Role of N Doping in the Reduction of Titania Nanostructures.

The effect of N-doping of titania (TiO2) nanoparticles (NPs) on their reduction through neutral O vacancy (Ovac) formation is investigated using all electron density functional theory-based calculations, including hybrid density functionals, and taking the bipyramidal anatase (TiO2)84 NP as a realistic model. The location of the N dopant is systematically analyzed, including O substitution in the (TiO2)84 structure and N occupying interstitial regions. Our computational study concludes that interstitial N doping is more favorable than N substituting O atoms and confirms that the presence of N reduces the energy gap. In the N-doped NP, Ovac formation is more favored than in undoped NP but less than in the N-doped bulk, which has important consequences.

Adsorption of Prototypical Asphaltenes on Silica: First-Principles DFT Simulations Including Dispersion Corrections.

In this work, we explore the interaction between some prototypical asphaltene and porphyrin molecules with a fully hydroxylated (0001) surface of α-quartz by means of theoretical calculations based on the density functional theory (DFT) under periodic boundary conditions. The influence of dispersion forces, adsorption geometries, and size of the side chain is analyzed. The inclusion of London dispersion forces is overriding as they increase the interaction by about 1 order of magnitude. All of the considered molecules strongly interact with the hydroxylated surface and prefer to adsorb in a parallel position instead of vertically. It is also found that adsorption energy always increases with larger side chains because dispersion interactions also augment. Interestingly, in the case of porphyrin, the less stable isomer in the gas phase is the preferred one after adsorption, which is substantiated by a differential stabilization induced by the surface. Finally, we present a comparative study of the adsorption of these model molecules in terms of energy per area unit and energy per interacting π electron.

Potassium and Water Coadsorption on TiO2(110): OH-Induced Anchoring of Potassium and the Generation of Single-Site Catalysts.

Potassium deposition on TiO2(110) results in reduction of the substrate and formation of loosely bound potassium species that can move easily on the oxide surface to promote catalytic activity. The results of density functional calculations predict a large adsorption energy (∼3.2 eV) with a small barrier (∼0.25 eV) for diffusion on the oxide surface. In scanning tunneling microscopy images, the adsorbed alkali atoms lose their mobility when in contact with surface OH groups. Furthermore, K adatoms facilitate the dissociation of water on the titania surface. The K-(OH) species generated are good sites for the binding of gold clusters on the TiO2(110) surface, producing Au/K/TiO2(110) systems with high activity for the water-gas shift.