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

Nozomi Takagi; Kazuya Ishimura; Hiroki Miura; Tetsuya Shishido; Ryoichi Fukuda; Masahiro Ehara; Shigeyoshi Sakaki

doi:10.1021/acsomega.8b02890

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

ACS Omega. 2019 Feb 4;4(2):2596-2609. doi: 10.1021/acsomega.8b02890. eCollection 2019 Feb 28.

Authors

Nozomi Takagi¹, Kazuya Ishimura², Hiroki Miura^{1

3}, Tetsuya Shishido^{1

3}, Ryoichi Fukuda¹, Masahiro Ehara^{1

2}, Shigeyoshi Sakaki^{1

4}

Affiliations

¹ Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan.
² Institute for Molecular Science, Okazaki 444-8585, Japan.
³ Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1, Minami-osawa, Hachioji, Tokyo 192-0397, Japan.
⁴ Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan.

Abstract

Density functional theory calculations here elucidated that Cu₃₈-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 Cu₃₈ surface with small ΔG°^‡ and very negative Gibbs reaction energy (ΔG°) to form ONNO species adsorbed to Cu₃₈. 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 N₂O adsorbed on Cu₃₈ and CO₂ molecule in the gas phase. N₂O dissociates from Cu₃₈, and then it is readsorbed to Cu₃₈ in the most stable adsorption structure. N-O bond cleavage of N₂O easily occurs with small ΔG°^‡ and significantly negative ΔG° to form the N₂ molecule and the O atom adsorbed on Cu₃₈. The O atom reacts with the CO molecule to afford CO₂ and regenerate Cu₃₈, which is rate-determining. N₂O species was experimentally observed in Cu/γ-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 Cu₃₈ are discussed in terms of the charge-transfer interaction with Cu₃₈ and Frontier orbital energy of Cu₃₈.