(202c) The Role of Surface Chemical Reactivity in Artificial Photocatalysis

Surface chemical reactivity is the engine that drives heterogeneous catalytic reactions. However, the degree at which innate surface chemical reactivity determines the overall catalytic performance in heterogeneous photocatalytic reduction of CO₂ and water-splitting reactions remains elusive. A combination of the first principles quantum chemical modeling and experiments has been applied to investigate the role of surface chemical reactivity in promoting overall catalytic activity and controlling the selectivity in photocatalytic CO₂ reduction towards the production of CH₃OH, CH₄, and H₂ production. Photocatalytic reduction of CO₂ for the production of CH₃OH, CH₄, H­₂ over TiO₂, SiC, GaN:ZnO, CdS, GaP, and Bi₂S₃ was investigated.  We found that the energetics for CO₂ and H₂O dissociation, the stability of organic intermediates, and the chemical nature of surface bound atomic hydrogen collectively contribute to the selectivity of the reaction.  For instance, organic intermediates are found to be stable on SiC due to its greater surface chemical reactivity towards carbon.  This enhanced stability correlates well with SiC ability to selectively produce CH₄. On the contrary, moderate surface chemical reactivity towards oxygen and hydrogen, in case of Bi₂S₃ and CdS, appears to correlate with their ability to selectively produce CH₃OH.  Results suggest that CH3OH production hinges upon inhibiting the second C-O bond-breaking step.  Finally, the catalysts with low surface chemical reactivity towards oxygen and hydrogen – TiO₂ and GaN – preferentially produce H2 since all organic intermediates are unstable and rapidly consumed in reverse reactions.