Zhitao Xiong

Releasing 17.8 wt% H2 from lithium borohydride ammoniate

Release of ca. 17.8 wt% of hydrogen was observed from the Co-catalyzed lithium borohydride ammoniate, Li(NH3)4/3BH4 (with equivalent protic and hydridic hydrogen atoms, composed of solid Li(NH3)BH4 and liquid Li(NH3)2BH4), in the temperature range of 135 to 250 °C in a closed vessel. The low NH3 equilibrium vapor pressure of the ammoniate in the vessel results in the retention of the majority of NH3 in the vicinity of LiBH4, and thus, creates an environment favorable for the direct dehydrogenation rather than deammoniation. The dehydrogenation is a two-step process forming the intermediates Li4BN3H10 and LiBH4. The final solid residue is a mixture of BN and Li3BN2. The presence of nanosized Co catalyst effectively promote the hydrogen release.

Synthesis, Structure and Dehydrogenation of Magnesium Amidoborane Monoammoniate

Magnesium amidoborane monoammoniate (Mg(NH(2)BH(3))(2) x NH(3)) which crystallizes in a monoclinic structure (space group P2(1)/a) has been synthesized by reacting MgNH with NH(3)BH(3). Dihydrogen bonds are established between coordinated NH(3) and BH(3) of [NH(2)BH(3)](-) in the structure, promoting stoichiometric conversion of NH(3) to H(2).

Hydrogen Sorption from the Mg(NH2)2-KH System and Synthesis of an Amide–Imide Complex of KMg(NH)(NH2)

The interaction between KH and Mg(NH(2))(2) is investigated. Results from temperature-programmed desorption measurements on samples of [Mg(NH(2))(2)][KH](x) (x=0.5, 1.0, and 2.0) indicated that dehydrogenation from [Mg(NH(2))(2)][KH] occurred through a two-step reaction with an onset temperature as low as 60 °C. Accompanied by hydrogen release, K(2)Mg(NH(2))(4) and MgNH successively developed at lower temperatures, whereas KMg(NH)(NH(2)) developed at higher temperatures. However, when dehydrogenation was conducted under isothermal and near-equilibrium conditions, a single-step reaction that led to the formation of KMg(NH)(NH(2)) was observed. KMg(NH)(NH(2)) is a new amide-imide complex. The synthesis of KMg(NH)(NH(2)) can be achieved either by dehydrogenation of the [Mg(NH(2))(2)][KH] mixture or by thermal decomposition of the [K(2)Mg(NH(2))(4)][Mg(NH(2))(2)] mixture.

Lithium Imide Synergy with 3d Transition‐Metal Nitrides Leading to Unprecedented Catalytic Activities for Ammonia Decomposition

Alkali metals have been widely employed as catalyst promoters; however, the promoting mechanism remains essentially unclear. Li, when in the imide form, is shown to synergize with 3d transition metals or their nitrides TM(N) spreading from Ti to Cu, leading to universal and unprecedentedly high catalytic activities in NH3 decomposition, among which Li2NH-MnN has an activity superior to that of the highly active Ru/carbon nanotube catalyst. The catalysis is fulfilled via the two-step cycle comprising: 1) the reaction of Li2NH and 3d TM(N) to form ternary nitride of LiTMN and H2, and 2) the ammoniation of LiTMN to Li2NH, TM(N) and N2 resulting in the neat reaction of 2 NH3⇌N2+3 H2. Li2NH, as an NH3 transmitting agent, favors the formation of higher N-content intermediate (LiTMN), where Li executes inductive effect to stabilize the TM-N bonding and thus alters the reaction energetics.

Growth of Crystalline Polyaminoborane through Catalytic Dehydrogenation of Ammonia Borane on FeB Nanoalloy

CAS [KGCX2-YW-806, KJCX2-YW-H21, 2009AA05Z108, 2010CB631304]; US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]

Effects of triphenyl phosphate on the hydrogen storage performance of the Mg(NH2)2-2LiH system

Mg(NH2)2–2LiH is an attractive system because of its high reversible hydrogen capacity (∼5.6 wt%) and suitable thermodynamic parameters that allow operation below 100 °C. However, a relatively high kinetic barrier in the hydrogen desorption blocks its application at low temperature. In this work, a small amount of additive, triphenyl phosphate (TPP), was introduced and its effects on hydrogen desorption/absorption in the Mg(NH2)2–2LiH system were studied. Experimental results showed that TPP can prevent aggregation/crystallization during the cycling tests and thus achieve an enhanced kinetic performance. Complete dehydrogenation and hydrogenation can be successfully carried out at temperatures below 150 °C. Moreover, a significant reduction of the entropy change of hydrogen desorption (ΔSdes) was found in the TPP-doped system compared with the pristine Mg(NH2)2–2LiH system, thought to be due to the persistence of amorphous Mg(NH2)2 in the TPP-doped sample during dehydrogenation and hydrogenation cycling, thereby greatly affecting the equilibrium hydrogen pressure.

Effects of Al-based additives on the hydrogen storage performance of the Mg(NH2)2–2LiH system

The Mg(NH2)2-2LiH composite is a promising hydrogen storage material due to its relatively high reversible hydrogen capacity (~5.6 wt%) and suitable thermodynamic properties that allow hydrogen sorption conducting at temperatures below 90 °C. However, the presence of a severe kinetic barrier inhibits its low-temperature operation. In the present work, Li3AlH6 was introduced to the Mg(NH2)2-2LiH system. Experimental results show that a 3.2% mol Li3AlH6-modified Mg(NH2)2-2LiH sample released hydrogen at a rate ca. 4.5 times as fast as that of the Li3AlH6-free sample at 140 °C. The enhancement of desorption kinetics was simultaneously demonstrated by activation energy (Ea) of ca. 96.3 ± 9 kJ mol(-1) which was significantly decreased by 31 kJ mol(-1) from that of the Li3AlH6-free sample. The interaction of Li3AlH6 and Mg(NH2)2 during ball milling results in the formation of LiAl(NH)2, LiNH2 and Mg3N2. LiAl(NH)2 was actually the active species for the enhancement of dehydrogenation/re-hydrogenation kinetics of the system.

Effective thermodynamic alteration to Mg(NH2)2–LiH system: achieving near ambient-temperature hydrogen storage

A strategy for thermodynamic improvement of 2Mg(NH2)2–3LiH composite via “stabilizing” the dehydrogenated product LiNH2 was developed. By introducing LiI, LiBr and LiBH4 to the composite, hydrogen release at 0.1 MPa equilibrium pressure is thermodynamically allowed at 333, 320 and 337 K, respectively, which are ten degree Celsius lower than the pristine composite. Moreover, the dehydrogenation kinetics and reversibility are significantly improved.

Improved kinetics of the Mg(NH2)2–2LiH system by addition of lithium halides

The Mg(NH2)2–2LiH composite is a promising on-board hydrogen storage material due to its high reversible hydrogen capacity and suitable thermodynamic properties. However, the severe kinetic barrier inhibits its low temperature operation. In the present work, the additive effects of lithium halides on the Mg(NH2)2–2LiH system were studied systematically. Experimental results showed that, among all those lithium halides, the LiBr doped Mg(NH2)2–2LiH composite exhibited the best dehydrogenation performance. The hydrogen sorption and desorption rates of the Mg(NH2)2–2LiH–0.2LiBr sample are ∼3 and 2 times, respectively, faster than that of the pristine sample at 140 °C. At the same time, enhanced kinetics for hydrogen desorption was observed from an activation energy (Ea) of ca. 92 ± 9 kJ mol−1 which was significantly decreased by 35 kJ mol−1 compared with the pristine sample. Subsequently, a plausible mechanism for the modified dehydrogenation/re-hydrogenation process was proposed.

Ambient temperature hydrogen desorption from LiAlH4-LiNH2 mediated by HMPA

By adding a small amount of HMPA approximately 2.5 equivalent or ca. 8.1 wt% H2 can be released from the LiAlH4–LiNH2 mixture at ambient temperature, which is ∼370 °C lower than the corresponding solid-state dehydrogenation. NMR characterizations reveal the formation of intermediates, such as [AlH] and [AlN] species, during the dehydrogenation. LiNH2 induces the enrichment of [AlH] species in the liquid phase which further reacts with LiNH2 to produce H2, Li2NH and Al.