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Notes Catalytic Investigation of Quasi-Two-Dimensional Palladium Nanoparticles Encapsulated in Graphite A Ä gnes Mastalir,*,† Ju¨rgen Walter,‡ Ferenc Notheisz,† and Miha´ly Barto´k† Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, University of Szeged, H-6720 Szeged, Do´ m te´ r 8, Hungary, and Department of Materials Science and Processing, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan Received December 22, 2000
Introduction Graphite intercalation compounds (GICs) are formed by inserting layers of atoms or molecules of a guest species between layers of the graphite host.1 Graphite can be intercalated by a variety of compounds, including alkali metals and metal halides.2-5 Alkali metal GICs were extensively studied as catalysts in the ortho-para hydrogen conversion,6 in the hydrogenation of alkenes and alkynes,3,7 and in NH3 and Fischer-Tropsch syntheses.5,8 GICs containing metal halides as intercalates may be used as catalysts, organic reagents, or precursor materials of finely dispersed metal particles intercalated in graphite.9,10 As reported previously, transition metal GICs (graphimets), prepared by reduction of the corresponding metal chloride GICs,11 were found to be efficient catalysts in a number of heterogeneous organic reactions.11-16 Nevertheless, in addition to their interlayer metal content, the graphimets were found to contain a considerable amount of ultrafine metal particles on their surface.10,15,16 Accordingly, the catalytic activities of graphimets were different from those of other transition metal-graphite systems, e.g., nanofiber-supported metals.17-19 As reported †
University of Szeged. Osaka University. * Corresponding author: e-mail:
[email protected].
‡
(1) Dresselhaus, M. S. Mater. Sci. Eng. 1988, B1, 259. (2) Boersma, M. A. M. Catal. Rev.sSci. Eng. 1974, 10, 243. (3) Boersma, M. A. M. Catalytic Properties of Graphite Intercalation Compounds; Advanced Materials in Catalysis; Academic Press: New York, 1977; p 67. (4) Tamaru, K. Adv. Catal. 1969, 20, 327. (5) Tamaru, K. Catal. Rev.sSci. Eng. 1970, 4, 161. (6) Inokuchi, H.; Wakayama, N.; Kondow, T.; Mori, Y. J. Chem. Phys. 1967, 46, 837. (7) Ichikawa, M.; Soma, M.; Onishi, T.; Tamaru, K. J. Catal. 1968, 9, 418. (8) Ichikawa, M.; Sudo, M.; Soma, M.; Onishi, T.; Tamaru, K. J. Am. Chem. Soc. 1969, 91, 1538. (9) Csuk, R.; Gla¨nzer, B. I.; Fu¨rstner, A. Adv. Organomet. Chem. 1988, 28, 85. (10) Setton, R.; Be´guin, F.; Piroelle, S. Synth. Met. 1982, 4, 299. (11) Lalancette, J. M. U.S. Patent No. 3,847,963, 1974. (12) Aika, K.; Yamaguchi, T.; Onishi, T. Appl. Catal. 1986, 23, 129. (13) Kikuchi, E.; Ino, T.; Morita, Y. J. Catal. 1979, 57, 27. (14) Csuk, R. Nachr. Chem. Technol. Lab. 1987, 35, 828. (15) Kira´ly, Z.; Mastalir, A Ä .; Berger, F.; De´ka´ny, I. Langmuir 1997, 13, 465. (16) Kira´ly, Z.; Mastalir, A Ä .; Berger, F.; De´ka´ny, I. Langmuir 1998, 14, 1281. (17) Rodriguez, N. M.; Kim, M.-S.; Baker, R. T. K. J. Phys. Chem. 1994, 98, 13108.
recently, Pd-graphite compounds prepared by the reduction of PdCl2-GICs had most of their Pd content encapsulated inside the graphite host,20 and the thickness of such interlayer Pd particles was as low as that of Pt nanosheets formed between graphite layers.21 The unique structural properties of such Pd-graphites suggested that they would find applications in heterogeneous catalytic transformations. In fact, a Pd-graphite sample with a similar structure, prepared from highly oriented pyrolytic graphite,22 has already been tested as a catalyst in the liquid-phase Heck reaction,23 although no experimental details have been published as yet. In contrast, no gasphase reactions have been investigated on these novel materials so far. Accordingly, the aim of the current work was to study the catalytic activity of a representative Pdgraphite in the gas-phase transformations of different alkene substrates. Experimental Section The sample was prepared as follows: PdCl2 was mixed in an ampule with crystallized natural graphite flakes from Kropfmu¨hl, Germany. After evacuation, chlorine gas was introduced, and the mixture was heated at 773 K for 7 days. The PdCl2-GIC was reduced in a H2 stream of 300 cm3/min at 623 K for 24 h. The metal content of the sample was 37%.20 The sample was characterized by XRD (Rigaku powder diffractometer, 40 kV, Cu KR radiation) TEM (Philips C-10 transmission electron microscope, LaB6 cathode, 100 kV, Cu grids), and selected area electron diffraction (SAED) measurements (camera lengths: 0.8 and 2 m; accelerating voltage: 200 kV). The catalytic test reactions were carried out in a static recirculation reactor system at 298 K, for the reactants 1-butene, cis-2-pentene, and cyclohexene. The mass of catalyst used was 5 mg. Before the reactions, the sample was pretreated in 13 kPa of H2 at 298 K for 1 h. The reaction mixture consisted of 2 kPa of alkene, 2 kPa of H2, and 66 kPa of argon. The product distribution was determined by means of capillary GC.
Results and Discussion Previous XRD measurements revealed the presence of PdCl2 and metallic Pd in the precursor PdCl2-GIC and the reduced product, respectively. The fact that the signals of graphite were not given by the precursor or the reduced compound was evidence that the starting material was fully intercalated and that the Pd particles formed after reduction were situated between the graphite layers.20 TEM bright field images revealed nanosized Pd particles, most of them fully encapsulated in graphite. The particles were unevenly distributed in the graphite lattice, with diameters in the range 5-540 nm. The thickness of the Pd nanoparticles was previously estimated to be 0.92.20 nm, which corresponded to 2-5 layers of Pd.20 The (18) Chambers, A.; Nemes, T.; Rodriguez, N. M.; Baker, R. T. K. J. Phys. Chem. B 1998, 102, 2251. (19) Baker, R. T. K.; Laubernds, K.; Wootsch, A.; Paa´l, Z. J. Catal. 2000, 193, 165. (20) Walter, J.; Shioyama, H. Phys. Lett. A 1999, 254, 65. (21) Shirai, M.; Igeta, K.; Arai, M. Chem. Commun. 2000, 623. (22) Walter, J. Adv. Mater. 2000, 12, 31. (23) Walter, J.; Heiermann, J.; Dyker, G.; Hara, S.; Shioyama, S. J. Catal. 2000, 189, 449.
10.1021/la001787+ CCC: $20.00 © 2001 American Chemical Society Published on Web 05/12/2001
Notes
Figure 1. TEM bright field image of Pd particles encapsulated in graphite. Magnification: 64 000.
low thickness of the Pd nanoparticles was attributed to the template effect of graphite. Accordingly, the Pd content of the reduced sample may be present as quasi-twodimensional nanoparticles encapsulated inside the graphite host. It is important to stress that there was no chargetransfer interaction between the Pd particles and graphite, which indicated that Pd-graphite cannot be considered a “true” intercalation compound.23 Instead, it may be more appropriately regarded as a dispersion of Pd particles encapsulated inside the graphite host. SAED patterns obtained at small camera lengths revealed the presence of (220), (111), (311), and (200) fcc Pd reflections whereas those taken at a camera length of 2 m clearly indicated the occurrence of mixed cubic and hexagonal reflections. Most of the Pd nanoparticles observed by TEM were intermediate states between the two geometrical forms, and it is assumed that such particles may change from hexagonal to cubic structure. For the Pd-graphite, a nearly commensurate superstructure was observed, and a 4% mismatch with respect to the bulk material was obtained. As depicted in Figure 3, the transformation of 1-butene on Pd-graphite was completed by a reaction time of 180 min, which suggested a pronounced catalytic activity. The main reaction pathway was hydrogenation, leading to the predominant formation of butane. Isomerization of the reactant also took place and the yield of trans-2-butene was always higher than that of the cis isomer. The product selectivities obtained in the initial stage of the reaction were very close. Nevertheless, as the reaction progressed, a proportion of the isomerization products were hydrogenated to butane, and this reaction was more apparent for the cis isomer. Accordingly, Pd-graphite has a substantial hydrogenation activity. That is supported by the comparative SH/SI data collected in Table 1 (obtained under the same conditions), which reflect a significant difference between Pd-graphite and a traditional SiO2supported Pd catalyst. As may be seen in Table 1, there was no appreciable difference between the selectivities of the isomerization
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Figure 2. TEM bright field image of Pd particles encapsulated in graphite. Magnification: 180 000.
Figure 3. Transformation of 1-butene on Pd-graphite. Table 1. Reaction of 1-Butene on Pd-Graphite and Pd/SiO2 Catalysts samplea
TOF [s-1]b
SH/SIc
Scis/Stransd
Pd-graphite 3% Pd/SiO2
0.086 0.181
1.680 0.294
0.327 0.322
a m ) 5 mg, T ) 298 K, p 1-butene ) 2 kPa, conversion ) 100%. Calculated on the basis of the total metal content. c SH/SI ) selectivity of hydrogenation/selectivity of isomerization. d Scis/Strans ) selectivity of cis-2-butene/selectivity of trans-2-butene. b
products obtained for the two samples. Furthermore, the activity of Pd-graphite was comparable with that of the supported catalyst. The slight difference in activity may be related to the quasi-two-dimensional character of the Pd nanoparticles of Pd-graphite, which means that the particles were less bulky than those of the supported
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Figure 4. Transformation of cis-2-pentene on Pd-graphite.
catalyst. Accordingly, for Pd-graphite, hydrogenation is more likely to take place on the edges of the Pd sheets intercalated in graphite. For the transformation of cis-2-pentene on Pd-graphite, the main reaction product was trans-2-pentene, and the conversion was considerably lower than that observed for 1-butene. Figure 4 indicates that the hydrogenation was less significant, but pentane still comprised a substantial part of the product mixture, which also contained a small amount of 1-pentene formed by double-bond isomerization. As revealed by Figure 4, the product selectivities displayed hardly any difference throughout the reaction time interval investigated. The decreased catalytic activity of Pd-graphite observed for the internal alkene cis-2pentene suggested that only part of its Pd content was available for the reactant molecules; i.e., access to the active Pd particles was limited. This is supported by the
Notes
observation of a complete lack of activity on Pd-graphite for cyclohexene, which is a cyclic nonplanar alkene and the largest of the substrate molecules investigated. Since the presence of surface Pd particles on Pdgraphite was previously excluded,23 it follows that the reactions of both 1-butene and cis-2-pentene took place in the interlamellar space of the graphite. The longer carbon chain of cis-2-pentene as compared with that of 1-butene made diffusion of the reactant molecules into the graphite slower, and thus the reaction rate decreased. According to Figures 3 and 4, around half of the active sites available for 1-butene were inaccessible for cis-2-pentene, which may be related to the different structures of the two alkenes. Furthermore, the evidence that cyclohexene was not transformed at all on Pd-graphite indicated that the occurrence of an interlamellar reaction was prevented by the increased size and the nonplanar character of the reactant. This is in accordance with previous results for Pd-graphimet, which suggested that cyclohexene was unable to enter between the graphite layers.24,25 As a consequence, Pd-graphite displays a molecular sieving effect, and the sample may be regarded as a shape-selective catalyst. It is worth mentioning that no evidence of an interlamellar reaction has been reported for any other transition metal-graphite system so far. This is probably due to the fact that the catalytic activities of these systems, including graphimets, were basically determined by their surface metal content. To gain more insight into the catalytic behavior of Pd-graphite, further experimental work is in progress. Acknowledgment. Financial support by the Hungarian Academy of Sciences through Grant OTKA T 026430 is gratefully acknowledged. A.M. thanks the Bolyai Ja´nos Foundation for its contribution. LA001787+ (24) Mastalir, A Ä .; Notheisz, F.; Barto´k, M.; Haraszti, T.; Kira´ly, Z.; De´ka´ny, I. Appl. Catal. 1996, 144, 237. (25) Mastalir, A Ä .; Kira´ly, Z.; De´ka´ny, I.; Barto´k, M. Colloids Surf. 1998, 141, 397.
