Site Selectivity for CO Adsorption and Stripping on Stepped and Kinked Platinum Surfaces in Alkaline Medium

Abstract

This work reports experimental evidence that the adsorption of CO on Pt surfaces composed by (111) terraces and steps and/or kinks in alkaline media occurs faster on sites with (100) symmetry, followed by (110) sites. On the other hand, the (111) terrace has the lowest adsorption rate. CO electrooxidation demonstrates the existence of a close relationship between the preferential site CO occupancy and the peak multiplicity observed in CO stripping voltammetry. For the stepped Pt(554) and Pt(544) and kinked Pt(875) single crystal surfaces, the CO stripping process takes place at high potentials (∼0.80 V vs RHE) when only the (110) or (100) step sites are blocked by CO. However, when the terrace sites with (111) symmetry are fully occupied, new CO stripping peaks appear at lower potentials (<0.68 V vs RHE). For all surfaces, it is observed that the first released sites after partial CO oxidation are (111) terrace sites, followed by the step (110) sites and sites with (100) symmetry. The results of partial CO oxidation suggest that the diffusion of adsorbed CO from sites with (110) or (100) symmetry toward unoccupied (111) terrace sites is negligible. However, CO diffusion from terrace sites to step or kink sites cannot be discarded during the growth of the adsorbed CO adlayer, because of the preferential site occupancy for these latter sites. Due to the fact that the CO oxidation process on stepped Pt surfaces in alkaline media occurs at different potentials on different sites, the activation energies for CO oxidation on the different sites have been estimated. The results for full CO coverage and CO decorated stepped surfaces are in good agreement, indicating that the oxidation of CO on the different sites is not coupled. In the absence of CO on terrace sites, in situ FTIR spectroscopy shows that CO molecules on the (110) step sites are essentially linearly bonded, while on the (100) step sites both linearly and bridge bonded CO are observed. The comparison of these spectra with those obtained when a full coverage is attained shows that the band frequencies for CO on step sites are highly coupled with those on the terrace sites.