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Nanoporous Gold – Understanding the Origin of the Reactivity of a 21st Century Catalyst Made by Pre-Columbian Technology Juergen Biener, Monika M Biener, Robert J. Madix, and Cynthia M. Friend ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01586 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on September 17, 2015
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ACS Catalysis
Nanoporous Gold – Understanding the Origin of the Reactivity of a 21st Century Catalyst Made by PreColumbian Technology Juergen Biener,*,† Monika M. Biener,† Robert J. Madix, and Cynthia M. Friend*, ‡ †
Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National
Laboratory, Livermore, California 94550, United States. ‡
Department of Chemistry and Chemical Biology, Harvard University, Cambridge,
Massachusetts 02138, United States, and School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States. Abstract: Nanoporous gold (np-Au), a three-dimensional nanoporous bulk material, is made by selective corrosion of Ag from Ag-Au alloys, a technique already applied by the Pre-Columbian cultures of South America. Nanoporous gold is actually a Au-rich Ag-Au alloy which, specifically the Ag0.03Au0.97 composition, combines high reactivity and selectivity for a wide variety of oxidation reactions, from simple CO oxidation to complex oxygen-assisted coupling reactions. Its catalytic reactivity is surprising as np-Au is a non-supported Au catalyst with relatively large feature sizes in the order of tens of nanometers, thus breaking the generally accepted notion that gold must be in the form of small particles (~few nanometers) to be an active catalyst. The ease of sample preparation in combination with high reactivity, selectivity, and long-term stability suggest that nanoporous gold has the potential to bring Au catalysis 1
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ACS Catalysis
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closer to practical applications. In this perspective, we provide a critical review of the current understanding of the origin of the high catalytic activity of nanoporous gold in context of morphology and surface composition. KEYWORDS: Nanoporous gold, energy-efficient catalysis, selective oxidation, activation, ozone INTRODUCTION: The current interest in gold catalysis goes back to the 1980s when it was first reported that nano-sized Au clusters supported on reducible metal oxides show high catalytic reactivity towards low-temperature carbon monoxide oxidation.1-2 This result diverged significantly from the relatively inactive behavior observed previously for gold supported on alumina or silica.3-6 At about the same time it was discovered that atomic oxygen on the surface of Au single crystals was easily reduced by CO at room temperature.7 Clearly, the stage was set for a renewed interest in catalysis by gold. Further, even at this time a more general picture of the surface reactivity of metallic gold with chemisorbed atomic oxygen began to emerge, indicating that it might be exploited for oxygen-assisted reactions if activation of dioxygen were possible.8 Since then much progress has been made in both understanding the effects of size and shape of nano-sized Au particles as well as their interaction with supports,9 and in utilizing the high reactivity and selectivity of supported Au catalysts for more complex chemical transformations.10 All these studies consistently showed that the catalytic activity of supported Au nanoparticles is limited to very small particles (
