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In-situ spectroelectrochemical surface studies of fundamental fuel cell catalysis

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Title: In-situ spectroelectrochemical surface studies of fundamental fuel cell catalysis
Author(s): Kutz, Robert
Director of Research: Wieckowski, Andrzej
Doctoral Committee Chair(s): Wieckowski, Andrzej
Doctoral Committee Member(s): Dlott, Dana D.; Gruebele, Martin; Kenis, Paul
Department / Program: Chemistry
Discipline: Chemistry
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): ethanol methanol formic acid fuel cell platinum catalyst catalysis vibrational spectroscopy sum frequency generation (SFG) broadband sum frequency generation (BB-SFG) sum frequency generation
Abstract: Electrochemical analysis and broadband sum frequency generation (BB-SFG) vibrational spectroscopy have been applied to a variety of fuel cell relevant systems. Primarily, ethanol oxidation was studied due to the potential of ethanol as an easily-transported and manufactured fuel with a higher molecular energy density of 12 electrons per molecule. Additionally, the reaction intermediate carbon monoxide was studied in-depth to better understand its spectroscopic behavior, and investigations were made into the characterization of a cobalt octaethylporphyrin complex by SFG spectroscopy. Ethanol electrooxidation as studied in acidic electrolytes yields certain key insights. The results include the first observation of adsorbed acetate and co-adsorbed sulfuric acid anions with SFG spectroscopy. Surface-adsorbed intermediates such as CO on Pt atop sites and acetate are observed in both H2SO4 and HClO4, but only H2SO4 shows evidence of adsorbed (bi)sulfate species or adsorption of CO to bridge sites. Studies performed with isotopically-labeled ethanol (12CH313CH2OH) demonstrate evidence of a methyl fragment (–12CHx) that is difficult to oxidize further to CO. Studied in alkaline electrolytes, the differences in the ethanol oxidation mechanism become more fully apparent. C–C bond cleavage and CO formation occur as early as 0.05 V vs. RHE. Furthrmore, CO is oxidized at ~0.45 V, which is 0.2 V lower than in acidic media. Isotopically-labeled ethanol (12CH313CH2OH) is once again used to study the formation of CO and other single-carbon intermediates. Surface-adsorbed 12CO and 13CO are observed in entirely different applied potential domains. 13CO molecules formed from the alpha carbon on the ethanol molecule, show the behavior expected from studies of CO-saturated alkaline media. 12CO is formed from the beta carbon of the ethanol molecule and is observed at unusually high potentials. The strongly adsorbed –CHx intermediate that is formed from the beta carbon after carbon-carbon bond cleavage is not oxidized from the Pt surface until the electrode potential is swept past 0.65 V. Further studies were also applied to methanol, acetaldehyde, and formic acid. The former two molecules were studied in an attempt to better understand the ethanol oxidation mechanism in alkaline media. Methanol oxidation in alkaline electrolytes produced a sustained CO signal and these results required clarification with the use of fast-scan cyclic voltammetry. Acetaldehyde oxidation in alkaline electrolytes shows unique CO behavior that is not observed in ethanol oxidation in the same electrolyte, adding further evidence that acetaldehyde is not a long-lived intermediate in ethanol oxidation in NaOH. Formic acid studies show the adsorbed formate radical appear on Pt(111) surfaces as soon as the CO molecule is oxidatively removed. Studies of spectroscopic behavior of CO were conducted to better construct an experimental framework for modeling CO adsorption. Here it is shown that both pH and CO coverage have different effects on the frequency of the CO vibrational band. Characterization of the Co-OEP macrocycle was conducted using Raman and BB-SFG spectroscopy. BB-SFG is shown to be a useful tool for in-situ observation of key molecular vibrations.
Issue Date: 2012-09-18
URI: http://hdl.handle.net/2142/34314
Rights Information: Copyright 2012 Robert Brien Kutz
Date Available in IDEALS: 2012-09-18
Date Deposited: 2012-08
 

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