Development of CO2-to-Methanol Hydrogenation Catalyst by Focusing on the Coordination Structure of the Cu Species in Spinel-Type Oxide Mg1–xCuxAl2O4 : Associate Professor Ryuji Kikuchi, Department of Chemical System Engineering, and other researchers.

Shohei Tada, Fumito Otsuka, Kakeru Fujiwara, Constantinos Moularas, Yiannis Deligiannakis, Yuki Kinoshita, Sayaka Uchida, Tetsuo Honma, Masahiko Nishijima, Ryuji Kikuchi

Abstract
Dispersion of metallic Cu nanoparticles on a metal oxide support increases the number of exposed metallic Cu sites and/or Cu-support interfacial sites, resulting in good catalytic performance for CO₂-to-methanol hydrogenation. However, the formation of highly dispersed Cu nanoparticles is challenging because they are easily sintered. Here, we studied Cu nanoparticle formation by a simple deposition–reduction technique using Cu-doped MgAl₂O₄ (Mg_1–xCu_xAl₂O₄). Mg_1–xCu_xAl₂O₄ possessed the following three types of Cu²⁺ species: short O–Cu octahedrally coordinated [CuO₆]_s, elongated O–Cu octahedrally coordinated [CuO₆]_el, and tetrahedrally coordinated [CuO₄]_t. The former two are found in the inverse-spinel-type Mg_1–xCu_xAl₂O₄, while the other is found in the normal-spinel-type Mg_1–xCu_xAl₂O₄. Additionally, by focusing on the difference in the reducibility of the Cu²⁺ species, we clarified that their fraction is related to Cu loading. For low Cu loading (x < 0.3), Mg_1–xCu_xAl₂O₄ mainly contained the [CuO₆]_s species. On the other hand, for high Cu loading (x ≥ 0.3), the fraction of the [CuO₆]_el and [CuO₄]_t species increased. Notably, among the prepared catalysts, H₂-reduced Mg_0.8Cu_0.2Al₂O₄ (x = 0.2) had the largest number of exposed metallic Cu sites, resulting in its good catalytic performance. Hence, the H₂ reduction of [CuO₆]_s is essential for forming metallic Cu nanoparticles on metal oxides.



ACS Catalysis : https://pubs.acs.org/doi/10.1021/acscatal.0c02868