Probing the Limits of d-Band Center Theory: Electronic and Electrocatalytic Properties of Pd-Shell–Pt-Core Nanoparticles

Abstract

Theoretical DFT calculations suggest that chemisorption energy, activation barrier, and energy of dissociation of small molecules on metal surface can be correlated to the d-band center of gravity of that metal. This holds true for many systems and reactions, but there are also reports where significant discrepancies were found. Here we present the critical assessment of applicability of the d-band center theory to nonuniform catalytic systems, such as core–shell nanoparticles. For Pt-core–Pd-shell nanoparticles we found a significant enhancement of catalytic activity toward formic acid oxidation, which was assigned to observed changes of density of states close to the Fermi level, in general in agreement with d-band center theory. However, at the same time the changes in d-band center for Pt-core–Pd-shell nanoparticles were contrary to those predicted by theory due to incorporation of Pt valence electrons to the overall band structure, which shifted the d-band center in the direction opposite to that predicted by the theory. Our data stress the role of experimental determination of electronic properties of catalytic systems when explaining the observed catalytic activity, as real systems can be more complicated than the one used for theoretical calculations. As a result the changes in d-band center energy must be used with care for explanation of the observed changes in catalytic activity. We show that for nonuniform systems density of states close to the Fermi level is a better predictor of chemisorption strength and catalytic activity than the d-band center.