Effect of ZnO doping on surface and catalytic properties of manganese oxides supported on alumina

Abstract

The effect of ZnO doping (1.42–12.58 wt.%) on surface and physicochemical properties of manganese oxides supported on alumina has been investigated using nitrogen adsorption at −196 °C and catalytic decomposition of H 2O 2 reaction at 30–50 °C. The pure and doped solids were subjected to thermal treatment at 400–1000 °C. The thermal products were characterized via X-ray diffraction (XRD)-technique. The results revealed that the specific surface areas of various samples decreased monotonically as a function of both the extent of zinc oxide doping and the precalcination temperature. Moreover, the activation energy of sintering, Δ E s, was found to decrease by increasing the concentration of ZnO to 5.44 wt.% and increased with increasing the amount of dopant to 12.58 wt.%. The pure and doped mixed solids preheated at 400 °C produced crystalline β-MnO 2 phase and its pattern intensity increased with increasing ZnO concentration up to 5.44 wt.%. Crystalline Mn 2O 3 phase was detected at 600 and 800 °C and its pattern intensity decreased with increasing the dopant concentration. Calcination of samples at 1000 °C showed the formation of crystalline γ-Al 2O 3, Mn 3O 4, ZnAl 2O 4, and ZnMn 2O 4 phases. The catalytic activities of the pure and doped mixed solids increased with increasing the reaction temperature and calcination temperature up to 600 °C. The ZnO doping led to the decrease of the catalytic activity of manganese oxides supported on alumina system. No measurable activity was observed for samples precalcined at 1000 °C because of formation of inactive compounds and/or sintering process. The changes in the catalytic activity of pure and doped solids were attributed to the change in oxidation states.