The review article presents an atomic-level discussion of the fundamental steps involved in hydrogen electro-adsorption on transition-metal electrodes. It also discusses the basic events involved in hydrogen physisorption and chemisorption under low-pressure gas-phase conditions in order to identify characteristics that are unique to the electrochemical environment. The electrochemical environment that comprises the solid electrode, the liquid electrolyte solution, and the electrostatic field creates favorable conditions for the electro-adsorption of two different hydrogen species, the under-potential and over-potential adsorbed hydrogen (H-UPD and H-OPD). The electro-adsorption of H-UPD and H-OPD occurs in two different potential ranges. The electro-adsorption of H-UPD is characteristic of Pt group metals only, while the electroadsorption of H-OPD takes place at all electrode materials at which H-2(g) can be electrolytically generated. The existence of H-UPD and H-OPD is supported by spectroscopic and thermodynamic results. A thermodynamic analysis of the electro-adsorption of H-UPD results in the determination of thermodynamic state functions that govern the process, namely the standard Gibbs energy, enthalpy and entropy of electro-adsorption, Delta(ec-ads)G degrees (H-UPD), Delta Hec-ads degrees (H-UPD), and Delta Sec-ads degrees (H-UPD), the Gibbs energy of later interactions, omega(H-UPD), and the M-H-UPD surface bond energy, E degrees(M-H-UPD). Thermodynamic data for the electroadsorption of H-UPD on polycrystalline electrodes (Pt(poly) and Rh(poly)) and on Pt(111) in several aqueous electrolyte (H2SO4, HClO4, and NaOH) solutions are reported. Their analysis and the analysis of Delta Hec-ads degrees (H-UPD) and E degrees (M-H-UPD) allow us to compare on a thermodynamic basis H-UPD to H-chem, and to identify surface adsorption sites possibly occupied by H-UPD and H-OPD adatoms.
