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.     

CuO–Ru0.3@Co3O4 nanocomposites act as a tandem catalyst for dehydrogenation of ammonia borane and hydrogenation of nitrobenezene

The development of catalysts with high activity for tandem reaction are all the ways pursued by chemists. Herein, CuO–Ru_0.3@Co₃O₄ has been synthesized and used as efficient tandem catalyst to promote the release of hydrogen from hydrolytic dehydrogenation of ammonia borane (AB) to catalyze the hydrogenation of nitrobenezenes (NBs). The catalyst exhibits the TOF of 29.87 min^?1 and provides the apparent activation energy of 45.2 kJ mol^?1 for the hydrolytic dehydrogenation of AB. Additionally, benefited from the magnetic separation capability, up to 99% of its initial catalytic activity is retained after four catalytic cycles.     

Coupling ultralow-content ruthenium with nickel hydroxide via corrosion engineering for highly efficient hydrogen generation from ammonia borane

Efficient and controllable release of hydrogen from solid hydrogen storage materials is a promising way to produce hydrogen safely and on-demand. The development of economical, highly active, easily recyclable catalysts is critical for practical applications, which remains a great challenging. Herein, the easily controllable and cost-effective corrosion strategy is ingeniously developed to simply prepare ultralow-content ruthenium coupled with nickel hydroxide on nickel foam (Ru–Ni–NF). After experiencing the spontaneous oxidation-reduction reactions between the reactive NF and Ru³⁺, ultrafine Ru nanoparticles decorated nickel hydroxide nanosheets are in situ intimately grown on porous NF networks. The optimal Ru–Ni–NF catalyst exhibits the excellent performance for catalytic hydrolysis of ammonia borane with a high turnover frequency (TOF) of 539.6 mol_H2 mol_Ru^?1 min^?1 at 298 K and a low apparent activation energy of 36.4 kJ mol^?1, due to the synergistic effect between Ru nanoparticles and nickel hydroxide nanosheets. Furthermore, the Ru–Ni–NF catalyst possesses easy separation and outstanding durability, which is superior to powdered catalysts. This study provides a facile and economical strategy for the preparation of ultralow-content noble metal supported metal foam-type catalysts for dehydrogenation of ammonia borane.     

Ruthenium(0) nanoparticles supported on silica coated Fe3O4 as magnetically separable catalysts for hydrolytic dehydrogenation of ammonia borane

Ruthenium(0) nanoparticles supported on bare or silica-coated magnetite are prepared by impregnation of ruthenium(III) ions followed by their reduction with aqueous solution of sodium borohydride on the surface of support. These magnetically isolable catalysts are used in hydrogen generation from the hydrolysis of ammonia borane at room temperature. They conserve their initial catalytic activity even after the fifth reuse in the hydrolysis reaction. Ruthenium(0) nanoparticles supported on bare magnetite and silica-coated magnetite provide turnover frequency values of 29 min^?1 and 127 min^?1 and in hydrolytic dehydrogenation of ammonia borane at 25.0 ± 0.1 °C. Thus, coating of the surface of magnetite with silica results in a significant enhancement in catalytic activity of ruthenium(0) nanoparticles in hydrogen generation from the hydrolysis of ammonia borane.     

Alumina nanofiber-stabilized ruthenium nanoparticles: Highly efficient catalytic materials for hydrogen evolution from ammonia borane hydrolysis

Effective catalysts for hydrogen generation from ammonia borane (AB) hydrolysis should be developed for the versatile applications of hydrogen. In this study, ruthenium nanoparticles (NPs) supported on alumina nanofibers (Ru/Al₂O₃-NFs) were synthesized by reducing the Ru(Ш) ions impregnated on Al₂O₃-NFs during AB hydrolysis. Results showed that the Ru NPs with an average size of 2.9 nm were uniformly dispersed on the Al₂O₃-NFs support. The as-synthesized Ru/Al₂O₃-NFs exhibited a high turnover frequency of 327 mol H₂ (mol Ru min)^?1 and an activation energy of 36.1 kJ mol^?1 for AB hydrolysis at 25 °C. Kinetic studies showed that the AB hydrolysis catalyzed by Ru/Al₂O₃-NFs was a first-order reaction with regard to the Ru concentration and a zero-order reaction with respect to the AB concentration. The present work reveals that Ru/Al₂O₃-NFs show promise as a catalyst in developing a highly efficient hydrogen storage system for fuel cell applications.     

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.     

Boron-doped Cobalt nanoparticles anchored to different activated carbon supports as recyclable catalysts for enhanced alkyl-substituted amine boranes dehydrogenation

Aiming at easily recoverable and regenerated catalyst development for efficient hydrogen production from alkyl-substituted amine boranes, boron (B)-doped cobalt (Co) nanoparticles with similar composition and particle size were anchored on two different activated carbon supports: granule and pellet. The effect of different independent variables such as type active carbon support (granule or pellet), alkyl-substituted amine boranes (ammonia borane-AB, methyl amine borane-MEAB and ethylenediamine borane-EDAB), recyclability cycle on hydrogen generation rate as dependent variable were investigated via Analysis of Variance (ANOVA). In addition, compared with ammonia borane, alkyl-substituted ones showed slower hydrogen generation properties in presence catalysts: AB > MEAB > EDAB. Among B-doped Co catalysts supported with different activated carbon supports, granule type activated carbon supported one showed best catalytic performance of derivatives of borane compounds dehydrogenation, and the hydrogen generation rate (2.49–0.44 L H₂ min^?1 g^?1_Co) and TOF values (7338.52–1451.96 mol_H2 mol^?1_catmin^?1). In the bargain, granule catalysts performed good recyclability activity, maintains its high hydrogen yields and its activity only decreased % 71 even after 5 repetitive cycles.     

Graphitic nanofibers supported NiMn bimetallic nanoalloys as catalysts for H2 generation from ammonia borane

Bimetallic nickel manganese nanoalloy-decorated graphitic nanofibers were prepared using electrospinning. The introduced catalysts were explored as an effective and inexpensive catalyst for H₂ generation from ammonia borane using hydrolysis. Standard techniques were used to determine the morphology and chemical composition of the nanofibers. Characterization indicated successful formation of bimetallic nickel-manganese-decorated graphitic nanofibers. Introduced effective catalysts showed a high reusability for H₂ generation using ammonia borane hydrolysis at low concentrations and temperatures. All formations of the introduced catalysts demonstrated a higher catalytic activity in H₂ generation than nickel-decorated carbon nanofibers. Samples composed of 55 wt% nickel and 45 wt% manganese showed the best catalytic activity compared with other formulations. Initial turnover frequency (TOF) of this sample was 58.2 min⁻¹, twice the TOF of the manganese-free catalyst. Kinetics and thermodynamics revealed that the catalyst concentration followed the pseudo-first order reaction while the ammonia borane concentration follow the pseudo-zero order reaction, providing activation energy of 38.9 kJ mol⁻¹.     

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.     

Enhanced ammonia dehydrogenation over Ru/La(x)-Al2O3 (x = 0–50 mol%): Structural and electronic effects of La doping

Ru (1.0 wt% loaded)-based catalysts supported on La(x)-Al₂O₃ (x = 0, 1, 5, 10, and 50 mol%) were synthesized and characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) measurement, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and temperature programmed reduction (TPR). The as-prepared La(x)-Al₂O₃ materials were found to have increased amounts of the LaAlO₃ phase as the La doping level (x) increased from 0 to 50 mol%. In addition to metal-to-support interactions between Ru and Al₂O₃, the newly formed LaAlO₃ phase in the Ru catalysts was proposed to interact strongly with Ru active sites based on the XRD, H₂-TPR and XPS results. The Ru/La(x)-Al₂O₃ catalysts were active for the dehydrogenation of ammonia, and among them, the Ru/La(10)-Al₂O₃ and Ru/La(50)-Al₂O₃ (or Ru/LaAlO₃) catalysts exhibited superior performance with >96% conversions of ammonia at 550 °C. When an increased Ru content (2.0 wt%) was impregnated onto La(10)-Al₂O₃, the dehydrogenation activity was significantly improved with nearly 100% conversion (>95%) of ammonia at 500 °C. This catalyst further displayed an enhanced thermal stability towards ammonia decomposition with the GHSV_NH3 of 10,000 mL/g_cat h at 550 °C for >120 h. The incorporated element La is thought to play an important role in enhancing metal-support interaction, ultimately facilitating ammonia dehydrogenation even at low temperatures.     

Highly monodisperse RuCo nanoparticles decorated on functionalized multiwalled carbon nanotube with the highest observed catalytic activity in the dehydrogenation of dimethylamine−borane

Herein we report the discovery of a superior dimethylamine?borane dehydrogenation catalyst, more active than the prior best heterogeneous catalyst (Celik B.; Y?ld?z Y.; Sert H.; Erken E.; Koskun Y.; and Sen F.; RSC Advances 2016, 6, 24097–24102) reported to date for the dehydrogenation of dimethylamine?borane. The new catalyst system consists of well dispersed ruthenium-cobalt nanomaterials decorated on functionalized multiwalled carbon nanotube (3.72 ± 0.37 nm) prepared by the use of the ultrasonic double reduction method. The morphology and structure composition of catalytically effective, recyclable and reproducible catalysts were fully characterized by using different analytical techniques such as UV–Vis, XPS, TEM, XRD and HR-TEM-EELS analyses. This catalyst showed best ever catalytic activity with a very high turnover frequency (775.28 h^?1) and low E_a value of 13.72 ± 2 kJ/mol for DMAB dehydrocoupling in the ambient conditions yielding 100% of the cyclic product (Me₂NBH₂)₂ at room temperature. Besides, RuCo@?-MWCNT catalyst exhibits great reusability, high catalytic performance and excellent durability for dimethylamine-borane dehydrogenation reaction.     

Tuning the alloy degree for Pd-M/Al2O3 (M=Co/ Ni /Cu) bimetallic catalysts to enhance the activity and selectivity of dodecahydro-N-ethylcarbazole dehydrogenation

Hydrogen is a promising candidate to substitute the fossil fuels. However, the efficient hydrogen storage technologies restrict the commercial applications. Developing new catalysts with high activity and selectivity is important for the dehydrogenation reaction in N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) hydrogen storage system. In this work, a series of Pd-M/Al₂O₃ (M = Co, Ni and Cu) bimetallic catalysts are synthesized successfully and show good performance in the dehydrogenation reaction of 12H-NECZ than the commercial Pd/Al₂O₃ catalyst. The Pd₁Co₁/Al₂O₃ catalyst (Practical Pd content = 2.4136 wt%) showed the highest catalytic performance with 95.34% H₂ release amount, TOF of 230.5 min⁻¹ and 85.4% selectivity of NECZ. Combined with the characterization analysis, it can be proposed that the dehydrogenation performance of 12H-NECZ is dependent on the alloy phases, reasonable electronic structures and nanoparticle size of catalysts. The fine-tuned alloy degree and appropriate nanoparticle size of Pd₁Co₁/Al₂O₃ bring the 17.7% increase of H₂ release amount and 99.5% increase of NECZ selectivity than those of Pd/Al₂O₃. For the bimetallic catalysts, the enhancement of selectivity of NECZ is mainly from the increase of the kinetic constant of rate-limiting step.     

A review on the catalysts used for hydrogen production from ammonia borane

Boron compounds have recently attracted attention in hydrogen production since they contain many hydrogen atoms. Among these compounds, ammonia borane, which has high hydrogen density (in weight basis), can be used to produce hydrogen through a hydrolysis reaction. However, since the ammonia borane solution is highly resistant to hydrolysis under ambient conditions, there is a need for active and stable catalysts to accelerate the reaction. In this review paper, unsupported and carbon-based supported metal catalysts used for hydrogen production through the hydrolysis of ammonia borane are presented. Noble metal catalysts (Ru, Rh, Pd, Pt and their binary and ternary alloys) and non-noble metal catalysts (Co, Ni, Fe, Cu and their binary and ternary alloys) were examined. The activation energy of reaction and turnover frequency (TOF) values were compared for these catalysts. Among the unsupported catalysts, it was concluded that the multi-metal catalyst systems (binary, ternary and quaternary) have higher catalytic activity than a single use of the same metals. In addition, the comparison showed that the supported catalysts are more resistant to catalytic cycles and suitable for long-term use. It was observed that CNT supported Rh (TOF = 706 mol H₂ mol cat⁻¹ min⁻¹) and graphene supported Ru (TOF = 600 mol H₂ mol cat⁻¹ min⁻¹) catalysts are the most active catalysts for the hydrogen generation from the ammonia borane at room temperature.     

Oxygen vacancy-engaged interfacial charge transfer modulation for upgrade Rh-catalyzed hydrogen and oxygen productions

Developing efficient modulation strategies to upgrade the catalytic activity and reusability of Rh-catalyzed hydrogen evolution from ammonia borane (AB) hydrolysis are definitely profitable but remains a grand challenge. Here, we develop a stepwise activation strategy to produce highly active and reusable Rh/CoFe₂O₄-SB-H₂ with abundant oxygen vacancies and strong electronic metal-support interaction through stepwise reduction of Rh/CoFe₂O₄ precursor using sodium borohydride and H₂ as the reducing agents. Under ultrasonic irradiation, Rh/CoFe₂O₄-SB-H₂ with an ultralow Rh loading of 0.20 wt% can be utilized as an excellent catalyst for hydrogen production from room-temperature AB hydrolysis with a high turnover frequency (TOF) of 1894 min⁻¹. The TOF value could be further promoted to 15,570 min⁻¹ in the alkaline ultrasonic environment. The catalyst has a superior reusability with 75% maintaining activity of initial one in the 10^th cycle. The strong electronic metal-support interaction, rich oxygen vacancies and ultrasound irradiation promote the oxidative cleavage of the O–H bonds in attracted H₂O and thus account for high performance toward hydrogen production from AB. This catalyst can also be utilized as an active catalyst for oxygen generation from H₂O₂ decomposition. The developed strategies can be applied to upgrade the performance of other reducible metal oxides supported metal catalysts toward catalytic applications.     

Ruthenium supported on nitrogen-doped porous carbon for catalytic hydrogen generation from NH3BH3 hydrolysis

In this paper, ruthenium supported on nitrogen-doped porous carbon (Ru/NPC) catalyst is synthesized by a simple method of in situ reduction using ammonia borane (AB) as reducing agent. The composition and structure of Ru/NPC catalyst are systematically characterized. This catalyst can efficiently catalyze the hydrolysis of AB. The hydrogen production reaction is completed within about 90 s at a temperature of 298 K and the maximum rate of hydrogen production is 3276 ml·s⁻¹·g⁻¹ with a reduced activation energy of 24.95 kJ·mol⁻¹. The turnover frequency (TOF) for hydrogen production is about 813 mol_H2·mol_Ru⁻¹·min⁻¹. Moreover, this catalyst can be recycled with a well-maintained performance. After five cycles, the maximum rate of hydrogen generation is maintained at 2206 ml·s⁻¹·g⁻¹, corresponding to 67.3% of the initial catalytic activity. Our results suggest that Ru/NPC prepared by in situ reduction is a highly efficient catalyst for hydrolytic dehydrogenation of AB.     

Hydrogen liberation from the dehydrocoupling of dimethylamine–borane at room temperature by using novel and highly monodispersed RuPtNi nanocatalysts decorated with graphene oxide

Addressed herein, we reported the fabrication of the graphene oxide (GO) supported monodispersed ruthenium–platinum–nickel (RuPtNi) nanomaterials (3.40 ± 0.32 nm) to be utilized as a catalyst in the process of dimethylamine borane (DMAB) dehydrogenation. The nanoparticles were fabricated through the ultrasonication method by co-reducing the Ru³⁺, Pt²⁺ and Ni²⁺ cations and then the nanomaterials were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), inductively coupled plasma optical emission spectrometry (ICP-OES), and X-ray photoelectron spectroscopy (XPS). The fabricated nanomaterials showed outstanding efficiency and remarkable reusability in addition to their record catalytic activity at low temperatures and with extreme low concentrations. They had a significantly high turnover frequency (TOF) (727 h^?1) and low activation energy (E_a) (49.43 ± 2 kJ mol^?1) for DMAB dehydrocoupling. To the best of our knowledge, RuPtNi@GO NPs become a very promising candidate as the best catalyst ever.     

Ammonia borane as hydrogen storage materials

Ammonia borane is an appropriate solid hydrogen storage material because of its high hydrogen content of 19.6% wt., high stability under ambient conditions, nontoxicity, and high solubility in common solvents. Hydrolysis of ammonia borane appears to be the most efficient way of releasing hydrogen stored in it. Since ammonia borane is relatively stable against hydrolysis in aqueous solution, its hydrolytic dehydrogenation can be achieved at an appreciable rate only in the presence of suitable catalyst at room temperature. Metal(0) nanoparticles have high initial catalytic activity in releasing H₂ from ammonia borane. Thermodynamically instable metal(0) nanoparticles can kinetically be stabilized against agglomeration either by using ligands in solution or by supporting on the surface of solid materials with large surface area in solid state. Examples of both type of stabilization are presented from our own studies. The results show that metal(0) nanoparticles dispersed in solution or supported on suitable solid materials with large surface area can catalyze the release of H₂ from ammonia borane at room temperature. Dispersion of metal(0) nanoparticles, stabilized in liquid phase by anions or polymers, seems advantageous as providing more active sites compared to the metal nanoparticles supported on a solid surface. However, the supported metal nanoparticles are found to be more stable against agglomeration than the ones dispersed in liquid phase. Therefore, metal nanoparticles supported on solid materials have usually longer lifetime than the ones dispersed in solution. Examples are given from the own literature to show how to improve the catalytic activity and durability of metal nanoparticles by selecting suitable stabilizer or supporting materials for certain metal. For the time being, nanoceria supported rhodium(0) nanoparticles are the most active catalyst providing a turnover frequency of 2010 min^?1 in releasing H₂ from ammonia borane at room temperature.     

Hydrogen production from the methanolysis of ammonia borane by Pd-Co/Al2O3 coated monolithic catalyst

The aim of this study is to enable high hydrogen production yield from catalytic methanolysis of ammonia borane (AB) in the presence of a cordierite type ceramic monolithic. The monolithic channel surfaces were coated with Al₂O₃ by wash-coating method and then this layer was impregnated with 1 wt%Pd-2 wt%Co bimetallic catalyst. SEM-EDX and multi-point BET analysis were used in order to characterize the catalyst. The experimental studies were conducted in a continuous flow type reactor, which was used for the first time in this study. The reactions were carried on low temperature (40 °C), and with various AB feed concentrations and flow rates. It was found that the highest hydrogen production yield (88.5%) was obtained from AB flow rate of 3.3 mL/min, and AB feed concentration of 0.1 wt%. It was concluded that Pd-Co/Al₂O₃ coated monolithic, which is a stable, active and low-cost catalyst, was a very promising catalyst for on-board hydrogen production from the methanolysis of ammonia borane.     

Rhodium(0), Ruthenium(0) and Palladium(0) nanoparticles supported on carbon-coated iron: Magnetically isolable and reusable catalysts for hydrolytic dehydrogenation of ammonia borane

We report the synthesis of magnetically isolable ruthenium(0), rhodium(0), and palladium(0) nanoparticles, supported on carbon-coated magnetic iron particles, and their employment as catalysts in hydrolysis of ammonia borane. Carbon-coated iron (C–Fe) particles are obtained by co-processing of iron powders with methane in a radio frequency thermal plasma reactor. The impregnation of ruthenium(III), rhodium(III) and palladium(II) ions on the carbon-coated iron particles followed by aqueous solution of sodium borohydride leads to the formation of respective metal(0) nanoparticles supported on carbon-coated iron, M⁰/C–Fe NP (M = Ru, Rh, and Pd) at room temperature. M⁰/C–Fe NPs are characterized using the ICP-OES, XPS, TEM, and EDX techniques and tested as catalysts for hydrolysis of ammonia borane at 298 K. The results reveal that Rh⁰/C–Fe, Ru⁰/C–Fe, Pd⁰/C–Fe catalysts provide turnover frequency of 83, 93, and 29 min^?1, respectively, in this industrially important reaction. More importantly, these magnetically separable metal(0) nanoparticles show very high reusability with no noticeable activity loss in subsequent runs of hydrolysis evolving 3.0 equivalent H₂ per mole of ammonia borane.