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

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.

Atom Pairing Enhances Sulfur Resistance in Low-Temperature SCR via Upshifting the Lowest Unoccupied States of Cerium

“…As shown in Figure a, although Ce 1+1 /TiO 2 exhibits a lower NH 3 conversion rate than Ce 1 /TiO 2 , it shows excellent selectivity, as manifested in the extremely low production of NO x . The data prove that Ce 1+1 /TiO 2 has a suitable oxidation ability to selectively oxidize NH 3 to NH 2 , which is required for efficient SCR. ,, On the contrary, the oxidation ability of Ce 1 /TiO 2 is strong enough to cleave all three N–H bonds of one NH 3 molecule and produce a large amount of NO x , resulting in poor SCR activity and N 2 selectivity …”

Catalytic Oxidation of Toluene over Pt/CeO2 Catalysts: A Double-Edged Sword Effect of Strong Metal–Support Interaction

“…The Ce 3d XPS spectra (Figure 2d) consist of Ce 3+ (V 0 and U 0 peaks centered at 885.0 and 903.0 eV) and Ce 4+ (the rest six bands) mixed valence state. 31,32 As displayed in Table S1, the Ce 3+ /Ce 4+ ratio of the catalysts increases with the NaBH 4 amount; the sequence is Pt/CeO 2 -1 (24.5%) > Pt/CeO 2 -0.5 (22.2%) > Pt/CeO 2 -0.25 (20.1%) > Pt/CeO 2 (18.6%). Furthermore, in O 1s XPS spectra (Figure S2), three peaks centered around 529.1, 531.1, and 533.0 eV could be clearly observed, which is ascribed to lattice oxygen (O L ), vacancy oxygen (O V ), and adsorbed oxygen species (O A ), respectively.…”