Unraveling the Complete Mechanism of the NH₃-Selective Catalytic Reduction of NO over CeO₂

CeO2-based oxides, with promising redox properties, exhibit application potential for the selective catalytic reduction (SCR) of nitrogen oxide (NOx) with NH3 (NH3-SCR). Despite decades of research, the underlying mechanisms governing the SCR activity remain unclear, and the catalytic paths of fast SCR (Fast_SCR) and standard SCR (Std_SCR) on the CeO2 surfaces are still under debate. Understanding the complete SCR reaction mechanism is crucial for the design and synthesis of efficient SCR catalysts. We perform density functional theory (DFT) simulations, synthesize CeO2 model catalysts for in situ spectroscopy experiments (in situ drifts, in situ Raman, in situ NAP-XPS, and in situ EPR) and SCR activity evaluation experiments to reveal the complete mechanism for NH3-SCR over CeO2. We find that the Std_SCR and the fast-SCR mechanisms share the same NO reduction path but go through two different adsorbed-hydrogen (H*) removal processes. For the NO reduction reaction, NH3 dissociation to NH2* and H* is catalyzed by the coupled [O* + O-vac] species. The NH2* then combines with NO to generate the NH2NO active intermediate, which further dissociates to N-2 and H2O. In the Fast_SCR H* removal process, NO2 reacts with H* and *NH3 to generate H2O and NH2NO. For the Std_SCR, the catalytic species of O* is consumed to complete the H* removal. Our experimental-theoretical joint study further provides design principles of oxide catalysts for NO removal based on the atomic-level understanding of the catalytic mechanisms.