Coordinative unsaturation catalysis

Catalysis by Metals. Metals are among the most important and widely used industrial catalysts (69,70). They offer activities for a wide variety of reactions (Table 1). Atoms at the surfaces of bulk metals have reactivities and catalytic properties different from those of metals in metal complexes because they have different ligand surroundings. The surrounding bulk stabilizes surface metal atoms in a coordinatively unsaturated state that allows bonding of reactants. Thus metal surfaces offer an advantage over metal complexes, in which there is only restricted stabilization of coordinative... 

A review22 with 100 references, is given on the chemistry of coordinatively unsaturated tripalladium and triplatinum clusters and on the relation between this chemistry and that which occurs on a Pt surface during chemisorption and catalysis. 

Therefore, the data indicate that Co-Mo-S can be considered as a M0S2 structure with Co atoms located in edge positions. As discussed below, these Co atoms play a direct role in the catalysis. Furthermore, it is generally accepted that the HDS reaction involves adsorption on sulfur vacancies. The low sulfur coordination number (large coordinative unsaturation) estimated from the Co EXAFS may, in fact, reflect that active sites (vacancies) are associated with the Co atoms. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

The difference between the close-packed crystal faces, such as (110) in bcc or (111) in fee crystals, and the open faces that prevail in the surface of nano particles, is of relevance to heterogeneous catalysis, because most adsorbates are more strongly bonded to the coordinatively unsaturated metal atoms in the open faces. 

Vacancies were later called coordinately unsaturated sites (cus). This is more in line with terminology used in organometallic chemistry. In view of the present understanding of the nature of the active sites, SBMS or Co(Ni)-Mo-S, the following discussion describes mechanisms in terms of catalysis by organometallic complexes. The references available on this topic are too numerous to mention, and the mechanisms are very well understood. A particularly useful reference is the book by Candlin, Taylor, and Thompson (90), although there are many others that can be consulted. 

Zeolites may behave as Lewis acids at Al,+ sites, or as Br0nsted-Lowry acids by means of absorbed H+ ions. Because they have relatively open structures, a variety of small molecules may be accommodated within the —O—Al—O—Si— framework. These molecules may then be catalyzed to react by the acidic centers. Coordinatively, unsaturated oxide ions can act as basic sites, and in some catalytic reactions both types of centers are believed to be important. Catalysis by zeolites is discussed further in Chapter 15. 

A recent development concerns the use of polyanions of the type [XMi 039M (0H2)]". In this type, the M atom easily becomes coordinatively unsaturated by dehydration (255). The resulting dehydrated anion, [XMhOjqM ]", can be considered an inorganic metalloporphyrin analog (322, 364, 365). Oxidation catalysis by these polyanions is described in Sections VIII and IX. Here, the catalytic performance and stability are compared with that of metalloporphyrin. 

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