NH₃-SCR over V–W/TiO₂ Investigated by Operando X-ray Absorption and Emission Spectroscopy

V–W/TiO 2 -based catalysts, which are used for the removal of NO x from the exhaust of diesel engines and stationary sources via selective catalytic reduction with NH 3 (NH 3 -SCR), were studied by operando X-ray absorption spectroscopy (XAS) and emerging photon-in/photon-out techniques. In order to minimize the influence of highly X-ray absorbing tungsten and the fluorescence of titanium, we used a high-energy-resolution fluorescence setup that is able to separate efficiently the V Kβ 1,3 emission lines and additionally allows to record valence-to-core (vtc) X-ray emission lines. High-energy resolution fluorescence-detected XAS (HERFD-XAS) and vtc X-ray emission spectroscopy (vtc-XES) proved to be the only way to perform an operando V K edge X-ray spectroscopic study on industrially relevant V–W/TiO 2 catalysts so far. The V–W/TiO 2 and V/TiO 2 samples synthesized by incipient wetness impregnation and grafting exhibited high activity toward NH 3 -SCR. Raman spectroscopy showed that they mainly contained highly dispersed, isolated, and polymeric V-oxo species. HERFD-XAS and XES identified redox cycling of vanadium species between V 4+ and V 5+ . With respect to most of the potential NH 3 adsorption complexes, density functional theory calculations further showed that vtc-XES is more limited than surface-sensitive techniques such as infrared spectroscopy; hence, a combination of X-ray techniques with IR or similar spectroscopies is required to unequivocally identify the mechanism of NH 3 -SCR over vanadia-based catalysts.