Catalytic reactions studied with surface-enhanced infrared spectroscopy
Olsen, Charles William
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Permalink
https://hdl.handle.net/2142/21883
Description
Title
Catalytic reactions studied with surface-enhanced infrared spectroscopy
Author(s)
Olsen, Charles William
Issue Date
1989
Doctoral Committee Chair(s)
Masel, Richard I.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Engineering, Chemical
Language
eng
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.
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