Structure and Properties of TMS Catalytic Materials

The catalytic activity of TMS catalysts is related to defects in their crystal lattice that occur at the surface. However, the properties and stability of these defects are determined by the bulk atomic and electronic structure. It is therefore particularly important to know which phases are stable in a catalytic environment and what are their crystal structures. This knowledge was incomplete when the Massoth article was written (1). The subsequent development of this knowledge formed the basis on which new understanding of the fundamental origins of the catalytic effects in the TMS was developed. 

For the binary phases, there are two main classes of TMS structures  

Isotropic sulfides dominate the Group VIII sulfides. Many different structures exist in this group, such as pyrite (RuS2), pentlandites (Ni.3S2), and others. The coordination of the metals in this group by sulfur is generally sixfold, octahedrally arranged, but some special structures, such as CoySs, can also contain mixed sixfold and fourfold tetrahedral coordination. Some structures, such as PdS and PtS, also contain metals in distorted sixfold coordination. 

Layered sulfides predominate in Groups IV to VII except for MnS, which is isotropic. The main structural feature of this class is a strong chemical anisotropy that greatly affects their catalytic properties. In the molybdenite structure (MoS2), the metal is in trigonal prismatic coordination. Other layered sulfides are octahedrally coordinated or distorted octahedrally coordinated (ReS2) with the metal surrounded by six sulfur atoms. It should also be noted that some TMS sulfides have structures that fall in between isotropic and layered sulfides. Rh2S3 is an example and others are completely amorphous, such as IrSt and OsSr. 

However, because they require high-temperature synthesis, they cannot be prepared in a dispersed form and their catalytic activity is low. 

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