Metal-oxide nanostructure catalysts for thermal and photothermal catalytic CO2 hydrogenation

Abstract

CO2 hydrogenation represents one of the most practical solutions for mitigating anthropogenic carbon into chemicals/fuels. The overall reactivity in these reactions is often linked to the nature of kinetics relevant surface intermediary steps. Herein, light-induced photoactivation and materials engineering strategies were employed to particularly tailor the sluggish reaction steps and improve the catalytic performance of two CO2 hydrogenation reactions, namely CO2 methanation and methanol synthesis.

The light-assisted CO2 methanation over NiOx/La2O3@TiO2 (NLT) was studied using isotopic-assisted in-situ diffuse reflectance infrared transform spectroscopic (DRIFTS) technique. It was shown that one of the important surface reactions – formate (HCO2*) conversion - can be promoted under visible light illumination. The La2O3 promoter and the broad-band localised surface plasma resonance (LSPR) property of Ni nanostructure, which acts as a vital adsorption site for CO2 activation and enables the generation of high-energy electrons, respectively, could play a primary role in this promotion.

Next, the effect of photoexcitation on light-assisted methanol production from CO2 hydrogenation over a Cu/ZnO/Al2O3 (CZA) catalyst, where HCO2* conversion also plays an important role, was studied. Interestingly, significant methanol enhancement was only observed under the photoexcitation of both Cu and ZnO, whilst CO production was improved irrespective of the spectral range. This was revealed to be governed by the photo-generated electrons and their subsequent interfacial transfer which were responsible for the simultaneous transformations in surface chemistry and catalytic reactions.

Further, CeO2 was studied as a promoter for Cu/ZnO based catalyst where enhanced methanol production was observed at dark condition. Lastly, a dopant strategy (Mg/La doping in ZnO) was adopted to electronically promote the Cu-ZnO interaction to testify its role in light-assisted methanol production. The photothermal catalytic methanol production over promoted CZA was improved benefiting from the enriched interfacial oxygen vacancies which could help channel the photo-generated electrons to the adsorbed HCO2* species.

Overall, the study demonstrated the potential of photoexcitation and materials engineering strategy in improving CO2 hydrogenation reaction over oxide supported Ni and Cu catalysts. Significantly, the mechanistic understanding at the molecular level is critical for the design of catalysts that can better harness the potential of photoexcitation.