Sum frequency generation spectroscopic investigation of TiOx/Pt(111):: Surface active sites and reaction paths probed by formate

The catalytic behavior of ultrathin titanium oxide films deposited on platinum (111) surfaces is investigated through sum frequency generation (SFG) spectroscopy of the adsorption and decomposition of formate. TiOx films of 7 angstrom or less in thickness are prepared on Pt(111) substrates by evaporation of titanium under oxygen, and samples are examined as-prepared or after annealing at 873 or 973 K. Formate is adsorbed to the samples by exposure to 1000 langmuir of formic acid at 250 K. The frequency of the stretching vibration (v(C-D)) of the deuterated formate C-D bond is found to be sensitive to the interaction between the adsorption sites and formate. SFG observations under different polarization combinations reveal that most formate is adsorbed to the surface with a bridge configuration, and that only a small amount of monodentate formate is present on the surface of TiOx/Pt(111) annealed at 973 K. Investigation of the effect of annealing temperature, time, and atmosphere on formate v(C-D) frequency and intensity, in combination with low-energy electron diffraction (LEED) observations, reveals three kinds of surface active sites: defect sites, disordered titanium ion sites, and ordered 5-fold coordinated Till sites. These adsorption sites are responsible for v(C-D) responses at frequencies of 2134-2184, 2195, and 2210-2226 cm(-1), respectively. The defect sites are suggested to be located at the boundary between the disordered phase and the ordered phase of titanium oxide. Upon heating, formate adsorbed to defect sites undergoes decomposition at temperatures below 250 K, while formate adsorbed to ordered 5-fold coordinated Ti4+ sites decomposes via reaction paths: dissociation via monodentate formate as an intermediate, and diffusion onto defect sites before dissociation. The temperature of direct decomposition of bridge formate adsorbed to Ti4+ sites appears to be related to the formate v(C-D) frequency, with lower frequencies corresponding to higher decomposition temperatures.