Carbon-carbon bond formation on titanium oxide surfaces: Dependence of activity on titanium oxidation state.

摘要

Surface science has made significant contributions to the understanding of a number of fundamental topics pertinent to the field of heterogeneous catalysis. One such area is in the understanding of the relationship between surface structure and reactivity. Previous work on metal oxide surfaces has examined structure-reactivity relationships based on the coordination environment of the surface metal cations. In this work, the dependence of reactivity on the oxidation state of Ti cations on reduced TiO{dollar}sb2{dollar} (001) surfaces is examined, focusing on reactions involving carbon-carbon bond formation.; This work is divided into two sections, investigating the reactions of oxygenates and unsaturated hydrocarbons on reduced TiO{dollar}sb2{dollar} (001) surfaces. The predominant reaction observed for carbonyl compounds on reduced TiO{dollar}sb2{dollar} surfaces was reductive bimolecular coupling to form symmetric olefins. The yield of bimolecular coupling products was found to correlate directly with the average extent of surface reduction. This requirement for gas-solid reductive bimolecular coupling is different than that proposed for this reaction with titanium-based liquid-solid reagents, where Ti{dollar}sp0{dollar} centers are thought to be the active site. While the site requirements for the gas-solid and liquid-solid reactions appear to conflict, evidence implicating a common pinacolate reaction intermediate was uncovered in this research.; The reactivity of a variety of alkynes on reduced TiO{dollar}sb2{dollar} (001) surfaces was examined. The predominant species observed was the aromatic product produced by cyclotrimerization. The yield of cyclotrimerization product on surfaces containing Ti cations in varying amounts was closely correlated to the population of Ti(+2) cations. This site requirement for cyclization can be explained if the reaction involves a metallacyclopentadiene intermediate, commonly observed for this reaction when carried out with organometallic complexes in solution. Studies of the reactivity of allene on reduced TiO{dollar}sb2{dollar} (001) surfaces are also presented. Steady-state reaction experiments investigating the cyclotrimerization of methylacetylene on reduced TiO{dollar}sb2{dollar} surfaces demonstrated that the cyclotrimerization reaction could be executed catalytically, with experiments exhibiting up to 20 turnovers without deactivation of the catalyst. This represents the first example of catalytic carbon-carbon bond formation on an oxide surface in ultra-high vacuum.