Low-temperature carbon monoxide oxidation with Au-Cu meatball-like cages prepared by galvanic replacement.

Owing to intrinsic scientific interest and its immense importance in catalysis technology, the oxidation of carbon monoxide has attracted much attention in the past few decades. The reaction is effectively catalyzed by certain noble metals, such as gold, platinum, palladium, and rhodium. Among all of these metals, gold exhibits an extraordinarily high activity because CO molecules adsorb strongly to it. CO molecules can reversibly adsorb onto a gold surface at low temperatures (T<150 K). However, dissociative adsorption of oxygen, another important step in CO oxidation, is strongly hindered at temperatures below 400 8C. This high dissociation barrier is attributed to weak coupling of oxygen to the gold surface due to filled d-states, and CO oxidation takes place readily only when oxygen is provided in atomic form. Therefore, gold nanoparticles are usually immobilized onto oxide supports that play a key role in oxygen adsorption and activation. In addition to oxide supports, certain metals, such as copper, can also enable oxygen adsorption and activation, making copper an important oxidation catalyst and a suitable support for gold catalysts. Motivated by this intrinsic advantage, Mou and co-workers have successfully prepared Au–Cu alloy nanoparticles by reducing HAuCl4 and Cu(NO3)2·3H2O with NaBH4 step by step, and reported that the synergism in coadsorption of CO and O2 significantly enhanced the catalytic activity towards CO oxidation. They further studied the effect of different Au/Cu ratios on the CO oxidation reaction, and found that a catalyst with a Au/Cu ratio of 3:1 offered the best performance. Their studies have shown that it is possible to prepare effective CO oxidation catalysts by constructing Au–Cu bimetallic nanostructures, and many researchers have devoted their efforts on this subject thus far. However, most studies focus on the fabrication of Au–Cu alloy nanoparticles by a coreduction method. This method usually results in the formation of spherical Au–Cu alloy nanoparticles. The composition of these two metals in the alloy nanoparticles is relatively difficult to control, because most alloy nanoparticles possess fixed compositions and it is difficult to realize simultaneous reduction of different metal ions by the co-reduction method. However, the composition and structure of the Au–Cu alloy nanoparticles usually changes during CO oxidation. Au3Cu1 alloy nanoparticles have been found to segregate into a Au core decorated with tiny CuOx patches during reaction with CO molecules. In contrast to alloy nanoparticles, Au nanoparticles supported on a Cu cage, named Au–Cu bimetallic cages, offer a relatively high surface area and tunable compositions, which are desirable properties for the application in the oxidation of CO. Recent investigations have shown that galvanic replacement reactions are an effective approach for the preparation of noble-metal hollow nanostructures, using Ag or Pd nanomaterials with various morphologies as sacrificial templates. Theoretically, Cu nanocrystals should be a more promising choice as sacrificial templates than Ag nanocrystals, because of the lower costs, milder reaction conditions, and stronger driving force during the galvanic replacement reaction. Therefore, the use of Cu nanocrystals as sacrificial templates for the preparation of Au–Cu hollow nanomaterials should be an effective way of generating high-efficiency catalysts for CO oxidation. Because the final morphology of the products is determined by the Cu nanocrystals, the preparation of Cu nanocrystals with new morphologies is a very important research area. Up to now, various methods to prepare Cu nanostructures with different morphologies have been developed. For example, Cu nanowires have been prepared by a number of methods, including electrochemical deposition, chemical vapor deposition(CVD), and templating. Cu nanocubes have also been prepared, by a solution reduction process. However, preparing well-defined Cu nanocrystals in aqueous solution remains a great challenge owing to the difficulty of reducing Cu to Cu. Several approaches towards Cu nanocrystals have been reported, but some of the materials were found to consist of Cu2O rather than Cu, in addition to poor control over the morphology. Among the factors that prevent the fabrication of Cu nanocrystals, the biggest obstacle is the lack of a rational approach to control the morphology and size. Recently, our group reported an effective method to prepare Cu nanowires and nanocubes. The morphology and size of Cu nanocrystals was controlled by simply adjusting the amount of glucose (reducing agent) and hexadecylamine (HDA). In this [a] C. Zhou, X. Jiang, Prof. Y. Yin, Prof. M. Jin Frontier Institute of Science and Technology Xi’an Jiaotong University Xi’an, Shaanxi 710054 (PR China) Fax: (+86)029-83395131 E-mail : [email protected] [b] Dr. L. Yang Department of Chemistry Liaoning University Shenyang, Liaoning 110036 (PR China) E-mail : [email protected] [c] Prof. Y. Yin Department of Chemistry University of California Riverside, CA 92521 (USA) [] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300401. Part of a Special Issue on “Shaping Nanostructures for Applications in Energy Conversion and Storage“. To view the complete issue, visit : http://onlinelibrary.wiley.com/doi/10.1002/cssc.v6.10/issuetoc