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Ammonia borane decomposition in the presence of cobalt halides
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 ) 1 2 9 5 5 e1 2 9 6 4
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Ammonia borane decomposition in the presence of cobalt halides R. Chiriac a, F. Toche a, U.B. Demirci a,b,*, O. Krol a, P. Miele b a
Universite´ Lyon 1, CNRS, UMR 5615, Laboratoire des Multimate´riaux et Interfaces, 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France b Institut Europe´en des Membranes, Universite´ Montpellier 2, CNRS, UMR 5253, Laboratoire des Agre´gats Interfaces et Mate´riaux pour l’Energie, Place Euge`ne Bataillon, F-34095 Montpellier Cedex 5, France
article info
abstract
Article history:
In the present work we studied the effect of cobalt halides (CoF2, CoCl2, CoBr2 and CoI2; also
Received 24 May 2011
denoted CoX2) on the thermal decomposition of ammonia borane NH3BH3 (AB) over the
Received in revised form
range 25e220 C. The reaction was followed by thermogravimetric analysis and differential
1 July 2011
scanning calorimetry. The gaseous products analyzed by gas chromatography and mass
Accepted 7 July 2011
spectrometry, and the solid by-products identified by elemental analysis: powder X-ray
Available online 10 August 2011
diffraction, infrared and X-ray photoelectron spectroscopies. Compared to pristine AB, the
Keywords:
and the content of unwanted borazine in the H2 stream, whereas the presence of CoI2 or
presence of CoCl2 and CoBr2 reduces both the induction period of the AB decomposition Ammonia borane
CoF2 has little or no effect, respectively. We propose that the positive effect of CoX2 comes
Chemical hydrogen storage
from their electronic and steric effects, and that CoCl2 is the compound which shows the
Cobalt halides
best properties relative to these effects. Herein, the roles of Co and X are discussed and
Hydrogen release
a revised mechanism of the AB dehydrocoupling initiation is proposed.
Thermolysis
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Ammonia borane (AB; NH3BH3) is hydrogen-dense, and when pristine, stores 19.6 wt% of hydrogen. This is one of the highest material-based absolute gravimetric hydrogen storage capacities (GHSC) for a material having a potential in chemical hydrogen storage, and this makes it particularly promising [1]. Hydrogen storage is one of the key issues for the development of a near-future hydrogen economy and one of the challenges is that any suitable material, from a technological viewpoint liberates at least an equivalent of 11 wt% of hydrogen [2] at temperatures lower than 85 C (target
for 2015), to be considered for light-duty vehicular applications [3]. AB does not fulfill the aforementioned criterion. It can dehydrogenate by hydrolysis (Eq. (1)) but, through this route, the material-based excess GHSC of AB-2H2O is only 9.0 wt% and the highest material-based net GHSC has been reported to be 7.8 wt% (because of the catalyst weight and the uncompleted conversion of AB) [4]: NH3 BH3 ðaq or sÞ þ 2 H2 OðlÞ/NHþ 4 ðaqÞ þ BO2 ðaqÞ þ 3 H2 ðgÞ
(1)
Today, this dehydrogenation route does not seem to be convenient or suitable because of the unexpected low material-
* Corresponding author. Universite´ Lyon 1, CNRS, UMR 5615, Laboratoire des Multimate´riaux et Interfaces, 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France. Tel.: þ33 0 4 67 14 91 60; fax: þ33 0 4 67 14 91 19. E-mail addresses: [email protected], [email protected] (U.B. Demirci). 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.2011.07.040
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based net GHSC and other issues, for example, ammonia (NH3) release [4,5]. AB can also dehydrogenate by thermolysis (Eq. (2)e(5)) [6], where upon heating, AB first melts and then starts its decomposition in three steps: NH3 BH3 ðsÞ/NH3 BH3 ðlÞ
(2)
NH3 BH3 ðlÞ/½NH2 BH2 ðsÞ þ H2 ðgÞ
(3)
½NH2 BH2 ðsÞ/½NHBHðsÞ þ H2 ðgÞ
(4)
½NHBHðsÞ/ BNðsÞ þ H2 ðgÞ
(5)
Unfortunately, this four-step process requires high temperatures (w100, 100e120, 150e200 and >500 C, respectively) because of a long induction period. These temperatures are overly elevated for the applications envisaged. Furthermore, unwanted gaseous side-products evolve. The formation of diborane (B2H6) and borazine (BZ; B3N3H6) have been reported [6,7]. The double challenge is thus to decrease the dehydrogenation temperatures while preventing the sideproducts from forming. Decreasing the temperature of AB dehydrogenation while drastically decreasing, or even avoiding, the formation of unwanted side-products is feasible. This can be done through several distinct approaches: nanoscaffolding (e.g. mesoporous silica SBA-15) [8]; catalysis (e.g. iridium pincer complex, (POCOP)Ir(H)2 with POCOP ¼ [h3-1,3-(OPtBu2)2C6H3]) of AB dissolved in organic medium (e.g. THF) or in ionic liquids [9]; solidesolid homogeneous catalysis (e.g. H2PtCl6) [10,11]; doping with hydrides of MOFs [12e15], and, chemical modification (e.g. synthesis of sodium amidoborane NaNH2BH3) [16,17]. Unsurprisingly, nanoscaffolding, dissolving, catalyzing and modifying AB all lead to a decrease in the materialbased GHSC because the weight of porous host, solvent, catalyst and new element has to be considered. Of these approaches, solidesolid homogeneous catalysis is perhaps the most hydrogen dense, simplest, and most convenient, and this is especially the case when commercial salts are used. For instance, the utilization of H2PtCl6 [11], NH4Cl [18], NH4NO3 [19], NiCl2 [20], CoCl2 [10,20], and FeCl3 [10,21] have already been reported. The present work is a further contribution to the utilization of metal salt additives (herein, cobalt halides). In previous work [10], we observed that AB decomposition in the presence of CoCl2 loaded at 10 wt% occurs at lower temperatures than for pristine AB and with much less BZ formation. This positive effect is attributed to the Lewis acidity of Co and the subsequent reduction of the induction period. Typically, the Lewis site destabilizes one AB molecule and, by chain reaction, the whole 3D network of the hydrogen bonds between adjacent AB molecules. However, the effect/role of the chloride anions, if any, is unknown. That is why, the present study is focused on better understanding this phenomenon, and so CoF2, CoCl2, CoBr2, and CoI2 were considered. The AB dehydrogenation in their presence was followed by thermal and calorimetric analyses; the hydrogen purity was assessed by gas chromatography (GC) and mass spectrometry (MS); and the solid by-products were characterized by ICP-AES, powder XRD
(X-ray Diffraction), IR and XPS (X-ray Photoelectron Spectroscopy). It was mainly observed that CoCl2 is the most effective in reducing the induction period of the AB decomposition while CoF2 is inert. Our main results are now reported and discussed.
2.
Experimental method
2.1.
Materials
AB (Sigma Aldrich, 97%), cobalt fluoride CoF2 (Strem Chemicals, anhydrous 99%), cobalt chloride CoCl2 (Acros Organics, anhydrous 97%), cobalt bromide CoBr2 (Sigma Aldrich, 99%), and cobalt iodide CoI2 (Sigma Aldrich, 99.99%) were used as received and handled in an argon-filled glove box. Promoted AB samples were prepared as follows. AB and 20 wt% of cobalt halide CoX2 were mixed together and ground in a mortar in the glove box. The content of CoX2 was fixed at 20 wt% in order to have enough material for the various characterizations later performed. By ICP-AES, it was found that the loading was consistent with the targets in that we obtained values of 20 1 wt%. The samples are hereafter denoted CoX2-AB.
2.2.
Thermogravimetric and calorimetric analysis
Thermogravimetric analysis (TGA) measurements were performed with TGA/SDTA Mettler Toledo 851e (SDTA: Simultaneous Differential Thermal Analysis) under the following conditions: sample mass 2e3 mg, aluminum crucible of 100 ml with a pinhole (B ¼ 670 mm), heating rate of 1 C min1, temperature range of 25e220 C, and atmosphere of N2 (50 ml min1). Sample mass loss and associated thermal effects were obtained by TGA/SDTA. In order to integrate the different mass loss steps, the TGA first derivation (mass loss rate) was used. TGA was calibrated over the range of temperature studied, i.e. 25e220 C min1. The melting points of five compounds (phenyl salicylate, naphthalene, benzoic acid, indium and tin) obtained from the DTA signals were used for the sample temperature calibration. The calibration gave a straight line with a regression coefficient of 1. Calcium oxalate monohydrate was used for the sample mass calibration and thus the experiment showed a difference in the 1st mass loss step of 3.5% compared to the theoretical value (experimental conditions: 2e3 mg of sample, heating rate of 1 C min1, temperature range 25e220 C). Note that the AB samples undergo a voluminous swelling, which may cause an artifact on the TGA profile or a contact with the internal walls of the TGA furnace. Hence, the mass was limited to 2e3 mg. The samples were analyzed by TGA three times to ensure the reproducibility of the results. The thermal study of the decomposition process was also investigated by differential scanning calorimetry (DSC) with a DSC1 Mettler Toledo device. The experimental conditions were identical to those used in TGA. For the kinetic study, heating rates of 1, 3, 5 and 7 C min1 were applied. The DSC was also calibrated over the range of temperature studied, i.e. 25e220 C. The melting points and enthalpies of a few standards were used for the calibration in terms of heat flow (indium and zinc), temperature and tau lag (indium and tin). As for the TGA experiments,
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the samples were analyzed by DSC three times to ensure the reproducibility of the results. The errors in the data reported are as follows: for temperature, 2 C (depending on how the user uses the software); for mass loss, 3.5% of the mass loss value; for DH from DSC, 2 kJ mol1 (depending on how the user selects the area to integrate).
2.3.
Gas analysis
The gas stream (H2 and side-products) was analyzed with a portable micro-chromatograph mGC M200 from Agilent M Series, which has 2 columns and 1 micro-thermal conductivity detector (m-TCD). H2 was separated on a molecular sieve ˚ ) and quantified with the TCD column (12 m 0.32 mm, 5 A detector. Another OV1 column (10 m 0.15 mm i.d.) separated BZ (and other possible gases like ammonia and diborane), its identification being realized by coupling the mGC with a mass selective detector (MSD). The mGC/MSD is commercialized by S.R.A. Instruments. The OV1 column temperature was fixed at 90 C while the molecular sieve one at 70 C. The mGC runtime was 60 and 120 s for H2 and BZ, respectively, while the injection time was fixed at 200 ms; the head column pressure was fixed at 27.6 psi for the molecular sieve column and 30.8 psi for the OV1 one. Helium and argon were used as the carrier gases for the separation of BZ and H2, respectively. To quantify the amounts of H2 released, the mGC/MSD was calibrated with a gas sample of known concentration, 2000 ppm.
2.4.
Solid by-products and cobalt analysis
To obtain large amounts of solid by-products after AB decomposition in the presence of CoX2, 200 mg of sample were heated up to 100 C or 200 C (heating rate of 1 C min1) in an alumina furnace under nitrogen flow (100 ml min1). The fresh and heated samples were all handled under argon. The as-obtained solid by-products (Band N-based polymers þ cobalt-based grey/black particles) were analyzed by X-ray diffraction (XRD, PANalytical X’pert pro MPD powder ˚ )), attenuated total diffractometer, CuKa radiation (l ¼ 1.5406 A reflectance Fourier transform infrared spectroscopy (IR, FTIR Nicolet 380), and elemental analysis (ICP-AES performed at “Service Central d’Analyses du CNRS”, in Solaize, France). The surface state of the cobalt-based grey/black particles was tentatively analyzed by X-ray photoelectron spectroscopy (XPS) on a PHI Quantera SXM spectrometer equipped with an Al Ka (hn ¼ 280 eV, 47.7 W); the spectrometer binding energy (Eb) scale was calibrated using the position of C 1s (284.8 eV) core level (XPS performed at “Science et Surface”, in Ecully, France).
3.
Results
3.1.
Thermal decomposition of CoX2-AB
The thermal decomposition of AB in the presence of CoX2 was followed by TGA and DSC, and then compared to the thermal decomposition of pristine AB. The release of the evolved gases was followed by the ATG-mGC-MS coupling. Only H2 and BZ were detected in our analytical conditions (except for CoF2-AB;
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see below). The amount of H2 was found to be around 13.0 1.3 wt% (i.e. 2.0 0.2 equiv. H2). Particular attention was paid to NH3 release, if any, and that is why we added a trap containing an aqueous solution of NH3-complexing CuSO4 at the outlet of the TGA furnace which the generated gases passed through. Finally, as an additional test, the gas stream (N2 þ evolving gases) was passed through 50 mL of deionized water (Millipore milli-Q with a resistivity > 18 MU cm) and the pH of the water was measured during the whole experiment (error of 0.05). In a subsequent experiment, a trap consisting of activated carbon (Rotilabo #HC24.1, Roth) was placed between the outlet of the TGA furnace and the deionized water. Fig. 1 presents the results obtained for CoF2-AB. As for pristine AB [10], the decomposition in the presence of CoF2 is a two-step process. The evolution of H2 starts at w50 C but the amounts remain negligible up to w90 C. Over the range 90e120 C, the sample mass loss is 12.3 wt% (for CoF2-AB), or 15.4 wt% if only AB is considered. The latter value is much higher than the 6.5 wt% of the first equivalent H2 (Eq. (2)) because of the formation of BZ. This can also be observed by the evolution of the pH of water that sharply increases to basic values when BZ starts to evolve. This is due to the fact that BZ hydrolyzes and forms basic borates [22]. Note that BZ can be trapped by activated carbon, which is an alternative approach to purify the H2 stream. The second main H2 evolution occurs at temperatures higher than w120 C. It is accompanied by the release of a greater amount of BZ (34.5 1.3 wt% over the whole temperature range). This is characterized by a mass loss of almost 50 wt% at 200 C. Interestingly by mGC-MS, we observed that monofluoroborazine (B3N3H5F, FBZ) evolves at temperatures higher than 100 C. The thermal decomposition of AB in the presence of CoF2 was also followed by DSC. It shows two successive endothermic processes (overall enthalpy of 3.2 kJ mol1) assigned to AB melting, followed by an exothermic one (14.6 kJ mol1) characteristic of AB decomposition. The exothermic peak is split into two successive peaks, which is consistent with the shoulders observed at w110 C for both H2 and BZ, also observed on the TGA curves. CoF2 may have an effect on the dehydrocoupling of some of AB molecules in close vicinity at temperatures above 100 C and such a process may occur in parallel to the decomposition of most of the AB that are not close to the salt. Compared to the thermal behavior of pristine AB, that of CoF2AB is lowered by few degrees and this suggests that CoF2 has negligible effect on reducing the induction period of AB decomposition. In previous work [10], we studied the thermal decomposition of CoCl2-AB (10 wt%). In the present work (Fig. 2), we reproduced these experiments but the CoCl2 amount was doubled i.e. 20 wt%. The results obtained in both studies are consistent. Typically, CoCl2 has a positive effect on the decomposition of AB: reducing the induction period, with H2 starting to evolve at w50 C. By TGA and DSC, it was observed that AB decomposes in 2 steps over the range 50e100 C with a loss (only AB is considered) of 13.1 wt%. Then, the decomposition continues in 3 (overlapped) steps, with a global loss of w35.0 wt% over the range 50e200 C. BZ is detected from 75 C but evolves the most above w110 C. Compared to the release of BZ from pristine AB, the amounts are drastically reduced
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Fig. 1 e CoF2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
Fig. 2 e CoCl2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
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(7.8 1.3 wt% versus 32.6 1.3 wt%). We also measured the variations of the pH of deionized water placed at the outlet of the TGA furnace. A DpH of w1.2 was found confirming the evolution of BZ. No NH3 was detected by mGC-MS. Fig. 3 shows the results obtained for CoBr2-AB. As for CoCl2, CoBr2 has a positive effect on the induction period of AB decomposition. H2 starts to evolve at w40 C and the amount exponentially increases up to 95 C; the mass loss (only AB is considered) is 8.1 wt%. BZ is detected at temperatures below 100 C. Then, the decomposition of AB shows two successive dehydrogenations over the range 120e200 C, where most of the evolved BZ is seen. The DSC results show that the decomposition of AB in the presence of CoBr2 is similar to that of CoCl2-AB but the peaks are shifted to higher temperatures. Typically, two main exothermic peaks are observed over the range 60e120 C (5.3 and 26.1 kJ mol1) but, in detail, the first one is actually composed of at least 3 overlapped peaks. These indicate the reduced capacity of CoBr2 allowing the decomposition of AB in milder conditions and a simpler decomposition mechanism. At 200 C, the mass loss is w38 wt%. This value is higher than that reported for CoCl2, which implies that CoBr2 is also less efficient in avoiding BZ evolution (16.5 1.3 wt% BZ). Fig. 4 shows the results obtained for CoI2-AB. AB in the presence of CoI2 decomposes at temperatures higher than 40 C, with 2H2 peaks centered at w61 and w73 C. The main dehydrogenation step occurs over the range 75e120 C and is followed by 2 others at higher temperatures. The mass losses (only AB is considered) are w1.3, w1.6, w5.3 and w10.8 wt% at 61, 73, 100 and 120 C, respectively. By DSC, we observe the
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first main exothermic peak is centered at 98 C (14.6 kJ mol1), which is also preceded by several small peaks (total enthalpy of 19.6 kJ mol1). Two other exothermic peaks are observed at temperatures higher than 125 C. In fact, the DSC curve is quite similar to that obtained with CoBr2. In addition, as for CoBr2, this is an indication of the complexity of the decomposition mechanisms. BZ evolution (14.0 1.3 wt% BZ) mainly occurs at temperatures higher than 110 C and only a small amount is detected over the range 75e100 C. If we now compare the aforementioned results, the following observations can be made. First, the TGA data reveal that, over the range 25e100 C, CoCl2 is the most efficient in reducing the induction period and thus in decreasing the decomposition temperature of AB. It appears to be more efficient than NH4NO3 [19] or FeCl3 [24] reported elsewhere, but a more detailed comparison (to these materials or to others) may be irrelevant given the discrepancies in the experimental conditions and also the high CoCl2 loading in our case. CoBr2 comes next in terms of efficiency and then CoI2, and finally, CoF2 which is rather inefficient. Second, the DSC results are consistent with the TGA ones. Third, the presence of CoCl2 and CoBr2 has a positive effect on lowering the amount of released BZ (Figure S1). Compared to the amount of BZ released from pristine AB (32.6 1.3 wt%), there is a significant decrease even though it is still not enough to avoid its formation. With CoF2, there is no effect on the BZ evolution, and in fact, it is even higher than that of pristine AB. Third, the evolution of the pH of deionized water placed at the outlet of the TGA furnace can be used to qualitatively assess the impact of CoX2 on the decrease in the amount of BZ. In the presence of
Fig. 3 e CoBr2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
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Fig. 4 e CoI2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
CoCl2, CoBr2, CoI2 and CoF2, a DpH of w1.2, w2.0, w2.6 and 3.0 were measured, respectively. This further helps explain the better efficiency of CoCl2, compared to the other halides.
3.2.
Kinetic analyses
A kinetic study using DSC was performed to determine the apparent activation energies for the dehydrogenation mechanism using the Kissinger equation [23]: ln ß=T2p ¼ lnðAR=Ea Þ Ea =RTp
(6)
where, b is the heating rate, Tp the temperature where the maximum reaction rate peaks, A the pre-exponential factor, and R the gas constant. Heating rates of 1, 3, 5 and 7 C min1 were applied to collect the Tp values. Then, ln (b/T2p) as a function of 1/Tp was plotted (Figure S2). The slope Ea/R permitted the calculation of Ea while the intercept of ln (AR/Ea) was used to determine A. Finally, with the as-obtained Ea and
A values, the specific rate constant k at a given temperature were calculated on the basis of the Arrhenius plot: k ¼ AexpðEa =RTÞ
(7)
Table 1 shows the values of Ea, A, and k at 80, 85 and 100 C for CoX2-AB as well as for pristine AB. As can be seen from these results, there is a reduction in the energy barrier for the samples containing CoCl2, CoBr2 and CoI2, the reduction being the most significant for the chloride (a decrease of 41 kJ mol1 compared to pristine AB). At 85 C, the rate constants for CoCl2-AB, CoBr2-AB, and CoI2-AB are 5.7, 2.8 and 2.3 times than that of pristine AB, respectively, whereas, at 100 C, they are 3.3, 2.3 and 2.0 times higher, respectively. We have confirmed that CoF2 is rather inactive in the decomposition of AB, as the values Ea, A, and k are similar to those of pristine AB. We compared our values of Ea, A, and k at 80 C with those reported by He et al. [20,24] for pristine AB (i.e. 129.0 kJ mol1, 1.00 1017 min1, and 8.14 103 min1) and for 3.3 wt% CoCl2-AB (i.e. 117.3 kJ mol1, 3.83 1015 min1, and
Table 1 e Kinetic parameters calculated from Kissinger and Arrhenius equations. Ea kJ mol1 Pristine AB CoF2-AB CoCl2-AB CoBr2-AB CoI2-AB
149 150 108 134 139
A min1 4.7 7.9 2.8 8.6 3.7
1019 1019 1014 1017 1018
k at 80 C min1 4.28 5.16 2.93 1.30 1.03
103 103 102 102 102
k at 85 C min1 8.68 1.05 4.91 2.45 2.00
103 102 102 102 102
k at 100 C min1 6.49 7.98 2.11 1.50 1.31
102 102 101 101 101
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1.68 102 min1). However, our results are slightly different. In fact, with respect to pristine AB, the apparent activation energy calculated here is lower by 20 kJ mol1 while the A value is 100 times higher. These differences can be rationalized by differences in calibration, heating rates (0.5e3 C min1 elsewhere) and DSC apparatus type. In a recent study, He et al. [24] have reported an Ea of 98.8 kJ mol1 for 3.2 wt% FeCl3-AB.
3.3.
B- and N-based solid by-products analysis
Though AB is known to dehydrogenate into amorphous polymers, i.e. polyaminoborane and polyiminoborane with the structural units [NH2BH2] and [NHBH], respectively (Eq. (3) and Eq. (4)), the solid by-products recovered upon heating at either 100 or 200 C were analyzed by XRD. No diffraction peaks (patterns not reported) were observed for the samples containing CoCl2, CoBr2 and CoI2, which is indicative of the dehydrogenation of AB and the subsequent formation of the polymers [18]. However, in the case of the sample containing CoF2, at 100 C, both starting materials, i.e. AB and CoF2, diffracted (Figure S3), confirming that CoF2 has no effect on the reduction of the induction period. The IR spectra of CoX2-AB heated at 100 C are presented in Figure S4. Comparison with the spectrum of pristine AB once again reveals that CoF2 has no effect on decreasing the decomposition temperature of AB as the spectra are very similar. In the case of the heat-treated CoBr2- and CoI2-AB, there are slight changes, indicative of the dehydrogenation of AB. On the other spectra, one can distinguish the broadening of the bands in the NH and BH stretching and bending regions. The IR spectra of CoX2-AB heat-treated at 200 C are shown in Figure S5, and confirm the formation of polymers with the structural unit [NHBH] [25] or [BNHy<2] [10]. Note that possible bands belonging to the BF bond should have been observed at <1200 cm1, but they were certainly overlapped by bands of NH and BH bonds. The B and N content was determined by ICP-AES. B:N molar ratios of 0.93, 1.00 and 1.05 were found for CoCl2-, CoBr2- and CoI2-AB, respectively. However, for heated CoF2-AB, a molar ratio of 2.13 was found whereas no NH3 was detected. The origin of such a deviation is unknown as yet. To sum up, we can say that, firstly, the XRD and IR results confirm that amorphous polymers are formed as solid byproducts, that secondly, CoF2 does not have an effect on the decrease of the induction period of AB decomposition, and that finally no bands related to cobalt-based compounds are observed.
3.4.
Cobalt analysis upon heating
The state of Co was only considered on heating at 100 C to observe its behavior during the initiation of the AB decomposition. For the first analysis, we considered the appearance of the samples before and after heating. Note that the color of the fresh samples, AB-CoF2, AB-CoCl2, AB-CoBr2 and AB-CoI2, is light pink, light blue, light blue-green and black, respectively, while that of the salts is pink, blue, green and black. This is consistent with a color dilution in the white matrix of AB. Upon heating, the sample color changes to grey which is indicative of a reduction of cobalt and thus the formation of
a new cobalt-based material. After that, cobalt was analyzed by XRD and XPS in each of the heated samples. It is well known that cobalt cations reduce into an amorphous cobalt-based compound in the presence of reducing agent such as sodium borohydride (NaBH4) or AB [20,26]. Hence, the solid by-products recovered after heating were analyzed by XRD (patterns not reported) and indeed no diffraction peaks were observed, except those of remaining CoF2 (Figure S3). Besides this, the presence of Co and halides X was verified by ICP-AES. The Co content was found to be in good agreement with the content in the fresh samples. The molar ratio X:Co was w2 when X was Cl, I and Br. However, it was confirmed that some F from CoF2 evolved in the form of FBZ while heating CoF2-AB: an F:Co molar ratio of 1.2 was determined. The samples, and especially the amorphous cobalt-based compound, were then analyzed by XPS. The first analysis of the heat-treated samples was unsuccessful in analyzing Co in the XPS conditions, because of its dilution in the B- and Nbased oligo-/polymer matrix. We therefore washed the samples with toluene and tetrahydrofuran under sonication to dissolve the oligo-/polymers and the remaining AB (in the case of CoF2). In doing so, the Co-based compounds were extracted and recovered owing to their magnetism. They were finally dried under vacuum, and analyzed by XPS. The analysis of the Co-based compound from CoI2-AB was not done because of its high magnetism and the subsequent risk for the XPS apparatus. There is therefore no XPS data for this material. Table 2 shows the binding energies (BE) found for the samples and the likely bonds attributed to them [27,28]. The presence of adsorbed B- and N-based compounds with BN, BH and NH bonds was observed. The B1s BE of about 190 eV and the N1s BE of about 398.5 eV suggest the formation of chains along BN bonds. Contamination with water was confirmed through the presence of BO bonds, indicative of borates [11]. For Co, the high content of surface B- and N-based compounds did not enable detection of its presence in our
Table 2 e Binding energies (eV) observed for the Co-based compounds extracted and recovered after heating at 100 C. CoF2-AB
Co 2p3/2 F 1s
783.3 685.6 689.0
Cl 2p3/2 Br 3d5/2 B 1s
CoCl2-AB
CoBr2-AB
n.d.a
n.d.a
198.0 199.4 189.8
190.1 192.0
68.3 190.0 191.9
398.4 400.5 532.5
398.5 400.4 532.3
192.7 N 1s O 1s
400.5 532.7
Likely bonds and/or compounds CoF CoF BF Chloride CoCl CoBr BN BH and/or BO BO and/or BF NB NH BO
a Not detected (because of an analyzed depth of 3 nm).
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operating conditions (analyzed depth of about 3 nm) for both CoCl2-AB and CoBr2-AB. As CoF2 is inactive the unreacted AB was easily removed and CoF2 was detected through the BEs of Co 2p3/2 and F 1s. The BEs found for the halides have been less straightforward to ascribe. The F 1s BE confirmed the presence of CoF2 and showed the formation of BF bonds, which is in agreement with the evolution of FBZ. The Cl 2p3/2 BE of 199.4 eV is that of CoCl in CoCl2, implying that all CoCl2 had not been totally reduced. An additional BE at lower energy, i.e. 198 eV, was observed; this implies that the electron density of Cl had increased, which may be due to a partial reduction of Co2þ by AB, its decomposition intermediates or evolving H2. This is consistent with the Coþ state reported for AB-reduced CoCl2 under heating [20]. In fact, we rather think that reduction by H2 occurs as there are no traces of BCl3 and NH4Cl, which normally form when AB reduces the metal salt [11]. These two compounds can be detected by MS and XRD, respectively. Finally, the Br 3d5/2 BE of 68.3 eV is attributed to CoBr bonds.
4.
Discussion
The dehydrogenation of AB is complicated by the Hdþ/Hd bonding network between the Hdþ from the NH3 groups and the Hd from the BH3 of adjacent AB molecules. It is a process involving 3 complex steps. The first is the induction when the hydrogen network is disrupted. An amorphous phase of AB
forms and AB isomerizes to diammoniate of diborane (DADB, [(NH3)2BH2]þBH4]. The formation of DADB is the nucleation step. It is a reactive intermediate that initiates the AB dehydrogenation and thus the growth step (head-to-tail dehydrocoupling of AB molecules through activation of BH and NH bonds) [18,29]. Reducing the induction period is important in lowering the dehydrogenation temperature of AB, and this may be successfully done by adding chemical additives (e.g. DADB, NH4Cl or hydrides), metal hollow spheres, transition metal salts (e.g. H2PtCl6 or NiCl2) or organometallic complexes (e.g. as those used in alkane dehydrogenation) [9e21]. With respect to the transition metal salts (MXa), it is expected that the metal cation Maþ initiates the AB dehydrocoupling (this may be assimilated to a radical formation such as Maþ/NþH2eBH2) and retains a BN unit during the basic steps of AB dehydrogenation (this may be assimilated to a polymerization) [29]. It has been suggested that Maþ and then B of BH2 of Maþ/NþH2eBH2 act as Lewis sites and activate the AB molecules through their NH3 moiety [29,30]. For chlorides, the M electronegativity may be the key factor for activating the first AB molecule [10]. In our work, we have observed, by TGA and DSC, that in the presence of CoCl2, CoBr2 and CoI2 the induction period is reduced, but the effect is less pronounced for CoBr2 and CoI2. However, with CoF2, the dehydrogenation takes place as for pristine AB; i.e. there is no effect. Elsewhere it has been suggested that Coþ is the active species that leads to a reduction of the induction period [20]. Our XPS results, and particularly
Fig. 5 e CoX2-AB thermolytic decomposition: plot of the mass loss in % from the TGA as a function of the electronegativity of X (a), of the radius of X (b), of the melting point of the salt (c), and of the enthalpy of formation of CoX2 (d).
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Fig. 6 e Reaction of AB with Co of CoX2.
the BEs found for the halides, support this assumption. In our conditions, the halides thus have a role in the ability of the metal to activate the AB dehydrocoupling. It is also important to note that the solid by-products are adsorbed on Coþ. Compared to the decomposition of pristine AB, the content of BZ that evolves over the range 100e200 C is decreased for both CoCl2 and CoBr2. However, when CoF2 and CoBr2 are used, there is no improvement. Hence, we suppose that the Coþ species favors AB dehydrocoupling while decreasing ring closing of AB trimers. To better understand the role of the halides, a few properties have been considered and the mass loss (in %) found by TGA at 100 C was plotted as a function of these different properties (Fig. 5). The main one is electronegativity (Fig. 5a). As can be seen here, the plot has a volcano shape, whose top is given by CoCl2. This suggests that the optimum electronegativity is in the range 1e1.5. The second property regarded is the halide ionic radius (Fig. 5b). Note that whatever the radius considered (i.e. atomic, covalent or ionic), the same trend was observed. The as-obtained plot also shows a volcano shape and peaks for the radius of Cl. The third property is the salt melting point (Fig. 5c), which is characteristic of the thermal stability of the salt. A volcano shape is also observed with an optimum value around 700 C. The fourth and final property is the enthalpy of formation of CoX2 (Fig. 5d). This has been considered even if the value for CoI2 was not found. The plot confirms the same trends clearly visible on the three others. From the results and analyses reported heretofore, we believe that the ability of CoX2 in reducing the induction
period is driven by both electronic and steric effects. The electronic effects are controlled by the electronegativity of X and the subsequent strength of the CoX bond. This rationalizes the inefficiency of CoF2 compared to CoBr2 and CoCl2. With respect to the steric effects, a larger size, for example with iodine, is detrimental to the diffusion of the reducing agents towards Co2þ. Therefore, the result is that the reduction of Co2þ to Coþ is kinetically less favored and this makes the CoX2 less efficient. CoCl2 therefore presents well-balanced electronic and steric properties. To sum up, Fig. 6 shows a more accurate explanation of the mechanism for the AB dehydrocoupling in the presence of CoX2. Though Co2þ from CoF2 is hardly reduced because of the electronic effects when the sample CoF2-AB is heated, the fluorides react with the B of AB and are able to substitute a few hydrides as shown by the evolution of FBZ at high temperatures. This has not been observed for the other halides but remains possible. With regard to application prospects, gaseous side-products with halide elements may be detrimental and therefore should be avoided.
5.
Conclusion
Metal salts are able to reduce the induction period of AB decomposition. In the present work, cobalt halides CoX2, where X is F, Cl, Br or I, were used to assess their efficiency in this reaction and to better understand the roles of both Co and X. It was observed that the best results are obtained with
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CoCl2, followed by CoBr2 and CoI2. However, CoF2 is inactive and does not contribute to improve the decomposition features of AB. The differences are explained in terms of electronic and steric effects where typically, the higher the electronegativity of the halide X the less reactive the CoX2, and the larger the radius of X the less reactive the CoX2. Therefore, CoCl2 presents the best compromise in terms of electronic and steric effects. In fact, to be efficient in reducing the induction period of AB decomposition, Co2þ has to be reduced to Coþ. The role of Coþ is then to activate the AB dehydrogenation by creating a germ, accelerating the AB polymerization and stabilizing the as-formed polymers, which reduces the BZ emissions. An updated mechanism has consequently been proposed.
Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ijhydene.2011.07.040.
[11]
[12]
[13]
[14]
[15]
[16]
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lu E, Yu¨ru¨m Y, Vezirog lu N. A review of hydrogen [1] Fakiog storage systems based on boron and its compounds. Int J Hydrogen Energy 2004;29:1371e6. [2] The U.S. Department of Energy has set severe criteria for hydrogen storage in regards to light-duty vehicular applications. The system-based net gravimetric hydrogen storage capacity (GHSC) is one of them. For example, it is expected to be 4.5 and 5.5 wt% by 2010 and 2015 respectively. It is nevertheless important to note that such a capacity does take into account the weight of the storage system taken as a whole (AB, tank, valves, pipes, etc). Here, we suppose that the weight of the material AB would be half of the weight of the storage system and that 100 % of the H2 expected from AB would be recovered. [3] U.S. Department of Energy. Office of Energy Efficiency and Renewable Energy and The FreedomCAR and Fuel Partnership, Targets for onboard hydrogen storage systems for light-duty vehicles. available at: http://www1.eere. energy.gov; September 2009 (accessed May, 2011). [4] Demirci UB, Miele P. Hydrolysis of solid ammonia borane. J Power Sources 2010;195:4030e5. [5] Brockman A, Zheng Y, Gore J. A study of catalytic hydrolysis of concentrated ammonia borane solutions. Int J Hydrogen Energy 2010;35:7350e6. [6] Sit V, Geanangel RA, Wendlandt WW. The thermal dissociation of NH3BH3. Thermo Chim Acta 1987;113:379e82. [7] Baitalow F, Baumann J, Wolf G, Jaenicke-Ro¨ßler K, Leitner G. Thermal decomposition of BeNeH compounds investigated by using combined thermoanalytical methods. Thermo Chim Acta 2002;391:159e68. [8] Gutowska A, Li L, Shin Y, Wang CM, Li XS, Linehan JC, et al. Nanoscaffold mediates hydrogen release and the reactivity of ammonia borane. Angew Chem 2005;117:3644e8. [9] Denney MC, Pons V, Hebden TJ, Heinekey DM, Goldberg KI. Efficient catalysis of ammonia borane dehydrogenation. J Am Chem Soc 2006;128:12048e9. [10] Benzouaa R, Demirci UB, Chiriac R, Toche F, Miele P. Metal chloride-doped ammonia borane thermolysis: positive effect
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on induction period as well as hydrogen and borazine release. Thermo Chim Acta 2010;509:81e6. De Benedetto S, Carewska M, Cento C, Gislon P, Pasquali M, Scaccia S, et al. Effect of milling and doping on decomposition of NH3BH3 complex. Thermo Chim Acta 2006; 441:184e90. Si XL, Sun LX, Xu F, Jiao CL, Li F, Liu SS, et al. Sawada. Improved hydrogen desorption properties of ammonia borane by Ni-modified metal-organic frameworks. Int J Hydrogen Energy 2011;36:6698e704. Srinivas G, Ford J, Zhou W, Yildirim T. Zn-MOF assisted dehydrogenation of ammonia borane: enhanced kinetics and clean hydrogen generation. Int J Hydrogen Energy; 2011. doi: 10.1016/j/ijhydene.2011.04.008. Gangal AC, Edla R, Iyer K, Biniwale R, Vashistha M, Sharma P. Effect of zeolites on thermal decomposition of ammonia borane. Int J Hydrogen Energy; 2011. doi:10.1016/j.ijhydene. 2011.04.011. Zhang Y, Shimoda K, Miyaoka H, Ichikawa T, Kojima Y. Thermal decomposition of alkaline-earth metal hydride and ammonia borane composites. Int J Hydrogen Energy 2010;35: 12405e9. Xiong Z, Yong CK, Wu G, Chen P, Shaw W, Karkamkar A, et al. High-capacity hydrogen storage in lithium and sodium amidoboranes. Nat Mater 2008;7:138e41. Sandra FPR, Demirci UB, Chiriac R, Moury R, Miele P. A simple preparation method of sodium amidoborane, highly efficient derivative of ammonia borane dehydrogenating at low temperature. Int J Hydrogen Energy 2011;36:7423e30. Heldebrant DJ, Karkamkar A, Hess NJ, Bowden M, Rassat S, Zheng F, et al. The effects of chemical additives on the induction phase in solid-state thermal decomposition of ammonia borane. Chem Mater 2008;20:5332e6. He´lary J, Salandre N, Saillard J, Poullain D, Beaucamp A, Autissier D. A physico-chemical study of an NH3BH3-based reactive composition for hydrogen generation. Int J Hydrogen Energy 2010;34:169e73. He T, Xiong Z, Wu G, Chu H, Wu C, Zhang T, et al. Nanosized Co- and Ni-catalyzed ammonia borane for hydrogen storage. Chem Mater 2009;21:2315e8. He T, Wang J, Wu G, Kim H, Proffen T, Wu A, et al. Growth of crystalline polyaminoborane through catalytic dehydrogenation of ammonia borane on FeB nanoalloy. Chem Eur J 2010;16:12814e7. Yoshizaki T, Watanabe H, Nakagawa T. Kinetics and mechanisms of hydrolysis of borazine derivatives in aqueous dioxane. Inorg Chem 1968;7:422e9. Kissinger HE. Reaction kinetics in differential thermal analysis. Anal Chem 1957;29:1702e6. He T, Wang J, Liu T, Wu G, Xiong Z, Yin J, et al. Quasi in situ Mo¨ssbauer and XAS studies on FeB nanoalloy for heterogeneous catalytic dehydrogenation of ammonia borane. Catal Today 2011;170:6975e81. Baumann J, Baitalow F. Thermal decomposition of polymeric aminoborane (H2BNH2)x under hydrogen release. Thermochim Acta 2005;430:9e14. Demirci UB, Miele P. Cobalt in NaBH4 hydrolysis. Phys Chem Chem Phys 2010;12:14651e65. NIST XPS Database. available at: http//srdata.nist.gov/xps/ Default.aspx (accessed April, 2011). La Surface XPS Database. available at: http://www.lasurface. com/database/elementxps.php (accessed May, 2011). Alcaraz G, Sabo-Etienne S. Coordination and dehydrogenation of amineeboranes at metal menters. Angew Chem Int Ed 2010;49:2e12. Bowden ME, Gainsford GJ, Robinson WT. Room-Temperature Structure of ammonia borane. Aust J Chem 2007;60:149e53.
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Ammonia borane decomposition in the presence of cobalt halides R. Chiriac a, F. Toche a, U.B. Demirci a,b,*, O. Krol a, P. Miele b a
Universite´ Lyon 1, CNRS, UMR 5615, Laboratoire des Multimate´riaux et Interfaces, 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France b Institut Europe´en des Membranes, Universite´ Montpellier 2, CNRS, UMR 5253, Laboratoire des Agre´gats Interfaces et Mate´riaux pour l’Energie, Place Euge`ne Bataillon, F-34095 Montpellier Cedex 5, France
article info
abstract
Article history:
In the present work we studied the effect of cobalt halides (CoF2, CoCl2, CoBr2 and CoI2; also
Received 24 May 2011
denoted CoX2) on the thermal decomposition of ammonia borane NH3BH3 (AB) over the
Received in revised form
range 25e220 C. The reaction was followed by thermogravimetric analysis and differential
1 July 2011
scanning calorimetry. The gaseous products analyzed by gas chromatography and mass
Accepted 7 July 2011
spectrometry, and the solid by-products identified by elemental analysis: powder X-ray
Available online 10 August 2011
diffraction, infrared and X-ray photoelectron spectroscopies. Compared to pristine AB, the
Keywords:
and the content of unwanted borazine in the H2 stream, whereas the presence of CoI2 or
presence of CoCl2 and CoBr2 reduces both the induction period of the AB decomposition Ammonia borane
CoF2 has little or no effect, respectively. We propose that the positive effect of CoX2 comes
Chemical hydrogen storage
from their electronic and steric effects, and that CoCl2 is the compound which shows the
Cobalt halides
best properties relative to these effects. Herein, the roles of Co and X are discussed and
Hydrogen release
a revised mechanism of the AB dehydrocoupling initiation is proposed.
Thermolysis
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Ammonia borane (AB; NH3BH3) is hydrogen-dense, and when pristine, stores 19.6 wt% of hydrogen. This is one of the highest material-based absolute gravimetric hydrogen storage capacities (GHSC) for a material having a potential in chemical hydrogen storage, and this makes it particularly promising [1]. Hydrogen storage is one of the key issues for the development of a near-future hydrogen economy and one of the challenges is that any suitable material, from a technological viewpoint liberates at least an equivalent of 11 wt% of hydrogen [2] at temperatures lower than 85 C (target
for 2015), to be considered for light-duty vehicular applications [3]. AB does not fulfill the aforementioned criterion. It can dehydrogenate by hydrolysis (Eq. (1)) but, through this route, the material-based excess GHSC of AB-2H2O is only 9.0 wt% and the highest material-based net GHSC has been reported to be 7.8 wt% (because of the catalyst weight and the uncompleted conversion of AB) [4]: NH3 BH3 ðaq or sÞ þ 2 H2 OðlÞ/NHþ 4 ðaqÞ þ BO2 ðaqÞ þ 3 H2 ðgÞ
(1)
Today, this dehydrogenation route does not seem to be convenient or suitable because of the unexpected low material-
* Corresponding author. Universite´ Lyon 1, CNRS, UMR 5615, Laboratoire des Multimate´riaux et Interfaces, 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France. Tel.: þ33 0 4 67 14 91 60; fax: þ33 0 4 67 14 91 19. E-mail addresses: [email protected], [email protected] (U.B. Demirci). 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.2011.07.040
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based net GHSC and other issues, for example, ammonia (NH3) release [4,5]. AB can also dehydrogenate by thermolysis (Eq. (2)e(5)) [6], where upon heating, AB first melts and then starts its decomposition in three steps: NH3 BH3 ðsÞ/NH3 BH3 ðlÞ
(2)
NH3 BH3 ðlÞ/½NH2 BH2 ðsÞ þ H2 ðgÞ
(3)
½NH2 BH2 ðsÞ/½NHBHðsÞ þ H2 ðgÞ
(4)
½NHBHðsÞ/ BNðsÞ þ H2 ðgÞ
(5)
Unfortunately, this four-step process requires high temperatures (w100, 100e120, 150e200 and >500 C, respectively) because of a long induction period. These temperatures are overly elevated for the applications envisaged. Furthermore, unwanted gaseous side-products evolve. The formation of diborane (B2H6) and borazine (BZ; B3N3H6) have been reported [6,7]. The double challenge is thus to decrease the dehydrogenation temperatures while preventing the sideproducts from forming. Decreasing the temperature of AB dehydrogenation while drastically decreasing, or even avoiding, the formation of unwanted side-products is feasible. This can be done through several distinct approaches: nanoscaffolding (e.g. mesoporous silica SBA-15) [8]; catalysis (e.g. iridium pincer complex, (POCOP)Ir(H)2 with POCOP ¼ [h3-1,3-(OPtBu2)2C6H3]) of AB dissolved in organic medium (e.g. THF) or in ionic liquids [9]; solidesolid homogeneous catalysis (e.g. H2PtCl6) [10,11]; doping with hydrides of MOFs [12e15], and, chemical modification (e.g. synthesis of sodium amidoborane NaNH2BH3) [16,17]. Unsurprisingly, nanoscaffolding, dissolving, catalyzing and modifying AB all lead to a decrease in the materialbased GHSC because the weight of porous host, solvent, catalyst and new element has to be considered. Of these approaches, solidesolid homogeneous catalysis is perhaps the most hydrogen dense, simplest, and most convenient, and this is especially the case when commercial salts are used. For instance, the utilization of H2PtCl6 [11], NH4Cl [18], NH4NO3 [19], NiCl2 [20], CoCl2 [10,20], and FeCl3 [10,21] have already been reported. The present work is a further contribution to the utilization of metal salt additives (herein, cobalt halides). In previous work [10], we observed that AB decomposition in the presence of CoCl2 loaded at 10 wt% occurs at lower temperatures than for pristine AB and with much less BZ formation. This positive effect is attributed to the Lewis acidity of Co and the subsequent reduction of the induction period. Typically, the Lewis site destabilizes one AB molecule and, by chain reaction, the whole 3D network of the hydrogen bonds between adjacent AB molecules. However, the effect/role of the chloride anions, if any, is unknown. That is why, the present study is focused on better understanding this phenomenon, and so CoF2, CoCl2, CoBr2, and CoI2 were considered. The AB dehydrogenation in their presence was followed by thermal and calorimetric analyses; the hydrogen purity was assessed by gas chromatography (GC) and mass spectrometry (MS); and the solid by-products were characterized by ICP-AES, powder XRD
(X-ray Diffraction), IR and XPS (X-ray Photoelectron Spectroscopy). It was mainly observed that CoCl2 is the most effective in reducing the induction period of the AB decomposition while CoF2 is inert. Our main results are now reported and discussed.
2.
Experimental method
2.1.
Materials
AB (Sigma Aldrich, 97%), cobalt fluoride CoF2 (Strem Chemicals, anhydrous 99%), cobalt chloride CoCl2 (Acros Organics, anhydrous 97%), cobalt bromide CoBr2 (Sigma Aldrich, 99%), and cobalt iodide CoI2 (Sigma Aldrich, 99.99%) were used as received and handled in an argon-filled glove box. Promoted AB samples were prepared as follows. AB and 20 wt% of cobalt halide CoX2 were mixed together and ground in a mortar in the glove box. The content of CoX2 was fixed at 20 wt% in order to have enough material for the various characterizations later performed. By ICP-AES, it was found that the loading was consistent with the targets in that we obtained values of 20 1 wt%. The samples are hereafter denoted CoX2-AB.
2.2.
Thermogravimetric and calorimetric analysis
Thermogravimetric analysis (TGA) measurements were performed with TGA/SDTA Mettler Toledo 851e (SDTA: Simultaneous Differential Thermal Analysis) under the following conditions: sample mass 2e3 mg, aluminum crucible of 100 ml with a pinhole (B ¼ 670 mm), heating rate of 1 C min1, temperature range of 25e220 C, and atmosphere of N2 (50 ml min1). Sample mass loss and associated thermal effects were obtained by TGA/SDTA. In order to integrate the different mass loss steps, the TGA first derivation (mass loss rate) was used. TGA was calibrated over the range of temperature studied, i.e. 25e220 C min1. The melting points of five compounds (phenyl salicylate, naphthalene, benzoic acid, indium and tin) obtained from the DTA signals were used for the sample temperature calibration. The calibration gave a straight line with a regression coefficient of 1. Calcium oxalate monohydrate was used for the sample mass calibration and thus the experiment showed a difference in the 1st mass loss step of 3.5% compared to the theoretical value (experimental conditions: 2e3 mg of sample, heating rate of 1 C min1, temperature range 25e220 C). Note that the AB samples undergo a voluminous swelling, which may cause an artifact on the TGA profile or a contact with the internal walls of the TGA furnace. Hence, the mass was limited to 2e3 mg. The samples were analyzed by TGA three times to ensure the reproducibility of the results. The thermal study of the decomposition process was also investigated by differential scanning calorimetry (DSC) with a DSC1 Mettler Toledo device. The experimental conditions were identical to those used in TGA. For the kinetic study, heating rates of 1, 3, 5 and 7 C min1 were applied. The DSC was also calibrated over the range of temperature studied, i.e. 25e220 C. The melting points and enthalpies of a few standards were used for the calibration in terms of heat flow (indium and zinc), temperature and tau lag (indium and tin). As for the TGA experiments,
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the samples were analyzed by DSC three times to ensure the reproducibility of the results. The errors in the data reported are as follows: for temperature, 2 C (depending on how the user uses the software); for mass loss, 3.5% of the mass loss value; for DH from DSC, 2 kJ mol1 (depending on how the user selects the area to integrate).
2.3.
Gas analysis
The gas stream (H2 and side-products) was analyzed with a portable micro-chromatograph mGC M200 from Agilent M Series, which has 2 columns and 1 micro-thermal conductivity detector (m-TCD). H2 was separated on a molecular sieve ˚ ) and quantified with the TCD column (12 m 0.32 mm, 5 A detector. Another OV1 column (10 m 0.15 mm i.d.) separated BZ (and other possible gases like ammonia and diborane), its identification being realized by coupling the mGC with a mass selective detector (MSD). The mGC/MSD is commercialized by S.R.A. Instruments. The OV1 column temperature was fixed at 90 C while the molecular sieve one at 70 C. The mGC runtime was 60 and 120 s for H2 and BZ, respectively, while the injection time was fixed at 200 ms; the head column pressure was fixed at 27.6 psi for the molecular sieve column and 30.8 psi for the OV1 one. Helium and argon were used as the carrier gases for the separation of BZ and H2, respectively. To quantify the amounts of H2 released, the mGC/MSD was calibrated with a gas sample of known concentration, 2000 ppm.
2.4.
Solid by-products and cobalt analysis
To obtain large amounts of solid by-products after AB decomposition in the presence of CoX2, 200 mg of sample were heated up to 100 C or 200 C (heating rate of 1 C min1) in an alumina furnace under nitrogen flow (100 ml min1). The fresh and heated samples were all handled under argon. The as-obtained solid by-products (Band N-based polymers þ cobalt-based grey/black particles) were analyzed by X-ray diffraction (XRD, PANalytical X’pert pro MPD powder ˚ )), attenuated total diffractometer, CuKa radiation (l ¼ 1.5406 A reflectance Fourier transform infrared spectroscopy (IR, FTIR Nicolet 380), and elemental analysis (ICP-AES performed at “Service Central d’Analyses du CNRS”, in Solaize, France). The surface state of the cobalt-based grey/black particles was tentatively analyzed by X-ray photoelectron spectroscopy (XPS) on a PHI Quantera SXM spectrometer equipped with an Al Ka (hn ¼ 280 eV, 47.7 W); the spectrometer binding energy (Eb) scale was calibrated using the position of C 1s (284.8 eV) core level (XPS performed at “Science et Surface”, in Ecully, France).
3.
Results
3.1.
Thermal decomposition of CoX2-AB
The thermal decomposition of AB in the presence of CoX2 was followed by TGA and DSC, and then compared to the thermal decomposition of pristine AB. The release of the evolved gases was followed by the ATG-mGC-MS coupling. Only H2 and BZ were detected in our analytical conditions (except for CoF2-AB;
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see below). The amount of H2 was found to be around 13.0 1.3 wt% (i.e. 2.0 0.2 equiv. H2). Particular attention was paid to NH3 release, if any, and that is why we added a trap containing an aqueous solution of NH3-complexing CuSO4 at the outlet of the TGA furnace which the generated gases passed through. Finally, as an additional test, the gas stream (N2 þ evolving gases) was passed through 50 mL of deionized water (Millipore milli-Q with a resistivity > 18 MU cm) and the pH of the water was measured during the whole experiment (error of 0.05). In a subsequent experiment, a trap consisting of activated carbon (Rotilabo #HC24.1, Roth) was placed between the outlet of the TGA furnace and the deionized water. Fig. 1 presents the results obtained for CoF2-AB. As for pristine AB [10], the decomposition in the presence of CoF2 is a two-step process. The evolution of H2 starts at w50 C but the amounts remain negligible up to w90 C. Over the range 90e120 C, the sample mass loss is 12.3 wt% (for CoF2-AB), or 15.4 wt% if only AB is considered. The latter value is much higher than the 6.5 wt% of the first equivalent H2 (Eq. (2)) because of the formation of BZ. This can also be observed by the evolution of the pH of water that sharply increases to basic values when BZ starts to evolve. This is due to the fact that BZ hydrolyzes and forms basic borates [22]. Note that BZ can be trapped by activated carbon, which is an alternative approach to purify the H2 stream. The second main H2 evolution occurs at temperatures higher than w120 C. It is accompanied by the release of a greater amount of BZ (34.5 1.3 wt% over the whole temperature range). This is characterized by a mass loss of almost 50 wt% at 200 C. Interestingly by mGC-MS, we observed that monofluoroborazine (B3N3H5F, FBZ) evolves at temperatures higher than 100 C. The thermal decomposition of AB in the presence of CoF2 was also followed by DSC. It shows two successive endothermic processes (overall enthalpy of 3.2 kJ mol1) assigned to AB melting, followed by an exothermic one (14.6 kJ mol1) characteristic of AB decomposition. The exothermic peak is split into two successive peaks, which is consistent with the shoulders observed at w110 C for both H2 and BZ, also observed on the TGA curves. CoF2 may have an effect on the dehydrocoupling of some of AB molecules in close vicinity at temperatures above 100 C and such a process may occur in parallel to the decomposition of most of the AB that are not close to the salt. Compared to the thermal behavior of pristine AB, that of CoF2AB is lowered by few degrees and this suggests that CoF2 has negligible effect on reducing the induction period of AB decomposition. In previous work [10], we studied the thermal decomposition of CoCl2-AB (10 wt%). In the present work (Fig. 2), we reproduced these experiments but the CoCl2 amount was doubled i.e. 20 wt%. The results obtained in both studies are consistent. Typically, CoCl2 has a positive effect on the decomposition of AB: reducing the induction period, with H2 starting to evolve at w50 C. By TGA and DSC, it was observed that AB decomposes in 2 steps over the range 50e100 C with a loss (only AB is considered) of 13.1 wt%. Then, the decomposition continues in 3 (overlapped) steps, with a global loss of w35.0 wt% over the range 50e200 C. BZ is detected from 75 C but evolves the most above w110 C. Compared to the release of BZ from pristine AB, the amounts are drastically reduced
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Fig. 1 e CoF2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
Fig. 2 e CoCl2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
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(7.8 1.3 wt% versus 32.6 1.3 wt%). We also measured the variations of the pH of deionized water placed at the outlet of the TGA furnace. A DpH of w1.2 was found confirming the evolution of BZ. No NH3 was detected by mGC-MS. Fig. 3 shows the results obtained for CoBr2-AB. As for CoCl2, CoBr2 has a positive effect on the induction period of AB decomposition. H2 starts to evolve at w40 C and the amount exponentially increases up to 95 C; the mass loss (only AB is considered) is 8.1 wt%. BZ is detected at temperatures below 100 C. Then, the decomposition of AB shows two successive dehydrogenations over the range 120e200 C, where most of the evolved BZ is seen. The DSC results show that the decomposition of AB in the presence of CoBr2 is similar to that of CoCl2-AB but the peaks are shifted to higher temperatures. Typically, two main exothermic peaks are observed over the range 60e120 C (5.3 and 26.1 kJ mol1) but, in detail, the first one is actually composed of at least 3 overlapped peaks. These indicate the reduced capacity of CoBr2 allowing the decomposition of AB in milder conditions and a simpler decomposition mechanism. At 200 C, the mass loss is w38 wt%. This value is higher than that reported for CoCl2, which implies that CoBr2 is also less efficient in avoiding BZ evolution (16.5 1.3 wt% BZ). Fig. 4 shows the results obtained for CoI2-AB. AB in the presence of CoI2 decomposes at temperatures higher than 40 C, with 2H2 peaks centered at w61 and w73 C. The main dehydrogenation step occurs over the range 75e120 C and is followed by 2 others at higher temperatures. The mass losses (only AB is considered) are w1.3, w1.6, w5.3 and w10.8 wt% at 61, 73, 100 and 120 C, respectively. By DSC, we observe the
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first main exothermic peak is centered at 98 C (14.6 kJ mol1), which is also preceded by several small peaks (total enthalpy of 19.6 kJ mol1). Two other exothermic peaks are observed at temperatures higher than 125 C. In fact, the DSC curve is quite similar to that obtained with CoBr2. In addition, as for CoBr2, this is an indication of the complexity of the decomposition mechanisms. BZ evolution (14.0 1.3 wt% BZ) mainly occurs at temperatures higher than 110 C and only a small amount is detected over the range 75e100 C. If we now compare the aforementioned results, the following observations can be made. First, the TGA data reveal that, over the range 25e100 C, CoCl2 is the most efficient in reducing the induction period and thus in decreasing the decomposition temperature of AB. It appears to be more efficient than NH4NO3 [19] or FeCl3 [24] reported elsewhere, but a more detailed comparison (to these materials or to others) may be irrelevant given the discrepancies in the experimental conditions and also the high CoCl2 loading in our case. CoBr2 comes next in terms of efficiency and then CoI2, and finally, CoF2 which is rather inefficient. Second, the DSC results are consistent with the TGA ones. Third, the presence of CoCl2 and CoBr2 has a positive effect on lowering the amount of released BZ (Figure S1). Compared to the amount of BZ released from pristine AB (32.6 1.3 wt%), there is a significant decrease even though it is still not enough to avoid its formation. With CoF2, there is no effect on the BZ evolution, and in fact, it is even higher than that of pristine AB. Third, the evolution of the pH of deionized water placed at the outlet of the TGA furnace can be used to qualitatively assess the impact of CoX2 on the decrease in the amount of BZ. In the presence of
Fig. 3 e CoBr2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
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Fig. 4 e CoI2-AB thermolytic decomposition as a function of the temperature: results of the TGA (a, b), DSC (b), GC (a, c), and pH (d).
CoCl2, CoBr2, CoI2 and CoF2, a DpH of w1.2, w2.0, w2.6 and 3.0 were measured, respectively. This further helps explain the better efficiency of CoCl2, compared to the other halides.
3.2.
Kinetic analyses
A kinetic study using DSC was performed to determine the apparent activation energies for the dehydrogenation mechanism using the Kissinger equation [23]: ln ß=T2p ¼ lnðAR=Ea Þ Ea =RTp
(6)
where, b is the heating rate, Tp the temperature where the maximum reaction rate peaks, A the pre-exponential factor, and R the gas constant. Heating rates of 1, 3, 5 and 7 C min1 were applied to collect the Tp values. Then, ln (b/T2p) as a function of 1/Tp was plotted (Figure S2). The slope Ea/R permitted the calculation of Ea while the intercept of ln (AR/Ea) was used to determine A. Finally, with the as-obtained Ea and
A values, the specific rate constant k at a given temperature were calculated on the basis of the Arrhenius plot: k ¼ AexpðEa =RTÞ
(7)
Table 1 shows the values of Ea, A, and k at 80, 85 and 100 C for CoX2-AB as well as for pristine AB. As can be seen from these results, there is a reduction in the energy barrier for the samples containing CoCl2, CoBr2 and CoI2, the reduction being the most significant for the chloride (a decrease of 41 kJ mol1 compared to pristine AB). At 85 C, the rate constants for CoCl2-AB, CoBr2-AB, and CoI2-AB are 5.7, 2.8 and 2.3 times than that of pristine AB, respectively, whereas, at 100 C, they are 3.3, 2.3 and 2.0 times higher, respectively. We have confirmed that CoF2 is rather inactive in the decomposition of AB, as the values Ea, A, and k are similar to those of pristine AB. We compared our values of Ea, A, and k at 80 C with those reported by He et al. [20,24] for pristine AB (i.e. 129.0 kJ mol1, 1.00 1017 min1, and 8.14 103 min1) and for 3.3 wt% CoCl2-AB (i.e. 117.3 kJ mol1, 3.83 1015 min1, and
Table 1 e Kinetic parameters calculated from Kissinger and Arrhenius equations. Ea kJ mol1 Pristine AB CoF2-AB CoCl2-AB CoBr2-AB CoI2-AB
149 150 108 134 139
A min1 4.7 7.9 2.8 8.6 3.7
1019 1019 1014 1017 1018
k at 80 C min1 4.28 5.16 2.93 1.30 1.03
103 103 102 102 102
k at 85 C min1 8.68 1.05 4.91 2.45 2.00
103 102 102 102 102
k at 100 C min1 6.49 7.98 2.11 1.50 1.31
102 102 101 101 101
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1.68 102 min1). However, our results are slightly different. In fact, with respect to pristine AB, the apparent activation energy calculated here is lower by 20 kJ mol1 while the A value is 100 times higher. These differences can be rationalized by differences in calibration, heating rates (0.5e3 C min1 elsewhere) and DSC apparatus type. In a recent study, He et al. [24] have reported an Ea of 98.8 kJ mol1 for 3.2 wt% FeCl3-AB.
3.3.
B- and N-based solid by-products analysis
Though AB is known to dehydrogenate into amorphous polymers, i.e. polyaminoborane and polyiminoborane with the structural units [NH2BH2] and [NHBH], respectively (Eq. (3) and Eq. (4)), the solid by-products recovered upon heating at either 100 or 200 C were analyzed by XRD. No diffraction peaks (patterns not reported) were observed for the samples containing CoCl2, CoBr2 and CoI2, which is indicative of the dehydrogenation of AB and the subsequent formation of the polymers [18]. However, in the case of the sample containing CoF2, at 100 C, both starting materials, i.e. AB and CoF2, diffracted (Figure S3), confirming that CoF2 has no effect on the reduction of the induction period. The IR spectra of CoX2-AB heated at 100 C are presented in Figure S4. Comparison with the spectrum of pristine AB once again reveals that CoF2 has no effect on decreasing the decomposition temperature of AB as the spectra are very similar. In the case of the heat-treated CoBr2- and CoI2-AB, there are slight changes, indicative of the dehydrogenation of AB. On the other spectra, one can distinguish the broadening of the bands in the NH and BH stretching and bending regions. The IR spectra of CoX2-AB heat-treated at 200 C are shown in Figure S5, and confirm the formation of polymers with the structural unit [NHBH] [25] or [BNHy<2] [10]. Note that possible bands belonging to the BF bond should have been observed at <1200 cm1, but they were certainly overlapped by bands of NH and BH bonds. The B and N content was determined by ICP-AES. B:N molar ratios of 0.93, 1.00 and 1.05 were found for CoCl2-, CoBr2- and CoI2-AB, respectively. However, for heated CoF2-AB, a molar ratio of 2.13 was found whereas no NH3 was detected. The origin of such a deviation is unknown as yet. To sum up, we can say that, firstly, the XRD and IR results confirm that amorphous polymers are formed as solid byproducts, that secondly, CoF2 does not have an effect on the decrease of the induction period of AB decomposition, and that finally no bands related to cobalt-based compounds are observed.
3.4.
Cobalt analysis upon heating
The state of Co was only considered on heating at 100 C to observe its behavior during the initiation of the AB decomposition. For the first analysis, we considered the appearance of the samples before and after heating. Note that the color of the fresh samples, AB-CoF2, AB-CoCl2, AB-CoBr2 and AB-CoI2, is light pink, light blue, light blue-green and black, respectively, while that of the salts is pink, blue, green and black. This is consistent with a color dilution in the white matrix of AB. Upon heating, the sample color changes to grey which is indicative of a reduction of cobalt and thus the formation of
a new cobalt-based material. After that, cobalt was analyzed by XRD and XPS in each of the heated samples. It is well known that cobalt cations reduce into an amorphous cobalt-based compound in the presence of reducing agent such as sodium borohydride (NaBH4) or AB [20,26]. Hence, the solid by-products recovered after heating were analyzed by XRD (patterns not reported) and indeed no diffraction peaks were observed, except those of remaining CoF2 (Figure S3). Besides this, the presence of Co and halides X was verified by ICP-AES. The Co content was found to be in good agreement with the content in the fresh samples. The molar ratio X:Co was w2 when X was Cl, I and Br. However, it was confirmed that some F from CoF2 evolved in the form of FBZ while heating CoF2-AB: an F:Co molar ratio of 1.2 was determined. The samples, and especially the amorphous cobalt-based compound, were then analyzed by XPS. The first analysis of the heat-treated samples was unsuccessful in analyzing Co in the XPS conditions, because of its dilution in the B- and Nbased oligo-/polymer matrix. We therefore washed the samples with toluene and tetrahydrofuran under sonication to dissolve the oligo-/polymers and the remaining AB (in the case of CoF2). In doing so, the Co-based compounds were extracted and recovered owing to their magnetism. They were finally dried under vacuum, and analyzed by XPS. The analysis of the Co-based compound from CoI2-AB was not done because of its high magnetism and the subsequent risk for the XPS apparatus. There is therefore no XPS data for this material. Table 2 shows the binding energies (BE) found for the samples and the likely bonds attributed to them [27,28]. The presence of adsorbed B- and N-based compounds with BN, BH and NH bonds was observed. The B1s BE of about 190 eV and the N1s BE of about 398.5 eV suggest the formation of chains along BN bonds. Contamination with water was confirmed through the presence of BO bonds, indicative of borates [11]. For Co, the high content of surface B- and N-based compounds did not enable detection of its presence in our
Table 2 e Binding energies (eV) observed for the Co-based compounds extracted and recovered after heating at 100 C. CoF2-AB
Co 2p3/2 F 1s
783.3 685.6 689.0
Cl 2p3/2 Br 3d5/2 B 1s
CoCl2-AB
CoBr2-AB
n.d.a
n.d.a
198.0 199.4 189.8
190.1 192.0
68.3 190.0 191.9
398.4 400.5 532.5
398.5 400.4 532.3
192.7 N 1s O 1s
400.5 532.7
Likely bonds and/or compounds CoF CoF BF Chloride CoCl CoBr BN BH and/or BO BO and/or BF NB NH BO
a Not detected (because of an analyzed depth of 3 nm).
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operating conditions (analyzed depth of about 3 nm) for both CoCl2-AB and CoBr2-AB. As CoF2 is inactive the unreacted AB was easily removed and CoF2 was detected through the BEs of Co 2p3/2 and F 1s. The BEs found for the halides have been less straightforward to ascribe. The F 1s BE confirmed the presence of CoF2 and showed the formation of BF bonds, which is in agreement with the evolution of FBZ. The Cl 2p3/2 BE of 199.4 eV is that of CoCl in CoCl2, implying that all CoCl2 had not been totally reduced. An additional BE at lower energy, i.e. 198 eV, was observed; this implies that the electron density of Cl had increased, which may be due to a partial reduction of Co2þ by AB, its decomposition intermediates or evolving H2. This is consistent with the Coþ state reported for AB-reduced CoCl2 under heating [20]. In fact, we rather think that reduction by H2 occurs as there are no traces of BCl3 and NH4Cl, which normally form when AB reduces the metal salt [11]. These two compounds can be detected by MS and XRD, respectively. Finally, the Br 3d5/2 BE of 68.3 eV is attributed to CoBr bonds.
4.
Discussion
The dehydrogenation of AB is complicated by the Hdþ/Hd bonding network between the Hdþ from the NH3 groups and the Hd from the BH3 of adjacent AB molecules. It is a process involving 3 complex steps. The first is the induction when the hydrogen network is disrupted. An amorphous phase of AB
forms and AB isomerizes to diammoniate of diborane (DADB, [(NH3)2BH2]þBH4]. The formation of DADB is the nucleation step. It is a reactive intermediate that initiates the AB dehydrogenation and thus the growth step (head-to-tail dehydrocoupling of AB molecules through activation of BH and NH bonds) [18,29]. Reducing the induction period is important in lowering the dehydrogenation temperature of AB, and this may be successfully done by adding chemical additives (e.g. DADB, NH4Cl or hydrides), metal hollow spheres, transition metal salts (e.g. H2PtCl6 or NiCl2) or organometallic complexes (e.g. as those used in alkane dehydrogenation) [9e21]. With respect to the transition metal salts (MXa), it is expected that the metal cation Maþ initiates the AB dehydrocoupling (this may be assimilated to a radical formation such as Maþ/NþH2eBH2) and retains a BN unit during the basic steps of AB dehydrogenation (this may be assimilated to a polymerization) [29]. It has been suggested that Maþ and then B of BH2 of Maþ/NþH2eBH2 act as Lewis sites and activate the AB molecules through their NH3 moiety [29,30]. For chlorides, the M electronegativity may be the key factor for activating the first AB molecule [10]. In our work, we have observed, by TGA and DSC, that in the presence of CoCl2, CoBr2 and CoI2 the induction period is reduced, but the effect is less pronounced for CoBr2 and CoI2. However, with CoF2, the dehydrogenation takes place as for pristine AB; i.e. there is no effect. Elsewhere it has been suggested that Coþ is the active species that leads to a reduction of the induction period [20]. Our XPS results, and particularly
Fig. 5 e CoX2-AB thermolytic decomposition: plot of the mass loss in % from the TGA as a function of the electronegativity of X (a), of the radius of X (b), of the melting point of the salt (c), and of the enthalpy of formation of CoX2 (d).
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Fig. 6 e Reaction of AB with Co of CoX2.
the BEs found for the halides, support this assumption. In our conditions, the halides thus have a role in the ability of the metal to activate the AB dehydrocoupling. It is also important to note that the solid by-products are adsorbed on Coþ. Compared to the decomposition of pristine AB, the content of BZ that evolves over the range 100e200 C is decreased for both CoCl2 and CoBr2. However, when CoF2 and CoBr2 are used, there is no improvement. Hence, we suppose that the Coþ species favors AB dehydrocoupling while decreasing ring closing of AB trimers. To better understand the role of the halides, a few properties have been considered and the mass loss (in %) found by TGA at 100 C was plotted as a function of these different properties (Fig. 5). The main one is electronegativity (Fig. 5a). As can be seen here, the plot has a volcano shape, whose top is given by CoCl2. This suggests that the optimum electronegativity is in the range 1e1.5. The second property regarded is the halide ionic radius (Fig. 5b). Note that whatever the radius considered (i.e. atomic, covalent or ionic), the same trend was observed. The as-obtained plot also shows a volcano shape and peaks for the radius of Cl. The third property is the salt melting point (Fig. 5c), which is characteristic of the thermal stability of the salt. A volcano shape is also observed with an optimum value around 700 C. The fourth and final property is the enthalpy of formation of CoX2 (Fig. 5d). This has been considered even if the value for CoI2 was not found. The plot confirms the same trends clearly visible on the three others. From the results and analyses reported heretofore, we believe that the ability of CoX2 in reducing the induction
period is driven by both electronic and steric effects. The electronic effects are controlled by the electronegativity of X and the subsequent strength of the CoX bond. This rationalizes the inefficiency of CoF2 compared to CoBr2 and CoCl2. With respect to the steric effects, a larger size, for example with iodine, is detrimental to the diffusion of the reducing agents towards Co2þ. Therefore, the result is that the reduction of Co2þ to Coþ is kinetically less favored and this makes the CoX2 less efficient. CoCl2 therefore presents well-balanced electronic and steric properties. To sum up, Fig. 6 shows a more accurate explanation of the mechanism for the AB dehydrocoupling in the presence of CoX2. Though Co2þ from CoF2 is hardly reduced because of the electronic effects when the sample CoF2-AB is heated, the fluorides react with the B of AB and are able to substitute a few hydrides as shown by the evolution of FBZ at high temperatures. This has not been observed for the other halides but remains possible. With regard to application prospects, gaseous side-products with halide elements may be detrimental and therefore should be avoided.
5.
Conclusion
Metal salts are able to reduce the induction period of AB decomposition. In the present work, cobalt halides CoX2, where X is F, Cl, Br or I, were used to assess their efficiency in this reaction and to better understand the roles of both Co and X. It was observed that the best results are obtained with
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CoCl2, followed by CoBr2 and CoI2. However, CoF2 is inactive and does not contribute to improve the decomposition features of AB. The differences are explained in terms of electronic and steric effects where typically, the higher the electronegativity of the halide X the less reactive the CoX2, and the larger the radius of X the less reactive the CoX2. Therefore, CoCl2 presents the best compromise in terms of electronic and steric effects. In fact, to be efficient in reducing the induction period of AB decomposition, Co2þ has to be reduced to Coþ. The role of Coþ is then to activate the AB dehydrogenation by creating a germ, accelerating the AB polymerization and stabilizing the as-formed polymers, which reduces the BZ emissions. An updated mechanism has consequently been proposed.
Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ijhydene.2011.07.040.
[11]
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