Teng He

Synthesis of sodium amidoborane (NaNH2BH3) for hydrogen production

The prospect of building a future energy system on hydrogen has stimulated much research effort in developing hydrogen storage technologies. One of the potential materials newly developed is sodium amidoborane (NaNH2BH3) which evolves ∼7.5 wt% hydrogen at temperatures as low as 91 °C. In this paper, two methods of synthesizing pure NaNH2BH3 were reported. One method is by reacting NaH and ammonia borane in THF at low temperatures, and the other is by reacting NaNH2 and ammonia borane in THF at ambient temperature. Non-isothermal testing on the thermolysis of solid NaNH2BH3 showed that hydrogen evolution was composed of two exothermic steps. More than 1 equiv. H2 was evolved rapidly at temperatures below 87 °C. After evolving 2 equiv. H2, NaH was identified in solid products and coexisted with amorphous BN.

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.

Breaking scaling relations to achieve low-temperature ammonia synthesis through LiH-mediated nitrogen transfer and hydrogenation

Ammonia synthesis under mild conditions is a goal that has been long sought after. Previous investigations have shown that adsorption and transition-state energies of intermediates in this process on transition metals (TMs) scale with each other. This prevents the independent optimization of these energies that would result in the ideal catalyst: one that activates reactants well, but binds intermediates relatively weakly. Here we demonstrate that these scaling relations can be broken by intervening in the TM-mediated catalysis with a second catalytic site, LiH. The negatively charged hydrogen atoms of LiH act as strong reducing agents, which remove activated nitrogen atoms from the TM or its nitride (TMN), and as an immediate source of hydrogen, which binds nitrogen atoms to form LiNH2. LiNH2 further splits H2 heterolytically to give off NH3 and regenerate LiH. This synergy between TM (or TMN) and LiH creates a favourable pathway that allows both early and late 3d TM-LiH composites to exhibit unprecedented lower-temperature catalytic activities.

Hydrogen Storage Properties of Ca(BH4)2–LiNH2 System

Ca(BH(4))(2) is one of the promising candidates for hydrogen storage materials because of its high gravimetric and volumetric hydrogen capacity. However, its high dehydrogenation temperature and limited reversibility has been a hurdle for its practical applications. In an effort to overcome these barriers and to adjust the thermal stability, we make a composite system Ca(BH(4))(2)-LiNH(2). Interaction of Ca(BH(4))(2) and LiNH(2) leads to decreased dehydrogenation temperatures and increased hydrogen desorption capacity in comparison to pristine Ca(BH(4))(2). More than 7 wt% of hydrogen can be detached at a temperature as low as approximately 178 degrees C from the cobalt-catalyzed Ca(BH(4))(2)-4 LiNH(2) system.

Covalent triazine framework-supported palladium nanoparticles for catalytic hydrogenation of N-heterocycles

A covalent triazine framework (CTF) with high surface area, large amount of nitrogen functionalities, and high porosity and basicity was employed as a support for palladium nanoparticles (NPs). A well-dispersed Pd/CTF-1 catalyst with uniform distribution of Pd particles was successfully synthesized in the present study. The as-prepared 4% Pd/CTF-1 catalyst showed a markedly improved activity in the hydrogenation of N-heterocyclic compounds compared to the activated carbon (AC)-supported catalyst, i.e., the Pd/CTF-1 catalyst exhibits ca. 3.6 times faster reaction than Pd/AC in the hydrogenation of N-methylpyrrole. Characterization of Pd/CTF indicated electron donation from the N in CTF to the metallic Pd NPs, showing intensified electronic interaction between the Pd NPs and CTF support, which is responsible for the enhanced activities for the catalytic hydrogenation of N-heterocycles.

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]

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.

Alkali and alkaline-earth metal borohydride hydrazinates: synthesis, structures and dehydrogenation

Four new borohydride hydrazinates, including NaBH4·NH2NH2, LiBH4·1/2NH2NH2, LiBH4·1/3NH2NH2 and Mg(BH4)2·3NH2NH2, were synthesized. NaBH4·NH2NH2 and Mg(BH4)2·3NH2NH2 possess monoclinic and trigonal structures, respectively, while LiBH4·1/2NH2NH2 and LiBH4·1/3NH2NH2 exhibit orthorhombic and monoclinic structures. The effects of composition on the dehydrogenation of hydrazinates were investigated. It is demonstrated that cations with high Pauling electronegativity hold hydrazine strongly in the vicinity of borohydride and result in direct dehydrogenation at elevated temperatures. Specifically, Mg(BH4)2 hydrazinates can directly generate hydrogen upon heating it under a flow of Ar; on the other hand, the Li and Na counterparts lost part or all of the hydrazine components under the same condition. In addition, reducing NH2NH2 content in the complexes leads to improved dehydrogenation properties. Mechanistic investigation of Mg(BH4)2 hydrazinates using isotopic labelling indicates that hydrogen desorption is via homogeneous dissociation of N-N bond of NH2NH2 followed by the establishment of B-N bond and combination of H(δ+) (N) and H(δ-) (B).

Covalent triazine-based framework as an efficient catalyst support for ammonia decomposition

The covalent triazine-based framework (CTF), a new type of nitrogen-containing microporous polymer, was employed as a catalyst support for ammonia decomposition. Either in terms of NH3 conversion rate or turnover frequency, Ru/CTF-1 has a highly enhanced performance compared to Ru/CNTs, which rank as one of the best un-promoted catalysts reported so far. The compositional and structural information of Ru/CTF-1 and Ru/CNTs catalysts have been characterized by ICP, N2 physisorption, XRD, TEM, XPS, and NH3-TPD techniques. Ru particles on CTF-1 and CNTs are ca. 3 nm in diameter and have a similar degree of dispersion. However, the binding energy of Ru 3p electrons is ca. 0.6 eV less for Ru/CTF-1 than that for Ru/CNTs showing significant increase in electron density in the former, which is likely due to the interaction between the nitrogen-rich groups of CTF-1 and the Ru nanoparticles. Moreover, the presence of CTF-1 enhances the chemisorption of NH3, which, together with the increased electron density of Ru, may facilitate the competitive chemisorption of NH3 and recombinative desorption of adsorbed nitrogen via lowered activation energy and thus, enable faster reaction rate.

Metathesis of alkali-metal amidoborane and FeCl3 in THF

Metathesis of LiNH2BH3 and FeCl3 in THF solution was investigated in detail. Instead of formation of expected Fe amidoborane i.e., 3LiNH2BH3 + FeCl3 → 3LiCl + Fe(NH2BH3)3, 1.5 equiv. H2/LiNH2BH3 together with LiCl and a black precipitate was produced as a result of salt metathesis and reduction of Fe3+ by BH3. The hydrogen was desorbed in two steps involving a homogeneous interaction of the two starting chemicals to form [Fe(H2NBH2)3] precipitate and subsequent solid-state dissociation of [Fe(H2NBH2)3] to yield a polymeric product, [Fe(HNBH)3]n, respectively. FTIR evidenced the persistence of B–H and N–H stretches in the above two solid products and following the dissociation of [Fe(H2NBH2)3] to release 1 equiv. H2/LiNH2BH3 the B–N bond strengthened. Mossbauer and XAFS both indicated that Fe atoms in these solids are in very similar chemical environments, linking to the neighbouring N and B atoms and bearing slightly positive charge. Most likely, H2NBH2 in [Fe(H2NBH2)3] binds to Fe as a π-bound ligand. The mechanism of salt metathesis and reduction of Fe3+ was confirmed based on simulation work on the homogeneous reaction process.