Catalytic Ammonia Synthesis over Pure, Defective, and Metal-Doped Rutile TiO₂: A Periodic DFT Study

In this work, we aim to describe the energetics associated with the formation of ammonia from N₂ interacting with doped hydroxylated rutile TiO₂(110) surfaces with the vacant O_2c site, following the reaction N₂ + 3H₂O → 2NH₃ + 3/2O₂. The water molecules interact with the surface, creating exposed Ti-OH groups that can transfer hydrogen to the adsorbed N₂ molecule. Two metal dopants are evaluated: Mo and Ta. For both metals, calculations show a dramatic decrease in the energy of most intermediates during the entire mechanism, leading to more favorable reaction mechanisms. Nonetheless, it is worth noting that when the Ti_6c site of the vacant site is doped with either Mo or Ta, there is a stronger effect on the energetics than doping on the exposed Ti_5c sites. The effect of increasing the concentration of metal dopants on the vacant site was also investigated. In this case, calculations indicate that a higher percentage of the dopant on the surface results in a more substantial decrease in the energy of most intermediates, suggesting that increasing the dopant content could be beneficial for the catalytic process.