Simultaneous thermal and photonic activation has provided interesting opportunities for solar upgrading of catalytic processes [1]. Photothermal catalysis works at the interface between photochemical processes, in which photon energy is converted into chemical energy, and thermal catalysis, with the catalyst activated by temperature. This combined catalysis is particularly promising for the activation of small, unreactive molecules at moderate temperatures compared to thermal catalysis and with higher reaction rates than those attained in photocatalysis. CO₂ is an archetype of this kind of molecule: its sustainable conversion into fuels or chemicals is one of the major challenges of modern chemistry. Among the many suggested catalyst formulations, those featuring metals dispersed on semiconductors show promise. These systems offer multiple photonic and thermal pathways for electron transfer, which can be exploited to regulate both the rate and selectivity of the reaction. We report here the photothermal catalytic hydrogenation of CO₂ using systems with selectivities: Cu/ZnO/Al₂O₃ and Ni/TiO₂. Cu/ZnO/Al₂O₃ is a common catalytic system for methanol synthesis that, containing a photocatalytically active phase like ZnO, shows great promise for photothermal activation. In turn, Ni is a low-cost alternative to noble metals for hydrogenation reactions. Ni/TiO₂ catalysts can achieve high performance and, interestingly, it has been found that metal support interactions can strongly modify the selectivity under photothermal conditions, changing the proportion of CO and CH₄ generated [2].

The main products of the thermal reaction over Cu/ZnO/Al₂O₃ are CH₃OH and CO. Cu content modulates the reaction: increasing Cu content reduces CO₂ conversion, while intermediate Cu loadings lead to higher CH₃OH selectivities. Light exerts a positive effect on activity and selectivity, improving CO₂ conversion and favouring CH₃OH production. The observed changes cannot be explained by a thermal effect of light, which suggests it also induces charge transfer processes that favour CH₃OH production. From in-situ XAS during TPR, it is deduced that increasing Cu wt.% leads to a faster reduction and lower amount of Cu+ intermediate species. NAP-XPS under reaction conditions shows light-induced charge transfer between Zn and Cu centres. Analysis of the surface species suggests that CO₂ adsorbs via carbonate/bicarbonate species and that CO₂ hydrogenation proceeds through the formation of CO₂^-  species upon charge transfer from Cu centres.

Good dispersion of Ni over TiO₂ is confirmed by the presence of clusters of Ni of about 5 nm seen by XEDS mapping for the catalyst with the higher metal loading. Ex-situ Ni 2p XPS spectra of the samples afer reaction show in the two samples a major contribution of Ni²⁺ with a small amount of metallic Ni, consistent with metal passivation in air. Similar Ni²⁺ contributions have been related to Ni²⁺ interacting with TiO₂ supports or coordinatively unsaturated Ni²⁺ in NiO. The activity increases with the metal content. Thus, at 350 °C and under UV irradiation the conversion of CO₂ for 10%Ni/TiO₂ is more than 10 times higher than for the catalysts lower loading. At this temperature the enhancement of the activity by UV light is modest, but at 150 °C reaches more than 8%. The Ni content has high impact on selectivity. At 250 °C, the CH₄ selectivity is about 72 % for 10%Ni/TiO₂, while it is lower with lower metal loading, which yields mainly CO [3].

This work demonstrates that the combination of light and heat has a synergistic effect on catalytic CO₂ hydrogenation: light produces both thermal and photonic effects that modify the reaction mechanism. Photothermal activation gives rise to new mechanistic routes and new possibilities for selectivity control.