In Situ Identification of Non-Specific Adsorption of Alkali Metal Cations on Pt Surfaces and Their Catalytic Roles in Alkaline Solutions
Li Jiao, Ershuai Liu, Sanjeev Mukerjee, Qingying Jia
Abstract
Alkali metal cations (AM+) have profound influences on catalytic activities of platinum (Pt) in aqueous solutions, but the underlying mechanisms remain unclear mainly because of limited knowledge of the interactions between AM+ and Pt surfaces within the interface. Herein, in situ X-ray absorption spectroscopy (XAS) coupled with ab initio FEFF calculations rule out specific adsorption of AM+ and hydroxyl (OHad) but not interfacial water with the oxygen facing toward the Pt surface (H2O↓ad) within the hydrogen underpotential deposition (HUPD) potential region in alkaline solutions. We further show that AM+ greatly affects the HUPD peaks and the kinetics of the hydrogen evolution/oxidation reactions (HER/HOR) of Pt in alkaline solutions. We accordingly propose that H2O↓ad is specifically adsorbed on undercoordinated Pt surfaces and bonded with the water molecules coordinated to AM+, forming the sandwich structure of Pt–H2O↓ad···[(H2O)x–AM]+ across the interface; and the exchange between the H2O↓ad···[(H2O)x–AM]+ and Had yields the sharp HUPD peaks. The AM+ weakens the Pt–H2O↓ad bond via noncovalent interactions and in turn positively shifts the HUPD peaks and hinders the HOR by delaying the oxidative removal of Had. The AM+ promotes the HER of Pt by facilitating the removal of the OHad in the form of OHad···[(H2O)x–AM]+ subsequent to the dissociation of H2O↓ad.