Communication pubs.acs.org/JACS
Solid-Solution Alloying of Immiscible Ru and Cu with Enhanced CO Oxidation Activity Bo Huang,† Hirokazu Kobayashi,† Tomokazu Yamamoto,‡,§ Syo Matsumura,‡,§,∥ Yoshihide Nishida,⊥ Katsutoshi Sato,⊥,# Katsutoshi Nagaoka,⊥ Shogo Kawaguchi,¶ Yoshiki Kubota,∇ and Hiroshi Kitagawa*,†,∥,⊗ †
Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan § Kyushu University and the Ultramicroscopy Research Center, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan ∥ INAMORI Frontier Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan ⊥ Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan # Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan ¶ Japan Synchrotron Radiation Research Insitute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ∇ Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan ⊗ Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ‡
S Supporting Information *
metastable state by means of high-temperature quenching. However, there are few reports on the solid-solution structured alloy NPs where the constituent elements cannot mix, even above melting point in bulk phase, like oil and water. Here we report on novel Ru−Cu solid-solution alloy NPs where Ru and Cu do not mix each other at all, even in high-temperature liquid phase (Figure S1).15 Ru is in a hexagonal close-packed (hcp) structure in bulk state, whereas Cu is in a face-centered cubic (fcc) structure. It appears to be difficult to form solid-solution structures from constituent elements with different structures, such as bodycentered cubic Fe with fcc-Cu16 or fcc-Au with hcp-Co,17 compared to the combination of elements with the same structure. We have focused our attention on the ruthenium acetylacetonate Ru(acac)3 precursor and its ability to form fcc structured Ru NPs.18 Here, by using Ru(acac)3 and solutionphase coreduction of Ru and Cu precursors, we successfully achieved the atomic level mixing of Ru and Cu for the first time (Figure 1). Furthermore, the obtained Ru0.5Cu0.5 solid-solution NPs exhibited higher catalytic CO oxidation activity than the fcc-Ru NPs, which are known as the best CO oxidizing monometallic catalysts. The Ru0.5Cu0.5 NPs were synthesized by the polyol method under inert atmosphere. First, a mixture of 796.8 mg (2.0 mmol) Ru(acac)3 and 399.4 mg (2.0 mmol) copper acetate monohydrate (Cu(OAc)2·H2O) was dissolved in 30 mL of diethylene glycol (DEG) (solution A). Second, 880 mg of polyvinylpyrrolidone (PVP) K30 was dissolved in 300 mL of DEG, followed by degassing with liquid N2 three times over 3 days under vacuum (solution B). Solution B was kept at 220 °C under N2 atmosphere, with vigorous stirring. Solution A was
ABSTRACT: We report on novel solid-solution alloy nanoparticles (NPs) of Ru and Cu that are completely immiscible even above melting point in bulk phase. Powder X-ray diffraction, scanning transmission electron microscopy, and energy-dispersive X-ray measurements demonstrated that Ru and Cu atoms were homogeneously distributed in the alloy NPs. Ru0.5Cu0.5 NPs demonstrated higher CO oxidation activity than fcc-Ru NPs, which are known as one of the best monometallic CO oxidation catalysts.
B
imetallic nanoparticles (NPs) have received much attention as key materials in a wide range of applications such as catalytic,1 optical,2 and magnetic3 properties. Besides the size and morphology4 of bimetallic NPs, the core/shell5 or solid-solution6 structure is a critical parameter that determines the intrinsic properties of the NPs. In particular, the solidsolution structure, in which the constituent elements are randomly and homogeneously distributed in NPs, is of great significance; it directly affects the electronic states and the properties of bimetallic NPs by changing compositions and/or combinations of constituent elements.7 Many investigations on the solid-solution NPs have been performed mainly by the combination of miscible elements in bulk phase at ambient condition.8 Recently, nonequilibrium solid-solution alloy NPs such as Pd−Ru,9 Pd−Rh,10 Pt−Rh,11 Pt−Au,12 Au−Ni,13 and Cu−Rh14 have been reported with the development of synthetic methods such as nonequilibrium solution-phase or hydrogen-process method.7 In general, the nonequilibrium solid-solution NPs are often obtained by the elements that are not miscible at room temperature but are miscible at high temperature. Bulk alloys of this type can also be prepared as a © XXXX American Chemical Society
Received: February 3, 2017
A
DOI: 10.1021/jacs.7b01186 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Figure 1. Schematic representation of Ru−Cu solid-solution alloy NPs where Ru and Cu do not mix each other above melting point in bulk phase.
added dropwise via syringe to solution B over 5 min. The reaction mixture was stirred for 5 min, then cooled to room temperature and the inert condition was lifted. The resulting black solution was washed three times with EtOH to reduce the amount of PVP in the crude product and to remove other byproducts. Finally, a black powder was collected by centrifugation. As reference for the measurement of catalytic properties, fcc-Ru NPs were synthesized using a similar method to that described in a previous report (see Experimental Details).18 Both samples were characterized by transmission electron microscopy (TEM) (Figure S2a,b). The mean diameters of Ru0.5Cu0.5 NPs and fcc-Ru NPs were estimated to be 9.2 ± 2.5 and 5.9 ± 0.9 nm, respectively. X-ray fluorescence (XRF) measurements of Ru0.5Cu0.5 NPs revealed that the atomic ratio of Ru to Cu was 0.54:0.46 (Table S1). From energy-dispersive X-ray (EDX) measurement, the atomic ratio of Ru to Cu was estimated to be 0.55:0.45. To investigate the structure of the synthesized Ru0.5Cu0.5 NPs, synchrotron powder X-ray diffraction (XRD) measurements were carried out (Figure S3). The XRD pattern of Ru0.5Cu0.5 NPs shows the same fcc structure as those of fcc-Ru NPs and bulk Cu. The diffraction peaks of Ru0.5Cu0.5 NPs are located at the middle of the peak positions of Ru NPs and bulk Cu, suggesting that the elements Ru and Cu are atomically mixed to form an alloy structure. We then performed Rietveld refinements on Ru0.5Cu0.5 NPs, fcc-Ru NPs, and bulk Cu (Figures 2, S4, S5, Tables S2−S4). These refinements revealed that Ru0.5Cu0.5 NPs have an fcc structure (91.2%), with a small amount of hcp structure (8.8%) as a minor phase. The lattice constant of Ru0.5Cu0.5 NPs was estimated to be 3.736 Å for the fcc phase, whereas those of Ru NPs and bulk Cu were 3.850(2) Å and 3.613 Å (Figures S4 and S5). Assuming that the lattice constant follows Vegard’s law, the Ru:Cu atomic ratio of Ru0.5Cu0.5 NPs was calculated to be 0.52:0.48. These results strongly suggest the formation of the Ru0.5Cu0.5 solid-solution nanoalloy. To clarify the alloyed state in the obtained Ru0.5Cu0.5 NPs, high-angle annular dark-field scanning TEM (HAADF-STEM) and EDX elemental mapping of Ru and Cu were carried out (Figure 3). Figure 3b−d shows the mappings of Ru, Cu, and their overlay, respectively. These mappings give a direct view of the homogeneous distribution of Ru and Cu atoms in every particle. The EDX line scanning analysis shown in Figure 3f also demonstrates that Ru and Cu atoms are homogeneously well distributed in the particle. Similarly, the distributions of Ru and Cu were investigated for several particles; they showed the same results (Figure S6). From the high-resolution (HR) HAADF-STEM image of Ru0.5Cu0.5 NPs, the distance of the
Figure 2. Synchrotron XRD pattern of Ru0.5Cu0.5 NPs (black dots) at 303 K and calculated pattern (red line). The bottom lines show the difference profile (gray) and the fitting curves of the fcc (blue) and hcp components (green). The radiation wavelength was 0.5787 Å.
Figure 3. (a) HAADF image, (b) Cu-K STEM-EDX map, and (c) RuL STEM-EDX map obtained from a group of Ru0.5Cu0.5 NPs; (d) reconstructed overlay image of the maps shown in panels b and c (red, Cu; green, Ru); (e) HR-STEM image; (f) compositional line profiles of Cu (red) and Ru (green) for the Ru0.5Cu0.5 NP recorded along the arrow shown in the STEM image (e).
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DOI: 10.1021/jacs.7b01186 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society
activity. These results demonstrated the enhanced activity of Ru0.5Cu0.5 NPs over the best monometallic catalyst of fcc-Ru NPs. It should be noticed that the mean diameter of Ru0.5Cu0.5 NPs (9.2 ± 2.5 nm) is larger than that of fcc-Ru NPs (5.9 ± 0.9 nm). It was reported that fcc-Ru NPs show better CO oxidation activity with particle size from 2.4 to 5.4 nm.18 This has been considered to relate to defects near the surface region of fcc-Ru NPs.21 In addition, considering that a physical mixture of fccRu NPs and Cu NPs on γ-Al2O3 (green curve) exhibited the same activity as fcc-Ru, the significantly enhanced CO oxidation activity of Ru0.5Cu0.5 NPs is considered to originate from the atomic level alloying of Ru and Cu. It is known that the CO oxidation reaction is related to chemisorbed species of O and CO coadsorption (Langmuir− Hinshelwood mechanism).22 Considering that Cu is known to have high oxygen affinity,23 whereas Ru has a relatively strong CO adsorption capability,20,24 the bifunctional active sites on the surface of Ru0.5Cu0.5 NPs provide the enhanced CO oxidation activity. In addition, the change in the electronic state of Ru followed by atomic level alloying may also contribute to the high catalytic activity. A theoretical calculation to clarify the mechanism is currently in progress. In summary, for the first time, we synthesized and characterized Ru0.5Cu0.5 solid-solution NPs, where Ru and Cu are completely immiscible, even above melting point in bulk phase. STEM-EDX mapping and PXRD of Ru0.5Cu0.5 solidsolution NPs demonstrated that Ru and Cu are randomly and homogeneously distributed in the fcc-NPs. The Ru(acac)3 precursor, likely forming the fcc structured Ru NPs, plays an important role in the atomic-level mixing of Ru and Cu. The novel Ru0.5Cu0.5 NP demonstrated excellent CO oxidation activity; it was better than the fcc-Ru NP, which is known as the best CO oxidizing monometallic catalyst. These new findings provide valuable information for the future development of higher performance materials, even with inexpensive and the most abundant elements.
lattice fringe was determined to be 2.1 Å, which is in agreement with the {111} lattice plane of a fcc-Ru0.5Cu0.5 alloy (Figure 3e). Hence, we successfully synthesized Ru0.5Cu0.5 solidsolution NPs where Ru and Cu are homogeneously mixed at the atomic level. It was interesting to note that the solidsolution structure of Ru0.5Cu0.5 NPs was stable up to 300 °C (Figure S7). We next performed the control experiment for the Ru0.5Cu0.5 solid-solution NPs by using RuCl3, which is widely used as a precursor for synthesizing hcp-Ru. The XRD pattern revealed that the obtained NPs are composed of multiple components (Figure S8). HAADF-STEM images and EDX mapping showed Ru-rich alloy NPs, including a low amount of Cu atoms, with aggregation of each particle (Figure S9). From EDX measurements, the atomic ratio of Ru to Cu was estimated to be 0.82:0.18. The incomplete RuCu alloy is considered to mainly originate from the combination of precursors providing different structures of hcp-Ru and fcc-Cu. The Ru(acac)3 precursor that is likely to cause fcc structured Ru NPs plays an important role for good homogeneous mixing of Ru and Cu atoms in the particle. Ru is best known as a high-performance catalyst for the CO oxidation reaction.19,20 Cu is one of the most abundant and inexpensive metallic elements. These features motivated us to explore the CO oxidation activity of the novel Ru0.5Cu0.5 solidsolution NPs. To measure the catalytic properties, the Ru0.5Cu0.5 NPs were supported on γ-Al2O3. For comparison, fcc-Ru NPs/γ-Al2O3, Cu NPs/γ-Al2O3 (Figure S2c), and a physical mixture of Ru+Cu NPs/γ-Al2O3 were also prepared (see Experimental Details). The metal loading amounts of all the samples were ca. 1 wt %, determined by XRF-ray fluorescence measurements. Before carrying out catalytic tests, all samples were heated at 100 or 300 °C under H2 flow to remove any surface oxidation of NPs (see Experimental Details). Their structures remained unchanged, before and after the H2 treatment (Figures S10 and S11). As shown in Figure 4, the temperatures corresponding to a 50% conversion (T50) of CO to CO2 for Ru0.5Cu0.5 (red curve) and fcc-Ru (black curve) catalysts were 122 and 141 °C, respectively. On the other hand, Cu NPs (blue curve) had poor CO oxidation
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01186. Experimental details, bulk phase diagram, TEM images, PXRD patterns, Rietveld refinements, line profiles, and STEM-EDX mapping images (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Hiroshi Kitagawa: 0000-0001-6955-3015 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Core Research for Evolutional Science and Technology (CREST) and ACCEL from the Japan Science and Technology Agency (JST), and Grants-in-Aid for JSPS Fellows (No. 27-2581) from the Japan Society for the Promotion of Science (JSPS). STEM observations were performed as part of a program conducted by the Advanced
Figure 4. Temperature dependence of CO conversion of Ru0.5Cu0.5 (red), fcc-Ru (black), Cu NPs (blue), and fcc-Ru+Cu (green) NPs supported on γ-Al2O3. C
DOI: 10.1021/jacs.7b01186 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Characterization Nanotechnology Platform sponsored by the MEXT of the Japanese Government.
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