Mechanistic insight into the enhanced NO reduction by CO over a pre-reduced CuxOy-CeO2 multiphase catalyst

Abstract

The CuxOy(9)-CeO2(1) catalyst contained with the low-valent Cu species was synthesized by a two-step method and employed for NO reduction by CO (NO+CO reaction), which was distinguished from the traditional CuxCe1−xO2 solid solution oxide catalyst because it was a variety of interaction of composite heterogeneous catalyst rather than pure solid solutions or loaded catalysts. The CuxOy(9)-CeO2(1)-pre catalyst after CO pretreatment showed almost 100% NO conversion and also early 100% selectivity to N2 at 150 °C in a reaction atmosphere with a CO to NO concentration ratio of 10, which were markedly superior to CuxOy(9)-CeO2(1)-no pre without CO pretreatment, Cu2O, and CeO2 catalysts. The analysis showed multi-electron cycling systems, including the transformation between Cu+ and Cu0, Cu+ and Cu2+, and the interaction between Cu+/Cu2+ and Ce3+/Ce4+ conducive to improving the REDOX performance of the catalyst. In addition, CO pretreatment was beneficial to producing and stabilizing surface synergistic oxygen vacancies (SSOVs) in the Cu-□-Ce system to promote activity. The in-situ DRIFTS analysis revealed that the NO+CO reaction followed the Mars van Krevelen (MvK) mechanism with Cu+ as the core species. During CO pretreatment, CO would react with the lattice oxygen on the catalyst surface to form the apparent surface oxygen vacancies (SOVs, created on a single metal atom) and SSOVs (mainly on a multi-metal catalyst) for continuous adsorption and reaction. NO could adsorb on oxygen vacancies and dissociate into [N] and [O] atoms, which interacted with CO and eventually generated N2 and CO2, respectively, or filled oxygen vacancies to complete the catalytic cycle. In this process, the formation of SSOVs and the adsorption and dissociation of NO on oxygen vacancies played essential roles.