Irreversibly adsorbed silver on Pt(111) and transformation of the electrosorption behaviour induced by thermal annealing

The electrosorption behaviour of a Pt(111) electrode modified by irreversibly adsorbed silver has been studied in sulfuric and perchloric acid solutions taking advantage of the absence of silver in the solution for determining silver coverage from hydrogen adsorption site blocking. Silver surface layers, with coverages ranging from submonolayers to bilayers, were prepared by spontaneous or forced adsorption from silver solutions. They were found to be similar to those reported for underpotentially deposited silver. Whatever the amount of irreversibly adsorbed silver and the deposition technique used, no sign of surface alloying was detected from the behaviour of the substrate after silver stripping. After the silver pre-covered Pt(111) surface was subjected to thermal annealing, the voltammetric profile recorded in sulfuric acid undergoes a dramatic change with the formation of a narrow reversible adsorption state at 0.28 V. Its amplitude is directly connected to the amount of silver present at the surface before annealing. The profile is shown to depend also on the composition of the cooling atmosphere. The electrochemical stripping occurs, in this case, with a disruption of the two-dimensional long range order of the Pt(111) substrate for a polarisation in a potential range where normally the surface structure of the substrate is preserved. This is interpreted as resulting from electrodissolution of a silver alloy formed in the uppermost layer of Pt(111). The role of the nature of the anion in the various surface conditions achieved with this system is presented briefly.