Characterizations of Core-Shell Nanoparticle Catalysts for Methanol Electrooxidation
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Characterizations of Core -Shell Nanoparticle Catalysts for Methanol Electrooxidation Mathew M. Maye, Jin Luo, Wai-Ben Chan, Li Han, Nancy Kariuki, H. Richard Naslund a, Mark H. Engelhard b, Yuehe Lin b, Randoll Sze, and Chuan-Jian Zhong* Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902. a) Department of Geological Sciences, State University of New York at Binghamton, b) Binghamton, NY 13902. Environmental and Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99352.
ABSTRACT This paper describes the results of an investigation of the structure and composition of core-shell gold and alloy nanoparticles as catalytically active nanomaterials for potential fuel cell catalysis. Centered on the electrocatalytic methanol oxidation, we show three sets of results based on electrochemical, surface, and composition characterizations. First, electrochemical studies have revealed that the nanostructured catalysts are active towards the electrooxidation of methanol and carbon monoxide. Second, X-ray photoelectron spectroscopy (XPS) data have shown that the organic encapsulating shells can be effectively removed electrochemically or thermally, which involves the formation of oxides on the nanocrystals. Thirdly, direct current plasma - atomic emission spectrometry (DCP-AES) has revealed insights for the correlation of the composition of alloy nanoparticles with the catalytic activities. Implications of these results to the design of nanostructured catalysts will also be discussed.
INTRODUCTION Catalysis plays a vital role in chemical processing, environmental protection and fuel cell technology. The use of gold nanoparticles as catalysts has attracted increasing interest [1-3], since the pioneer work of Haruta [4], which has demonstrated high catalytic activities for CO and hydrocarbon oxidation at gold nanoparticles of less than ~10 nm diameter that are supported on oxides. We have recently shown that the preparation of nanoscale gold catalysts can be achieved using core-shell assembled nanoparticles that consist of metal or alloy nanocrystal cores and organic molecular wiring or linkage to define the interparticle spatial property [5-6]. This approach is important in addressing fundamental issues related to size, shape, aggregation, poisoning, and surface engineering of nanoparticles in fuel cell catalysis. The focus of this work is to characterize the catalytic structure and composition at gold and alloy nanoparticle assemblies using electrochemical, XPS and DCP-AES techniques. We studied decanethiolate-capped gold and alloy nanoparticles of 2-nm (Au2-nm) and 5-nm (Au5-nm) core sizes assembled on planar substrates using 1,9-nonanedithiol (NDT) and 11mercaptoundecanoic acid (MUA) as molecular wiring agents as a model system. XPS was employed to detect the identity of active surface species and to analyze the elemental
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composition or oxidation states of the nanomaterials, from which we derive structural information about the surface rec
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