A novel nanocomposite of Co doped Al₂O₃ nanosheets supporting Pt nanoparticles (Pt/Co–Al₂O₃) was prepared. By merely utilizing focused UV-Vis-IR illumination from a 500 W Xe lamp, highly efficient photothermocatalytic CO₂ reduction by CH₄ (CRM) on Pt/Co–Al₂O₃ is achieved. This demonstrates very high production rates of CO (r_CO, 89.41 mmol min⁻¹ g⁻¹) and H₂ (r_H2, 75.60 mmol min⁻¹ g⁻¹), and high light-to-fuel efficiency (η, 27.2%), enhanced by 5.4, 6.0, and 6.2 times as compared to those of Al₂O₃ nanosheets supporting Pt nanoparticles (Pt/Al₂O₃). Pt/Co–Al₂O₃ also demonstrates good catalytic durability as the side-reactions of coke deposition are significantly blocked. In striking contrast, Pt/Al₂O₃ quickly deactivates due to its high coke deposition rate (r_C), 15.3 times higher than that of Pt/Co–Al₂O₃. The significantly promoted photothermocatalytic performance arises from a synergetic effect between Pt and Co–Al₂O₃. The formation of a Pt/Co–Al₂O₃ interface significantly promotes CH₄ dissociation to carbon species as compared to Pt/Al₂O₃, thus increasing the catalytic activity. The active oxygen of Co–Al₂O₃ participates in the oxidation of carbon species as the rate-determining step of CRM, thus not only increasing the catalytic activity, but also blocking carbon species being polymerized to coke. The photothermocatalytic CRM on Pt/Co–Al₂O₃ follows a mechanism of light-driven thermocatalysis. The intense absorption across the whole sunlight spectrum of Co–Al₂O₃ induced by Co doping substantially intensifies the photothermal conversion of Pt/Co–Al₂O₃, thus improving the catalytic activity. A novel photoactivation is discovered to considerably increase the catalytic activity due to the oxidation of carbon species being promoted upon illumination.