Hydrated Protons at Solid Water-Metal Interfaces

Abstract

Understanding the nature of hydrated protons at an electrolyte/electrode interface is fundamentally important to a wide range of scientific research, including heterogeneous catalysis, corrosion, and electrochemical processes in acidic environment. This thesis aims to elucidate the nature of hydrated protons at the water–metal interface in ultra-high vacuum (UHV) conditions. I studied the formation and stability of hydrated protons on a Pt surface by the coadsorption of atomic hydrogen and water layer, and examined the spatial distribution of hydrated protons at the water‒Pt interface with counterions. Chapter I introduces the simulation of the electrochemical interface under UHV, known as the UHV model study. Simulation of the electrochemical interface in UHV conditions is useful for exploring the electric double layer (EDL) outside of an electrochemical cell without an applied electrochemical potential. UHV modeling allows the study of aqueous interfaces at the molecular level. Previous UHV modeling studies have been limited to obtaining the elemental, structural, and electronic information of surfaces. The present work demonstrates that simulation of the EDL, along with surface spectroscopic techniques, can provide valuable information about the water–metal interface at the molecular level. The basic principle of experimental methods was discussed in Chapter II. Reflection-absorption IR spectroscopy can provide the information about the molecular structure of surface species. Cs+ reactive ion scattering (RIS) and low energy sputtering (LES) reveal the identities of the neutral and ionic species on the surface, respectively. Kelvin probe was introduced to estimate the distance of charge separation. In addition, instruments equipped in the UHV chamber are also described. Chapter III presents the study on the nature of hydrated protons by the coadsorption of atomic hydrogen and water layer on a Pt(111) surface. Spectroscopic evidence obtained by mass spectrometry and reflection absorption infrared spectroscopy showed that the adsorbed hydrogen atoms ionize into multiply hydrated proton species (H5O2+, H7O3+, and H9O4+) on the surface, rather than into H3O+. Then, upon the addition of a water overlayer, the metal-bound hydrated protons spontaneously evolved into three-dimensional fully hydrated proton structures via proton transfer along the water overlayer. The stability of the hydrated protons on the Pt surface and their bulk dissolution behavior suggest the possibility that the surface hydrated protons are a key intermediate in the electrochemical interconversion between the adsorbed H atoms and H+(aq) in water electrolysis and hydrogen evolution reactions. In Chapter IV, the spatial distribution of hydrated protons and chloride ions is studied by the coadsorption of HCl and H2O on a Pt surface. An amorphous solid water (ASW) film was overlaid on the ions to simulate the electrochemical interface in UHV. The experimental results showed that H+ and Cl- ions on the Pt surface have the thermodynamic preference to reside near the Pt surface rather than diffuse out into the ASW film. The vertical average distance of H+ ions from the Cl- ions, which specifically adsorbed on the Pt surface, was estimated to be about one water layer. The migration of protons to the hydration sphere can be attributed to the stability of fully solvated structure. The distribution of H+ and Cl- ions near the Pt surface exhibits the consistence of the specifically adsorbed (Cl-) and non-specifically adsorbed (H+) ions in the EDL.