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Dynamic Measurement of Hydrogen Storage/Release Properties of Mg Doped with Pd Nanoparticles Using a Tapered-Element Oscillating Microbalance under Flow Conditions Xiaochun Xu, Jian Zheng, and Chunshan Song* Clean Fuels and Catalysis Program, The Energy Institute, and Department of Energy & Geo-Environmental Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802 Received March 14, 2005. Revised Manuscript Received July 5, 2005
A tapered-element oscillating microbalance (TEOM) has been applied to measure the hydrogen storage/release properties of magnesium without and with palladium nanoparticle dopant under continuous flow conditions. Compared with those measured by a volumetric method, TEOM provides more accurate and dynamic information on the hydrogen absorption and release behavior under continuous flow conditions in real time, which is important for the study of hydrogen storage materials and for the proper design of hydrogen storage devices for practical applications.
Introduction Hydrogen storage is considered to be one of the key technological options for the advancement of hydrogen energy-based power systems such as fuel cells in transportation, stationary, and portable applications.1 For practical application, a reliable measurement of hydrogen absorption/release data is important for the proper design of hydrogen storage devices. Three different techniques, that is, the volumetric method,2,3 the gravimetric method,4,5 and thermal desorption spectroscopy (TDS),6,7 have been applied for studying hydrogen storage/release in solids. The volumetric method measures the pressure drop due to hydrogen absorption after applying a hydrogen pressure to the specimen contained in a constant volume. Similarly, the pressure increase or gas blowoff due to desorption can be measured. For volumetric measurement, there is an accumulation of the error. Furthermore, any leakage or temperature instability of the apparatus may give rise to considerable experimental errors. The gravimetric method measures the weight changes of the specimen due to absorption or desorption * Corresponding author. Fax: 814-865-3248. E-mail:
[email protected]. (1) U.S. Department of Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program. http://www.eere.energy.gov/hydrogenandfuelcells/storage/, 2005. (2) Fan, Y.-Y.; Liao, B.; Liu, M.; Wei, Y.-L.; Lu, M. Q.; Cheng, H.M. Carbon 1999, 37, 1649. (3) Bogdanoviæ, B.; Schwickardi, M. J. Alloys Compd. 1997, 253254, 1. (4) Stro¨bela, R.; Jo¨rissena, L.; Schliermannb, T.; Trappc, V.; Schu¨tzd, W.; Bohmhammele, K.; Wolfe, G.; Garchea, J. J. Power Sources 1999, 84, 221. (5) Pinkerton, F. E.; Wicke, B. G.; Olk, C. H.; Tibbetts, G. G.; Meisner, G. P.; Meyer, M. S.; Herbst, J. F. J. Phys. Chem. B 2000, 104, 9460. (6) Mommer, N.; Hirscher, M.; Cuevas, F.; Kronmu¨ller, H. J. Alloys Compd. 1998, 266, 255. (7) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Klang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377.
of hydrogen. However, data obtained by the gravimetric method are influenced by effects associated with flow patterns, bypassing, and aerodynamic factors such as buoyancy. Gas flow rate smaller than 100 mL/min is required for a reliable measurement.8 Furthermore, this technique is not sensitive to the nature of gases absorbed or desorbed because it is based only on weighing. At high temperature or high pressure, the mass resolution of the gravimetric apparatus is significantly reduced. In addition, the response time is 10-30 s, which is not suitable for short-duration and kinetics study. Thermal desorption spectroscopy (TDS) measures only the hydrogen desorption in high vacuum utilizing mass spectrometry. In our laboratory, we are applying a technique using a tapered-element oscillating microbalance (TEOM), where the solid sample is examined as a packed bed, through which the gas passes. This technique circumvents most of the above problems associated with the volumetric method, gravimetric method, or TDS. The main feature of the TEOM technique is an oscillating element that is based on inertial forces, instead of weight, to measure the amount adsorbed or desorbed. Therefore, errors associated with leakage, flow patterns, bypassing, and aerodynamic factors can be eliminated. Furthermore, mass resolution of TEOM is as high as 1 µg and not reduced at higher temperature and higher pressure. Consequently, the TEOM technique allows accurate and reproducible measurements. In addition, the fast response time of up to 0.1 s makes the shortresidence-time measurement and kinetic study possible and reliable. At static conditions, the performance of the TEOM will improve further.8 However, there is little information in the literature regarding the measure(8) Liu, C. Rupprecht & Patashnick, TEOM measurement. Personal Communication.
10.1021/ef050062g CCC: $30.25 © 2005 American Chemical Society Published on Web 08/11/2005
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Figure 1. Simplified diagram of the oscillating tapered element with a sample bed.
ment of hydrogen absorption/release in metal hydride by TEOM. In this work, hydrogen storage/release properties of magnesium and palladium-doped magnesium were investigated by the TEOM technique. For comparison, the hydrogen storage/release properties were also measured by a volumetric method.
Figure 2. Comparison of hydrogen absorption properties of Pd-doped magnesium measured by the TEOM technique and the volumetric method at 573 K under 400 psi. Samples used in this paper are magnesium powder (Aldrich, 99.8%, 325 mesh) and palladium nanoparticle-doped magnesium. The nanoparticle palladium-doped magnesium was prepared by an in-situ catalyst deposition method reported elsewhere.10 The doped amount of Pd is 0.5 mol %, and the particle size of Pd estimated from XRD diffraction patterns is around 7 nm.10 Hydrogen absorption was carried out at 573 K and 400 psi. Hydrogen release was measured at 613 K and ambient pressure.
Experimental Section An integrated flow system designed on the basis of a Rupprecht & Patashnick TEOM 1500-pulse mass analyzer (with a 100-mg sample cell) was used for measurement of hydrogen storage and release properties. The active element of the TEOM consists of a tube constructed of a material having a special taper. A feedback system maintains the oscillation of the tapered tube. The natural frequency will change in relation to the mass in the tapered tube. The mass change is then determined by the change of the oscillating frequency through eq 1:9
∆M ) K0
(
1 1 f12 f02
)
(1)
where ∆M is the weight change, K0 is the spring constant, f0 is the oscillation frequency at the start of the experiment, and f1 is the oscillation frequency at a given time during the experiment. K0 is relatively constant over a large temperature range and changes slightly with temperature. To overcome any variation in K0 with experimental conditions, experiments include background runs to determine baseline performance of the TEOM over the range of experimental conditions. The background data is then used to subtract out any variability. A simplified diagram of the oscillating element in a TEOM system is shown in Figure 1. Gases are fed to the unit using Brooks mass flow controllers. The total pressure in the oscillating element is controlled by a backpressure regulator. Quartz wool was used at the top and the bottom of the sample bed to keep the hydrogen storage materials firmly packed, a condition which is essential for the stable measurement. Both the feed gas and purge gas were hydrogen, and the gas flow rate was kept at 100 mL/min. About 30 mg of sample was used in each test. During the test, the amount of hydrogen absorbed from the stream of gas flowing over the sample is below 1%. A detailed description of the TEOM apparatus is available in the literature.9 (9) Zhu, W.; Graaf, J. M. v. d.; Broeke, L. J. P. v. d.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1998, 37, 1934.
Results and Discussion 1. Hydrogen Absorption Measurement. The hydrogen absorption performance of Pd-doped magnesium was measured by TEOM at 573 K under 400 psi and compared with that measured by the volumetric method under comparable conditions in Figure 2. The hydrogen storage capacity measured by TEOM is almost the same as that measured by the volumetric method, which indicates the accuracy of the two measurement methods. However, the hydrogen absorption rate measured by TEOM is faster than that measured by the volumetric method. When hydrogen absorption is measured by the volumetric method, the Pd-doped magnesium is sealed in a vessel with high-pressure hydrogen gas. Therefore, hydrogen storage is carried out in static status and the hydrogen concentration gradient exists in the adsorption bed, which results in the slow mass transfer in the absorption bed and the slow hydrogen absorption rate. On the other hand, when hydrogen absorption is measured by TEOM, a high flow rate forces hydrogen gas to flow through the absorption bed, which improves the mass transfer in the absorption bed and accelerates the hydrogen absorption rate. Another important feature is that the response time for TEOM is very fast, as short as 0.1 s, whereas the response time for the volumetric method is much longer. TEOM makes the fast measurement and kinetic study possible and reliable. Chen et al.11 also reported that the TEOM provided more accurate data on the catalyst deactivation kinetics than the conventional microbalance. 2. Hydrogen Release Measurement. Figure 3 compares the hydrogen release properties of Pd-doped (10) Xu, X. C.; Zheng, J.; Song, C. S. Appl. Catal. A: General, submitted for publication. (11) Chen, D.; Gronvold, A.; Rebo, H. P.; Moljord, K.; Holmen, A. Appl. Catal. A: General 1996, 137, L1.
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Figure 3. Comparison of hydrogen release properties of Pddoped magnesium measured by the TEOM technique and the volumetric method at 613 K under ambient pressure.
Figure 4. Effect of nanoparticle palladium catalysts on the hydrogen release properties of magnesium hydride measured by TEOM at 613 K under ambient pressure.
magnesium measured by TEOM and the volumetric method at 613 K under atmosphere pressure. Similarly, the hydrogen release rate measured by the TEOM technique is also faster than that measured by the volumetric method. Due to the elimination of the hydrogen concentration gradient in the absorption bed and the fast response time, the TEOM technique provides more accurate and detailed information on the hydrogen absorption and release kinetic than the volumetric method or other gravimetric methods, which is important for the study of next-generation highperformance hydrogen storage materials and for the proper design of hydrogen storage devices for practical applications. 3. Effect of Nanosized Pd Catalysts on Hydrogen Release. The effect of nanoparticle palladium catalysts on the hydrogen release properties of magnesium hydride was measured by the TEOM technique at 613 K under ambient pressure. The results are shown in Figure 4. Apparently, the rate of hydrogen release for palladium-doped magnesium hydride is much faster than that for the undoped magnesium hydride, which is in agreement with that measured by the volumetric method.10 While the Pd-doped magnesium readily releases hydrogen, the undoped magnesium needs 5 min of incubation time before it releases hydrogen. At 10 min, magnesium hydride and Pd-doped magnesium hydride release 0.76 wt % and 4.03 wt % hydrogen, respectively. After 40 min of hydrogen release, Pd-doped magnesium hydride releases almost all the absorbed
hydrogen, and the undoped magnesium hydride only releases about 50% of the absorbed hydrogen. This is most likely due to hydrogen spillover with the aid of palladium nanoparticles. Conclusions The tapered-element oscillating microbalance (TEOM) technique provides dynamic and more accurate information on the hydrogen absorption and release properties of magnesium than the volumetric method or other gravimetric methods, which is important for the study of hydrogen storage materials and for the proper design of hydrogen storage devices for practical applications. The results with the TEOM technique show that doping of magnesium with nanoparticles of Pd significantly increase the speed of the hydrogen absorption and hydrogen release, particularly in the first few minutes. This is most likely due to hydrogen spillover with the aid of palladium nanoparticles. Acknowledgment. We are pleased to acknowledge the financial support by a Seed Grant from the Hydrogen Energy Center at the Pennsylvania State University. C. Song also thanks the U.S. Department of State and the U.S.-U.K. Fulbright Commission for a Fulbright Distinguished Scholar award for a sabbatical stay at the Department of Chemical Engineering, Imperial College London, University of London, U.K. EF050062G
