Boosting effect of encapsulated polyoxometalates in the photocatalytic CO2 reduction by MOF-545

Achieving efficient photocatalytic CO2 reduction is a current complex challenge, requiring the development of strategies that optimize not only the capture of photons but also the photoinduced charge separation and electron transfer processes. In this pursuit, we have immobilized polyoxometalates (POMs), specifically [SiW12O40]4(SiW12) and [W10O32]4- (W10), within the Zr-based porphyrinic metal-organic framework (MOF) MOF-545 catalytic material with the purpose of maximizing its CO2 photoreduction activity. The resulting SiW12@MOF-545 and W10@MOF-545 composites were fully characterized by various techniques (IR spectroscopy, powder X-ray diffraction, N2 adsorption isotherms, HADDF-STEM) to confirm the POM’s incorporation via impregnation. Highresolution TEM images of sections of W10@MOF-545 crystals prepared by ultramicrotomy confirm the location of POMs inside the MOF channels. These characterizations were complemented by simulations in order to locate the POM into the MOF’s cavities and identify host/guest interactions. In photocatalytic conditions, i.e. under visiblelight irradiation and in CH3CN/TEOA 20:1 solution, the two SiW12@MOF-545 and W10@MOF-545 composites reduced CO2 to formate with 100 % selectivity at rates of 669 and 1238 mu mol gMOF first 2 h. Remarkably, W10@MOF-545 showed around a 3-fold increase in activity compared to its POM-free counterpart. DFT calculations suggest that both POM guests can accept photoexcited electrons from the porphyrin linkers of MOF-545, allowing increased lifetime of the photogenerated holes in the MOF upon illumination, thus boosting TEOA oxidation by the porphyrinic MOF for subsequent CO2 reduction. Moreover, the calculations unveil the origin of the observed superior overall catalytic activity of W10@MOF-545 over SiW12@MOF-545 due to stronger thermodynamic driving force for charge separation, providing rational guidelines for future design of efficient photocatalysts.