Exploring the CO2 conversion into hydrocarbons via a photocatalytic process onto M-doped titanate nanotubes (M = Ni and Cu)

A combined theoretical and experimental work was performed to assess the carbon dioxide (CO2) evolution reaction into short chain hydrocarbons. The theoretical calculations were performed by using Density Functional Theory (DFT) at the DFT + U level. The reaction mechanisms were elucidated with the string method by comparing the photocatalytic behavior of the pristine Ti-NT surface, previously synthesized in our group, and the M-doped Ti-NT (M-Ti-NT, where M = Cu, Ni) systems. For the pristine material, the results showed lower adsorption energies of the CO2 molecule (-0.27 eV), as compared to that obtained with the M-doped Ti-NT systems. Ni-Ti-NT showed an enhancement in photocatalytic performance with respect to the other surfaces, by yielding small activation energies throughout the reaction path. On the experimental side, Ti-NT and M-Ti-NT (M = Cu, Ni) materials were characterized through several techniques to assess their structural, morphological, textural, and optoelectronic properties. The photocatalytic CO2 reduction was evaluated under wavelength illumination between 440-540 nm. The liquid solar fuel identified products were HCOOH, CH2O, and CH3OH, showing a different distribution among photocatalysts which correlates with the position of the conduction band of the photocatalysts. Doping with Cu and Ni of the Ti-NT structure enhances the carriers' density which improves the photoactivity mainly in the case of Ni-Ti-NT. The photocatalytic experimental results agree with the theoretical calculations.