Adsorption of formic acid on rutile TiO2 (110) revisited: An infrared reflection-absorption spectroscopy and density functional theory study

Abstract

Adsorption of formic acid on rutile TiO2 (110) revisited: An infrared reflection-absorption spectroscopy and density functional theory study. Journal of Chemical Physics AbstractFormic acid (HCOOH) adsorption on rutile TiO 2 (110) has been studied by s-and p-polarized infrared reflection-absorption spectroscopy (IRRAS) and spin-polarized density functional theory together with Hubbard U contributions (DFT+U) calculations. To compare with IRRAS spectra, the results from the DFT+U calculations were used to simulate IR spectra by employing a three-layer model, where the adsorbate layer was modelled using Lorentz oscillators with calculated dielectric constants. To account for the experimental observations, four possible formate adsorption geometries were calculated, describing both the perfect (110) surface, and surfaces with defects; either O vacancies or hydroxyls. The binding energy was found to be E bind = 1.84 eV for the bridging bidentate formate species, which bonds with its IRRAS spectra measured on surfaces prepared to be either reduced, stoichiometric, or to contain surplus O adatoms, were found to be very similar. By comparisons with computed spectra, it is concluded that formate binds to in-plane Ti 5c atoms rather than to O vacancy sites. The results emphasize the importance of protonation and reactive surface hydroxylseven under UHV conditions -as reactive sites in e.g. catalytic applications.

Infrared spectroscopy study of adsorption and photodecomposition of formic acid on reduced and defective rutile TiO2 (110) surfaces

“…The crystal was mounted in the UHV chamber as delivered and cleaned by repeated Argon ion sputtering (1 lA ion current at 1 keV for 10 min at 323 K) and annealing cycles (973 K for 20 min), 7,10,15,27,28 which resulted in a reduced crystal (denoted r-TiO 2 ) that has been reported to have a O vacancy surface coverage of the order h % 0.05. 7,27,29 A rough surface was also prepared, where the final annealing cycle was omitted (denoted sp-TiO 2 ).…”

“…This dosage time yields a formic acid coverage of approximately 1 monolayer. 15 UV irradiation of the samples was done using the 365 nm emission line from a Hg(Xe) lamp (Oriel Instruments) in combination with a narrow band pass filter (Oriel Instruments, FWHM ¼ 10 nm). A water filter was used to remove the IR part of the radiation.…”

“…15 Briefly, it consists of a vacuum-pumped FTIR spectrometer (Bruker IFS66v/S) equipped with an external HgCdTe liquid nitrogen cooled detector connected to an UHV chamber via KBr windows thus allowing the entire beam path to be under vacuum. The incident angle of the IR light is 85 with respect to the surface normal of the sample.…”

“…20 Here, we use, The majority of adsorbed formate ions bond through the two oxygen atoms to two Ti 5c atoms along the troughs in the [100] direction in a bridging bidentate fashion, thus creating a (2 Â 1) overlayer on the rutile surface. 7,15 Formate has also been reported to bind to oxygen vacancy sites in the O ridges along the [001] direction, whereby the O-C-O molecular plane is rotated into the [1 10] direction, 13 or to adsorb as monodentate hydrogen bonded species oriented perpendicular to the [001] direction to protonated two-fold coordinated bridging O 2c atoms. 15,24,25 Aizawa et al 26 also reported on another adsorption structure, where the formate binds to an O vacancy to form a monodentate structure.…”

“…9,10 Grazing incidence infrared reflection-absorption spectroscopy (IRRAS) has been used in a few studies to study adsorbate structures on TiO 2 . [12][13][14][15][16][17][18][19] IRRAS has the advantage of being surface sensitive and capable of identifying adsorbed molecular species and their coordination, based on their unique fingerprint spectra, using different polarizations of the IR light and crystal orientations. In general, one distinguishes between measurements on metal and semiconductor surfaces.…”