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Understanding the structure of well-defined anatase TiO 2 surfaces is critical for deciphering site-specific thermal and photoreaction mechanisms on anatase TiO 2 . Using ultrahigh vacuum scanning tunneling microscopy (STM), we have studied the atomic structure of anatase TiO 2 (001) epitaxial thin films grown by oxygen plasma assisted molecular beam epitaxy. Bright rows of the (1×4) reconstructed surface are resolved as three types of features with different sizes. High-resolution STM images taken from the same area at different bias voltages show that these individual features are originated from combinations of two basic atomic building blocks. We propose a modified added molecule model for the anatase TiO 2 (001) surface structure.

Direct imaging of site-specific reactions of individual molecules as a function of temperature has been a long-sought goal in molecular science. Here, we report the direct visualization of molecular coupling of formaldehyde on reduced rutile TiO 2 (110) surfaces as we track the same set of molecules using scanning tunneling microscope (STM) when the temperature is increased from 75 to 170 K. Our previous study showed that formaldehyde preferably adsorbs at bridgingbonded oxygen (O b ) vacancy (V O ) defect site. Herein, images from the same area as the temperature is increased show that V O -bound formaldehyde couples with Ti-bound formaldehyde forming a diolate intermediate. Consequently, exposure of formaldehyde at room temperature leads to diolate as the majority species on the surface and no V O -bound formaldehyde is observed. The diolate species are the key reaction intermediates in the formation of ethylene reported in previous ensemble-averaged studies.

The formation and coupling of methylene upon dissociation of formaldehyde on reduced TiO 2 (110) are studied using variable temperature scanning tunneling microscopy (STM). In agreement with prior studies, formaldehyde preferably adsorbs on the bridging-bonded oxygen vacancy (V O ) defect site. V O -bound formaldehyde couples with Ti-bound formaldehyde forming a diolate species, which appears as the majority species on the surface at 300 K. Here, STM images directly visualize a low-temperature coupling reaction channel. Two V O -bound formaldehyde molecules can couple and form Tibound species, which desorbs above ∼215 K. This coupling reaction heals both V O sites indicating the formation and the desorption of ethylene. We also directly observed the diffusion of methylene groups to nearby empty V O sites upon dissociation of the C−O bond in V O -bound formaldehyde, which suggests that the ethylene formation occurs via coupling of the methylene groups. Statistical analysis shows that the sum of visible reaction products on the surface can only account for a half of the consumption of the initial V O coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposure between 215 and 300 K. ■ INTRODUCTIONChemical reactions of aldehyde on reducible metal oxides are important in numerous catalytic reactions in the applications related to the degradation of toxic compounds, air purification, and chemical synthesis. 1−3 For instance, reductive coupling of carbonyl species has proven to be a versatile strategy for the functionalization of vicinal carbon atoms in organic molecules. 3−5 Adsorption and reaction of various oxygenated hydrocarbons on well-defined surfaces have been extensively studied to provide unparalleled understanding of the reaction mechanisms. 6−10 Production and conversion of simple C1 molecules such as methane, methanol, formaldehyde and formic acid are involved in many of these reactions. Hence the reactivity of formaldehyde has been studied on many metal 11−17 and metal oxide surfaces 18−24 . On metal surfaces, formaldehyde generally dehydrogenates to form CO via various intermediates such as formyl, methoxy, and formate. The coupling of aldehydes to form alkenes has been identified on TiO 2 , 6,9,18,23 UO 2 , 20 V(100), 11 and Mo(110). 12 On these surfaces, the coupling reaction proceeds through the formation of diolates via C−C coupling of two aldehydes. 6,9,12,18,20 A distinct low-temperature pathway was determined on V(100), which occurs via the coupling of methylene groups formed upon dissociation of the C−O bond in adsorbed formaldehyde. 11 On reduced TiO 2 (110) surfaces, several ensemble-averaged experimental studies reported the C−C coupling of CH 2 O forming a diolate (−OCH 2 CH 2 O−) species, followed by ejection of the olefin at high temperature (∼600 K). 18,23 There is currently a controversy regarding the role of the defects including the bridging-bonded oxygen (O b ) vacancy (V O ) and Ti interstitial in this coupling reaction. 6,18,...

We have studied the dynamic relationship between acetone and bridge-bonded oxygen (Ob) vacancy (VO) defect sites on the TiO2(110)-1 × 1 surface using scanning tunneling microscopy (STM) and density function theory (DFT) calculations. We report an adsorbate-assisted VO diffusion mechanism. The STM images taken at 300 K show that acetone preferably adsorbs on the VO site and is mobile. The sequential isothermal STM images directly show that the mobile acetone effectively migrates the position of VO by a combination of two acetone diffusion channels: one is the diffusion along the Ob row and moving as an alkyl group, which heals the initial VO; another is the diffusion from the Ob row to the five-coordinated Ti(4+) row and then moving along the Ti(4+) row as an acetone, which leaves a VO behind. The calculated acetone diffusion barriers for the two channels are comparable and agree with experimental results.