Photocatalytic reduction of CO 2 for the production of synthesis gas, hydrocarbons, alcohols, or aldehydes from only CO 2 , H 2 O, and solar radiation may be a promising route to recycle CO 2 waste at the source of its production, reduce atmospheric CO 2 through direct removal from Earth's atmosphere, and/or produce CH 4 rocket fuel on Mars. In this study, we present the first studies of high-temperature (350 °C) photocatalytic reduction of CO 2 in the gas phase over a suite of semiconductors with a range of surface chemical reactivity and bulk electronic properties, namely, SiC, Si, Pt/TiO 2 , and GaN. The results indicate that catalysts that exhibit high Debye temperatures and elevated surface reactivity may enable the production of new reaction intermediates that facilitate C−O cleavage and full CO 2 reduction to CH 4 . Elevated photocatalyst surface reactivity also correlated with dramatically improved selectivity between hydrogenation (CH 4 production) and H 2 evolution, for example, 1:8, 1:71, 1:400, and 1:1000+ for SiC, Si, Pt/TiO 2 , and GaN, respectively. Tests at low (250 °C) and high (350 °C) temperatures over top-performing catalysts, SiC and Si, show that both C−O cleavage and H 2 O dissociation may be thermally promoted, leading to enhanced CH 4 , CO, and H 2 production, further indicating surfacechemical and thermal-input contributions.
