(348g) Two Distinct Types of Surface-Bound Atomic Hydrogen and Their Role in Dictating Product Selectivity in Artificial Photosynthesis Reactions

Understanding the surface reaction mechanisms, the nature of surface reaction sites, and the chemical physics behind differently-promoted hydrogen transfer steps in the photocatalytic reduction of CO₂ and H₂O is critical to enable the systematic design and improvement of catalysts for the production of higher hydrocarbons and oxygenates from solar energy. Our recent studies indicate that the product distribution, e.g., H₂ and CO vs. CH₄, in the photocatalytic reduction of CO₂ is connected to two distinct types of hydrogen transfer steps â?? electrochemically-coupled H-transfer and excited-electron mediated H-transfer. Presence of protonic hydrogen (H⁺) on photocatalyst surface correlated with electrochemically-coupled H-transfer steps to preferentially produce H₂. The electrochemical steps for CO production in this case likely follow similar chemical physics. On the contrary, when metallic hydrogen (H⁰) is encountered, results suggest that the transfer of H⁰ to surface-bound organic fragments occur via excited electron induced vibrational excitation of the uncharged H⁰, resulting in production of CH₄. A clear connection was observed between the presence of H⁰ on surface and CH₄ evolution, and conversely, the presence of H⁺ on surface and H₂ evolution. In our photoreaction experiments, we were able to modify the catalyst selectivity from CH₄ to H₂ evolution by transforming the adsorbed metallic H⁰ into protonic H⁺ by incorporating more electronegative reaction sites into the photocatalyst surface. Theoretical calculation results suggested a shift from excited electron-mediated H⁰ transfer phenomena to electrochemical H-transfer phenomena resulting in H₂ evolution. Understanding the electronic nature of adsorbed hydrogen on photocatalyst surface and the nature of surface reaction sites will potentially lend insights into tuning the overall photocatalytic product distribution, which may lead to the sustainable production of higher hydrocarbons and oxygenates from CO₂ and H₂O using solar radiation.