Hydrogen Storage on Platinum Nanoparticles Doped on

11086

J. Phys. Chem. C 2007, 111, 11086-11094

Hydrogen Storage on Platinum Nanoparticles Doped on Superactivated Carbon Yingwei Li and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: April 12, 2007; In Final Form: May 20, 2007

The equilibrium and kinetics of hydrogen storage on Pt nanoparticles doped on AX-21 superactivated carbon were studied. The Pt/AX-21 sample was prepared by ultrasound-assisted impregnation of H2PtCl6 in a solution of acetone. The dispersion of platinum was determined by CO/H2 chemisorption and high-resolution transmission electron microscopy (HRTEM). Hydrogen adsorption isotherms and kinetics were measured at 298-348 K and up to 10 MPa. HRTEM images showed that Pt was well dispersed with a uniform particle size ∼2 nm. CO chemisorption results indicated that the dispersion was 58%. By doping 5.6 wt % Pt, the hydrogen storage capacity of AX-21 was enhanced to 1.2 wt % at 298 K and 10 MPa. Furthermore, the isotherm was totally reversible and rechargeable at 298 K. The overall isosteric heats of adsorption (-23 to -15 kJ/mol) and the apparent activation energy for surface diffusion (7.6 kJ/mol) were determined from the temperature dependence of, respectively, equilibrium isotherm and diffusion time constant. Diffusion of the spiltover H atoms on carbon surface at room temperature was a slow process, which resulted in slow hydrogen uptake and desorption rates especially at higher pressures (higher surface concentrations). Desorption appeared to follow a reverse spillover process. The simple mechanistic model that we reported previously was capable of interpreting all experimental data in this study.

1. Introduction With increasing concerns on environmental pollution and energy crisis worldwide from the current use of fossil fuels, hydrogen has been proposed as an alternative fuel because it is clean and easily produced.1 The possible utilization of hydrogen as an energy carrier for fuel-cell-powered vehicles requires an effective hydrogen storage method. Currently, there are several candidate storage technologies including gas compression, liquefaction, metal hydride, chemical hydride, and adsorbent material. 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 Since the first claim of hydrogen storage properties in carbon nanotubes by Dillon et al.3 and the development of new carbon materials with high surface areas, hydrogen storage in carbon nanostructures has attracted considerable attention in recent years. Very high hydrogen adsorption capacities have been reported in various carbon nanostructures.4,5 However, the large amounts of hydrogen stored in carbon materials were still disputed because of the difficulties in obtaining reproducible adsorption capacities in different laboratories.1 On the basis of numerous theoretical studies and careful experimental validation, recent reports have claimed that at ambient temperature the hydrogen storage capacities in known carbon nanostructures are all well below 1 wt %.6,7 This discrepancy in the hydrogen storage abilities of carbon nanostructures is considered to be due to difficulties in accurate measurements, impurities in the carbon samples and in the hydrogen gas, and poor understanding of hydrogen sorption mechanism.8-10 Recently, we have demonstrated that hydrogen storage by hydrogen spillover is a promising approach to enhancing the hydrogen storage capacities in nanostructured materials includ* Corresponding author. E-mail address: [email protected]. Fax: (734) 764-7453.

ing carbon nanostructures, zeolites, and metal-organic frameworks.11-16 In the storage system by spillover, hydrogen molecules are initially dissociated on metal nanoparticles, and subsequently hydrogen atoms migrate onto the nearby surface of the receptor by spillover and surface diffusion.17 Although the detailed mechanism of atomic hydrogen sorption on the surface of the receptors is yet unclear, it is understood that the interactions between the spiltover hydrogen atoms and the surface sites on the receptors are much stronger than that in the physical adsorption system, where hydrogen molecules are adsorbed by van der Waals interactions.18-20 Consequently, the hydrogen adsorption capacities in nanostructured materials can be increased substantially by hydrogen spillover. There are currently two ways to introduce metal particles that are capable of dissociating hydrogen molecules onto receptors for hydrogen storage by spillover. One is through physical mixing of the receptor material and a supported metal catalyst (such as Pd-doped active carbon).11-16,21 In this case, the initial migration of hydrogen atoms from the metal Pd particles to the active carbon support is primary spillover. Subsequent transport of atomic hydrogen from the carbon support to the receptor can be considered as secondary spillover.12,22 Secondary spillover requires intimate contacts between the two unlike materials because there exist tremendous physical/energy barriers for transfer of hydrogen atoms from one material to another. It was found that creating “bridges” between these two materials was helpful for improving the contacts and hence facilitating secondary spillover.13,15,16,23 However, the physical mixing process involves many factors, such as sample amounts, mortar size, grinding time and intensity, etc., that will affect the particle sizes and the contacts between particles. This would result in different connectivities between the particles, leading to poor reproducibility in sample and storage capacity. Another way to introduce metal onto the receptor material is by chemical-doping method. Chemical doping has been developed extensively,

10.1021/jp072867q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007

H2 Storage on Pt Nanoparticles Doped on AX-21 particularly in catalysis. Contrary to physical mixing, chemical doping produces identical samples and is more reproducible. There are several reports that show the enhancements in hydrogen storage capacities by doping transition metal (e.g., Ni, Pd) in carbon nanostructures (such as CNTs, active carbon, carbon nanofibers, etc.).24-28 However, the reversible hydrogen capacities are all below 1.0 wt % at high pressure and room temperature. Kim et al. reported a 2.8 wt % hydrogen capacity on Ni nanoparticle-dispersed multiwalled carbon nanotubes at a moderate temperature. But they reported that the hydrogen adsorbed on the material could only be released at temperatures higher than 340 K (340-520 K).20 In this paper, we selected Pt as the metal for hydrogen dissociation and developed an effective doping method that enabled nanosized Pt to be highly dispersed on a superactivated carbon (AX-21), a commercially available material. Low- and high-pressure hydrogen isotherms and rates of adsorption/ desorption were measured at different temperatures and pressures. The overall heats of adsorption of hydrogen on carbon and apparent activation energy for surface diffusion of the spiltover hydrogen were calculated. By doping ∼6 wt % Pt, the reversible and rechargeable storage capacity of AX-21 was enhanced by a factor of 2, to ∼1.2 wt % at 298 K and 10 MPa. Furthermore, the technique is simple, and the storage measurement is reproducible. The experimental results are interpreted satisfactorily by using our mechanistic model by hydrogen spillover. 2. Experimental Methods 2.1. Synthesis. The AX-21 superactivated carbon was obtained from Anderson Development Company. Because AX21 adsorbs a large amount of moisture from the ambient air, it was fully dried by degassing in vacuum at 393 K for 12 h before doping. Typically, 200 mg of well-dried AX-21 carbon was dispersed in 20 mL of acetone and was stirred for 0.5 h in a 125 mL Erlenmeyer flask at room temperature. A 2 mL acetone solution containing 26 mg H2PtCl6 (Aldrich, 99.9%) was slowly added dropwise to the above solution under vigorous agitation for about 10 min. The Erlenmeyer flask containing the slurry was subjected to ultrasounication (100 W, 42 kHz) at room temperature for 1 h and was then magnetically agitated at room temperature for 24 h. After being dried in an oven at 333 K overnight to evaporate most of the acetone solvent, the impregnated carbon sample was transferred to a quartz boat, which was slid into a horizontal quartz tube. The sample was further dried in a He flow at 393 K for 2 h to remove the residual acetone and also the moisture adsorbed on the sample. Then the He flow was switched to H2 and the temperature was increased to 573 K at a heating rate of 1 K/min and held for 2 h. After slowly cooling to room temperature in H2, the sample was purged with flowing He and was stored under He atmosphere before further measurement. The main phenomena responsible for ultrasound actions are cavitation and acoustic streaming, the effects of which on adsorption and desorption from solution have been studied.29 When applied to impregnation of metal particles, ultrasound tends to create finer particles and better dispersion, although this phenomenon is not understood. It might also create more intimate contacts between the particles and the substrate. This subject is under further study in our laboratory. 2.2. Characterization. Powder X-ray diffraction (XRD) data was 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°/min and a step size of 0.02° in 2θ. The mean crystallite

J. Phys. Chem. C, Vol. 111, No. 29, 2007 11087 TABLE 1: Surface Areas, Pore Volumes, and Pore Diameters of AX-21 and Pt/AX-21 sample

BET SA (m2/g)

Langmuir SA (m2/g)

pore volume (cm3/g)a

median pore diameter (Å)a

AX-21 Pt/AX-21

2880 2518

4032 3678

1.27 1.22

20.3 13.8

a

From H-K analysis.

size of Pt particles was calculated from the Scherrer equation: D ) 0.89 λ/(B cos(θ)). The composition data of samples were acquired through scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis. High-resolution transmission electron microscopy (HRTEM) images of the materials were obtained on a JEOL 2010F analytical electron microscope equipped with EDX analysis operated at 200 kV. Brunauer-Emmett-Teller (BET) surface area and pore volume were measured on a Micromeritics ASAP 2010 sorptometer using nitrogen adsorption at 77 K. CO chemisorption were measured at 308 K on a Micromeritics ASAP 2020 instrument. The sample was degassed in vacuum at 623 K (350 °C) for at least 12 h prior to the gas adsorption measurements. 2.3. Hydrogen Isotherm Measurements. Low-pressure H2 adsorption isotherms (0-1 atm) were measured with a standard static volumetric technique (using Micromeritics ASAP 2020). Hydrogen adsorption at 298 K and pressures greater than 0.1 MPa and up to 10 MPa were measured using a static volumetric technique with a specially designed Sievert’s apparatus. The apparatus was previously tested to prove to be leakfree and proven for accuracy through calibration by using LaNi5, AX-21, zeolites, and IRMOFs at 298 K. All isotherms matched the known values. Approximately 200-300 mg of sample was used for each high-pressure isotherm measurement in this study. Before measurements, the samples were degassed in vacuum at 623 K (350 °C) for at least 12 h. 3. Results and Discussion 3.1. Characterization of Carbon Nanostructures. The BET surface areas, pore volumes, and median pore diameters of pure AX-21 and the Pt-doped AX-21 sample are shown in Table 1. The superactivated carbon had a BET surface area of 2880 m2/g and a total pore volume of 1.27 cm3/g. It can be seen that the BET surface area and pore volume decreased slightly upon doping a small amount of Pt. The decrease in surface area and pore volume can be attributed to blocking or filling of the micropores and mesopores of AX-21 by Pt particles. EDX analysis showed that the content of Pt doped on the AX-21 carbon was about 5.6 wt %, in agreement with the stoichiometry in the synthesis. HRTEM images of the Pt/AX-21 sample are shown in Figure 1. It can be seen that the Pt nanoparticles were highly dispersed on the surface of the superactivated carbon with rather uniform sizes around 2 nm. As shown in Figure 1d, at a higher resolution the black spots of Pt particles were distributed widely over the carbon surface, and the microstructures of the carbon began to emerge. Powder XRD patterns for pure AX-21 and the Pt/AX-21 sample are shown in Figure 2. The XRD pattern of the superactivated carbon showed an amorphous structure. The XRD pattern of the Pt-doped AX-21 sample showed two large reflections at 2θ ) 39.8° (111) and 46.3° (200), characteristic of the cubic platinum metal structure [JCPDS 4-802].30 The platinum metal reflections are broad, which indicates that the presence of small platinum crystals is enough to affect the XRD

11088 J. Phys. Chem. C, Vol. 111, No. 29, 2007

Li and Yang

Figure 3. Equilibrium isotherms of CO on pure AX-21 (2) and Pt/ AX-21 ()) at 308 K.

Figure 1. HRTEM images of the Pt/AX-21 sample.

Figure 4. Equilibrium isotherms of H2 on pure AX-21 (9) and Pt/ AX-21 ()) at 298 K.

Figure 2. X-ray diffraction patterns of pure AX-21, and the Pt/AX21 sample.

signal. It is worthy to note that in the XRD pattern of the Pt/ AX-21 sample, no platinum oxide (PtO) phase was detected (the three strongest peaks from PtO should appear at 2θ ) 34.8, 42.5, and 54.9°).30 This indicated that the sample was fully reduced, and a short expose of the sample to air did not oxidize the Pt metal significantly, or the pretreatment (evacuation at 350 °C for 12 h) was able to reduce any oxides on the Pt nanoparticles because of the reducing surfaces of the carbon substrate. The mean crystallite size of the cubic phase was calculated to be 3.0 nm by using the Scherrer equation (compared to 2 nm from TEM). The calculated size from broadening of the XRD peaks can be larger than that observed from the TEM images. This can be attributed to the main problem with the conventional wide-angle XRD measurements.31 As the grain size decreases, especially below ∼3 nm, the peak intensities decrease relative to the background, giving rise to broad and poorly resolved peaks, as can be seen in Figure 2. This makes it difficult to get a precise value for the

full peak width at half-maximum used in the Scherrer equation. Thus, it was suggested that wide-angle XRD would be more suitable for assessing the platinum particles dispersed in a carbon matrix with an average diameter greater than 3 nm.31 The dispersion of platinum of the Pt/AX-21 sample was determined by using static volumetric CO and H2 chemisorption methods. The amounts of chemisorbed CO or H2 on the samples were obtained by the isotherm extrapolation method introduced by Benson and Boudart in which the isotherm from low pressures is extrapolated to zero pressure to determine the monolayer surface coverage of the sample.32,33 The CO adsorption isotherms on pure AX-21 and the Pt/AX-21 sample at 308 K are shown in Figure 3. The amount of chemisorbed CO on pure AX-21 was zero, indicating that the adsorption of CO on AX-21 is physical adsorption. The obtained CO chemisorbed amount at zero pressure on the Pt/AX-21 sample was about 2.6 cm3/g. With the assumption of 0.7 CO molecule per surface Pt atom,34,35 the dispersion of Pt on AX-21 was calculated to be ∼58%. This indicated a high dispersion of Pt on AX-21 by using our doping technique. The low-pressure H2 isotherms on pure AX-21 and Pt/AX21 at 298 K are presented in Figure 4. The adsorption of H2 on pure AX-21 is also by physical adsorption because the adsorption capacity at zero pressure by extrapolating the isotherm was

H2 Storage on Pt Nanoparticles Doped on AX-21

Figure 5. High-pressure hydrogen isotherms at 298 K for pure AX21 (9), and the Pt/AX-21 sample: adsorption (O), and desorption (2).

0 wt %. The chemisorption amount at zero pressure on Pt/AX21 was ∼0.015 wt %. With the assumption of an adsorption of one H atom per surface Pt,34 the dispersion of Pt on AX-21 was calculated to be ∼52%. Dawody et al. investigated the platinum dispersion on Pt/BaO/Al2O3 catalysts by using various techniques, such as static volumetric CO and H 2 chemisorption, dynamic CO chemisorption, dissociation of N2O, and TEM.33 They concluded that CO and H2 chemisorption was reliable for determining the dispersion of Pt. In addition, the dispersion obtained by H2 chemisorption was slightly lower than that by CO chemisorption. Our results are in good agreement with theirs. It has been suggested that TEM could be unsuitable for determining the dispersion of metal because there might exist small particles that are difficult to be detected by TEM as well as due to overlap of metal particles and support.33 It is worth noting that the slope of the H2 isotherm of the Pt/AX-21 sample was much greater than that of pure AX-21. However, the CO adsorption isotherms of pure AX-21 and the Pt/AX-21 sample were almost parallel at pressures > 10 Torr, as shown in Figure 3. The large enhancement of H2 adsorption on AX-21 is attributed to hydrogen spillover, that is, hydrogen atoms migrated from Pt metal particles to carbon.22 Because of the spillover phenomenon, H2 adsorption method alone would not be reliable for determining the dispersion of metal on supported catalysts.32 Our results showed that CO did not dissociate on Pt particles or spillover. Therefore, it would be most suitable for measuring the dispersion of Pt on supported catalysts, although a combination of different methods was suggested to obtain the dispersion of metals.34,36 3.2. High-Pressure Hydrogen Isotherms at 298 K. Highpressure hydrogen isotherms at 298 K for pure AX-21 and Pt/ AX-21 are presented in Figure 5. As shown in Figure 5, AX21 had a hydrogen storage capacity of ∼0.6 wt % at 298 K and 10 MPa. This value agreed with the reported data on the H2 uptakes on AX-21 under the same conditions.37-39 Furthermore, repeated H2 adsorption measurements on our high-pressure H2 adsorption system yielded the same value. This indicates that our apparatus and measurement procedure are highly accurate and reproducible. By doping 5.6 wt % Pt on AX-21, the hydrogen uptakes have been significantly enhanced at all pressures, as shown in Figure 5. The maximum hydrogen storage capacity was ∼1.2 wt % at 10 MPa. In comparison with pure AX-21, it is remarkable that the hydrogen adsorption amount of AX-21

J. Phys. Chem. C, Vol. 111, No. 29, 2007 11089 carbon has been enhanced by a factor of 2. This significant enhancement cannot be attributed to the differences in the surface area and pore volume because both the surface area and pore volume of the Pt/AX-21 sample were lower than that of pure AX-21 (Table 1). Hydrogen adsorption on Pt metal could not be a reason for the enhancement. Even assuming 100% dispersion of Pt on AX-21 and one H atom per Pt, the hydrogen adsorption amount on 6 wt % Pt in the doped sample amounted to only 0.03 wt %. Furthermore, if the individual contributions of 6 wt % Pt metal and the AX-21 support (94% in the doped sample) were considered additive, the expected hydrogen uptake of the Pt/AX-21 sample would be slightly lower than 0.6 wt %, that is, the storage capacity of pure AX-21. Therefore, it is apparent that H2 adsorption on Pt metal cannot account for the large enhancement by metal doping. This large enhancement was clear evidence of spillover of atomic hydrogen from the Pt nanoparticles to the AX-21 receptor.12 The enhancements in hydrogen adsorption capacity by metal doping in carbon nanostructures have also been reported elsewhere.24-28 Zielinski et al. found that the capacity of activated carbon (0.1%) could be significantly enhanced to 0.53% at 30 bar and room temperature by doping 1 wt % nickel.27 However, Anson et al. observed a very small contribution of spillover effect by metal doping to the total hydrogen capacity at high pressure and room temperature.25 That could be attributed to the high loading of metal (from 13 to 50 wt %) on carbon structures, which led to low surface areas of materials and also the metal particles. It was shown that the surface area of MAXSORB activated carbon (similar to AX-21) was reduced from 2112 to 199 m2/g with the impregnation of ∼49 wt % Pd. At the same time, the particle sizes of Pd were around 42 nm.25 In our work, the high dispersion of the Pt nanoparticles on the AX-21 carbon by using ultrasound led to the high storage capacity. Highly dispersed Pt will have a large metal surface area that enables the maximum contacts with the carbon structures and also with hydrogen molecules. Therefore, it could facilitate the dissociation of hydrogen and the diffusion of atomic hydrogen on the carbon surface. Reversibility was evaluated by measuring the desorption branch down to 1 atm. It can be seen from Figure 5 that the desorption branch nearly followed the adsorption branch, although there appeared to be a slight hysteresis. Zielinski et al. also observed that the hydrogen adsorbed on Ni-doped activated carbon desorbed easily at room temperature.27 The sample was then evacuated to a pressure of 1 Pa (7.5 × 10-3 Torr) for 12 h at 298 K and total desorption occurred. The second adsorption isotherm was in complete agreement with the first adsorption isotherm. These results showed that hydrogen adsorption in the Pt/AX-21 sample was fully reversible. 3.3. Apparent Heats of Adsorption. The isosteric heat of adsorption can be calculated from the adsorption isotherms for a particular gas-adsorbent system at different temperatures by using the Clausius-Clapeyron equation:

∆Hads ) R

( ) d ln P d(1/T)

(1)

n

The low-pressure hydrogen adsorption isotherms on the Pt/ AX-21 sample at 298, 323, and 348 K are shown in Figure 6. It can be seen that the H2 adsorption amounts at all pressures up to 1 atm decreased with an increase in temperature. 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. Such plots are presented in Figure 7. The heats of


Figure 6. Low-pressure H2 equilibrium adsorption isotherms on the Pt/AX-21 sample at 298 ()), 323 (9), and 348 K (4).

Figure 7. Plots of ln(P) vs T-1 at different H2 adsorption amounts at low pressures (60 kJ/mol). That is because we calculated the heats of adsorption at H2 adsorption amounts > 0.02 wt %, which were far beyond the capacity for H-Pt anneal, as shown in Figure 6. The absolute value decreased sharply with adsorption amount (or surface coverage) but became leveled off at relatively high-surface coverage. On the basis of assuming that the hydrogen atoms first adsorbed on the sites close to the Pt particles, the much higher absolute values of heats of adsorption implied the presence of local sites around Pt particles where H atoms are preferentially adsorbed more strongly than at the sites away from the Pt centers. Recently, it has been reported that atomic hydrogen can be strongly adsorbed at defect sites on carbon materials.40-42 Yoo et al. studied the atomic hydrogen storage in Pd-modified carbon nanotubes (CNTs) at 1 atm and 573 K.42 The defect sites on CNTs as adsorption sites of atomic hydrogen were prepared by oxidation pretreatment using a La catalyst at 873 K. In their best case, 1.0 wt % hydrogen was stored in the defective CNT modified with Pd at 1 atm and 573 K. In our study, the use of ultrasound for doping Pt could possibly have created some defect sites on AX-21 around Pt particles because of the high energy of ultrasound. In addition, defects could be created nearby the Pt

Li and Yang particles by the H2 reduction procedure at 573 K because of the possible methanation of carbon catalyzed by Pt. Thus, the higher heats of adsorption at low coverage could be due to the defect sites on carbon surrounding the Pt particles. It is known that Pt is an excellent catalyst for the carbon-H2 reaction (forming methane) where the Pt particles catalyze the gasification reaction by channeling and tunneling actions, both creating intimate contacts between the Pt particle and the carbon.43,44 Such intimate contacts would obviously facilitate spillover. In our previous work, we have studied the heats of adsorption of hydrogen on IRMOF-8 by measuring the temperature dependence of the equilibrium isotherm.19 In comparison with the heats of adsorption of hydrogen on AX-21 from this work, IRMOF-8 has higher heats of adsorption toward hydrogen. This could be attributed to the higher bonding energies on some of the IRMOF-8 sites. Ab initio molecular orbital calculations have shown that the hydrogen adsorption sites on IRMOF-8 were energetically heterogeneous, and the bonding energies between hydrogen atoms and the O2, C3, and C4 sites on the organic linker of IRMOF-8 were in the range of -40.3 to -49.6 kJ/ mol.19 From the data shown in Figure 5, the presence of Pt nanoparticles increased the amount adsorbed by approximately a factor of 2, and the heats of adsorption were also substantially higher than that on pure carbon. It is known that the ClausiusClapeyron equation gives the overall heats of adsorption. Therefore, in this study the obtained heats of adsorption from the Clausius-Clapeyron equation were approximately the overall values of the bonding energies of H2 on carbon, H atoms on Pt metal, and various carbon sites, including some very energetic graphitic edges sites. H atoms bond more favorably to surfaces than H2 because H has a free electron. The interactions between the spiltover hydrogen atoms and the carbon sites will be much stronger than that in the physical adsorption system, where hydrogen molecules are adsorbed on carbon by van der Waals and electrostatic interactions. Thus, the overall heats of adsorption can be significantly enhanced by spillover.18-20 The overall heats of adsorption on the Pt/ AX-21 were in the range of -23 to -15 kJ/mol, compared to only -6 kJ/mol for the pure AX-21.45 This comparison indicates that the majority of the adsorbed hydrogen on the Pt/AX-21 sample was the spiltover hydrogen. Upon exposure to hydrogen, H2 first adsorbed quickly on the carbon sites via physical adsorption; the spiltover H atoms would subsequently displace these weakly adsorbed molecules from these carbon sites. For this reason, we treated the entire isotherms in the kinetics study, although Robell et al.22 used only the “net adsorption,” that is, the increased amounts of adsorption due to Pt, in treating their kinetic data. 3.4. Adsorption Rates and Apparent Activation Energy for Surface Diffusion. It has been widely suggested that surface diffusion of atomic hydrogen could be the rate-determining step for the hydrogen adsorption process by spillover.22,46-54 Thus, the kinetics of hydrogen adsorption via hydrogen spillover may be characterized by a surface diffusivity (D) or diffusion time constant (D/R2, where R is the characteristic radius of sphere for diffusion). From the diffusion time constants at different temperatures, the activation energy for surface diffusion may be obtained. To calculate the activation energy for surface diffusion (Ea, kJ/mol), the adsorption rates of hydrogen on the Pt/AX-21 sample were measured. Figure 8 shows the uptake rates of hydrogen at various temperatures and at ∼80 Torr. It is seen that the Pt/AX-21 sample adsorbed hydrogen very rapidly at


J. Phys. Chem. C, Vol. 111, No. 29, 2007 11091

Figure 8. Hydrogen adsorption kinetics at different temperatures on the Pt/AX-21 sample (P ) ∼80 Torr).

all temperatures at this low pressure. It took less than 10 s to reach equilibrium adsorption capacity at all temperatures studied. The adsorption rates increased with temperature. The effect of temperature on the adsorption rates can be interpreted by the increase of diffusivity of atomic hydrogen on the surface of activation carbon. The governing equation for surface diffusion of hydrogen atoms on the activated carbon can be expressed as

( )

∂u ∂2u )D 2 ∂t ∂r

(2)

u ) Cr

(3)

where r, C, and D denote the radial distance from the center of the Pt source, the surface concentration of the spiltover hydrogen atoms, and the diffusion coefficient of the spiltover hydrogen, respectively.55 The initial and boundary conditions are such that the surface is initially clean, and after time zero hydrogen is equilibrated with the point source of Pt and undergoes diffusion to the carbon sphere. As suggested in our previous discussions on the diffusion of hydrogen on the IRMOF-8 structure, we will use the available simplified solution for this case where the source is on the outer boundary rather than at the center.19 The same initial and boundary conditions for the simplified case are used here. The approximate solution for small times, or when Mt/M∞ < 0.3, is

()

Mt 4 Dt ) M∞ xπ R2

1/2

(4)

where Mt and M∞ denote the total amounts of the spiltover hydrogen at time t and at equilibrium with Pt, respectively.55 From the kinetic data presented in Figure 8, D/R2 values at various temperatures can be calculated from eq 4. Surface diffusion is an activated process, and the temperature dependence of surface diffusivity can be correlated by the Eyring equation:

( )

D ) D0 exp -

Ea RT

(5)

where D0 is the surface diffusivity at zero loading, Ea is the activation energy for surface diffusion, R is the gas constant,

Figure 9. Determination of activation energy: plot of ln(D/R2) vs T-1 at pressure of ∼80 Torr.

and T is absolute temperature. D0, Ea, and R are all constants for a given system. Therefore, D is only a function of temperature T. The apparent activation energy Ea can be obtained by Arrhenius plot of ln(D/R2) against 1/T. The Arrhenius plot is shown in Figure 9. Good linearity of the plot is observed. The value of Ea for hydrogen adsorption on the Pt/AX-21 sample was calculated to be 7.6 kJ/mol from the slope of the plot. This value of Ea for surface diffusion was around one half to one third of the absolute values of the heats of adsorption (15-23 kJ/mol). Therefore, comparing with literature our results on the activation energy and heats of adsorption are reasonable.52,53 Yoo et al. investigated the atomic hydrogen storage in carbon nanotubes promoted by metal catalysts.42 They obtained much higher activation energies for hydrogen adsorption over Pdpromoted carbon nanotubes. The values varied from 46.6 to 129.8 kJ/mol, depending on the annealing temperatures at 523773 K. The results were ascribed to the adsorption of hydrogen atoms on the defects of carbon nanotubes.42 In addition, the high activation energies of hydrogen on the surface of carbon nanotubes could be attributed to the much higher adsorption temperatures used in their experiments (573 K) than ours (298348 K). As the temperature is further increased, more and stronger bonds between hydrogen atoms and more energetic sites on carbon could be formed. The high activation energies indicated very strong interactions of atomic hydrogen with the surface sites. As a result, the surface diffusion of hydrogen atoms on the surface of carbon nanotubes and the desorption of hydrogen from the adsorption sites will be difficult. The temperature-programmed desorption (TPD) experiments revealed that the hydrogen desorption temperatures from the carbon nanotubes were 700-900 K.42 However, our hydrogen isotherms showed that the adsorption was fully reversible even at 298 K. Therefore, the activation energy for hydrogen adsorption on the Pt/AX-21 sample should be much lower than that obtained by Yoo et al. on the Pd-promoted carbon nanotubes. Figure 10 shows the adsorption and desorption kinetics of hydrogen on the Pt/AX-21 sample at higher and various pressures (5-100 atm) and 298 K. In comparison with the adsorption kinetics at low pressures (Figure 8), the adsorption was much slower at high pressures (5-100 atm). As shown in Figure 10a, the adsorption rates decreased steadily with increasing pressure. The desorption rates were relatively faster than the adsorption rates for the similar pressures (Figure 10b). In addition, the adsorption and desorption rates at similar pressures


Li and Yang

Figure 10. Rates of adsorption (a) and desorption (b) at different pressures on the Pt/AX-21 sample (T ) 298 K).

Figure 11. D/R2 as a function of H2 uptakes at 298 K for the Pt/AX21 sample. 2, adsorption; 9, desorption.

were much faster than that observed on the bridged IRMOF-8 sample.19 It can be seen from Figure 10a that >10% of the total hydrogen adsorption capacity at each pressure was reached within 0.5 h. The slower adsorption/desorption rates on the bridged IRMOF-8 can be due to the slow diffusion of hydrogen atoms on the surface of IRMOF-8 because of the stronger bonding energies and also the higher activation energy for surface diffusion. The D/R2 values for both adsorption and desorption on the surface of the Pt/AX-21 sample at different pressures estimated from eq 5 are plotted as a function of hydrogen adsorption capacity or surface concentration. The plots are shown in Figure 11. As seen in Figure 11, the D/R2 values dropped sharply at low loadings of hydrogen (at low pressures) and became nearly constant at >0.4 wt % hydrogen capacity. A similar trend was also observed for the desorption. These results would suggest that the main mechanism for hydrogen desorption on Pt/AX-21 was through reverse spillover, as discussed in our previous paper on the bridged IRMOF-8 sample.19 3.5. Hydrogen Adsorption Model and Equilibrium Isotherm. In our work on hydrogen spillover on the bridged IRMOF-8,19 we proposed a mechanistic model for the equilibrium isotherm for the spillover system. The bridged IRMOF-8 involved Pt supported on activated carbon (Pt/AC), which was brought into contact (via carbon bridges) with the IRMOF-8 particles. Thus, the model involved a Pt center, a primary receptor area (of AC) with radius R1, and a secondary receptor (of IRMOF-8) with radius R2. In that work, the diffusion time

Figure 12. Diffusion distance R as a function of H2 uptake. Open circles indicate the data derived from Figure 11 assuming constant D.

constant, D/R2, also declined with the amount adsorbed (q) in a similar manner as that shown in Figure 11. By assuming that the surface diffusivity was not dependent on surface concentration, a cubic relationship was determined between R2 and q, R2 ∝ q1/3. The resulting isotherm was19

q)

k1xPH2 1 + k2xPH2 - k3xPH2

(6)

From the TEM images of the Pt/AX-21 sample, we can assume that the surface of carbon is covered with a certain number of identical active zones associated with the platinum centers. These active zones consist of platinum centers and the surrounding most active adsorption carbon sites for hydrogen atoms at which hydrogen atoms are adsorbed strongly and the adsorption rates are fast. The presence of these active carbon zones surrounding the Pt centers is evident because the heats of adsorption of hydrogen at low surface coverages were much higher than that at high coverages, as discussed in section 3.3. A similar observation was also noted by Robell et al. in their study of H2 on Pt/carbon.22 Let the radius of the active zones be R1. The value of R1 is a constant, which depends on the composition and the structure of the sample. Each zone is covered with hydrogen atoms in equilibrium with gaseous molecular hydrogen through a platinum center. The surface concentration of H atoms on the carbon zone depends only on


J. Phys. Chem. C, Vol. 111, No. 29, 2007 11093 carbon site is approximately 8.2 × 10-2 nm2 (average of basal plane and two edge sites on graphite). From the BET surface area and the assumption of one H per C site, the maximum storage on AX-21 is 5.1 wt %. Thus, our results were far below the maximum amount. Optimization of the doping method to allow better dispersion and more carbon surface to contact with the metal particles should be helpful to further increase the adsorption capacity by spillover. 4. Conclusions

Figure 13. Hydrogen spillover model (b) fits to high-pressure H2 adsorption isotherm data (4) at 298 K on Pt/AX-21.

H2 pressure at a constant temperature (298 K). The equilibrated zone serves as the source of hydrogen atoms for surface diffusion on the carbon surface surrounding the zone. By contrast, the rate of diffusion of hydrogen atoms on the carbon surface surrounding the equilibrated carbon zone is very slow, as indicated by the data. Let the average distance for surface diffusion on the carbon surface away from the equilibrated carbon zone be R2. The maximum value of R2 should be the radius of the area that is equal to the total reachable surface area of the carbon divided by the number of Pt particles. As such, the limitation of R2 should depend on the total concentration of Pt in the sample and the dispersion of Pt particles on the carbon surface. The limiting R2 value is 35 nm. Moreover, from Figure 11 and by assuming D being independent of surface concentration, Figure 12 is obtained. Figure 12 shows that a cubic relationship also holds between R2 and q for the H2 on Pt/AX-21 system. These similarities indicate that our proposed model can be applied to interpret the adsorption data for Pt/ AX-21. In principle, all of the constants involved in eq 6 could be measured independently.19 The shape of the isotherm is determined by the relative magnitudes of the two terms in the denominator, k2xPH2 versus k3xPH2. The isotherm will be concave in shape (i.e., concave toward the pressure axis) if k2xPH2 > k3xPH2 or k2 > k3. The isotherm can become convex (i.e., against the pressure axis) if k2xPH2 < k3xPH2 or k2 < k3. The seemingly linear isotherms obtained in this paper can be explained by this isotherm. An attempt to fit the real isotherm of H2 on Pt/AX-21 by using eq 6 indicated a good agreement between the experimental data and the theoretical model, as shown in Figure 13. The theoretical equation to fit the experimental data can be expressed as

q)

2.076 × 10-4xPH2

1 - 1.432 × 10-4xPH2

(7)

where the units for the two constants and PH2 are (Pa)-1/2 and Pa, respectively, and q is expressed in wt %. The nearly linear isotherm should become concave at higher pressures and will level off at very high pressures based on our model. The maximum amount of hydrogen storage for Pt/AX21 can be estimated as follows. The average area per surface

An effective doping method has been developed to highly disperse nanoparticles of Pt on the AX-21 superactivated carbon. CO chemisorption at 308 K showed that the dispersion of Pt was ∼58% when the doping amount was ∼5.6 wt %. By spillover, the rechargeable and reversible hydrogen storage capacity in AX-21 carbon was enhanced by a factor of 2, to 1.2 wt % at 298 K and 10 MPa. The isotherm was nearly linear and showed no saturation up to 10 MPa. The enhancement of hydrogen adsorption amount can be interpreted by the dissociation of hydrogen molecules on the Pt surface and the subsequent surface diffusion and adsorption of atomic hydrogen on the carbon surface. The overall heat of adsorption of H2 on the Pt/ AX-21 material decreased with surface coverage and fell in the range -23.3 to -15.1 kJ/mol at low pressures (0-1 atm). The apparent activation energy of surface diffusion was 7.6 kJ/mol. The diffusion time constant (D/R2) decreased with increasing pressure (or surface concentration), indicating that the average diffusion distance (R) increased with surface concentration. The mechanistic model for hydrogen spillover that we proposed previously could be used to interpret all experimental results. Acknowledgment. The authors acknowledge the funding provided by the U.S. 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, 353-358. (2) Dillon, A. C.; Heben, M. J. Appl. Phys. A 2001, 72, 133-142. (3) Dillon, A. C.; Johns, K. M.; Bekkedahl, T. A.; Klang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377-379. (4) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127-1129. (5) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. J. Phys. Chem. B 1998, 102, 4253-4256. (6) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B. Appl. Phys. Lett. 1999, 74, 2307-2309. (7) Shiraishi, M.; Takenobu, T.; Kataura, H.; Ata, M. Appl. Phys. A 2004, 78, 947-953. (8) Zu¨ttel, A. Mater. Today 2003, 6, 24-33. (9) Yang, R. T. Carbon 2000, 38, 623-626. (10) Becher, M.; Haluska, M.; Hirscher, M.; Quintel, A.; Skakalova, V.; Dettlaff-Weglikovska, U.; Chen, X.; Hulman, M.; Choi, Y.; Roth, S.; Meregalli, V.; Parrinello, M.; Strobel, R.; Jorissen, L.; Kappes, M. M.; Fink, J.; Zuttel, A.; Stepanek, I.; Bernier, P. C. R. Phys. 2003, 4, 1055-1062. (11) Lueking, A.; Yang, R. T. J. Catal. 2002, 206, 165-168. (12) Lueking, A.; Yang, R. T. Appl. Catal., A 2004, 265, 259-268. (13) Lachawiec, A. J.; Qi, G. S.; Yang, R. T. Langmuir 2005, 21, 11418-11424. (14) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 726-727. (15) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 8136-8137. (16) Li, Y. W.; Yang, R. T. J. Phys. Chem. B 2006, 110, 17175-17181. (17) Conner, W. C., Jr.; Falconer, J. L. Chem. ReV. 1995, 95, 759788. (18) Yang, F. H.; Yang, R. T. Carbon 2002, 40, 437-444. (19) Li, Y. W.; Yang, F. H.; Yang, R. T. J. Phys. Chem. C 2007, 111, 3405-3411. (20) Kim, H. S.; Lee, H.; Han, K. S.; Kim, J. H.; Song, M. S.; Park, M. S.; Lee, J. Y.; Kang, J. K. J. Phys. Chem. B 2005, 109, 8983-8986. (21) Srinivas, S. T.; Rao, P. K. J. Catal. 1994, 148, 470-477. (22) Robell, A. J.; Ballou, E. V.; Boudart, M. J. Phys. Chem. 1964, 68, 2748-2753.

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