Structural analysis of manganese oxides supported on SiO₂ for benzene oxidation with ozone

Catalytic oxidation of benzene with ozone was carried out over SiO₂-supported manganese oxides (Mn/SiO₂) prepared by impregnation methods using two kinds of manganese precursors, manganese nitrate and acetate. The structure of manganese oxides were characterized by X-ray diffraction and X-ray absorption measurements and effects of the manganese oxide structures on their catalytic properties were investigated. EXAFS studies revealed that by using manganese nitrate precursor in catalyst preparation, aggregated manganese oxides Mn₂O₃ were formed, whereas highly dispersed manganese oxides were formed when manganese acetate precursor was used. In both cases, the structures were not strongly affected by the manganese loading levels. The average oxidation state estimated form the absorption edge in the XANES region was also almost unchanged by changing the manganese loading for both catalysts. The catalytic activities of Mn/SiO₂ for benzene oxidation with ozone at 343 K were much higher for the catalysts prepared from manganese acetate precursors than those prepared from manganese nitrate precursor at any given manganese loading levels, indicating that highly dispersed manganese oxides on SiO₂ exhibited much higher activities for the reaction than the aggregated manganese oxides on SiO₂. The product distribution and the efficiency for ozone utilization were not so much affected by the manganese oxide structures.