Atomic Ordering-Induced Ensemble Variation in Alloys Governs Electrocatalyst On/Off States

The catalytic behavior of a material
is influenced by ensemblesthe
geometric configuration of atoms on the surface. In conventional material
systems, ensemble effects and the electronic structure are coupled
because these strategies focus on varying the material composition,
making it difficult to understand the role of ensembles in isolation.
This study introduces a methodology that separates geometric effects
from the electronic structure. To tune the Pd ensemble size on the
catalyst surface, we compared the reactivity of structurally different
but compositionally identical Pd3Bi intermetallic and solid
solution alloys. Pd3Bi intermetallics display no reactivity
for methanol oxidation (MOR), while their solid solution counterparts
show significant reactivity (0.5 mA cmPd–2). Intermetallics form smaller ensembles (1, 3, 4, and 5 atoms across
all low-energy facets), whereas solid solution Pd3Bi has
several facets that support larger Pd ensembles, with an average size
of 5.25 atoms and up to 6 atoms. A partially ordered Pd3Bi (a mixed phase of intermetallic and solid solution) alloy shows
intermediate MOR activity (0.1 mA cmPd–2), confirming that methanol oxidation activity tracks with the average
ensemble size. All Pd3Bi alloys maintained similar electronic
structures, as confirmed by X-ray photoelectron spectroscopy (XPS)
valence band spectroscopy and X-ray absorption near edge structure
(XANES) measurements, indicating that reactivity differences arise
from variations in the ensemble size induced by differences in the
atomic ordering. Our findings offer an approach for designing materials
with controllable active site configurations while maintaining the
catalyst’s electronic structure, thereby enabling more efficient
catalyst design.