Influence of the conditions of pretreatment of Mn-containing catalyst on its redox properties
Among the oxides of transition metals, manganese oxides are the most promising for developing effective catalysts for the deep oxidation of hydrocarbons and carbon monoxide. This is due to the high potential of the oxidizing ability of manganese oxides with oxidation states of +4 and +3, the propensity MnOx to thermal activation due to the formation of defective spinel Mn3O4, and a synergistic effect in oxidative activity when MnOx is added to noble metals and some oxides of transition metals (Cu, Fe, and etc.). Often, the redox and catalytic properties of the supported catalysts depend on the preparation mode and the conditions of heat treatment that control both the composition and the defectiveness of the structure of the catalytically active metal and its interaction with the support. For the preparation of modern MnOx-containing catalysts, a nontraditional method of synthesis such as solution combustion synthesis, proposed by for the first time Kingsley and Patil [citation], is ofpractical interest. The principle of supporting and anchoring the active component consists of mixing saturated solutions of salts of catalytically active metals and organic fuel with reducing properties (for example, urea), depositing them on the supports, and initiating a self-propagating burning reaction. In this paper, the regularities in the formation of redox properties of MnOx-containing catalysts during thermal treatment in inert and oxidizing media at 200 and 450 °C were studied. The catalyst was prepared by co-impregnation with glycine on a ceramic honeycomb support with a low specific surface. The treatment temperatures were chosen taking into account the temperature range in which the catalytic activity of MnOx-based systems is manifested in hydrocarbon oxidation reactions. Analysis of the TPR-H2 data showed that during pretreatment in argon at 450 °C, the reactive oxygen species that can be active in the butane oxidation was removed from the surface sample and the active component was characterized by the Mn3O4 phase. Pretreatment in oxygen (100 vol. %) promoted the formation of the most oxidized states of manganese, which is unlikely for the conditions of catalyst preparation by the method of solution combustion synthesis. Based on the results of TPR-H2, it was concluded that the most correct pretreatment conditions for characterizing the freshly prepared MnOx-containing catalyst is pretreatment in argon at 200 °C. However, under oxidizing conditions of catalytic tests of the MnOx-containing catalyst, oxidation of the MnOx phases existing in the initial catalyst could occur, and these phases make a significant contribution to the activity of the catalyst in hydrocarbon oxidation reactions. These phases of MnO2/Mn2O3 were identified in the catalyst after its preliminary treatment under oxidizing conditions with an oxygen content of 10 vol. %.