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A combined electrochemical and x-ray photoelectron spectroscopic approach to electrocatalysis

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Title: A combined electrochemical and x-ray photoelectron spectroscopic approach to electrocatalysis
Author(s): Rigsby, Matthew A.
Director of Research: Wieckowski, Andrzej
Doctoral Committee Chair(s): Wieckowski, Andrzej
Doctoral Committee Member(s): Scheeline, Alexander; Masel, Richard I.; Bailey, Ryan C.
Department / Program: Chemistry
Discipline: Chemistry
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): fuel cells X-ray photoelectron spectroscopy (XPS) electrocatalysis platinum rhodium underpotential deposition copper silver electronic effects
Abstract: Photoelectron spectroscopy and electrochemical techniques have been used to study commercial and model electrocatalysts for fuel cell applications. The overlying goal of this work is to gain a better understanding of the effects that electronic structure changes have on electrocatalytic systems. As a starting point, commercial Pt/Ru alloy nanoparticles of different compositions were investigated as catalysts for the electrooxidation of methanol and formic acid. In the course of the study of electrooxidation of methanol, it was found to be very difficult, if not impossible, to separate the effects of the bifunctional mechanism and the electronic structure effects that might play a role in the activity. However, due to the difference in reaction pathways, data for electrooxidation of formic acid, in conjunction with photoelectron spectroscopy, confirm a definitive contribution from electronic structure on the reactivity, demonstrating a direct connection with theory in heterogeneous catalysis. Another commercially available catalyst, nanoparticle Pt3Co alloy, was studied as an electrocatalyst for the oxygen reduction reaction in both acidic and alkaline media. X-ray photoelectron spectroscopy and electrochemical methods indicated that the as-received catalyst consisted of both metallic cobalt and cobalt oxides, and that a dissolution of the cobalt oxides occurred during electrochemical treatments in acidic media to form a Pt-skin structure, whereas the catalyst was stable in alkaline media. The Pt3Co catalyst was found to have better activity than Pt black catalysts in acidic media, but worse activity in alkaline media. Photoelectron spectroscopic results showed that the core-level binding energy of the platinum shifts to higher values in the alloy. This result indicates that an electronic effect is the probable reason for the improved activity of the Pt-skin catalyst. Attention was then turned to model catalysts, which provide a simplified method of studying fundamental electrocatalyst properties. To this end, platinum was deposited on a Rh(111) surface in order to obtain a range of coverage. The resulting Pt-decorated electrodes were probed by photoelectron spectroscopy in order to determine the shift in the core-level binding energy of platinum as a function of coverage. The platinum core-levels were found to shift to higher binding energy, in a manner consistent with electronic effects induced by lattice strain, charge transfer, and orbital rehybridization. In order to provide more information on the nature of electronic effects in metal-metal bonds, the core-level binding energies of underpotentially-deposited copper and silver on a Rh(111) electrode surface were examined by X-ray photoelectron spectroscopy. Through XPS, the examined surfaces were found to contain carbon, oxygen, and sulfur or chlorine species, which confirmed the presence of anions at the Rh(111) surface, in addition to the deposited metals. The core-level binding energies for underpotentially-deposited copper and silver displayed negative shifts with respect to both the bulk deposits and clean metal references. These negative shifts were explained in terms of contributions from charge transfer, orbital rehybridization, and lattice strain.
Issue Date: 2010-05-20
URI: http://hdl.handle.net/2142/16267
Rights Information: Copyright 2010 Matthew Aaron Rigsby
Date Available in IDEALS: 2010-05-20
Date Deposited: May 2010
 

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