Photocatalysts can absorb light and convert the exciton into an electron–hole pair, so the electron can drive a reduction reaction and/or the hole can drive an oxidation reaction. Photocatalysts are usually inorganic semiconductors (e.g., TiO2). Dr. Xin-Ping Wu, a postdoctoral scholar working with Don Truhlar and Laura Gagliardi, has proposed that metal–organic frameworks (MOFs) containing cerium would also be good photocatalysts. MOFs are crystalline materials with some attractive properties for photocatalysis, including well separated inorganometallic nodes that can serve as catalytic centers and – since MOFs are nanoporous –including high accessibility of potential catalytic sites to reagents; however, the highest unoccupied and the lowest unoccupied crystal orbitals (HOCO and LUCO) of MOFs are usually localized solely on their linkers, so most MOFs have poor charge separation capabilities, which leads to short lifetimes of excited states and limits possible photocatalytic activity. But Ce has low-lying empty 4f orbitals, and Ce-MOFs can have the LUCO on Ce in the node, thereby separating the HOCO and LUCO onto different subsystems (the linker and the node) in the MOF, as shown in the figure on the right. This anticipated result was confirmed by quantum chemical calculations on a substituted UiO-66 MOF, with Zr (for stability) and Ce (for photocatalytic activity) as the metals in the nodes. With purposefully designed linkers, absorption is in the visible and the absolute orbital energies are well situated for water splitting or CO2 reduction. The work shows the power of theory in engineering the electronic properties of functional nanoporous materials.
The work is published in the Journal of the American Chemical Society [doi.org/10.1021/jacs.8b03613]. The research was supported by the Nanoporous Materials Genome Center (NMGC) [NMGC main page], which is a Computational Chemical Sciences Center in the Department of Chemistry at the University of Minnesota.
June 2, 2018