Silica-Supported Au−CuO x Hybrid Nanocrystals as Active and Selective Catalysts for the Formation of Acetaldehyde from the Oxidation of Ethanol J. Chris Bauer, † Gabriel M. Veith, ⊥ Lawrence F. Allard, ∥ Yatsandra Oyola, † Steve H. Overbury, †,‡ and Sheng Dai* ,†,‡,§ † Chemical Sciences Division, ⊥ Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37821, United States ‡ Center for Nanophase Material Sciences, ∥ Physical Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States ABSTRACT: The selective oxidation of ethanol with molecular O 2 is increasingly becoming an important process to develop fine chemicals because it can be obtained from renewable biomass feedstock while reducing the demand on fossil fuels. We have synthesized silica-supported Au−Cu alloy nanoparticles, and through an oxidative dealloying process, we have developed Au−CuO x hybrid catalysts for the selective oxidation of ethanol into acetaldehyde. Using a combination of XRD, XPS, and HR-STEM experiments, we have confirmed that the active catalyst is a Au core with a thin CuO x shell. Oxidation of the AuCu/SiO 2 alloy catalyst at 300 °C was found to produce the most active and stable catalyst for ethanol conversion (∼90%) with the highest selectivity (∼80−90%) at a reaction temperature of 200 °C for 50 h on-stream. TEM and XRD results show that Au−CuO x /SiO 2 catalysts calcined at 300 and 500 °C are also more resistant to sintering during pretreatment and catalytic conditions than pure gold supported on silica. Furthermore, the silica-supported Au−CuO x catalysts (calcined at 300 and 500 °C) were also found to be more active and selective in the dehydrogenation of ethanol to form acetaldehyde. It is likely that the increased interfacial contact between the Au and CuO x forms the most active site on the catalyst and is responsible for the enhanced catalytic properties when compared with pure Au/SiO 2 . KEYWORDS: Au catalysis, AuCu alloy, ethanol oxidation, acetaldehyde, dehydrogenation, heterostructure ■ INTRODUCTION The selective oxidation of biomass-derived ethanol to produce fine and specialty chemicals, such as acetaldehyde, ethylene, butadiene, acetic acid, etc., may potentially become an important process in the chemical industry. 1,2 Bioethanol is already in production around the world, and its availability is expected to increase in the next few years which may help to reduce the demand on fossil fuels as a feedstock. 1 It is also derived from biomass feed stock, which is a renewable resource. Furthermore, the selective oxidation of ethanol, as well as other alcohols, has increasingly become important as an alternative pathway to produce fine chemicals. 3 Unfortunately, the use of toxic and expensive 4 oxidants, such as oxygenates, permanganates, and peroxides, often provides poor selectivity and requires extra separation and waste treatment steps. To avoid potential ecological contamination from inorganic oxidizers, an intense research effort has focused on the development of a solid catalyst capable of activating molecular O 2 as an inexpensive and clean oxidant. Supported metal catalysts have emerged as very active catalysts for the oxidation of alcohols. 5 For example, palladium nanoparticles supported on hydroxypatite (HAP) have received a lot of attention because of the high turnover numbers for the oxidation of 1-phenylethanol at atmospheric pressure in O 2 and is considered as the standard. 6 Pd and Pt nanoparticles have also been found to be highly active for aqueous-phase ethanol oxidation. 7 Unfortunately, these metals exhibit poor product selectivity because they require high reaction temperatures. Prati and co-workers were among the first to report the use of heterogeneous gold catalyst supported on carbon to oxidize ethane-1,2-diol and propane-1,2-diol to glycolic and lactic acids using molecular oxygen. 8−10 They later discovered that medium- sized Au particles were the most active on carbon support, as opposed to the smaller particles showing more activity on oxide supports during the oxidation of ethylene glycol. 11 Biella demonstrated that Au/SiO 2 was capable of oxidizing a variety of primary and secondary aliphatic alcohols to aldehydes and ketones in the gas phase. 12 Hutchings reported on the oxidation of glycerol using Au, Pd, and Pt supported on carbon supports and found that Au could obtain 100% selectivity toward glyceric Received: April 9, 2012 Revised: September 25, 2012 Published: October 18, 2012 Research Article pubs.acs.org/acscatalysis © 2012 American Chemical Society 2537 dx.doi.org/10.1021/cs300551r | ACS Catal. 2012, 2, 2537−2546