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The selective catalytic reduction (SCR) of NO with NH 3 was systematically investigated over a series of supported vanadia catalysts to obtain additional insight into this important industrial reaction. The influence of surface vanadia coverage, promoters (surface tungsten oxide, niobium oxide, and sulfate species), and the specific oxide support (TiO 2 , Al 2 O 3 , and SiO 2 ) was examined. The molecular structures of the surface metal oxide species were determined by in situ Raman spectroscopy, and the corresponding surface acidity properties were monitored with infrared spectroscopy employing pyridine adsorption. The redox properties of the surface metal oxide species were probed with the sensitive methanol oxidation reaction and temperature-programmed reduction. The SCR reactivity of the various catalysts was determined over a wide temperature range. The current findings suggest that a dual-site (a surface vanadia redox site and an adjacent nonreducible metal oxide site) mechanism is required for the efficient selective catalytic reduction of NO with NH 3 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 appear to depend on the specific structure of the surface vanadia species. The SCR selectivity toward N 2 formation also varies with the immediate environment of the surface vanadia species. The selectivity depends on the specific oxide support (TiO 2 > Al 2 O 3 > SiO 2 ), temperature (decreases at higher temperature due to oxidation of NH 3 and NO to N 2 O), and surface concentration of redox sites (decreases with the concentration of pairs of redox sites). The SCR reaction is not related to the properties of the terminal V= =O bond since in situ Raman studies during SCR, employing V= = 18 O, demonstrate that this bond is relatively stable under reaction conditions (possessing a lifetime that is ∼10 times the characteristic reaction time). Thus, the bridging V-O-support bond appears to be involved in the rate-determining step.

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Alumina-Supported Manganese Oxide Catalysts. Part 2. Surface Characterization and Adsorption of Ammonia and Nitric Oxide.-The title catalysts, prepared by impregnation with Mn acetate, are characterized by in situ IR spectroscopy and temp.-programmed reaction and desorption (TPRD) with regard to SCR of NO with NH3. On the catalyst, exhibiting two Lewis acid-type coordinatively unsaturated Mn ions on the surface, most likely in the +3 state, NH3 coordination is stable up to near 473 K. In the absence of O2 no NH3 oxidation is observed. The interaction with NO is very weak. Mn3+ species are probably able to bind both one NO and one NH3 molecule. In the absence of O2, reaction of NO with NH3 is observed in the IR cell in the range 300-423 K. The presence of oxygen has a strong effect on the rate of the SCR reaction. It plays a role in the catalyst reoxidation, promotion of NO absorption enables the abstraction of H from adsorbed NH3 and thus activates reaction with NO and it oxidizes gas-phase NO to NO2. -(KAPTEIJN, F.; SINGOREDJO, L.; VAN DRIEL, M.; AN-DREINI, A.; MOULIJN, J. A.; RAMIS, G.; BUSCA, G.; J. Catal. 150 (1994) 1, 105-116; Dep. Chem. Eng., Delft Univ. Technol., NL-2628 BL Delft, Neth.; EN)

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