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Self-ignition combustion synthesis of LaNi5 at different hydrogen pressures
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 6 0 4 e8 6 0 9
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Self-ignition combustion synthesis of LaNi5 at different hydrogen pressures Naoto Yasuda, Tohru Tsuchiya, Shino Sasaki, Noriyuki Okinaka, Tomohiro Akiyama* Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
article info
abstract
Article history:
In this paper, we describe the self-ignition combustion synthesis (SICS) of LaNi5 utilizing the
Received 21 February 2011
hydrogenation heat of metallic calcium at different hydrogen pressures, and focus on the
Received in revised form
effect of hydrogen pressure on the ignition temperature and the initial activation of
24 March 2011
hydrogenation. In the experiments, La2O3, Ni, and Ca were dry-mixed, and then heated at
Accepted 3 April 2011
0.1, 0.5, and 1.0 MPa of hydrogen pressure until ignition due to the hydrogenation of
Available online 30 April 2011
calcium. The products were recovered after natural cooling for 2 h. The results showed that the ignition temperature lowered with hydrogen pressure. The products changed from
Keywords:
bulk to powder with hydrogen pressure. This was probably caused by volume expansion
Hydrogen storage alloy
due to hydrogenation at higher pressure. The product obtained at 1.0 MPa showed the
Combustion synthesis
highest hydrogen storage capacity under an initial hydrogen pressure of 0.95 MPa. The
LaNi5
results of this research can be applied as an innovative production route for LaNi5 without
Calcio-thermic reduction
the conventional melting of La and Ni.
Gas-solid reaction
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Activation treatment
1.
Introduction
Nickel rare-earth metal compounds, so-called AB5-type hydrogen storage alloys, have been extensively studied owing to their good hydrogenation properties and potential applications such as in hydrogen storage, chemical heat pumps, hydrogen purification, and Ni-metal hydride batteries [1,2]. Hydrogen storage alloys are conventionally produced using a melting method that requires several time- and energy-consuming processes, such as heat treatment, pulverization, and activation treatment. The product requires the treatment of melting and homogenization at high temperature for extended periods. Post-treatment of pulverization and activation is also necessary for increasing the surface area of the product and improving its reactivity with hydrogen. Specifically, so-called activation treatment [3e5] involves repeated procedure of hydrogenation and
reserved.
dehydrogenation by heating and vacuuming. This is the most energy consuming procedure in the production system. To solve this problem, one of the authors recently proposed hydriding combustion synthesis (HCS) and self-ignition combustion synthesis (SICS) as innovative manufacturing processes for Mgbased and TiFe-based hydrogen storage alloys [6e9]. These processes were deemed considerably attractive from the perspective of minimized operating time and energy savings in comparison to the conventional melting method. The significant benefits of SICS have been reported on Mg-based and Ti-based alloys. However, no papers have reported on nickel rare-earth-based alloys of SICS from technical difficulties; in general, the fine powders of rare-earth metal are easily oxidized or hydroxylated in air. In a previous report, we proposed that SICS utilizing the hydrogenation heat of metallic calcium for the production of
* Corresponding author. Tel.: þ81 11 706 6842; fax: þ81 11 726 0731. E-mail address: [email protected] (T. Akiyama). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.017
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nickel rare-earth-based alloys such as LaNi5 [10]. The proposed method resulted in the production of a hydrogen storage alloy of LaNi5 using nickel and La2O3 powders as raw materials and calcium grains as both the reducing agent and the heat source. The SICS product obtained in hydrogen improved the initial hydrogenation properties in comparison to the product obtained in argon at a temperature of 298 K and an initial hydrogen pressure of 4.1 MPa. However, the initial activity at hydrogen pressures lower than 1.0 MPa is much more important for practical use owing to the Japanese regulation of the use of high-pressure gas. In general, the activation treatment requires more hydrogen pressure than the equilibrium hydrogen pressure of hydrogen storage alloys [11]. The newly developed alloy is strongly required to store as much hydrogen as possible at pressures closer to the equilibrium pressure. However, no papers have reported on such optimization from the kinetics and equilibrium theory perspectives. Therefore, the purpose of this study is to synthesize a LaNi5 hydrogen storage alloy by SICS at different hydrogen pressures and to evaluate the hydrogenation properties, in which initial activation behavior at less than 1.0 MPa in hydrogen pressure was mainly examined. The effect of hydrogen pressure during SICS on the ignition temperature and particle size distribution were also examined in the experiments. The results obtained will support the possibility of a new LaNi5 production process based on SICS for practical use.
2.
Experiment
A detailed explanation of both the apparatus and the preparation of samples is provided elsewhere [8,10]; only the framework of the experiments is described here. A watercooled chamber with a reactor and a graphite heater was used. The reactor had two R-type thermocouples for controlling the temperature inside the reactor and for measuring the sample temperature. The pressure in the reactor was also maintained at a constant value by using the on-off controller. A heating electric power of 7.5 kW was sufficient for uniform heating of the sample to 1358 K. La2O3 and Ni powders used were both 99.9% pure and were less than 45 mm and 3e5 mm in size, respectively. Small chips of metallic calcium were 99.5% pure and were 1e3 mm in size. In the experiments, 40 g of the reagents were first mixed in a molar ratio of La2O3:Ni:Ca ¼ 1:10:6, where the amount of Ca was twice as much as the theoretical value, for accelerating the reduction of La2O3. Then, each sample was placed in a carbon crucible, which was carefully covered by a carbon sheet to prevent the sample from sticking between the crucible and the product. An R-type thermocouple placed inside a protective alumina tube was introduced into the center of the sample. Table 1 summarizes the heating conditions for the selfignition mode during combustion synthesis. The furnace was evacuated at a pressure of 20 Pa using a rotary pump and was replaced by 99.999% pure argon. After repeating the procedure of vacuuming and replacing to achieve substitution four times, we finally charged hydrogen to the desired pressure. In the experiments, samples were uniformly heated by
Table 1 e Heating conditions for self-ignition mode during combustion synthesis. Run No 1 2 3
Thermal Power [kW]
Hydrogen Pressure [MPa]
7.5 7.5 7.5
0.1 0.5 1.0
using the graphite heater at an electric power of 7.5 kW, and changes in the sample temperature were monitored. As soon as an exothermic reaction was observed, the heating of the samples was immediately stopped and the samples were naturally cooled in the same atmosphere for 2 h. The products were polished in order to remove the carbon sheet from the surface of them. The bulk product was first crushed to less than 2.8 mm in size using a tungsten mortar. Then, the product powders obtained were washed with an aqueous solution of 5 mass% acetic acid in order to remove the residue of calcium and the by-product of calcium oxide and were finally dried in a desiccator in air at 353 K for 12 h. The phases of products were identified by X-ray diffraction (XRD), and the surfaces of the products were observed using a scanning electron microscope (SEM). The specific surface areas of the products were also determined by the BET method. The particle size distributions of products were measured by the laser diffraction scattering method. Shortly after the washing treatment, the products were placed in two stainless steel reactors for the measurement of the pressure-composition-isotherm (PCT) property and the initial hydrogenation kinetics, which were measured using Sieverts’ method. It should be noted that each measurement was performed using different samples after the washing treatment. The PCT properties of the products were evaluated after repeating the procedure of vacuuming/pressurizing with 4.1 MPa of hydrogen, five times; these experimental conditions were sufficient to activate and to stabilize the LaNi5 alloy. During the measurement of the initial hydrogenation kinetics, the other reactor filled with products was first evacuated for more than 1 h at 298 K using a turbo-molecular pump, and then hydrogen, with a pressure of 0.95 MPa, was introduced.
3.
Results and discussion
Fig. 1 shows the changes in the sample temperatures with time and their quadratic differential values during SICS experiments at different hydrogen pressures. The temperature of all samples jumped at around 0.2e0.3 ks owing to the exothermic reaction. As soon as the exothermic reaction occurred, the electric heating of the sample was turned off, and then, each sample was naturally cooled in the furnace. The ignition point has a maximum value in quadratic temporal differentiations of temperature. These values remarkably decreased from 890 to 620 K with hydrogen pressure, because the hydrogenation reaction of calcium (Ca þ H2 / CaH2) was more enhanced by the higher hydrogen pressure.
Temperature [K]
d2T/(dt)2 [K/s2]
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earth oxide by calcium has been proposed for the production of rare-earth-based materials [12,13]. Ohtsuka and Tanabe et al. [14,15] reported the formation mechanism of LaNi5 by the RD process. They elucidated that the CaNi5 compound was generated as an intermediate product and that the melting of calcium enhanced significantly the growth rate.
60 800K
40
620K
20
890K
0 -20 1500
1283K 1280K
1300
Run 1
1218K
Run 2
1100 900
I.P.-1
700
I.P.-2
Run 3
I.P.-3
500
I.P.: Ignition point
300 0
0.2
0.4
0.6
0.8
1
Time [ks] Fig. 1 e Changes in sample temperature with time and their secondary differentiations during SICS experiments at different hydrogen pressures of 0.1 MPa (Run 1), 0.5 MPa (Run 2), and 1.0 MPa (Run 3). Note that the ignition point decreased with hydrogen pressure because the calcium (Ca D H2 5 CaH2) was enhanced by higher hydrogen pressure.
The maximum temperature lowered with increasing pressure; however, the values of that at Runs 1 and 2 were almost equal. Fig. 2 shows the effect of hydrogen pressure during SICS on ignition and maximum temperature of the products. The broken line represents the boundary of thermodynamically stable regions of Ca and CaH2. The diagram indicates that La2O3 can be reduced by CaH2 because all the products are synthesized at the stability region of CaH2. The maximum temperature at Run 1 was close to the broken line. This result indicates that the increase in the sample temperature at Run 1 was inhibited by the endothermic reaction; a part of the calcium must be decomposed again (CaH2 / Ca þ H2). According to a literature search using a major database, reduction-diffusion (RD) process based on the reduction of rare-
Ca + H2
(1)
3CaðlÞ þ La2 O3 ðsÞ/2LaðlÞ þ 3CaOðsÞ
(2)
CaNi5 ðsÞ þ LaðlÞ/LaNi5 ðs; lÞ þ CaðlÞ
(3)
We estimated that the reaction mechanism in SICS was similar to that of the RD method; however, the CS reactions were completed in only a few seconds. CaðsÞ þ H2 ðgÞ/CaH2 ðs; lÞ
(4)
CaH2 ðs; lÞ þ 5NiðsÞ/CaNi5 ðsÞ þ H2 ðgÞ
(5)
3CaH2 ðlÞ þ La2 O3 ðsÞ/2LaðlÞ þ 3CaOðsÞ þ H2 ðgÞ
(6)
CaNi5 ðsÞ þ LaðlÞ þ H2 ðgÞ/LaNi5 ðs; lÞ þ CaH2 ðs; lÞ
(7)
When the amount of calcium was twice that of the chemical equivalent value, the overall reaction could be expressed by the following equation: 1=2La2 O3 þ5Niþ3Caþ3=2H2 ¼ LaNi5 þ3=2CaOþ3=2CaH2 þ479kJ
CaH2
1300
14
1100
12 10
900 700
Ignition temp. Maximum temp.
500
(8)
After the SICS process, the powdered product was obtained at Run 3 without the pulverization process while the bulk products were obtained at Runs 1 and 2. The bulk products obtained at Runs 1 and 2 were crushed to less than 2.8 mm in size using a tungsten mortar. The product powders were washed with an aqueous solution of 5 mass% acetic acid. Fig. 3 shows particle size distributions of the products after the post-washing treatment, measured by the laser diffraction scattering method. The refractive index of LaNi5 was taken from Knyazev’s paper [16]. The results of Runs 1 and 2 showed two peaks; Runs 1 and 2 had a maximum value at around 15 mm and 60 mm, respectively. The larger peak was most likely
Frequency [%]
Temperature [K]
1500
CaðlÞ þ 5NiðsÞ/CaNi5 ðsÞ
Run 1 Run 2 Run 3
Run 3; D50= 16.7
8
Run 2; D50= 28.1 Run 1; D50= 45.1
6 4 2
300 0.0
0.3 0.6 0.9 Hydrogen Pressure [MPa]
1.2
Fig. 2 e Effect of hydrogen pressure during SICS on ignition and maximum temperature of the products. This result showed that La2O3 was reduced by CaH2 because all the products were synthesized at the thermodynamically stable region of CaH2.
0 1
10
100
1000
Particle size [ m]
Fig. 3 e Particle size distributions of the SICSed products with post-washing treatment, measured by the laser diffraction method. Run 3 showed only sharp peak with Poisson distribution.
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Pressure [MPa]
10 Run 1 Run 2 Run 3
1
0.1
0.01 0
0.4
0.8
1.2
1.6
Stored hydrogen [mass %] Fig. 6 e PCT curves of the three products, measured by Sievert’s method at 298 K. The equilibrium pressure of the products obtained at any hydrogen pressure was constant. Fig. 4 e SEM images and specific surfaces of the SICSed products after post-washing treatment. While the bulk products were obtained at Runs 1 and 2, the powdered product was obtained at Run 3 without pulverization treatment. Note that many cracks were induced at Run 3 by rapid volume expansion due to hydrogenation.
attributed to powder sintering, which decreased with hydrogen pressure in the experiments. Interestingly, Run 3 showed only a sharp peak with Poisson distribution. This result was explained by volume expansion due to hydrogenation during SICS; higher hydrogen pressure was effective for the generation of only fine powder without any sintering. This means that the product at Run 3 did not require any postpulverization treatment of the product, which would offer quite an attractive benefit. Fig. 4 shows the SEM images and specific surfaces of the SICSed products after the post-washing treatment. Note that many cracks were induced at Run 3 by rapid volume expansion due to hydrogenation. This can probably be explained by the fact that Run 3 produced quite large specific surface area of
1.6
Ni
LaNi5
Stored hydrogen [mass%]
Run 1 Intensity [a.u.]
the product in comparison to Runs 1 and 2. Therefore, the product at Run 3 absorbed hydrogen well during the cooling process. Fig. 5 shows the XRD patterns of the three products after the post-washing treatment. The major peaks of the all products were exactly indexed to the LaNi5 phase, although the peak observed at 45 suggests the existence of un-reacted nickel. The product obtained at Run 3 showed low and broadened intensity as compared with the other products. This result implies the existence of a hydride phase in which peaks were broadening due to hydrogenation [17,18]. Fig. 6 shows the PCT curves of the three products, measured by Sievert’s method at 298 K. Their PCT properties were evaluated after repeating the procedure of vacuuming/ pressurizing with hydrogen at 4.1 MPa five times. This procedure was sufficient to activate and stabilize the LaNi5 alloy. The equilibrium pressure of all the products was almost the same. We judged that the SICSed product was the LaNi5 phase from the results of the XRD analyses and the PCT measurements. In contrast, maximum hydrogen storage capacity slightly decreased with hydrogen pressure during
Run 2
Initial hydrogen pressure: 0.95 MPa Temperature: 298 K 1.2
0.8
Run 1 Run 2 Run 3 Reagent
0.4
Run 3 0 0
20
30
40
50
60
70
80
90
2 [degree]
Fig. 5 e XRD patterns of the three products after washing treatment, showing LaNi5 peaks as the major phase. The low and broadened intensity of the product obtained at Run 3 was probably caused by hydrogenation during the cooling process.
10
20 30 Time [ks]
40
50
Fig. 7 e Initial hydriding curves of reagent and SICSed LaNi5, measured by Sievert’s method at a temperature of 298 K and an initial hydrogen pressure of 0.95 MPa. Note that the products obtained at Run 3 showed the highest hydrogen storage capacity even under hydrogen pressure of less than 1.0 MPa.
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Table 2 e Results of the measurement of the initial hydrogenation kinetics and the ratios between equilibrium compositions in the measurement and maximum hydrogen storage capacities at PCT measurement.
Run 1 Run 2 Run 3 Reagent
P0 [MPa]
Peq [MPa]
Ceq [mass%]
Cmax [mass%]
Ceq/Cmax [mass%] []
0.95 0.95 0.95 0.95
0.51 0.55 0.39 0.87
1.02 0.95 1.31 0.17
1.51 1.48 1.42 1.54
0.68 0.64 0.92 0.11
P0: Initial hydrogen pressure [MPa]; Peq: Equilibrium hydrogen pressure [MPa]; Ceq: Equilibrium composition [mass%]; Cmax: Maximum hydrogen storage capacity at PCT measurement [mass%].
SICS. This was most likely attributed to the lower ignition temperature; the reaction heat was not high enough to synthesize pure LaNi5. The purity of the product could be improved by optimizing the experimental conditions, such as the sample amounts and/or heating rate. Fig. 7 shows the initial hydriding curves of the SICSed LaNi5 at a temperature of 298 K and an initial hydrogen pressure of 0.95 MPa, together with the pure LaNi5 reagent as a reference. The SICSed products used in this experiment were different from that used in PCT measurement in order to evaluate the initial hydrogenation kinetics. The reagent, with a diameter of 250e500 mm, did not absorb any hydrogen under the experimental conditions. The product obtained at Run 3 showed a different tendency as compared with the other SICSed products. It had the highest hydrogen storage capacity, even under less than 1.0 MPa, although it showed the lowest maximum hydrogen storage capacity (see Fig. 6 or Table 2). Table 2 shows the results of the measurement of the initial hydrogenation kinetics and the ratios between equilibrium compositions in this measurement and maximum hydrogen storage capacities at PCT measurement. P0, Peq and Ceq denote initial hydrogen pressure, Equilibrium hydrogen pressure and Equilibrium composition at kinetics measurement, respectively. In contrast, Cmax denotes Maximum hydrogen storage capacity at PCT measurement. The product obtained at Run 3 showed a 1.31 mass% in hydrogen storage capacity, which is equivalent to 92% of its maximum capacity. Inui et al. reported the effect of the lattice defects on hydrogen sorption properties [11]. The results showed that the absorption pressure of the LaNi5 alloy drastically decreased between the first and second cycles due to the formation of severe cracks and the introduction of lattice defects, such as a-type dislocations. The extent of decrease in the absorption pressure due to the formation of cracks depended on the particle size of alloy powder. These were the reasons why activation treatment required much higher hydrogen pressure than equilibrium pressure; the absorption pressure in the first cycle was much higher than its equilibrium pressure. The difference of initial activity in our experiments can be explained by a similar mechanism. It was estimated that it was not possible to activate the LaNi5 reagent since its particle size was large as compared to the SICSed LaNi5. On the other hand, the product obtained at Run 3 was easily activated since
its particle size was small and lattice defects were introduced owing to hydrogenation during SICS.
4.
Conclusion
The self-ignition combustion synthesis (SICS) of LaNi5 hydrogen storage alloys at different hydrogen pressures was comparatively studied and the following conclusions were derived. 1. The overall reaction eq. (8) of 1/2La2O3 þ 5Ni þ 3Ca þ 3/ 2H2 ¼ LaNi5 þ 3/2CaO þ 3/2CaH2 þ 479 kJ consists of four elemental equations from eqs. (4) to (7), including the hydrogenation of calcium (Ca þ H2 / CaH2 eq. (4)). The ignition point decreased with the hydrogen pressure, because the hydrogenation rate of calcium increased with the higher hydrogen pressure. 2. The shape of the product also depended on the hydrogen pressure. With increasing hydrogen pressures from 0.1 to 1.0 MPa, the product shape changed from bulk to powder. This is probably explained by the volume expansion due to the hydrogenation at the higher hydrogen pressure. 3. The initial activity of the product depended on the hydrogen pressure charged in the production. The product obtained at 1.0 MPa showed 1.31 mass% in hydrogen storage capacity when an initial hydrogen pressure of 0.95 MPa was charged. This value reached 92% of its maximum hydrogen storage capacity. The results also showed that the SICS at relatively high hydrogen pressure offers many benefits for lowering ignition temperature, minimizing operation time and energy, obtaining fine powdered product directly, and improving initial activity at hydrogen pressure of only 1.0 MPa.
Acknowledgments This research was partly supported by the Ministry of the Environment, Japan, under the project “Development of Hydrogen Freezer Using Industrial Waste Heat or Solar Heat for Reducing CO2 Emission” (FY 2008- FY 2010).
references
[1] Van Mal HH, Buschow KHJ, Miedema AR. Hydrogen absorption in LaNi5 and related compounds: experimental observations and their explanation. J Less Common Met 1974;35:65e76. [2] Willems JJG, Buschow KHJ. From permanent magnets to rechargeable hydride electrodes. J Less Common Met 1987; 129:13e30. [3] Belkbir L, Ge´rard N, Percheron-Gue´gan A, Achard JC. Kinetics of hydrogen absorption and desorption by ternary LaNi5-type intermetallic compounds. Int J Hydrogen Energ 1979;4: 541e57. [4] Sandrock GD, Goodell PD. Surface poisoning of LaNi5, FeTi and (Fe, Mn)Ti by O2, Co and H2O. J Less Common Met 1980; 73:161e8.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 6 0 4 e8 6 0 9
[5] Uchida H, Ohtani Y, Ozawa M, Kawahata T, Suzuki T. Surface processes of H2 in the initial activationof LaNi5. J Less Common Met 1991;172-174:983e96. [6] Akiyama T, Isogai H, Yagi J. Hydriding combustion synthesis for the production of hydrogen storage alloy. J Alloys Compd 1997;252:L1e4. [7] Li L, Akiyama T, Yagi J-I. Activation behaviors of Mg2NiH4 at different hydrogen pressures in hydriding combustion synthesis. Int J Hydrogen Energ 2001;26:1035e40. [8] Wakabayashi R, Sasaki S, Saita I, Sato M, Uesugi H, Akiyama T. Self-ignition combustion synthesis of TiFe in hydrogen atmosphere. J Alloys Compd 2009;480:592e5. [9] Yasuda N, Wakabayashi R, Sasaki S, Okinaka N, Akiyama T. Self-ignition combustion synthesis of TiFe1-xMnx hydrogen storage alloy. Int J Hydrogen Energ 2009;34:9122e7. [10] Yasuda N, Sasaki S, Okinaka N, Akiyama T. Self-ignition combustion synthesis of LaNi5 utilizing hydrogenation heat of metallic calcium. Int J Hydrogen Energ 2010;35:11035e41. [11] Inui H, Yamamoto T, Hirota M, Yamaguchi M. Lattice defects introduced during hydrogen absorption-desorption cycles and their effects on PeC characteristics in some intermetallic compounds. J Alloys Compd 2002;330-332:117e24.
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[12] Li Z, Yasuda K, Itagaki K. Calciothermic reduction of mischmetal oxide and formation of RmNi5. J Alloys Compd 1993;193:26e8. [13] Takeya K, Tsugita Y, Okajima Y, Sakai T, Miyamura H, Kuriyama N, et al. Hydrogen storage alloy powders produced by a reduction-diffusion process and their electrode properties. J Alloys Compd 1993;192:167e9. [14] Ohtsuka M, Kim DY, Itagaki K. Formation of RExNiy (RE ¼ La, Ce, Pr or Nd) compounds by calciothermic reduction of La2O3, CeO2, Pr6O11 and Nd2O3. J Alloys Compd 1995;230: 46e52. [15] Tanabe T, Asaki Z. Formation mechanism of LaNi5 in the reduction-diffusion process. Metall Mater Trans B 1998;29: 331e8. [16] Knyazev Y, Kuz’min Y, Kuchin A. Optical properties of RNi5 intermetallic compounds (R ¼ Y, La, Ce). Optic Spectros 2007; 102:408e12. [17] Kisi EH, Buckley CE, Gray EM. The hydrogen activation of LaNi5. J Alloys Compd 1992;185:369e84. [18] Nakamura Y, Oguro K, Uehara I, Akiba E. X-ray diffraction peak broadening and degradation in LaNi5-based alloys. Int J Hydrogen Energ 2000;25:531e7.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Self-ignition combustion synthesis of LaNi5 at different hydrogen pressures Naoto Yasuda, Tohru Tsuchiya, Shino Sasaki, Noriyuki Okinaka, Tomohiro Akiyama* Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
article info
abstract
Article history:
In this paper, we describe the self-ignition combustion synthesis (SICS) of LaNi5 utilizing the
Received 21 February 2011
hydrogenation heat of metallic calcium at different hydrogen pressures, and focus on the
Received in revised form
effect of hydrogen pressure on the ignition temperature and the initial activation of
24 March 2011
hydrogenation. In the experiments, La2O3, Ni, and Ca were dry-mixed, and then heated at
Accepted 3 April 2011
0.1, 0.5, and 1.0 MPa of hydrogen pressure until ignition due to the hydrogenation of
Available online 30 April 2011
calcium. The products were recovered after natural cooling for 2 h. The results showed that the ignition temperature lowered with hydrogen pressure. The products changed from
Keywords:
bulk to powder with hydrogen pressure. This was probably caused by volume expansion
Hydrogen storage alloy
due to hydrogenation at higher pressure. The product obtained at 1.0 MPa showed the
Combustion synthesis
highest hydrogen storage capacity under an initial hydrogen pressure of 0.95 MPa. The
LaNi5
results of this research can be applied as an innovative production route for LaNi5 without
Calcio-thermic reduction
the conventional melting of La and Ni.
Gas-solid reaction
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Activation treatment
1.
Introduction
Nickel rare-earth metal compounds, so-called AB5-type hydrogen storage alloys, have been extensively studied owing to their good hydrogenation properties and potential applications such as in hydrogen storage, chemical heat pumps, hydrogen purification, and Ni-metal hydride batteries [1,2]. Hydrogen storage alloys are conventionally produced using a melting method that requires several time- and energy-consuming processes, such as heat treatment, pulverization, and activation treatment. The product requires the treatment of melting and homogenization at high temperature for extended periods. Post-treatment of pulverization and activation is also necessary for increasing the surface area of the product and improving its reactivity with hydrogen. Specifically, so-called activation treatment [3e5] involves repeated procedure of hydrogenation and
reserved.
dehydrogenation by heating and vacuuming. This is the most energy consuming procedure in the production system. To solve this problem, one of the authors recently proposed hydriding combustion synthesis (HCS) and self-ignition combustion synthesis (SICS) as innovative manufacturing processes for Mgbased and TiFe-based hydrogen storage alloys [6e9]. These processes were deemed considerably attractive from the perspective of minimized operating time and energy savings in comparison to the conventional melting method. The significant benefits of SICS have been reported on Mg-based and Ti-based alloys. However, no papers have reported on nickel rare-earth-based alloys of SICS from technical difficulties; in general, the fine powders of rare-earth metal are easily oxidized or hydroxylated in air. In a previous report, we proposed that SICS utilizing the hydrogenation heat of metallic calcium for the production of
* Corresponding author. Tel.: þ81 11 706 6842; fax: þ81 11 726 0731. E-mail address: [email protected] (T. Akiyama). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.017
8605
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nickel rare-earth-based alloys such as LaNi5 [10]. The proposed method resulted in the production of a hydrogen storage alloy of LaNi5 using nickel and La2O3 powders as raw materials and calcium grains as both the reducing agent and the heat source. The SICS product obtained in hydrogen improved the initial hydrogenation properties in comparison to the product obtained in argon at a temperature of 298 K and an initial hydrogen pressure of 4.1 MPa. However, the initial activity at hydrogen pressures lower than 1.0 MPa is much more important for practical use owing to the Japanese regulation of the use of high-pressure gas. In general, the activation treatment requires more hydrogen pressure than the equilibrium hydrogen pressure of hydrogen storage alloys [11]. The newly developed alloy is strongly required to store as much hydrogen as possible at pressures closer to the equilibrium pressure. However, no papers have reported on such optimization from the kinetics and equilibrium theory perspectives. Therefore, the purpose of this study is to synthesize a LaNi5 hydrogen storage alloy by SICS at different hydrogen pressures and to evaluate the hydrogenation properties, in which initial activation behavior at less than 1.0 MPa in hydrogen pressure was mainly examined. The effect of hydrogen pressure during SICS on the ignition temperature and particle size distribution were also examined in the experiments. The results obtained will support the possibility of a new LaNi5 production process based on SICS for practical use.
2.
Experiment
A detailed explanation of both the apparatus and the preparation of samples is provided elsewhere [8,10]; only the framework of the experiments is described here. A watercooled chamber with a reactor and a graphite heater was used. The reactor had two R-type thermocouples for controlling the temperature inside the reactor and for measuring the sample temperature. The pressure in the reactor was also maintained at a constant value by using the on-off controller. A heating electric power of 7.5 kW was sufficient for uniform heating of the sample to 1358 K. La2O3 and Ni powders used were both 99.9% pure and were less than 45 mm and 3e5 mm in size, respectively. Small chips of metallic calcium were 99.5% pure and were 1e3 mm in size. In the experiments, 40 g of the reagents were first mixed in a molar ratio of La2O3:Ni:Ca ¼ 1:10:6, where the amount of Ca was twice as much as the theoretical value, for accelerating the reduction of La2O3. Then, each sample was placed in a carbon crucible, which was carefully covered by a carbon sheet to prevent the sample from sticking between the crucible and the product. An R-type thermocouple placed inside a protective alumina tube was introduced into the center of the sample. Table 1 summarizes the heating conditions for the selfignition mode during combustion synthesis. The furnace was evacuated at a pressure of 20 Pa using a rotary pump and was replaced by 99.999% pure argon. After repeating the procedure of vacuuming and replacing to achieve substitution four times, we finally charged hydrogen to the desired pressure. In the experiments, samples were uniformly heated by
Table 1 e Heating conditions for self-ignition mode during combustion synthesis. Run No 1 2 3
Thermal Power [kW]
Hydrogen Pressure [MPa]
7.5 7.5 7.5
0.1 0.5 1.0
using the graphite heater at an electric power of 7.5 kW, and changes in the sample temperature were monitored. As soon as an exothermic reaction was observed, the heating of the samples was immediately stopped and the samples were naturally cooled in the same atmosphere for 2 h. The products were polished in order to remove the carbon sheet from the surface of them. The bulk product was first crushed to less than 2.8 mm in size using a tungsten mortar. Then, the product powders obtained were washed with an aqueous solution of 5 mass% acetic acid in order to remove the residue of calcium and the by-product of calcium oxide and were finally dried in a desiccator in air at 353 K for 12 h. The phases of products were identified by X-ray diffraction (XRD), and the surfaces of the products were observed using a scanning electron microscope (SEM). The specific surface areas of the products were also determined by the BET method. The particle size distributions of products were measured by the laser diffraction scattering method. Shortly after the washing treatment, the products were placed in two stainless steel reactors for the measurement of the pressure-composition-isotherm (PCT) property and the initial hydrogenation kinetics, which were measured using Sieverts’ method. It should be noted that each measurement was performed using different samples after the washing treatment. The PCT properties of the products were evaluated after repeating the procedure of vacuuming/pressurizing with 4.1 MPa of hydrogen, five times; these experimental conditions were sufficient to activate and to stabilize the LaNi5 alloy. During the measurement of the initial hydrogenation kinetics, the other reactor filled with products was first evacuated for more than 1 h at 298 K using a turbo-molecular pump, and then hydrogen, with a pressure of 0.95 MPa, was introduced.
3.
Results and discussion
Fig. 1 shows the changes in the sample temperatures with time and their quadratic differential values during SICS experiments at different hydrogen pressures. The temperature of all samples jumped at around 0.2e0.3 ks owing to the exothermic reaction. As soon as the exothermic reaction occurred, the electric heating of the sample was turned off, and then, each sample was naturally cooled in the furnace. The ignition point has a maximum value in quadratic temporal differentiations of temperature. These values remarkably decreased from 890 to 620 K with hydrogen pressure, because the hydrogenation reaction of calcium (Ca þ H2 / CaH2) was more enhanced by the higher hydrogen pressure.
Temperature [K]
d2T/(dt)2 [K/s2]
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earth oxide by calcium has been proposed for the production of rare-earth-based materials [12,13]. Ohtsuka and Tanabe et al. [14,15] reported the formation mechanism of LaNi5 by the RD process. They elucidated that the CaNi5 compound was generated as an intermediate product and that the melting of calcium enhanced significantly the growth rate.
60 800K
40
620K
20
890K
0 -20 1500
1283K 1280K
1300
Run 1
1218K
Run 2
1100 900
I.P.-1
700
I.P.-2
Run 3
I.P.-3
500
I.P.: Ignition point
300 0
0.2
0.4
0.6
0.8
1
Time [ks] Fig. 1 e Changes in sample temperature with time and their secondary differentiations during SICS experiments at different hydrogen pressures of 0.1 MPa (Run 1), 0.5 MPa (Run 2), and 1.0 MPa (Run 3). Note that the ignition point decreased with hydrogen pressure because the calcium (Ca D H2 5 CaH2) was enhanced by higher hydrogen pressure.
The maximum temperature lowered with increasing pressure; however, the values of that at Runs 1 and 2 were almost equal. Fig. 2 shows the effect of hydrogen pressure during SICS on ignition and maximum temperature of the products. The broken line represents the boundary of thermodynamically stable regions of Ca and CaH2. The diagram indicates that La2O3 can be reduced by CaH2 because all the products are synthesized at the stability region of CaH2. The maximum temperature at Run 1 was close to the broken line. This result indicates that the increase in the sample temperature at Run 1 was inhibited by the endothermic reaction; a part of the calcium must be decomposed again (CaH2 / Ca þ H2). According to a literature search using a major database, reduction-diffusion (RD) process based on the reduction of rare-
Ca + H2
(1)
3CaðlÞ þ La2 O3 ðsÞ/2LaðlÞ þ 3CaOðsÞ
(2)
CaNi5 ðsÞ þ LaðlÞ/LaNi5 ðs; lÞ þ CaðlÞ
(3)
We estimated that the reaction mechanism in SICS was similar to that of the RD method; however, the CS reactions were completed in only a few seconds. CaðsÞ þ H2 ðgÞ/CaH2 ðs; lÞ
(4)
CaH2 ðs; lÞ þ 5NiðsÞ/CaNi5 ðsÞ þ H2 ðgÞ
(5)
3CaH2 ðlÞ þ La2 O3 ðsÞ/2LaðlÞ þ 3CaOðsÞ þ H2 ðgÞ
(6)
CaNi5 ðsÞ þ LaðlÞ þ H2 ðgÞ/LaNi5 ðs; lÞ þ CaH2 ðs; lÞ
(7)
When the amount of calcium was twice that of the chemical equivalent value, the overall reaction could be expressed by the following equation: 1=2La2 O3 þ5Niþ3Caþ3=2H2 ¼ LaNi5 þ3=2CaOþ3=2CaH2 þ479kJ
CaH2
1300
14
1100
12 10
900 700
Ignition temp. Maximum temp.
500
(8)
After the SICS process, the powdered product was obtained at Run 3 without the pulverization process while the bulk products were obtained at Runs 1 and 2. The bulk products obtained at Runs 1 and 2 were crushed to less than 2.8 mm in size using a tungsten mortar. The product powders were washed with an aqueous solution of 5 mass% acetic acid. Fig. 3 shows particle size distributions of the products after the post-washing treatment, measured by the laser diffraction scattering method. The refractive index of LaNi5 was taken from Knyazev’s paper [16]. The results of Runs 1 and 2 showed two peaks; Runs 1 and 2 had a maximum value at around 15 mm and 60 mm, respectively. The larger peak was most likely
Frequency [%]
Temperature [K]
1500
CaðlÞ þ 5NiðsÞ/CaNi5 ðsÞ
Run 1 Run 2 Run 3
Run 3; D50= 16.7
8
Run 2; D50= 28.1 Run 1; D50= 45.1
6 4 2
300 0.0
0.3 0.6 0.9 Hydrogen Pressure [MPa]
1.2
Fig. 2 e Effect of hydrogen pressure during SICS on ignition and maximum temperature of the products. This result showed that La2O3 was reduced by CaH2 because all the products were synthesized at the thermodynamically stable region of CaH2.
0 1
10
100
1000
Particle size [ m]
Fig. 3 e Particle size distributions of the SICSed products with post-washing treatment, measured by the laser diffraction method. Run 3 showed only sharp peak with Poisson distribution.
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Pressure [MPa]
10 Run 1 Run 2 Run 3
1
0.1
0.01 0
0.4
0.8
1.2
1.6
Stored hydrogen [mass %] Fig. 6 e PCT curves of the three products, measured by Sievert’s method at 298 K. The equilibrium pressure of the products obtained at any hydrogen pressure was constant. Fig. 4 e SEM images and specific surfaces of the SICSed products after post-washing treatment. While the bulk products were obtained at Runs 1 and 2, the powdered product was obtained at Run 3 without pulverization treatment. Note that many cracks were induced at Run 3 by rapid volume expansion due to hydrogenation.
attributed to powder sintering, which decreased with hydrogen pressure in the experiments. Interestingly, Run 3 showed only a sharp peak with Poisson distribution. This result was explained by volume expansion due to hydrogenation during SICS; higher hydrogen pressure was effective for the generation of only fine powder without any sintering. This means that the product at Run 3 did not require any postpulverization treatment of the product, which would offer quite an attractive benefit. Fig. 4 shows the SEM images and specific surfaces of the SICSed products after the post-washing treatment. Note that many cracks were induced at Run 3 by rapid volume expansion due to hydrogenation. This can probably be explained by the fact that Run 3 produced quite large specific surface area of
1.6
Ni
LaNi5
Stored hydrogen [mass%]
Run 1 Intensity [a.u.]
the product in comparison to Runs 1 and 2. Therefore, the product at Run 3 absorbed hydrogen well during the cooling process. Fig. 5 shows the XRD patterns of the three products after the post-washing treatment. The major peaks of the all products were exactly indexed to the LaNi5 phase, although the peak observed at 45 suggests the existence of un-reacted nickel. The product obtained at Run 3 showed low and broadened intensity as compared with the other products. This result implies the existence of a hydride phase in which peaks were broadening due to hydrogenation [17,18]. Fig. 6 shows the PCT curves of the three products, measured by Sievert’s method at 298 K. Their PCT properties were evaluated after repeating the procedure of vacuuming/ pressurizing with hydrogen at 4.1 MPa five times. This procedure was sufficient to activate and stabilize the LaNi5 alloy. The equilibrium pressure of all the products was almost the same. We judged that the SICSed product was the LaNi5 phase from the results of the XRD analyses and the PCT measurements. In contrast, maximum hydrogen storage capacity slightly decreased with hydrogen pressure during
Run 2
Initial hydrogen pressure: 0.95 MPa Temperature: 298 K 1.2
0.8
Run 1 Run 2 Run 3 Reagent
0.4
Run 3 0 0
20
30
40
50
60
70
80
90
2 [degree]
Fig. 5 e XRD patterns of the three products after washing treatment, showing LaNi5 peaks as the major phase. The low and broadened intensity of the product obtained at Run 3 was probably caused by hydrogenation during the cooling process.
10
20 30 Time [ks]
40
50
Fig. 7 e Initial hydriding curves of reagent and SICSed LaNi5, measured by Sievert’s method at a temperature of 298 K and an initial hydrogen pressure of 0.95 MPa. Note that the products obtained at Run 3 showed the highest hydrogen storage capacity even under hydrogen pressure of less than 1.0 MPa.
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Table 2 e Results of the measurement of the initial hydrogenation kinetics and the ratios between equilibrium compositions in the measurement and maximum hydrogen storage capacities at PCT measurement.
Run 1 Run 2 Run 3 Reagent
P0 [MPa]
Peq [MPa]
Ceq [mass%]
Cmax [mass%]
Ceq/Cmax [mass%] []
0.95 0.95 0.95 0.95
0.51 0.55 0.39 0.87
1.02 0.95 1.31 0.17
1.51 1.48 1.42 1.54
0.68 0.64 0.92 0.11
P0: Initial hydrogen pressure [MPa]; Peq: Equilibrium hydrogen pressure [MPa]; Ceq: Equilibrium composition [mass%]; Cmax: Maximum hydrogen storage capacity at PCT measurement [mass%].
SICS. This was most likely attributed to the lower ignition temperature; the reaction heat was not high enough to synthesize pure LaNi5. The purity of the product could be improved by optimizing the experimental conditions, such as the sample amounts and/or heating rate. Fig. 7 shows the initial hydriding curves of the SICSed LaNi5 at a temperature of 298 K and an initial hydrogen pressure of 0.95 MPa, together with the pure LaNi5 reagent as a reference. The SICSed products used in this experiment were different from that used in PCT measurement in order to evaluate the initial hydrogenation kinetics. The reagent, with a diameter of 250e500 mm, did not absorb any hydrogen under the experimental conditions. The product obtained at Run 3 showed a different tendency as compared with the other SICSed products. It had the highest hydrogen storage capacity, even under less than 1.0 MPa, although it showed the lowest maximum hydrogen storage capacity (see Fig. 6 or Table 2). Table 2 shows the results of the measurement of the initial hydrogenation kinetics and the ratios between equilibrium compositions in this measurement and maximum hydrogen storage capacities at PCT measurement. P0, Peq and Ceq denote initial hydrogen pressure, Equilibrium hydrogen pressure and Equilibrium composition at kinetics measurement, respectively. In contrast, Cmax denotes Maximum hydrogen storage capacity at PCT measurement. The product obtained at Run 3 showed a 1.31 mass% in hydrogen storage capacity, which is equivalent to 92% of its maximum capacity. Inui et al. reported the effect of the lattice defects on hydrogen sorption properties [11]. The results showed that the absorption pressure of the LaNi5 alloy drastically decreased between the first and second cycles due to the formation of severe cracks and the introduction of lattice defects, such as a-type dislocations. The extent of decrease in the absorption pressure due to the formation of cracks depended on the particle size of alloy powder. These were the reasons why activation treatment required much higher hydrogen pressure than equilibrium pressure; the absorption pressure in the first cycle was much higher than its equilibrium pressure. The difference of initial activity in our experiments can be explained by a similar mechanism. It was estimated that it was not possible to activate the LaNi5 reagent since its particle size was large as compared to the SICSed LaNi5. On the other hand, the product obtained at Run 3 was easily activated since
its particle size was small and lattice defects were introduced owing to hydrogenation during SICS.
4.
Conclusion
The self-ignition combustion synthesis (SICS) of LaNi5 hydrogen storage alloys at different hydrogen pressures was comparatively studied and the following conclusions were derived. 1. The overall reaction eq. (8) of 1/2La2O3 þ 5Ni þ 3Ca þ 3/ 2H2 ¼ LaNi5 þ 3/2CaO þ 3/2CaH2 þ 479 kJ consists of four elemental equations from eqs. (4) to (7), including the hydrogenation of calcium (Ca þ H2 / CaH2 eq. (4)). The ignition point decreased with the hydrogen pressure, because the hydrogenation rate of calcium increased with the higher hydrogen pressure. 2. The shape of the product also depended on the hydrogen pressure. With increasing hydrogen pressures from 0.1 to 1.0 MPa, the product shape changed from bulk to powder. This is probably explained by the volume expansion due to the hydrogenation at the higher hydrogen pressure. 3. The initial activity of the product depended on the hydrogen pressure charged in the production. The product obtained at 1.0 MPa showed 1.31 mass% in hydrogen storage capacity when an initial hydrogen pressure of 0.95 MPa was charged. This value reached 92% of its maximum hydrogen storage capacity. The results also showed that the SICS at relatively high hydrogen pressure offers many benefits for lowering ignition temperature, minimizing operation time and energy, obtaining fine powdered product directly, and improving initial activity at hydrogen pressure of only 1.0 MPa.
Acknowledgments This research was partly supported by the Ministry of the Environment, Japan, under the project “Development of Hydrogen Freezer Using Industrial Waste Heat or Solar Heat for Reducing CO2 Emission” (FY 2008- FY 2010).
references
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