Identification of Surface Species on Titania-Supported Manganese, Chromium, and Copper Oxide Low-Temperature SCR Catalysts
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Abstract
TiO2-supported transition metal oxides (Mn, Cr, and Cu) for the SCR of NO with NH3 have been synthesized by wet impregnation. The adsorption and coadsorption of NH3, NO, and O2, in conjunction with in situ FT-IR spectroscopy, was used to elucidate the reaction mechanism as the samples were heated from 323 to 673 K. While Cr was the only transition metal that generated significant amounts of Brønsted acidity, strong Lewis acid sites were present over all of the materials. The peak strength corresponding to the δs(NH3) coordinated to Lewis acid sites decreased in the following order: Ti > Mn > Cr ∼ Cu. Similarly, the peak strength corresponding to the δas(NH3) coordinated to Lewis acid sites decreased as follows: Mn > Cr ∼ Cu. Exposing the catalysts to oxygen before the introduction of NO did not impact the adsorption of NO as nitrates on the catalysts, suggesting that labile lattice oxygen plays an important role in the formation of nitrates. Three types of nitrates were observed after the adsorption of NO. Monodentate and bidentate nitrates formed on the surface of all the materials tested, while bridged nitrates only formed on CrOx/TiO2. The in situ FTIR data collected resulted in the development of a reaction mechanism for MnOx/TiO2. A combination of moderately strong monodentate and bidentate nitrate species, along with a split in the symmetric deformation of NH3 coordinated to Lewis acid sites, appear to be important for high activity and selectivity. The peak resulting from the vibrational mode of ammonia adsorbed on Lewis acid sites, which is located at ∼1170 cm-1, is believed to be important in facilitating hydrogen abstraction to form amide species that react with bidentate nitrates (1620 cm-1). It is proposed that the reaction mechanism proceeds through the formation of nitrosamide and azoxy species, which most likely possess lifetimes as reaction intermediates that are too brief for detection. In contrast to MnOx/TiO2, the apparent participation of Brønsted acid sites for CrOx/TiO2 suggests that a different reaction pathway is involved for this catalyst.