Catalytic Hydrolysis of Ammonia Borane by Cobalt Nickel Nanoparticles Supported on Reduced Graphene Oxide for Hydrogen Generation

Well dispersedmagnetically recyclable bimetallic CoNi nanoparticles (NPs) supported on the reduced graphene oxide (RGO) were synthesized by one-step in situ coreduction of aqueous solution of cobalt(II) chloride, nickel (II) chloride, and graphite oxide (GO) with ammonia borane (AB) as the reducing agent under ambient condition.TheCoNi/RGONPs exhibits excellent catalytic activity with a total turnover frequency (TOF) value of 19.54molH 2 mol catalystmin and a low activation energy value of 39.89 kJmol at room temperature. Additionally, the RGO supported CoNi NPs exhibit much higher catalytic activity than themonometallic and RGO-free CoNi counterparts. Moreover, the as-prepared catalysts exert satisfying durable stability and magnetically recyclability for the hydrolytic dehydrogenation of AB, whichmake the practical reusing application of the catalysts more convenient.The usage of the low-cost, easy-getting catalyst to realize the production of hydrogen under mild condition gives more confidence for the application of ammonia borane as a hydrogen storage material. Hence, this general method indicates that AB can be used as both a potential hydrogen storage material and an efficient reducing agent, and can be easily extended to facile preparation of other RGO-based metallic systems.


Introduction
Hydrogen, as a globally accepted clean and environmentally friendly fuel, has attracted widespread research concerns and interests [1].However, for the widespread use of hydrogen as an energy carrier to achieve sustainable mobility, developing a compact, safe, and affordable means of storing hydrogen remains a major obstacle [2].Chemical hydrogen storage is thought to be one of the most promising approaches to meet this challenge, because of its considerably high gravimetric and volumetric hydrogen density [3][4][5][6].In recent years, ammonia borane (AB) was discovered as an efficient and safe storage medium for H 2 due to the high capacity (19.6 wt.% and 0.145 kg H 2 /L) and stability in ambient atmosphere [7][8][9][10][11].In the presence of a suitable catalyst, three moles of hydrogen can be released from one mole of AB at room temperature.The hydrolysis of AB can be represented as follows [12][13][14]: So, the discovery of suitable catalysts becomes the main issue of the practical application of AB as an on-board hydrogen medium.With the trend of nanocatalysts applied in the hydrolysis of AB system, transition metal nanoparticles (NPs) catalysts are considered to be a promising option because of their novel structure, electronic properties, and magnetic performances [15][16][17][18].So far, not only nonnoble metal NPs [12,[19][20][21][22][23][24] and noble metal NPs [13,20,[25][26][27][28][29], but also their composites [14,[30][31][32][33][34][35][36][37][38] have been tested for hydrolytic dehydrogenation of AB; among them platinum [20,25,26] shows the highest activity.However, concerning the element abundance and related economic issues, it is clearly a desired goal to prepare low-cost catalysts with high catalytic activity for the terminal practical application of this reaction system in the fuel cell.The catalytic performance of the metal NPs is highly dependent on the dispersion of the active metals.Thus, the aggregation of metal NPs during the catalytic process due to the high surface energies and magnetic properties is the main impediment to restraint their development [36].To solve this problem, various surfactants and substrates are successfully utilized to obtain catalysts with preferable dispersity [12][13][14].Graphene, a single layer of sp 2 carbon lattices, could be an ideal substrate for growing and anchoring metal NPs with good dispersion due to its high specific surface area and large density of free electrons [37][38][39].
In this work, we have discovered that the CoNi/RGO exhibits excellent catalytic activity in hydrolysis of AB for hydrogen generation.Separation of this catalyst from reactant liquid is quite simple by using a magnet, as CoNi/RGO has an excellent magnetic property.This mild and rapid strategy provides great potential for application of ammonia borane as an on-board hydrogen medium.

Experimental
2.1.Graphite Oxide (GO) Preparation.GO was made by a modified Hummers method [40,41].Briefly, natural graphite powder (325 mesh) was placed into an 80 ∘ C solution of concentrated H 2 SO 4 (30 mL), K 2 S 2 O 8 (2.5 g), and P 2 O 5 (2.5 g).The mixture was carefully diluted with distilled water and filtered using a 0.2 micron Nylon Millipore filter to remove the residual acid.The product was dried at 80 ∘ C under ambient condition overnight.The preoxidized graphite was put into cold concentrated H 2 SO 4 , then KMnO 4 was added gradually under stirring, and the temperature of the mixture was kept below 20 ∘ C for 2.5 h.The mixture was stirred at 35 ∘ C for 4 h.Afterwards, 250 mL of deionized water was added and the suspension was stirred at 100 ∘ C for another 2 h.Subsequently, additional 300 mL of deionized water was added.Shortly after that, 7 mL of 30% H 2 O 2 was added to the mixture to terminate the reaction.The suspension was then repeatedly centrifuged and washed first with 5% HCl solution and then with water.Exfoliation of graphite oxide to GO was achieved by ultrasonication of the dispersion for 30 min [42].

In Situ Synthesis of RGO Supported
CoNi NPs (Co/Ni = 0.5/0.5)and Their Catalytic Activities toward the Hydrolysis of AB (Catalysts/AB = 0.05).8 mL aqueous solution containing CoCl 2 (6.01 mg), NiCl 2 (6.60 mg), and GO solution (1.77 g containing 0.68‰ GO) was kept in a 25 mL twonecked round-bottom flask.One neck was connected to a gas burette and the other was connected to a pressureequalization funnel to introduce AB in 2 mL of aqueous solution containing 34.3 mg AB (1 mmol).The reactions were started when the aqueous AB solution was added to the flask with vigorous stirring.The evolution of gas was monitored using the gas burette.After the hydrogen generation reaction was completed, 34.3 mg AB (1 mmol) was added to the flask and the evolution of gas was monitored.A water bath was used to control the temperature of the reaction solution.
For comparison, RGO supported CoNi NPs with different Co/Ni ratio, GO reduced by AB, RGO-free CoNi reduced by AB, and CoNi/RGO reduced by NaBH 4 were synthesized using the same method.

Different Supported Carbon Materials.
Sets of experiments with different supported carbon materials (such as activated charcoal and graphite powder) were performed at room temperature.All the experiments were performed in the same way as described in Section 2.2.

Kinetic Studies of Hydrolytic Dehydrogenation of AB Catalyzed by CoNi/RGO NPs.
Temperature was varied at 25 ∘ C, 30 ∘ C, 35 ∘ C, and 40 ∘ C, while the ratio of the concentration of CoNi (0.05 mmol) and AB (1 mmol) was kept constant at 0.05 to obtain the activation energy (  ).

Stability Test.
For stability test, catalytic reactions were repeated 5 times by adding other equivalents of AB (1 mmol) into the mixture after the previous cycle.The molar ratio of catalyst/AB was kept at 0.05.

Catalyst Characterization.
Transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED) were observed using FEI Tecnai G20 U-Twin TEM instrument operating at 200 kV.Powder X-ray diffraction (XRD) studies were performed on a Rigaku RINT-22005 X-ray diffractometer with a Cu K source (40 kV, 20 mA).X-ray photoelectron spectroscopy (XPS) measurement was performed with a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer.FTIR spectra were collected at room temperature by using a Thermo Nicolet 870 instrument using KBr discs in the 500-4000 cm −1 region.Raman spectrometer was carried out using a confocal Raman microscope (LabRAM HR).

Results and Discussion
3.1.Synthesis and Characterization.Chemical reduction methods are widely employed for the synthesis of NPs in solution phase in the presence of surfactant [43], whose presence on the surface diminishes the activity to some extent by blocking some of the active sites when NPs are employed as a catalyst [44].Surfactant-free NPs are difficult to synthesize because growth of in situ generated nuclei cannot be halted.Herein, RGO supported CoNi NPs were successfully synthesized without using any external surfactant.We employed AB itself (much milder than NaBH 4 ) as the reducing agent in our reactions.
When AB was added to the precursor solution containing CoCl 2 and NiCl 2 at room temperature, reaction started after an induction period of 55 min, while with the precursor solution with graphene oxide (GO), the induction period time decreased to 3.5 min, as shown in Figure S1 (see Figure S1 in Supplementary Materials available online The microstructures of the CoNi/RGO were characterized by transmission electron microscopy (TEM), highresolution TEM (HRTEM), energy-dispersive X-ray spectrometer (EDS), and selective area electron diffraction (SAED) (Figure 1).The GO sheets shown in Figure 1(a) are transparent and corrugated together.As shown in Figures 1(b) and 1(c), the NPs were well dispersed on RGO, which helps to prevent the agglomeration.A close examination of the catalysts by HRTEM (Figure 1(d)) indicates that the dspacing of the particle lattice is ∼0.203 nm, which is consistent with the (111) plane of cubic Ni (JCPDS number 04-0850).The EDS spectrum of the specimen shows the presence of Co and Ni (Figure 1(e), which was taken from the specially marked area in the TEM image), with the atomic ratio for Co : Ni being detected to be 0.02 : 0.02, which is in good agreement with the appointed atomic ratio (1 : 1).Therefore, the Co NPs may be in amorphous phase.
Figure 2S shows the power XRD pattern of the Co/RGO and Ni/RGO.The diffraction peak attributed to Ni (111) is observed.However, no diffraction peak of Co exists, which may be caused by the amorphous phase of Co.Therefore, the peak at around 44.10 ∘ in the power XRD pattern of the as-prepared CoNi/RGO (Figure 2) corresponds to the Ni (111).Furthermore, the most intense peak at around 11.5 ∘ that corresponds to the (001) reflection of GO disappeared and a new peak at around 25.1 ∘ is observed in the as-synthesized RGO supported CoNi NPs, indicating that GO is successfully reduced to RGO.
CoNi/RGO was further characterized by X-ray photoelectron spectroscopy (XPS) to investigate the surface nature of the CoNi NPs and RGO (Figure 3).Compared with the peaks of GO (Figure 3    Two peaks centered at 1316 and 1577 cm −1 appear in the Raman spectra of the GO and RGO supported CoNi NPs (Figure 4(a)), corresponding to the  and  bands of the carbon products, respectively.The intensity ratio of the  to  band (  /  ) is generally accepted to reflect the degree of graphitization of carbonaceous materials and defect density.After loading of the CoNi NPs, the   /  of GO is increased from 1.14 to 1.61.The relative changes in the  to  peak intensity ratio confirm the reduction of GO during the in situ fabrication.Figure 4(b) displays the FTIR spectra of GO and CoNi/RGO NPs.After the formation of RGO supported CoNi NPs, the disappearance of C=O peak at 1716 cm −1 , C-OH peak at 1399 cm −1 , and C-O peak at 1064 cm −1 of GO further indicates that the GO was reduced to RGO during the process.1, while CoNi/RGO show better activity than pure Co NPs or pure Ni NPs supported on RGO, indicating the positive effect of metal interaction in the bimetallic CoNi NPs on hydrogen generation from hydrolysis of AB.As shown in Figure S3, compared with the NPs reduced by NaBH 4 , the as-synthesized NPs generated by AB exhibit higher catalytic activities, which confirms that it is possible to achieve much more control over the nucleation and growth process of the RGO by choosing weaker reducing agents.shown in Figure S4, their catalytic activities are inferior to that of Co 0.5 Ni 0.5 /RGO NPs, highlighting the dominant factor of RGO in facilitating hydrolysis of AB in our system.

Catalytic Activities for Hydrolysis of AB.
In order to get the activation energy (  ) of the AB hydrolysis catalyzed by CoNi/RGO NPs, the hydrolytic reactions at different temperatures in the range of 298-313 K were carried out.The values of the rate constant  at different temperatures were calculated from the slope of the linear part of each plot from Figure 6(a).The Arrhenius plot of ln  versus 1/ for the catalyst is plotted in Figure 6(b), from which the apparent activation energy was determined to be approximately 39.89 kJ/mol, being lower than most of the reported   values (Table 1), indicating the superior catalytic performance of the as-synthesized CoNi/RGO catalysts.

Reusability and Recycle
Ability.The reusability of the catalysts is crucial in the practical application.The recyclability of CoNi/RGO catalysts up to the fifth run for hydrolysis of AB is shown in Figure 7.The as-synthesized CoNi/RGO catalysts retain 68% of their initial activities in the hydrolysis of AB in the fifth run, indicating their superior recycle stabilities.The observed activity loss is likely to result from the precipitation of metaborates to the catalyst surface [19].Moreover, the in situ synthesized CoNi/RGO are magnetic and thus can be separated from the reaction solution by an external magnet (Figure 7 inset), which makes the practical recycling application of nanocatalysts more convenient.

Conclusion
In summary, we have developed a facial in situ one-step method for the synthesis of magnetic RGO supported bimetallic CoNi NPs using AB as a reductant.
Figure2Sshows the power XRD pattern of the Co/RGO and Ni/RGO.The diffraction peak attributed to Ni (111) is observed.However, no diffraction peak of Co exists, which may be caused by the amorphous phase of Co.Therefore, the peak at around 44.10 ∘ in the power XRD pattern of the as-prepared CoNi/RGO (Figure2) corresponds to the Ni (111).Furthermore, the most intense peak at around 11.5 ∘ that corresponds to the (001) reflection of GO disappeared and a new peak at around 25.1 ∘ is observed in the as-synthesized RGO supported CoNi NPs, indicating that GO is successfully reduced to RGO.CoNi/RGO was further characterized by X-ray photoelectron spectroscopy (XPS) to investigate the surface nature of the CoNi NPs and RGO (Figure3).Compared with the peaks of GO (Figure3(a)), the intensities of oxygen containing functional groups (such as -C-O, -C=O, and -COO) in Co/RGO (Figure 3(b)) decrease significantly, which also reveal the reduction of GO to RGO.Figure 3(c) shows the peaks of Co 2p.There are three peaks whose peak tops are 778.6,781.7, and 786.0 eV, which stand for zero valent Co and oxidized Co [45]. Figure 3(d) shows the peaks of Ni 2p.There are two peaks at 853.2 and 870.0 eV which are attributed to zero valent Ni, while the other two peaks at 856.6 eV and 874.0 eV stand for oxidized Ni [46].The formation of the

Figure 3 (
Figure2Sshows the power XRD pattern of the Co/RGO and Ni/RGO.The diffraction peak attributed to Ni (111) is observed.However, no diffraction peak of Co exists, which may be caused by the amorphous phase of Co.Therefore, the peak at around 44.10 ∘ in the power XRD pattern of the as-prepared CoNi/RGO (Figure2) corresponds to the Ni (111).Furthermore, the most intense peak at around 11.5 ∘ that corresponds to the (001) reflection of GO disappeared and a new peak at around 25.1 ∘ is observed in the as-synthesized RGO supported CoNi NPs, indicating that GO is successfully reduced to RGO.CoNi/RGO was further characterized by X-ray photoelectron spectroscopy (XPS) to investigate the surface nature of the CoNi NPs and RGO (Figure3).Compared with the peaks of GO (Figure3(a)), the intensities of oxygen containing functional groups (such as -C-O, -C=O, and -COO) in Co/RGO (Figure 3(b)) decrease significantly, which also reveal the reduction of GO to RGO.Figure 3(c) shows the peaks of Co 2p.There are three peaks whose peak tops are 778.6,781.7, and 786.0 eV, which stand for zero valent Co and oxidized Co [45]. Figure 3(d) shows the peaks of Ni 2p.There are two peaks at 853.2 and 870.0 eV which are attributed to zero valent Ni, while the other two peaks at 856.6 eV and 874.0 eV stand for oxidized Ni [46].The formation of the
Figure 5(a) shows the composition effect of RGO supported Co 1− Ni  on AB hydrolysis ( = Co:Co + Ni), with the molar ratio for catalysts to AB being 0.05.Obviously, by changing the molar ratio, the NPs demonstrate different catalytic activities.As a result, Co 0.5 Ni 0.5 /RGO displays the best catalytic performance in the hydrolysis of AB, generating a stoichiometric amount of hydrogen (H 2 /AB = 3.0) in the shortest time (3.07 min) with a turnover frequency (TOF) value of 19.54 mol H 2 mol catalyst −1 min −1 , which is higher than the most reported nonnoble metal-based NPs and even many noble metalbased NPs, as shown in Table

Figure 6 :
Figure 6: (a) Plots of volume of hydrogen generated versus time for Co 0.5 Ni 0.5 /RGO catalyzed hydrolysis of AB at four different temperatures in the range of 298-313 K, CoNi/AB = 0.05 and (b) Arrhenius plot obtained from the data of Figure 6(b).

Figure 7 :
Figure 7: (a) Hydrogen generation from hydrolysis of ammonia borane (0.10 M, 10 mL) catalyzed by Co 0.5 Ni 0.5 /RGO from the 1st to 5th cycle, CoNi/AB = 0.05, and (inset) photographs of the Co 0.5 Ni 0.5 /RGO before (left) and after (right) the magnetic separation; (b) percentage of initial catalytic activity of Co 0.5 Ni 0.5 /RGO in successive runs after the reuse for the hydrolysis of AB.

Table 1 :
Catalytic activity of different catalysts used for the hydrolytic dehydrogenation of AB.
CatalystTOF (mol H 2 ⋅mol catalyst −1 ⋅min −1 )   (kJ mol −1 ) ) value of 39.89 kJ mol −1 .The RGO supported CoNi NPs exhibit much higher catalytic activity than the graphite powder and activated charcoal supported or monometallic and RGO-free CoNi counterparts.Compared with catalysts reduced by NaBH 4, the CoNi/RGO catalysts generated by the milder reducing agent AB exhibit higher catalytic activities.Moreover, the CoNi/RGO catalysts show good durable stability and magnetic recyclability for the hydrolytic dehydrogenation of AB, which makes the practical recycling application of the catalyst more convenient.This simple synthetic method can be extended to other RGObased bimetallic systems for more application.
The CoNi/RGO exhibit superior catalytic activity, with a turnover frequency (TOF) value of 19.54 H 2 mol catalyst −1 min −1 and an activation energy (