Characterization of surface species

Characterization of Surface Species by SERS. Before presenting the results obtained with 1, the spectral features which have proven to be useful in identifying surface species will be reviewed. Both in solution and by SERS, pyridines show a ring mode in the Raman spectrum near 1600 cm-. When the ring nitrogen is protonated, this band disappears and is replaced by a band near 1640 cm-1. The... 

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]. 

Figure 10 Typical ultrahigh vacuum system used for surface science studies of catalytic reactions on model systems. The combined development of new preparation methods for realistic catalytic samples and in situ spectroscopies for the molecular level characterization of surface species during catalysis promises to advance the basic understanding of catalytic processes...

To understand the electrocatalytic steps, a more detailed characterization of surface species and of the activity of surface sites for prospective catalysts is needed. Some adsorption results only on Pt, discussed earlier, indicate the existence of two differnet catalytic sites that contribute to benzene exchange reactions with deuterium (795,194). Adsorption and reaction on two different catalytic sites is not unknown in heterogeneous, gas phase catalytic reductions of olefinic and cyclic hydrocarbons (560). 

Structural Characterization of Surface Species and Surface Sites by Conventional Optical Spectroscopies... 

Carbon-13. Carbon-13 solid state NMR is particularly difficult on non-enriched samples because of the low density of the sites, in addition to the small natural abundance (1.1%) and weak sensitivity of carbon-13 (Tc/Yh = 0-25). The acquisition of NMR spectra is normally realized with sequences implying a polarization transfer from the protons to the carbons, cross polarization (CP, Scheme 2) under the Hartman-Hahn optimal conditions coh = YhBh = YcBc = c- This has the advantage to greatly enhance the intensity of the carbon signals. However, in the case of the characterization of surface species, it is still necessary to enrich samples with carbon-13 (10-100%). Yet, despite these problems, it has become a crucial technique for the identification of surface species. 

FTIR is especially useful for the characterization of surface species and the state of adsorbed molecules. A study of supported oxide catalysts involving FTIR and adsorption measurements is reported by Rives and his co-workers and the use of FTIR as an ancillary technique is referred to in several other papers. NMR measurements have been carried out in the context of image analysis (NMRI) of pore structures (Ewing et al), determination of dlffusivities of adsorbed species (Bahceli et al) and the pore structural analysis of wet materials (Smith and Davis). 

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