Magnetically recyclable Co/ZnO@NiFe2O4nanoparticles as highly active and reusable catalysts for hydrazine monohydrate hydrogen generation

Article abstract of DOI:10.1039/d0cy01829e

This research investigates the improvement of novel and cost-effective highly magnetic nanoparticle (NP) catalysts for hydrazine monohydrate dehydrogenation, which are viewed as some of the greatest solid chemical hydrogen storage materials because of their high gravimetric hydrogen storage capacity. The improvement of a new catalytic material, Co supported on ZnO@NiFe₂O₄NPs, was also studied. The experimental results indicate that the resultant Co/ZnO@NiFe₂O₄NPs (2.0 wt% Co) exhibits excellent catalytic performance, 100% H₂selectivity and recyclability for hydrogen generation from hydrazine monohydrate (N₂H₄·H₂O) at 25 °C in alkaline solution conditions. The turnover frequency (TOF) value was approximately 4445.37 h^?1, which is higher than that obtained with 0.5 wt% Co NPs (TOF value 997.42 h^?1). Moreover, Co/ZnO@NiFe₂O₄demonstrates outstanding catalytic performance and excellent recycle stability through high hydrazine monohydrate dehydrogenation. The superior catalytic properties may be the result of electronic interactions between Co, Zn and Ni NPs. The tested catalysts were easily recovered by utilizing a permanent magnet and were reused for up to 15 cycles without losing their characteristic catalytic activity. In summary, Co/ZnO@NiFe₂O₄NP catalysts have been successfully developed and show a good future application potential.

Full text of DOI:10.1039/d0cy01829e

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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Magnetically recyclable Co/ZnO@NiFe O Nano particle catalyst as a highly
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active and reusable catalyst for hydrazine monohydrate hydrogen generation
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Samikannu Prabu and Kung-Yuh Chiang*
Graduate Institute of Environmental Engineering, National Central University, Tao-Yuan city,
32001, Taiwan
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Corresponding author: Prof. Kung-Yuh Chiang Email: kychiang@ncu.edu.tw
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Abstract
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This research investigates that the improvement of novel and cost-effective highly magnetic
Nano particle (NP) catalysts for hydrazine monohydrate dehydrogenation, which is viewed as one
of the greatest solid chemical hydrogen storage materials because of its high gravimetric hydrogen
storage capacity. The improvement of a new catalytic material Co supported on ZnO@NiFe O
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NPs was also studied. The experimental results indicated the resultant Co/ZnO@NiFe O NPs (2.0
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wt% Co) exhibits excellent catalytic performance, 100% H selectivity and recyclability for
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hydrogen generation from hydrazine monohydrate (N H .H O) at 25 C in alkaline solution
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conditions. The turnover frequency (TOF) value was approximately 4445.37 h that it was higher
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than that of 0.5 wt% of Co NPs condition (TOF value 997.42 h ). Moreover, Co/ZnO@NiFe O
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also demonstrates outstanding catalytic performance and excellent recycle stability through high
hydrazine monohydrate dehydrogenation. The superior catalytic properties were possibly the
result of the electronic interaction between Co, Zn and Ni NPs. The tested catalysts were easily
recovered by utilizing a permanent magnet and reused up to 15 recycles without losing their
characteristic catalytic activity. In summary, Co/ZnO@NiFe O NPs catalyst has been
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successfully developed and showed a good application potential in the future.
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1. Introduction
H is safe and has attracted abundant importance as a worldwide acknowledged clean energy
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carrier.1,2 Secure capacities and effective release of H are the constraining factors in H energy
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applications. The hydrogen capacity in a compacted structure and its effective transformation are
key factors that give hydrogen potential as an energy transporter.3,4 While there have been
tremendous efforts to develop suitable hydrogen storage technologies, such as hydrocarbon steam
reforming, pyrolysis, biomass gasification, fuel cell, electrolysis, plasma technology and releasing
materials over the last few years. The effective storage and production of hydrogen are still two
key issues in the “Hydrogen Economy. The nearness of a suitable catalyst is fundamental to
hydrogen generation at an adequate reaction rate. Therefore, safe and efficient chemical hydrogen
storage materials that appropriately release hydrogen under mild conditions are desperately
required.5
Hydrazine monohydrate (N H ·H O), which is a liquid at room temperature and contains high
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hydrogen capacity (8.0 wt%), is considered a promising hydrogen transporter attributable to its
benign handling, easy recharging, and CO-free hydrogen production.6-8 More significantly, only
N and H are produced utilizing the complete decomposition reaction method (eqn (1)). This
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method is not appropriate for recycling.9-11 To maximize the N H ·H O ability as a suitable
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hydrogen storage material, the fragmented (relying on the nature of the catalyst, temperature and
alkalinity), NH produced from the decomposition reaction through eqn (2) makes this process
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unsuitable.
N H → N (g) + 2H (g) ……………. (1)
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3N H → 4NH (g) + N (g) …………. (2)
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Other than the catalyst composition, the structure and properties of the support materials are
fundamental for improving the catalytic effect. Many transition metals and noble metal-containing
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Ni-based NP catalysts are efficient in hydrous hydrazine dehydrogenation (e.g., NiIr, PtNi,
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NiMoB, RhNi, NiPt, NiFe, NiCo, NiCu, NiRh. Zhang et al. manufactured a graphene-
supported Rh-Ni catalyst. This catalyst showed total hydrogen selectivity (100%), which
furthermore established considerably higher catalytic effectiveness than that of only Rh-Ni Nano
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particle catalysts while supporting NaOH at 25 C. Tong et al. produced an fcc-In Ni alloy Nano
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particle catalyst which showed great catalytic efficiency for hydrogen generation from hydrazine
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monohydrate at 20 C. The fcc-In Ni alloy activation energy NRs was 38.9 ± 1.0 kJ mol .
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However, the greater part of the metal Nano particles exhibit long-term stability because it could
accumulate into bunches would cause a significant loss in catalytic activity. However, it should be
stabilized against aggregation into bigger particles and monometallic NPs are synthetically
inactive towards hydrazine decomposition at room temperature.20
To control and stabilize the metal NPs catalyst size, organic agents are broadly used but the
active sites on the metal NP surfaces might be hindered by surface ligands, which are unfavorable
near catalytic reactions. Graphene supports,21 multi-wall carbon Nano tubes,22,23 zeolites,24
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alumina, and polymers have been widely used as supporting materials in the preparation of
metal NPs with controllable size and size dispersal. Carbonaceous supporting materials, if well
dispersed with metal Nano particles with high external surface area, produce favorable catalytic
reactions with metal oxides. Among them, Ce-based metal oxides show promising selectivity and
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activity for N H dehydrogenation due to their high stability and redox properties.
Contrasted
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with unadulterated CeO , doping transition metal oxides such as Y O , ZrO , MoO La O and
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TiO into CeO could change the synergistic basic and electronic effects.
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Ideally the catalyst should be re-utilized for low-cost, effective hydrazine dehydrogenation.32
Catalysts are reused for recycling in the accompanying new reaction. Magnetic NPs can be simply
isolated from the reaction solution using an outward permanent magnet. Associated with other
magnetic materials, NiFe O NPs possess outstanding high powerful strength, environmental
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stability and stress-free preparation in the magnetic NP field. Zinc is the most well-known
inorganic coating material for metal NPs because it protects the core material and filters
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agglomeration. Ag, Cu, Pd and Ru NPs supported on SiO -CoFe O were recently effectively
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synthesized as exceptionally well-organized catalysts.
Cobalt is more affordable than other transition metals. Contrasted with other transition metal
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NPs, Co has been studied less for catalytic decomposition of hydrazine in aqueous solution. Co
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containing catalysts, particularly when Co is the prevailing catalyst substance, were discovered
as humbly active catalysts for hydrogen generation from hydrazine monohydrate dehydrogenation.
Encouraged by Co/ZnO@NiFe O NPs structural merits, this research investigated incorporating
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highly active, high stability Co species into NiFe O437,38 on the atomic scale to manufacture
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twofold Ni-Co catalysts for hydrous hydrazine decomposition. The resultant catalysts possess the
following alluring highlights, high Co and NiFe O species dispersion facilitating catalytic activity
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improvement for sodium-ion batteries, water oxidation processes, adsorption and photocatalysts.39-
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The majority of these low-cost catalytic systems suffer from less activity and reusability
performance. The development of highly active and reusable heterogeneous catalysts based on
low-cost magnetic Co/ZnO@NiFe O NPs is a preferred objective for this significant reaction.
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The catalytic activities of Co NPs catalysts are still not satisfactory. Accomplishing high activity
of Co/ZnO@NiFe O NPs catalysts is of great scientific interest.
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For the first time the novel preparation and synthesis of Co NPs supported on magnetic
ZnO@NiFe O Nano particle catalysts is used in aqueous hydrous hydrazine catalytic
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dehydrogenation. The as-synthesized catalysts utilize higher catalytic activities than most of the
described Co-based NPs in decomposition of hydrazine monohydrate. As attractively recyclable
catalysts, the presented catalysts can be separated from the reaction solution by an external
permanent magnet, exhibiting excellent recycling stability. Our report specifies kinetic study
results on hydrazine monohydrate catalytic decomposition depending on the catalyst effect and
temperatures.
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2. Experimental
Materials and methods
Nickel nitrate hexahydrate (Ni(NO ) .6H O, Alfa Aesar, 98%), Iron nitrate hexahydrate
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(Fe(NO ) .6H O, Showa chemicals, 99.9%), Cobalt nitrate hexahydrate (Co(NO ) .6H O, Showa
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chemicals, 99.9%), Sodium hydroxide (NaOH, Shimakyu’s pure Chemicals, 95%), Znic sulfate
(ZnSO , Alfa Aesar, 98A%, Hydrous hydrazine (N H ·H O, Aldrich, 38%), Hydrochloric acid
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(HCl, Alfa Aesar, 38%), Ethanol (C H OH, Alfa Aesar, 95%). Distilled water was used
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throughout this study. All chemicals were of sufficient purity and used without any further
purification.
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Nickel ferrite Nano particle (NF NPs) preparation
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The nickel ferrite (NF, NiFe O ) Nano particles were prepared as previously reported with some
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significant alterations. Briefly, Ni(NO ) .6H O and Fe(NO ) .9H O were taken in a 1:2 ratio
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3 3
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(0.01:0.02 M) and dissolved in 100 mL of ethanol. The reactants were consistently stirred using a
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magnetic stirrer until completely dissolved. To another vessel, 50 mL of sodium hydroxide (2.0
mmol) was gradually added to the previous salt solution. After the complete addition of NaOH
o
solution, the mixture was continually stirred (500 rpm) using a magnetic stirrer for 1 h at 25 C.
The precipitate was collected later, washed with DI water and ethanol three times. The obtained
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brown color precipitate was dried in an oven at 105 C overnight and then ground and calcined at
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750 C for 3 h.
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ZnO@NiFe O Nano particle preparation
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In a typical synthetic procedure, ZnO@NiFe O Nano particles were synthesized using direct
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precipitation methods using NiFe O zinc nitrate and NaOH as precursors. Point five grams of
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NiFe O , 0.2 g zinc sulfate (ZnSO ) and 50 ml H O were added to a beaker. This mixture was
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stirred for 15 min at 25 C. The 1M NaOH solution was slowly added into the reaction mixture at
room temperature under vigorous stirring at 500 rpm. This resulted in the establishment of a brown
suspension. The brown color precipitate was centrifuged at 5000 rpm for 15 min and washed with
distilled water. This solution was then washed with absolute alcohol three times. The obtained
o
product was dried in an oven at 105 C overnight and then ground and calcined at 750°C in an air
atmosphere for 1 hour.
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Co/ZnO@NiFe O Nano particle preparation
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ZnO@NiFe O (100 mg) was added to a Co(NO ) .6H O (5.65 mg) solution in 30 mL H O in a
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50 mL beaker. This slurry was continually stirred at room temperature for 1 h. The resulting solid
Co-ZnO@NiFe O product was centrifuged at 6000 rpm for 10 min and washed three times with
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distilled water. and the solution was then washed with alcohol 3 times. The obtained product was
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dried in an oven at 105 C overnight and then calcined at 750 °C in air atmosphere for 3 hours, as
shown in Scheme 1. The 750 °C temperature was preferred for annealing the fabricated materials
to remove all of the unreacted nitrate originators from the materials.
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Scheme 1. A schematic illustration of the preparation of magnetic Co/ZnO@NiFe O4
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nanoparticles.
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Characterization of materials
Powder X-ray diffraction (XRD) pattern was obtained utilizing a Bruker D8 PHASER X-ray
diffractometer with GO monochromatized Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 5°
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min (40 kV, 20 mA). Transmission electron microscopy (TEM, JEM-2010) prepared with
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energy dispersed X-ray spectroscopy (EDX) and selected area electron diffraction (SAED) and
energy dispersive X-ray detector (EDX) for elemental analysis to obtain the detailed
microstructure information of the synthesized magnetic catalysts. X-ray photoelectron
spectroscopy (XPS) analysis was performed on an Rigaku- NANO-Viewer X-ray photoelectron
spectrometer with achromatized Al Kα source (15 kV, 150 W). The Ar sputtering experiments
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were carried out in a circumstantial vacuum environment 5×10 Pa and sputtering acceleration
voltage 3 kV. Inductively coupled plasma atomic emission spectroscopy (ICP-OES) was adopted
to detect the detail components of the samples. Thermogravimetric and differential thermal
analysis (TGA-DTA) was employed to clarify the thermal behavior of the typical as nanoparticle
samples.
Experimental methods for N H dehydrogenation
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The Co/ZnO@NiFe O catalytic effect in hydrazine monohydrate decomposition was
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determined by H +N gas generation volumetric measurement. Typically, the sodium hydride
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(NaOH, 50 mg) mixture, as-synthesized Co/ZnO@NiFe O catalyst (50 mg), and DI water (4.5
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mL) were placed in a single neck conical flask. This flask was placed in a water bath maintained
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at room temperature (25 C) followed by vigorous stirring 10 min to achieve thermal equilibrium.
Next, 2.0 mmole of N H .H O solution was injected into the reaction bottle through its septum
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using a 1.0 mL gastight syringe. The gas products were first passed through an HCl (0.1M) trap to
ensure NH absorption. The evolved hydrogen gas volume was measured by recording the water
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displacement in a gas burette (Fig. 1). The reaction was completed when no more gas was
generated. The molar ratio (λ) was then calculated as λ=n(H + N ) / n(N H ) using the total H
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and N volume. Finally, the catalytic decomposition of hydrazine in aqueous solution was
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performed starting with 5 mL of 2.0 mmole hydrous hydrazine solution and 50 mg
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[Co/ZnO@NiFe O ] at various temperatures (25 C, 30 C, 35 C, 40 C and 45 C) to obtain the
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activation energy.
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Fig. 1 The experimental setup for hydrogen generation.
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Recyclability test
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After the hydrogen generation reaction was cultivated, the catalyst was isolated utilizing a
magnet. The black precipitates were washed with DI water and ethanol before the following
reusability test. For the Co/ZnO@NiFe O NPs catalyst recycle stability test, the catalytic
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decomposition reaction was done over the same catalyst for fifteen cycles by adding another
equivalent of hydrazine monohydrate (2 mmol) into the mixture after the previous cycle.
Calculation method
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1) The turnover frequency (TOF) reported in this work is calculated from the equation as follows:
The TOF described here is a superficial TOF value based on the number of (Co NPs) atoms
in the catalysts calculated from equation (3) as follows:
TOF = (P VH /n RTt) …………… (3)
metal
atm
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However, Patm is the atmospheric pressure (101325 Pa), V is the volume of generated hydrogen
gas when the conversion reached 50% of the reaction, R is the universal gas constant (8.3145 m3
-1
-1
Pa mol . K ), T is the reaction temperature (298-318 K), n
is the total number of moles of
metal
(Co) atoms in the catalyst and t is the time when the conversion reached 50% of the reaction.
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2) The relationship between the temperature (k) and the initial reaction rate k (conversion = 50%)
was followed by Arrhenius behavior. The Arrhenius’ reaction rate equation (4) can be written
as follows:
ln k = lnA – E /RT
(4)
a
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Where A is the reaction constant, E is the activation energy, R is the universal gas constant,
a
and T is the temperature (k).
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3) The H selectivity for N H ∙H O (α) can be obtained using the equation (5) as follows:
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휆 ― 1
푛(퐻2 + 푁2)
푛(푁2퐻4)
1
훼 =
[휆 =
(3 ≤ 휆 ≤ 3
)
]
(5)
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The molar ratio (λ) of generated gas to hydrazine, i.e., n(N +H )/n(N H ). Consequently, the
2 2 2 4
hydrogen (H ) selectivity (α) could be calculated.
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3. Result and discussion
Magnetic Co/ZnO@NiFe O NPs characterization
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Magnetic nickel ferrite (NiFe O ) and Co/ZnO@NiFe O Nano particle preparation using co-
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precipitation methods followed by ZnO and Co coating of the resulting Co/ZnO@NiFe O Nano
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particles. Fig. 2 shows the X-ray diffraction pattern of the NiFe O , ZnO, ZnO@NiFe O and
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Co/ZnO@NiFe O synthesized. Bragg reflections in the 2θ range of 10-90 (NiFe O4-18.45,
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30.31,35.75, 37.38, 53.78, 57.40, 63.02 for (111), (220), (311), (222), (422), (511), (440) and ZnO-
31.76, 34.48, 36.29, 47.53, 56.68, 62.94, 66.46, 67.91, 69.10 for (100), (002), (101), (102), (110),
(103), (200), (112), (201), respectively) are assigned to a single NiFe O (JCPDS card no.10-0325)
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and ZnO phase (JCPDS card no.043-0002). One can conclude that the ZnO shell is crystalline and
the magnetic NiFe O NPs are reserved after the ZnO layer. The Nano crystalline nature of
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Co/ZnO@NiFe O NPs prepared using the present method leads to superficial broadening of the
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NP catalyst Bragg peaks. The XRD pattern of ZnO@Fe O NPs shows the same pattern as the
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ZnO sample (ZnO as merged in Fe O ) because ZnO phase could be dominated. The main
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ZnO@Fe O NP diffraction peaks are also observed (as shown in Fig. S1). Table 1 indicated that
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the average crystalline sizes of the particles calculated by half-width of peak using Scherrer’s
equation. Based on the calculation results by Scherers equation, the crystallinite sizes of Fe O and
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ZnO estimated about 6.55 and 21.96 nm from the peak broading of the (311) and (101) peaks,
respectively. The particle size of the NPs catalysts is also estimated from High Resolution
Transmission Electron Microscope (HRTEM) micrographs using Java-based Image J software.
All modified catalysts (NiFe O and Co/ZnO@ NiFe O NPs) exhibit different crystalline sizes
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4
2
4
and particle size are all very small as well.
Table 1 Crystallite size and particle size of NPs catalysts.
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S.No
Samples
ZnO
Crystallite size (nm)
Particle size (nm)
1
2
3
4
5
6
.
.
.
.
.
.
6.55
-
NiFe O
18.34
45.14
21.96
15.64
18.32
7.94
73.53
-
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ZnO@Fe O
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4
Fe O
2
4
Co/ZnO@ NiFe O
5.49
9.17
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Used Co/ZnO@ NiFe O
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ZnO@NiFe O
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ZnO
NiFe O
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 
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Fig. 2 NiFe O , ZnO, ZnO@NiFe O and Co/ZnO@NiFe O NPs catalyst XRD patterns.
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Fig. 3 shows TEM images of Co/ZnO@NiFe O NPs with a cobalt load of 2.0 wt% taken with
2 4
different magnifications and the TEM-EDX spectrum taken from the lattice fringes (0.201 nm) of
Co/ZnO@NiFe O NPs in Fig. 3a). The mean Co/ZnO@NiFe O NPs particle size was established
2
4
2
4
at 5.49 nm, respectively (Fig. S2). As presented in Fig. 3b, c, an observable lattice fringe is
observed in Co/ZnO@NiFe O NPs. The lattice fringe of pure Co NPs was measured to be ~0.221
2
4
nm, which located between the Co, indicating that Co is in a crystalline alloy state. TEM-EDX
spectrum of Co/ZnO@NiFe O NPs with a cobalt load of 2.0 wt% indicating that cobalt is the only
2
4
element detected in the sample in addition to the Co/ZnO@NiFe O NPs (Co, O, Ni, Fe, Zn)
2
4
1
2

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2
2
2
41
42
43
framework elements (Fig. 3d). Furthermore, it is well known that the Co NP method structure can
improve the NP catalyst electronic conductivity. In order to further verify the spreading of an
element in Co/ZnO@NiFe O NPs, element mapping for the TEM images are shown in Fig. S3.
2
4
2
44
2
2
2
2
45
46
47
48
Fig. 3 TEM images of Co/ZnO@NiFe O NPs with the scale bar of (a) 50 nm, (b) 5 nm and TEM
2 4
images of Co/ZnO@NiFe O NPs with a Co loading of 2.0 wt%, (c) show the corresponding size
2
4
distribution and SAED patterns (d) TEM-EDX spectrum of Co/ZnO@NiFe O NPs.
2
4
1
3

Catalysis Science & Technology
Page 14 of 38
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2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
49
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60
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64
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68
69
70
The surface morphology of catalyst is one of the important parameters that affect the hydrogen
geration efficiency. The NPs catalysts were investigated by FE-SEM image as shown in Fig. S4
The FE-SEM images shows that the ZnO and Co/ZnO@NiFe O NPs were formed in a very
2
4
uniform manner in the form of cubes and a few of them appeared spherically. In order to better
discern the composition of the sample, elemental mapping analysis has been done. Figure S6
(C)~(F) illustrates the Energy-dispersive X-ray spectroscopy (EDX) mapping analysis of Co, Ni,
C, Fe, and O for the Co/ZnO@NiFe O NPs sample. The results confirm the presence of all these
2
4
elements in the sample which distributed quasi uniformly in the studied section. This result
demonstrates the successful preparation of the powder sample. The results specify that the particles
contain Co, Zn, Fe, Ni and O elements, and the elements consistently isolated in the material. The
high-angle annular dark-field scanning TEM (HAADF-STEM) measurement revealed well-
dispersed ultrafine Co NPs on the substrate (Fig. 4a). The elemental chemical mappings (Fig. 4b-
e) show that the Co, Zn, Fe and Ni elements were uniformly distributed in the Co/ZnO@NiFe O4
2
NPs catalysts. Meanwhile, the presence of the Co, Zn, Fe, Ni and O peaks as displayed in Fig. 4f
signals that it showed the successful functionalization of catalyst. Taking into consideration the EDS
and XPS calculation results, it could be clearly proved Co is doped in the Co/ZnO@NiFe O NPs. It
2
4
is further demonstrated that the materials contain detached; NiFe O NPs catalysts with 7.94 nm
2
4
(Fig. S5a, c). The particles are generally spherical in shape and demonstrate small size distribution.
Images of inaccessible NPs at high magnification (HRTEM, Fig. S5d), further reproduce the
crystallinity and phase transparency of the as-synthesized NiFe O NPs. The well-defined lattice
2
4
fringes of 0.210 nm Nano crystal specify good crystallinity (Fig. S5d). In addition, the chemical
composition was obtained using energy-dispersive X-ray spectroscopy (EDX, Fig. S5e): the peak
1
4

Page 15 of 38
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2
2
71
72
ratio is in good arrangement with the probable elemental composition. The results specify that the
particles contain Fe, Ni and O elements, and the elements consistent isolated in the material.
2
2
73
74
Fig. 4 HAADF-STEM image (a), elemental mappings (b-e) and EDX spectrum (f) of the
2
2
75
76
Co/ZnO@NiFe O catalyst.
2
4
1
5

Catalysis Science & Technology
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2
2
77
78
Fig. 5 (a) X-ray photoelectron (XPS) spectrum of Co/ZnO@NiFe O NPs. (a) Co 2p , (b) Fe
2
4
3/2
2
2
2
79
80
81
2p , (c) Ni 2p , (d) Zn 2p and (e) O1s. All of these samples are analyzed after Ar sputtering
3/2 3/2 3/2
for 50s.
1
6

Page 17 of 38
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DOI: 10.1039/D0CY01829E
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
00
01
02
To comprehend the valence state and Co/ZnO@NiFe O NPs catalyst surface structure, an X-ray
2 4
photoelectron spectroscopy (XPS) examination was completed. The Co is the main element
distinguished notwithstanding the ZnO@NiFe O NPs catalyst elements (Zn, Ni, Fe, O). To
2
4
examine the effect of presenting Co on the valence and electronic states of the catalyst, the XPS
measurements were subsequently completed. The total XPS survey spectrum specified the
presence of Co, Zn, Ni, Fe and O (Fig. S7). Two peaks at 781.27 eV and 797.48 eV with an energy
3
+
difference of 16.21 eV were attributed to Co 2p and 2p of Co in the Co/ZnO@NiFe O NPs
3
/2
1/2
2
4
(Fig. 5a); As shown in Fig. 5b, in the Fe 2p region, the two main peaks at BE of 709.79 eV and
723.91 eV correspond to Fe 2p and Fe 2p spin-orbit peaks (difference 14.13 eV), respectively.
3
/2
1/2
2
+
3+
Due to the small chemical shift between Fe and Fe , the Fe 2p peaks are relatively broad.
0
Moreover, a small peak centered at ~708 eV confirms the existence of zero-valent Fe . The Ni 2p
XPS spectrum (Fig. 5c) demonstrates two peaks centered at 855.07 eV and 872.76 eV (difference
2
+
17.69 eV), which can be accepted to metallic Ni and Ni , respectively. Meanwhile, the peak at
2
+
862.25 eV is the satellite peak for Ni , while the peaks at 1022.28 and 1045.61 eV were typical
2
+
XPS peaks for the Zn ion (difference 23.3 eV) (Fig. 5d). The binding O 1S spectrum in Fig. 5e
has two peaks at 530.73 eV and 529.25 eV (difference 1.48 eV), corresponding to Fe-OH, Fe-O
and Zn-O, respectively (Table S1). The peaks lower binding energy of 529.25 eV corresponding
to Zn-O, probably to oxygen on a split interstitial configuration or oxide near Zn vacancies (Fig.
5e). The results are a good agreement. Oxidized Zn was inactive for the decomposition of aqueous
hydrazine, therefore the Co, Ni and Fe metal is mainly active center in the present reaction.
3
3
03
04
1
7

Catalysis Science & Technology
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3
05
Catalytic activities for decomposition of hydrazine in aqueous solution
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
In advance of experimenting with Co/ZnO@NiFe O NPs catalytic activity, a control
2 4
investigation is necessary to check whether the support material catalyzes hydrazine monohydrate.
For this determination, we achieved hydrous hydrazine decomposition in the presence of
ZnO@NiFe O at 25 °C and found that the host catalyst is catalytically less active in
2
4
decomposition of hydrazine. The catalytic effect of the prepared catalysts (such as
Co/ZnO@NiFe O , ZnO@NiFe O , ZnO@Fe O , ZnO, and NiFe O NPs), the physical mixture
2
4
2
4
2
4
2
4
of Co and ZnO@NiFe O and supported ZnO supported Co NPs has been investigated for
2
4
o
hydrogen generation from decomposition of hydrazine at 25 C. As shown in Fig. 6, no effective
hydrogen generation during 13 min test was observed for ZnO, indicating that ZnO is inactive for
decomposition of hydrazine. Both NiFe O , ZnO and Fe O show poor catalytic performance but
2
4
2
4
ZnO@NiFe O activity is better than that of NiFe O . Co/Zno@NiFe O NPs exhibit excellent
2
4
2
4
2
4
catalytic performance to release a stoichiometric amount of hydrogen from hydrazine in 3.5 min
-1
with turnover frequency (TOF) value of 4445.37 h relatively high values for Co-based catalysts
for hydrogen generation from decomposition of hydrazine. Hydrogen generation begins
immediately without the induction period and proceeds until the complete consumption of
hydrazine monohydrate. This perception shows that the reduction of cobalt (III) ions to the metallic
Co on the ZnO@NiFe O NP surface is fast. Compared to the corresponding ZnO@NiFe O NPs,
2
4
2
4
the Co NP and ZnO@NiFe O physical mixture shows less catalytic activity toward this reaction,
2
4
indicating that Co/ZnO@NiFe O NP fabrication is important for obtaining a suitable environment
2
4
around the Co NPs for effective hydrazine dehydrogenation catalysts. The optimal amount of
Co/ZnO@NiFe O NPs solid solutions is 50 mg (Fig. S9). The continuous increase in
2
4
Co/ZnO@NiFe O NPs would lead to catalytic activity conquest, possibly because the active Co
2
4
1
8

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3
3
3
3
3
3
3
3
3
3
28
29
30
31
32
33
34
35
36
37
38
NP sites on a superficial level are probably going to be blocked as well as secured by the Nano
ZnO@NiFe O NPs solid solution abundance. In this research, the metallic active sites were
2
4
provided by Co/ZnO@NiFe O catalysts. It implied that the tested metals could be presented as
2
4
metal oxides due to the calcination controlled in air atmosphere. There are some factors could be
influenced on the active sites and catalytic activity of Co/ZnO@NiFe O catalysts, such as metallic
2
4
Co, Ni and Fe proportions, catalyst preparation method, reduction temperature, and the nature of
support. The active sites and catalytic activity will affect the performances of the catalyst in the
hydrogen generation of aqueous hydrazine decomposition. Meanwhile, the structure of the Co
nanoparticle is core-shell resulting in a good catalytic activity. To further discussion of catalytic
metallic activity, it is crucial point to understand the specific role of the spiked metal characteristics
and structure in determining the overall catalytic activity.
3
3
3
3
3
3
3
3
3
3
3
3
39
40
41
42
43
44
45
46
47
48
49
50
The Co/ZnO@NiFe O NPs catalyst with Co load of 2.0 wt% (Fig. 7a) shows the greatest
2
4
catalytic effect for hydrogen generation among all the Co/ZnO@NiFe O NPs catalysts with
2
4
different Co weight. Such a rare consequence from the metal-support may be qualified for
synergetic interaction between Co and ZnO@NiFe O . Synergetic interaction between Co and
2
4
ZnO@NiFe O presumably results from the electron-rich environment of the Co starts from the
2
4
facile electron transfer from catalytic Co support, which leads to the developed Co/ZnO@NiFe O
2
4
NP catalytic activity associated with the equivalent mono counterparts. Further experimental work
is necessary to illuminate the comprehensive mechanism. In conclusion, hydrogen generation from
o
hydrazine monohydrate decomposition was considered at room temperature (25 C) using
-1
ZnO@NiFe O supported Co NPs. Co/ZnO@NiFe O NPs provided a TOF value of 4445.37 h
2
4
2
4
(Fig. 7b) with an activation energy value of 44.84 kJ/mole. The as-synthesized Co/ZnO@NiFe O4
2
NPs show acceptable durable stability and magnetically recyclability for the decomposition of
1
9

Catalysis Science & Technology
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3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
hydrazine via 15 recycle stability test. The number of metallic active sites on the basic carrier can
not only improve the durability of the catalyst, but also improve the selectivity of hydrogen.
Compared with the corresponding single-metal, nanoalloys have the advantages have the
advantages of reconfigurable electronic structure, variable composition ratio, which show unique
properties different from bulk alloy and metal elements. Because of the nano-size effect and a large
number of kinks, the nano-alloy obtains higher specific surface energy and rich metallic active
sites through electronic configuration reorganization, which greatly promotes its catalytic
performance. The optimal catalytic properties were obtained in the catalyst with a nominal alloy
composition of Co/ZnO@NiFe O NPs. The prepared catalyst compositions are included 0.026
2
4
mmol of Ni, 0.076 mmol of Fe, and 0.052 mmol of Co are determined by ICP-OES.
In order to further evaluate the Co/ZnO@NiFe O NPs pre-catalyst performance, an initial TOF
2
4
-1
value was as high as 4445.37 h for the decomposition of hydrazine, due to the interfacial
interaction between active metal NPs and supporter. Moreover, the prepared catalyst performed
the first in situ experiments to study the electronic structure changes of the pre-catalyst during
decomposition. With a metal oxide and metal NPs as a pre-catalyst, the catalytically active
component of the catalyst is the zero-valent metal. This is because a part of the metal oxide can be
reduced by hydrazine.17,19,29,43 Consequently, such metal oxide catalysts are mainly metal-based
oxides, because metal oxides can be easily reduced to metal(0) by hydrazine at room temperature
(25 ℃ ). Co/ZnO@NiFe O is a promising catalyst for hydrogen generation from aqueous
2
4
3
3
70
71
hydrazine and for developing a greatly efficient hydrogen storage method for fuel cell applications.
2
0

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3.0
2.5
2.0
1.5
1.0
0.5
0.0
NiFe O4
2
ZnO@NiFe O4
2
Co/ZnO@NiFe O4
2
2
ZnO@Fe O4
ZnO
0
2
4
6
8
10
12
14
Time (min)
3
72
o
3
3
3
73
74
75
Fig. 6 Hydrogen generation from hydrous hydrazine decomposition (N H = 2 mmole, T =25 C)
2 4
in the presence of as-synthesized NiFe O , ZnO, Co/ZnO@NiFe O , ZnO@NiFe O and
2
4
2
4
2
4
ZnO@Fe O . (Co+Ni+Fe/N H = 0.02).
2
4
2
4
2
1

Catalysis Science & Technology
Page 22 of 38
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(a)
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
Co Wt %
0
1
1
2
2
.5
.0
.5
.0
.5
0
2
4
6
8
10
12
14
16
Time (min)
3
76
4
4
3
3
2
2
1
1
500
000
500
000
500
000
500
000
(b)
100
9
9
5
0
85
8
7
7
6
6
5
0
5
0
5
0
5
500
0
50
0.5
1.0
1.5
Different wt% of Co
2.0
2.5
3
3
3
3
77
78
79
80
Fig. 7 (a) Hydrogen generation from hydrous hydrazine decomposition (2 mmol, 5 mL) by the
Co/ZnO@NiFe O NPs with different Co load (0.50, 1.0, 1.5, 2.0, 2.5 wt%) and (b) TOF values
2
4
of the catalyst with corresponding cobalt load.
3
81
2
2

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3
3
3
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4
82
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86
87
88
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90
91
92
93
94
95
96
97
98
99
00
The great dispersal of fine Co NPs and their synergistic combination with ZnO@NiFe O NPs, in
2 4
the presence of NaOH induces important properties in the development of catalytic effect. To
characterize the NaOH quantity effect on the catalytic performance. The hydrolytic decomposition
reactions of Co/ZnO@NiFe O NPs with various amounts of NaOH (1-10 mole) were considered
2
4
(Fig. 8a). It has been established that hydrazine decomposition catalytic activity increases with the
amount of NaOH until the value reaches 7 mole. Any additional increase in the amount of NaOH
has no effect on the catalyst performance. Catalytic performance can be significantly decreased by
adding excessive NaOH in the catalytic process. Surprisingly, it was displayed that complete
hydrous hydrazine decomposition with 100% hydrogen selectivity was achieved within 3.5 min,
-1
corresponding to a TOF value of 4445.37 h (Fig. 8b). According to the analysis results, it can be
concluded that the catalytic performance of the catalyst prepared with NaOH was superior to that
of the one prepared with NaOH, although an equal amount of NaOH was added in the catalytic
process, indicating that NaOH affects not only the catalytic process but also the synthesis of the
catalyst. The conceivable reason for the NaOH improvement effects are described underneath: the
+
+
-
addition of NaOH could proficiently decrease the attention of unattractive N H (N H + OH →
2
5
2
5
N H + H O) and support the rate responsible deprotonation step (N H → N H * + H*). These
2
4
2
2
4
2
3
were critical steps for explaining the NaOH effect that it could assist the final removal of N-H
4
4-46
bonds to produce H and suppress NH byproduct production as well.
2
3
2
3

Catalysis Science & Technology
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(a)
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
1
3
5
7
1
M
M
M
M
0 M
0
3
6
9
12
15
18
21
Time (min)
4
01
4
4
3
3
2
2
1
1
500
000
500
000
500
000
500
000
(b)
100
9
5
0
9
85
8
7
7
6
6
5
0
5
0
5
0
5
5
00
0
50
0
1
2
3
4
5
6
7
8
9
10 11
Mole of NaOH
4
4
02
03
Fig. 8 (a) Time course plots for hydrogen generation from decomposition of N H in aqueous
2
4
4
4
4
04
05
06
solution and (b) the corresponding H selectivity and TOF value over the Co/ZnO@NiFe O NPs
2 2 4
o
with different concentrations of NaOH at 25 C (Co+Ni+Fe/N H = 0.02).
2
4
2
4

Page 25 of 38
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4
4
4
4
4
4
4
4
4
4
4
4
07
08
09
10
11
12
13
14
15
16
17
18
Temperature effect on the decomposition reaction
The gas generation reaction rate finished the Co/ZnO@NiFe O NPs catalyst significantly
2
4
depending on the temperature. In another set of experiments gas generation from catalytic
decomposition of aqueous hydrazine was conducted at different temperatures ranging from 25 to
o
45 C. The gas generation rate increases with increasing temperature, as estimated. The catalytic
o
reactions are completed in 3.5, 3.0, 2.52, 2.0 and 1.4 min at 25, 30, 35, 40 and 45 C, respectively
(Fig. 9a), which resemble the TOF values of 4445.37, 5712.26, 6133.96, 8291.73, and 11159.14
-1
h (Fig. 9b). The Arrhenius plot of lnk vs. 1/T for the catalyst is shown in Fig. 9c, the basis for the
hydrazine monohydrate decomposition TOF values at different temperatures. Activation energy
(Ea) of 44.84 kJ/mol is provided, which is lower than the previously described values for hydrazine
monohydrate (Table S2).
(a)
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
o
2
3
3
4
4
5 C
o
0 C
o
5 C
o
0 C
o
5 C
0
1
2
3
4
5
Time (min)
4
19
2
5

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1
2000
0000
(b)
100
9
9
8
8
7
7
6
6
5
5
1
0
5
0
5
0
5
0
5
8
6
4
2
000
000
000
000
0
50
2
5
30
35
Temperature ( C)
40
45
o
4
20
Equation
Weight
y = a + b*x
No Weighting
(c)
4
.2
Residual Sum of 0.00601
Squares
Pearson's r
-0.99634
0.99027
Adj. R-Square
Value
Standard Error
4
3
3
3
3
3
.0
.8
.6
.4
.2
.0
Intercept
Slope
21.01952
0.86645
C
-5393.04481
267.02192
Ea=44.84 kJ/mol
0
.00310
0.00315
0.00320
0.00325
0.00330
0.00335
-1
1
/T (k )
4
4
21
22
Fig. 9 Time course plots for hydrogen generation and the corresponding TOF values from an
4
4
23
24
aqueous solution of (a,b) N H and in the presence of NaOH (7 M) at different temperatures, (c)
2
4
corresponding Arrhenius plots.
2
6

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4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Hydrogen generation from hydrazine mechanism
The catalyst surface shows the complete N H to N and H decomposition (pathway-I (supportive
2
4
2
2
information)) is more desired for useful applications. To recognize this, the N-N and N-H bond
cleavage arrangement should be achieved. As shown in Fig. 10, it is suggested that the primary
stage for complete N H decomposition is the coordination of N H molecules with the catalyst
2
4
2
4
surface.47-49 The second stage is N-H bond cleavage as opposed to N-N bond cleavage. From the
thermodynamic perspective the N-N bond cleavage is simpler in contrast with N-H bond cleavage
because of its lower bond cleavage energy (286 kJ/mol for N-N bond and 360 kJ/mol for N-H
bond) which will prompt the N and NH arrangement (equation-2). The catalyst assumes a key
2
3
characteristic to bring down the N-H cleavage energy barrier. Previous researchers specified that
strong metal-H bonds (M-H) can encourage N-H bond cleavage. For this specimen, since the M-
H bond is stronger than M-N, the N-H cleavage energy carrier becomes lower than that of the N-
N fraction.50 The designed M-H, and H N-NH , HN-NH , H-N=N-H and N=N-H become
2
2
2
reinforced with the metal surface central to N and H generation when the N-H bond fraction is
2
2
exclusively actuated.
2
7

Catalysis Science & Technology
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4
4
41
42
Fig. 10 Systematic decomposition of hydrazine monohydrate into H and N over a catalyst
2
2
4
4
4
4
4
4
4
4
4
43
44
45
46
47
48
49
50
51
surface.
Stability and Reusability of Magnetically Separable Co/ZnO@NiFe O NPs Catalysts in
2
4
hydrous hydrazine decomposition
Catalyst recycling stability is essential for its primary application. The advantage of our novel
Co/ZnO@NiFe O NPs catalyst lies in the comfort of it being magnetically separated from the
2
4
reaction mixture and washed with DI water and ethanol for recycling (Co/ZnO@NiFe O NPs)
2
4
(Fig. 11). This procedure was repeated 15 times, which demonstrates practical Co/ZnO@NiFe O4
2
NP recycling application. A reusability test shown in Fig. 12a demonstrates that the
2
8

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DOI: 10.1039/D0CY01829E
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Co/ZnO@NiFe O NPs catalysts still exhibit high catalytic performance and maintain 100% of
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their initial catalytic activity in hydrazine monohydrate decomposition for 15 cycles. The TOF
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values of 4445.37, 4144.73, 3883.17, 3651.79, 3446.42, 3262.93, 3097.99 and 2949.49 h were
developed in different cycles, respectively (Fig. 12b).
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Characterization of the sequestered catalyst from the 7 and 15 catalytic run using XRD reveals
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that the crystallinity of the magnetic Co/ZnO@NiFe O NPs is reserved at the end of the 7 and
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15 catalytic runs. XRD established the seventh and fifteen cyclic usage showing no significant
change in the catalyst phase structure (as shown in Fig. S12). As obviously perceived from the
TEM image in Fig. S13a, there is no obvious aggregation of the Co NPs on ZnO@Fe O NPs,
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representing that the ZnO@NiFe O NPs can stabilize the Co NPs during the reaction.
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Consequently, the main purpose for better durability can be attributed to the high dissolution of
Co NPs on the ZnO@NiFe O NP surface. As shown in Fig. S13b, c, an observable lattice fringe
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is observed in Co/ZnO@NiFe O NPs. The lattice fringe of pure Co NPs was measured at
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0.274~0.313 nm, located between the Co, representing that Co is crystalline in nature. Fig. S13d
shows the TEM-EDX spectra Co/ZnO@NiFe O NPs. The EDX spectral peaks of Co, Zn, Fe, Ni
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and O, respectively. The contrast line profile is plotted under the corresponding image (Fig. S13e)
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and the Co/ZnO@NiFe O NPs composition after the 15 recycling run, signifying that the
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Co/ZnO@NiFe O NPs structure remains unaffected after consumption. The average d-spacing
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distance is measured at 0.274~0.313 nm. The insignificant decrease in the Co NP activity observed
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in the 15 catalytic run is recognized as the increase in mean size of the Co NPs from 5.49 nm
(fresh catalyst) to 9.17 nm (used catalyst) (Fig.S14). Therefore, the Co/ZnO@NiFe O NPs
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catalyst exhibits extremely high stability and durability toward hydrazine decomposition. We
concluded that the perceived activity loss strength occurs from the destruction of active sites by
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