Identification of surface hydronium: Coadsorption of hydrogen fluoride and water on platinum (111)

Abstract

The adsorption of HF and its coadsorption with water were studied on Pt(111) by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) as a step in the UHV modeling of the acidic aqueous electrolyte/electrode interface. Anhydrous HF adsorbs without dissociation. HF coadsorbed with water reacts to form several phases distinguishable by TPD. HREELS spectra show that the reaction forms the H 3O + ion. The stoichiometries of thermal desorption identify an acid monohydrate phase ([H 3O +][F −]) and fully hydrated phases with stoichiometries of HF · 5H 2O in the monolayer and HF · 8H 2O in the multilayer. To the as-yet-unknown extent that low-temperature measurements are relevant to normal aqueous electrochemistry, these results indicate that even such classically “non-specifically” adsorbed ions as H + and F − interact sufficiently strongly with Pt surfaces to displace some water from their inner solvation shells. These data also show that Brönsted acid-base chemistry can be carried out and spectroscopically observed in low temperature monolayers in UHV, and point the way towards UHV studies of such pH-dependent phenomena as corrosion and electrocatalysis.