Investigations on Formation and Structure of MnOx/SiO2 Catalysts

Abstract

Abstract MnOx/SiO2 catalysts with different manganese loadings, prepared by using the pore volume impregnation technique, have been studied by using the temperature-programmed reduction (TPR) and microscopic laser-Raman spectroscopy techniques in order to characterize the dispersion of the manganese on the support and the nature and formation of the manganese oxide species. A highly inhomogeneous distribution of manganese over the SiO2 surface has been found, consisting of unloaded “white” surfaces, transition zones, MnOx “islands” and well crystallized manganese oxide spots. A MnOx-monolayer species has been identified which consisted of Mn2+ and Mn3+ cations bonded together and to the SiO2 support by means of O-atoms. The presence of this phase in the transition zone diminished by increasing the manganese loading of the catalyst and could not be unequivocally identified on the islands and crystal spots, but at the same time the amount of dispersed and crystallized MnO2 and Mn2O3 increased. A model for the formation of MnOx -species is proposed.