Xiangdong Kang

An Amorphous Carbon Nitride Photocatalyst with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Hydrogen Generation

Amorphous carbon nitride (ACN) with a bandgap of 1.90 eV shows an order of magnitude higher photocatalytic activity in hydrogen evolution under visible light than partially crystalline graphitic carbon nitride with a bandgap of 2.82 eV. ACN is photocatalytically active under visible light at a wavelength beyond 600 nm.

Ammonia Borane Destabilized by Lithium Hydride: An Advanced On-Board Hydrogen Storage Material†

An advanced hydrogen storage material, with potential for on-board application, is readily prepared by mechanically milling a 1:1 ammonia borane/lithium hydride (AB/LiH) mixture. The material possesses a H capacity of around 10 wt %, higher than the 2015 DOE gravimetric H capacity target, and can rapidly release over 7 wt % pure H2 at around 100 °C.

Selective breaking of hydrogen bonds of layered carbon nitride for visible light photocatalysis

Selective breaking of the hydrogen bonds of graphitic carbon nitride can introduce favorable features, including increased band tails close to the band edges and the creation of abundant pores. These features can simultaneously improve the three basic processes of photocatalysis. As a consequence, the photocatalytic hydrogen-generation activity of carbon nitride under visible light is drastically increased by tens of times.

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.

Improved hydrogen storage property of Li–Mg–B–H system by milling with titanium trifluoride

The Li–Mg–B–H system that is prepared from 2LiH + MgB2 or 2LiBH4 + MgH2 possesses high hydrogen capacity and relatively favorable thermodynamics, but it is greatly restricted in practical hydrogen storage applications by problematic H-exchange kinetics. In the present study, TiF3 was mechanically milled with a 2LiH + MgB2 mixture and examined with respect to its effect on reversible dehydrogenation of the Li–Mg–B–H system. Experimental study showed that TiF3 is highly effective for promoting the two-step dehydrogenation reaction in the Li–Mg–B–H system. Compared to the neat 2LiH + MgB2 sample, the 2LiH + MgB2 + 0.01TiF3 sample exhibits significantly reduced dehydrogenation temperature and markedly enhanced dehydriding rate at both steps. Furthermore, the catalytic enhancement arising upon adding TiF3 additive was observed to persist well in the hydrogenation/dehydrogenation cycles. Based on the results of phase analysis and a series of designed experiments, the mechanism underlying the observed property improvement is discussed.

A Comparative Study of the Structural, Electronic, and Vibrational Properties of NH3BH3 and LiNH2BH3: Theory and Experiment

Herein, we systematically investigate the structural, electronic, and vibrational properties of ammonia borane (NH(3)BH(3), AB) and lithium amidoborane (LiNH(2)BH(3,) LAB) through both density functional calculations and experiments. AB and LAB samples are generated and their vibrational spectra are obtained by using Fourier transformed infrared spectroscopy (FTIR). The measured vibrational spectra are in good agreement with the calculated ones. Our calculations show that the Li-related vibration modes are primarily found in the low-frequency region (<1000 cm(-1)), and that the intermolecular interactions significantly influence the vibrational spectra. Electronic structure calculations provide insights into the differences between the binding natures of AB and LAB and their influence on the vibrational properties of these compounds.

A novel three-step method for preparation of a TiB2-promoted LiBH4–MgH2 composite for reversible hydrogen storage

The reversible dehydrogenation properties of the 2LiBH(4)-MgH(2) composite can be effectively improved by incorporating heterogeneous nucleation agents, typically transition metal borides. A careful study of the 2LiBH(4)-MgH(2) composite with a titanium trifluoride (TiF(3)) additive finds that using the conventional one-step milling method renders only a partial conversion from TiF(3) to titanium boride (TiB(2)) through an intermediate of titanium hydride (TiH(2)). Based on a fundamental understanding of the reaction processes of the system, we developed a three-step preparation method, which involves pre-milling the LiBH(4)-TiF(3) mixture, isothermal treatment and milling together with MgH(2). A combination of phase/chemical state/microstructural analyses using X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy techniques shows that the newly developed method can effectively promote the formation of TiB(2) and meanwhile, ensure a homogeneous dispersion of TiB(2) nanoparticles in the composite matrix. As a consequence, the composite sample prepared by the new method exhibits a favorable combination of high hydrogen capacity, fast reaction kinetics and satisfactory cyclic stability.

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).