Vibrational spectroscopy is the most powerful non-destructive method for surface studies. Spectra of surface functional groups and adsorbed test molecules provide information on the nature of active sites, their strength and concentration. At low-temperatures it is possible to see the spectra of CO, NO, H2 N2, CHF3 or other simple molecules that do not adsorb at room temperature and characterize surface OH-groups, electron-accepting or electron-donating sites. Variable temperature spectroscopy enables us to study thermodynamics of surface processes and get information about the mechanism of catalytic reactions. To trap unstable intermediates of catalytic processes we can follow spectra evolution with temperature and their structure can be clarified using the isotopic substitution.
Some adsorption products cannot be stabilized at low temperatures, but arise at the surface as a result of thermal excitation.
So, besides the usual C-bonded structure CO forms with the cations in zeolites energetically less favorable O-bonded species. Such linkage isomerism was established for some other adsorbed species, such as cyanide ion CNproduced by HCN dissociation.
FTIR spectra are sensitive to lateral interactions between the adsorbed species, which shift the bands of test molecules or complicate their contours. Co-adsorption of acidic and basic molecules leads to mutual enhancement of adsorption. Acidity of surface sites can be increased by adsorbed acidic molecules, this explains the superacidity of oxides doped with
SO4 2-. By means of isotopic dilution this static interaction can be distinguished from the dynamic one. The latter, or resonance dipole-dipole interaction, accounts for the vibrational energy exchange in the adsorbed layer. Its spectral
manifestation provides additional information on the geometry of surfaces.
Quantitative spectral analysis of surface sites is not possible without the knowledge of absorption coefficients of test molecules. Quantum chemical calculations and electrostatic approach predict the correlation between the frequency shifts on adsorption and the absorption coefficients, in a fair agreement with the experimental data.