Surface species synthesis

The field of surface-mediated synthesis of metal carbonyl clusters has developed briskly in recent years [4-6], although many organometallic chemists still seem to be unfamiliar with the methods or consider themselves ill-equipped to carry them out. In a typical synthesis, a metal salt or an organometallic precursor is brought from solution or the gas phase onto a high-area porous metal oxide, and then gas-phase reactants are brought in contact with the sample to cause conversion of the surface species into the desired products. In these syntheses, characteristics such as the acid-base properties of the support influence fhe chemisfry, much as a solvenf or coreactant influences fhe chemisfry in a convenfional synfhesis. An advanfage of... 

Anchored organometallic complexes of tungsten were prepared by the interaction of W(C4H7)4 with surface OH groups, and the resulting surface species play a role as precursors for the synthesis of different surface tungsten compounds (Scheme 7.11).231... 

For Example 14-16, all for NH3 synthesis, the authors implied that a surface reaction is the rate-determining step. For given steps in these three cases the logL values are similar. Either Step 8 or Step 12 could be rate determining but the reacting surface species are probably not N2 and H2, and therefore these steps can probably be ruled out. Almost certainly none of the steps in Table VII are rate determining in NH3 synthesis. 

The evidence, produced already in the early 1980s, that monometallic surface species like Os " or Rh carbonyl fragments, incapsulated into the surface of silica or alumina, may have the necessary mobility to react with each other, since they return quite easily under a CO atmosphere to the original clusters Os3(CO)i2 and Rh6(CO) respectively [26, 27, 31], was the origin of so-called surface mediated organometallic synthesis. 

However, over the past decade, advances in, and in particular the availability of sophisticated instrumentation, and in the understanding of the instrumental techniques and the hosts and guests to which they are applied, mean that this need no longer be the case. A recent example in which a gamut of carefully chosen techniques, including such basic but essential measurements as elemental analyses, has led to the same precise characterization of surface species as has been the mainstay of molecular compounds is the study of the synthesis, characterization and reactivity of tantalum hydrides on silica, and their involvement in the dissociation of dinitrogen [203]. 

Finally, the surface-mediated synthesis of ruthenium carbonyl complexes has also been used to prepare supported ruthenium particles. Using silica as a reaction medium and conventional salts, apart from Ru3(CO)i2, mononuclear Ru(CO)j, and high nuclearity carbonyl-derived species can be obtained by CO reductive carbonylation [127, 128]. This opens new routes to preparing tailored supported ruthenium particles. 

In addition, surface-mediated synthesis results a powerful tool in the preparation of surface carbonyl species (homo- or heteronuclear) that are further precursors of supported, tailored metallic catalysts. 

Scheme 12.3 Synthesis of rare-earth metal (Ln) surface complexes, that is, heterogenized Ln single-sites, on silica Do = donor ligands, L = monoanionic ligands and M = Al, Mg, Zr or Si M = Si refers to tethered surface species.

Scheme 12.10 Synthesis of scandium methyl surface species by consecutive ligand exchange (cf route C in Scheme 12.3).

Iron has a rich surface coordination chemistry that forms the basis of its important catalytic properties. There are many catalytic applications in which metallic iron or its oxides play a vital part, and the best known are associated with the synthesis of ammonia from hydrogen and nitrogen at high pressure (Haber-Bosch Process), and in hydrocarbon synthesis from CO/C02/hydrogen mixtures (Fischer-Tropsch synthesis). The surface species present in the former includes hydrides and nitrides as well as NH, NH2, and coordinated NH3 itself. Many intermediates have been proposed for hydrogenation of carbon oxides during Fischer-Tropsch synthesis that include growing hydrocarbon chains. 

EXAFS has been very useful in the study of catalysts, especially in investigating the nature of metal clusters on surfaces of the supported metal catalysts (Kulkarni et al, 1989 Sinfelt et al, 1984). A variety of systems has been examined already and there is still considerable scope for investigation in this area. Since EXAFS gives bond distances and coordination numbers and is absorber-selective, it is possible to study one metal at a time (Fig. 2.12). Thus, an EXAFS investigation of sulphided Co—Mo— Al20j and related catalysts has shown the nature of the reactive surface species (Kulkarni Rao, 1991). Cu/ZnO catalysts have revealed certain unusual features suggesting the complex nature of the species involved in methanol synthesis (Arunarkavalli et al, 1993). Time-resolved EXAFS is of considerable value for the study of catalysts (Sankar et al, 1992). 

A similar reaction of great commercial importance is the synthesis of ammonia from the diatomic elements. The catalysts that are used commercially in this reaction are mixtures of iron and iron oxide with the oxides of potassium and aluminum. Indicating the adsorbed species by the subscript (or), the mechanism for the surface-catalyzed synthesis of ammonia is thought to involve the steps... 

By characterizing various zeolite catalysts under the same reaction conditions, the authors found weaker MAS NMR signals of alkoxy species for the less active zeolites HY and HZSM-5 than for the more active zeolite H-beta (250). This observation suggests that the alkoxy species observed under steady-state conditions act as reactive surface species in the MTBE synthesis from isobutylene and methanol on acidic zeolite catalysts. 

The chemisorption studies of Parris and Klier (43) using the Cu/ZnO catalyst have been mentioned earlier. Carbon monoxide was irreversibly bonded at room temperature to the surface of the binary catalysts that were also active in methanol synthesis however, this irreversible adsorbate could be desorbed as CO, which indicates that it was not a surface carbonate but rather a strongly bonded carbonyl-type CO. Infrared studies of this chemi-sorbate are lacking and it would be very desirable to determine the structure of this surface species. 

Of particular interest are phthalocyanines with strong absorption in the near infrared. A considerable effort has gone into the synthesis of new compounds and the development of methods for conversion of known phthalocyanines into morphologies that are infrared sensitive. This lias been difficult because the absorption spectra and photogeneration efficiencies depend on the chemical and crystal structure, particle size and morphology, as well as the presence of absorbed surface species (Sappok, 1978 Whitlock et al., 1992 Kubiak et al., 1995). As a consequence, specific methodologies must be developed for the synthesis, purification, and treatment of each compound. Methods of fabrication must also be designed to ensure that the desired characteristics are retained or induced (Mayo et al., 1994 Yao et al., 1995). 

These results provide direct evidence that chemical reactions occur and that new chemical bonds are formed during active ion bombardment of organic surfaces. Also, the use of active ions for chemical synthesis via reaction of beams with surface species or by selective "chemical sputtering" is suggested. 

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