X-ray Fluorescence Investigation of Ordered Intermetallic Phases as Electrocatalysts towards the Oxidation of Small Organic Molecules

• Published in: Chemistry : a European journal Vol. 16; no. 46; pp. 13689 - 13697
• Main Authors: Liu, Yi, Lowe, Michael A., Finkelstein, Ken D., Dale, Darren S., DiSalvo, Francis J., Abruña, Héctor D.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 10-12-2010; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Active control; Chemistry; Electrocatalysts; Fluorescence; fluorescence spectroscopy; Formic acid; fuel cells; intermetallic phases; Intermetallics; Nanoparticles; Platinum; X-ray fluorescence
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201001211

The composition of ordered intermetallic nanoparticles (PtBi and PtPb) has been quantitatively studied by in situ X‐ray fluorescence (XRF) during active electrochemical control in solutions of supporting electrolyte and small organic molecules (SOMs). Because the Pt Lβ1,2 lines and the Bi Lα1,2 lines are only separated by 200 eV, an energy‐dispersive detector and a multiple‐channel analyzer (MCA) were used to record the major fluorescent emission lines from these two elements. The molar ratios of platinum to the less‐noble elements (Bi, Pb) in the nanoparticles dramatically changed as a function of the applied upper limit potentials (Eulp) in cyclic voltammetric (CV) characterization. Similar to previous investigations for bulk intermetallic surfaces, the less‐noble elements leached out from the surfaces of the intermetallic nanoparticles. For PtBi nanoparticles, the ratios of fluorescence intensities of Pt/Bi in the samples were 0.42, 0.96, and 1.36 for Eulp=+0.40, +0.80, and 1.20 V, respectively, while cycling the potential from −0.20 V to the Eulp value for 10 cycles. The leaching‐out process of the less‐noble elements occurred at more negative Eulp values than expected. After cycling to relatively positive Eulp values, nonuniform PtM (M=Bi of Pb) nanoparticles formed with a Pt‐rich shell and intermetallic PtM core. When the supporting solutions contained active fuel molecules in addition to the intermetallic nanoparticles (formic acid for PtBi, formic acid and methanol for PtPb), kinetic stabilization effects were observed for Eulp=+0.80 V, in a way similar to the response of the bulk materials. It was of great importance to quantitatively explore the change in composition and structure of the intermetallic nanoparticles under active electrochemical control. More importantly, this approach represents a simple, universal, and multifunctional method for the study of multi‐element nanoparticles as electrocatalysts. This is, to our knowledge, the first report of nondestructive, quantitative characterization of bimetallic or multi‐elemental nanoparticles electrocatalysts under active electrochemical control.
Simple and effective investigation: The composition of ordered intermetallic nanoparticles (PtBi and PtPb) has been quantitatively studied by in situ X‐ray fluorescence during active electrochemical control in solutions of supporting electrolyte and small organic molecules (see picture). This approach represents a simple, universal, nondestructive, and multifunctional method for the study of multi‐element nanoparticles as electrocatalysts.