Solvation and Ionization of Hydroxyl Groups in Water-Ice Layers on Silver(110)

摘要

Solvation and ionization of hydroxyl groups on Ag(110) were examined with isotopeexchange experiments involving OD and (18)OD, and temperature programmed desorption (TPD). Water adsorption onto oxygen-covered Ag(110) gives rise to the well known alpha, beta, gamma, and delta peaks in TPD. The alpha state represents multilayer water, the gamma state an (OH)/H2O complex, and the delta state OH groups. The identity of the beta state is the subject of this paper. Surface solvation of OH groups involves as many as 17 water molecules (beta and gamma) in the limit of zero OH coverage and decreases linearly to 6 water molecules for 0.16 ML of OH. The solvating molecules are essentially all beta-state molecules as the gamma-state contains only 0.5 water molecules per OH group. The maximum coverage of solvating water molecules and OH groups is 1.04 ML in good agreement with a perfect bilayer coverage of 1.18 ML on Ag(110). From this we identify the beta-state as an extended surface bilayer structure. Isotope exchange experiments demonstrated proton mobility in the OD/H2O adlayer at temperatures of 150 K and below as well as migration of (18)OD groups from the surface and into alpha-state, multilayer water molecules. This constitutes desorption of hydroxide ion into the water-ice multilayers; the analogy between this reaction and electrochemical reduction of silver oxide is discussed.