and Hofmann—wavefront analysis supports Eley-Rideal/redox mechanisms. In 1982 and 1983, Fiolitakis and Hofmann231,232 carried out wavefront analysis to analyze the dependence of the microkinetics of the water-gas shift reaction on the oxidation state of CuO/ZnO. They observed three important mechanisms after treatment of the catalyst surface with different H20/H2 ratios. These included two Eley-Rideal mechanisms which converted the reactants via adsorbed intermediates, and a redox mechanism that regulated the oxygen activity, as shown in Scheme 56. The authors indicated that different mechanisms could be dominating at different sections along the length of the fixed bed reactor. 

The kinetics of selective CO oxidation over the Cu Cej r02, nanostructured catalysts can be well described by employing Mars and van Krevelen type of kinetic equation derived on the basis of a redox mechanism ... 

This equilibrium has a buffer-like effect stabilizing the presence of cationic copper species in the structure even in a highly reductive atmosphere. The above scheme of copper oxide-ceria interactions indicates clearly that the catalyst is mutually promoted, i.e., both copper and ceria cooperate in the redox mechanism. 

In our studies we have demonstrated that the redox mechanism that was used to model dynamic behavior of CO oxidation is consistent with a kinetic model of the selective CO oxidation obtained under steady-state mode of operation [62], We propose the following tentative scheme (Figure 7.15) for the selective CO oxidation over the CuolCe(J902 v catalyst CO and H2 adsorb on the... 

The amount of the products formed over the studied catalysts, in the presence and absence of molecular O2, are listed in Table III. It is evident that the formation of the oxidation products is associated with the gas phase oxygen supply. Then, as the reaction rates in the mixture of reactant and in separate steps differ (19), these data exclude the participation of lattice oxygen in the partial oxidation of methane via a two step redox mechanism as main reaction pathway proving the occurrence of a "concerted mechanism". 

The crystal structure and the sintering behavior of hexaaluminates was widely investigated. The relation of sintering resistance to anisotropic ion diffusion in the layered alumina phase was clarified to a large extent. Other evidence suggests that combustion activity is obtained through a redox mechanism associated with reversible variation of oxidation state of the transition metal ions in the structure. Mn was the best and most stable active component. However, fundamental and applied studies are needed to better clarify the redox mechanism of the reaction and how it is related to the chemical and structural features of the Mn-containing layered-alumina phase. This could also provide useful information for the development of an optimum catalyst composition,... 

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

For a large group of metal oxide catalysts, it has been proved that a redox mechanism occurs, as originally proposed by Mars and van Krevelen [204], The oxidation of the hydrocarbons, methanol, etc. is effected by oxygen supplied by the catalyst, very often by oxygen contained in the crystal lattice a vacancy results which is reoxidized by oxygen from the gas phase, viz. 

The redox mechanism applies not only to allylic oxidation of olefins and to the oxidation of aromatic hydrocarbons, but also to the oxidation of methanol and sulphur dioxide, as well as the oxidation of ammonia to nitrogen. Only in the case of ethylene oxidation and oxyhydration of olefins do catalysts act according to another mechanism. The latter processes seem to be always low temperature reactions, occurring below 300° C, whereas redox mechanisms are possible above this temperature (e.g. 400—500°C). 

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