Heterogeneous single metal site

Single Metal Site Heterogeneous Catalysts and the Design of New Catalysis 17... 

The reduction of unsaturated C-C bonds constitutes one of the largest applications of heterogeneous catalysis [48]. On the surface of a metal particle, hydrogen is either cleaved homolytically to give M-H units or is adsorbed in the intact form. The metal sites that are responsible for either activation path of Hj on the catalyst surface as well as the chemical-physical characteristics of the adsorbed H2 are still a matter of debate. In this respect, molecular systems mimicking the activation of Hj on a single metal site without the cooperation of a solvent are expected to provide valuable information on the mechanism of Hj adsorption/release over heterogeneous metal catalysts. 

As is shown in this book, the activation of dihydrogen by single metal sites encompasses a broad spectrum of research fields, from biochemistry to chemistry and spectroscopy, hence to catalysis, industrial processes, and theoretical analyses. The studies outlined in this chapter are indicative of the range of ongoing applications and challenges of T 2-H2 metal complexes in homogeneous and heterogeneous catalysis. 

The splitting of N2 at a single metal site is remarkable, since generally multimetaUic cooperations are invoked in the context of enzymatic, organo-metaUic, and heterogeneous N2 activation. In monometaUic Schrock-Hke systems (see Scheme 33 in Section 3.3.2), the addition of external proton and electron sources is required, while no NH3 addition is formed on addition of H2 A mechanism for the surface reaction was later proposed on... 

Thus it appears from the knowledge gained from the chemistry of metal complexes of n-bonded thiophenes that n adsorption on highly (triply) unsaturated metal sites, which has been frequently invoked in the heterogeneous literature, may in itself be a peripheral situation, rather than a crucial phenomenon directly related to the actual HDS reactions taking place, since a number of other pathways involving the more abundant singly or doubly CUS are available under desulfurization conditions. 

Irrespective of the phase system (homogeneous, aqueous biphasic, heterogeneous single-site), the hydrogenation mechanisms of thiophene (T) or BT catalyzed by metal complexes comprise the usual steps of H2 oxidative addition, f-C,C coordination of the substrate, hydride transfer to form dihydrobenzothienyl, and elimination of DHBT by hydride/dihydrobenzothienyl reductive coupling (Scheme 2) [8],... 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

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