Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Sendai , Japan

Transcription

1 Materials Transactions, Vol. 52, No. 5 (2011) pp to 1052 #2011 The Japan Institute of Metals Synthesis of Palladium Nanoparticles and Palladium/Spherical Carbon Composite Particles in the Solid Liquid System of Palladium Oxide Alcohol by Microwave Irradiation Yoshihiro Sekiguchi*, Yamato Hayashi and Hirotsugu Takizawa Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Sendai , Japan Palladium nanoparticles were synthesized in the solid liquid system of palladium oxide alcohol by microwave irradiation. They were compared with those produced by a conventional heating method. We also used various alcohol solvents and compared the products obtained. The products contained particles that had diameters of several nanometers. In these measurements, microwave heating produced smaller particles than conventional heating because it provided homogeneous and direct heating. Additionally, Pd/spherical carbon (SC) composite particles could be prepared by the same method. For microwave heating, SC particles can support palladium particles without calcination, which is due to selective heating by microwaves. [doi: /matertrans.m ] (Received December 20, 2010; Accepted February 15, 2011; Published May 1, 2011) Keywords: palladium, alcohol, nanoparticles, microwave, solid-liquid system 1. Introduction The ratio of surface atoms to inner atoms generally increases with decreasing particle diameter. 1) Quantum size effects appear when particle diameters are of the order of nanometers. Noble metal nanoparticles such as platinum and palladium catalyze oxidation of carbon monoxide and hydrocarbons 2) as well as hydrogenation dehydrogenation of organic compounds. 3,4) Consequently, they are used in many applications including automobile catalysts 5) and chemical synthesis. They reduce the load on the environment and enhance productivity. Hence, synthesis of noble metal nanoparticles has been intensively researched. Various methods have been developed for synthesizing noble metal nanoparticles, including gas evaporation, 6) metal salt reduction, 7) and metal complex decomposition. However, most of these methods have problems associated with them such as the use of expensive vacuum chambers, toxic inorganic metal salts, and expensive organometallic compounds. 8) Thus, cheaper methods with lower environmental loads are required. Over the last few decades, microwave heating has received much interest for synthesizing nanoparticles. Tu and Liu synthesized Pt, Ir, Rh, Pd, Au, and Ru nanoparticles in aqueous methanol solution or ethylene glycol from noble metal chlorides by microwave heating. 9) They point out that microwave heating has the advantages of being rapid and homogeneous. Therefore, large energy reductions are expected because synthesis is completed rapidly. 10) Furthermore, microwave heating achieves fast and homogeneous nucleation, making it possible to achieve narrower particle diameter distributions than conventional heating. 11) Ishikawa et al. heated platinum oxide in aqueous ethanol solution by microwaves and prepared ca. 20 nm platinum nanoparticles without using capping or dispersing agents. 12) In this method, the metal source is an oxide, which is less toxic and is cheaper than other starting materials. Moreover, *Graduate Student, Tohoku University it does not emit toxic anions. In addition, alcohols are inexpensive and low toxicity organic materials. In the present study, we synthesized palladium nanoparticles by irradiating a solid liquid system consisting of palladium oxide alcohol with microwaves. We investigated the dependence on the alcohol used in this synthesis. Furthermore, we examined carbon-supported catalysts by adding spherical glassy carbon and preparing composite particles. 2. Experimental 2.1 Compounds and reactors Palladium oxide (II) (PdO, 99.9%, Kojundo Chemical Lab. Co., Ltd.) was used as the starting material. Ethanol (EtOH, 99.5%, Wako Pure Chemical Industries, Ltd.), 1-propanol (1-PrOH, 99.5%, Wako Pure Chemical Industries, Ltd.), 2- propanol (2-PrOH, 99.7%, Wako Pure Chemical Industries, Ltd.), 1-butanol (1-BtOH, 99%, Wako Pure Chemical Industries, Ltd.) were used as solvents. Spherical glassy carbon (SC; Engineered Carbons Inc.) was used as the support. A 2.45 GHz microwave reactor ( Reactor, Shikoku Instrumentation Co., Ltd.) was used for microwave heating. In this experiment, a fluorescence optic-fiber temperature sensor (FL-2000, Anritsu Meter Co., Ltd.) was used to measure the temperature. A hot stirrer (DP-2S, Iuchi Seieido Co., Ltd.) was used for conventional heating and K-type thermocouples were used to measure the temperature in this experiment. 2.2 Preparation of starting suspension 0.1 g PdO powder and zirconia balls were added to 100 ml alcohol solvent and wet ball milled for 24 h. A black suspension was obtained that turned dark brown. To prepare Pd/SC composite particles, SC was added after ball milling and it was dispersed by ultrasonic agitation. 2.3 Synthesis of palladium nanoparticles The starting suspension and a magnetic stirrer were placed

2 Synthesis of Palladium Nanoparticles and Palladium/Spherical Carbon Composite Particles 1049 Fig. 1 Experimental setup for microwave heating. in a 300 ml three-necked flask. The flask was placed in a microwave reactor with a Dimroth condenser (see Fig. 1). Microwave irradiation or conventional heating was performed for min and the suspension was then refluxed at the boiling point of each solvent. 2.4 Characterization of products The obtained suspension was dropped onto Au-coated glass substrates and observed by scanning electron microscopy (SEM; LEO1420, LEO Electron Microscopy Ltd.). The suspension was also dropped into a polypropylene cell and the average particle diameter was measured by dynamic light scattering (DLS). In addition, the samples were dropped onto carbon-coated Cu microgrids and observed by field-emission transmission electron microscopy (FE-TEM; HF-2000, Hitachi High- Technologies Co.). Air-dried samples were characterized by powder X-ray diffraction (XRD; Cu K, RINT-2000PC, Rigaku Co.). Fig. 2 XRD patterns of (a) starting material and samples synthesized by 300 W microwave heating in 1-PrOH for (b) 60, (c) 120, (d) 180, (e) 240, and (f) 300 min. Table 1 Crystallite diameters of samples synthesized by 300 W microwave heating in 1-PrOH for various times. Time, t/min Crystallite diameter, d C /nm Results and Discussion 3.1 Dependence on reaction time Figure 2 shows XRD patterns of samples synthesized by microwave heating using 1-PrOH as the solvent. The PdO peak intensities decrease with time and face-centered cubic metallic Pd was obtained at 180 min. Table 1 shows the crystallite diameter calculated by the Debye Scherrer equation 13) for the (111) peak in these patterns. Additionally, Fig. 3 shows a FE-TEM image and an electron diffraction pattern of the products obtained after heating for 300 min. The electron diffraction pattern reveals that the sample has been reduced to metal Pd. The FE-TEM image shows that the primary particles have diameters in the range 5 10 nm. This result is supported by the crystallite diameters and the Debye Scherrer rings in the electron diffraction pattern. 14) Fig. 3 TEM image and electron diffraction of sample synthesized by microwave heating in 1-PrOH for 300 min. 3.2 Dependence on microwave power Figure 4 shows XRD patterns of samples synthesized by 300 or 700 W microwave heating for 120 min in 1-PrOH and Fig. 5 shows temperature profiles for the first 2 min. In these profiles, achieving temperatures were over 103 C, which is higher than original boiling point of 1-PrOH (97 C). In case of polar solvents such as alcohols, solvents achieve higher temperature than conventional boiling point under microwave irradiation, without stirring. 15) This is called superheating, 16) and similar phenomenon was observed in these experiments despite of stirring. At boiling point, solvent needed only about 100 W experimentally. So it is hard for heat of vaporization to remove redundant heat of strong microwave. The XRD pattern in Fig. 4 for 700 W heating reveals that PdO was completely reduced, whereas that obtained for 300 W heating for the same reaction temperature indicates the presence of some residual starting material. This difference is attributed to different heating rates. The solvent will reach the reaction temperature faster when a higher heating power is used, but the time to reach the reaction temperature differs by only 1 min for 300 and 700 W heating. As

3 1050 Y. Sekiguchi, Y. Hayashi and H. Takizawa Table 2 Crystallite and average diameters of samples synthesized by microwave heating in 1-PrOH for 180 min. Power, P/W Crystallite diameter, d C /nm Average diameter, d A /nm Fig. 4 XRD patterns of (a) starting material and samples synthesized by microwave heating in 1-PrOH for 120 min at (b) 300 and (c) 700 W. Fig. 6 XRD patterns of (a) starting material and samples synthesized by microwave heating in (b) in EtOH at 78 C for 480 min, (c) in 1-PrOH at 97 C for 180 min, (d) in 2-PrOH at 84 C for 10 min, and (e) in n-btoh at 117 C for 40 min. method and the average diameters obtained by DLS. In this table, the crystallite and average diameters differ greatly. This suggests that secondary particles (with average diameters of a few hundreds of nanometers) contain many primary particles (with crystallite diameters of several nanometers). Fig. 5 Temperature profiles for the first 2 min during microwave heating at (a) 300 and (b) 700 W. mentioned above, some researchers have reported that microwave heating accelerates reactions. 9 11) For example, Komarneni et al. synthesized ceramic powders by a microwave-hydrothermal process and found that this method enhanced the crystallization kinetics of the ceramics by one or two orders of magnitude. 17) In addition, in the present case, microwave irradiation is likely to not only maintain the solvent at boiling point but also promote reduction. In other words, the reaction is promoted by using a higher power even though the solvent temperature remains the same. Bogdol et al. reported selectivity oxidation of alcohols into corresponding carbonyl compounds by solid oxidant (MagtrieveÔ) under microwave irradiation. 18) In the experiments, oxidant was heated up to about 360 C by microwave. In the present work, the amount of solvent is far major than solid state (PdO) and we did not observed extreme thermal gradient. But it is possible that PdO absorbed microwave and heated up selectively, then thermal gradient promotes reduction on solid-liquid interfaces. Table 2 shows the crystallite diameters obtained by the above-mentioned 3.3 Dependence on solvent Figure 6 shows the reaction times of samples synthesized in various solvents by 300 W microwave heating. Except for EtOH, all the alcohols reduced the raw materials to metal palladium. EtOH was unable to completely reduce the raw materials even after a reaction time of 8 h. Table 3 shows the reduction times; they increase in the order 2-PrOH < 1-BtOH < 1-PrOH < EtOH (see Fig. 6). 2-PrOH is the only secondary alcohol in these solvents; it is oxidized to ketone (acetone) by palladium oxide. PdO þ CH 3 CH(OH)CH 3! Pd þ CH 3 COCH 3 þ H 2 O G r ¼ 102 kj/mol 19{21Þ ð1þ On the other hand, when primary alcohols are oxidized, they become aldehydes, and finally, carboxylic acids. For example, in EtOH, PdO þ CH 3 CH 2 OH! Pd þ CH 3 CHO þ H 2 O G r ¼ 81:4 kj/mol 19{21Þ ð2þ PdO þ CH 3 CHO! Pd þ CH 3 COOH G r ¼ 159:27 kj/mol 19{21Þ ð3þ As Table 3 shows, the primary alcohols have similar standard Gibbs energies of formation, so that their G r (eq. (2)) are also similar. 20,22) For the oxidation of alcohol

4 Synthesis of Palladium Nanoparticles and Palladium/Spherical Carbon Composite Particles 1051 Table 3 Standard Gibbs energies of formation of alcohols and aldehydes. Number of Standard Gibbs energy of formation, G f /kj mol 1 carbon atoms Alcohol Aldehyde Þ Þ Þ Þ Þ Þ Table 4 Reaction times, boiling point, reaction temperatures, crystallite diameters, and average diameters of samples synthesized by 300 W microwave heating in three solvents. Solvent 1-propanol 2-propanol 1-butanol Reaction time, t/min Boiling point, T/ C Reaction temperature, T/ C Crystallite diameter, d C /nm Average diameter, d A /nm (the first step of this reaction), 2-PrOH has a lower G r (eq. (1)). It is thus thermodynamically favored, so that 2- PrOH has the highest reaction rate. In the case of primary alcohols, G r for the reaction between PdO and aldehydes (eq. (3)) (the second step of the reaction) are lower than those between PdO and 2-PrOH (eq. (1)). Thus, even though the aldehydes have high reductivities, the production of aldehydes is slow because of slow reaction between PdO and primary alcohols (eq. (2)), which is the first step of the reaction. Consequently, G r is high caused by the low aldehyde concentrations, as expressed by: G r ¼ G r þ RT ln Q; where Q is the reaction ratio. Additionally, many palladium catalysts oxidize primary alcohol to aldehydes but not aldehydes to carboxylic acids. 23) Therefore, aldehydes is not likely to reduce PdO in this system. For these reasons, the reaction rates were lower for a primary alcohol than for a secondary alcohol. Moreover, the boiling points of the primary alcohols decrease with decreasing carbon chain length as 1-BtOH (117 C) > 1-PrOH (97 C) > EtOH (78 C). The reactions in this experiment were performed at boiling point. The reaction rate generally increases with increasing reaction temperature, as predicted by the Arrhenius equation (k ¼ A expð E a =RTÞ). Therefore, reduction is promoted by using an alcohol with a high boiling point (i.e., with a long alkyl chain). In these experiments, we did not attempt to reduce PdO at the same temperature (e.g., 70 C) using different primary alcohols, but we expect that there will be no notable differences in their reaction rates because they have similar G r. Table 4 shows the reaction times, the crystallite diameters, and the average particle diameters of the products. In this table, the crystallite and average diameters increase in the order 2-PrOH < 1-PrOH < n-btoh (i.e., the same order as their boiling points). The average diameter (of the secondary particles) generally increases at higher temperatures since particles in a colloidal dispersion system tend to aggregate at higher temperatures. A similar phenomenon is considered to be occurred in this experiment. 3.4 Comparison with conventional heating Figure 7 shows XRD patterns of products synthesized by conventional heating at the boiling point (97 C). The reaction proceeded in the similar way as for microwave heating. Table 5 shows the crystallite and average diameters of the products. These results reveal that conventional heating produces primary and secondary particles with larger diameters than microwave heating. Fig. 7 XRD patterns of (a) starting material and samples synthesized by conventional heating in 1-PrOH (b) for 60 (c) and 180 min. Table 5 Crystallite and average diameters of samples synthesized by microwave and conventional heating in 1-PrOH for 180 min. Method Microwave Conventional Crystallite diameter, d C /nm Average diameter, d A /nm Microwave heating heats molecules in the solvent homogeneously 11) and directly. 12) For example, Tu et al. synthesized noble metal colloids by microwave heating and oil-bath heating. 9) They found that microwave heating produces particles with smaller average sizes and narrower size distributions than oil-bath heating because its heats homogeneously and directly. In contrast, in conventional heating, the bottom of the flask is heated by a hot plate whose temperature exceeds the boiling point of the solvent (about 200 C) causing particles to aggregate at the bottom of the flask. Microwave heating has the advantage that it suppresses aggregation. 3.5 Palladium/spherical glassy carbon composite particles Figure 8 shows SEM images of composite particles prepared by microwave and conventional heating in three different solvents. These images suggest that palladium nanoparticles are supported on SC particles. More palladium particles are supported on SC prepared by microwave heating than by conventional heating, especially in n-btoh.

5 1052 Y. Sekiguchi, Y. Hayashi and H. Takizawa We prepared palladium nanoparticles by microwave irradiation of the solid liquid system, palladium oxide alcohol. The primary particles of the products are several nanometers in diameter. We were able to synthesize palladium nanoparticles inexpensively and with a low environmental load. Increasing the microwave power promotes reduction of PdO. Secondary alcohols are more active than primary alcohols due to their higher reactivities. In addition, longer primary alcohols are more active than shorter primary alcohols due to their higher boiling points. In this experiment, microwave heating produced smaller particles than conventional heating. Moreover, we synthesized composite particles by adding SC. It is possible to prepare composite particles in a single step by microwave heating. Therefore, microwave heating is expected to be used to synthesize supporting catalysts in the future. Microwave heating can prepare both fine Pd particles and Pd/SC composite particles easier than conventional methods because it provides direct, rapid, and local heating. Acknowledgement This study was supported by Suzuki Foundation. REFERENCES Fig. 8 SEM images of samples synthesized with spherical glassy carbon (a) (b) in 1-PrOH for 180 min, (c) (d) in 2-PrOH for 10 min, (e) (f) in n- BtOH for 40 min; (a) (c) (e) are synthesized by conventional heating and (b) (d) (f) are synthesized by microwave heating. It is generally difficult to load metal particles onto carbon, especially without calcination. Few palladium particles are supported on SC synthesized by conventional heating. In contrast, many more composite particles synthesized by microwave heating can load palladium onto SC. Since carbonaceous materials generally absorb microwaves and are self-heated by Joule heat loss, SC could be expected to be locally heated. In this experiment, palladium particles are possible to adhere to SC without additional heat treatment. For these reasons, microwave heating has many advantages for preparing supported catalysts. 4. Conclusion 1) W. P. Halperin: Rev. Mod. Phys. 58 (1986) ) M. Skoglundh, H. Johansson, L. Löwendahl, K. Jansson, L. Dahl and B. Hirschauer: Appl. Catal. B 7 (1995) ) G. C. Bond: Catalysis by metals, (Academic Press, London, 1962) pp ) P. Tetenyi and K. Schachter: Acta. Chim. Acad. Sci. Hung. 56 (1968) ) P. Granger, C. Dujardin, J.-F. Paul and G. Leclercq: J. Mol. Catal. A: Chem. 228 (2005) ) R. Uyeda: J. Cryst. Growth 24 (1974) ) J. M. Thomas: Pure Appl. Chem. 60 (1988) ) Y. Hayashi, H. Takizawa, M. Inoue, K. Niihara and K. Suganuma: IEEE Trans. Electron. Packag. Manuf. 28 (2005) ) W. Tu and H. Liu: J. Mater. Chem. 10 (2000) ) S. Komarneni, D. Li, B. Newalkar, H. Katsuki and A. S. Bhalla: Langmuir 18 (2002) ) T. Yamamoto, Y. Wada, T. Sakata, H. Mori, M. Goto, S. Hibino and S. Yanagida: Chem. Lett. 33 (2004) ) D. Ishikawa, Y. Hayashi and H. Takizawa: J. Nanosci. Nanotechnol. 8 (2008) ) P. Scherrer: Nachr. Ges. Wiss. Göttingen 26 (1918) ) M. Brust, J. Fink, D. Bethell, D. J. Schiffrin and C. Kiely: J. Chem. Soc. Chem. Commun. (1995) ) R. N. Gedye, F. E. Smith and K. C. Westaway: Can. J. Chem. 66 (1988) ) D. R. Baghurst and D. M. P. Mingos: J. Chem. Soc. Chem. Commun. (1992) ) S. Komarneni, R. Roy and Q. H. Li: Mater. Res. Bull. 27 (1992) ) D. Bogdal, M. Lukasiewics, J. Pielichowski, A. Miciak and Sz. Bednarz: Tetrahedron 59 (2003) ) D. D. Wagman, W. H. Evans, V. B. Parker, R. H. Schumm, I. Halow, S. M. Bailey, K. L. Churney and R. L. Nuttall: J. Phys. Chem. Ref. Data 11 (1982) Suppl ) R. C. Wilhoit and B. J. Zwolinski: J. Phys. Chem. Ref. Data 2 (1973) Suppl ) J. Nell and H. St. C. O Neill: Geochim. Cosmochim. Acta 60 (1996) ) E. Buckley and J. D. Cox: Trans. Faraday Soc. 63 (1967) ) M. B. Smith and J. March: March s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 6th Edition, (John Wiley & Sons, Inc., New Jersey, 2007) pp