Photocatalytic Efficiency for CO₂ Reduction of Co and Cluster Co₂O₂ Supported on g-C₃N₄: A Density Functional Theory...

It is well known from experimental results that a single atom of cobalt supported on g-C₃N₄ is an efficient photocatalyst for the reduction of CO₂ to CO, with a better photocatalytic activity than g-C₃N₄. In this work, we investigate the performance as catalysts for the CO₂ reduction of single atoms of cobalt and Co₂O₂ clusters supported on graphitic carbon nitride (g-C₃N₄). Employing density functional theory plus Hubbard (DFT + U) calculations, we investigate in detail the reduction mechanisms to CO and HCOOH for the first time. We find that deposition of cobalt on g-C₃N₄ decreases the work function of g-C₃N₄ to 6.6 eV and provides a better candidate for the reduction reaction. In addition, we find that the preferred product of CO₂ reduction on Co@g-C₃N₄ is CO, with a rate-determining barrier of 0.97 eV, while on Co₂O₂@g-C₃N₄, CO₂ reduces to formate with a rate-determining barrier of 0.44 eV. We determine the creation of CO₂ from COOH to only take place on Co₂O₂@g-C₃N₄, with a (relatively high) barrier of 2.27 eV. In order to obtain more easily the transition state energies of the reactions mentioned above, we applied machine learning methods to search for high-importance descriptors for these quantities, in the case of single transition metal atoms supported on C₃N₄. Interestingly, our results show that our quantities of interest are closely related to the adsorption energies of products and normalized valence electrons of the products of the elementary reactions as well as those of the metal atoms. The former of these two sets of features can be straightforwardly obtained via DFT, while the latter energies are extensively tabulated. Our results offer guidance for the design of catalysts and photocatalysts for CO₂ reduction on single-metal atoms supported on C₃N₄.