Hydroxide Ligands Cooperate with Catalytic Centers in Metal–Organic Frameworks for Efficient Photocatalytic CO2 Reduction

Abstract

Converting CO2 into fuels via photochemical reactions relies on highly efficient and selective catalysts. We demonstrate that the catalytic active metal center can cooperate with neighboring hydroxide ligands to boost the photocatalytic CO2 reduction, by comparing six cobalt-based metal-organic frameworks (MOFs) with different coordination environments at the typical reaction condition (photosensitizer, electron donor, water, and visible light). In pure CO2 at 1.0 atm, the MOFs bearing -OH− ligands neighboring the open Co centers showed CO selectivities and turnover frequencies (TOFs) up to 98.2% and 0.059 s−1, respectively. More importantly, their TOFs reduced only ca. 20% when the CO2 partial pressure was reduced to 0.1 atm, while other MOFs reduced by at least 90%. Periodic density functional theory calculations and isotope tracing experiments showed that the -OH− ligands serve not only as strong hydrogen-bonding donors to stabilize the initial Co−CO2 adduct, but also local proton sources to facilitate the C−O bond breaking.