Heterogeneous process oxidative coupling

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

Kinetic modeling. - The discussion xesented so far clearly shows that the oxidative coupling of methane is a complex process involving homogeneous and heterogeneous reactions. 

Earlier and continuing work from our laboratories has shown that the introduction of small quantities of tetrachloromethane (TCM) into the feedstream for methane partial oxidation or oxidative coupling produces benefrcial increases in the selectivides to hydrocarbons, particularly ethylene, with many heterogeneous catalysts [19]. Consequently it is of interest to examine the effect of this additive with other oxidation processes, such as the conversion of CO to COj. 

The application of heterogeneous catalysts to the methane conversion process has recently been examined by Lunsford (ref. 9) with MgO and Mo/Si02, Somorjai (ref. 10) with Mo/Si0 2 and V/SiO2 and Sofranko (ref. 11), the latter of whom has examined various supported oxides for the oxidative coupling of methane. 

On the other hand, the oxidative coupling reaction of CH4 in the presence of O2, even when performed in membrane type reactors,188 is mainly catalysed by metal oxides catalysts.185 Also, oligomerisation, aromatisa-tion, and the partial oxidation apply non-metallic heterogeneous catalysts (such as zeolites). The reader is therefore directed to some excellent reviews on these subjects.189,190 At this point, it is perhaps relevant to introduce the formation of carbon nanofibres or nanotubes from methane, these being catalysed by metal nanoparticles, but at this moment this is not considered as a Cl chemistry reaction. Again we direct the attention of the reader to some reviews on this type of process.191 192... 

However, despite all these observations, it is still unclear which additional coupling steps take place after the nucleation of an oligomeric layer on the electrode. In the literature, experimental data have been presented to support the view that, with the formation of an oxidized prefilm , the process gradually transforms into one in which the chain growth can take place on the surface immediately by coupling of the monomer cation radical and the oxidized chain in the film [67]. However, the assumption of heterogeneous coupling of monomers... 

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