Modeling the role of Ta-dopant and co-catalytic water for activation of CO2 on anatase TiO2(101)

Herein, we have performed DFT＋U calculations to investigate the adsorption and activation of CO₂ on a Ta-doped anatase TiO₂(101) and compared it to that on a pristine anatase TiO₂(101). Replacing Ti with a higher valence Ta-dopant results in an excess electron, which localizes on the neighboring Ti atom. We find that there is a slight increase in adsorption energy of CO₂ with Ta-doping, however, there is no significant change in the barriers for CO₂ activation. Further, presence of a water molecule enhances the adsorption energy and stabilizes the CO₂ molecule having both linear and bent configurations (activated). This stabilization is more pronounced for a activated CO₂ molecule, which is attributed to the strong H-bonding. Apart from being a reactant, water also acts as a co-catalyst and significantly reduces the barrier for CO₂ activation. This is true for both anatase TiO₂(101) and Ta-doped anatase TiO₂(101). We have also investigated the activation of CO₂ via hydrogen and two electron transfer, which constitutes the limiting step in the photochemical reduction of CO₂ to CH₄. Our calculations show that the barriers for CO₂ activation via the two-electron pathway are similar for both anatase TiO₂(101) and Ta-doped TiO₂(101). However, presence of a water molecule on Ta-doped TiO₂(101), not only stabilizes the CO₂ molecule but also reduces the barrier for CO₂ activation dramatically by acting as a co-catalyst.