Hydrogen bonding and surface interactions in protic solvents. Coadsorption of ammonia and hydrogen fluoride with water on silver(110)

Abstract

Coadsorption of water with either ammonia or hydrogen fluoride on an Ag(110) substrate has been studied to examine the relationships between intermolecular hydrogen bonding near the surface and preferential adsorption at the surface. The experiments were conducted in ultrahigh vacuum (UHV) with facilities for thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS). Adsorption was performed at 110 K for ammonia and water and at 90 K for hydrogen fluoride and water. Ammonia and water are simply coadsorbed at 110 K with no evidence of specific hydrated complexes nor of ionization to NH₄⁺. Coadsorbed water enhances the population of the chemisorbed state of ammonia, which we estimate to have a saturation coverage of 0.12 monolayer (ML) without water, through a mechanism of dielectric screening. At coverages approaching one monolayer and higher, water stabilizes the bulk of ammonia, increasing the average desorption temperature by as much as 21 K from 134 K. Stabilization energies estimated from these temperature shifts are of the order of 5 kJ mol^–1, much less than typical hydrogen bond energies, ca. 25 kJ mol^–1. In contrast, hydrogen fluoride and water form a well defined monohydrate HF · H₂O, as evidenced by the coincident thermal desorption curves of both species. The monohydrate behaves as an adlayer azeotrope and thermal desorption measurements can be used to follow the sublimation temperature as a function of adlayer composition. From these data so-called bubble-point and dew-point curves can be developed by analogy with normal binary solutions.