Wang, Chuan-Bao; Cai, Yeping; Wachs, Israel E.

Langmuir (1999), 15(4), 1223-1235 CODEN: LANGD5; ISSN: 0743-7463. English.

Reaction-induced spreading of bulk metal oxides onto surfaces of oxide supports during alc. oxidation, a new phenomenon occurring at temps. 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 compds. 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 .mchgt. ethanol > 2-butanol, water .mchgt. 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 alc. 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 alc. 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 alc. oxidation reactions. These new findings have important fundamental implications for synergetic effects of metal oxide catalysts composed of phys. mixts. and for com. applications.