Effect of zeolites on thermal decomposition of ammonia borane

Chemical hydrides due to their light weight and high storage capacity are considered to be promising hydrogen storage materials for both mobile and stationary applications. Ammonia Borane (AB) is a novel material with very high hydrogen content (19.6 wt %) per mass. The decomposition of AB takes place in three steps at desorption temperatures of about 100 °C, 140 °C and above 1000 °C respectively releasing 1 mol of hydrogen in each step. The major obstacle towards the use of AB as a hydrogen store is its irreversibility and slow kinetics. With the additives the decomposition temperature could be reduced and the kinetics can be improved. Effect of addition of Zeolites on decomposition of Ammonia Borane at different temperatures is reported in this paper. It was observed that the kinetics behaviour is greatly affected by addition of Zeolites with considerable reduction in the induction or warm-up period.     

Catalytic dehydrogenation of ammonia borane in non-aqueous medium

Dehydrogenation of Ammonia Borane (NH₃BH₃, AB) catalyzed by transition metal heterogeneous catalysts was carried out in non-aqueous solution at temperatures below the standard polymer electrolyte membrane (PEM) fuel cell operating conditions. The introduction of a catalytic amount (∼2 mol%) of platinum to a solution of AB in 2-methoxyethyl ether (0.02–0.33 M) resulted in a rapid evolution of H₂ gas at room temperature. At 70 °C, the rate of platinum catalyzed hydrogen release from AB was the dehydrogenation rate which was 0.04 g s⁻¹ H₂ kW⁻¹.     

Chemical kinetics of Ru-catalyzed ammonia borane hydrolysis

Ammonia borane (AB) is a candidate material for on-board hydrogen storage, and hydrolysis is one of the potential processes by which the hydrogen may be released. This paper presents hydrogen generation measurements from the hydrolysis of dilute AB aqueous solutions catalyzed by ruthenium supported on carbon. Reaction kinetics necessary for the design of hydrolysis reactors were derived from the measurements. The hydrolysis had reaction orders greater than zero but less than unity in the temperature range from 16 °C to 55 °C. A Langmuir–Hinshelwood kinetic model was adopted to interpret the data with parameters determined by a non-linear conjugate-gradient minimization algorithm. The ruthenium-catalyzed AB hydrolysis was found to have activation energy of 76 ± 0.1 kJ mol⁻¹ and adsorption energy of −42.3 ± 0.33 kJ mol⁻¹. The observed hydrogen release rates were 843 ml H₂ min⁻¹ (g catalyst)⁻¹ and 8327 ml H₂ min⁻¹ (g catalyst)⁻¹ at 25 °C and 55 °C, respectively. The hydrogen release from AB catalyzed by ruthenium supported on carbon is significantly faster than that catalyzed by cobalt supported on alumina. Finally, the kinetic rate of hydrogen release by AB hydrolysis is much faster than that of hydrogen release by base-stabilized sodium borohydride hydrolysis.     

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.     

Room temperature hydrolytic dehydrogenation of ammonia borane catalyzed by Co nanoparticles

Amorphous and well dispersed Co nanoparticles (less than 10 nm) have been in situ synthesized in aqueous solution at room temperature. The as-synthesized Co nanoparticles possess high catalytic activity (1116 L mol⁻¹ min⁻¹) and excellent recycling property for the hydrogen generation from aqueous solution of ammonia borane under ambient atmosphere at room temperature. The present low-cost catalyst, high hydrogen generation rate and mild reaction conditions (at room temperature in aqueous solution) represent a promising step toward the development of ammonia borane as a viable on-board hydrogen-storage and supply material.     

Mo remarkably enhances catalytic activity of Cu@MoCo core-shell nanoparticles for hydrolytic dehydrogenation of ammonia borane

Ammonia borane (AB) has been identified as one of the most promising candidates for chemical hydrogen storage. However, the practical application of AB for hydrogen production is hindered by the need of efficient and inexpensive catalysts. For the first time, we report that the incorporation of Mo into Cu@Co core-shell structure can significantly improve the catalytic efficiency of hydrogen generation from the hydrolysis of AB. The Cu_0.81@Mo_0.09Co_0.10 core-shell catalyst displays high catalytic activity towards the hydrolysis dehydrogenation of AB with a turnover frequency (TOF) value of 49.6 molH₂ mol_cat^?1 min^?1, which is higher than most of Cu-based catalysts ever reported, and even comparable to those of noble-metal based catalysts. The excellent catalytic performance is attributed to the multi-elements co-deposition effect and electrons transfer effect of Cu, Mo and Co in the tri-metallic core-shell NPs.     

Ammonia borane decomposition in the presence of cobalt halides

In the present work we studied the effect of cobalt halides (CoF₂, CoCl₂, CoBr₂ and CoI₂; also denoted CoX₂) on the thermal decomposition of ammonia borane NH₃BH₃ (AB) over the range 25-220 °C. The reaction was followed by thermogravimetric analysis and differential scanning calorimetry. The gaseous products analyzed by gas chromatography and mass spectrometry, and the solid by-products identified by elemental analysis: powder X-ray diffraction, infrared and X-ray photoelectron spectroscopies. Compared to pristine AB, the presence of CoCl₂ and CoBr₂ reduces both the induction period of the AB decomposition and the content of unwanted borazine in the H₂ stream, whereas the presence of CoI₂ or CoF₂ has little or no effect, respectively. We propose that the positive effect of CoX₂ comes from their electronic and steric effects, and that CoCl₂ is the compound which shows the best properties relative to these effects. Herein, the roles of Co and X are discussed and a revised mechanism of the AB dehydrocoupling initiation is proposed.     

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.     

Ruthenium nanoparticles immobilized in montmorillonite used as catalyst for methanolysis of ammonia borane

Ammonia borane (AB) is an intriguing molecular crystal material with extremely high hydrogen density. In the present study, we prepared ruthenium (Ru) nanoparticles immobilized in montmorillonite (MMT) and examine its catalytic effect on the methanolysis reaction of AB. The Ru/MMT catalyst was prepared by cation-exchange method followed by hydrogen reduction at elevated temperatures. Property examinations found that the Ru/MMT catalyst was highly effective and robust for promoting the methanolysis reaction of AB. For example, the methanolysis system employing Ru/MMT catalyst exhibited an average hydrogen generation rate of 29 L min⁻¹ g⁻¹ (Ru). The catalyst at its twentieth usage retained 95% of its initial activity and ensured 100% conversion of AB. Kinetics studies found that the methanolysis reaction of AB employing Ru/MMT catalyst follows first-order kinetics with respect to AB concentration and catalyst amount, respectively.     

Hydrolysis of solid ammonia borane

Ammonia borane NH₃BH₃ is a promising hydrogen storage material by virtue of a theoretical gravimetric hydrogen storage capacity (GHSC) of 19.5 wt%. However, stored hydrogen has to be effectively released, one way of recovering this hydrogen being the metal-catalyzed hydrolysis. The present study focuses on CoCl₂-catalyzed hydrolysis of NH₃BH₃ with the concern of improving the effective GHSC of the system NH₃BH₃-H₂O. For that, NH₃BH₃ is stored as a solid and H₂O is provided in stoichiometric amount. By this way, an effective GHSC of 7.8 wt% has been reached at 25 °C. To our knowledge, it is the highest value ever reported. Besides, one of the highest hydrogen generation rates (HGRs, 21 ml(H₂) min⁻¹) has been found. In parallel, the increases of the water amount and temperature have been studied and the reaction kinetics has been determined. Finally, it has been observed that some NH₃ release, what is detrimental for a fuel cell. To summarize, high performances in terms of GHSCs and HGRs can be reached with NH₃BH₃ and since research devoted to this boron hydride is at the beginning we may be confident in making it viable in a near future.     

Effect of maleic acid on onset temperature and H2 release kinetics for thermal dehydrogenation of ammonia borane

Ammonia borane (AB, NH₃BH₃) has received great attention as an attractive hydrogen storage candidate because it has high hydrogen contents and releases hydrogen under mild operating conditions. Despite the favorable properties, AB thermolysis has several drawbacks such as long induction period, slow kinetics, and relatively high onset temperature, compared to hydrolysis approach. In this study, hydrogen release properties from AB were investigated in the addition of maleic acid (C₄H₄O₄, MA). Using thermogravimetric analysis, temperature programmed reaction with mass spectrometry, and FTIR analyses, the solid and gaseous products generated by thermolysis of the AB-MA mixture were characterized to understand the reaction mechanism. It was found that with the addition of MA, hydrogen yield and release kinetics were enhanced, while the onset temperature reduced significantly to ~60 °C. It is likely that the hydrolysis between O–H bonds in MA and B–H bonds in AB was initiated, and the heat released from the hydrolysis triggers the thermolysis of AB. It was also confirmed that a combination of the two additives (MA and boric acid) enables a further increase of H₂ yield while the onset temperature remains at ~60 °C. Our results suggest that MA is a promising additive to improve AB dehydrogenation.     

Methanolysis of ammonia borane catalyzed by magnetically isolable RHODIUM(0) nanoparticles

This article reports the preparation and employment of rhodium (0) nanoparticles (Rh⁰NPs) on the surface of magnetite nanospheres, denoted as Rh⁰@Fe₃O₄, as magnetically isolable nanocatalyst in the methanolysis of ammonia borane (MAB). The monodispersed Fe₃O₄ nanospheres are fabricated by a simple technique and used as nanosupport for Rh⁰NPs which are well stabilized and homogeneously distributed on the surface of nanospheres with a mean particle size of 2.8 ± 0.5 nm. The as-synthesized Rh⁰@Fe₃O₄ has a remarkable TOF value of 184 min⁻¹ in the MAB to produce H₂ gas in RT. Most of all, Rh⁰@Fe₃O₄ nanocatalyst can be reused, evolving 3.0 mol of H₂ gas for a mole of AB, keeping 100% of its initial activity even in the fourth reuse of MAB at 25 °C. Recovery of the Rh⁰@Fe₃O₄ nanocatalyst can be accomplished by simply approaching an external magnet, which eliminates many laborious catalyst removal steps in catalytic reactions. Reported are the outcomes of kinetic investigation, done by altering the concentration of substrate and catalyst together with temperature. Kinetic studies reveal that the catalytic MAB shows dependence on the concentration of reactants and temperature.     

Pd nanoparticles supported on MIL-101 as high-performance catalysts for catalytic hydrolysis of ammonia borane

Well dispersed ultrafine Pd NPs have been immobilized in the framework of MIL-101, and tested for the catalytic hydrolysis of ammonia borane. The powder XRD, N₂ adsorption–desorption, TEM, and ICP-AES were employed to characterize the Pd@MIL-101 catalyst. The as-synthesized Pd@MIL-101 exhibit the highest catalytic activity toward hydrolysis of AB among the Pd-based nano-catalysts ever reported, with the TOF value of 45 mol H₂ min⁻¹ (mol Pd)⁻¹.     

Catalytic hydrolysis of ammonia borane via magnetically recyclable copper iron nanoparticles for chemical hydrogen storage

In this work, a series of new Cu_1−xFe_x alloy nanoparticles (NPs) have been successfully in situ synthesized by a very simple method and used as catalysts for hydrogen generation from the aqueous solution of ammonia borane (AB) under ambient atmosphere at room temperature. The prepared nanoalloys exhibit excellent catalytic activity, especially for Cu_0.33Fe_0.67 sample outperform the activity of monometallic counterparts, and even of Cu@Fe core–shell NPs. By using an external magnet, these catalysts can be readily separated from the solution for recycle purpose, and can keep the high activity even after 8 times of recycle under ambient atmosphere. The hydrolysis activation energy for the Cu_0.33Fe_0.67 alloy NPs was measured to be approximately 43.2 kJ/mol, which is lower than most of the reported activation energy values for the same reaction using many different catalysts except for some noble-metal containing catalysts, indicating the superior catalytic performance of Cu_0.33Fe_0.67 nanocatalysts.     

Zn-MOF assisted dehydrogenation of ammonia borane: Enhanced kinetics and clean hydrogen generation

We report controllable and enhanced hydrogen release kinetics at reduced temperatures in ammonia borane (AB) catalyzed by Zn-MOF-74. AB is loaded into the unsaturated Zn-metal coordinated one-dimensional hexagonal open nanopores of MOF-74 (ABMOF) via solution infiltration. The ABMOF system provides clean hydrogen by suppressing the release of detrimental volatile byproducts such as ammonia, borazine and diborane. These byproducts prevent the direct use of AB as a hydrogen source for polymer electrolyte membrane fuel cell applications. The H₂ release temperature, kinetics, and byproduct generation are dependent on the amount of AB loading. We show that nanoconfinement of AB and its interaction with the active Zn-metal centers in MOF are important in promoting efficient and clean hydrogen generation.     

Amino-group and space-confinement assisted synthesis of small and well-defined Rh nanoparticles as efficient catalysts toward ammonia borane hydrolysis

Hydrogen evolution from ammonia borane (AB) hydrolysis is of great importance considering the ever-increasing demand for green and sustainable energy. However, the development of a facile and efficient strategy to construct high-performance catalysts remains a grand challenge. Herein, we report an amino-group and space-confinement assisted strategy to fabricate Rh nanoparticles (NPs) using amino-functionalized metal-organic-frameworks (UiO-66-NH₂) as a NP matrix (Rh/UiO-66-NH₂). Owing to the coordination effect of amino-group and space-confinement of UiO-66-NH₂, small and well-distributed Rh NPs with a diameter of 3.38 nm are successfully achieved, which can be served as efficient catalysts for AB hydrolysis at room temperature. The maximum turnover frequency of 876.7 min^?1 is obtained by using the Rh/UiO-66-NH₂ with an optimal Rh loading of 4.38 wt% and AB concentration of 0.2 M at 25 °C, outperforming most of the previously developed Rh-based catalysts. The catalyst is also stable in repetitive cycles for five times. The high performance of this catalyst must be ascribed to the structural properties of UiO-66-NH₂, which enable the formation of small and well-dispersed Rh NPs with abundant accessible active sites. This study provides a simple and efficient method to significantly enhance the catalytic performance of Rh for AB hydrolysis.     

Noncatalytic hydrothermolysis of ammonia borane

Hydrolysis of ammonia borane (AB) is attractive as a chemical method for hydrogen storage. The use of catalysts is, however, usually required. In the present paper, two new methods for releasing hydrogen from AB and water are investigated which do not involve any catalyst. One method is based on combustion of AB mixtures with nanoscale aluminum powder and gelled water. It is shown experimentally that these mixtures, upon ignition, exhibit self-sustained combustion with hydrogen release from both AB and water. The other method involves external heating of aqueous AB solutions to temperatures 120 °C or higher, under argon pressure to avoid water boiling. To clarify the reaction mechanism, isotopic experiments using D₂O instead of H₂O were conducted. It is shown that heating AB/D₂O solution to temperatures 117–170 °C releases 3 equiv. of hydrogen per mole AB, where 2–2.1 equiv. originate from AB and 0.9–1 equiv. from water. The prospects of both methods for hydrogen storage are discussed.     

Improvement of reaction kinetics by metal chloride on ammonia and lithium hydride system

Ammonia NH₃ and lithium hydride LiH system releases hydrogen even at room temperature to form lithium amide LiNH₂. LiNH₂ is recycled back to NH₃ and LiH below 300 °C under hydrogen H₂ flow condition. However, the reaction rate of the system is slow for a practical application. In this work, various kinds of transition metal chlorides were examined as a potential catalyst to improve the kinetics. For hydrogen desorption reaction, the reaction kinetics of titanium chloride TiCl₃ dispersing LiH was about 8 times faster than the raw LiH, suggesting that TiCl₃ possessed an excellent catalytic effect. In the case of the regeneration reaction, the reaction kinetics was also improved by the addition of TiCl₃. It was mainly caused by physical effects in contrast to the hydrogen desorption process, in other words, the small crystallite and/or particle were formed by the milling with the additive.     

Magnetically recyclable Ni@h-BN composites for efficient hydrolysis of ammonia borane

The magnetic Ni@h-BN composites containing the uniform Ni nanoparticles supported on h-BN nanosheets have been prepared via a facile solvothermal method. The as-prepared samples show high catalytic performance for H₂ generation from the ammonia borane aqueous solution, especially for the Ni@h-BN with 25.0 wt% Ni content. Moreover, the Ni@h-BN composites possess a good ferromagnetic property at room temperature, endowing them with rapid magnetic separation to recycle. The kinetics of the hydrolysis of ammonia borane over the Ni@h-BN composites were further investigated in detail. It is found that the hydrogen generation was highly dependent on the catalyst amount and the reaction temperature. The activation energy of the hydrolysis reaction of ammonia borane is found to be 47.3 kJ mol^?1 over the Ni@h-BN with 25.0 wt% Ni content. Considering the good catalytic activities for H₂ release, the Ni@h-BN composites are expected to find important application in fuel cells and the related fields.