Oxide formation factor

This equation defines the oxide formation factor m (nm V ), that is, the layer thickness increases linearly with the applied potential. 

Figure 1.35 Zr02 film orientation angle and oxide formation factor as a function of the Zr substrate orientation [17].

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

At lower temperatures (260°C) higher operating pressures (5 bar) and high C2H4 to 02 ratios (Fig. 8.42) ethylene oxide formation and C02 formation both exhibit electrophobic behaviour over the entire Uwr range 47 Both rates vary by a factor of 200 as UWr is varied by 0.6 V (p varies between 3 and 0.015). The selectivity to ethylene oxide exhibits two local maxima 47 More interestingly, acetaldehyde appears as a new product47... 

In a second paper from Kirillov et al., synthetic aspects of liquid phase precipitation reactions are discussed in relation to the factors responsible for metal-oxide formation. In particular the Pechini reaction, a sol-gel process, was examined. A good reference section is provided to introduce the reader to previous literature. The authors stress that more work needs to be done to establish what factors are important for producing the highest quality metal-oxide powders from such reactions. The conclusion specifically lists the criteria needed in order to accomplish this task. 

Because these two currents are equal (and opposite), the same amount of reaction will occur at either electrode. We see how an electrode reaction must also occur at the cathode as well as the desired oxidative formation of alumina at the anode. (The exact nature of the reaction at the anode will depend on factors such as the choice of electrode material.)... 

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... 

When the pressure of C02 in a carbonate-oxide system is equal to the equilibrium pressure pe, no net reaction occurs. When p < pe, the thermodynamic driving force favors oxide formation conversely, when p > pe, carbonate formation is favored. In the actual system the favored reaction may not occur, however, because kinetic factors prevent it. Particularly when p is not far from pe, the reaction may not proceed because some rate-limiting process, such as nucleus formation, is proceeding too slowly. The resulting spurious equilibria15 give rise to hysteresis effects, i.e., decomposition stops for some p < pe, recombination stops for some p > pe. It is for this reason that this work relies largely on thermodynamic methods for the calculation of equilibrium pressures. 

High-temperature conditions such as those occurring in gas turbines or rocket propulsion units present severe conditions of operation to any metallic component. In such conditions, the hot gases attack the metal surface causing oxide formation (in the case of oxygen attack). In order for the metal to be serviceable under these conditions, oxidation must cease immediately a thin adherent film has formed on the metal surface, i.e. the film is protective. What factors govern the rate of scale formation in metal/hot gas reactions ... 

The presence of an oriented oxide and the type of orientation are determined to a marked extent by the nature of the metal surface. Two factors in particular, the presence or absence of contaminating materials and the topography of the metal surface, have a very strong influence on the oxide formation. The presence of a contaminant in many cases leads to the formation of a randomly oriented polycrystalline oxide rather than an oriented one. A faceted or terraced surface can lead to the formation of orientations which are different from those found on a macroscopically smooth surface. It is obviously important to have well prepared and characterized surfaces if epitaxial studies are to have any real meaning. Despite this, there has been a notable lack of attention to this point and some authors have made no indication at all of the nature of the metal surface. 

There is much still to be learned about conditions required for formation of fer-ryl or other hypervalent iron complexes, the actual structure of the complexes under different circumstances, the kinetics and mechanisms by which they react, and the overall consequences to lipid oxidation. The factors that appear to be most important include the following ... 

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