The structural properties of amides and imides as hydrogen storage materials

Ichikawa, Takayuki; Isobe, Shigehito

doi:10.1524/zkri.2008.1029

Cited by 15 publications

(8 citation statements)

References 66 publications

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“…A lot of research, both computational and experimental, has been done on this system, particularly to understand its structure and phase transition. 9,10 In this context, the elementary steps of the reaction have recently attracted considerable interest. 11 In particular, the possibility of quantum delocalization of the protons over several possible sites has been reported, as a consequence of the relatively flat potential energy landscape in the highly symmetric structure.…”

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confidence: 99%

Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li₂NH

Ludueña

Sebastiani

2010

J. Phys. Chem. Lett.

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We analyze the possibility of quantum delocalization in lithium imide (Li 2 NH) in the condensed phase using ab initio path-integral molecular dynamics. Our results provide evidence that the effective potential felt by the protons in the material has a toroidal shape. The virtually flat potential may lead to quantum delocalization of the NH protons over the torus. SECTION Molecular Structure, Quantum Chemistry, General Theory U nder ambient conditions, most atoms can normally be approximated as point particles, and this approximation yields a good level of accuracy for many properties. However, in the case of hydrogen, nuclear quantum effects can be observable due to its low atomic mass. In some cases, hydrogen has been reported to exhibit quantum properties, such as tunneling and quantum delocalization. 1-5 These effects might also play a role in industrially relevant situations, such as hydrogen storage. 6 Besides this special aspect, understanding the nature of quantum effects of hydrogen in bulk materials is a matter of high fundamental and general relevance.As of today, no production-ready material has been found to meet the required per-weight capacity of hydrogen storage. [6][7][8] In this work, we focus on the particular case of lithium imide (Li 2 NH), which is a promising hydrogen storage system due to its low weight. The lithium imide/amide system can undergo a reversible hydrogenation with high H storage densities (6.5%) via the reaction Li 2 NH þ H 2 S LiNH 2 þ LiH. A lot of research, both computational and experimental, has been done on this system, particularly to understand its structure and phase transition. 9,10 In this context, the elementary steps of the reaction have recently attracted considerable interest. 11 In particular, the possibility of quantum delocalization of the protons over several possible sites has been reported, as a consequence of the relatively flat potential energy landscape in the highly symmetric structure. 12 In our previous publication, 10 we observed that the effect of nuclear quantum delocalization for the hydrogen atoms is considerably reduced compared to a full delocalization of each proton on the six octahedral sites around its nitrogen atom. In the present work, we analyze the potential energy landscape of the protons in the condensed phase by means of ab initio path-integral molecular dynamics simulations (PIMD). 13,14 We show that this surface is surprisingly flat within a torusshaped volume, which might lead to coherent delocalization at low temperatures.We model the initial configuration of the Li 2 NH protons as delocalized particles, which are distributed over six crystallographically symmetric sites, using the setup presented in our previous work. 10 Such a proton configuration is plausible because of the low potential barrier between these sites. 12 As reported previously, 10 the equilibrated structure at 300 K features a somewhat disordered Li lattice. The NH bonds form angles of 30°from a preferred N-N axis 15 (Figure 1, left), and the protons remain in ...

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confidence: 99%

Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li₂NH

Ludueña

Sebastiani

2010

J. Phys. Chem. Lett.

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“…Comparatively, the Mn 4 N-LiH, Mn 4 N-CaH 2 , and Mn 4 N-BaH 2 samples started to gain weight at ∼400 K and ∼14.6, 6.4, and 4.7 wt % increases were achieved as temperature was increased to 660 K (Figure S3), which are equivalent to ∼0.7, 0.4, and 1.1 mol of fixed N per mole of Mn, respectively (see Figure 2a). The Mn 4 N-NaH and Mn 4 N-KH samples, on the other hand, show very little weight change in and BaNH) species, 25 which allow CaH 2 or BaH 2 to accommodate some N absorbed by Mn 4 N-CaH 2 and Mn 4 N-BaH 2 samples. For Na or K, however, there are no stable imide phases 25 because of their small lattice enthalpies (see Table S2).…”

Section: Resultsmentioning

confidence: 99%

“…Indeed, the crystalline phase of Li 2 NH was clearly observed on a Mn 4 N-LiH sample heated in N 2 (Figure S6). Similarly, there are stable Ca–N–H (Ca 2 NH and CaNH) and Ba–N–H (Ba 2 NH and BaNH) species, which allow CaH 2 or BaH 2 to accommodate some N absorbed by Mn 4 N-CaH 2 and Mn 4 N-BaH 2 samples. For Na or K, however, there are no stable imide phases because of their small lattice enthalpies (see Table S2).…”

Section: Resultsmentioning

confidence: 99%

“…Similarly, there are stable Ca–N–H (Ca 2 NH and CaNH) and Ba–N–H (Ba 2 NH and BaNH) species, which allow CaH 2 or BaH 2 to accommodate some N absorbed by Mn 4 N-CaH 2 and Mn 4 N-BaH 2 samples. For Na or K, however, there are no stable imide phases because of their small lattice enthalpies (see Table S2). The similar diffraction peaks of Mn 4 N (Figure S5) and specific surface areas of the Mn 4 N-AHs samples (Table S4) indicate the dispersion/active site number may not be the main reason for different activities.…”

Section: Resultsmentioning

confidence: 99%

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Alkali and Alkaline Earth Hydrides-Driven N₂ Activation and Transformation over Mn Nitride Catalyst

et al. 2018

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Early 3d transition metals are not focal catalytic candidates for many chemical processes because they have strong affinities to O, N, C, or H, etc., which would hinder the conversion of those species to products. Metallic Mn, as a representative, undergoes nitridation under ammonia synthesis conditions forming bulk phase nitride and unfortunately exhibits negligible catalytic activity. Here we show that alkali or alkaline earth metal hydrides (i.e., LiH, NaH, KH, CaH 2 and BaH 2 , AHs for short) promotes the catalytic activity of Mn nitride by orders of magnitude. The sequence of promotion is BaH 2 > LiH > KH > CaH 2 > NaH, which is different from the order observed in conventional oxide or hydroxide promoters. AHs, featured by bearing negatively charged hydrogen atoms, have chemical potentials in removing N from Mn nitride and thus lead to significant enhancement of N 2 activation and subsequent conversion to NH 3 . Detailed investigations on Mn-LiH catalytic system disclosed that the active phase and kinetic behavior depend strongly on reaction conditions. Based on the understanding of the synergy between AHs and Mn nitride, a strategy in the design and development of early transition metals as effective catalysts for ammonia synthesis and other chemical processes is proposed.

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“…[8][9][10][11][12][13] Regarding the special case of lithium imide (Li 2 NH), the exact structure of the material has not yet been fully determined. [14] A recent combined neutron and X-ray diffraction study [15] provided several different possible proton positions. Furthermore, the possibility of quantum delocalization of the protons has been reported.…”

Section: Introductionmentioning

confidence: 99%

Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

Ludueña¹,

Wegner²,

Bjålie³

et al. 2010

ChemPhysChem

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Amides and imides of alkali metals are a very promising class of materials for use as a hydrogen-storage system, as they are able to store and release hydrogen via a chemical route at controllable temperatures and pressures. We critically revise the present picture of the atomic structure of the lightest member (LiNH(2)/Li(2)NH) by using a combined computational and experimental approach. Specifically, ab initio path integral molecular dynamics simulations and solid-state (1)H NMR techniques are combined. The results show that the presently assumed local structure might be inconsistent or at least incomplete and needs considerable revision. In particular, the Li atoms turn out to be more mobile and more disordered than suggested by structural data obtained from X-ray scattering. Also, the configuration of the hydrogen atoms, which is accessible via the NMR experiment and the corresponding first-principles calculations, is different from the previously assumed data. The computed and experimentally observed (1)H NMR parameters are in very good mutual agreement and illustrate the unusual chemical environment of the hydrogen atoms in this system. Incorporating our results on the new lithium data, we show that the effect of nuclear quantum delocalization for the hydrogen atoms is considerably reduced compared to the perfect crystal structure.

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The structural properties of amides and imides as hydrogen storage materials

Cited by 15 publications

References 66 publications

Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li₂NH

Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li₂NH

Alkali and Alkaline Earth Hydrides-Driven N₂ Activation and Transformation over Mn Nitride Catalyst

Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

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The structural properties of amides and imides as hydrogen storage materials

Cited by 15 publications

References 66 publications

Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li2NH

Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li2NH

Alkali and Alkaline Earth Hydrides-Driven N2 Activation and Transformation over Mn Nitride Catalyst

Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

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Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li₂NH

Possibility of Coherent Delocalized Nuclear Quantum States of Protons in Li₂NH

Alkali and Alkaline Earth Hydrides-Driven N₂ Activation and Transformation over Mn Nitride Catalyst