Understanding oxygen reduction on tantalum oxyphosphate and tantalum oxide supported platinum by X-ray absorption spectroscopy

The Pt activity for the oxygen reduction reaction (ORR) is improved when Pt nanoparticles are supported on nanoscale layers of tantalum oxyphosphate (nominally, TaOPO ₄) on Vulcan carbon (VC) and heated at high temperature (660 °C) in reducing conditions. We attempt to explain the increased activity of the Pt-TaOPO ₄/VC "HT" by comparison to other less-active electrocatalysts comprising Pt on tantalum oxide (Pt-Ta ₂O ₅/VC) and Pt-TaOPO ₄/VC heated to 200 °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 Δμ 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 Δμ XANES analysis at the Pt L ₂ edge suggests that interfacial hydrogen exists between the Pt and the support for Pt-Ta ₂O ₅/VC and Pt-TaOPO ₄/VC "HT" at potentials > 0.3 V vs RHE even after surface H is removed. More importantly, in the most active sample, Pt-TaOPO ₄/VC "HT", the onset potential for O(H) adsorption is highest in the Δμ XANES. The XAS and microscopy/EDS results show that the Pt-TaOPO ₄/VC "HT" is Pt-Ta ₂O ₅/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-TaOPO ₄/VC "HT" facilitates proton conduction to the Pt particles and hence moves the equilibrium for OH adsorption on the Pt to higher potentials.