Model systems potential step

Demonstrating that a redox transformation of a contaminant involves mediated electron transfer requires meeting several criteria (i) the overall reaction must be energetically favorable, (ii) the mediator must have a reduction potential that lies between the bulk donor and the terminal acceptor so that both steps in the electron transfer chain will be energetically favorable, and (iii) both steps in the mediated reaction must be kinetically fast relative to the direct reaction between bulk donor and terminal acceptor. Most evidence for involvement of mediators in reduction of contaminants comes from studies with model systems, because natural reducing media (such as anaerobic sediments) consist of more redox couples than can be characterized readily. Although this is an active area of research, we can identify a variety of likely mediator half-reactions (see Table 16.5). 

The sequence of elementary steps shown in Fig. 13.2 suggests that one can formulate the problem of carbon poisoning in terms of the selectivity associated with the formation of C-0 vs. C-C bonds on Ni. In order to prevent carbon-induced deactivation, a catalyst should be able to selectively oxidize C atoms (and CH fragments) rather than form C-C bonds. This elementary step mechanism was the basis for the DFT calculations that focused on the identification of catalysts (mainly Ni-containing alloys), which preferentially oxidize C atoms rather than form C-C bonds [15, 16]. In these DFT calculations, the potential energy surfaces for the formation of C-C and C-0 bonds were calculated for different Ni alloys. The alloy model system used in these calculations contained mainly Ni, with some Ni atoms displaced by another atom in the surface layer. While we have examined a number of different alloys, we will focus our discussion on the alloy material (Sn/Ni). We note that this alloy material has also been studied by others previously [35, 38, 41, 49, 50]. 

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