Investigation into the Competitive and Site-Specific Nature of Anion Adsorption on Pt Using In Situ X-ray Absorption Spectroscopy

In situ X-ray absorption spectroscopy along with electrochemical measurements (CV and RDE) and previously published EQCN data provide further understanding of the nature of chloride poisoning on different faces/sites of carbon supported platinum clusters (1-2 nm) in acidic medium (HClO4). Chloride is shown to adsorb in 3-fold sites on the Pt(111) faces at the investigated Cl- concentrations (10(-3) and 10(-2) M). Atop chloride was found to be present within a narrow potential range (0.4-0.7 V RHE) when compressed adlayers of Cl- are formed on the Pt(111) faces forcing some Cl- to exist in atop/bridged sites. The interplay of anionic (Cl-, Br-, OH-, and HSO4-) adsorption on the different surfaces of Pt are also considered. For example O/OH can easily displace atop chloride on the edges/corners but not the Cl- at the Pt(111) sites, and therefore Cl- dramatically raises the overpotential for water activation at the Pt(111) sites. Chloride also drastically alters the ORR causing an increase of the overpotential by similar to 85 mV for every 10-fold increase in chloride concentration with a total 150-200 mV increase in the overpotential at large concentrations at the Pt(111) sites. Finally Cl- ions cannot displace the bisulfate overlayer on the Pt(111) faces after it is formed at lower potentials; however, once the bisulfate adsorption is disturbed at higher potentials, the bisulfate cannot displace the Cl- adsorption. These relative anion adsorption preferences can help to explain the different dependencies of the important ORR on anion adsorption, and suggests that the effect of Cl- poisoning might be quite dependent on the Pt particle size.