(351a) Deciphering the Chemical and Physical Mechanisms Active in Elevated Temperature Photocatalytic Synthesis of Hydrocarbons and Ammonia

The photocatalytic synthesis of hydrocarbons, oxygenates, and NH₃ from CO₂, N₂, NO/NO₂, and H₂O is a truly sustainable route to fuels and building block chemicals production. Unfortunately, the community still lacks the ability to rationally improve overall rates of reaction and control product distributions due to a lack of reaction mechanism understanding. Endemic issues with organic contamination in experimental studies has further contributed to this lack of progress. Our combined computational/experimental studies of photocatalytic CO₂ reduction by H₂O in organic-free reaction conditions at an appreciable temperature range (gas phase reaction at ~100-400^oC+) have shed significant light upon the surface chemical and electronic properties that affect these reaction mechanisms. Moreover, studies have also isolated new H-transfer mechanisms. Both of these developments have the power to significantly aid in the design of these materials. The new H-transfer mechanisms and an understanding of the role of surface chemistry have further enabled control over the selectivity of surface-bound atomic H between hydrogenation and H₂ evolution -- a critical efficiency aspect of the system. Additionally, key carbon-containing reaction intermediates that allow for carbon-carbon coupling have been isolated as well. Catalysts shown to be active at elevated temperatures indicate semiconductors with a high Debye temperature, robust bulk bonding, and elevated surface chemical reactivity may lend additional tunable features for photocatalytic synthesis mechanisms. With the ultimate aim of producing higher hydrocarbons, oxygenates, and ammonia photocatalytically from simple thermodynamic-minimum reactants, these studies have mapped the course forward for materials selection and mechanistic understanding.