Article pubs.acs.org/JPCC
Two-Peak Mystery of LiNH2−NaH Dehydrogenation Is Solved? A Study of the Analogous Sodium Amide/Lithium Hydride System Ankur Jain,*,† Hiroki Miyaoka,† and Takayuki Ichikawa*,†,‡ †
Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan
‡
S Supporting Information *
ABSTRACT: Alkali metal amide and imide systems especially NaNH2 and Li2NH have recently been proposed as a catalyst for NH3 cracking. This finding ignites the interest of researchers worldwide to study amide−imide/amide−hydride systems. Although these systems have shown catalytic properties toward NH3 cracking and have also been proposed as a hydrogen storage material, the decomposition mechanism of sodium amide is still unclear due to the complexity and low melting point of NaNH2. Herein, pure NaNH2 and its composite with lithium hydride as well as sodium hydride in a molar ratio 1:1 has been investigated, and a detailed mechanism associated with their decomposition has been suggested. The sodium amide−lithium hydride system is found to be analogous to lithium amide−sodium hydride. It desorbs 4 wt % H2 through cation exchange in a temperature range of 25−200 °C followed by NH3 mediated reaction in the temperature range 200−400 °C. The desorbed product can be rehydrogenated at around 200 °C under 2.0 MPa. An important intermediate, a double cation amide, i.e., Li3Na(NH2)4, is formed during the decomposition process. The mechanism for the formation of this double cation amide and successive H2 desorption is proposed herein. The two-peak mystery in the analogous LiNH2−NaH system remained unknown for almost a decade and is cracked herein during this study. We successfully propose our model based on NH3 mediated reaction for the decomposition of sodium amide−alkali hydride composite system.
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INTRODUCTION Metal−N−H system was proposed as an efficient hydrogen storage media by Chen et al. in 2002.1 After the first representative of this family, lithium amide−hydride system was to be known to store up to 10.4 wt % of hydrogen via following conversion reactions:
reactivity, low melting point, and a high enthalpy. Recent reports15−17 on the utilization of Na/NaNH2 system as a catalyst to crack NH3 to produce H2 efficiently, accelerate the studies toward sodium amide based systems. The decomposition of NaNH2 was reported by Titherley in 1894 as follows:9
Li3N + 2H 2 → Li 2NH + LiH + H 2 ↔ LiNH 2 + 2LiH (1)
The conversion reaction of amide−hydride to imide−hydride occurred in favorably moderate P−T window with reasonable high H2 capacity of 6.5 wt %. Since this invention of the above system, many researches have been undertaken by some experts in hydrogen community, and it is very well established that alkali amides and metal hydrides can react with each other and release H2 at much lower temperature than the individual decomposition temperature of amide and hydride. Two contemporary mechanisms based on ion migration2 and NH3 mediated reaction3 were proposed for this improvement; since then, several analogous double cation amide systems were studied, and new compounds with enhanced properties were developed. These were explored by varying cation combinations of amide and hydride, e.g., Li−Mg−N−H,4−6 Li−Ca−N− H,7 Mg−Ca−N−H,8 etc. Whereas LiNH2 and Mg(NH2)2 based amide systems attract the attention of H2 community, the efforts toward sodium amide (NaNH2) based system can be counted on fingers9−14 due to its modest H2 content, high © XXXX American Chemical Society
NaNH 2 → NaH + 0.5N2 + 0.5H 2
(2)
NaNH 2 → Na + 0.5N2 + H 2
(3)
Excess pressure than the dissociation pressure of NaH allows the first reaction to proceed whereas lower pressure allows the decomposition of NaH.10 In both cases, ammonia is also produced in addition to N2 and H2. Except these preliminary reports, only some scattered reports on NaNH2−MgH2,11 NaNH2−CaH2,12 NaNH2−LiAlH4,13 etc. are available in the literature without detailed investigation. To the best of our knowledge, no reports on the effect of alkali hydrides on NaNH2 does exist in the literature. Thus, as a first attempt, we choose the NaNH2−LiH system to be investigated. In this work, we prepared the NaNH2−LiH compound using ball milling and studied its decomposition properties. During our studies, a double cation amide Received: October 21, 2016 Revised: November 23, 2016 Published: November 23, 2016 A
DOI: 10.1021/acs.jpcc.6b10611 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Li3Na(NH2)4 has been found as an intermediate compound that has to be quite a different point from the other amide− imide systems. While exploring the literature, we found two approaches for the synthesis of this mixed cation amide phase. Jacobs and Harbrecht, in 1982,18 reported the formation of Li3Na(NH2)4 and LiNa2(NH2)3 mixed phases by direct reaction of ammonia with a mixture of Li and Na metals. The other approach proposes solid state synthesis using LiNH2 and NaNH2, which actually eliminated the direct use of supercritical ammonia.19 This method also produces a mixture of Li3Na(NH2)4 and LiNa2(NH2)3 phases. Although these two reports proposed two double cation amides having high H2 content of 7.5 and 6.0 wt %, respectively, these were limited to synthesis and structural investigations of these compounds. There are no reports in the literature on the reaction mechanism of these materials. Thus, we prepared the Li3 Na(NH2 )4 compound via a different method using mechanical milling of starting materials LiNH2 and NaNH2 in a 3:1 ratio. Then it was studied for its hydrogenation characteristics.
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Figure 1. TDMS profile for NaNH2 decomposition. Scan rate 5 °C min−1.
an adsorbed state. We have also tried milling of NaNH2 under an Ar atmosphere for 2 h to remove the adsorbed N2, and then measured the TDMS of the milled sample (Figure S1, Supporting Information), but the N2 signal still exists, which confirms the bonding state of nitrogen but not the adsorbed state. So some nitrogen deficient (NaN1−δH2) structure could be formed after nitrogen evolution at 160 °C, although it has a structure very similar to the parent NaNH2 structure. Further heating to 200 °C resulted in the phase transition and subsequent melting of sodium amide, as indicated by two consecutive endothermic peaks at 160 and 193 °C, respectively. Although we have tried to characterize the sample after the phase transition event (Figures S2a and S2b, Supporting Information), the structure is found to be similar to that of the starting material (NaNH2), which must be due to the reversible nature of phase transition; this reversibility has been confirmed by DSC measurement performed up to 170 °C (Figure S3, Supporting Information). Immediately after melting, NH3 evolution is started at 225 °C, which continues until the end of studied temperature, i.e., 400 °C. In addition to NH3, N2 is also evolved out at a higher temperature with MS peak position at 340 °C. This could be generated due to a side reaction at this temperature. However, it does not have any significant effect on decomposition mechanism as the amount of this N2 is negligibly small as observed from the almost similar peak areas of both N 2 evolution peaks and the negligible contribution toward weight loss according to the TG experiment (Figure S1, Supporting Information). The decomposition process has been tracked using XRD and IR after different thermal/gas evolving events. The decreasing intensity of IR peaks corresponding to NaNH2 stretching modes concluded that NaNH2 gradually decomposes into Na metal and release NH3 with the traces of N2.
EXPERIMENTAL SECTION
The starting material NaNH2 (≥95%, Aldrich) and LiH (≥99.4%, Aldrich) were milled in a 1:1 molar ratio using a Fritsch P7 ball milling apparatus. Twenty Cr steel balls (SUJ-2, 7 mm diameter) were used for a 500 mg sample. The milling was performed under 0.1 MPa at 400 rpm for 2 h with 1 h milling and 30 min rest pattern. The double cation amide phase Li3Na(NH2)4 was prepared by using mechanical milling with the starting material NaNH2 (≥95%, Aldrich) and LiNH2 (≥95%, Aldrich) in a 1:3 molar ratio. The milling conditions were kept the same as that of the NaNH2/LiH system. Due to the high sensitivity of materials toward oxygen and moisture, all the handling of samples was done in a high purity Ar (99.9999%) filled glovebox (Miwa MFG, MP-P60W). The oxygen and moisture contents were maintained at
