Oxygen surface structure

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

In this section, we will investigate the surface structure of the electrode in the potential range before a surface or bulk oxide starts forming, and will restrict ourselves to the adsorption of atomic oxygen only (not OH ) [Jacob and Scheffler, 2007]. Furthermore, in our simulations, we assume a single-crystal Pt(lll) electrode, which will be compared with the experimental CV curve (Fig. 5.9) for poly crystalline Pt. This simplification is motivated by the fact that our interest here is to describe the general behavior of the system only. 

For the a-Pt02 system, we find that above an electrode potential of 1.2 V, the (001) surface with bulk composition is most stable and shows only minor relaxation effects (denoted as (OOl)-O in Fig. 5.11a). This surface structure corresponds to experimental UHV measurements of surface oxides on Pt(llO), supported by DFT calculations [Li et al., 2004]. In the case of a very thin surface layer, the layer composition might even be PtO. Increasing the electrode potential above 2.0 V would cause stronger interactions with the surrounding water dipoles and lead to a o -PtO2(011) surface with an enrichment of oxygen (as 0 ) on the surface. 

For the /3-Pt02 system, we find the same general that increasing the electrode potential leads to surface structures with more oxygen, although the transition... 

Hernandez J, Solla-Gullon J, Herrero E. 2004. Gold nanoparticles synthesized in a water-in-oil microemulsion Electrochemical characterization and effect of the surface structure on the oxygen reduction reaction. J Electroanal Chem 574 185-196. 

Markovic NM, Tidswell IM, Ross PN. 1994. Oxygen and hydrogen peroxide reduction on the gold(lOO) surface in alkaline electrolyte the roles of surface structure and hydroxide adsorption. Langmuir 10 1-4. 

Figure 5.11 Variation in the catalytic activity of an Mg(0001) surface when exposed to a propene-rich propene- oxygen mixture at room temperature. The surface chemistry is followed by XPS (a), the gas phase by mass spectrometry (b) and surface structural changes by STM (c, d). Initially the surface is catalytically active producing a mixture of C4 and C6 products, but as the surface concentrations of carbonate and carbonaceous CxHy species increase, the activity decreases. STM images indicate that activity is high during the nucleation of the surface phase when oxygen transients dominate. (Reproduced from Ref. 39).

If the tip is contaminated, its apex is most likely attached to a hydrogen molecule or H atoms. As a consequence, the conductance of this tip should be much lower than that of a clean tungsten tip. Since this conductance change has not been reported, it can be concluded that the reduced 0—0 distance is not the effect of a contaminated tip. Surface-tip interactions are evaluated by calculating the interaction between the reacted surface and a tungsten cluster at low distance. Here, the calculations indicate that there is no substantial relaxation due to interactions between the two leads. Consequently, the only possibility left is that the electronic surface structure somehow changes the appearance of the oxygen positions. 

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