Morphology of WS2 nanoclusters in WS2/C hydrodesulfurization catalysts revealed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging

Tungsten disulfide (WS2) nanoclusters supported on high surface area graphite are studied by use of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging. It is found that HAADF-STEM, in contrast to high-resolution transmission electron microscopy (HRTEM), enables ready observation of all the WS2 nanoclusters and gives direct insight into their morphology. Such measurements may thus provide a more detailed and quantitative basis for establishing activity correlations. The S-W-S layers of the WS2 nanocluster structure are found to be oriented parallel to the graphite surface and with an apparent topotactic orientation relationship. Whereas many WS2 nanoclusters are observed to be irregular in shape, hexagonal and more often triangular or truncated triangular shapes are also encountered. The shape variations within a particular sample are larger than the shape variations between different samples each prepared by a particular sulfiding procedure. The vast majority of nanoclusters contains only a single S-W-S layer even after sulfiding at 1073 K. (C) 2004 Elsevier Inc. All rights reserved.