Mechanistic insights into NH3-assisted selective reduction of NO on CeO2: a first-principles microkinetic study on selectivity and activity

Abstract

To understand the activity- and selectivity-limiting factors of selective catalytic reduction of NO with NH₃ (NH₃-SCR) catalyzed by CeO₂-based oxides, a molecular-level mechanistic exploration was performed on CeO₂(110) using a first-principles microkinetic study. Herein, the favored reaction pathway for N₂ formation on CeO₂(110) is unveiled, which includes three key subprocesses. (i) NH₃ adsorbs on the Ce_cus site and dissociates into *NH₂ assisted by O_lat; (ii) *NH₂ preferentially couples with NO adsorbed on O_lat (ONO#), forming *NH₂NO on the Ce_cus site; (iii) *NH₂NO undergoes dehydrogenation into *NHNO, which can be easily anchored by O_vac and can then decompose into N₂. The quantitative microkinetic results show that the transfer of NHNO from Ce_cus to O_vac, rather than the further conversion of N₂O to N₂ on O_vac, emerges as the N₂ selectivity-determining step on CeO₂, in which O_vac plays a key role. The number of O_vac is an important factor determining the N₂ selectivity of CeO₂-based catalysts. The sensitivity analysis reveals that NH₂NO formation, i.e., *NH₂ + ONO# → *NH₂NO + O#, is the rate-determining step for NH₃-SCR on the CeO₂ catalyst; accordingly, enhancing NH₃ adsorption could be an effective strategy to boost the catalytic activity of CeO₂ for NH₃-SCR. In general, creating O_vac on CeO₂ and introducing components (e.g., WO₃) with strong NH₃ adsorption would be efficient for designing CeO₂-based catalysts with superior N₂ selectivity and activity. These results could provide a consolidated theoretical basis for understanding and optimizing CeO₂-based catalysts for NH₃-SCR.