Noble Metal-free Bimetallic Cobalt/Manganese Oxide Catalyst for Hydrogen Generation by Decomposition of Hydrous Hydrazine

Article abstract of DOI:10.1002/bkcs.11887

Bimetallic cobalt and manganese oxide (Co/MnO) is prepared by annealing the spinel structure of CoMn₂O₄ under hydrogen environment. This noble metal-free catalyst is applied for hydrogen generation from aqueous hydrazine solution. X-ray powder diffraction, transmission electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy results indicated that the phase separation of metallic cobalt and manganese monoxide from CoMn₂O₄ occurs during annealing process. Co/MnO exhibits catalytic activity with the TOF value of 14.2 h^?1 and 100% selectivity without generation of ammonia at 343 K. It is found that metallic cobalt plays role on the active site for hydrogen generation.

Full text of DOI:10.1002/bkcs.11887

Article
DOI: 10.1002/bkcs.11887
BULLETIN OF THE
Y. Kim and K.-Y. Kwon
KOREAN CHEMICAL SOCIETY
Noble Metal-free Bimetallic Cobalt/Manganese Oxide Catalyst for
Hydrogen Generation by Decomposition of Hydrous Hydrazine
*
Youngyong Kim and Ki-Young Kwon
Department of Chemistry, Gyeongsang National University and RINS, Jinju-si 52828, South Korea.
*E-mail: kykwon@gnu.ac.kr
Received September 4, 2019, Accepted September 18, 2019
Bimetallic cobalt and manganese oxide (Co/MnO) is prepared by annealing the spinel structure of
CoMn2O4 under hydrogen environment. This noble metal-free catalyst is applied for hydrogen generation
from aqueous hydrazine solution. X-ray powder diffraction, transmission electron microscopy, energy-
dispersive spectroscopy, and X-ray photoelectron spectroscopy results indicated that the phase separation
of metallic cobalt and manganese monoxide from CoMn2O4 occurs during annealing process. Co/MnO
exhibits catalytic activity with the TOF value of 14.2 h−1 and 100% selectivity without generation of
ammonia at 343 K. It is found that metallic cobalt plays role on the active site for hydrogen generation.
Keywords: Bimetallic catalyst, Cobalt, Manganese, Hydrogen, Hydrazine
Introduction
separated metallic cobalt and manganese oxide (Co/MnO).
It is noteworthy that our sample is a platinum-group metals
free catalyst for hydrogen generation from aqueous hydra-
zine. For comparison, we prepared two additional catalysts
which are metallic cobalt and manganese oxide (MnO),
respectively. We found that catalytic activity originated
from metallic cobalt and MnO acts as a catalytic support of
metallic cobalt. Our bimetallic catalyst (Co/MnO) shows
higher catalytic activity than monometallic cobalt catalyst.
We propose that the improved catalytic property is come
from the synergistic electric effect between metallic Co and
MnO involved in charge transfer steps.
Hydrogen gas is considered as a clean energy source due
to no generation of greenhouse gases during combustion
process.1,2 Besides the development of economic and easy
hydrogen generation, advanced hydrogen storage materials
is essential for implementing hydrogen economy.3 So far,
the liquid hydrogen storage materials are attracting lots of
interest due to safe storing under ambient conditions and
easy transportation using the existing fuel infrastructure.4,5
Among these candidates, hydrous hydrazine (N2H4ꢀH2O)
is considered as a promising chemical owing to the high
content of hydrogen (8.0 wt %) and the advantage of
COx-free H2 production.6 Hydrazine monohydrate can be
decomposed via two pathways and generate different
products.7,8
Experimental
Synthesis of Spinel CoMn2O4 and Co/MnO. CoMn2O4
was synthesized by a one-step hydrothermal method. An
aqueous 10 mL of 10 M NaOH was added into 50 mL of
aqueous solution containing Co(NO3)2ꢀ6H2O (1 mmol) and
Mn(ClO4)2 (2 mmol). After stirring above solution at room
temperature for 10 min, the mixture was transferred into a
N2H4 ! N2 + 2H2
3N2H4 ! 4NH3 + N2
ð1Þ
ð2Þ
To maximize hydrogen generation from hydrazine, the
reaction pathway should follow the pathway (1), whereas
the unwanted pathway (2) must be avoided. The rare transi-
tion metal based catalysts such as Ir,9 Pt,10,11 Rh12,13 have
shown the high catalytic activity for hydrazine decomposi-
tion in aqueous solution. Previous research efforts have led
to the development of bimetallic catalysts, which are pri-
marily composed of noble metal and non-noble transition
metal.10,14–16 For examples, CoIr-supported γ-Al2O3 (TOF:
ꢁ
Teflon-lined autoclave reactor and kept at 160 C for 6 h.
Black-green precipitates were collected by centrifugation
and washed three times with DI water followed by drying
the sample under vacuum for overnight. Finally, the metal-
lic cobalt and manganese monoxide (Co/MnO) was
ꢁ
obtained by annealing CoMn2O4 at 500 C for 3 h under
hydrogen gas condition (4% H2, 96% Ar). For comparison,
monometallic Co, and MnO were prepared by the same
procedures as mentioned above by adding one metal pre-
cursor [either Co(NO3)2ꢀ6H2O or Mn(ClO4)2].
27.76 h−1 at 25 ꢁC),15 CoPt-supported CeO (TOF:
194.8 h−1 at 25 C under 0.5 M NaOH condition), NiPt-
14
ꢁ
MnOx supported on NPC-900 (TOF: 120 h−1 at room tem-
perature)17 were reported. Here, we reported a cobalt and
manganese bimetallic catalyst composed of the phase
Characterization.
X-ray diffraction patterns were
recorded using a Bruker D8 Advance X-ray diffractometer
(Billerica, MA, USA), with Cu Kα radiation. The X-ray
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photoelectron spectroscopy (XPS) results were obtained on
a X-ray photoelectron spectrometer K-Alpha (Thermo
Scientific, Tokyo, Japan) in which monochromated Al Kα
radiation (hν = 1486.6 eV) is illuminated as the light
source. The X-ray anode was run at 25 W and the voltage
was maintained at 15 kV. The pass energy was fixed at
23.5 eV to ensure sufficient resolution to determine peak
positions accurately. The binding energies were calibrated
by using the C 1 s peak at 284.8 eV. The morphologies of
samples were characterized by transmission electron micros-
copy (TEM, Tecnai G2 F20S-TWIN, FEI, USA). The com-
position of the sample was analyzed by energy-dispersive
spectroscopy (EDS) fitted on the TEM. ICP-AES was per-
formed to determine the composition of Co/MnO catalyst.
Catalytic Tests. The catalytic dehydrogenation from
hydrazine monohydrate was tested in a two-necked round-
bottom test tube, where one neck was connected to a gas
burette to measure the volume of the released gas, other
neck was used to inject hydrazine monohydrate. The reac-
ꢁ
tion temperature (70 C) was controlled using water bath.
The reaction was initiated by injecting 5 mL of N2H4ꢀH2O
solution (0.12 mol/L) into the test tube containing 10 mg of
catalyst. The volume of generated gas was measured using
a gas barrette after either passing or bypassing a 1.0 M HCl
trap. Hydrazine conversion (XN2H4) was calculated by ratio
of the generated volume of the gas to the theoretical total
volume of gas if the hydrazine is totally decomposed to N2
and H2. Turnover frequency (TOF) is calculated at 50%
hydrazine conversion as following equation.15
Figure 1. XRD patterns of CoMn2O4 (a) and Co/MnO (b).
shift toward approximately 0.4–0.6 higher angle. The shift of
diffraction angles to positive value and decreased lattice
parameters of MnO was previously reported when Mn2+ ions
[rion (Mn2+) = 0.83 Å] are substituted to smaller Co2+ ions
[rion(Co2+) = 0.745 Å].23 Therefore, we think that the small
amounts of Co2+ ions still remain in MnO during phase segre-
gation process from the spinel structure of CoMn2O4 into
metallic cobalt and MnO.
TOF = ðPV=RTÞ=ð3nMetal tÞ
Here, P is the atmospheric pressure, V is the volume of
generated gases (H2 + N2), R is universal gas constant, T is
the reaction temperature, nMetal is the mole number of metal
in catalyst, and t is the reaction time for the 50% conver-
sion of hydrazine monohydrate.
Figure 2 showed the XPS spectra to characterize the
chemical nature of the Co, Mn, and O and investigate the
effect of hydrogen treatment of the sample in annealing
process. The Co 2p spectra of CoMn2O4 can deconvoluted
by six species composing two pairs of spin-orbital doublets
characteristic of Co2+ and Co3+ and two shake-up satellite,
which are denoted “sat” (Figure 2(a)). Two Co 2p3/2 and
Co 2p1/2 peaks located at around 780.6 and 796.2 eV are
accompanied by two major shake-up satellite peaks (around
786.6 and 803 eV) in Figure 2(a) and (b), suggesting the
abundant presence of the Co2+ in CoMn2O4 and Co/MnO.
Particularly, the intensity of satellite peaks of Co/MnO is
higher than that of CoMn2O4. It was reported that the
enhancement satellite intensity around 786 eV indicates the
presence of Co with the oxidation state of +2 24. Therefore,
we believe that the cobalt atoms on the surface of Co/MnO
are mostly oxidized to Co2+ forming thin CoO shell at
ambient condition although XRD patterns showed only the
presence of zerovalent oxidation state.25 The Mn 2p XPS
Results and Discussion
Structural Characteristics. Figure 1 showed the X-ray
powder diffraction (XRD) patterns of the precipitates before
and after annealing. For the precipitates before annealing, the
dominant spinel structure of CoMn2O4 (JCPDS No.01-077-
0471)18,19 as well as small amount of CoO(OH) (JCPDS
No.01-072-2280) was presented. It was known that the
CoO(OH) can be formed under the strong aqueous basic solu-
tion.20,21 After being annealed under hydrogen gas environ-
ment (4% H2/96% Ar) at 500 ꢁC for 3 h, the XRD patterns of
the CoMn2O4 was dramatically changed into a simplified form
which assigned to MnO (JCPDS No.00-007-0230) and metal-
lic Co (JCPDS No.00-001-1255) (Figure 1(b)). This indicates
that the mixed phases containing CoO(OH), and CoMn2O4
reduced to metallic Co and MnO under hydrogen treatment at
500 ꢁC. While diffraction angles of metallic cobalt are exactly
matched with literature data,22 the two theta values of MnO
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Co/MnO Catalyst for Hydrogen Generation by Decomposition Hydrazine
KOREAN CHEMICAL SOCIETY
Figure 2. X-ray photoelectron spectra for the CoMn2O4 and
Co/MnO. (a) Co 2p XPS spectrum of the CoMn2O4, (b) Co 2p
XPS spectrum of the co/MnO, (c) Mn 2p XPS spectrum of the
CoMn2O4, and (d) Mn 2p XPS spectrum of the Co/MnO.
spectrum of CoMn2O4 exhibits two major peaks at binding
energies of 642.0 and 653.8 eV which correspond to Mn
2p1/2 and Mn 2p3/2, respectively. After being annealed, the
two signals of Mn 2p3/2 and 2p1/2 centered at 641.6 and
653.2 eV which correspond to the shift to approximately
0.4–0.6 lower energy. These result suggested that Mn with
oxidation state of 3+/4+ reduced to Mn2+ during an
annealing process under hydrogen condition.26 In addition,
the spin energy separation between Mn 2p1/2 and Mn 2p3/2
is 11.6 eV. This value is commonly referred to represent
the oxidation state of Mn2+.27
Figure 3. TEM image and EDS element mapping images of the
CoMn2O4 and the Co/MnO. The HAADF image of (a) CoMn2O4,
and EDS maps of the (b) cobalt (red), (c) manganese (purple),
(d) oxygen (green), and (e) overlapping of Co, Mn, and O, TEM
images of (f) Co/MnO, (g) TEM image of boxed region in (f) and
corresponding EDS maps of the (h) cobalt (red), (i) manganese
(purple), (j) oxygen (green), and (k) overlapping of Co,
Mn, and O.
TEM and EDX experiments (Figure 3) were applied to
characterize the morphology and elemental composition of
the CoMn2O4 and Co/MnO. Figure 3(a) represents the
TEM image of CoMn2O4, which is rhombus shape.
EDX data of CoMn2O4 (Figure 3(a)–(e)) indicates that
Mn and Co are well dispersed in the entire area of
CoMn2O4 crystallites. After annealing the CoMn2O4 at 500
ꢁC for 3 h under hydrogen atmosphere, the rhombus shape
of CoMn2O4 was changed into formless morphology which
is composed of round shape darker particles and less darker
framework (Figure 3(f)). Importantly, EDS analyses dem-
onstrated that cobalt is abundant in round shape particles,
and manganese is abundant in shapeless framework.
Figure 3(g) shows the enlarged TEM image of the area
marked by the red square in Figure 3(b). Figure 3(h)–
(k) represents the elemental mapping images of Co/MnO.
While Co is mainly located in the particle (in comparison
with Figure 3(i)), Mn is placed in the framework (Figure 3
(i)). Therefore, we assigned that the particle and framework
correspond to metallic cobalt and MnO, respectively. It
should be noticed that there is certain amount of cobalt in
framework although majority amount of cobalt exist in par-
ticle. EDS Quantification indicated that the ratio of cobalt
to manganese is approximately 10%. The presence of
cobalt in framework (assigned to MnO) well matched with
the XRD pattern shift of MnO toward higher angle which
originated from incorporation of smaller size of cobalt ion
than that of manganese ion. Oxygen is distributed in both
the particle and framework (Figure 3(j)). Particularly, in
particle, oxygen is sparse in the core area while dense in
the outer area. This result seems to be the oxidation of the
surface of metallic cobalt in the air. This surface oxidation
phenomena of cobalt is supported by the presence of Co2+
in XPS data shown in Figure 2. However, the presence of
any cobalt oxide such as CoO does not appear in XRD data
(Figure 1). Therefore, we concluded that the thickness of
oxide layers in cobalt nanoparticles is very thin or the oxide
layers exhibit amorphous phase. In framework, oxygen and
cobalt are evenly distributed in entire area indicating the
presence of MnO. Figure 3(k) represent the elemental map-
ping with Co, Mn, and O overlaid. We clearly reconfirmed
the presence of oxide layer on the outer shell of cobalt par-
ticles (Figure 3(k)). From these results, we conclude that
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Co/MnO Catalyst for Hydrogen Generation by Decomposition Hydrazine
KOREAN CHEMICAL SOCIETY
metallic cobalt and MnO containing 10% cobalt are formed
from mainly the spinel structures of CoMn2O4 through
annealing process under hydrogen environment.
Catalytic Performances. Hydrazine decomposition experi-
ments were carried out to confirm the catalytic activities for
hydrogen generation. Figure 4(a) and (b) exhibits the hydra-
zine conversion (XN2H4) as a function of reaction time using
bimetallic catalysts (CoMn2O4, Co/MnO) and monometallic
catalysts (metallic Co, MnO) in absence and presence of
NaOH, respectively. While the Co/MnO and metallic Co
showed catalytic activity, the CoMn2O4 and MnO exhibited
almost no catalytic performance. Therefore, we concluded
that metallic cobalt plays as an active site for hydrazine
decomposition. Interestingly, Co/MnO (TOF: 4.7 h−1)
exhibit higher catalytic activity than unimetallic Co (TOF:
h−1) when the same amount of catalysts are used. This phe-
nomenon can be considered that MnO in the catalyst plays
role in promoting the catalytic activity. Xia et al. reported
the decomposition of hydrazine using NiPt nanoparticles/
MnOx in which the ratio of NiPt to MnOx significantly
affect the catalytic activity.17 Recently, Co/MnO or
CoO/MnO are applied for the anode material for lithium-ion
batteries where the morphology of metallic cobalt and elec-
trical properties are significantly dependent on the molar
ratio of precursor cobalt ions to manganese ions.28,29 In
addition, it is known that charge transfer step in decomposi-
tion of hydrazine is important in determining reaction kinet-
ics.17 Therefore, we believe that optimal electronic structures
of Co/MnO has superior kinetic property compared to Co.
To study the effect of pH on the rate of hydrazine decom-
position, the reaction was carried out under 0.12 M NaOH
solution (Figure 4(b)). The catalytic activities of Co/MnO
and metallic Co are improved approximately three times in
the presence of NaOH. It was reported that OH– ions in
hydrazine solution decrease the concentration of undesirable
N2H5 (N2H5 + OH– ! N2H4 + H2O) acting as pro-
moters.30 The TOF of the Co/MnO and metallic Co in pres-
ence of 0.12 M NaOH exhibit 14.2, 2.5 h−1, respectively.
These value is comparable to the previous noble metal-free
catalysts (Ni0.5Fe0.5 (3.2 h−1 at 70 ꢁC),31 Ni0.60Pd0.40 (5.5 h−1
+
+
32
33
at 70 C), Ni50Fe50-CT (9 h−1 at 70 C), Ni1.5Fe1.0-alloy/
ꢁ
ꢁ
34
(MgO)3.5 (16.5 h−1 at 70 C), and NiFe/Cu (35.3 h−1 at 70
ꢁ
ꢁC)33). Figure 4(c) shows the conversion plots with/without
using the HCl trap under 0.12 M NaOH. The conversion
kinetics are almost same regardless of the HCl trap usage
indicating that little ammonia is produced. In addition, the
generated gases obtained during the decomposition reaction
were analyzed by GC (Figure 4(d)). Figure 4(d) shows GC
results before the injection of hydrazine solution and after the
reaction of hydrazine decomposition, respectively. After reac-
tion, H2 gas was detected. Importantly, no other peaks such
as ammonia were detected except for H2 peak.
Conclusion
In summary, Co/MnO was prepared by an annealing spinel
CoMn2O4 under hydrogen gas condition. In comparison to
CoMn2O4, Co/MnO catalysts exhibited the catalytic activ-
ity for the hydrazine decomposition and the 100%
Figure 4. N2H4 conversion as a function of time over the prepared
samples at different concentration of NaOH (a) 0 M NaOH,
(b) 0.12 M NaOH, (c) 0.12 M NaOH using the co/MnO with/
without ammonia trap, and (d) GC profile for gas product of (c).
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Co/MnO Catalyst for Hydrogen Generation by Decomposition Hydrazine
KOREAN CHEMICAL SOCIETY
14. O. Song-Il, J.-M. Yan, H.-L. Wang, Z.-L. Wang, Q. Jiang,
Int. J. Hydrog. Energy 2014, 39, 3755.
15. N. Firdous, N. K. Janjua, I. Qazi, M. H. Sarwar Wattoo, Int.
J. Hydrog. Energy 2016, 41, 984.
selectivity for hydrogen. In addition, the catalytic activity
of Co/MnO is higher than that of metallic Co under usage
of same amount of catalysts. We believe that the improved
catalytic property originated from the synergistic electric
effect between metallic Co and MnO involved in charge
transfer steps.
16. L. He, Y. Huang, A. Wang, Y. Liu, X. Liu, X. Chen,
J. J. Delgado, X. Wang, T. Zhang, J. Catal. 2013, 298, 1.
17. B. Xia, T. Liu, W. Luo, G. Cheng, J. Mater. Chem. A 2016,
4, 5616.
18. S. E. Colley, R. G. Copperthwaite, G. J. Hutchings,
S. P. Terblanche, M. M. Thackeray, Nature 1989, 339, 129.
19. P. W. Menezes, A. Indra, N. R. Sahraie, A. Bergmann,
P. Strasser, M. Driess, ChemSusChem 2015, 8, 164.
20. Y. Juan, S. Quaresma, M. Sen, J. M. F. Ferreira, P. Norby,
Key Eng. Mater. 2005, 280-283, 713.
Acknowledgments. This research was supported by Basic
Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Edu-
cation (NRF-2019R1I1A3A01059837) and by the Korea
Basic Science Institute (KBSI) National Research Facili-
ties & Equipment Center (NFEC) grant funded by the Min-
istry of Education (2019R1A6C1010042).
21. H. Chen, M. Yang, S. Tao, M. Ren, G. Chen, Cryst. Growth
Des. 2016, 16, 6286.
22. V. A. De La Peña O’Shea, M. C. Alvarez-Galvan,
J. M. Campos-Martin, J. L. G. Fierro, Catal. Lett. 2005,
100, 105.
References
1. J. A. Turner, Science 2004, 305, 972.
23. C. Li, X. Han, F. Cheng, Y. Hu, C. Chen, J. Chen, Nat.
Commun. 2015, 6, 7345.
24. W. Ni, S. Liu, Y. Fei, Y. He, X. Ma, L. Lu, Y. Deng,
J. Mater. Chem. A 2016, 4, 7746.
25. Y. Su, Y. Zhu, H. Jiang, J. Shen, X. Yang, W. Zou, J. Chen,
C. Li, Nanoscale 2014, 6, 15080.
2. M. B. Ley, L. H. Jepsen, Y.-S. Lee, Y. W. Cho,
J. M. Bellosta von Colbe, M. Dornheim, M. Rokni,
J. O. Jensen, M. Sloth, Y. Filinchuk, J. E. Jørgensen,
F. Besenbacher, T. R. Jensen, Mater. Today 2014, 17, 122.
3. J. D. Holladay, J. Hu, D. L. King, Y. Wang, Catal. Today
2009, 139, 244.
26. D. P. Dubal, D. S. Dhawale, R. R. Salunkhe,
C. D. Lokhande, J. Electrochem. Soc. 2010, 157, A812.
27. B. Liu, X. Hu, H. Xu, W. Luo, Y. Sun, Y. Huang, Sci. Rep.
2004, 4, 4229.
4. R. Lan, J. T. S. Irvine, S. Tao, Int. J. Hydrog. Energy 2012,
37, 1482.
5. T. C. Johnson, D. J. Morris, M. Wills, Chem. Soc. Rev. 2010,
39, 81.
28. T. Kokubu, Y. Oaki, E. Hosono, H. Zhou, H. Imai, Adv.
Funct. Mater. 2011, 21, 3673.
29. F. Dang, Y. Oaki, T. Kokubu, E. Hosono, H. Zhou, H. Imai,
Chem. Asian J. 2013, 8, 760.
30. W. X. Yin, Z. P. Li, J. K. Zhu, H. Y. Qin, J. Power Sources
2008, 182, 520.
6. M. Zheng, R. Cheng, X. Chen, N. Li, L. Li, X. Wang,
T. Zhang, Int. J. Hydrog. Energy 2005, 30, 1081.
7. J. Prasad, J. L. Gland, Langmuir 1991, 7, 722.
8. A. A. Konnov, J. De Ruyck, Combust. Flame 2001, 124, 106.
9. P.-X. Zhang, Y.-G. Wang, Y.-Q. Huang, T. Zhang, G.-S. Wu,
J. Li, Catal. Today 2011, 165, 80.
31. K. V. Manukyan, A. Cross, S. Rouvimov, J. Miller,
A. S. Mukasyan, E. E. Wolf, Appl. Catal., A 2014, 476, 47.
32. S. K. Singh, Y. Iizuka, Q. Xu, Int. J. Hydrog. Energy 2011,
36, 11794.
33. S. K. Singh, A. K. Singh, K. Aranishi, Q. Xu, J. Am. Chem.
Soc. 2011, 133, 19638.
10. S. N. Oliaee, C. Zhang, S. Y. Hwang, H. M. Cheung,
Z. Peng, J. Phys. Chem. C 2016, 120, 9764.
11. M. Liu, Y. Zheng, S. Xie, N. Li, N. Lu, J. Wang, M. J. Kim,
L. Guo, Y. Xia, Phys. Chem. Chem. Phys. 2013, 15, 11822.
12. Z. Deng, X. Lu, Z. Wen, S. Wei, Y. Liu, D. Fu, L. Zhao,
W. Guo, Phys. Chem. Chem. Phys. 2013, 15, 16172.
13. J. Wang, W. Li, Y. Wen, L. Gu, Y. Zhang, Adv. Energy
Mater. 2015, 5, 1401879.
34. L. He, Y. Huang, A. Wang, X. Wang, X. Chen, J. J. Delgado,
T. Zhang, Angew. Chem. Int. Ed. 2012, 51, 6191.
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