Synthesis of Bimetallic AuPt Clusters with Clean Surfaces via Sequential Displacement-Reduction Processes

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• 作者：Alexandra M. Landry ; Enrique Iglesia
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：August 23, 2016
• 年：2016
• 卷：28
• 期：16
• 页码：5872-5886
• 全文大小：783K
• 年卷期：0
• ISSN：1520-5002

文摘

We report the synthesis of bimetallic AuPt nanoparticles (3.3–4.3 nm) of uniform size and composition using colloidal methods and reagents containing only C, H, O, and N. These clusters were dispersed onto SiO₂ and treated at low temperatures in the presence of reductants to remove all surface residues without concomitant agglomeration, thus leading to bimetallic structures suitable for mechanistic inquiries into bimetallic effects on surface reactivity. Synthesis protocols exploit and generalize galvanic displacement-reduction (GDR) processes previously used to prepare AuPd clusters; these routes promote bimetallic mixing but become more challenging for systems (e.g., AuPt) with smaller reduction potential differences and less favorable mixing enthalpies than AuPd. These hurdles are addressed here through procedural modifications that inhibit the formation of large Au-rich clusters, which compromise size and compositional uniformity. In doing so, we extend GDR techniques to endothermic alloys with elements of more similar redox properties. Higher temperatures and lower Au³⁺ precursor concentrations promoted metal mixing and inhibited homogeneous and heterogeneous nucleation. Cluster size and compositional uniformity were confirmed by UV–visible spectroscopy during and after colloid formation, transmission electron microscopy, and high-angle annular dark-field (HAADF) imaging with energy-dispersive X-ray spectroscopy (EDS). Particle-by-particle EDS analysis and HAADF imaging demonstrated the prevalence of GDR processes in AuPd bimetallic cluster assembly. These methods also showed that size-dependent intracluster diffusion during AuPt cluster formation, driven by unfavorable AuPt mixing thermodynamics, leads to Au surface enrichment, thus promoting autocatalytic Au deposition. This rigorous mechanistic comparison of AuPt and AuPd systems provides essential guidance and specific control variables and procedures for the synthesis of other bimetallic systems based on the redox potential differences and mixing thermodynamics of their two components.