Characterization of Al2O3-Supported Manganese Oxides by Electron Spin Resonance and Diffuse Reflectance Spectroscopy

Abstract

Alumina-supported manganese oxides, used as catalysts for the selective catalytic reduction of NO, were characterized by combined electron spin resonance and diffuse reflectance spectroscopies. Upon impregnation of the acetate precursor solution, the [Mn(H2O)6]^2+ complex interacts strongly with surface hydroxyls of the y-Al2O3. Evidence was obtained that this anchoring reaction proceeds at a Mn/OH = 1/2 ratio up to 4.5 wt % Mn loading, leading to a highly dispersed oxidic manganese layer. At higher loadings, the precursor complex is deposited on the surface concurrently. Upon drying at 383 K, part of the manganese is oxidized to higher oxidation states (Mn^3+ and Mn^4+ ), while a further increase in (average) oxidation state takes place upon calcination at 573 K. After calcination, the manganese species are present as a mixture of Mn^2+ ,Mn^3+ , and Mn^4+ . At low loadings (<1 wt %), approximately equal amounts of these three oxidation states are present, whereas Mn 3+ becomes the predominant species at higher loadings. ESR reveals that at low loadings, almost all the manganese is present as isolated species, while at 4.5 wt % Mn loading, still more than 70% of the manganese is isolated. The decrease of the fraction of isolated manganese species at higher loadings is accompanied by a decreased selectivity toward N2 production in the selective catalytic reduction of NO. The fraction Mn^2+ is present in an axially distorted octahedral coordination.