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Title:Catalytic reactions studied with surface-enhanced infrared spectroscopy
Author(s):Olsen, Charles William
Doctoral Committee Chair(s):Masel, Richard I.
Department / Program:Chemical and Biomolecular Engineering
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Physical
Engineering, Chemical
Abstract:Reflection absorption infrared spectroscopy is used to investigate ammonia, methylamine, dimethylamine and trimethylamine adsorption of Pt(111). The RAIR spectra show molecular adsorption in all cases with the subsequent formation of low symmetry surface complexes.
The most significant aspect of the data is the significantly enhanced IR absorption intensities observed. They are 2-3 orders of magnitude larger than expected from gas phase absorption coefficients and surface densities. Representative integrated absorbencies for 1 $\times\ 10\sp{15}$ molecules/cm$\sp2$ on Pt(111) are 49% cm$\sp{-1}$ for ammonia, 52 % cm$\sp{-1}$ for methylamine, 28% cm$\sp{-1}$ for dimethylamine and 12% cm$\sp{-1}$ for trimethylamine. Interestingly, the observed enhancements show a strong dependence on the methods of sample preparation, but this behavior only affects the first monolayer which absorbs. This suggests the enhancements are a surface effect and hence, this is the first observation of surface enhanced infrared spectroscopy.
A benefit of the large IR intensities observed is that it allows the observation of rotational splittings in the spectra. An analysis of the rotational structure for methylamine adsorbed on Pt(111) yields a rotational constant for the ground state, A$\rm\sb{o}$, of 5.8 cm$\sp{-1}$. This can be compared to a rotational constant of 3.44 cm$\sp{-1}$ for gaseous methylamine. This marks the first observation of rotation in an adsorbed molecule with RAIRS.
The intensities observed here are not as yet accounted for by present theory. A plausible explanation consistent with this work involves hydrogen bonding. The molecules adsorb and form ordered clusters on the surface as a result of hydrogen bonding when possible. The hydrogen bonded chains then act collectively to produce the intensity enhancements. The details of the collective motions and subsequent enhancements, however, are not yet clear.
Issue Date:1989
Rights Information:Copyright 1989 Olsen, Charles William
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI8924914
OCLC Identifier:(UMI)AAI8924914

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