Photo- and Electrocatalytic Reduction of CO 2 over Metal-Organic Frameworks and Their Derived Oxides: A Correlation of the Reaction Mechanism with the Electronic Structure.

A Ce/Ti-based bimetallic 2-aminoterephthalate metal-organic framework (MOF) was synthesized and evaluated for photocatalytic reduction of CO 2 in comparison with an isoreticular pristine monometallic Ce-terephthalate MOF. Owing to highly selective CO 2 adsorption capability, optimized band gaps, higher flux of photogenerated electron-hole pairs, and a lower rate of recombination, this material exhibited better photocatalytic reduction of CO 2 and lower hydrogen evolution compared to Ce-terephthalate. Thorough probing of the surface and electronic structure inferred that the reducibility of Ce 4+ to Ce 3+ was due to the introduction of an amine functional group into the linker, and low-lying Ti(3 d ) orbitals in Ce/Ti-2-aminoterephthalate facilitated the photoreduction reaction. Both the MOFs were calcined to their respective oxides of Ce 1- x Ti x O 2 and CeO 2 , and the electrocatalytic reduction of CO 2 was performed over the oxidic materials. In contrast to the photocatalytic reaction mechanism, the lattice substitution of Ti in the CeO 2 fluorite cubic structure showed a better hydrogen evolution reaction and consequently, poorer electroreduction of CO 2 compared to pristine CeO 2 . Density functional theory calculations of the competitive hydrogen evolution reaction on the MOF and the oxide surfaces corroborated the experimental findings.