Active Sites at Finite Coverages – an STM Investigation of the Dissociation of NO on Ru(0001)

Abstract

Abstract We have investigated the role of active sites on a partially adsorbate covered surface. The decomposition of NO molecules on a Ru surface served as model reaction. Experiments were performed in an ultra-high vacuum chamber, by dosing NO on a Ru(0001) single crystal surface at 300 K. The distributions of N and O atoms resulting from the NO decomposition were investigated by scanning tunneling microscopy. We find massive changes compared to the zero-coverage limit that has been studied before. Whereas atomic steps are active sites for the decomposition of the NO molecules at low coverages, the flat terraces also become active at higher coverages. Mobile O atoms on the terraces, formed by the initial dissociation of the NO molecules, are responsible for this effect. Furthermore, narrow terraces fill with N and O atoms at a higher rate than wide terraces, also contrasting the low coverage result. The effect can be understood by the blocked surface diffusion of NO precursors to the steps by N atoms accumulating at the steps. On the approximately half covered surface striking mesoscopic concentration patterns are observed, connected with a demixing of the N and O atoms. Here two effects contribute, the different surface diffusion rates of N and O atoms away from the steps, and the different interactions between the two types of atoms. At saturation NO forms a molecular state, with one NO molecule per unit cell of the (2×2) mixed structure of N and O atoms.