Langmuir 2006, 22, 637-641
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Sum Frequency Generation Studies of Surfaces of High-Surface-Area Powdered Materials Mohsen S. Yeganeh,* Shawn M. Dougal, and Bernard G. Silbernagel ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801 ReceiVed July 12, 2005. In Final Form: October 6, 2005 We report the first observation of sum frequency generation (SFG) photons on high-surface-area powders, critically important materials in heterogeneous catalysis. We utilize SFG in total internal reflection (TIR) geometry and show that the TIR-SFG approach markedly reduces the destructive interference associated with nonlinear optical spectroscopy of small particle surfaces, making SFG studies of high-surface-area powders possible. The index of refraction of materials and their distance from the TIR-SFG prism are key parameters in generating and detecting the sum frequency signal. We find that TIR-SFG is highly sensitive to capillary condensation. To demonstrate the capability of the TIR-SFG technique, we measure the thermodynamics of methanol adsorption and desorption on high-surface-area SiO2.
1. Introduction In situ characterization of catalytic reactions under actual process conditions has been a monumental goal of chemical industries for many years. Achieving this goal is particularly challenging with heterogeneous catalysts, because only the surface can take part in the reaction. For these systems, the ideal in situ probe would follow only the surface processes, ignoring the bulk of the catalytic material, the gas- and liquid-phase reactants, and the products. Vibrational probes, such as IR and Raman spectroscopy, are of interest in catalytic studies given their sensitivity to chemical entities and intermolecular interactions.1 However, due to the intrinsic sensitivity of these probes to the bulk of materials, differentiation of the adsorbed molecule from its gas phase is difficult. This is a particularly problematic issue with molecules that have a similar vibrational frequency in their adsorbed and gas phase. In addition, IR and Raman spectroscopy generally are not suitable for obtaining the molecular orientation and arrangement at a solid surface. Sum frequency generation (SFG) spectroscopy is also a vibrational probe, similar to IR spectroscopy. It provides structural/compositional information and, moreover, possesses intrinsic surface sensitivity.2 Over the past decade, SFG spectroscopy has been proven to be a unique tool to study solid/ solid,3 liquid/solid,4 and gas/solid5 interfaces. SFG spectroscopy has also achieved some success in catalytic applications.6-8 Despite the applicability of SFG spectroscopy to many systems, this probe has been applied only to model catalytic materials with a planar geometry. Although it is important to understand interfacial phenomena on flat substrates, most practical systems, * Corresponding author. E-mail:
[email protected]. (1) See for example, Burcham, L. J.; Briand, L. E.; Wachs, I. E. Langmuir 2001, 17, 6175. (2) Shen, Y. R. Nature 1989, 337, 519. (3) Yeganeh, M. S.; Qi, J.; Yodh, A. G.; Tamargo, M. C. Phys. ReV. Lett. 1992, 68, 3761. (4) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1992, 103, 3292. (5) Hatch, S. R.; Polizzotti, R. S.; Dougal, S.; Rabinowitz, P. Chem. Phys. Lett. 1992, 196, 97. (6) Miragliotta, J.; Polizzotti, R. S.; Rabinowitz, P.; Cameron, S. D.; Hall, R. B. Chem. Phys. 1990, 143, 123. (7) Owrutsky, J. C.; Culver, J. P.; Li, M.; Kim, Y. R.; Sarisky, M. J.; Yeganeh, M. S.; Yodh, A. G.; Hochstrasser, R. M. J. Chem. Phys. 1992, 97, 4421. (8) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942.
including heterogeneous catalysts, are high-surface-area powders. Thus, the investigation of interfaces involving powders and irregular geometries is critical. Recently, second harmonic generation (SHG)sa similar technique to SFGshas been used to detect adsorbed molecules on micron-sized spherical materials.9-11 Detection of SFG signals from the surface of 1-5 µm sodium dodecyl sulfate powders has also been reported.12 The fundamental reason for selecting relatively large particles in these studies is that the dimension of these particles is much greater than the wavelength of light used. Consequently, SF photons generated at the surface of the opposite sides of a particle do not fully destructively interfere. However, for small particles (e.g.,
