Filling metal–organic framework mesopores with TiO2 for CO2 photoreduction

Abstract

Metal–organic frameworks (MOFs)^1–3 are known for their specific interactions with gas molecules^4,5; combined with their rich and ordered porosity, this makes them promising candidates for the photocatalytic conversion of gas molecules to useful products⁶. Attempts to use MOFs or MOF-based composites for CO₂ photoreduction^6–13, however, usually result in far lower CO₂ conversion efficiency compared to state-of-the-art solid-state or molecular catalysts^14–18, even when facilitated by sacrificial reagents. Here, we create ‘molecular compartments’ inside a MOF by growing TiO₂ inside different pores of MIL-101 and its derivatives (Fig. 1). This compartmentalization allows for synergy between the light-absorbing/electron-generating TiO₂ units and the catalytic metal clusters in the MOFs, and therefore facilitates photocatalytic CO₂ reduction, concurrent with production of O₂. An apparent quantum efficiency for CO₂ photoreduction of 11.3% at 350 nm, is observed in a composite, 42%-TiO₂-in-Cr-MIL-101-NO₂. TiO₂ units in one type of compartment in this construct 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.