Electrochemical and NMR Studies of Fuel Cell Catalysts and Carbon Monoxide Surface Diffusion
Chung, Jong Ho
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https://hdl.handle.net/2142/84285
Description
Title
Electrochemical and NMR Studies of Fuel Cell Catalysts and Carbon Monoxide Surface Diffusion
Author(s)
Chung, Jong Ho
Issue Date
2007
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, Inorganic
Language
eng
Abstract
Surface diffusion of CO adsorbed on Pt nanoparticle catalysts was measured at the solid/liquid interface at and near room temperature, 253 K--293K, by using electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy and a spin labeling pulse sequence. 2H NMR data indicate that the electrolyte remains in a liquid-like state at temperatures where the experiments were performed. For the fully CO covered samples, the exchange between different CO populations, driven by a chemical potential gradient, is suggested as the main mechanism for CO diffusion. For partially CO covered samples, the diffusion activation energies (Ed) and pre-exponential factors (D 0) show the CO coverage dependence. Ed decreases linearly with increasing CO coverage. This indicates that the CO-CO repulsive interactions make a strong influence on the coverage dependence of the activation energy. The pre-exponential factor, D0, shows the exponential decrease as the CO coverage increases. This exponential decrease can be explained assuming a free site hopping mechanism as the main pathway for CO surface diffusion. The diffusion measurement was also carried out for CO adsorbed on small carbon supported Pt nanoparticles. For fully and partially covered samples, diffusion coefficients (DCO) show a particle size dependence. That is, for the CO adsorbed on small Pt nanoparticles, the diffusion is slower as compared to CO on large Pt nanoparticles. However, the root-mean-square-displacement values calculated from the diffusion coefficients, show that CO motions are fast enough to cover the whole particle in 1s. The properties of CO on Ru were also studied by using 13C NMR. 13C NMR spectra of CO adsorbed on Ru nanoparticles show a symmetrically broadened peak having a large isotropic shift as compared to the CO adsorbed on supported Ru catalysts. The CO adsorbed on Ru was metallic and the motional narrowing of the spectrum, and the deviation from Korringa behavior, all indicate the occurrence of the CO surface diffusion. Fermi level local density of states for CO 5sigma and 2pi* shows that the CO adsorption bond is weaker on Ru than on Pt. 195Pt NMR spectra of three carbon supported Pt nanoparticles whose size is 1.6, 2.6, and 4.8 nm in diameter show size dependence in line shapes. Although the variation in the NMR spectra with different particle size can be explained as due to the effect of the layer by layer variation of the s-type and d-type local density of states, the difference in the surface electronic properties of these Pt/C catalysts due to size variations was negligible.
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