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.     

A study of catalytic hydrolysis of concentrated ammonia borane solutions

Ruthenium catalyzed ammonia borane (AB) hydrolysis using aqueous solutions in the range of 5–25 wt% were experimentally studied. The experimental apparatus also included a means for gaseous ammonia sequestration. In addition, the effects of aging on reaction rates and total H₂ conversion were investigated using AB solutions that were stored in a limited-air environment for a maximum of 67 days. The present work provides data on total H₂ conversion, chemical kinetics, solution density, pH value, byproduct solubility, ammonia generation, and long-term storage stability of concentrated aqueous AB solutions.     

Hydrogen production from hydrolysis of ammonia borane with limited water supply

Effect of limited water supply to hydrolysis of ammonia borane for hydrogen evolution is studied over the cases in which the initial molar ratio of water to ammonia borane (H₂O/AB) is set at 1.28, 2.57 and 4.50. The conversion efficiency of ammonia borane to hydrogen is estimated from the accumulated volume of produced hydrogen gas and the quantitative analysis of hydrolysate by solid-state ¹¹B NMR. Characteristics of hydrogen evolution are significantly influenced by both water dosage and injection rate of water. In the case that water is a limiting agent, namely, H₂O/AB = 1.28, less hydrogen is produced than that predicted stoichiometrically. In contrast, conversion efficiency of ammonia borane reaches nearly 100% for the case with H₂O/AB = 4.50. Injection rate of water to ammonia borane also affect profoundly the produced volume and production rate of hydrogen, if water is used as a limiting agent in the hydrolysis of ammonia borane. Nonetheless, boric acid and metaboric acid are found to be the dominant products in the hydrolysate from XRD, FT-IR and solid-state ¹¹B NMR analysis. The hydrogen storage capacity using limited water supply in this work could reach as high as about 5.33 wt%, based on combined mass of reactants and catalyst.     

Low-cost CuFeCo@MIL-101 as an efficient catalyst for catalytic hydrolysis of ammonia borane

Trimetallic nanoparticles of non-noble Cu–Fe–Co with different molar ratios were successfully immobilized in the metal-organic frameworks (MIL-101) via an easy impregnation–reduction process. XRD, TEM, XPS, ICP-MS and BET methods were used to characterize the catalyst. Comparing to their bimetallic counterparts, Cu₆Fe_0.8Co_3.2@MIL-101 demonstrates the best catalytic performance for dehydrogenation of ammonia borane by hydrolysis at 298 K Cu₆Fe_0.8Co_3.2@MIL-101 shows a total turnover frequency (TOF) value of 23.2 mol_H2 mol_catalyst⁻¹ min⁻¹ and an activation energy value of 37.1 kJ mol⁻¹. The enhancement of catalytic activity was attributed to a synergistic effect among copper, cobalt and iron nanoparticles supported on MIL-101. In addition, the catalyst still exhibits good stability and magnetic recyclability after seven cycles. The low-cost catalyst has good prospect for application in the field of hydrogen storage.     

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.     

Hydrogen generated from hydrolysis of ammonia borane using cobalt and ruthenium based catalysts

Ammonia borane (NH₃BH₃, AB), one kind of promising hydrogen storage materials, is hydrolyzed to produce hydrogen in presence of HCl, Co/IR-120 and Ru/IR-120 catalysts. The kinetics analysis of the AB hydrolysis shows that hydrogen production is of the first-order reaction in regard to both concentrations of ammonia borane and catalysts initially present, respectively. The hydrolyzate of ammonia borane after hydrogen evolution is also characterized with XRD, FT-IR and ¹¹B NMR. Boric acid (H₃BO₃) is found to be the dominant product in the hydrolyzate. Besides, the produced gas is discovered to contain both hydrogen and ammonia according to the GC–MS analysis and the indophenol colorimetric analysis. A possible reaction pathway on hydrogen generation from hydrolysis of ammonia borane is, accordingly, proposed based on the existence of boric acid, hydrogen and ammonia in the products. The total life cycle of ammonia borane is also proposed to illustrate formation of different intermediates during the AB hydrolysis for hydrogen generation and a possible regeneration scheme of the spent ammonia borane.     

Anchoring and space-confinement effects to synthesize ultrasmall Pd nanoparticles for efficient ammonia borane hydrolysis

Hydrogen production from ammonia borane (AB) hydrolysis catalyzed by efficient heterogenous catalysts is regarded as a compelling strategy to meet the increasing requirement for clean energy. Palladium (Pd) nanoparticle (NP)-based catalysts have stimulated intensive attention for AB hydrolysis, while their catalytic performances still need to be significantly improved. By exploiting sodium hydroxide and three-dimensional (3D) architecture of interconnected porous carbon nanosheets (IPCNs) as a NP carrier, a simple yet efficient strategy is developed to synthesize uniformly distributed ligand-free Pd NPs (2.17 nm in diameter) for hydrogen generation from AB hydrolysis. The particle size and spatial dispersion control of Pd NPs on the IPCNs surface supply an abundance of surface-active sites and thus remarkable improve the catalytic performance for AB hydrolysis to hydrogen evolution. Specifically, the achieved Pd/IPCNs reveals an extremely high catalytic activity with a turnover frequency (TOF) of 122.8 min⁻¹ toward AB hydrolysis, which is higher than that of many reported Pd-based catalysts. This simple, straightforward and efficient method is of significant importance for preparing metal NP catalysts with high catalytic activities for catalytic applications.     

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.     

Carbon nanotube-graphene hybrid supported platinum as an effective catalyst for hydrogen generation from hydrolysis of ammonia borane

In this study, we report the results of a kinetic study on the hydrogen (H₂) generation from the hydrolysis of ammonia borane (NH₃BH₃) catalyzed by Platinum supported on carbon nanotube-graphene hybrid material (Pt/CNT-G). Synthesized catalyst was characterized by TGA, XRD, CP-OES, TEM and SEM-EDX techniques. Characterization studies have shown that the CNT-G hybrid support material provides desired distribution of the Pt particles on the support material. The effect of various parameters such as catalyst loading, reaction temperature, effect of NaOH and the effect of NH₃BH₃ concentration are also determined. Experimental results showed that the Pt/CNT-G catalyst exhibited high catalytic activity on NH₃BH₃ hydrolysis reaction to release H₂. It has been found that Pt/CNT-G catalyst shows low activation energy of 35.34 kJ mol⁻¹ for hydrolysis reaction of NH₃BH₃. Pt/CNT-G catalyst also exhibited high catalytic activity with turnover frequency (TOF) of 135 (mol_H2/mol_cat.min). Therefore, the synthesized Pt/CNT-G catalyst is a potential candidate for enhanced H₂ generation through NH₃BH₃ hydrolysis.     

A bottom-up approach to prepare cobalt-based bimetallic supported catalysts for hydrolysis of ammonia borane

Catalysis is an important research topic in the field of hydrogen generation by hydrolysis of boron-based hydrides. A typical example is hydrolysis of ammonia borane (AB, NH₃BH₃), 1 mol of which is able to liberate up to 3 mol H₂ at temperatures lower than 80 °C. However, the presence of a catalyst, generally metal-based, is necessary. The present work was thus conducted in this framework. Herein, we propose a bottom-up approach to prepare cobalt-based bimetallic supported catalysts. The general idea was: first, to screen cobalt-based bimetallic nanoparticles and select the best combination, which was found to be CoCu with a weight ratio 70:30 – its reactivity was discussed in terms of electronic and geometric effects; second, to prepare Ni foam-supported CoCu through a 2-stage process – CoCu/Ni showed a hydrogen generation rate of ca. 25 mL min⁻¹, which almost 5 times better than that observed for the monometallic counterparts Co/Ni and Cu/Ni; third, to propose a new concept of CoCu supported catalysts using a plastic film (light, easy to handle and to prepare) – it showed to be stable and, despite a low hydrogen generation rate (because most of the nanoparticles were embedded in the film), totally converted AB. Our main results are reported and discussed herein.     

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.     

A highly efficient Co (0) catalyst derived from metal-organic framework for the hydrolysis of ammonia borane

A Co (0) catalyst is synthesized by the reduction of a metal-organic framework (MOF) precursor Co₂(bdc)₂(dabco) (bdc = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). The amorphous catalyst exhibits highly efficient activity in the hydrolysis of ammonia borane (AB). The dehydrogenation of a 0.32 M aqueous AB solution completes in 1.4 min under room temperature. The porous structure in the MOF is proposed to play a key role. The catalytic effective Co (0) sites are stabilized by the organic molecules, which used to coordinate to Co (II) in the MOF precursor. This work implies that MOFs, with their large surface area and ample pore structures, may serve as ideal precursors for highly efficient heterogeneous catalysts.     

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.     

Ionic liquid microemulsion mediated synthesis of Pt/TiO2 nanocomposites for ammonia borane hydrolysis

Tunable microstructure of microemulsion self-assembly systems has the prominent application as nanoreactors in reaction processes. Scientific and technological interest in microemulsions derived from the confinement effect of polar or nonpolar domains for hydrophilic/hydrophobic molecules. Benefiting from the tunable nature, ionic liquids-based self-assembly systems have been demonstrated as a versatile platform for catalytic reaction, drug delivery, functional materials fabrication and among others. In this work, ionic liquids microemulsions were designed by using ionic liquids as apolar phase and surfactant. The phase equilibrium and microstructure were studied through phase diagram, conductivity method and dynamic light scattering. Ionic liquids-in-water microemulsions were proposed as nanoreactor for fabrication of nanosized platinum/titanium dioxide composites. The size, morphology and catalytic performance can be tuned by the microemulsion structure through its confinement effect. The hydrolysis of ammonia borane catalyzed by these composites exhibits better catalytic activity with higher turnover frequency and lower activation energy of the reaction. It is believed that the nanoreactors constructed from versatile ionic liquids will provide new guidance in designing advanced scientific applications.     

Hexagonal boron nitride supported ruthenium nanoparticles as highly active catalysts for ammonia borane hydrolysis

Ammonia borane (AB) hydrolysis is a comparative strategy for developing the sustainable hydrogen economy. Considering the hydrolysis cannot occur kinetically at low temperature, a suitable catalyst is indispensable. In this work, the dispersed ruthenium nanoparticles are stabilized on hexagonal boron nitride (h-BN) via an adsorption-in situ reduction procedure. Various characterization techniques are adopted for elucidating the structure-performance relationship of the obtained catalysts for the hydrolytic dehydrogenation of AB. In the presence of the resultant Ru/h-BN catalysts, the corresponding turnover frequency (1177.5 min^?1) in alkaline solution at 303 K and the apparent activation energy (24.1 kJ mol^?1) are superior to most literature previously reported. Our work provides a facile fabrication method for metal-based catalysts, which are highly promising in chemical storage material hydrolysis.     

Facile and eco-friendly synthesis of porous carbon nanosheets as ideal platform for stabilizing rhodium nanoparticles in efficient hydrolysis of ammonia borane

Carbon materials have been demonstrated as excellent carriers for preparing supported metal nanocatalysts in catalytic applications. However, numerous chemical activators including strong acids and bases were applied, leading to the entire process dangerous and hazardous. Eco-friendly, economic, and convenient synthesis of carbon materials with desired properties as supports for metal nanoparticle (NP) stabilization to boost performance is important but remains challenging. Here, we developed a facile and eco-friendly strategy to synthesize porous carbon nanosheets (PCNs) with ultrahigh specific surface area (2575.1 m²/g) via pyrolysis the mixture of potassium oxalate and glucose. The resultant PCNs can be used as ideal platform for in-situ distribution of small Rh NPs (Rh/PCNs) as efficient catalysts in hydrogen production from ammonia borane (AB) under ambient conditions. Specifically, Rh/PCNs displayed high activity for AB hydrolysis, with a turnover frequency (TOF) of 513.2 min⁻¹. Small and well-distributed Rh NPs on PCNs with large catalytically active surface atoms are contributed to the high catalytic property of Rh/PCNs for the reaction. Present study has demonstrated that the PCNs is a superior catalyst support for preparing a series of metal NPs in other catalytic applications beyond hydrolysis reaction.     

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.     

Hydrogen generation by hydrolysis of ammonia borane with a nanoporous cobalt-tungsten-boron-phosphorus catalyst supported on Ni foam

In this study, quaternary cobalt-tungsten-boron-phosphorus porous particles supported on Ni foam (Co-W-B-P/Ni), which are prepared through ultrasonification-assisted electroless deposition route, have been investigated as the catalyst for hydrogen generation (HG) from hydrolysis of ammonia borane (NH₃BH₃, AB). Compared with Ni-supported binary Co-B and ternary Co-W-B catalysts, the as-synthesized Co-W-B-P/Ni shows a higher HG rate. To optimize the preparation parameters, the molar ratio of NaBH₄/NaH₂PO₂·H₂O (B/P) and the concentration of Na₂WO₄·2H₂O (W) have been investigated and the catalyst prepared with B/P value of 1.5 and W concentration of 5 g L⁻¹ shows the highest activity. The results of kinetic studies show that the catalytic hydrolysis of AB is first order with respect to the catalyst and AB concentrations. By using the quaternary catalyst with a concentration of 0.5 wt % AB, a HG rate of 4.0 L min⁻¹ g⁻¹ is achieved at 30 °C. Moreover, the apparent activation energy for the quaternary catalyst is determined to be 29.0 kJ mol⁻¹, which is comparable to that of noble metal-based catalysts. These results indicate that the Co-W-B-P/Ni is a promising low-cost catalyst for on-board hydrogen generation from hydrolysis of borohydride.     

Rapid preparation of Ru(0) nanoparticles stabilized with water-soluble biopolymer for hydrogen production from ammonia borane: Development of battery-like hydrolysis media

The biohybrid Na-Alg@Ru catalyst was prepared as a result of stabilizing Ru(0) nanoparticles with biopolymer chains of sodium alginate. The in-situ prepared Ru(0) nanoparticles had an average particle size of 1.023 ± 0.097 nm. The monodisperse Ru(0) nanoparticles prepared with a very practical, inexpensive and rapid method were used as a catalyst in hydrogen production by the hydrolysis reaction of ammonia borane (AB). The Na-Alg@Ru catalyst containing 3 mg Ru(0) metal catalyzed the hydrolysis of 50 mM AB with 100% yield, and the activation energy (E_a) of the reaction was estimated as 61.05 kJ mol⁻¹. In addition, the Na-Alg@Ru nanoparticles were prepared with acrylamide as p(AAm)/Na-Alg@Ru hydrogel films suitable for use in hydrogen production in fuel cells, which represents a battery-like environment, and used for hydrogen production from AB. Thus, it was shown that the catalysts prepared in a few nm size could easily be used in battery-like environments.