Hydrogen Storage Properties of Carbons Doped with Ruthenium

Jul 22, 2008 - Catalytic Effects of TiF3 on Hydrogen Spillover on Pt/Carbon for Hydrogen Storage. Hao Chen and Ralph T. Yang. Langmuir 0 (proofing),...
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12486

J. Phys. Chem. C 2008, 112, 12486–12494

Hydrogen Storage Properties of Carbons Doped with Ruthenium, Platinum, and Nickel Nanoparticles Lifeng Wang and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: April 10, 2008; ReVised Manuscript ReceiVed: May 23, 2008

Hydrogen adsorption properties of two different high surface area carbon materials of templated carbon (TC) and superactivated carbon (AX-21) doped with three different metals (Ru, Pt, and Ni) have been studied. The equilibrium hydrogen storage capacities followed the order of Ru/C > Pt/C > Ni/C on both TC and AX-21. Ru doped on templated carbon (Ru/TC) showed a hydrogen storage capacity of 1.43 wt % at 298 K and 10.3 MPa. Furthermore, a thermal reduction method was applied to increase the contacts between the Ru metal particles and the carbon support, and in turn, to facilitate hydrogen spillover. The ruthenium-doped templated carbon by thermal reduction (Ru/TC-T) showed the highest hydrogen storage of 1.56 wt % at 298 K and 10.3 MPa, resulting in an enhancement factor of 2 compared with that of the undoped carbon. These experimental results were interpreted by using a simple mechanistic model for hydrogen spillover. 1. Introduction Hydrogen storage is one of the key issues for the realization of fuel-cell powered vehicles using hydrogen as the energy carrier.1 There are currently several candidate hydrogen storage technologies including liquid or high-pressure H2 gas, chemical hydride, metal hydride, and porous adsorbents. However, at the present time none is capable of satisfying the DOE criteria of size, recharge, kinetics, cost, and safety requirements for personal transportation vehicles.2,3 Among these hydrogen storage materials, nanostructured and porous carbon materials, including carbon nanotubes, graphite nanofibers, activated carbon, and graphite, have received considerable research interest due to their light weight, high surface areas, and relative chemical stabilities.4–7 However, it has become clear that carbon materials cannot store a sufficient amount of H2 required for transportation applications merely by physical adsorption at ambient temperature.8,9 An effective way to enhance hydrogen storage on adsorbents materials is by hydrogen dissociation followed by spillover. We have demonstrated recently that the hydrogen storage capacities at 298 K in nanostructured and porous materials including carbon, zeolites, and metal-organic frameworks could be enhanced significantly by exploiting hydrogen spillover.10–16 Hydrogen spillover is defined as the dissociative chemisorption of hydrogen on metal nanoparticles, and subsequent migration of hydrogen atoms onto adjacent surfaces of a receptor via spillover and surface diffusion.17–19 Evidence of atomic hydrogen spillover was first observed indirectly during studies of ethylene hydrogenation via heterogeneous catalysis.20 Voluminous literature on the evidence of atomic hydrogen stillover has been reviewed.21 More recently, direct evidence of spillover of atomic hydrogen has been reported. Inelastic neutron scattering studies have provided evidence of atomic hydrogen spillover from Pt to carbon at room temperature.22,23 Infrared spectroscopic study showed evidence of spillover of atomic hydrogen from Au nanoparticles to TiO2 also at room temperature.24 A number of theoretical studies have been done on the mechanism and energetics of spillover. The most recent DFT study by Chen et * Corresponding author. Fax: (734)764-7453. E-mail: [email protected].

al. illustrated the facile pathways for spillover from a Pt particle onto a graphene basal plane via physisorption of H atoms.25 Hydrogen storage by hydrogen spillover could be achieved through physical mixing of a receptor with a supported metal catalyst or chemical doping of metal onto the receptor.10–31 The physical mixing process was generally influenced by many factors, such as sample amounts, grinding time and intensity, and contacts between the particles, thus leading to poor reproducibility in the sample and storage capacity. Compared with physical mixing, chemical doping produces identical samples and is more reproducible. Several studies have showed enhancements in hydrogen storage capacities by doping transition metal on carbon nanotubes, activated carbons, and carbon nanofibers,26–30 but their reversible hydrogen capacities were less than 1.0 wt % at room temperature and high pressure. Recently, Yang et al. reported a higher hydrogen storage capacity of 1.2 wt % on a Pt doped on AX-21 sample at 298 K and 10 MPa.31 Although encouraging progress has been achieved, further enhancements in hydrogen storage capacity and an understanding of the different factors in hydrogen storage by hydrogen spillover are needed. There are mainly three factors for hydrogen storage enhancement by hydrogen spillover. One is the metal used as hydrogen dissociation source. Doping by different metals such as Pt, Ni, and Pd has been reported, but there has been no direct comparison of different metals on hydrogen storage capacities for the same carbon. The second factor is the different support as the spillover hydrogen receptor. It is expected that a receptor with a higher surface area would provide more adsorption sites for hydrogen atoms. A microporous carbon with a BET surface area of 3200 m2/g was recently claimed to have the highest hydrogen storage capacity at 77 K and 20 bar.32 However, no high surface area carbon (>3500 m2/g) has been applied to hydrogen storage at room temperature, especially for hydrogen spillover. Recent template synthesis developed by Jaroniec and Ryoo provided a route to high surface area materials.33 The third factor and the least understood one is the contact between the source and receptor. Because of the existence of tremendous physical and energy barriers for transfer of hydrogen atoms from one material to another, an intimate contact should facilitate

10.1021/jp803093w CCC: $40.75  2008 American Chemical Society Published on Web 07/22/2008

Carbons Doped with Ru, Pt, and Ni Nanoparticles

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Figure 1. X-ray diffraction patterns of templated carbon TC (A), Ni/ TC (B), Pt/TC (C), and Ru/TC (D).

Figure 2. Nitrogen isotherms of templated carbon TC (]), Ru/TC (O), Pt/TC (µ), and Ni/TC (0).

the spillover and hence enhance the hydrogen storage capacity. Bridge building and plasma assisted doping techniques have been developed in our group to improve the contact between the source and receptor and thus result in a large enhancement in hydrogen storage capacity.12–15,34 To improve hydrogen storage capacities from the three aspects, herein we report the hydrogen adsorption properties of three different metals, Ru, Pt, and Ni, which are doped on two different high surface area carbon materials of templated carbon (TC) and superactivated carbon (AX-21). It was found that the hydrogen adsorption capacities followed the sequence of Ru/C > Pt/C > Ni/C on both templated carbon and superactivated carbon. The metals doped on templated carbon (metal/TC) exhibit higher hydrogen storage capacity than their counterparts doped on superactivated carbon AX-21 (metal/AX-21) due to the higher surface area of the templated carbon. Furthermore, to enhance the contact between the metal and support, we applied an effective thermal reduction method to hydrogen storage by spillover. In our case, the ruthenium doped on ultrahigh surface area templated carbon by thermal reduction (Ru/TC-T) showed the highest hydrogen storage of 1.56 wt % at 298 K and 10.3 MPa, which was enhanced by a factor of 2 compared with that of the plain carbon. These experimental results were interpreted by using our mechanistic model for hydrogen spillover.

Figure 3. TEM images of Ru/TC (A), Pt/TC (B), and Ni/TC (C); scale bar ) 20 nm.

2. Experimental Methods 2.1. Synthesis. Zeolite EMC-2. EMC-2 was synthesized according to an established procedure.35 Typically, a gel with a composition of 1.00SiO2:0.10Al2O3:0.22Na2O:0.087(18-crown6):4.00H2O was aged for 24 h at room temperature, then sealed in a Teflon-lined autoclave at 383 K for 13 days. The product was washed with water and dried overnight and calcined at 873 K for 6 h.

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Figure 4. High-pressure hydrogen isotherms at 298 K for templated carbon TC (]), Ru/TC (O), Pt/TC (4), and Ni/TC (0).

Templated Carbon (TC) from EMC-2. TC derived from zeolite EMC-2 was prepared according to a procedure similar to that proposed by Kyotani and Parmentier.36,37 EMC-2 was vacuumed in a flask for 12 h at 473 K, then furfuryl alcohol (FA) was introduced into the flask at a reduced pressure. The mixture of EMC-2 and FA was stirred overnight in a He flow and then filtered. The polymerization of FA in EMC-2 was carried out by heating the composite under a flow of He at 353 K for 24 h, then at 423 K for 8 h. The EMC-2/PFA composite was placed in a vertical quartz tube and heated to 973 K under a N2 flow. When the temperature reached 973 K, propylene gas (2% in N2 by volume; flow rate 150 cm3/min) was passed through the tube for 15 h. After the CVD treatment, the composite was further heated at 1173 K for 2 h under a flow of N2. The obtained EMC-2/carbon composite was treated in HF solution (40%) for 24 h and subsequently refluxed by concentrated HCl solution for 4 h to dissolve the template of EMC-2. The resulting TC was collected by filtration and washed with distilled water. Six Weight Percent Ru, Pt, and Ni Doped on TC. (1) Ru/ TC: Typically, 200 mg of well-dried TC was dispersed in 20 mL of acetone and was stirred for 0.5 h in a flask at room temperature. Next 5 mL of acetone solution was mixed with 800 mg of Ru(NO)(NO3)x(OH)y aqua (1.5 wt % Ru content), then the mixture was slowly added to the above solution under vigorous agitation. The mixture was subjected to ultrasonication (100 W, 42 kHz) for 1 h and then magnetically agitated at room temperature for 24 h. After being dried in an oven at 333 K overnight, the impregnated TC sample was transferred to a horizontal quartz tube and further dried in a He flow at 393 K for 2 h to remove the residual acetone in the sample. Then the He flow was switched to H2 and the temperature was increased to 573 K and held for 3 h. After being cooled to room temperature in H2, the sample was purged with flowing He and stored under He atmosphere before further measurement. (2) Pt/TC: Pt/TC was prepared by using a similar procedure to that for Ru/TC except that 5 mL of acetone solution containing 26 mg of H2PtCl6 was used as the Pt precursor. (3) Ni/TC: Ni/TC was prepared by using a similar procedure to that for Ru/TC except that 5 mL of acetone solution containing 60 mg of Ni(NO3)2 · 6H2O was used as the Ni precursor. (4) Ru/TCT: Ru/TC-T was prepared by using a similar procedure to that for Ru/TC. The difference was that the sample was treated at 1173 K for 1 h under a N2 atmosphere instead of the treatment at 573 K for 3 h under a H2 atmosphere.

Wang and Yang Six Weight Percent Ru, Pt, and Ni Doped on AX-21. Ru/ AX-21, Pt/AX-21, and Ni/AX-21 were prepared by using a similar procedure to their counterparts doped on TC except that superactivated carbon AX-21 was used as the support instead of TC. The AX-21 sample was supplied by Anderson Development Co. 2.2. Characterization. Powder X-ray diffraction (XRD) data were recorded on a Rigaku Miniflex diffractometer at 30 kV, 15 mA for Cu KR (λ ) 0.1543 nm) radiation, with a scan speed of 2 deg/min and a step size of 0.02° in 2θ. High-resolution transmission electron microscopy (HRTEM) images of the materials were obtained on a JEOL 3011 analytical electron microscope equipped with EDX analysis operated at 300 kV. Nitrogen adsorption and low-pressure H2 adsorption isotherms (0-1 atm) were measured with a standard static volumetric technique (Micromeritics ASAP 2020). Hydrogen adsorption at 298 K and pressures greater than 0.1 MPa and up to 10 MPa was measured by using a static volumetric technique with a specially designed Sievert’s apparatus. The apparatus was previously proven to be leak-free and accurate through calibration by using LaNi5, AX-21, zeolites, and MOFs at 298 K. Approximately 200 mg of sample was used for each highpressure isotherm measurement in this study. Before measurements, the samples were degassed in vacuum at 350 °C for at least 12 h. 3. Results and Discussion 3.1. Hydrogen Storage Properties of Ru/TC, Pt/TC, and Ni/TC. XRD. Powder X-ray diffraction patterns of TC and Ru/ TC, Pt/TC, and Ni/TC samples are shown in Figure 1.The plain TC (Figure 1A) shows a strong peak at 2θ ) 6.3°, indicating that the structural ordering of zeolite template has been replicated in the carbon. For Ru/TC, Pt/TC, and Ni/TC samples, the low-angle XRD patterns all exhibit a similar peak position to that of TC at 2θ ) 6.3° although the peak intensity slightly decreases. This indicates that the microstructure of TC is retained in the metal/TC samples after the doping treatments. Furthermore, the wide-angle XRD patterns show that Ru/TC, Pt/TC, and Ni/TC exhibit the typical peaks assigned to Ru, Pt, and Ni, respectively. Correspondingly, Ni/TC (Figure 1B) exhibits two reflections at 44.6° (111) and 51.9° (200) characteristic of the metallic nickel. Pt/TC (Figure 1C) shows two peaks at 39.8° (111) and 46.3° (200) characteristic of the metallic platinum metal. Ru/TC (Figure 1D) shows a weak peak at around 44° that corresponds to the (101) diffraction of metallic Ru. The particle sizes of Ru, Pt, and Ni calculated from the Scherrer equation are approximately 2, 3, and 5 nm, respectively. These results confirm that these nanosized metals have been successfully doped on TC by applying our doping method. Nitrogen Isotherms. Nitrogen adsorption isotherms are shown in Figure 2.The isotherms of TC and metal/TC samples all exhibit the typical I curve, thus revealing the presence of microporosity in the samples, in agreement with the XRD results. The BET surface area and pore volume of TC are 3839 m2/g and 1.83 cm3/g, respectively. These textural properties are comparable to those of reported zeolite templated carbon with high surface areas.36,37 After doping metals on TC, the BET surface areas and pore volumes of Ru/TC, Pt/TC, and Ni/TC samples are 3004, 3126, 3091 m2/g and 1.49, 1.53, and 1.53 cm3/g, respectively. The BET surface area and pore volume of metal/TC samples are lower than that of plain TC. This should be due to the increased weight and micropore blocking caused by the doped metal particles. However, it is rather encouraging that the surface area was decreased only so slightly after doping

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Figure 5. Low-pressure H2 adsorption isotherms for Ru/TC (A), Pt/TC (B), and Ni/TC (C) at 298 (open symbols) and 323 K (solid symbols); calculated isosteric heats of adsorption of H2 (D) for Ru/TC (O), Pt/TC (4), and Ni/TC(0).

of the nanosized particles, indicating that the majority of the pores remained open after doping. TEM. High-resolution TEM images of the metal/TC samples are shown in Figure 3. For each sample, the nanosized black spots of Ru (1-3 nm), Pt (2-5 nm), and Ni (4-8 nm) particles were well dispersed on the surface of TC, in agreement with the XRD results. These results further confirm the metals have been successfully doped on the carbon support. It may be noted that the particle density on the surface cannot be obtained from the TEM images because the TEM image is a three-dimensional image (as the electron beam penetrates through the carbon substrate) superimposed on a two-dimensional picture. High-Pressure Hydrogen Isotherms. High-pressure hydrogen isotherms at 298 K for plain TC, Ru/TC, Pt/TC, and Ni/TC samples are presented in Figure 4. As shown in Figure 4, TC has a hydrogen storage capacity of 0.8 wt % at 298 K and 10.3 MPa. By doping 6.0 wt % metal on TC, the hydrogen uptakes on Ru/TC, Pt/TC, and Ni/TC at 10 MPa were enhanced to 1.43, 1.33, and 1.12 wt %, respectively. It can be seen that all the metal/TC samples exhibit much higher hydrogen adsorption capacities than the plain TC sample up to 10 MPa. The enhanced hydrogen storage capacity should not be attributed to the differences in surface area because the metal/TC samples have lower surface areas than that of TC, as evident from nitrogen adsorption results. The enhancement of hydrogen storage should be due to the spillover of atomic hydrogen from metal particle to TC. The Ru, Pt, and Ni metals are all effective as a hydrogen dissociation source, and the enhanced hydrogen storage by metal (Ni, Pd, Pt, etc.) doped on carbon materials (CNTs, active

carbon, carbon nanofiber, etc.) has been observed.26–31 In our case, the hydrogen storage capacities follow the order of Ru/ TC > Pt/TC > Ni/TC. The maximum hydrogen uptake reached 1.43 wt % at 10.3 MPa on Ru/TC. In comparison with plain TC, it is remarkable that the hydrogen adsorption amount of Ru/TC has been enhanced by a factor of 1.8. It is noted that metal hydrides do not form on these metals under our conditions. Apparent or OWerall Heats of Adsorption. In this work, the heats of adsorption of H2 on the Ru/TC, Pt/TC, and Ni/TC samples were calculated from the H2 adsorption isotherms at 298 and 323 K by using the Clausius-Clapeyron equation, as shown in Figure 5. The isosteric heats of adsorption were determined by evaluating the slope of the plot of ln(P) versus (1/T) at the same adsorption amount. It can be seen that the H2 adsorption amounts at all pressures up to 1 atm decrease with an increase in temperature. Figure 5D shows that the absolute values of heat of adsorption decrease sharply with adsorption amount for each sample. The heats of adsorption are >20 kJ/ mol at low surface coverage and level off to ∼8-11 kJ/mol at relatively high surface coverage. The high values of heats of adsorption at low surface coverage can be attributed to the strong adsorption of H atoms on the metal particles, as well as the H atoms on the strongest sites on carbon. Recently, it has been reported that atomic hydrogen can be strongly adsorbed at defect sites on carbon materials and some defects can be generated during the catalyst preparation.38–40 In our case, defective sites could be created nearby the metal particles by the H2 reduction procedure at high temperature due to the possible gasification (methanation) of carbon catalyzed by the metal. Previous

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Figure 6. Rates of adsorption at different end pressures on Ru/TC (A), Pt/TC (B), and Ni/TC (C) at 298 K; (D) D/R2 as a function of H2 uptakes at 298 K for Ru/TC (O), Pt/TC (4), and Ni/TC (0).

Figure 7. H2 adsorption isotherms (normalized by BET surface area) of pure Ru (O), Pt (4), and Ni (0) powders at 298 K.

research has proved that group VIII metal particles such as Ru, Pt, and Ni could catalyze the graphite hydrogenation by tunneling and channeling actions.41,42 Energy heterogeneity on surfaces of carbon is well-known, but poorly understood. Besides defect sites, edge sites (i.e., armchair and zigzag edge sites of graphite crystallites) are also strong sites for adsorption. Surface oxygen complexes are still other strong sites.

Figure 8. High-pressure hydrogen isotherms at 298 K for 3 wt % Ru/TC (]), 6 wt % Ru/TC (O), and 8 wt % Pt/TC (4).

It is known that the Clausius-Clapeyron equation yields the overall heats of adsorption. In this case, the heats of adsorption are approximately the overall values of the bonding energies of H2 on carbon, H atoms on metal, and various carbon sites. As a first-order analysis, we take the heat of adsorption at high H2 adsorption amount as an indicator of the adsorption strength of hydrogen atoms on the surface of the TC. As shown in Figure 5D, the heats of adsorption at high H2 adsorption amount on the Ru/TC, Pt/TC, and Ni/TC samples are about 11.1, 10.7, 8.8

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Figure 11. High-pressure hydrogen isotherms at 298 K for AX-21 (]), Ru/AX-21 (O), Pt/AX-21 (4), and Ni/AX-21 (0).

Figure 9. Rates of adsorption at 298 K and 103 atm on 3, 6, and 8 wt % Ru/TC samples (A) and D/R2 as a function of surface Ru particle density for 3, 6, and 8 wt % Ru/TC samples at 298 K and 103 atm (B). Inset in panel B: normalized plot where the units for (D/R2)(1/N) are 10-6 µm2/s.

Figure 10. Nitrogen isotherms of AX-21 (]), Ru/AX-21 (O), Pt/AX21 (4), and Ni/AX-21 (0) at 77 K.

kJ/mol, respectively. The relative higher heat of adsorption on Ru/TC than Pt/TC and Ni/TC suggests more H atoms favorably bonded to Ru/TC, in agreement with high-pressure hydrogen adsorption results. It is worthy noticing that the heats of adsorption on Ru/TC, Pt/TC, and Ni/TC samples are all higher than that of hydrogen physisorption on activated carbon,

Figure 12. High-pressure hydrogen isotherms at 298 K for thermally reduced Ru/TC-T: adsorption (]) and desorption (4).

indicating the strong interactions between the spiltover H and TC. For comparison, the heats of adsorption for H2 molecules on carbons are ∼5 kJ/mol, and are, for H2 on boron-substituted carbon, ranging from 12.47 kJ/mol at low coverage to ∼11 kJ/ mol at high coverages.43 While in physisorption, hydrogen molecules are adsorbed on carbon by van der Waals and electrostatic interactions, the heats of adsorption for the spillover system are overall values for a complex process involving a number of steps. Adsorption Kinetics and Hydrogen Adsorption Model. Figure 6 shows the adsorption kinetics of hydrogen on the Ru/TC, Pt/ TC, and Ni/TC samples at various end pressures (5-100 atm) and 298 K. As shown in Figure 6A-C, the adsorption rates on the three samples all decrease steadily with increasing pressure. In comparison with the adsorption kinetics on our previous IRMOF-8,16 the adsorption rates on Ru/TC, Pt/TC, and Ni/TC samples at similar pressures are much faster. The slower adsorption rates on the bridged IRMOF-8 can be attributed to the slow diffusion of hydrogen atoms on its surface because of the much stronger bonding energies on IRMOF-8. As suggested in our previous discussions on IRMOF-8, the kinetics of hydrogen adsorption via hydrogen spillover is limited by the surface diffusion step and it could be characterized by a surface diffusivity (D) and diffusion time constant (D/R2, where R is the characteristic radius for diffusion from each metal particle).

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The approximate solution to the diffusion equation for small times, or when Mt/M∞ < 0.3, is

( )

Mt 4 Dt ≈ M∞ √π R2

1⁄2

(1)

where Mt and M∞ represent the total adsorbed amounts of spiltover hydrogen at time t and at equilibrium, respectively.44 The D/R2 values for the adsorption on the surface of Ru/TC, Pt/TC, and Ni/TC at different pressures calculated from eq 1 are plotted as a function of hydrogen adsorption capacity or surface concentration. It can be clearly observed that the diffusion time constant, D/R2, drops sharply at higher loadings of hydrogen (at higher pressures) and becomes nearly constant at >0.4 wt % hydrogen capacity (Figure 6D). Similar trends were also observed in our previous studies on IRMOF-8 and Pt/AX-21. This result indicates that the diffusion distance R is increasing with surface concentration. The concentration dependence for surface diffusion has been discussed in detail and explained satisfactorily by models proposed by Higashi et al. and Yang et al.45–47 Surface diffusion is the result of a hopping process for both physical adsorption and chemisorption of small molecules or atoms. Within the monolayer coverage, the surface diffusivity usually increases with surface concentration. In addition, the D/R2 value of Ru/TC is slightly higher than that of Pt/TC and Ni/TC. This should be due to a smaller diffusion distance, R, on the Ru/TC sample, as evident from the TEM results and a relatively higher Ru dispersion on Ru/TC. The definition of R will be discussed in the following hydrogen adsorption model. In our previous work, a mechanistic model for the equilibrium isotherm for the spillover system has been proposed for bridged IRMOF-8. The bridged IRMOF-8 was prepared by doping a commercial Pt/AC catalyst (5 wt % Pt supported on activated carbon) on the IRMOF-8 particles through a carbon bridge building technique. In that work, the model involved a Pt center doped on a primary receptor area of activated carbon (AC) with radius R1, and a secondary receptor of IRMOF-8 with radius R2. For the metal-doped carbon system, a similar but simpler model could be formulated for the equilibrium isotherm. For the metal-doped carbon, we assume for each particle an average area of radius R is reachable for the spiltover hydrogen. The Pt particle is covered with hydrogen atoms in equilibrium with gaseous molecular hydrogen. The surface concentration of H atoms on Pt (CPt) depends only on H2 pressure at a constant temperature (298 K). Let us assume that there is an equilibrium constant K1 that relates the surface concentration of H on Pt to that on the carbon support (CAC):

K1 )

CAC CPt

(2)

The total amount adsorbed, q, is given by:

q ) qPt + πR2K1qPt

(3)

where R is the reachable radius for spillover from each metal particle. K1 is a partition coefficient that depends on the surface chemistry of the carbon and the metal, and is measurable in principle. The equilibrium adsorption amount of H atoms on the Pt particle, qPt, was given by Langmuir for dissociative chemisorption:48

qPt )

k1√PH2 1 + k2√PH2

(4)

Thus, the equilibrium isotherm for the metal-doped carbon system is

q)

k3√PH2 1 + k2√PH2

(5)

where k3 ) 1 + πk1K1R2. It is seen from eq 3 that the total hydrogen adsorption amount on metal/TC is proportional to qm, the hydrogen adsorption amount on metal; thus, hydrogen adsorption on different metals was further investigated. Figure 7 shows the hydrogen isotherms (normalized by BET surface area) of pure Ru, Pt, and Ni powders. It is seen that the hydrogen adsorption amount per unit surface area of metal powder follows the order Ru > Pt > Ni. Hydrogen adsorption on different metals has been studied extensively. Kubicka investigated the hydrogen adsorption on Ru, Pt, and Re powders and found a higher adsorption amount on Ru than Pt and Re.49 It is noted that the calculated H/M ratios are 1.3, 1.2, and 1.0 for Ru, Pt, and Ni, respectively. The coverage in excess of 1 has been reported by many authors. Kaldor and Cox found the hydrogen can adsorb on various unsupported metals in amounts much exceeding the unity stoichiometry.50 Dalla Betta found the H/Ru ratio of 1.27 and Kubicka observed that H/Ru ) 1.5 on ruthenium powder;49,51 Tsuchiya et al.52 and Vannice et al.53 found H/Pt > 1 on unsupported platinum. On the basis of the hydrogen adsorption ability of Ru, Pt, and Ni metals and inferred from eq 5, the total hydrogen adsorption amounts on metal/TC would follow the sequence of Ru/TC > Pt/TC > Ni/TC. The proposed mechanistic model fits well with the experimental results. Hydrogen Adsorption on Ru/TC with Different Ru Loadings. Hydrogen storage capacities of Ru/TC samples with different Ru loadings (3, 6, and 8 wt %) were compared, as shown in Figure 8. The hydrogen storage capacities follow the order of 6 wt % Ru/TC > 8 wt % Ru/TC > 3 wt % Ru/TC. Nitrogen isotherms indicate that BET surface area of the 8 wt % Ru/TC sample (2723 m2/g) is less than that of the 6 wt % Ru/TC sample (3004 m2/g). The 8 wt % Ru/TC sample with a lower surface area provides fewer hydrogen adsorption sites and hence leads to the reduced storage capacity compared with 6 wt % Ru/TC, while for 3 wt % Ru/TC the lower amount of Ru metals as ahydrogen dissociation source should be mainly responsible for the reduced storage capacity. Figure 9 shows the adsorption kinetics of hydrogen on the 3, 6, and 8 wt % Ru/TC samples at 103 atm and 298 K. As shown in panels A and B of Figure 9, the adsorption rates on the three samples increased with increasing loading amount. The D/R2 values for the adsorption on the surface of Ru/TC samples at 103 atm calculated from eq 1 are plotted as a function of surface metal particle density (N). The surface metal particle density for the 3, 6, and 8 wt % Ru/TC were 3.32 × 103/µm2, 3.82 × 103/µm2, and 4.30 × 103/µm2 (or at ratios of 1:1.15:1.30), based on the hydrogen chemisorption results and the assumption of cubic particles. As shown in Figure 9B, the D/R2 values increased with surface metal particle density. For the 3, 6, and 8 wt % Ru/TC samples, D should be a constant due to the same metal and TC surface, thus D/R2 should depend on surface diffusion distance R that is related to the total surface metal particle density on the carbon surface. As discussed in the hydrogen storage model, the value of R should be the radius of the area that is equal to the total reachable surface area of the carbon divided by the number of Ru particles. Thus R depends on the density of metal particles on the carbon surface. When normalized by 1/N, the plot of (D/R2)(1/N) vs N (Figure 9B inset) is indeed flat or horizontal, as expected and explained above.

Carbons Doped with Ru, Pt, and Ni Nanoparticles From data in Figure 9B, one may obtain values for R, the average diffusion distance, which would range from 0.008 to 0.01 µm. Thus, the values for D are of the order of 2 × 10-16 cm2/s, which is in fair agreement with those estimated for hydrogen on carbon thin films.54,55 3.2. Hydrogen Storage Properties of Ru/AX-21, Pt/AX21, and Ni/AX-21. We also compared the hydrogen storage capacities of metal doped on large surface area superactivated carbon AX-21 as for consideration of hydrogen spillover receptor effect. Figure 10 shows the nitrogen isotherms of AX21 and metal/AX-21 samples at 77 K. All samples exhibit a rapid increase at the low relative pressure (P/P0 < 0.02), indicating the presence of micropores in the samples. The BET surface area and pore volume of AX-21 are 2850 m2/g and 1.40 cm3/g. The BET surface area and pore volume of Ru/AX-21, Pt/AX-21, and Ni/AX-21 are 2538, 2545, 2570 m2/g and 1.26, 1.28, 1.30 cm3/g, respectively. It is noted that the BET surface areas of metal/AX-21 samples were slightly reduced from 2850 to about 2500 m2/g, while the BET surface areas of metal/TC samples were more reduced from 3839 to about 3000 m2/g. This was due to the smaller and narrowly distributed micropore size of TC (1 nm) than that of AX-21 (1-2 nm), based on DFT analysis. The small and narrowly distributed micropores could be easily blocked by the metal particles. However, the BET surface areas of metal/TC samples are still higher than those of metal/AX-21. High-pressure hydrogen isotherms at 298 K for AX-21, Ru/ AX-21, Pt/AX-21, and Ni/AX-21 are presented in Figure 11. As shown in Figure 11, AX-21 has a hydrogen storage capacity of 0.6 wt % at 298 K and 10 MPa. This value is in agreement with literature data.56,57 By doping 6.0 wt % metal on AX-21, the hydrogen uptakes on Ru/AX-21, Pt/AX-21, and Ni/AX-21 at 10 MPa were enhanced to 1.30, 1.22, and 1.02 wt %, respectively. It can be seen that all the metal/AX-21 samples exhibit much higher hydrogen adsorption capacities than the plain AX-21 sample up to 10 MPa. The enhancement of hydrogen storage was due to spillover of atomic hydrogen from metal particles to AX-21. It is noted that the hydrogen storage capacities follow the same order as that of metal/TC (Ru/AX21 > Pt/AX-21 > Ni/AX-21), but these metal/AX-21 samples exhibit lower hydrogen storage capacities than their corresponding metal/TC samples. This should be due to the larger surface area of TC than AX-21. The larger surface area of receptor would provide more hydrogen adsorption sites and thus lead to the higher capacity. To further enhance the hydrogen storage capacity, efforts are being taken to avoid the significant reduction of the surface area of metal/TC. 3.3. Hydrogen Storage Properties of Ru/TC-T. In hydrogen spillover systems, intimate contact between the source and the receptor is one of the key factors for facilitating spillover. Bridge building and plasma treatment techniques have been developed to enhance spillover through improved contacts between the source and the receptor.12–15,34 Recently, a thermal reduction method was applied to prepare Ru catalysts for hydrogenation of monoaromatics, and it was suggested that intimate Ru-support contacts improved the catalytic activities.58 In our work, we consider thermal reduction as a possible means for facilitating hydrogen spillover and hence enhancing hydrogen storage. Ru/TC prepared by hydrogen reduction had the highest hydrogen storage capacity among aforementioned catalysts. Thus we prepared ruthenium doped on templated carbon (Ru/TC-T) by thermal reduction and compared the hydrogen storage capacity of Ru/TC-T with that of Ru/TC prepared by hydrogen reduction. The hydrogen isotherm at 298 K for Ru/TC-T is

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12493 presented in Figure 12. As shown in Figure 12, Ru/TC-T had a hydrogen storage capacity of 1.56 wt % at 298 K and 10.3 MPa. This value was higher than that of Ru/TC (1.43 wt %). During the thermal reduction process, it is suggested that multiple bonds could be formed between the Ru metal surface and carbon support.59,60 The formed Ru-carbon contacts (Ru carbene complexes) would facilitate the spiltover hydrogen transfer. To verify the assumption, the Ru/TC-T sample was calcined in air at 573 K for 10 min then reduced by hydrogen at 573 K for 2 h. The hydrogen storage capacity of the treated Ru/TC-T was reduced to less than 1.0 wt % at 10 MPa and 298 K. Calcination in air would remove the Ru-carbon contacts thus leading to the decreased storage capacity. Boudart et al. also reported reduced hydrogen adsorption capacity on a platinized carbon after the carbon contaminant on the platinum surface was burned off.61 These results indicate that the enhanced hydrogen storage capacity in Ru/TC-T was related to the intimate contacts between the Ru particles and the carbon support. Furthermore, hydrogen desorption down to 1 atm was measured on the Ru/TC-T sample. It was seen from Figure 12 that the desorption branch nearly followed the adsorption branch, although a slight hysteresis was present. The sample was then evacuated to a pressure of 1 Pa for 12 h at 298 K and total desorption occurred. The second adsorption isotherm was in agreement with the first adsorption isotherm, indicating reversible adsorption. 4. Conclusions In this study, we have investigated hydrogen storage properties of Ru, Pt, and Ni doped on two high surface area carbon materials. The hydrogen storage capacities follow the orders of Ru/C > Pt/C > Ni/C on both templated carbon (TC) and superactivated carbon (AX-21). For hydrogen spillover, Ru is a more effective hydrogen dissociation source, and the ultrahigh surface area templated carbon is an ideal receptor. Ru/TC showed a higher hydrogen storage capacity of 1.43 wt % at 298 K and 10.3 MPa. These results are in agreement with a simple model for the spillover on metal-doped carbons. Furthermore, a direct high-temperature thermal reduction method was applied to increase the contacts between the Ru metal and the carbon support. The resulting Ru/TC-T sample showed a further increased hydrogen storage capacity of 1.56 wt % at 298 K and 10.3 MPa. Acknowledgment. The authors acknowledge the funding provided by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy within the Hydrogen Sorption Center of Excellence (HS CoE). References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (2) Dillon, A. C.; Heben, M. J. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133. (3) Zu¨ttel, A. Mater. Today 2003, 6, 24–33. (4) Dillon, A. C.; Johns, K. M.; Bekkedahl, T. A.; Klang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (5) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. J. Phys. Chem. B 1998, 102, 4253. (6) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (7) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 2307. (8) Yang, R. T. Carbon 2000, 38, 623. (9) Shiraishi, M.; Takenobu, T.; Kataura, H.; Ata, M. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 947. (10) Lueking, A.; Yang, R. T. J. Catal. 2002, 206, 165. (11) Lueking, A.; Yang, R. T. Appl. Catal., A 2004, 265, 259. (12) Lachawiec, A. J.; Qi, G. S.; Yang, R. T. Langmuir 2005, 21, 11418.

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