Enhanced Hydrogen Storage on Pt-Doped Carbon by Plasma

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Enhanced Hydrogen Storage on Pt-Doped Carbon by Plasma Reduction Zhao Wang and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: NoVember 2, 2009; ReVised Manuscript ReceiVed: December 29, 2009

Hydrogen adsorption properties of a superactived carbon (AX-21) doped with Pt nanoparticles by using plasma reduction were studied and were compared with that by using traditional H2 reduction. The hydrogen storage capacity was significantly increased by plasma reduction. The H2 storage capacity on Pt-doped AX-21 at 298 K and 10 MPa was increased from 1.19 wt % (by H2 reduction) to 1.46 wt % on the sample obtained by plasma reduction. The plasma-reduced sample produced 1.5-3 nm Pt particles that were highly dispersed on carbon, and most interestingly, the metal particles were recessed into the carbon substrate. Both isosteric heats of adsorption and the activation energies for spillover were decreased by plasma reduction, which was evidence that the energy barrier for H spillover was lowered by plasma treatment, resulting in faster rates as well as higher spillover capacities. Mechanisms for plasma reduction and for edge recession of the metal particles into carbon are proposed. 1. Introduction Increasing global energy demands, limited fossil fuel reserves, and efforts to reduce greenhouse gas emissions have motivated research on alternative transportation fuels. Hydrogen as a clean and efficient energy carrier has been proposed as an alternative energy source. A major challenge for commercialization of hydrogen fuel cells is storage of enough hydrogen onboard to give a driving range matching that by current internal combustion engines.1 The U.S. Department of Energy (DOE) has established a comprehensive set of targets for onboard hydrogen storage and delivery.2 Many investigations have been conducted to search for viable hydrogen storage materials.3–7 Carbon-based materials, including carbon nanotubes (CNTs), graphite nanofibers (GNFs), activated carbons (ACs), templated carbons (TCs), graphite, and metal-organic frameworks (MOFs), have been the major candidates due to their light weight and high surface areas for hydrogen storage.8–16 Doping of transition metals (Pd, Pt, Ni, etc.) can be used to enhance the storage capacity of carbons via a hydrogen spillover mechanism, which has been validated experimentally by many groups.17–20 On the basis of molecular orbital calculations and experimental results, Yang et al. proposed to use the hydrogen spillover approach for hydrogen storage at ambient temperature.21–26 The phenomenon of spillover has been studied within the context of catalysis (i.e., at elevated temperatures) for nearly a half century. Hydrogen spillover is defined as the transport of an active hydrogen species from metal nanoparticles onto adjacent surfaces of a receptor via spillover and surface diffusion.27–29 There are a number of factors that control the spillover process and hence the storage capacity. To increase the binding energy of the receptor surface with the spilled over hydrogen is a promising approach that we have taken recently.30–33 Another approach is to maximize the contacts between the metal nanoparticles and the receptor. Thus, the metal doping and dispersing steps are important. * To whom correspondence should be addressed. Fax: (734) 764-7453. E-mail: [email protected].

Different preparation conditions can result in variation in metal particle size, morphology, and the metal-support interactions, and then impact on the spillover behaviors. After doping of a metal precursor, the reduction step is normally conducted using flowing hydrogen at elevated temperatures or using chemical reducing agents, such as NaBH4, formaldehyde, and hydrazine. However, the resulting metal particles were usually dispersed with a broad size distribution or a weak connectivity between metal particles and the receptor. Plasma treatment techniques are useful and effective in preparing highly dispersed, supported metal catalysts as well as improving the metal-support interactions.34–39 Jang and co-workers40 found plasma treatment can significantly change the structure of a Ni precursor supported on alumina and the interaction between them. Kostecki and co-workers41 reported a microwave plasma chemical vapor deposition technique to prepare uniformly dispersed Pt supported on carbon films. Shim et al.42 applied hydrogen plasma directly for the preparation of carbon-supported Pt catalysts. The sizes of the obtained platinum particles supported on carbon were in the range of 3-5 nm. More recently, Liu et al. reported a simple and energy-efficient glow discharge plasma reduction route for preparing nanosized metal-doped materials using inert gas as the plasma forming gas.36,43,44 His group also applied a cold plasma to control the metal-support interface of NiO-loaded photocatalysts.45 A preliminary work showed enhanced hydrogen spillover on Pt/AC (where AC is activated carbon) by Ar plasma reduction.46 This work is an in-depth study of the hydrogen storage properties of a Pt-doped AX-21 (where AX-21 is a commercially available superactivated carbon) by various metal dispersing and reduction techniques, including plasma reduction. We show significant enhancement in spillover storage by plasma reduction. Possible mechanisms for the plasma reduction process and enhancement in spillover are proposed. The enhancement in spillover is attributed to stronger anchoring and multiple bonds between the Pt metal and carbon support, which would lower the energy barrier for spillover from metal to carbon. We also showed that plasma reduction enabled nanosized Pt to be highly dispersed on carbon.

10.1021/jp910466x  2010 American Chemical Society Published on Web 03/08/2010

Enhanced Hydrogen Storage on Pt-Doped Carbon

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Figure 1. Schematic of plasma treatment setup.

2. Experimental Methods 2.1. Sample Preparation and Plasma Treatment. Platinum (6 wt %) Doped on AX-21. The AX-21 superactivated carbon was obtained from Anderson Development Company. Typically, 200 mg of well-dried AX-21 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 of H2PtCl6 (Aldrich, 99.9%) was slowly added dropwise to the above solution under vigorous stirring for 10 min. The mixed slurry was subjected to ultrasonication (100 W, 42 kHz) at room temperature for 1 h and then magnetically stirred at room temperature for 24 h. After being dried in an oven at 333 K overnight, the impregnated AX-21 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. The sample was then split into two portions. One portion was to be reduced by hydrogen (and the other portion by plasma). For hydrogen reduction, 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. The sample prepared this way was designated as Pt/AX-21-H. Some Pt/AX-21-H was further treated with H2O/Ar at 800 °C for 0.5 h (by bubbling Ar through H2O at room temperature). It was designated as Pt/AX-21-H-Ar. Plasma Reduction. The other portion was reduced by glow discharge plasma. The plasma setup is shown in Figure 1. The sample (about 0.2 g) was loaded on a quartz boat that was placed in the glow discharge cell. About 0.2 g of water was added in droplets to wet the sample. When the pressure was adjusted to the range of 100-200 Pa, the glow discharge plasma was generated by applying 5000 V to the electrodes using a dc highvoltage generator (DL-200, Tianda Cutting and Welding Inc., Tianjin, China) with Ar as the plasma-forming gas. Details of the plasma equipment and the treatment procedure have been described elsewhere.43,44 The time of each plasma treatment was 10 min, and each sample was treated 12 times. The sample prepared this way was designated as Pt/AX-21-P. 2.2. Characterization. High-resolution transmission electron microscopy (HRTEM) images of the samples were obtained on a JEOL 3011 analytical electron microscope equipped with EDX (energy-dispersive X-ray spectroscopy) analysis operated at 300 kV. The X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos Axis Ultra XPS spectrometer. Nitrogen adsorption and low-pressure H2 adsorption isotherms (0-1 atm) were measured with a standard static volumetric technique (Micromeritics ASAP 2020). Hydrogen adsorption isotherms at 298 K and pressures greater than 0.1 MPa (up to 10 MPa) were 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.47 Approximately

Figure 2. Nitrogen isotherms of AX-21 (4), Pt/AX-21-P (]) (plasmareduced), and Pt/AX-21-H (O) (H2-reduced) at 77 K.

200 mg of sample was used for each high-pressure isotherm measurement in this study. Before measurements, the samples were degassed in vacuum at 473 K for at least 12 h. 3. Results and Discussion Nitrogen Isotherms. Nitrogen adsorption isotherms are shown in Figure 2. The isotherms of AX-21, Pt/AX-21-H, and Pt/AX-21-P samples all exhibited the typical Type I curve, thus revealing the presence of microporosity in the samples. The BET surface area and pore volume of the AX-21 carbon sample were 2805 m2/g and 1.43 cm3/g, respectively. After doping Pt metals, the BET surface areas and pore volumes of Pt/AX-21-H and Pt/AX-21-P samples were reduced to 2497 and 2568 m2/g and 1.27 and 1.29 cm3/g, respectively. This should be due to the increased weight and micropore blocking caused by the Pt metal particles. It is worth noting that the surface area decreased slightly after doping of the Pt particles, indicating that most of the porosity of the carbon support remained open after doping; that is, the plasma treatment did not cause structure collapse. XPS. Figure 3a shows the Pt 4f XPS spectra of Pt/AX-21-P. Two symmetrical peaks were observed for the sample, and no other peaks could be deconvoluted. This indicates that these metal species were in a single chemical state. The binding energy of the metal ion is 74.5 and 71.2 eV for the Pt 4f5/2 and Pt 4f7/2 transitions, which can be assigned to metallic Pt 4f5/2 and Pt 4f7/2, respectively.48,49 The result confirmed that the supported metal ions had been reduced to their metallic states. To analyze the influence to plasma reduction by adding water, Pt/AX-21 was also prepared by plasma treatment without water. Figure 3b shows the Pt 4f XPS spectra of this sample. The two peaks (with binding energies of 76.4 and 73.0 eV) are assigned to the Pt4+ ion. It indicated that Pt ions could not be reduced by plasma treatment without water. Thus, addition of water was necessary for metal reduction in Ar plasma. We propose that the mechanism of plasma reduction may be related to an indirect process. Marignier et al.,50 in 1985, successfully obtained cobalt and nickel nanoparticles from colloidal solutions irradiated by r-rays for the first time. Many noble metals, some non-noble metals, and bimetallic alloys51–56 were then prepared by the irradiation technique. The radiolytic reduction mechanism has been studied.57,58 The primary radicals and molecules produced in water upon electron-beam pulse are

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Figure 4. High-pressure hydrogen isotherms at 298 K for the H2reduced Pt/AX-21-H sample, adsorption (b) and desorption (O), and the plasma-reduced Pt/AX-21-P sample, adsorption (9) and desorption (0).

Figure 3. Pt 4f XPS spectra for plasma-reduced Pt/AX-21: (a) with water added to carbon and (b) without water.

shown by eq 1.59,60 The solvated electrons eaq- and H• atoms are indeed the strongest reducing agents that are responsible for reduction of the metal ions to the metallic state through reactions 2 and 3. The Pt4+ was reduced to Pt0 by the reducing agents, as indicated below. • + • • H2O f eaq, H3O , H , OH , H2, H2O2, HO2

(1)

+ 0 eaq + M f M

(2)

H• + M+ f M0 + H+

(3)

Hydrogen Storage at 298 K and 10 MPa. The high-pressure H2 isotherms for both adsorption and desorption branches were measured with a Sievert’s apparatus. Details of the system were given elsewhere.47 High-pressure hydrogen isotherms at 298 K for Pt/AX-21-H (H2-reduced) and Pt/AX-21-P (plasma-reduced) samples are presented in Figure 4. As shown in Figure 4, Pt/ AX-21-H had a hydrogen storage capacity of 1.19 wt % at 298

Figure 5. Low-pressure H2 equilibrium adsorption isotherms at 298 K: Pt/AX-21-P (b) (H2-reduced) and Pt/AX-21-H (9) (plasma-reduced).

K and 10 MPa. For the Pt/AX-21-P sample, the hydrogen uptake at 10 MPa was enhanced to 1.46 wt %. It can be seen that the plasma-treated samples had much higher hydrogen adsorption capacities. The enhanced hydrogen storage capacity could not be attributed to the differences in surface areas or pore sizes because the plasma-treated samples had similar surface areas and pore size distributions. Clearly, higher dispersion and stronger anchoring on Pt/AX-21-P were responsible for this enhanced capacity. This result will be further discussed along with TEM results. Low-Pressure H2 Isotherms. Figure 5 shows the lowpressure H2 isotherms on the Pt-doped carbons by H2 reduction and plasma reduction. Plasma treatment resulted in a lower “knee” (at P ∼ 40 Torr) and a steeper slope in the isotherm. From the adsorbed amount of hydrogen extrapolated to zero pressure, the dispersion of Pt metal on AX-21 could be calculated according to Benson-Boudart method.61 Using the assumption of one H per surface Pt atom, the metal dispersions were 45% and 24%, for Pt/AX-21-H (H2-reduced) and Pt/AX21-P (plasma-reduced), respectively. The dispersion of Pt/AX-



Figure 6. TEM images of the samples: (a, b) Pt/AX-21-P (plasma-reduced), (c, d) Pt/AX-21-H (H2-reduced), and (e, f) Pt/AX-21-H-Ar (H2reduced, followed by reaction with H2O/Ar at 800 °C).

21-P appeared to be very low due to the recessed Pt particles into the carbon after plasma treatment. Further discussion will be given along with TEM results. TEM. TEM images of the Pt/AX-21-P and Pt/AX-21-H are shown in Figure 6. Obviously, Pt was highly dispersed on the

carbon with very small particle sizes over the sample that was reduced by plasma. The particle size on Pt/AX-21-P was 1.5-3 nm, whereas it was 2-7 nm over the H2-reduced Pt/AX-21-H. The plasma-reduced metal also showed a narrow size distribution; that is, plasma reduction can be an excellent way for

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Figure 7. Schematic of metal/carbon interface structures by the (a) traditional H2 reduction method and (b) Ar plasma reduction method.

controlling the particle size. Figure 6c shows clear aggregation of the particles on the Pt/AX-21-H sample. A significant difference between Pt/AX-21-H and Pt/AX-21-P was found in the TEM images. Many Pt particles were observed to be deposited on the outer edges of the carbon surfaces on the H2-reduced Pt/AX-21-H (Figure 6c,d). Most interestingly, on the plasma-reduced sample, all of the Pt particles that were on the edges were recessed into the carbon (Figure 6a,b). This edge recession phenomenon suggests that carbon has been gasified during the recession process. Pt particles, along with other transition-metal particles, are excellent catalysts for carbon gasification reactions. During the catalyzed gasification reactions, edge recession is one of the most observed and studied mode of catalytic actions.62–66 To verify that this mode of action was operative during the plasma reduction step, the Pt/AX21-H-Ar sample (Pt/AX-21-H treated with H2O/Ar at 800 °C for 0.5 h, by bubbling Ar through H2O) was prepared and studied by TEM. As shown in Figure 6e,f, the Pt particles on the edges were indeed all recessed into the carbon by catalyzed carbon gasification (with water, to form CO and H2). Thus, a similarity in the reaction mechanism was already evident between plasma treatment and the catalyzed carbon gasification process. The mechanism of the Pt-catalyzed carbon gasification reaction (typically at a high temperature, such as 800 °C) is well-understood.62–67 As shown previously, the driving force for particle recession is the adhesion forces between the front faces of the metal particle and the edge carbon. The catalytic action by metal involves the following sequential steps: dissolution of carbon into the metal at the leading edge of the metal particle, diffusion of carbon in the metal to the opposite side of the metal particle, where it is exposed to the reactive gas, and gasification of the carbon on the exposed metal surface. The same explanation of the edge recession behavior of Pt is applicable to the plasma reduction step where water was added to the carbon sample. The key step in the metal-catalyzed gasification is the dissolution of carbon in the metal, and at below 800 °C, the solubility is too low70 to induce significant gasification rates. Although the temperature of the Ar gas in the glow discharge plasma was “cold” (i.e., at near ambient temperature),68,69 the results above suggested that the local temperature of the Pt nanoparticles was quite high, for example, near 800 °C, causing catalytic edge recession by carbon gasification.62–67 Local heating of the nanoparticles could be caused by the high-energy and high-temperature electrons generated in the Ar plasma. Such a reaction could also occur in a plasma atmosphere.68,69 Figure 7 illustrates spillover on the two types of metal/carbon interfacial structures. On the Pt particles recessed into carbon by plasma treatment, the exposed surface area of the metal particles was reduced due to recession into the carbon. This can explain the result of low-pressure H2 isotherms showing

Wang and Yang an apparent, lower metal dispersion on AX-21 by plasma treatment. Meanwhile, the stronger anchoring and the more intimate contacts between the metal and carbon led to a lower energy barrier for the spillover of H atoms from Pt to carbon. This will be shown in the results on heats of adsorption and activation energy for spillover. As suggested and demonstrated in our previous work,30–33 the connectivity between the dissociation metals and the receptors also played an important role in determining the hydrogen storage capacities by spillover. Intimate contacts will facilitate the spillover and surface diffusion of hydrogen atoms and hence improve the storage capacities. Apparent or Overall Heats of Adsorption. The use of the Clausius-Clapeyron equation would yield the overall heats of adsorption. The overall heats of adsorption of H2 on the Pt/ AX-21-H and Pt/AX-21-P samples were calculated from the H2 isotherms at 298 and 323 K by using the Clausius-Clapeyron equation, as shown in Figure 8. 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 decreased with an increase in temperature. As shown in Figure 8c, the heats of adsorption at high H2 loadings on the Pt/AX-21-P and Pt/AX-21-H samples were 8.4 and 9.3 kJ/mol, respectively, and at low H2 loadings, were 22.8 and 25.2 kJ/mol. The heats of adsorption on Pt/AX21-P were lower than that on Pt/AX-21-H. However, it shows that the isosteric heat of adsorption on Pt/AX-21-H decreased faster than that on Pt/AX-21-P. The lower values of heats of adsorption on the plasma-treated sample can be attributed to the lower energy barrier of hydrogen spillover from the Pt particle to carbon due to the stronger anchoring between the Pt particle and carbon, hence a lower energy barrier for spillover from metal to carbon. This result also indicates that the metal-carbon spillover step is important in the storage process. The slower decline of the isosteric heat of adsorption on Pt/ AX-21-P was likely due to the fact that more hydrogen was spilled over on the plasma-reduced sample. Adsorption Rates and Apparent Activation Energy for Surface Diffusion. The surface diffusion of atomic hydrogen on carbon as well as the metal-carbon spillover step are both important in the overall spillover process. To understand the mechanism, activation energies (∆E) for spillover were obtained for Pt/AX-21-P and Pt/AX-21-H. The ∆E values for spillover were calculated from the temperature dependence of the spillover uptake rates in different pressure increase steps. The uptake rates for Pt/AX-21-P and Pt/AX21-H at various temperatures during the pressure step from 0 to 80 Torr were measured, and the results are shown in Figure 9. The rates for both Pt/AX-21-P and Pt/AX-21-H at 298 K were high at low pressures. From these rate data, estimates of the surface diffusion time constants, D/R2, where D is the surface diffusivity and R is an average radius of diffusion for spillover, were estimated from the solution of the diffusion equation.14,30 To obtain the activation energy for spillover, the following temperature dependence can be correlated by the Eyring equation

( )

D ) D0 exp -

Ea

R′T

(4)

where R′ is the gas constant, T is the absolute temperature, and Ea is the difference in energy between the states



Figure 9. Hydrogen adsorption kinetics at different temperatures on the samples (P ) 0-80 Torr): (a) Pt/AX-21-P (plasma-reduced) and (b) Pt/AX-21-H (H2-reduced).

Figure 8. Low-pressure H2 adsorption isotherms for Pt/carbon at 298 K (solid symbols) and 323 K (open symbols): (a) Pt/AX-21-P (plasmareduced) and (b) Pt/AX-21-H (H2-reduced). (c) Calculated isosteric heats of adsorption of H2: Pt/AX-21-H (b) and Pt/AX-21-P (9).

Figure 10. Determination of activation energy (or energy barrier) for spillover: plot of ln(D/R2) vs T-1 at pressure ) 0-80 Torr.

corresponding to adsorption at the ground vibrational level of the bond and to free mobility on the surface. Thus, plots of log(D/R2) versus 1/T yielded the activation energies.

The results for ∆E values are given in Figure 10. The values of ∆E of Pt/AX-21-P and Pt/AX-21-H were 6.5 and 7.6 kJ/ mol, respectively. The lower ∆E for the plasma-reduced Pt/

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Figure 11. Rates of adsorption on Pt/AX-21-P (plasma-reduced) and Pt/AX-21-H (H2-reduced) at 298 K and various pressure increasing steps. Pressure steps: (a) 0-4.8 atm for Pt/AX-21-P, (b) 0-4.8 atm for Pt/AX-21-H, (c) 51.7-71.3 atm for Pt/AX-21-P, and (d) 51.4-71.1 atm for Pt/AX-21-H.

AX-21-P is direct manifestation of the lower energy barrier for spillover because of the better anchoring by plasma treatment. Figure 11 compares the adsorption rates on the two samples during high pressure steps and 298 K. It is obvious that the uptake rates for high-pressure ranges (>1 atm) on Pt/AX-21-P were higher than that on Pt/AX-21-H. These results are in agreement with the ∆E data. 4. Conclusion We have investigated Pt doped on AX-21 by plasma reduction for hydrogen storage. By plasma reduction, the hydrogen storage capacity on carbon was significantly enhanced at 298 K and 10 MPa. Plasma reduction produced 1.5-3 nm Pt particles that were highly dispersed on carbon and were recessed into the carbon substrate. The rates of spillover were also increased by plasma reduction. The lower activation energy for spillover reflects the lowered energy barrier for spillover due to the better and deeper anchoring by plasma reduction. Acknowledgment. Funding provided by the U.S. DOE Energy Efficiency and Renewable Energy Hydrogen Sorption Center of Excellence and the NSF (Grant No. CBET0753008) is acknowledged. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (2) U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program, MultiYear RD&D Plan, 2005. (3) Dillon, A. C.; Johns, K. M.; Bekkedahl, T. A.; Klang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (4) Poirier, E.; Chahine, R.; Beńard, P.; Lafi, L.; Dorval-Douville, G.; Chandonia, P. A. Langmuir 2006, 22, 8784. (5) Yang, Z.; Xia, Y.; Mokaya, R. J. Am. Chem. Soc. 2007, 129, 1673. (6) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M. Science 2003, 300, 1127. (7) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Nature 2002, 420, 302. (8) Dillon, A. C.; Heben, M. J. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133. (9) Frost, H.; Snurr, R. Q. J. Phys. Chem. C 2007, 111, 18794. (10) Benard, P.; Chahine, R. Scr. Mater. 2007, 56, 803.

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