Mixed water/OH structures on Pd(111)

Chemisorbed O and water react on Pd(111) at low temperatures to form a mixed OH/H2O layer with a (root 3 x root 3)R30 degrees registry. Reaction requires at least two water molecules to each 0 before the (2 x 2)0 islands are consumed, the most stable OH/water structure being a (OH + H2O) layer containing 0.67 ML of oxygen, formed by the reaction 3H(2)O + O -> 2(H2O + OH). This structure is stabilised compared to pure water structures, decomposing at 190 K as OH recombines and water desorbs. The (root 3 x root 3)R30 degrees - (OH + H2O) phase cannot be formed by O/H reaction and is distinct from the (root 3 x root 3)R30 degrees structure formed by O/H coadsorption below 200 K. Mixed OH/water structures do not react with coadsorbed H below 190 K on Pd(111), preventing this phase catalyzing the low temperature H-2/O-2, reaction which only occurs at higher temperatures. (c) 2006 Elsevier B.V. All rights reserved.