The surface science of titanium dioxide

The interaction of water and methanol with well-defined (1 × 1) terminated surfaces of anatase-TiO2(101) were investigated with temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). For water, three desorption states were observed in the TPD spectra at 160, 190, and 250 K. The three desorption peaks were assigned to multilayer water, water adsorbed to 2-fold-coordinated O, and water adsorbed to 5-fold-coordinated Ti, respectively. The TPD spectra for methanol were more complicated. For methanol, five desorption peaks were observed in the TPD spectra at 135, 170, 260, 410, and 610 K. The five desorption peaks were assigned to multilayer methanol, methanol adsorbed to 2-fold-coordinated O, methanol adsorbed to 5-fold-coordinated Ti, methoxy adsorbed to 5-fold-coordinated Ti, and methoxy adsorbed to Ti at step edges, respectively. The XPS results indicated that the adsorbed water and methanol were predominantly bound to the surface in a molecular state, with no evidence for diss...

Experimental Investigation of the Interaction of Water and Methanol with Anatase−TiO2(101)

Organic Reactions at Well-Defined Oxide Surfaces.

http://pubs.acs.org/doi/pdf/10.1021/cr400357r

Catalytic reactions of acetylene: a feedstock for the chemical industry revisited.

Organic chemistry on solid surfaces

The Effects of Bulk Titania Crystal Structure on the Adsorption and Reaction of Aliphatic Alcohols

Previous studies have demonstrated that the binding and reactivity of aliphatic alcohols on TiO2 surfaces are controlled by local coordination environments at the binding sites. As a result, there ...

Influence of Surface Hydroxyls on the Adsorption and Reaction of Ethanol on Polycrystalline Titania

Previous temperature programmed desorption (TPD) experiments on reduced TiO2 (001) surfaces have demonstrated that alkynes are converted to the corresponding aromatic products with high selectivity. This reaction also represents the first example of catalytic assembly of carbon–carbon bonds on a metal oxide surface in ultrahigh vacuum. Although the catalytic formation of carbon–carbon bonds on single crystal surfaces is a rarity, many important catalytic processes involve carbon–carbon bond formation, and it is therefore worthwhile to consider how such reactions might be studied directly using the tools of surface science. Steady-state experiments involving the production of trimethylbenzene from methylacetylene at low pressure (10−9–10−5 mbar) conditions have demonstrated multiple turnovers of the catalyst and no significant catalyst deactivation at temperatures between 290 and 500 K. A four-step kinetic model is proposed, which contains three nonactivated steps for alkyne adsorption/reaction to form the aromatic, followed by the final step, aromatic desorption. This model captures the measured temperature and pressure dependence of the reaction rate.

Steady-state catalytic C–C bond formation on reduced TiO2 surfaces

Cyclotrimerization of alkynes to form aromatics can be carried out catalytically with low valent transition metal complexes in solution, with supported and single-crystal metal catalysts, and with partially reduced TiO2 surfaces. In each of these cases the reaction has been proposed to proceed via formation of a metallacyclopentadiene intermediate, i.e., by association of a pair of acetylene molecules to form a C4 ligand bound to the catalytic site. At least one group of authors has proposed that on late-transition metal surfaces a pair of C4 ligands combine to form the C8 ligand, cyclooctatetraene (COT). In this alternative mechanism, benzene is then formed by the elimination of a C2 fragment from the C8 intermediate. This reaction channel was previously examined on Pd(111) and Cu(110), and was found to be a minor or nonexistent channel on each, reinforcing the case for direct addition of a third acetylene molecule to the metallacyclopentadiene intermediate to form the aromatic product. We have examined ...

Investigation of Cyclooctatetraene on Reduced TiO2(001) as a Possible Intermediate in Alkyne Cyclotrimerization

The steady-state reaction of a molecular beam of methylacetylene impinging on a reduced TiO2(001) crystal was examined under UHV (10−9 to 10−8 mbar) conditions. Cyclotrimerization to form trimethylbenzene took place at steady-state. The reaction is truly catalytic; more than 20 turnovers could be obtained without deactivation of the single crystal catalyst. Experiments examining the cyclotrimerization rate as a function of pressure showed that, in the region from 3.0×10−9 to 2.4×10−8 mbar, the reaction was approximately first order in alkyne partial pressure. Temperature variation experiments showed a small decrease in rate as temperature was increased, indicating a slightly negative overall activation energy between 400 K and 535 K. A four-step kinetic model of the cyclotrimerization reaction was constructed using information derived from TPD experiments. These results demonstrate that steady-state catalytic studies of carbon-carbon bond forming reactions, characterized by an overall decrease in number of moles, can be performed on single crystal metal oxides under UHV conditions.

Victor S. Lusvardi

Papers

The Effects of Bulk Titania Crystal Structure on the Adsorption and Reaction of Aliphatic Alcohols

Influence of Surface Hydroxyls on the Adsorption and Reaction of Ethanol on Polycrystalline Titania

Steady-state catalytic C–C bond formation on reduced TiO2 surfaces

Investigation of Cyclooctatetraene on Reduced TiO2(001) as a Possible Intermediate in Alkyne Cyclotrimerization

Catalytic formation of carbon-carbon bonds in ultrahigh vacuum: Cyclotrimerization of alkynes on reduced TiO2 surfaces