Surface oxide diagram

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

The Pourbaix diagram indicates the possibility of attack by solutions of pH values above about 9-5, but the position of this limit is influenced by temperature, by the constitution of the solution, and by the surface condition of the metal. Corrosion will ensue if the surface oxide can be dissolved this will invariably take place if the pH exceeds 12, and may occur even at pH values between 10 and 12. 

The surface phase diagram of vanadium oxides on Rh(l 11) has been investigated in a series of papers of our group [4, 18, 19, 90, 101-103]. It is characterized by pronounced polymorphism and many different oxide structures have been detected as a function of coverage and growth temperature. The vanadium oxide structures for coverages up to the completion of the first monolayer formed on Rh(l 11) under the different preparation conditions may be subdivided into highly oxidized phases... 

Abstract This chapter first explains the natural flotability of some minerals in the aspect of the crystal structure and demonstates the collectorless flotaiton of some minerals and its dependence on the h and pH of pulp. And then the surface oxidation is analysed eletrochemically and the relations of E to the composition of the solutions are calculated in accordance with Nemst Equation. The E h-pH diagrams of several minerals are obtained. Thereafter, electrochemical determination such as linear potential sweep voltammetry (LPSV) and cyclic voltammetry (CV) and surface analysis of surface oxidation applied to the sulphide minerals are introduced. And recent researches have proved that elemental sulfur is the main hydrophobic entity which causes the collectorless flotability and also revealed the relation of the amount of sulfur formed on the mineral surfaces to the recoveries of minerals, which is always that the higher the concentration of surface sulphur, the quicker the collectorless flotation rate and thus the higher the recovery. 

The h-pH diagrams of surface oxidation of arsenopyrite and pyrite are shown in Fig. 2.16 and Fig. 2.17, respectively. Figure 2.16 shows that jBh-pH area of self-induced collectorless flotation of arsenopyrite is close to the area forming sulphur. The reactions producing elemental sulphur determine the lower limit potential of flotation. The reactions producing thiosulphate and other hydrophilic species define the upper limit of potential. In acid solutions arsenopyrite demonstrates wider potential region for collectorless flotation, but almost non-floatable in alkaline environment. It suggests that the hydrophobic entity is metastable elemental sulphur. However, in alkaline solutions, the presence of... 

Fig. 7.2). The second structure, however, was found to play an important role in the overall phase diagram. This structure is called a surface oxide since the outermost layers of the material are in an oxide form while the bulk of the material is a pure metal. It can be seen from Fig. 7.5 that at most temperatures, there is a range of pressures spanning several orders of magnitude for which this surface oxide structure is more stable than either the bulk oxide or the clean metal surface. This phase diagram strongly suggests that the working catalyst under industrial conditions is a surface oxide rather than bare Ag.

The example above illustrates how constructing a phase diagram is relatively straightforward once a list of candidate structures has been specified. At the same time, the complexity of the surface oxide structure in Fig. 7.6 is an excellent example of why generating the relevant candidate structures is often far from straightforward. The structure shown in Fig. 7.6 was based on the best experimental data available on this ordered surface phase that were available at the time of Li, Stampfl, and Scheffler s1 calculations. Since then, however, additional experiments and DFT calculations have indicated that the structure of the true surface oxide is somewhat different than the one shown in Fig. 7.6 and, moreover, other surface oxide phases with similar stabilities also exist. 

Figure 7.6 Several of the structures considered by Li, Stampfl, and Scheffler1 in constructing their phase diagram for 02/ Ag( 111). The top left panel shows a top view of a structure with both surface and subsurface O atoms (large and small dark spheres, respectively). The top right panel shows a side view of the same structure. The DFT calculations do not predict this structure to be thermodynamically favored. The bottom panel shows a top view of the (4 x 4) surface oxide, which has a complex arrangement of Ag atoms (light spheres) and O atoms (dark spheres) on top of an Ag(lll) substrate (unfilled circles). DFT calculations predict this structure to be favored at certain temperatures and pressures. (Reprinted by permission from the source cited in Fig. 7.5.)...

The production of corrosion-resistant materials hy alloying is well established, hut the mechanisms are noi lull) understood. It is known, of course, that elements like chromium, mckcl. titanium, and aluminum depend for their corrosion resistance upon a tenacious surface oxide layer (passive film). Alloying elements added for the purpose of passivation must be in solid solution. The potential of ion implantation is promising because restrictions deriving from equilibrium phase diagrams frequently do not applv li e., concentrations of elements beyond tile limits of equilibrium solid solubility might he incorporated). This can lead to heretofore unknown alloyed surfact-s which are very corrosion resistant... 

Keywords Atomic scale characterization surface structure epoxidation reaction 111 cleaved silver surface oxide STM simulations DFT slab calculations ab initio phase diagram free energy non-stoichiometry oxygen adatoms site specificity epoxidation mechanism catalytic reactivity oxametallacycle intermediate transition state catalytic cycle. 

Reuter K, Scheffler M (2003) First-principles atomistic thermodynamics for oxidation catalysis surface phase diagrams and catalytically interesting regions. Phys Rev Lett 90(4) 046103... 

Fig. 5.13. DFT-GGA surface phase diagram of stable surface structures at a Pd(lOO) surface in constrained equilibrium with an O2 and CO environment. The stability range of bulk PdO is the same as the one shown in Fig. 5.12. Phases involving the (. 5 X. 5)R27° surface oxide exhibit a largely increased stability range, which now comprises gas phase conditions representative of technological CO oxidation catalysis (po2 = Pco = 1 atm. and 300 K < T < 600 K, marked by the black line, as in Fig. 5.12) (from [46])...

The kinetics of tiiis reaction were also found to follow a Langmuir-Hinshelwood type behavior, with competitive adsorption of C2H4 and oxygen. A comparative kinetic diagram for all the supported Rh catalysts is given in Fig. 4. The dashed lines indicate an abrupt drop in the ethylene combustion rate which is probably due to surface oxide formation. The same behavior was also observed in the electrochemical promotion (NEMCA) experiments, as briefly discussed below and further described elsewhere [22]. 

A simplified diagram of the fate of heavy metals in a wetland environment is presented in Figure 12.1. Partitioning of metals in wetlands is subject to seasonal flooding, drainage cycles, and hydroperiod. Drained wetland soils contain a surface-oxidized horizon and a low pH. Periods of flooding result in anaerobic soil conditions with a shift in pH to a near-neutral state. 

M. Pourbaix [5] has devised a compact summary of thermodynamic data of potential - pH diagrams related to corrosion behavior of any metal in water. These diagrams are now available for most common metals. Diagram (Fig. 1.4) shows specific conditions of potentiai and pH under which the metal either does not react (immunity) or is able to react to form specific oxides or complex ions. These data indicate the conditions under which diffusion-barrier films may form on the electrode surface. The diagram outlines the nature of stoichiometric compounds into which any less stable compounds... 

Passivity and alloy dissolution are covered in detail in other chapters of this volume. However, Pourbaix diagrams can be used to suggest the influence of alloying on passivation or oxide film formation. It is possible for one element in an alloy to enrich in a surface oxide layer if the oxide of that metal is stable in an E/pH region where the other elements are not stable. This results in an effective extension of the passivity region for the base metal of the alloy. An example of this is stainless steel. The... 

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