Photo-thermal Catalytic CO₂ Methanation by RuO_<i>x</i>@MIL-101(Cr) with 9.2% Apparent Quantum Yield under Visible Light Irradiation

Solar-assisted gaseous CO₂ hydrogenation to CH₄ is a potential strategy for favoring the transition to net zero emissions. Here, we report the development of a series of efficient metal-organic frameworks with MIL-101(Cr or Fe) topology decorated with RuO_<i>x</i> nanoparticles (ca. 0.2-2 wt %) as heterogeneous photocatalysts for the selective methanation of CO₂ by H₂ under simulated sunlight irradiation. The activity of RuO_<i>x</i>(1 wt %)@MIL-101(Cr) is between 3 and 50 times higher than related MOF-based photocatalysts under similar reaction conditions. Among the different photocatalysts, the optimized RuO_<i>x</i>(2 wt %)@MIL-101(Cr) photocatalyst showed 98.1% CO₂ conversion with 98.8% CH₄ selectivity reaching a production rate of 7.85 mmol g^-1 h^-1 with 720 mW cm^-2 at 200 °C. Further, this photocatalyst exhibited a record apparent quantum yield of 9.2% at 600 nm and 200 °C after subtracting thermal activity contribution compared to any previous MOF- or other heterogeneous-based photocatalyst reported so far. The photocatalyst retained its activity and integrity upon reuse for about 110 h. Transient photocurrent, electrochemical impedance, photoluminescence, and laser flash photolysis spectroscopies together with additional photocatalytic experiments suggest the occurrance of dual photochemical and photothermal reaction pathways. The photocatalytic CO₂ methanation reaction mechanism was further investigated using <i>operando</i> Fourier transform infrared spectroscopy.