Rapid release of 1.5 equivalents of hydrogen from CO2-treated ammonia borane

This work addresses the accelerated dehydrogenation of ammonia borane (AB, NH₃BH₃) in two separate processes of CO₂ pre-treatment of AB and dehydrogenation of the treated AB. Decoupling these two processes can still keep the dehydrogenation activity of CO₂-treated AB and eliminate the purification step of H₂ from gas phase. When AB is exposed to 1.38 MPa of carbon dioxide (CO₂) at 70 °C, it shows the most favorable and controllable operating condition for the CO₂ pre-treatment. The pre-treatment enhances not only the rate but also the amount of hydrogen release at the dehydrogenation step; 1.5 mol H₂ per mol of AB rapidly desorbs at 85 °C in 1 h, corresponding to 10.1 wt.% of hydrogen with regard to pristine AB. Also, our observations show that the fast dehydrogenation resulted from the CO₂ pre-treatment is preserved for more than four days of storage. The degree of dehydrogenation is further confirmed by ATR-FTIR spectroscopic and elemental analyses of the solid product. The spectra display the N–H stretching mode involving π-bonded nitrogen (sp² N) at ca. 3434 cm⁻¹,while the atom ratio of H:B is found to be 2.84:1. Based on the hydrogen release measurements, spectroscopic observations and elemental analyses, we deduce that the predominant solid product of dehydrogenation of CO₂-treated AB at 85 °C is a polymer with an empirical formula of (NBH₃)_n. It corresponds to the solid product after 1.5 equivalent hydrogen release of AB.     

Mechanistic studies of ammonia borane dehydrogenation

Ammonia borane (NH₃BH₃, AB) has received extensive attention as a potential hydrogen storage medium, however hydrogen release mechanisms from AB are not well understood. AB follows different reaction routes if the dehydrogenation occurs in solvent or solid state, but a comparative study for AB dehydrogenation in these two states is not available. In this work, a detailed study of AB dehydrogenation mechanism in diglyme and solid state is presented, and a comprehensive reaction network for both cases is proposed. The experimental and DFT results suggest that two main reaction pathways occur; one involves cyclization of monomers which results in faster dehydrogenation at lower temperature, while the other involves propagation to acyclic intermediates which requires higher temperature to carry out the cyclization step. AB dehydrogenation in solid state was experimentally found to be initiated by B–N bond cleavage and not by direct dehydrogenation, which agrees with high level CCSD(T)/MP2 calculations reported previously. It was found that diglyme plays a significant role in hindering B–N bond cleavage of AB which facilitates the cyclization pathway. In solid state, experiments including labeled AB (ND₃BH₃) mapped out the source of hydrogen (from hydridic or protonic ends), and a clear difference in the degree of dehydrogenation from the two ends is demonstrated.     

Hydrogen generation from the dehydrogenation of ammonia–borane in the presence of ruthenium(III) acetylacetonate forming a homogeneous catalyst

Starting with ruthenium(III) acetylacetonate a homogeneous catalyst is formed which catalyzes the release of 1 equivalent of hydrogen gas from the dehydrogenation of ammonia–borane in toluene solution at low temperature in the range 50–65 °C. Mercury poisoning experiments showed that the catalytic dehydrogenation of ammonia–borane starting with ruthenium(III) acetylacetonate is a homogeneous catalysis. The final product obtained after the catalytic dehydrogenation of ammonia borane was thoroughly characterized by using ¹¹B Nuclear Magnetic Resonance and Infrared spectroscopies. The homogeneous catalyst formed from the reduction of ruthenium(III) acetylacetonate provides 950 turnovers (TTO) over 58 h and 27 (mol H₂)(mol Ru)⁻¹(h)⁻¹ value of initial turnover frequency (TOF) in hydrogen generation from the dehydrogenation of ammonia–borane at 60 °C before deactivation. Kinetics of this homogenous catalytic dehydrogenation of ammonia–borane was studied depending on the catalyst concentration, substrate concentration, and temperature. The hydrogen generation was found to be first order with respect to both the substrate concentration and catalyst concentration. The activation parameters of this reaction were also determined from the evaluation of the kinetic data: activation energy; E_a = 48 ± 2 kJ mol⁻¹, the enthalpy of activation; ΔH^# = 45 ± 2 kJ mol⁻¹ and the entropy of activation ΔS^# = −152 ± 5 J mol⁻¹ K⁻¹.     

Improved dehydrogenation properties of the combined Mg(BH4)2·6NH3–nNH3BH3 system

A combined strategy via mixing Mg(BH₄)₂·6NH₃ with ammonia borane (AB) is employed to improve the dehydrogenation properties of Mg(BH₄)₂·6NH₃. The combined system shows a mutual dehydrogenation improvement in terms of dehydrogenation temperature and hydrogen purity compared to the individual components. A further improved hydrogen liberation from the Mg(BH₄)₂·6NH₃–6AB is achieved with the assistance of ZnCl₂, which plays a crucial role in stabilizing the NH₃ groups and promoting the recombination of NH^δ+?HB^δ−. Specifically, the Mg(BH₄)₂·6NH₃–6AB/ZnCl₂ (with a mole ratio of 1:0.5) composite is shown to release over 7 wt.% high-pure hydrogen (>99 mol%) at 95 °C within 10 min, thereby making the combined system a promising candidate for solid hydrogen storage.     

Strategic synthesis of graphene supported trimetallic Ag-based core–shell nanoparticles toward hydrolytic dehydrogenation of amine boranes

We reported the synthesis and characterization of two trimetallic (Ag@CoFe, and Ag@NiFe) core–shell nanoparticles (NPs), and their catalytic activity toward hydrolytic dehydrogenation of ammonia borane (AB) and methylamine borane (MeAB). The as-synthesized trimetallic core–shell NPs were obtained via a facile one-step in situ procedure using methylamine borane as a reducing agent and graphene as the support under ambient condition. The as-synthesized NPs are well dispersed on graphene, and exhibit higher catalytic activity than the catalysts with other conventional supports, such as the SiO₂, carbon black, and γ-Al₂O₃. Additionally, compared with NaBH₄ and AB, the as-synthesized Ag@CoFe/graphene NPs reduced by MeAB exhibit the highest catalytic activity, with the turnover frequency (TOF) value of 82.9 (mol H₂ min⁻¹ (mol Ag)⁻¹), and the activation energy (E_a) value of 32.79 kJ/mol. Furthermore, the as-prepared NPs exert good durable and magnetically recyclability for the hydrolytic dehydrogenation of AB and MeAB. Moreover, this simple strategic synthesis method can be easily extended to the facile preparation of other graphene supported multi-metal core–shell NPs.     

Hydrolytic dehydrogenation of ammonia borane and methylamine borane catalyzed by graphene supported Ru@Ni core–shell nanoparticles

Ru@Ni core–shell nanoparticles (NPs) supported on graphene have been synthesized by one-step in situ co-reduction of aqueous solution of ruthenium (III) chloride, nickel (II) chloride, and graphene oxide (GO) with ammonia borane (AB) as the reducing agent under ambient condition. The as-synthesized NPs exhibit much higher catalytic activity for hydrolytic dehydrogenation of AB than the monometallic, bimetallic alloy (RuNi/graphene), and graphene-free core–shell (Ru@Ni) counterparts. Additionally, the Ru@Ni/graphene NPs facilitate the hydrolysis of AB, with the turnover frequency (TOF) value of 340 mol H₂ min⁻¹ (mol Ru)⁻¹, which is among the highest value reported on Ru-based NPs so far, and even higher than the reversed Ni@Ru NPs. Furthermore, the as-prepared NPs exert satisfied durable stability and magnetically recyclability for the hydrolytic dehydrogenation of AB and methylamine borane (MeAB). Moreover, this simple synthetic method can be extended to other Ru-based bimetallic core–shell systems for more applications.     

Efficient and highly rapid hydrogen release from ball-milled 3NH3BH3/MMgH3 (M = Na,K, Rb) mixtures at low temperatures

The stoichiometric reactions of ammonia borane (NH₃BH₃, AB) and selected alkali or alkaline-earth metal hydrides produce metal amidoboranes, which possess dehydrogenation property advantages over their parent AB. However, the losses of hydrogen capacity and chemical energy in the preparation process make metal amidoboranes less energy-effective for hydrogen storage application. In the present study, by combining the M⁺–Mg²⁺ double cations remarkably lowers the reactivity of the alkali metal hydrides toward AB. As a result, the starting Mg-based ternary hydrides MMgH₃ (M = Na, K, Rb) and AB phases are largely stable in the mechanical milling process, but transform to the corresponding mixed-cation amidoboranes in the subsequent heating process. Importantly, when the post-milled 3AB/MMgH₃ mixtures are isothermally heated at above 60 °C using water bath, the formation and decomposition processes of the mixed-cation amidoboranes can be favorably combined, giving rise to rapid and efficient dehydrogenation performances at the mild temperatures (60–80 °C). The results acquired may provide a generalized reactions coupling strategy for designing and synthesis other potentially efficient hydrogen storage system.     

Monodisperse nickel–palladium alloy nanoparticles supported on reduced graphene oxide as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane

Addressed herein is the catalysis of reduced graphene oxide-supported monodisperse NiPd alloy nanoparticles (NPs) (rGO-NiPd) in the hydrolytic dehydrogenation of ammonia borane (AB). This is the first example of the use of NiPd alloy NPs as catalyst in the hydrolytic dehydrogenation of AB. Monodisperse NiPd alloy NPs (3.5 nm) were synthesized by co-reduction of nickel(II) acetate and palladium(II) acetylacetonate in oleylamine (OAm) and borane-tert-butylamine complex (BTB) at 100 °C. The current recipe allowed to control the composition of NiPd alloy NPs and to study the composition-controlled catalysis of rGO-NiPd in the hydrolytic dehydrogenation of AB. Among the all compositions tested, the Ni₃₀Pd₇₀ was the most active one with the turnover frequency of 28.7 min⁻¹. The rGO-Ni₃₀Pd₇₀ were also durable catalysts in the hydrolytic dehydrogenation of AB providing 3650 total turnovers in 35 h and reused at six times without deactivation. The detailed reaction kinetics of hydrolytic dehydrogenation of AB revealed that the reaction proceeds first order with respect to the NiPd concentration and zeroth order with respect to the AB concentration. The apparent activation energy of the catalytic dehydrogenation of AB was also calculated to be E_a^app = 45 ± 2 kJ*mol⁻¹.     

Carbon-supported Ni3B nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report the preparation of Ni₃B and carbon-supported Ni₃B (denoted as Ni₃B/C) nanoparticles, and their catalytic performance for hydrogen generation from hydrolytic dehydrogenation of ammonia borane (NH₃BH₃, AB). Ni₃B and Ni₃B/C were prepared via a chemical reduction and crystallization in tetraethylene glycol solution. The obtained Ni₃B catalysts are in well-defined crystalline state and Ni₃B/C catalysts have a high dispersion in the carbon. The hydrogen generation measurement shows that the carbon-supported Ni₃B presents enhanced catalyst activity during hydrolytic dehydrogenation of AB. Among the as-prepared Ni₃B/C catalysts, Ni₃B/C with 34.25 wt% Ni₃B loading displays the best catalytic activity, delivering a high hydrogen release rate of 1168 mL min⁻¹ g⁻¹ and the lower activation energy of 46.27 kJ mol⁻¹. The kinetic results show that the hydrolysis is a first-order reaction in catalyst concentration, while it is a zero-order in AB concentration. Furthermore, the Ni₃B/C is a recyclable catalyst under mild reaction conditions, indicating that the carbon-supported Ni₃B is a promising catalyst for AB hydrolytic dehydrogenation.     

Synthesis of Cu@FeCo core–shell nanoparticles for the catalytic hydrolysis of ammonia borane

Non-noble Cu@FeCo core–shell nanoparticles (NPs) containing Cu cores and FeCo shells have been successfully in situ synthesized via a facile chemical reduction method. The NPs exerted composition-dependent activities towards the catalytic hydrolysis of ammonia borane (NH₃BH₃, AB). Among them, the Cu_0.3@Fe_0.1Co_0.6 NPs showed the best catalytic activity, with which the max hydrogen generation rate of AB can reach to 6674.2 mL min⁻¹ g⁻¹ at 298 K. Kinetic studies demonstrated that the hydrolysis of AB catalysed by Cu_0.3@Fe_0.1Co_0.6 NPs was the first order with respect to the catalyst concentration. The activation energy (Ea) was calculated to be 38.75 kJ mol⁻¹. Furthermore, the TOF value (mol of H₂. (mol of catalyst. min)⁻¹) of Cu_0.3@Fe_0.1Co_0.6 NPs was 10.5, which was one of the best catalysts in the previous reports. The enhanced catalytic activity was largely attributed to the preferable synergistic effect of Cu, Fe and Co in the special core–shell structured NPs.     

Combination of two H-enriched B–N based hydrides towards improved dehydrogenation properties

A new hydrogen storage system NaZn(BH₄)₃?2NH₃-nNH₃BH₃ (n = 1–5) was synthesized via a simple ball milling of NaZn(BH₄)₃?2NH₃ and NH₃BH₃ (AB) with a molar ratio from 1 to 5. Dehydrogenation results revealed that NaZn(BH₄)₃?2NH₃-nAB (n = 1–5) showed a mutual dehydrogenation improvement in terms of significant decrease in the dehydrogenation temperature and preferable suppression of the simultaneous evolution of by-products (i.e. NH₃, B₂H₆ and borazine) compared to the unitary compounds (NaZn(BH₄)₃?2NH₃ and AB). Specially, the NaZn(BH₄)₃?2NH₃-4AB sample is shown to reach the maximum hydrogen purity (99.1 mol %) and favorable dehydrogenation properties rapidly releasing 11.6 wt. % of hydrogen with a peak maximum temperature of 85 °C upon heating to 250 °C. Isothermal dehydrogenation results revealed that 9.6 wt. % hydrogen was liberated from NaZn(BH₄)₃?2NH₃-4AB within 80 min at 90 °C. High-resolution in-situ XRD and Fourier transform infrared (FT-IR) measurements indicated that the significant improvements on the dehydrogenation properties in NaZn(BH₄)₃?2NH₃-4AB can be attributed to the interaction between the NH₃ group from NaZn(BH₄)₃?2NH₃ and AB in the mixture, resulting a more activated H^δ+···^−δH combination. The research on the reversibility of the spent fuels of NaZn(BH₄)₃?2NH₃-4AB showed that regeneration could be partly achieved by reacting them with hydrazine in liquid ammonia. These aforementioned favorable dehydrogenation properties demonstrate the potential of the combined systems to be used as solid hydrogen storage material.     

Enhanced dehydrogenation of ammonia borane by reaction with alkaline earth metal chlorides

The study of ammonia borane (AB) with controllable dehydrogenations is an active research topic for solid-state hydrogen storage materials. The present work shows that tuning the reactivity of both B–H and N–H bonds in AB by alkaline earth metal chlorides not only results in a significantly decrease in the onset dehydrogenation temperature to 40 °C but also suppresses undesirable volatile by-products due to the incorporation of alkaline earth metal chlorides in the AB dehydrogenation process. These results provide further insights into the promotion of hydrogen release from amidoboranes and related borohydride ammine complexes.     

Palladium nanoparticles supported on chemically derived graphene: An efficient and reusable catalyst for the dehydrogenation of ammonia borane

Chemically derived graphene (CDG) was prepared by hydrazine hydrate reduction of graphene oxide and used as support for palladium nanoparticles (Pd NPs) generated ex situ with controllable particle size and dispersion. The Pd NPs supported on CDG were well characterized by using a combination of advance analytical techniques and employed as catalyst in the dehydrogenation and hydrolysis of ammonia borane (AB) in organic solvents and aqueous solutions, respectively. Monodisperse Pd NPs of 4.5 nm were prepared from the reduction of palladium(II) acetylacetonate by tert-butylamine borane in the presence of oleylamine. They were readily impregnated on CDG which has BET surface area of 500 m² g⁻¹. Pd NPs retain their particle size dispersion and stability when supported on chemically derived graphene. The resulting materials are highly active and stable catalyst for the dehydrogenation and hydrolysis of AB. In addition to their high activity and stability, these Pd NPs are also reusable catalyst in both dehydrogenation and hydrolysis of AB preserving 85% and 95% of initial activity after 5th and 10th runs, respectively.     

Synergistic catalysis of MCM-41 immobilized Cu–Ni nanoparticles in hydrolytic dehydrogeneration of ammonia borane

Bimetallic Cu–Ni nanoparticles (NPs) were successfully immobilized in MCM-41 using a simple liquid impregnation-reduction method. All the resulting composites Cu–Ni/MCM-41 catalysts with various contents of Cu–Ni, and in particular Cu_0.2Ni_0.8/MCM-41 sample, outperform the activity of monometallic Cu and Ni counterparts and pure bimetallic Cu_0.2Ni_0.8 NPs in hydrolytic dehydrogeneration of ammonia borane (AB) at room temperature. The Cu_0.2Ni_0.8/MCM-41 catalyst exhibits excellent catalytic activity with a total turnover frequency (TOF) value of 10.7 mol H₂ mol catalyst⁻¹ min⁻¹ and a low activation energy value of 38 kJ mol⁻¹ at room temperature. In addition, Cu_0.2Co_0.8/MCM-41 also exhibits excellent activity with a TOF value as high as 15.0 mol H₂ mol catalyst⁻¹ min⁻¹. This obtained activity represents the highest catalytic active of Cu-based monometallic and bimetallic catalysts up to now toward the hydrolytic dehydrogeneration of ammonia borane (AB). The unprecedented excellent activity has been successfully achieved thanks to the strong bimetallic synergistic effects among the Cu–Ni (or Co) NPs of the composites.     

Improved low-temperature hydrogen generation from NH3BH3 and TiO2 composites pretreated with water

Ammonia borane (AB, NH₃BH₃) is nontoxic easily transportable solid hydride with high stability in air. In this work we demonstrate that simple mixing of AB with TiO₂ (anatase) allows for hydrogen gas to be generated at temperatures as low as 80 °C. No losses of hydrogen have been observed during preparation of hydride-containing composites. It was shown that the adsorption of water vapor on TiO₂ and the increase of TiO₂ loading considerably accelerated the rate of AB decomposition. The experimentally observed formation of B–O chemical bonds and the elevated heat emission suggest strong interaction of AB with the adsorbed water species on TiO₂ surface. It has been found that this interaction proceeds at a higher rate comparing with binary AB/H₂O systems. The heat being released in the process is thought to contribute to overcoming the activation barrier in the dehydrogenation of ammonia borane to produce hydrogen gas.     

Destabilization of LiBH4 by nanoconfinement in PMMA–co–BM polymer matrix for reversible hydrogen storage

Destabilization of LiBH₄ by nanoconfinement in poly (methyl methacrylate)–co–butyl methacrylate (PMMA–co–BM), denoted as nano LiBH₄–PMMA–co–BM, is proposed for reversible hydrogen storage. The onset dehydrogenation temperature of nano LiBH₄–PMMA–co–BM is reduced to ∼80 °C (ΔT = 340 and 170 °C as compared with milled LiBH₄ and nanoconfined LiBH₄ in carbon aerogel, respectively). At 120 °C under vacuum, nano LiBH₄–PMMA–co–BM releases 8.8 wt.% H₂ with respect to LiBH₄ content within 4 h during the 1st dehydrogenation, while milled LiBH₄ performs no dehydrogenation at the same temperature and pressure condition. Moreover, nano LiBH₄–PMMA–co–BM can be rehydrogenated at the mildest condition (140 °C under 50 bar H₂ for 12 h) among other modified LiBH₄ reported in the previous literature. Due to the hydrophobicity of PMMA–co–BM host, deterioration of LiBH₄ by oxygen and humidity in ambient condition is avoided after nanoconfinement. Although the interaction between LiBH₄ and the pendant group of PMMA–co–BM leads to a reduced hydrogen storage capacity, significant destabilization of LiBH₄ is accomplished.     

Effect of boric acid on thermal dehydrogenation of ammonia borane: Mechanistic studies

Ammonia borane (AB, NH₃BH₃) is a promising hydrogen storage material for use in proton exchange membrane (PEM) fuel cell applications. In this study, the effect of boric acid on AB dehydrogenation was investigated. Our study shows that boric acid is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for AB thermolysis. With heating, boric acid forms tetrahydroxyborate ion along with some water released from boric acid itself. It is believed that this ion serves as Lewis acid which catalyzes AB dehydrogenation. Using boric acid, we obtained high H₂ yield (11.5 wt% overall H₂ yield, 2.23 H₂ equivalent) at 85 °C, PEM fuel cell operating temperatures, along with rapid kinetics. In addition, only trace amount of NH₃ (20–30 ppm) was detected in the gaseous product. The spent AB solid product was found to be polyborazylene-like species. The results suggest that the addition of boric acid to AB is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.     

Dehydrogenation characteristics of ammonia borane via boron-based catalysts (Co–B,Ni–B,Cu–B) under different hydrolysis conditions

In the present study, dehydrogenation characteristics of ammonia borane (NH₃BH₃) catalyzed via boron-based catalysts under different hydrolysis conditions were investigated. A series of boron-based catalysts (Co_1−x–B_x, Ni_1−x–B_x, and Cu_1−x–B_x, x: 0.25, 0.50, 0.75) were prepared by sol–gel method. Gels were calcinated at different temperatures (250 °C, 350 °C, and 450 °C) in order to obtain the boron-based catalysts. XRD characterizations revealed that Co–B, Ni–B, and Cu–B crystalline structures were formed during calcination at 450 °C. Hydrogen generation measurements were performed in order to determine the optimum composition of the boron-based catalyst. The maximum hydrogen generation rates were 7607 ml min⁻¹ gcat⁻¹, 3869 ml min⁻¹ gcat⁻¹ and 1178 ml min⁻¹ gcat⁻¹ for Co_0.75B_0.25, Ni_0.75B_0.25 and Cu_0.75B_0.25, respectively. Furthermore, the hydrolysis of NH₃BH₃ was performed at 20 °C, 40 °C, 60 °C and 80 °C under magnetic stirring (750 rpm), ultrasonic irradiation and non-stirring in order to determine how these parameters effect hydrolysis. Activation energies (E_a) were calculated by evaluation of the kinetic data. Under ultrasonic irradiation, the E_a in the presence of Co_0.75B_0.25, Ni_0.75B_0.25 and Cu_0.75B_0.25 were 40.85 kJ mol⁻¹, 43.19 kJ mol⁻¹ and 48.74 kJ mol⁻¹, respectively, which compares favorably with results reported in the literature. Thus, the catalytic activities of the boron-based catalysts were found to be Cu < Ni < Co and the best reaction condition for the catalytic hydrolysis of NH₃BH₃ was determined to be non-stirring < magnetic stirring < ultrasonic irradiation.     

Nanoporous Ni-based catalysts for hydrogen generation from hydrolysis of ammonia borane

We report nanoporous Ni, Ni–Fe, and Ni–Pt as catalysts for hydrogen generation from hydrolytic dehydrogenation of ammonia borane (NH₃BH₃, AB). The Ni and Ni–Fe nanoparticles with diameters of 20–25 nm were synthesized by a colloidal method in starch-containing aqueous solution. They exhibited considerable in situ catalytic performance but severely lost activity after separating from the reaction solution. Nanoporous Ni_1−xPt_x (x = 0.01, 0.08 and 0.19) with particle size below 5 nm was prepared from the isolated Ni nanoparticles through a replacement reaction. After centrifugation, drying, washing, and annealing, the obtained nanoporous Ni–Pt could attain remarkable activity, high hydrogen generation rate and efficiency, and low activation energy.     

One-step synthesis of graphene supported Ru nanoparticles as efficient catalysts for hydrolytic dehydrogenation of ammonia borane

Ru nanoparticles supported on graphene have been synthesized via a one-step procedure using methylamine borane as reducing agent. Compared with NaBH₄ and ammonia borane, the as-prepared Ru/graphene NPs reduced by methylamine borane exhibit superior catalytic activity towards the hydrolytic dehydrogenation of ammonia borane. Additionally, the Ru/graphene NPs exhibit higher catalytic activity than its graphene free counterparts, and retain 72% of their initial catalytic activity after 4 reaction cycles. A kinetic study shows that the catalytic hydrolysis of ammonia borane is first order with respect to Ru concentration, the turnover frequency is 100 mol H₂ min⁻¹ (mol Ru)⁻¹. The activation energy for the hydrolysis of ammonia borane in the presence of Ru/graphene NPs has been measured to be 11.7 kJ/mol, which is the lowest value ever reported for the catalytic hydrolytic dehydrogenation of ammonia borane.