This study reports a comprehensive investigation into the active sites and reaction mechanism of the selective catalytic reduction of NO by NH3 (NH3–SCR) over sulfate-loaded ceria (S1/CeO2). Catalyst characterization and density functional theory (DFT) calculations reveal that SO42– and S2O72– species are the dominant sulfate species on the S1/CeO2 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 L3-edge, ultraviolet–visible (UV–vis), and infrared (IR) spectroscopies, along with online analysis of outlet products (operando spectroscopy). The Ce4+(OH–) species, possibly located adjacent to the sulfate species, are reduced by NO + NH3 to produce N2, H2O, and Ce3+ species through an NO+ intermediate (RHC). The Ce3+ species are subsequently reoxidized by O2 (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 Ce4+(OH–), initially generating NO+, followed by the formation of Ce3+ and gaseous HONO. HONO then reacts with NH3 to produce N2 and H2O via NH4NO2 intermediates.
Qian et al. (Mon,) studied this question.