Design strategies of highly selective nickel catalysts for H2 production via hydrous hydrazine decomposition: a review

Design strategies of highly selective nickel catalysts for H2 production via hydrous hydrazine... Abstract Hydrazine, a widely used liquid propellant, has the potential to be employed as a hydrogen source in certain instances and has therefore attracted considerable attention; consequently, the complete decomposition of hydrazine with 100% H2 selectivity under mild conditions has become the current research focus for catalyst design. In this review, the strategies for the design of efficient catalysts are summarized for complete hydrazine decomposition. The first part of this review introduces the mechanism of hydrazine decomposition, while the second part illustrates the key factors influencing the H2 selectivity of nickel catalysts, including the effects of alloying, alkali promoter addition and strong metal–support interactions. Finally, the critical elements of catalyst design employed in industrial applications are analyzed. hydrazine, selectivity, H2, catalyst, nickel INTRODUCTION Hydrazine (N2H4) is an energy-rich molecule, which can decompose into a mixture of N2, H2 and NH3 [1]. In the presence of efficient catalysts, the decomposition rate can be enhanced even at low temperatures. During the decomposition process, the chemical energy in N2H4 can be converted into kinetic energy. One such application of N2H4 decomposition is in orbiting satellites, which use hydrazine to control their altitude and orbit adjustment propulsion. The thermal or catalytic decomposition of hydrazine occurs via two routes according to Eqs. (1–4) [1,2]: Complete: N2H4(l) → N2(g) + 2H2(g) \begin{eqnarray} {\Delta _{\rm{r}}}H^\circ ( {298} ) &=& - 50.6{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}},{\Delta _{\rm{r}}}G^\circ ( {298} )\nonumber\\ &=& - 149.4{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (1) Incomplete: N2H4(l) → 4/3NH3(g) + 1/3N2(g) \begin{eqnarray} {\Delta _{\rm{r}}}H^\circ ( {298} ) &=& - 111.9{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}},{\Delta _{\rm{r}}}G^\circ ( {298} )\nonumber\\ &=& - 171.3{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (2) However, subsequent reactions also need to be considered. The produced H2 can further react with N2H4 according to the following reaction: \begin{eqnarray} {{\rm{N}}_{\rm{2}}}{{\rm{H}}_{\rm{4}}}( {\rm{l}} ) + {{\rm{H}}_2}( {\rm{g}} ) &\to & 2{\rm{N}}{{\rm{H}}_3}( {\rm{g}} ){\, \Delta _{\rm{r}}}H^\circ ( {298})\nonumber\\ &=& - 142.5{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (3) Furthermore, decomposition of the NH3 component in the produced mixture can form H2 and N2: \begin{eqnarray} {\rm{N}}{{\rm{H}}_3}( {\rm{g}} ) &\to & 1/2{{\rm{N}}_2}( {\rm{g}} ) + 3/2{{\rm{H}}_2}( {\rm{g}} )\, {\Delta _{\rm{r}}}H^\circ ( {298} )\nonumber\\ &=& + 45.9\,{\rm{kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (4) During a catalytic decomposition process, the above-mentioned parallel reactions during catalytic decomposition lead to a superimposition of the reaction products [1,3]. For each equation mentioned above, more than one elementary step is performed during the reaction process, which leads to hydrazine decomposition containing many elementary steps, hence the difficulty in describing them. To simplify the research that has been conducted using various catalytic systems, the above reactions are unified by one equation, where x represents the H2 selectivity: \begin{eqnarray} 3{{\rm{N}}_2}{{\rm{H}}_4}( {\rm{l}} ) &\to & 4(1 - x){\rm{N}}{{\rm{H}}_3}( {\rm{g}} )\nonumber\\ &&+\, ( {1 + 2x}){{\rm{N}}_2}( {\rm{g}} ) + 6x{{\rm{H}}_2}( {\rm{g}}).\nonumber\\ \end{eqnarray} (5) Depending on the application, it is desirable to design catalysts with controlled hydrogen selectivity. For example, the specific impulse of a chemical rocket exhibits a maximum in the H2 selectivity range of 30–40% when hydrazine is used as a space propellant. The production of CO-free hydrogen is currently a hot topic, and the catalytic decomposition of hydrazine is proposed for this purpose [4–6]. If complete N2H4 decomposition proceeds through equation (1), the hydrogen released is calculated to be 12.5 wt.%. This value is significantly larger than the target set by the Department of Energy (DOE) for onboard hydrogen systems (5.5 wt.%) [7]. Therefore, N2H4 qualifies as a mobile hydrogen source meeting the DOE target. Additionally, during the complete decomposition of N2H4, three equivalent moles of gaseous products are generated leading to increased volumes of emitted gas, which prevents the solubility of ammonia in water, thus providing a driving force in submarine rescue systems when employing hydrazine for ballast water expulsion. In such cases, it is critical that catalyst design yields high selectivity, especially in mild conditions. Another important property of hydrazine is the formation of hydrous hydrazine when hydrazine reacts with water, similar to ammonia. The monohydrate of hydrazine (N2H4·H2O) has a much lower freezing point (−51.7°C) compared with N2H4 (1.4°C), which makes the former a suitable propellant when employed across a wider temperature range. The water molecule may potentially lead to a partial slowdown of the decomposition rate. Although N2H4·H2O is still considered to be a dangerous chemical, it is far safer to handle compared to explosive N2H4 and, thus, more suitable for investigation in fundamental research. In 2009, Xu and co-workers proposed the idea of using hydrous hydrazine as a potential candidate for chemical hydrogen storage [8]. Since then, there have been a number of reports concerning the decomposition of hydrous hydrazine compared with its anhydrous counterpart. Hydrazine, being the simplest diamine, has applications beyond chemical propellants and is widely employed as a precursor in the synthesis of organic molecules containing N–N bonds, especially in the preparation of pesticides and pharmaceuticals [9,10]. Additionally, hydrazine is a strong reducing agent that is used to control corrosion in boilers, reduce noble metal catalysts or graphene oxide, and to hydrogenate unsaturated bonds in organic compounds [11]. In the case of handling spent hydrazine, complete decomposition of hydrazine into hydrogen and nitrogen is considered the most suitable route to circumvent the production of ammonia. The development of efficient and selective catalysts for H2 generation from hydrazine is an important topic considering the applications described above. From a thermodynamic perspective, the Gibbs free energy of equation (2) is lower than that of the equation (1). That is to say, hydrazine decomposition to ammonia is thermodynamically more favorable. As shown in Fig. 1, the major thermodynamic products of hydrazine decomposition are NH3 and N2 at low temperature. Therefore, it is challenging to avoid its undesirable decomposition to ammonia. Figure 1. View largeDownload slide Thermodynamic products versus temperature for the decomposition of 3 mol hydrazine (0.1 MPa). Figure 1. View largeDownload slide Thermodynamic products versus temperature for the decomposition of 3 mol hydrazine (0.1 MPa). In this review, we briefly introduce the mechanism of hydrazine decomposition to demonstrate the principles of catalyst design. Thereafter, catalytic systems are presented that focus on the selective decomposition of N2H4 or N2H4·H2O under mild conditions. To date, multiple strategies in catalyst design have been reported, including pure metallic nanoparticles and supported catalysts. These catalytic systems are divided into three types according to their influencing factors on H2 selectivity, including alloying effects, alkali promotion and metal–support interactions. The principal reasons for the tunable selectivity are discussed both experimentally and theoretically, which may provide guidance for further catalyst design. In addition to inspiring further research on hydrazine decomposition, we hope that this review will stimulate research on other related N-containing reactions, such as NH3 or NH3BH3 decomposition. MECHANISM CONSIDERATION Early research focused on mechanistic studies via adsorption experiments over Ir, Pt and Rh model metallic surfaces [12–16]. For example, it has been observed that the initial step of hydrazine decomposition over an Ir surface is the dissociative adsorption of the N2H4 molecule, which produces the adatoms of nitrogen, hydrogen, and NH2 species [12]. The adsorbed hydrogen atoms display good mobility and can further react with another hydrogen atom to produce molecular hydrogen or with an NH2 radical to produce NH3. Under mild conditions in the presence of an Ir surface, NH3 is the main product because of the ease by which NH2 combines with adsorbed hydrogen. Conversely, the main intermediate is NH in the presence of an Rh surface using temperature-programed surface reaction experiments, which generate N2 and H2 as the main products at low N2H4 coverage and N2 and NH3 at high N2H4 coverage [14]. Isotopic-labeled 15N2H4 was employed to obtain further information regarding the intermediates during the decomposition process [17,18]. The ratio of 15N to 14N in the produced N2 is small, which illustrates that the produced N–N comes from the same hydrazine molecule and therefore abrogates the hypothesis of prior N–N cleavage during the N2H4 decomposition process [17]. With the development of computational technologies, theoretical calculations have been performed over noble metal and non-noble metal surfaces. Further detailed reaction pathways have been proposed over various modeling surfaces, such as Ir(111) [19], Rh(111) [20,21], Fe (211) [22], Ni [23] and Cu [24] surfaces. However, the adsorption conformations of hydrazine and the intermediates show significant differences in these reports. The hydrazine decomposition mechanism is still contested over different metallic catalysts. Therefore, tailoring of selectivity through catalyst design is still determined experimentally. Essentially, the reaction route is determined by the sequence of N–N bond and N–H bond cleavage [1]. The N–N bond cleavage energy in gaseous N2H4 is 286 kJ mol−1, and the N–H bond energy is 360 kJ mol−1 when the entire bond breaks into N and H atoms. From a thermodynamic viewpoint, the first step relating to N–N bond cleavage appears to be easier, which consequently leads to the production of N2 and NH3. However, the bond energy of N–H is lowered to 276 kJ mol−1 if N2H4 partially breaks into N2H2 and H adatoms are observed over a Ni (100) surface [25], which can potentially lead to large quantities of H2 being produced. In another case, the H–Pt bond is stronger than the N–Pt bond on Pt(111), leading to the barrier to N–H cleavage being much lower than that to N–N cleavage [15]. As a result, regardless of the higher N–H bond energy compared with the N–N bond, the first bond to break in hydrazine adsorbed on platinum is an N–H bond instead of the N–N bond. Nitrogen forms and desorbs close to 40°C through an intramolecular process [15]. Motivated by these findings, the SiO2-supported Ni, Pd and Pt catalysts developed by our group displayed excellent H2 selectivity in gas phase hydrazine decomposition, even at room temperature [3]. According to these reports, the higher H2 selectivity on Ni-, Pd- and Pt-based catalysts is because of the lower barrier to N–H cleavage, which may be related to the stronger M–H bond. Furthermore, de Medeiros et al. correlated H2 selectivity with the enthalpy of adsorption of hydrazine. Hydrazine decomposition over catalysts possessing low enthalpies of adsorption mainly produces N2 and H2. On catalysts having high enthalpies of adsorption (520 kJ mol−1), the reaction products are N2 and NH3 [26]. To summarize the most plausible mechanism, general reaction pathways for hydrazine decomposition are proposed in Fig. 2, showing NH3, N2 and H2 formation (Fig. 2a), and NH3 and N2 formation (Fig. 2b). Because the theory of hydrazine decomposition is not comprehensive enough, catalyst design for H2 generation from hydrazine is mainly based on experimental exploration. It should be noted that the decomposition of hydrazine has been studied in the vapor phase as well as in solution. Although the mechanism is very similar in both cases, it is sometimes difficult to exclude solvent and surface interference in the latter case [27]. Therefore, we indicate which phase the reaction is in hereafter. Figure 2. View largeDownload slide Reaction pathways for hydrazine decomposition. (a) NH3, N2 and H2 formation. (b) NH3 and N2 formation. Figure 2. View largeDownload slide Reaction pathways for hydrazine decomposition. (a) NH3, N2 and H2 formation. (b) NH3 and N2 formation. CATALYTIC SYSTEMS Metallic catalysts are one of the most widely used catalytic systems in heterogeneous catalysis for industrial applications. These catalysts are often composed of active metals, supports and other promoters. There are many factors that can influence catalytic performance, including the choice of metal and support composition, the exposed facet of the active metal, the interaction between the metal and the support, and the properties of the additional promoters. From a molecular point of view, the surface structure and electronic properties are the main influencing factors for heterogeneous catalysts as the majority of the reactions occur on the surface. For hydrazine decomposition, changes to the sequence of N–N bond and N–H bond cleavage are the major strategy to change selectivity. Among various catalytic systems, transition metals (e.g. Ir, Ru and Ni) are the most efficient candidates for N–H bond activation and exhibit high activity for hydrazine decomposition under mild conditions. However, H2 selectivity is limited to below 10% over Ru and Ir catalysts. To date, monometallic catalysts have failed to achieve the complete catalytic decomposition of hydrous hydrazine to H2 without generating NH3. Hence, modification of the catalysts is necessary to promote H2 selectivity. In this section, methods that have been explored for the promotion of H2 formation will be introduced, including alloying of a second metal, alkali addition and then introduction of strong metal–support interactions. Alloy effect Bimetallic catalysts often exhibit enhanced activity, selectivity and stability compared with their parent metals because of their distinct electronic and chemical properties. Therefore, the development of bimetallic catalysts provides opportunities to obtain novel catalysts with improved catalytic performance in many industrial applications. To further understand the origins of their novel properties, bimetallic catalysts have been extensively studied in fundamental research. The geometric and electronic changes of the bimetallic surface have been shown to be the reason for their extraordinary properties in many reactions, such as hydrocarbon reforming, as reported previously in the literature [28–30]. In this section of the review, we will focus on summarizing the investigation of bimetallic catalysts for hydrous hydrazine decomposition to date. In 1979, an Al2O3-supported Ir–Ni catalyst was studied for its performance in gas hydrazine decomposition at 27°C [31]. Although Ir is considered to be the most active metal for gas hydrazine decomposition, its selectivity for hydrogen is poor at low temperatures. Alloying Ni to Ir led to an increase in H2 production through complete decomposition via equation (1). The adsorption experiment results have demonstrated that the surface bonding of NH and N adspecies to Ir is weakened by the addition of Ni atoms, which originate from the formation of the Ir–Ni alloy. In 2009, Xu and co-workers reported the excellent performance of Rh–Ni bimetallic nanoparticles in hydrogen production via hydrous hydrazine decomposition [32], which inspired the following research on bimetallic catalysts. The catalytic performances of various two-component transition metal catalysts are summarized in Table 1. The Ni-based bimetallic catalysts with or without supports provide the highest H2 selectivity, which stimulated interest in investigating the reason for their excellent performance. To date, nanoparticles composed of three types of metals have been reported, including NiFeCu [33], NiFePd [34] and NiFeMo [35]. Additionally, other novel materials have been tested in the decomposition of hydrous hydrazine, such as Co0.85Se/graphene hybrid nanosheets [36]; Ni–B [37], Co–B [38,39], Fe–B [40] and Rh–Ni–B [41] materials; Ni–Rh and Ni–Pt nanoparticles immobilized on a metal–organic framework (MOF); and MOF-derived carbon dots and nitrogen-doped porous carbon [42–44]. These materials have exhibited good performance in hydrous hydrazine decomposition, although further discussion is not included in this review. Table 1. Comparison of hydrogen selectivity (%) of hydrous hydrazine decomposition over different bimetallic catalysts. Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] View Large Table 1. Comparison of hydrogen selectivity (%) of hydrous hydrazine decomposition over different bimetallic catalysts. Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] View Large To study the reasons why the second metal promotes catalytic performance, structural configuration and surface adsorption experiments were performed. It is well documented that the arrangement of the two metals can result in three structure types: alloy, core-shell, and heterogeneous. For Pt-promoted Ni catalysts, X-ray diffraction (XRD) results illustrate that the main diffraction peaks show only the existence of metallic Ni [45]. The diffraction peaks shift to lower angles after doping with Pt, demonstrating the formation of a Ni–Pt alloy in the bimetallic catalysts. The formation of the Ni–Pt alloy has been further confirmed by the co-reduction of Ni and the second metal, illustrated by H2-temperature-programed reduction (TPR) data. Furthermore, extended X-ray absorption fine structure (EXAFS) experiments have been employed to study the Pt oxidation status and its coordination environment. The results indicate that the majority of the Pt species exist in the form of a Ni–Pt alloy, which leads to an electronic transfer from Ni to Pt. Therefore, the adsorption of H2 and NH3 is significantly weakened on the bimetallic surface compared with the parent Ni or Pt catalysts, which has been proven by H2-temperature-programmed desorption (TPD), microcalorimetry and NH3-TPD experiments. In addition to the three bimetallic structure systems, the diverse arrangements of the surface and sub-surface metal atoms of the binary system show a significant influence on the surface adsorption and electronic properties of the alloy catalysts [28,30]. Further studies on Ni–Ir catalysts were performed to investigate the influence exerted by the arrangement of the surface atoms, and revealed that pretreating temperatures significantly influenced their catalytic performance. As shown in Fig. 3, the low-temperature-reduced catalyst (Ni–Ir/Al2O3-300R) showed >99% H2 selectivity. In contrast, the analogous high-temperature-reduced catalysts (Ni–Ir/Al2O3-500R and Ni–Ir/Al2O3-700R) showed a considerable reduction in H2 selectivity [58]. The results of XRD, EXAFS and H2-TPR confirmed alloy formation for all three catalysts. However, the coordination number of the Ni–Ir bond increased in the order of Ni–Ir/Al2O3-300R < Ni–Ir/Al2O3-500R < Ni–Ir/Al2O3-700R, which demonstrated the relocation of Ir atoms from the surface of the Ni–Ir alloy particle into the core as a function of increasing reduction temperature. Consequently, the atom arrangement at the surface changed from a Ni–Ir alloy into a Ni-rich surface, leading to a decrease in H2 selectivity during hydrous hydrazine decomposition. Figure 3. View largeDownload slide Structural model of bimetallic Ni–Ir catalysts for hydrous hydrazine decomposition. Reprinted with permission from [58]. Copyright 2013 Elsevier B.V. Figure 3. View largeDownload slide Structural model of bimetallic Ni–Ir catalysts for hydrous hydrazine decomposition. Reprinted with permission from [58]. Copyright 2013 Elsevier B.V. Furthermore, the surface structure was characterized for Pt-, Ir- and Au-doped Ni bimetallic catalysts reduced at equivalent temperatures. Correlating with their hydrous hydrazine decomposition catalytic performance, it was concluded that the formation of a surface alloy is necessary for the purpose of obtaining high H2 selectivity [58]. Provided a homogeneous alloy is formed, such as a Pt–Ni catalyst, the Niδ+ can be observed because of the partial electron transfer from Ni to a nearby Pt atom. Conversely, for Au–Ni catalysts, segregation of Au on the surface is detected. This structural change greatly influences the surface adsorption properties and catalytic performance of the catalyst. The surface structure of the alloy catalyst plays an important role in the generation of H2 from hydrous hydrazine decomposition and may promote future rational catalyst design to obtain good activity and superior selectivity. Strong basic promoter For heterogeneous catalysts, non-active components are typically added as promoters to adjust selectivity or enhance activity. For example, the addition of alkali promoters is crucial for ammonia synthesis. In the 1970s, strong basic components, e.g. NaOH and KOH, were found to improve H2 selectivity for hydrous hydrazine decomposition over Ir catalysts at low temperatures [1]. However, these additives could not catalyze this reaction themselves and led to a decrease in the reaction rate. Recently, the development of non-noble metal catalysts for hydrous hydrazine decomposition to improve H2 generation has gained significant attention. It is important to note that strong basic additives are necessary for such catalysts to obtain high H2 selectivity in mild conditions. These strong basic promoter additives can be divided into two types: i) alkalis (e.g. NaOH) added separately and ii) basic supports combined with the active component during preparation. Xu and co-workers developed the bimetallic Ni–Fe nanocatalyst, which could catalyze the complete decomposition of hydrous hydrazine with the assistance of NaOH [46]. Later, many multi-component catalysts were found to catalyze this reaction with the addition of an alkali, such as Fe–B/MWCNT [40] or NiMoB–La(OH)3 [68]. In our research on Raney Ni catalysts [69], the addition of NaOH was precisely controlled. Before the addition of NaOH to the Raney Ni catalyst, H2 selectivity reached only 80% during the decomposition of hydrous hydrazine. As shown in Fig. 4, a small amount of NaOH (0.05 mol L−1) could significantly promote the selectivity to 91%. Further increasing the concentration of NaOH to above 0.5 mol L−1, the H2 selectivity reached over 99%. In addition, the Raney Ni catalyst could be easily collected and reused after the reaction because of its magnetic properties, as shown in the insets of Fig. 4. The apparent activation energies were calculated for H2 (pathway 1) and NH3 (pathway 2) generation, respectively. The results indicated an increase in the activation energy for pathway 2 after the addition of NaOH, which confirmed the inhibition of NH3 production with the existence of NaOH. Figure 4. View largeDownload slide H2 selectivity improvement as a function of strong basic promoter concentration in hydrous hydrazine decomposition on Raney Ni catalyst. Insets: magnetic separation property of Raney Ni catalyst after reaction of (a) reactants with catalyst after reaction and (b) with a magnet to separate catalyst. Reprinted with permission from [69]. Copyright 2013 American Institute of Chemical Engineers. Figure 4. View largeDownload slide H2 selectivity improvement as a function of strong basic promoter concentration in hydrous hydrazine decomposition on Raney Ni catalyst. Insets: magnetic separation property of Raney Ni catalyst after reaction of (a) reactants with catalyst after reaction and (b) with a magnet to separate catalyst. Reprinted with permission from [69]. Copyright 2013 American Institute of Chemical Engineers. Recently, several supported metal catalysts were shown to catalyze the decomposition of hydrous hydrazine with nearly 100% H2 selectivity. This high selectivity could be attributed to the existence of strong basic sites over the catalysts. In the study of hydrotalcite-derived Ni/Al2O3 catalysts, H2 selectivity reached 93% at 30°C, which was significantly higher than the impregnated counterpart (67%) [70]. The CO2–TPD results revealed large concentrations of strong basic sites in the hydrotalcite-derived Ni/Al2O3, which are thought to originate from the O2− species in the Ni–O–Al structure. The strong basic sites inhibit the production of NH3 and also influence the electronic properties of Ni particles, which consequently improves H2 selectivity during hydrous hydrazine decomposition in mild conditions. In summary, the promoting effect can be attributed to two aspects: i) inhibition of generation of the basic byproduct NH3 and ii) electronic perturbation on the surface of the active metal because of the existence of vicinal strong basic sites. However, the addition of alkalis may not be the best choice to improve H2 selectivity via hydrous hydrazine decomposition when taking into account environmental issues. Strong metal–support interaction effect Widely used industrialized heterogeneous catalysts often consist of metal nanoparticles supported on large surface area oxides. The importance of these oxides has been increasingly recognized over recent decades, following the discovery of metal–support interactions. In addition to dispersing metallic particles, the oxide support also influences the catalytic properties of the metal catalysts through geometric or electronic effects. The representative oxides include CeO2, TiO2 and Fe2O3. To investigate the influence of the support for Ni catalysts in hydrous hydrazine decomposition, two types of Ni/CeO2 catalysts were prepared using co-precipitation and impregnation methods, denoted as Ni/CeO2-CP and Ni/CeO2-IMP, respectively [71]. The two Ni/CeO2 catalysts exhibited quite different activity and selectivity toward hydrous hydrazine decomposition because of their significant structural differences. The Ni particles were highly dispersed in the co-precipitated sample over high Ni loading, which led to improvements in activity for the reaction. In contrast, Ni/CeO2-IMP resulted in obvious aggregation of Ni particles because of the small surface area of the CeO2 support, resulting in significantly lower activity. Furthermore, H2 selectivity reached over 99% for the Ni/CeO2-CP catalyst, considerably higher than for the impregnated counterpart (65%), although the Ni particle sizes were very similar on both samples. Electron microscopy images, H2-TPR and Raman spectroscopy results indicated that Ni particles were partially surrounded by amorphous CeO2 in Ni/CeO2-CP (Fig. 5). The co-precipitation method induced the existence of a Ni–O–Ce structure. However, for Ni/CeO2-IMP, the Ni particles resided on CeO2 with little interaction between Ni and CeO2 [71]. This geometric variation of the surface structure led to an electronic change of the exposed Ni. Using CO as an adsorption probe, Fourier Transform Infrared Spectroscopy (FT-IR) results illustrated that the exposed Ni species in Ni/CeO2-CP differs from metallic Ni. The presence of a weak CO adsorption band on the Niδ+ site could be observed, indicating changes to the electronic properties of the exposed Ni species on Ni/CeO2-CP because of the modification of vicinal CeO2 [71]. In particular, the CeO2 support not only played a role in dispersing Ni particles, but also behaved as a promoter or modifier to improve hydrogen selectivity of Ni in the decomposition of hydrous hydrazine. Figure 5. View largeDownload slide Structural models of Ni/CeO2-CP and Ni/CeO2-IMP. Reprinted with permission from [71]. Copyright 2015 American Chemical Society. Figure 5. View largeDownload slide Structural models of Ni/CeO2-CP and Ni/CeO2-IMP. Reprinted with permission from [71]. Copyright 2015 American Chemical Society. On the basis of results outlined above, we synthesized other Ni–O–M structures on varying metal oxide supports. Using the co-precipitation method, a series of supported Ni catalysts were prepared, including Ni/MgO, Ni/La2O3 and Ni/ZrO2 [71]. These catalysts had one thing in common: a strong metal–support interaction existed between Ni and the oxide support. Similar to the Ni/CeO2–CP sample, these catalysts all exhibited >80% H2 selectivity for hydrous hydrazine decomposition, which further confirmed the promotion effect of the oxide supports, as long as a Ni–O–M structure formed. This finding may inspire the future design of more efficient catalysts for the decomposition of hydrous hydrazine using economically viable facile preparation methods. CONCLUSION AND OUTLOOK Different hydrazine usage demands have made hydrazine decomposition selectivity increasingly important. This focus of this review has been to analyze factors that influence improvements to hydrogen selectivity in mild conditions. As Ni-based catalysts have been proven to be the most active candidates, strategies to further improve their performance have been analyzed, including the choice of metal composition, alkali promoter addition and the tailoring of metal–support interactions. Our intent is to encourage the development of more highly selective catalysts for H2 production via hydrous hydrazine decomposition. Although many attempts, both experimental and theoretical, have been made to reveal the key factors that affect the selectivity of metallic catalysts, the detailed mechanism is still not fully understood. Therefore, studies on hydrazine decomposition are necessary to better understand the reasons for selectivity differences. Additionally, such studies may further inspire the exploration of other reactions including N–N bonds and N–H bonds, such as NH3 synthesis or decomposition, and provide opportunities for the development of novel catalytic systems. Furthermore, the majority of the catalysts reported in the literature to date have only been successful at the laboratory scale and have shown limitations in industrial application. As is already known, this reaction is an exothermal process, and the amount of heat released and the distribution of gaseous products (N2, H2 and NH3) depends on the reactive conditions and the selectivity of the catalysts. Therefore, heat and mass transfer should be considered during the design of catalysts. In additional, the stabilities of the currently available catalysts are not sufficient to meet the demands of industrial production. Therefore, many engineering and technological problems need to be investigated and resolved prior to their industrial application. FUNDING This work was supported by the National Natural Science Foundation of China (21103173 and 21476226), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100), the National Key Projects for Fundamental Research and Development of China (2016YFA0202804) and the Youth Innovation Promotion Association of the Chinese Academy of Sciences. Conflict of interest statement. None declared. REFERENCES 1. Batonneau Y , Kappenstein CJ , Keim W . Catalytic decomposition of energetic compounds: gas generators and propulsion . Handbook of Heterogeneous Catalysis . Weinheim : Wiley-VCH , 2008 . 2. Lucien HW . Thermal decomposition of hydrazine . J Chem Eng Data 1961 ; 6 : 584 – 6 . https://doi.org/10.1021/je60011a030 Google Scholar CrossRef Search ADS 3. Zheng M , Cheng R , Chen X et al. A novel approach for CO-free H production via catalytic decomposition of hydrazine . 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Design strategies of highly selective nickel catalysts for H2 production via hydrous hydrazine decomposition: a review

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Oxford University Press
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© The Author(s) 2017. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.
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2095-5138
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2053-714X
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Abstract

Abstract Hydrazine, a widely used liquid propellant, has the potential to be employed as a hydrogen source in certain instances and has therefore attracted considerable attention; consequently, the complete decomposition of hydrazine with 100% H2 selectivity under mild conditions has become the current research focus for catalyst design. In this review, the strategies for the design of efficient catalysts are summarized for complete hydrazine decomposition. The first part of this review introduces the mechanism of hydrazine decomposition, while the second part illustrates the key factors influencing the H2 selectivity of nickel catalysts, including the effects of alloying, alkali promoter addition and strong metal–support interactions. Finally, the critical elements of catalyst design employed in industrial applications are analyzed. hydrazine, selectivity, H2, catalyst, nickel INTRODUCTION Hydrazine (N2H4) is an energy-rich molecule, which can decompose into a mixture of N2, H2 and NH3 [1]. In the presence of efficient catalysts, the decomposition rate can be enhanced even at low temperatures. During the decomposition process, the chemical energy in N2H4 can be converted into kinetic energy. One such application of N2H4 decomposition is in orbiting satellites, which use hydrazine to control their altitude and orbit adjustment propulsion. The thermal or catalytic decomposition of hydrazine occurs via two routes according to Eqs. (1–4) [1,2]: Complete: N2H4(l) → N2(g) + 2H2(g) \begin{eqnarray} {\Delta _{\rm{r}}}H^\circ ( {298} ) &=& - 50.6{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}},{\Delta _{\rm{r}}}G^\circ ( {298} )\nonumber\\ &=& - 149.4{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (1) Incomplete: N2H4(l) → 4/3NH3(g) + 1/3N2(g) \begin{eqnarray} {\Delta _{\rm{r}}}H^\circ ( {298} ) &=& - 111.9{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}},{\Delta _{\rm{r}}}G^\circ ( {298} )\nonumber\\ &=& - 171.3{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (2) However, subsequent reactions also need to be considered. The produced H2 can further react with N2H4 according to the following reaction: \begin{eqnarray} {{\rm{N}}_{\rm{2}}}{{\rm{H}}_{\rm{4}}}( {\rm{l}} ) + {{\rm{H}}_2}( {\rm{g}} ) &\to & 2{\rm{N}}{{\rm{H}}_3}( {\rm{g}} ){\, \Delta _{\rm{r}}}H^\circ ( {298})\nonumber\\ &=& - 142.5{\rm{\, kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (3) Furthermore, decomposition of the NH3 component in the produced mixture can form H2 and N2: \begin{eqnarray} {\rm{N}}{{\rm{H}}_3}( {\rm{g}} ) &\to & 1/2{{\rm{N}}_2}( {\rm{g}} ) + 3/2{{\rm{H}}_2}( {\rm{g}} )\, {\Delta _{\rm{r}}}H^\circ ( {298} )\nonumber\\ &=& + 45.9\,{\rm{kJ}}\,{\rm{mo}}{{\rm{l}}^{ - 1}}. \end{eqnarray} (4) During a catalytic decomposition process, the above-mentioned parallel reactions during catalytic decomposition lead to a superimposition of the reaction products [1,3]. For each equation mentioned above, more than one elementary step is performed during the reaction process, which leads to hydrazine decomposition containing many elementary steps, hence the difficulty in describing them. To simplify the research that has been conducted using various catalytic systems, the above reactions are unified by one equation, where x represents the H2 selectivity: \begin{eqnarray} 3{{\rm{N}}_2}{{\rm{H}}_4}( {\rm{l}} ) &\to & 4(1 - x){\rm{N}}{{\rm{H}}_3}( {\rm{g}} )\nonumber\\ &&+\, ( {1 + 2x}){{\rm{N}}_2}( {\rm{g}} ) + 6x{{\rm{H}}_2}( {\rm{g}}).\nonumber\\ \end{eqnarray} (5) Depending on the application, it is desirable to design catalysts with controlled hydrogen selectivity. For example, the specific impulse of a chemical rocket exhibits a maximum in the H2 selectivity range of 30–40% when hydrazine is used as a space propellant. The production of CO-free hydrogen is currently a hot topic, and the catalytic decomposition of hydrazine is proposed for this purpose [4–6]. If complete N2H4 decomposition proceeds through equation (1), the hydrogen released is calculated to be 12.5 wt.%. This value is significantly larger than the target set by the Department of Energy (DOE) for onboard hydrogen systems (5.5 wt.%) [7]. Therefore, N2H4 qualifies as a mobile hydrogen source meeting the DOE target. Additionally, during the complete decomposition of N2H4, three equivalent moles of gaseous products are generated leading to increased volumes of emitted gas, which prevents the solubility of ammonia in water, thus providing a driving force in submarine rescue systems when employing hydrazine for ballast water expulsion. In such cases, it is critical that catalyst design yields high selectivity, especially in mild conditions. Another important property of hydrazine is the formation of hydrous hydrazine when hydrazine reacts with water, similar to ammonia. The monohydrate of hydrazine (N2H4·H2O) has a much lower freezing point (−51.7°C) compared with N2H4 (1.4°C), which makes the former a suitable propellant when employed across a wider temperature range. The water molecule may potentially lead to a partial slowdown of the decomposition rate. Although N2H4·H2O is still considered to be a dangerous chemical, it is far safer to handle compared to explosive N2H4 and, thus, more suitable for investigation in fundamental research. In 2009, Xu and co-workers proposed the idea of using hydrous hydrazine as a potential candidate for chemical hydrogen storage [8]. Since then, there have been a number of reports concerning the decomposition of hydrous hydrazine compared with its anhydrous counterpart. Hydrazine, being the simplest diamine, has applications beyond chemical propellants and is widely employed as a precursor in the synthesis of organic molecules containing N–N bonds, especially in the preparation of pesticides and pharmaceuticals [9,10]. Additionally, hydrazine is a strong reducing agent that is used to control corrosion in boilers, reduce noble metal catalysts or graphene oxide, and to hydrogenate unsaturated bonds in organic compounds [11]. In the case of handling spent hydrazine, complete decomposition of hydrazine into hydrogen and nitrogen is considered the most suitable route to circumvent the production of ammonia. The development of efficient and selective catalysts for H2 generation from hydrazine is an important topic considering the applications described above. From a thermodynamic perspective, the Gibbs free energy of equation (2) is lower than that of the equation (1). That is to say, hydrazine decomposition to ammonia is thermodynamically more favorable. As shown in Fig. 1, the major thermodynamic products of hydrazine decomposition are NH3 and N2 at low temperature. Therefore, it is challenging to avoid its undesirable decomposition to ammonia. Figure 1. View largeDownload slide Thermodynamic products versus temperature for the decomposition of 3 mol hydrazine (0.1 MPa). Figure 1. View largeDownload slide Thermodynamic products versus temperature for the decomposition of 3 mol hydrazine (0.1 MPa). In this review, we briefly introduce the mechanism of hydrazine decomposition to demonstrate the principles of catalyst design. Thereafter, catalytic systems are presented that focus on the selective decomposition of N2H4 or N2H4·H2O under mild conditions. To date, multiple strategies in catalyst design have been reported, including pure metallic nanoparticles and supported catalysts. These catalytic systems are divided into three types according to their influencing factors on H2 selectivity, including alloying effects, alkali promotion and metal–support interactions. The principal reasons for the tunable selectivity are discussed both experimentally and theoretically, which may provide guidance for further catalyst design. In addition to inspiring further research on hydrazine decomposition, we hope that this review will stimulate research on other related N-containing reactions, such as NH3 or NH3BH3 decomposition. MECHANISM CONSIDERATION Early research focused on mechanistic studies via adsorption experiments over Ir, Pt and Rh model metallic surfaces [12–16]. For example, it has been observed that the initial step of hydrazine decomposition over an Ir surface is the dissociative adsorption of the N2H4 molecule, which produces the adatoms of nitrogen, hydrogen, and NH2 species [12]. The adsorbed hydrogen atoms display good mobility and can further react with another hydrogen atom to produce molecular hydrogen or with an NH2 radical to produce NH3. Under mild conditions in the presence of an Ir surface, NH3 is the main product because of the ease by which NH2 combines with adsorbed hydrogen. Conversely, the main intermediate is NH in the presence of an Rh surface using temperature-programed surface reaction experiments, which generate N2 and H2 as the main products at low N2H4 coverage and N2 and NH3 at high N2H4 coverage [14]. Isotopic-labeled 15N2H4 was employed to obtain further information regarding the intermediates during the decomposition process [17,18]. The ratio of 15N to 14N in the produced N2 is small, which illustrates that the produced N–N comes from the same hydrazine molecule and therefore abrogates the hypothesis of prior N–N cleavage during the N2H4 decomposition process [17]. With the development of computational technologies, theoretical calculations have been performed over noble metal and non-noble metal surfaces. Further detailed reaction pathways have been proposed over various modeling surfaces, such as Ir(111) [19], Rh(111) [20,21], Fe (211) [22], Ni [23] and Cu [24] surfaces. However, the adsorption conformations of hydrazine and the intermediates show significant differences in these reports. The hydrazine decomposition mechanism is still contested over different metallic catalysts. Therefore, tailoring of selectivity through catalyst design is still determined experimentally. Essentially, the reaction route is determined by the sequence of N–N bond and N–H bond cleavage [1]. The N–N bond cleavage energy in gaseous N2H4 is 286 kJ mol−1, and the N–H bond energy is 360 kJ mol−1 when the entire bond breaks into N and H atoms. From a thermodynamic viewpoint, the first step relating to N–N bond cleavage appears to be easier, which consequently leads to the production of N2 and NH3. However, the bond energy of N–H is lowered to 276 kJ mol−1 if N2H4 partially breaks into N2H2 and H adatoms are observed over a Ni (100) surface [25], which can potentially lead to large quantities of H2 being produced. In another case, the H–Pt bond is stronger than the N–Pt bond on Pt(111), leading to the barrier to N–H cleavage being much lower than that to N–N cleavage [15]. As a result, regardless of the higher N–H bond energy compared with the N–N bond, the first bond to break in hydrazine adsorbed on platinum is an N–H bond instead of the N–N bond. Nitrogen forms and desorbs close to 40°C through an intramolecular process [15]. Motivated by these findings, the SiO2-supported Ni, Pd and Pt catalysts developed by our group displayed excellent H2 selectivity in gas phase hydrazine decomposition, even at room temperature [3]. According to these reports, the higher H2 selectivity on Ni-, Pd- and Pt-based catalysts is because of the lower barrier to N–H cleavage, which may be related to the stronger M–H bond. Furthermore, de Medeiros et al. correlated H2 selectivity with the enthalpy of adsorption of hydrazine. Hydrazine decomposition over catalysts possessing low enthalpies of adsorption mainly produces N2 and H2. On catalysts having high enthalpies of adsorption (520 kJ mol−1), the reaction products are N2 and NH3 [26]. To summarize the most plausible mechanism, general reaction pathways for hydrazine decomposition are proposed in Fig. 2, showing NH3, N2 and H2 formation (Fig. 2a), and NH3 and N2 formation (Fig. 2b). Because the theory of hydrazine decomposition is not comprehensive enough, catalyst design for H2 generation from hydrazine is mainly based on experimental exploration. It should be noted that the decomposition of hydrazine has been studied in the vapor phase as well as in solution. Although the mechanism is very similar in both cases, it is sometimes difficult to exclude solvent and surface interference in the latter case [27]. Therefore, we indicate which phase the reaction is in hereafter. Figure 2. View largeDownload slide Reaction pathways for hydrazine decomposition. (a) NH3, N2 and H2 formation. (b) NH3 and N2 formation. Figure 2. View largeDownload slide Reaction pathways for hydrazine decomposition. (a) NH3, N2 and H2 formation. (b) NH3 and N2 formation. CATALYTIC SYSTEMS Metallic catalysts are one of the most widely used catalytic systems in heterogeneous catalysis for industrial applications. These catalysts are often composed of active metals, supports and other promoters. There are many factors that can influence catalytic performance, including the choice of metal and support composition, the exposed facet of the active metal, the interaction between the metal and the support, and the properties of the additional promoters. From a molecular point of view, the surface structure and electronic properties are the main influencing factors for heterogeneous catalysts as the majority of the reactions occur on the surface. For hydrazine decomposition, changes to the sequence of N–N bond and N–H bond cleavage are the major strategy to change selectivity. Among various catalytic systems, transition metals (e.g. Ir, Ru and Ni) are the most efficient candidates for N–H bond activation and exhibit high activity for hydrazine decomposition under mild conditions. However, H2 selectivity is limited to below 10% over Ru and Ir catalysts. To date, monometallic catalysts have failed to achieve the complete catalytic decomposition of hydrous hydrazine to H2 without generating NH3. Hence, modification of the catalysts is necessary to promote H2 selectivity. In this section, methods that have been explored for the promotion of H2 formation will be introduced, including alloying of a second metal, alkali addition and then introduction of strong metal–support interactions. Alloy effect Bimetallic catalysts often exhibit enhanced activity, selectivity and stability compared with their parent metals because of their distinct electronic and chemical properties. Therefore, the development of bimetallic catalysts provides opportunities to obtain novel catalysts with improved catalytic performance in many industrial applications. To further understand the origins of their novel properties, bimetallic catalysts have been extensively studied in fundamental research. The geometric and electronic changes of the bimetallic surface have been shown to be the reason for their extraordinary properties in many reactions, such as hydrocarbon reforming, as reported previously in the literature [28–30]. In this section of the review, we will focus on summarizing the investigation of bimetallic catalysts for hydrous hydrazine decomposition to date. In 1979, an Al2O3-supported Ir–Ni catalyst was studied for its performance in gas hydrazine decomposition at 27°C [31]. Although Ir is considered to be the most active metal for gas hydrazine decomposition, its selectivity for hydrogen is poor at low temperatures. Alloying Ni to Ir led to an increase in H2 production through complete decomposition via equation (1). The adsorption experiment results have demonstrated that the surface bonding of NH and N adspecies to Ir is weakened by the addition of Ni atoms, which originate from the formation of the Ir–Ni alloy. In 2009, Xu and co-workers reported the excellent performance of Rh–Ni bimetallic nanoparticles in hydrogen production via hydrous hydrazine decomposition [32], which inspired the following research on bimetallic catalysts. The catalytic performances of various two-component transition metal catalysts are summarized in Table 1. The Ni-based bimetallic catalysts with or without supports provide the highest H2 selectivity, which stimulated interest in investigating the reason for their excellent performance. To date, nanoparticles composed of three types of metals have been reported, including NiFeCu [33], NiFePd [34] and NiFeMo [35]. Additionally, other novel materials have been tested in the decomposition of hydrous hydrazine, such as Co0.85Se/graphene hybrid nanosheets [36]; Ni–B [37], Co–B [38,39], Fe–B [40] and Rh–Ni–B [41] materials; Ni–Rh and Ni–Pt nanoparticles immobilized on a metal–organic framework (MOF); and MOF-derived carbon dots and nitrogen-doped porous carbon [42–44]. These materials have exhibited good performance in hydrous hydrazine decomposition, although further discussion is not included in this review. Table 1. Comparison of hydrogen selectivity (%) of hydrous hydrazine decomposition over different bimetallic catalysts. Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] View Large Table 1. Comparison of hydrogen selectivity (%) of hydrous hydrazine decomposition over different bimetallic catalysts. Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] Fe Co Ni Fe 0 [46] 100 [46–48] Co 0 [46] 18 [46] Ni 100 [46–48] 18 [46] Ru 99 [49] Rh 30 [32] 20 [32] 100 [32,50–53] Pd 0 [54] 7 [54] 100 [55] Ir 0 [56] 100 [57] 100 [56,58] Pt 0 [46] 72 [59] 100 [45,60–66] Au 80 [58] Cu 0 [46] 15 [46] 100 [67] View Large To study the reasons why the second metal promotes catalytic performance, structural configuration and surface adsorption experiments were performed. It is well documented that the arrangement of the two metals can result in three structure types: alloy, core-shell, and heterogeneous. For Pt-promoted Ni catalysts, X-ray diffraction (XRD) results illustrate that the main diffraction peaks show only the existence of metallic Ni [45]. The diffraction peaks shift to lower angles after doping with Pt, demonstrating the formation of a Ni–Pt alloy in the bimetallic catalysts. The formation of the Ni–Pt alloy has been further confirmed by the co-reduction of Ni and the second metal, illustrated by H2-temperature-programed reduction (TPR) data. Furthermore, extended X-ray absorption fine structure (EXAFS) experiments have been employed to study the Pt oxidation status and its coordination environment. The results indicate that the majority of the Pt species exist in the form of a Ni–Pt alloy, which leads to an electronic transfer from Ni to Pt. Therefore, the adsorption of H2 and NH3 is significantly weakened on the bimetallic surface compared with the parent Ni or Pt catalysts, which has been proven by H2-temperature-programmed desorption (TPD), microcalorimetry and NH3-TPD experiments. In addition to the three bimetallic structure systems, the diverse arrangements of the surface and sub-surface metal atoms of the binary system show a significant influence on the surface adsorption and electronic properties of the alloy catalysts [28,30]. Further studies on Ni–Ir catalysts were performed to investigate the influence exerted by the arrangement of the surface atoms, and revealed that pretreating temperatures significantly influenced their catalytic performance. As shown in Fig. 3, the low-temperature-reduced catalyst (Ni–Ir/Al2O3-300R) showed >99% H2 selectivity. In contrast, the analogous high-temperature-reduced catalysts (Ni–Ir/Al2O3-500R and Ni–Ir/Al2O3-700R) showed a considerable reduction in H2 selectivity [58]. The results of XRD, EXAFS and H2-TPR confirmed alloy formation for all three catalysts. However, the coordination number of the Ni–Ir bond increased in the order of Ni–Ir/Al2O3-300R < Ni–Ir/Al2O3-500R < Ni–Ir/Al2O3-700R, which demonstrated the relocation of Ir atoms from the surface of the Ni–Ir alloy particle into the core as a function of increasing reduction temperature. Consequently, the atom arrangement at the surface changed from a Ni–Ir alloy into a Ni-rich surface, leading to a decrease in H2 selectivity during hydrous hydrazine decomposition. Figure 3. View largeDownload slide Structural model of bimetallic Ni–Ir catalysts for hydrous hydrazine decomposition. Reprinted with permission from [58]. Copyright 2013 Elsevier B.V. Figure 3. View largeDownload slide Structural model of bimetallic Ni–Ir catalysts for hydrous hydrazine decomposition. Reprinted with permission from [58]. Copyright 2013 Elsevier B.V. Furthermore, the surface structure was characterized for Pt-, Ir- and Au-doped Ni bimetallic catalysts reduced at equivalent temperatures. Correlating with their hydrous hydrazine decomposition catalytic performance, it was concluded that the formation of a surface alloy is necessary for the purpose of obtaining high H2 selectivity [58]. Provided a homogeneous alloy is formed, such as a Pt–Ni catalyst, the Niδ+ can be observed because of the partial electron transfer from Ni to a nearby Pt atom. Conversely, for Au–Ni catalysts, segregation of Au on the surface is detected. This structural change greatly influences the surface adsorption properties and catalytic performance of the catalyst. The surface structure of the alloy catalyst plays an important role in the generation of H2 from hydrous hydrazine decomposition and may promote future rational catalyst design to obtain good activity and superior selectivity. Strong basic promoter For heterogeneous catalysts, non-active components are typically added as promoters to adjust selectivity or enhance activity. For example, the addition of alkali promoters is crucial for ammonia synthesis. In the 1970s, strong basic components, e.g. NaOH and KOH, were found to improve H2 selectivity for hydrous hydrazine decomposition over Ir catalysts at low temperatures [1]. However, these additives could not catalyze this reaction themselves and led to a decrease in the reaction rate. Recently, the development of non-noble metal catalysts for hydrous hydrazine decomposition to improve H2 generation has gained significant attention. It is important to note that strong basic additives are necessary for such catalysts to obtain high H2 selectivity in mild conditions. These strong basic promoter additives can be divided into two types: i) alkalis (e.g. NaOH) added separately and ii) basic supports combined with the active component during preparation. Xu and co-workers developed the bimetallic Ni–Fe nanocatalyst, which could catalyze the complete decomposition of hydrous hydrazine with the assistance of NaOH [46]. Later, many multi-component catalysts were found to catalyze this reaction with the addition of an alkali, such as Fe–B/MWCNT [40] or NiMoB–La(OH)3 [68]. In our research on Raney Ni catalysts [69], the addition of NaOH was precisely controlled. Before the addition of NaOH to the Raney Ni catalyst, H2 selectivity reached only 80% during the decomposition of hydrous hydrazine. As shown in Fig. 4, a small amount of NaOH (0.05 mol L−1) could significantly promote the selectivity to 91%. Further increasing the concentration of NaOH to above 0.5 mol L−1, the H2 selectivity reached over 99%. In addition, the Raney Ni catalyst could be easily collected and reused after the reaction because of its magnetic properties, as shown in the insets of Fig. 4. The apparent activation energies were calculated for H2 (pathway 1) and NH3 (pathway 2) generation, respectively. The results indicated an increase in the activation energy for pathway 2 after the addition of NaOH, which confirmed the inhibition of NH3 production with the existence of NaOH. Figure 4. View largeDownload slide H2 selectivity improvement as a function of strong basic promoter concentration in hydrous hydrazine decomposition on Raney Ni catalyst. Insets: magnetic separation property of Raney Ni catalyst after reaction of (a) reactants with catalyst after reaction and (b) with a magnet to separate catalyst. Reprinted with permission from [69]. Copyright 2013 American Institute of Chemical Engineers. Figure 4. View largeDownload slide H2 selectivity improvement as a function of strong basic promoter concentration in hydrous hydrazine decomposition on Raney Ni catalyst. Insets: magnetic separation property of Raney Ni catalyst after reaction of (a) reactants with catalyst after reaction and (b) with a magnet to separate catalyst. Reprinted with permission from [69]. Copyright 2013 American Institute of Chemical Engineers. Recently, several supported metal catalysts were shown to catalyze the decomposition of hydrous hydrazine with nearly 100% H2 selectivity. This high selectivity could be attributed to the existence of strong basic sites over the catalysts. In the study of hydrotalcite-derived Ni/Al2O3 catalysts, H2 selectivity reached 93% at 30°C, which was significantly higher than the impregnated counterpart (67%) [70]. The CO2–TPD results revealed large concentrations of strong basic sites in the hydrotalcite-derived Ni/Al2O3, which are thought to originate from the O2− species in the Ni–O–Al structure. The strong basic sites inhibit the production of NH3 and also influence the electronic properties of Ni particles, which consequently improves H2 selectivity during hydrous hydrazine decomposition in mild conditions. In summary, the promoting effect can be attributed to two aspects: i) inhibition of generation of the basic byproduct NH3 and ii) electronic perturbation on the surface of the active metal because of the existence of vicinal strong basic sites. However, the addition of alkalis may not be the best choice to improve H2 selectivity via hydrous hydrazine decomposition when taking into account environmental issues. Strong metal–support interaction effect Widely used industrialized heterogeneous catalysts often consist of metal nanoparticles supported on large surface area oxides. The importance of these oxides has been increasingly recognized over recent decades, following the discovery of metal–support interactions. In addition to dispersing metallic particles, the oxide support also influences the catalytic properties of the metal catalysts through geometric or electronic effects. The representative oxides include CeO2, TiO2 and Fe2O3. To investigate the influence of the support for Ni catalysts in hydrous hydrazine decomposition, two types of Ni/CeO2 catalysts were prepared using co-precipitation and impregnation methods, denoted as Ni/CeO2-CP and Ni/CeO2-IMP, respectively [71]. The two Ni/CeO2 catalysts exhibited quite different activity and selectivity toward hydrous hydrazine decomposition because of their significant structural differences. The Ni particles were highly dispersed in the co-precipitated sample over high Ni loading, which led to improvements in activity for the reaction. In contrast, Ni/CeO2-IMP resulted in obvious aggregation of Ni particles because of the small surface area of the CeO2 support, resulting in significantly lower activity. Furthermore, H2 selectivity reached over 99% for the Ni/CeO2-CP catalyst, considerably higher than for the impregnated counterpart (65%), although the Ni particle sizes were very similar on both samples. Electron microscopy images, H2-TPR and Raman spectroscopy results indicated that Ni particles were partially surrounded by amorphous CeO2 in Ni/CeO2-CP (Fig. 5). The co-precipitation method induced the existence of a Ni–O–Ce structure. However, for Ni/CeO2-IMP, the Ni particles resided on CeO2 with little interaction between Ni and CeO2 [71]. This geometric variation of the surface structure led to an electronic change of the exposed Ni. Using CO as an adsorption probe, Fourier Transform Infrared Spectroscopy (FT-IR) results illustrated that the exposed Ni species in Ni/CeO2-CP differs from metallic Ni. The presence of a weak CO adsorption band on the Niδ+ site could be observed, indicating changes to the electronic properties of the exposed Ni species on Ni/CeO2-CP because of the modification of vicinal CeO2 [71]. In particular, the CeO2 support not only played a role in dispersing Ni particles, but also behaved as a promoter or modifier to improve hydrogen selectivity of Ni in the decomposition of hydrous hydrazine. Figure 5. View largeDownload slide Structural models of Ni/CeO2-CP and Ni/CeO2-IMP. Reprinted with permission from [71]. Copyright 2015 American Chemical Society. Figure 5. View largeDownload slide Structural models of Ni/CeO2-CP and Ni/CeO2-IMP. Reprinted with permission from [71]. Copyright 2015 American Chemical Society. On the basis of results outlined above, we synthesized other Ni–O–M structures on varying metal oxide supports. Using the co-precipitation method, a series of supported Ni catalysts were prepared, including Ni/MgO, Ni/La2O3 and Ni/ZrO2 [71]. These catalysts had one thing in common: a strong metal–support interaction existed between Ni and the oxide support. Similar to the Ni/CeO2–CP sample, these catalysts all exhibited >80% H2 selectivity for hydrous hydrazine decomposition, which further confirmed the promotion effect of the oxide supports, as long as a Ni–O–M structure formed. This finding may inspire the future design of more efficient catalysts for the decomposition of hydrous hydrazine using economically viable facile preparation methods. CONCLUSION AND OUTLOOK Different hydrazine usage demands have made hydrazine decomposition selectivity increasingly important. This focus of this review has been to analyze factors that influence improvements to hydrogen selectivity in mild conditions. As Ni-based catalysts have been proven to be the most active candidates, strategies to further improve their performance have been analyzed, including the choice of metal composition, alkali promoter addition and the tailoring of metal–support interactions. Our intent is to encourage the development of more highly selective catalysts for H2 production via hydrous hydrazine decomposition. Although many attempts, both experimental and theoretical, have been made to reveal the key factors that affect the selectivity of metallic catalysts, the detailed mechanism is still not fully understood. Therefore, studies on hydrazine decomposition are necessary to better understand the reasons for selectivity differences. Additionally, such studies may further inspire the exploration of other reactions including N–N bonds and N–H bonds, such as NH3 synthesis or decomposition, and provide opportunities for the development of novel catalytic systems. Furthermore, the majority of the catalysts reported in the literature to date have only been successful at the laboratory scale and have shown limitations in industrial application. As is already known, this reaction is an exothermal process, and the amount of heat released and the distribution of gaseous products (N2, H2 and NH3) depends on the reactive conditions and the selectivity of the catalysts. Therefore, heat and mass transfer should be considered during the design of catalysts. In additional, the stabilities of the currently available catalysts are not sufficient to meet the demands of industrial production. Therefore, many engineering and technological problems need to be investigated and resolved prior to their industrial application. FUNDING This work was supported by the National Natural Science Foundation of China (21103173 and 21476226), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100), the National Key Projects for Fundamental Research and Development of China (2016YFA0202804) and the Youth Innovation Promotion Association of the Chinese Academy of Sciences. Conflict of interest statement. None declared. REFERENCES 1. Batonneau Y , Kappenstein CJ , Keim W . Catalytic decomposition of energetic compounds: gas generators and propulsion . Handbook of Heterogeneous Catalysis . Weinheim : Wiley-VCH , 2008 . 2. Lucien HW . Thermal decomposition of hydrazine . J Chem Eng Data 1961 ; 6 : 584 – 6 . https://doi.org/10.1021/je60011a030 Google Scholar CrossRef Search ADS 3. Zheng M , Cheng R , Chen X et al. A novel approach for CO-free H production via catalytic decomposition of hydrazine . 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National Science ReviewOxford University Press

Published: Sep 29, 2017

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