Pt-Ni Nanoalloys for H2 Generation from Hydrous Hydrazine

Hydrous hydrazine (N2H4∙H2O) is a candidate for a hydrogen carrier for storage and transportation due to low material cost, high hydrogen content of 8.0%, and liquid stability at room temperature. Pt and Pt nanoalloy catalysts have been welcomed by researchers for the dehydrogenation of hydrous hydrazine recently. Therefore, in this review, we give a summary of Pt nanoalloy catalysts for the dehydrogenation of hydrous hydrazine and briefly introduce the decomposition mechanism of hydrous hydrazine to prove the design principle of the catalyst. The chemical characteristics of hydrous hydrazine and the mechanism of dehydrogenation reaction are briefly introduced. The catalytic activity of hydrous hydrazine on different supports and the factors affecting the selectivity of hydrogen catalyzed by Ni-Pt are analyzed. It is expected to provide a new way for the development of high-activity catalysts for the dehydrogenation of hydrous hydrazine to produce hydrogen.


Introduction
Hydrogen energy, as a kind of green energy with abundant reserves, wide sources, and high energy density, has shown an excellent application prospect in fuel cells and as a substitute for fossil fuels [1][2][3][4]. In the process of utilization, it is a key to storage and transportation [5,6]. Hydrogen storage methods can be roughly divided into physical methods [7,8] and chemical methods [9,10], which have made some progress in a certain extent. Based on the liquid organic hydride, hydrogen storage technology in chemical methods, owing to large hydrogen storage capacity, high energy density, and safe and convenient liquid storage and transportation [11][12][13], has attracted the attention of scholars. The chemical nature of hydrazine (N 2 H 4 ) is relatively active and can react with strong oxidants. However, hydrazine is likely to explode if it is left in the air for a long time or under high temperature heating. Therefore, hydrazine is an extremely dangerous substance and is not suitable as a hydrogen storage material [14,15], but hydrous hydrazine (N 2 H 4 ·H 2 O), as a liquid hydrogen storage material with excellent hydrogen storage content (8.0 wt%)-the only by-product when completely decomposed is N 2 , which has no pollution to the environment-has always been considered to have great prospects [16][17][18][19].
The decomposition process of hydrous hydrazine is often accompanied by side reactions that produce ammonia, which greatly reduces the hydrogen production efficiency of hydrous hydrazine [20].

Mechanism of H 2 Production from Hydrous Hydrazine through Pt Nanoalloys
The reaction pathway of hydrous hydrazine [38] depends on the breaking of the N-N bond and N-H bond. In theory, the N-N bond energy is 286 KJ mol −1 and the N-H bond total cleavage energy is 360 KJ mol −1 [39]. Thus, the N-N bond seems to be more easily fractured. However, if N 2 H 4 breaks down to N 2 H 2 and H 2 , the N-H bond energy is 276 KJ mol −1 , and the H-metal bond energy is stronger than the N-metal bond, which makes the N-H bond more likely to break. Zhang's team [40] summarized most of the literature on the decomposition of N 2 H 4 and proposed a relatively complete mechanism of N 2 H 4 ( Figure 1). The effective hydrogen storage component of hydrous hydrazine is hydrazine (N 2 H 4 ), and its decomposition reaction can be carried out according to the following two competitive paths [41,42]: complete decomposition, H 2 NNH 2 → N 2 + 2H 2 , ∆H = −95.4 kJ mol −1 (1) and incomplete decomposition, from the application of hydrogen storage, it is necessary to selectively promote the complete decomposition of N 2 H 4 into H 2 and N 2 , and effectively inhibit the formation of NH 3 by-products.
The key to the development of N 2 H 4 ·H 2 O controllable hydrogen technology lies in the development of cheap catalysts with high catalytic activity and high hydrogen production selectivity [43].

Ni-Pt Nanoalloying
In the past reports, many transition metal nano-catalysts have been synthesized by coprecipitation [51], chemical reduction, combustion synthesis [52], galvanic displacement [53], impregnation [54], and alloying [55]. Among them, alloying is an effective strategy to improve catalytic activity and hydrogen selectivity [55]. Fixing the catalyst nanoparticles on the basic carrier can not only improve the durability of the catalyst, but also improve the selectivity of hydrogen [56]. Compared with the corresponding single-metal, nanoalloys have the advantages of reconfigurable electronic structure, variable composition ratio, and adjustable selectivity, which show unique properties different from bulk alloy and metal elements [56]. Because of the nano-size effect and a large number of kinks, the nanoalloy obtains higher specific surface energy and rich active sites through electronic configuration reorganization, which greatly promotes its catalytic performance, further evidence that it has obvious superiority and broad application prospects in chemical catalysis. On the one hand, nanoalloy catalytic materials cannot only catalyze the reforming of syngas, but also catalyze various coupling reactions, which has promoted the development of chemical industry, Early studies focused on the mechanism of adsorption on the surfaces of Ir [44], Pt [45,46], and Rh [47,48] model metals. For hydrogen production from hydrous hydrazine, changing the order of the rupture of N-N and N-H bonds is the main strategy to change the selectivity. Transition metals (such as Ir, Ru, and Ni) are the most effective N-H bond activation candidates in various catalytic systems and exhibit high hydrazine decomposition activity under mild conditions. Nevertheless, the selectivity of H 2 is limited to less than 10% as compared to Ru and Ir catalysts [49]. So far, single-metal catalysts have failed to achieve complete catalytic decomposition from hydrazine to H 2 without forming NH 3 . The Pt-based catalysts have strong catalytic activity [50]. Therefore, the modification of the catalyst is necessary to improve the selectivity of H 2 . Bimetallic catalysts, due to their unique electronic and chemical properties, exhibit enhanced activity, selectivity, and stability compared with the parent metals. For Pt-promoted nickel catalysts, the results of x-ray diffraction (XRD) in Zhang's experiment [40] showed that the main d fractionation peaks only show the presence of Ni. The shift of the diffraction peak to a smaller angle after the substitution of Pt indicates that the co-reduction of Ni-Pt with the second metal in the bimetallic catalyst further confirms the formation of Ni-Pt alloy, as indicated by temperature programmed reduction (TPR) data. In addition, the results of x-ray absorption fine structure (EXAFS) experiments with extended x-ray show that most Pt species exist in Ni-Pt alloys, which leads to electron transfer from Ni to Pt.

Ni-Pt Nanoalloying
In the past reports, many transition metal nano-catalysts have been synthesized by co-precipitation [51], chemical reduction, combustion synthesis [52], galvanic displacement [53], impregnation [54], and alloying [55]. Among them, alloying is an effective strategy to improve catalytic activity and hydrogen selectivity [55]. Fixing the catalyst nanoparticles on the basic carrier can not only improve the durability of the catalyst, but also improve the selectivity of hydrogen [56]. Compared with the corresponding single-metal, nanoalloys have the advantages of reconfigurable electronic structure, variable composition ratio, and adjustable selectivity, which show unique properties different from bulk alloy and metal elements [56]. Because of the nano-size effect and a large number of kinks, the nanoalloy obtains higher specific surface energy and rich active sites through electronic configuration reorganization, which greatly promotes its catalytic performance, further evidence that it has obvious superiority and broad application prospects in chemical catalysis. On the one hand, nanoalloy catalytic materials cannot only catalyze the reforming of syngas, but also catalyze various coupling reactions, which has promoted the development of chemical industry, energy, environment, medicine, and other industries. Nanoalloy catalytic materials cannot only enhance the activity through the surface structure, but also improve the performance through the mechanical effect, greatly enriching the theoretical system of nano-catalysis [57]. The nanoalloys (NAs) are obviously different from bulk alloy or single-metal, with tunable composition and proportion, variable structure, reconfigurable electrical characteristic structure, and optimized performance, which give NAs a fascinating prospect in the field of catalysis.
Based on the previously reported literature, we found that most of the papers were based on the dehydrogenation of hydrous hydrazine on different supports supported with different proportions of Ni-Pt bimetallic catalysts [58][59][60]. The noble metal Pt has high reduction potential and stability, so it is easier to prepare new structures [61]. They provide excellent catalytic properties that are closely related to Fermi's electrons. The base metal, Ni, readily gives up electrons and is oxidized to a lower reduction potential. Ni is one of the most important catalysts for industrial catalysis because of its excellent catalytic selectivity and cost efficiency [62]. Compared with the single-metal, the activity and stability of the Pt-Ni catalyst were improved significantly [57]. Figure 2 shows the etching mechanism of Pt-Ni nanoparticles (NPs). Noble/base metal-mixed NAs (NBMNAs) [63][64][65] are promising catalysts that combine the high catalytic activity of noble metal Pt and the high selectivity of base metal Ni. At the same time, a high utilization ratio of noble metal atoms can be obtained. The electronic structures of Pt and Ni are reorganized by alloying, which is an effective way to reduce the irreversible oxidation or leakage of base metals and to improve the stability of catalysts. The proper distribution of metal elements helps to form the Pt surface layer of noble metals to protect the catalysts. At the same time, the performance of the catalyst is improved because of the mechanical action of the alloy base and the precious metal.
Catalysts 2020, 10, x FOR PEER REVIEW  4 of 21 energy, environment, medicine, and other industries. Nanoalloy catalytic materials cannot only enhance the activity through the surface structure, but also improve the performance through the mechanical effect, greatly enriching the theoretical system of nano-catalysis [57]. The nanoalloys (NAs) are obviously different from bulk alloy or single-metal, with tunable composition and proportion, variable structure, reconfigurable electrical characteristic structure, and optimized performance, which give NAs a fascinating prospect in the field of catalysis. Based on the previously reported literature, we found that most of the papers were based on the dehydrogenation of hydrous hydrazine on different supports supported with different proportions of Ni-Pt bimetallic catalysts [58][59][60]. The noble metal Pt has high reduction potential and stability, so it is easier to prepare new structures [61]. They provide excellent catalytic properties that are closely related to Fermi's electrons. The base metal, Ni, readily gives up electrons and is oxidized to a lower reduction potential. Ni is one of the most important catalysts for industrial catalysis because of its excellent catalytic selectivity and cost efficiency [62]. Compared with the single-metal, the activity and stability of the Pt-Ni catalyst were improved significantly [57]. Figure 2 shows the etching mechanism of Pt-Ni nanoparticles (NPs). Noble/base metal-mixed NAs (NBMNAs) [63][64][65] are promising catalysts that combine the high catalytic activity of noble metal Pt and the high selectivity of base metal Ni. At the same time, a high utilization ratio of noble metal atoms can be obtained. The electronic structures of Pt and Ni are reorganized by alloying, which is an effective way to reduce the irreversible oxidation or leakage of base metals and to improve the stability of catalysts. The proper distribution of metal elements helps to form the Pt surface layer of noble metals to protect the catalysts. At the same time, the performance of the catalyst is improved because of the mechanical action of the alloy base and the precious metal.

Hydrous Hydrazine
Hydrogen storage materials with a large energy storage density are the radical condition for practical application; in the required temperature range, the energy hydrogen storage density of a large number of metal alloy hydrogen storage materials and physical adsorption materials studied in the past can hardly meet the demand of practical application [66]. Therefore, in recent years, lightweight hydrogen storage substances with ultra-high hydrogen storage capacity have become a research hotspot.
In liquid phase hydrogen storage carriers, hydrous hydrazine has been widely studied by scholars because of outstanding hydrogen storage density, easy storage and transportation, and convenient use N2H4. There are two decomposition paths for N2H4: complete decomposition to produce H2 and N2, and incomplete decomposition to produce N2 and NH3. The former reaction path produces H2 for human use, while the latter produces NH3 that can poison Nafion membranes and fuel cell catalysts, so the latter reaction must be avoided. So far, the catalytic decomposition of hydrous hydrazine has reached 100% catalytic selectivity [67][68][69][70][71]. The catalytic kinetic is still very

Hydrous Hydrazine
Hydrogen storage materials with a large energy storage density are the radical condition for practical application; in the required temperature range, the energy hydrogen storage density of a large number of metal alloy hydrogen storage materials and physical adsorption materials studied in the past can hardly meet the demand of practical application [66]. Therefore, in recent years, light-weight hydrogen storage substances with ultra-high hydrogen storage capacity have become a research hotspot.
In liquid phase hydrogen storage carriers, hydrous hydrazine has been widely studied by scholars because of outstanding hydrogen storage density, easy storage and transportation, and convenient use N 2 H 4 . There are two decomposition paths for N 2 H 4 : complete decomposition to produce H 2 and N 2 , and incomplete decomposition to produce N 2 and NH 3 . The former reaction path produces H 2 for Catalysts 2020, 10, 930 5 of 21 human use, while the latter produces NH 3 that can poison Nafion membranes and fuel cell catalysts, so the latter reaction must be avoided. So far, the catalytic decomposition of hydrous hydrazine has reached 100% catalytic selectivity [67][68][69][70][71]. The catalytic kinetic is still very slow, which enormously blocks the practical application of the system. Thence, it is urgent to develop highly selective and efficient e at room temperature.
Hence the decomposition of hydrous hydrazine tends to produce NH 3 rather than H 2 , which not only reduces the yield of hydrogen, but also complicates the separation process due to the low toxicity of Nafion membrane and the fuel cell catalyst to NH 3 [72][73][74]. Therefore, it is of great significance to develop a highly efficient and selective hydrous hydrazine catalyst for hydrogen evolution. In recent years, metal nano-catalysts have made great progress in the catalytic decomposition of hydrous hydrazine. The Ir/Al 2 O 3 catalyst was the first reported active catalyst for the decomposition of hydrous hydrazine catalyzed by highly dispersed metal particles [75]. In order to develop high-property catalysts for hydrogen-selective dehydrogenation of hydrous hydrazine, bimetallic alloys based on nickel and noble metals Rh, Pt, or Ir can completely decompose hydrous hydrazine to produce hydrogen selectively [76][77][78][79]. The H 2 selectivity of bimetallic NPs is strongly dependent on the metal ratio, and the alloy nano-catalyst with stable surface activity can achieve 100% hydrogen selectivity, which is more active than the corresponding single-metal nano-catalyst [80]. It is reasonable to consider that the coordination in bimetallic catalysts can adjust the bond mode of reactants, stabilize the reaction intermediates on the catalyst surface, and improve the catalytic activity and stability. These bimetallic nano-catalysts have good hydrogen selectivity for the dehydrogenation of hydrous hydrazine, but the chemical kinetics are still exceedingly low [81].

H 2 Selectivity in Hydrous Hydrazine Dehydrogenation
Although bimetallic nano-catalysts have a good hydrogen selectivity for the decomposition of hydrous hydrazine, chemical kinetics are still intensely low [82]. In subsequent studies, Zhu et al. [83] found that the presence of basic additives, such as NaOH, was beneficial to increase the catalytic activity and selectivity of metal catalysts for hydrogen production from hydrous hydrazine. When NaOH is added at 0.5 M, the selectivity of Ni 45 Pt 55 increases from 61% to 86%, and the selectivity of Ni 50 Ir 50 increases from 7% to 95% at 25 • C [84]. The alkaline solution makes the surface of the catalyst highly alkaline, which helps to inhibit the formation of alkaline selectivity, and thus favors the formation of H 2 . In addition, with the increase of the reaction temperature, the catalytic performance is significantly improved [85]. In the presence of NaOH, highly dispersed surfactant Ni-Pt NPs were synthesized, in which NaOH plays an important role in the formation and stability of small-size NPs. The crystalline porous structure of metal organic frameworks (MOFs) can restrict the migration and aggregation of metal NPs. The MOF-supported bimetallic NPs [86] have recently been reported as the catalysts of high properties for the selective decomposition of hydrazine.

Catalytic Effect of Different Supports on Hydrous Hydrazine
Extensively used industrial heterogeneous catalysts are usually composed of metal nanoparticles supported on a large surface area. In recent decades, with the discovery of metal-carrier interactions, the significance of the oxides has been recognized to an increasing extent. Apart from dispersing metal particles, oxide supports also play a role in influencing the catalytic performance of metal catalysts through electronic or geometric effects. Representative oxides include CeO 2 , TiO 2 , and Fe 2 O 3 . The strategies to further improve its performance, including the choice of metal composition and the adjustment of metal-carrier interaction, have been analyzed [87]. In heterogeneous catalysis, support in the nano-catalyst may play a key role in the whole process. On the one hand, support can be used to disperse and reduce the size of the metal nanoparticles during the synthesis process, thereby exposing more active sites and leading to the improvement of catalytic performance. On the other hand, coupling metal nanoparticles with a support may introduce additional synergistic effects, so inherently changing the physical and chemical properties of the interface between them. Therefore, the catalytic activity and selectivity can be adjusted by changing different supports.

TiO 2 -Decorated Ti 3 C 2 T x
In 2020, Ni-Pt nanoparticles (NPs) dispersed on TiO 2 -decorated Ti 3 C 2 T x (denoted as DT-Ti 3 C 2 T x ) nanosheets were prepared (Lu et al.) [88] by a facile wet chemical reduction method and used as an effective catalyst for dehydrogenation of N 2 H 4 as shown in Scheme 1. The oxygen-rich functional groups on the surface of DT-Ti 3 C 2 T x not only contributed to the formation and fixation of monodisperse Ni-Pt NPs, but also enhanced the synergy between metal NPs and the MXene carrier. In all the tested samples, the best Ni 0.8 Pt 0.2 /DT-Ti 3 C 2 T x nano-catalyst showed 100% H 2 selectivity and the optimal catalytic performance. The turnover frequency (TOF) value of the selective dehydrogenation of N 2 H 4 at 323 K was 1220 h −1 . Ni-Pt alloy NPs were evenly dispersed on DT-Ti 3 C 2 T x nanosheets with an average size of 2.8 nm. DT-Ti 3 C 2 T x in Ni-Pt/DT-Ti 3 C 2 T x acts as an electron donor for Ni-Pt NPs to enhance the electron density of Ni-Pt NPs. Appropriate oxidation of Ti 3 C 2 T x MXene can augment the number of oxygen-containing functional groups on the surface of Ti 3 C 2 T x , which plays an important role in fixing the small particle size Ni-Pt NPs and promoting the synergy between Ni-Pt NPs and DT-Ti 3 C 2 T x . The Ni-Pt/DT-Ti 3 C 2 T x catalyst shows strong activity, 100% H 2 selectivity, and excellent durability in the production of hydrogen from hydrazine and hydrazinoborane in aqueous solution. In addition, DT-Ti 3 C 2 T x supports other bimetallic NPs and also shows splendid catalytic activity, emphasizing the generality of DT-Ti 3 C 2 T x as a carrier.

TiO2-Decorated Ti3C2Tx
In 2020, Ni-Pt nanoparticles (NPs) dispersed on TiO2-decorated Ti3C2Tx (denoted as DT-Ti3C2Tx) nanosheets were prepared (Lu et al.) [88] by a facile wet chemical reduction method and used as an effective catalyst for dehydrogenation of N2H4 as shown in Scheme 1. The oxygen-rich functional groups on the surface of DT-Ti3C2Tx not only contributed to the formation and fixation of monodisperse Ni-Pt NPs, but also enhanced the synergy between metal NPs and the MXene carrier. In all the tested samples, the best Ni0.8Pt0.2/DT-Ti3C2Tx nano-catalyst showed 100% H2 selectivity and the optimal catalytic performance. The turnover frequency (TOF) value of the selective dehydrogenation of N2H4 at 323 K was 1220 h −1 . Ni-Pt alloy NPs were evenly dispersed on DT-Ti3C2Tx nanosheets with an average size of 2.8 nm. DT-Ti3C2Tx in Ni-Pt/DT-Ti3C2Tx acts as an electron donor for Ni-Pt NPs to enhance the electron density of Ni-Pt NPs. Appropriate oxidation of Ti3C2Tx MXene can augment the number of oxygen-containing functional groups on the surface of Ti3C2Tx, which plays an important role in fixing the small particle size Ni-Pt NPs and promoting the synergy between Ni-Pt NPs and DT-Ti3C2Tx. The Ni-Pt/DT-Ti3C2Tx catalyst shows strong activity, 100% H2 selectivity, and excellent durability in the production of hydrogen from hydrazine and hydrazinoborane in aqueous solution. In addition, DT-Ti3C2Tx supports other bimetallic NPs and also shows splendid catalytic activity, emphasizing the generality of DT-Ti3C2Tx as a carrier. Scheme 1. Schematic diagram for the preparation of Ni-Pt/DT-Ti3C2Tx; copyright (2020), Elsevier.

CeO2
In 2018, Luo et al. [89] reported excellent catalytic properties for the co-reduction composite of Ni-Pt nanoparticles supported on CeO2 nanospheres and the generation of hydrogen from hydrazine basic solutions at ambient temperature. As can be seen from Figure 3a, Ni-CeO2 has almost no catalytic activity. However, when Ni-CeO2 is alloyed with Pt, Ni-Pt-CeO2 catalysts with different metal ratios show strong catalytic activity and selectivity. As can be seen from Figure 3b, the dehydrogenation effect of the Ni-Pt alloy catalyst on hydrous hydrazine is different with different supports, and the catalytic performance of Ni-Pt-CeO2 is the best, which indicates that the CeO2 nanosphere is one of the key factors to promote the catalytic performance. Through the interaction between Ni-Pt and CeO2, the Ni5Pt5-CeO2 catalyst has remarkable catalytic activity, 100% hydrogen selectivity, and hydrazine dehydrogenation selectivity, and the TOF value is 416 h −1 at room temperature. Scheme 1. Schematic diagram for the preparation of Ni-Pt/DT-Ti 3 C 2 T x ; copyright (2020), Elsevier.

CeO 2
In 2018, Luo et al. [89] reported excellent catalytic properties for the co-reduction composite of Ni-Pt nanoparticles supported on CeO 2 nanospheres and the generation of hydrogen from hydrazine basic solutions at ambient temperature. As can be seen from Figure 3a, Ni-CeO 2 has almost no catalytic activity. However, when Ni-CeO 2 is alloyed with Pt, Ni-Pt-CeO 2 catalysts with different metal ratios show strong catalytic activity and selectivity. As can be seen from Figure 3b, the dehydrogenation effect of the Ni-Pt alloy catalyst on hydrous hydrazine is different with different supports, and the catalytic performance of Ni-Pt-CeO 2 is the best, which indicates that the CeO 2 nanosphere is one of the key factors to promote the catalytic performance. Through the interaction between Ni-Pt and CeO 2 , the Ni 5 Pt 5 -CeO 2 catalyst has remarkable catalytic activity, 100% hydrogen selectivity, and hydrazine dehydrogenation selectivity, and the TOF value is 416 h −1 at room temperature. Catalysts 2020, 10, x FOR PEER REVIEW 7 of 21 Wang et al. [90] prepared the Ni50Pt50/CeO2 catalyst, as shown in Scheme 2, in 2019 and then transferred it to a sealed polytetrafluoroethylene-lined autoclave for aging treatment. Under an H2 atmosphere, the resulting black sediment is finally reduced at a high temperature to obtain a tar electronic catalyst. According to ICP-AES elemental analysis, the actual composition of the target Ni50Pt50/CeO2 catalyst is 50.3 mol% Ni50.4Pt49.6/49.7 mol% CeO2, which is close to the originally designed atomic ratio. In order to better understand the phase evolution of the catalyst during preparation, they combined phase/microstructure/chemical state analysis of samples collected at different stages. It was found that the preparation of the catalyst involved the structure of [(CH3)4N] 2PtCl6 and CeNi0.5Ox in the coprecipitation step and their evolution in the consecutive reduction and aging process. Preventing [(CH3)4N]2PtCl6 corrosion of metal Pt during aging is an essential procedure in the preparation process. It has a profound impact on the cracking performance of the catalyst on the composition, microstructure characteristics, and corresponding catalyst of metal Pt. Their study clearly describes for the first time the phase/structure evolution during the preparation of Ni50Pt50/CeO2 catalyst. This may lay the foundation for future rational design and control of the synthesis of high-performance catalysts to promote the production of chemical hydrides such as N2H4•H2O. Scheme 2. Schematic illustration of the preparation process of Ni50Pt50/CeO2 catalyst by coprecipitation method; copyright (2019), Elsevier.
A simple impregnation-reduction method was used to prepare a monolithic catalyst consisting of Ni-Pt/CeO2 nanoparticles and granular activated carbon (GAC) by Wang et al. [91] in 2018, as Wang et al. [90] prepared the Ni 50 Pt 50 /CeO 2 catalyst, as shown in Scheme 2, in 2019 and then transferred it to a sealed polytetrafluoroethylene-lined autoclave for aging treatment. Under an H 2 atmosphere, the resulting black sediment is finally reduced at a high temperature to obtain a tar electronic catalyst. According to ICP-AES elemental analysis, the actual composition of the target Ni 50 Pt 50 /CeO 2 catalyst is 50.3 mol% Ni 50.4 Pt 49.6 /49.7 mol% CeO 2 , which is close to the originally designed atomic ratio. In order to better understand the phase evolution of the catalyst during preparation, they combined phase/microstructure/chemical state analysis of samples collected at different stages. It was found that the preparation of the catalyst involved the structure of [(CH 3 ) 4 N] 2 PtCl 6 and CeNi 0.5 O x in the coprecipitation step and their evolution in the consecutive reduction and aging process. Preventing [(CH 3 ) 4 N] 2 PtCl 6 corrosion of metal Pt during aging is an essential procedure in the preparation process. It has a profound impact on the cracking performance of the catalyst on the composition, microstructure characteristics, and corresponding catalyst of metal Pt. Their study clearly describes for the first time the phase/structure evolution during the preparation of Ni 50 Pt 50 /CeO 2 catalyst. This may lay the foundation for future rational design and control of the synthesis of high-performance catalysts to promote the production of chemical hydrides such as N 2 H 4 ·H 2 O. Wang et al. [90] prepared the Ni50Pt50/CeO2 catalyst, as shown in Scheme 2, in 2019 and then transferred it to a sealed polytetrafluoroethylene-lined autoclave for aging treatment. Under an H2 atmosphere, the resulting black sediment is finally reduced at a high temperature to obtain a tar electronic catalyst. According to ICP-AES elemental analysis, the actual composition of the target Ni50Pt50/CeO2 catalyst is 50.3 mol% Ni50.4Pt49.6/49.7 mol% CeO2, which is close to the originally designed atomic ratio. In order to better understand the phase evolution of the catalyst during preparation, they combined phase/microstructure/chemical state analysis of samples collected at different stages. It was found that the preparation of the catalyst involved the structure of [(CH3)4N] 2PtCl6 and CeNi0.5Ox in the coprecipitation step and their evolution in the consecutive reduction and aging process. Preventing [(CH3)4N]2PtCl6 corrosion of metal Pt during aging is an essential procedure in the preparation process. It has a profound impact on the cracking performance of the catalyst on the composition, microstructure characteristics, and corresponding catalyst of metal Pt. Their study clearly describes for the first time the phase/structure evolution during the preparation of Ni50Pt50/CeO2 catalyst. This may lay the foundation for future rational design and control of the synthesis of high-performance catalysts to promote the production of chemical hydrides such as N2H4•H2O.
A simple impregnation-reduction method was used to prepare a monolithic catalyst consisting of Ni-Pt/CeO2 nanoparticles and granular activated carbon (GAC) by Wang et al. [91] in 2018, as Scheme 2. Schematic illustration of the preparation process of Ni 50 Pt 50 /CeO 2 catalyst by co-precipitation method; copyright (2019), Elsevier.
A simple impregnation-reduction method was used to prepare a monolithic catalyst consisting of Ni-Pt/CeO 2 nanoparticles and granular activated carbon (GAC) by Wang et al. [91] in 2018, as shown in Figure 4. The study found that the catalyst activity, H 2 selectivity, and atmosphere can be easily adjusted by transforming the annealing temperature. The optimum Ni-Pt/CeO 2 /GAC catalyst can completely decompose N 2 H 4 ·H 2 O into H 2 in the presence of 1M NaOH at moderate temperature, with 100% selectivity. Significantly, by utilizing this monolithic catalyst, they have formed an N 2 H 4 ·H 2 O-based H 2 generation (HG) system with a material hydrogen capacity of up to 6.54 wt%.
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 21 shown in Figure 4. The study found that the catalyst activity, H2 selectivity, and atmosphere can be easily adjusted by transforming the annealing temperature. The optimum Ni-Pt/CeO2/GAC catalyst can completely decompose N2H4•H2O into H2 in the presence of 1M NaOH at moderate temperature, with 100% selectivity. Significantly, by utilizing this monolithic catalyst, they have formed an N2H4•H2O-based H2 generation (HG) system with a material hydrogen capacity of up to 6.54 wt%.

Al2O3
In 2012, Pt-modified Ni/Al2O3 catalysts were synthesized by Zhang et al. [92] and studied in the dissociation of hydrous hydrazine, as shown in Figure 5. Compared to Ni/Al2O3, TOF was found to be 7 times stronger in Ni-Pt, and the selectivity to H2 was improved to 98%. The results show that the formation of Pt-Ni alloy weakens the adsorption state, including the interaction between H2 and NHx and Ni atoms on the surface. On the Ni-PtX/Al2O3 catalyst, the weakening effect can explain the increase of reaction rate and H2 selectivity.

Al 2 O 3
In 2012, Pt-modified Ni/Al 2 O 3 catalysts were synthesized by Zhang et al. [92] and studied in the dissociation of hydrous hydrazine, as shown in Figure 5. Compared to Ni/Al 2 O 3 , TOF was found to be 7 times stronger in Ni-Pt, and the selectivity to H 2 was improved to 98%. The results show that the formation of Pt-Ni alloy weakens the adsorption state, including the interaction between H 2 and NH x and Ni atoms on the surface. On the Ni-Pt X /Al 2 O 3 catalyst, the weakening effect can explain the increase of reaction rate and H 2 selectivity.
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 21 shown in Figure 4. The study found that the catalyst activity, H2 selectivity, and atmosphere can be easily adjusted by transforming the annealing temperature. The optimum Ni-Pt/CeO2/GAC catalyst can completely decompose N2H4•H2O into H2 in the presence of 1M NaOH at moderate temperature, with 100% selectivity. Significantly, by utilizing this monolithic catalyst, they have formed an N2H4•H2O-based H2 generation (HG) system with a material hydrogen capacity of up to 6.54 wt%.

Al2O3
In 2012, Pt-modified Ni/Al2O3 catalysts were synthesized by Zhang et al. [92] and studied in the dissociation of hydrous hydrazine, as shown in Figure 5. Compared to Ni/Al2O3, TOF was found to be 7 times stronger in Ni-Pt, and the selectivity to H2 was improved to 98%. The results show that the formation of Pt-Ni alloy weakens the adsorption state, including the interaction between H2 and NHx and Ni atoms on the surface. On the Ni-PtX/Al2O3 catalyst, the weakening effect can explain the increase of reaction rate and H2 selectivity.

Graphene Oxide (GO)
In 2018, Xu et al. [93] used the wet chemical reduction method, as shown in Figure 6a, to fix the ultrafine and uniform dispersion bimetallic Pt-Ni nanoparticles (NPs) onto a new three-dimensional N-doped graphene network (NGNs), which was fabricated by cross-linking graphene oxide (GO) with melamine-formaldehyde resin (MFR) and N-dope graphene layers. As can be seen from Figure 6b, the scanning electron microscope of NGNs-850 and Pt 0.5 Ni 0.5 NGNs-850 showed a porous 3D structure with randomly opened stomatal layers. The evenly dispersed Pt 0.5 Ni 0.5 NPs on NGNs-850 can be seen from the transmission electron microscope (TEM) and high-angle annular dark-field scanning image of Pt 0.5 Ni 0.5 NPs on Figure 6c-f Pt 0.5 Ni 0.5 /NGNs-850. The average particle size is 2.2 nm. The main purpose of this work is to show that the prepared Pt 0.5 Ni 0.5 /NGNs-850 catalyst has very high catalytic activity for hydrous hydrazine to produce hydrogen, and its catalytic activity is 943 h −1 at 303 K. The high property of the catalyst is mainly due to the ultra-small size effect, the powerful synergy between Pt and Ni atoms, as well as the excellent porous structure of the 3D network. This was achieved by increasing the interaction between the graphene surface and the metallic species through N-bonding and graphene cross-linking.

Graphene Oxide (GO)
In 2018, Xu et al. [93] used the wet chemical reduction method, as shown in Figure 6a, to fix the ultrafine and uniform dispersion bimetallic Pt-Ni nanoparticles (NPs) onto a new three-dimensional N-doped graphene network (NGNs), which was fabricated by cross-linking graphene oxide (GO) with melamine-formaldehyde resin (MFR) and N-dope graphene layers. As can be seen from Figure  6b, the scanning electron microscope of NGNs-850 and Pt0.5Ni0.5 NGNs-850 showed a porous 3D structure with randomly opened stomatal layers. The evenly dispersed Pt0.5Ni0.5 NPs on NGNs-850 can be seen from the transmission electron microscope (TEM) and high-angle annular dark-field scanning image of Pt0.5Ni0.5 NPs on Figure 6(c-f) Pt0.5Ni0.5/NGNs-850. The average particle size is 2.2 nm. The main purpose of this work is to show that the prepared Pt0.5Ni0.5/NGNs-850 catalyst has very high catalytic activity for hydrous hydrazine to produce hydrogen, and its catalytic activity is 943 h -1 at 303 K. The high property of the catalyst is mainly due to the ultra-small size effect, the powerful synergy between Pt and Ni atoms, as well as the excellent porous structure of the 3D network. This was achieved by increasing the interaction between the graphene surface and the metallic species through N-bonding and graphene cross-linking. In 2020, Lu et al. [94] reported a low Ni-Pt-containing bimetallic catalyst immobilized on a new MIL-101/rGO composite prepared by a simple and easy impregnation-reduction method (Figure 7). It was discovered that the synthesized Ni-Pt/MIL catalyst has the best catalytic performance and 100% hydrogen selectivity for the hydrogen evolution of hydrous hydrazine at 323 K alkalinity. The turnover frequency value (TOF) is 960 h −1 . In addition, the Ni-Pt/MIL catalyst also shows excellent durability. In 2020, Lu et al. [94] reported a low Ni-Pt-containing bimetallic catalyst immobilized on a new MIL-101/rGO composite prepared by a simple and easy impregnation-reduction method (Figure 7). It was discovered that the synthesized Ni-Pt/MIL catalyst has the best catalytic performance and 100% hydrogen selectivity for the hydrogen evolution of hydrous hydrazine at 323 K alkalinity. The turnover frequency value (TOF) is 960 h −1 . In addition, the Ni-Pt/MIL catalyst also shows excellent durability.

Figure 7.
A low Pt-containing Ni-Pt nano-catalyst immobilized on a metal organic framework (MOF)/rGO composite has been synthesized for hydrogen production from hydrous hydrazine and hydrazine borane; copyright (2020), Elsevier.
Xu et al. [95] fixed the ultrafine bimetallic Pt-Ni nanoparticles (NPs) with an average size of 1.8 nm to zirconia/porous carbon/graphene oxide (ZrO2/C/rGO) by the wet chemical reduction method using the template for a metal-organic skeleton shown in the Scheme 3. Unexpectedly, hydrogen is generated from hydrazine, and an extremely high turnover frequency value can be provided to 1920 h − 1 at 323 K. The high enhanced catalytic activity can be attributed to the small size effect, the strong synergistic effect between Pt and Ni atoms, and the strong metal-support interaction. Cheng et al. [96] have successfully synthesized ultrafine monodisperse Ni-Pt alloy NPs by conjugating nickel acetylacetonate and platinum acetylacetonate with borane-tert-butylamine (BTB) in 90 °C oleylamine (Figure 8). The composition of Ni-Pt NPs is controlled by changing the initial molar ratio of the metal precursor. Ni-Pt NPs were deposited on graphene, and the catalytic dehydrogenation of hydrazine alkaline solution was tested under ambient conditions. Among all the tested catalysts, Ni84Pt16/graphene showed 100% hydrogen selectivity and had obvious high catalytic activity. The TOF value at 50 °C was 415 h −1 and the TOF value at 25 °C 133 h −1 , higher than Xu et al. [95] fixed the ultrafine bimetallic Pt-Ni nanoparticles (NPs) with an average size of 1.8 nm to zirconia/porous carbon/graphene oxide (ZrO 2 /C/rGO) by the wet chemical reduction method using the template for a metal-organic skeleton shown in the Scheme 3. Unexpectedly, hydrogen is generated from hydrazine, and an extremely high turnover frequency value can be provided to 1920 h −1 at 323 K. The high enhanced catalytic activity can be attributed to the small size effect, the strong synergistic effect between Pt and Ni atoms, and the strong metal-support interaction.
Catalysts 2020, 10, x FOR PEER REVIEW 10 of 21 Figure 7. A low Pt-containing Ni-Pt nano-catalyst immobilized on a metal organic framework (MOF)/rGO composite has been synthesized for hydrogen production from hydrous hydrazine and hydrazine borane; copyright (2020), Elsevier.
Xu et al. [95] fixed the ultrafine bimetallic Pt-Ni nanoparticles (NPs) with an average size of 1.8 nm to zirconia/porous carbon/graphene oxide (ZrO2/C/rGO) by the wet chemical reduction method using the template for a metal-organic skeleton shown in the Scheme 3. Unexpectedly, hydrogen is generated from hydrazine, and an extremely high turnover frequency value can be provided to 1920 h − 1 at 323 K. The high enhanced catalytic activity can be attributed to the small size effect, the strong synergistic effect between Pt and Ni atoms, and the strong metal-support interaction. Cheng et al. [96] have successfully synthesized ultrafine monodisperse Ni-Pt alloy NPs by conjugating nickel acetylacetonate and platinum acetylacetonate with borane-tert-butylamine (BTB) in 90 °C oleylamine (Figure 8). The composition of Ni-Pt NPs is controlled by changing the initial molar ratio of the metal precursor. Ni-Pt NPs were deposited on graphene, and the catalytic dehydrogenation of hydrazine alkaline solution was tested under ambient conditions. Among all the tested catalysts, Ni84Pt16/graphene showed 100% hydrogen selectivity and had obvious high catalytic activity. The TOF value at 50 °C was 415 h −1 and the TOF value at 25 °C 133 h −1 , higher than Cheng et al. [96] have successfully synthesized ultrafine monodisperse Ni-Pt alloy NPs by conjugating nickel acetylacetonate and platinum acetylacetonate with borane-tert-butylamine (BTB) in 90 • C oleylamine (Figure 8). The composition of Ni-Pt NPs is controlled by changing the initial molar ratio of the metal precursor. Ni-Pt NPs were deposited on graphene, and the catalytic dehydrogenation of hydrazine alkaline solution was tested under ambient conditions. Among all the tested catalysts, Ni84Pt16/graphene showed 100% hydrogen selectivity and had obvious high catalytic activity. The TOF value at 50 • C was 415 h −1 and the TOF value at 25 • C 133 h −1 , higher than most reported values. It is believed that the metal composition-controlled synthesis of ultrafine alloy NPs with a limited size distribution strongly promotes the practical application of aqueous hydrazine as a hydrogen storage material.
Catalysts 2020, 10, x FOR PEER REVIEW 11 of 21 most reported values. It is believed that the metal composition-controlled synthesis of ultrafine alloy NPs with a limited size distribution strongly promotes the practical application of aqueous hydrazine as a hydrogen storage material.

La2O3
Wang et al. [97] reported the catalytic decomposition of concentrated hydrous hydrazine solutions over La2O3-supported Ni-Pt alloy catalysts. The Ni-Pt/La2O3 catalyst synthesized by onestep coprecipitation method has high catalytic activity and 100% selectivity. In particular, their research demonstrated that a system consisting of concentrated N2H4•H2O solutions, a high capability Ni-Pt/La2O3 catalyst, and an adequate amount of alkali accelerator can produce 6% of the material's hydrogenation capacity. As shown in Figure 9, the catalytic performance of the Ni-Pt/La2O3 catalyst is highly dependent on Pt content. The Pt/La2O3 catalyst was completely deactivated, and the Ni/La2O3 catalyst showed worse catalytic activity and moderate dehydrogenation selectivity. Compared with the Ni/La2O3 catalyst, the catalytic activity of the Ni-Pt/La2O3 catalyst with Pt/Ni Mole ratio of 1/9 was increased by 40 times, and the selectivity of H2 was increased from 72% to 100%. When the molar ratio of Pt/Ni increased to 2/3, the catalyst showed the best catalytic property. In the presence of 3.0 M NaOH, it can completely decompose N2H4•H2O in 1.5 min at 30 °C. Assuming that all the nickel atoms are involved in the catalytic reaction, the average reaction rate on the Ni60pt40/La2O3 catalyst can reach 448 h −1 . Studies have shown that the best Ni60Pt40/La2O3 catalyst can complete the decomposition reaction of N2H4•H2O in the presence of 3.0 M NaOH at 30 °C to generate hydrogen at a rate of 448 h −1 .

La 2 O 3
Wang et al. [97] reported the catalytic decomposition of concentrated hydrous hydrazine solutions over La 2 O 3 -supported Ni-Pt alloy catalysts. The Ni-Pt/La 2 O 3 catalyst synthesized by one-step coprecipitation method has high catalytic activity and 100% selectivity. In particular, their research demonstrated that a system consisting of concentrated N 2 H 4 ·H 2 O solutions, a high capability Ni-Pt/La 2 O 3 catalyst, and an adequate amount of alkali accelerator can produce 6% of the material's hydrogenation capacity. As shown in Figure 9, the catalytic performance of the Ni-Pt/La 2 O 3 catalyst is highly dependent on Pt content. The Pt/La 2 O 3 catalyst was completely deactivated, and the Ni/La 2 O 3 catalyst showed worse catalytic activity and moderate dehydrogenation selectivity. Compared with the Ni/La 2 O 3 catalyst, the catalytic activity of the Ni-Pt/La 2 O 3 catalyst with Pt/Ni Mole ratio of 1/9 was increased by 40 times, and the selectivity of H 2 was increased from 72% to 100%. When the molar ratio of Pt/Ni increased to 2/3, the catalyst showed the best catalytic property. In the presence of 3.0 M NaOH, it can completely decompose N 2 H 4 ·H 2 O in 1.5 min at 30 • C. Assuming that all the nickel atoms are involved in the catalytic reaction, the average reaction rate on the Ni 60 pt 40 /La 2 O 3 catalyst can reach 448 h −1 . Studies have shown that the best Ni 60 Pt 40 /La 2 O 3 catalyst can complete the decomposition reaction of N 2 H 4 ·H 2 O in the presence of 3.0 M NaOH at 30 • C to generate hydrogen at a rate of 448 h −1 .
In this paper [98], the supported Ni@Ni-Pt La 2 O 3 catalyst with core-shell structure was synthesized by coprecipitation and galvanic displacement method. The catalyst has high catalytic activity and 100% selectivity for hydrous hydrazine hydrogenation under mild conditions, which is superior to most reported hydrous hydrazine decomposition catalysts. The good catalytic performance of the Ni@Ni-Pt La 2 O 3 catalyst is related to the electronic and geometric modification induced by Pt on the surface of the catalyst. When the reaction temperature was raised from 30 • C to 60 • C, the reaction rate increased about 6 times, as shown in Figure 10. Interestingly, there was no effect of the temperature on H 2 selectivity. The decomposition of N 2 H 4 ·H 2 O over the Ni@Ni-Pt La 2 O 3 catalyst has 100% H 2 selectivity in the temperature range of 30-60 • C. In this paper [98], the supported Ni@Ni-Pt La2O3 catalyst with core-shell structure was synthesized by coprecipitation and galvanic displacement method. The catalyst has high catalytic activity and 100% selectivity for hydrous hydrazine hydrogenation under mild conditions, which is superior to most reported hydrous hydrazine decomposition catalysts. The good catalytic performance of the Ni@Ni-Pt La2O3 catalyst is related to the electronic and geometric modification induced by Pt on the surface of the catalyst. When the reaction temperature was raised from 30 °C to 60 °C, the reaction rate increased about 6 times, as shown in Figure 10. Interestingly, there was no effect of the temperature on H2 selectivity. The decomposition of N2H4•H2O over the Ni@Ni-Pt La2O3 catalyst has 100% H2 selectivity in the temperature range of 30-60 °C.   In this paper [98], the supported Ni@Ni-Pt La2O3 catalyst with core-shell structure was synthesized by coprecipitation and galvanic displacement method. The catalyst has high catalytic activity and 100% selectivity for hydrous hydrazine hydrogenation under mild conditions, which is superior to most reported hydrous hydrazine decomposition catalysts. The good catalytic performance of the Ni@Ni-Pt La2O3 catalyst is related to the electronic and geometric modification induced by Pt on the surface of the catalyst. When the reaction temperature was raised from 30 °C to 60 °C, the reaction rate increased about 6 times, as shown in Figure 10. Interestingly, there was no effect of the temperature on H2 selectivity. The decomposition of N2H4•H2O over the Ni@Ni-Pt La2O3 catalyst has 100% H2 selectivity in the temperature range of 30-60 °C.

CN
Highly dispersed Pt-Ni nanoparticles (NPs) were synthesized on the surface of carbon nanodots (CNDs) with metal-organic framework ZIF-8 as a raw material [99] (Scheme 4). The obtained Pt-Ni-CND catalyst has 100% H 2 selectivity and distinguished activity for the decomposition of N 2 H 4 ·H 2 O at an ambient temperature. Carbon nanotubes (CNTs) were used as the support of NPs to promote the kinetics of electron transfer and mass transfer in the catalytic reaction. The introduction of water-soluble CNDs was able to effectively anchor NPs, control the small size of NPs, and guard against the aggregation in solution. Compared with the traditional water-soluble NPs, due to the lack of long-chain organic groups on CNDs, the NPs immobilized on CNDs have more catalytic sites for the contact between reactants and NPs. The results show that the synthesized NP-CND catalyst has high activity for hydrous hydrazine decomposition at room temperature.
soluble CNDs was able to effectively anchor NPs, control the small size of NPs, and guard against the aggregation in solution. Compared with the traditional water-soluble NPs, due to the lack of longchain organic groups on CNDs, the NPs immobilized on CNDs have more catalytic sites for the contact between reactants and NPs. The results show that the synthesized NP-CND catalyst has high activity for hydrous hydrazine decomposition at room temperature. In this paper [100], a layered nanostructure Ni-Pt/N-doped carbon peroxide is synthesized without three steps, which can solve these problems simultaneously (Scheme 5). N-doped carbon substrate and catalytic active Ni-Pt nanoparticles were formed by the chelation of metal precursors with dopamine and the thermal decomposition of the complexes in reducing atmosphere. Due to the use of silicon dioxide templates and dopamine precursors, the N-doped carbon substrate has a layered macroporous structure. This, coupled with the evenly dispersed tiny Ni-Pt nanoparticles on the carbon substrate, provides opportunities for rich and accessible activity. Because of these favorable properties, the Ni-Pt/N-doped carbon catalyst can completely and rapidly produce hydrogen in basic N2H4•H2O solution at 50 °C at a rate of 1602 h −1 , which is superior to existing N2H4•H2O decomposition catalysts.  In this paper [100], a layered nanostructure Ni-Pt/N-doped carbon peroxide is synthesized without three steps, which can solve these problems simultaneously (Scheme 5). N-doped carbon substrate and catalytic active Ni-Pt nanoparticles were formed by the chelation of metal precursors with dopamine and the thermal decomposition of the complexes in reducing atmosphere. Due to the use of silicon dioxide templates and dopamine precursors, the N-doped carbon substrate has a layered macroporous structure. This, coupled with the evenly dispersed tiny Ni-Pt nanoparticles on the carbon substrate, provides opportunities for rich and accessible activity. Because of these favorable properties, the Ni-Pt/N-doped carbon catalyst can completely and rapidly produce hydrogen in basic N 2 H 4 ·H 2 O solution at 50 • C at a rate of 1602 h −1 , which is superior to existing N 2 H 4 ·H 2 O decomposition catalysts.
the aggregation in solution. Compared with the traditional water-soluble NPs, due to the lack of longchain organic groups on CNDs, the NPs immobilized on CNDs have more catalytic sites for the contact between reactants and NPs. The results show that the synthesized NP-CND catalyst has high activity for hydrous hydrazine decomposition at room temperature. In this paper [100], a layered nanostructure Ni-Pt/N-doped carbon peroxide is synthesized without three steps, which can solve these problems simultaneously (Scheme 5). N-doped carbon substrate and catalytic active Ni-Pt nanoparticles were formed by the chelation of metal precursors with dopamine and the thermal decomposition of the complexes in reducing atmosphere. Due to the use of silicon dioxide templates and dopamine precursors, the N-doped carbon substrate has a layered macroporous structure. This, coupled with the evenly dispersed tiny Ni-Pt nanoparticles on the carbon substrate, provides opportunities for rich and accessible activity. Because of these favorable properties, the Ni-Pt/N-doped carbon catalyst can completely and rapidly produce hydrogen in basic N2H4•H2O solution at 50 °C at a rate of 1602 h −1 , which is superior to existing N2H4•H2O decomposition catalysts. In 2019, Wan et al. [101] designed a series of Ni-Pt/CNTs catalysts and prepared Ni-Pt/CNTs catalysts by wet chemical reduction method (Scheme 6). The crystal structure, chemical state, and morphology of the catalyst were characterized in detail. In these catalysts, Ni 0.4 Pt 0.6 /CNTs have higher catalytic activity, a turnover frequency at 50 • C of 1725.3 h −1 , and a low Ea of 36.3 KJ mol −1 . Ni 0.4 Pt 0.6 /CNTs is a promising catalyst for the production of hydrogen from hydrazine hydrolysate.
Cheng et al. [102] reported a simple Ni-Pt-MnOx synthesis method to support nitrogen-doped porous carbon (NPC) from annealed metal-organic skeleton MOFs at different temperatures in Ar. Interestingly, the produced (Ni 3 Pt 7 ) 0.5 -(MnOx) 0.5 /NPC-900 exhibitors showed better catalytic activity for the dehydrogenation of hydrazine in alkaline solution, with primary turnover frequencies of 706 and 120 h −1 at 323 K and ordinary temperature, respectively. Because of the synergistic electronic effect of Ni-Pt and MnOx and the small-scale size and lofty dispersion of the Ni-Pt-MnOx nano-catalyst, NPC, as an excellent carrier, may have excellent catalytic performance, large pore volume, high specific surface area, high nitrogen content and conductivity, and the high density of metal-n active sites between Ni-Pt-MnOx and NPC. As shown in Figure 11, the presence of OHlimits the formation of undesirable N 2 H 5 + in aqueous solutions and promotes the cleavage of the N-H bond during the determination of the hydrazine dehydrogenation reaction. The decomposition of (Ni 3 Pt 7 ) 0.5 -(MnOx) 0.5 /NPC-900 in alkaline solution may be concerned with the following steps: first, target N 2 H 4 adsorption may be intensified due to the illustrious surface area and porosity of NPC-900; second, N 2 H 4 decomposition into N 2 and H 2 , the rate of desorption of N 2 and H 2 molecules generated; third, desorption of the resultant N 2 and H 2 molecules as shown in Figure 11. Furthermore, the charge transfer process was involved in the above three steps, which can be promoted by the NPC-900 porous network with high conductivity and graphitization, as well as the introduction of nitrogen atoms and MnOx. This can explain the splendid catalytic performance of (Ni 3 Pt 7 ) 0.5 -(MnOx) 0.5 /NPC-900 catalyst.
Catalysts 2020, 10, x FOR PEER REVIEW 14 of 21 In 2019, Wan et al. [101] designed a series of Ni-Pt/CNTs catalysts and prepared Ni-Pt/CNTs catalysts by wet chemical reduction method (Scheme 6). The crystal structure, chemical state, and morphology of the catalyst were characterized in detail. In these catalysts, Ni0.4Pt0.6/CNTs have higher catalytic activity, a turnover frequency at 50 °C of 1725.3 h −1 , and a low Ea of 36.3 KJ mol −1 . Ni0.4Pt0.6/CNTs is a promising catalyst for the production of hydrogen from hydrazine hydrolysate. Cheng et al. [102] reported a simple Ni-Pt-MnOx synthesis method to support nitrogen-doped porous carbon (NPC) from annealed metal-organic skeleton MOFs at different temperatures in Ar. Interestingly, the produced (Ni3Pt7)0.5-(MnOx)0.5/NPC-900 exhibitors showed better catalytic activity for the dehydrogenation of hydrazine in alkaline solution, with primary turnover frequencies of 706 and 120 h −1 at 323 K and ordinary temperature, respectively. Because of the synergistic electronic effect of Ni-Pt and MnOx and the small-scale size and lofty dispersion of the Ni-Pt-MnOx nanocatalyst, NPC, as an excellent carrier, may have excellent catalytic performance, large pore volume, high specific surface area, high nitrogen content and conductivity, and the high density of metal-n active sites between Ni-Pt-MnOx and NPC. As shown in Figure 11, the presence of OHlimits the formation of undesirable N2H5 + in aqueous solutions and promotes the cleavage of the N-H bond during the determination of the hydrazine dehydrogenation reaction. The decomposition of (Ni3Pt7)0.5-(MnOx)0.5/NPC-900 in alkaline solution may be concerned with the following steps: first, target N2H4 adsorption may be intensified due to the illustrious surface area and porosity of NPC-900; second, N2H4 decomposition into N2 and H2, the rate of desorption of N2 and H2 molecules generated; third, desorption of the resultant N2 and H2 molecules as shown in Figure 11. Furthermore, the charge transfer process was involved in the above three steps, which can be promoted by the NPC-900 porous network with high conductivity and graphitization, as well as the introduction of nitrogen atoms and MnOx. This can explain the splendid catalytic performance of (Ni3Pt7)0.5-(MnOx)0.5/NPC-900 catalyst.

Summary
Chemical hydrogen storage materials need to be efficiently catalyzed by a suitable catalyst to release hydrogen efficiently over a wide temperature range and to meet environmental standards [103]. The research results show that hydrous hydrazine can meet the above requirements. The Figure 11.
The possible mechanism of decomposition of hydrazine catalyzed by (Ni 3 Pt 7 ) 0.5 -(MnOx) 0.5 /NPC-900 in the absence of NaOH (pathway a) and in the presence of NaOH (pathway b); copyright (2016), Journal of Materials Chemistry A.

Summary
Chemical hydrogen storage materials need to be efficiently catalyzed by a suitable catalyst to release hydrogen efficiently over a wide temperature range and to meet environmental standards [103]. The research results show that hydrous hydrazine can meet the above requirements. The developed Pt-based catalysts mainly include Ni-Pt alloy catalysts [104], Ni-Pt-supported catalysts [105], and Ni-Pt-YOx-supported multi-metal catalysts [102]. Among them, Ni-Pt-supported catalysts and Ni-Pt-YOx-supported multi-metal catalysts have high practicality. The Ni-Pt alloy effect, the strong interaction between Ni-Pt and the support, and the synergistic effect between the metal oxide and Ni-Pt are the reasons for the high activity and hydrogen selectivity of the catalyst. In the future, the development direction of catalysts will tend to be low-cost, low reaction temperature, and low operating costs to meet the needs of large-scale hydrous hydrazine catalytic hydrogen production. On the one hand, the presence of Pt and Ni in the Pt-Ni alloy state is an important factor for the high activity of the catalyst; on the other hand, the carrier rGO also plays a supporting and synergistic role, making the hydrazine molecule preferentially break the N-H bond on the catalyst surface to promote the decomposition reaction to proceed completely [106]. The introduction of metal Ce can significantly improve the catalytic activity and hydrogen selectivity of the catalyst to catalyze the decomposition of hydrous hydrazine to produce hydrogen. The study also found that only the proper amount of Ce can be introduced to ensure the high activity of the catalyst, the high dispersion of Ni-Pt-CeOx, and the strong interaction between Ni-Pt-CeOx and the support. The excellent activity of the catalyst is due to the synergistic electronic effect between Ni-Pt alloy particles and MnOx.

Conclusions
The research progress of Pt-based catalysts for catalytic hydrogen production from hydrous hydrazine decomposition is reviewed. The decomposition of hydrous hydrazine includes two ways: complete decomposition and incomplete decomposition. Its hydrogen content is as high as 8.0 wt%, which has a good application prospect. Pt-based catalysts play an important role in catalyzing the selective decomposition of hydrous hydrazine to produce hydrogen. They are mainly divided into Pt-support catalysts, Pt-Ni alloy catalysts, Pt-Ni-supports catalysts, and Pt-Ni-YOx-support multi-metal catalysts (Y is Mn, Ce). Due to the synergistic effect between the metal oxides YOx and Pt-Ni, and the strong interaction between the metal or metal composite and the carrier, the Ni-based catalyst exhibits high catalytic activity and has a good effect on hydrogen in the production of hydrous hydrazine 100% selectivity. In addition, the existence of basic additives is helpful to improve the catalytic activity and selectivity of metal catalysts. We look forward to providing some reference for improving the hydrogen economy of the catalyst and other applications.