Anodic oxides coverage

Thinning of the anodic oxide coverage is found at sharp 90° edges of the substrate. This effect has been ascribed to oxide stress, because similar results are found for low-temperature thermal oxidation under conditions where viscous flow is not present. For oxide thicknesses in excess of about 100 nm, cracks develop in... 

Table 3.1 Peak potential, charge passed and corresponding oxide coverage for the various oxide regions in the anodic sweep of the Pt cyclic voltammogram...

Carbon Monoxide The presence of CO in a H2-rich fuel has a significant effect on anode performance because CO affects Pt electrodes catalysts. The poisoning is reported to arise from the dual site replacement of one H2 molecule by two CO molecules on the R surface (40, 41). According to this model, the anodic oxidation current at a fixed overpotential, with (ico) and without (in2) CO present, is given as a function of CO coverage (0co) by Equation (5-11) ... 

The rest of the chapter has been devoted to special topics and in materials science there are many possibilities. Those selected include the mechanism of the flotation of minerals in which the addition of a certain organic to the solution causes a specific mineral to become hydrophobic so that it is exposed to air bubbles, the bubbles stick to it and buoy the mineral up to the surface, leaving unwanted minerals on the bottom of the tank. It turns out that the mechanism of this phenomenon involves a mixed-potential concept in which the anodic oxidation of the organic collector, often a xanthate, allows it to form a hydrophobic film upon a semiconducting sulfide or oxide, but only if there is a partner reaction of oxygen reduction. This continues until there is almost full coverage with the dixanthate, and the surface is thereby made water-repelling. 

The investigation of anodic oxide on various metals shows that at first usually amorphous structures are formed with a dense coverage of the terraces with grains, which change to nano-crystallites with time. The extent and the rate of this change depend on the system under study. This crystallization occurs for Cr within hours [127], whereas Cu keeps the amorphous grain structure for a very few minutes only and develops a well-ordered, faceted, crystalline layer covering the whole electrode surface [128, 129], In the next section, the details of the structure of layers formed on Cu are discussed, followed by a summary of some other more reactive metals like Ni and Cr. 

The cathodic hydrogenation and anodic oxidation of benzene have been investigated by DEMS at Au(lll) and Au(332) electrode surfaces electrodeposited with ultrathin Pd films. The use of submonolayer Pd films was to eliminate possible interferences by (sub-surface) hydrogen absorbed in crystalline or thick-film Pd. The Au(332) surface was selected because, at fairly low submonolayer coverages (e.g., 0.14 monolayer), Pd preferentially deposits on the step sites as the coverage is increased (e.g., 0.82 ML), the... 

Relative atomic concentrations (at%) determined by XPS of anodically oxidized and roughened aluminium sheets coated with ferf-butyl methacrylate based polymer films and the degrees of coverage 0 calculated according to equation (1)... 

V Eli- This is due to passivation of active sites required for H2 dissociation by a relatively small OH coverage. A complete oxide film at Pt will, however, react with H2 in an autocatalytic way, depending on opening up holes in the oxide which exposes metal sites where the dissociation of H2 can occur. The oxide film is then stripped in a mixed cathodic/anodic (oxidation of H) process. 

From the results of Pajkossy [1994] at Pt in the presence of Cl ion, it seems clear that dispersion of the double-layer capacitance is a direct result of Cl" anion adsorption effects. Similar conclusions arise in the case of Cl ion-containing solutions at Au (Pajkossy [1994], an Germain et al. [2004]). Also to be noted is the fact that in the oxide region at Pt, no anomalous dispersion behavior arises despite the possibility of roughness of the oxide film itself. In the presence of extensively formed, anodic oxide films at Pt or Au, CT ion chemisorption (as well as that of HSOj ion) is much diminished, although at low O coverages the surface oxidation of Pt and Au is competitively inhibited (Conway and Novak [1981]). 

Equation 4.32 shows that the oxidation current depends on the anode surface coverage by HCOOH and CO, Ohcooh and 0co, respectively, and it relates therefore, to the oscillatory phenomena discussed earlier. Experimentally, Lovic et al. determined a Tafel slope of 150 mV dec on Pt/C in both H2SO4 and HCIO4 up to an anode potential of 200 mV vs. SCE (i.e., 440 mV vs. SHE), with further increase (almost doubling) at higher potentials [144]. It was shown that various Oco values could simulate to some extent the measured experimental current densities [144]. Furthermore, the porous nature of the electrode with mass transfer and ionic conductivity related limitations effects also the apparent Tafel slope, inducing what is referred to as multiple Tafel slope behavior [146]. 

It is generally recognized [1 —16] now that chemisorbed carbonaceous species are formed during the anodic oxidation of many organic fuels. These species play an important role in our understanding of electrocatalysis. The techniques for their study, their properties and coverage of the surface under different conditions are discussed in this chapter. The role of adsorbed species in the oxidation of various fuels is dealt with in chapter X. 

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