Filling metal-organic framework mesopores with TiO₂ for CO₂ photoreduction

Metal-organic frameworks (MOFs)^1-3 are known for their specific interactions with gas molecules^4,5; this, combined with their rich and ordered porosity, makes them promising candidates for the photocatalytic conversion of gas molecules to useful products⁶. However, attempts to use MOFs or MOF-based composites for CO₂ photoreduction^6-13 usually result in far lower CO₂ conversion efficiency than that obtained from state-of-the-art solid-state or molecular catalysts^14-18, even when facilitated by sacrificial reagents. Here we create 'molecular compartments' inside MOF crystals by growing TiO₂ inside different pores of a chromium terephthalate-based MOF (MIL-101) and its derivatives. This allows for synergy between the light-absorbing/electron-generating TiO₂ units and the catalytic metal clusters in the backbones of MOFs, and therefore facilitates photocatalytic CO₂ reduction, concurrent with production of O₂. An apparent quantum efficiency for CO₂ photoreduction of 11.3 per cent at a wavelength of 350 nanometres is observed in a composite that consists of 42 per cent TiO₂ in a MIL-101 derivative, namely, 42%-TiO₂-in-MIL-101-Cr-NO₂. TiO₂ units in one type of compartment in this composite are estimated to be 44 times more active than those in the other type, underlining the role of precise positioning of TiO₂ in this system.