Photothermal catalytic CO2 conversion to CO

Modern society is heavily dependent on liquid hydrocarbon fuels and is expected to remain so for the foreseeable future. The conversion of atmospheric carbon dioxide to carbon monoxide (RWGS) via photothermal conversion using suitable photocatalysts is a promising technology for the production of fuels and platform chemicals with minimal use of fossil fuels. The ternary catalyst CZA (Cu/ZnO/Al2O3) has been utilized with a composition of 50% Cu, 30% ZnO, and 20% Al2O3. The use of metal nanoparticles in conjunction with a semiconductor is employed due to the synergistic effects of different modes of photoactivation. The catalyst has been characterized using XRD, BET, and in-situ UV-vis spectroscopy. In particular, the latter enabled the determination of the reflectance spectrum of CZA, revealing the plasmon resonance of metallic copper at ca. 590 nm and the band-gap energy of ZnO under the reaction conditions. Activation tests were conducted at ambient pressure to determine the dependence of the CO reaction rate on various parameters and the differences between catalyst performances in dark and light conditions. All results showed an increased production rate under visible light irradiation. The highest photo-enhancement was measured at 230°C and H2/CO2=1, with a relative gain of 2.8, from 0.23 to 0.62 mmol/gcat/min. These reaction conditions were therefore carried out in all subsequent tests. The calculated decrease in the activation energies from 83 to 50 kJ/mol under light irradiation indicated a hot electron-induced reaction mechanism, where the electrons generated by the Cu LSPR (or potentially transferred from excited ZnO) transiently occupy the orbital states of the surface CO2 to promote its activation and subsequent processes. The significant role of photon-electron excitations has been highlighted by varying the light wavelength. This suggested that shorter wavelengths (blue photons) are necessary to produce a substantial population of hot electrons with enough energy to transfer to the LUMO of CO2, and are the ones with the highest apparent quantum yield. The kinetic analysis showed a change in the reaction order with respect to H2 from 0.7 to -2.6, indicating an increase in the surface coverage of adsorbed H* and so proving the alteration of the reaction mechanism upon light irradiation. In situ DRIFTS enabled the identification of reaction intermediates under both dark and light conditions. The results indicated that more intermediates are converted to CO upon illumination, freeing up reactive sites and indicating an increase in reactivity. The CO2 and H2 desorption test revealed no CO formation when only carbon dioxide was fed, suggesting that a redox mechanism is not the primary pathway for the RWGS reaction. In conclusion photothermal RWGS by the industrially-relevant CZA catalyst resulted in activation under mild reaction conditions, kinetic benefits that exceed those resulting solely from temperature increases and enabled the discovery and analysis of some new phenomena occurring under simultaneous heat and light activation.