Alexey Tsyganenko
St. Petersburg State University, Russia
Title: FTIR spectroscopy for the studies of catalysts and catalytic reaction mechanisms
Biography
Biography: Alexey Tsyganenko
Abstract
IR spectroscopy became a classical method of solid catalysts characterization. The paper deals with the advances in the studies of mechanisms, establishing the structure of intermediates and the nature of active sites of the reactions catalyzed by oxides and zeolites using FTIR spectroscopy at variable temperatures. Variable temperature spectroscopy broadens the number of test molecules for acid sites. At low temperatures, besides ammonia, pyridine and nitriles, we can use CO, NO, H2 or other molecules that do not adsorb at 300 K. Low-temperature adsorption of weak CH proton-donating molecules such as CHF3, enables one to characterize the basicity of surface electron-donating sites. Carrying out simultaneous measurements of spectra, pressure and temperatures one can obtain thermodynamic characteristics of surface species, while spectrokinetic data provide information about the height of activation barriers. To trap the unstable intermediates of catalytic reaction, 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 processes at the surface of atmospheric aerosol particles. Th e structure of intermediates can be clarifi ed using isotopic substitution and finally the detailed mechanism of catalytic processes could be established. Some adsorption products, however, 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. Th ese species have the excess of energy and can be considered as an activated state, which can play a role of intermediate in catalytic reactions. Surface isomeric states were established for some other adsorbed species, such as cyanideion CN- produced by HCN dissociation. The linkage isomerism can be explained by an electrostatic model, or quantum mechanical calculations. Th e strength of surface sites can be aff ected by lateral interactions between the adsorbed species, which modifi es the catalytic properties of solids and shift the bands of test molecules, distorting the data on surface acidity. Co-adsorption of acidic and basic molecules leads to mutual enhancement of adsorption. Th is can be evidenced by protonation of bases, such as NH3 or 2,5-dimethylpyridine (DMP) on silanol groups in the presence of SO2 or NO2. Th is eff ect suggests an explanation of the promoting action of these gases in the reactions catalyzed by Brønsted sites. Besides the above eff ect of induced Brønsted acidity, induced basicity in the presence of adsorbed bases has also been detected spectroscopically. Lewis acidity can also be infl uenced by adsorbed acidic molecules. Th is eff ect was illustrated by CO adsorption on CaO pre-exposed to CO2, SO2, SO3, showing higher electron accepting ability of salts as compared with oxides. It is consistent with superacidity of oxides doped with (SO4)2- and explains much higher Lewis acidity of cationic sites in zeolites than that of oxides of the same elements. 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 coeffi cients, in a fair agreement with the published data on CO adsorption on ionic surfaces.