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-ra