- Email: [email protected]
Preparation of catalytically active Au nanoparticles by sputter deposition and their encapsulation in metal-organic framework of Cu3(BTC)2
Materials Letters 261 (2020) 127124
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Preparation of catalytically active Au nanoparticles by sputter deposition and their encapsulation in metal-organic framework of Cu3(BTC)2 Kiyoshi Matsuyama a,⇑, Tomohiro Tsubaki b, Takafumi Kato c, Tetsuya Okuyama d, Hiroyuki Muto e a
Department of Life, Environment and Applied Chemistry, Faculty of Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku, Fukuoka 811-0295, Japan Department of Biochemistry and Applied Chemistry, National Institute of Technology, Kurume College, 1-1-1 Komorino, Kurume, Fukuoka 830-8555, Japan c Department of Chemical Engineering, Fukuoka University, 8-19-1 Nanakuma Jonan-ku, Fukuoka 814-0180, Japan d Department of Materials Science and Engineering, National Institute of Technology, Kurume College, 1-1-1 Komorino, Kurume, Fukuoka 830-8555, Japan e Institute of Liberal Arts and Sciences, Toyohashi University of Technology, 1-1 Hibarigaoka Tenpaku-cho, Toyohashi, Aichi 441-8580, Japan b
a r t i c l e
i n f o
Article history: Received 25 August 2019 Received in revised form 24 November 2019 Accepted 4 December 2019 Available online 5 December 2019 Keywords: Au nanoparticle Sputter deposition Metal-organic frameworks Cu3(BTC)2
a b s t r a c t Gold nanoparticles (Au NPs) in liquid poly (ethylene glycol) (PEG) were prepared via sputter deposition. This is an easy and simple preparation method for Au NPs, and does not require a chemical reaction or additional stabilizers. A metal-organic framework (MOF), Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylic acid), was synthesized in the liquid PEG containing the Au NPs to obtain Au@Cu3(BTC)2. Liquid PEG acted as the capture medium for sputter deposition and reaction medium for the formation of Au@Cu3(BTC)2. Images obtained through scanning transmission electron microscopy (STEM) clearly demonstrated that the Au NPs were successfully immobilized inside the pores of Cu3(BTC)2. The resulting Au@Cu3(BTC)2 exhibited high activity for CO oxidation. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Gold nanoparticles (Au NPs) have been widely used in heterogeneous catalysis since Haruta et al. found that Au NPs that support metal oxides are highly reactive for various important catalytic reactions [1]. Many controlled synthesis methods of Au NPs and nanoclusters have been developed for catalytic applications [2]. Combining sputter deposition with liquid poly(ethylene glycol) (PEG), as the capture medium, is among the most effective methods of forming Au NPs [3,4]. This is an easy and simple method for preparing Au NPs in liquid PEG, and does not require a chemical reaction or additional stabilizers. PEG is economical and environmentally friendly, and the low vapor pressure of the solvent is enough to permit sputtering. However, the catalytic activity of Au NPs prepared via sputter deposition has not yet been reported because the recovery of dispersed Au NPs in liquid PEG is practically difficult. On the other hand, noble metal NPs, as heterogeneous, active catalysts, can be deposited and immobilized on porous metal oxides [5–7]. Au NPs supported by metal oxides show high catalytic activity. The Au-oxide interfacial perimeter plays an important role [8]. Au NPs immobilized inside the pores of metal-organic
⇑ Corresponding author. E-mail address: [email protected] (K. Matsuyama). https://doi.org/10.1016/j.matlet.2019.127124 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
frameworks (MOFs) have attracted immense attention as functional materials for heterogeneous catalysis [9–12]. MOFs are well-designed metal-organic nanoporous hybrid materials realized by connecting organic linkers and metal ions in a coordination network [13]. The loading of metal NPs into the pores of MOFs enhances the catalytic performances of NPs [14–16]. In this study, the catalytic performance of Au NPs prepared via sputter deposition and using liquid PEG as the capture medium was determined. Au NPs were immobilized inside the pores of the MOF, Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylic acid), to recover Au NPs from liquid PEG. Cu3(BTC)2 was prepared in the liquid PEG containing Au NPs as the reaction media. First, PEG was removed by supercritical carbon dioxide (CO2) drying, following which ultrafine Au NPs were thoroughly dispersed into Cu3(BTC)2 and Au@Cu3(BTC)2 to evaluate their catalytic performance for CO oxidation.
2. Materials and methods Au NPs in liquid PEG 600 were prepared by sputter deposition using a sputter coater (SC-701 MkII Eco., Sanyu Electronics Co. Ltd.). The synthetic methodologies of Au NPs have been described in previous studies [4]. Au was deposited under reduced pressure and a voltage of 800 kV. Liquid PEG 600 (5 cm3) was placed on a glass Petri dish horizontally set in the sputter coater. The surface
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K. Matsuyama et al. / Materials Letters 261 (2020) 127124
Fig. 1. (a) Au NPs in liquid PEG 600 prepared using sputter deposition (left side: pristine PEG 600, right side: Au NPs in liquid PEG 600), (b) pristine Cu3(BTC)2, (c) Au@Cu3(BTC)2, (d) SEM, and (e) HR-TEM images of Au@Cu3(BTC)2.
of PEG 600 was at a distance of 30 mm from the target Au foil. The sputtering time at which PEG 600 containing Au NPs was obtained was 30 min. To obtain Au@Cu3(BTC)2, Cu3(BTC)2 was synthesized in PEG 600 containing Au NPs as the reactant and solvent. The synthetic methodologies of Cu3(BTC)2 in PEG 600 have been described in previous studies [17]. In the experiment, BTC (0.3 mol) and copper(II) acetate (Cu(OAc)2H2O, 0.54 mol) were loaded into PEG 600 containing Au NPs, followed by the addition of 0.5 cm3 triethylamine. The mixture was stirred at 25 °C for 48 h. The product obtained after centrifugation was soaked in absolute ethanol, wherein the solvent was replaced every 24 h for 72 h, to exchange the occluded solvent from ethanol. The ethanol-containing samples were then placed in a high-pressure cell (50 cm3) to undergo supercritical CO2 drying [18,19]. The pristine Cu3(BTC)2 was
synthesized without the Au NPs in pure PEG solution under the same conditions for preparation of Au@Cu3(BTC)2. The structures of Au@Cu3(BTC)2 were analyzed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX) analysis (JEOL, JEM-ARM200CF). The samples were prepared by dipping carboncoated copper grids into the sample solution and drying the grids. The sections were observed using an STEM system operating at 200 kV. The catalytic activity of Au@Cu3(BTC)2 was CO oxidation, as observed using a fixed bed plug-flow reactor. Pure CO (1 vol%), O2 (20 vol%), and He (79 vol%) were supplied through mass flow controllers and mixed with each other, and the final reactant gas (100 ml min 1) was passed through the catalyst bed. Au@Cu3 (BTC)2 was diluted using glass sand (30–50 mesh) and the mixture
Fig. 2. (a) N2 adsorption-desorption isotherms and (b) XRD pattern of pristine Cu3(BTC)2 and Au@Cu3(BTC)2.
K. Matsuyama et al. / Materials Letters 261 (2020) 127124 Table 1 BET surface areas and pore volumes of pristine Cu3(BTC)2 and Au@Cu3(BTC)2. Materials
BET surface area SBET [m2 g 1]
Pore volume Vp [cm3 g 1]
Pristine Cu3(BTC)2 Au@Cu3(BTC)2
105 87
0.230 0.217
was passed through a reactor made of stainless-steel pipes (inner diameter 15 mm, length 50 mm). The reaction products were analyzed through gas chromatography (GC-8A, Shimadzu). 3. Results and discussion Before preparing Au@Cu3(BTC)2, Au NPs in liquid PEG 600 were prepared using sputter deposition. The sample turned dark red in color and no precipitate was observed (Fig. 1(a)), indicating the successful formation of Au NPs in PEG 600. TEM image of Au NPs is shown in Fig. S1 (see the Supplementary information). Au@Cu3(BTC)2 was synthesized in PEG 600 containing Au NPs as the reactant and solvent. The color of Au@Cu3(BTC)2 was slightly dark compared to pristine Cu3(BTC)2 due to the Au NPs being dispersed in Cu3(BTC)2 (Fig. 1(b, c)). The amount of Au in Cu3(BTC)2 was 1.1 wt%, according to X-ray fluorescence (XRF). Scanning electron microscopy (SEM) and HR-TEM images of Au@Cu3(BTC)2 are shown in Fig. 1(d, e). Au@Cu3(BTC)2 exhibited an irregular form, with particle sizes of approximately 1 lm. The HR-TEM images clearly demonstrated the uniform distribution of Au NPs throughout the interior cavities of Cu3(BTC)2. N2 isotherms and X-ray powder diffraction (XRD) patterns of Cu3(BTC)2 and Au@Cu3(BTC)2 are shown in Fig. 2. The Brunauer–Emmett–Teller (BET) surface areas and pore volumes are summarized in Table 1. The pristine Cu3(BTC)2 prepared in PEG show the low surface area compared with that prepared by conventional hydrothermal method. The residual PEG may accelerate the aggregation of Cu3(BTC)2 particles and the collapse of Cu3(BTC)2 pores caused by the capillary pressure at the liquid–vapor interface of the solvent during the solvent evacuation process. The N2-accessible surface area of Au@Cu3(BTC)2 was slightly reduced compared to pristine Cu3(BTC)2 because Au NPs were embedded into the Cu3(BTC)2 network and occupied some of the pores. Similar XRD patterns of Cu3(BTC)2 were reported by Xue et al. [17], indicating that the crystal structure of Cu3(BTC)2
3
was not dependent on the addition of Au NPs. Moreover, Au NPs might be too small to detect via XRD. All the diffraction peaks of Cu3(BTC)2 are in good agreement with that of the simulated pattern [20]. The structure of Au@Cu3(BTC)2 was characterized by STEM with elemental mapping data (Fig. 3(a–d)). The HR-TEM image showed that the Au NPs had sizes in the range of 3–4 nm (Fig. 3 (a)). Fig. 3(b) is a HAADF-STEM image. Fig. 3(c) and (d) show the Au-L and Cu-K STEM-EDX maps, respectively, which provide visual evidence that Au NPs were successfully immobilized into Cu3(BTC)2. To investigate the catalytic properties of Au@Cu3(BTC)2, CO oxidation reactions were performed (Fig. 3(e)). The pristine Cu3(BTC)2 shows lower catalytic activity below 200 °C (Fig. 3(e)). Ye and Liu reported that Cu3(BTC)2 acts as a catalyst for CO oxidation and the temperature required for 100% conversion of CO is 240 °C [21]. They also indicated that the catalytic activity can be improved by loading palladium dioxide (PdO2) NPs onto Cu3(BTC)2 and decreasing the temperature for 100% conversion to 220 °C. On the other hand, Au@Cu3(BTC)2 shows high catalytic activity, and the temperature for the 100% conversion of CO is less than 190 °C. On the other hand, the CO conversion property of physical mixture of Au NPs and Cu3(BTC)2 differs from Au@Cu3(BTC)2 as shown in Fig. 3(e). The physical mixture of Au NPs and Cu3(BTC)2 show the same conversion of Cu3(BTC)2, because the Au NPs may be agglomerated each other. 4. Conclusions Au NPs were successfully synthesized via sputter deposition and using liquid PEG 600 as the capture medium. The prepared Au NPs were loaded into the MOF of Cu3(BTC)2. STEM images clearly demonstrated the uniform distribution of Au NPs in Cu3(BTC)2. The loading of Au NPs onto Cu3(BTC)2 accelerated the catalytic CO oxidation reaction. Such Au NPs are promising heterogeneous catalysts. CRediT authorship contribution statement Kiyoshi Matsuyama: Conceptualization, Methodology, Writing - original draft, Supervision. Tomohiro Tsubaki: Data curation. Takafumi Kato: Methodology. Tetsuya Okuyama: Methodology, Investigation. Hiroyuki Muto: Methodology, Investigation.
Fig. 3. (a) HR-TEM image, (b) HAADF-STEM image, (c) Au-L STEM-EDX map, (d) Cu-K STEM-EDX map of Au@Cu3(BTC)2, and (e) conversion-temperature curve for CO oxidation over pristine Cu3(BTC)2 and Au@Cu3(BTC)2.
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K. Matsuyama et al. / Materials Letters 261 (2020) 127124
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was partly supported by Advanced Characterization Platform of the Nanotechnology Platform Japan (JPMXP09-A-19KU-0259) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We are grateful to Dr T.Toriyama of Kyushu University for his helpful support in transmission electron microscopy analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.127124. References [1] M. Haruta, Size- and support-dependency in the catalysis of gold, Catal. Today 36 (1) (1997) 153–166, https://doi.org/10.1016/S0920-5861(96)00208-8. [2] J. Zhao, R. Jin, Heterogeneous catalysis by gold and gold-based bimetal nanoclusters, Nano Today 18 (2018) 86–102, https://doi.org/10.1016/ j.nantod.2017.12.009. [3] Y. Hatakeyama, J.-I. Kato, T. Mukai, K. Judai, K. Nishikawa, Effect of adding a thiol stabilizer on synthesis of Au nanoparticles by sputter deposition onto poly(ethylene glycol), Bull. Chem. Soc. Jpn. 87 (7) (2014) 773–779, https://doi. org/10.1246/bcsj.20140023. [4] Y. Hatakeyama, T. Morita, S. Takahashi, K. Onishi, K. Nishikawa, Synthesis of gold nanoparticles in liquid polyethylene glycol by sputter deposition and temperature effects on their size and shape, J. Phys. Chem. C 115 (8) (2011) 3279–3285, https://doi.org/10.1021/jp110455k. [5] J.A. Martens, J. Jammaer, S. Bajpe, A. Aerts, Y. Lorgouilloux, C.E.A. Kirschhock, Simple synthesis recipes of porous materials, Microporous Mesoporous Mater. 140 (1) (2011) 2–8, https://doi.org/10.1016/j.micromeso.2010.09.018. [6] K. Matsuyama, S. Tanaka, T. Kato, T. Okuyama, H. Muto, R. Miyamoto, H.-Z. Bai, Supercritical fluid-assisted immobilization of Pd nanoparticles in the mesopores of hierarchical porous SiO2 for catalytic applications, J. Supercrit. Fluids 130 (2017) 140–146, https://doi.org/10.1016/j.supflu.2017.07.032. [7] Y. Yang, K. Chiang, N. Burke, Porous carbon-supported catalysts for energy and environmental applications: a short review, Catal. Today 178 (1) (2011) 197– 205, https://doi.org/10.1016/j.cattod.2011.08.028.
[8] Y.-G. Wang, D. Mei, V.-A. Glezakou, J. Li, R. Rousseau, Dynamic formation of single-atom catalytic active sites on ceria-supported gold nanoparticles, Nat. Commun. 6 (2015) 6511, https://doi.org/10.1038/ncomms7511. [9] A. Dhakshinamoorthy, A.M. Asiri, H. Garcia, Metal organic frameworks as versatile hosts of Au nanoparticles in heterogeneous catalysis, ACS Catal. 7 (4) (2017) 2896–2919, https://doi.org/10.1021/acscatal.6b03386. [10] H.-L. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, Au@ZIF-8: CO oxidation over gold nanoparticles deposited to metal–organic framework, J. Am. Chem. Soc. 131 (32) (2009) 11302–11303, https://doi.org/10.1021/ja9047653. [11] H. Liu, Y. Liu, Y. Li, Z. Tang, H. Jiang, Metal–organic framework supported gold nanoparticles as a highly active heterogeneous catalyst for aerobic oxidation of alcohols, J. Phys. Chem. C 114 (31) (2010) 13362–13369, https://doi.org/ 10.1021/jp105666f. [12] N. Tsumori, L. Chen, Q. Wang, Q.-L. Zhu, M. Kitta, Q. Xu, Quasi-MOF: exposing inorganic nodes to guest metal nanoparticles for drastically enhanced catalytic activity, Chemistry 4 (4) (2018) 845–856, https://doi.org/10.1016/j. chempr.2018.03.009. [13] H. Furukawa, K.E. Cordova, M. O’Keeffe, O.M. Yaghi, The chemistry and applications of metal-organic frameworks, Science 341 (6149) (2013) 1230444, https://doi.org/10.1126/science.1230444. [14] K. Matsuyama, M. Motomura, T. Kato, T. Okuyama, H. Muto, Catalytically active Pt nanoparticles immobilized inside the pores of metal organic framework using supercritical CO2 solutions, Microporous Mesoporous Mater. 225 (2016) 26–32, https://doi.org/10.1016/j.micromeso.2015.12.005. [15] Q. Yang, Q. Xu, H.-L. Jiang, Metal–organic frameworks meet metal nanoparticles: synergistic effect for enhanced catalysis, Chem. Soc. Rev. 46 (15) (2017) 4774–4808, https://doi.org/10.1039/C6CS00724D. [16] L. Chen, Q. Xu, Metal-organic framework composites for catalysis, Matter 1 (1) (2019) 57–89, https://doi.org/10.1016/j.matt.2019.05.018. [17] Z. Xue, J. Zhang, L. Peng, B. Han, T. Mu, J. Li, G. Yang, Poly(ethylene glycol) stabilized mesoporous metal–organic framework nanocrystals: efficient and durable catalysts for the oxidation of benzyl alcohol, ChemPhysChem 15 (1) (2014) 85–89, https://doi.org/10.1002/cphc.201300809. [18] K. Matsuyama, N. Hayashi, M. Yokomizo, T. Kato, K. Ohara, T. Okuyama, Supercritical carbon dioxide-assisted drug loading and release from biocompatible porous metal–organic frameworks, J. Mater. Chem. B 2 (43) (2014) 7551–7558, https://doi.org/10.1039/C4TB00725E. [19] K. Matsuyama, Supercritical fluid processing for metal–organic frameworks, porous coordination polymers, and covalent organic frameworks, J. Supercrit. Fluids 134 (2018) 197–203, https://doi.org/10.1016/j.supflu.2017.12.004. [20] W. Wong-Ng, J.A. Kaduk, D.L. Siderius, A.L. Allen, L. Espinal, B.M. Boyerinas, I. Levin, M.R. Suchomel, J. Ilavsky, L. Li, I. Williamson, E. Cockayne, H. Wu, Reference diffraction patterns, microstructure, and pore-size distribution for the copper(II) benzene-1,3,5-tricarboxylate metal organic framework (CuBTC) compounds, Powder Diffr. 30 (3) (2015), https://doi.org/10.1017/ s0885715615000330. [21] J.Y. Ye, C.J. Liu, Cu3(BTC)2: CO oxidation over MOF based catalysts, Chem. Commun. 47 (7) (2011) 2167–2169, https://doi.org/10.1039/C0CC04944A.
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Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Preparation of catalytically active Au nanoparticles by sputter deposition and their encapsulation in metal-organic framework of Cu3(BTC)2 Kiyoshi Matsuyama a,⇑, Tomohiro Tsubaki b, Takafumi Kato c, Tetsuya Okuyama d, Hiroyuki Muto e a
Department of Life, Environment and Applied Chemistry, Faculty of Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku, Fukuoka 811-0295, Japan Department of Biochemistry and Applied Chemistry, National Institute of Technology, Kurume College, 1-1-1 Komorino, Kurume, Fukuoka 830-8555, Japan c Department of Chemical Engineering, Fukuoka University, 8-19-1 Nanakuma Jonan-ku, Fukuoka 814-0180, Japan d Department of Materials Science and Engineering, National Institute of Technology, Kurume College, 1-1-1 Komorino, Kurume, Fukuoka 830-8555, Japan e Institute of Liberal Arts and Sciences, Toyohashi University of Technology, 1-1 Hibarigaoka Tenpaku-cho, Toyohashi, Aichi 441-8580, Japan b
a r t i c l e
i n f o
Article history: Received 25 August 2019 Received in revised form 24 November 2019 Accepted 4 December 2019 Available online 5 December 2019 Keywords: Au nanoparticle Sputter deposition Metal-organic frameworks Cu3(BTC)2
a b s t r a c t Gold nanoparticles (Au NPs) in liquid poly (ethylene glycol) (PEG) were prepared via sputter deposition. This is an easy and simple preparation method for Au NPs, and does not require a chemical reaction or additional stabilizers. A metal-organic framework (MOF), Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylic acid), was synthesized in the liquid PEG containing the Au NPs to obtain Au@Cu3(BTC)2. Liquid PEG acted as the capture medium for sputter deposition and reaction medium for the formation of Au@Cu3(BTC)2. Images obtained through scanning transmission electron microscopy (STEM) clearly demonstrated that the Au NPs were successfully immobilized inside the pores of Cu3(BTC)2. The resulting Au@Cu3(BTC)2 exhibited high activity for CO oxidation. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Gold nanoparticles (Au NPs) have been widely used in heterogeneous catalysis since Haruta et al. found that Au NPs that support metal oxides are highly reactive for various important catalytic reactions [1]. Many controlled synthesis methods of Au NPs and nanoclusters have been developed for catalytic applications [2]. Combining sputter deposition with liquid poly(ethylene glycol) (PEG), as the capture medium, is among the most effective methods of forming Au NPs [3,4]. This is an easy and simple method for preparing Au NPs in liquid PEG, and does not require a chemical reaction or additional stabilizers. PEG is economical and environmentally friendly, and the low vapor pressure of the solvent is enough to permit sputtering. However, the catalytic activity of Au NPs prepared via sputter deposition has not yet been reported because the recovery of dispersed Au NPs in liquid PEG is practically difficult. On the other hand, noble metal NPs, as heterogeneous, active catalysts, can be deposited and immobilized on porous metal oxides [5–7]. Au NPs supported by metal oxides show high catalytic activity. The Au-oxide interfacial perimeter plays an important role [8]. Au NPs immobilized inside the pores of metal-organic
⇑ Corresponding author. E-mail address: [email protected] (K. Matsuyama). https://doi.org/10.1016/j.matlet.2019.127124 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
frameworks (MOFs) have attracted immense attention as functional materials for heterogeneous catalysis [9–12]. MOFs are well-designed metal-organic nanoporous hybrid materials realized by connecting organic linkers and metal ions in a coordination network [13]. The loading of metal NPs into the pores of MOFs enhances the catalytic performances of NPs [14–16]. In this study, the catalytic performance of Au NPs prepared via sputter deposition and using liquid PEG as the capture medium was determined. Au NPs were immobilized inside the pores of the MOF, Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylic acid), to recover Au NPs from liquid PEG. Cu3(BTC)2 was prepared in the liquid PEG containing Au NPs as the reaction media. First, PEG was removed by supercritical carbon dioxide (CO2) drying, following which ultrafine Au NPs were thoroughly dispersed into Cu3(BTC)2 and Au@Cu3(BTC)2 to evaluate their catalytic performance for CO oxidation.
2. Materials and methods Au NPs in liquid PEG 600 were prepared by sputter deposition using a sputter coater (SC-701 MkII Eco., Sanyu Electronics Co. Ltd.). The synthetic methodologies of Au NPs have been described in previous studies [4]. Au was deposited under reduced pressure and a voltage of 800 kV. Liquid PEG 600 (5 cm3) was placed on a glass Petri dish horizontally set in the sputter coater. The surface
2
K. Matsuyama et al. / Materials Letters 261 (2020) 127124
Fig. 1. (a) Au NPs in liquid PEG 600 prepared using sputter deposition (left side: pristine PEG 600, right side: Au NPs in liquid PEG 600), (b) pristine Cu3(BTC)2, (c) Au@Cu3(BTC)2, (d) SEM, and (e) HR-TEM images of Au@Cu3(BTC)2.
of PEG 600 was at a distance of 30 mm from the target Au foil. The sputtering time at which PEG 600 containing Au NPs was obtained was 30 min. To obtain Au@Cu3(BTC)2, Cu3(BTC)2 was synthesized in PEG 600 containing Au NPs as the reactant and solvent. The synthetic methodologies of Cu3(BTC)2 in PEG 600 have been described in previous studies [17]. In the experiment, BTC (0.3 mol) and copper(II) acetate (Cu(OAc)2H2O, 0.54 mol) were loaded into PEG 600 containing Au NPs, followed by the addition of 0.5 cm3 triethylamine. The mixture was stirred at 25 °C for 48 h. The product obtained after centrifugation was soaked in absolute ethanol, wherein the solvent was replaced every 24 h for 72 h, to exchange the occluded solvent from ethanol. The ethanol-containing samples were then placed in a high-pressure cell (50 cm3) to undergo supercritical CO2 drying [18,19]. The pristine Cu3(BTC)2 was
synthesized without the Au NPs in pure PEG solution under the same conditions for preparation of Au@Cu3(BTC)2. The structures of Au@Cu3(BTC)2 were analyzed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX) analysis (JEOL, JEM-ARM200CF). The samples were prepared by dipping carboncoated copper grids into the sample solution and drying the grids. The sections were observed using an STEM system operating at 200 kV. The catalytic activity of Au@Cu3(BTC)2 was CO oxidation, as observed using a fixed bed plug-flow reactor. Pure CO (1 vol%), O2 (20 vol%), and He (79 vol%) were supplied through mass flow controllers and mixed with each other, and the final reactant gas (100 ml min 1) was passed through the catalyst bed. Au@Cu3 (BTC)2 was diluted using glass sand (30–50 mesh) and the mixture
Fig. 2. (a) N2 adsorption-desorption isotherms and (b) XRD pattern of pristine Cu3(BTC)2 and Au@Cu3(BTC)2.
K. Matsuyama et al. / Materials Letters 261 (2020) 127124 Table 1 BET surface areas and pore volumes of pristine Cu3(BTC)2 and Au@Cu3(BTC)2. Materials
BET surface area SBET [m2 g 1]
Pore volume Vp [cm3 g 1]
Pristine Cu3(BTC)2 Au@Cu3(BTC)2
105 87
0.230 0.217
was passed through a reactor made of stainless-steel pipes (inner diameter 15 mm, length 50 mm). The reaction products were analyzed through gas chromatography (GC-8A, Shimadzu). 3. Results and discussion Before preparing Au@Cu3(BTC)2, Au NPs in liquid PEG 600 were prepared using sputter deposition. The sample turned dark red in color and no precipitate was observed (Fig. 1(a)), indicating the successful formation of Au NPs in PEG 600. TEM image of Au NPs is shown in Fig. S1 (see the Supplementary information). Au@Cu3(BTC)2 was synthesized in PEG 600 containing Au NPs as the reactant and solvent. The color of Au@Cu3(BTC)2 was slightly dark compared to pristine Cu3(BTC)2 due to the Au NPs being dispersed in Cu3(BTC)2 (Fig. 1(b, c)). The amount of Au in Cu3(BTC)2 was 1.1 wt%, according to X-ray fluorescence (XRF). Scanning electron microscopy (SEM) and HR-TEM images of Au@Cu3(BTC)2 are shown in Fig. 1(d, e). Au@Cu3(BTC)2 exhibited an irregular form, with particle sizes of approximately 1 lm. The HR-TEM images clearly demonstrated the uniform distribution of Au NPs throughout the interior cavities of Cu3(BTC)2. N2 isotherms and X-ray powder diffraction (XRD) patterns of Cu3(BTC)2 and Au@Cu3(BTC)2 are shown in Fig. 2. The Brunauer–Emmett–Teller (BET) surface areas and pore volumes are summarized in Table 1. The pristine Cu3(BTC)2 prepared in PEG show the low surface area compared with that prepared by conventional hydrothermal method. The residual PEG may accelerate the aggregation of Cu3(BTC)2 particles and the collapse of Cu3(BTC)2 pores caused by the capillary pressure at the liquid–vapor interface of the solvent during the solvent evacuation process. The N2-accessible surface area of Au@Cu3(BTC)2 was slightly reduced compared to pristine Cu3(BTC)2 because Au NPs were embedded into the Cu3(BTC)2 network and occupied some of the pores. Similar XRD patterns of Cu3(BTC)2 were reported by Xue et al. [17], indicating that the crystal structure of Cu3(BTC)2
3
was not dependent on the addition of Au NPs. Moreover, Au NPs might be too small to detect via XRD. All the diffraction peaks of Cu3(BTC)2 are in good agreement with that of the simulated pattern [20]. The structure of Au@Cu3(BTC)2 was characterized by STEM with elemental mapping data (Fig. 3(a–d)). The HR-TEM image showed that the Au NPs had sizes in the range of 3–4 nm (Fig. 3 (a)). Fig. 3(b) is a HAADF-STEM image. Fig. 3(c) and (d) show the Au-L and Cu-K STEM-EDX maps, respectively, which provide visual evidence that Au NPs were successfully immobilized into Cu3(BTC)2. To investigate the catalytic properties of Au@Cu3(BTC)2, CO oxidation reactions were performed (Fig. 3(e)). The pristine Cu3(BTC)2 shows lower catalytic activity below 200 °C (Fig. 3(e)). Ye and Liu reported that Cu3(BTC)2 acts as a catalyst for CO oxidation and the temperature required for 100% conversion of CO is 240 °C [21]. They also indicated that the catalytic activity can be improved by loading palladium dioxide (PdO2) NPs onto Cu3(BTC)2 and decreasing the temperature for 100% conversion to 220 °C. On the other hand, Au@Cu3(BTC)2 shows high catalytic activity, and the temperature for the 100% conversion of CO is less than 190 °C. On the other hand, the CO conversion property of physical mixture of Au NPs and Cu3(BTC)2 differs from Au@Cu3(BTC)2 as shown in Fig. 3(e). The physical mixture of Au NPs and Cu3(BTC)2 show the same conversion of Cu3(BTC)2, because the Au NPs may be agglomerated each other. 4. Conclusions Au NPs were successfully synthesized via sputter deposition and using liquid PEG 600 as the capture medium. The prepared Au NPs were loaded into the MOF of Cu3(BTC)2. STEM images clearly demonstrated the uniform distribution of Au NPs in Cu3(BTC)2. The loading of Au NPs onto Cu3(BTC)2 accelerated the catalytic CO oxidation reaction. Such Au NPs are promising heterogeneous catalysts. CRediT authorship contribution statement Kiyoshi Matsuyama: Conceptualization, Methodology, Writing - original draft, Supervision. Tomohiro Tsubaki: Data curation. Takafumi Kato: Methodology. Tetsuya Okuyama: Methodology, Investigation. Hiroyuki Muto: Methodology, Investigation.
Fig. 3. (a) HR-TEM image, (b) HAADF-STEM image, (c) Au-L STEM-EDX map, (d) Cu-K STEM-EDX map of Au@Cu3(BTC)2, and (e) conversion-temperature curve for CO oxidation over pristine Cu3(BTC)2 and Au@Cu3(BTC)2.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was partly supported by Advanced Characterization Platform of the Nanotechnology Platform Japan (JPMXP09-A-19KU-0259) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We are grateful to Dr T.Toriyama of Kyushu University for his helpful support in transmission electron microscopy analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.127124. References [1] M. Haruta, Size- and support-dependency in the catalysis of gold, Catal. Today 36 (1) (1997) 153–166, https://doi.org/10.1016/S0920-5861(96)00208-8. [2] J. Zhao, R. Jin, Heterogeneous catalysis by gold and gold-based bimetal nanoclusters, Nano Today 18 (2018) 86–102, https://doi.org/10.1016/ j.nantod.2017.12.009. [3] Y. Hatakeyama, J.-I. Kato, T. Mukai, K. Judai, K. Nishikawa, Effect of adding a thiol stabilizer on synthesis of Au nanoparticles by sputter deposition onto poly(ethylene glycol), Bull. Chem. Soc. Jpn. 87 (7) (2014) 773–779, https://doi. org/10.1246/bcsj.20140023. [4] Y. Hatakeyama, T. Morita, S. Takahashi, K. Onishi, K. Nishikawa, Synthesis of gold nanoparticles in liquid polyethylene glycol by sputter deposition and temperature effects on their size and shape, J. Phys. Chem. C 115 (8) (2011) 3279–3285, https://doi.org/10.1021/jp110455k. [5] J.A. Martens, J. Jammaer, S. Bajpe, A. Aerts, Y. Lorgouilloux, C.E.A. Kirschhock, Simple synthesis recipes of porous materials, Microporous Mesoporous Mater. 140 (1) (2011) 2–8, https://doi.org/10.1016/j.micromeso.2010.09.018. [6] K. Matsuyama, S. Tanaka, T. Kato, T. Okuyama, H. Muto, R. Miyamoto, H.-Z. Bai, Supercritical fluid-assisted immobilization of Pd nanoparticles in the mesopores of hierarchical porous SiO2 for catalytic applications, J. Supercrit. Fluids 130 (2017) 140–146, https://doi.org/10.1016/j.supflu.2017.07.032. [7] Y. Yang, K. Chiang, N. Burke, Porous carbon-supported catalysts for energy and environmental applications: a short review, Catal. Today 178 (1) (2011) 197– 205, https://doi.org/10.1016/j.cattod.2011.08.028.
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