IRREVERSIBLY ADSORBED SILVER ON PT(111) AND TRANSFORMATION OF THE ELECTROSORPTION BEHAVIOR INDUCED BY THERMAL ANNEALING

The electrosorption behaviour of a Pt(111) electrode modified by irrev ersibly adsorbed silver has been studied in sulfuric and perchloric ac id solutions taking advantage of the absence of silver in the solution for determining silver coverage from hydrogen adsorption site blockin g. Silver surface layers, with coverages ranging from submonolayers to bilayers, were prepared by spontaneous or forced adsorption from silv er solutions. They were found to be similar to those reported for unde rpotentially deposited silver. Whatever the amount of irreversibly ads orbed silver and the deposition technique used, no sign of surface all oying was detected from the behaviour of the substrate after silver st ripping. 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 th e amount of silver present at the surface before annealing. The profil e is shown to depend also on the composition of the cooling atmosphere . The electrochemical stripping occurs, in this case, with a disruptio n of the two-dimensional long range order of the Pt(111) substrate for a polarisation in a potential range where normally the surface struct ure of the substrate is preserved. This is interpreted as resulting fr om 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.