Understanding Oxygen Reduction on Tantalum Oxyphosphate and Tantalum Oxide Supported Platinum by X-ray Absorption Spectroscopy

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• 作者：Anna Korovina ; Yannick Garsany ; Albert Epshteyn ; Andrew P. Purdy ; Karren More ; Karen E. Swider-Lyons ; David E. Ramaker
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：August 30, 2012
• 年：2012
• 卷：116
• 期：34
• 页码：18175-18183
• 全文大小：487K
• 年卷期：v.116,no.34(August 30, 2012)
• ISSN：1932-7455

文摘

The Pt activity for the oxygen reduction reaction (ORR) is improved when Pt nanoparticles are supported on nanoscale layers of tantalum oxyphosphate (nominally, TaOPO₄) on Vulcan carbon (VC) and heated at high temperature (660 掳C) in reducing conditions. We attempt to explain the increased activity of the Pt-TaOPO₄/VC 鈥淗T鈥?by comparison to other less-active electrocatalysts comprising Pt on tantalum oxide (Pt-Ta₂O₅/VC) and Pt-TaOPO₄/VC heated to 200 掳C in air. Our toolbox for this analysis contains the rotating disk electrode methodology for characterization of the ORR and high-angle annular dark-field scanning transmission electron microscopy with high-resolution energy-dispersive X-ray spectroscopy (EDS) capabilities. The adsorption of molecular species on the Pt and Ta is determined from the 螖渭 XANES (X-ray absorption near-edge structure) adsorbate isolation technique of X-ray absorption spectroscopy (XAS) data for electrocatalsyts in situ, whereby H and OH adsorption products from water activation can be used to infer how an electrocatalyst would react for oxygen reduction. The 螖渭 XANES analysis at the Pt L₂ edge suggests that interfacial hydrogen exists between the Pt and the support for Pt-Ta₂O₅/VC and Pt-TaOPO₄/VC 鈥淗T鈥?at potentials > 0.3 V vs RHE even after surface H is removed. More importantly, in the most active sample, Pt-TaOPO₄/VC 鈥淗T鈥? the onset potential for O(H) adsorption is highest in the 螖渭 XANES. The XAS and microscopy/EDS results show that the Pt-TaOPO₄/VC 鈥淗T鈥?is Pt-Ta₂O₅/VC with polyphosphate groups directly associated with the Pt. From the body of results, we surmise that the nanoscale layer of polyphosphate on the Pt-TaOPO₄/VC 鈥淗T鈥?facilitates proton conduction to the Pt particles and hence moves the equilibrium for OH adsorption on the Pt to higher potentials.