Bridging the pressure gap in water and hydroxyl chemistry on metal surfaces: The Cu(110) case

We report the first measurements on the,quantitative partitioning of water between its molecular and dissociated forms at a gas-metal interface under elevated water pressures and temperatures. By means of synchrotron-based in situ photoelectron spectroscopy, mixed H2O and OH phases on Cu(110) at H2O pressures up to 1 Torr in the 275-520 K temperature range are studied. In increasing order of stability, three phases with H2O/OH ratios of 2:11 1:1, and 0:1 were observed. It was found that surprisingly large quantities of molecular water are present on the surface up to 428 K in 1 Torr H2O. A detailed comparison with previous ultrahigh vacuum (UHV) studies shows that the observed species, phases, and chemical kinetics under UHV compare very well with our results at elevated pressures and temperatures. The stability of the hydrogen-bonded H2O-OH complex at the surface, and its influence on the adsorption -desorption and dissociation kinetics, constitutes the essential link between our results and those obtained under UHV conditions.