CHARACTERIZATION OF TIO(2) SURFACES ACTIVE FOR NOVEL ORGANIC SYNTHESES

Quantitative distributions were determined for the oxidation states of titanium cations on TiO2(001) surfaces reduced by argon ion bombardme nt and reoxidized by thermal treatment. Information about the inhomoge neity of these distributions within the near-surface region sampled by XPS was obtained by angle-resolved measurements in which the position of the analyzer axis was varied with respect to the surface normal. T hese experiments demonstrated that (1) sputtering with 2 keV ions prod uced a surface containing Ti cations in the 1+, 2+, 3+, and 4+, but no t 0 oxidation states; (2) these were reoxidized in a roughly sequentia l fashion as the surface was annealed to 750 K, at which temperature r eoxidation to Ti4+ was complete; and (3) the average oxidation state o f the topmost layers of the ion-bombarded surface was slightly higher than that determined from XPS spectra collected normal to the surface. Since the surfaces reduced by argon ion bombardment are active for or ganic assembly processes, including reductive coupling of aldehydes to form symmetric olefins, these surface characterization results demons trate that Ti0 sites are not required to effect reductive coupling, in contrast to conclusions from previous slurry-phase studies. These obs ervations enhance the possibility that reductive carbonyl coupling mig ht be rendered catalytic, since a smaller cycle of oxidation states is required than previously recognized.