Alexey Tsyganenko

Abstract:

Surfaces and interfaces are everywhere: in the ground, seawater, atmosphere, space, and even in the living organisms. Vibrational spectroscopy is the most powerful non-destructive method for surface characterization. Spectra of surface functional groups and adsorbed test molecules provide information on the nature of active sites, their strength and concentration. Variable temperature spectroscopy data enable us to study thermodynamics of surface processes and measure the energy or entropy of adsorption. At low-temperatures it is possible to see the spectra of CO, NO, H2 N2 or other simple molecules that do not adsorb at room temperature. Using low-temperature adsorption of weak CH proton-donating molecules such as CHF3, we can characterize the basicity of surface electron-donating sites. Carrying out simultaneous measurements of spectra, pressure and temperatures one can obtain spectrokinetic data and get information about the height of activation barriers of surface reactions. To trap the unstable intermediates of catalytic processes we can follow spectra evolution with temperature and observe the chain of reactant transformations. In particular, the method can be applied to the studies of photocatalytic reactions, modeling the reactions at the surface of atmospheric aerosol particles. The structure of intermediates can be clarified using isotopic substitution, then the detailed mechanism of catalytic processes could be established. Some adsorption products cannot be stabilized at low temperatures, but arise at the surface as a result of thermal excitation. So, CO forms with the cations in zeolites two kinds of complexes. Besides the usual C-bonded structure the energetically less favorable O-bonded species arise and exist in thermodynamic equilibrium with usual form. Such linkage isomerism was established for some other adsorbed species, such as cyanide ion CN- produced 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 is consistent with superacidity of oxides doped with SO42-. By means of isotopic dilution this static interaction can be distinguished from the dynamic one. The latter, called also as resonance dipole-dipole (RDD) interaction, accounts for the vibrational energy exchange in the adsorbed layer. Its spectral manifestation provide 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.