Tailoring reactivity of small Ag/Pt bimetallic cluster for ORR: A comprehensive study using density functional theory approach

Abstract

Bimetallic clusters, as the name suggests, are known to act as bifunctional catalyst. Therefore, it is possible to tune the composition of the catalyst to achieve the best performance for a specific reaction. In the first part of this work, we explore the geometric and electronic structures of various Ag-Pt bimetallic clusters and find the right composition for ORR, one of the most sought-after reactions in the field of energy materials. We have considered Ag-Pt bimetallic clusters in both free and on Al₂O₃(0001) support. The results reveal that the equilibrium structures of the bimetallic Ag_nPt_m (n+m ≤ 12 atoms) clusters are governed by a balance of competing interactions (Pt-Pt vs Ag-Pt vs cluster-support interactions). Whilst the isolated decamer cluster with 5:5 or 6:4 composition (Ag₆Pt₄ and Ag₅Pt₅) shows atomically segregated structures, Ag rich clusters (Ag₉Pt_1–3) prefer to form core-shell pattern where Pt forms the core surrounded by Ag atoms. In general, these clusters adopt pseudo-planar structure on alumina support following the surface template. Analysis of the electronic structure shows that Al₂O₃ surface induces significant broadening in the energy states of the bimetallic cluster, which in turn facilitates higher mixing between d-states of the Ag and Pt in the complex. Finally, the d-band centre descriptor model has been utilized to underscore the chemical reactivity of these bimetallic clusters. Remarkably, the Ag₆Pt₄@Al₂O₃ cluster with d-band centre at −2.68 eV is found to be in “just right” zone for ORR. This is further corroborated by the reduction (∼ 25 %) in oxidation reaction enthalpy of the Ag₆Pt₄@Al₂O₃ than standard Pt(111) catalytic surface.