Article pubs.acs.org/JPCC
Hydrogen Desorption Behavior of Calcium Amidoborane Obtained by Reactive Milling of Calcium Hydride and Ammonia Borane F. Leardini,*,†,§ J. R. Ares,† J. Bodega,† M. J. Valero-Pedraza,‡ M. A. Bañares,‡ J. F. Fernández,† and C. Sánchez† †
Grupo Mire, Dpto. Física de Materiales, Universidad Autonoma de Madrid, Cantoblanco, 28049, Madrid, Spain Catalytic Spectroscopy Laboratory, Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie 2, Cantoblanco, 28049, Madrid, Spain
‡
S Supporting Information *
ABSTRACT: Metal amidoboranes have been regarded as potential candidates for hydrogen storage applications, due to their high gravimetric capacities and their relatively low hydrogen desorption temperatures. Here, we report about the synthesis of calcium amidoborane (CaAB) by means of reactive milling of ammonia borane and calcium hydride. Structural properties and hydrogen desorption behavior of samples, prepared with different milling times and after aging for long periods in an inert atmosphere, have been investigated by X-ray powder diffraction, thermal programmed desorption, and differential scanning calorimetry. It has been observed that CaAB spontaneously decomposes during aging at room temperature giving rise to the formation of an amorphous phase product. The observed sample metastability has a strong effect on the hydrogen desorption behavior of CaAB and give place to complex changes in the hydrogen desorption kinetics as well as in desorption enthalpies.
1. INTRODUCTION The use of hydrogen as a fuel for the future requires the development of safe, compact, and light hydrogen storage systems. This requirement has motivated a great research activity in hydrogen storage materials that are able to evolve gaseous hydrogen by thermolysis at mild pressure and temperature conditions.1 Ammonia borane (AB hereafter), with chemical formula NH3BH3, is a solid with one of the highest hydrogen mass capacity. It can release two H 2 molecules per formula unit (namely, about 13 wt %) via a two-step thermolysis at temperatures at ca. 100 and 150 °C, respectively. These reactions are moderately exothermic (ΔH ≈ −20 kJ/mol H2), although these are stabilized by high activation barriers. These impose sluggish dehydrogenation kinetics below 85 °C.2−6 Besides, small quantities of gaseous poisons, such as diborane and borazine, are released during the thermolysis of AB.2,3,6 These factors, along with the irreversibility of AB thermolysis, prevent its utilization for hydrogen storage. Alkali amidoboranes such as LiNH2BH3 and NaNH2BH3 (LiAB and NaAB, hereafter) present different and improved dehydrogenation characteristics with respect to AB while maintaining high hydrogen gravimetric densities.7−13 In particular, it has been observed that H2 release from LiAB and NaAB is less exothermic than from AB (about −3 to −5 kJ/mol H2, respectively).7 This trend shows that chemical substitution of H atoms in NH3 groups by Li or Na produces a stabilizing effect with respect to hydrogen release. This effect © 2012 American Chemical Society
would facilitate the rehydrogenation of these compounds, although reversibility by direct solid−gas reaction has not been achieved until now. Relatively less attention has been paid to alkaline earth amidoboranes such as calcium amidoborane (Ca(NH2BH3)2 (CaAB hereafter)). As far as we know, there are only three reports on the synthesis and characterization of this compound by using different synthetic routes. The first one used a wetchemical method by reaction of one equivalent of CaH2 and two of AB in tetrahydrofuran (THF) solution.14 This synthesis route has been recently used to prepare CaAB and investigate its chemoselective reduction properties.15 A second approach consists in the synthesis by reactive milling of both reactants in an inert atmosphere.8 Finally, CaAB has been obtained by deammonation of calcium amidoborane−ammoniate −(Ca(NH2BH3)2·2NH3)−, i.e., (CaAB·2NH3) under dynamic flow and at temperatures below 100 °C.16 The second approach, i.e., reactive milling, has the advantage of being more environmentally friendly and does not require removing the THF nor NH3 adducts from the compound. There are few data about hydrogen desorption properties of CaAB,8,14,16 and these give nonreproducible results. This lack of reproducibility could partially stand on the different synthetic routes used to prepare CaAB samples but also to the different Received: July 20, 2012 Revised: October 19, 2012 Published: November 1, 2012 24430
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detection sensitivity of the TPD system (further details of calibration experiments are given in the Supporting Information). DSC experiments have been done in a TA Instruments calorimeter (mod. Q100). To avoid exposure to air, the samples used for TPD and DSC analyses were placed into Al pans hermetically sealed in an inert atmosphere. This hermetical sealing produces a delay between H2 desorption from the samples and its detection at the mass spectrometer, thus giving rise to an apparent shift of the TPD peaks as compared to DSC ones.
experimental conditions used to analyze H2 desorption. In the first work,14 kinetics of H2 release was investigated at isothermal conditions at different temperatures up to 160 °C. The second paper reports a temperature-programmed desorption (TPD) curve of hydrogen release showing two (maybe three) desorption events at temperatures lower than those for AB.8 In the third report,16 a combined differential scanning calorimetry (DSC) and TPD analysis was used to investigate the thermolysis of CaAB·2NH3. An endothermic event related to deammoniation of that compound was observed below 100 °C, thus giving rise to the formation of CaAB. An exothermic event followed by an endothermic one is observed on increasing temperature. Both peaks occur concomitantly to a broad H2 desorption peak (which maybe the superposition of two adjacent peaks) with a maximum at 151 °C. Finally, a recent work investigates the thermal decomposition of CaH2/AB composites obtained by ball milling.17 The formation of CaAB is not reported in that work, although X-ray powder diffraction (XRPD) data did reveal the presence of two peaks, which match those of CaAB (at 2θ values of about 22.6 and 26.9°). However, the amount of CaAB phase seems to be very low, and the samples are mainly composed of CaH2 and AB. A single value of about 3.5 kJ/mol has been reported until now for the enthalpy of H2 desorption from CaAB.14 However, that value was estimated from an isothermal DSC measurement performed at a temperature at which H2 desorption was incomplete (according to the kinetic data reported in that work). Therefore, it is not clear whether desorption enthalpy was properly determined. It must be emphasized that the enthalpy of H2 desorption is a key factor to assess the reversible hydrogenation of the compound. We now aim to investigate the dehydrogenation behavior of CaAB by means of quantitative DSC and TPD measurements. These measurements allow the determination of the amount of H2 evolved from the samples and the heat involved in that process, from which the enthalpy of CaAB decomposition can be obtained. In addition, we investigate the stability of CaAB samples upon aging for long periods in an inert atmosphere as well as in air.
3. RESULTS 3.1. Synthesis and Structural Properties of CaAB. As previously stated, CaAB has been synthesized by milling one equivalent of CaH2 and two of AB according to reaction: CaH 2 + 2NH3BH3 → Ca(NH 2BH3)2 + 2H 2
(1)
A high-energy shaker mill has been used instead of the lowenergy planetary one previously used.8 As a matter of fact, highenergy milling reduces reaction times, although it may produce higher amorphization of the resulting powders. Reaction 1 has been followed as a function of milling time. To this aim, the milling vial was opened (in Ar atmosphere) from time to time, and small aliquots of sample were taken for XRPD characterization. Parts a−d of Figure 1 show the diffraction patterns obtained for milling times of 0.5, 6, 12, and 18 h. After 0.5 h of milling (Figure 1a), the sample only exhibits the Bragg peaks of CaH2 and AB phases. These diffraction peaks have a broad width, indicating a reduction of the crystallite size induced by milling. In addition, a bump is observed in the 12−25° range, which has been ascribed to the Kapton film used to cap the sample holder. In fact, other samples with high crystallinity also present this bump with the same intensity (see Figure S2 of the Supporting Information). Diffraction peaks of the CaAB phase are already visible after 6 h of milling (Figure 1b), and their intensity shows a substantial increase in the sample milled for 12 h (Figure 1c). This increase is accompanied by a decrease in CaH2 and AB abundances, as expected. On the other hand, the bump appearing in the 12− 25° range shows a clear rise in intensity when increasing milling time. This feature indicates the formation of an amorphous phase in the sample induced by milling. Subsequent milling up to 18 h (Figure 1d) does not result in a higher yield of CaAB phase. On the contrary, a slight increase of the amorphous bump is observed, whereas the intensities of CaAB, CaH2, and AB peaks diminish. The obtained samples are rich in CaAB phase, although some proportion of CaH2 and AB still remains. Incomplete conversion of CaH2 and AB into CaAB was also observed in previous work after 30 h of milling in a planetary mill.8 It is very likely that the initial stages of milling do not result in CaAB formation due to the milling time needed to activate CaH2 particles. Therefore, the use of some lubricant (e.g., a metal oxide) during milling could help to refine the powders,19 thus accelerating the kinetics of CaAB formation and increasing the yield of the synthesis reaction. The structural stability of CaAB-rich samples (i.e., milled for 18 h) has been also investigated by means of XRPD. Figure 1e shows the diffraction pattern of a sample stored in Ar atmosphere during 25 weeks after its preparation. This pattern still shows the presence of CaAB, AB, and CaH2 phases. In addition, there is an increase in the bump related to the presence of an amorphous phase (see Supporting Information for further details). This structural modification produced
2. EXPERIMENTAL METHODS NH3BH3 (97% wt purity) and CaH2 (95% wt purity) have been provided by Sigma Aldrich. Upon receiving the compounds, these have been stored in an Ar glovebox, without exposing them to air, and have been used without further purification. Milling of the materials has been done in an Spex 8000 shaker mill placed inside the glovebox. Stainless steel vial and balls have been used with balls to powder mass ratio of 10:1. To avoid an excessive heating of the materials, milling has been accomplished by 30 min steps, allowing for a period of at least 30 min between successive milling steps. It must be noted that the synthesis of CaAB generates hydrogen gas, and therefore, it produces a pressure rise inside the milling vial. In our preparations, overpressure did not exceed 1 bar. XRPD measurements have been performed with Cu Kα radiation in a Siemens D5000 apparatus. The powders were mounted in a special sample holder capped with a Kapton film to avoid exposure to air. All diffraction patterns were taken at the same conditions (angular range 10−70°, steps of 0.02° and a time of 8 s per step). TPD experiments have been accomplished in the experimental system described elsewhere.18 Several calibration experiments have been performed to determine the hydrogen 24431
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moisture. However, these observations contrast with the results reported in ref 14, which address that no changes are detected after exposing CaAB to air during two days. Such remarkable difference should be ascribed to differences in the CaAB synthesis methods. In fact, the previous report used a chemical synthesis method producing very large crystallites and the presence of THF molecules intercalated in the compound. By contrast, the present results and those of ref 8 used reactive milling, giving rise to the formation of smaller crystallites and without THF molecules, which may react more easily with air moisture. 3.2. Analysis of the Hydrogen Thermal Desorption Process from CaAB. Hydrogen thermal desorption properties of CaAB-rich samples have been investigated by means of DSC and TPD measurements performed in parallel. In this way, it is possible to quantify the amount of hydrogen desorbed in each desorption event as well as the heat involved in it, thus allowing the calculation of the corresponding enthalpy. This information has been complemented with the monitoring of borazine and diborane desorption through TPD data as well as with gravimetric measurements done before and after desorption runs. The DSC and TPD profiles of CaAB-rich samples stored in Ar over different periods are shown in Figure 2. The first sample (Figure 2a) was obtained after 12 h of milling and 3 weeks of storage before performing desorption experiments. This sample presents three hydrogen desorption peaks at 85, 95, and 180 °C; its desorption profile is rather similar to that previously reported for a CaAB-rich sample obtained by reactive milling.8 The total amount of hydrogen released in the first two peaks is about 4.4 ± 0.3 wt %, whereas in the third peak it is close to 4.6 ± 0.3 wt %. On the other hand, the DSC profile reveals that the first two desorption peaks are exothermic, whereas the third one is slightly endothermic. The second sample (Figure 2b) was obtained by 18 h of milling and 2 weeks storing before desorption experiments. The hydrogen desorption profile is qualitatively similar to that of the first sample, although it only shows two H2 desorption events: an exothermic H2 desorption peak at about 85 °C and another endothermic one at 180 °C. The amount of H2 released in the exothermic peak is slightly higher than for the first sample, namely, of 4.8 ± 0.3 wt %. By contrast, the amount of H2 released in the endothermic peak decreases to 3.2 ± 0.1 wt %. The third and fourth samples are similar to the second one but have been aged in Ar for longer times, namely, 12 and 25 weeks, respectively. The corresponding hydrogen desorption profiles of these samples are shown in parts c and d of Figure 2. In both cases, a single broad hydrogen desorption peak is observed. This desorption peak is exothermic and no signs of endothermic hydrogen desorption peaks are found in the DSC curves. Integration of hydrogen desorbed flows shows that the amounts of H2 released after 12 and 25 weeks are respectively equal to 7.3 ± 0.3 and 6.7 ± 0.3 wt %. In all cases the weight loss calculated by integrating the hydrogen desorption profiles is in good accordance with the results obtained by gravimetric measurements of the samples before and after desorption runs. This implies that no significant desorption of other molecules is taking place. In fact, no traces of borazine or diborane were detected in TPD experiments. These results contrast with those observed in AB samples, which exhibit a mass loss much higher than that due to H2 desorption because of sample volatilization and borazine and diborane emission.3 As a consequence, substitution of H
Figure 1. Diffraction patterns of CaH2:2NH3BH3 powders milled for: (a) 0.5 h, (b) 6 h, (c) 12 h, and (d) 18 h. XRPD patterns of a sample milled for 18 h and aged during 25 weeks in Ar atmosphere without exposing it to air and briefly exposed to air for a few hours are shown in (e) and (f), respectively. Symbols indicate the Bragg peaks of CaAB (§), AB (*), CaH2 (#), and Ca(OH)2 (&) phases.
during samples storage is accompanied by significant changes of the H2 desorption properties. The next section is devoted to present and discuss this point. The stability of the obtained powders upon air exposure presents a quite different behavior. Figure 1f shows a XRPD pattern of a sample milled for 18 h and exposed to air for a few hours by producing a leak in the Kapton film covering the sample holders. It can be seen that the sample completely transforms into Ca(OH)2 and AB, indicating that CaAB quickly reacts with air moisture according to Ca(NH 2BH3)2 + 2H 2O → Ca(OH)2 + 2NH3BH3
(2)
Diffraction patterns of samples milled for 0.5 and 6 h and briefly exposed to air also present Ca(OH)2 and AB phases, exclusively. This reflects the fact that Ca compounds interact strongly with air moisture to yield calcium hydroxide. Reference 8 already describes that CaAB is extremely sensitive to air 24432
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Figure 2. DSC (upper graphs) and TPD (lower graphs) profiles of CaAB samples obtained by high energy milling and stored in Ar atmosphere during different times. (a) CaAB sample obtained by 12 h of milling and stored during 3 weeks; CaAB sample obtained by 18 h of milling and stored during 2 weeks (b), 12 weeks (c), and 25 weeks (d). The experiments have been performed by applying a constant heating rate of 0.5 °C/min.
NaAB decomposition profiles, it is expected that reaction 3 originates from two consecutive reactions each one desorbing a half of the total H2 yield
atoms in NH3 groups by Ca seems to produce a strong effect in stabilizing the compound.
4. DISCUSSION Hydrogen desorption from CaAB samples should follow the overall reaction Ca(NH 2BH3)2 → Ca(NBH)2 + 4H 2
(3)
Ca(NH 2BH3)2 → Ca(NHBH 2)2 + 2H 2
(3i)
Ca(NHBH 2)2 → Ca(NBH)2 + 2H 2
(3ii)
It is very likely that the first exothermic H2 desorption event observed in parts a and b of Figure 2 is associated to reaction 3i, whereas the endothermic one observed at higher temperatures is due to reaction 3ii. The intermediate peak observed in Figure 2a should be related to H2 desorption from residual AB and CaH2 through reaction 1 or from the direct decomposition of AB. On increasing aging time the reaction mechanism changes drastically giving rise to a single exothermic desorption event; the possibility of two superposed peaks should not be disregarded. It seems that the complex evolution of H2 desorption kinetics observed in CaAB samples are also related to the structural modifications experienced by the samples upon aging them for long periods. Rather similar results have already been observed in preheated AB samples.20 In those experiments, AB samples were subjected to different thermal treatments and the effect of these treatments on the hydrogen thermal desorption profiles was investigated. It was deduced that heating AB samples at 80 °C for periods of about 3 h leads to the formation of different oligomers or polymers of AB and that these structural modifications have a strong effect on the kinetics of H2 desorption from the samples. Same conclusions are reached on analyzing the thermodynamic characteristics of the desorption process. An estimation of the enthalpies of H2 desorption from the samples can be obtained by analyzing the TPD and DSC profiles in Figure 2. According to that estimation, the enthalpy of endothermic H2 release observed at higher temperatures in parts a and b of Figure 2 is of about 2 ± 1 kJ/mol H2. The H2 amount desorbed in this process decreases with increasing aging time and completely disappears from the TPD profiles of parts c and d of Figure 2. The enthalpies of the exothermic desorption events are respectively equal to −12, −15, −25, and −23 kJ/mol H2 in parts a−d of Figure 2, respectively. The estimated errors in these values are 2 kJ/mol H2. These results show that H2 desorption from the samples becomes more exothermal when
which in turn can take place through one or several steps, as discussed below. Reaction 3 predicts a yield for the desorbed H2 of about 8.1 wt %, similar to those values obtained in TPD and gravimetric measurements with aged samples. However, the present results show that the total amount of H2 released by the aged samples decreases from an initial value of about 9 wt % to a value close to 6.7 wt % on increasing the aging time. This fact suggests that during the aging time itself CaAB sample slowly decomposes (following eq 3) and spontaneously loses H2 at room temperature. This hypothesis is in agreement with the favorable thermodynamics observed in DSC experiments (exothermic H2 release) and with the low kinetic barriers for H2 desorption deduced from TPD experiments (the onset for H2 release is slightly above 50 °C). In addition, our experimental results show that spontaneous decomposition during the aging time of the samples is accompanied by the formation of an amorphous phase observed by XRPD and that is related to CaAB decomposition. Indeed, it has been reported that the product of CaAB decomposition (Ca(NBH)2), as well as those of LiAB (LiNBH) and NaAB (NaNBH), have amorphous structures.7,8 The observed metastability of the samples at room temperature suggests that they should be stored at lower temperatures (in a freezer, for instance) to improve their stability. The use of high-pressure storage vessels is also recommended to avoid problems with pressure build up due to hydrogen desorption. The spontaneous decomposition of CaAB during aging is also accompanied by several changes in the hydrogen desorption properties of CaAB. Concerning the kinetics of H2 desorption, TPD profiles show a complex evolution of the reaction kinetics, passing from a three-peak shape to a single broad desorption peak. This change in the reaction mechanism is also observed in the DSC profiles. In line with AB, LiAB, and 24433
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the CaAB samples at room temperature. This spontaneous decomposition is accompanied by a loss of H2 capacity as well as the formation of an amorphous phase product detected by XRPD. Moreover, CaAB shows complex changes in the kinetics and thermodynamics of hydrogen desorption after aging it for long periods in an inert atmosphere at room temperature. These results imply that H2 desorption from CaAB is very sensitive to the structural properties of the samples.
increasing aging time. At the same time, the onset temperatures for hydrogen desorption observed in TPD experiments shift to lower temperatures. These two observations imply that CaAB samples become increasingly unstable with aging. It seems plausible to think that the change in the hydrogen desorption thermodynamics is also related to the structural modifications exhibited in XRPD measurements. These measurements show the formation of an amorphous phase product during room temperature decomposition of the samples. It is expected that this product has a polymerlike structure which may be in a metastable state. In other words, an incomplete polymerization may occur due to the kinetic barriers for molecule diffusivity along the solid phases at room temperature. When heating these samples in the DSC and TPD apparatus complete polymerization may occur during hydrogen desorption, but the final structure and properties of the stable polymer will depend on the initial polymerization state of the samples. This process should be analogous to that occurring in other polymerization reactions. In particular, it has been observed a dependence of the onset temperatures and the reaction enthalpy on the initial polymerization state in somewhat similar reactions.21 It is possible that sample contamination with residual moisture inside the glovebox has an influence on the abovementioned process. Sample contamination inside the glovebox would represent a small fraction of the samples, since there are no signs of Ca(OH)2 phase in the XRPD patterns of the samples aged in the glovebox (parts c−e of Figure 2). However, even a small fraction of hydroxide groups can have a role on the polymerization reaction. To elucidate that question we performed additional TPD and DSC measurements with a CaAB sample aged in the glovebox and intentionally exposed to air (see Figure S3 in the Supporting Information). These measurements showed that the onset for hydrogen desorption takes place at lower temperatures than for the samples stored in the glovebox. In addition, the obtained desorption enthalpy per mol of desorbed hydrogen is more exothermal for the air exposed sample (i.e., −30 kJ/molH2). These two effects support that hydroxide groups can influence the polymerization reaction, thus altering the thermodynamics of hydrogen desorption, even if they are in a small concentration. It must be emphasized that the obtained reaction enthalpies for hydrogen release from CaAB imply that the reversible hydrogenation of the compound is thermodynamically prevented, as occurs in AB. Finally, it must be noted that the present results can explain the lack of reproducibility in the TPD and DSC profiles obtained in previous reports.8,14,16 In previous works CaAB samples were obtained by using different synthetic routes and, therefore, they would present different microstructural properties. According to the present results, such differences can give rise to significant changes in the kinetics and thermodynamics of H2 desorption from CaAB. These effects could also be relevant when analyzing the thermal desorption behavior of other similar compounds, such as LiAB or NaAB.
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ASSOCIATED CONTENT
S Supporting Information *
Calibration experiments of the TPD system, XRPD patterns of CaAB samples, TPD and DSC measurements of air exposed CaAB samples and DSC baseline. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address §
Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro, 2, 00185, Roma, Italy.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Experimental support from Fernando Moreno, Marine Ponthieu, and Maria José de la Mata is gratefully acknowledged. J.R.A., J.F.F., C.S., and M.A.B. also thank Spanish MICINN for financial support under Contracts MAT2011-22780 and CTQ2011-25517, and M.J.V.P. acknowledges her FPI fellowship.
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REFERENCES
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5. CONCLUSIONS High-energy reactive milling of CaH2 and AB has been used to obtain CaAB samples. They turn out to be quite air moisture sensitive and, after exposure to air for relatively short times, transform into Ca(OH)2 and AB. By contrast, they exhibit a metastable behavior when stored in an inert atmosphere for long times (several weeks). The sample metastability is mainly characterized by a slow spontaneous decomposition suffered by 24434
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