Dehydrogenation Kinetics and Modeling Studies of 2LiBH4 + MgH2

Publication Date (Web): June 6, 2012 ... Low-Temperature Reversible Hydrogen Storage Properties of LiBH4: A Synergetic Effect of .... kinetics and mod...
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Dehydrogenation Kinetics and Modeling Studies of 2LiBH4 + MgH2 Enhanced by NbF5 Catalyst S. T. Sabitu and A. J. Goudy* Department of Chemistry, Delaware State University, Dover, Delaware 19901, United States ABSTRACT: Although LiBH4−MgH2 composite shows excellent hydrogen storage capacity, it still has the problem of sluggish kinetics. To improve the hydrogen desorption kinetics, mixtures of LiBH4−MgH2 (2:1) were doped with 4 mol % NbF5, Nb2O5, and Mg2Ni. The hydrogen desorption kinetics of these mixtures were compared using constant pressure thermodynamic forces in which the ratio of the equilibrium plateau pressure to the opposing plateau was the same for all of the reactions studied. The results showed NbF5 to be vastly superior to the other catalysts for improving the kinetics of the composite mixture with the hydrogen desorption rates being in the order: NbF5 ≫ Nb2O5 > Mg2Ni. Desorption enthalpies that were obtained from van’t Hoff plots indicated that the thermodynamic stabilities of the catalyzed mixtures were in the order: NbF5 < Nb2O5 < Mg2Ni. Modeling studies indicated that chemical reaction at the phase boundary was the most likely process controlling the reaction rates for the catalyzed mixtures. system, indicating that the kinetics of the LiBH4−MgH2 composite was significantly improved by the introduction of Mo. Jiang and Liu2 reported that the dehydrogenation kinetics of LiBH4−MgH2 (2:1) was enhanced by the addition of a CuCl2 catalyst. They also suggested that hydrogen backpressure plays a role in the formation of MgB2, which is a determining factor for the reaction pathway and dehydrogenation kinetics of the LiBH4−MgH2 (2:1) composite. Zhou et al.1 showed that a reactive composite of LiBH4-xLa2Mg17 could be successfully prepared mechanically by reacting LiBH4 with MgH2 and LaH3. They reported that the MgH2 and LaH3 additives provided a synergetic thermodynamic and kinetic destabilization on the dehydrogenation/hydrogenation of LiBH4. It should be noted that in none of these studies was an attempt made to compare the intrinsic reaction rates of the catalyzed LiBH4−MgH2 composite with other materials using constant pressure thermodynamic forces. This is important because without constant pressure driving forces, results will vary largely as the conditions change and comparisons would be meaningless. The importance of this unique technique was first demonstrated by Goudy and coworkers when they analyzed the kinetic behavior of a series of LaNi5-based intermetallic hydrides.13−15 The kinetics of other materials such as MgH2,16 NaAlH4,17 CaBH2/LiBH4,18 and LiNH2/MgH219 has also been studied using this method. Sabitu et al.16 compared the intrinsic dehydriding kinetics of MgH2 ballmilled with TiH2, Mg2Ni, and Nb2O5 using constant pressure thermodynamic driving forces and found that the reaction rates

1. INTRODUCTION The complex hydride, LiBH4, has attracted considerable attention as a potential hydrogen storage material due to its high gravimetric and volumetric hydrogen capacities.1,2 It has a gravimetric density of 18.5 wt % and a volumetric hydrogen density of 121 kg/m3.1−10 However, its usage for on-board hydrogen storage applications is limited due to both thermodynamic and kinetics deficiencies.7 In the past years, attempts have been made to reduce the desorption enthalpy and improve the hydrogenation/dehydrogenation kinetics of the LiBH4 system by alloying with transition metals, oxides, carbon, amides, and halides.1,3,4,6,9 Vajo et al.11 reported that the addition of MgH2 to LiBH4, as shown in eq 1, can lower the dehydrogenation enthalpy by 25 kJ/mol H2. 2LiBH4 + MgH2 → 2LiH + MgB2 + 4H 2 (1) Other studies have shown that hydrogen release does not proceed directly according to reaction 1 but with an intermediate reaction step.2,4 This slow intermediate step made it possible to destabilize reaction 1 thermodynamically but not kinetically.4 Vajo et al.11,12 reported that several catalysts such as TiCl3, VCl3, NiCl2, and TiF3 could improve the reaction kinetics of the LiBH4−MgH2 system. Fan et al.9 doped Nb2O5 into a LiBH4−MgH2 composite and found that Nb2O5 decreases the activation energy. In terms of kinetics, at 400 °C, the addition of Nb2O5 significantly improved the dehydrogenation/hydrogenation kinetics due to the formation of NbH2, which facilitates hydrogen diffusion. Xia et al.3 reported that the hydrogen storage properties of LiBH4−MgH2 composite were enhanced by the catalytic effect of MoCl3. The activation energy for hydrogen desorption from the composite mixture with MoCl3 was lower than that of pure LiBH4−MgH2 © XXXX American Chemical Society

Received: March 19, 2012 Revised: May 12, 2012

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are in the order Nb2O5 > Mg2Ni > TiH2. Because these catalysts were effective for MgH2, it is possible that they could also be effective in improving the desorption kinetics of the LiBH4−MgH2 (2:1) system. Therefore an attempt was made to study the kinetic behavior of the LiBH4−MgH2 (2:1) system after adding Nb2O5, Mg2Ni, and also NbF5 to the mixture. An attempt was also made to determine the process that controls the rate of hydrogen desorption from the mixtures.

2. EXPERIMENTAL DETAILS The starting materials used in this research were obtained commercially from Sigma Aldrich. They were used as received in powder form without further purification. Sample handling, weighing, and loading were performed in a vacuum atmospheres argon-filled glovebox that was capable of achieving Nb2O5 > NbF5. The two Nb-based catalysts were both able to lower the onset temperature to Nb2O5 > NbF5, which is the same trend that was seen in the onset temperatures that were obtained from the TPD curves in Figure 1. This indicates that the catalysts are effective in destabilizing the reaction mixtures. Desorption kinetics experiments were carried out on the lower plateau at 450 °C. The kinetics measurements were all done at the same of constant pressure thermodynamic driving force. The theoretical basis for this concept is that the Gibbs free energy (ΔG) is the driving force for a spontaneous reaction at constant temperature and pressure. For these reactions, ΔG = ΔG° + RT ln(Pm/Pop), where Pm is the mid-plateau pressure and Pop is the applied or opposing hydrogen pressure above the sample. If the pressure ratio (Pm/Pop) is kept the same for all samples, then ΔG will be the same. The procedure for keeping the pressure ratios constant was as follows: First, the hydrogen pressure in the reactor was adjusted to a value slightly higher than that of the midplateau pressure (Pm) of the lower plateau to ensure that only the hydrogen-rich phase was present initially and the reactor was sealed off. Then, the pressure in the remaining system (Pop) was the adjusted to a value such that the ratio between the midplateau pressure and opposing pressure (Pm/Pop) was a small whole number. This ratio is defined as the N value. In these experiments, the N value was set to 3 for all of the sample mixtures. For example, at the temperature used in these experiments, Pm for the lower plateau in the NbF5-catalyzed sample was 14 atm. Therefore, the Pop necessary for N = 3 in this sample was 4.67. This represents the first time that this technique has been applied to the kinetic study of 2LiBH4−MgH2 system. C

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⎡ ⎞⎛ ⎛ aRCs° ⎢ 1 ⎛ 1 r3 ⎞ r2 ⎞ ⎜⎜ − 1 ⎟⎟⎜1 − c ⎟ + R ⎜1 − c ⎟ CA ⎢⎣ 3 ⎝ kg De ⎠⎝ 2D′eA ⎝ R3 ⎠ R2 ⎠ +

⎤ rc ⎞⎥ 1 ⎛⎜ ⎟ 1 − R ⎠⎥⎦ aksCs° ⎝

(2)

where t is the reaction time, D′e and D′eA are diffusivity terms, kg is a mass transfer coefficient, ks is a rate constant, ‘a’ is a stoichiometric coefficient, C°s and CA are molar concentration terms, rc is the radial position of the unreacted core boundary, and R is the radius of the hydride particle. Smith and Goudy15 analyzed their kinetic data using two other equations based on the SCM, and they are shown below t = 1 − (1 − XB)1/3 (3) τ

Figure 5. Modeling results for 2LiBH4 + MgH2 + 4 mol % NbF5 at 450 °C.

where

τ=

ρB R bksCAg

t = 1 − 3(1 − XB)2/3 + 2(1 − XB) τ

(4)

where τ=

ρB R2 6bDeCAg

where t is the time at a specific point in the reaction, XB is the fraction of the metal reacted, R is the initial radius of the hydride particles, ‘b’ is a stoichiometric coefficient of the metal, CAg is the gas-phase concentration of reactant, De is the effective diffusivity of hydrogen atoms in the hydride, ρB is the density of the metal hydride, and ks is a rate constant. Equations 3 and 4 are similar to eq 2 in a number of ways. The reaction time is inversely proportional to the rate constant in eq 3 and inversely proportional to the diffusivity in eq 4. On the basis of this, it is evident that a model based on eq 3 will have chemical reaction at the phase boundary controlling the reaction rate. A model based on eq 4 is one in which the overall reaction rate is controlled by diffusion. Equations 3 and 4 were fitted to the kinetic data for each of the reaction sample mixtures to determine which kinetic model best describes the reactions in this study. Figure 5 shows three curves for the 2LiBH4−MgH2 + 4 mol % NbF5 system. One is an experimental curve taken from Figure 4; a second curve was a theoretical curve that was calculated from eq 4 with diffusion controlling the overall reaction; and a third curve was calculated from eq 3 with chemical reaction at the phase boundary controlling the rate. To determine the theoretical curves, it was first necessary to determine a value for τ. This was accomplished through a series of statistical data analyses to determine a value that resulted in the closest correspondence between the experimental and theoretical data. As shown in Figure 5, the data generated from the model with chemical reaction at the phase boundary controlling the overall rate fit the experimental data better than the data generated from the model with diffusion controlling the overall reaction rate. This is true during the first 80% of the reaction. Kinetic modeling studies were also attempted on the other mixtures as well as a sample of pure MgH2. The results for the Nb2O5 and Mg2Ni catalyzed mixtures are given in Figures 6 and 7, respectively. The modeling results for these

Figure 6. Modeling results for 2LiBH4 + MgH2 + 4 mol % Nb2O5 at 450 °C.

Figure 7. Modeling results for 2LiBH4 + MgH2 + 4 mol % Mg2Ni at 450 °C.

systems also show that chemical reaction at the phase boundary is also the likely rate-controlling process. Figures 8 and 9 contain modeling curves for the uncatalyzed mixture and the pure MgH2 sample. The results for these two systems are not clear. Neither of the theoretical curves is a good fit with the experimental data. Because diffusion and chemical reaction at the phase boundary are always occurring simultaneously in these reactions, it could be that the kinetics in the case of MgH2 and the uncatalyzed 2LiBH4−MgH2 mixture are under the mixed control of both processes. The fitting of data with eqs 3 D

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Figure 10. XRD plots for (a) NbF5, (b) 2LiBH4 + MgH2 ball milled with 4 mol % NbF5, and (c) 2LiBH4 + MgH2 manually mixed with 4 mol % NbF5.

Figure 8. Modeling results for 2LiBH4 + MgH2 uncatalyzed at 450 °C.

catalyzed mixtures show that the hydrogen desorption enthalpies and onset temperatures are in the order: Mg2Ni > Nb2O5 > NbF5. This indicates that NbF5 is the most effective additive for thermodynamically destabilizing the 2LiBH4− MgH2 composite. The results also show the hydrogen desorption rates are in the order: Mg2Ni < Nb2O5 ≪ NbF5. All of these findings indicate that NbF5 is vastly superior to the other materials for catalyzing the 2LiBH4/MgH2 system. The modeling studies based on the kinetics data indicate that chemical reaction at the phase boundary is the likely ratecontrolling process in all of the catalyzed mixtures. However, in the case of MgH2 and the uncatalyzed 2LiBH4−MgH2 mixture, the results indicate that their kinetics may be under the mixed control of more than one process.

Figure 9. Modeling results for MgH2 at 450 °C.

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and 4 can provide the values of the rate constant. In these model equations, 1/τ = k, where k is the rate constant. Values of k for each catalyzed mixture are listed in Table 1. It can be seen that the rate constants increase as the times required for 90% reaction completion decreases. The NbF5-catalyzed mixture has the largest rate constant, the smallest reaction time, the smallest enthalpy, and the lowest onset temperature. All of these findings indicate that NbF5 is vastly superior to the other materials for catalyzing the 2LiBH4−MgH2 system. The finding that reaction at the phase boundary controls reaction rates indicates that the catalysts employed may not merely coat the surface of the hydride particles during ballmilling but are rather alloyed mechanically into the 2LiBH4− MgH2 phase. To determine if this was the case, we obtained some XRD patterns for the NbF5 catalyst and for the 2LiBH4− MgH2 mixture that was ball-milled with the NbF5 catalyst or manually mixed with NbF5 catalyst. Figure 10 contains the patterns. It can be seen that the pattern for the 2LiBH4−MgH2 mixture manually mixed with the NbF5 catalyst contains some small reflections that correspond to the NbF5 reflections. However, the ball-milled mixture does not contain any of the NbF5 reflections. This is a good indication that the NbF5 in the ball-milled mixture is part of the phase.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by grants from the Department of Energy and the U.S. Department of Transportation.

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4. CONCLUSIONS The results of this study clearly demonstrate that the hydrogen storage properties of a 2LiBH4−MgH2 composite can be significantly improved by the addition of several catalysts. Comparisons of the van’t Hoff plots and TPD analyses of the E

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