Complex reactions on a convertible catalyst surface: A study of the S-O-Cu system

Abstract

The interaction of clean and partially oxidized Cu(110) with sulphur was studied by scanning tunneling mi-croscopy and density functional theory calculations in the low-coverage range. On the clean Cu surface in-dividual S atoms adsorb in the troughs between the Cu atom rows. Hollow sites are preferred, but long-bridgesites are occasionally occupied as well. The majority of adsorbed S, however, seems to be involved in the for-mation of highly mobile CuxSyclusters of various sizes. The clusters preferentially attach to steps thus changingthe step morphology completely. Some of the clusters form aggregates on the terraces. On the partially oxidizedsurface similar clusters form and cause long-range mass transport to steps. Additionally, nanowires form in [001]direction on and along the surface oxide stress domains. These nanowires have a complex composition, exhibitdifferent corrugations and appear sometimes as three-dimensional needles. Occasionally theyflip their directionby 90°, but doing so they partially decompose. Finally, annealing of the S-O-Cu surface leads to consumption ofthe surface oxide stripes indicating loss of oxygen presumably via SO2formation. Simultaneously, linear sulphurchains suspended between the [001]-O-Cu-O- chains form in [110] direction. The surprising multitude of pro-cesses and products even at low-pressure, low-temperature conditions in the comparatively simple S-O-Cusystem highlights the difficulty of controlling reactivity and selectivity on such convertible catalyst surfaces.