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 fin e structure (EXAFS and NEXAFS) showed that the manganese oxide was highly d ispersed on the surface of the supports. The EXAFS spectra suggested that t he 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 N EXAFS spectra for the catalysts showed a chemical shift to lower energy, an d 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 react ion. The catalysts were tested for their ozone decomposition reactivity and reaction rates were found to have a fractional order dependency (n < 1) wi th ozone partial pressure. The apparent activation energies for the reactio n were found to be low (3-15 kJ/mol). The support was found to influence th e desorption step (a reduction step) and this effect manifested itself in t he preexponential factor of the rate constant for desorption. Trends for th is preexponential factor correlated with the NEXAFS trends and reflected th e ease of electron donation from the adsorbed species to the active center.