Using scanning tunneling microscopy we studied the adsorption and reactions of water on a Cu(110) surface partially covered with oxygen in the O(2 × 1) phase formed by Cu–O chains. The experiments were carried out from low (77 K) to higher temperatures to sequentially open reaction channels that require increased activation energy. At 77 K, water adsorbs at the edges and on top of the Cu–O chains. Heating to 155 K caused water molecules to react with the oxygen atoms in the chains to produce OH. These hydroxyl groups form elongated H-bonded structures mixed with excess unreacted water molecules. The Cu atoms freed in the reaction form small clusters at the location of the initial Cu–O chains. Heating to 180 K leads to a second reaction that desorbs all excess water (i.e., water not H-bonded with OH), leaving H2O–OH zigzag chains along [110] directions. At the low oxygen coverage studied here (<0.12 ML) the extent of the partial water dissociation reaction is stoichiometrically determined by the amount of preadsorbed oxygen (H2O + O → 2OH). A third annealing to 280 K resulted in desorption of all water and hydroxyl species and the reappearance of Cu–O chains. After heating to 280 K, the oxygen coverage returns to nearly the same value as that of the initially surface.

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