Unraveling the Complete Mechanism of the NH<sub>3</sub>-Selective Catalytic Reduction of NO over CeO<sub>2</sub>
Weibin Chen, Shenzhen Xu
Abstract
CeO 2 -based oxides, with promising redox properties, exhibit application potential for the selective catalytic reduction (SCR) of nitrogen oxide (NOx) with NH 3 (NH 3 -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 CeO 2 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 CeO 2 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 NH 3 -SCR over CeO 2 . 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, NH 3 dissociation to NH 2 * and H* is catalyzed by the coupled [O* + O vac ] species. The NH 2 * then combines with NO to generate the NH 2 NO active intermediate, which further dissociates to N 2 and H 2 O. In the Fast_SCR H* removal process, NO 2 reacts with H* and *NH 3 to generate H 2 O and NH 2 NO. 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.