Dehydrogenation of Ammonia Borane by Metal Nanoparticle Catalysts

Perspective pubs.acs.org/acscatalysis

Dehydrogenation of Ammonia Borane by Metal Nanoparticle Catalysts Wen-Wen Zhan,† Qi-Long Zhu,† and Qiang Xu*,†,‡ †

National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan Graduate School of Engineering, Kobe University, Nada Ku, Kobe, Hyogo 657-8501, Japan

‡

ABSTRACT: Ammonia borane (AB), having a high hydrogen density of 19.6 wt %, has attracted much attention as a promising chemical hydrogen storage material. In the past few years, a number of highly active metal nanoparticle (NP) catalysts, which are easy to handle and separate, have been developed for AB dehydrogenation. In this Perspective, we summarize new progress in the development of metal NP catalysts, which are categorized into monometallic and heterometallic catalysts, with excellent activity and high recyclability for different AB dehydrogenation pathways, including solvolysis (hydrolysis and methanolysis) in protic solvents and dehydrocoupling in nonprotic solvents, and we survey the corresponding methods for the regeneration of AB. Moreover, the merits and drawbacks of solvolysis and dehydrocoupling are discussed. KEYWORDS: ammonia borane dehydrogenation, heterogeneous catalysis, metal nanoparticle, hydrolysis, methanolysis, dehydrocoupling, chemical hydrogen storage

1. INTRODUCTION

hydrogen storage methods are mature enough for industrial applications under mild conditions. Chemical hydrogen storage materials, where covalently bound hydrogen exists in either liquid or solid form, have attracted much attention due to their generally high hydrogen densities.7 Among them, B−N compounds have great potential to achieve the requirements of transportation applications.1b,9 Ammonia borane (AB), as the simplest B−N compound, exhibits the advantages of high hydrogen content (19.6 wt %), low molecular weight (30.7 g mol−1), and high stability in solutions, making it a highly promising candidate for hydrogen storage.7a,10 AB is a colorless solid under ambient conditions, with a density of 0.74 g cm−3 and a high melting point of 112− 114 °C. It is soluble in polar solvents like water and methanol. Since the first AB synthesis report by Shore and Parry in 1955,11 many methods with considerable yields have been developed.12 AB was first reported to release hydrogen by thermal treatment.13 In the last decades, the thermolysis of AB has been widely investigated due to the high amount of released hydrogen. Although neat AB possesses 3 equiv of H2, only 2 equiv could be released at a temperature of 200 °C. To decrease the temperature of AB decomposition, various approaches, including nanoconfinement,13b,14 catalysis,15 dispersion in organic and ionic liquids,16 and the use of derivatives (e.g., metal amidoboranes)17 have been developed, which not only reduce the temperature of AB thermolysis but also lower

Nowadays, the ever-increasing demand for energy along with the rapid development of industrial production and population growth provokes the development of alternative energy strategies, which are effective, renewable, and do not cause further damage to the environment. Hydrogen, which can be renewably produced from a variety of (nonfossil) feedstocks, is a globally accepted clean energy carrier. It has a much higher gravimetric energy density than petroleum (120 kJ g−1 for hydrogen vs 44 kJ g−1 for petroleum) and can be readily used to operate high-energy-efficient fuel cells that produce water as the only byproduct, making it the perfect alternative energy carrier of the future.1 Secure and efficient hydrogen storage is the key problem to address among all issues related to the applications of hydrogen energy.2 When employed as an energy carrier in portable electronic devices and vehicles, hydrogen fuel cells should have the highest possible energy content combined with the smallest possible volume and mass. According to the updated targets for on-board hydrogen storage systems set by the U.S. Department of Energy (DOE), the minimum gravimetric and volumetric capacities are 5.5 wt % and 40 g L−1 for the year 2020, ultimately increasing to 7.5 wt % and 70 g L−1, respectively.3 In order to fulfill this criterion, a large amount of research has been performed on hydrogen storage materials, providing different solutions.4 Physical (compressed hydrogen gas, cryocompressed hydrogen storage, and hydrogen adsorbents5) and chemical storage systems (e.g., metal hydrides,6 organic hydrides,7 borane−nitrogen (B−N) compounds,7 and aqueous solution of hydrazine8) have been investigated.4 However, no © 2016 American Chemical Society

Received: August 3, 2016 Revised: August 31, 2016 Published: September 2, 2016 6892

DOI: 10.1021/acscatal.6b02209 ACS Catal. 2016, 6, 6892−6905

Perspective

ACS Catalysis

water-dispersible efficient nanocatalyst for AB hydrolysis at room temperature.27 Around 80% of the initial TOF value (80 min−1) was retained after the fifth catalytic run. Various solid supports, such as metal−organic frameworks (MOFs), carbon materials, SiO2, γ-Al2O3, and so on, have also been used to fabricate well-dispersed noble-metal NPs with controllable size and morphology. Our group successfully immobilized ultrafine Pt nanoparticles inside the pores of MIL101 and avoided NP aggregation on the external framework surfaces by using a double solvent method (DSM).28 The welldispersed Pt NPs with an average size of 1.8 ± 0.2 nm exhibited high activity in the hydrolysis of AB. Three equivalents of H2 were released in 3 min (Pt/AB = 0.0029) at room temperature, corresponding to a TOF of 414 min−1. The Pt@MIL-101 catalyst also showed high durability, and the Pt NPs retained good distribution and ultrafine morphology after the catalytic reaction. Recently, carbon materials such as graphene, carbon nanotubes (CNTs), and porous carbons have been widely used as the most appealing supports due to their good physical and chemical properties.29 Cheng et al. reported a graphenesupported Ru NP catalyst highly active in AB hydrolysis.30 Due to the narrow size distribution of Ru NPs and the synergistic effect with graphene, the prepared Ru/graphene composite showed high performance with an initial TOF of 600 min−1 and a total TOF of 500 min−1. Duan et al. employed CNTs treated by acid oxidation and high temperature as a support for Pt NPs, which combined the merits of defect-rich and oxygen group deficient surfaces with the unique textural properties of supports and the optimum particle size of Pt NPs (Figure 1).

the amount of unwanted byproducts. However, hydrogen release by thermal AB dehydrogenation still requires high temperatures, making reaction control somewhat difficult.9a Another route for AB dehydrogenation is solvolysis in protic solvents, including hydrolysis (eq 1)10,18 and methanolysis (eq 2),12b,19 as well as dehydrocoupling in nonprotic solvents (eqs 3 and 4).9a H3N·BH3(aq) + 2H 2O(l) → NH4 ·BO2 (aq) + 3H 2(g) (1)

H3N·BH3(sol) + 4CH3OH(l) → NH4 ·B(OCH3)4 (sol) + 3H 2(g)

(2)

nH3N·BH3(sol) → (NH 2BH 2)n (s or sol) + nH 2(g)

(3)

nH3N·BH3(sol) → (NHBH)n (s or sol) + 2nH 2(g)

(4)

δ−

In both solvolysis reactions, the H bound to the B atom reacts with the Hδ+ of the protic solvent, which can stoichiometrically produce 3 equiv of H2. The aqueous and methanolic solutions of AB are stable under neutral or weakly basic conditions.20 When proper catalysts are used, hydrogen release can occur at ambient conditions. Therefore, efficient, economical, and easily prepared catalysts for AB solvolysis are highly desired. To date, significant developments have been achieved for the heterogeneous catalytic dehydrogenation of AB with metal nanoparticle (NP) catalysts.21 Regarding dehydrocoupling, more efforts have been devoted to homogeneous catalysts than to heterogeneous ones. 22 Several comprehensive reviews focused on the topic of heterogeneous catalytic AB dehydrogenation were published before 2014,10,21b,c and since then, hundreds of papers have been published. In this Perspective, we mainly survey the progress achieved in the last three years in developing highly efficient metal NP catalysts, including monometallic and heterometallic ones, for the solvolytic dehydrogenation of AB. A brief overview is also given for AB dehydrocoupling with metal NP catalysts. We also discuss the merits and drawbacks of heterogeneous catalytic AB dehydrogenation, which may help future research in this area.

2. SOLVOLYSIS OF AB 2.1. Hydrolytic Dehydrogenation of AB. Since our first report of AB hydrolysis catalyzed by noble metals,18a numerous studies have been conducted on monometallic and heterometallic catalysts.23 2.1.1. Monometallic Catalysts. A number of noble-metal catalysts, including Pt, Rh, Ru, Ag, and Pd, display high activities in the hydrolytic dehydrogenation of AB.23b,24 In order to improve catalytic activity and durability, stabilizers and supports have been used to control the size and morphology of metal NPs.25 Ö zkar et al. employed noble-metal NPs with different stabilizers as catalysts for the hydrolytic dehydrogenation of AB. Well-dispersed Rh NPs within the framework of a polyaminoborane-based polymeric support were prepared by reduction of [Rh(μ-Cl)(1,5-cod)]2 (cod = 1,5-η2-cyclooctadiene) with N2H4BH3 without using other solid supports and pretreatment techniques.26 The stabilized Rh NPs provided a turnover frequency (TOF) of 130 min−1 for catalytic AB hydrolysis and showed high durability. The authors also reported Ru NPs (stabilized by the dihydrogen phosphate anion) with an average particle size of 2.9 ± 0.9 nm acting as a

Figure 1. (a) TOF as a function of Pt particle size over Pt nanocatalysts. Reaction conditions: 30 °C, nPt/nAB = 4.7 × 10−3 and CAB = 0.01 g mL−1. (b) Possible pathway for the hydrolytic dehydrogenation of AB over two kinds of Pt particles immobilized on close CNT supports. Note: higher (yellow) and lower (brown) binding energy of Pt 4f. Reproduced with permission from ref 31. Copyright 2014 The Royal Society of Chemistry. 6893


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ACS Catalysis The above composite showed excellent performance in catalytic AB hydrolysis with a TOF of 567 min−1.31 Yao et al. developed a CNT-supported Ru NP catalyst that efficiently catalyzed the hydrolytic dehydrogenation of AB.32 The ultrafine Ru NPs (2.3 nm) of Ru/CNT helped achieve an activity of up to 706 min−1 at 25 °C. Introduction of magnetic materials into supports may allow easy separation of the supported metal NP catalysts from the liquid-phase reaction media.33 A Ag/SiO2−CoFe2O4 catalyst with a Ag loading of 0.98 wt % exhibited high catalytic activity in AB hydrolysis with a TOF of 264 min−1 and was easily separable using an external magnet.33a Kaya et al. have also reported Pd and Ru NP AB hydrolysis catalysts supported by SiO2−CoFe2O4 showing high activity and easy separability with TOFs of 254 and 172 min−1, respectively.33b,c Noble-metalbased catalysts on other supports, such as TiO2, CeO2, hydroxyapatite (HAP), and so forth, have also shown high catalytic activities in AB hydrolysis.34 Some representative noble -metal-based catalysts for the hydrolytic dehydrogenation of AB are listed in Table 1,35−43 in which we can see the activity order for the best monometallic noble and non-noble catalysts reported thus far. Although noble-metal-based catalysts show significant activities for H2 generation by hydrolysis of AB, practical applications require the development of low-cost metal catalysts.44−59 Nanoparticle catalysts based on first-row transition metals, such as Fe, Co, Ni, and Cu have shown high AB hydrolysis activities.18,23,60 As in the case of noblemetal catalysts, supports are critical for the high catalytic performance of non-noble-metal catalysts. Polymers have also been used to stabilize non-noble-metal NPs. Our group prepared dendrimer-encapsulated Co NPs through the complexation of Co2+ cations with the internal tertiary amine of sixth-generation hydroxyl-terminated poly(amidoamine) dendrimers followed by reduction with a mixture of AB and NaBH4 as the reducing agent.54 Well-dispersed Co NPs with an average size of approximately 1.6 nm displayed high performance in the catalytic hydrolysis of AB. We also reported highly dispersed Ni NPs in MSC-30 and MCM-41 synthesized via a chemical vapor deposition (CVD) method.47 The 18.2 wt % Ni@MSC-30 catalyst showed a TOF as high as 30.7 min−1. Sun et al. reported a Ni/CNT hybrid catalyst for AB hydrolysis.50 They used both thin and thick CNTs as supports and investigated their influence on the catalytic performance of Ni NPs. By probing the electronic structure of Ni-based NPs deposited on CNTs using scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM), a strong interfacial interaction between Ni NPs and thin CNTs due to the formation of C−O−Ni bonds was observed (O binding site came from the partly oxidized surface of thin CNTs), which benefits the suitable binding energy between the Ni NPs and the reactant, leading to superior catalytic performance in AB hydrolysis. 50 In addition, continuous-flow hydrogen generation from AB hydrolysis, which is desired for practical application, was carried out with non-noble-metal catalysts.60k,l RGO sheet-supported magnetic Co@g-C3N4 NP catalyst, where g-C3N4 could protect Co NPs from aggregating or leaching, showed high activity and durability for AB hydrolysis (Figure 2).60l In the continuousflow processes, the magnetism of Co NPs and the shape of rGO nanosheets achieved effective momentum transfer in the external magnetic field, which was an important supplement to the high catalytic activity.

Table 1. Catalytic Activities of Selected Monometallic Catalysts for the Hydrolytic Dehydrogenation of AB catalyst

temp (°C)

catalyst/AB molar ratio

TOF (min−1)

ref

Rh/CeO2 Rh/CNT Pt/CNT Ru/graphene Ru/CB Pt@MIL-101 Ru/CeO2 Rh/graphene Ru@MIL-53(Al) Ag/SiO2−CoFe2O4 Ru@MIL-53(Cr) Rh@TiO2 Pd/SiO2−CoFe2O4 Ru@nanodiamond Ru@MIL-101 Ru/SiO2−CoFe2O4 Ru/HAP Rh@PAB Pd/CeO2 metastable Ru NP Pd/RGO Ni@3D-(N)GF Cu/SiO2−CoFe2O4 Co/PEI-GO Ni@MCM-41 Ni/CNT Co/PSMA Ni/CNT Ni@mesoSiO2 Ni/ZIF-8 Co/graphene G6-OH(Co60) Ni NPs/C PEG stabilized Fe Co@N−C-700 skeletal Ni amorphous Fe

25 25 30 25 25 rt 25 25 rt 25 rt 25 25 25 25 30 25 25 25 25 25 rt 25 25 rt 25 25 rt 25 rt 25 25 30 25 25 30 rt

0.0008 0.0025 0.0047 0.002 0.00425 0.0029 0.00095 0.004 0.004 0.0028 0.004 0.0033 0.012 0.00194 0.00567 0.0005 0.011 0.0025 0.004 0.009 0.0031 0.111 0.016 0.0254 0.02 0.00425 0.016 0.013 0.013 0.0425 0.125 0.12

2010 706 567 500 429.5 414 361 325 266.9 264 260.8 260 254 229 187 172 137 130 29 21.8 6.25 41.7 40 39.9 30.7 26.2 25.7 23.53 18.5 14.2 13.9 10 8.8 6.4 5.6 5.3 3.125

35 32 31 30 36 28 37 24f 38 33a 38 34e 33b 39 40 33c 34b 26 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

To further improve the activity of non-noble-metal catalysts for AB hydrolysis, postmodified supports have been used. An amine-capped Co NP catalyst with high catalytic activity for the hydrolytic dehydrogenation of AB was reported by the Lu group.46 Exploiting amine-rich polyethylenimine (PEI) and graphene oxide (GO) as NP supports, they found that PEI deposited on GO could control the morphology, spatial distribution, and surface active sites of Co NPs. Due to the synergistic effect between amine groups and Co NPs, Co/PEIGO exhibited very high AB hydrolysis activity (TOF = 39.9 min−1).46 Moreover, Shaabani et al. employed a threedimensional N-doped graphene framework (3D-(N)GF) with immobilized Ni NPs as a catalyst,44 which showed efficient performance and high stability in AB hydrolysis. The high dispersion of Ni NPs over the 3D-(N)GFs and the consolidated interaction between the 3D-(N)GFs and Ni NPs facilitated the dissociation of hydrogen molecules and the subsequent migration of hydrogen atoms on the 3D-(N)GFs, providing a TOF as high as 41.7 min−1, which is among the highest values ever reported for non-noble monometallic catalysts.44 6894


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components that can enhance catalytic performance.67 Efficient heterometallic catalysts have been extensively used for hydrolytic AB dehydrogenation.68 By selecting different compositions, control of the electronic structure and surface metal sites could be achieved, affecting catalytic activity by influencing the interactions between substrates and catalysts as well as the formation of intermediates. In particular, Rakap reported that a polyvinylpyrrolidone (PVP)-stabilized PdRh catalyst provided a TOF as high as 1333 min−1.69 However, for noble-metal-based catalysts, the cost is one of the biggest limitations for practical applications. In combination with nonnoble metals, the obtained lower-cost bimetallic or multimetallic catalysts could exhibit considerable catalytic activities for AB hydrolysis, making the industrialization of AB dehydrogenation for chemical hydrogen storage more feasible. Some representative heterometallic catalysts with different compositions are listed in Table 2,69−103 in which we can see Table 2. Catalytic Activities of Selected Bimetallic and Multimetallic Catalysts for the Hydrolytic Dehydrogenation of AB

Figure 2. Simplified diagram of (a) a batch reactor and (b) a continuous-flow slurry-bed reactor for AB hydrolysis. Abbreviations: R, reactor; P, pump; T, tank; TE, thermoelement; TS, thermosensor; V, valve. (c) and (d) TEM images of rGO sheets supported magnetic Co@g-C3N4 NPs. Reproduced with permission from ref 60l. Copyright 2016 American Chemical Society.

The oxidation of non-noble-metal catalysts in air and under the catalytic reaction conditions is a big source of concern. The introduction of metalloid elements like boron (B) and phosphorus (P) seems to be effective for improving the stability of non-noble-metal catalysts.61 Co−B materials have been reported, showing high catalytic activity and durability in AB hydrolysis, while some of the reported Co−B materials have not been well-characterized.62 Figen et al. reported Co−B catalysts prepared by the sol−gel reaction of boron oxide with cobalt chloride hexahydrate in the presence of citric acid. This amorphous Co−B catalyst retained 90% of its initial activity after four runs.63 The Peng group reported a Co−P catalyst with an urchin-like hollow structure, which showed high magnetic recyclability and durability in the hydrolysis of AB.64 Rakap prepared a Co−P/TiO 2 catalyst by electrolytic deposition, which effectively promoted the release of hydrogen and displayed good reusability after six runs of catalytic AB hydrolysis.65 Fu et al. reported that the easily prepared crystalline Ni2P NPs with an average size of less than 12 nm exhibited high catalytic activity and sustainability with an initial TOF of 20.2 min−1 based on Ni under ambient conditions.66 The kinetics of AB hydrolysis catalyzed by Ni2P were investigated under different temperatures, providing an activation energy of 44.6 kJ mol−1. In addition, their investigation of the mechanism, based on density functional theory (DFT) calculations, suggested that the combination of Ni2P surface and substrate molecules significantly enhanced the dehydrogenation activity under ambient conditions by reducing the reaction barrier at room temperature.66 To date, many efforts have been devoted to developing efficient non-noble-metal-based catalysts for AB hydrolysis, but the catalytic activity gap between non-noble and noble-metal catalysts still exists, and further efforts are desired. 2.1.2. Heterometallic Catalysts. Heterometallic catalysts, including bimetallic and multimetallic catalysts, have attracted much attention due to the synergistic effect between

a b

6895

catalyst

temp (°C)


TOF (min−1)

ref

PdRh@PVP RuNi/TiO2 RuCo/Ti3C2X2 RuNi/Ti3C2X2 PtNi@PVP PtAuNi RhNi/graphene Pd@Co/graphene RuRh@PVP PtCo/PEI-GO Ru@Co/graphene Ru@Ni/graphene RuCuNi/CNT PtRu@PVP RuCuCo@MIL-101 Ag@CoNiFe/graphene Ag@Co/graphene NiAgPd/C AgPd@UIO-66-NH2 NiMo/graphene AuNi@MIL-101 PtRu CuNi/CMK-1 Pd@Co@MIL-101 AuCo/NXC-1 AuCo/CNT CoPd/C annealed CuPd/RGO PdNi/rGO AuCo@MIL-101 Ag-doped Ni/MIL-101 CuCo@MIL-101 CoNi/RGO SnPd/C Au@Co CoNi/graphene PtNi@SiO2

25 25 25 25 rt 25 25 rt 25 25 25 25 25 25 25 25 25 21 25 25 rt 25 25 30 rt 25 25 25 25 rt 25 rt 25 25 rt 25 30

0.003 0.001 0.02 0.02 0.0078 0.013 0.002a 0.002 0.003 0.0027 0.004a 0.004a 0.0015a 0.003 0.0012a 0.05 0.05 0.012 0.0125 0.05 0.017 0.001 0.072 0.011 0.02 0.02 0.024 0.01 0.017 0.017 0.034 0.05 0.0275 0.02 0.02 0.036

1333 914b 896b 825b 511 496 420b 408.9b 386 377.83 344b 340b 311.15b 308 214.2b 118.5b 102.4b 93.8 90 66.7 66.2 59.6 54.8 51 42.1 36.05 35.7 29.9 28.3 23.5 20.2 19.6 19.54 13.64 13.6 13.49 5.54

69 70 71 72 73 74 75 76 23h 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 68c 96 97 98 99 100 101 102 103

Catalyst/AB molar ratio calculated based on only noble metals. TOF value calculated based on only noble metals. DOI: 10.1021/acscatal.6b02209 ACS Catal. 2016, 6, 6892−6905

Perspective

ACS Catalysis the activity order for the best heterometallic catalysts reported thus far. It should be noted that some of the reported multimetallic NPs lack compositional and structural characterization. Therefore, the TOF values given for those catalysts may have potentially erroneous issues. Various non-noble-metal-involved heterometallic catalysts have demonstrated high activities in AB hydrolysis. These catalysts possess either alloy or core−shell structures. Very recently, Mori, Yamashita, and co-workers reported that RuNi alloy NPs with an average particle size of 2.3 nm well-dispersed on TiO2 showed significant activity for catalytic AB hydrolysis due to the synergistic effect between the two metals (Figure 3).70 A significantly higher total turnover number (TTO) of

For further cost reduction, noble-metal-free heterometallic NP catalysts with alloy structures have also attracted considerable attention.105 Noble-metal-free alloy catalysts have been supported on porous materials, graphene, and so on. The Kleitz group prepared highly dispersed CuNi alloy NPs supported on mesoporous materials by a simple incipient wetness method.90 Mesoporous carbons (MCNS, CMK-1) and mesoporous silica (MCM-48) were chosen as supports for CuNi alloy NPs. The chemical composition and size of metal NPs, which had a significant influence on the catalytic properties, were readily controlled by adjusting the metal loading and ratio of metal precursors. Among all catalysts, Cu0.5Ni0.5/CMK-1 with an average particle size of 6.7 nm showed the best catalytic activity in the hydrolysis of AB (Figure 4), having a TOF of 54.8 min−1, while CuNi/MCM-48

Figure 3. (A) HR-TEM image of Ru1Ni0.3/TiO2 P25. (B) Time courses for hydrogen production from AB using different Ru/Ni molar ratios. Reproduced with permission from ref 70. Copyright 2016 American Chemical Society.

approximately 153 000 over 8 h was achieved, corresponding to a TOF of 914 molH2 min−1 molRu−1, at room temperature. NPs of Pt alloys with non-noble metals have been widely studied as AB hydrolysis catalysts due to their superior activities. The Lu group supported well-dispersed PtCo NPs with a size of 2.3 nm on PEI-decorated GO, and the resulting composites showed excellent activities due to the anchoring effect of PEI for precursor metal ions.77 They observed a significant dependence of the synergistic effect on the Pt/Co ratio, with Pt0.17Co0.83/ PEI-GO being the most active catalyst (TOF = 377.83 min−1). Moreover, a series of monodispersed Pt-M alloy NPs (M = Fe, Co, Ni) were prepared by the coreduction of metal precursors with NaBH4 in the presence of polyvinylpyrrolidone (PVP) by Sun et al.73 PtFe, PtCo, and PtNi NPs with different component ratios displayed different kinetics in the catalytic AB hydrolysis. Among all catalysts, Pt/Ni (4:1) displayed the best performance with a very high TOF (511 min−1). Alloy NPs based on other noble metals (like Au, Pd, and Rh) and non-noble metals have also been employed as catalysts. Recently, our group reported MOF-immobilized Au-M (M = Ni and Co) alloy NPs as catalysts for the hydrolysis of AB.88,96 A liquid-phase concentration-controlled reduction (CCR) strategy was used to control the size and location of Au-M NPs during the reduction of metal precursors, which were introduced into the MOF pores using the DSM. The obtained Au-M NPs showed high catalytic activities for AB hydrolysis due to the synergistic effect between two metals, which was dependent on the Au/M ratio. CoPd and SnPd NPs synthesized via a high-temperature solution-phase method in the presence of oleylamine and trioctylphosphine showed high catalytic activities.94,100 Besides, other alloy catalysts composed of noble and non-noble metals (e.g., bimetallic RhNi/graphene, trimetallic RuCuNi/CNT, NiAgPd/C, etc) are also highly active in the catalytic hydrolysis of AB.75,80,85,104

Figure 4. (a) HRTEM, STEM-HAADF images, EDS element mapping and (b) corresponding metal particle size distribution histogram of Cu0.5Ni0.5/CMK-1 with metal loading of 4.7 wt %. (c) Time course of hydrogen evolution from AB hydrolysis over Cu0.5Ni0.5/CMK-1 with different metal particle sizes. (d) Mass activity and area activity as a function of metal particle size in AB hydrolysis reaction catalyzed by Cu0.5Ni0.5/CMK-1 catalysts (T = 25 °C, Metal/ AB = 0.072). Reproduced with permission from ref 90. Copyright 2015 American Chemical Society.

showed a much lower activity in this reaction than the CuNi NPs on carbon supports.90 Yamashita et al. reported a CeO2supported FeNi alloy catalyst, which was also efficient for the hydrolytic AB dehydrogenation.106 The highly dispersed and partially oxidized amorphous FeNi NPs were stabilized by strong interaction with the CeO2 support due to Ni−O−Ce and Fe−O−Ce bonding. The amorphous character was essential to attain the high activity of FeNi NPs by providing more surface active sites. CuCo alloy NPs immobilized in MIL101 via DSM also showed high activity, with a TOF of 19.6 min−1 and high durability.98 Moreover, a series of Co−B and Co−B−P NPs doped with secondary transition metals, such as Cu, Cr, Mo, and W, where dopant transition metals can act as an atomic barrier to efficiently limit the aggregation and 6896


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ACS Catalysis increase the number of active sites on the NP surface, have demonstrated activity for catalytic AB hydrolysis.107 In addition to heterometallic alloy NPs, core−shell metal NPs also display distinct chemical and physical properties due to their controllable surface metal sites and tunable electronic structures.67a In particular, one-pot reduction methods for efficient preparation of core−shell heterometallic catalysts are highly desirable. Weak reductants such as AB, methyl ammonia borane (MeAB), etc. have been used to fabricate core−shell metal NPs, where the noble metal forms the core and the nonnoble metal forms the shell, due to the differences in reduction potentials.101 When using weak reductants, noble-metal NPs are first formed as the seed. Subsequently intermediate M−H species are formed, which benefit the reduction of non-noble metals to obtain core−shell nanostructures. Our group reported the preparation of Au@Co NPs utilizing their different reduction potentials (EoCo2+/Co = −0.28 eV vs SHE, and EoAu3+/Au = +0.93 eV vs SHE).101 During the reduction process, Au NPs were obtained prior to Co reduction and acted as seeds for building the subsequently reduced Co shell. The Au@Co NPs were more active than their alloy and monometallic counterparts as catalysts for AB hydrolysis. This method was also employed to immobilize core−shell metal NPs in porous MOFs and carbons. Jiang et al. reported Pd@Co core−shell NPs confined inside MIL-101 as an AB dehydrogenation catalyst.91 As shown in Figure 5a, the precursor Pd2+ and Co2+ readily diffused into the pores of MIL-101 via DSM and were then reduced by AB, forming core−shell Pd@Co NPs. Due to the confinement effect of MIL-101, Pd@Co@MIL-101 showed a high catalytic activity for AB hydrolysis with a TOF of 51 min−1. In contract, Pd@ Co/MIL-101 with most of metal NPs located on the MOF surfaces and alloy PdCo@MIL-101 reduced by NaBH4 showed lower activities, highlighting the critical role of electronic structure modification-induced synergistic effects between Pd and Co species in the core−shell structure and the confinement effect of MIL-101.91 Zhang et al. used in situ synthesized graphene-supported Pd@Co NPs as catalysts for the subsequent hydrolytic dehydrogenation of AB.76 The activities of Pd@Co/graphene catalysts were highly dependent on the Pd/Co ratio: the catalyst with a ratio of 1:9 exhibited the highest activity (916 L mol−1 min−1). In addition, there are several reports on in situ synthesized Ru-based core−shell NPs showing high catalytic activities for H2 generation from AB.78,79,108 Recently, some works have focused on multilayer core−shell metal NPs as catalysts, owing to their possibly more tunable electronic structure and a larger number of surface active sites compared to their bimetallic counterparts.109 One-pot reduction of metal salts is also a general way to prepare multilayer core−shell metal NPs due to the corresponding intrinsic differences in reduction potentials. The characterization of multilayer core−shell NPs is mainly based on TEM, EDS, EELS, and XPS measurements, which provide information on elemental distribution. Our group synthesized magnetically recyclable Au/Co/Fe core−shell NPs, composed of a Au core, a Co-rich interlayer, and a Fe-rich shell based on the results of TEM, EDS, and EELS measurements.110 The prepared trilayer core−shell NPs exhibited much better activity for the hydrolysis of AB than Co@Fe, Au@Fe, Au@Co, and other monometallic counterparts. Wang et al. also reported trilayer core−shell metal NPs with a Ag@Co@Ni architecture, determined by elemental mapping and cross-sectional composi-

Figure 5. (a) Synthesis of Pd@Co@MIL-101, Pd@Co/MIL-101, and PdCo@MIL-101 catalysts by different procedures and reducing agents. (b) Plots of time vs volume of hydrogen generated from the catalytic hydrolysis of AB (0.875 mmol in 20 mL water) over Pd@Co@MIL101, Pd@Co/MIL-101, PdCo@MIL-101, Pd@MIL-101, and Co@ MIL-101 catalysts at 30 °C. The same Pd/Co = 0.3 (molar ratio) is applied, and the total amount of (Pd+Co) is fixed to be (Pd+Co)/AB = 0.011 (molar ratio) for all catalysts involving bimetallic NPs. The amount of Pd in Pd@MIL-101 and Co in Co@MIL-101 is equal to their respective amount in Pd@Co@MIL-101. Reproduced with permission from ref 91. Copyright 2014 Wiley-VCH.

tional line profile characterization.109c During the reduction process, silver NPs were first generated due to the higher reduction potential than the latter metal (EoAg+/Ag = +0.80 eV vs SHE, EoCo2+/Co = −0.28 eV vs SHE, and EoNi2+/Ni = −0.26 eV vs SHE), and Co was generated second, owing to its lower relative magnetic permeability than Ni, which made Co closer to the nonmagnetic Ag in a magnetic field. [email protected]@Ni0.48 showed the highest performance for AB hydrolysis among the trilayer core−shell NPs with different ratios. In addition, core−shell noble-metal-free heterometallic catalysts have also shown high catalytic activities for AB hydrolysis. Among them, catalysts with a Cu core attract the most interest. Our group first reported the synthesis of Cu@M (M = Co, Fe, Ni) NPs by a simple one-pot reduction method.111 Cu1@Co5 NPs, interconnected to form a nanochain-like structure, were the most active among all Cu@M catalysts. By following a similar procedure, various catalysts based on metal NPs with a Cu core, such as bimetallic Cu@Co, trimetallic Cu@FeCo, Cu@FeNi, and Cu@CoNi, and even tetrametallic Cu@FeCoNi have been obtained and showed high activities for AB hydrolysis.112 2.1.3. Mechanism of Metal-Catalyzed AB Hydrolysis. For metal NP-catalyzed hydrolysis of AB, some plausible mechanisms have been proposed. (1) Our group assumed the 6897


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ACS Catalysis formation of activated complex species as the rate-determining step, which are attacked by H2O, leading to a concerted cleavage of the B−N bond and hydrolysis of the resulting BH3 intermediate to produce BO2− along with H2 release.18b,44 (2) Fu proposed an almost self-powered process based on DFT calculations, which involves the formation of BH3OH− and NH4+ followed by the attack of adjacent H2O to generate H2 (Figure 6).66 (3) Referring to the mechanism of metal-

Figure 7. Ammonia borane hydrogen cycle. Reproduced with permission from ref 12b (with minor modification). Copyright 2007 American Chemical Society.

catalysts are listed in Table 3,119−129 in which we can see the activity order for the best catalysts for AB methanolysis Table 3. Catalytic Activities of Selected Catalysts for the Methanolytic Dehydrogenation of AB

Figure 6. Plot of energy changes versus reaction coordinate calculated for Ni2P-catalyzed hydrolysis of AB. Reproduced with permission from ref 66. Copyright 2015 Wiley-VCH.

catalyzed borohydride (BH4−) hydrolysis,113 Jagirdar and Chen suggested the attack of H2O on a transient M−H bond resulting in the release of H2.114 Na and Ma also proposed a mechanism similar to that of BH4− hydrolysis, in which the rate-determining step involves breaking the O−H bond in water.115 (4) Very recently, using Pt/CNT as catalyst, Chen et al. proposed a mechanism based on thermodynamic and kinetic analyses and FTIR (Fourier Transform infrared spectroscopy) measurements, and suggested the cleavage of the O−H bond in water as the rate-determining step.116 A Langmuir−Hinshelwood kinetic model was developed, which fits well with the experimental data. 2.2. Methanolytic Dehydrogenation of AB. In addition to AB hydrolysis, studies on H2 release by AB methanolysis over various catalysts have been reported. Despite the lower hydrogen capacity (3.9%), AB methanolysis has some advantages over AB hydrolysis in the following aspects: (1) the solubility of AB in methanol is higher, (23 wt % at 23 °C), and the solution is more stable at ambient conditions;20 (2) AB methanolysis can be catalyzed at temperatures below 0 °C by proper catalysts, which makes practical applications possible in cold weather;117 (3) pure H2 can be produced by methanolysis of AB without NH3 release;118 (4) the byproduct of AB methanolysis (i.e., ammonium tetramethoxyborate) is easily reconverted into AB by reacting with NH4Cl and lithium aluminum hydride at room temperature (Figure 7).12b A series of highly efficient catalysts have been reported for H2 production by AB methanolysis, such as precatalyst metal salts, including RuCl3, RuCl2, PdCl2, CoCl2, and NiCl2,12b and other noble or non-noble-metal-based catalysts. Some representative

a

catalyst

temp (°C)


TOF (min−1)

ref

Rh/CC3-R-homo Rh/CC3-R-hetero Rh/nanoSiO2 Rh/nanoHAP Ru/MMT Ru/graphene CuPd/C CoPd/C PVP-stabilized Pd PVP-stabilized Ni Cu−Cu2O−CuO/C Co−Co2B Ni−Ni3B Co−Ni−B

25 25 25 25 25 25 25 25 25 25 25 rt rt rt

0.0199 0.0199 0.00245 0.00203 0.003 0.003 0.072 0.027 0.005 0.02 0.04 0.2 0.2 0.2

215.3 65.5 168a 147a 118.1 99.4 53.2 27.7 22.3 12.1 24 7.5 5 10

119 119 120 121 122 123 124 125a 126 127 128 129 129 129

TOF value calculated based on initial hydrogen release.

reported thus far. Among these catalysts, noble-metal Rh- and Ru-based catalysts were the most active ones for AB methanolysis. Supported or stabilized metal NP catalysts are more active, controllable, and recyclable than metal salts. The Wang group prepared a Ru/MMT (montmorillonite) nanocomposite, where the Ru NPs are intercalated between silica layers and embedded on the MMT surface.122 The obtained Ru catalyst showed excellent catalytic activity for AB methanolysis with a TOF of 118.1 min−1 and satisfactory durability due to the strong immobilization of Ru clusters in the MMT interlayers. The Ru NPs with an average size of about 1.87 nm highly dispersed on exfoliated graphene oxide can afford an overall hydrogen generation rate of 24 400 ± 500 mL min−1 g−1 by catalytic AB methanolysis, with a TOF of 99.4 min−1 at 25 °C.123 The catalyst also demonstrated satisfactory durability, retaining 73.2% of the initial hydrogen generate rate on the 15th run. Very recently, our group first reported a Rh catalyst supported by an organic molecular cage.119 As a support for Rh NPs, the porous organic molecule CC3-R, belonging to the class of [4 + 6] cycloimine cages, could be utilized to homogenize the heterogeneous catalyst in solution due to its 6898


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ACS Catalysis high solubility in dichloromethane (Figure 8). Compared with Rh/CC3-R-hetero, which was obtained by drying Rh/CC3-R-

Figure 9. (A,B) TEM images of the 7 nm Co48Pd52 NPs formed from the reduction of the 0.35 mmol Co(acac)2 and 0.15 mmol PdBr2. (C) Plot of time versus volume of hydrogen generated from methanolysis of AB catalyzed by different CoPd/C catalysts and (D) plot of catalytic activity versus mole fraction of cobalt (NP = 15 mg, [AB] = 200 mM, T = 25 ± 1 °C). Reproduced with permission from ref 125a. Copyright 2012 American Chemical Society.

Figure 8. Schematic illustration of preparation of homogenized heterogeneous catalyst and its heterogeneous counterpart. Reproduced with permission from ref 119. Copyright 2015 American Chemical Society.

homo with nitrogen and redispersion in methanol solution, the solution of Rh/CC3-R-homo in methanol/dichloromethane could combine the merits of both homo- and heterogeneous catalysts, resulting in a much higher catalytic activity for AB methanolysis with a TOF as high as 215.3 min−1, which is the highest value ever reported for this reaction. Besides, the Rh/ CC3-R-homo catalyst could be easily recovered by drying in a stream of N2 and washing with water and methanol, thus exhibiting excellent durability and reusability.119 Similarly to the hydrolytic dehydrogenation of AB, low-cost non-noble-metal-based materials could be employed as catalysts for the methanolysis of AB.125 In the past few years, high catalytic activities of non-noble monometallic NPs, non-noblemetal-based bimetallic NPs, metal oxides, among others, have been demonstrated. Monodispersed 7 nm CoPd NPs with controlled compositions were prepared by the reduction of cobalt acetylacetonate and palladium bromide in the presence of oleylamine and trioctylphosphine (Figure 9).125a After deposition on Ketjen carbon, CoPd alloy NPs were tested as catalysts for AB methanolysis. Among the CoPd/C catalysts with different ratios, Co48Pd52/C was the most active, having a TOF of 27.7 min−1. In their later work, the authors replaced Co by Cu, and found CuPd NPs to be more active than CoPd NPs in catalytic AB methanolysis,124 with Cu48Pd52 NPs exhibiting the highest activity (TOF = 53.2 min−1). Yao et al. reported mesoporous Cu nanocatalysts with diverse morphologies, which were prepared by reducing mesoporous CuO nanostructures with corresponding morphologies.130 Mesoporous Cu with flower-like nanostructure was most active, having a TOF of 2.14 min−1 and an activation energy of 34.2 ± 1.2 kJ mol−1. Moreover, the flower-like Cu nanostructures also exhibited high durability in recyclability experiments. In addition, copper and copper oxide composites were also reported to catalyze hydrogen release from AB by methanolysis. Zahmakiran et al. found that Cu−Cu2O-CuO nanocomposites supported on activated carbon were active in AB methanolysis, having a TOF of 24 min−1.128 As described above, efficient heterogeneous catalysts, including noble-metal- and non-noble-metal-based ones, have

been developed for the solvolytic dehydrogenation of AB, making it highly useful for practical applications. Decay of catalytic activity is often observed in the durability test of AB solvolysis, due to several potential reasons. The aggregation of metal nanoparticles during the catalytic reaction can decrease the number of active sites. Studies indicated that metal catalysts with a more easily changed nanoparticle size exhibit lower durability.131 For the catalysts containing non-noble metals, oxidation of the latter can cause catalyst deactivation.60c,132 In addition, the byproducts of AB solvolysis (borate species) can be adsorbed on the surface of metal NPs and cover the metal active sites, resulting in a decrease of activity compared to initial values.131,133 Moreover, regeneration of AB from the byproducts of AB hydrolysis and the relatively low hydrogen generation rate of AB methanolysis are still pending issues.

3. DEHYDROCOUPLING OF AB Metal-catalyzed dehydrocoupling of AB in nonprotic solvents, including organic ones, such as tetrahydrofuran (THF), tetraglyme, among others, and ionic liquids, is also a potential way for hydrogen generation from AB. As shown in Figure 10, metal-catalyzed reaction pathways for AB dehydrocoupling have been proposed,134 and various oligomeric products, mainly aminoboranes (H 2 NBH 2 ) n , and iminoboranes (HNBH)n, are typically produced.1b

Figure 10. Proposed metal-catalyzed reaction pathways for AB dehydrocoupling. Reproduced with permission from ref 134. Copyright 2011 The Royal Society of Chemistry. 6899


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ACS Catalysis

aluminum hydride at room temperature (Figure 11a).12b However, the recycling of reductants to lithium aluminum

Although many homogeneous catalysts have shown high activities for this reaction, several heterogeneous catalysts, which are easily reused, have been developed. Burrel et al. reported heterogeneous catalytic AB dehydrocoupling by Pt, Pd, and Ru catalysts in 2-methoxyethyl ether.135 The Pt catalyst exhibited the highest activity with 1.5 equiv of H2 extracted within 30 min (Pt/AB = 0.02) and close to two equivalents of H2 extracted eventually at 70 °C. Kang et al. prepared a Pd catalyst using a tetraglyme (TG)-mediated route, which showed high activity for AB dehydrocoupling, releasing 2.3 equiv of H2 in 1 h at 85 °C (Pd/AB = 0.019).136 When the Pd NPs were embedded in the channels of SBA-15 (Santa Barbara Amorphous-15), 2 equiv of H2 were released in 1 h, with borazine and polyborazylene as products, giving an initial TOF of 9760 h−1.137 Ö zkar et al. reported several highly active heterogeneous catalysts for AB dehydrocoupling. The tertbutylammonium octanoate-stabilized Rh NPs138 and oleylamine-stabilized Pd nanoclusters139 were employed as catalysts in THF solution at room temperature. In the case of the Rh catalyst, 1.4 equiv of H2 were released (initial TOF = 342 h−1) with polymeric B−N and B = N species as products, while the Pd catalyst could release 2 equiv of H2 (initial TOF = 240 h−1) with polyborazylene and insoluble linear B−N polymers as products. The authors also reported the high activity of B−N polymer-embedded Fe NPs, which showed a TOF of 202 h−1 at 40 °C.140 Some heterogeneous catalysts could be generated in situ during catalytic AB dehydrocoupling.141 Manners et al. reported that dehydrocoupling of amine boranes is a heterogeneous catalytic process when using the rhodium precatalyst [Rh(cod)Cl]2.142 A Ru nanocatalyst (Run cluster) was also formed in situ during the dehydrocoupling of AB in THF at 25 °C using the homogeneous Ru(cod) (cot) precatalyst (cot =1,3,5-η3-cyclooctatriene), releasing more than 1.0 equiv of H2 at the complete conversion of AB to polyaminoborane and polyborazylene units and having an initial TOF of 35 h−1 at room temperature.143 Moreover, heterogeneous catalytic systems could be established in an ionic liquid medium using homogeneous precatalysts. [Rh(cod)Cl]2 can be used as a precatalyst for AB dehydrocoupling in imidazolium chloride ionic liquids, which undergoes a heterogeneous catalytic process.134 Although the hydrogen productivity and release rate of AB dehydrocoupling are lower than those of AB solvolysis, the recent progress in the efficient regeneration of dehydrocoupling byproducts amplifies the importance of catalytic AB dehydrocoupling.144 In addition, various heterogeneous metal catalysts have been developed to catalyze dehydrocoupling of AB derivatives, RxNH3−xBH3 (R = alkyl, aryl), making them potential alternatives to AB as chemical hydrogen storage materials.142,145

Figure 11. AB regeneration cycle from byproducts of (a) methanolysis, (b) hydrolysis, and (c) dehydrocoupling, respectively.

hydride is still a big challenge for industrial application. Their later work demonstrated that trimethoxyborane (B(OMe)3), which can be produced by the reaction of NH4·BO2 (the byproduct of AB hydrolysis) with H2O and methanol (eqs 5 and 6), could be reconverted to AB using a similar strategy (Figure 11b).146d,147 Therefore, the regeneration of AB from hydrolytic byproducts requires more energy than that from the methanolytic ones. It should be noted that AB and sodium borohydride (SB) share borates as the spent fuel, and SB can be employed as the starting material for regenerating AB.148 Thus, we can infer the recycling process of spent fuel in the AB hydrolysis system from the SB hydrolysis system.12c According to the literature reports on recycling borates to SB, this process still needs improvements.149

4. REGENERATION OF AB For the practical application of AB as a hydrogen storage material, methods for its efficient regeneration, which depend on the dehydrogenation strategy, are needed.146 As mentioned above, the byproducts of AB dehydrogenation are mainly borate species for solvolysis and BNHx for dehydrocoupling. Borate species, the byproducts of AB solvolysis, are so stable that much energy is required for regenerating AB from them. The Ramachandran group reported that AB can be regenerated from the byproducts of its methanolysis (i.e., ammonium tetramethoxyborate) by reacting with NH4Cl and lithium

NH4 ·BO2 + 2H 2O → NH4B(OH)4

(5)

NH4B(OH)4 + 3MeOH → B(OMe)3 + NH3 + 4H 2O (6)

In the last several years, many efforts have been devoted to the regeneration of AB from BNHx, which mainly consists of three major steps: digestion (H+ addition), reduction (H− addition), and ammoniation (NH3 addition), in addition to the necessary recycling steps.146 Many digesting (i.e., strong acids, alcohols, and amines) and reducing agents (i.e., transition metal hydrides and main group hydrides) have been developed to 6900


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ACS Catalysis regenerate AB.144,146,150 However, most of the regeneration systems still have the problem of recycling the reducing agents, which is important to complete the regeneration cycle. Recently, Yu et al. have developed a complete AB regeneration cycle using Bu3SnH for the reductive dechlorination of BCl3 and Et2PhN as a “helper ligand” to generate Et2PhN·BH3, resulting in a high yield of AB at ambient temperature.151 In addition, polyborazylene can be converted to AB nearly quantitatively by treatment with hydrazine in liquid ammonia at 40 °C.144c As the above-mentioned processes are multistep and energy-consuming, new high-yielding and energy-efficient processes for the regeneration of AB are highly desired.

heterogeneous catalytic hydrogen generation by AB dehydrogenation, including solvolysis in protic solvents (hydrolysis and methanolysis) and dehydrocoupling in nonprotic solvents, achieved in the recent few years. Metal NP catalysts, including monometallic and heterometallic ones, have shown excellent catalytic activity and recycling performance in AB dehydrogenation. We have also summarized the methods for regenerating AB and analyzed the advantages and issues of solvolysis and dehydrocoupling. Although great progress has been achieved in heterogeneous catalytic dehydrogenation, further efforts are needed for solving pending issues, such as breaking the strong B−O bonds in byproducts of AB solvolysis, reducing NH3 release, and increasing the hydrogen productivity of AB dehydrocoupling. Furthermore, some new methods, (e.g., fabricating effective photocatalysts) have been developed for promoting the hydrolysis of AB.153 We are looking forward to new developments of metal nanoparticle-catalyzed AB dehydrogenation and its practical applications.

5. ADVANTAGES AND ISSUES OF AB DEHYDROGENATION Recently, tremendous progress has been achieved in the solvolytic dehydrogenation of AB. In addition to the improved catalytic performance obtained using noble-metal catalysts, lowcost non-noble-metal-based catalysts have also shown considerable activities for hydrogen release.148 However, there are still several issues related to AB solvolysis. The first one is the storage irreversibility and cost factor. The regeneration of AB from byproducts of solvolysis, especially hydrolysis, is costineffective, with undesired byproducts of the recycling process that cannot be converted to the main reactants. For hydrolysis, excess water is required for the hydration of borates due to the limited solubilities of AB and metaborate in water, which is another issue that obstructs the practical use of AB hydrolysis.22a In this case, the best solution may be storing AB in the solid state, with on-demand release of H2 by injecting the amounts of water required for hydrolysis. For example, a material-based excess gravimetric hydrogen storage capacity (GHSC) of 7.8 wt % has been reported for addition of a minimum amount of H2O to AB.152 In addition, the generation of NH3 from NH4+ during the hydrolysis of AB, which is detrimental to fuel cells, is also one of the drawbacks limiting the use of AB. Despite these issues, the high stability of AB in water and methanol and its fast hydrogen release rate using catalysts under ambient conditions make AB solvolytic dehydrogenation a promising hydrogen generation system, especially for cases that require a convenient and reliable hydrogen source. Heterogeneous catalytic AB dehydrocoupling in nonprotic solvents can achieve hydrogen release at relatively lower temperatures, which makes up the shortcomings of the hightemperature requirement in thermolysis. Compared with AB solvolysis, the progress in developing efficient methods for AB regeneration from dehydrocoupling byproducts is more significant. Many systems have been exploited to regenerate AB from a variety of hydrogen-depleted fuel forms. However, most of these systems offer incomplete regeneration cycles, and the ammoniation reaction proceeds with a relatively low yield, low energy efficiency and requires high temperature, indicating that the development of efficient strategies for the recycling of AB is still a big issue. More importantly, the relatively lower hydrogen productivity and release rate of dehydrocoupling are crucial issues to overcome in future research.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to the editor for the kind invitation and to the reviewers for valuable suggestions. We would like to acknowledge the contribution from all researchers in this area and the fine work of the talented and dedicated graduate students, postdoctoral fellows, and colleagues who have worked with us and whose names can be found in the references. The authors thank AIST and Kobe university for financial support. W.W.Z. thanks the China Scholarship Council (CSC) for a scholarship of a joint Ph.D. program.

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REFERENCES

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Dehydrogenation of Ammonia Borane by Metal Nanoparticle Catalysts

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