In Situ and Real-Time Monitoring of Oxide Growth in a Few Monolayers at Surfaces of Platinum Nanoparticles in Aqueous Media

Abstract

The electrochemical oxidation behaviors of the surfaces of platinum nanoparticles, one of the key phenomena in fuel cell developments, were investigated in situ and in real time, via time-resolved hard X-ray diffraction and energy dispersive X-ray absorption spectroscopy. Combining two complementary structural analyses, dynamical and inhomogenous structural changes occurring at the surfaces of nanoparticles were monitored on an atomic level with a time resolution of less than 1 s. After oxidation at 1.4 V vs RHE (reversible hydrogen electrode) in a 0.5 M H(2)SO(4) solution, longer Pt-O bonds (2.2-2.3 A that can be assigned to OHH and/or OH species) were first formed on the surface through the partial oxidation of water molecules. Next, these species turned to shorter Pt-O bonds (2.0 A, adsorbed atomic oxygen), and atomic oxygen was incorporated into the inner part of the nanoparticles, forming an initial monolayer oxide that had alpha-PtO(2)-like local structures with expanded Pt-Pt bonds (3.1 A). Finally, quasi-three-dimensional oxides with longer Pt-(O)-Pt bonds (3.5 A, precursor for beta-PtO(2)) grew on the surface, at almost 100 s after oxidation. Despite the very complex oxidation mechanism on the atomic level, XANES analysis indicated that the charge transfer from platinum to the adsorbed oxygen species was almost constant and rather small, that is, about 0.5 electrons per oxygen, up to two monolayers of oxygen. This means that ionic polarization hardly develops at this stage of the surface platinum's "oxide" growth.

In situ X-ray probing reveals fingerprints of surface platinum oxide

“…This appears to be the case in the Pt/Rh(111) system studied here, for which lower ORR activity and increased H 2 O 2 formation in comparison to Pt(111) was found 41 . L 3 -edge spectra of pure Pt nanoparticles 6,[8][9][10][11][12] exhibit a strong increase and broadening of the white-line, similar to what we observe on Pt/Rh(111), already at lower onset potentials of around 0.8 V but with less intensity since bulk Pt atoms also contribute significantly to the L-edge peak. The similarities between our data and previously reported spectra of Pt nanoparticles suggest a re-interpretation of the latter, with the generally observed white-line increase being due to oxide formation rather than OH or O chemisorption.…”

Pseudo-Single-Crystal Electrochemistry on Polycrystalline Electrodes: Visualizing Activity at Grains and Grain Boundaries on Platinum for the Fe2+/Fe3+ Redox Reaction

“…This is particularly true for the Fe 2+/3+ redox process. 47,50 Based on our recent work, 21,[24][25][26][27][28][29]33,51 SECCM provides a powerful method to investigate whether structural effects hold for the oxidation of Fe 2+ on polycrystalline platinum.…”

Understanding the electrocatalysis of oxygen reduction on platinum and its alloys

“…142 X-Ray absorption spectroscopy (XAS) has provided more direct evidence of this phenomenon on Pt nanoparticles. 35,[143][144][145][146] Friebel et al recently reinterpreted these earlier XAS experiments to suggest that subsurface oxides could form on Pt nanoparticles as early as 0.8 V. 147 In summary, surface roughening and subsurface oxide formation are likely to be detrimental to catalyst stability, and thus strategies are required to mitigate these processes.…”