Low-temperature photothermal CO2-to-CO conversion from flue gas using a g-C3N4/TiO2/MXene heterojunction with 100% selectivity

Abstract

Via a sol–gel route, we synthesized a ternary type-II/Schottky heterojunction g-C₃N₄/TiO₂/MXene (GTM) photothermal catalyst that harnessed residual heat from industrial flue gas together with sunlight to achieve efficient CO₂ reduction without external energy input. Under low-temperature photothermal conditions (80 °C + sunlight), GTM achieved a CO production rate of 349.3 μmol g⁻¹ h⁻¹ with 100% selectivity, representing enhancements by 12.7 times over photocatalysis (27.5 μmol g⁻¹ h⁻¹) and 81.2 times over thermocatalysis (4.3 μmol g⁻¹ h⁻¹), evidencing a pronounced photothermal synergy. Integrated characterization and density functional theory (DFT) analysis clarified the origin of this synergy: under illumination, aided by the excellent photothermal conversion capability and high electrical conductivity of MXene, the catalyst surface heated rapidly and photogenerated electrons from g-C₃N₄ and TiO₂ migrated swiftly to MXene, where CO₂ was activated into a bent adsorption configuration, strengthened adsorption and lowered the formation barrier of the key intermediate COOH*. Thermal input, on the one hand, excited localized surface plasmon resonance (LSPR) effect in MXene, generated abundant hot carriers; on the other hand, it accelerated surface molecular motion, enhanced H₂O adsorption/dissociation, lowered the free energy of the rate-determining step, and promoted CO desorption. The complementary advantages of the three components and the superposition of light and heat effects markedly boosted the CO₂-to-CO rate under mild conditions, offering a feasible pathway for industrial flue-gas treatment and carbon resource utilization.