Oxygen reduction reaction platinum surface

In a catalytic reaction, all steps do not equally depend on the surface structure. So, for example, the rate of simple desorption processes is often not markedly affected by the structure of the surface. In catalysis, therefore, reactions are classified into "structure sensitive" and "structure insensitive" [5], usually on the basis of the variation of reactivity with particle size. As an example, the electrocatalytic oxygen reduction at platinum (which is of importance for fuel cells) will be mentioned, where a decrease of specific activity with increasing particle size was reported [6,7]. In a theoretical analysis [8], the kinetics was treated on the (111), (10 0), and (211) facets of several transition metals, and the results were combined with simple models for the geometries of catalytic nanoparticles. Thus, the experimentally observed trend could be well reproduced. 

More recent reviews on the electro catalysis of oxygen reduction in PEFC have been given in [23,49]. Surface chemistry of platinum, particularly the coverage of the metal surface with oxygen containing species above a potential of 750 mV vs. RHE caused difficulties in the kinetic description of the oxygen reduction reaction. 

Cai Y, Adzic RR (2011) Platinum monolayer electrocatalysts for the oxygen reduction reaction improvements induced by surface and subsurface modification of cores. Adv Phys Chem 11 16. doi 10.1155/2011/530397, Article ID 530397... 

It should also be noted that the platinum surface is very sensitive to the presence of species in solution and to electrode pre-treatments (anodization, pre-reduction). Damjanovic etal reported a very strong dependence of the reaction pathway on the purity of the solution. They concluded that the oxygen reduction reaction occurred without hydrogen peroxide intermediate formation on a pre-reduced platinum electrode, and therefore that the production of hydrogen peroxide was effective only on sites affected by the presence of adsorbed impurities. 

Lastly, another effect by alloying platinum with a foreign transition metal is discussed in the literature. The galvanic effect comes from the fact that the transition metals are more easily oxidisable than platinum. They allow for the protection of the platinum surface, preventing it from oxidation, and only metallic platinum is active for the oxygen reduction reaction. 

Johnston, C.M., Choi, J., Kim, YS. and Zelenay, P. (2(X)6) Towards understanding ruthenium crossover effects the oxygen reduction reaction on Ru-modified platinum surfaces. 209th Electrochemical Society meeting, Denver, Colorado, May 07-May 12, Abs. no. 1123. 

---