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We have employed elevated temperature scanning tunneling microscopy to elucidate the reactivity and surface structure of the ͑1 3 2͒ reconstructed TiO 2 ͑110͒ surface. We find two distinctly different ordered surface reconstructions depending upon the level of bulk reduction of the crystal (degree of nonstoichiometry). On the near stoichiometric surface reactivity to oxygen is low and attributed to the formation of the stable Ti 2 O 3 type termination. For heavily reduced crystals a cross-linked ͑1 3 2͒ reconstruction forms with high activity to oxygen resulting in a well-defined cyclic reaction (TiO 2 growth). [S0031-9007(99)09062-6] PACS numbers: 68.35.Bs, 82.65.Jv, 82.65.My The ability to prepare conducting TiO 2 samples in UHV has lead to a wealth of studies on its surface structure and chemistry [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, there has been relatively little work exploiting the effect of varying the bulk stoichiometry on surface structure and reactivity [8]. The link between surface structure and bulk defect formation is strong in reducible d 0 metal oxides (TiO 2 , V 2 O 5 , MoO 3 , and WO 3 ) where defects can cluster and form crystallographic shear planes (CS) within a crystal [1,14,15]. These have recently been shown to terminate at the surface in a well-ordered array of half-height steps [16][17][18][19] on TiO 2 ͑110͒.There are at present two structural models considered to be in broad agreement with the wide range of techniques that have been used to probe the TiO 2 ͑110͒-͑1 3 2͒ reconstruction. The first, proposed by Onishi et al., is for an added row of stoichiometry Ti 2 O 3 which grows upon the ͑1 3 1͒ terrace in an oxygen ambient as a result of transport of Ti n1 interstitials to the surface [2,3,20]. More recently, a second added row model has been proposed by Pang et al. on the basis of atomic resolution STM data and theoretical calculations in which the added rows consist of strings of the fully reduced bulk termination ͑Ti 3 O 5 ͒ [9]. However, there are a number of reports in the literature of ͑1 3 2͒ rows that have been stabilized by cross-linking every few tens of Å [5,6,8]. Most notably these crosslinks are reported to be more prevalent after annealing a reduced surface in oxygen. Structural models proposed to explain the cross-links were based on a missing row reconstruction that has largely been rejected because of conflicting evidence by other techniques.In this Letter we propose that both models are, in essence, correct as we show that there are indeed two forms of the ͑1 3 2͒ reconstruction on the surface, the straight ͑1 3 2͒, and the cross-linked ͑1 3 2͒. With the aid of time resolved reoxidation scanning tunneling microscopy (STM) experiments on lightly and heavily reduced TiO 2 ͑110͒ crystals we show that the surface termination is dictated by bulk nonstoichiometry. These observations are also compatible with, and contribute to, recent reports of the reoxidation behavior of TiO 2 ͑110͒ [5,6,13]. The ability to image the reoxidation process in situ is ess...

We have used scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) to investigate the thermal stability of Pd(111) islands and thin films on TiO2(110)-(1 × 2). Two new nano-structures were observed to form on the surface of the Pd by annealing to 973 K. Atomic resolution STM images show a “pinwheel” super-structure. One domain has a unit cell with respect to the Pd(111) whereas the other domain has a unit cell. Coexisting with this phase is a structure consisting of zigzag rows that run along the close-packed directions of the Pd(111) islands. This has a rectangular unit cell incommensurate with both the substrate TiO2(110) and the Pd(111) islands. STM shows these two structures merge with no noticeable domain barriers or steps, suggesting a close relationship between the two. LEED shows several distinct, overlapping patterns that can be identified with the surface structures observed; TiO2(110)-(1 × 2), Pd(111)-(1 × 1), the hexagonal pinwheel structure, and the rectangular zigzag unit cell. XPS at normal and grazing emission show the encapsulating layer to be composed of TiO x with Ti predominantly in ∼2+ or ∼3+ oxidation states. The proposed models are structurally consistent with the LEED and STM data and have stoichiometries of TiO and TiO1.4, chemically consistent with the XPS spectra. The STM images of the zigzags bear a strong similarity to structures seen for annealed Pt islands on TiO2(110)-(1 × 1) and TiO x supported on Pt(111), while the pinwheel structure is similar to annealed Cr on Pt(111). We discuss the similarities of our structures to those seen before for these related systems.

In the future we will be phasing out the use of fossil fuels in favour of more sustainable forms of energy, especially solar derived forms such as hydroelectric, wind and photovoltaic. However, due to the variable nature of the latter sources which depend on time of day, and season of the year, we also need to have a way of storing such energy at peak production times for use in times of low production. One way to do this is to convert such energy into chemical energy, and the principal way considered at present is the production of hydrogen. Although this may be achieved directly in the future via photocatalytic water splitting, at present it is electrolytic production which dominates thinking. In turn, it may well be important to store this hydrogen in an energy dense liquid form such as methanol or ammonia. In this brief review it is emphasised that CO2 is the microscopic carbon source for current industrial methanol synthesis, operating through the surface formate intermediate, although when using CO in the feed, it is CO which is hydrogenated at the global scale. However, methanol can be produced from pure CO2 and hydrogen using conventional and novel types of catalysts. Examples of such processes, and of a demonstrator plant in construction, are given, which utilize CO2 (which would otherwise enter the atmosphere directly) and hydrogen which can be produced in a sustainable manner. This is a fast‐evolving area of science and new ideas and processes will be developed in the near future.

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