Novel Preparation of Palladium Nanoclusters Using Metal Nitrates

Pd(OAc)2 was purchased from the N. E. Chemcat Co. .... Figure 1 shows XRD patterns of several Pd clusters prepared by using .... However, the total nu...
4 downloads 0 Views 160KB Size
Download PDF
Langmuir 2002, 18, 1849-1855

1849

Novel Preparation of Palladium Nanoclusters Using Metal Nitrates and Their Catalysis for Oxidative Acetoxylation of Toluene in the Presence of Molecular Oxygen Kohki Ebitani, Kwang-Min Choi, Tomoo Mizugaki, and Kiyotomi Kaneda* Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan Received August 27, 2001. In Final Form: November 13, 2001 Anion ligands-preserved cationic giant Pd clusters were prepared by treatment of palladium carbonyl acetate cluster, Pd4(CO)4(OAc)4‚2AcOH (PCA), with metal nitrates, e.g. Cu(NO3)2, and Fe(NO3)3, in the presence of 1,10-phenanthroline (phen). Extensive characterization of the Pd clusters was made by XRD, XPS, XAFS, TPD, FE-SEM, ion-chromatography, and CO adsorption. It was revealed that the inner part of the clusters was an exclusively metallic form, whereas both Pd cations (Pd2+ and Pd+) and Pd0 species were located on the same surface of the clusters to form specific ensemble sites. Further, the particle size and surface fraction of Pd0, Pd+, and Pd2+ of the giant Pd clusters could be tuned by changing the amount of the metal nitrate; the largest particle size of about 35 Å and the highest surface fraction of the Pd2O could be obtained when the Cu(NO3)2/PCA ratio was 0.10. The 8-shell giant Pd clusters of about 35 Å in diameter having a large number of surface Pd cations showed a high catalytic activity for the oxidative acetoxylation of toluene to give benzyl acetates in the presence of molecular oxygen. Furthermore, the Pd cluster could be immobilized on a neutral TiO2 surface by keeping its original cluster size and high catalytic activity. The prominent catalysis of this Pd cluster can be ascribed to the ensemble sites composed of metallic Pd atoms and neighboring Pd cations on its surface.

Introduction The chemistry of polynuclear metal clusters in the nanometer size range has become a facilitating field because of the unique electronic, optic, magnetic, and catalytic functions of the clusters, which bring new technologies in many industrial areas.1-15 Control over * To whom correspondence should be addressed. Phone and fax: +81-6-6850-6260, E-mail: [email protected]. (1) (a) Kno¨zinger, H. Cluster Models for Surface and Bulk Phenomena; Plenum: New York, 1992. (b) Shriver, D. F.; Kaesz, H. D.; Adams, R. D. The Chemistry of Metal Cluster Complexes; VCH Publishers. Inc: New York, 1990. (c) Schmid, G. Aspects of Homogeneous Catalysis, Ugo, R., Eds.; Kluwer Academic Publishers: Netherlands, 1990; Vol. 7, p 1. (d) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (e) Schmid, G. Chem. Rev. 1992, 92, 1709. (f) Schmid, G. Polyhedron 1988, 7, 2321. (g) Schmid, G.; Morun, B.; Malm, J.-O. Angew. Chem., Int. Ed. Engl. 1989, 28, 778. (h) Klabunde, K. J.; Mohs, C. In Chem. Adv. Mater. Interrante, L.V., Hampden-Smith, M. J. Eds.; Wiley-VCH: New York, 1998; p. 271. (i) Schmid, G.; Ba¨umle, M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. Rev. 1999, 28, 179. (j) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. Rev. 2000, 29, 27. (k) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (2) (a) Heiz, U.; Vanolli, F.; Sanchez, A.; Schneider, W.-D. J. Am. Chem. Soc. 1998, 120, 9668. (b) Abbet, S.; Sanchez, A.; Heiz, U.; Schneider, W.-D.; Ferrari, A. M.; Pacchioni, G.; Ro¨sch, N. J. Am. Chem. Soc. 2000, 122, 3453. (3) (a) Cai, Y. Q.; Bradshaw, A. M.; Guo, Q.; Goodman, D. W. Surf. Sci. 1998, 399, L357. (b) Stone, P.; Bennett, R. A.; Poulston, S.; Bowker, M. Surf. Sci. 1999, 433-435, 501. (4) (a) Tan, B. J.; Klabunde, K. J.; Tanaka, T.; Kanai, H.; Yoshida, S. J. Am. Chem. Soc. 1988, 110, 5951. (b) Tan, B. J.; Klabunde, K. J.; Sherwood, P. M. A. J. Am. Chem. Soc. 1991, 113, 855. (c) Klabunde, K. J.; Li, Y. X.; Tan, B. J. Chem. Mater. 1991, 3, 30. (5) (a) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401. (b) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995, 267, 367. (c) Reetz, M. T.; Quaiser, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2240. (d) Kolb, U.; Quaiser, S. A.; Winter, M.; Reetz, M. T. Chem. Mater. 1996, 8, 1889. (e) Reetz, M. T.; Winter, M.; Tesche, B. Chem. Commun. 1997, 147. (6) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (7) (a) Toshima, N. Supramol. Sci. 1998, 5, 395. (b) Harada, M.; Asakura, K.; Ueki, Y.; Toshima, N. J. Phys. Chem. 1992, 96, 9730. (c) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217. (8) Teranishi, T.; Hori, H.; Miyake, M. J. Phys. Chem. B 1997, 101, 5774.

the particle size of the metal clusters is one of the challenging tasks and at present there is some progress being made in the size regulation of the giant metal clusters.4,5,10,11-14 The giant metal clusters have been usually prepared and isolated in the presence of stabilizing ligands such as surfactants,5 organic polymers,6-9 and organic bases.10-15 For this reason, the above ligands can preserve the structure of the giant metal clusters from agglomeration of the clusters themselves. Transition metal compounds have been extensively used as catalysts for many organic reactions.16,17 Their oxidation (9) Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 2001, 105, 2515. (10) Bradley, J. S.; Tesche, B.; Busser, W.; Masse, M.; Reetz, M. T. J. Am. Chem. Soc. 2000, 122, 4631. (11) (a) Vargaftik, M. N.; Zagorodnikov, V. P.; Stolarov, I. P.; Moiseev, I. I.; Likholobov, V. A.; Kotchubey, D. I.; Chuvilin, A. L.; Zaikovsky, V. I.; Zamaraev, K. I.; Timofeeva, G. I. J. Chem. Soc., Chem. Commun. 1985, 937. (b) Vargaftik, M. N.; Zagorodnikov, V. P.; Stolarov, I. P.; Moiseev, I. I.; Kochubey, D. I.; Likholobov, V. A.; Chuvilin, A. L.; Zamaraev, K. I. J. Mol. Catal. 1989, 53, 315. (c) Moiseev, I. I. In Catalytic Oxidation. Principles and Applications; Sheldon, R. A.; van Santen, R. A., Eds.; World Scientific: Singapore, 1995; p. 203. (d) Vargaftik, M. N.; Kozitsyna, N. Yu.; Cherkashina, N. V.; Rudyi, R. I.; Kochubei, D. I.; Novgorodov, B. N.; Moiseev, I. I. Kinet. Catal. 1998, 39, 740. (e) Moiseev, I. I.; Vargaftik, M. N. In Catalysis by Di- and Polynuclear Metal Cluster Complexes; Adams, R. D.: Cotton, F. A., Eds.; Wiley-VCH: New York, 1998; p 395. (12) (a) Schmid, G.; Harms, M.; Malm, J.-O.; Bovin, J.-O.; van Ruitenbeck, J.; Zandbergen, H. W.; Fu, W. T. J. Am. Chem. Soc. 1993, 115, 2046. (b) Schmid, G.; Emde, S.; Maihack, V.; Meyer-Zaika, W.; Peschel, St. J. Mol. Catal. A: Chemical 1996, 107, 95. (c) Junges, U.; Schu¨th, F.; Schmid, G.; Uchida, Y.; Schlo¨gl, R. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1631. (13) (a) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891. (b) Lin, Y.; Finke, R. G. J. Am. Chem. Soc. 1994, 119, 8335. (c) Watzky, M. A.; Finke, R. G. Chem. Mater. 1997, 9, 3083. (d) Aiken, J. D.; Finke, R. G. J. Am. Chem. Soc. 1998, 120, 9545. (14) (a) Kaneda, K.; Fujie, Y.; Ebitani, K. Tetrahedron Lett. 1997, 38, 9023. (b) Ebitani, K.; Fujie, Y.; Kaneda, K. Langmuir 1999, 15, 3557. (15) Reetz, M. T.; Koch, M. G. J. Am. Chem. Soc. 1999, 121, 7933. (16) Tsuji, J. Organic Synthesis with Palladium Compounds; SpringerVerlag: Berlin, 1980. Heck, R. F. Palladium Reagents in Organic Syntheses, Academic Press: London, 1985.

10.1021/la011359j CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002

1850

Langmuir, Vol. 18, No. 5, 2002

state plays an important role in their unique catalysis.18,19 This generation of multi-functions, originated from a coorperative action between zerovalent metal and metal cations on the cluster surface, is a promising strategy for designing high performance catalysts. Nanostructured transition metal colloids have been prepared either by the aerobic oxidation of a ligand-stabilized metal cluster, i.e. (C8H17)4N+Br--stabilized Co cluster,20 or hydrolysis/ condensation of metal salts in the presence of surfactants,15,21 where the colloidal surfaces are fully covered with metal cations in the absence of metallic species. Pioneering works for the mixed-valence states on the surface in ligand-stabilized giant palladium clusters have been independently reported by Moiseev11(a) and Schmid.12(a) Full characterization of giant metal clusters, e.g. a determination and control of the surface oxidation states, however, is still a matter of the greatest importance to create their unique catalysis. We now present a novel synthetic method for the giant palladium clusters containing both Pd0 and cationic Pd species on their surface by chemical reactions of the small Pd cluster with metal nitrates. Treatment of the Pd4phen2(CO)2(OAc)4 with copper nitrate under an O2 atmosphere afforded anion ligand-preserved giant Pd clusters with particle sizes between 24 and 35 Å and the metallic and cationic Pd species to exist on the same cluster surface. An advantage of this preparation method is that the particle size and the surface oxidation state of giant Pd clusters could be controlled only by selecting the amount of metal nitrates. We also found that the Pd nanocluster with about a 35 Å diameter involving a large number of surface Pd cations, acted as a high performance catalyst for the oxidative acetoxylation of toluene in the presence of molecular oxygen.22-27 Further, the giant Pd cluster could be immobilized on a neutral TiO2 surface by keeping its original cluster size and high catalytic activity. This paper describes the novel preparation and characterization of giant Pd clusters having mixed-valence with Pd0, (17) (a) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidation of Organic Compounds; Academic Press: New York, 1981. (b) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; Wiley-Interscience: New York, 1992. (c) Augustine, R. L. Heterogeneous Catalysis for the Synthetic Chemist; Marcel Dekker: New York, 1996. (18) (a) Mori, K.; Yamaguchi, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Chem. Commun. 2001, 461. (b) Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2000, 122, 7144. (c) Ebitani, K.; Nagashima, K.; Mizugaki, T.; Kaneda, K. Chem. Commun. 2000, 869. (d) Matsushita, T.; Ebitani, K.; Kaneda, K. Chem. Commun. 1999, 265. (19) (a) Thomas, J. M.; Raja, R.; Sankar, G.; Bell, R. B. Nature 1999, 398, 227. (b) Raja, R.; Sanker, G.; Thomas, J. M. J. Am. Chem. Soc. 1999, 121, 11926. (20) Reetz, M. T.; Quaiser, S. A.; Winter, M.; Becker, J. A.; Scha¨fer, R.; Stimming, U.; Marmann, A.; Vogel, R.; Konno, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 2092. (21) Harriman, A.; Thomas, J. M.; Millward, G. R. New J. Chem. 1987, 11, 757. (22) (a) Bryant, D. R.; McKeon, J. E.; Ream, B. C. J. Org. Chem. 1968, 33, 4123. (b) Bryant, D. R.; McKeon, J. E.; Ream, B. C. Tetrahedron Lett. 1968, 3371. (23) Starchevskii, M. K.; Vargaftik, M. N.; Moiseev, I. I. Kinet. Katal. 1979, 20, 1163. (24) (a) Benazzi, E.; Mimoun, H.; Cameron, C. J. J. Catal. 1993, 140, 311. (b) Benazzi, E.; Cameron, C. J.; Mimoun, H. J. Mol. Catal. 1991, 69, 299. (25) (a) Tanielyan, S.; Augustine, R. L. Chem. Ind; Marcel Dekker: New York, 1996; p 485. (b) Tanielyan, S.; Augustine, R. L. J. Mol. Catal. 1994, 90, 267. (c) Tanielyan, S.; Augustine, R. L. J. Mol. Catal. 1994, 87, 311. (26) Recent examples for the palladium-catalyzed oxidation of alcohols in the presence of molecular oxygen: (a) Kaneda, K.; Fujii, M.; Morioka, K. J. Org. Chem. 1996, 61, 4502. (b) Noronha, G.; Henry, P. M. J. Mol. Catal., A: Chemical 1997, 120, 75. (c) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185. (d) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. J. Org. Chem. 1999, 64, 6750. (e) ten Blink, G.-J.; Isabel, W. C. E. A.; Sheldon, R. A. Science 2000, 287, 1636. (f) Nishumura, T.; Kakiuchi, N.; Inoue, M.; Uemura, S. Chem. Commun. 2000, 1245.

Ebitani et al.

Pd+, and Pd2+. A prominent oxidation catalysis of the present giant Pd clusters is also discussed in relation to the mixed-valence of Pd atoms on the cluster surface. Experimental Section General Procedures. All organic reagents were purified by the standard procedures before use.28 1, 10Phenanthroline (phen) was obtained from Nacalai Tesque Co. Ltd. and used after recrystalization. Pd(OAc)2 was purchased from the N. E. Chemcat Co. Ltd. TiO2 was supplied from the Catalysis Society of Japan as reference catalysts of JRC-TIO-2.14(b) The BET surface area and pore volume of JRC-TIO-2 were 14 m2g-1 and 3.3 × 10-2 cm3g-1, respectively. The main phase was anatase, and the diameter of the crystallite was 38 nm. The amount of gas absorption was volumetrically measured by a gas buret directly connected to the rotary vacuum pump. GC analysis was performed with a Shimadzu GC-8A gas chromatograph with a FID and a 2 m column of PEFF. The amount of acetate anion in the solution was determined by Shimadzu LC-6A ion chromatograph using an aqueous solution of boric acid (6 mM), mannitol (18 mM), and tris(hydroxymethyl)aminomethane (7.5 mM) as an eluent. Synthesis and Immobilization of Giant Pd Clusters. A representative example is the synthesis of the largest giant Pd cluster, 35 Å in diameter. A solution of Pd(OAc)2 (0.40 g; 1.78 mmol) in AcOH (40 mL) was stirred at 50 °C for 2 h in a continuous CO flow, yielding 0.24 g of Pd4(CO)4(OAc)4‚2AcOH (PCA) as a yellow precipitate.29 The obtained PCA (0.31 mmol) was stirred in AcOH (5 mL) in the presence of phen (0.62 mmol) at room temperature for 30 min under air to give the Pd4phen2(CO)2(OAc)4 cluster30,31 [Calculated for Pd4C34H28N4O10, %: C, 37.9; H, 2.6; N, 5.2. Found, %: C, 34.1; H, 2.7; N, 5.2]. Then, 0.031 mmol of Cu(NO3)2‚3H2O was added to the mixture and heated at 90 °C under an atmospheric O2. After 25 min, a black solid was precipitated. The precipitate was washed with AcOH for several times and dried in vacuo to yield the Pd cluster (0.065 g).32 [X-ray photon-spectroscopy (XPS): Pd 3d5/2 ) 334.5-335.2 eV, O 1s ) 531.5 eV, N 1s ) 399.4 eV, The XPS peak positions are referred to C 1s at 284.6 eV. Atomic ratio by XPS: C/Pd ) 0.6, N/Pd ) 0.12. Copper signals were not appreciable on the clusters.] It is notable that the time to form the Pd cluster precipitates became shorter when using a large excess of copper nitrate. In this case, further treatment in the solution resulted in a complete conversion of the cluster to soluble monomeric Pd complexes. On the other hand, when the Cu(NO3)2/PCA ratio is 0.10, the Pd (27) Pd-catalyzed aerobic acetoxylations via multistep electron transfer: (a) Ba¨ckvall, J.-E.; Hopkins, R. B.; Grennberg, H.; Mader, M. M.; Awasthi, A. K. J. Am. Chem. Soc. 1990, 112, 5160. (b) Grennberg, H.; Simon, V.; Ba¨ckvall, J.-E. J. Chem. Soc., Chem. Commun. 1994, 265. (c) Hansson, S.; Heumann, A.; Rein, T.; Akermark, B. J. Org. Chem. 1990, 55, 975. (d) Ayanyos, A.; Szabo´, K.-J.; Ba¨ckvall, J.-E. J. Org. Chem. 1998, 63, 2523. (28) Purification of Laboratory Chemicals, 3rd ed.; Perrin, D. D., Armarego, W. L. F., Eds.; Pergamon Press: Oxford, 1988. (29) Moiseev, I. I.; Stromnova, T.; Vargaftik, M. N.; Ja Mazo, G.; Kuz’mina, L. G.; Struchkov, Y. T. J. Chem. Soc., Chem. Commun. 1978, 27. (30) Moiseev, I. I. J. Organomet. Chem. 1995, 488, 183. (31) An optimum mole ratio of Pd to phen in the synthesis of giant Pd clusters was found to be 2. Use of Pd4phen4(CO)2(OAc)4 instead of Pd4phen2(CO)2(OAc)4 did not afford giant Pd clusters. (32) Pd(phen)(OAc)2 was formed in the residual solution. This monomeric Pd complex was inactive for the acetoxylation under our reaction conditions. See; (a) S. B. Halligudi, S. B.; Kahn, N. H.; Kureshy, R. I.; Suresh, E.; Venkatsubramanian, K. J. Mol. Catal. A: Chemical 1997, 124, 147. (b) Weintraub, P. M.; King, C.-H. R. J. Org. Chem. 1997, 62, 1560.

Novel Preparation of Palladium Nanoclusters

Langmuir, Vol. 18, No. 5, 2002 1851

cluster precipitate, viz. 8-shell cluster, was fairly stable in the solution under preparation conditions even after 1 day. A typical example for the immobilization of the Pd cluster on the metal oxides is described for Pd cluster/ TiO2. TiO2 (JRC-TIO-2, 1.30 g)14(b) was added into the acetic acid solution of Pd4phen2(CO)2(OAc)4 and Cu(NO3)2‚3H2O, and then stirred at 90 °C under an atmospheric O2 (5 wt % as Pd cluster). The solid compound of 1.4 g was obtained after filtration, washing with acetic acid, and drying under a vacuum. Treatment of the Pd cluster (0.053 g ) 0.44 mmol Pd) with an atmospheric hydrogen at room temperature in a water (1.5 mL) resulted in an absorption of hydrogen (H2/ Pd ) 0.1 mol/mol). Analysis of the water filtrate by ionchromatography showed that the 0.075 mmol OAc anions was released from the solid sample. The measurement of TPD profile of the Pd cluster by Q-mass spectrometer revealed desorption peaks of HOAc (m/e ) 60) and NO2 (m/e ) 46) at 220 and 150-210 °C, respectively. Characterization. X-ray diffraction (XRD) was measured on the X’pert diffractionmeter (Phillips Co., Ltd.). The particle size of the Pd clusters was calculated by the line broadening of the most intense diffraction peak using the Scherrer equation. XPS measurements of the cluster were performed on an ESCA-2000 (Shimadzu Co., Ltd.) and a MODEL-5500 MT (ULVAC-PHI Co., Ltd.). FESEM studies were conducted by a Hitachi S-5000L microscope (18.0 kV). The images of the clusters were recorded without sputtering the samples. The Pd K-edge XAFS was conducted in a transmission mode at EXAFS facilities installed on the BL-10B line of the PF at High Energy Accelerator Research Organization, Tsukuba, Japan. The detailed procedure for data analysis was described elsewhere.14(b),33 To determine the surface oxidation state of the Pd cluster, CO uptakes on the Pd clusters were volumetrically measured at 0 °C using a MKS Baratron type 627 absolute pressure transducer. Since CO2 is evolved during the CO uptake,34 both oxidation of CO to CO2 (Pd2O + CO f 2Pd0 + CO2) and adsorption of CO (Pd0 + CO f Pd-COads) occurred by treatment of the Pd cluster with CO. The amount of the evolved CO2 (NCO2) would correspond to the number of the Pd cations, e.g. Pd2O, reducible by a CO molecule at the cluster surface.35 Since CO could adsorb both on newly formed Pd0 species and the Pd0 sites initially present on the surface (NPd0), NPd0 can be estimated by the following equation,

Figure 1. XRD patterns of the Pd clusters prepared using various Cu(NO3)2/PCA ratios. (a) 0.05, (b) 0.10, and (c) 0.30.

Figure 2. Dependences of (a) particle size and (b) percentage of Pd2O species on the surface of the giant Pd cluster as a function of the Cu(NO3)2/PCA ratio.

Results and Discussion

NPd0 ) NCO - 3 NCO2 where NCO is the total amount of CO uptake. The above calculation is based on the assumption that the adsorption stoichiometry of CO and surface Pd0 is unified. Catalytic Reaction. A typical procedure for the acetoxylation of toluene in the presence of molecular oxygen is as follows. Acetic acid (4 mL) and toluene (5 mmol) were added to a reaction vessel containing the Pd cluster (0.035 g). Then, the heterogeneous mixture was stirred at 90 °C for 3 h under an atmospheric pressure of O2. The catalyst was separated by filtration and the GC analysis showed a 94% yield of benzyl acetates (1:2 ) 4.1:1) (Scheme 1).

Characterization. Figure 1 shows XRD patterns of several Pd clusters prepared by using different amounts of copper nitrate. The peak positions and relative peak intensities of the Pd clusters were identical to those of Pd metal. The particle sizes of the Pd clusters estimated from the peaks at 40.2° are depicted in Figure 2 (a). The size of the cluster could be controlled by the amount of copper nitrate; the largest size was ca. 35 Å for the Pd cluster prepared in a Cu(NO3)2/PCA mole ratio of 0.10. This largest particle size of the cluster corresponds to that of the giant metal cluster with an eight-shell as the magic number.1(c) The surface analysis of the above Pd clusters by XPS proved the presence of both cationic and metallic Pd

Scheme 1

1852

Langmuir, Vol. 18, No. 5, 2002

Ebitani et al.

Figure 3. (a) Fourier transform (FT) of k3-weighted EXAFS of the 8-shell Pd cluster and (b) its inverse FT of the main peak appearing at 1.8-2.9 Å in (a). The dotted line in (b) shows the result of a curve-fitting analysis using Pd-Pd shell parameters in the range of 4-12 Å-1. Phase shift was not corrected in (a).

species. As mentioned in the Experimental Section, treatment of the Pd clusters with a CO molecule at 0 °C led to both CO2 evolution and CO adsorption on metallic Pd species; the cationic Pd species are reduced to Pd0 species during the exposure to CO. The evolution of CO2 ascertains the presence of oxygen species on the Pd cluster surface. Since the reactivity of the surface oxygen species was high enough to oxidize a CO molecule even at 0 °C, the Pd sites responsible for the CO oxidation would be Pd2O species on the cluster surface.36 The ratio of the Pd2O species to the surface Pd atoms can be evaluated from the amounts of the formed CO2 and the CO uptake, which is depicted in Figure 2(b). It is well demonstrated that the fraction of the surface Pd2O species also strongly depends on the amount of copper nitrate used in the preparation of the Pd clusters; the Pd cluster obtained in a Cu(NO3)2/PCA ratio of 0.10 had the highest fraction of the Pd2O species of about 23%. The Fourier transform (FT) of k3-weighted EXAFS of this Pd cluster and its inverse FT are shown in Figure 3. The curve-fitting analysis of the inverse FT of a main peak in Figure 3(a) could be completed by using a single Pd-Pd shell in the metallic form to give a distance (R) and coordination number (N) of 2.75 Å and about 10, respectively. The R value was the same as that of the Pd foil (2.75 Å), whereas the N was smaller than that of the foil (12). The above results indicate that the Pd cluster has a small particle size and its inner part is composed of a metallic Pd atom. Conclusively, the change in the oxidation state of Pd atoms with respect to the Cu(NO3)2/PCA ratio (Figure 2(b)) occurred on the cluster surface. Some parts of surface Pd cations exist as the Pd2O evidenced by the above CO method, where the others are metallic Pd species and divalent Pd cations stabilized by anion ligands such as OAc- and NO3-. The amount of OAc- and NO3- anions on the Pd clusters was determined by TPD measurement, XPS, and ion(33) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1988, 84, 2987. (34) The evolution of CO2 during the CO treatment was evidenced by separated experiment using a photoacaustic spectrometer. (35) The amount evolved CO2 after treatment of PdO with CO at 0 °C was 1.2% compared to total Pd atoms. (36) A Pd+ species has been already reported on palladium-loaded silicoaluminophosphate molecular sieves. See; Choo, H.; Prakash, A. M.; Zhu, Z.; Kevan, L. J. Phys. Chem. B 2000, 104, 3608; Yu, J.-S.; Lee, C. W.; Kevan, L. J. Phys. Chem. 1994, 98, 5736.

Figure 4. FE-SEM image of the 8-shell Pd cluster/TiO2 (JRCTIO-2). The white points show the Pd cluster particles of ∼40 Å. The content of the Pd cluster is 2.5 wt %.

chromatography. In the cuboctahedral structure model of the 8-shell cluster composed of 2060 Pd atoms, the number of the surface Pd atoms is 640. The analysis with ionchromatography showed that the amount of OAc- on the largest Pd cluster corresponded to 56% of the surface Pd atoms. Also, a percentage of NO3- calculated by XPS was 57% of the surface Pd atoms. Assuming two anionic ligands are coordinated to one Pd2+ species on the surface of the cluster, about 56% of the Pd atoms of the surface are bound to both anionic ligands. Correspondently, the residual 44% of the surface Pd atoms could be composed of Pd2O and Pd0 species. Based on the surface estimation method using CO uptakes, the Pd atoms on the cluster surface have both +1 and 0 states with about 23% and 20%, respectively. The above results lead us to propose a composition of the giant Pd cluster with a 35 Å diameter as Pd∼2060(NO3)∼360(OAc)∼360O∼80 and its structure as illustrated in Scheme 2(a).37 This giant Pd cluster would have a cuboctahedral structure with an 8-shell cluster and about 80% of the surface would be composed of the (37) The giant Pd cluster could be obtained by the following equation. [Pd4phen2(CO)2(OAc)4] + 9[Cu(NO3)2‚3H2O]+ 172AcOH + 137O2 f 0.05[Pd∼2060(NO3)∼360(OAc)∼360O∼80] + 180[Pdphen(OAc)2] + 77[Pd(OAc)2] + 9[Cu(OAc)2] + 113H2O + 180CO2.

Novel Preparation of Palladium Nanoclusters

Langmuir, Vol. 18, No. 5, 2002 1853

Scheme 2. Proposed Structure Models of (a) 8-shell Pd Cluster, Pd2060(NO3)360(OAc)360O80, and (b) 5-shell Pd Cluster. The Anionic Ligands in the outer Sphere Are Omitted.

cationic Pd species, i.e. Pd2+X2 and Pd2O, where X denotes OAc- and NO3- anions. Presumably, the Pd2+ species coordinated with both OAc- and NO3- might be mainly located at the corner/edge sites (30% of the surface Pd atoms) and the Pd2O species could be situated on the face of the cluster. Scheme 2(b) shows a proposed structure of the Pd cluster with a small diameter. The diameter of 27 Å corresponds to the 5-shell cluster composed of 561 Pd atoms.11 We also found that the use of Fe(NO3)3 instead of Cu(NO3)2 could afford the giant Pd clusters.38 The particle of about 35 Å in diameter was obtained with a mole ratio of Fe(NO3)3 to PCA of 0.07, where the fraction of the surface Pd2O species exhibited the maximum of 22%. The composition of this 8-shell Pd cluster was Pd∼2060(NO3)∼250(OAc)∼470O∼80. The ratio of OAc- to NO3anions was different from that obtained for the 8-shell Pd cluster using Cu(NO3)2. However, the total number of anionic species situated on the cluster surface was constant, irrespective of the metal nitrates.39 The particle size and the surface oxidation state of the giant Pd clusters can be controlled by changing the amount of the metal nitrates. The feature in cluster formation is considered as follows. The metal nitrates promote disproportionation of monovalent Pd ions of Pd4phen2(CO)2(OAc)4 complex to yield Pd0 and Pd2+ cation species. The Pd0 species aggregate to form relatively small Pd assemblies. Since the amounts of Pd0 and Pd2+ species increase with increasing amounts of the metal nitrates, the small assembly of Pd0 species is overspread with generated Pd0 species, which leads to growth of a larger Pd0 cluster. Subsequently, the Pd0 atoms located at the corner/edge sites of the larger cluster surface are oxidized by the metal nitrate to give Pd2+ cations, which are stabilized by anionic ligands such as OAc- and NO3-. On the other hand, the Pd2O surface species might be formed by the oxidation of the Pd0 ions by molecular oxygen.40 Additional increases in the amount of metal nitrates resulted in a deep oxidation of the surface Pd species and also, an exfoliation of Pd2+ ions from the cluster surface, which result in a formation of smaller metallic clusters together with a decomposition into the monomeric Pd2+ complex.32 Correspondingly, both the particle size and the surface oxidation state of the Pd cluster exhibited maxima with respect to the amount of the metal nitrates used. (38) The use of Li(NO3) instead of Cu(NO3)2 or Fe(NO3)3 did not afford the Pd cluster. (39) The low optimum ratio of Fe(NO3)3 to PCA is due to a strong ability of the Fe3+ cation for the disproportionation of Pd+ ions and for the oxidation of the surface Pd0 species. (40) The Pd cluster could not be obtained by the treatment of Pd4phen2(CO)2(OAc)4 with Cu(NO3)2 under Ar instead of oxygen atmosphere.

Figure 5. Effect of mole ratios of copper nitrate to PCA on the acetoxylation of toluene using the Pd cluster. Reaction conditions: Pd (0.29 mmol), toluene (5 mmol), AcOH (4 mL), 90 °C, 3 h, O2 atmosphere. Table 1. Acetoxylation of toluene with Various Pd Catalysts in the Presence of Molecular Oxygena catalysts

convn(%)

total yield of 1 + 2(%)

1:2

Pd clusterb Pd cluster/TiO2 5% Pd/Carbon 5% Pd/Al2O3 Pd(OAc)2

96 93 19 21 0

94 88 trace 0 0

4.1:1 5.0:1 -

a Reaction conditions: Pd(0.25 mmol), toluene (5 mmol), AcOH (4 mL), 90 °C, 3 h, O2 atmosphere. b Pd (0.29 mmol).

We tried to immobilize the 8-shell Pd cluster on the TiO2 surface. As seen in Figure 4, the FE-SEM image of the 8-shell Pd cluster on the TiO2 surface shows the distribution of the Pd particles with ∼40 Å on the surface; there are assemblies of the Pd cluster particles around the edges of the TiO2 crystallines. On the other hand, the previously reported 5-shell Pd561 cluster particles, which might be covered with cationic Pd species,11 uniformly dispersed around the TiO2 surface.14(b) Based on the unique ensemble surface structure of the present 8-shell Pd cluster, it is supposed that anionic ligands at the corner/ edge of the cluster surface could be strongly attached to coordinatively unsaturated Ti4+ cations around the edge of the TiO2 crystal.2(b) Conclusively, the surface of the 8-shell Pd cluster is composed of Pd0, Pd2O, and PdX2 where X represents OAcand NO3- anion ligands. This is the first example for the precise determination of the surface oxidation state of the giant Pd cluster. Furthermore, the giant Pd cluster can be immobilized on the TiO2 surface by keeping its original structure. A unique oxidation catalysis of the giant Pd cluster based on the novel surface state is described below. Acetoxylation of Toluene by Giant Pd Cluster Catalysts. Acetoxylation is one of the powerful methods to functionalize various kinds of C-H bonds in hydrocarbons; the acetoxyl group can be facilely transformed into other functional ones. Generally, acetoxylation of toluene occurs by using a homogeneous Pd(OAc)2 complex in acetic acid under an oxygen atmosphere to give benzyl acetates, where additives of KOAc and Sn(OAc)2 are indispensable.22 In heterogeneous Pd catalysts, Pd particles on charcoal or SiO2 as a support interacting with

1854

Langmuir, Vol. 18, No. 5, 2002

Ebitani et al.

Scheme 3. Proposed Reaction Mechanism of Acetoxylation of Toluene Catalyzed by the Surface of the 8-shell Giant Pd Cluster

excess amounts of Sn(2-ethyl-hexanoate)2 and KOAc exhibit catalytic activity.24,25 It is said that the efficient acetoxylation by Pd catalysts cannot be attained without any additives. To explore the potential catalytic ability of our giant Pd clusters based on the unique surface structure having Pd0, Pd+, and Pd2+ species, the oxidative acetoxylation of toluene using molecular oxygen was performed. We found that the catalytic activity of the Pd clusters was strongly sensitive to the amount of the metal nitrate used in the preparation. Figure 5 depicts a typical result using Cu(NO3)2. Clearly, the maximum catalytic activity was obtained in the case of the 8-shell Pd cluster with the highest fraction of Pd2O and the divalent Pd species.41,42 The 8-shell Pd cluster prepared using Fe(NO3)3 also had high catalytic activity for this acetoxylation. The high catalytic activity of the giant Pd cluster might be due to the oxidation state of the surface Pd atoms as well as the particle size. It is interesting that this giant Pd cluster can efficiently catalyze the acetoxylation of toluene using molecular oxygen without any additives such as Sn compounds, KOAc, and reoxidizing reagents.43 The 8-shell Pd cluster showed a much higher catalytic activity than conventional Pd catalysts, e.g. Pd/Carbon, Pd/Al2O3, and Pd(OAc)2. The 8-shell Pd cluster immobilized on the TiO2 surface also had high catalytic activity for the acetoxylation of toluene.44 These results are summarized in Table 1. Here, we speculate on a possible reaction mechanism for the acetoxylation of toluene catalyzed by the 8-shell Pd cluster. This is shown in Scheme 3. An ensemble of the cationic Pd species (Pd+ and Pd2+) and Pd0 on the giant Pd cluster surface plays an important role in this reaction. The initial step is a π-bond interaction between toluene and the cationic Pd2+ species. The second step would be the rupture of the methyl C-H bond of toluene by a (41) The acetoxylation did not proceed when the reaction using the Pd cluster was carried out under Ar atmosphere instead of oxygen. This Pd cluster was also effective for the acetoxylation of p-methoxytoluene; p-methoxybenzyl acetate was formed with 85% yield at 90 °C for 24 h. (42) Cu(OAc)2 instead of Cu(NO3)2 did not give active Pd clusters forthe oxidative acetoxylation of toluene. (43) The particle size of the Pd cluster did not change appreciably after the acetoxylation reaction. (44) During the acetoxylation, no Pd leaching was observed by the ICP method whose detection limit is 24 ppb.

neighboring Pd0 to form a π-benzyl Pd adduct together with formation of a Pd-H species. This would be followed by an attack of an OAc anion on a benzylic position to yield benzyl acetates and H2O. The facile interaction of toluene on the surface of a giant Pd cluster can be explained by the geometry of the Pd atoms; a distance between the Pd0 and neighboring Pd2+ cations of the 8-shell Pd cluster is 2.75 Å obtained by EXAFS, which fits well with about a 2.9 Å distance between the center of benzene ring to the methyl carbon of toluene molecule.45 Moiseev et al. has already pointed out an importance of such a multiple interaction in the acetoxylation of toluene promoted by the Pd black.23 Subsequently, the hydride species on the Pd atoms readily reacts with oxygen species of the neighboring Pd2O to give H2O, regenerating the Pd0 species. The Pd2O species could be easily reformed by the dissociative adsorption of molecular oxygen on the surface of Pd0 species. In Vide supra, in the conventional oxidative acetoxylation catalyzed by Pd catalyst systems, an addition of potassium and tin compounds is indispensable.22-25 The Sn2+ additives promote the reduction step of Pd2+ to Pd0 and the resulting Sn4+ oxide may serve as an oxygen reservoir in oxygen-poor regimes.24 The potassium ones render Pd particles more electropositive, which becomes susceptible to interaction with an electron-donating substrate, e.g. toluene. Since the cationic Pd atoms already exist on the surface of the Pd cluster in the present catalyst system, addition of potassium compounds is not required to facilitate the interaction with toluene. Furthermore, the tin compound is also unnecessary because the Pd2O species acted as an oxygen reservoir through an easy interconversion between Pd+ and Pd0 species. Conclusions The novel synthesis and characterization of giant Pd clusters containing Pd0, Pd+, and Pd2+ species on the same surface were extensively described. In particular, the 8-shell cationic giant Pd cluster acted as an efficient catalyst for the oxidative acetoxylation of toluene using (45) The distance between the center of the benzene ring and methyl group of the toluene was calculated by the PM3 semiempirical method as implemented in the MOPAC version 97 system.

Novel Preparation of Palladium Nanoclusters

molecular oxygen, which could be further immobilized on the surface of a TiO2 support without changing its original structure. The present study clearly shows that mixed-valence states on the surface of Pd clusters lead to a prominent and unique catalysis. Additionally, utilization of such metal cluster catalysts would give a clue to understanding the roles of the surface oxidation state for the supported metal particles in many organic transformations. We are continuously studying the preparation of high performance catalysts based on the well-ordered atomic structure on the surface of nanoscaled metal clusters aimed at environmentally benign chemical processes. Acknowledgment. This work was supported by the Grant-in-Aid for Scientific Research from the Ministry of

Langmuir, Vol. 18, No. 5, 2002 1855

Education, Culture, Sports, Science, and Technology of Japan (11450307 and 11750676). We are grateful to the Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University for scientific support provided by the “Gas-Hydrate Analyzing System (GHAS)” and for the Lend-Lease Laboratory System. The X-ray absorption experiments have been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 97G006). The authors thank Prof. Masaharu Nomura at KEK-PF for helpful technical advice and Dr. Hisao Yoshida at Nagoya University for his help in XAFS measurements. K.-M. C. thanks the Panasonic Scholarship Foundation. LA011359J