Scanning tunneling microscopy investigations of cluster sizes of molybdenum-based catalysts on graphite

Abstract

Surface morphologies of molybdenum-based catalysts supported on graphite were investigated with a scanning tunneling 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 Å in length and 60 Å in width, while CoOx formed round and needle-shaped clusters, about 200 Å in size. Oxidized Co-Mo was observed to form ringtype structures which were uniformly 100 Å 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 Å × 150 Å). 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 Å) 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 Å. 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.