Catalysts, general oxygen diffusion

Platinum is generally acknowledged as the most effective catalyst for the electroreduction of oxygen in a wide range of conditions (e.g. fuel cells). In the instance of aqueous HC1 electrolysis, the basic drawback is corrosion or deactivation of the catalyst during cell shutdown, owing to chemical attack from HC1 and chlorine that diffuse across the membrane. 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

Intermediate 1 could also be stabilised by proton transfer from oxygen to give 3 in Scheme 11.9. The proton acceptor B could be solvent water or a general base catalyst. The reaction will only be catalysed if the rate of breakdown of 1 to regenerate reactants is faster than the rate of proton transfer. In this case, such catalysis would be independent of the base strength of the catalyst B as proton transfer would invariably be thermodynamically favourable and hence occur at the maximum diffusion-controlled rate. If proton transfer to solvent is thermodynamically favourable, such that proton donation to 55.5 M water is faster than to, say, 1 M added base, any observed catalysis by base must represent transition state stabilisation by hydrogen bonding, or a concerted mechanism. 

The potential difference developed between aluminium and stainless steel is about the same as that developed between aluminium and copper. The cathodic reaction is easier on copper oxide than that on the highly protective passive oxide of stainless steels. Then, it is not the difference of potential between anode and cathode which counts, but the facility and rate of every reaction. A bare metal is generally a much better cathode than one covered with an oxide. Aluminium is more active than zinc in the electrochemical series. Practically, zinc protects aluminium which becomes covered with an oxide film.20 All more noble metals accelerate corrosion similarly, except when a surface film (e.g., on lead) acts as a barrier to diffusion of oxygen or when the metal is a poor catalyst for reduction of oxygen. 

POMs exhibit two dominant modes of reactivity in redox processes. There are well-documented variations for each mode. Mode 1, which is the most frequent one when 02 is used as the oxidant, involves initial substrate oxidation (Equation (26)) followed by reoxidation of the reduced POM (Equation (27)). The net reaction is Equation (28). Mode 2, which is most frequent with oxygen donor oxidants including peroxides, involves initial activation of the oxidant, OX, by the POM with formation of a POM-OX complex (Equation (29)). There are three general fates of these complexes. They can directly react with substrate to form product (Equation (30)), transform to another complex, [POM-OX] (Equation (31)) which then oxidizes the substrate (Equation (32)), or form freely diffusing oxidizing intermediates that are not bound to the POM catalyst... 

If the TPR profiles for the NM/CeOa catalysts and the bare support, also included in Figure 4.3, are compared, a common high temperature feature centred at 1090 K may be noted. This peak is generally interpreted as due to the bulk reduction of ceria (61, and references there in). In agreement with several earlier studies (73,110,283), the position of this peak does not seem to be modified by the presence of any supported metal. This observation is typically interpreted in terms of a kinetic model (205) which assumes that the high temperature reduction process is controlled by the slow bulk diffusion of the oxygen vacancies created at the surface of the oxide. 

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