A Multitechnique Study of CO Adsorption on the TiO2 Anatase (101) Surface

The adsorption of carbon monoxide on the anatase TiO2 (101) surface was studied with infrared reflection absorption spectroscopy (IRRAS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory (DFT). The IRRAS data reveal only one CO band at ∼2181 cm–1 for both stoichiometric and reduced TiO2 (101) surfaces. From TPD, an adsorption energy of 0.37 ± 0.03 eV is estimated for the isolated molecule, which shifts to slightly smaller values at higher coverages. Combining STM imaging and controlled annealing of the sample confirms the adsorption energies estimated from TPD and the slight repulsive intermolecular interaction. CO molecules desorb from electron-rich, extrinsic donor defect sites at somewhat higher temperatures. Confronting the experimental results with DFT calculations indicates that the anatase (101) surface does not contain any significant concentration of subsurface oxygen vacancies in the near-surface region. Comparison with CO adsorption on the rutile TiO2 (110) surface shows that the tendency for excess electron localization in anatase is much weaker than in rutile.