Solar Gas-Phase CO2 Hydrogenation by Multifunctional UiO-66 Photocatalysts

Solar-assisted CO₂ conversion into fuels and chemical products involves a range of technologies aimed at driving industrial decarbonization methods. In this work, we report on the development of a series of multifunctional metal-organic frameworks (MOFs) based on nitro- or amino-functionalized UiO-66(M) (M: Zr or Zr/Ti) supported RuO_x NPs as photocatalysts, having different energy band level diagrams, for CO₂ hydrogenation under simulated concentrated sunlight irradiation. RuO_x(1 wt %; 2.2 ± 0.9 nm)@UiO-66(Zr/Ti)-NO₂ was found to be a reusable photocatalyst, to be selective for CO₂ methanation (5.03 mmol g^-1 after 22 h;, apparent quantum yield at 350, 400, and 600 nm of 1.67, 0.25, and 0.01%, respectively), and to show about 3-6 times activity compared with previous investigations. The photocatalysts were characterized by advanced spectroscopic techniques like femto- and nanosecond transient absorption, spin electron resonance, and photoluminescence spectroscopies together with (photo)electrochemical measurements. The photocatalytic CO₂ methanation mechanism was assessed by operando FTIR spectroscopy. The results indicate that the most active photocatalyst operates under a dual photochemical and photothermal mechanism. This investigation shows the potential of multifunctional MOFs as photocatalysts for solar-driven CO₂ recycling.