Coordinative chemisorption

Continuity equation electrochemical reactor, 30 311 mass transport, 30 312 Continuous-flow stirred-tanlt reactor, 31 189 Continuous reactor, 33 4-5 Continuous stirred-tank reactor, 27 74-77 ControUed-atmosphere studies, choice of materials for construction, 31 188 Conversion theory, 27 50, 51 Coordinatimi number, platinum, 30 265 Coordinative bonding, energy of, 34 158 Coordinative chemisorption on silicon, 34 155-158... 

The question of the possibility of coordinative chemisorption of molecules on a tetracoordinated silicon atom of Si02 is still extensively discussed in the literature. The assumption of the existence of such adsorption is sometimes used to explain various transformations of molecules on the surface of silica gel. Even this circumstance itself draws attention to this problem. 

Fig. 10. Cluster model of the coordinative chemisorption of an NH3 molecule on (a) the face and (b) the edge of a silicon-oxygen tetrahedron in Si02.

CO <7-coordinated complexes (AaG° = +9, +7 and +6 kJ moP for A, A and B species, respectively), in agreement with the increased strength of the coordinative chemisorption with respect to the plain polarization of the CO molecule. 

B.J.H. methods) (iii) the average diameter (T.E.M.) and/or the dispersion (chemisorption of probe molecule) of the metallic particle. EXAFS will also provide average coordination numbers, which decrease sharply as the particle size decreases. 

The hydrogen molecule does not chemisorb onto clean sintered gold surfaces at or above 78 K [147] but on unsintered films, a small amount of H2 is chemisorbed if gold surface atoms of low coordination number are present [148]. Stobinski [149] found that H2 can also chemisorb on thin sintered Au films if the surface is covered at low temperatures with a small amount of gold equivalent to 1-3 Au monolayers prior to H2 exposure. This suggests a fundamental role of surface Au atoms of low coordination number in the chemisorption process. Deuterium molecules also chemisorb in a similar fashion on gold films at 78 K and isotope effects were... 

It is of tremendous practical importance that at many metal surfaces chemisorption of a large variety of molecules can occur. This can easily be explained from a discussion on a molecular level as follows. At the surface of a crystal the co-ordination number of the metal atoms is lower than in the bulk of the crystal. Fig. 3.4 illustrates this for Ni. In the bulk the coordination number of all Ni atoms is 12, whereas on the three faces in Fig. 3.4 these numbers are only 8, 7 and 9 for the (100), (110) and (111) surfaces, respectively. So-called free valences exist at the surface. The numbers between brackets are the Miller indices of the surfaces. 

Active catalyst sites can consist of a wide variety of species. Major examples are coordination complexes of transition metals, proton acceptors or donors in a solution, and defects at the surface of a metallic, oxidic, or sulphidic catalyst. Chemisorption is one of the most important techniques in catalyst characterization (Overbury et al., 1975 Bartley et al, 1988 Scholten et at, 1985 Van Delft et al, 1985 Weast, 1973 and Bastein et al., 1987), and, as a consequence, it plays an essential role in catalyst design, production and process development. 

Figure 1.4 Proposed steps in the chemisorption of OH on/in Pt, starting with arrays of OH groups over the uppermost metal atom layer, increasing the coordination number of the adsorbed OH by place exchange, and next generating a mixed, metal/oxygen overlayer while further oxidizing to form O atoms. From Conway et al. [1990].

Of crucial significance in deciding between various models have been estimates of the number of copper atoms required to transform the surface into a (2 x 3)N phase. This was the approach adopted by Takehiro et al 2 in their study of NO dissociation at Cu(110). They concluded that by determining the stoichiometry of the (2 x 3)N phase that there is good evidence for a pseudo-(100) model, where a Cu(ll0) row penetrates into the surface layer per three [ll0]Cu surface rows. It is the formation of the five-coordinated N atoms that drives the reconstruction. The authors are of the view that their observations are inconsistent with the added-row model. The structure of the (2 x 3)N phase produced by implantation of nitrogen atoms appears to be identical with that formed by the dissociative chemisorption of nitric oxide. 

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. 

The rather low coordination in the (100) and (110) surfaces will clearly lead to some instability and it is perhaps not surprising that the ideal surface structures shown in Figure 1.2 are frequently found in a rather modified form in which the structure changes to increase the coordination number. Thus, the (100) surfaces of Ir, Pt and Au all show a topmost layer that is close-packed and buckled, as shown in Figure 1.3, and the (110) surfaces of these metals show a remarkable reconstruction in which one or more alternate rows in the <001 > direction are removed and the atoms used to build up small facets of the more stable (111) surface, as shown in Figure 1.4, These reconstructions have primarily been characterised on bare surfaces under high-vacuum conditions and it is of considerable interest and importance to note that chemisorption on such reconstructed surfaces can cause them to snap back to the unreconstructed form even at room temperature. Recently, it has also been shown that reconstructions at the liquid-solid interface also... 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

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