High light-to-fuel efficiency and CO2 reduction rate achieved on a unique nanocomposite of Co/Co doped Al2O3 nanosheets with UV-Vis-IR irradiation

A unique nanocomposite of Co nanoparticles supported on Co doped Al2O3 nanosheets (Co/Co–Al2O3) was synthesized by a facile method. For photothermocatalytic CO2 reduction with CH4 under focused UV-visible-infrared (UV-vis-IR) irradiation, Co/Co–Al2O3 exhibits high light-to-fuel efficiency (27.1%) and production rates of CO and H2 (43.46 and 39.42 mmol min−1 g−1). Even with λ > 690 nm focused vis-IR irradiation, it still exhibits high light-to-fuel efficiency (23.2%) and production rates of CO and H2 (24.62 and 18.97 mmol min−1 g−1). It is found that supporting Co nanoparticles on Co doped Al2O3 nanosheets significantly improves the photothermocatalytic durability compared to a reference catalyst of Co nanoparticles on Al2O3 nanosheets (Co/Al2O3). The improved durability is attributed to the fact that O atoms from Co doped Al2O3 in Co/Co–Al2O3 participate in the oxidation of C* species formed by both the complete dissociation of CH4 and the disproportionation of the produced CO as side-reactions of carbon deposition, thus significantly reducing the carbon deposition rate. The high photothermocatalytic activity is attributed to effective light-driven thermocatalysis, substantially boosted by new photoactivation: Co nanoparticles in Co/Co–Al2O3 act as thermocatalytic components. Both Co nanoparticles and Co doped Al2O3 in Co/Co–Al2O3 act as light absorbers due to their strong absorption over the entire solar spectral region. Their efficient photothermal conversion together with the IR heating effect of the focused irradiation causes an elevation in the surface temperature of Co/Co–Al2O3, thus triggering the thermocatalytic reaction. The focused irradiation substantially reduces the activation energy for CO2 reduction by CH4 on Co/Co–Al2O3, thus considerably promoting the catalytic activity. The photoactivation also promotes catalyst durability due to the inhibition of carbon deposition by CO disproportionation.