Group members
Wenjie Tang, Nathan Froemming, and Travis Powell
Motivation
At present, the kinetics of Pt catalyzed oxygen reduction is slow. Better catalysts with low material costs need to be developed. According to the model proposed for the bulk system, a good catalyst should have both a low dissociation barrier and a low atomic oxygen binding energy. Metals with a balance between these two things, such as Pt, are promising candidates as good catalysts.
Metal nanoparticles are good candidates for new catalysts because their catalytic properties are different from bulk metals, and are sensitive to particle size, shape and composition. However, the factors that affect the activity of the nanoparticles have not been well investigated.
Study of shell/core nanoparticles
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 materials 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.
A typical oxygen dissociation reaction:
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.
