Redox-Half-Cycle Dynamics of NH 3 −SCR on Sulfate-Modified CeO 2 Revealed by Modulation Excitation Spectroscopy

This study reports a comprehensive investigation into the active sites and reaction mechanism of the selective catalytic reduction of NO by NH 3 (NH 3 –SCR) over sulfate-loaded ceria (S1/CeO 2 ). Catalyst characterization and density functional theory (DFT) calculations reveal that SO 4 2– and S 2 O 7 2– species are the dominant sulfate species on the S1/CeO 2 catalysts under the experimental conditions. The reduction/oxidation half-cycles (RHC/OHC) were investigated using modulation excitation in situ X-ray absorption near-edge structure (XANES) at the Ce L 3 -edge, ultraviolet–visible (UV–vis), and infrared (IR) spectroscopies, along with online analysis of outlet products ( operando spectroscopy). The Ce 4+ (OH – ) species, possibly located adjacent to the sulfate species, are reduced by NO + NH 3 to produce N 2, H 2 O, and Ce 3+ species through an NO + intermediate (RHC). The Ce 3+ species are subsequently reoxidized by O 2 (OHC). The RHC is promoted by the sulfate species. The results from IR spectroscopy suggest that the RHC begins with the reaction between NO and Ce 4+ (OH – ), initially generating NO +, followed by the formation of Ce 3+ and gaseous HONO. HONO then reacts with NH 3 to produce N 2 and H 2 O via NH 4 NO 2 intermediates.