Adsorbed intermediate

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

Formation of the first layer (a monolayer) of passivating oxide film on a denuded metal surface occurs very simply by the loss of protons from the adsorbed intermediate oxidation products, such intermediates being common to both dissolution and passivation processes . Thus for example, the first oxidative step in the anodic oxidation of nickel is the formation of the unstable adsorbed intermediate NiOH by... 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

Participation of adsorbed intermediates can also be shown by the prolonged decay of the potential 011 interruption of the current (Conway and Vijh, 1967a) or by measurement of the time-dependence of the formation of products by carrying out the reaction with pulses of potential of controlled duration (Fleischmann et al., 1966). Thus the formation of ethane in the Kolbe reaction of acetate ions in acid solutions is initially proportional to the square of time as would be predicted for the rate of the step (27) (Fleischmann et al., 1965). 

Power law expressions are useful as long as the approximate orders of reactant concentration are constant over a particular concentration course. A change in the order of the reaction corresponds to a change in the surface concentration of a particular reactant. A low reaction order usually implies a high surface concentration, a low reaction order, and a low surface reaction of the corresponding adsorbed intermediates. In order to deduce (Eq. (1.17b)) the rate of surface carbon hydrogenation, the power law of Eq. (1.18) has been used. 

Assuming that the catalytic reaction takes place in a flow reactor under stationary conditions, we may use the steady state approximation to eliminate the fraction of adsorbed intermediate from the rate expressions to yield ... 

Let us look at the limiting cases, starting from the complete rate expression in Eq. (111). If the conversions from adsorbed intermediate into either product or reactant are fast, then the denominator approaches 1 and the catalyst surface will be mostly empty, 6r s 0. In this case the rate expression becomes equal to that of the uncatalyzed reaction, and the order in the reactant becomes +1. Under such conditions it is beneficial to increase the reactant concentration since the surface is mainly unoccupied, and the catalyst inefficiently used. 

This example illustrates several points First, reaction mechanisms, adsorbed intermediates and transition states can nowadays be investigated very well by computational... 

These results are all consistent with a mechanism which involves the comhination of O2, a and Eta in the formation of an adsorbed intermediate, Ig, which can then branch out to evolve EtO (leaving an oxygen adatom behind on the surface) or to produce CO2 + H2O. The oxygen adatom thus inherent to EtO production must itself go on to make either CO2 or H2O, which sets a theoretical upper-limit on the selectivity of 6/7. This agrees within experimental error with the maximum selectivity observed on both high surface area catalysts (2,35,37) and single-crystals (26,27). 

The reaction mechanisms associated with pathways (A) and (B) may share In common such adsorbed Intermediates as peroxide, super-oxide and their corresponding protonated forms. The distinction between pathways (A) and (B) can become therefore a matter of whether an Intermediate such as peroxide (H02 ) desorbs In significant quantities or not for example,... 

Thus, worldwide efforts have focused on the elucidation of the reaction mechanism. For this purpose, knowledge about the following items is vital (1) identification of reaction products and the electrode kinetics of the reactions involved, (2) identification of adsorbed intermediate species and their distribution on the electrode surface, and (3) dependence of the electrode kinetics of the intermediate steps in the overall and parasitic reactions on the structure and composition of the electrocatalyst. It is only after a better knowledge of the reaction mechanisms is obtained that it will be possible to propose modifications of the composition and/or structure of the electrocatalyst in order to significantly increase the rate of the reaction. 

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

Figure 6. SNIFTIR spectra of the adsorbed intermediates involved in the oxidation of 0.1 M CHjOH in 0.5 M HCIO4 on a smooth Pt electrode (p-polarized light modulation potential AE = 0.3 V averaging of 128 interferograms). Electrode potential (mV/RHE) (1) 370, (2) 470, (3) 570, (4) 670, (5) 770.

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

A so-called direct pathway involving a more weakly adsorbed perhaps even partially dissolved intermediate. Likely candidates for such intermediates are formaldehyde and formic acid. The oxidation mechanism of formic acid is discussed in Section 6.3. The idea is that the formation of a strongly adsorbed intermediate is circumvented in the direct pathway, though in practice this has appeared difficult to achieve (the dashed line in Fig. 6.1). Section 6.4 will discuss this in more detail in relation to the overall reaction mechanism for methanol oxidation. 

Willsau J, Heitbaum J. 1986. Analysis of adsorbed intermediates and determination of surface potential shifts by DEMS. Electrochim Acta 31 943-948. 

Batista EA, Iwasita T. 2006. Adsorbed intermediates of formaldehyde oxidation and their role in the reaction mechanism. Langmuir 22 7912-7916. 

FIG. 10 Extraction rate profiles of Ni(II) with dithizone and phen in which an interfacially adsorbed intermediate complex is formed. 

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