Atomic\nOrdering-Induced Ensemble Variation in Alloys\nGoverns Electrocatalyst On/Off States

The catalytic behavior of a material\nis influenced by ensemblesthe\ngeometric configuration of atoms on the surface. In conventional material\nsystems, ensemble effects and the electronic structure are coupled\nbecause these strategies focus on varying the material composition,\nmaking it difficult to understand the role of ensembles in isolation.\nThis study introduces a methodology that separates geometric effects\nfrom the electronic structure. To tune the Pd ensemble size on the\ncatalyst surface, we compared the reactivity of structurally different\nbut compositionally identical Pd<sub>3</sub>Bi intermetallic and solid\nsolution alloys. Pd<sub>3</sub>Bi intermetallics display no reactivity\nfor methanol oxidation (MOR), while their solid solution counterparts\nshow significant reactivity (0.5 mA cm<sub>Pd</sub><sup>–2</sup>). Intermetallics form smaller ensembles (1, 3, 4, and 5 atoms across\nall low-energy facets), whereas solid solution Pd<sub>3</sub>Bi has\nseveral facets that support larger Pd ensembles, with an average size\nof 5.25 atoms and up to 6 atoms. A partially ordered Pd<sub>3</sub>Bi (a mixed phase of intermetallic and solid solution) alloy shows\nintermediate MOR activity (0.1 mA cm<sub>Pd</sub><sup>–2</sup>), confirming that methanol oxidation activity tracks with the average\nensemble size. All Pd<sub>3</sub>Bi alloys maintained similar electronic\nstructures, as confirmed by X-ray photoelectron spectroscopy (XPS)\nvalence band spectroscopy and X-ray absorption near edge structure\n(XANES) measurements, indicating that reactivity differences arise\nfrom variations in the ensemble size induced by differences in the\natomic ordering. Our findings offer an approach for designing materials\nwith controllable active site configurations while maintaining the\ncatalyst’s electronic structure, thereby enabling more efficient\ncatalyst design.