Hydrogenation Reaction Pathways in the Systems Li3N–H2, Li3N–Mg

Article pubs.acs.org/JPCC

Hydrogenation Reaction Pathways in the Systems Li3N−H2, Li3N− Mg−H2, and Li3N−MgH2−H2 by in Situ X‑ray Diffraction, in Situ Neutron Diffraction, and in Situ Thermal Analysis Gereon Behrendt,‡ Christian Reichert,† and Holger Kohlmann*,‡ ‡

Inorganic Chemistry, University of Leipzig, Johannisallee 29, 04103 Leipzig, Germany Department of Chemistry, Saarland University, 66123 Saarbrücken, Germany

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ABSTRACT: The phase diagram Li−Mg−N−H offers ample opportunities for potential hydrogen storage systems. Three systems based on lithium nitride, Li3N, were investigated by time-resolved in situ methods (thermal analysis, Xray and neutron diffraction) at temperatures up to 703 K and hydrogen gas pressures up to 9.4 MPa. Pure lithium nitride reacts in a one-step reaction to lithium amide according to Li3N + 2H2 → LiNH2 + 2LiH at 1.0 MPa hydrogen pressure. Equimolar mixtures of lithium nitride with magnesium hydride, both at 1.5 and 9.4 MPa hydrogen gas pressure, react in the same way up to 543 K, i.e., magnesium hydride does not participate in the reaction. At higher temperatures, lithium magnesium nitride is formed according to the endothermic reaction LiNH2 + MgH2 → LiMgN + 2H2 at moderate (≤1.5 MPa) and via the exothermic reaction Li3N + MgH2 → LiMgN + 2LiH at higher hydrogen gas pressures (5.0 MPa). Mixed imide Li2Mg(NH)2 is formed when an excess of Li3N is used in the reaction. The hydrogenation of mixtures of lithium nitride with magnesium starts with the formation Li2NH and Li4NH, followed by the mixed imide Li2Mg(NH)2 at higher temperatures and finally the formation of Mg3N2 and LiH. Deuterides react accordingly.

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INTRODUCTION Hydrogen is a promising energy carrier in a postfossil energy supply, especially for mobile applications like vehicles or portable devices. In contrast to the compressed gas (CGH2), liquid hydrogen (LH2) or adsorption techniques, chemical hydrogen storage is based on the fixation and release of hydrogen in reversible chemical reactions. Light metal hydrides like lithium hydride, LiH, or alkaline boron hydrides offer large gravimetric values of hydrogen uptake due to their low molar masses, but are normally too stable for reversible reactions because of their highly exothermic formation. However, this “thermodynamic sink” can be tackled partly by coupling with a thermodynamically favorable side reaction.1−4 Besides weight and volume efficiency, the reversibility of the hydrogenation-dehydrogenation cycle is one of the key issues for identifying suitable storage materials. Light-weight compounds, such as nitrides have attracted much interest in that regard. The only alkaline metal nitride, which is thermodynamically stable under standard conditions, is lithium nitride, Li3N. Lithium nitride, usually appearing as the hexagonal α-Li3N (space group P6/mmm, no. 191) or the high-pressure modification β-Li3N (P63/mmc, no. 194), was found to serve as a reversible hydrogen store via lithium imide and amide according to the equation

nitride requires harsh conditions (>600 K, 1 mPa); however, lithium imide allows for reversible storage of about 6.5 wt % hydrogen through the reaction to lithium amide.5,6 In situ investigations (synchrotron or neutron, up to 533 K and 0.5 MPa) elucidated the reaction pathways of hydrogenation and dehydrogenation and proved a whole range of nonstoichiometric phases to form at low to medium hydrogen pressures,7−10 following the simplified reaction equation Li3N + x H 2 → (1 − 2x)Li3N + Li3N0.5H 0.5(NH)0.5 → Li4 − 2xN1 − xH1 − x(NH)x + (2x − 1)LiH → LiNH 2 + 2LiH + (x − 2)H 2

and avoiding production of ammonia during the decomposition of LiNH2. At higher hydrogen pressures simpler mechanisms were discovered.9 The introduction of Mg into the Li2NH/LiNH2 system promised to ease hydrogen release because of the lower formation enthalpy of MgH2 (−74 kJ/mol vs LiH with −90 kJ/ mol) and led to the Li−Mg−N−H system. For example, the hydrogen desorption temperatures of lithium amide, LiNH2, were lowered by 50 K by partial cation substitution of lithium by magnesium;11 however, kinetic hindrance still make rather

Li3N + 2H 2 ⇄ Li 2NH + LiH + H 2 ⇄ LiNH 2 + 2LiH

Received: May 14, 2016 Revised: June 3, 2016

Strictly speaking, only the second step is reversible and thereby useful for storage, because the back reaction from imide to © XXXX American Chemical Society

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resolution was set to 4 min per pattern for the experiment on Li3N and to 3 min for the experiment on Li3N-MgD2. The sample was held inside a sapphire gas pressure cell and heated by a two-sided laser system (2 × 40 W).19,22 Deuterium gas pressure was controlled by an automatic gas pressure regulation system and the temperature was measured with a calibrated pyrometer (Keller HCWPL 11AF3). The actual neutron wavelength was determined by a Rietveld refinement of a measurement of a NIST640b silicon standard and found to be 186.77(2) pm for the experiment on Li3N and 186.70(3) pm for the experiment on Li3N-MgD2. Rietveld refinements were carried out with the program FullProf and pseudo-Voigt as profile functions.23,24 Debye−Waller factors were constrained to be the same for like atoms in one phase. Regions excluded from the Rietveld refinements were due to sapphire contribution from the container or because of detector failure (vide inf ra). For more details on the high-intensity diffractometer D20, see ref 25. Special care must be taken when handling devices under high pressure and/or temperature. Mechanical shielding for the case of disintegration due to mechanical failure, which can be extremely harmful, is mandatory.

high temperature necessary. Different strategies to circumvent this problem are described: nanoconfinement of Li2Mg(NH)2;12 hydrogenation of ball-milled LiMgN to LiNH2 and MgH2, requiring high H2 pressures;13 mechanically activated composites of Mg(NH2)2•4LiH•LiNH2;14 and composites of Mg(NH2)2•2LiH.15 Some details of the reaction pathways in both Li−N−H and Li−Mg−N-H systems remain unclear, especially at higher hydrogen gas pressures, making new in situ investigations desirable. In this context, new possibilities for in situ investigations of solid−gas reactions have arisen by current instrumental developments.16−19 For the characterization of hydrogenation reactions in situ thermal analysis (differential scanning calorimetry (DSC) under hydrogen gas pressure) was proven to be a very useful tool.20,21 To the best of our knowledge, this technique was not applied as yet to the Li−N− H and Li−Mg−N-H systems. For in situ neutron diffraction the development of a single crystal based sapphire gas pressure cell allows for the collection of high quality powder diffraction data in time-resolved experiments with good time resolution.19,22 We have therefore performed in situ investigations (thermal analysis, X-ray and neutron powder diffraction) in the systems Li3N−H2, Li3N−Mg−H2 and Li3N−MgH2−H2 in order to contribute further to the elucidation of reaction pathways. An improved understanding of hydrogen uptake and release and effects underlying reversibility is of great importance for the application of materials as hydrogen stores.

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RESULTS AND DISCUSSION

In Situ Neutron Diffraction Study of the Deuteration of Li3N. The hydrogenation reaction of lithium nitride was followed in situ by neutron powder diffraction. In order to avoid strong incoherent scattering of 1H, deuterium was used instead. A sapphire-based single crystal gas pressure cell provided access to high quality diffraction data on the high intensity diffractometer D20 at the Institut Laue-Langevin (ILL) in Grenoble, France.19 In the beginning of the experiment, neutron diffraction patterns evidence both α- and β-modifications of Li3N and a small amount of Li2O to be present. Upon heating under 1.0 MPa of deuterium gas pressure lithium amide starts to form after about 50 min at a temperature around 470 K (Figure 1). While lithium nitride is

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MATERIALS AND METHODS Due to air sensitivity of most compounds, all handlings were carried out in an argon filled glovebox. Li3N (ABCR, 99.6%) was used for the in situ experiment on the deuteration by neutron powder diffraction of the pure compound, Li3N (Sigma-Aldrich, 80 mesh, 99.5%) for the in situ experiment on the deuteration by neutron powder diffraction of the mixture Li3N-MgD2 (see In Situ Neutron Diffraction Study of the Deuteration of Equimolar Mixtures section) and all DSC experiments (vide inf ra). MgH2 (Alfa Aesar, 98%) was used for the DSC experiments (vide inf ra). MgD2 was synthesized from magnesium powder (ABCR, 99.8%) under 10 MPa deuterium gas pressure and 670 K in a hydrogen resistant autoclave for several days. The product was ground and the procedure repeated until colorless MgD2 was yielded single phase according to X-ray powder diffraction. Hydrogen (Air Liquide, 99.999%) and deuterium gas (Air Liquide, 99.8% isotopic purity) were used for the respective reactions. X-ray powder diffraction data were collected on a Huber Guinier G670 camera with image plate system using Mo−Kα1 radiation. Due to moisture- and air-sensitivity, the samples were enclosed between kapton foils in apiezon grease. Hydrogenation experiments were carried out in a differential scanning calorimeter (DSC) Q1000 from TA Instruments equipped with a gas pressure cell at various hydrogen gas pressures in crimped aluminum pans with sample amounts of approximately 10−30 mg of the intimately mixed powders. DSC experiments were repeated once or twice for each sample without opening the cell in order to investigate the reversibility of the hydrogenation reactions. The heating rate for all experiments was 10 K/min. In situ neutron powder diffraction experiments up to 547 K and 9.43 MPa deuterium gas pressure were carried out at the Institut Laue-Langevin in Grenoble, France at the high flux diffractometer D20 in high resolution mode. The time

Figure 1. In situ neutron powder diffraction data of the deuteration of Li3N taken on the diffractometer D20 at the Institute Laue-Langevin, Grenoble, France (λ = 186.77(2) pm) in a single crystal sapphire gas pressure cell19,22 at 1.0 MPa deuterium gas pressures and varying temperatures. Intensities are given in false colors, and the y scale is linear with time (total 628 min = 10.5 h). Estimated standard uncertainty of the temperatures and deuterium gas pressures given are σ(T) = 2 K and σ(p) = 0.1 MPa, respectively. Phases: α Li3N (P6/ mmm), β Li3N (P63/mmc), ▲ LiND2, ⊡ LiD, ⬠ Li2O, ‡ single crystal reflection. B

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Figure 3. In situ neutron powder diffraction data of the deuteration of equimolar mixtures of Li3N and MgD2 taken on the diffractometer D20 at the Institute Laue-Langevin, Grenoble, France (λ = 186.70(3) pm) in a single crystal sapphire gas pressure cell19,22 at 1.5 MPa deuterium gas pressures and varying temperatures. Intensities are given in false colors, and the y scale is linear with time (total 671 min = 11.2 h). Estimated standard uncertainty of the temperatures and deuterium gas pressures given are σ(T) = 2 K and σ(p) = 0.1 MPa, respectively. Phases: α Li3N (P6/mmm), β Li3N (P63/mmc), ▲ LiND2, ⊡ LiD, ● MgD2, ‡ single crystal reflection.

Figure 2. Phase fractions from Rietveld analysis of neutron diffraction data during the in situ deuteration of Li3N under 1.0 MPa deuterium gas pressure shown in Figure 1.

being consumed, LiND2 + 2LiD form (see introduction), but also more lithium oxide (Figure 2). It could later be shown that the starting material contains some lithium hydroxide, probably reacting with the lithium hydride to lithium oxide at elevated temperatures. Therefore, another source for Li3N was used for later experiments (vide inf ra). After 12 h reaction time, only 6 mol % of lithium nitride remains unreacted. The deuteration of lithium nitride proceeds without intermediate phases directly to LiND2 + 2LiD, in accordance with earlier observations that the quite complicated reaction pathway at lower hydrogen pressures becomes much simpler for higher hydrogen pressures.9 The phase contents determined from neutron diffraction (Figure 2) are in agreement with the reaction Li3N + 2D2 → LiND2 + 2LiD, taking into account the proposed reaction of lithium deuteride with lithium hydroxide. In Situ Neutron Diffraction Study of the Deuteration of Equimolar Mixtures Li3N + MgD2. Similar in situ neutron powder diffraction studies as described for pure Li3N (vide supra) were performed on equimolar mixtures of Li3N + MgD2 under various deuterium gas pressures with a time resolution of 3 min (Figure 3). Horizontal blank lines correspond to interruption of the experiment due to technical problems with the laser heating system. After 150 min reaction time at 469 K Li3N starts to take up deuterium gas (1.5 MPa) to form LiND2 + 2LiD (Figure 3). Unfortunately, Rietveld refinement did not yield satisfactory results for any data set due to systematic intensity errors, possibly due to technical problems with the radial collimator. Therefore, the reaction progress was monitored by the intensities of strong reflections of the respective phases. Figure 4 shows the development of such reflection intensities during an isothermal section (476 K) of the experiment for 72 min. The diffraction intensities clearly reveal that while Li3N is being consumed, LiND2 + 2LiD form following a parabolic time law typical for solid−gas reactions.26 In a second run, a higher deuterium gas pressure (9.43 MPa) was chosen, which yielded similar results (Figure 5). After 6 h the reaction mixture, LiND2, LiD, and remaining unreacted Li3N were kept at 563 K. Lowering the deuterium gas pressure at this temperature to 130 kPa did not yield in decomposition of the products. From these in situ experiments we may conclude that the reaction pathway is the same for both deuterium gas pressures, 1.5 and 9.4 MPa, and that at temperatures below 545 K magnesium deuteride do not participate in the reaction.

Figure 4. Development of the intensities of individual reflections during an isothermal section (476 K) in the in situ neutron powder diffraction experiment on the deuteration of equimolar mixtures of Li3N and MgD2 (see Figure 3).

In Situ Thermal Analysis Studies (DSC) and X-ray Diffraction of the Hydrogenation of Mixtures Li3N + MgH2. The hydrogenation of mixtures Li3N + MgH2 was followed in situ by thermal analysis under hydrogen gas pressure, in order to map thermodynamic behavior and relate results to that of in situ neutron diffraction (vide supra). Figure 6 shows a typical DSC diagram for the reaction of equimolar mixtures of Li3N and MgH2 under 1.5 MPa hydrogen pressure with a distinct exothermic peak around 500 K and an endothermic signal at 620 K that might be overlapped by another exothermic peak. The fact that there are no signals in the second cycle reveals complete conversion in the first run, and the flat baseline upon cooling in both cycles proves irreversibility under the applied conditions. In Figure 7, X-ray powder diffraction patterns of several DSC experiments are shown. Irrespective of the applied hydrogen gas pressure (that varied between 1.0 and 5.0 MPa) and the time for which the maximum temperature of 703 K was held, C

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Figure 5. In situ neutron powder diffraction data of the deuteration of equimolar mixtures of Li3N and MgD2 taken on the diffractometer D20 at the Institute Laue-Langevin, Grenoble, France (λ = 186.70(3) pm) in a single crystal sapphire gas pressure cell19,22 at 9.4 MPa deuterium gas pressures and varying temperatures. Intensities are given in false colors, and the y scale is linear with time (total 363 min = 6.1 h). Estimated standard uncertainty of the temperatures and deuterium gas pressures given are σ(T) = 2 K and σ(p) = 0.1 MPa, respectively. Phases: α Li3N (P6/mmm), β Li3N (P63/mmc), ▲ LiND2, ⊡ LiD, ● MgD2, ‡ single crystal reflection.

Figure 7. X-ray powder diffraction patterns on equimolar mixtures of Li3N and MgH2 after treatment in in situ DSC experiments at different hydrogen pressures for 2 and 10 h, respectively; maximum temperature 703 K; λ = 70.93 pm (Mo).

Figure 8. X-ray powder diffraction pattern on equimolar mixtures of Li3N and MgH2 after treatment in in situ DSC experiments at 1.5 MPa hydrogen pressure; maximum temperature 543 K; λ = 70.93 pm (Mo).

Figure 6. In situ DSC diagram of the hydrogenation of equimolar mixtures of Li3N and MgH2 under 1.5 MPa hydrogen pressure.

first reaction as seen by an exothermic signal in a DSC experiment (Figure 6) is the hydrogenation of lithium nitride to lithium amide according to Li3N + 2H2 → LiNH2 + 2LiH. At higher temperatures, lithium amide reacts with magnesium hydride to form lithium magnesium nitride according to LiNH2 + MgH 2 → LiMgN + 2H 2 (Δ r H = +30 kJ/mol 27), corresponding to the second endothermic signal in the DSC experiment (Figure 6). This second step is thus a dehydrogenation reaction, which should be highly susceptible to pressure changes. Indeed higher hydrogen pressures seem to suppress this reaction and instead an exothermic signal is detected by DSC (Figure 9. left). LiMgN is still identified in the reaction product by X-ray diffraction, however. We propose an alternative route to LiMgN in this case, i.e., Li3N + MgH2 → LiMgN + 2LiH (ΔrH = −131 kJ/mol). Varying the stoichiometric composition of the starting materials gives similar results in case of higher MgH2 contents; however, a Li3N excess yields mainly the mixed imide Li2Mg(NH)2. These results are similar to dehydrogenationrehydrogenation studies on ball-milled mixtures of magnesium hydride and lithium nitride. Here, the same hydrides, nitrides,

they all point to the same reactions products: a mixture of mainly LiMgN and LiH with smaller amounts of LiNH2, Li2NH, and Li4NH. In order to resolve the processes corresponding to the different DSC signals, the same experiment was carried out with a maximum temperature of 543 K, thus stopping the DSC run shortly after the first peak maximum in Figure 6. This temperature corresponds to the highest attained in the in situ neutron diffraction experiment. X-ray characterization of this sample gave completely different results (Figure 8): Instead of the mixed nitride LiMgN, now only LiNH2 and Li2NH were found alongside unreacted MgH2. Due to their high structural similarity, the powder diffraction patterns might also fit to the mixed imides Li2Mg(NH)2 and Li2Mg2(NH)3 instead of lithium amide and imide, but this would be in contrast to the NPD results and the finding that MgD2 is not consumed at this stage of the reaction (see Figure 2). These DSC results in combination with X-ray and neutron diffraction reveal the following reaction pathway for the hydrogenation of equimolar mixtures of Li3N + MgH2. The D

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reveals that the formation of Li2NH starts at about 440 K, Li2Mg(NH)2 at 490 K and Mg3N2 at 550 K, and is largely in agreement with DSC results (vide supra). The reaction pathway thus reveals that the higher the temperature, the more magnesium rich phases are formed, i.e., magnesium is at first part of multinary imides and later will form nitrides.

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CONCLUSIONS Hydrogenation reaction pathways in potential hydrogen storage systems based on lithium nitride, Li3N, were investigated by in situ methods. Time resolved in situ neutron powder diffraction experiments on the deuteration of pure lithium nitride at 1.0 MPa deuterium gas pressure reveals a one-step reaction to lithium amide according to Li3N + 2 D2 → LiND2 + 2LiD. Similar results are observed for the deuteration of equimolar Li3N + MgD2 mixtures, both at 1.5 and 9.4 MPa deuterium gas pressure, i.e., magnesium deuteride does not participate in the reaction up to the highest achieved temperature of 543 K (Figure 11). In situ thermal analysis (DSC) and X-ray

Figure 9. Comparison of in situ DSC diagram of the hydrogenation of equimolar mixtures of Li3N and MgH2 under 1.5 MPa hydrogen pressure (left) and Li3N and Mg under 5.0 MPa hydrogen pressure (right), first cycles only.

and imides are found after hydrogenation with varying phase contents, depending on reaction conditions.28−30 Analogous DSC and X-ray diffraction experiments for the deuterated compounds yield very similar results. In Situ Thermal Analysis Studies (DSC) and X-ray Diffraction of the Hydrogenation of Mixtures Li3N + Mg. Replacing magnesium hydride by magnesium in mixtures with lithium nitride reveals a more complex behavior. Magnesium is involved in the reaction from the start as a combined DSC and X-ray diffraction study shows. At high hydrogen pressure, the DSC yields similar results as for the Li3N/MgH2 mixture, and at 1.5 MPa the strong exothermic signal at 620 K seems to be absent (Figure 9). Rietveld analysis of X-ray diffraction (Figure 10) shows that for both low and high pressure, a mixture of Figure 11. In situ X-ray powder diffraction on an equimolar mixture of Li3N and Mg at 1.5 MPa hydrogen pressure; λ = 70.93 pm (Mo); Phases: α - Li3N (P6/mmm), β - Li3N (P63/mmc), x - Mg, U Li2Mg(NH)2, T - Mg3N2, ⊡ - LiH.

diffraction suggest that at higher temperatures lithium magnesium nitride is formed according to the endothermic reaction LiNH2 + MgH2 → LiMgN + 2H2 at moderate (≤1.5 MPa) and via the exothermic reaction Li3N + MgH2 → LiMgN + 2LiH at higher hydrogen gas pressures (5.0 MPa). An excess of Li3N in the reactions favor the formation of the mixed imide Li2Mg(NH)2. Rather complex behavior was found for the hydrogenation of mixtures Li3N + Mg. The reaction starts with the formation Li2NH and Li4NH, followed by the mixed imide Li2Mg(NH)2 at higher temperatures and finally the formation of Mg3N2 and LiH.

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Figure 10. X-ray powder diffraction pattern of an equimolar mixtures of Li3N and Mg after treatment in in situ DSC experiments at 1.5 MPa hydrogen pressure and Rietveld analysis (phase content see text; λ = 70.93 pm (Mo)).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: +49 341 97 36201. Author Contributions

phases is formed: 30% (14%) Li2NH, 24% (27%) Li4NH, 12% (17%) Mg3N2, 12% (24%) LiH, 6% (19%) Li2Mg(NH)2, 15% (0%) LiOH, 1% (0%) Mg for 1.5 MPa (5.0 MPa) hydrogen pressure. Following the reaction of Li3N + Mg under 1.5 MPa hydrogen pressure with in situ X-ray diffraction up to 600 K

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS We acknowledge the Deutsche Forschungsgemeinschaft (DFG, Grant No. KO 1803/4-1) for financial support. We thank the Institute-Laue Langevin for provision of beamtime and Dr. Thomas Hansen for support of the neutron diffraction experiments at the high-intensity powder diffractometer D20.

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