Development of CO2-to-Methanol Hydrogenation Catalyst by Focusing on the Coordination Structure of the Cu Species in Spinel-Type Oxide Mg1–xCuxAl2O4

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 CO 2 -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 MgAl 2 O 4 (Mg 1– x Cu x Al 2 O 4 ). Mg 1– x Cu x Al 2 O 4 possessed the following three types of Cu 2+ species: short O–Cu octahedrally coordinated [CuO 6 ] s, elongated O–Cu octahedrally coordinated [CuO 6 ] el, and tetrahedrally coordinated [CuO 4 ] t . The former two are found in the inverse-spinel-type Mg 1– x Cu x Al 2 O 4, while the other is found in the normal-spinel-type Mg 1– x Cu x Al 2 O 4 . Additionally, by focusing on the difference in the reducibility of the Cu 2+ species, we clarified that their fraction is related to Cu loading. For low Cu loading ( x < 0.3), Mg 1– x Cu x Al 2 O 4 mainly contained the [CuO 6 ] s species. On the other hand, for high Cu loading ( x ≥ 0.3), the fraction of the [CuO 6 ] el and [CuO 4 ] t species increased. Notably, among the prepared catalysts, H 2 -reduced Mg 0.8 Cu 0.2 Al 2 O 4 ( x = 0.2) had the largest number of exposed metallic Cu sites, resulting in its good catalytic performance. Hence, the H 2 reduction of [CuO 6 ] s is essential for forming metallic Cu nanoparticles on metal oxides.