(36f) CO2 Hydrogenation over Inverse CeOx-Cu Catalysts

Anthropogenic CO₂ emissions are a major global concern, and this has led to great research traction in chemical transformation of CO₂ into fuels and commodity chemicals. Some of these efforts have been focused towards the production of methanol, which is particularly challenging owing to the stability of CO₂ and the prevalence of competitive side reactions. Graciani et al.¹ reported one of the most promising catalysts for CO₂ hydrogenation to methanol. They observed that a CeO_x/Cu(111) catalyst promoted the formation of unstable carboxylate intermediate species that readily converted into methanol. Inspired by their findings, we synthesized supported inverse CeO_x-Cu catalysts in an attempt to increase the number of interfacial sites for an improved hydrogenation of CO₂. First, a Cu/SiO₂ catalyst was synthesized by the strong electrostatic adsorption method. Then, after reduction, ceria (CeO_x) was added onto the Cu nanoparticles by controlled surface reactions. Higher CeO_x loadings were attained by successive cyclic depositions accompanied with intermediate reduction steps.

Interestingly, XPS results revealed a 100% abundance of Ce(III) after the first CeO_x cycle, which is attributed to very small CeO_x nanoclusters. This value decreased progressively with the number of cycles (Figure 1) due to an increased CeO_x particle size. The Ce(III)/Ce(IV) ratio plays a critical role in the distribution of oxygen vacancies and adsorption sites over the ceria catalysts. At high CeO_x loadings, STEM images revealed the presence of ceria nanorods near the Cu nanoparticles (Figure 2), an effect ascribed to the preferential anchorage of cerium to the silanol groups rather than the Cu surface. Remarkably, when studied in the hydrogenation of CO₂, these catalysts not only provided methanol, but also ethanol and 2-propanol.

1. Graciani, J., Mudiyanselage, K., Xu, F., Baber, A.E., Evans, J., Senanayake, S.D., Stacchiola, D.J., Liu, P., Hrbek, J., Fernandez Sanz, J., Rodriguez, J.A. Science 345 (6196), 546 (2014).