Mechanisms of catalytic NO reduction mediated by gas-phase metal clusters

Catalytic conversion of NO and CO into N₂ and CO₂ is imperative under the weight of the increasingly stringent emission regulations, and oxide-supported platinum group metals (PGMs, e.g., Pd, Rh, and Pt) play pivotal roles in driving this catalytic reaction. Maximizing the utilization of PGMs and developing noble-metal-free catalysts are promising strategies to face these economic and environmental issues. The key to achieving this goal lies in obtaining a fundamental understanding of the nature of active sites as well as the mechanisms that govern such catalytic conversion, while it is extremely challenging due to the complexity of real-world catalysts. Gas-phase reactions on atomic clusters under well-defined conditions are widely studied to generate active species that compositionally and functionally resemble the active sites in real-world catalysts. Moreover, cluster reactions in combination with quantum-chemical calculations can provide crucial mechanistic insights at a strictly molecular level. This review summarizes recent advances (over the past decade) in the catalytic reduction of NO by CO mediated by gas-phase metal clusters. The following three topics are discussed: (1) the adsorption and activation of NO on metal clusters, (2) the mechanisms for the selective reduction of NO to N₂O by CO, and (3) the mechanisms for driving NO reduction to N₂ by CO. These findings parallel the catalytic behaviors observed in related condensed-phase catalysts, and this knowledge could be pivotal for guiding the design of advanced catalytic materials in the future.