Nanostructured Zinc Oxide Nanorods with Copper Nanoparticles as a Microreformation CatalystThis work was supported by the National Natural Science Council, Ministry of Education, Taiwan, and AOARD under AFSOR, US

The use of hydrogen for energy generation has attracted significant attention in recent years as a clean, sustainable, and transportable alternative fuel, and this interest has consequently sparked a rapid global development of hydrogen fuel cells for electric power generation. Catalytic reformation of hydrocarbons, with careful attention to avoid storage and safety issues, is currently the predominant process for hydrogen generation. One of the leading and most promising techniques for hydrogen generation is catalytic reformation of methanol. Cu/ZnO-based catalysts are, therefore, of great importance for industrial scale catalytic production of reformate hydrogen. Owing to their wide commercial relevance, Cu/ZnO-based catalysts, prepared by several preparation routes, are being extensively investigated, and substantial improvements in their efficiency of catalytic activity brought about by addition of suitable promoter/ support, combination with effective component, and implementation of new preparation techniques, have been reported. 7–12] Unfortunately, use of Cu/ZnO-based heterostructures as reforming catalysts is still lacking to date. This inspired us to design a core–shell nanostructured catalyst consisting of a ZnO nanorod (NR) core and an outer shell of copper nanoparticles (NPs), that is, NR@NPs, for achieving high efficiency of catalytic conversion. The idea of using microreformers is also highly attractive for several applications, such as on-board hydrogen sources for small vehicles and portable fuel cells. However, two key issues have hindered the realization of microreformers for catalysis, namely, poor adhesion between the catalyst layer and the microchannels and poor utilization of catalyst layer deposited in the form of thick film. Notwithstanding, several approaches investigated to overcome these issues, catalyst immobilization, and its efficient utilization inside the microchannel remains a challenge. Most of these approaches involve a two-step process, wherein active catalysts are prepared in the first step, followed by its immobilization on the surface of the microchannels in the second step. Herein, we report a simple and reliable method for integrating in-situ synthesis of catalyst and its immobilization for microreformer applications. The ZnO NR arrays were first grown on a microchannel reactor using a simple template-free aqueous approach. A simple mixture of copper salts, aqueous media, and ZnO NR arrays at low temperature subsequently resulted in spontaneous formation of cable-like nanostructures. As the ZnO NR@Cu NP nanocomposites were synthesized in-situ directly on the microreactor, the arrayed ZnO@Cu nanocomposites were strongly anchored onto the microchannel. The strong mechanical anchorage of nanostructured catalysts on the surface of microchannel was shown by the observation that no material loss occurred after sonication in the water for several hours. The interaction between Cu NPs and ZnO NRs was studied by several analytical techniques, including electron microscopy (EM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and temperature-programmed reduction (TPR). The structure of the microreformer design based on the ZnO NR@Cu NP nanocomposite is illustrated in the Supporting Information (Scheme S1), along with photographs comparing the microchannels before and after the deposition of the ZnO NR@Cu NP nanocomposite (Supporting Information, Figure S1). One of the most significant advantages of the core–shell nanocomposites, which are clearly distinct from traditional catalysts, is the large surface area they offer for effective surface contact between the reactants and catalysts. Figure 1a shows a cross-sectional SEM image of vertically aligned ZnO NRs grown on the inner surface of the microchannel. The size of the NRs range from 35 to 50 nm in diameter and around 5 mm in height, as determined directly from the SEM micrograph. The TEM image of a typical NR is shown in Figure 1b, indicating an uneven surface with stacking faults (marked with arrows), which is shown in greater detail in Figure 1c. Figure 1d shows a typical TEM image of ZnO@Cu hybrid nanocomposites at a copper decoration concentration of 2 mm. Close attachment of Cu NPs on the ZnO NR cores can be clearly observed. More detailed TEM images with EDX elemental mapping of copper and zinc are shown in Figure 2, which further confirms the attachment of Cu NPs on the ZnO NR cores. Furthermore, a high-resolution TEM image (Figure 1e) yielded the spacing of the {111} lattice [*] Dr. L.-C. Chen, Dr. K.-H. Chen Center for Condensed Matter Sciences, National Taiwan University Taipei 106 (Taiwan) E-mail: [email protected]