A comparison between water adsorbed on Rh(111) and Pt(111), with and without predosed oxygen

Abstract

The adsorption of water and its interactions with oxygen on Rh(111) were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS), and low energy electron diffraction (LEED); and comparison was made with similar data for Pt(111). On Rh(111) water absorbs molecularly in hydrogen-bonded clusters; no evidence for dissociation was seen on the clean surface. Reaction of water with adsorbed oxygen on Rh(111) produces hydrated surface hydroxyls. While the gross features of adsorption and hydroxyl formation are similar to those previously reported on Pt(111), significant differences in detail were found. In particular, the complex librational and OH-stretching regions of the HREELS spectra for H 2 O/Rh(111), more closely resemble those for other noble metal surfaces than the sharp, single feature observed for Pt(111). HREELS peaks at 970, 1020 and 1950 cm −1 seen for H 2 O/Pt(111) were absent on Rh(111). The middle (3a 1 ) molecular orbital for molecular water on Rh(111) is shifted towards the Fermi level, while on Pt(111) the spacing between the three orbitals is the same as in water vapor. Comparison with spectral data for bulk phases suggests that water on Pt(111) exists primarily in a state with O-O nearest neighbor distances closer to those of liquid water than of ice, allowing better match with the Pt(111) surface mesh. Additional minority species account for the additional EELS peaks specific to Pt(111). Water on Rh(111) is a mixture of ice-like water and water similar to the majority species on Pt(111). The structural differences lead to different chemistry. On both surfaces adsorbed oxygen and water react to yield a surface phase which evolves water upon heating to 210 K. On Pt(111) this phase contains OH but no H 2 O. On Rh (111) this phase contains both OH and H 2 O in association. The differences in the interactions between water and the (111) surfaces of these two catalytically and electrochemically similar metals may help explain electrochemical effects peculiar to the (111) face of Pt.