Abstract
Quantitative distributions were determined for the oxidation states of titanium cations on TiO2(001) surfaces reduced by argon ion bombardment and reoxidized by thermal treatment. Information about the inhomogeneity 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. These experiments demonstrated that (1) sputtering with 2 keV ions produced a surface containing Ti cations in the 1+, 2+, 3+, and 4+, but not 0 oxidation states; (2) these were reoxidized in a roughly sequential fashion as the surface was annealed to 750 K, at which temperature reoxidation to Ti4+ was complete; and (3) the average oxidation state of 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 organic assembly processes, including reductive coupling of aldehydes to form symmetric olefins, these surface characterization results demonstrate that Ti0 sites are not required to effect reductive coupling, in contrast to conclusions from previous slurry-phase studies. These observations enhance the possibility that reductive carbonyl coupling might be rendered catalytic, since a smaller cycle of oxidation states is required than previously recognized.