Structure-Activity Relationships of Hydrothermally Aged Titania-Supported Vanadium-Tungsten Oxide Catalysts for SCR of NO_xEmissions with NH₃

Supported V₂O₅-WO₃/TiO₂materials are employed as selective catalytic reduction (SCR) catalysts for NO_xemission control from power plants. Fresh SCR catalysts usually receive exposure to harsh treatments in the industry to accelerate catalyst activation (calcination in air at 650 °C) and catalyst aging (hydrothermal aging at 650 °C) in a way that represents various points in the catalyst/product lifetime. The present study investigates the catalyst structural and chemical changes occurring during such harsh treatments. Three series of supported V₂O₅-WO₃/TiO₂catalysts were prepared by incipient-wetness impregnation of aqueous ammonium metavanadate and metatungstate precursors. The catalysts were subsequently dried and calcined at 550 °C in O₂, 650 °C in O₂, and hydrothermal conditions (10% O₂, 8% H₂O, 7% CO₂, and 75% N₂) at 650 °C. The resulting catalysts were physically characterized by numerous techniques (in situRaman;in situIR;in situhigh-field-high-spinning solid-state⁵¹V MAS NMR;in situelectron paramagnetic resonance; X-ray diffraction; Brunauer, Emmett, and Teller surface area; and inductively coupled plasma) and chemically probed with adsorbed ammonia, SCR-TPSR, and the SCR reaction. The surface WO_xsites on the TiO₂support behave as a textural promoter that stabilizes the TiO₂(anatase) phase from sintering and transforming to the undesirable crystalline TiO₂(rutile) phase that can lead to formation of a Ti_1-xV_xO₂(rutile) solid solution with reduced V⁴⁺cations (∼7-15%). The surface VO_xsites are mostly oligomerized as surface V⁵⁺O_xsites (∼50-85% oligomers) and the extent of oligomerization tends to increase with surface WO_xcoverage and calcination temperature. A major difference between the calcined and hydrothermally treated catalysts was the low concentration of surface NH₃^*species on Lewis acid sites for the hydrothermally treated catalysts, yet the SCR activity was almost comparable for both catalysts. This finding suggests that surface NH₄⁺*, primarily associated with the surface VO_xsites, are able to efficiently perform the SCR reaction. Given that multiple catalyst parameters were simultaneously varying during these treatments, it was difficult to correlate the SCR activity with any single catalyst parameter. A correlation, however, was found between the SCR TOF/activity and the sum of the surface NH₃^*and NH₄⁺* species, which is dominated by the surface NH₄⁺* species.