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 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 (Mg1-xCuxAl2O4). Mg(1-)xCu(x)Al(2)O(4) 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 Mg1-xCuxAl2O4, while the other is found in the normal-spinel-type Mg1-xCuxAl2O4. 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), Mg1-xCuxAl2O4 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, H-2-reduced Mg0.8Cu0.2Al2O4 (x = 0.2) had the largest number of exposed metallic Cu sites, resulting in its good catalytic performance. Hence, the H-2 reduction of [CuO6](s) is essential for forming metallic Cu nanoparticles on metal oxides.