Surface chemistry of the non-basal planes of cobalt: The structure,stability, and reactivity of Co(101̄2)-CO

The structural and chemisorptive properties of the stepped, non-unique, (101̄2) surface of cobalt have been investigated by standard LEED/Auger/Δφ/thermal desorption methods. The clean surface is well-ordered, unreconstructed, and reversibly undergoes the predicted structural changes on cycling through the phase transition. CO chemisorption is rapid and non-dissociative at 300 K, leading ultimately to a (3 × 1) structure with a COCO spacing of 3.8 Å. Heating of the adlayer can, depending on the conditions, lead to competitive desorption and dissociation reactions. The data suggest that the transition state to desorption is mobile whereas that for dissociation is localised. Dissociation is accompanied by diffusion of oxygen into the bulk and formation of a very well-ordered (2 × 3) carbon structure. This structure is interpreted in terms of epitaxial growth of the (001) plane of Co₃C. The carbide surface is still capable of chemisorbing a substantial amount of CO, but cannot dissociate it. Some other ordered phases of the CoCCO system are also observed, and an attempt is made to interpret them in a consistent way. The CO chemistry of the (101̄2) surface is very different from that of the basal plane.