Catalysis of Cu Cluster for NO Reduction by CO: Theoretical Insight into the Reaction Mechanism

Density functional theory calculations here elucidated that Cu-38-catalyzed NO reduction by CO occurred not through NO dissociative adsorption but through NO dimerization. NO is adsorbed to two Cu atoms in a bridging manner. NO adsorption energy is much larger than that of CO. N-O bond cleavage of the adsorbed NO molecule needs a very large activation energy (Delta G degrees double dagger). On the other hand, dimerization of two NO molecules occurs on the Cu-38 surface with small Delta G degrees double dagger and very negative Gibbs reaction energy (Delta G degrees double dagger) to form ONNO species adsorbed to Cu-38. Then, a CO molecule is adsorbed at the neighboring position to the ONNO species and reacts with the ONNO to induce N-O bond cleavage with small Delta G degrees double dagger and very negative.G degrees, leading to the formation of N2O adsorbed on Cu-38 and CO2 molecule in the gas phase. N2O dissociates from Cu-38, and then it is readsorbed to Cu-38 in the most stable adsorption structure. N-O bond cleavage of N2O easily occurs with small Delta G degrees double dagger and significantly negative Delta G degrees to form the N-2 molecule and the O atom adsorbed on Cu-38. The O atom reacts with the CO molecule to afford CO2 and regenerate Cu-38, which is rate-determining. N2O species was experimentally observed in Cu/gamma-Al2O3-catalyzed NO reduction by CO, which is consistent with this reaction mechanism. This mechanism differs from that proposed for the Rh catalyst, which occurs via N-O bond cleavage of the NO molecule. Electronic processes in the NO dimerization and the CO oxidation with the O atom adsorbed to Cu-38 are discussed in terms of the charge-transfer interaction with Cu-38 and Frontier orbital energy of Cu-38.