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

[Image: see text] 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 (ΔG°(‡)). On the other hand, dimerization of two NO molecules occurs on the Cu(38) surface with small ΔG°(‡) and very negative Gibbs reaction energy (ΔG°) 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 ΔG°(‡) and very negative ΔG°, leading to the formation of N(2)O adsorbed on Cu(38) and CO(2) molecule in the gas phase. N(2)O dissociates from Cu(38), and then it is readsorbed to Cu(38) in the most stable adsorption structure. N–O bond cleavage of N(2)O easily occurs with small ΔG°(‡) and significantly negative ΔG° to form the N(2) molecule and the O atom adsorbed on Cu(38). The O atom reacts with the CO molecule to afford CO(2) and regenerate Cu(38), which is rate-determining. N(2)O species was experimentally observed in Cu/γ-Al(2)O(3)-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).