Electron transfer effects in ozone decomposition on supported manganese oxide

Manganese oxide catalysts supported on Al2O3, ZrO2, TiO2, and SiO2 supports were used to study the effect of support on ozone decomposition kinetics. In-situ laser Raman spectroscopy, temperature-programmed oxygen desorption, surface area measurements, and extended and near-edge X-ray absorption fine structure (EXAFS and NEXAFS) showed that the manganese oxide was highly dispersed on the surface of the supports. The EXAFS spectra suggested that the manganese active centers on all of the surfaces were surrounded by five oxygen atoms. These metal centers were found to be of a monomeric type for the Al2O3-supported catalyst and multinuclear for the other supports. The NEXAFS spectra for the catalysts showed a chemical shift to lower energy, and an intensity change in the L-edge features which followed the trend Al2O3 > ZrO2 > TiO2 > SiO2. The trends provided insights into the positive role of available empty d-states required in the reduction step of a redox reaction. The catalysts were tested for their ozone decomposition reactivity and reaction rates were found to have a fractional order dependency (n < 1) with ozone partial pressure. The apparent activation energies for the reaction were found to be low (3-15 kJ/mol). The support was found to influence the desorption step (a reduction step) and this effect manifested itself in the preexponential factor of the rate constant for desorption. Trends for this preexponential factor correlated with the NEXAFS trends and reflected the ease of electron donation from the adsorbed species to the active center.