Explore the Catalytic Reaction Mechanism in the Reduction of NO by CO on the Rh₇⁺ Cluster: A Quantum Chemical Study

Abstract

Rhodium has been proved to possess unique reactivity to convert NO_<i>x</i> into N₂ with high conversion efficiency and selectivity. In this study, we have carried out DFT calculations on the reaction mechanism in the reduction of NO by CO on the surface of the Rh₇⁺ cluster. The calculated results suggest that the reaction proceeds via three steps. First, the NO and CO are adsorbed on the Rh₇⁺ cluster, then the adsorbed NO decomposes to N and O atoms. The O atom reacts with the adsorbed CO leading to the formation of CO₂ molecule. Second, another NO is adsorbed on the rhodium cluster and decomposes to N and O atoms, then the two N atoms couple with each other to yield N₂ molecule. Finally, the second CO can be adsorbed on the Rh₁ or Rh₇ atom of the Rh₇⁺ cluster and oxidized to CO₂ molecule. On the basis of present calculations from gas-phase Gibbs free energy profiles, the reaction path related to CO adsorption on the Rh₇ atom is energetically more favorable. The second adsorbed NO generating N and O atoms in the second step is the rate-limiting step of whole catalytic cycle. The high activation barrier (TS₆₇) of 56.6 kcal/mol can be driven by large exergonic reaction. Our work would provide some valuable fundamental insights into the reaction mechanism between NO and CO on the rhodium surface, which is vitally important to decrease NO emissions in automotive exhaust gas