Hydrolytic Dehydrogenation of Ammonia Borane Attained by Ru-Based Catalysts: An Auspicious Option to Produce Hydrogen from a Solid Hydrogen Carrier Molecule

Chemical hydrogen storage stands as a promising option to conventional storage methods. There are numerous hydrogen carrier molecules that afford satisfactory hydrogen capacity. Among them, ammonia borane has attracted great interest due to its high hydrogen capacity. Great efforts have been devoted to design and develop suitable catalysts to boost the production of hydrogen from ammonia borane, which is preferably attained by Ru catalysts. The present review summarizes some of the recent Ru-based heterogeneous catalysts applied in the hydrolytic dehydrogenation of ammonia borane, paying particular attention to those supported on carbon materials and oxides.


Introduction
Energy demand has constantly increased in the last decades, which is closely linked to the expanding population and increasing prosperity. Currently, about 80% of the world's energy supply comes from fossil fuels (i.e., coal, oil, and natural gas). However, their utilization is inevitably associated with the emission of hazardous gases, which drives the current global warming crisis. Currently, nearly 100% of the total CO 2 emissions originate from the combustion and processing of fossil fuels [1], and its concentration in the atmosphere, which has experienced a great increase since the start of the Industrial Revolution, is nowadays higher than 400 ppm [2].
Hence, the tremendous concerns about the environmental issues related to the use of fossil fuels, together with their finite nature, is fostering the search toward the deployment of renewable energy sources. Such an ambitious goal has been the central focus of many investigations. In this sense, the use of hydrogen has tremendous hope for the use of renewable energies in different industrial applications and the transport sector. This is reflected in the constantly increasing number of publications in which its supremacy as an outstanding energy vector is highlighted [3][4][5][6][7].
The use of hydrogen, produced from renewable energy sources, has significant benefits. Its use has zero emission of greenhouse gases and it produces only water as a by-product. Additionally, it has a high energy storage capacity on a gravimetric basis (120 MJ/kg), which is much greater than those of gasoline (44.4 MJ/kg), diesel (45.4 MJ/kg), biodiesel oil (42.2 MJ/kg), and natural gas (53.6 MJ/kg) [8]. However, against all the benefits of hydrogen, its low volumetric energy density (i.e., 0.01 MJ/L at standard temperature and pressure conditions (STP)), which is much lower than those of common fuels (gasoline (34.2 MJ/L), diesel (34.6 MJ/L), biodiesel oil (33 MJ/L), and natural gas (0.0364 MJ/L)) limits its utilization as a fuel at ambient conditions. There are different options to increase  [31][32][33] Seeking and exploring new hydrogen storage options are in continuous progress and significant advances related to reversible hydrogen storage have been recently achieved [34][35][36][37]. In this review, we cover some of the most relevant recent strategies on hydrogen production from NH 3 BH 3 (ammonia borane, AB), which is the simplest nitrogen boron hydrogen compound [38][39][40], and one of the most fruitfully investigated solid-state hydrogen-rich molecules.
AB is a white crystalline solid at room temperature, which was first prepared by Shore and Parry in 1955 [41], and it has received tremendous attention due to several advantages compared to other hydrogen carrier molecules such as its high hydrogen content (19.6 wt. %; each equivalent yielding up to 3 equivalents of hydrogen), low molecular weight (30.87 g mol −1 ), its stability in solid-state, and high solubility in water. Furthermore, it is nonexplosive and non-flammable under standard conditions. AB has a high melting point of 112 • C and a density of 0.74 g cm −3 [38].
AB has an equal number of protic H δ+ (N-H) and hydridic H δ− (B-H) hydrogens in intra-and intermolecular interactions. It has heteropolar (N-H···H-B) and homopolar (B-H···H-B) dihydrogen interactions, which are the origin of the intra-and intermolecular dehydrogenation of AB [42]. It also has a strong B-N bond, so that the release of hydrogen is more favored than the dissociation into NH 3 and BH 3 under most conditions [43]. AB can be synthesized in three different ways (i.e., Lewis acid-Lewis base exchange, saltmetathesis followed by hydrogen release, and isomerization of the diammonate of diborane ([H 2 B(NH 3 ) 2 ] + [BH 4 ] − )) [38].
AB solvolysis can afford 3 equivalents of molecular hydrogen at moderate temperatures upon utilization of a proper catalyst, so this is the preferred option. The dehydrogenation of amine-borane adducts catalyzed by transition-metals dates back to the late 1980s [45], but its application in the production of hydrogen is still drawing great interest in the research community, as can be seen in the increasing number of publications reported per year (see Figure 1). Both homogeneous and heterogeneous catalytic systems have been explored, but the advantages of heterogeneous catalysts make these systems preferred from a practical point of view. There is vast literature reported on the hydrolytic dehydrogenation of AB, highlighting those contributions made by Yamashita et al. [46][47][48][49][50][51][52][53][54], Özkar et al. [55][56][57][58], and Xu et al. [44,[59][60][61][62].
Among the heterogeneous catalysts, systems based on ruthenium nanoparticles (NPs) have shown outstanding performances. Ru-based catalysts usually achieve complete AB dehydrogenation, producing ~3 equivalents of hydrogen in short reaction times, and with thermo-controllable reaction rates. The hydrolysis rate is also frequently related to the amount of catalyst and AB, but the hydrolysis rate is frequently found as zero-order relation or quasi-zero-order relation with the concentration of AB [63].
Among the heterogeneous catalysts, systems based on ruthenium nanoparticles (NPs) have shown outstanding performances. Ru-based catalysts usually achieve complete AB dehydrogenation, producing~3 equivalents of hydrogen in short reaction times, and with thermo-controllable reaction rates. The hydrolysis rate is also frequently related to the amount of catalyst and AB, but the hydrolysis rate is frequently found as zero-order relation or quasi-zero-order relation with the concentration of AB [63].
It is worth mentioning that despite the intense efforts devoted to unveiling the mechanism of the hydrolytic dehydrogenation of ammonia borane, there are still some aspects that remain unclear (e.g., rate-determining steps, the order of bond cleavages, etc.) [64]. For instance, Xu et al. postulated that the interaction between AB molecules and the surface of the metal active phase gives rise to the formation of activated complex species, which are attacked by a molecule of H 2 O, leading to the concerted dissociation of the B-N bond and the hydrolysis of the BH 3 intermediate to form BO 2 , releasing H 2 [65]. Fu et al. proposed a mechanism that proceeds via an almost self-powered process that involves the formation of BH 3 OH − and NH 4 + , followed by the attack of adjacent H 2 O to generate H 2 [66]. Na et al. suggested that the mechanism is very similar to that of the hydrolytic dehydrogenation of sodium borohydride, and proceeds via dissociative adsorption of ammonia borane on Ru surface [67]. More recently, Liu et al. claimed that the hydrogen production from AB attained by noble metal catalysts occurs via the following steps: (1) AB molecules interact with the surface of the catalyst to form a complex; (2) A molecule of H 2 O attacks to AB-catalyst complex; and (3) AB and H 2 O each lose a hydrogen atom to form H 2 . Such a mechanism is illustrated in Figure 2 [63].
For instance, Xu et al. postulated that the interaction between AB molecules and the surface of the metal active phase gives rise to the formation of activated complex species, which are attacked by a molecule of H2O, leading to the concerted dissociation of the B-N bond and the hydrolysis of the BH3 intermediate to form BO2, releasing H2 [65]. Fu et al. proposed a mechanism that proceeds via an almost self-powered process that involves the formation of BH3OH -and NH4 + , followed by the attack of adjacent H2O to generate H2 [66]. Na et al. suggested that the mechanism is very similar to that of the hydrolytic dehydrogenation of sodium borohydride, and proceeds via dissociative adsorption of ammonia borane on Ru surface [67]. More recently, Liu et al. claimed that the hydrogen production from AB attained by noble metal catalysts occurs via the following steps: (1) AB molecules interact with the surface of the catalyst to form a complex; (2) A molecule of H2O attacks to AB-catalyst complex; and (3) AB and H2O each lose a hydrogen atom to form H2. Such a mechanism is illustrated in Figure 2 [63]. It is evident that the catalyst's nature plays a crucial role in controlling the whole reaction. Most of the studies reported on Ru-based catalysts for the hydrolytic dehydrogenation of AB are focused on elucidating the role of the features of the metal active phase (i.e., size, morphology, incorporation of a second and third metal in the nanoparticles, etc.), while less attention has been paid to the properties of the support. We divided this manuscript into several sections, which contain a review of representative catalytic systems based on monometallic Ru NPs and supports of a diverse nature, which have been used for the hydrolytic dehydrogenation of AB. As a summary, Table 2 includes representative examples of Ru-based catalysts supported on carbon materials, oxides, metal organic frameworks (MOF), and some other less explored supports, together with the turnover frequency values achieved (TOF; in molH2. molRu −1 . min −1 ) and the calculated activation energy (Ea; in kJ mol −1 ).  It is evident that the catalyst's nature plays a crucial role in controlling the whole reaction. Most of the studies reported on Ru-based catalysts for the hydrolytic dehydrogenation of AB are focused on elucidating the role of the features of the metal active phase (i.e., size, morphology, incorporation of a second and third metal in the nanoparticles, etc.), while less attention has been paid to the properties of the support. We divided this manuscript into several sections, which contain a review of representative catalytic systems based on monometallic Ru NPs and supports of a diverse nature, which have been used for the hydrolytic dehydrogenation of AB. As a summary, Table 2 includes representative examples of Ru-based catalysts supported on carbon materials, oxides, metal organic frameworks (MOF), and some other less explored supports, together with the turnover frequency values achieved (TOF; in mol H2 ·mol Ru −1 ·min −1 ) and the calculated activation energy (Ea; in kJ mol −1 ).

Hydrolytic Dehydrogenation of Ammonia Borane (AB) over Carbon Material-Supported Catalysts
Several catalytic supports have been explored for the synthesis of highly efficient catalysts for the hydrolytic dehydrogenation of AB. Carbon materials are one of the most intensively studied and the resulting catalysts have given very interesting results.
Akbayrak and Özkar explored the performance of catalysts supported on multiwalled carbon nanotubes (Ru(0)@MWCNT) by evaluating the activity of in-situ formed Ru nanoparticles (NPs) [72]. The resulting NPs had an average particle size range of 1.4-3.0 nm and were well-dispersed on the support. The effect of the Ru content was evaluated by checking the activity (in mL of H 2 /min) of catalysts with metal contents of 0.73, 1.47, 1.91, 2.26, and 2.83 wt. %. Among those investigated, the sample with 1.91 wt. % displayed the best activity, with a TOF = 329 min −1 (mol H2 ·mol Ru −1 ·min −1 ). That catalyst was evaluated during four consecutive reaction cycles and it preserved 41% of its initial activity. Doe et al. also checked the performance of MWCNT-supported Ru catalysts with Ru(NH 3 ) 6 Cl 3 as the metal precursor and using electrostatic adsorption (EA) and incipient wetness impregnation (IWI) methods [98]. Additionally, catalysts based on activated carbon and SiO 2 were prepared as reference materials. The Ru NPs were located on the external surface after both EA and IWI, and a smaller average size was achieved for the EA (2 and 3 nm for Ru/MWCNT-EA and Ru/MWCNT-IWI, respectively). Checking the performance of three sets of catalysts supported on MWCNTs, activated carbon, and SiO 2 with various average NPs size, it was observed that in all cases the catalysts with larger NPs attained higher reaction rates (expressed as turnover rates in mol H2 ·mol surface Ru −1 ·s −1 ), and higher TOF values were achieved for the Ru/MWCNT catalysts. Among those evaluated in that study, Ru/MWCNTs-EA produced the highest initial TOF value and the lowest activation energy, which was attributed to the hydrogen spillover taking place on metal NPs supported on CNTs.
Cheng et al. reported a simple method for the preparation of a Ru/graphene catalyst synthesized from graphene oxide and RuCl 3 using a one-step co-reducing approach with methylamine borane (MeAB) [68]. The resulting catalyst was compared to those synthesized by using different reducing agents, namely AB and NaBH 4 . It was observed that the sample prepared with MeAB provided better results than those using AB or NaBH 4 , which was attributed to a better control over nucleation and growth processes. It was determined that the average NPs size was 1.2, 1.7, and 2.0 nm for the catalysts reduced with MeAB, AB, and NaBH 4 , respectively, suggesting that the size of the NPs increased as the reducing agent became stronger. The catalyst reduced with MeAB had a TOF of 100 mol H2 ·mol Ru −1 ·min −1 , and activation energy of 11.7 kJ mol −1 , and it retained 72% of its initial activity after four reaction cycles. The same research group followed a very similar Energies 2021, 14, 2199 6 of 20 one-step co-reducing approach using ascorbic acid to synthesize Ru/graphene catalysts, which resulted in much higher TOF values of 600 mol H2 ·mol Ru −1 ·min −1 and activation energy of 12.7 kJ mol −1 [78], which pointed out the importance of the reducing agent in controlling the final catalytic performance. The high TOF value achieved by Ru/graphene was attributed to the narrow size distribution of the Ru NPs and the utilization of graphene as a suitable support.
In the study of Ma and co-workers, [75] ligand-free Ru NPs supported on carbon black were prepared in-situ from the reduction of the metal precursor (i.e., RuCl 3 ) by AB concomitantly with its hydrolysis. The resulting catalyst had an average nanoparticle size of 1.7 nm and showed a TOF of 429.5 mol H2 ·mol Ru −1 ·min −1 . In that case, the reusability of the catalyst was checked during five consecutive reaction runs, after which it preserved 43.1% of its initial activity. The results of the characterization of the spent catalyst indicated that there were neither metal leaching nor aggregation of the NPs, so that the activity loss was attributed to an increasing concentration of the reaction products (i.e., metaborate and Cl − ions) and their adsorption on the surface of the NPs. Furthermore, it was postulated that the increase in the viscosity in the solution after several reaction cycles could impede the diffusion of the reactant molecules and their collision with the Ru active sites.
Sun et al. studied the performance of poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Ru NPs loaded onto bamboo leaf-derived porous carbon (Ru/BC) [80]. In that case, the NPs were synthesized by in-situ reduction with AB using RuCl 3 ·nH 2 O as a metal precursor and with various PVP content (i.e., 0, 1, 3, 5, or 10 mg), which was used to avoid the agglomeration of the NPs. The catalysts with the best activity among those investigated (i.e., Ru/BC stabilized with 1 mg of PVP) displayed a TOF of 718 mol H2 ·mol Ru −1 ·min −1 , and retained nearly 56% of the initial catalytic activity after 10 consecutive reaction cycles. That stability was much better than that of the PVP-free catalyst, which indicated the important role of PVP in enhancing the recyclability of the catalysts by preventing the agglomeration of the NPs. The same research group also investigated the performance of nitrogen-doped (N-doped) porous carbon materials [81]. The support was prepared by hydrothermal treatment of hydrochloride semicarbazide and glucose, and it was subsequently loaded with the metal precursor. The resulting Ru/NPC catalyst completed the dehydrogenation reaction in 90 s at room temperature, reaching a TOF of 813 mol H2 ·mol Ru −1 ·min −1 . The stability of the catalyst was checked by performing five consecutive runs, after which 67.3% of the initial activity was preserved, and the activity decay was attributed to the agglomeration of the NPs.
Yamashita et al. addressed the preparation of highly efficient Ru/carbon catalysts prepared by pyrolysis of a supported Ru complex (i.e., tri(2,2-bipyridyl) ruthenium (II) chloride hexahydrate) [79]. In that study, the Ru catalysts were prepared by impregnating a commercial activated carbon with the metal precursor and subsequent decomposition of the metal complex by carrying out a heat treatment at temperatures ranging from 600 to 1000 • C (catalysts denoted as Ru/C(600)-(1000)). Transmission Electron Microscopy (TEM) analysis confirmed the formation of Ru NPs even for the lowest temperatures used for the decomposition of the metal complex. The average NP size strongly depended on the decomposition temperature and ranged from 3.8 to 13.5 nm for the Ru/C catalysts. Two additional reference samples were also synthesized: Ru/C(imp) prepared by the same protocol using Ru(NO)(NO 3 ) 3 as the metal precursor, and the Ru/SiO 2 catalyst prepared from Ru(bpy) 3 2+ and commercial fumed silica, with NP size of 3.3 and 2.0 nm, respectively. The results of the catalytic activity indicated that complete conversion of AB was attained with Ru/C(800), Ru/C(900) and Ru/C(1000), whereas it was not achieved with Ru/C(600), Ru/C(700), Ru/C(imp), and Ru/SiO 2 . That observation led the authors to conclude that under the experimental conditions used, relatively large NPs were preferred for the reaction. Among those investigated, Ru/C(800) showed the most promising activity with an average TOF number of 670 mol H2 ·mol Ru −1 ·min −1 . That catalyst achieved 100% of conversion even after four consecutive reaction runs, but the reaction rate was progressively sluggish. A summary of the results of the catalytic activity of the materials assessed in that work is  Figure 3. It was also concluded that the electronic properties of Ru NPs played an important role in controlling the catalytic performance. It was claimed that Ru/C(800) had an optimum proportion of oxidized Ru species, which are important in the reaction.
Ru/C(600), Ru/C(700), Ru/C(imp), and Ru/SiO2. That observation led the authors to conclude that under the experimental conditions used, relatively large NPs were preferred for the reaction. Among those investigated, Ru/C(800) showed the most promising activity with an average TOF number of 670 molH2. molRu −1 . min −1 . That catalyst achieved 100% of conversion even after four consecutive reaction runs, but the reaction rate was progressively sluggish. A summary of the results of the catalytic activity of the materials assessed in that work is plotted in Figure 3. It was also concluded that the electronic properties of Ru NPs played an important role in controlling the catalytic performance. It was claimed that Ru/C(800) had an optimum proportion of oxidized Ru species, which are important in the reaction. Gao et al. [70] studied the performance of Ru/nanodiamonds in the hydrolytic dehydrogenation of AB. That study was motivated by a wide variety of oxygen functional groups present in the nanodiamonds, which were expected to serve as anchoring points for the Ru NPs. Commercial nanodiamonds, with an average diameter of 5-10 nm were used as the support and RuCl3 as the metal precursor, for the synthesis of catalysts with a metal content of 3.22, 4.82, 6.21, and 8.05 wt. %. The TOF values showed a volcano-type tendency, the best catalyst being that with a Ru content of 6.21 wt. % (229 molH2. molRu −1 . min −1 ). The recyclability tests performed with that sample indicated that total conversion of AB was achieved during four consecutive runs, but the TOF values decreased during the cycles, which was related to the increase of the NPs from 3.7 to 5.1 nm and the increasing viscosity and concentration of metaborate in the reaction solution.
Most of the catalysts used for the hydrolytic dehydrogenation of AB displayed good reusability and attained total conversion during several consecutive reaction runs. However, the reaction is frequently sluggish and longer reaction times are needed to produce 3 equivalents of H2 per mole of AB, so the poor stability of the catalysts under reaction conditions is one of the most addressed issues.
The stability of the catalysts was shown to improve upon utilization of supports with abundant surface functional groups, which are known to increase the stability and reusability of the catalysts [8]. That was the case of Fan et al., who developed catalysts consisting of ultrafine and highly dispersed Ru NPs supported on N-doped carbon nanosheets formed by a hierarchically porous carbon material (HPCM) [76]. Ru/HPCM had an average NP size of 1.41 nm and narrow size distribution, which ranged from 0.6 nm to 2.0 nm. Gao et al. [70] studied the performance of Ru/nanodiamonds in the hydrolytic dehydrogenation of AB. That study was motivated by a wide variety of oxygen functional groups present in the nanodiamonds, which were expected to serve as anchoring points for the Ru NPs. Commercial nanodiamonds, with an average diameter of 5-10 nm were used as the support and RuCl 3 as the metal precursor, for the synthesis of catalysts with a metal content of 3.22, 4.82, 6.21, and 8.05 wt. %. The TOF values showed a volcano-type tendency, the best catalyst being that with a Ru content of 6.21 wt. % (229 mol H2 ·mol Ru −1 ·min −1 ). The recyclability tests performed with that sample indicated that total conversion of AB was achieved during four consecutive runs, but the TOF values decreased during the cycles, which was related to the increase of the NPs from 3.7 to 5.1 nm and the increasing viscosity and concentration of metaborate in the reaction solution.
Most of the catalysts used for the hydrolytic dehydrogenation of AB displayed good reusability and attained total conversion during several consecutive reaction runs. However, the reaction is frequently sluggish and longer reaction times are needed to produce 3 equivalents of H 2 per mole of AB, so the poor stability of the catalysts under reaction conditions is one of the most addressed issues.
The stability of the catalysts was shown to improve upon utilization of supports with abundant surface functional groups, which are known to increase the stability and reusability of the catalysts [8]. That was the case of Fan et al., who developed catalysts consisting of ultrafine and highly dispersed Ru NPs supported on N-doped carbon nanosheets formed by a hierarchically porous carbon material (HPCM) [76]. Ru/HPCM had an average NP size of 1.41 nm and narrow size distribution, which ranged from 0.6 nm to 2.0 nm. Such ultrafine NPs were confined into the micropores and mesopores of the support, affording numerous active sites and stabilizing the NPs from sintering under reaction conditions. X-ray Photoelectron Spectroscopy (XPS) analysis confirmed the presence of pyridinic, pyrrolic, and graphitic nitrogen. The importance of both pyridinic and graphitic nitrogen in enhancing catalytic performance was pointed out in that study. Ru/HPCM was evaluated during eight consecutive reaction cycles, after which it retained 50% of its initial activity. After that, the average NP size of the spent catalyst slightly increased to 1.47 nm and the partial activity decay was related to the catalyst loss and the passivation effect of metaborate ions formed along the recycling tests. The same group also synthesized Rubased catalysts supported on N-doped bagasse-derived carbon materials (BC-hs) [73]. That biomass residue, which has abundant negative oxygen and nitrogen functional groups, was suitable for the interaction with Ru 3+ cations of the metal precursor. Catalysts with various metal contents (i.e., 2.5, 3.5, 4.5, and 5.5 wt. %) were synthesized by in-situ reduction with AB, achieving homogeneously dispersed ultrafine Ru NPs. It was observed that the best-performing catalyst preserved 80% of its TOF value after five runs, demonstrating the suitability of the BC-hs support to stabilize the metal NPs. The partial loss of the activity was attributed to changes in the NP size and loss of the catalyst during the separation and washing steps.
Ma et al. used a support based on a N-doped porous carbon material (NC-Fe) using a facile pyrolysis of a porous organic polymer (POP) synthesized from ferrocene carboxaldehyde and melamine as the starting materials [69]. The resulting Ru catalyst (Ru/NC-Fe) achieved the total conversion of AB during five cycles, but the reaction was considerably sluggish. That catalyst contained γ-Fe 2 O 3 , so it was easily recovered with a magnet. The activity decay was attributed to both the metaborate formed in the reaction and the agglomeration of the metal NPs.
Liu et al. used a N-enriched hierarchically macroporous-mesoporous carbon support that was synthesized by a co-template evaporation-induced self-assembly approach with SiO 2 nanospheres as a macroporous hard template and F127 as a mesoporous soft template and a soft nitriding by the low-temperature thermolysis of urea [99]. The resulting support, denoted as hPCN, was loaded with Ru NPs (Ru@hPCN), and reference catalysts based on pure macroporous and pure mesoporous support were also prepared (Ru@macroPCN and Ru@mesoPCN, respectively). Ru@hPCN exhibited the best activity and total dehydrogenation was finished in 120 s, while longer reaction times were required for the reference samples and for a commercial Ru/C catalyst (160, 280, and 880 s, for Ru@macroPCN, Ru@mesoPCN, and Ru/C, respectively). Such better performance of Ru@hPCN was also evidenced by the higher TOF values achieved at 60 • C (1850, 1258, 902, and 308 min −1 , for Ru@hPCN, Ru@macroPCN, Ru@mesoPCN, and Ru/C, respectively). The reusability of Ru@hPCN was evaluated during four consecutive reaction cycles and it was observed that the NP size increased from 0.7 nm to 2.7 nm. Additionally, the amount of surface N species decreased from 13.4 at. % to 2.3 at. %, which suggested that the N-enriched species contributed to improving the catalytic performance by dissociating the electropositive H δ+ from water molecules and the breakage of the B-N bonds.
Not only N-doped carbon materials, but also other N-containing carbon-based supports were shown to be effective in stabilizing Ru NPs for the hydrolytic dehydrogenation of AB. For instance, Zheng et al. used hierarchical porous graphitic carbon nitride (g-C 3 N 4 ) nanosheets to anchor ultrafine Ru NPs [100]. In that case, the supports were prepared from melamine and various amounts of NH 4 Cl as a dynamic gas template. The NPs were encapsulated into the network of the g-C 3 N 4 by the reduction of the metal precursor with NaBH 4 to achieve a final metal content of 1.91 wt. %. The characterization of the catalyst indicated that the resulting NPs had a size that ranged from 1.9 to 5.1 nm and they exhibited uniform dispersion onto the support. Among the synthesized supports, that prepared with a melamine to NH 4 Cl mass ratio of 1:3 exhibited the highest surface area (S BET of 59 m 2 g −1 ) and its counterpart Ru catalyst was selected to assess the performance in the dehydrogenation of AB. That catalyst displayed satisfactory recyclability even after four consecutive reaction cycles. The activity loss observed from the fifth cycle was attributed to a particle aggregation of the NPs and to the accumulation of NH 4 BO 2 species in the reaction solution, which increases its viscosity and blocks the active sites of the catalyst.
Tang et al. also explored the suitability of Ru/g-C 3 N 4 catalysts for the dehydrogenation of AB [77]. In that case, the catalysts were prepared from urea and RuCl 3 . Catalysts with various metal loadings (i.e., 4.10, 3.28, 2.46, and 1.64 wt. %) were synthesized. The time needed for the reaction to be completed decreased to 3.5 min for the catalyst with 3.28 wt. %, which showed the highest TOF value (459.3 mol H2 ·mol Ru −1 ·min −1 ). Concerning the recyclability of Ru/g-C 3 N 4 , it preserved 50% of the initial catalytic activity after Yamashita et al. also studied the performance of g-C 3 N 4 supported Ru catalysts [101]. In that case, carbon/g-C 3 N 4 composites with various carbon contents (C(x)/g-C 3 N 4 ; "x" is the initial carbon weight per cent of 0.1, 0.5, 1.0, 2.0, and 4.0 wt. %, respectively) were synthesized from glucose and dicyandiamide by a simple experimental procedure. The obtained supports were subsequently impregnated with RuCl 3 ·3H 2 O to obtain Ru NPs after reduction with H 2 gas at 300 • C. The incorporation of carbon was reported to extend the absorption of the materials to the visible region of 480-800 nm compared to the pristine g-C 3 N 4 , so the resulting Ru/C(x)/g-C 3 N 4 were interesting photocatalysts for the dehydrogenation of AB under visible-light irradiation. Additionally, the incorporation of carbon served to achieve smaller Ru NPs than those achieved in the raw g-C 3 N 4 , which also affected the catalytic activity. Figure 4 contains information on the catalytic activity displayed by Ru/C(x)/g-C 3 N 4 . It was observed that the addition of moderated carbon contents in the catalysts enhanced the activity compared to that displayed by Ru/g-C 3 N 4 , which was related to the smaller NPs shown in those samples (Figure 4a). It was also seen that the reaction rate of all the materials improved under visible light irradiation (Figure 4b), achieving the fastest reaction rate with Ru/C(1.0)/g-C 3 N 4 . solution, which increases its viscosity and blocks the active sites of the catalyst.
Tang et al. also explored the suitability of Ru/g-C3N4 catalysts for the dehydrogenation of AB [77]. In that case, the catalysts were prepared from urea and RuCl3. Catalysts with various metal loadings (i.e., 4.10, 3.28, 2.46, and 1.64 wt. %) were synthesized. The time needed for the reaction to be completed decreased to 3.5 min for the catalyst with 3.28 wt. %, which showed the highest TOF value (459.3 molH2. molRu −1 . min −1 ). Concerning the recyclability of Ru/g-C3N4, it preserved 50% of the initial catalytic activity after the fourth run, and the activity loss was related to the increase of the size of the NPs from 2.8 nm to 4.1 nm and the adsorption of B species on the surface of the NPs.
Yamashita et al. also studied the performance of g-C3N4 supported Ru catalysts [101]. In that case, carbon/g-C3N4 composites with various carbon contents (C(x)/g-C3N4; "x" is the initial carbon weight per cent of 0.1, 0.5, 1.0, 2.0, and 4.0 wt. %, respectively) were synthesized from glucose and dicyandiamide by a simple experimental procedure. The obtained supports were subsequently impregnated with RuCl3.3H2O to obtain Ru NPs after reduction with H2 gas at 300 °C. The incorporation of carbon was reported to extend the absorption of the materials to the visible region of 480-800 nm compared to the pristine g-C3N4, so the resulting Ru/C(x)/g-C3N4 were interesting photocatalysts for the dehydrogenation of AB under visible-light irradiation. Additionally, the incorporation of carbon served to achieve smaller Ru NPs than those achieved in the raw g-C3N4, which also affected the catalytic activity. Figure 4 contains information on the catalytic activity displayed by Ru/C(x)/g-C3N4. It was observed that the addition of moderated carbon contents in the catalysts enhanced the activity compared to that displayed by Ru/g-C3N4, which was related to the smaller NPs shown in those samples (Figure 4a). It was also seen that the reaction rate of all the materials improved under visible light irradiation (Figure 4b), achieving the fastest reaction rate with Ru/C(1.0)/g-C3N4. Interesting results were also achieved by Fan et al., who synthesized phosphorusdoped carbon-supported Ru catalysts (Ru/PPC) [74]. In that study, the supports were prepared from hypercrosslinked polymer networks of triphenylphosphine and benzene, and they were subsequently used for the synthesis of Ru catalysts with various metal contents (i.e., 1.5, 2.5, 3.5, and 4.5 wt. %). Small and well-distributed Ru NPs were attained in all  Interesting results were also achieved by Fan et al., who synthesized phosphorusdoped carbon-supported Ru catalysts (Ru/PPC) [74]. In that study, the supports were prepared from hypercrosslinked polymer networks of triphenylphosphine and benzene, and they were subsequently used for the synthesis of Ru catalysts with various metal contents (i.e., 1.5, 2.5, 3.5, and 4.5 wt. %). Small and well-distributed Ru NPs were attained in all the materials, the smallest for Ru/PPC with 3.5 wt. % of Ru (average size of 1.13 nm). The catalytic activity was shown to be dependent on the metal content, achieving TOF values of 174, 325, 413, and 290 mol H2 ·mol Ru −1 ·min −1 for catalysts with 1.5, 2.5, 3.5, and 4.5 wt. % of Ru, respectively. The stability of the most active material was evaluated during four consecutive cycles, observing that its activity gradually decreased after the first run. Such loss of activity was attributed to the NP sintering (from 1.13 to 2.47 nm) as well as the catalyst loss in the separation and washing steps.

Hydrolytic Dehydrogenation of AB over Oxide-Supported Catalysts
As in most catalytic reactions, carbon materials are the most fruitfully studied supports. However, interesting results have also been achieved with oxide-supported catalysts, with silica (SiO 2 ) and alumina (Al 2 O 3 ) the most investigated. Representative examples of such systems are summarized in this section. SiO 2 has been shown to serve as a suitable support for the dehydrogenation of AB. SiO 2 , with various structures and morphologies, have been nicely utilized for the development of well-performing catalysts. For instance, Zhu et al. reported on Ru NPs confined in SBA-15 (Ru@SBA-15) by using a double solvent approach (with hexane and water) [85]. Metal loadings of 0.5, 1.1, 2.1, 3.2, and 4.0 wt. % were used, which resulted in NPs with an average size of 2.0 ± 0.6 nm, 2.2 ± 0.6 nm, 3.0 ± 0.8 nm, and 3.7 ± 0.7 nm, respectively. Among investigated, Ru@SBA-15 with a metal content of 2.1 wt. % displayed the best activity, which was also superior to those catalysts with non-confined NPs supported on SBA-15 and SiO 2 . It also showed good durability after five consecutive reaction runs at room temperature.
Chen et al. designed Ru catalysts supported on cubic 3D cage-type mesoporous silica SBA-1 functionalized with carboxylic acid (Ru/S1B-C10) [84]. The carboxylic acid was incorporated by co-condensation of tetraethyl orthosilicate (TEOS) and carboxyethylsilanetriol sodium salt in the presence of poly(acrylic acid) and hexadecylpyridinium chloride. The -COOH groups were uniformly distributed within the mesopores of the support, which assisted the preparation of well-dispersed and small Ru NPs. Catalysts with metal loadings from 0.5 to 2.0 wt. % were synthesized with both -COOH free support (SIB-C0) and -COOH containing support (S1B-C10). It was observed that while the average NP size observed in Ru/S1B-C0 catalysts increased from 2.8 to 4.6 nm when the metal loading increased from 0.5 to 2.0 wt. %, it only increased from 2.0 to 3.2 nm in Ru/SIB-C10 for similar metal contents. Catalysts with 1 wt. % of Ru displayed better performance for both SIB-C0 and SIB-C10 supports than materials with other Ru loadings. Among those investigated, Ru(1)@S1B-C10 had the highest TOF value (202 mol H2 ·mol Ru −1 ·min −1 ), which was attributed to the nanosized Ru particles and their good dispersion as well as the effect of the pore confinement of the support. The reusability of the best-performing catalysts was evaluated during five reaction cycles and even though the total conversion was achieved in all cases, the reaction rates decreased with an increase in the number of cycling tests, which was related to a partial metal leaching and restricted access of reactants to the Ru active sites originated by the adsorption of metaborate on the surface of the NPs.
Yao el at. prepared core-shell Ru@SiO 2 catalysts with various metal contents (i.e., 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, and 10.0 wt. %), which consisted of Ru NPs of~2 nm embedded in the center of spherical SiO 2 particles of~25 nm [83]. Among those investigated, Ru@SiO 2 with 6 wt. % of Ru loading exhibited the best performance for the production of H 2 from AB, with a TOF of 200 mol H2 ·mol Ru −1 ·min −1 . A much slower reaction rate was observed for a supported Ru/SiO 2 catalyst used as a reference sample, which was related to the easy aggregation occurring in Ru/SiO 2 . The Ru@SiO 2 core-shell catalyst displayed good recycling stability for five cycles, but the reaction was progressively sluggish during the cycles.
Özkar et al. explored the activity of catalysts formed by Ru NPs loaded on magnetic silica-coated cobalt ferrite (Ru(0)/SiO 2 -CoFe 2 O 4 ) [82]. That catalyst showed a moderated activity, with a TOF of 172 mol H2 ·mol Ru −1 ·min −1 at room temperature, but excellent recyclability during 10 reaction runs, preserving 94% of the initial activity. After each reaction cycle, the spent catalyst was isolated using a permanent magnet, and no metal leaching was detected.
Onat et al. [86] also developed SiO 2 -based magnetic core-shell catalysts. In that study, Ru NPs were loaded on amino functionalized silica-covered magnetic NPs (Fe 3 O 4 @SiO 2 -NH 2 -Ru), and the resulting catalysts were evaluated in the dehydrogenation of AB. It was claimed that the amine group served to increase the electron transfer from the coreshell Fe 3 O 4 @SiO 2 structure to the surface of the Ru NPs, which resulted in enhanced catalytic activity in the production of H 2 . The developed catalyst had a TOF value of 617 mol H2 ·mol cat −1 ·min −1 and showed good stability during eight reaction cycles. Al 2 O 3 has also been applied for the synthesis of Ru-based catalysts for the dehydrogenation of AB. For instance, Metin el al. reported an easy method for the synthesis of nearly monodisperse Ru NPs, which consisted in the thermal decomposition and simultaneous reduction of the metal precursor (ruthenium(III) acetylacetonate (Ru(acac) 3 ) in the presence of oleylamine (serving as a stabilizer and reducing agent) and benzylether (used as solvent) [87]. The as-synthesized NPs (with an average size of 2.5 nm) were subsequently loaded on γ-Al 2 O 3 to build a Ru@Al 2 O 3 catalyst with 1 wt. % of Ru. That material had a TOF of 39.6 mol H2 ·mol Ru −1 ·min −1 . It was observed that the TOF value increased to 83.3 mol mol H2 ·mol Ru −1 after carrying out a treatment of the catalyst with acetic acid, which removed the organics from the surface of the NPs. The treated catalysts exhibited great stability even after 10 consecutive reaction runs, and the TOF only decreased 10% of the initial value.
Chen el at. developed a microporous crystalline γ-Al 2 O 3 with a large surface area (large surface, 349 m 2 g −1 ) prepared from a microporous covalent triazine framework (CTF-1, surface area of 697 m 2 g −1 ) as a template [88]. Ru-based catalysts with 1, 2, and 5 wt. % were prepared from RuCl 3 and they were evaluated in the dehydrogenation of AB. The high surface area and the hierarchical pore structures of the micropores developed in the synthesized γ-Al 2 O 3 was claimed to be responsible for the better performance achieved by the materials studied in that work compared to those reported elsewhere for Ru/γ-Al 2 O 3 catalysts (TOF as high as 256.8 mol H2 ·mol Ru −1 ·min −1 was obtained for Ru/γ-Al 2 O 3 with 2 wt. %; while values of one order of magnitude lower were attained for Ru/γ-Al 2 O 3 reported in other studies).
Fan et al. evaluated the performance of catalysts formed by Ru NPs supported on Al 2 O 3 nanofibers with an average length of 200-300 nm and a BET surface area of 300 m 2 g −1 (Ru/Al 2 O 3 -NFs) [89]. Catalysts with metal contents from 2.52 to 4.91 wt. % were prepared in-situ by reducing the metal precursor with AB. It was observed that the time needed to achieve a total conversion of AB decreased with increasing the metal content. The stability test indicated that AB conversion was completed even during five consecutive runs, but the reaction rate decreased along the cycles, which was attributed to an increase of the average NPs size from 2.9 to 3.1 nm.
Lee et al. investigated the effect of the crystal phase of Ru on the dehydrogenation of AB by using face-centered cubic (fcc) structures and hexagonal close-packed (hcp) structured Ru NPs loaded on γ-Al 2 O 3 [102]. Catalysts with 1 wt. % of Ru and different sizes of both fcc and hcp Ru NPs were prepared. The size and the crystal phase were controlled by adjusting the experimental conditions, in terms of amount and type of metal precursor, solvent, and amount of PVP. It was observed that under the experimental conditions used in that study, fcc NPs were achieved using Ru(acac) 3 while RuCl 3 originated hcp NPs. XPS analysis suggested that fcc Ru NPs were more easily oxidized than the hcp counterpart since a larger relative proportion of Ru 4+ was detected (i.e., [Ru 4+ ]/[Ru 0 ] of 28.2 and 19.2% for fcc and hcp Ru/γ-Al 2 O 3 , respectively). The catalytic activity of fcc and hcp NPs with different sizes (i.e., 2.4, 3.5, 3.9, and 5.4 nm) was evaluated by monitoring the H 2 generation profiles at 25 • C (see Figure 5). As can be seen, hcp NPs displayed better performance regardless of the size of the NPs. It was also seen that the difference between hcp and fcc NPs became smaller as the NPs increased. Additionally, the opposite tendency was seen for fcc and hcp NPs: the catalytic activity of fcc Ru/γ-Al 2 O 3 enhanced with increasing NP size, while hcp displayed worse performance with increasing sizes. Density functional theory (DFT) calculations were done to determine the adsorption energy of O 2 molecules on the (001) crystal plane of fcc and hcp Ru to get information about their easy oxidation. The results obtained suggest that the fcc Ru was easily oxidized than hcp Ru, which was consistent with the experimental results observed in that study. Thus, the authors of that study ascribed the worse performance of smaller NPs with fcc crystal phases to their higher degree of oxidation, while the tendency observed for hcp was attributed to size effects. consistent with the experimental results observed in that study. Thus, the authors of that study ascribed the worse performance of smaller NPs with fcc crystal phases to their higher degree of oxidation, while the tendency observed for hcp was attributed to size effects.
Some examples of the use of other oxides such as titania (TiO2) [90,103], ceria (CeO2) [104,105], and hafnia (HfO2) [91] as catalytic supports of Ru NPs for the hydrolytic dehydrogenation of AB can also be found in the literature.

Other Supported Ru Catalysts
Carbon materials and oxides are the most investigated supports for the hydrolytic dehydrogenation of AB, but there are other interesting materials that have also attracted great interest in the last years. Among them, the utilization of MOF should be highlighted because of the attention drawn not only for this application, but for many other catalytic reactions [106][107][108]. They have been shown to be suitable to embed metal NPs as well as to stabilize them by incorporating additional functionalization. This was the case of Chen et al., who achieved well-dispersed Ru NPs immobilized within the pores of amine-functionalized MIL-53 by using an in-situ impregnation-reduction method [94]. It was claimed that the amino groups were located at the acid linkers in MIL-53(Al)-NH2, and served as Lewis bases, thus stabilizing the Ru precursor (i.e., RuCl3) during the impregnation step. Some examples of the use of other oxides such as titania (TiO 2 ) [90,103], ceria (CeO 2 ) [104,105], and hafnia (HfO 2 ) [91] as catalytic supports of Ru NPs for the hydrolytic dehydrogenation of AB can also be found in the literature.

Other Supported Ru Catalysts
Carbon materials and oxides are the most investigated supports for the hydrolytic dehydrogenation of AB, but there are other interesting materials that have also attracted great interest in the last years. Among them, the utilization of MOF should be highlighted because of the attention drawn not only for this application, but for many other catalytic reactions [106][107][108]. They have been shown to be suitable to embed metal NPs as well as to stabilize them by incorporating additional functionalization. This was the case of Chen et al., who achieved well-dispersed Ru NPs immobilized within the pores of amine-functionalized MIL-53 by using an in-situ impregnation-reduction method [94]. It was claimed that the amino groups were located at the acid linkers in MIL-53(Al)-NH 2 , and served as Lewis bases, thus stabilizing the Ru precursor (i.e., RuCl 3 ) during the impregnation step. An amino-free catalyst was prepared for comparison purposes. The average NP size was determined to be 1.22 nm, smaller than the mean diameter of the pores of the amino functionalized MIL-53(Al) (2.07 nm) so that they were embedded in the framework of the MOF, while larger NPs were located on the external surface. Ru/MIL-53(Al)-NH 2 catalyst displayed better activity than the amino-free counterpart, and also better durability and reusability. Such better performance exhibited by Ru/MIL-53(Al)-NH 2 was attributed to the amino groups, which assisted the formation and stabilization of small Ru NPs. Xia et al. also used MIL-53 as support of Ru NPs for the hydrolytic dehydrogenation of AB. In that study, Ru NPs were deposited on MIL-53(Cr) and MIL-53(Al) by the impregnation method with RuCl 3 . Catalysts with Ru contents of 0. 19 [92]. The MOF selected in that study was claimed to be a 3D framework with three different kind of cages as well as with thermal and chemical stability in water. Ru catalysts loaded on other supports (i.e., carbon black, SiO 2 , γ-Al 2 O 3 , and GO) were prepared for comparison purposes. Ru/MIL-96 was the most active catalysts checked in that study, with a TOF of 231 mol H2 ·mol Ru −1 ·min −1 . However, that material did not show suitable stability, since it only retained 65% of its initial activity after the fifth run, which was attributed to the increasing NP size and viscosity of the solution.
Zhu et al. explored the suitability of a N-containing microporous organic framework (POF) as a scaffold to anchor Ru NPs [95]. The resulting catalyst, which was denoted as Ru/PAF-72, has an average NP size of 1-2 nm and showed a TOF of 294 mol H2 ·mol Ru −1 ·min −1 . The most remarkable aspect of that system was its great stability even after 10 consecutive reaction cycles. For comparison purposes, the stability of a catalyst based on Ru NPs loaded onto carbon black was also evaluated, showing a significant activity decay after only four cycles.
Additionally, some other supports, which are less frequently used in catalysts, have served to develop Ru catalysts for the hydrolytic dehydrogenation of AB. For instance, Cai et al. developed a sophisticated catalyst based on Ru NPs loaded a natural polyphenolic polymer (bayberry tannin, BT) immobilized on collagen fibers (CF). The catalysts, denoted as CF-BT-Ru, were synthesized by immobilization of BT, crosslinking of glutaraldehyde, and subsequent chelation of Ru 3+ (final metal loadings of 0.58, 1.03, 1.57, and 2.12 wt. %) [71]. The procedure used is shown in Figure 6. Various reference samples were also prepared for comparison purposes (i.e., CF-BT-Ru, CF-Ru, and Ru-carbon material (Ac, GO, CNTs and g-C 3 N 4 )). It was observed that the presence of BT enhanced the dispersion of the metal active phase, due to the interaction with the phenolic groups, so that the BT containing catalyst has much smaller Ru NPs than the BT-free counterpart (i.e., 2.6 ± 0.6 and 6.5 ± 0.5 nm, respectively). XPS characterization indicated that the catalyst contained positively charged nitrogen and neutral amine groups. Such neutral amine groups were responsible for the stabilization of the Ru NPs by providing some of their electrons. The six catalysts evaluated showed total conversion of AB, but different reaction rates, with CF-BT-Ru being the most active with a TOF of 322 mol H2 ·mol Ru −1 ·min −1 . In an attempt to gain insights into the less explored effect of the composition of the support, Zhao et al. [96]  , respectively. According to that observation, it was postulated that the Mg/Al ratio, which is related to the relative acidity of the material and the support-Ru interaction, was a key aspect in controlling the final catalytic performance. It was found that Ru/Mg 2 Al-LDH-h had more Brønsted acid sites and it showed a weaker interaction with Ru species, which enabled the existence of Ru in the metallic state, thus explaining the better activity exhibited by that catalyst. That sample preserved its stability during the first four cycles, but it slightly decayed from the fourth to the tenth consecutive run, which was attributed to the increased viscosity of the solution, which impedes the diffusion of AB molecules, thus hindering their interaction with the Ru active sites.
tion method (support denoted as Mg4Al-LDH-p). After that, Ru catalysts were prepared from RuCl3.H2O. The results of the catalytic activity indicated that the activity followed the order Ru/Mg2Al-LDH-h > Ru/Mg3Al-LDH-h > Ru/Mg4Al-LDH-(h) > Ru/Mg4Al-LDH-(p), with TOF values of 85.7, 63.3, 42.7, and 40.1 molH2. molRu −1 . min −1 , respectively. According to that observation, it was postulated that the Mg/Al ratio, which is related to the relative acidity of the material and the support-Ru interaction, was a key aspect in controlling the final catalytic performance. It was found that Ru/Mg2Al-LDH-h had more Brønsted acid sites and it showed a weaker interaction with Ru species, which enabled the existence of Ru in the metallic state, thus explaining the better activity exhibited by that catalyst. That sample preserved its stability during the first four cycles, but it slightly decayed from the fourth to the tenth consecutive run, which was attributed to the increased viscosity of the solution, which impedes the diffusion of AB molecules, thus hindering their interaction with the Ru active sites. Sun et al. also explored the performance of MgAl-LDH-supported Ru catalysts for the hydrolytic dehydrogenation of AB [97]. In this case, supports with a composition of Mg0.5Al1-LDHs, Mg1Al1-LDHs, Mg2Al1-LDHs, and Mg3Al1-LDHs were prepared and loaded with Ru NPs. Among those samples, Ru/Mg1Al1-LDHs showed better activity, completing the hydrogen release in 130 s, compared to the other catalysts that needed longer reaction times (180, 140, and 230 s, for Ru/Mg0.5Al1-LDHs, Ru/Mg2Al1-LDHs, and Ru/Mg3Al1-LDHs). In this case, such tendency was also attributed not only to the higher content of Brønsted acid sites in Ru/Mg1Al1-LDHs, but also to the higher purity of Mg1Al1-LDHs, which did not contain boehmite (AlO(OH)) and hydromagnesite (Mg5(- Sun et al. also explored the performance of MgAl-LDH-supported Ru catalysts for the hydrolytic dehydrogenation of AB [97]. In this case, supports with a composition of Mg 0.5 Al 1 -LDHs, Mg 1 Al 1 -LDHs, Mg 2 Al 1 -LDHs, and Mg 3 Al 1 -LDHs were prepared and loaded with Ru NPs. Among those samples, Ru/Mg 1 Al 1 -LDHs showed better activity, completing the hydrogen release in 130 s, compared to the other catalysts that needed longer reaction times (180, 140, and 230 s, for Ru/Mg 0.5 Al 1 -LDHs, Ru/Mg 2 Al 1 -LDHs, and Ru/Mg 3 Al 1 -LDHs). In this case, such tendency was also attributed not only to the higher content of Brønsted acid sites in Ru/Mg 1 Al 1 -LDHs, but also to the higher purity of Mg 1 Al 1 -LDHs, which did not contain boehmite (AlO(OH)) and hydromagnesite (Mg 5 (-CO 3 ) 4 (OH) 2 ·4H 2 O). Ru/Mg 1 Al 1 -LDHs exhibited good stability during 10 reaction runs, and 58.1% of the initial activity remained after those cycles. DTF calculations were conducted to get information on the promotion effect of MgAl-LDHs catalysts. It was determined that Ru/MgAl-LDHs catalysts have the beneficial electronic properties to accelerate the H 2 O dissociation to form O-H bonds, activating the H 2 O molecules during the hydrolytic dehydrogenation of AB.

Conclusions and Perspectives
There is great hope for the potential of hydrogen as an energy vector, which motivates the search for alternatives that overcome the limitations that are frequently related to its storage. Chemical hydrogen storage stands up as a promising option and there are several hydrogen carrier molecules that afford satisfactory hydrogen capacity. Among them, ammonia borane has drawn much attention, and research to exploit the potential of ammonia borane as a hydrogen storage material has been intensified in the last years.
This review summarized some of the recent Ru-based heterogeneous catalysts applied in the hydrolytic dehydrogenation of ammonia borane. A perusal of the most frequently used catalysts is included, paying particular attention to those heterogeneous catalysts with carbon materials and oxides as supports. Among the vast diversity of supported Ru catalysts studied thus far, carbon material-based catalysts frequently attain the best performance.
Most of the investigations deal with the optimization of the properties of the active phase such as the nanoparticle size and morphology, while the effect of the composition of the support is less explored. Most of the catalysts experienced deactivation during few consecutive reaction cycles, which is linked to several factors: (i) aggregation of the nanoparticles; (ii) metal leaching; and (iii) accumulation of metaborate in the solution and change in the viscosity. It is expected that such phenomena involved in the deactivation of the catalysts could be partially averted by using encapsulated or nanoconfined metal catalysts upon selection of a suitable porous material serving as a host for the metal active phase. Most studies include information about the NP size and metal content of the spent catalysts, but no information has been reported about the concentration of metaborate species and viscosity of the solution before and after the reaction, which would help to verify such possible reasons for deactivation that are frequently mentioned but never confirmed. Some other interesting works evidenced the formation of different B-containing products such as B(OH) 3 , BO 2 − , B(OH) 4 − , and polyborates [64,65,[109][110][111]. Some studies have aimed at enhancing the cyclability of the catalysts by stabilizing the metallic phases using strong metal-support interaction, using, for example, a support with abundant functional groups. Nitrogen-containing supports have been widely studied, but the resulting catalysts are still lacking in stability during the cycles. Hence, the insufficient stability of the assessed catalysts is frequently the weakest point indicated in the literature for the heterogeneous catalysts used in the dehydrogenation of ammonia borane. The regeneration of the catalysts after reaction would therefore be a very interesting issue to be considered in future works.
No attention is paid to the actual cornerstone for the successful application of this hydrogen carrier molecule in the hydrogen storage scenario, which is the regeneration of ammonia borane from the products obtained in its decomposition reaction. The AB regeneration problem, which was pointed out long back [112], remains unsolved nowadays and studies dealing with the regeneration of AB are still sparse [113]. The nature of the by-products of the hydrolysis of AB, which are mainly borate species, complicates the regeneration of AB, since the stable B-O bond formed in the by-product are strong and they are not easily reconverted to the B-H bond present in AB molecules, so a strong reducing agent would be needed. Thus, multi-step reactions are required for the regeneration of AB. For instance, Liu et al. proposed a regeneration process that implied the conversion of boric acid to trimethyl borate (B(OCH 3 ) 3 by esterification with methanol. Then, B(OCH 3 ) 3 can react with NaH, generating NaBH 4 and, finally, AB was formed upon reaction of NaH with ammonia sulfate in THF [114]. Gagare et al. aimed at regenerating AB from NH 4 B(OMe) 4 using LiAlH 4 as a reducing agent and NH 4 Cl as an ammonia source [115]. Vasiliu et al. reported on the regeneration of AB from a more simplified and energy-efficient process that involved minimum reaction steps [116]. However, their starting point was not the regeneration of the by-products originating from the production of 3 equivalents of H 2 per molecule of AB, but polyborazylene was used, which is formed upon generation of two equivalents of H 2 per molecule of AB (partially spent AB). In that case, polyborazylene was converted to AB nearly quantitatively by 24 h treatment with N 2 H 4 in liquid NH 3 at 40 • C. More recently, Sharma et al. [117] studied the regeneration of AB using a digestion-based approach in the presence of methanol and subsequent regeneration with the reducing agent (i.e., LiAlH 4 ).
There is still plenty of room for improvement in both the design and development of stable and reusable catalysts as well as in the processes involved in the real application of ammonia borane as a hydrogen storage material, especially for those on-board systems. One of the main issues to be tackled is the high cost of ammonia borane compared to other hydrogen storage systems, so finding cost-effective ways for the synthesis and regeneration of ammonia borane is highly desirable.
The present review contains only information on monometallic Ru-based catalysts, which have been shown to be the most effective to attain suitable catalytic behavior, but there are other compositions that have also displayed interesting results.