Mechanism of NH₃-SCR over P/CeO₂ 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 NH₃ (NH₃-SCR) over phosphate-loaded ceria (P/CeO₂). Catalyst characterization and density functional theory calculations reveal that H₃PO₄ and H₂P₂O₆ species are the dominant phosphate species on the P/CeO₂ 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 L₃-edge, ultraviolet-visible, and infrared (IR) spectroscopies together with online analysis of outlet products (operando spectroscopy). The Ce⁴⁺(OH^-) species, possibly adjacent to the phosphate species, are reduced by NO + NH₃ to produce N₂, H₂O, and Ce³⁺ species (RHC). The Ce³⁺ species is reoxidized by aqueous O₂ (OHC). The results from IR spectroscopy suggest that the RHC initiates with the reaction between NO and Ce⁴⁺(OH^-) to yield Ce³⁺ and gaseous HONO, which then react with NH₃ to produce N₂ and H₂O via NH₄NO₂ intermediates.