Comparison of catalytic properties of supported metal oxides for benzene oxidation using ozone

Abstract

Catalytic oxidation of gaseous benzene using ozone over SiO₂-supported metal oxides (oxides of Mn, Fe, Co, Ni, and Cu) was performed at 343 K. The supported metal oxides were prepared from metal nitrates via impregnation followed by calcination at 873 K. X-ray diffraction studies revealed that aggregated metal oxides were formed on the supports. The quantities of the exposed metal oxides on SiO₂ were determined using temperature-programmed desorption of formic acid species adsorbed on the catalysts. The turnover frequencies (TOFs) for ozone decomposition and catalytic oxidation of benzene with ozone were also compared for the supported metal oxides. The TOFs for ozone decomposition with the supported metal oxides decreased in the order: Cu > Co ~ Ni > Fe > Mn. Conversely, the SiO₂-supported Mn oxide (Mn/SiO₂) catalyst exhibited the highest activity for catalytic oxidation of benzene with ozone and the highest efficiency for ozone utilization. FTIR studies revealed that benzene ring cleavage continuously proceeded on the Mn/SiO₂ catalyst, and oxygen-containing species, which were readily oxidized to CO₂ and CO in the presence of ozone, accumulated on the catalyst surface. The Fe-, Co-, Ni-, and Cu-oxides suffered from catalyst deactivation due to the build-up of by-product compounds, which was attributed to their low activity for oxidation of these substances. These results suggest that the most important steps in benzene oxidation with ozone on metal oxides are the formation and oxidation of by-product compounds. The higher activity of the Mn oxide catalyst for benzene oxidation is ascribed to the facile oxidation of these by-product compounds.