NMR as a probe for molecular adsorption on metal surfaces in electrochemical environments and at electrified interfaces
Wu, Jianjun
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Permalink
https://hdl.handle.net/2142/19399
Description
Title
NMR as a probe for molecular adsorption on metal surfaces in electrochemical environments and at electrified interfaces
Author(s)
Wu, Jianjun
Issue Date
1996
Doctoral Committee Chair(s)
Wieckowski, Andrzej
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Chemistry, Physical
Engineering, Chemical
Language
eng
Abstract
The $\sp{13}$C and $\sp{15}$N NMR spectra of surface adsorbed cyanide on a platinum electrode have been obtained for the first time. The in situ electrostatic field effect on $\sp{13}$C and $\sp{15}$N chemical shifts and the temperature effect on their relaxation times over a wide temperature range have also been investigated. T$\sb1$ measurements for both $\sp{13}$C and $\sp{15}$N show a single exponential decay of the magnetization, indicating a predominance of only one surface adsorbed species. T$\sb1$ rates of $\sp{13}$C is proportional to temperature from 10K to 273K, with a Korringa relationship T$\sb1\cdot$T $\sim$ 138 s$\cdot$K. A complex Korringa behavior is observed for $\sp{15}$N: over the temperature range from 50K to 200K, T$\sb1\cdot$T is $\sim$ 4300 s$\cdot$K, much larger than that of $\sp{13}$C; but at above 200K, T$\sb1\cdot$T is $\sim$ 3000 s$\cdot$K, indicating a small structural rearrangement. The above T$\sb1\cdot$T results show that the conduction electron of platinum is contributed to the T$\sb1$ relaxations and the carbon atom is attached to the platinum surface. T$\sb2$ measurements of $\sp{13}$C in deuterated/non-deuterated electrolytes show that the proton in H$\sb2$O contributes to T$\sb2$ relaxation. Temperature-dependent T$\sb2$ relaxation rates in D$\sb2$O provide information on surface diffusion. It has been confirmed that T$\sb1$, T$\sb2$, peak shift and breadth of $\sp{13}$CN on platinum, acquired in a sealed cell at room temperature, are the same as those obtained in an ENMR-cell under full potentiostatic control, within experimental error. It has also been found that both $\sp{13}$C and $\sp{15}$N surface resonances respond to the applied electric field. Further, the adsorption of CH$\sb3$CN on platinum in these electrochemical environments has been investigated by choosing specific nuclei as probe for the relaxation time measurements.
The results presented in this thesis show that surface NMR spectra of adsorbates at electrified interfaces can be obtained even weaker signals than $\sp{13}$C, such as $\sp{15}$N. The electrochemical NMR provides a direct and microscopic insight into the electronic structure, dynamics and the nature of the adsorbate-metal chemical bond at such an interface. This may greatly help in understanding the nature of metal-adsorbate interactions in electrochemical systems.
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