Mechanism of NH 3 -SCR over P/CeO 2 Catalysts Investigated by Operando Spectroscopies.

This study reports a comprehensive investigation into the active sites and reaction mechanism for the selective catalytic reduction of NO by NH 3 (NH 3 -SCR) over phosphate-loaded ceria (P/CeO 2 ). Catalyst characterization and density functional theory calculations reveal that H 3 PO 4 and H 2 P 2 O 6 species are the dominant phosphate species on the P/CeO 2 catalysts under the experimental conditions. The reduction/oxidation half-cycles (RHC/OHC) were investigated using in situ X-ray absorption near-edge structure for Ce L 3 -edge, ultraviolet-visible, and infrared (IR) spectroscopies together with online analysis of outlet products ( operando spectroscopy). The Ce 4+ (OH - ) species, possibly adjacent to the phosphate species, are reduced by NO + NH 3 to produce N 2 , H 2 O, and Ce 3+ species (RHC). The Ce 3+ species is reoxidized by aqueous O 2 (OHC). The results from IR spectroscopy suggest that the RHC initiates with the reaction between NO and Ce 4+ (OH - ) to yield Ce 3+ and gaseous HONO, which then react with NH 3 to produce N 2 and H 2 O via NH 4 NO 2 intermediates.