Article ID Journal Published Year Pages File Type
61520 Journal of Catalysis 2012 10 Pages PDF
Abstract

The activity of oxygen reduction electrocatalysts is governed by the Sabatier principle and follows a Volcano curve as a function of the oxygen-binding energy. Density functional theory calculations show that the oxygen-binding energy decreases in steps of about 10 kJ/mol in a series of core–shell Pd3M@Pd3Pt (M = Ni, Co, Fe, Mn, and Cr) electrocatalysts, leading to a gradual, Volcano-like variation in the oxygen reduction activity. A series of carbon-supported PdM@PdPt (M = Ni, Co, Fe, and Cr) nanoparticles with similar particle sizes were prepared by an exchange reaction between PdM nanoparticles and an aqueous solution of PtCl42-. The variation in the surface electronic structure of the core–shell structures was evaluated by Pt 4f7/2 X-ray photo-electron spectroscopy and by CO-stripping voltammetry and agrees with the first principle calculations. At 0.85 V, the PdM@PdPt/C core–shell electrocatalysts show a 6-fold variation in activity, following the Volcano trend predicted by the calculations. The Pt mass activity of the Volcano-optimal PdFe@PdPt/C catalyst is an order of magnitude higher than the activity of commercial 3.0-nm Pt/C catalysts. The core–shell catalysts also display a high methanol tolerance, which is important for use in direct methanol fuel cells. Calculated Pt–M segregation energies suggest that the Pd3M@Pd3Pt core–shell structures are stable, in particular in the presence of 1/4 ML CO. Adsorption of oxygen-containing species may induce surface segregation of the 3d transition metal, except for the Volcano-optimal ORR catalyst, Pd3Fe@Pd3Pt.

Graphical abstractVariation of the 3d transition metal M in PdM@PdPt/C core–shell catalysts gradually changes the electronic structure of the surface Pt atoms, as evidenced by the Pt 4f7/2 binding energies. The gradual change in electronic structure causes a gradual change in the measured CO-stripping peak position and in the calculated oxygen affinity. The variation in oxygen affinity leads to a Volcano-like variation in the measured oxygen reduction activity, as expected from Sabatier’s principle.Figure optionsDownload full-size imageDownload high-quality image (227 K)Download as PowerPoint slideHighlights► A series of PdM@PdPt/C core–shell catalysts with similar particle sizes were prepared. ► Pt XPS and CO-stripping peaks vary gradually over the series, in agreement with DFT. ► Oxygen-binding energies change in steps of 10 kJ/mol, leading to a Volcano curve for the predicted ORR activity. ► PdM@PdPt/C catalysts show a 6-fold variation in ORR activity, following the predicted trend.

Related Topics
Physical Sciences and Engineering Chemical Engineering Catalysis
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