  A palladium-based (Pd-based) core@shell catalyst can be modified to achieve
the desired oxygen adsorption properties by selecting an appropriate core
composition, surface alloying, and compressive strain. Herein, we present the
effects of compressive strain, core composition, and surface alloying in
Pd3Ni@PdIr(111), Pd3CuNi@PdIr(111), and Pd3Cu@PdIr(111) alloy-core@alloy-shell
catalysts on dioxygen adsorption. Using experimental lattice parameters for the
unstrained catalysts, -1% to -5%, the strain was systematically introduced. The
calculated dioxygen-adsorption energies for the surface Pd and surface Ir atoms
reveal that the Pd3CuNi@PdIr catalyst has the lowest dioxygen-adsorption energy
at a given compressive strain. Bader charge calculations show that the
Pd3CuNi@PdIr catalyst surface is the most charge depleted. The d-band model
displays an intermediate d-band center downshift for the surface Pd atoms, and
the highest downshift for the surface Ir atoms. Due to synergism between charge
depletion, the d-band center shift, and the surface alloy effect, the
Pd3CuNi@PdIr catalyst has the lowest dioxygen-adsorption energy. The
relationship between the experimentally obtained catalyst-surface mass activity
and the theoretically calculated d-band center of the surface Pd and the
surface Ir is volcano shaped, with the Pd3CuNi@PdIr catalyst at the apex of the
volcano. The catalytic activities of these catalysts were observed to follow
the order: Pd3CuNi@PdIr > Pd3Cu@PdIr > Pd3Ni@PdIr. This work sheds light on the
importance of ligand and strain effects, as well as surface alloying for the
fine-tuning of alloy-core@alloy-shell-catalysts during the rational design of
catalysts from first principles.
