The Role of Fe2O3 Species in Depressing the Formation of N2O in the Selective Reduction of NO by NH3 over V2O5/TiO2-Based Catalysts

Promotion of 2.73% Fe2O3 in an in-house-made V2O5-WO3/TiO2 (VWT) and a commercial V2O5-WO3/TiO2 (c-VWT) has been investigated as a cost effective approach to the suppression of N2O formation in the selective catalytic reduction of NO by NH3 (NH3-SCR). The promoted VWT and c-VWT catalysts all gave a significantly decreased N2O production at temperatures >400 degrees C compared to the unpromoted samples. However, such a promotion led to the loss in high temperature NO conversion, mainly due to the oxidation of NH3 to N-containing gases, particularly NO. Characterization of the unpromoted and promoted catalysts using X-ray diffraction (XRD), NH3 adsorption-desorption, and Raman spectroscopy techniques could explain the reason why the promotion showed much lower N2O formation levels at high temperatures. The addition of Fe2O3 to c-VWT resulted in redispersion of the V2O5 species, although this was not visible for 2.73% Fe2O3/VWT. The iron oxides exist as a highly-dispersed noncrystalline-Fe2O3 in the promoted catalysts. These Raman spectra had a new Raman signal that could be tentatively assigned to Fe2O3-induced tetrahedrally coordinated polymeric vanadates and/or surface V-O-Fe species with significant electronic interactions between the both metal oxides. Calculations of the monolayer coverage of each metal oxide and the surface total coverage are reasonably consistent with Raman measurements. The proposed vanadia-based surface polymeric entities may play a key role for the substantial reduction of N2O formed at high temperatures by NH3 species adsorbed strongly on the promoted catalysts. This reaction is a main pathway to greatly suppress the extent of N2O formation in NH3-SCR reaction over the promoted catalysts.