Junhong Luo

Promoted hydrogen release from ammonia borane by mechanically milling with magnesium hydride: a new destabilizing approach

Ammonia borane (NH(3)BH(3), AB) is an intriguing molecular crystal with an extremely high hydrogen capacity and moderate thermal stability. In the present study, we show a simple but effective approach for destabilizing AB for promoted hydrogen release at moderate temperatures. It is found that mechanically milling with magnesium hydride (MgH(2)) can dramatically improve the dehydrogenation properties of AB, on both the kinetic and thermochemical aspects. For the mechanically milled AB/0.5MgH(2) material, over 8 wt% hydrogen can be released from AB within 4 h at around 100 degrees C without undesired volatile by-products. Moreover, the dehydrogenation reaction of the AB/0.5MgH(2) sample becomes significantly less exothermic than that of neat AB. In situ X-ray diffraction results demonstrate that the MgH(2) additive well maintains its phase stability during the ball-milling and the subsequent heating processes. Meanwhile, Raman spectroscopy and in situ(11)B NMR studies show that the MgH(2) additive exerts considerable influence on the chemical bonding state and decomposition process/products of AB. Combined phase/structure analyses results suggest that MgH(2) exerts effect via developing solid phase interaction with AB.

Synthesis, formation mechanism, and dehydrogenation properties of the long-sought Mg(NH2BH3)2 compound

The synthesis of magnesium amidoborane, Mg(NH2BH3)2 (MgAB), has been attracting considerable interest since the recognition of the potential of metal amidoboranes as promising hydrogen storage media. But so far, all the efforts for synthesizing Mg(NH2BH3)2 using mechanochemical or wet chemistry methods have been frustrated. In this paper, we report a successful synthesis of MgAB using ammonia borane (AB) and magnesium hydride (MgH2) or magnesium (Mg) powder as starting materials. It was found that the post-milled 2AB/MgH2 and 2AB/Mg mixtures undergo solid-phase reactions under mild temperatures (≤70 °C), resulting in the formation of a new crystalline phase. A combination of X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transformation infrared (FTIR), and solid-state 11B MAS NMR characterizations, in conjunction with weight loss monitoring and the designed metathesis experiment, strongly indicates that the new crystalline phase is the long-sought MgAB. Our study using the solid-state 11B NMR technique further revealed that the formation of MgAB involves a reaction mechanism that is distinct from other metal amidoboranes. MgH2 or Mg may actually react with the mobile phase of AB (AB*), rather than with the starting normal AB. The examination of its properties revealed that MgAB is a stable compound at room temperature and can release ∼10 wt% H2 of high purity at temperature below 300 °C, suggesting that it is a promising hydrogen storage material.

Renewed Insight into the Promoting Mechanism of Magnesium Hydride on Ammonia Borane

Our previous study found that mechanically milling with magnesium hydride (MgH(2)) could dramatically improve the dehydrogenation property of ammonia borane (AB). Meanwhile, it appears that the MgH(2) additive maintains its phase stability in the milling and subsequent heating process. In an effort to further the mechanistic understanding of the AB/MgH(2) system, we reinvestigated the property and structure evolution in the hydrogen release process of the AB/0.5MgH(2) sample. Property examination using volumetric method and synchronous thermal analyses showed that the AB/0.5 MgH(2) sample releases approximately 13.8 wt % hydrogen after being heated at 300 degrees C. This hydrogen amount is in excess of that available from AB, indicative of the participation of a faction of MgH(2) in the dehydrogenation process of AB. Structural and chemical state analyses using Fourier transformation infrared spectroscopy and solid-state (11)B nuclear magnetic resonance techniques further showed that part of MgH(2) participates in the dehydrogenation process of AB from the first step, resulting in the formation of Mg-B-N-H intermediate species. The incorporation of Mg in AB is believed to be a crucial event leading to dehydrogenation property improvements, particularly for the release of the last equivalent of H(2) in AB at relatively moderate temperature. These findings have provided renewed insight into the promoting mechanism of MgH(2) on the hydrogen release from AB.

Facile solid-phase synthesis of the diammoniate of diborane and its thermal decomposition behavior

The recent mechanistic finding of the hydrogen release pathways from ammonia borane (AB) has sparked new interest in the chemistry and properties of the diammoniate of diborane (DADB), an ionic isomer of AB. We herein report a facile one-step solid-phase synthesis route of DADB using inexpensive starting materials. Our study found that mechanically milling a 1 : 1 NaBH(4)/NH(4)F powder mixture causes the formation of crystalline DADB via a NH(4)BH(4) intermediate. The produced DADB can be readily separated from the sodium fluoride (NaF) by-product by a purification procedure using liquid ammonia at -78 °C. The thermal decomposition behavior of DADB was studied using synchronous thermal analyses, particularly in comparison with AB. It was found that the decomposition steps and products of DADB are similar to those of AB. But meanwhile, DADB exhibits a series of advantages over AB that merit its potential hydrogen storage application, such as lower dehydrogenation temperature, free of foaming and lack of an induction period in the thermal decomposition process. Our study further found that the volatile non-hydrogen products from DADB can be effectively suppressed by milling with MgH(2).

A simple and efficient approach to synthesize amidoborane ammoniates: case study for Mg(NH2BH3)2(NH3)3 with unusual coordination structure

Metal amidoborane ammoniates are a new type of promising hydrogen storage material consisting of metal cation, [NH2BH3](-) anionic unit and NH3 ligand. Herein, we report a new reactive ball milling approach for preparation of magnesium amidoborane ammoniate, Mg(NH2BH3)(2)(NH3)(3). Our study found that mechanically milling the NH3BH3-MgH2 mixture in a 2 : 1 molar ratio under NH3 atmosphere can readily produce Mg(NH2BH3)(2)(NH3)(3). Its crystal structure was successfully determined by a combination of X-ray diffraction (XRD) analysis and first-principles calculations. This compound possesses a novel ordered structure with alternating layers of Mg(NH3)(6)(2+) hexamminemagnesium cations and Mg(NH2BH3)(4)(2-) complex anions, in which Mg2+ exhibits both VI and IV coordinations. Property measurements found that Mg(NH2BH3)(2)(NH3)(3) can release 7 equivalents (10.6 wt%) of H-2 upon heating to 300 degrees C in a closed system. A combination of XRD, Fourier transformation infrared spectroscopy (FTIR), and solid-state B-11 MAS NMR techniques has been employed to characterize Mg(NH2BH3)(2)(NH3)(3) and its dehydrogenation product. A series of control experiments have also been conducted to gain insight into the formation mechanism of Mg(NH2BH3)(2)(NH3)(3).