Hydrogen Storage on Carbon Doped with Platinum Nanoparticles

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Hydrogen Storage on Carbon Doped with Platinum Nanoparticles Using Plasma Reduction Yingwei Li and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109

Chang-jun Liu and Zhao Wang Key Laboratory for Green Chemistry, School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, People’s Republic of China

The doping of metal on carbon can be significantly improved by plasma treatment in two ways: increased dispersion and stronger interactions between metal particles and the substrate, both leading directly to enhanced hydrogen spillover and, thereby, increased hydrogen storage. Thus, the hydrogen storage capacity of activated carbon at 298 K was increased almost 3-fold by doping with 3 wt % of platinum, using a plasma treatment. Without plasma treatment but with H2 reduction, the increase in hydrogen storage was only 54% by doping with 3 wt % of platinum. 1. Introduction Hydrogen energy is considered as an alternative energy for fossil fuels, because it is clean and is easily produced.1 An effective hydrogen storage technique is one of the key barriers to the realization of fuel-cell powered vehicles using hydrogen as the energy carrier. There are currently several candidate storage technologies for hydrogen; however, at the present time, none are capable of satisfying the U.S. Department of Energy (DOE) criteria of size, recharge, kinetics, cost, and safety requirements for personal transportation vehicles.2,3 For H2sorbent materials, nanostructured carbons (including carbon nanotubes, graphite nanofibers, activated carbon, and graphite) have been the major candidates, because of their light weight and high surface areas.4-6 However, recent reports have indicated that, at ambient temperature, carbon nanostructures cannot store the sufficient amount of H2 required for transportation applications merely by physical adsorption.7-9 Hydrogen spillover is a well-documented phenomenon in catalysis.10,11 Recently, we have demonstrated that hydrogen storage by hydrogen spillover is a promising approach to enhance the hydrogen storage capacities in nanostructured materials including carbon nanostructures, zeolites, and metalorganic frameworks.12-18,32 A phenomenological model has been formulated by Yang and co-workers to explain the almost-linear isotherms that are observed for the spillover systems.19 There are currently two ways to introduce metal particles that are capable of dissociating H2 onto high-surface-area receptors for hydrogen storage by spillover. One is by physical mixing the receptor material with a supported metal catalyst (such as palladium-doped activated carbon).12-20,32 In this case, we have demonstrated that additional nanobridges are needed for spillover. Another way to introduce metal onto the receptor is via chemical doping. In contrast to physical mixing, chemical doping could potentially produce identical samples and is more reproducible. Several reports have shown enhancements in hydrogen storage by doping transition metals (e.g., nickel, palladium) in carbon nanostructures (such as carbon nanotubes (CNTs), active carbon, carbon nanofibers).21-26 In these studies, * To whom correspondence should be addressed. Fax: (734) 7647453. E-mail address: [email protected].

the preparation of the metal-doped carbons was done by reduction of the corresponding metal salts impregnated on the carbon materials at elevated temperatures. The resulting metal particles were usually heterogeneously dispersed with a broad size distribution. That resulted in low utilization ratios of the metals, and, hence, low enhancement factors for hydrogen storage by hydrogen spillover. Recently, a new plasma-assisted reduction method has received much attention for the preparation of nanosized metaldoped materials.27-31 Its application in catalyst preparation has been previously reviewed.27 This method produces metal particles with uniform sizes that are highly dispersed on the surface of the support. The high dispersion of metals reduced by plasma would enhance the utilization efficiency of metals for dissociation of hydrogen for spillover. Here, we present the idea of using plasma treatment to enhance hydrogen storage, and we report the hydrogen adsorption results on a platinum-doped carbon material that has been reduced using an argon glow discharge plasma.32 It was observed that the plasma-reduced platinum-carbon (Pt/C) exhibited significantly higher hydrogen adsorption capacity than the Pt/C sample via traditional H2 reduction. The reversible hydrogen uptake at 298 K and 10 MPa on the Pt/C sample was enhanced by a factor of 3 via a simple plasma treatment. The plasma technique is simple, and the storage capacities are reversible at 298 K. 2. Experimental Methods 2.1. Sample Preparation and Plasma Treatment. The activated-carbon (AC) sample was obtained from Norit Americas, Inc. (grade SX ULTRA). The Brunauer-Emmett-Teller (BET) surface area was 1200 m2/g, and the ash content was 5 wt %. Platinum was doped on the carbon via standard incipient wetness impregnation, using an aqueous solution of H2PtCl6. The doped sample was further dried in a helium flow at 393 K for 2 h, to remove the residual moisture from the sample. The helium flow was then switched to H2 and the temperature was increased to 523 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 helium and stored under a helium atmosphere before further measurement. This sample was designated as Pt/

10.1021/ie0712075 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/31/2007

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AC-H (H2 reduction). The platinum amount was 3 wt % (dryweight basis). Another doped sample (activated carbon doped with platinum, Pt/AC) was treated with argon glow-discharge plasma, without H2 reduction. Details of the plasma setup has been given elsewhere.27,30 The sample (∼0.4 g), which was loaded on a quartz boat, was placed in the glow discharge chamber, which was a horizontal quartz tube (inner diameter (id) of 35 mm) with two stainless steel electrodes (outer diameter (od) of 30 mm). The system was connected to an argon source and was evacuated with a vacuum pump. When the argon pressure was adjusted to 50-100 Pa, the glow discharge plasma was generated by applying 900 V to the electrodes, using a highvoltage amplifier (Trek, 20/20B), with argon (>99.999%) as the plasma-forming gas. The current was in the range of 1-2 mA. The signal input for the high-voltage amplifier was supplied by a function/arbitrary waveform generator (Hewlett-Packard, model 33120A) with a 100 Hz square wave. The plasma treatment time was 10 min, and each sample was treated six times, with manual mixing of the sample between treatments to ensure even exposure to the plasma. The temperature of the plasma gas was measured using infrared imaging (Ircon, model 100PHT), which indicated that the treatment was conducted at ambient temperature. The plasma-treated sample was designated as Pt/AC-P (plasma reduction). 2.2. Characterization. Powder X-ray diffraction (XRD) data were recorded on a Rigaku Miniflex diffractometer using Cu KR radiation (λ ) 0.1543 nm) at 30 kV and 15 mA, with a scan speed of 2° 2θ/min and a step size of 0.02° 2θ. Highresolution transmission electron microscopy (HRTEM) images of the materials were obtained on an analytical electron microscope (JEOL, model 2010F) that was equipped for energydispersive X-ray (EDX) analysis and operated at 200 kV. 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 of >0.1 MPa and up to 10 MPa was measured using a static volumetric technique with a specially designed Sieverts apparatus. The apparatus was previously tested to prove that it was leak free, and its accuracy was proven through calibration, 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 highpressure isotherm measurement in this study. Before the measurements, the samples were degassed under vacuum at 300 °C for at least 12 h. 3. Results and Discussion Powder XRD patterns for activated carbon, Pt/AC-H (H2 reduction), and Pt/AC-P (plasma reduction) samples are shown in Figure 1. The XRD pattern of the activated carbon showed a reflection at 2θ ) 26.7°, which indicated the existence of a small amount of graphitic carbon. By doping 3 wt % platinum, the diffraction peaks for activated carbon were reduced somewhat. However, no platinum phase was detected, because there were no reflections at 2θ ) 39.8° (111) and 46.3° (200), which are characteristic reflections of the cubic platinum metal structure [JCPDS File Card No. 4-802].33 This could be due to the low content of platinum in the samples, because the absence of the Pt XRD lines at low platinum loadings has been widely observed for similar platinum loadings (e.g., Hinz et al. also found that no XRD peaks from platinum metal were visible at platinum loadings up to 3 wt %34). In addition, the relatively high dispersions of metals at low loadings would be another

Figure 1. X-ray diffraction (XRD) patterns of activated carbon (AC), H2 reduction (Pt/AC-H), and plasma reduction (Pt/AC-P).

reason for the absence of the XRD peaks from the metals, as also suggested in other papers.35,36 The small peaks at 2θ ≈ 21°, 36.8°, and 39.5° could be assigned to the impurities in the sample (i.e., 5 wt % ash content in the carbon). HRTEM images of the Pt/AC samples are shown in Figure 2. The dark spots (which indicate platinum particles), which are easily visible, were dispersed on the entire surface of the carbon of the hydrogen-reduced sample (Pt/AC-H). The sizes of the platinum particles were 2-4 nm for the Pt/AC-H sample. However, visible platinum particles were scarce on the Pt/AC-P sample (via plasma reduction). The largest particles were ∼2 nm, as indicated by circles in Figure 2B. This would suggest that platinum was highly dispersed on the carbon with very small particle sizes. High-pressure hydrogen isotherms at 298 K for pure carbon, Pt/AC-H, and Pt/AC-P are shown in Figure 3. As shown in Figure 3, activated carbon had a hydrogen storage capacity of ∼0.3 wt % at 298 K and 10 MPa. The hydrogen uptake at 10 MPa was enhanced to ∼0.5 wt % by doping 3 wt % Pt and using H2 reduction. The effects of plasma treatment are shown by the Pt/AC-P sample; the hydrogen uptake reached ∼0.9 wt % at 10 MPa. In comparison with the original activated carbon, it is remarkable that the hydrogen adsorption amount of activated carbon was enhanced by a factor of ∼3. The hydrogen adsorption capacity on the Pt/AC sample reduced by plasma was ∼1.8 times larger than that by H2 reduction. This significant enhancement can only be attributed to the spillover of atomic hydrogen from the platinum nanoparticles to the carbon receptor.12-26,32 A high dispersion of metal particles will lead to a high storage capacity, because highly dispersed metal will have a large metal surface area, which 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. As suggested and demonstrated in our previous work,12,14-19,32 the connectivity between the dissociation metals and the receptors also had an important role on determining the hydrogen storage capacities by spillover. Intimate contacts will facilitate the spillover and surface diffusion of H atoms and, hence, improve the storage capacities. Therefore, the large difference in the storage capacities between the two doped

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Figure 2. Transmission electron microscopy (TEM) images of (A, C, and E) the Pt/AC-H samples, and (B, D, and F) the Pt/AC-P samples. Panels A and B represent as-synthesized samples; panels C-F represent samples after sintering at 500 °C (in argon) for 3 h.

samples could be caused by different interaction strengths/ contacts between the platinum particles and carbon. To verify this, the two samples were subjected to a sintering test. The doped samples were heated at 500 °C in argon for 3 h. The samples after the sintering test were characterized by HRTEM and low-pressure hydrogen adsorption measurements. As shown in Figures 2C and 2E, very large platinum particles (∼6 nm in size) were observed on the heat-treated Pt/AC-H sample. This showed that the platinum particles were sintered upon heating at 500 °C for 3 h. However, for the plasma-treated sample, no significant changes in the size of the platinum particles were observed, as can be seen from Figures 2D and 2F). The result suggested the thermal stability of the Pt/AC-P sample, as also confirmed by low-pressure H2 adsorption measurements. Figure

4 shows the low-pressure H2 isotherms on the two doped samples before and after sintering test. It can be observed that the hydrogen adsorption capacity at zero pressure, by extrapolating the isotherm, was clearly reduced on the Pt/AC-H sample after heat treatment, showing considerable sintering. However, no significant difference was observed between the Pt/AC-P samples before and after heat treatment. These results indicated stronger interactions between the platinum particles and the carbon on the plasma-treated sample. The stronger interactions enabled more intimate connectivities between the platinum and the carbon, thereby facilitating more spillover of hydrogen from the platinum to the surface of the carbon. An electron reduction mechanism has been proposed for the plasma reduction, because an abundance of high-energy elec-

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Figure 3. High-pressure hydrogen isotherms at 298 K for (0) activated carbon, ([) H2-reduced Pt/AC-H, and the plasma-treated Pt/AC-P samples ((O) adsorption and (2) desorption).

to the observed experimental transmission electron microscopy (TEM) results that are shown in Figure 2. Therefore, some platinum nanoparticles on the Pt/AC-P sample could be inactive for the dissociative adsorption of H2. These nanoparticles could be in the form of amorphous metal clusters, which have been widely observed on plasma-treated samples.31,38-40 However, the mechanism for the plasma reduction is not understood. The actual forms of the platinum metals on the carbon surface are not known. Further studies are underway to understand the reasons for the decrease in the H2 chemisorption amount upon plasma treatments. Reversibility was evaluated on the Pt/AC-P sample by measuring the desorption branch down to 1 atm. Figure 3 shows that the desorption branch almost followed the adsorption branch, although there appeared to be a slight hysteresis. 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 (not shown here) was in complete agreement with the first adsorption isotherm. These results showed that hydrogen adsorption in the Pt/AC-P sample was fully reversible. 4. Conclusions In conclusion, we have shown that the use of plasma is promising to dope highly dispersed metal nanoparticles on nanostructures for hydrogen storage. By doping with 3 wt % platinum, assisted by plasma reduction, the reversible hydrogen storage capacity in carbon has been enhanced by a factor of 3 at 298 K and 10 MPa. The excellent storage nature in the plasma-reduced sample can be attributed to the strong interactions between the platinum particles and carbon support that facilitate the spillover of H atoms from the platinum to the carbon surface. Acknowledgment

Figure 4. Low-pressure H2 equilibrium adsorption isotherms on (9) the as-synthesized H2-reduced Pt/AC-H, and (b) the plasma-treated Pt/AC-P samples; open symbols represent samples that were subjected to sintering at 500 °C (in argon) for 3 h ((0) Pt/AC-H and (O) Pt/AC-P).

trons is one characteristic for all types of plasma, which is independent of the type of plasma-forming gas.31 As the only reducing species in the plasma, these electrons most likely serve as the reducing agents. Thus, plasma reduction could be caused by a direct transfer of electrons from the plasma to the metal ions. In addition, under the effects of glow discharge, the carbon would be modified by plasma species, especially in the presence of water. Some “dangling bonds” would be formed on the surface of the carbon that will help the formation of smaller metal particles.31,36 In addition, the dangling bonds could also strengthen the interactions between the platinum and the carbon support that prevent the particles from sintering. The detailed mechanism is far from being understood. Further studies are being undertaken to understand the reduction mechanism by plasma. It is surprising to note that the H2 chemisorption amounts on the Pt/AC-P sample were less than that on the Pt/AC-H sample. The H2 chemisorption amounts on platinum were obtained using the Benson-Boudart method, in which the isotherm from low pressures is extrapolated to zero pressure.37 The calculated dispersions of platinum on the Pt/AC-P sample would be lower than that on the Pt/AC-H sample if all the platinum nanoparticles could adsorb hydrogen. The calculations would be contradictory

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ReceiVed for reView September 7, 2007 ReVised manuscript receiVed October 15, 2007 Accepted October 19, 2007 IE0712075