In situ XRD Study of the Reduction of Mixed Mn-Co Oxide
The reducibility of oxides is an essential characteristic of catalyst of different reactions. First of all, it is important for oxidation reactions following the Mars-van Krevelen mechanism. In this mechanism, a weakly bound surface oxygen atom is added to forming the oxygenated compound, leaving behind an oxygen vacancy on the surface. Then, the molecular oxygen react with the surface, dissociate and refill the vacancy. In this case, reducibility of oxide catalyst measures the tendency of oxide to lose oxygen or to donate it to an adsorbed species. Secondly, reduction can be an activation stage of Fischer-Tropsch synthesis, stream reforming and watergas shift reaction. The catalysts activation leads to transformation from oxide state to metallic, depending on the reductive conditions the state of active sites can be changed significantly. Mncontaining oxides can effectively catalyze the oxidation of hydrocarbons, CO and chlorcarbons. Cooperative utilize of Mn and another oxide leads to synergetic effect – increase in catalytic activity as compared with simple oxides. In double metal oxide catalysts, the formation of mixed oxides is possible. The purpose of this work was to elucidate influence of redox properties of solid solutions on the catalytic performance. A series of catalysts based on mixed Mn-Co oxides with different molar ratios of cations have been prepared by coprecipitation. Reduction mechanism of Mn-containing catalysts with hydrogen was studied by a TPR-H2, in situ XRD, and in situ XPS, XANES. It was shown that the mechanism of reduction of Mn-Co oxides with hydrogen differs significantly from the processes occurring on simple oxides. The reduction of Mn-Co oxides occurs in two steps: (Mn,Co)3O4 transforms to (Mn,Co)O solid solutions, then the reduction of solid solutions (Mn,Co)O to metallic cobalt Co and MnO proceeds. Correlations between the redox properties and the catalytic activity in the CO oxidation reaction have been found.