Relationship between the surface coverage of spectator species and the rate of electrocatalytic reactions

Relationships between the surface coverage of spectator (blocking) species and the rate of the hydrogen oxidation reaction (HOR), the oxygen reduction reaction (ORR), and the bulk oxidation of dissolved CO on Pt(100) and Pt(111) single crystals in acidic electrolytes has been probed by cyclic voltammetry, in situ surface X-ray scattering (SXS), and ex situ scanning tunneling microscopy (STM) techniques. It is shown that the surface coverage by spectator species during the HOR and the ORR are the same as for the corresponding coverage obtained in the inert (Ar-saturated) environment. This observation is consistent with the proposition that the availability of active sites for H-2 and O-2 is determined almost entirely by the coverage of adsorbates from the supporting electrolyte and not by the active intermediates. Related electrochemical-SXS studies undertaken for bulk CO oxidation reveal that the maximum rate above the ignition potential is reached on a surface that is covered by approximate to 90% of an ordered CO adlayer. The nature of the active sites in this case is determined by a combination of electrochemical and STM results. It is found that the active sites in this potential region are steps, which appear to be active sites for OH adsorption. To get insight into the relationship between the diffusion-limiting current and the surface coverage by the inactive CO adlayer, we introduce the concept of a partially blocked electrode surface with active and inactive areas. On the basis of the calculations and experimental results, it is proposed that the active sites for given electrochemical reactions on Pt electrodes are arrays of adsorbate-free nanoscale patches embedded in an inactive adlayer of nonreactive molecular species.