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

Supported V2O5–WO3/TiO2 materials are employed as selective catalytic reduction (SCR) catalysts for NOx emission 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/TiO2 catalysts 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 situ Raman; in situ IR; in situ high-field–high-spinning solid-state 51V MAS NMR; in situ electron 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 WOx sites on the TiO2 support 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 Ti1–xVxO2 (rutile) solid solution with reduced V4+ cations (∼7–15%). The surface VOx sites are mostly oligomerized as surface V5+Ox sites (∼50–85% oligomers) and the extent of oligomerization tends to increase with surface WOx coverage 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 VOx sites, 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.