| Summary: | A comprehensive mechanistic insight into the photocatalytic reduction of CO<sub>2</sub> by H<sub>2</sub>O is indispensable for the development of highly efficient and robust photocatalysts for artificial photosynthesis. This work presents first-principles mechanistic insights into the adsorption and activation of CO<sub>2</sub> in the absence and presence of H<sub>2</sub>O on the (001), (010), and (110) surfaces of tantalum nitride (Ta<sub>3</sub>N<sub>5</sub>), a photocatalysts of significant technological interest. The stability of the different Ta<sub>3</sub>N surfaces is shown to dictate the strength of adsorption and the extent of activation of CO<sub>2</sub> and H<sub>2</sub>O species, which bind strongest to the least stable Ta<sub>3</sub>N<sub>5</sub>(001) surface and weakest to the most stable Ta<sub>3</sub>N<sub>5</sub>(110) surface. The adsorption of the CO<sub>2</sub> on the Ta<sub>3</sub>N<sub>5</sub>(001), (010), and (110) surfaces is demonstrated to be characterized by charge transfer from surface species to the CO<sub>2</sub> molecule, resulting in its activation (i.e., forming negatively charged bent CO<sub>2</sub><sup>−δ</sup> species, with elongated C–O bonds confirmed via vibrational frequency analyses). Compared to direct CO<sub>2 </sub>dissociation, H<sub>2</sub>O dissociates spontaneously on the Ta<sub>3</sub>N<sub>5</sub> surfaces, providing the necessary hydrogen source for CO<sub>2</sub> reduction reactions. The coadsorption reactions of CO<sub>2</sub> and H<sub>2</sub>O are demonstrated to exhibit the strongest attractive interactions on the (010) surface, giving rise to proton transfer to the CO<sub>2</sub> molecule, which causes its spontaneous dissociation to form CO and 2OH<sup>−</sup> species. These results demonstrate that Ta<sub>3</sub>N<sub>5</sub>, a narrow bandgap photocatalyst able to absorb visible light, can efficiently activate the CO<sub>2</sub> molecule and photocatalytically reduce it with water to produce value-added fuels.
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