Intensities of IR stretching bands as a criterion of polarization and activation of adsorbed molecules in heterogeneous acid-base catalysis
Zusammenfassung der Projektergebnisse
A new spectral criterion for the evaluation of the chemical activation on catalyst surfaces and the characterization of active sites was developed in collaboration between the Zelinsky Institute of the Russian Academy of Sciences in Moscow and the Fritz Haber Institute of the Max Planck Society in Berlin. The initial hypothesis was that the intensities of stretching vibrations in IR spectra, which reflect the change of the dipole moment during vibration, should indicate the polarization of the respective bond through interaction with the surface and thus its chemical activation. Hence, IR band intensities could be an additional criterion of chemical activation besides the well-known frequency shifts of IR stretching bands. Polarization is the main source of chemical activation in acid-base catalysis, and the interaction of hydrocarbons with oxidic surfaces was selected as a test case for the hypothesis. The intensity distribution in the C-H stretching regime of small hydrocarbons (i.e., alkanes and alkenes) adsorbed on zeolites was investigated systematically by diffuse reflectance IR spectroscopy. The framework type and the nature of the cation (alkali, alkaline earth, zinc, proton) were varied. The results consistently showed that the intensities of the C-H stretching bonds are sensitive to the cation type and the framework type. Further, the relative intensities changed up to an order of magnitude with respect to the gas phase spectra while the frequencies shifted only a few percent. On zeolites as well as on Ga2U3 it could be shown that adsorption and polarization of adsorbed molecules is followed by heterolytic dissociation of those bonds that are activated according to analysis of the IR spectra. It was attempted to determine absolute extinction coefficients by combining transmittance IR spectroscopy with a gas-volumetric method (to measure the adsorbed amount). Sulfated zirconia and zeolite materials were investigated, and most of these materials, even if pressed into very thin wafers, scattered the IR radiation too strongly in the C-H stretching regime to allow for accurate determination of the extinction coefficients of adsorbed hydrocarbons. More reliable results were obtained when adsorbing alkenes on suitable zeolites and analyzing the C=C stretching regime, where scattering was less pronounced. Also in this case, a correlation of the intensity of the respective band and the cation polarizing power was obtained. The results demonstrate that IR intensities of stretching vibrations can be used as a criterion of chemical activation of individual bonds in molecules upon their interaction with the surface of a catalyst and in turn, can also be used to characterize surface sites.
Projektbezogene Publikationen (Auswahl)
- Effect of Mn and Fe on the Reactivity of Sulfated Zirconia toward H2 and n-Butane: A Diffuse Reflectance IR Spectroscopic Investigation. Langmuir 21 (2005) 10564-10572
B.S. Klose, F.C. Jentoft, R. Schlögl, I.R. Subbotina, V.B. Kazansky
- Intensities of IR Stretching Bands as a Criterion of Polarization and Initial Chemical Activation of Adsorbed Molecules in Acid Catalysis. Ethane Adsorption and Dehydrogenation by Zinc Ions in ZnZSM-5 Zeolite. Journal of Physical Chemistry B 109 (2005) 2103-2108
V.B. Kazansky, E.A. Pidko
- In Situ Spectroscopic Investigation of Activation, Start-up and Deactivation of Promoted Sulfated Zirconia Catalysts. Catalysis Today 116 (2006) 121-131
B.S. Klose, F.C. Jentoft, P. Joshi, A. Trunschke, R. Schlögl, I.R. Subbotina, V.B. Kazansky
- Intensities of C-H IR Stretching Bands of Ethane and Propane Adsorbed by Zeolites as a New Spectral Criterion of Their Chemical Activation via Polarization Resulting from Stretching of Chemical Bonds. Journal of Physical Chemistry B 110 (2006) 17468-17477
V.B. Kazansky, I.R. Subbotina, F.C. Jentoft, R. Schlögl
- Intensities of Combination IR Bands as an Indication of the Concerted Mechanism of Proton Transfer from Acidic Hydroxyl Groups in Zeolites to the Ethylene Hydrogen-Bonded by Protons. Journal of Catalysis 240 (2006) 66-72
V.B. Kazansky, I.R. Subbotina, F. Jentoft
- Interplay of Bonding and Geometry of the Adsorption Complexes of Light Alkanes within Cationic Faujasites. Combined Spectroscopic and Computational Study. Journal of Physical Chemistry B 110 (2006) 22618-22627
E.A. Pidko, J. Xu, B.L. Mojet, L. Lefferts, I.R. Subbotina, V.B. Kazansky, R.A. van Santen
- Unusual Infrared Spectrum of Ethane Adsorbed by Gallium Oxide. Journal of Physical Chemistry B 110 (2006) 7975-7978
V.B. Kazansky, I.R. Subbotina, A.A. Pronin, R. Schlögl, F.C. Jentoft
