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 ∼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.