Hydrogen Storage Properties of Isocyanide-Stabilized Palladium

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Langmuir 2006, 22, 1880-1884

Hydrogen Storage Properties of Isocyanide-Stabilized Palladium Nanoparticles Shintaro Horinouchi,† Yoshinori Yamanoi,† Tetsu Yonezawa,† Toshihiro Mouri,‡ and Hiroshi Nishihara*,† Department of Chemistry, School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and Toyota Motor Corporation, 1 Toyota-cho, Toyota, Aichi 471-8572, Japan ReceiVed September 29, 2005. In Final Form: December 8, 2005 Monodispersed palladium nanoparticles protected with n-octyl isocyanide were prepared, and their hydrogen absorption behavior was evaluated. The formation of the nanoparticles has been confirmed by means of 1H NMR and elemental analysis. Fourier transform infrared (FT-IR) showed that three distinct bands (2156, 1964, and 1611 cm-1) assigned to mono-, double-, and triple-bridged isocyanide ligands on the palladium surface. The average diameter of the particles was estimated to be 2.1 ( 0.7 nm from observation by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) analysis revealed that the particles contained Pd(0) with little amounts of Pd(II) or Pd(IV), in sharp contrast to the thiol- or phosphine-stabilized palladium nanoparticles. The absorption and desorption of hydrogen were reversible, and the reactions were much faster for the nanoparticles than for the bulk palladium metal, whereas the storage capacity was almost the same, 0.6 wt %.

Introduction Hydrogen is the most ideal substance for fuel-cell-powered automobile applications by virtue of its light weight, high abundance, and environmental friendliness. However, hydrogen storage remains a great challenge because of its high explosiveness and the high cost of the storage process.1,2 Among the wide range of industrial applications for hydrogen fuel cells, much attention has been focused on the Pd-H system in ultrafine structured Pd because Pd is a typical face-centered cubic metal that can absorb a large amount of hydrogen.3 However, to activate the Pd(0) surface, in many cases it is necessary to preheat bulk materials with hydrogen (annealing treatment). The past decade has seen growing interest in nanoparticles, a new class of materials with chemical and physical properties different from conventional bulk materials or atoms.4 Sizecontrolled palladium nanoparticles can be excellent candidates for practical hydrogen storage materials. Since the proportion of activated atoms on the surface increases as the particle size decreases, greater hydrogen storage capacity and a higher hydrogen absorption/desorption rate are expected as the particle size decreases. Recently, several nanometer-sized Pd clusters (with grain sizes of less than 100 nm) have been developed for hydrogen storage,5,6 and a theoretical model was proposed for hydrogen storage in a cuboctahedral Pd13 cluster.7 Since the pioneering work by Brust et al.,8 metal clusters protected by organic monolayers (e.g., thiolates) have gained widespread * To whom correspondence should be addressed. Tel: +81-3-5841-4346. Fax: +81-3-5841-8063. E-mail: [email protected]. † University of Tokyo. ‡ Toyota Motor Corporation. (1) For representative reviews, see (a) David, E. J. Mater. Proc. Technol. 2005, 162-163, 169. (b) Special issue on hydrogen storage materials. MRS Bull. 2002, 27, 675. (c) Padro´, C. E. G.; Lau, F. AdVances in Hydrogen Energy; Kluwer Academic/Plenum Publishers: New York, 2000. (2) For recent representative reports on hydrogen storage, see (a) Ramachandran, R.; Menon, R. K. Int. J. Hydrogen Energy 1998, 23, 593. (b) Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414, 332. (c) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (d) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (e) Ward, M. D. Science 2003, 300, 1104. (3) For representative reviews, see (a) Lewis, F. A. The Palladium Hydrogen System; Academic Press: London, 1967. (b) Hydrogen in Metals II; Alefeld, G., Vo¨ikl, J., Eds.; Springer: Berlin, Heidelberg, 1978. (4) Nanoparticles; Schmid, G., Ed.; Wiley-VCH: Weinheim, Germany, 2004.

attention as promising candidates for nanoscale functional materials. This monolayer-stabilizing particle may be the best material for hydrogen storage because the organic stabilizing layer is very thin. However, since less attention has been given to the hydrogen storage properties of thiol-, phosphine-, or isocyanide-stabilized palladium nanoparticles, the behaviors of these particles in a hydrogen atmosphere have yet to be elucidated. In this paper, we report the preparation of monodispersed palladium nanoparticles protected with n-octyl isocyanide, and we evaluate the hydrogen absorption behavior of these particles from measurements by TEM, XPS, and pressure-composition isotherm (PC-isotherm) curves. Experimental Section Materials. All materials were obtained from commercial sources and used as received without additional purification. Ultrapure water was purified with a Millipore system (resistivity higher than 18.2 MΩ cm). n-Octyl isocyanide was prepared from n-octylamine and chloroform in a 50% NaOH aqueous solution in the presence of benzyltriethylammonium chloride as a phase-transfer catalyst.9 The starting material for the palladium nanoparticle, PdCl2(CH3CN)2, was synthesized as reported.10 (5) (a) Kishore, S.; Nelson, J. A.; Adair, J. H.; Eklund, P. C. J. Alloys Compd. 2005, 389, 234. (b) Hirai, N.; Takashima, M.; Tanaka, T.; Hara, S. Sci. Technol. AdV. Mater. 2004, 5, 181. (c) Suleiman, M.; Jisrawi, N. M.; Dankert, O.; Reetz, M. T.; Ba¨htz, C.; Kirchheim, R.; Pundt, A. J. Alloys Compd. 2003, 356-357, 644. (d) Bekyarova, E.; Hashimoto, A.; Yudasaka, M.; Hattori, Y.; Murata, K.; Kanoh, H.; Kasuya, D.; Iijima, S.; Kaneko, K. J. Phys. Chem. B 2005, 109, 3711. (e) Yamauchi, M.; Kitagawa, H. Synth. Met. 2005, 153, 353. (6) For representative examples of other nanosized materials, see (a) Isobe, Y.; Yamauchi, M.; Ikeda, R.; Kitagawa, H. Synth. Met. 2003, 135-136, 757. (b) Bogdanoviæ, B.; Felderhoff, M.; Kaskel, S.; Pommerin, A.; Schlichte, K.; Schu¨th, F. AdV. Mater. 2003, 15, 1012. (c) Shao, H.; Wang, Y.; Xu, H.; Li, X. J. Solid State Chem. 2005, 178, 2211. (d) Yavari, A. R.; LeMoulec, A.; de Castro, F. R.; Deledda, S.; Friedrichs, O.; Botta, W. J.; Vaughan, G.; Klassen, T.; Fernandez, A.; Kvick, Å. Scr. Mater. 2005, 52, 719. (e) Shao, H.; Xu, H.; Wang, Y.; Li, X. J. Solid State Chem. 2004, 177, 3626. (f), Callejas, M. A.; Anso´n, A.; Benito, A. M.; Maser, W.; Fierro, J. L. G.; Sanjua´n, M. L.; Martı´nez, M. T. Mater. Sci. Eng. B 2004, 108, 120. (g) Janot, R.; Rougier, A.; Aymard, L.; Lenain, C.; HerreraUrbina, R.; Nazri, G. A.; Tarascon, J. M. J. Alloys Compd. 2003, 356-357, 438. (7) Watari, N.; Ohnishi, S.; Ishii, Y. J. Phys.: Condens. Matter 2000, 12, 6799. (8) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (9) Weber, W. P.; Gokel, G. W.; Ugi, I. K. Angew. Chem., Int. Ed. Engl. 1972, 11, 530.

10.1021/la052657+ CCC: $33.50 © 2006 American Chemical Society Published on Web 01/19/2006

Hydrogen Absorption of Isocyanide-Protected Pd Characterization. NMR: 1H NMR studies of the nanoparticles and their precursors were recorded on a Bruker DRX-500 spectrometer at room temperature. IR: Fourier transform infrared (FT-IR) spectra were recorded on a Jasco FT/IR-620v spectrometer. Elemental Analysis: Elemental analysis of the products was performed on a Yanaco MT-6 C, H, N corder at the Elemental Analysis Center of The University of Tokyo. Transmission electron microscopy (TEM): TEM images were recorded at 200 kV using a Hitachi HF-2000 equipped with an AMT-CCD camera. TEM samples of palladium nanoparticles were prepared at room temperature by depositing chloroform-dispersed particles onto a carbon film supported by a copper grid. The size distribution of the nanoparticles was obtained by manually measuring the diameters of more than 500 particles. X-ray photoelectron spectroscopy (XPS): XPS experiments were performed using a Ulvac φTHI-5700 and a Mg KR anode (1253.6 eV). The energy calibration peak of C1s 248.8 eV was used in this experiment. All XPS spectra were measured with a pass energy (PE) of 29.35 eV and a step size of 0.125 eV. Synthetic Procedure of n-Octyl Isocyanide-Protected Palladium Nanoparticle (1). Palladium nanoparticle 1 was prepared by a modification method based on the literature using n-octyl isocyanide as a stabilizer.11 Under a nitrogen atmosphere, PdCl2(CH3CN)2 (2.59 g, 10.0 mmol) and [N(n-C8H17)4]Br (surfactant: 21.9 g, 40.0 mmol) were dissolved in dehydrated tetrahydrofuran (THF) (300 mL) and stirred until an orange-yellow solution was formed. Superhydride (LiBEt3H, 30 mL, 30.0 mmol, 1.0 M THF solution) was subsequently added to the solution, and immediately the solution turned dark brown. The mixture was vigorously stirred for 30 min. Then n-octyl isocyanide (2.78 g, 20 mmol) was added into the obtained dispersion. The mixture was stirred for another 15 min, and the solvent was removed by evaporation until only a small volume remained. Methanol (100 mL) was added, and the precipitate was collected on a poly(tetrafluoroethylene) (PTFE) membrane filter. Reprecipitation with methanol was repeated to ensure the removal of any unbound isocyanide and surfactant in the product. Finally, the nanoparticles were characterized by 1H NMR, FT-IR, UV-vis, elemental analysis, and TEM. All processes were carried out under a nitrogen atmosphere. 1: 0.51 g (32%).12 1H NMR (CD2Cl2, 500 MHz) δ 2.32 (t, J ) 7.2 Hz, 2H), 1.63 (quin, J ) 7.4 Hz, 2H), 1.42-1.38 (m, 2H), 1.38-1.10 (m, 8H), 0.90-0.80 (t, J ) 7.0 Hz, 3H). IR (KBr) 2923, 2855, 1964, 1622, 1460, 1339, 1027, 906 cm-1. Hydrogen Storage. Samples for these measurements were prepared by putting palladium nanoparticles 1 into a special cell made of stainless steel (Figure S1, Supporting Information) under a nitrogen atmosphere and sealing the cell. Ultrahigh-purity hydrogen gas (99.999% pure) was used for the storage studies. The weight storage data were corrected at all pressures. The number of storage isotherms for bulk palladium (99.9%+ pure, supplied by Aldrich) were verified using the conventional constant volume apparatus for PC-isotherm measurement, and the results were found to be in good agreement with those reported in the literature.3

Results and Discussion Previously, we and another group reported the preparation of alkanethiol-protected palladium nanoparticles.11,13 We also (10) Komiya, S. Synthesis of Organometallic Compounds: A Practical Guide; John Wiley & Sons: New York, 1996; p 285. (11) Quiros, I.; Yamada, M.; Kubo, K.; Mizutani, J.; Kurihara, M.; Nishihara, H. Langmuir 2002, 18, 1413. (12) Elemental analysis of carefully washed particles showed that the material was composed of 26.27 wt % carbon, 4.47 wt % hydrogen, and 3.17 wt % nitrogen. Therefore, the particle’s palladium content was estimated to be 66.09 wt %. The yield was calculated by dividing the weight of palladium in the isolated particle by the weight of palladium produced from the complete reduction of PdCl2(CH3CN)2. The palladium nanoparticle with a diameter of 2.1 nm () 2r) is comprised of 331 palladium atoms () FPd[4/3(πr3)/Ar(Pd)], where FPd ) 12.02 g/cm3 and Ar(Pd) (atomic mass) ) 106.4). From the elemental analysis, a 2.1 nm Pd nanoparticle contains 33.91 wt % of n-octyl isocyanide. Therefore, the average chemical composition was calculated to be Pd331(C8H17NC)129. (13) (a) Murayama, H.; Ichikuni, N.; Negishi, Y.; Nagata, T.; Tsukuda, T. Chem. Phys. Lett. 2003, 376, 26. (b) Zelakiewicz, B. S.; Lica, G. C.; Deacon, M. L.; Tong, Y.-Y. J. Am. Chem. Soc. 2004, 126, 10053.

Langmuir, Vol. 22, No. 4, 2006 1881 Scheme 1. Formation of Isocyanide-Protected Pd Nanoparticle 1

prepared trialkyl phosphine-protected palladium nanoparticles (see Supporting Information). However, none of these compounds showed any hydrogen storage ability at all. Partial oxidation of these particle surfaces was observed by XPS.14,15 From the above results and on the basis of the hydrogen storage mechanism that begins from uptake on the Pd(0) surface and the formation of the Pd-H bond, we took an approach to keep the palladium surface in a zero oxidation state using an electron-donating stabilizer. This strategy turned our attention to the synthesis of palladium nanoparticles protected with alkyl isocyanide, in the expectation that such particles would maintain the zero valence of the particle surface. Isocyanide (R-NtC) has a strong ability to coordinate with transition metals such as palladium, and the formation of the Pd-CtN-R interface is the key to enhancing monolayer stability against oxidation on the surface. The coordination bond is explained by the two kinds of molecular orbital interaction (see Supporting Information): (i) electron donation from the σ orbital of isocyanide to the empty metal orbital, and (ii) back-donation from the filled metal orbital to the empty π* orbital of isocyanide. Both donation and back-donation are important factors governing the strength of the bond that forms between Pd and isocyanide.16 Back-donation interaction usually increases when the oxidation state of palladium decreases. The bond between the metal and the phosphorus atom can also be described as a donation of the tertiary phosphine (PR3) lone pair to the metal and a back-donation from the metal to the empty 3d orbital of phosphorus. However, the π-acceptor strength of PR3 is weaker than that of CtN-R.17 Palladium nanoparticles protected with n-octyl isocyanide were prepared by using a one-phase system based on a modification method in the literature (Scheme 1).11,18,19 The palladium nanoparticles were purified to remove excess unbound stabilizer molecules by washing with MeOH, and were isolated as black powder. The average particle diameter was 2.1 ( 0.7 nm from observation by TEM image and diameter distribution as shown in Figure 1. IR spectroscopy is a useful tool for the analysis of (14) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Physical Electronics Division, Perkin-Elmer Corp.: Eden Prairie, MN, 1984. (15) The number of phosphine on the palladium surface was too small (PPh3: core size 2.1 nm, Pd309(PPh3)12; P(C8H17)3: core size 2.2 nm, Pd448[P(C8H17)3]27). The surface on these particles is sparsely covered with stabilizer and is easily oxidized. (16) (a) Yamamoto, Y. Coord. Chem. ReV. 1980, 32, 193. (b) Hartley, F. R. In ComprehensiVe Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, 1982. (17) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1999. (18) There have been a few investigations of the isocyanide-protected palladium nanoparticles. See (a) Yonezawa, T.; Imamura, K.; Kimizuka, N. Langmuir 2001, 17, 4701. (b) Wang, Z.; Shen, B.; He, N. Mater. Lett. 2004, 58, 3652. (19) For other isocyanide-protected transition-metal nanoparticles, see (a) Ontko, A. C.; Angelici, R. J. Langmuir, 1998, 14, 1684. (b) Horswell, S. L.; Kiely, C. J.; O’Neil, I. A.; Schiffrin, D. J. J. Am. Chem. Soc. 1999, 121, 5573. (c) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B 2001, 105, 941. (d) Lee, C.; Kim, S. I.; Yoon, C.; Gong, M.; Choi, B. K.; Kim, K.; Joo, S.-W. J. Colloid Interface Sci. 2004, 271, 41.

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Figure 3. Possible coordination structures for isocyanide on Pd nanoparticle surfaces.

Figure 1. (a) TEM micrograph of nanoparticle 1 and (b) size distribution of Pd cores.

Figure 2. FT-IR of n-octyl isocyanide (a) and Pd nanoparticle 1 (b).

isocyanide bonding geometry because of the strong NtC stretching vibration.20 Free octyl isocyanide exhibited a strong NtC stretching vibration at 2147 cm-1 (Figure 2(a)), whereas palladium particle 1 showed three salient infrared spectral features: a NC-stretching absorption at 2156 cm-1, characteristic of a terminal isocyanide ligand; another at ∼1964 cm-1 (21001700 cm-1), characteristic of a doubly bridged isocyanide ligand; and a third at ∼1611 cm-1 (1700-1500 cm-1), ascribable to a triply bridged isocyanide ligand (Figure 2b).19 The 1H NMR data for the derivatized particles also indicate the presence of isocyanide on the palladium surface (see Supporting Information). The NMR spectrum is similar to that of the free isocyanide except that some line broadening is evident. Line broadening in the 1H NMR of thiol- or phosphine-derived Pd nanoparticles has been attributed to the metal center’s effect on the ligand and has been observed to decrease for nuclei further away from the metal center. A smaller degree of broadening was observed in the case of isocyanide-stabilized Pd nanoparticles. This suggests that the N atom in isocyanide does not directly coordinate to Pd.21 Isocyanides have several coordination structures for transition metals, as shown in Figure 3.22 Although it is not clear which coordination form would be more reliable at present, it is possible that n-octyl isocyanide binds to palladium mainly in the bonding geometries a, b, and d, which are remarkably stable on the surface and provide dense surface stabilization. An XPS study was performed to ascertain the chemical state of bulk palladium and nanoparticle 1. The corresponding high(20) (a) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; Wiley: New York, 1997; Part B (Applications in Coordination, Organometallic, and Bioorganic Chemistry), p 115. (b) Swanson, S. A.; McClain, R.; Lovejoy, K. S.; Alamdari, N. B.; Hamilton, J. S.; Scott, J. C. Langmuir 2005, 21, 5034. (c) Cotton, F. A.; Zingales, F. J. Am. Chem. Soc. 1961, 83, 351. (d) Queau, R.; Poilblanc, R. J. Catal. 1972, 27, 200. (21) If N atoms in isocyanide coordinate to Pd, the NMR peak of R-protons should be observed as a broaden peak. See Supporting Information for details.

Figure 4. XPS spectra showing the positions of the 3d5/2 and 3d7/2 peaks characteristic of bulk palladium (Bulk Pd) and nanoparticle 1 (Pd-OcNC).

resolution XPS Pd 3d spectra are shown in Figure 4, revealing the presence of Pd(3d5/2) and Pd(3d7/2) peaks. The binding energy values of these samples are similar to that reported for metallic Pd in the literature.23 It is thus confirmed that the oxidized compounds were not contained at all on the surface of either the nanoparticle or the bulk palladium because there was no peak around 336 eV corresponding to Pd(II) species. As compared with the XPS of the bulk palladium, the peak of nanoparticle 1 was shifted to a lower binding energy; this shift can be explained by the stabilizer’s electron negativity. It is thought that this effect results from the electron donation of the isocyanide group. This is one of the important reasons to choose the isocyanide ligand as the palladium nanoparticle stabilizer for hydrogen storage (Vide infra) because other stabilizers for the palladium surface, such as phosphines or thiols, gave partial oxidation. Palladium nanoparticles 1 were put into a cell (stainless steel, entire length 37.5 cm; see Supporting Information) under argon for hydrogen absorption measurement. Figure 5 shows the hydrogen absorption curves of bulk palladium and Pd nanoparticle 1. The vertical axis in this figure represents the hydrogen absorption. Because the total palladium amount in nanoparticle 1 is lower than that in bulk palladium, the hydrogen absorption curve of nanoparticle 1 is rather noisy. At 373 K, nanoparticle 1 absorbed hydrogen completely in a few seconds. The hydrogen absorption rate of nanoparticle 1 was much faster than that of bulk palladium, although this sample showed hydrogen absorption at the same level (∼0.6 wt %) as that for the bulk palladium. The thermodynamic aspects of metal-hydrogen interactions are usually understood from PC-isotherms. Figure 5c,d shows (22) (a) Robertson, M. J.; Angelici, R. J. Langmuir 1994, 10, 1488. (b) Henderson, J. I.; Feng, S.; Bein, T.; Kubiak, C. P. Langmuir 2000, 16, 6183. (c) Murphy, K. L.; Tysoe, W. T.; Bennett, D. W. Langmuir 2004, 20, 1732. (23) Brun, M.; Berthet, A.; Bertolini, J. C. J. Electron Microsc. Relat. Phenom. 1999, 104, 55.

Hydrogen Absorption of Isocyanide-Protected Pd

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Figure 5. Hydrogen absorption curves of (a) bulk palladium and (b) palladium nanoparticle 1 under a starting hydrogen pressure of 3 MPa at 373 K, and hydrogen storage isotherms for the nanoparticle sample compared with that for the bulk Pd sample in the pressure range of 0-10 MPa (pressure axis on log scale) for (c) bulk palladium and (d) palladium nanoparticle 1.

the PC-isotherms of the bulk palladium and nanoparticle 1 with the pressure axis plotted on a logarithmic scale.24 The reactor was heated at different temperatures, and the hydrogen absorption PC-isotherms for palladium nanoparticle 1 showed the general features expected for bulk palladium. That is, each isotherm showed three distinct regions, which, in the bulk, have been identified with the R-phase, the (R+β)-phase that marks the miscibility gap, and the β-phase at the highest pressures.25 In particular, the hydrogen absorption isotherms for nanoparticle 1 showed the following distinct characteristics: (1) the plateau region of the isotherm (constant pressure two-phase field) is a little narrower than that of bulk Pd, and (2) the plateau of 1 narrows as the temperature falls, which is in sharp contrast to the isotherms of bulk palladium. For most hydrogen storage materials, low cyclability is usually a critical issue. As we can see in Figure 5c,d, the absorption curves coincided with each other during several absorptiondesorption cycles. This observation shows that Pd nanoparticle 1 has a high stability for hydrogen storage. The p-T dependence is expressed by the van’t Hoff relation:

ln p ) -∆H/RT + ∆S/R where ∆H and ∆S are the changes in standard entropy and enthalpy during the hydrogen storage process, respectively. The Arrhenius plot of this equation reveals that the apparent activation energy for the hydrogen storage process through the Pd nanoparticle remains unchanged within the experimental temperature and pressure ranges. The exact activation energy of 1 (24) Schlapbach, L. Hydrogen in Intermetallic Compounds I & II; SpringerVerlag: Berlin, 1988 and 1992. (b) Chen, J.; Li, S.-L.; Tao, Z.-L.; Shen, Y.-T.; Cui, C.-X. J. Am. Chem. Soc. 2003, 125, 5284. (25) The R and β phases are determined by the concentration of hydrogen in a hydride metal, such as PdHx. The low-concentration phase (i.e., x < 0.1) is called the R-phase, while the β-phase is formed when x is higher than 0.6. Please refer to ref 7.

was calculated as 36.1 kJ mol-1, which was close to that obtained from Pd bulk (36.9 kJ mol-1) (see Supporting Information).26 It is worth pointing out that the absorption and desorption of hydrogen were reversible and that the hydrogen storage rate at 100 °C of palladium nanoparticle 1 was about twelve times faster than that of bulk palladium. The higher reaction rate can be ascribed to the large surface area, and the unchanged hydrogen storage ability can be attributed to the fact that the absorption site number is the same between nanoparticle 1 and bulk.

Conclusions Palladium nanoparticles stabilized by n-octyl isocyanide (1) were prepared by using a one-phase system with a stabilizerexchange process. This work showed that the kinetics of hydrogen storage is significantly faster in the palladium nanoparticle than it is in the bulk. The storage capacity of the nanoparticles was found to lie around 0.6 wt %, which is almost the same as the storage capacity of bulk palladium. It has been suggested that the increase in the hydrogen absorption rate is due to the higher electron-donating nature of n-octyl isocyanide. The results of XPS and the pressure compositions of isomers show that hydrogen can be absorbed inside the nanoparticle, but these absorbing sites have not been examined in the present stage. This material offers promising applications toward catalysis reactions with a certain degree of reusability. Acknowledgment. The present work was partially supported by Grants-in-Aid from the Ministry of Education, Science, Sports, and Culture, Japan. Supporting Information Available: Scheme S1: Formation of alkanethiol-protected Pd nanoparticles 2 and 3; Scheme S2: Formation of tertiaryphosphine-protected palladium nanoparticles 4 and 5; Figure (26) Sun, Y.; Tao, Z.; Chen, J.; Herricks, T.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 5940.

1884 Langmuir, Vol. 22, No. 4, 2006 S1: A photograph of the special stainless cell used for measurement of hydrogen storage; Figure S2: Bonding isocyanide to palladium; Figure S3: XPS spectra of palladium nanoparticles; Figure S4: Ratios of Pd(0), Pd(II), and Pd(IV) components on the surfaces of palladium nanoparticles 2-5 calculated from XPS; Figure S5: 1H NMR of (a)

Horinouchi et al. n-octyl isocyanide and (b) Pd nanoparticle 1; Figure S6: Van’t Hoff plots for (a) Pd nanoparticle 1 and (b) bulk palladium. This material is available free of charge via the Internet at http://pubs.acs.org. LA052657+