Dissociation of CH4 on Ni(1 1 1) and Ru(0 0 0 1)

Recently steps and surface imperfections were proven by Dahl et al. [Phys. Rev. Lett. 83 (1999) 1814] to have an all-dominating effect on the N2 dissociation on Ru(0 0 0 1). In this paper the dissociative sticking of CH4 on the close-packed surfaces of Ni and Ru has been investigated in order to clarify to what extent a similar effect is present in these systems. The apparent activation energies found were 74±10 and 51±6 kJ/mol on Ni(1 1 1) and Ru(0 0 0 1) respectively. On neither Ru(0 0 0 1) nor Ni(1 1 1) did we observe a significant decrease of the overall sticking coefficient when the steps were passivated with Au. On Ni(1 1 1) sputtering the surface at 500 K with no subsequent annealing increased the initial sticking but at higher coverages this effect vanished. We interpret these results as due to the steps on both surfaces being blocked by carbon species leaving them inaccessible to impinging CH4 molecules. We conclude that in these types of experiments steps play a minor role for CH4 dissociation on Ru(0 0 0 1) and Ni(1 1 1). In a temperature programmed oxidation reaction we observe that carbon from the steps of Ru(0 0 0 1) leaves the surface at lower temperatures than carbon from the terraces. Finally we have observed a large promotion of the sticking probability of CH4 on Ru(0 0 0 1) by increasing the surface temperature at a fixed gas temperature thus lending support to a mechanism where the dissociation of methane takes place over Ru atoms displaced normal to the surface.