Metal oxide bulk doping

In catalysis, oxides with well defined acidic and basic properties are used in different forms that have found application in numerous catalytic applications in the gas-solid and liquid-solid heterogeneous catalysis [3, 46, 47], Among the most used oxide materials in catalysis, we And (i) bulk oxides (one component metal oxides) (ii) doped and moditied oxides (iii) supported metal oxides (dispersed active oxide component onto a support oxide component) (iv) bulk and supported binary metal oxides to quaternary metal oxides (mixed oxide compositions) (v) complex oxides (e.g., spinels, perovskites, hexa-aluminates, bulk and supported hydrotalcites, pillared clays, bulk and supported heteropolyacids, layered silicas, etc.). 

However, the correlation of the electrical properties of the bulk phase with the catalytic properties of the essentially heterogeneous catalyst surface is a classical difficulty. This may be one of the reasons why no general correlation between these properties is found when a variety of different metal oxide catalysts is compared. A close relationship is often shown, on the other hand, when a particular catalyst is modified or doped with minor amounts of an additional metal oxide. It is very likely that the correlation is successful in this case, because the nature of surface sites is not essentially changed. 

Enhancement of reactivity of incendiary components has been claimed by the introduction of impurity states, particularly into metallic oxides (Refs 56 86). Impurity states have a twofold effect they disturb the lattice structure of the oxide (and[ so increase the diffusivity of the reactants), and they disturb the electronic distribution on the surface as well as in the bulk. The argument is made that by doping the oxide, or by controlling the formation temp, one may change an oxide from an n-type to a p-type semiconductor and hence cause it to become a better electron acceptor, and vice versa... 

In technological applications, mixed, doped, or multi-metal oxides play an important role, for example, Mo-V-Te-Nb oxide [15] is used for selective oxidation of propane to acrylic acid. For some complex oxides, the bulk oxide structures and distribution of phases are often unknown and there is little knowledge of the atomic surface structure and composition, extent of hydroxylation, type and density of defects, and the location of dopants (homogeneously distributed, concentrated at the surface, grain boundaries, or interfaces). 

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