English

• 1. Introduction

  H₂ is a clean and renewable fuel. So it will play a vital role in transition from conventional fossil fuels to renewable energy sources on the way towards a sustainable energy future. Like H₂ production from ammonia borane, water splitting is also a promising startegy [1-3]. The safe storage and transportation of H₂ has become the most important challenge. Ammonia borane (AB), the simplest type of B–N hydride, with a molecular weight of only 30.7 g/mol and a hydrogen storage capacity of up to 19.6 wt%, which was successfully synthesized in 1955 by Shore and Parry [4]. Ammonia borane become the most promising hydrogen carrier materials because it has some excellent physical and chemical properties, such as remain stable solid form at ambient conditions, high hydrogen content, eco-friendly, and high solubility in most solvents. AB coulde released ultrapure hydrogen via dehydrogenation reaction, hydrolysis reaction and methanolysis reaction.

  2. Synthesis of AB

  In the 1950s, with the help of diethyl ether, Shore and Parry [4,5] synthesized AB through lithium borohydride and diammoniate of diborane. The reaction process is expressed as follows:

  (1)

  (2)

  Yields of AB are only 45% of this process, which is far from enough for large-scale commerce applications of AB. Enhancing the yields of AB are one of the main goals. Heldebrant et al. [6] through metathesis of NH₄X and MBH₄ salts in liquid NH₃ to synthesize AB and it is synthesized efficiency reach 99%. The reaction process is as follows:

  (3)

  So far, the high efficiency of AB has been achieved, which greatly promotes the wide application of AB.

  Since 1995, the synthesis efficiency of aminoborane has increased from an initial 45% to 99% (prepared by the reaction of NH₄X + MBH₄ in liquid ammonia, where X represents halogen and M represents alkali metal). Ramachandran from Purdue University in the United States and his scientific research team have been committed to the study of the preparation reaction of aminoborane for many years. In 2015, they further optimized the reaction conditions, so that SBH (NaBH₄) and (NH₄)₂SO₄ could react to prepare aminoborane in an open environment in the presence of THF(C₄H₈O), and the reaction yield could reach more than 90%. This greatly reduces the price of commercial ammoniborane and improves the practicality of large-scale application of AB [7-9].

  3. Thermal dehydrogenation of AB

  One method of releasing hydrogen from AB is thermal dehydrogenation [5-7]. The process of thermal dehydrogenation of AB as follows (Fig. 1a):

  (4)

  (5)

  (6)

  Figure 1

  

  Figure 1.  (a) TPDS/MS profiles of neat AB powder and AB loaded over Co-P-B/Ni catalyst, by using THF based solution. The scanned temperature was between 50 ℃ and 200 ℃ at heating rate of 2 ℃/min. The hydrogen and borazine with atomic mass of 2 and 80, respectively, have been highlighted in the figure. Copied with permission [12]. Copyright 2012, Elsevier. (b) The release of H₂ between 75 ℃ and 175 ℃ (covering the two-step dehydrogenation of AB). Copied with permission [14]. Copyright 2018, American Chemical Society. (c) Temperature and H₂ equivalent profiles for AB with two additives (bottom AB: MA = 1:1, top AB: BA = 1:1). MA: maleic acid (C₄H₄O₄). BA: boric acid. Copied with permission [15]. Copyright 2020, Elsevier.

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  This indicates that AB is stable at a lower temperature (< 90 ℃), and the release of H₂ requires a higher temperature, which limits the applicable of AB. So, looking for a catalyst material that catalyzes the ammonia borane dehydrogenation reaction at room temperature or about room temperature has been a major challenge. Experimental and theoretical efforts have been made to achieve this goal.

  3.1 Experimental research

  In order to make full use of H₂, the dehydrogenation of ammonia borane should be realized in the near proton exchange membrane (PEM) fuel cell operating temperatures (−85 ℃) [10,11]. Co-P-B coating on Ni (Fig. 1a) [12] and silicon nanoparticles [13] catalysts were used as catalysts to study thermal decomposition of ammonia borane and can achieve the maximum hydrogen production rate at around 80 ℃. At the same time, for safety, we should ensure that the mixture of AB and catalyst is stable below 50 ℃ and does not produce H₂. Therefore, it is very important to raise the triggering temperature of catalyst to about 60 ℃. By heating NH₃BD₃ to about 200 ℃ until two H₂ molecules are released, hydrogen (HD) was released mainly from heteropolar hydrogen-deuterium interactions (N−H^δ+··· ^δ+D−B) and homopolar dihydrogen interactions (N−H^δ+···^δ+H−N), and homopolar dideuterium interactions (B−D^δ−···^δ− D−B) is negligible (Fig. 1b) [14]. As shown in Fig. 1c [15], using this configuration, a mixture of AB: MA = 1:1 was placed at the bottom, while another mixture of AB: BA = 1:1 was added at the top, the H₂ equivalent was improved to 2.11 at 60 ℃. This also increases the H₂ yield and lowers the temperature required for the reaction.

  Cu metal has better catalytic activity for AB thermal catalytic process. As shown in Figs. 2a and b, the study have shown that CuCl₂ has the highest catalytic activity at 85 ℃ among FeCl₂, CoCl_2, NiCl₂, CuCl₂, and ZnCl₂ [16]. Moreover, Salinas-Torres et al. [17] added Cu atom in La_0.7Sr_0.3CoO₃ and realized the highest catalytic activity among, when containing 2.72 wt% of Cu, turnover frequency (TOF) reaches 843 L H₂/h. In this process, the closer interaction Cu-Co might be favoring the catalytic activity. The catalytic process is shown in Fig. 2c.

  Figure 2

  

  Figure 2.  Mass losses at 85 ℃ as a function of (a) the electronegativity and (b) the electron affinity of M (i.e., Fe, Co, Cu, Zn and Pt) of MCl₂ (Insets showed the plots when Ni is taken into consideration). Copied with permission [16]. Copyright 2012, Elsevier. (c) Schematic diagram of AB dehydrogenation catalyzed by Cu-doped La-Sr-Co-O compound. Copied with permission [17]. Copyright 2019, Elsevier.

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  3.2 Theoretical calculations

  Tong et al. [18] construct two catalyst structures of AB-Pd₂/MgO and AB-Pd₄/MgO to investigated dehydrogenation mechanism of ammonia borane by DFT/UB3LYP method (Fig. 3a). The calculation results are shown in Fig. 3a and rate determining step (RDS) barrier reach 9.4 and 12.6 kcal/mol, respectively (Fig. 3b). Zhou et al. [19] calculated the catalysts of Fe₂, FeCu, and Fe₁₂Cu to catalyze dehydrogenation of AB through Gaussian 03 Program and for Fe₁₂Cu catalyst, overall exothermic and barrierless were achieved, as shown in Fig. 3c. Kuang et al. [20] calculated various possible paths that pristine and defective h-BN sheets catalyst material catalyzed the dehydrogenation of AB to H₂, indicating that defective was conducive to improving the catalytic activity. The defective h-BN sheets with VB and V₃B+N vacancies are excellent metal-free catalysts for facilitating the hydrogen release of AB [20]. As shown in Figs. 3d and e, the energy barriers to release the H₂ are 0.77 and 0.84 eV, respectively. This is a lower activation barrier than N-doped graphene catalysts. Wu et al. [21] showed that single Pt atoms supported on oxidized graphene (Pt₁/Gr-O) catalyst can catalyze AB hydrolysis. The rate-limiting barrier is 16.1 kcal/mol, and the catalyst can spontaneously recover after H₂ release. The production process of the first H₂ molecule is shown in Fig. 3f. Based on the Kissinger method and isoconversional model-free fitting method of AB, and solid-state kinetics model-based method, Gangal and Sharma [22] obtained a new mechanism, based on the fundamental kinetic equations and isothermal experimental data.

  Figure 3

  

  Figure 3.  (a) Optimized structures on the Pd₂/MgO(100) and Pd₄/MgO(100)surface for the stepwise dehydrogenation pathway. Copied with permission [18]. Copyright 2013, Elsevier. (b) Schematic Gibbs free energy profiles for the concerted and stepwise dehydrogenation pathways: on the Pd₂/MgO surface and on the Pd₄/MgO surface. Copied with permission [18]. Copyright 2013, Elsevier. (c) The reaction pathways for the dehydrogenation reactions between Fe₁₂Cu dimer and AB molecule. Copied with permission [19]. Copyright 2016, Elsevier. (d, e) Energetic profiles for the reaction between AB and defective h-BN sheets. Copied with permission [20]. Copyright 2016, Elsevier. (f) The relative energy profiles of the releasing of the first hydrogen molecule from NH₃BH₃ catalyzed by Pt₁/Gr-O. Copied with permission [21]. Copyright 2018, Chinese Physical Society.

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  We focus on the thermal decomposition of AB catalyzed by two-dimensional (2D) materials or nanoclusters (Fig. 4). Using 2D MgSiP₂ structure [23] as the catalyst, we achieved the dissociation of 6 H atoms in AB, and 1 mol AB could generate 3 mol H₂ without the participation of H₂O molecules in the reaction. AB dehydrogenation is realized on the surface of Os/P₃C monatomic structure [24] catalyst, and the calculation shows that the doping of O atom can greatly reduce the activation barrier. The reaction process of AB thermolysis dehydrogenation was simulated by Fe₂₂@Co₅₈ core-shell structure and Fe₃₆Co₄₄ bimetallic nanocluster catalyst [25,26].

  Figure 4

  

  Figure 4.  (a) The structure of adsorbed intermediate states on the 2D MgSiP₂ structure and Gibbs free energy on the BN alternating dehydrogenation. Copied with permission [23]. Copyright 2022, Elsevier. (b) The complete free energy (ΔG) correction diagram and related structure diagrams for the dehydrogenation of AB molecules on the surface of Os/P₃C and Os/P₃C_O. Copied with permission [24]. Copyright 2022, Elsevier. (c) Fe₂₂@Co₅₈ core-shell structure and (d) Fe₃₆Co₄₄ bimetallic nanoclusters catalyzed dehydrogenation of AB by thermal decomposition. Copied with permission [25,26]. Copyright 2022, Elsevier.

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  4. Hydrolysis and methanolysis of AB

  4.1 Hydrolysis of AB

  The hydrolysis of AB is one of the safe and stable methods to prepare H₂. The hydrolysis reaction path with break of B-N bond can be divided into four types (Eqs. 7–10) [25,27,28]. There is no difference in the formation process of the first H₂, and the difference mainly comes from the formation of the second and three H₂, as shown in the following formula:

  (7)

  (8)

  (9)

  (10)

  In the other case that the B-N bond is not broken, the first H₂ molecule is produced as follows [29]:

  (11)

  Taking Pt₁Co₁ as an example, the possible mechanism for the AB dehydrogenation reaction [30] is shown in Scheme 1.

  Scheme 1

  

  Scheme 1.  (a–d) The possible mechanism for the AB dehydrogenation reaction on PtCo NP alloy.

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  4.1.1   Experimental research

  Hydrolysis reaction is an effective catalytic process of AB dehydrogenation, and a variety of catalyst materials can show excellent catalytic activity (Table 1) [27-67]. Among them, Ru_0.5Ni_0.5/g-C₃N₄ catalyst has the lowest activation barrier energy (E_a), reaching 14.1 kJ/mol. It can be found that the presence of noble metal Ru can greatly improve the reactivity of the catalyst and the reaction rate. The RuP@NHMCs catalyst has the largest TOF of 4307 mL mol_catal ⁻¹ min⁻¹ in the presence of 0.2 mol/L NaOH. This enhancement effect confirms that NaOH can behave as the catalyst promoter to facilitate the hydrolysis of AB at room temperature.

  Table 1

  

  Table 1.  Activities of catalysts in the hydrolysis of AB at room temperature.

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  From Table 1, it can be seen that element P plays an important role in the catalytic process. The comparison of catalytic TOF of CoNiP/GO and NiCo-NC shows that the catalytic rate is increased by nearly 4 times. With the addition of P element, the catalytic activity barrier of CoP nanoparticles and Co/C is decrease significantly. In addition, the transition metal Ni also has a good catalytic activity. Comparing Ru_0.5Ni_0.5/g-C₃N₄ and Ru/CS, it can be found that due to the addition of Ni, the reactivity activation barrier becomes 1/3 of the previous one. Therefore, non-metals and transition metals also play an important role in the design of catalysts.

  4.1.2   Theoretical calculations

  Theoretical simulation can describe the structure and energy change of the reaction process more specifically. From the perspective of computational simulation, Peng and co-workers [27] described the formation process of the first H₂ molecule in detail with Ni₂P-catalyzed, as shown in Fig. 5. The remaining two H₂ molecules are formed by dropping the H atom which attached to B atom. The step-by-step reaction process can be expressed as follows (Eqs. 12–15):

  (12)

  (13)

  (14)

  (15)

  Figure 5

  

  Figure 5.  (a) S_N2 reaction mechanism of AB dehydrogenation process. Copied with permission [28]. Copyright 2017, the Royal Society of Chemistry. (b) Plot of energy changes versus reaction coordinate calculated for Ni₂P-catalyzed hydrolysis of AB. Copied with permission [27]. Copyright 2015, Wiley. (c) Calculation of AB hydrolysis reaction catalyzed by NiCo₂P₂ and Ni₃P₂. Copied with permission [28]. Copyright 2017, the Royal Society of Chemistry.

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  The total reaction can be expressed as Eq. 16, and the whole reaction requires the participation of 4 H₂O molecules. After crossing the barrier of 0.12 eV (due to the binding of AB with the active atom of the catalyst), the whole reaction can occur spontaneously at room temperature.

  (16)

  The break of the B-N bond in AB can also be achieved by OH⁻ attack, which is known as the S_N2 reaction mechanism. Hou et al. [28] simulated the formation of the first H₂ molecule by S_N2 reaction mechanism on Ni₃P₂(0001) and NiCo₂P₂(0001) surface catalysts.

  As shown in Fig. 6, among the catalyst structures of Pt₁/Co₃O₄, Pt₁/CeO₂, Pt₁/ZrO₂, and Pt₁/ graphene, Pt₁/Co₃O₄ has the largest TOF (1220 min⁻¹) and the smallest E_a (37.4 kJ/mol) [68] and Pt₁/Co₃O₄ still maintained good catalytic activity after 15 cycles. As shown in Fig. 6e, after the adsorption of AB by Pt₁/Co₃O₄, the 5d electron of Pt atom interacts strongly with H atom, resulting in obvious elongation of B-H bond.

  Figure 6

  

  Figure 6.  (a–d) Catalytic performance of Pt₁ SACs in hydrolytic dehydrogenation of AB at 25 ℃. (e) Local partial density of state (LPDOS) projected of Pt₁/Co₃O₄ on the Pt₁ 5d orbitals with the Fermi level set at zero and the local configurations for AB adsorption. Copied with permission [68]. Copyright 2019, American Chemical Society.

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  Using non-noble metal Fe and Co elements as main components, we completed the calculation of the catalytic hydrolysis reaction of AB catalyzed by Fe₃₆Co₄₄ bimetallic nanoclusters [26]. The path diagram is shown in Figs. 7a and b. Meanwhile, the different catalytic effects of different activation and coordination environments were analyzed in this work. We have completed the complete calculation process of AB hydrolysis to prepare 3 mol H₂ using Fe₂₂@Co₅₈ core-shell as catalyst material (Fig. 7c), and it was found that there were new reaction paths (Eq. 17) beyond the paths shown in Eq. 16 (Fig. 5c) [25].

  (17)

  Figure 7

  

  Figure 7.  (a, b) Reaction paths of Fe₃₆Co₄₄ Bimetallic nanoclusters catalyzed AB hydrolysis. Copied with permission [26]. Copyright 2022, Elsevier. (c) Fe₂₂@Co₅₈ core-shell catalytic hydrolysis of AB to prepare 3 mol H₂. Copied with permission [25]. Copyright 2021, Elsevier.

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  In the case that the B-N bond is not broken, Li et al. [29] gave the catalytic reaction process on the Pt/Ni surface catalyst from the perspective of calculation simulation (Fig. 8a). From the calculation, it can be found that the incorporation of Pt atom reduces the reaction rate-determination step activation barrier (0.75 vs. 0.88 eV). Wu and co-workers [34] calculated the first H₂ formation process of Co(111), CoO(200), and CoO/CoP surfaces respectively from the perspective of calculation. And the high activity of CoP-CoO/surfaces was revealed (Fig. 8b). On the doped Ni(0001) surface, the rate-determining step of the AB hydrolysis reaction is the dissociation of H₂O molecule. Therefore, by means of the first-principles calculation, Li et al. studied the dissociation process of H₂O molecule [39] and the reaction path shown in Fig. 8c.

  Figure 8

  

  Figure 8.  (a) Simulated pathways of hydrolytic dehydrogenation of AB at Pt centre on atomic Pt substituted Ni surface (top) and at Ni centre on pristine Ni surface (bottom). Copied with permission [29]. Copyright 2017, American Chemical Society. (b) Energy profiles of AB and H₂O molecules dissociated on Co(111), CoO(200), and CoO/CoP surfaces. Copied with permission [34]. Copyright 2021, Elsevier. (c) Dissociation process and energy changes of H₂O molecules on fcc-Ni(111), hcp-Ni(0001), and hcp-CuNi(0001) model surfaces. Copied with permission [39]. Copyright 2022, Elsevier.

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  Therefore, the specific path of AB hydrolysis reaction is different for different catalysts, and this process needs to be further studied.

  4.2 Methanolysis of AB

  Liberation of NH₃ and inefficacy in recycling the hydrolysis product, metaborate become the mianly problems for hydrolysis of AB. Using methanol instead of water is a better choice, since that Ammonia borane has high solubility in methanol (23 wt% at 23 ℃), methanolysis produces pure H₂ gas with no ammonia contamination, and the methanolysis product, ammonium tetramethoxyborate, is recyclable. The technology of ammonia borane methanolysis is another prominent technology that suitable for application in on-board hydrogen production. The methanolysis product of NH₄B(OCH₃)₄ can be converted back to AB by reaction with LiAlH₄ in the presence of NH₄Cl [69-72]. The process of Methanolysis of AB is shown in Eq. 18:

  (18)

  Methanolysis of AB occurs only in the presence of a suitable catalyst. Various catalytic systems consisting of homogeneous and heterogeneous metal catalysts, such as metal cations, monometallic nanoparticles (NPs), alloys, and core-shell catalysts have been studied for catalytic AB methanolysis. Among them, noble metal catalysts such as Pd [70,73-75], Pt [76], Ru [77-81], Rh [82-89] and Cu [90-92] nanoparticles have been determined to have superior catalytic activities for H₂ release from AB methanolysis. The results are shown in Table 2. Ru/Immobilized in Montmorillonite catalyst had the lowest activation barrier, indicating that the noble metal has good catalytic activity. However, their high cost and less content hinder their widespread use for practical applications. Therefore, development of cost-saver, efficient, and reusable supported catalysts is very vital to overcome aforementioned problems for on-board applications. So far, there are few reports on computational simulation of methanolysis reaction, which needs to be supplemented by researchers in subsequent studies.

  Table 2

  

  Table 2.  Activities of catalysts in the methanolysis of AB at room temperature.

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  As shown in Table 2, TOF and E_a of methanolysis of AB are listed. It can be found that Ru/MMT [77] has the lowest E_a, indicating that noble metal catalyst has high catalytic activity. CuNi NPs [92] also has a low E_a, indicating that transition metal elements also can serve as a good methanolysis of AB catalyst.

  5. Photocatalysis

  Photocatalysis is a clean reaction process. It is an important research direction to realize the dehydrogenation process of AB under visible light (λ > 420 nm) by reducing the band gap of semiconductor catalyst.

  5.1 Experimental research

  Visible light is the most abundant and convenient clean energy, which is constantly available and very easy to obtain. Therefore, visible light catalysis is an important research direction. For example, Au and Ag [93] are excellent photocatalytic materials with excellent LSPR in the ultraviolet and near-infrared regions. Au nanoparticles deposited on TiO₂ [94] catalysts can be used as electron traps to prevent electron-hole recombination, thus improving catalytic activity.

  Oxides are often important photocatalytic materials. under visible light irradiation (λ > 420 nm), MoO_3−x [95,96] and WO_3−x nanowires [97] could catalyze ammonia borane to generate H₂ (Figs. 9a and c), which opens up new prospects for efficient heterogeneous catalysis of plasma semiconductor nanostructures. Through the load of the noble metal Rh, the activity of Rh/WO_3-x [98] catalysts were further improved with a TOF as high as 805.0 mol_H2 mol_Rh⁻¹ min⁻¹ and an apparent activation energy of 45.2 kJ/mol (Fig. 9b).

  Figure 9

  

  Figure 9.  (a) Time course of H₂ evolution from NH₃BH₃ aqueous solution at room temperature for different samples: in the dark and under visible light irradiation (λ > 420 nm). Copied with permission [95]. Copyright 2014, Wiley. (b) Structure model of Rh/WO_3−x hybrid nanowire catalysts. Copied with permission [98]. Copyright 2019, Royal Society of Chemistry. (c) H₂ evolution rate from ammonia borane over different catalysts: comparison of no catalyst, commercial WO₃ and WO_3−x-160 in dark and under light irradiation; comparison between WO_3-x and WO_3−x-120 in dark and under light irradiation. Copied with permission [97]. Copyright 2015, Wiley.

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  As shown in Fig. 10, in Pd/H₂Ti₃O₇ composite nanowires catalyst [99], excited electrons accumulate to Pd under light conditions, so the existence of Pd promotes the separation of electron-hole and prevents electron-hole recombination. This synergy enhanced the catalytic activity of Pd/H₂Ti₃O₇ composite nanowires. Monometallic Co and bimetallic CuCo and FeCo catalysts supported by g-C₃N₄ are catalysts for transition metal nanoclusters [100]. After electrons are activated in g-C₃N₄ under light, they transfer to metal atoms. The metal atom becomes the center of catalytic activity. Taking CoAu/g-C₃N₄ catalyst [101] as an example, under the influence of the Mott-Schottky effect, electrons are transferred from the conduction band of g-C₃N₄ to metal and a potential barrier is formed to prevent electron countercurrent. Under the influence of light, more electrons are excited into the metal, increasing the separation of charge.

  Figure 10

  

  Figure 10.  (a) Schematic of the proposed mechanism for the hydrolysis of ammonia borane on Pd/H₂Ti₃O₇ composite nanowires. Copied with permission [99]. Copyright 2015, Elsevier. Comparison of TOF between (b) CoM/g-C₃N₄ (M = Cu, Fe, Ni) and (c) M/g-C₃N₄ (M = Co, Ni) catalysts. Copied with permission [100]. Copyright 2016, Royal Society of Chemistry. (d) Schematic view of Mott-Schottky type contact based on Au-Co hybrid nanoparticles. Copied with permission [101]. Copyright 2014, American Chemical Society.

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  With favorable transfer properties, better dispersion, CdS-TiO₂/carbon nanocomposite with high surface area [102] has become a good catalyst for ammonia borane dehydrogenation. Using non-noble metal nanoparticle supported on a chromium-based MOF (MIL-101) [103], such as Cu/MIL-101, can efficient catalysts H₂ production in the hydrolysis of AB. The electrons generated by illumination concentrate on the surface of Cu atoms, making Cu atom become a good reduction catalytic site, and MIL-101 becomes a good oxidation active site. Co and Ni NPs supported by these C₃N₄ species [104], where TOF of Co/C₃N₄ reaches 93.8 min⁻¹, which exceeded the values of all the reported heterogeneous noble metal-free catalysts, has excellent catalytic activity. Co and Ni NPs supported by seven photoactive and non-photoactive MOFs [105], Co-based catalysts had the highest activities among the reported noble-metal-free catalysts at 298 K. The TOF value is 117.7 min⁻¹ and the activity almost as good as that of noble metal catalyst.

  Under light conditions, NiCu alloy loaded carbon nitride nanosheets (Ni_xCu_y/CNS) [106] realized the hydrolysis process of AB (Figs. 11a and b). Under the effect of localized surface plasmon resonance (LSPR), charge accumulates from Cu to Ni, and the catalytic reaction with Ni as the active site achieves the break of B-H bond in AB and O-H bond in H₂O. In Au-Co nanoparticles (Au-Co@CN), electrons flow from g-C₃N₄ and Co atoms to Au atoms, and thus Au act as active atoms (Figs. 11a and b). For Co/P_3.59CN nanocluster catalyst [107], electrons accumulate towards Co atoms, which makes Co atoms behave more like noble metals with higher activity (Figs. 11c and d). At 298 K, TOF reaches to 67.09 mL_H⁻¹ min_Co⁻¹. The comparison between Ru/g-C₃N₄ and Ru/C/g-C₃N₄ shows that [108], when Ru/C(1.0)/g-C₃N₄, average Ru NP size is the smallest, and the maximum TOF reaches 196.4 h⁻¹. The TOF of RuP₂-/g-C₃N₄ [109] could reach 175 min⁻¹ under light conditions (Fig. 11e). At the same time, the change of P atom ratio will greatly affect Schottky barriers and promote electron movement (Figs. 11c and d). Oxide based Co/V₂O₅ [110] and Ti₂O₃ [111] have a narrow band gap, which can realize the catalytic process of ammonia borane dehydrogenation under light conditions. In Co/V₂O₅, photogenerated electrons transfer from V₂O₅ to Co atoms. It is an important research direction to realize AB dehydrogenation at low temperature. Through a reduction transformation method, nanoscale Ti₂O₃ particles with high chemical stability and narrow band gap are prepared, realizing a rapid production of 2.0 equiv. of hydrogen from AB at ambient temperature (Fig. 12a) [105,111]. In NiCoP/TiO₂ [112] catalyst, under visible light NiCoP work as sensitizer to absorb light and generate electron-hole pair while TiO₂ work as electron trapper. So positively charged NiCoP surface and negatively charged TiO₂ surface are achieved under visible light, and the H₂ generation rate from hydrolysis of AB was increased to 2.0 folds, with E_a reduced from 52.76 kJ/mol to 25.89 kJ/mol. However, if 2.5%Pt is added to TiO₂, the reaction rate will increase to 0.55 mL/s (Fig. 12b). In different proportions of CuNi/TiO₂-CdS, Cu_0.45Ni_0.55/TiO₂-CdS catalyst [113] had the fastest hydrogen evolution rate with a high conversion frequency (TOF) of 25.9 mol_H mol_cat⁻¹ min⁻¹ at 25 ℃ and low activation energy of 32.8 kJ/mol (Fig. 12c). The band gap of TiO₂ structure modified by Fe³⁺ and graphene decreases [108,114], and the light absorption spectrum line has an obvious red shift (Fig. 12d). When added graphene concentration is 1% and Fe³⁺ concentration is 2%, H₂ production efficiency is the highest, reaching 1235.32 µmol min⁻¹ g_cat⁻¹ (Fig. 11d).

  Figure 11

  

  Figure 11.  (a) The plausible mechanism of ammonia borane hydrolysis on NiCu/CNS. Copied with permission [106]. Copyright 2020, American Chemical Society. (b) Activity comparison in the dark and under visible light irradiation with or without NaHCO₃ as a hole scavenger over NiCu/CNS. Copied with permission [106]. Copyright 2020, American Chemical Society. (c) Hydrogen evolution curves and TOF for the AB hydrolysis catalyzed by Co/P₀CN, Co/P_2.16CN, Co/P_3.59CN, Co/P_5.26CN, and Co/P_8.49CN under visible-light irradiation ([AB] = 170 mmol/L, 5 mL, n_Co/n_AB = 0.02, and T = 298 K). Copied with permission [107]. Copyright 2021, The Royal Society of Chemistry. (d) Structural model of Co/PCN catalyst. Copied with permission [107]. Copyright 2021, The Royal Society of Chemistry. (e) Schematic of the photocatalytic hydrolysis mechanism of AB using the prepared RP₂/CN. Copied with permission [109]. Copyright 2020, Elsevier.

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  Figure 12

  

  Figure 12.  (a) Hydrogen release from AB under different photothermal and thermal reaction conditions, with or without Ti₂O₃ photothermal materials (RT: room temperature). Copied with permission [111]. Copyright 2020, Wiley-VCH GmbH. (b) Schematic diagram of AB hydrolysis catalyzed by NiCoP nanostructure. Copied with permission [112]. Copyright 2019, Elsevier. (c) Schematic diagram of charge separation and transfer in the Cu_0·45Ni_0·55/TiO₂CdS NTs system under visible-light irradiation. Copied with permission [113]. Copyright 2019, Elsevier. (d) Photocatalytic hydrolysis of ammonia borane to produce hydrogen. Copied with permission [114]. Copyright 2021, American Chemical Society.

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  Au-Ru alloy [115] and Ag@Pd core-shell nanocubes [116] catalysts show significant light-boosting effect. Among them, the TOF of Au-Ru alloy catalyst reaches 240 mol_H mol_cat⁻¹ min⁻¹. Under the influence of light, electron transfer can be realized. For example, in Co-CeVO₄/CeO₂ catalyst [117], electrons are transferred from CeO₂ to CeVO₄ and then to Co atoms, thus forming electron-hole centers, respectively (Fig. 13).

  Figure 13

  

  Figure 13.  (a) Proposed mechanism for the enhanced catalytic activity of Ag@Pd core-shell catalyst for AB hydrolysis under light illumination. Copied with permission [116]. Copyright 2020, American Chemical Society. (b) Mechanism for the visible-light-accelerated hydrolytic dehydrogenation of AB nanocatalyzed by Au₁Ru₁@Dendrimer1. Copied with permission [115]. Copyright 2020, American Chemical Society. (c) Possible mechanism for the H₂ production from an aqueous NH₃BH₃ solution over Co-CeVO₄/CeO₂ strong electronic interactions and rich oxygen vacancies under visible-light irradiation. Copied with permission [117]. Copyright 2021, American Chemical Society.

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  5.2 Theoretical calculations

  TiN-Pt nanohybrid can catalyze the dehydrogenation of AB under light conditions [48]. The calculated study shows that the barrier of H₂O dissociation in AB hydrolysis reaction is 0.3 and 0.5 eV under illumination and no illumination, respectively (Fig. 14a), indicating that illumination reduces the activation barrier.

  Figure 14

  

  Figure 14.  (a) Schematic representation of the free energy plots showing the energy barriers associated with the formation of the reaction transition state (TS) of the RDS during NH₃BH₃ dehydrogenation reaction under light and dark conditions. Copied with permission [48]. Copyright 2020, American Chemical Society. (b) The plausible mechanism of ammonia borane hydrolysis on NiCu/CNS and activity comparison in the dark and under visible light irradiation with or without NaHCO₃ as a hole scavenger over NiCu/CNS. Copied with permission [106]. Copyright 2020, American Chemical Society. (c) Ni (Ni₆₄, gray blue) and NiCu (Cu: brown, Ni₃₂Cu₃₂) catalyzed AB hydrolysis and energy change. Copied with permission [106]. Copyright 2020, American Chemical Society.

  DownLoad: Full-Size Img PowerPoint

  Theoretical calculations show that the addition of Cu atoms greatly reduces the dissociation barrier of H₂O, which is the RDS of AB hydrolysis, and the migration barrier of OH, which enables the reaction process to be carried out at lower temperatures. NiCu alloy loaded carbon nitride nanosheets (Ni_xCu_y/CNS) catalyst [106], under the influence of LSPR, electrons are transferred from Cu to Ni atoms, which makes Ni atoms activated and accelerated break of O−H bond from H₂O and B−H bond from AB. Subsequently, the redox reactions separately proceed in the electron-rich metal and the hole-rich CNS. In this case, (1) M−H⁻ and M−H⁺ will combine with each other to release hydrogen molecules (H⁻H⁺); (2) Two M−H⁺ intermediates will also get two electrons from electron-rich Ni sites to form hydrogen molecules (H⁺H⁺); (3) Two M−H⁻ will also get two holes from hole-rich CNS sites at the interface between the metal and the CNS carrier to form hydrogen molecules (H⁻H⁻). These intermediates M−BH₂⁺NH₃ react with the M−OH⁻ forming a H₃NBH₂OH species. Eventually, one molecule of AB and two molecules of H₂O produce three H₂ molecules and one molecule of NH₄BO₂ (Figs. 14b and c).

  Whether single metal or binary metal clusters are attached to g-C₃N₄ surface (Cu/CNS, Ni/CNS and Cu_0.5Ni_0.5/CNS) [118], the band gap is greatly reduced and the transfer of photogenerated electrons and holes is increased, thus improving the photocatalytic activity. The calculated bandgap of pure CNS, Cu/CNS, Ni/CNS and Cu_0.5Ni_0.5/CNS were 1.18, 0.74, 0.54 and 0.36 eV respectively. β-SiC exhibits high catalytic activity when loaded with Pt and Ru clusters. By comparing the band structure of Pt/β-SiC and Ru/β-SiC with β-SiC [119]. Pure β-SiC is a wide band gap semiconductor catalyst, and the band gap of the Pt/β-SiC and Ru/β-SiC are less than the pure β-SiC. This is in good agreement with the experimental results (Fig. 15).

  Figure 15

  

  Figure 15.  Band structure, total density of states (TDOS), projected density of states (PDOS) of Pt/β-SiC and Ru/β-SiC. (a) Pt/β-SiC, (b) Ru/β-SiC. (c) UV-vis diffuses reflectance spectra and band-gap calculation of β-SiC, Pt/β-SiC, and Ru/β-SiC composite nanowires. Copied with permission [119]. Copyright 2019, Elsevier.

  DownLoad: Full-Size Img PowerPoint

  6. Effect of electric field

  The electric field affects the distribution and transfer of charge, which is an important factor in catalytic reactions. A built-in electric field was formed on the surface of PtNi Alloy nanoparticles on Al₂O₃ (Al₂O₃-PtNi) catalyst, which promoted the process of AB dehydrogenation. As shown in Fig. 16a, the flow direction of electrons promotes the fracture of B-H bond and N-H bond, respectively [120].

  Figure 16

  

  Figure 16.  (a) The adsorption configuration of AB molecule on PtNi (111) surface and the proposed mechanism for the effect of Pt-Ni dipole on the hydrolysis of AB. Copied with permission [120]. Copyright 2020, Elsevier. (b) Under the action of applied electric field, the negative and cationic loaded BC₃ catalyzes AB activation process. Copied with permission [121]. Copyright 2021, Elsevier. (c) The state of N-H bond in AB under different electric field (E_Field) on Pt-M/BC₃N₂ diatomic catalyst structure. Copied with permission [122]. Copyright 2022, Elsevier.

  DownLoad: Full-Size Img PowerPoint

  With the help of the First-principles calculation simulation, activation of B-H bond in AB was achieved on BC₃ surface [121] by applied axial electric field (Fig. 16b). The applied electric field can act as a switch for the activation of B-H and N-H bonds in AB, and the dehydrogenation of AB can be realized under the action of the applied axial electric field. For Pt-M /BC₃N₂ (M = Al, Zn, Hf, Y) diatomic catalysts, B-H bond and N-H bond break process have different switching electric field (Fig. 16c) [122].

  7. Photo-piezoelectric synergy catalysis

  Photopiezoelectric co-catalysis has been widely used in dye degradation and water splitting [123-128]. The photo-piezoelectric synergy effect can improve catalytic activity, and since the dehydrogenation of AB involves both oxidation and reduction, the separation of electrons and holes becomes critical. In Ag₂O-BaTiO₃ hybrid photocatalyst, the built-in electric field is formed through continuous ultrasonic excitation to continuously separate photocarriers and improve the catalytic activity [129]. In the case of NIPS compounds (Ni₂P, Ni₁₂P₅ and Ni₃P) [130], illumination promotes electron-hole separation and migration to two different surfaces to form oxidation and reduction active sites, respectively, catalyzing the dehydrogenation of AB.

  MoS₂ is superior piezocatalyst and Doped with metal atoms could changes the band gap of MoS₂, allowing MoS₂ to be activated by visible light to generate electron-hole pairs (Fig. 17) [131]. Co-doped [132] and Sr-doped [133] MoS₂ two-dimensional materials can simultaneously form oxidation and reduction catalytic sites by forming a built-in electric field to promote electron-hole separation under the photo-piezoelectric co-catalytic action, and can catalyze the break of B-H bond and N-H bond in AB to generate H₂ in the visible region.

  Figure 17

  

  Figure 17.  (a) Schematic illustration of charge carrier generation in a Ag₂O nanoparticle when excited by a photon. (b) The separation of electrons and holes in Ag₂O nanoparticles attached to the two opposite surfaces of a BaTiO₃ nanocube that have opposite polarization charges due to piezoelectric effect and the corresponding tilting in the bands. Copied with permission [129]. Copyright 2015, American Chemical Society. (b) Possible mechanism for the hydrogen evolution from NH₃BH₃ in the alkaline aqueous solution over nickel phosphides under visible light irradiation. Copied with permission [130]. Copyright 2020, Wiley. (c) Reaction mechanism of piezo-photocatalytic H₂ production of Co-doped MoS₂. Copied with permission [132]. Copyright 2020, Royal Society of Chemistry. (d) Mechanism of hydrogen release from NH₃BH₃ solution by piezoelectric catalysis and piezo-enhanced NIR photocatalysis. Copied with permission [133]. Copyright 2021, China Academic Journal Electronic Publishing House.

  DownLoad: Full-Size Img PowerPoint

  8. Regeneration of AB

  AB regeneration is an important part of AB dehydrogenation reaction, which determines the promotion of application scope of AB dehydrogenation reaction. Up to now, AB regeneration and recycling are realized by means of intermediates and alcohols [69,72,134]. AB was regenerated from methanolysis intermediate by following borate → B(OH)₃ → B(OMe)₃ → NaBH₄ → NH₃BH₃ (Fig. 18). The cost of regeneration is lower, and more environmentally friendly. And it could realize high yields (≥95%) and very high purity (≥98%).

  Figure 18

  

  Figure 18.  Ammonia-borane hydrogen cycle. Copied with permission [69]. Copyright 2007, American Chemical Society.

  DownLoad: Full-Size Img PowerPoint

  Whether thermolysis, hydrolysis or methanolysis, AB can be regenerated by reaction byproducts. Sutton et al. [135] showed that the byproduct "BNH" after thermal decomposition of ammoniborane could react with N₂H₄ to regenerate BH₃NH₃ and realize the reaction cycle. Because the by-products of hydrolysis and alcoholysis have B-O bonds (NH₄BO₂ and NH₄B(OCH₃)₄), and B-O bonds are more stable than B-N bonds, it is more difficult to prepare BH₃NH₃ from the byproducts of hydrolysis and alcoholysis than from the products of thermal decomposition. Hausdorf et al. showed that AB can be recycled from the by-product of "BNH" and the reaction material can be saved as much as possible [136]. Through the byproduct of thermal decomposition reaction "BNH" to achieve AB regeneration, the reaction process is further simplified as [135]:

  (19)

  AB can been regenerated by NH₄BO₂, a byproduct of the hydrolysis reaction, and the reaction process is as follows [137]:

  (20)

  (21)

  (22)

  Also, for methanolysis, AB can been regenerated by NH₄B(OCH₃)₄, a byproduct of the methanolysis reaction, and the reaction process is as follows [69]:

  (23)

  9. Summary and prospect

  The high hydrogen capacity of 19.6 wt% has made AB an attractive material for chemical hydrogen storage. In this paper, we describe AB dehydrogenation from the aspects of thermal decomposition, hydrolysis, methanolysis, photocatalysis and photo-piezoelectric synergy catalyzes. The adaptive relationship between different catalyst structures and catalytic reaction types was explored, which provided important theoretical support for the design of AB dehydrogenation catalyst. The convenient generation and transportation of H₂ will expand the application scenarios of hydrogen energy and make important contributions to solving the energy crisis and environmental protection.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This study were funded by the Natural Science Basic Research Program of Shaanxi (Nos. 2022JQ-108 and 2022JQ-096) and the National Natural Science Foundation of China (No. 22104079).

  
  
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  Figure 1  (a) TPDS/MS profiles of neat AB powder and AB loaded over Co-P-B/Ni catalyst, by using THF based solution. The scanned temperature was between 50 ℃ and 200 ℃ at heating rate of 2 ℃/min. The hydrogen and borazine with atomic mass of 2 and 80, respectively, have been highlighted in the figure. Copied with permission [12]. Copyright 2012, Elsevier. (b) The release of H₂ between 75 ℃ and 175 ℃ (covering the two-step dehydrogenation of AB). Copied with permission [14]. Copyright 2018, American Chemical Society. (c) Temperature and H₂ equivalent profiles for AB with two additives (bottom AB: MA = 1:1, top AB: BA = 1:1). MA: maleic acid (C₄H₄O₄). BA: boric acid. Copied with permission [15]. Copyright 2020, Elsevier.

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  Figure 2  Mass losses at 85 ℃ as a function of (a) the electronegativity and (b) the electron affinity of M (i.e., Fe, Co, Cu, Zn and Pt) of MCl₂ (Insets showed the plots when Ni is taken into consideration). Copied with permission [16]. Copyright 2012, Elsevier. (c) Schematic diagram of AB dehydrogenation catalyzed by Cu-doped La-Sr-Co-O compound. Copied with permission [17]. Copyright 2019, Elsevier.

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  Figure 3  (a) Optimized structures on the Pd₂/MgO(100) and Pd₄/MgO(100)surface for the stepwise dehydrogenation pathway. Copied with permission [18]. Copyright 2013, Elsevier. (b) Schematic Gibbs free energy profiles for the concerted and stepwise dehydrogenation pathways: on the Pd₂/MgO surface and on the Pd₄/MgO surface. Copied with permission [18]. Copyright 2013, Elsevier. (c) The reaction pathways for the dehydrogenation reactions between Fe₁₂Cu dimer and AB molecule. Copied with permission [19]. Copyright 2016, Elsevier. (d, e) Energetic profiles for the reaction between AB and defective h-BN sheets. Copied with permission [20]. Copyright 2016, Elsevier. (f) The relative energy profiles of the releasing of the first hydrogen molecule from NH₃BH₃ catalyzed by Pt₁/Gr-O. Copied with permission [21]. Copyright 2018, Chinese Physical Society.

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  Figure 4  (a) The structure of adsorbed intermediate states on the 2D MgSiP₂ structure and Gibbs free energy on the BN alternating dehydrogenation. Copied with permission [23]. Copyright 2022, Elsevier. (b) The complete free energy (ΔG) correction diagram and related structure diagrams for the dehydrogenation of AB molecules on the surface of Os/P₃C and Os/P₃C_O. Copied with permission [24]. Copyright 2022, Elsevier. (c) Fe₂₂@Co₅₈ core-shell structure and (d) Fe₃₆Co₄₄ bimetallic nanoclusters catalyzed dehydrogenation of AB by thermal decomposition. Copied with permission [25,26]. Copyright 2022, Elsevier.

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  Scheme 1  (a–d) The possible mechanism for the AB dehydrogenation reaction on PtCo NP alloy.

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  Figure 5  (a) S_N2 reaction mechanism of AB dehydrogenation process. Copied with permission [28]. Copyright 2017, the Royal Society of Chemistry. (b) Plot of energy changes versus reaction coordinate calculated for Ni₂P-catalyzed hydrolysis of AB. Copied with permission [27]. Copyright 2015, Wiley. (c) Calculation of AB hydrolysis reaction catalyzed by NiCo₂P₂ and Ni₃P₂. Copied with permission [28]. Copyright 2017, the Royal Society of Chemistry.

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  Figure 6  (a–d) Catalytic performance of Pt₁ SACs in hydrolytic dehydrogenation of AB at 25 ℃. (e) Local partial density of state (LPDOS) projected of Pt₁/Co₃O₄ on the Pt₁ 5d orbitals with the Fermi level set at zero and the local configurations for AB adsorption. Copied with permission [68]. Copyright 2019, American Chemical Society.

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  Figure 7  (a, b) Reaction paths of Fe₃₆Co₄₄ Bimetallic nanoclusters catalyzed AB hydrolysis. Copied with permission [26]. Copyright 2022, Elsevier. (c) Fe₂₂@Co₅₈ core-shell catalytic hydrolysis of AB to prepare 3 mol H₂. Copied with permission [25]. Copyright 2021, Elsevier.

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  Figure 8  (a) Simulated pathways of hydrolytic dehydrogenation of AB at Pt centre on atomic Pt substituted Ni surface (top) and at Ni centre on pristine Ni surface (bottom). Copied with permission [29]. Copyright 2017, American Chemical Society. (b) Energy profiles of AB and H₂O molecules dissociated on Co(111), CoO(200), and CoO/CoP surfaces. Copied with permission [34]. Copyright 2021, Elsevier. (c) Dissociation process and energy changes of H₂O molecules on fcc-Ni(111), hcp-Ni(0001), and hcp-CuNi(0001) model surfaces. Copied with permission [39]. Copyright 2022, Elsevier.

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  Figure 9  (a) Time course of H₂ evolution from NH₃BH₃ aqueous solution at room temperature for different samples: in the dark and under visible light irradiation (λ > 420 nm). Copied with permission [95]. Copyright 2014, Wiley. (b) Structure model of Rh/WO_3−x hybrid nanowire catalysts. Copied with permission [98]. Copyright 2019, Royal Society of Chemistry. (c) H₂ evolution rate from ammonia borane over different catalysts: comparison of no catalyst, commercial WO₃ and WO_3−x-160 in dark and under light irradiation; comparison between WO_3-x and WO_3−x-120 in dark and under light irradiation. Copied with permission [97]. Copyright 2015, Wiley.

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  Figure 10  (a) Schematic of the proposed mechanism for the hydrolysis of ammonia borane on Pd/H₂Ti₃O₇ composite nanowires. Copied with permission [99]. Copyright 2015, Elsevier. Comparison of TOF between (b) CoM/g-C₃N₄ (M = Cu, Fe, Ni) and (c) M/g-C₃N₄ (M = Co, Ni) catalysts. Copied with permission [100]. Copyright 2016, Royal Society of Chemistry. (d) Schematic view of Mott-Schottky type contact based on Au-Co hybrid nanoparticles. Copied with permission [101]. Copyright 2014, American Chemical Society.

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  Figure 11  (a) The plausible mechanism of ammonia borane hydrolysis on NiCu/CNS. Copied with permission [106]. Copyright 2020, American Chemical Society. (b) Activity comparison in the dark and under visible light irradiation with or without NaHCO₃ as a hole scavenger over NiCu/CNS. Copied with permission [106]. Copyright 2020, American Chemical Society. (c) Hydrogen evolution curves and TOF for the AB hydrolysis catalyzed by Co/P₀CN, Co/P_2.16CN, Co/P_3.59CN, Co/P_5.26CN, and Co/P_8.49CN under visible-light irradiation ([AB] = 170 mmol/L, 5 mL, n_Co/n_AB = 0.02, and T = 298 K). Copied with permission [107]. Copyright 2021, The Royal Society of Chemistry. (d) Structural model of Co/PCN catalyst. Copied with permission [107]. Copyright 2021, The Royal Society of Chemistry. (e) Schematic of the photocatalytic hydrolysis mechanism of AB using the prepared RP₂/CN. Copied with permission [109]. Copyright 2020, Elsevier.

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  Figure 12  (a) Hydrogen release from AB under different photothermal and thermal reaction conditions, with or without Ti₂O₃ photothermal materials (RT: room temperature). Copied with permission [111]. Copyright 2020, Wiley-VCH GmbH. (b) Schematic diagram of AB hydrolysis catalyzed by NiCoP nanostructure. Copied with permission [112]. Copyright 2019, Elsevier. (c) Schematic diagram of charge separation and transfer in the Cu_0·45Ni_0·55/TiO₂CdS NTs system under visible-light irradiation. Copied with permission [113]. Copyright 2019, Elsevier. (d) Photocatalytic hydrolysis of ammonia borane to produce hydrogen. Copied with permission [114]. Copyright 2021, American Chemical Society.

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  Figure 13  (a) Proposed mechanism for the enhanced catalytic activity of Ag@Pd core-shell catalyst for AB hydrolysis under light illumination. Copied with permission [116]. Copyright 2020, American Chemical Society. (b) Mechanism for the visible-light-accelerated hydrolytic dehydrogenation of AB nanocatalyzed by Au₁Ru₁@Dendrimer1. Copied with permission [115]. Copyright 2020, American Chemical Society. (c) Possible mechanism for the H₂ production from an aqueous NH₃BH₃ solution over Co-CeVO₄/CeO₂ strong electronic interactions and rich oxygen vacancies under visible-light irradiation. Copied with permission [117]. Copyright 2021, American Chemical Society.

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  Figure 14  (a) Schematic representation of the free energy plots showing the energy barriers associated with the formation of the reaction transition state (TS) of the RDS during NH₃BH₃ dehydrogenation reaction under light and dark conditions. Copied with permission [48]. Copyright 2020, American Chemical Society. (b) The plausible mechanism of ammonia borane hydrolysis on NiCu/CNS and activity comparison in the dark and under visible light irradiation with or without NaHCO₃ as a hole scavenger over NiCu/CNS. Copied with permission [106]. Copyright 2020, American Chemical Society. (c) Ni (Ni₆₄, gray blue) and NiCu (Cu: brown, Ni₃₂Cu₃₂) catalyzed AB hydrolysis and energy change. Copied with permission [106]. Copyright 2020, American Chemical Society.

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  Figure 15  Band structure, total density of states (TDOS), projected density of states (PDOS) of Pt/β-SiC and Ru/β-SiC. (a) Pt/β-SiC, (b) Ru/β-SiC. (c) UV-vis diffuses reflectance spectra and band-gap calculation of β-SiC, Pt/β-SiC, and Ru/β-SiC composite nanowires. Copied with permission [119]. Copyright 2019, Elsevier.

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  Figure 16  (a) The adsorption configuration of AB molecule on PtNi (111) surface and the proposed mechanism for the effect of Pt-Ni dipole on the hydrolysis of AB. Copied with permission [120]. Copyright 2020, Elsevier. (b) Under the action of applied electric field, the negative and cationic loaded BC₃ catalyzes AB activation process. Copied with permission [121]. Copyright 2021, Elsevier. (c) The state of N-H bond in AB under different electric field (E_Field) on Pt-M/BC₃N₂ diatomic catalyst structure. Copied with permission [122]. Copyright 2022, Elsevier.

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  Figure 17  (a) Schematic illustration of charge carrier generation in a Ag₂O nanoparticle when excited by a photon. (b) The separation of electrons and holes in Ag₂O nanoparticles attached to the two opposite surfaces of a BaTiO₃ nanocube that have opposite polarization charges due to piezoelectric effect and the corresponding tilting in the bands. Copied with permission [129]. Copyright 2015, American Chemical Society. (b) Possible mechanism for the hydrogen evolution from NH₃BH₃ in the alkaline aqueous solution over nickel phosphides under visible light irradiation. Copied with permission [130]. Copyright 2020, Wiley. (c) Reaction mechanism of piezo-photocatalytic H₂ production of Co-doped MoS₂. Copied with permission [132]. Copyright 2020, Royal Society of Chemistry. (d) Mechanism of hydrogen release from NH₃BH₃ solution by piezoelectric catalysis and piezo-enhanced NIR photocatalysis. Copied with permission [133]. Copyright 2021, China Academic Journal Electronic Publishing House.

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  Figure 18  Ammonia-borane hydrogen cycle. Copied with permission [69]. Copyright 2007, American Chemical Society.

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  Table 1.  Activities of catalysts in the hydrolysis of AB at room temperature.

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  Table 2.  Activities of catalysts in the methanolysis of AB at room temperature.

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•