Unlocking solar photo-thermal catalytic CO2 methanation at mild conditions using porphyrin-based metal-organic frameworks

Catalytic CO 2 hydrogenation to CH 4 is an important field of research that aims to achieve net-zero emissions. Conventional processes use high temperatures (> 350 °C) and pressures (>20 bar) to overcome the high energy barriers of this kinetically demanding eight-electron reduction reaction. Even though some studies have reported the photocatalytic version of this reaction under milder reaction conditions, the temperatures commonly used are still between 200 and 300 °C and pressures up to 20 bar. This paper reports on an engineered mixed-metal porphyrinic metal-organic framework MIL-173(Zr/Ti) decorated RuO x nanoparticles to catalyse this reaction at low temperatures (125 to 175 °C) and pressure (1.5 bar) under simulated sunlight irradiation. The performance of this material at 175 °C (reaching up to 1.63 mmol CH 4 g −1 h −1 , with apparent quantum yields (AQYs) ranging from 3.2 at 400 nm to 1% at 700 nm) ranks it among the most active MOF-based photocatalysts reported so far under similar reaction conditions. These AQY values are also higher or comparable than to other reported solid photocatalysts, even when they operate at temperatures in the range between 200 and 250 °C. Insights about materials performance were experimentally obtained using several techniques, including nanosecond transient absorption spectroscopy, photoluminescence, transient photocurrent, electrochemical impedance spectroscopy and thermal imaging together with additional photocatalytic tests. RuO x @MIL-173(Zr/Ti) activity can be explained by the multifunctionality of this photocatalyst favoring chemisorption of reagents, a wide range of visible light absorption, efficient charge separation and photothermal performance. • RuOx species decorated Zr/Ti porphyrinic-based MOF photocatalyst has been synthesized. • The photocatalyst is active for CO 2 methanation at mild conditions (<175 °C, 1.5 bar). • Photothermal and photochemical pathways are proposed as a function of irradiation intensity.