SCANNING-TUNNELING-MICROSCOPY INVESTIGATIONS OF CLUSTER SIZES OF MOLYBDENUM-BASED CATALYSTS ON GRAPHITE

Surface morphologies of molybdenum-based catalysts supported on graphite were investigated with a scanning tunnelling microscope. The oxide phases of Mo, Co, and Mo-Co were observed to form distinct types of clusters on graphite. MoOx clusters appeared to be elliptical, with sizes ranging from 100 to 200 Angstrom in length and 60 Angstrom in width, while CoOx formed round and needle-shaped dusters, about 200 Angstrom in size. Oxidized Co-Mo was observed to form ring-type structures which were uniformly 100 Angstrom in diameter. This ring-type structure was attributed to the CoMoOx structure with Co decorating the edges of Mo. Sulfided phases of these catalysts were observed to form large slabs on graphite. Sulfided MoOx formed large islands (100 Angstrom x 150 Angstrom). These may be formed from the aggregation of small MoS2 crystallites. Besides forming large patches, CoS also formed needle-type clusters similar to CoOx, but smaller (120 Angstrom) in length. These clusters and patches are believed to be bulk Co9S8. Sulfided CoMoOx formed CoMoS islands similar to the needle-type clusters observed in CoS, but much larger in size, from 300 to 600 Angstrom. No MoS2 and CoS types of clusters were observed on this sample. No apparent movement of the clusters was observed during repeated scanning, suggesting that there is an interaction between the cluster and the graphite support.