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

Achieving efficient photocatalytic CO 2 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 [SiW 12 O 40 ] 4- (SiW 12 ) and [W 10 O 32 ] 4- (W 10 ), within the Zr-based porphyrinic metal-organic framework (MOF) MOF-545 catalytic material with the purpose of maximizing its CO 2 photoreduction activity. The resulting SiW 12 @MOF-545 and W 10 @MOF-545 composites were fully characterized by various techniques (IR spectroscopy, powder X-ray diffraction, N 2 adsorption isotherms, HADDF-STEM) to confirm the POM’s incorporation via impregnation. High-resolution TEM images of sections of W 10 @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 visible-light irradiation and in CH 3 CN/TEOA 20:1 solution, the two SiW 12 @MOF-545 and W 10 @MOF-545 composites reduced CO 2 to formate with 100% selectivity at rates of 669 and 1238 μmol g MOF -1 h -1 , respectively, during the first 2 h. Remarkably, W 10 @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 CO 2 reduction. Moreover, the calculations unveil the origin of the observed superior overall catalytic activity of W 10 @MOF-545 over SiW 12 @MOF-545 due to stronger thermodynamic driving force for charge separation, providing rational guidelines for future design of efficient photocatalysts. • Two POM@MOF composites were constructed by encapsulating W10 and SiW12 POMs in the hexagonal cavities of the porphyrinic MOF-545 • The optimal composite W10@MOF-545 exhibit excellent photocatalytic CO2 reduction into formate with a 4.2-fold increase in activity compared with the POM-free MOF • In these composites, the porphyrin linkers play the role of photosensitizers, the catalytic sites are the Zr6 clusters, the POMs act as electron reservoirs • DFT calculations showed that the POMs improve the photogenerated electron-hole separation and confirm the superior activity of W10.