Water-hydroxyl phases on an open metal surface: breaking the ice rules

Abstract

Hydroxyl is a key reaction intermediate in many surface catalyzed redox reactions, yet establishing the phase diagram for water/hydroxyl adsorption on metal surfaces remains a considerable challenge for interfacial chemistry. While the structures formed on close packed metal surfaces have been discussed widely, the phase diagram on more reactive, open metal surfaces, is complex and the H-bonding structures are largely unknown. Based on scanning tunnelling microscopy and density functional theory calculations, we report the phase diagram for water/hydroxyl on Cu(110), providing a complete molecular description of the complex hydrogen bonding structures formed. Three distinct phases are observed as the temperature is decreased and the water/hydroxyl ratio increased: pure OH dimers, extended 1H₂O:1OH chains, aligned along the close-packed Cu rows, and finally a distorted 2D hexagonal c(2 × 2) 2H₂O:1OH network. None of these phases obey the conventional ‘ice rules’, instead their structures can be understood based on weak H donation by hydroxyl, which favours H-bonding structures dominated by water donation to hydroxyl, and competition between hydroxyl adsorption sites. Hydroxyl binds in the Cu bridge site in the 1D chain structures, but is displaced to the atop site in the 2D network in order to accommodate water in its preferred atop binding geometry. The adsorption site and stability of hydroxyl can therefore be tuned simply by changing the surface temperature and water content, giving a new insight as to how the open metal template influences the water/hydroxyl structures formed and the activity of hydroxyl.