Mechanisms and products of oxide decomposition

The dissociation of an oxide may, in general, be represented as a two-step process, involving both electron transfer and dimerization 

The steps do not necessarily, however, proceed as simply as indicated but may also involve the formation of additional intermediates, such as adsorbed O , 02, etc., and/or the generation and migration of defects at a surface, interface or in the bulk. 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being 

Ag20 - Ag Cr02 - Cr203 3-Mn02 - a-Mn203 - /3-Mn304 U03 - U02-9 U3Og 

It is concluded [634] that, so far, rate measurements have not been particularly successful in the elucidation of mechanisms of oxide dissociations and that the resolution of apparent outstanding difficulties requires further work. There is evidence that reactions yielding molecular oxygen only involve initial interaction of ions within the lattice of the reactant and kinetic indications are that such reactions are not readily reversed. For those reactions in which the products contain at least some atomic oxygen, magnitudes of E, estimated from the somewhat limited quantity of data available, are generally smaller than the dissociation enthalpies. Decompositions of these oxides are not, therefore, single-step processes and the mechanisms are probably more complicated than has sometimes been supposed. 

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