Charge redistribution in core-shell nanoparticles to promote oxygen reduction.

Bimetallic core-shell nanoparticles are a class of near-surface alloy catalyst for which there is a high degree of control over size and composition. A challenge for theory is to understand the relationship between their structure and catalytic function and provide guidelines to design new catalysts that take advantage of material properties arising at the nanoscale. In this work, we use density functional theory to calculate the energetics of oxygen dissociative adsorption on 1 nm Pd-shell nanoparticles with a series of core metals. The barrier for this reaction and the binding energy of atomic oxygen is found to correlate well with the d-band level of the surface electrons. Noble metal cores lower the barrier and increase the binding, reducing the activity of the Pd-shell as compared to Pt. Reactive core metals such as Co and Mo, on the other hand, lower the d-band of the shell with respect to the Fermi level, giving the Pd-shelled particles oxygen reduction kinetics similar to that of Pt. While both ligand and strain effects determine the d-band center of the Pd shell, a greater surface relaxation reduces the strain in nanoparticles as compared to single-crystal near-surface alloys. Charge redistribution between core and shell then becomes an important factor for lowering the d-band center of Pd-shelled particles and increasing their activity for the oxygen reduction reaction.