Structure and chemical reactivity of adsorbed carboxylic acids on anatase TiO2(001)

The structure of the anatase TiO2(0 0 1) surface and its reactivity towards carboxylic acids were studied using in situ scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS) and temperature-programmed desorption (TPD). Following annealing in ultra-high vacuum, a (1 x 4) reconstruction developed that was visible in STM and LEED. At densest packing, a (2 x 4) overlayer of dissociated carboxylate groups was seen with STM. From the images, it was deduced that single carboxylates bind to positions centered atop (1 x 4) rows. TPD experiments revealed a range of reactivity depending on the preparation conditions. The fully oxidized anatase (0 0 1)-(1 x 4) surface bonded carboxylate groups strongly with no TPD products seen below 750 K. Surfaces that were not fully oxidized, or had been sputtered, converted formic acid to CO at 540 K and formaldehyde at 450 K. Sequential TPD experiments changed the surface activity, suppressing CO production, but not significantly affecting the sites responsible for formaldehyde production. The defective surface converted adsorbed acetate to CO at 535 K and ketene at 560 K. Sequential acetic acid TPD runs caused the reactivity towards both CO and ketene to diminish. These results are attributed to sputtering creating under-coordinated Ti4+ and Ti3+ sites that were revealed in XPS spectra and STM images. No bimolecular coupling reactions of acetic acid took place, indicating that the formaldehyde was produced via a reduction pathway. Both the location of the adsorbates and the lack of reaction products associated with fourfold coordinated Ti4+ raise doubts about current models of the (I x 4) reconstruction, (C) 2002 Elsevier Science B.V. All rights reserved.