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.材料的催化性能受原子系综（ensemble）——即表面原子的几何构型——的影响。在传统材料体系中，原子系综效应与电子结构相互耦合，这是因为传统调控策略均围绕改变材料组分展开，因此难以单独解析原子系综的作用机制。本研究提出了一种可将几何效应与电子结构解耦的研究方法。为调控催化剂表面钯（Pd）原子系综的尺寸，本研究选取结构各异但组分完全一致的Pd₃Bi金属间化合物与固溶体合金，对比二者的反应活性。Pd₃Bi金属间化合物对甲醇氧化反应（methanol oxidation, MOR）无活性，而同组分的固溶体合金则表现出显著的反应活性（0.5 mA·cm⁻² Pd）。金属间化合物的钯原子系综尺寸更小（所有低能晶面对应的系综原子数为1、3、4和5），而固溶体Pd₃Bi的多个晶面可支撑更大的钯原子系综，平均尺寸为5.25个原子，最大可达6个原子。部分有序的Pd₃Bi合金（金属间化合物与固溶体的混合相）则表现出中等强度的甲醇氧化反应活性（0.1 mA·cm⁻² Pd），这证实甲醇氧化反应活性与原子系综的平均尺寸呈正相关。所有Pd₃Bi合金的电子结构均保持一致，这一点通过X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）价带谱与X射线吸收近边结构（X-ray absorption near edge structure, XANES）测试得到证实，说明反应活性的差异源于原子有序度不同所诱导的原子系综尺寸变化。本研究成果为在维持催化剂电子结构的前提下，设计具有可控活性位点构型的催化材料提供了新思路，从而助力更高效的催化剂开发。