THE KINETIC-STUDY OF SPECIFIC ADSORPTION OF PHOSPHATE SPECIES ON PT(111) IN ACIDIC SOLUTIONS

The study of the adsorption/desorption mechanism of phosphate anions a t Pt(111) in acidic solution of pH 4.3 and 0.8 was performed by the po tential step method in order to reveal the kinetics of anion adsorptio n. The current-time curve due to phosphate adsorption/desorption showe d various decay features, being dependent on the potential region. The rate of current decay depended on pH, being faster in a lower pH solu tion. Specific adsorption processes were analyzed by the Langmuir and Elovich adsorption equations and also in terms of a two-dimensional nu cleation-growth mechanism in different adsorption/desorption regions. In the case of adsorption in 0.3 M phosphate buffer solution of pH 4.3 , random adsorption without interaction following the Langmuir adsorpt ion, takes place at low coverage, while random adsorption with repulsi ve force was observed at high coverage. In the desorption process, ran dom desorption with repulsive force takes place at high coverage, and the repulsive force disappears where random adsorption without interac tion takes place at medium coverage. When the surface coverage becomes further lower, the desorption mechanism changes dramatically into a t wo-dimensional nucleation-growth type, suggesting that an ordered adso rbate structure is formed after a rapid discharge process of anion ads orption. (C) 1997 Elsevier Science S.A.