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

Abstract

Supported V2O5–WO3/TiO2 materials are employed as selective catalytic reduction (SCR) catalysts for NO x 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 WO x 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–x V x O2 (rutile) solid solution with reduced V4+ cations (∼7–15%). The surface VO x sites are mostly oligomerized as surface V5+O x sites (∼50–85% oligomers) and the extent of oligomerization tends to increase with surface WO x 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 VO x 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.

Discovering Surface Structure and the Mechanism of Graphene Oxide-Triggered CeO2–WO3/TiO2 Catalysts for NO Abatement with NH3

“…There is no Raman characteristic peak for anatase TiO 2 above 900 cm –1 . A weak band at 998 cm –1 is assigned to mono-oxo surface W 6+ O 5 sites . The absence of detectable Raman bands originating from WO 3 (∼805 cm –1 ) and CeO 2 (∼460 cm –1 ) suggests that these species have a small crystalline size and good dispersion on the TiO 2 .…”

Hydrothermal Aging Treatment Activates V2O5/TiO2 Catalysts for NOx Abatement

“…The hydrothermal stability of VTi was better than that of V 2 O 5 /WO 3 –TiO 2 , which started to deactivate when hydrothermally aged at 600 °C for 24 h (the mass-specific rate constant at 250 °C decreased). After hydrothermal aging at 650 °C, the activity over 1V1WTi (with 1% V 2 O 5 and 1% WO 3 loaded on TiO 2 ) increased a little, while that over 1V8WTi (with 1% V 2 O 5 and 8% WO 3 loaded on TiO 2 ) decreased . The impact of WO 3 on the thermal stability of vanadia-based catalysts and the role of water in the aging process will be investigated in the future.…”

Impact of Hydration on Supported V2O5/TiO2 Catalysts as Explored by Magnetic Resonance Spectroscopy

“…This may well be due to the redistribution of surface vanadia to occupy more oligomeric configurations that are relatively resistant to hydrothermal treatment in a fashion similar to the oligomerization observed in the presence of tungsten and the concomitant increase in vanadium stability afforded by the presence of tungsten oxide. Indeed, the extent of oligomerization tends to increase with surface tungsten oxide coverage as well as calcination temperature . Hydrothermal aging trends suggest similar responses, but with some variability at very high tungsten concentrations.…”

“…Indeed, the extent of oligomerization tends to increase with surface tungsten oxide coverage as well as calcination temperature. 52 Hydrothermal aging trends suggest similar responses, but with some variability at very high tungsten concentrations. Such thermal aging practices may work cooperatively with tungsten oxide to enhance V 2 O 5 − WO 3 /TiO 2 SCR catalysts.…”