Determination of Oxygen Reduction on Platinic and Non-Platinic Electrodes and Sulfur Oxidation Reaction Mechanisms on Pt and Pt₃Co Electrodes using in situ X-Ray Absorption Spectroscopy

X-ray Absorption Spectroscopy (XAS) has been used to examine SnOx supported Au, ligand-stabilized Pt nanoparticles, and sulfur poisoned Pt cathodes in an electrochemical cell in 1M HClO4. Both Extended X-ray Absorption Fine Structure (EXAFS) analysis, as well as a difference technique utilizing the X-ray Absorption Near Edge Structure (XANES) region (delta μ) are performed. Experimental results are interpreted in part by comparison with full multiple scattering theoretical results in the XANES region, and particle modeling in the EXAFS region. Au nanoparticles supported on tin oxide (Au-SnOx) are active electrocatalysts for the four-electron oxygen reduction reaction (ORR) to water as opposed to Au/VC which is inactive for the ORR generating mostly peroxides. The reason for this is a support effect that SnOx imparts on Au. Analysis of in situ XAS data taken on the Au L3 and Sn K edges show that a bifunctional mechanism plays the dominant role in the ORR below 0.55V. O2 adsorbs and dissociates on the SnOx surface with concurrent electron transfer from the Au and then is reduced to water by electron transfer from Au. Triphenyl phosphate triphosphate (TPPTP) stabilized Pt nanoparticles exhibit a dramatic reduction in OH coverage which poisons the Pt surface. Pt L3 edge XAS data show that a 0.3ML of TPPTP lies on the Pt surface via a Pt-P link at 0.54V converting to Pt-O-P link at 1.0V. This causes a reduction in OH adsorption due to weakening of the Pt-O bond which enhances the surface specific ORR rate. Sulfur poisoning on Pt and Pt3Co cathodes has been investigated using XAS. It takes nearly 8 cycles to fully remove S from the surface as opposed to 3 cycles for Pt3Co. From XAS studies, it is shown that Co exerts a ligand effect on Pt making the S weakly bound to the surface as opposed to Pt/VC where S binds more tightly. Calculations from FEFF 8.0 along with delta μ signatures reveal sulfur oxidation reaction mechanisms on both catalysts. S leaves Pt after oxidizing to SO3 which leaves with water from the electrolyte as H2SO4.

Author

• Gatewood, Daniel

Language

• en

Date created

• 2009

Type of Work

• Dissertation

License

• All rights reserved

Rights statement

• In Copyright

GW Unit

• Chemistry

Degree

• Ph.D.

Advisor

• Ramaker, David E

Committee Member(s)

• Pietron, Jeremy J
• Sadtchenko, Vladislav
• Licht, Stuart
• Gillmor, Susan D

Persistent URL

•  https://scholarspace.library.gwu.edu/etd/8p58pd049