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Isotope Tracer Study of Hydrogen Spillover on Carbon-Based Adsorbents for Hydrogen Storage Anthony J. Lachawiec, Jr. and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109, USA ReceiVed February 2, 2008. ReVised Manuscript ReceiVed March 19, 2008 A composite material comprising platinum nanoparticles supported on molecular sieve templated carbon was synthesized and found to adsorb 1.35 wt % hydrogen at 298 K and 100 atm. The isosteric heat of adsorption for the material at low coverage was approximately 14 kJ/mol, and it approached a value of 10.6 kJ/mol as coverage increased for pressures at and above 1 atm. The increase in capacity is attributed to spillover, which is observed with the use of isotopic tracer TPD. IRMOF-8 bridged to Pt/C, a material known to exhibit hydrogen spillover at room temperature, was also studied with the hydrogen-deuterium scrambling reaction for comparison. The isotherms were reversible. For desorption, sequential doses of H2 and D2 at room temperature and subsequent TPD yield product distributions that are strong indicators of the surface diffusion controlled reverse spillover process.
Introduction The adsorption of hydrogen in nanostructured carbon materials has been studied for over a decade.1–3 Beginning with the discovery of the interesting behavior of single-walled nanotubes3 (SWNTs), carbons have been viewed as a potential solution for issues surrounding on-board hydrogen storage in vehicular applications.4 Recently, attention has been focused on the phenomenon of hydrogen spillover as a possible route toward increased capacity sorbents for ambient temperature storage.5–9 It is clear that, to advance practical adsorbent development, the mechanism and kinetics of hydrogen spillover must be understood. Evidence of atomic hydrogen spillover was first observed indirectly during studies of ethylene hydrogenation via heterogeneous catalysis.10 Khoobiar reported room-temperature formation of tungsten bronze upon exposure of the metal oxide to molecular hydrogen in the presence of alumina-supported platinum.11 The extensive work of Boudart and co-workers confirmed the effect with results obtained from hydrogen reduction of transition metal oxides mixed with Pt black12 and studies of hydrogen uptake by transition metals on carbon supports.13,14 Room-temperature spillover for carbon-supported platinum has been reported with evidence drawn from hydrogen uptake and benzene hydrogenation experiments.15 In several studies by Beck et al.,16,17 the isotope deuterium was used to *
[email protected]. (1) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (2) Noh, J. S.; Agarwal, R. K.; Schwarz, J. A. Int. J. Hydrogen Energy 1987, 12, 693. (3) Panella, B.; Hirscher, M.; Roth, S. Carbon 2005, 43, 2209. (4) Hydrogen and Our Energy Future; DOE/EE-0320; U.S. Department of Energy: Washington, DC, 2007; p 19. (5) Yang, F. H.; Yang, R. T. Carbon 2002, 40, 437. (6) Lueking, A. D.; Yang, R. T. J. Catal. 2002, 206, 165. (7) Zacharia, R.; Rather, S.; Hwang, S. W.; Nahm, K. S. Chem. Phys. Lett. 2007, 434, 286. (8) Li, Y.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 8136. (9) Anson, A.; Lafuente, E.; Urriolabeitia, E.; Navarro, R.; Benito, A. M.; Maser, W. K.; Martinez, T. M. J. Phys. Chem. B 2006, 110, 6643. (10) Sinfelt, J. H.; Lucchesi, P. J. J. Am. Chem. Soc. 1963, 85, 3365. (11) Khoobiar, S. J. Phys. Chem. 1964, 68, 411. (12) Boudart, M.; Vannice, M. A.; Benson, J. E. Z. Phys. Chem. 1969, 64, 171. (13) Boudart, M.; Aldag, A. W.; Vannice, M. A. J. Catal. 1970, 18, 46. (14) Robell, A. J.; Ballou, E. V.; Boudart, M. J. Phys. Chem. 1964, 68, 2748. (15) Srinivas, S. T.; Rao, P. K. J. Catal. 1994, 148, 470. (16) Beck, D. D.; White, J. M. J. Phys. Chem. 1984, 88, 174, 2764. (17) Beck, D. D.; Bawagan, A. O.; White, J. M. J. Phys. Chem. 1984, 88, 2771.
trace room-temperature spillover on Pt/TiO2. The flash TPD technique was extended to Pt/Al2O3 in the later work of Chen and White.18 More recently, temperature-programmed reduction of carbon-supported palladium demonstrated hydrogen consumption below 573 K in excess of the palladium hydride amount, a behavior attributed to hydrogen spillover.19 This is attractive especially because, on the basis of receptor site density calculations, carbon has been proven to be a better spillover hydrogen acceptor than alumina or silica.20 To our knowledge, sequential exposure studies using molecular hydrogen and deuterium have not been used to study spillover on carbonsupported platinum or hybrid materials consisting of a metal organic framework (MOF) receptor. We report the synthesis and hydrogen capacity for a composite with molecular sieve templated carbon as an atomic hydrogen receptor and platinum nanoparticles as the source. We have performed sequential dosing of H2 and D2 to this adsorbent at room temperature and report temperature-programmed desorption (TPD) results. TPD product distributions and HD formation indicate hydrogen spillover, which accounts for increased capacity.
Experimental Methods Sample Preparation and Characterization. Molecular sieves are often used as templates for organic precursors to produce high surface area carbons with a substantial degree of microporosity.21–23 These carbons have surface area and microporosity equivalent to or greater than commercial products such as MAXSORB,24 but production does not require the severe conditions of KOH activation of this and other superactivated carbons.25 In addition, the hydrocarbon deposition process can be tailored to adjust the size of the pore openings, which may influence the rate of spillover and hence the rate of approach to equilibrium.26 Templated carbon (TC) (18) Chen, H.-W.; White, J. M. J. Mol. Catal. 1986, 25, 355. (19) Ramos, A. L. D.; Aranda, D. A. G.; Schmal, M. Stud. Surf. Sci. Catal. 2001, 138, 291. (20) Bond, G. C.; Mallat, T. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1743. (21) Kyotani, T.; Nagai, T.; Inoue, S.; Tomita, A. Chem. Mater. 1997, 9, 609. (22) Kyotani, T.; Ma, Z.; Tomita, A. Carbon 2003, 41, 1451. (23) Su, F.; Zhao, X. S.; Lv, L.; Zhou, Z. Carbon 2004, 42, 2821. (24) Otowa, T.; Tanibata, R.; Itoh, M. Gas. Sep. Purif. 1993, 7, 241. (25) Lozano-Castello´, D.; Lillo-Ro´denas, M. A.; Cazorla-Amoro´s, D.; LinaresSolano, A. Carbon 2001, 39, 741. (26) Yang, R. T. Adsorbents: Fundamentals and Applications; Wiley & Sons: New York, 2003; pp 109-115.
10.1021/la800371j CCC: $40.75 2008 American Chemical Society Published on Web 05/10/2008
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was prepared according to the procedure of Ma et al., using Na-Y zeolite (powder, Aldrich 334448) as a template.27 CVD conditions were selected to produce the highest reported BET surface area, and the method consistently produced microporous carbon with 3400 ( 20 m2/g. Typical carbon yields of this process were 20% of the dried zeolite template. Platinum, the spillover source, was doped at 6 wt % loading by liquid-phase impregnation augmented by ultrasonication.28 Hydrogen hexachloroplatinate (IV) hydrate (Pt 38-40%, Aldrich 398322) was used as the platinum source and was dissolved in acetone (HPLC grade, Aldrich 270725) to facilitate impregnation. The procedure was modified slightly from that followed by Li et al.29 to allow for sonication of the impregnation slurry in a polyethylene bag. This step was performed to decrease sonic wave attenuation that occurs using laboratory glassware30 and subsequently improve metal-support interaction. Intimate contact between the source and the receptor is critical in determining the spillover rate.31 After evaporating the solvent and drying the carbon composite, it was reduced in a flow of pure hydrogen at 100 sccm following the temperature program as outlined previously.29 A Micromeritics ASAP 2010 analyzer was used to measure nitrogen adsorption isotherms at 77 K to characterize the sample by BET surface area measurement and pore size distriubtion. X-ray diffraction analysis was performed using a Rigaku Miniflex X-ray diffractometer (30 kV, 15mA, Cu KR radiation) scanning with a resolution of 0.02° in 2θ. Transmission electron microscopy (JEOL 3011 TEM, 300 kV accelerating voltage) was used to capture images of the platinum-doped carbon particles. In situ energy-dispersive X-ray spectroscopy (Edax Ultrathin Window Si-Li X-ray detector) confirmed the composition of the particles observed in the images. A composite material consisting of IRMOF-8 bridged to a commercial 5 wt % Pt/C catalyst (Strem Chemicals, Inc. 78-1600) was synthesized for measurement of spillover by deuterium tracer. Sucrose was used as the bridge precursor. Details of the procedure for generating the IRMOF-8, producing the bridges, and full characterization of the material have been reported elsewhere.32
Equilibrium Hydrogen Isotherms and Adsorption Rates. Hydrogen isotherms for the platinum-loaded material were measured at several temperatures and up to 1 atm using a Micromeritics ASAP 2010 Analyzer. The adsorption rates were automatically recorded using the built-in software and saved for later processing. The temperatures selected were 273, 293, and 323 K. Circulating baths were used to maintain the sample at 293 or 323 K, while an ice-waterfilled Dewar flask was adequate to maintain 273 K during the experiment. Isotherms for the material from atmospheric pressure to 100 atm were measured at 298 K with a custom Sievert’s apparatus.33 For deuterium isotherms, as in the case of hydrogen, the nonideality correction in the calculations was made using the compressibility factor calculated from the pure-component second and third virial coefficients at 298 K.34 Since rate data were collected manually in the high-pressure system, adsorption and desorption data collected for rate analysis were arranged such that the pressure steps reflected equal amounts adsorbed or desorbed in the interval. This allowed for direct comparison of the rates attributed to the spillover and adsorption or desorption process without requiring correction for varying volumes of gas. In all cases, prior to the isotherm measurements, in situ degassing was performed at 623 K for 10 h while under vacuum to 0.25 Pa in the high-pressure Sievert’s apparatus or to 13 µPa in the lowpressure analyzer. Free space measurements were performed after all hydrogen isotherm data were collected to minimize the effect of helium adsorption. Free space values were corrected for helium adsorption on carbon at room temperature using the method published by Sircar.34 Temperature Programmed Desorption. TPD was performed after transferring the material to a 316 SS tubular chamber. A bellows valve was installed at one end to allow sequential dosing and a filter disk (0.5 µm) was used to prevent particle intrusion. The dosing procedure was similar for all experiments, and the only changes were to the sequence of gases. After the sample was degassed and equilibrated for at least 2 h at room temperature in vacuum (13 µPa), it was isolated from the manifold. The manifold was charged with gas to the desired dosing pressure. The dosing procedure was performed at room temperature with sequential exposures to 0.4 atm of the desired gas for 5 min. The sample was evacuated for 1 min after each dose to remove the gas phase. The chamber typically reached 1.3 kPa during this step. After the second dose and 1 min evacuation, the sample was isolated and immediately cooled in a liquid nitrogen bath at 77 K. Prior to TPD, the sample was evacuated to 0.3 Pa and held at that pressure for 1 h at 77 K. Analysis of evolved gases during TPD experiments was performed with an AeroVac 1200 Magnetic Sector mass spectrometer (VTI, Inc.) operating at an accelerating voltage of 70 eV. An electron multiplier, operating at 1000 V, was used to increase the sensitivity to low current signals. A thin 316 SS tube was used to deliver evolved gas directly to the inlet of a molecular leak valve (Varian, Inc.). A sheath around the sampler was dynamically pumped with a mechanical vacuum pump to remove residual gas and ensure that the sample was composed solely of gas issuing from the surface of the adsorbent. The pumping rate was adjusted to maintain constant pressure in the sampling line during the TPD experiment. Heating was accomplished using an external heater. For templated carbon and 6 wt % Pt on TC, the upper limit of TPD was 650 K to prevent Pt sintering36,37 and carbon gasification.38 TPD was limited to 523 K for the IRMOF-8 bridged Pt/C material to prevent its decomposition. The controlled temperature was measured with a surface thermocouple, and the sample temperature was calculated using a calibrated offset. Rates studied were 10, 15, and 20 K/min. The sample was treated at 650 K (templated carbon receptor) or 523 K (IRMOF-8 receptor) and 13 µPa for 3 h between each TPD
(27) Ma, Z.; Kyotani, T.; Tomita, A. Carbon 2002, 40, 2367. (28) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (29) Li, Y.; Yang, R. T. J. Phys. Chem. C 2007, 111, 11086. (30) Schueller, B. S.; Yang, R. T. Ind. Eng. Chem. Res. 2001, 40, 4912. (31) Bond, G. C. Stud. Surf. Sci. Catal. 1983, 17, 1. (32) Li, Y.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 8136.
(33) Lachawiec, A. J., Jr.; Qi, G.; Yang, R. T. Langmuir 2005, 21, 11418. (34) Dymond, J. H.; Smith, E. B. The Virial Coefficients of Pure Gases and Mixtures; Clarendon Press: Oxford, 1980; pp 184-185. (35) Sircar, S. Fundamentals of Adsorption 7; Kaneko, K., Kanoh, H., Hanzawa, Y., Eds.; IK International: Chiba-City, Japan, 2002; pp 656-663. (36) Bett, J. A.; Kinoshita, K.; Stonehart, P. J. Catal. 1974, 35, 307. (37) Chu, Y. F.; Ruckenstein, E. Surf. Sci. 1977, 67, 517. (38) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J. Catal. 1979, 56, 390.
Figure 1. (a) N2 Isotherms at 77 K and BET plot (inset); (b) PSD and cumulative pore volume (inset) based on DFT method for AX-21 (O), TC (4), and 6 wt % Pt on TC (0). Filled markers represent desorption. Table 1. BET Surface Area and Pore Volume Characterization of Templated Carbon and 6 wt % Pt on TC Composite BET SA (m2/g) BET C value BET Vm (cc[STP]/g) total pore volume (cc/g) micropore volume (cc/g)
templated carbon
6 wt % Pt on TC
3400 170 777 1.5 1.2
2755 214 633 1.2 1.0
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Figure 2. Left: XRD of (a) Na-Y zeolite; (b) TC; (c) 6 wt % Pt on TC. Right: HRTEM image of 6 wt % Pt on TC illustrating Pt particle size relative to carbon particle (inset).
experiment to remove atomic hydrogen and deuterium that might have remained from prior exposure. Repeat experiments confirmed that atoms remaining after this treatment were sufficiently anchored to prevent interaction with subsequent doses. An empty chamber was sequentially dosed and analyzed to establish a background signal for subtraction. Undoped templated carbon was also examined to serve as a baseline for comparison of spillover response. Procedures for preparation, dosing, and analysis were identical to those previously described for these systems. Attention was given to the purity of all gases used in this study, as moisture and atmospheric contaminants can have effects on hydrogen adsorption. Ultrahigh-purity helium (99.995%) and hydrogen (99.995%) were passed through molecular sieve 5 Å beds to ensure the gases were dry and free of contaminants. Deuterium (UHP, 99.97%) was used without further purification.
Figure 3. (a) Equilibrium hydrogen isotherms at 298 K for TC (4) and 6 wt % Pt on TC (0) compared with AX-21. (b) Equilibrium hydrogen isotherms as a function of temperature used to calculate qst (inset). Filled markers represent desorption.
Results and Discussion Nitrogen adsorption isotherms for the materials are shown in Figure 1a, along with the correesponding BET surface area reduction. The isotherm for AX-21, indicated by a single solid line demonstrating no hysteresis, is included for comparison with templated carbon. While there are known issues associated with applying the BET method to ultrahigh surface area, microporous materials,39–41 it has recently been shown that carefully examining the reduced data can provide reasonable estimates of accessible area for comparison purposes.42 This is illustrated in the inset to Figure 1a, where we have limited BET calculations to P/Psat data collected below 0.15, compared to the traditional value of 0.30.43 The templated carbon demonstrates hysteresis behavior on desorption that has been noted by other authors for similar materials.23,27 As expected, the hysteresis loop is indicative of slit-shaped pores. Pore size distribution was calculated from the adsorption branch of the isotherm using the density functional theory (DFT) method, and the results are shown in Figure 1 b. The results are consistent with PSD for activated carbons with a significant quantity of micropores and some mesoporosity.44 Note that the final composite has a surface area similar to plain AX-21 (∼2850 m2/g) and retains some mesoporosity. The undoped material contained 1.2 cc/g micropore (39) Kaneko, K.; Ishii, C. Colloids Surf. 1992, 67, 203. (40) Kruk, M.; Jaroniec, M.; Gadkaree, K. Langmuir 1999, 15, 1442. (41) Kruk, M.; Jaroniec, M.; Ryoo, R.; Joo, S. H. J. Phys. Chem. B 2000, 104, 7960. (42) Walton, K. S.; Snurr, R. Q. J. Am. Chem. Soc. 2007, 129, 8552. (43) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982; Chapter 2. (44) Ryu, Z.; Zheng, J.; Wang, M.; Zhang, B. Carbon 1999, 37, 1257.
volume in pores below 20 Å, and this was reduced to 1.0 cc/g after platinum doping. These results suggest that the doping process likely causes some pores to be blocked by platinum particles. Despite this decrease in micropore volume, the PSD maintains a similar shape after doping. The properties of both plain and Pt-doped templated carbon are summarized in Table 1. Figure 2 compares the XRD patterns for Na-Y zeolite, TC, and 6 wt % Pt on TC. A peak at 2θ ) 6.6° in Figure 1b indicates that the carbon has copied the template structure.27,45 Peaks above 2θ ) 35° in Figure 1c represent (111), (200), (220), and (311) Pt crystal faces. The average crystallite size, as calculated by the Scherer equation, is 44 Å. This agrees well with HRTEM images of the platinum particles shown in the TEM image (right). These results support the earlier theory that some micropores are blocked by platinum particles given the size comparison. The high-pressure hydrogen isotherm at 298 K is shown in Figure 3a, along with that of plain TC and AX-21. The composite has a capacity of 1.35 wt % at 100 atm, a 68 % enhancement over plain TC. The increase in hydrogen capacity for TC over AX-21 is attributed to larger BET surface area and total pore volume. It is recognized that enhancement of hydrogen spillover on carbon-supported palladium catalysts synthesized using sonication as part of the impregnation step has been reported previously by (45) Matsuoka, K.; Yamagishi, Y.; Yamazaki, T.; Setoyama, N.; Tomita, A.; Kyotani, T. Carbon 2005, 43, 876.
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Table 2. Spillover Isotherm Model Parameters at Various Temperatures for 6 wt % Pt on TCa temperature (K)
k1 (mmol/g-atm1/2)
k2 (1/atm1/2)
k3 (1/atm1/2)
273 293 323
0.0493 0.0404 0.0229
0.165 0.199 0.182
0.760 0.798 0.812
a
Parameters k1, k2, k3 for the model: q ) k1PH2/(1 + k2PH2 - k3PH2).
Cheng et al.46 Their study employed an immersion sonication probe, which can deposit metal particles on the carbon from the sonication horn. These horns are frequently constructed from titanium alloys, which can adsorb substantial quantities of hydrogen even at room temperature.47,48 As a result, it is difficult to decouple the effect of enhancement due to spillover and that due to adsorption by metal particles from the probe. The enhancement of hydrogen adsorption shown here is ensured to be solely due to spillover from the platinum nanoparticles because of the external sonication technique, which ensures that no other metals are inadvertently deposited on the structure during this process. Figure 3 b shows low-pressure hydrogen adsorption measured at three temperatures. The data at each temperature were fit to the isotherm model for hydrogen spillover developed by Li and Yang.29 The results are plotted as the solid lines in Figure 3b, and the parameters for the models at each temperature are provided in Table 2. The Clapeyron equation was used to calculate the isosteric heat of adsorption, which decreased from ∼14 kJ/mol at very low loading to ∼10.6 kJ/mol as loading increased (Figure 3b, inset). This value is double that for hydrogen physisorption on activated carbon (∼5 kJ/mol), but all values are smaller than that measured for hydrogen adsorption at low coverage on Pt planes (40-68 kJ/mol).49–51 In order to support the theory of spillover, hydrogen atoms must be capable of moving from the metal surface to the support or receptor. Menon and Fromet identified weak, moderately strong, and strong hydrogen atoms chemisorbed to platinum that caused a distribution of signals in TPD spectra.52 Norton et al.51 observed a mobile adsorbed layer, and the apparent difference in the heat of adsorption for this situation implies that there is substantial energy available to assist the hopping of hydrogen atoms from Pt to the carbon receptor. In addition, transport of atoms from Pt to the receptor is supported by the large chemical potential gradient of hydrogen atoms between the two locations, despite the process itself being endothermic.53 This emphasizes the importance of a physical bridge or strong metal-support interaction (SMSI) as a diffusion path for atomic hydrogen, especially at room temperature.13,54,55 The equilibrium isotherm at 298 K for deuterium on 6 wt % Pt on TC was measured up to a pressure of 55 atm, and the results are presented in Figure 4, shown in comparison with hydrogen adsorption on the same material. The deuterium delivery pressure limited the maximum pressure in the study. Of note is the behavior that deuterium adsorbs in smaller quantity relative to hydrogen (46) Cheng, Z. X.; Yuan, S. B.; Fan, J. W.; Zhu, Q. M.; Zhen, M. S. Stud. Surf. Sci. Catal. 1997, 112, 261. (47) Dillon, A. C.; Gennett, T.; Alleman, J. L.; Jones, K. M.; Parilla, P. A.; Heben, M. J. Proceedings of the 2000 DOE/NREL Hydrogen Program ReView; Golden, CO; May 8-10, 2000. (48) Blackburn, J. L.; Parilla, P. A.; Gennett, T.; Hurst, K. E.; Dillon, A. C.; Heben, M. J. J. Alloys Compd., 2008, 454, 483. (49) Eley, D. D. Philos. Trans. R. Soc. London, Ser. A 1986, 318, 117. (50) Christmann, K.; Ertl, G.; Pignet, T. Surf. Sci. 1976, 54, 365. (51) Norton, P. R.; Davies, J. A.; Jackman, T. E. Surf. Sci. 1982, 121, 103. (52) Menon, P. G.; Froment, G. F. Stud. Surf. Sci. Catal. 1982, 11, 171. (53) Kiselev, V. F.; Krylov, O. V. Adsorption and Catalysis on Transition Metals and Their Oxides; Springer-Verlag: New York, 1989; p 391. (54) Neikam, W. C.; Vannice, M. A. J. Catal. 1972, 27, 207. (55) Fujimoto, K.; Toyoshi, S. Stud. Surf. Sci. Catal. 1981, 7, 235.
Figure 4. Equilibrium isotherms for hydrogen (0) and deuterium (O) on 6 wt % Pt on TC at 298 K. Filled markers represent desorption.
Figure 5. Rate of adsorption for hydrogen and deuterium on 6 wt % Pt on TC at 298 K and 0.4 atm, expressed as fraction adsorbed. Inset provided to clearly illustrate data for short times.
and that the deviation increases with increasing pressure. At the pressures used for TPD work (
