Selective Catalytic Reduction of NO with NH3over Supported Vanadia Catalysts

Abstract

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.

Fundamental Studies of Butane Oxidation over Model-Supported Vanadium Oxide Catalysts: Molecular Structure-Reactivity Relationships

“…The surface niobia species possess very weak Brønsted acid sites (detectable with ammonia adsorption, but not pyridine adsorption). The surface tungsten oxide species possess stronger Brønsted acid sites (detectable by both ammonia and pyridine adsorption), and they are predominantly present as surface Lewis acid sites (41). Thus, the presence of these adjacent acidic metal oxide sites to the surface vanadia species has a beneficial effect on butane oxidation TOF and maleic anhydride selectivity.…”

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

“…One of the intriguing questions for years is which molecular bond in the supported vanadium oxides is responsible for the oxidation activity in various catalytic oxidation reactions [200][201][202]. Three types of bonds can be distinguished: terminal V=O bonds, bridging V-O-V bonds, and V-O-support bonds (Fig.…”

Supported Tantalum Oxide and Supported Vanadia-tantala Mixed Oxides: Structural Characterization and Surface Properties

“…Vanadium oxide catalysts catalyze a number of industrially important processes. They have been used successfully in the selective oxidation of aliphatic and aromatic hydrocarbons (e.g., oxidation of methane to formaldehyde, 1 oxidation of o-xylene to phthalic anhydride [2][3][4] ), the selective oxidation of SO 2 in the production of sulfuric acid, 5 and the selective catalytic reduction (SCR) of nitric oxide with ammonia 6,7 . In addition, the photocatalytic activity of vanadium oxide catalysts has been reported.…”