Manipulating the Spin State of Co Sites in Metal-Organic Frameworks for Boosting CO₂ Photoreduction

Photocatalytic CO₂ reduction holds great potential for alleviating global energy and environmental issues, where the electronic structure of the catalytic center plays a crucial role. However, the spin state, a key descriptor of electronic properties, is largely overlooked. Herein, we present a simple strategy to regulate the spin states of catalytic Co centers by changing their coordination environment by exchanging the Co species into a stable Zn-based metal-organic framework (MOF) to afford Co-OAc, Co-Br, and Co-CN for CO₂ photoreduction. Experimental and DFT calculation results suggest that the distinct spin states of the Co sites give rise to different charge separation abilities and energy barriers for CO₂ adsorption/activation in photocatalysis. Consequently, the optimized Co-OAc with the highest spin-state Co sites presents an excellent photocatalytic CO₂ activity of 2325.7 μmol·g^-1·h^-1 and selectivity of 99.1% to CO, which are among the best in all reported MOF photocatalysts, in the absence of a noble metal and additional photosensitizer. This work underlines the potential of MOFs as an ideal platform for spin-state manipulation toward improved photocatalysis.