Defect Oxides and Sulfides in Catalysis

Nonstoichiometric metal oxides are effective catalysts for a variety of oxidation-reduction reactions (as might be expected) since the variable valence of the constituent ions enables the oxide to act as a sort of electron bank. Nonstoichiometric metal oxides resemble metals in that they can also catalyze hydrogenation and alkene isomerization reactions. However, on zinc oxide, for instance, these two processes are independent, whereas hydrogen must be present for isomerization to occur on metals. 

The mode of action of semiconducting metal oxides can be illustrated with reference to catalysis of the oxidation of carbon monoxide  

the two oxides consumed in reaction 6.9 are regenerated in reactions 6.10 and 6.11. 

Metal sulfides play an important role in catalyzing a wide variety of hydrogenations (e.g., of fats, coal, or olefins) and also desulfurization reactions, which are used in pretreatment of fossil fuels to reduce the emission of sulfur oxides during combustion (Section 8.5). Molybdenum disulfide, an important defect catalyst, can be made to function as an n-type (Moi+a S2) or p-type (Moi j S2) semiconductor by exposure to an appropriate mixture of H2S and hydrogen at temperatures on the order of 600 °C. The equilibrium 

A typical commercial hydrodesulfurization catalyst might contain 14% M0O3 and 3% CoO, on an alumina support. The oxides are converted to Co-doped M0S2 by exposure to H2S/H2 under carefully controlled conditions of temperature and partial pressures. 

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