SELECTIVE CATALYTIC REDUCTION OF NO WITH NH3 OVER SUPPORTED VANADIA CATALYSTS

The selective catalytic reduction (SCR) of NO with NH3 was systematica lly investigated over a series of supported vanadia catalysts to obtai n additional insight into this important industrial reaction, The infl uence of surface vanadia coverage, promoters (surface tungsten oxide, niobium oxide, and sulfate species), and the specific oxide support (T iO2, Al2O3, and SiO2) was examined, The molecular structures of the su rface metal oxide species were determined by in situ Raman spectroscop y, and the corresponding surface acidity properties were monitored wit h infrared spectroscopy employing pyridine adsorption, The redox prope rties of the surface metal oxide species were probed with the sensitiv e methanol oxidation reaction and temperature-programmed reduction. Th e SCR reactivity of the various catalysts was determined over a wide t emperature range, The current findings suggest that a dual-site (a sur face vanadia redox site and an adjacent nonreducible metal oxide site) mechanism is required for the efficient selective catalytic reduction of NO with NH3 over supported vanadia catalysts, The SCR reaction is sensitive to the immediate environment of the surface vanadia species: overall surface coverage of the metal oxide overlayer (factor of 5 in turnover frequency), nature of adjacent surface metal oxide species ( factor of 10 in turnover frequency) and oxide support ligands (factor of 3 in turnover frequency). The SCR reaction, however, does not appea r to depend on the specific structure of the surface vanadia species, The SCR selectivity toward N-2 formation also varies with the immediat e environment of the surface vanadia species, The selectivity depends on the specific oxide support (TiO2 > Al2O3 > SiO2), temperature (decr eases at higher temperature due to oxidation of NH3 and NO to N2O), an d surface concentration of redox sites (decreases with the concentrati on of pairs of redox sites), The SCR reaction is not related to the pr operties of the terminal V=O bond since in situ Raman studies during S CR, employing V=O-18, demonstrate that this bond is relatively stable under reaction conditions (possessing a lifetime that is similar to 10 times the characteristic reaction time), Thus, the bridging V-O-suppo rt bond appears to be involved in the rate-determining step. (C) 1996 Academic Press, Inc.