CO ₂ Adsorption and Photocatalytic Reduction Mechanisms on TiO ₂ -Terminated CaTiO ₃ (100): A Density Functional Theory Study

Photoreduction is an important approach aimed at reducing the CO₂ atmospheric content, which is responsible for global warming. The development of an efficient photocatalyst can strongly improve the efficiency and selectivity of the byproducts of such a process. Recently, CaTiO₃ has been used as an alternative semiconductor catalyst due to its attractive properties. In this study, we present first-principles electronic structure calculations to investigate the general reaction mechanism that leads to the main value-added HCOOH, CO, CH₃OH, and CH₄ byproducts, focusing on the reactions of the adsorption, activation, and reduction reactions of molecules on the TiO₂-terminated CaTiO₃ (100). Our results show that CO₂ can be activated by charge transfer of excess electrons, leading to a CO₂ ^·- anion that can give the formate (HCOO^-) intermediate by first reduction. However, the second hydrogenation leading to HCOOH is impeded by the prohibitive energy barrier; in particular, activated CO₂ can also easily undergo decomposition, which facilitates CO production. Afterward, we discuss the possible reaction mechanisms of CO photoreduction toward CH₃OH and CH₄ value-added products, taking into account the experimental evidence that only CO and CH₄ have been detected. The reaction pathway generally follows the most energetically convenient routes characterized by activated intermediates. Although CH₃OH could finally be produced, its strong adsorption and promoted decomposition to CH₃O + H on the surface could explain why it has not been detected, compared to the more volatile CH₄ molecule, ascribed by its nonpolar nature.