Copper oxide hollow spheres: Synthesis and catalytic application in hydrolytic dehydrogenation of ammonia borane

In this paper, CuO hollow microspheres with different shell thickness and porosity have been synthesized using carbonaceous saccharide microspheres as templates according to a modified literature method. These CuO hollow microspheres were characterized and their catalytic properties in the hydrolytic dehydrogenation of ammonia borane (NH₃BH₃, AB) were examined. A kinetic study indicated that a maximum hydrogen generation rate of 294 mL H₂ min^?1 (g catalyst)^?1 can be achieved at 45 ± 0.2 °C in the present system, which is comparable with that for other reported Cu based catalysts.     

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.     

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.     

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.     

Monodisperse PtCu alloy nanoparticles as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane

Pt-M alloy nanoparticles (NPs) with well-defined size and compositions exhibit dramatically catalytic performance in chemical reactions. In this work, monodisperse PtCu NPs with controlled size and compositions were synthesized by the co-reduction method in the presence of oleylamine. These NPs have excellent catalytic activities in the hydrolytic dehydrogenation of ammonia borane (AB) and their activities were composition dependent. Among the different-composition PtCu NPs, the Cu₅₀Pt₅₀ NPs exhibit the highest catalytic activity with an initial turnover frequency of 102.5 mol_(hydrogen)·mol_(catalyst)^?1·min^?1 and an apparent activation energy of 36 kJ·mol^?1, which demonstrate the validity of partly replacing Pt by a first-row transition metal on constructing high performance heterogeneous nanocatalysts for the hydrolytic dehydrogenation of AB.     

Co-SiO2 nanosphere-catalyzed hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage

Cobalt clusters-silica nanospheres (15-30 nm) were synthesized using a Co(NH₃)₆Cl₃ template method in a polyoxyethylene-nonylphenyl ether/cyclohexane reversed micelle system followed by in situ reduction in aqueous NaBH₄/NH₃BH₃ solutions. The cobalt clusters are located either inside or on the outer surface of the silica nanospheres as shown by the transmission electron microscope (TEM)/energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) measurements. The cobalt-silica nanospheres have a high catalytic activity for the hydrolysis of ammonia borane that generates a stoichiometric amount of hydrogen, and can be efficiently cycled and reused 10 times without any significant loss of the catalytic activity.     

Solvent-free synthesis of Rh/meso-Al2O3 via mechanochemistry for hydrolytic dehydrogenation of ammonia borane

An effective strategy synthesis of Rh/meso-Al₂O₃ catalysts was demonstrated by mechanochemistry for hydrolytic dehydrogenation of ammonia borane (AB). These catalysts are characterized systematically by N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the turnover frequency (TOF) and activation energy (E_a) are 246.8 mol_H2·mol_Rh^?1·min^?1 and 47.9 kJ mol^?1 for hydrolytic dehydrogenation of at 298 K catalyzed by Rh/Al₂O₃-CTAB-400, obviously higher than those previously reported catalysts. Furthermore, catalyst Rh/Al₂O₃-CTAB-400 can be recycled by simple centrifugal separation and the catalytic activity is still well maintained after five cycles. In addition, a plausible mechanism for hydrolytic dehydrogenation of AB has also been proposed. This mechanochemical synthesis method exhibits great application prospects for the preparation of heterogeneous catalysts.     

Oleylamine-stabilized Cu0.9Ni0.1 nanoparticles as efficient catalyst for ammonia borane dehydrogenation

Monodisperse CuNi nanoparticles are conveniently prepared by the reduction of cupric acetate and nickel(II) acetylacetonate in the presence of oleylamine and borane tributylamine under inert gas atmosphere. It is found that among the CuNi system, Cu_0.9Ni_0.1 shows the best performance for catalyzing the dehydrogenation of ammonia borane. In total, 2.5 equiv. of hydrogen per ammonia borane is generated even at room temperature with an initial turnover frequency value of 212.3 mol of H₂·(mol of Cu_0.9Ni_0.1)^?1·h^?1, which is comparable to the best Pd-based catalyst ever reported. The remarkable catalytic performance is attributed to the mild affiliation of oleylamine (OAm) to NPs, which not only stabilizes NPs to maintain good dispersion but also leaves sufficient surface active sites to facilitate the catalytic reaction. This low-cost and high catalytic performance catalyst makes it an exciting alternative towards the application of ammonia borane as a hydrogen storage material for fuel cell applications.     

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.     

Polydopamine-coated halloysite nanotubes supported AgPd nanoalloy: An efficient catalyst for hydrolysis of ammonia borane

AgPd alloy nanoparticles (NPs) supported on halloysite nanotubes (HNTs) coated polydopamine (PDA) successfully synthesized by one-pot hydrothermal route. XRD, TEM and XPS were employed to verify the alloy structure of the obtained AgPd NPs. The HAADF-STEM result revealed that the thickness of PDA coating was ～10 nm, which could be formed on the surface of HNTs, and the existence of PDA was beneficial to deposit AgPd alloys with high dispersibility on the surface of HNTs. AgPd/PDA-HNT nanocomposites were effective catalysts for the hydrolysis of ammonia borane at room temperature, and the reaction was completed within 160 s using Ag₃Pd₂/PDA-HNT as catalysts, with a high total turnover frequency (TOF) value of 90 mol_H2 mol_catalyst^?1 min^?1 and a low apparent activation energy (Ea) of 22.7 kJ mol^?1. After the sixth cycle, Ag₃Pd₂/PDA-HNT catalyst retained 72% of its initial activity and 100% conversion. The excellent catalytic properties, good durability and reusability, enabled Ag₃Pd₂/PDA-HNT to be an ideal catalyst in the practical applications.     

Group 4 oxides supported Rhodium(0) catalysts in hydrolytic dehydrogenation of ammonia borane

Rh³⁺ ions are first impregnated on Group 4 metal oxides (TiO₂, ZrO₂, HfO₂) in aqueous solution and, then reduced with aqueous solution of NaBH₄ to form rhodium(0) nanoparticles (NPs) on the oxide surface. The analyses reveal that Rh(0) NPs are highly dispersed on the surface of TiO₂, ZrO₂, HfO₂. Rh⁰/MO₂ (M: Ti, Zr, Hf) NPs have high activity and reusability in releasing H₂ from the hydrolysis of ammonia borane with an initial turnover frequency of 643, 198, and 188 min⁻¹, respectively, at 25.0 ± 0.1 °C. The reusability of Rh⁰/ZrO₂ and Rh⁰/HfO₂ catalysts is higher than that of the Rh⁰/TiO₂ catalyst.     

Hollow Ni-SiO2 nanosphere-catalyzed hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage

Nickel clusters contained within silica nanospheres (20-30 nm) were synthesized by using a Ni(NH₃)₆Cl₂ crystal template method in a polyoxyethylene-nonylphenyl ether/cyclohexane reversed micelle system followed by an in situ reduction in aqueous NaBH₄/NH₃BH₃ solutions. Metallic nickel clusters exist inside the SiO₂ nanospheres prepared by the method while oxidized nickel clusters prepared by the conventional impregnation method were supported on the outer surface of silica as shown in the results of transmission electron microscope (TEM)/energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) measurements. The nickel clusters inside of silica nanospheres show higher catalytic activity for hydrolysis of ammonia borane to generate stoichiometric amount of hydrogen than the supported nickel catalysts.     

Ruthenium supported on MIL-96: An efficient catalyst for hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage

For the first time, ultrafine Ru nanoparticles with mean diameter of 2 nm are successfully deposited on MIL-96 by using a simple liquid impregnation strategy, and tested for catalytic hydrolysis of ammonia borane. The powder X-ray diffraction, N₂ physical adsorption, transmission electron microscopy, energy-dispersive X-ray spectroscopy and inductively coupled plasma-atomic emission spectroscopy measurements are employed to characterized the Ru/MIL-96 catalysts. Thanks to the unique 3D structure of MIL-96, Ru NPs supported on MIL-96 exhibit much enhanced catalytic activity compared with other commercial supported materials and graphene, with the TOF value of 231 mol H₂ min⁻¹ (mol Ru)⁻¹, which is among the highest value ever reported. Moreover, this simple method can be extended to facile synthesis of other MOFs supported monometallic and polymetallic NPs for more application.     

Electrospun nickel doped titanium dioxide nanofibers as an effective photocatalyst for the hydrolytic dehydrogenation of ammonia borane

In this study, Ni-doped titanium dioxide (TiO₂) electrospun nanofibers are introduced as novel material for dehydrogenation of ammonia borane (AB) complex. Hydrolysis experiments with introduced catalytic nanofibers are prevailed to rapidly release hydrogen from AB complex. Typically, Ni nanoparticles (NPs) behave as a catalyst, meanwhile the incorporation of nickel NPs lead to decrease in the electrons/holes recombination rate in TiO₂ which resulted in the increase of active ions in the solution to a rapid evolution of hydrogen gas at room temperature. The utilized physiochemical analyses indicate that the introduced Ni-doped TiO₂ nanofibers have a smooth surface and uniform diameters along their lengths. Under sunlight irradiation, the hydrogen production rate in case of utilizing Ni-doped TiO₂ nanofibers is rapidly increased compared to the pristine TiO₂ nanofibers, the maximum hydrogen equivalent in case of the doped nanofibers is 2.6 while the pristine one is 1.4. Both formulations exhibit almost equal low activity in daylight as the observed hydrogen equivalent is 0.4. Overall, this study proposes cheap, stable and effective material for AB dehydrogenation at room temperature.     

In situ facile synthesis of bimetallic CoNi catalyst supported on graphene for hydrolytic dehydrogenation of amine borane

Well dispersed magnetically recyclable bimetallic Co_xNi_1−x (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) nanoparticles (NPs) supported on graphene have been synthesized via a facile in situ one-step procedure, using the mixture of sodium borohydride (NaBH₄) and methylamine borane (MeAB) as the reducing agent under ambient condition. These NPs were composition dependent for catalytic hydrolysis of amine boranes. Among all the CoNi/graphene catalysts tested, the Co_0.9Ni_0.1/graphene NPs exhibit the highest catalytic activity toward hydrolysis of AB with the turnover frequency (TOF) value of 16.4 (mol H₂ min⁻¹ (mol catalyst)⁻¹), being higher than that of most reported non-noble metal-based NPs, and even many noble metal-based NPs. Moreover, the activation energy (E_a) value is 13.49 kJ/mol, which is the second lowest value ever reported for catalytic hydrolytic dehydrogenation of ammonia borane, indicating the superior catalytic performance of the as-synthesized Co_0.9Ni_0.1/graphene catalysts. Additionally, Compared with other reducing agents, such as NaBH₄, AB, MeAB, and the mixture of NaBH₄ and AB, the as-synthesized Co_0.9Ni_0.1/graphene catalysts reduced by the mixture of NaBH₄ and MeAB exert the highest catalytic activity. The Co_0.9Ni_0.1 NPs supported on graphene exhibit higher catalytic activity than catalysts with other conventional supports, such as SiO₂, carbon black, and γ-Al₂O₃. Furthermore, the as-synthesized Co_0.9Ni_0.1/graphene NPs show good recyclability and magnetically reusability for the hydrolytic dehydrogenation of amine boranes, which make the practical reusing application of the catalysts more convenient.     

Bamboo fungus-derived magnetic porous carbon encapsulated nickel stabilized Rh nanoparticles as efficient catalysts for hydrolytic dehydrogenation of ammonia borane

Biomass-derived porous carbons are generally used as supports for metal nanoparticle (NP) stabilizations, while the strong hydrophilicity of such materials makes the as-prepared catalysts hard to be isolated after reaction, significantly affecting their potential applications. Herein, magnetic N-functionalized carbon (CN) encapsulated Ni composite (Ni@CN) prepared via pyrolysis of bamboo fungus pre-absorbed with nickel nitrate is exploited as a matrix to synthesize Rh/Ni@CN hybrid, which can be used as a magnetically recoverable catalytic material for hydrolytic dehydrogenation of ammonia borane (AB) to generate hydrogen. The Rh/Ni@CN (Rh loading: 0.84 wt%) exhibits an optimal activity (turnover frequency: 351 min⁻¹) for hydrogen evolution from hydrolytic dehydrogenation of AB. Most importantly, this catalyst can be simply isolated by a magnet and reused at least five times with complete conversion of AB to hydrogen. The strong interaction between the two metals and the small size of Rh NPs are responsible for the improved catalytic activity for hydrolytic dehydrogenation of AB. This work provides an eco-friendly and efficient strategy to fabricate excellent catalysts in catalytic applications.     

Efficient hydrolytic dehydrogenation of ammonia borane over ultrafine Ru nanoparticles supported on biomass-derived porous carbon

Ammonia borane hydrolysis is a promising strategy for developing sustainable hydrogen energy. However, this reaction is not kinetically feasible at ambient temperature, thus developing a proper catalyst is indispensable. In this work, Porous carbon is facilely prepared from cattail fibers by using K₂CO₃, and then used to stabilize Ru nanoparticles. The effects of different synthesis parameters for the biomass-derived carbon supports (e. g. K₂CO₃ dosage and calcination temperature) and various catalytic reaction conditions (e. g. the amounts of the catalysts, ammonia borane and NaOH, and reaction temperature) on the hydrolysis rate of ammonia borane are investigated. Benefitting from the interconnected hierarchical pores of the optimal porous carbon (p-C), which was prepared with a mass ratio of 6 : 1 for K₂CO₃ to cattail fibers and calcined at 873 K, and the high dispersion of Ru nanoparticles, the optimal Ru/p-C catalysts exhibit excellent catalytic performance. The corresponding apparent activation energy (28.8 kJ mol^?1) and turnover frequency (744.7 min^?1 in alkaline solution) are superior to many catalysts previously reported. This work offers a competitive catalyst for the hydrolytic dehydrogenation of chemical hydrogen storage materials.     

Theoretical exploration of the mechanisms on the iron complexes catalyzed ammonia borane dehydrogenation and polyaminoborane formation

Density functional theory study was carried out to provide mechanistic insights into the bifunctional iron complex [Fe(DCPE)(PhNCH₂)₂ (DCPE = 1,2-bis(dicyclohexylphosphino)ethane)] catalyzed AB dehydrogenation and polyaminoborane formation with the detailed mechanistic pathways. Computational results imply a favorable scenario that complies with a proton transfer pathway between the metal center and ligand, which could further lead to a kinetically feasible transition state with low energy barrier. Subsequent polyaminoborane formation involves the generation of nucleophiles, and the NH₂BH₂ moiety, which is featured with a nitrogen lone pair, plays a crucial role in the chain polyaminoborane formation. Natural bond orbital analysis (NBO), and energy decomposition analysis (EDA) were utilized to study the orbital interactions and intramolecular weak interactions were revealed by the independent gradient model based on Hirshfeld partition method (IGMH).