S. Beau Waldrup, University of South Carolina, Department of Chemical Engineering, Columbia, SC 29208 and Christopher Williams, Chemical Engineering, University of South Carolina, 2C02, Swearingen Engineering Center, 301 South Main Street; USC, Columbia, SC 29208.
Sum-Frequency spectroscopy (SFS) is a relatively new technique that is increasingly being utilized to investigate so-called "buried interfaces". The power of the nonlinear optical approach lies in the ability to selectively probe interfacial molecular vibrational properties, even in the presence of significant bulk phase species with similar (or identical) vibrational signatures. In this study, SFS has been used to probe the interface between a model catalyst support (Al2O3-coated CaF2) and liquid consisting of neat acetonitrile and acetonitrile in ethanol. Vibrational features associated with both CN stretching (~2242 and 2281 cm-1) and CH stretching (~2942 and 2989 cm-1) are observed for adsorbed nitrile. In the case of the neat liquid on the model catalyst support, the CH stretching vibrations have also been probed using various polarization combinations of the sum frequency, visible and infrared beams in order to determine the orientation of the nitrile with respect to the surface. The average molecular angle calculated (~39o) shows the off-normal alignment of acetonitrile on the surface of the model support. Finally, an attempt has been made to obtain qualitative information about the adsorption isotherm of acetonitrile on Al2O3/CaF2. The lack of curvature in the isotherm indicates the lack of affinity for the surface by acetonitrile in ethanol. In addition, a model catalyst (Pt/Al2O3/CaF2) has also been examined under these conditions, opening up avenues for studies of catalytic hydrogenation mechanisms.