ARTICLE pubs.acs.org/JPCC
Hydrogen Desorption from the NaNH2MgH2 System Drew A. Sheppard,* Mark Paskevicius, and Craig E. Buckley Department of Imaging and Applied Physics, Curtin University of Technology, GPO Box U 1987, Perth 6845, WA, Australia
bS Supporting Information ABSTRACT: Ball-milled sodium amide and magnesium hydride (NaNH2: MgH2 = 1:1 molar ratio) desorbs 3.3 wt % of hydrogen between 70 and 335 °C with three desorption events. X-ray diffraction indicates that the hydrogen desorption is associated with two unidentified magnesium-containing phases. Fourier transform infrared spectroscopy indicates that these two phases correspond to an imide and nitride, respectively. Analysis of the desorption products shows a large excess of NaH and MgH2 and that optimizing the starting reagents will increase both the kinetics and the amount of hydrogen desorbed from the system below 165 °C.
’ INTRODUCTION Lithium and magnesium amide have been extensively studied as hydrogen storage media due to their high hydrogen contents. Interest has focused on combining these amides with various hydrides (primarily LiH and MgH2),18 alanates,9,10 and borohydrides1113 to derive thermodynamically destabilized reactions. The use of lithium and magnesium amide in this role has led to the synthesis of previously unknown compounds, such as Li2Mg(NH)214 and Li3BN2H8.11 In addition, extensive summaries of hydrogen storage in metal amides have recently been published.15,16 In contrast, sodium amide (NaNH2, 5.1 wt % H2 content) has received little attention as a possible component in a hydrogen storage system. This may be due to the fact that, unlike lithium and magnesium, sodium imide is not known to exist, and this limits possible destabilization reaction pathways. Despite this, there are a few scattered studies of NaNH2-containing systems such as NaNH2NaBH4,12,17 NaNH2NaAlH4,9 and NaNH2LiAlH4.18,19 Prior to now, only two other publications have examined the NaMgNH system.20,21 Xiong et al.20 examined Mg(NH2)2 and NaH ball milled in the molar ratios of 1:1, 1:1.5, and 1:2, respectively. Temperature-programmed desorption with mass spectrometry (TPD-MS) performed on these samples revealed significant hydrogen evolution between 150 and 200 °C with N2 and NH3 evolution between 200 and 300 °C. Increasing the ratio of NaH in the system suppressed N2 and NH3 release but also reduced the hydrogen capacity. Upon desorption, an unknown phase (as well as a small amount of NaNH2) was identified that could not be attributed to any known MgNH, NaNH, nor NaMgNH compound. Dolotko et al.21 examined MgH2 and NaNH2 ball milled in a ratio of 3:2. TPD-MS up to 395 °C yielded 5 wt % hydrogen. Desorption began at ∼130 °C and was comprised of two distinct desorption processes. X-ray diffraction (XRD) revealed a number of competing solid state reactions that involved the formation and decomposition of Mg(NH2)2 and r 2011 American Chemical Society
NaMgH3 as intermediary phases with the overall decomposition reaction being: 2NaNH2 þ 3MgH2 f Mg3 N2 þ 2NaH þ 4H2
ð1Þ
Partial rehydrogenation was achieved at 395 °C under a pressure of 150 bar resulting in the formation of MgNH, believed to be mediated by the presence of NaH. Careful examination of the XRD pattern for the sample desorbed at 250 °C shows trace amounts of the same unknown phase first identified by Xiong et al.20 Our original interest in NaNH2 stems from the work of Lu et al. on the LiNH2/MgH2 system.22 When ball milled in a 1:1 ratio, the system was shown to undergo a dehydrogenation process according to (8.2 wt % hydrogen): LiNH2 þ MgH2 f LiMgN þ 2H2 f LiH 1 1 þ MgH2 þ MgðNH2 Þ2 r f LiMgN þ 2H2 ð2Þ 2 2 Although this system has been since shown to be more complex than first thought, forming Li2Mg(NH)2 as an intermediate phase,14 an equivalent system that substituted Na for Li would yield 6.2 wt % of hydrogen for full hydrogen desorption. The use of Na has the benefit of lower cost and higher abundance as compared to Li. Additionally, the identification and characterization of the unknown phase observed in the work of Xiong et al.20 and Dolotko et al.21 may lead to new destabilization pathways for other amide/imide/hydride systems.
’ EXPERIMENTAL SECTION All handling of chemicals and sealable milling canisters was undertaken in an argon-atmosphere glovebox to minimize oxygen (O2 < 5 ppm) and water (H2O < 5 ppm) contamination. Received: January 9, 2011 Revised: March 14, 2011 Published: March 31, 2011 8407
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The Journal of Physical Chemistry C NaNH2 (Sigma-Aldrich, g95%) and MgH2 (Sigma-Aldrich, g96.5%) were used as starting reagents. The reagents (5.00 g total) were ball milled for 3 h in a 1:1 molar ratio (NaNH2:MgH2) with a custom-made 316 stainless steel ball-milling canister (650 cm3 internal volume) attached to a Glen Mills Turbula T2C shaker mixer. The shaker mixer was operated at 160 rpm using a 70:1 ball to powder ratio (BTP) with 23 7.9 mm 316 stainless steel balls and 24 12.7 mm 316 stainless steel balls. The milling time was chosen to maximize the mixing of the reagents while limiting the degree of metathesis between NaNH2 and MgH2. An additional sample was ball milled in a 1:1 molar ratio (NaNH2:MgH2) for 24 h with 4 wt % TiCl3 to examine the effects of increased milling time and the addition of a catalyst on the decomposition reaction in the system. In this case, however, the BTP was maintained at 35:1 to minimize contamination from the milling media. XRD was performed using a Bruker D8 Advance diffractometer (Cu KR radiation) with a 2θ range of 1070°. The XRD instrument was equipped with a LynxEye 3° linear position sensitive detector (PSD) with 192 pixels. The instrumental line broadening is taken into account using a fundamental parameters approach in Topas (Bruker AXS) that was verified using a NIST 660a LaB6 reference standard. The samples were loaded into an XRD sample holder and sealed with a poly(methylmethacrylate) (PMMA) airtight bubble to prevent oxygen/moisture contamination during data collection. Fourier transform infrared spectroscopy (FTIR) was performed on a Perkin-Elmer Spectrum 100 FTIR Spectrometer using a resolution of 4 cm1. Measurements were performed by combining the samples with KBr and pressing them into thin discs within an argon glovebox. TPD-MS was performed on a PCT-Pro E&E (Hy-Energy) coupled to a quadrupole mass spectrometer residual gas analyzer (Stanford Research Systems RGA 300). For each measurement, approximately 30 mg of sample was outgassed at 3 107 bar and 25 °C overnight. While still under vacuum, the samples were heated up to 370 °C at a heating rate of 1 °C/min at which point the heating was halted, and the sample was allowed to cool to room temperature. The TPD-MS measurements were repeated but were, instead, halted at 165, 267, 335, and 370 °C, respectively, for which the samples shall be referred to as BM-165, BM-267, BM-335, and BM-370, respectively. FTIR and XRD were also performed on these samples to examine the progression of phase transformations during decomposition. Hydrogen desorption measurements were performed on a PCT-Pro E&E using 0.2 g of sample and a total system volume of 188.3 cm3. Hydrogen pressure measurements were recorded using an MKS model 628D pressure transducer with an accuracy of 0.12% in the pressure range of 05 bar. Hazards: It is noted here that the starting reagents NaNH2 and MgH2 are combustible on contact with air or water. However, the dehydrogenation products of these reagents react violently with air and water.
’ RESULTS AND DISCUSSION TPD-MS. A sample was prepared by ball milling a 1:1 molar ratio of NaNH2 and MgH2 for 3 h (referred to as BM-3h henceforth). XRD of BM-3h (Figure S1 in the Supporting Information) shows that the starting reagents are largely intact after 3 h of ball milling. However, a small amount of NaH is detectable as well as a small amount of NaOH, an impurity phase in the as-supplied
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Figure 1. Partial pressure of hydrogen, ammonia, and nitrogen evolved from BM-3 h (NaNH2:MgH2 = 1:1) during TPD-MS. The vertical lines A, B, C, and D correspond to samples BM-165, BM-267, BM-335, and BM-370, respectively, where subsequent TPD-MS measurements were halted.
NaNH2 reagent. The presence of NaH after 3 h of milling suggests that the metathesis reaction involving the exchange of NH2 and H between Na and Mg has begun. This is in agreement with previous studies on high energy milling of LiNH2 and MgH2.23 Dolotko et al.21 have also shown that a mixture of NaNH2 and MgH2 undergoes metathesis at 147 °C with negligible hydrogen release. Completion of the metathesis reaction is described by the following: 1 1 NaNH2 þ MgH2 f NaH þ MgH2 þ MgðNH2 Þ2 2 2
ð3Þ
Despite the presence of NaH in BM-3h, the expected Mg(NH2)2 product is absent from this XRD pattern as a consequence of high-energy milling-induced amorphization.5,24 To examine the dehydrogenation properties of BM-3h, TPD-MS was performed between room temperature and 370 °C. The partial pressures of hydrogen, ammonia, and nitrogen evolved from the sample, as a function of temperature, are shown in Figure 1. From Figure 1, it is evident that there are three significant hydrogen desorption events that appear between room temperature and 330 °C. Small quantities of hydrogen are evolved almost immediately after heating begins, and hydrogen evolution increases with temperature with a small hydrogen peak at 130 °C and a larger hydrogen evolution peak centered at 140 °C. Concurrent to this is a small but detectable evolution of ammonia also centered at 140 °C. At this temperature, ammonia comprises 1.6 mol % of the evolved gas with the remainder being hydrogen. Other than this peak, the ammonia content stays below 0.5 mol % up until 310 °C. There are two additional hydrogen evolution peaks at 191 and 230 °C, and hydrogen evolution begins to increase again above 280 °C where both hydrogen and ammonia show peaks at 315 °C. At this point, ammonia comprises 3.7 mol % of the gas stream. The peak ammonia release occurs at the same temperature as the peak hydrogen release, which may be an inherent property of the system. However, under the TPD-MS experimental conditions used here (i.e., vacuum conditions rather than flowing argon), it cannot be excluded that evolved H2 is acting as a carrier gas for high vapor pressure NH3. Beyond 335 °C, N2 (previously a negligible component of the desorbed gas stream), NH3, and H2 production begins to increase with N2 8408
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Figure 2. XRD patterns of (a) sample BM-165, (b) sample BM-267, (c) sample BM-335, and (d) sample BM-370. UP-A and UP-B refer to unknown phases that do not match any known crystallographic phase.
and NH3 combining to form 10 mol % of the gases evolved when the TPD-MS run ceased at 370 °C. It is unclear at this juncture whether or not the N2 is evolved directly from the sample or from the decomposition of NH3 at high temperature. The TPD-MS curve for BM-3h is distinctly different in shape to those previously reported for the NaMgNH system by Xiong et al.20 and Dolotko et al.21 and may indicate a different decomposition pathway. NaNH2 has been reported to be stable above its melting point, 210 °C,25 and decomposes between 400 and 500 °C with the release of N2 and H2.26 However, Chater et al.12 found that pure NaNH2 began to decompose at ∼275 °C, with the decomposition rate peaking at 350 °C, with the release of NH3 and negligible amounts of H2 and N2. The fact that predominantly hydrogen evolution occurs below 300 °C (as shown in Figure 1) for BM-3h indicates that the combination of NaNH2 and MgH2 in a 1:1 molar ratio results in a destabilized decomposition reaction. Phase Identification. To examine the evolution of phases during the desorption process, the TPD-MS measurements were repeated but were instead halted at 165 (Figure 1, line A, sample BM-165), 267 (Figure 1, line B, sample BM-267), 335 (Figure 1, line C, sample BM-335), and 370 °C (Figure 1, line D, sample BM-370), respectively. Their respective XRD patterns are shown in Figure 2. The XRD pattern of BM-165 (Figure 2a) reveals the sample primarily consists of MgH2, NaH, and a small amount of NaMgH3. A number of broad diffraction peaks could not be readily identified. To improve their crystallinity and prevent decomposition, BM-165 was placed under a H2 pressure of 200 bar and heated to 300 °C. Subsequent XRD (Figure S2 in the Supporting Information) revealed that a number of these peaks could be identified as Mg(NH2)2.27 Additionally, unidentified broad diffraction peaks originally at 2θ values of 28.9, 30.5, 35.0, 38.3, 39.7, 41.7, and 51.0°, respectively, have vanished after the heat treatment, while peaks for MgNH27 emerged. The lack of discernible hydrogen evolution during this annealing process
suggests that the phase associated with the unidentified peaks converted to MgNH without a detectable hydrogen release. The XRD pattern of BM-267 (Figure 2b) reveals that MgH2 and Mg(NH2)2 have been consumed and are not present in the sample, while the peaks associated with NaH and NaMgH3 have increased in intensity. Additionally, the peaks associated with the unidentified phase in BM-165 are now clearly discernible but with an obvious peak shift (Δ2θ ∼ 0.5° at 2θ = 35.5°). This unknown phase (referred to hence forth as UP-A) can be positively identified as the unknown phase first identified by Xiong et al.20 The XRD pattern of BM-335 (Figure 2c) shows an absence of NaMgH3, the formation of some Mg3N2, and a sharpening of the diffraction peaks associated with UP-A. Annealing UP-A to improve its crystallinity and gain further insight into its structure proved challenging. Increasing the annealing temperature under vacuum has the effect of decomposing both UP-A and NaH. To circumvent this, BM-3h was first evacuated for 6 days at 200 °C before being placed under 0.3 bar of H2 pressure. The temperature was then slowly raised to 300 °C and held at this temperature for a further 3 days. From the resulting XRD pattern (Supporting Information, Figure S3) of the product (hence forth termed BM-annealed), UP-A could be indexed as trigonal with possible space group P3c1 and lattice constants a = 6.11 Å and c = 17.90 Å. The XRD pattern of BM-370 (Figure 3d) shows the presence of Na metal, Mg3N2, and a second new unidentified phase (referred to as UP-B henceforth). The Na metal arises due to the high temperature and high vacuum conditions of the RGA measurement. These conditions result in the decomposition of NaH, and the low vapor pressure of Na results in some of the metal vaporizing and depositing on the inside walls of the sample container. To improve the crystallinity of UP-B, a portion of BM3h was evacuated at 200 °C for 18 h followed by evacuation at 280 °C for a further 18 h. From the resulting XRD pattern (Figure S4 in the Supporting Information), UP-B was determined to be monoclinic with possible space group C2 and lattice 8409
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constants a = 13.92 Å, b = 3.58 Å, c = 12.39 Å, and β = 115.8°. A summary of the phases identified via XRD during decomposition is presented in Table 1. A summary of the crystallographic information determined for UP-A and UP-B as well as MgNH and Mg3N2 (for comparison) is included in Table 2. To gain a further insight into the nature of the two unknown phases observed (UP-A and UP-B), FTIR was performed. FTIR
can be used to follow the conversion between amide, imide, and nitride phases by examining the change in the vibrational modes associated with the conversion from NH21 (amide) to NH2 (imide) and, finally, N3 (nitride). Figure 3 shows the FTIR patterns of MgH2, NaH, and NaNH2 as reference materials, in conjunction with patterns for sample BM-3h, sample BM-165, sample BM-267, sample BM-335, and sample BM-370. The FTIR patterns covering an extended wavenumber data range for each of the samples are available in the Supporting Information, Figure S5. After 3 h of milling, BM-3h shows that NaNH2 is largely intact (Figure 3c,d), while absorption peaks due to Mg(NH2)2 (3332, 3326, and 3277 cm1)28 are not observed. It is expected that minor Mg(NH2)2 absorption peaks may be obscured given the strong intensity of the NaNH2 absorption peaks after the lack of any major metathesis reaction. After desorption up to 165 °C, BM-165 (Figure 3e) shows broad absorption features between 3150 and 3300 cm1. These features are comprised of a number of poorly resolved peaks, but their position is consistent with those of the alkali and alkaline-earth amides.28,29 Broad absorption peaks seen in FTIR, like those here, have previously been attributed to the small crystallite size of the phase in question.29,30 After desorption up to 267 °C, sample BM-267 (Figure 3f) shows that the broad absorption features observed in BM-165 have almost completely disappeared and that a broad absorption peak centered at 3171 cm1 has formed. The presence of a single peak in the region between 3100 and 3300 cm1 is typical of an imide such as Li2NH29 and the alkaline-earth metal imides.28 NaNH2 has not been previously shown to form an imide, and the position of the absorption trough excludes the formation of pure magnesium imide (MgNH) (3251 and 3199 cm1).28 These FTIR data in conjunction with the XRD information for BM-267 suggest that UP-A is an imide that is either a previously unknown MgNH structure or an imide comprised of both Na and Mg. A further discussion of this is presented below. Further heating to 335 °C (sample BM-335, Figure 3g) shows a sharpening of the peak at 3171 cm1 akin to the sharpening of the diffraction peaks of UP-A in the XRD pattern. There are some additional changes in troughs in the range of 15001600 cm1 (Figure S5 in the Supporting Information), but the similarity of
Figure 3. FTIR spectra for (a) pure MgH2, (b) pure NaH, (c) pure NaNH2, (d) BM-3h, (e) sample BM-165, (f) sample BM-267, (g) sample BM-335, and (h) sample BM-370.
Table 1. Decomposition Products Determined with XRD as a Function of Temperature decomposition products determined from XRD sample name
temperature range (°C)
BM-3 h
23
BM-165
50165
BM-267 BM-335
165267 267335
BM-370
335370
NaNH2
Mg(NH2)2
MgH2
yes
yes
yes
NaMgH3
NaH
yes
yes
yes
yes
yes yes
yes yes
yes
UP-A
UP-B
Mg3N2
Na
yes
yes yes
yes
yes
Table 2. Crystallographic Information Derived for the Unknown Phases UP-A and UP-Ba lattice parameters phase
a
crystal type
a (Å)
b (Å)
R (o)
c (Å)
β (o)
γ (o)
unit cell volume (Å3)
UP-A
trigonal
6.11
6.11
17.90
90
90
120
579
UP-B
monoclinic
13.92
3.58
12.39
90
115.8
90
556
MgNH Mg3N2
hexagonal cubic
11.58 9.67
11.58 9.67
3.68 9.67
90 90
90 90
120 90
427 990
Crystallographic information on MgNH and Mg3N2 is included for comparison purposes. 8410
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Figure 4. TPD-MS of NaNH2 þ MgH2 ball-milled for 24 h with 4 wt % TiCl3 (BM-24h-TiCl3).
magnesium amide, magnesium imide, and sodium hydride in this spectral region makes definitive phase identification difficult. Upon further heating to 370 °C (sample BM-370, Figure 3h), all discernible features present in the FTIR pattern disappear. This is consistent with conversion of imide phases to their respective nitrides.22 In this case though, we know from XRD that our sample consists of Na metal, Mg3N2, and UP-B. Because of the large proportion of Mg3N2 in the sample, UP-B cannot be positively identified as a nitride. However, the FTIR spectrum can exclude the possibility of UP-B from being an amide or imide phase; hence, a nitride phase seems likely. Kinetic Enhancement of Hydrogen Desorption. Initial efforts to measure the hydrogen desorption properties of BM3h at 200 °C showed that the sample did not reach hydrogen equilibrium even after 100 h of desorption. To improve the kinetics of the system, a new sample was synthesized using 4 wt % TiCl3 as a catalyst, in addition to increasing the ball-milling time to 24 h (sample BM-24h-TiCl3). The TPD-MS curve of sample BM-24hTiCl3 (Figure 4) shows that the desorption peaks shift to lower temperatures. The small desorption peaks initially at 130 and 140 °C for BM-3h have merged and shifted to a single peak at 123 °C for BM-24h-TiCl3. The peaks previously at 191 and 230 °C, respectively, have merged into a single peak centered at 182 °C, while the peaks at 315 and 334 °C have shifted to 296 and 304 °C, respectively. The decrease in temperature for each desorption peak (of between 7 and 30 °C) indicates that the addition of TiCl3 combined with the increased ball-milling time results in a modest decrease in the activation energy of hydrogen desorption. This should translate to improved hydrogen desorption kinetics. Although a kinetic enhancement was observed in BM-24h-TiCl3, XRD (not shown) revealed that an appreciable amount of NaCl was formed during ball milling due to reduction of TiCl3. The generation of NaCl (and reduction of the quantity of Na to react with other phases) causes difficulties in analyzing the complex reaction progression as a function of temperature. Hydrogen Desorption. Figure 5 shows the isothermal kinetic desorption data at 200 °C of the original BM-3h (Figure 5a) as compared to BM-24h-TiCl3 (Figure 5b). As suggested by TPDMS, BM-24h-TiCl3 shows improved desorption kinetics over BM-3h. However, BM-24h-TiCl3 still does not reach equilibrium after ∼100 h of desorption, indicating that there are severe kinetic limitations in this system. Despite this, we can determine
Figure 5. Kinetic hydrogen desorption curves at 200 °C for successive steps of (a) BM-3h BM and (b) BM-24h-TiCl3. Lines joining the data points are included as a guide to the eye.
that the hydrogen desorption equilibrium pressure is greater than 1.15 bar at 200 °C (as compared to 0.28 bar observed by Xiong et al.20 for their NaMgNH system). The desorption of BM24h-TiCl3 reaches a much higher pressure on its first desorption step (1.95 bar), but because of the Na-poor nature of the sample (Na f NaCl), it is unclear if alternative reaction pathways involving Ti lead to this increased desorption pressure. Potassium hydride (KH) has recently been used to dramatically improve the hydrogen sorption kinetics of the Mg(NH2)2/ LiH31 system and may also prove advantageous here. Given the poor desorption kinetics, even for the catalyzed sample, it was not viable to collect a pressurecomposition temperature (PCT) curve. Instead, the quantity of hydrogen evolved between 24 and 350 °C (Figure 6) was assessed by measuring the pressure of hydrogen evolved into a large volume during TPD-MS. Temperatures above 350 °C were not examined due to the coproduction of NH3 and N2. Measurable hydrogen evolution begins at 70 °C and reaches 0.5 wt % at 165 °C (sample BM-165). Further heating to 267 °C (sample BM-267) yields a further 2.0 wt % of hydrogen. Lastly, heating between 267 and 335 °C evolves 0.8 wt % hydrogen. The total hydrogen desorption equates to 3.3 wt %, which is slightly more than half of the total hydrogen in the sample. The origin of the hydrogen evolution between 24 and 165 °C can be associated with the formation of small amounts of the imide-like UP-A phase. Similarly, the hydrogen evolution between 165 and 267 °C is associated with the complete decomposition of Mg(NH2)2 to the imidelike UP-A phase. However, the large quantity of NaH and NaMgH3 remaining after desorption to 267 °C indicates that they are in excess of that required for a pure UP-A/UP-B system. It is apparent that the formation of UP-A in this system is kinetically limited from the isothermal hydrogen desorption results in Figure 5. Similar kinetically limited behavior is also evident in the TPD-MS pattern (in Figure 1) that displays two major hydrogen desorption events up to 267 °C during the formation of only one decomposition product (UP-A). It is likely that the reaction to form UP-A is limited by the diffusion of reactants, thus requiring high temperatures for complete UP-A production over the course of two 8411
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Additionally, lithium is known to form both a stoichiometric amide phase with sodium—LiNa2(NH2)3—and a lithium-rich solid solution sodium amide phase—Li4xNax(NH2)4 for 0 e x e 1.33 There is some evidence that suggests that the NaMg imide is in fact a solid solution. UP-A is present in XRD patterns for both 165 and 267 °C in Figure 2a,b, respectively. However, there is a significant 0.5° 2θ shift in the peak positions of UP-A between these two XRD patterns. This significant shift could be due to compositional changes in the NaMg imide structure (UP-A) due to Na or Mg migration to or from the phase as a function of reaction and/or temperature. If the unknown phases herein are indeed solid solutions, then the synthesis, isolation, and characterization of a NaMgN solid solution series would present the opportunity to tailor the thermodynamics of hydrogen desorption by alterations to the composition of these phases and is part of ongoing research.
Figure 6. Excess wt % of hydrogen evolved from BM-3h between 24 and 350 °C. The lines marked A, B, and C correspond to samples BM165, BM-267, and BM-335.
major hydrogen desorption events. The diffusion limitation could be due to the presence of excess NaH and MgH2 that do not release hydrogen in this temperature range. These poor desorption kinetics suggest that an optimization of the starting reagents by altering the NaMgNH ratios could increase the hydrogen capacity, reduce the desorption temperature at which the hydrogen is released, and improve the hydrogen desorption kinetics. Lastly, the hydrogen desorption between 267 and 335 °C can primarily be assigned to the decomposition of NaMgH3 and the formation of some Mg3N2. It is evident that sodium is required in the system for the unknown phases (UP-A and UP-B) to form, as without it the decomposition reaction would form magnesium analogues [Mg(NH2)2, MgNH, and Mg3N2].5 However, the large quantity of NaH and MgH2 remaining after desorption to 267 °C is also an indication that the NaMgNH ratio is not optimized for a pure UP-A/UP-B system. The same decomposition pathway could be designed by altering the composition of the starting reagent while limiting the excess NaH and MgH2 in the system that degrade the gravimetric hydrogen content. Significance. Given that BM-267 and BM-335 contain an unknown, magnesium-containing imide (UP-A) and BM-370 likely contains a magnesium-containing nitride (UP-B), the possibility arises that the unknown phases are comprised of an NaMg imide and nitride, respectively. Determination of the Na content of the unknown phases would verify this possibility. Thus far, attempts to quantify the Na content using XRD by mixing the samples with a known amount of an internal standard have not been successful, as the action of mixing the internal standard further degraded the already poor crystallinity of the unknown phases. Because XRD suggests that NaH is in excess in the desorbed products, an alternative route to quantify the Na content of the unknown phases may be possible through the synthesis of single phases of UP-A and UP-B. This may reveal whether the unknown phases are NaMg solid solutions of the respective imide or nitride. The alternative is that these unknown phases are fixed ratio (Na:Mg) stoichiometric compounds. Li and Mg are known to form nonstoichiometric amides— LixMg2x(NH2)2þx for 0 < x < 126—a stoichiometric imide— Li2Mg(NH)214—and nonstoichiometric nitrides—(LixMg1x)3N2x, 0 < x < 0.2 and (LixMg1x)2N(2x)2/3, 0.5 < x < 0.6.32
’ CONCLUSIONS We have identified two new magnesium-containing phases that form during hydrogen desorption from a mixture of NaNH2 and MgH2 (1:1 molar ratio). XRD and FTIR suggest that these two phases are comprised of a magnesium-containing imide and nitride, respectively, but that they are structurally different to MgNH and Mg3N2. Further work needs to be done to isolate and determine the exact compositions of these unknown phases. The sample is capable of desorbing 3.3 wt % of hydrogen between 24 and 335 °C. During desorption, a large amount of NaH is formed that does not release hydrogen until the temperature exceeds 335 °C. This NaH is in excess and limits the wt % of hydrogen desorbed as a function of the mass of the sample. However, optimization of the starting reagent ratios should circumvent this issue. Hydrogen desorption shows severe kinetic limitations, even after the addition TiCl3 as a catalyst. ’ ASSOCIATED CONTENT
bS
Supporting Information. Additional XRD diffraction patterns and FTIR patterns. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge the facilities, scientific, and technical assistance with FTIR measurements of Peter Chapman, Department of Chemistry, Curtin University of Technology, Perth, Western Australia. C.E.B. acknowledges the financial support of the Australian Research Council for ARC Discovery Grant DP0877155 and ARC LIEF Grants LE0775551 and LE0989180, which enabled the XRD studies and temperature program desorption studies to be undertaken. ’ REFERENCES (1) Ichikawa, T.; Isobe, S.; Hanada, N.; Fujii, H. J. Alloys Compd. 2004, 365, 271–276. (2) Luo, W. J. Alloys Compd. 2004, 381, 284–287. (3) Nakamori, Y.; Kitahara, G.; Orimo, S. J. Power Sources 2004, 138, 309–312. 8412
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dx.doi.org/10.1021/jp200242w |J. Phys. Chem. C 2011, 115, 8407–8413
