Operando Spectroscopic Study of Reduction and Oxidation Half-Cycles in NH3–SCR over CeO2-Supported WO3

Operando spectroscopies (in situ Ce and W L3-edge X-ray absorption near-edge structure, ultraviolet–visible (UV–vis), and infrared (IR) spectroscopies, combined with online analysis of gas-phase products) were exploited to elucidate reduction/oxidation half-cycles in the selective catalytic reduction of NO with NH3 (NH3–SCR) over WO3-loaded CeO2. The Ce4+ species was reduced by NO + NH3 to yield N2 and Ce3+ species (reduction half-cycle), which was then reoxidized by O2 (oxidation half-cycle). The oxidation state of the W6+ species remained unchanged under redox conditions. IR and theoretical results indicated that the reduction half-cycle started with the reaction of W6+–OH and adjacent Ce4+–O with NO to afford Ce3+ species and gaseous HONO, which was converted to NO+ species on the catalyst. The NO+ species then reacted with NH3 to generate N2.