10 th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved. Microwave-assisted synthesis of Au, Ag and Au-Ag nanoparticles and their catalytic activities for the reduction of nitrophenol S. Albonetti a , M. Blosi b , F. Gatti a , A. Migliori d , L. Ortolani d , V. Morandi d , G. Baldi c , A. Barzanti c , M. Dondi b a Department of Industrial Chemistry and Materials, INSTM, Research Unit of Bologna, Viale Risorgimento 4, 40136 Bologna University, Italy b ISTEC-CNR, Institute of Science and Technology for Ceramics, National Research Council, Via Granarolo 64, 48018, Faenza, Italy c CERICOL, Via Pietramarina 123, 50053 Sovigliana Vinci, Firenze, Italy d IMM-CNR Sezione di Bologna - Via Gobetti, 101, 40129 Bologna, Italy Abstract A simple, microwave-assisted, strategy for producing Au/Ag concentrated sols by glucose reduction in water was developed. PVP stabilized Ag-Au bimetallic nanoparticles were characterized and their catalytic activity was studied in the reduction of 4-nitrophenol with NaBH 4 as a probe reaction. The Ag-Au nanoparticles were prepared by first optimizing the synthesis of Au colloid and then carrying out the deposition of a silver shell. Microwave heating has been shown to provide more homogeneous particle nucleation and shorter synthesis time than traditional heating. Prepared Au, Ag and Au/Ag nanocrystals function as effective catalyst for the reduction of p-nitrophenol in the presence of NaBH 4 otherwise unfeasible if only the strong reducing agent NaBH 4 is employed. Keywords: Au, Ag, microwave, bimetallic sols, nanoparticles, water media 1. Introduction Colloidal suspension of different metals has found applications in various fields, including catalysis, because a large fraction of the catalytically active metal sites in this case is exposed to the reactants [1-2]. In particular, alloy and core-shell nanoparticles have received special attention due to the possibility of tuning the optical and electronic (and thus catalytic) properties over a broad range by simply varying the composition [3- 5]. 2. Experimental procedure 2.1. Bimetallic synthesis All the chemical reagents used in this experiment were analytical grade (Sigma Aldrich). Au/Ag core-shell nanoparticles were prepared by a two-step method, characterized by the shell synthesis on the preformed core used as seeds of nucleation (Scheme 1). Au-core and Ag-core sols, exploited as seeds, were obtained separately through the reduction of HAuCl 4 or AgNO 3 by glucose in alkaline water. PVP-coated metal seeds were synthesized in 5 minutes at 70°C (for Ag) or at 90°C (for Au) by using microwave heating and following a patented procedure [6]. For each metal, the glucose amount and the solution pH were carefully optimized. Table 1 shows the characteristics corresponding