Atomic-resolution in situ transmission electron microscopy of a promoter of a heterogeneous catalyst.

Insight into the location, state, and function of a promoter in heterogeneous catalysis was obtained through atomic-resolution in situ transmission electron microscopy. In the most active ruthenium catalyst for ammonia synthesis known so far, the barium promoter is shown to be located in two different phases in the catalyst. The increased activity is suggested to be related to a two-dimensional barium-oxygen overlayer on the ruthenium crystals. The possibility for conducting such studies for other reactions could add substantially to our current understanding of heterogeneous catalysis. Heterogeneous catalysis plays an increasingly important role in environmental protection processes, in fuel upgrading, and in providing the majority of the chemical building blocks required by contemporary society. Most heterogeneous catalysts of industrial importance are multicomponent materials that are designed by trial-and-error experimentation. Application of even the most sophisticated physical-chemical characterization techniques is usually not sufficient to obtain a complete understanding of the structure of the active site, the reaction mechanism and kinetics, the structural dynamics, and the specific roles of all catalyst components.

Atomic-scale structure of single-layer MoS2 nanoclusters

We have studied using scanning tunneling microscopy (STM) the atomic-scale realm of molybdenum disulfide ( MoS2) nanoclusters, which are of interest as a model system in hydrodesulfurization catalysis. The STM gives the first real space images of the shape and edge structure of single-layer MoS2 nanoparticles synthesized on Au(111), and establishes a new picture of the active edge sites of the nanoclusters. The results demonstrate a way to get detailed atomic-scale information on catalysts in general.

Supported metal sulphide nanoclusters studied by HAADF-STEM.

The morphology of MoS(2) and WS(2) nanoclusters supported on high-surface area graphitic carbon was investigated using high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). Most of the MoS(2) (WS(2)) nanoclusters contain only a single S-Mo-S (S-W-S) layer and the most commonly encountered morphology is truncated triangular. This is in contrast to the hexagonal morphology of macroscopic MoS(2) (WS(2)) crystals. When in addition to molybdenum (tungsten), nickel is also present, the regular nanoclusters are truncated to a larger extent, indicating that Ni has influenced the morphology by the formation of so-called Ni-Mo-S (Ni-W-S) structures. For these structures, the additional truncations are observed to lead to dodecahedral-like shapes.

Surface, catalytic, and magnetic properties of small iron particles: The effect of chemisorption of hydrogen on magnetic anisotropy.

The superparamagnetic behavior of very small particles of metallic iron (ca 1.5 nm), with about half of their atoms at the surface, is changed reversibly by adsorption and desorption of hydrogen below the superparamagnetic transition temperature. The change after adsorption implies a lowering of the anisotropy energy barrier for the magnetic relaxation of iron and is ascribed to a change in crystalline shape. No such changes are observed for larger particles of iron (ca 8 nm) with about 10% of their atoms at the surface.