271258 Theoretical Insights On the Role of Defects in TiO2 Surface Chemistry
TiO2 has long been considered a prototypical metal oxide for catalytic processes. The nature and role of defects has been a rich area of study, both experimentally and theoretically. These defects, such as O vacancies or Ti interstitials, may lead to surface reduction and drastically change the surface chemistry of TiO2 through both direct (such as serving as reaction sites) or indirect (such as providing excess electrons to the lattice) means. Density functional theory (DFT) simulations were used to study several types of defects in TiO2surfaces. We elucidate the nature of these defects and quantify their role in surface reactivity.
By applying DFT+U we were able to selectively control the localization of electrons at Ti sites on rutile (110) surfaces; Ti4+ is reduced to Ti3+ when point defects (such as O vacancies or surface hydroxyls) form. DFT incorrectly predicts delocalized states while DFT+U correctly forms localized gap states upon O vacancy formation. Our results show that a manifold of nearly degenerate states may arise and that a Boltzmann-like distribution is appropriate for describing reduced TiO2. We have also examined the adsorption and reactivity of several adsorbates over reduced anatase and rutile surfaces. The transfer of electrons to adsorbates from Ti3+ centers is observed, indicating that defects such as vacancies (which create Ti3+) may enable electron transfer to adsorbates, and thus change surface chemistry in an indirect manner. We observed the transfer of these electrons to significantly affect the adsorption and reactivity (such as O2 reacting with surface hydroxyls) over the surface. Finally, surface reduction may result from hydroxyl formation, and these hydroxyls may also strongly influence or participate in surface reactions. We have developed an innovative experimental technique that creates surfaces with high hydroxyl coverage by photodecomposition of adsorbed carboxylic acids. Upon annealing to higher temperatures (relevant during catalytic cycles) these hydroxyls may potentially desorb as H2, desorb as H2O or diffuse into the bulk TiO2. The fate of these surface hydroxyls is modeled using DFT, and we show that entropic effects are crucial to determining the stability of surface hydroxyls over TiO2.
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