Phase Transformations in physical mixtures of Pd-Cu nanoparticles
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Phase Transformations in physical mixtures of Pd-Cu nanoparticles Vineetha Mukundan1, Jun Yin2, Chuan-Jian Zhong2 and Oana Malis1 1
Department of Physics, Purdue University, West Lafayette, IN 47907, USA Department of Chemistry, SUNY Binghamton, NY 13902, USA
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ABSTRACT The temperature induced structural transformations in physical mixtures of 1nm palladium and ultrafine (~0.5nm) copper nanoparticles supported on carbon were studied using in-situ real time synchrotron based x-ray diffraction. These nanoparticles were subjected to two-step thermal annealing from 25°C to 700°C. The Pd and Cu nanoparticles were found to coalesce forming alloy nanoparticles that subsequently undergo a structural phase transformation from ordered B2 to disordered fcc. The random alloy formed at the end of the thermal treatments was found to be copper-rich. INTRODUCTION Noble metals have important applications in fuel cell catalysis. Palladium, in particular is utilized as a catalyst in numerous chemical reactions used in industrial and laboratory processes. Pd is one of the most active metals for oxidation reduction reactions [1, 2]. Pd in combination with Cu have proven to exhibit high catalytic performance that make them promising candidates to replace the more expensive Pt as electrocatalyst for the cathode oxidation reduction reactions (ORR) in fuel cells like the polymer electrolyte fuel cell (PEFC) and direct methanol fuel cells [3,4]. The formation of PdCu alloy phase also has technological application in PdCu bi-layers used for hydrogen storage, where it was found to be H2S poisoning resistant [5]. The stability and activity of nanocatalysts are dependent on preparation methods and thermal treatments. In order to achieve optimal electrochemical performance for the catalysts, it is important to understand the effect of the synthesis process and the structural details that ensue. The synthesis procedure determines the size distribution of the nanoparticles and composition homogeneity (degree of alloying, size and composition of the bimetallic systems). To enhance the electrochemical efficiency the catalysts are typically heat treated in reducing atmospheres that promote alloy formation. PdCu bimetallic catalysts supported on carbon with molar ratios ranging from 1:9 to 9:1 were investigated by Wang et al. in the temperature range 300°C to 800°C [3]. They found that the maximum ORR activity occurs for bimetallic composition of Pd:Cu =1:1 [3, 6]. The (1:1) PdCu/C mass activity was also found to increase with the increase in heat treatment and particle size. In their study, the PdCu alloy nanoparticles have an average diameter of 3 nm that upon heat treatment changes to 5-10nm. The random solid solution alloy structure was found to have higher mass activity for electrochemical oxidation of methanol. Different compositions of Pd and Cu nanoparticles on Vulcan-XC carbon support annealed at 400°C were also investigated by Yin et al. [6]. Using high-angle annular dark field- scanning tunneling electron microscopy (HAADF-STEM) measureme
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