abstracts - Институт катализа им. Г.К. Борескова

OP-III-11THERMODYNAMIC CHARACTERISTICS AND INTEGRAL ABSORPTIONCOEFFICIENT OF ADSORBED COTsyganenko A.A, Kondratieva E.V., Yanko V.S.V.A. Fock Institute of Physics of St. Petersburg State University, St. Petersburg, RussiaE-mail: tsyg@photonics.phys.spbu.ruVibrational spectroscopy is widely used for establishing the structure of adsorbed speciesand the nature of surface active sites. Most of information is drawn from the positions of IRbands, while band intensity provides data about site concentration. To obtain the latter, theknowledge of absorption coefficients ε of adsorbed test molecules is needed. Values of εthemselves can be used as a measure of molecule perturbation in the field of surface site.Besides these direct spectroscopic parameters, FTIR studies at variable temperatures providemeans to measure the thermodynamic characteristics of adsorbed species for the case ofreversible molecular adsorption. Pressure and temperature dependences of the band intensityenable the enthalpy and entropy of adsorption to be determined for distinct adsorbed species.In the present work analysis of variable temperature spectral data for CO adsorbed onsilica and on some zeolites was carried out in order to verify structural predictions ofelectrostatic model and to elucidate its applicability to describe spectral manifestations ofmolecule interaction with surface site, in particular the influence of surface electric field ofionic solids onto the absorption coefficient of the adsorbed molecules.In accordance with the electrostatic model [1], CO adsorption on ionic sites can beadequately described as interaction of a quadrupole with a point charge. With anions COshould form side-on complexes with slightly lowered frequencies. With the cations linearcomplexes bound via carbon or oxygen atom should be anticipated. The former areenergetically more favorable and have CO stretching frequency shifted to higherwavenumbers, while for the latter opposite electrostatic field direction with respect tomolecule causes frequency lowering. Another prediction of theory deals with CO absorbance,which in axial electrostatic field of the cation should decrease for usual C-bonded species andincrease for O-bonded complexes. This result is in contradiction with some earlier works, e.g.[2], where the integral extinction coefficient of CO bound to Lewis acid sites was claimed torise with the growing strength of interaction. Cationic zeolites, where equilibrium betweentwo kinds of CO complexes with the same cations was recently established by means ofvariable temperature FTIR spectroscopy [3], could be excellent systems to verify thisprediction, if only extinction of both structures can be measured separately.332