MnO _x -Al₂O₃ catalysts for deep oxidation prepared with the use of mechanochemical activation: The effect of synthesis conditions on the phase composition and catalytic properties

Abstract

The catalytic activity of alumina-manganese catalysts in the oxidation of CO was studied. The MnO _x -Al₂O₃ catalysts were prepared by an extrusion method with the introduction of mechanically activated components (manganese oxide and its mixtures with aluminum oxide, aluminum hydroxide, and a mixture of a manganese salt with aluminum hydroxide) into a paste of aluminum hydroxide followed by thermal treatment in air or argon at 1000°C. In the majority of cases, the catalysts contained a mixture of the phases of β-Mn₃O₄ (Mn₂O₃), α-Al₂O₃, and δ-Al₂O₃. The presence of low-temperature δ-Al₂O₃ suggested the incomplete interaction of manganese and aluminum oxides. It was found that the catalytic activity of MnO _x -Al₂O₃ depends on the degree of interaction of the initial reactants, and its value is correlated with the amount of β-Mn₃O₄ in the active constituent. The intermediate thermal treatment of components at 700°C negatively affects the catalytic activity as a result of the formation of Mn₂O₃ and the coarsening of particles, which levels the results of mechanochemical activation. The greatest degree of interaction between Al- and Mn-containing components was reached in the selection of mechanochemical activation conditions by decreasing the size of grinding bodies, optimizing the time of mechanochemical activation, and using the mechanochemical activation of precursor mixtures. As a result of mechanochemical activation, the initial reactants were dispersed, the amounts of MnO₂ and Mn₂O₃ changed, and defects were formed; this strengthened the interaction of components and increased catalytic activity.