Assembling ultrafine TiO2 nanoparticles on UiO-66 octahedrons to promote selective photocatalytic conversion of CO2 to CH4 at a low concentration

[Display omitted] •Ultrafine TiO2 was uniformly assembled on UiO-66 via a facile two-step strategy.•The optimum composite with 81.3 % of TiO2 exhibits high CO2 uptake of 78.9 cm3 g−1.•The CO2 enrichment and exposure of sufficient catalytic sites were ensured.•The efficient and selective CH4 production was achieved under diluted CO2 gas condition.•The enhanced mechanism was discussed to present the superiority of designed structure. The improvement of CO2 conversion efficiency over semiconductor photocatalysts is usually restricted by their poor CO2 adsorption capacity. In this work, a zirconium-based organic framework (UiO-66) was introduced as an effective CO2 adsorbent to couple with TiO2 photocatalyst. The designed two-step strategy endowed the TiO2/UiO-66 composite with hierarchical porous structure, which ensured the sufficient exposed catalytic sites and a high CO2 uptake of 78.9 cm3 g−1. In consequence, a high CH4 production rate of 17.9 μmol g-1 h-1 with 90.4 % selectivity was achieved in a mild gas-solid system using water as electron donors. Interestingly, the photocatalytic efficiency reached the similar level to that of pure CO2 atmosphere even under diluted CO2 condition (≤ 2%). Further, the enhanced mechanism was discussed in detail to present the superiority of such designed composite structure from the aspects of CO2 enrichment and exposure of sufficient catalytic sites.