THE EFFECTS OF ORGANIC ADSORBATES ON THE UNDERPOTENTIAL DEPOSITION OF SILVER AN PT(111) ELECTRODES

Studies have been undertaken to determine the effects of competing organic adsorbates on the underpotential deposition of silver on Pt(111). The adsorbates studied are known to bind to Pt primarily through the hetero-atom (either nitrogen or sulfur) and include pyrazine, 2,2'-bipyridyl, 4,4'-bipyridyl, 4-phenylpyridine, 1,2-Bis(4-pyridyl)ethane, 2-mercaptopyridine, and 4-mercaptopyridine. The effects of the adsorbate layer on silver deposition are strongly dependent on the nature and structure of the co-adsorbed species. Adsorbates that bind primarily through a ring nitrogen atom were found to inhibit the deposition of the second, but not the first, silver monolayer. This may be explained by the formation of a Pt(111)/Ag/adsorbate structure in which the silver deposits underneath the adsorbate layer. These adsorbates also displayed a significant pH dependence Likely due to protonation of the binding atom. In contrast, the sulfur containing adsorbates inhibited all deposition processes at the electrode surf ace except that of bulk silver deposition. In this case there was a significant overpotential to bulk deposition. Moreover, a monolayer of electrodeposited silver could be displaced from the Pt surface upon exposure of the electrode to a solution of 2-mercaptopyridine. This behavior would indicate a higher bond strength between the sulfur atom and the Pt surface than that between the ring nitrogens and the Pt surface. These results are consistent with the expected strengths of adsorption.