Surface-Oxide Formation

The specific catalytic properties of polyciystalline and single crystal surfaces have prompted extensive research on their oxidation in electrochemical- and gas- pliase environments. Recent developments in fuel cell technology have renewed efforts to improve Pt-Ru electrocatalysis for both reformate hydrogen- and methanol-oxidation. In the following Section, we discuss the oxidation of single crystal surfaces in both UHV- and electro-chcmical-cnvi ronments. 

---

The behavior of Au faces in pH >7 solutions has been stud-ied.24,63,391 545 546 Surface oxide formation is a two-step process a peak at a less positive E has been explained by OH" specific adsorption24,63,188,391,485,547 that at a more positive E in terms of irreversible oxide formation by examining the negative moving i, E profile at various... 

Shi, H. and Stampfl, C. (2007) First-principles investigations of the structure and stability of oxygen adsorption and surface oxide formation at Au(lll). Physical Review B Condensed Matter, 76, 075327-1-075327-14. 

The lack of knowledge of precise values of the roughness factor makes it difficult to compare data reported from different studies. This applies in particular to the double-layer capacity data, the values of surface concentration of the adsorbates, and the rates of electrochemical reactions. Therefore, the question of how to determine the real surface of the electrode is of cmcial importance. A survey of various methods for determining roughness was given by Trasatti and Petrii. For noble metal electrodes, the charges of hydrogen deposition and surface oxide formation can be utilized in real-surface determination." ... 

More recent research has focused on the binary Ru sulfides and selenides. Schulenberg et al. showed that modifying a Ru/C with Se (via H2Se03) improved activity by a factor of three. It was concluded that the Se inhibited surface oxide formation that limits active sites with Ru/C. Both catalysts showed some H2O2 formation at lower potentials (e.g., 3% at... 

The first indications of surface oxide formation were obtained in the course of combustion studies. Bonnetain et al. (141) and Bonnetain (142) studied the kinetics of the graphite-oxygen reaction and concluded that oxygen was intermediately bonded to the periphery of the carbon layers. 

Table 6. Testing the independence of impedance parameters and potential sweep rate in surface oxide formation [19]. The observed values are in brackets...

Figure 4.12 Current-potential curve for platinum surface oxide formation and reduction in 0.5 M H2S04. (Reproduced with permission from Ref. 38.)...

Kim et al., 1971). These studies were carried out in aqueous medium, and it is therefore pertinent to ask whether a similar surface modification of platinum will take place in a non-aqueous solvent. Considering the fact that water is easily present at the 1 mM level in such solvents, one is forced to conclude that the conditions for surface oxide formation are favourable under most imaginable conditions. 

Surface-oxide formation begins after an adsorbate layer of oxygen or an 0-containing species on the electrode smface forms at more positive potentials (> 1.1 V). After time a bnlk-oxide continues growing. While different electrochemical techniques " show evidence for oxide formation, the exact stracture and thickness of this oxide is still unclear. " The conunon view is that oxide-growth first begins with the formation of a thin... 

Surface oxide formation undoubtedly is involved in the Fe(II)-dichromate titration curves, which Smith and Brandt found to be different when the direction of titration was reversed (Figure 15-2, right). Kolthoff and Tanaka found that the rate of oxidation with dichromate was slow, whereas the rate of reduction with Fe(II) was fast. Ross and Shain found the same sort of behavior and noted also that the rates of oxidation and reduction decreased in more dilute solutions. The oxidized surface in a dichromate solution may be largely covered with adsorbed dichromate, as chromium surfaces have been shown to be in some experiments with radio-chromium, so that it is relatively ineffective as an electron-transfer surface for the Fe(III)-Fe(II) system. 

I a is the applied anodic current density in the pulse, and 6 is the desired fractional coverage by the intermediate. The first and second terms on the right-hand side of Eq. (35) represent, respectively, the part of ip used to ionize the adsorbed H atoms on the surface and that for any other Faradaic process, for example, surface oxide formation, which may be occurring over the same range of potentials. By combining Eqs. (34) and (35),... 

In principle, the STM can work in air and liquid environments. However, most STM work has been done in ultra-high vacuum. This is because most sample surfaces in an air environment quickly develop an oxide layer. An ultra-high vacuum environment can prevent possible surface oxide formation and maintain a conductive sample surface. Also, low temperature is preferred because it reduces thermal drift and diffusion of atoms and helps to obtain a static surface image of atoms. However, an elevated temperature provides an environment for observing dynamic processes of atoms and molecules. 

Although the latter occurs of course always to some extent just on entropy grounds, there will at any rate be an enrichment of oxygen atoms in the easier to relax interstitial sites in the immediate near-surface fringe. And one could expect this deformation argument to hold even more for subsurface sites close to even lower coordinated atoms, i.e., interstitials in the vicinity of point defects, steps, or dislocations. The latter are in fact frequently believed to be the nucleation centers for surface oxide formation, and kinetic arguments like an easier O penetration are often put forward as explanation. Yet, we see that the thermodynamic deformation cost factor could also favor an initial oxygen accommodation close to such sites. 

PtO2 (752, 757), similar to some electrochemical oxygen layers. Figure 12 shows a possible structure of platinum oxides on various planes (752). The [1(X)] plane has a PtOj composition (752,757), while the bulk corresponds to a PtO oxide. Present information does not unambiguously point toward either surface oxide formation or chemisorption. Because of the apparent similarity of some surface oxygen species on catalysts and electrocatalysts, coordinated efforts in both fields using standardized techniques and procedures could resolve uncertainties. 

---