Mechanism of selective catalytic reduction on VOx/TiO2 catalyst by in situ ambient pressure X-ray photoelectron spectroscopy

Unveiling the relationship between NH3 adsorption/activation behaviors and SCR reaction mechanism is imperative for rational catalyst design, but is is still not established because of lacking methodology of in situ spectroscopies. Here, vanadia-based SCR catalysts were used to identify the electron transfer and dynamic evolutions of active sites in SCR reaction by in situ AP-XPS, Raman, and DRIFTS spectra. The activation of NH3 occurred at V5+--O sites, and then they reduced to V4+-OH species by proton-coupled electron transfer process at high temperature (350 degrees C). Reducibility of vanadium dominated the catalytic reaction and both lattice and surface oxygen can oxidize the V4+-OH to fulfill redox cycles. Whereas, at low temperature (150 degrees C), the adsorptions of NH3 on Bronsted acid sites was the rate-determine step of NH3 activations. NO was stored as nitrate/ nitrite with the help of surface chemisorbed oxygen. The NHx species reacted with adsorbed nitrite/nitrate to generate the NHx-NOx intermediate that subsequently decomposed into N2 and H2O.