Participation of lattice oxygen

Several recent contributions concerning the participation of lattice oxygen in selective oxidation processes have appeared and fully agree with the above concepts. They will be discussed in more detail below. 

In a second group of metal oxides, which are not easily reduced, the oxygen is strongly bound and the catalyst is generally in a fully oxidized state. Thus O2- is not reactive, but an adsorbed form of oxygen, much more weakly bound, is active. This leads only to combustion. Quite a number of these metals are non-transition metal oxides. 

A third group contains those metal catalysts which do not form specific crystal phases in an oxidized state. The common types of oxygen on the surface are then 02 (adsorbed) and O (adsorbed) which generally do not lead to selective oxidation. One of the exceptions is silver, which very probably catalyses the selective oxidation of ethylene by providing 02 on the surface. However, an active role of surface oxides, which may be formed particularly by the action of promotors, is not excluded. 

The participation of lattice oxygen is inherent to the redox mechanism, which is operative in many of the oxidation processes that are catalyzed by metal oxides. Reviewing the processess described in Sect. 2, participation of lattice oxygen appears to be the case for the majority of them, namely for the allylic (amm)oxidation of olefins, for the (amm)oxidation of aromatic hydrocarbons and for the oxidation of methanol, ammonia and sulphur dioxide. 

Two types of experiment are commonly used to give evidence of participation of lattice oxygen (a) experiments in the absence of gas phase oxygen and (b) experiments with labelled oxygen. 

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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". 

Mechanism. The mechanism outlined for the propene oxidation over metal oxides is, in general, fully applicable to bismuth molybdate. The occurrence of a symmetrical allyl intermediate and the participation of lattice oxygen is well established (Hucknall [160], Voge and Adams [343]). 

Sancier et al. (43) used oxygen-18 to examine the relative role of adsorbed versus lattice oxygen in propylene oxidation over a silica-supported bismuth molybdate catalyst as a function of temperature. At 400°C they observed the formation of predominantly acrolein[I60] rather than acrolein[I80], indicating significant participation of lattice oxygen. However, as the reaction temperature was decreased, the authors concluded that the role of adsorbed oxygen became more important. 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

Although catalytic oxidation of propylene has been found to be first order with respect to the olefin (5), a dependence on oxygen has also been reported (9, 54). Investigations of the participation of lattice oxygen in the oxidation process over mixed oxides, which were thought to contain antimony(V), antimony(III), and tin(IV), reported no support for the redox mechanism observed with bismuth molybdate. The matter of oxygen participation has also been considered by Christie et al. (53), who reported that the rates of... 

The rate constants have been assumed independent of the coverages also any participation of lattice oxygen from layers other than the first one was neglected. 

On and D represent lattice oxygen ions and surface anion vacancies, respectively, and ( ) designates a free adsorption site. The CO2 produced can remain adsorbed at the surface or be readsorbed later as a stable carbonate at the active surface, thus effecting the self-poisoning that is invariably observed. The direct participation of lattice oxygen in the oxidation reaction has been substantiated in this study by in-situ electrical conductivity measurements. 

Doomkamp, C., Clement, M., and Ponec, V. Activity and selectivity patterns in the oxidation of aUyl iodide on the period IV metal oxides the participation of lattice oxygen in selective and total oxidation reactions. Appl Catal. A Gen. 1999,188, 325-336. 

Characteristic features of catalysis by oxides are the involvement of acid-base properties of adsorbate and substrate, causing the hetero-lytic dissociation of molecules that would yield neutral species on metals, and the active participation of lattice oxygen as a reactant in oxidation reactions. The reactivity of oxide surfaces is often determined by point defects and the extent to which cations are exposed to the gas phase. 

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