The Nature Of Pt/TiO₂ Interfacial Interaction And Its Role In Catalytic Dissociation Of Methanol From First Principles Theory

The focus of this dissertation is to investigate the interaction of platinum and anatase in Pt/TiO2 as well as their catalytic activities toward methanol adsorption and dissociation using first principles based slab calculations. The results from this study will be helpful to understand the existing catalysts and useful to improve the design of new and highly efficient catalysts. Furthermore, the results of methanol adsorption and dissociation over Pt/TiO2 are essential to the understanding of the overall mechanism of hydrogen production from methanol.When a single Pt atom adsorbed on the perfect anatase TiO2(101) surface, it prefers the bridge site formed by two neighbor 2-coordinated oxygen (2cO) atoms along the [010] direction, whereas Pt adatom prefers occupying the oxygen vacancy site when Pt was deposited on the defective anatase surface. The growth of Pt cluster on the perfect surface is three-dimensional whereas on the defective surface it depends on the ratio of oxygen vacancy density and Pt adatom coverage as well as the deposition temperature. If the density of the oxygen vacancy on the anatase surface is higher than the coverage of the Pt adatoms, these adatoms will preferably occupy the oxygen vacancy sites until all the vacancies are filled. Further increasing the Pt adatom coverage beyond the density of oxygen vacancy, the additional Pt adatom will bind to the existing adatom in the vacancy site to form an adsorbed Pt dimer. However, the advantage of the oxygen vacancy as the nucleation center is expected to diminish eventually as the size of the cluster is increased. The clustering energy of supported Pt clusters on anatase TiO2 depends on the strength of interactions between Pt and the anatase surface when the number of Pt atoms in the cluster is less than four. As the cluster grows bigger, the Pt-Pt interaction dominates the contribution to the clustering energy.The strongest interaction between molecular methanol and Pt/perfect-TiO2 system occurs at the interface where methanol molecule interacts with Pt and TiO2 simultaneously. The synergistic effect maximized the stability of the molecularly adsorbed methanol on Pt/perfect-TiO2. On the clean perfect anatase TiO2(101) surface, methanol dissociation via O-H bond scission has a small activation barrier of 0.51 eV. As such, a equilibrium between molecularly adsorbed methanol and coadsorbed methoxy and hydroxyl is expected at room temperature. In contrast, the activation barrier for C-O bond cleavage is 2.56 eV. The presence of Pt clusters enhances the dissociative adsorption of methanol on the perfect anatase surface. On the clean defective surface, methanol dissociation at the oxygen vacancy site via either C-O or O-H bond breaking produces the same product state: coadsorption of methyl and hydroxyl. After the oxygen vacancy sites on the defective surface were occupied by Pt clusters, these active sites for methanol dissociation became unavailable. Consequently, methanol was forced on to less active site, making the dissociative adsorption on Pt/defective-TiO2 less favorable than that on the clean defective anatase surface.