Strain and Ligand Effects of Transition Metals in Pt Alloys on Oxygen Reduction Reaction

The strain and ligand effects on the catalytic activities have been an essential topic in heterogeneous catalysis.¹ The electronic properties of the outmost metal layer can be altered significantly by the compositions (usually consisting of transition metals) underneath. For instance, the hydrogen adsorption/desorption was greatly suppressed and the onset of oxide formation shifted to more positive potentials on the Pt-skin like surface in the Pt₃Ni(111) single crystals compared to Pt(111) surface.^{2, 3} The delayed formation of oxide layer caused by the weaker adsorption energies of oxygen containing species on the Pt/Pt-Ni surface was believed to be the main reason for the 10-fold enhancement in oxygen reduction reaction (ORR) activity. The combination of the compressive strain in the Pt-skin caused by the smaller Ni atoms in the sub-layers and core, and hybridization of the d-states of the Pt surface atoms with Ni atoms (ligand effect) resulted in the unique electronic properties of the Pt-skin surfaces.² During fuel cell operation, the transition metals are expected to leach out gradually and form thick Pt overlayers resulting in a lower performance in the hot, low pH and high potential environment.⁴ Thus, the relationship of electronic properties of Pt (strain and ligand effects) as a function of thickness of the Pt shell is of great interest to understand the activity decay mechanism of Pt alloys and design more active Pt alloys and core-shell catalysts.

In this study, the number of Pt layers was controlled by using the Cu-UPD-Pt-displacement technique,⁵ which has been successfully used to deposit Pt monolayer on various substrates. ⁶ Our experimental results clearly show that the ORR activity decreases with the thickness of the pure Pt shell and the activity enhancement disappears when the Pt shell is ~4 atomic layer thick on the 4.5 nm Pt₃Ni nanoparticles. The shell thickness dependent behavior is associated with the oxygen binding energies on the Pt shell. Density functional theory (DFT) calculations performed on nanoparticles are able to estimate the strain and ligand effects from the Pt₃Ni core with different Pt shell thickness and the results agree well with the experimental data.

References

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