Covalent surface bond

Despite the obvious beauty of this approach and the enormous research work dedicated to its technical realization there is no example of a covalently surface bonded hydroformylation catalyst in industry so far. The main difficulty that has been encountered is unacceptably high metal leaching from the support, mainly due to unfavorable ligand-metal complexation equilibria. Therefore, the best results have been reported for bidentate, covalently anchored ligands, with Rh leaching in the range 100-1000 ppb. Other drawbacks of the approach in... 

The bond length within the dissociating molecule increases upon adsorption. This process costs energy. The orbitals of adsorbate and surface need to have finite overlap in order for a covalent surface bond to form. The covalent interaction with the metal surface lowers the energy cost of dissociation. 

In Figure 10.1.17, the Cls XPS spectrum of the carbon-deposited film fluorinated at 175°C is compared with that of the corresponding fluorinated tubes after the HF washing. The former spectrum and the latter one give information on the external flat surface of the fluorinated film and the outer surface of the fluorinated tube, respectively. In spectrum (a), the most intense component is the peak assigned to covalent CF bond. On the other hand, spectrum (b) exhibits a clear peak at 284.4 eV, which corresponds to the sp2 carbon of the nanotube. These findings indicate selective fluorination of carbon tubes the covalent CF bonds were formed almost exclusively on the inner surfaces of nanotubes, but the outer surfaces retained their sp2 hybridization. 

The amine may enter into hydrogen bonding interaction with a surface hydroxyl (figure 9.23 a). The hydrogen bond formation is responsible for the fast adsorption of the silane molecules onto the silica surface, as discussed above. The basic amine function may abstract a proton from a silanol and form an ionic bond (figure 9.23 b). This type of interaction is much more stable than the first one. The hydrogen bonded molecules may self-catalyse the condensation of the silicon side of the silane molecule (figure 9.23 c). Thus, a covalent siloxane bond is formed. 

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