The Selective Catalytic Reduction of NOx with NH3 over Titania Supported Rhenium Oxide Catalysts

Titania supported vanadium oxide catalysts have been demonstrated to be very efficient catalysts for the selective reduction of NOx with NH3 and have found widespread industrial application for the control of NOx emissions from stationary sources (1). Characterization studies have demonstrated that supported vanadium oxide catalysts consist of a two-dimensional metal oxide overlayer on the oxide support substrate (2–5). The surface vanadia overlayer consists of redox and acid sites that are critical for the efficient selective catalytic reduction of NOx with NH3 (6–8). The titania support is superior to other oxide supports (e.g., alumina, silica, etc.) for the SCR of NO with NH3 primarily because of its ability to simultaneously enhance the redox properties of the surface vanadia species (5, 9, 10), stabilize surface acid sites in the surface vanadia overlayer (5–8), and remain resistant to corrosive gases such as SO2/SO3 that are typically present in industrial environments (1). Consequently, there have been numerous studies investigating the SCR of NO with NH3 with other surface metal oxides on titania: MoO3(11), WO3 (12–14), CrO3 (15), Nb2O5 (16, 17), and SOx (8, 18). The above supported metal oxide catalyst systems, however, have generally been less active and selective for the SCR of NO with NH3 than the efficient titania supported vanadia catalyst. A titania supported catalyst that possesses very active redox sites (19, 20) and surface acid sites in the metal oxide overlayer (21) is the titania supported rhenium oxide system. However, there have been no SCR of NO with NH3 studies on this catalytic system to date. The redox properties of titania supported metal oxide catalysts were probed by methanol oxidation and follow the trend Re∼VÀCr∼Mo≫Nb, W, and S (8, 19, 22, 23), which reveals that surface rhenia and vanadia species possess comparable redox activities toward methanol. The selectivites for redox products were in excess of 90% for both surface rhenia and vanadia species on titania, but formaldehyde was exclusively formed with vanadia and formaldehyde as well as methylformate (approximately 3:1 ratio) were formed with rhenia(9, 20). The distribution of surface Lewis and Brønsted acid sites for these two systems are also very similar as a function of surface coverage: the concentration of surface Lewis acid sites decreases and the concentration of surface Brønsted acid sites increases with coverage (21, 24). The molecular structures of the surface rhenia species on titania, as well as other supports, have been extensively characterized with Raman and IR spectroscopy and consist of isolated surface S–O–Re(==O)3 species (complete absence of Re–O–Re vibrations at ∼200 and ∼450 cm−1), where S is the support cation (25). Two slightly different isolated surface rhenia species are observed as a function of coverage and give rise to Re==O symmetric stretches at 1004–1006 and 1009–1011 cm−1. The isolated surface metal oxide species is unique for the titania supported rhenia system since other supported metal oxide systems typically contain a mixture of isolated and polymerized surface metal oxide species (3, 4, 19, 23, 25). For example, titania supported vanadia catalysts posses surface VO4 units that exhibit Raman bands at 1027–1031 cm−1 and 920–950 cm−1due to terminal V==O and polymeric V–O–V functionalities, respectively (3, 4). Thus, the high redox activity, the surface acidity characteristics, and the isolated nature of the surface rhenia species on titania allow this catalyst to serve as a model system that can provide additional insight into the nature of the surface sites involved in the selective catalytic reduction of NOx with NH3. The Re2O7/TiO2 catalysts were prepared by the incipientwetness impregnation of a 60 –70% aqueous solution of perrhenic acid, HReO4 (Alfa), on TiO2(Degussa P25, 50 m2/g). After the impregnation step, the sample was dried at room temperature overnight, followed by additional drying at 383 K overnight, and calcination at 723 K for 2 h. Characterization of the catalysts by Raman, IR, TPR, and BET were reported previously (25). The surface areas of the catalysts were not influenced by the calcination treatment. The acidity of the Re2O7/TiO2 catalysts were measured by pyridine adsorption using a diffuse-reflectance fourier transform infrared instrument. Only relative amounts of Lewis and Brønsted acid sites are reported. Details of the acidity measurements can be found elsewhere (8). The SCR of NO with NH3 was measured using a reactant gas mixture of O2 (2%), NO (500 ppm), NH3 (550 ppm), and balance He (supplied by UCAR–Union Carbide) fed through four mass controllers (Hi-Tec MFC 201). Analysis of the reactants and products were performed by a mass