Partial dissociation of water on Ru(0001) at low temperatures - Adsorption, structure formation and hydrogen passivation effects

Aiming at an improved molecular scale understanding of the interaction of water with Ru surfaces and electrodes, in particular with respect to thermally induced water dissociation, we have performed a systematic study of the low temperature adsorption and structure formation behavior of water on Ru(0001) by scanning tunneling microscopy (STM). Typical structures formed upon adsorption at below 100 K indicate the presence of molecularly adsorbed water. For adsorption at 120K, in contrast, the resulting structures are identical to phases which have been associated with partially dissociated adsorbed water, indicating that for continuous adsorption under these conditions thermally induced water dissociation is activated. This assignment is supported by the strongly hindered activity for further adsorption of H2O or CO (at 100 K) on surfaces where only half of the surface is covered by structures related to an adsorbed H2O/OH phase. This is attributed to passivation by a H adlayer on the remaining, apparently bare surface areas, which results from H2O dissociation. Consequences of these findings on the understanding of water interaction with Ru(0001) are discussed.