The influence of steps on the water-formation reaction on Ru(001)

The influence of well-defined steps on the water formation by reaction of surface oxygen with postadsorbed hydrogen on Ru(001) has been studied in the temperature range 300-600 K. The reaction rate for the stepped surface (8% step density) is considerably higher than for the flat surface, which is partly due to a higher sticking coefficient of hydrogen molecules. In agreement with the observations on flat Ru(001), the reaction rate vanishes when approaching an oxygen coverage of 0.25 ML. This is explained by structure sensitivity of the reaction rare: at 0.25 ML an unreactive O(2 x 2) superstructure is formed while at higher oxygen coverages a transformation into much more reactive O(2 x 1) domains occurs. At coverages close to 0.5 ML, the reaction slows down due to the vanishing sticking coefficient of hydrogen molecules on the O(2 x 1) areas. The reaction of coadsorbed oxygen and hydrogen atoms therefore occurs primarily at the rim of the reactive O(2 x 1) domains. At intermediate oxygen coverages a bimodal behavior of the reaction rate is found which is due to the formation of O(2 x 1) islands of different size, depending on the pretreatment of the layer. Under the reaction conditions used here the reaction rate is linearly dependent on the hydrogen background pressure, which indicates that the intermediate OH species is only short-lived.