Reaction-induced spreading of metal oxides onto surfaces of oxide supports during alcohol oxidation: Phenomenon, nature, and mechanisms

Reaction-induced spreading of bulk metal oxides onto surfaces of oxide supports during alcohol oxidation, a new phenomenon occurring at temperatures much lower than that required for thermal spreading, has been extensively investigated with Raman spectroscopic and-fixed-bed catalytic studies. The reaction-induced spreading kinetics were accelerated by reaction of gaseous components with metal oxides to form mobile complex compounds and found to depend on temperature, gaseous component, metal oxide, and oxide support. Increasing the reaction temperature increases the metal oxide spreading rate. The efficiencies of converting three-dimensional bulk metal oxides into two-dimensional surface metal oxide species by different gaseous components are methanol >> ethanol > 2-butanol, water >> oxygen. The high reaction-induced spreading efficiency of methanol is related to the high volatility and stability of its metal-methoxy complexes. Reaction-induced spreading of CrO3, MoO3, V2O5, Re2O7, and Cr2O3 during alcohol oxidation readily occurs on TiO2 and SnO2 supports but does not take place on SiO2 because of the low interaction energy between SiO2 and surface metal oxide species. Furthermore, reaction-induced spreading does not appear to be influenced by the oxidation state of the spreading metal oxides. The mechanism of reaction-induced spreading proceeds via the reaction of an alcohol with metal cations to form surface mobile and volatile metal-alkoxy complexes and their subsequent transport through surface diffusion and volatilization/ readsorption. The reaction-induced spreading of metal oxides is directly reflected in the catalytic properties of such mixed metal oxide materials since the surface metal oxide species are significantly more active than bulk metal oxides for alcohol oxidation reactions. These new findings have important fundamental implications for synergetic effects of metal oxide catalysts composed of physical mixtures and for commercial applications.