Mesoporous multiwalled carbon nanotubes as supports for monodispersed iron-boron catalysts: Improved hydrogen generation from hydrous hydrazine decomposition

Article abstract of DOI:10.1039/c2ta00015f

Multiwalled carbon nanotube supported iron-boron catalysts (Fe-B/MWCNTs) were developed via the chemical reduction of aqueous sodium borohydride with iron chloride for hydrolysis of hydrous hydrazine (N₂H₄). The effects of loading, N₂H₄ concentration, reaction temperature and supporting materials on the as-prepared Fe-B/MWCNTs were investigated. A high H₂ generation rate of 34.2 L h^-1 g^-1 catalyst for the 9.86 wt% Fe-B/MWCNTs is achieved with 97.0% H₂ selectivity at room temperature. Furthermore, the hydrolysis reaction is zero order with respect to N₂H₄ concentration in the range 0.1-5 mol L^-1, and the activation energy is 46.7 ± 1.3 kJ mol^-1 for 9.86 wt% Fe-B/MWCNTs. The 9.86 wt% Fe-B/MWCNTs are stable catalysts for N₂H₄ hydrolytic dehydrogenation as they provide 114 480 total turnovers in 30 h. The improved performance of Fe-B/MWCNTs is a promising step towards the application of N ₂H₄ as a chemical hydrogen storage material.

Full text of DOI:10.1039/c2ta00015f

Journal of
Materials Chemistry A
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PAPER
Mesoporous multiwalled carbon nanotubes as supports
for monodispersed iron–boron catalysts: improved
hydrogen generation from hydrous hydrazine
decomposition†
Cite this: J. Mater. Chem. A, 2013, 1,
58
3
Dong Ge Tong,*ab Wei Chu,* Ping Wu, Gui Fang Gu and Li Zhang
c
ab
ab
ab
Multiwalled carbon nanotube supported iron–boron catalysts (Fe–B/MWCNTs) were developed via the
chemical reduction of aqueous sodium borohydride with iron chloride for hydrolysis of hydrous
hydrazine (N
2 4 2 4
H ). The effects of loading, N H concentration, reaction temperature and supporting
ꢀ1
materials on the as-prepared Fe–B/MWCNTs were investigated. A high H
2
generation rate of 34.2 L h
selectivity at room temperature.
concentration in the range 0.1–5
mol L , and the activation energy is 46.7 ꢁ 1.3 kJ mol for 9.86 wt% Fe–B/MWCNTs. The 9.86 wt%
Fe–B/MWCNTs are stable catalysts for N hydrolytic dehydrogenation as they provide 114 480 total
turnovers in 30 h. The improved performance of Fe–B/MWCNTs is a promising step towards the
application of N as a chemical hydrogen storage material.
ꢀ1
g
catalyst for the 9.86 wt% Fe–B/MWCNTs is achieved with 97.0% H
2
Furthermore, the hydrolysis reaction is zero order with respect to N
2 4
H
Received 22nd August 2012
ꢀ
1
ꢀ1
Accepted 28th September 2012
2 4
H
DOI: 10.1039/c2ta00015f
www.rsc.org/MaterialsA
2 4
H
Therefore, to meet practical applications, it is crucial to develop
catalysts with high activity and H selectivity during hydrous
hydrazine decomposition. Recently, Xu’s group demonstrated
that Rh Ni, Ni0.93Pt0.07 or Ni0.95Ir0.05 nano-catalysts can achieve
00% conversion of hydrous hydrazine to hydrogen at room
1
Introduction
2
Hydrogen is a promising clean energy carrier for sufficient and
sustainable energy supplies. The main technical challenge for
widespread hydrogen applications is real-time production and
4
1
20–22
storage. Thus, the search for applicable hydrogen storage temperature but only with a low H generation rate.
More
2
1–7
materials is extremely important. Up to now, various kinds of recently, Xu’s group developed a highly efficient and low-cost
solid materials for hydrogen storage have been investigated and catalyst made from an alloy of non-precious nickel and iron
found to be promising. Typical examples are metal/complex nanoparticles, which also achieved 100% hydrogen selectivity
23
hydrides, metal nitrides and imides, carbon materials, inor- from hydrous hydrazine decomposition. However, these
1
–9
ꢀ1
ganic nanostructures, and metal–organic frameworks.
catalysts were only active at 343 K, and addition of 0.5 mol L
Recently, hydrous hydrazine (N H ) has attracted great atten-
2
4
NaOH was necessary for high selectivity. In 2012, Zhang’s group
tion as a promising hydrogen storage material owing to the high
developed a graphene supported rhodium–nickel catalyst, also
10–19
content of hydrogen (8.0%).
Hydrazine decomposes in two exhibiting 100% hydrogen selectivity from hydrous hydrazine
24
ways: (1) complete decomposition via eqn (1):
decomposition at room temperature in the presence of NaOH.
Meanwhile, Zhang and co-workers prepared supported nickel
catalysts derived from Ni–Al hydrotalcite for hydrogen genera-
N
2
H
4
/ N + 2H
2
2
(1)
(2)
and (2) incomplete decomposition via eqn (2):
/ 4NH + N
tion from the catalytic decomposition of N
2
H
4
under ambient
25
conditions with 93% H
2
selectivity. However, concerning the
3
N
2
H
4
3
2
.
element abundance and related economic issues, it is still a
desirable goal to prepare low-cost catalysts with high catalytic
activity and H selectivity for N H fuel cells.
a
Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education
2
2
4
Institutions, College of Materials and Chemistry & Chemical Engineering, Chengdu
University of Technology, Chengdu 610059, China. E-mail: tongdongge@163.com;
Low cost materials such as transition metal boride (M–B)
amorphous alloy catalysts attract much attention owing to their
Institute of green catalysis and synthesis, College of Materials and Chemistry & unique properties such as isotropic structure, high concentra-
Fax: +86-28-8407 9074
b
Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China
tion of coordinative unsaturated sites, and chemical
c
26–36
College of Chemical Engineering, Sichuan University, Chengdu 610065, China.
stability.
For example, we have synthesized Co–B honey-
E-mail: chuwei65@yahoo.com.cn; Fax: +86-28-8540 3397
comb and monodispersed Co–B nanospheres using solution
plasma techniques, which showed superior catalytic
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c2ta00015f
1
3
58 | J. Mater. Chem. A, 2013, 1, 358–366
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performances compared to those of commercial and reported system under O
2
for 5 min. The dispersion of MWCNTs in the
37,38
Rh nanoparticles for N H hydrolysis.
However, the transi- KCl solution was ltered through a polycarbonate membrane
2
4
tion metal boride (M–B) amorphous alloy catalysts produced by lter (GE Osmonics, pore size 100 nm). The collected MWCNTs
regular chemical reduction methods agglomerate severely were washed with de-ionized water to remove the KCl residues,
39
because of the exothermic nature of the reaction. Their cata- followed by three washings with absolute alcohol (EtOH).
lytic activity is generally poor because of the low surface-to- Finally, the obtained product was dried overnight under
volume ratio available aer agglomeration. Supporting mate- vacuum. Then the SPP-treated MWCNTs (1.0 g) were added to
ꢀ
1
rials, such as SiO
2 3
, carbon and nickel foam, provide a potential FeCl solution (0.025 mol L ) in a beaker and the solution was
route to decrease the agglomeration and improve the dispersion stirred for 24 h. Subsequently, the reducing agent, alkaline
40–45
ꢀ1
ꢀ1
of metal boride amorphous alloy catalysts.
Among them, NaBH solution (0.1 mol L NaBH + 0.01 mol L NaOH), was
4 4
MWCNTs has attracted much attention as an ideal substrate for added to FeCl3 dropwise with magnetic stirring. During the
high-performance catalysts owing to its unique structure and reduction reaction, the reactor was placed in an ice water bath
ꢂ
properties, such as high surface area, high thermal/electrical (0 C) to prevent a vigorous reaction. The resulting black
conductivity, light weight, excellent exibility and mechanical precipitate was ltered and washed repeatedly using distilled
43,46–48
strength.
Wei and co-workers prepared carbon nanotube deionized water. Five samples with different Fe–B loadings
supported Co–B amorphous alloy catalysts with good activities (3.33, 6.74, 9.86, 13.50 and 20.65 wt%) were prepared by
4
3
in the catalytic hydrolysis of NaBH
4
solution. Xu and Wang changing the reactant ratios. The Fe–B/MWCNTs catalysts were
ꢂ
et al. prepared RuB and NiB amorphous alloy catalysts sup- then dried in vacuum at 40 C. The Fe–B, 9.86 wt% Fe–B/KOH-
ported on carbon nanotubes with good performances in modied MWCNTs, and 9.86 wt% Fe–B on carbon (Vulcan XC-
47,48
gaseous benzene hydrogenation.
However, the amorphous 72C, Carbot, USA) (9.86 wt% Fe–B/C) catalysts were also
alloy nanoparticles loaded onto MWCNTs agglomerated to form prepared as comparisons, using the same procedure. For KOH-
large particles or dense layers because of the exothermic nature modied MWCNTs, potassium was impregnated onto the
43,46–48
of the reaction.
As a result, the catalytic activity is puried MWCNTs from a 10% KOH/ethanol solution, followed
hampered. Recently, Patel et al. developed an efficient route to by calcination at 773 K for 5 h in an Ar ow. The Fe/MWCNTs
avoid agglomeration by xing the Co–B amorphous alloy and Fe on carbon (Vulcan XC-72C, Carbot, USA) (Fe/C) were
nanoparticles in the mesopores of highly ordered silica particles prepared by reduction with N H using the above procedure.
2
4
49
(
MSP). Meanwhile, Fe–B amorphous alloy catalyst has attrac-
ted attention owing to its good catalytic activity and its low
cost. Therefore, we have attempted to combine a Fe–B amor-
50
2.2 Catalyst characterization
phous alloy with mesoporous MWCNTs to achieve a novel The Fe and B contents in the catalysts were determined by
catalyst with high activity. Herein, using ordered mesoporous inductively coupled plasma atomic emission spectrometry (ICP-
MWCNTs as the support, we obtained highly dispersed Fe–B AES; Irris, Advantage). The dry samples were dissolved in
catalysts that achieve a high hydrogen generation rate of 34.2 L boiling aqua fortis using a microwave digestion system. The
ꢀ1
ꢀ1
h
g
catalyst and up to 97.0% H selectivity for the decom- reproducibility was checked by at least three repeated runs and
2
position of N H4 at room temperature. The effects of Fe–B was found to be within acceptable limits (ꢁ0.5%). The amor-
2
loading, N H4 concentration, reaction temperature and sup- phous structure was veried by X-ray diffraction (XRD; Rigaku
2
porting materials on the as-prepared Fe–B/MWCNTs are also D/Max-RB with Cu-Ka radiation). The surface morphology was
investigated. To our knowledge, this is the rst report in which examined using transmission electron microscopy (TEM; JEOL
supported amorphous catalysts show such high hydrogen JEM 2100F, 200 kV). Samples were dispersed in ethanol, soni-
generation rates and high selectivity towards the formation of cated for 30 min to ensure adequate dispersion of the nano-
H
2
in N
2
H
4
decomposition.
structures, and one drop of the solution was evaporated onto a
00 mesh Cu grid, coated with a lacey carbon lm. Near-surface
3
elemental analysis was carried out using energy-dispersive X-ray
spectroscopy (EDAX). The surface electronic states were inves-
tigated using X-ray photoelectron spectroscopy (XPS; Perkin-
2
Experimental
2.1 Catalyst preparation
The ordered mesoporous multi-wall carbon nanotubes Elmer PHI 5000C ESCA using Al Ka radiation). All the binding
MWCNTs) (95% purity, diameter: 10–40 nm, length: 1–3 mm) energy (BE) values were calibrated using C1s ¼ 284.6 eV as a
were purchased from the Chengdu Organic Institute, Chinese reference. For the in situ XPS measurement, 10 mg of the cata-
Academy of Sciences. According to the provider, the MWCNTs lyst was collected from the N decomposition system at
(
2
H
4
ꢀ
1
were puried by reuxing in 6 mol L hydrochloric acid. Other intervals of 10 s and xed in a home-made in situ XPS reactor
reagents were obtained from Aldrich and were used without cell. Aer drying in an argon atmosphere, the sample was
further purication. Fe–B/MWCNTs catalysts were prepared by transferred to an analysis chamber and the XPS spectrum was
an impregnation–chemical reduction method. The walls of the recorded. The active surface area (SFe) was measured by
MWCNTs were made hydrophilic by the solution plasma hydrogen chemisorption on a Quantachrome CHEMBET 3000
51
ꢀ20
process (SPP) in a self-designed reactor reported previously. In system, assuming H/Fe(s) ¼ 1 and a surface area of 6.02 ꢃ 10
2
52
a typical experiment, 2 g of puried MWCNTs and 100 mL of
m
per Fe atom.
N
2
adsorption–desorption isotherms were
ꢀ
1
0
.05 mol L KCl were added into the solution plasma process measured at 77 K using a Quantachrome Nova 4000 analyzer.
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The pore size distribution and the average pore size were
determined using the BJH model. CO2 temperature-pro-
3
Results and discussion
grammed desorption (TPD) was performed as follows. First, the The compositions of the as-prepared Fe–B and Fe–B/MWCNTs
air on the sample surface was removed with a nitrogen stream samples were determined by ICP-AES (Table 1). Obviously, the
(
purity of 99.999%, treated with a Chrompack clean-oxygen relative contents of Fe and B have no change aer loading on
lter) at 573 K for 1 h and the surface was cooled to room the MWCNTs.
temperature in the same nitrogen stream. Then, CO was pre-
Fig. 1 shows the nitrogen absorption–desorption isotherms of
adsorbed by the catalyst at 383 K for 2 h. The CO
stream was Fe–B, MWCNTs and Fe–B/MWCNTs. A typical type-IV isotherm is
then replaced with the carrier gas, which was kept at 383 K for 2 observed for the MWCNTs with a hysteresis loop (in the range of

2
2
5
3
h to purge the gas and physically absorbed CO away from the P/P
0
¼ 0.25–0.95) indicating the mesoporous structure. The
catalyst surface. Aer the surface was cooled down to room well-dened step in the 0.05–0.25 P/P range represents the
temperature, CO2 desorption was carried out by raising the spontaneous lling of the mesopores due to capillary conden-
2
0
ꢀ
1
temperature at a speed of 10 K min up to 773 K, and the sation. Meanwhile, the gradual increase in volume with P/P
released CO
was determined by an on-line gas chromatograph (>0.25) is due to the multilayer adsorption on the outer surface of
GC) equipped with a thermal conductivity detector (TCD). the MWCNTs. Aer Fe–B loading, the height of the inection
0
2
(
Ultraviolet visible (UV-Vis) spectra were collected on a UV5800 point at low relative pressure signicantly decreases. This indi-
modal spectrophotometer. For the UV-Vis measurement, 0.5 g cates that most of the mesopores are blocked by Fe–B particles
para-dimethylaminobenzaldehyde (PDAB), 25 mL ethanol and for nitrogen absorption by either completely lling the pores or
ꢀ1
53
2
.5 mL hydrochloric acid (1 mol L ) were mixed together as the by accumulating over the opening of the pores. Furthermore,
chromogenic agent for N H . Then 0.3 mL solutions of 0.5 mol the N adsorption–desorption isotherms of unsupported Fe–B
hydrous hydrazine, 0.1 mL of chromogenic agent and 3.4 indicate the nonporous structure.
mL of de-ionized water were added and the sample solutions
The specic surface areas of Fe–B, MWCNTs and Fe–B/
2
4
2
ꢀ
1
L
were stored for 30 min before measurements. The room MWCNTs are shown in Table 1. It can be seen that the specic
temperature magnetic characterization of the sample was per- surface areas of Fe–B/MWCNTs are lower than that of MWCNTs,
formed using a vibrating sample magnetometer (MagLab-12, and depend on the Fe–B contents. Meanwhile, the loading of
Oxford).
Fe–B evidently decreases the pore volume of MWCNTs, while a
2.3 Hydrazine decomposition reaction
The hydrazine decomposition reaction was carried out at room
temperature. The hydrazine catalytic decomposition reaction,
which releases hydrogen (along with nitrogen), was initiated by
stirring a mixture of hydrazine monohydrate aqueous solution
ꢀ
1
(
200 mL, 0.01–10 mol L ), which was added by a pump to the
reaction vessel, and the as-prepared catalysts. The gas released
during the reaction was passed through a trap containing 1.0
ꢀ1
mol L hydrochloric acid to ensure the absorption of ammonia
volume monitored with a owmeter). The analysis of the gas
(
released during the reaction was performed with an on-line gas
chromatograph (HP Series6890) equipped with a Poropak N
column and a thermal conductivity detector. The recycled
catalyst was collected using a magnetic separation technique
and washed thoroughly (at least three times) with water and
absolute ethanol.
Fig. 1 Nitrogen adsorption–desorption isotherms of (a) MWCNTs, (b) 3.33 wt%
Fe–B/MWCNTs, (c) 6.74 wt% Fe–B/MWCNTs, (d) 9.86 wt% Fe–B/MWCNTs, (e)
13.50 wt% Fe–B/MWCNTs, (f) 20.65 wt% Fe–B/MWCNTs and (g) Fe–B.
Table 1 Physico-chemical properties of Fe–B, MWCNTs and Fe–B/MWCNTs
Average pore
volume/cm g
Average pore Average particle
diameter/nm sizes/nm
2
ꢀ1
3
ꢀ1
Catalysts
Fe-loading/wt% Compositions/at.%
S
BET/m g
Fe–B
MWCNTs
—
—
Fe0.675
—
B
0.325
24.7
—
—
—
—
226.1
143.3
126.7
113.5
96.4
0.44
0.35
0.31
0.28
0.24
0.19
0.27
4.1
4.0
3.9
3.7
3.6
3.4
3.6
3
6
9
1
2
9
.33 wt% Fe–B/MWCNTs
.74 wt% Fe–B/MWCNTs
.86 wt% Fe–B/MWCNTs
3.50 wt% Fe–B/MWCNTs
0.65 wt% Fe–B/MWCNTs
3.05
6.17
9.08
12.36
18.90
Fe0.672
Fe0.673
Fe0.672
Fe0.674
Fe0.673
Fe0.671
B
0.328
B
0.327
B
0.328
B
0.326
B
0.327
B
0.329
2.2
3.6
4.9
6.7
8.8
5.0
75.9
111.1
.86 wt% Fe–B/KOH-modied MWCNTs 9.07
3
60 | J. Mater. Chem. A, 2013, 1, 358–366
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slight decrease is observed in the average pore size (Table 1 and unsupported Fe–B at high and low magnications,
and ESI, Fig. S1†). These results further indicate that the respectively. From Fig. 2a, it can be seen that the unloaded
loaded Fe–B are able to block mesopores by either completely MWCNTs have a clear surface. With the addition of Fe–B, the

lling the pores from inside or positioning themselves on the surfaces of the MWCNTs become rounded (Fig. 2b–f). The Fe–B
pore face so that the pores becomes unavailable for nitrogen loaded MWCNTs are highly dispersed, while the unsupported
53
absorption.
Fe–B aggregate to form large particles (Fig. 2g). Owing to the
Fig. 2 and S2 (in the ESI†) present typical TEM images of irregular shape of Fe–B particles, the determination of the exact
MWCNTs, and Fe–B/MWCNTs with different loading amounts particle size is difficult. However, a rough estimation is
obtained by measuring 150 randomly chosen particles in the
magnied TEM images. The size distribution of Fe–B nano-
particles is shown in the histograms in Fig. S3.† It is clearly seen
that there are some Fe–B nanoparticles larger than the pore
sizes of the MWCNTs (Fig. S3† and Table 1), indicating that the
Fe–B nanoparticles are located on the surface of the MWCNTs
with some anchored to the pores. This is benecial for the
catalytic reaction as the Fe–B particles are fully located
inside the pores and are difficult to access compared with
the nanoparticles on the surface. The unique morphology of the
Fe–B/MWCNTs catalyst (Fig. 2b–g) is attributed to the impreg-
nation–reduction process. During impregnation, FeCl solution
3

lls most of the MWCNTs pores and is also absorbed on the
surface of MWCNTs. When FeCl in the pores is reduced by
NaBH solution, the Fe–B particles act as nucleation sites for the
3
4
further growth of Fe–B particles on the support surface. Since
large numbers of nucleation sites are available in the form of
regularly arranged pores, most of the particles do not grow
above 10 nm. However, we found some large particle sized Fe–B
in Fe–B/MWCNTs during the TEM studies (Fig. S2 and S4†).
Some of the pore channels on the surface of MWCNTs are not
3
impregnated with FeCl solution; thus, Fe–B particles agglom-
erate around these pores forming big particles with sizes 15–30
nm on the outer surface (Fig. S4†). Further work is necessary to
improve the homogeneity. The high degree of dispersion of Fe–
B nanoparticles is attributed to the large number of available
nucleation sites in the mesopores of MWCNTs. Compared with
previous attempts for the loading of amorphous metal borate
43,46–48
alloy on MWCNTs,
our current method based on the
ordered mesoporous MWCNTs allows us to prepare highly
dispersed Fe–B nanoparticles loaded onto MWCNTs. Further-
more, by varying the mass ratio of reactant and MWCNTs, the
particle sizes of Fe–B catalysts can be tuned (Fig. 2 and S3†).
Meanwhile, it seems that the Fe–B nanoparticles in Fig. 2f are
ordered and respect a specic arrangement, which is possibly
related to the regularly arranged mesopores of our MWCNTs
(Fig. 1a). But, the exact mechanism for such a unique arrange-
ment requires further study, which is currently under way.
Fig. 3 shows the XRD patterns of the samples. Fe–B is present
in the amorphous structure, since one broad peak at around
ꢂ
50
ꢂ
2
q ¼ 44.5 appears. For Fe–B/MWCNTs, the peak at 2q ¼ 25.0
43,46–48
ꢂ
is ascribed to MWCNTs,
and the broad peaks at 44.5 are
attributed to amorphous Fe–B. The small angle diffraction peak
ꢂ
(
2q ¼ 2.75 ) in the XRD pattern (Fig. S5†) further demonstrates
that MWCNTs have an ordered mesoporous structure. Upon
Fe–B particle loading, the intensity of the small angle XRD
peaks decreases.
Fig. 2 TEM images of (a) MWCNTs, (b) 3.33 wt% Fe–B/MWCNTs, (c) 6.74 wt%
Fe–B/MWCNTs, (d) 9.86 wt% Fe–B/MWCNTs, (e) 13.50 wt% Fe–B/MWCNTs, (f)
Fig. 4, 5 and S6† show the Fe 2p3/2, B 1s and overall XPS
spectra of Fe–B particles before and aer loading, respectively.
2
0.65 wt% Fe–B/MWCNTs and (g) Fe–B at high magnifications.
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As shown in Fig. 4a, Fe is present in the metallic state with a
54
binding energy of 706.7 eV; in contrast B (Fig. 5a) is present in
both the elemental state (188.4 eV) and the oxidized state (192.7
54
eV). No signicant oxidation of metallic Fe occurred during
the preparation process, indicating that the presence of B
effectively protects Fe from oxidation. As no oxidized B was
detected by XRD, this suggests that the amount of oxidized B
species was negligible. Furthermore, the binding energy of the B
species positively shis to 1.3 eV, suggesting that some elec-
trons are transferred from B to Fe, making Fe electron-enriched
in the supported Fe–B catalyst, as found in most other metal–
34–38,47–49,55
boron amorphous alloys.
The change in Fe binding
energies is not as much as that of B, as the amounts and the
atomic weights of Fe are greater than those of B in the Fe–B. As a
result, the binding energy of Fe was not as sensitive as that of B.
In the Fe–B/MWCNTs, the Fe peak shis to a lower binding
energy while the peak of the alloying B shis to a higher energy
compared to B in the unsupported Fe–B (Fig. 4b–f and 5b–f).
Fig. S7† is the typical Fe 2p3/2 spectra of Fe/MWCNTs and Fe/C.
Compared with Fig. 4 and S7,† the observed binding energy
shis for Fe in Fe–B/MWCNTs imply that the MWCNTs promote
the electrons transferring from B to Fe to form a more electron-
rich iron and increase the amount of iron in the alloying
Fig. 3 XRD patterns of (a) MWCNTs, (b) 3.33 wt% Fe–B/MWCNTs, (c) 6.74 wt%
Fe–B/MWCNTs, (d) 9.86 wt% Fe–B/MWCNTs, (e) 13.50 wt% Fe–B/MWCNTs, (f)
2
0.65 wt% Fe–B/MWCNTs and (g) Fe–B.
47,56–59
state.
Meanwhile, as the peak of the oxidized B species is
not observed (Fig. 5b–f and S6†), we infer that only a small
amount of Fe–B loading leads to oxidized B under the detection
limit of the XPS equipment.
During the hydrazine decomposition over Fe–B catalysts, the
on-line gas chromatographic analysis of the gas released
2 2
showed that the H /N ratio remains almost constant as the
reaction proceeds (Fig. S8†). This result indicates that the
competing reactions (1) and (2) proceed at the same time, which
20–25,29,30
is in agreement with the literature.
Therefore, under our
experimental conditions, the hydrolysis of hydrazine over Fe–B
amorphous alloy can be expressed by eqn (3):
Fig. 4 Fe 2p3/2 XPS spectra of (a) 3.33 wt% Fe–B/MWCNTs, (b) 6.74 wt% Fe–B/
MWCNTs, (c) 9.86 wt% Fe–B/MWCNTs, (d) 13.50 wt% Fe–B/MWCNTs, (e) 20.65
wt% Fe–B/MWCNTs and (f) Fe–B.
3
N H / (1 + 2x)N + 6xH + 4(1 ꢀ 2x)NH ,
(3)
2
4
2
2
3
25,29,30
where x is the H
2
selectivity.
The hydrogen generation
volumes of Fe–B, MWCNTs, and Fe–B/MWCNTs during the
hydrolysis ofN are shown inFig. S9.† Negligible activityfor H
2
2
H
4
generation is obtained by using bare MWCNTs (Fig. S9a†).
Meanwhile, the hydrogen generation rate increases with Fe–B
loading up to 9.86 wt% (Table 2). However, the hydrogen
generation rate tends to decrease with further increases in Fe–B
loading because of the growth of particle sizes at high loading
content. As a result, the active surface areas (SFe) decrease (Table
ꢀ
1
2
g
). The achieved highest hydrogen generation rate is 34.2 L h
ꢀ1
2
catalyst for 9.86 wt% Fe–B/MWCNTs with 97.0% H selec-
tivity, which is superior to those of the most active catalysts ever
reported for this reaction (Table 2). The reaction completeness
was further determined by UV-Vis spectroscopy (Fig. S10†). The
absence of the adsorption peak at 458 nm aer the completion of
hydrazine decomposition indicates 100% conversion of hydra-
zine. Furthermore, the unsupported Fe–B exhibits the lowest
Fig. 5 B 1s XPS spectra of (a) 3.33 wt% Fe–B/MWCNTs, (b) 6.74 wt% Fe–B/
MWCNTs, (c) 9.86 wt% Fe–B/MWCNTs, (d) 13.50 wt% Fe–B/MWCNTs, (e) 20.65
wt% Fe–B/MWCNTs and (f) Fe–B.
ꢀ1
ꢀ1
hydrogen generation rate of 0.89 L h
g
Fe–B with 35.1% H
2
selectivity, because of its poor dispersion (Fig. 2g).
3
62 | J. Mater. Chem. A, 2013, 1, 358–366
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Journal of Materials Chemistry A
a
2 4
Table 2 Catalytic parameters of different samples for N H decomposition
H
2
generation
2
ꢀ1
ꢀ1 ꢀ1
ꢀ1 ꢀ1 ꢀ1
ꢀ1
ꢀ1
Samples
S
Fe/m g
rate/L h
g
catalyst
m
R /mol
h
g
Fe TOF/s
H
2
Selectivity/% TTON
ATOF/s
Fe–B
MWCNTs
10.1
—
0.89
0
0.062
—
0.11
—
35.1
—
—
—
—
—
—
—
—
—
3
6
9
1
2
9
9
9
9
.33 wt% Fe–B/MWCNTs
.74 wt% Fe–B/MWCNTs
.86 wt% Fe–B/MWCNTs
3.50 wt% Fe–B/MWCNTs
0.65 wt% Fe–B/MWCNTs
.08 wt% Fe/MWCNTs
.08 wt% Fe/C
71.3
76.4
78.7
66.5
52.8
26.2
17.4
60.9
5.03
18.6
34.2
30.2
25.4
0.18
0.091
10.1
34.9
1.96
0.63
0.998
1.639
3.58
9.61
28.9
5.67
0.59
5.86
7.84
8.75
6.08
3.62
0.051
0.026
2.81
8.93
—
0.83
1.03
1.12
0.97
0.69
0.035
0.027
0.46
1.12
—
62.9
86.1
97.0
90.2
83.0
21.2
17.5
88.8
99.3
43.8
100
100
100
100
93.0
41.8
21.3
8.7
11 4480 1.06
—
—
—
—
—
—
—
—
—
—
—
—
18 360
—
—
—
—
—
—
—
—
—
—
—
—
0.17
.86 wt% Fe–B/C
.8 6wt% Fe–B/KOH–MWCNTs 78.7
b
Rh
—
—
—
—
—
—
90.2
81.1
24.3
b
NiFe
3
—
—
—
—
—
—
—
—
b
Ni0.95Ir0.05
Ni0.93Pt0.07
Rh4.32Ni/graphene
b
c
d
Ni–Al
2
O
3
-HT
—
—
e
Honeycomb-like Co–B
Co–B nanospheres
Regular Co–B
1.72
0.66
0.17
0.21
0.089
0.076
e
e
a
ꢀ1
b
20–23
c
Reaction conditions in this work: 0.5 g catalyst, 200 mL 0.5 mol L
N
2
H
4
aqueous solution, T ¼ 298 K. Reported by Xu’s group.
Reported by
24
d
25
e
37,38
Zhang’s group.
Reported by Zhang’s group.
Reported by Tong’s group.
Based on the above results, the improved catalytic activity and with N
selectivity for Fe–B/MWCNTs in comparison with Fe–B is was determined from the linear portion of the hydrogen gener-
attributed to the dispersed electron enriched Fe sites as shown by ation plot for each N concentration, and was used for con-
TEM and XPS. It was conrmed by the control experiments over structing the reaction rate versus N H concentration plot in a
2 4
H concentrations (Fig. S13†). Therefore, the reaction rate
H
2
2 4
H
2
4
Fe/MWCNTs and Fe/C. As shown in Table 2, all of the Fe–B logarithmic scale (Fig. 6). As the N H concentration increased
2
4
ꢀ
1
amorphous alloy catalysts had much better catalytic properties from 0.05 to 0.075 mol L , the hydrogen generation rate
than the 9.08 wt% Fe/MWCNTs and 9.08 wt% Fe/C catalysts. increases. The slope of the line in the rst part of the plot
ꢀ
1
Apparently, these electron enriched Fe sites in the Fe–B amor- (C(N
phous alloy catalysts are most favorable for the catalytic hydro- tion is rst order at low N
lysis of N as the excess electrons contribute to the hydrolysis attributed tothelargesurfaceareaofFe–B/MWCNTs, which leads
reaction by providing the charges required for hydrogen atoms to to a low concentration of N and is not enough to saturate the
detach from the catalyst surface. As a result of such an inter- catalytically active sites on the Fe–B/MWCNTs surface. But the
2
H
4
) ¼ 0.01–0.075 mol L ) is 1, indicating that decomposi-
2 4
H
concentration (Fig. 6). This is
2
H
4
2 4
H
23
ꢀ
1
action, the barrier for N–H dissociation is lowered, establishing slope of the line in the second part (C(N H ) ¼ 0.1–5 mol L ) is 0.
2
4
an easier way to produce hydrogen instead of ammonia because
further N–N bond cleavage needs more energy than the dissoci-
23–25
ation of N
2
H
3
to form N
2
and H
2
.
The catalytic role of electron
enriched Fe sites was further conrmed by the Fe2p3/2 binding
energy shis during the decomposition reaction (Fig. S11†).
Interestingly, there are signicant increases in the H selectivity
2
when varying the average particles size of Fe–B from 2.2 to 4.9
nm, while H selectivity decreases with the further increases in
2
the average particle sizes of Fe–B. This indicates that the particle
size distribution in an appropriate range of loaded Fe–B
enhances H selectivity of Fe–B/MWCNTs. Therefore, 9.86 wt%
2
Fe–B/MWCNTs with an average particle size of 4.9 nm were
selected in the subsequent experiments for further study.
2 4
To understand the effect of initial N H concentrations
(
C(N H )) on the hydrogen generation rate of 9.86 wt% Fe–B/
2
4
MWCNTs, the N H concentrations were varied from 0.01 to 10
2
4
ꢀ1
mol L . Fig. S12† shows the plots of generated hydrogen volume
versus time over 9.86 wt% Fe–B/MWCNTs at various N
concentrations under 293 K. The H
selectivity did not change 293 K.
Fig. 6 Plot of hydrogen generation rate versus N
2 4
H concentration (both in
2 4
H
logarithmic scale) for the decomposition of N over 9.86 wt% Fe–B/MWCNTs at
2 4
H
2
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ꢀ
1
2 4
These results indicate that when C(N H ) $ 0.1 mol L , the
reaction becomeszero order withrespect to N H concentrations.
2
4
However, the rate decreases gradually when C(N H ) is further
2
4
ꢀ
1
increased in the range of 7.5–10 mol L . This may be due to the
increase in solution viscosity, which may cause mass-transport
2 4
limitations of N H from the solution to the surface of the cata-
lyst. A similar phenomenon has been observed in the hydrolysis
44
of NaBH
4
over Co–B/C. The solution gravimetric hydrogen
densities (mass of hydrogen/mass of solution) at various N
H
2 4
concentrations are shown in Fig. S14.† The densities increase
with increasing N H concentrations and have maximum values
2
4
ꢀ
1
of 3.88 wt% at C(N H ) ¼ 10 mol L . The result indicates that our
2
4
2 4 2 4
N H system at high C(N H ) may be effective as a hydrogen
storagesystem for a fuel cell uninterrupted power supply (FCUPS)
or some emergency power sources aer optimizing the related Fig. 8 Time profiles for N
experimental parameters in the future.
2
H
4
decomposition by 9.86 wt% Fe–B supported on (a)
MWCNTs and (b) Vulcan XC-72 carbon.
Recently, NaOH has been reported to be a promoter for
23,24
selective cracking of the N–H bond in N H decomposition.
plotted in Fig. 7. The activation energy for the decomposition of
2
4
In our case, the addition of NaOH results in a slight increase in N H , calculated from the slope of the linear curve in Fig. 7 ,is
2
4
ꢀ
1
the H selectivity of catalysts from 97.0 to 99.1% for 9.86 wt% Fe– 46.7 ꢁ 1.3 kJ mol , which compares favorably with the repor-
2
ꢀ1
ꢀ1
B/MWCNTs until the molar ratio of NaOH/N
2
H
4
reaches 0.5, ted values of 49.3 ꢁ 3.2 kJ mol and 54.3 ꢁ 2.7 kJ mol on
25,38
aer which any further increases in the ratio are negligible (Table Ni–Al O and Co–B honeycomb structures, respectively.
2
3
S1†). But, the presence of alkaline additives has no inuence on
The catalytic performance of 9.86 wt% Fe–B/MWCNTs was
also compared with that of 9.86 wt% Fe–B/C for H production
(Fig. 8). The H generation rate
the hydrogen generation rate on the temperature was measured achieved by 9.86 wt% Fe–B/MWCNTs is about 2 times higher
the reaction activity of 9.86 wt% Fe–B/MWCNTs (Table S1†).
2
For determining the activation energy, the dependence of from the hydrolysis of N
2
H
4
2
ꢀ1
ꢀ1 ꢀ1
in 0.5 mol L aqueous N H solution containing 0.5 g 9.86 wt% than that of 9.86 wt% Fe–B/C (10.1 L h
g
catalyst), even
2
4
Fe–B/MWCNTs. From Table S2,† it can be seen that the average though the Vulcan XC-72C has a higher specic surface area
ꢀ1
ꢀ1
2
ꢀ1
hydrogen generation rate increases from 34.2 to 342.1 L h
catalyst when the solution temperature increases from 293 to dispersion of active components,
33 K. The H selectivity did not alter. The following Arrhenius sion over Vulcan XC-72C is lower than that over MWCNTs
equation: measured by H chemisorption as shown in Table 2. The high
g
(250 m g ). Generally, a high surface area is favorable for the
61–64
however the Fe–B disper-
3
2
2
dispersion of Fe–B on the MWCNTs could be attributed to the
unique ordered mesoporous structure of MWCNTs, which allow
Fe–B to be located inside the pores. As a result, the mesopores
restrict the growth of Fe–B particles, which is benecial for Fe–B
r ¼ k exp[ꢀE /(RT)]
(4)
0
a
is used to calculate the reaction rate, where r is the reaction rate
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
(mol h
g
), k
0
is the reaction constant (mol h
a
g ), E is the
dispersion. Furthermore, the various functional groups (e.g.
activation energy for the reaction, R is the gas constant and T is
51
–
COOH, –OH) on the SPP treated MWCNTs are also respon-
60
the solution temperature. An Arrhenius plot, in which ln R
m
is
3+
sible for anchoring Fe–B via the Fe during impregnation.
Thus, it can be concluded that ordered mesoporous MWCNTs
are more suitable for Fe–B loading than Vulcan XC-72C.
Recently, Zhang’s group proposed that the basic sites of the
plotted against the reciprocal of absolute temperature (1/T), is
25
support merit the H
2 2 4 2
selectivity of N H . The CO -TPD exper-
iments (Fig. S15†) indicate that the MWCNTs have almost no
60
CO2 desorption, revealing that there are no basic sites. To
conrm the inuence of the basic property of the supports,
KOH-modied MWCNTs was also applied as a support. One
peak centered at 387 K over KOH-modied MWCNTs is
observed, showing that basic sites are introduced onto
MWCNTs. As expected, the KOH-modied MWCNTs supported
Fe–B show a slight increase in H
2
selectivity from 97.0 to 99.3%
(Table 2) and the catalytic activity is also improved. Since the
KOH-modied MWCNTs do not alter the particle size and active
surface area of Fe–B signicantly in comparison with those of
Fe–B/MWCNTs (Table 1), we conclude that Fe–B loaded with
basic sites on the support can promote H
2
production and
Fig. 7 Plot of log R
B/MWCNTs in N
m
vs. 1/T to determine the activation energy of 9.86 wt% Fe–
decomposition.
2
H
4
catalytic activity of Fe–B during N decomposition.
2 4
H
3
64 | J. Mater. Chem. A, 2013, 1, 358–366
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boron oxide lm results in a decrease in SFe, as it blocks the Fe
active sites and the reduction of Fe electron density.
Furthermore, the facile magnetic separation of 9.86 wt%
37,38
Fe–B/MWCNTs is very important for its lifetime.
magnetic properties of the sample are shown in Fig. 10. For 9.86
wt% Fe–B/MWCNTs, the typical coercivity (H ), saturation
magnetization (M ), and remnant magnetization (M ) at 298 K
The
c
s
r
ꢀ
1
ꢀ1
are 80.1 Oe, 22.5 emu g , and 3.2 emu g , respectively. The
magnetic properties enable the solution to be effectively sepa-
rated by a magnet.
According to previous research, a hydrogen generation rate
ꢀ
1
of 1.0 L min can supply H2 for a 162 W proton exchange
65,66
membrane fuel cell (PEMFC) at 100% H
2
utilization.
Therefore, a reactor lled with 1.0 g 9.86 wt% Fe–B/MWCNTs
may produce more than 0.57 L within 1 min in the hydrolysis of
N
2
H
4
. In other words, it supplies H
2
for a 92.3 W PEMFC at
1
00% H utilization in the hydrolysis of N H .
2
2 4
4
Conclusions
In summary, Fe–B/MWCNTs catalysts were prepared via the
chemical reduction of aqueous sodium borohydride with iron
2 4
chloride for the catalytic hydrolysis of N H . The use of meso-
porous MWCNTs as supports leads to the dispersion of Fe–B
Fig. 9 (a) TEM image; (b) HRTEM image; (c) SAED pattern; and (d) particle size
distribution of 9.86 wt% Fe–B/MWCNTs after a lifetime test over 30 h.
catalysts, therefore increasing the catalytic activity. The 9.86 wt
%
Fe–B/MWCNTs exhibited the highest catalytic activity with an
ꢀ
1
ꢀ1
average hydrogen generation rate of 34.2 L h
g
catalyst. The
introduction of basic sites or the addition of KOH to the
N H solution helps increase H2 selectivity. The 9.86 wt%
2
4
2 4
Fe–B/MWCNTs is a stable catalyst in the hydrolysis of N H with
ꢀ1
an average TOF value of 1.06 s over 30 h, which is 94.2% of its
initial TOF. The good catalytic activity and lifetime of 9.86 wt%
Fe–B/MWCNTs make it a strong candidate for developing highly
2 4
efficient portable hydrogen generation systems using N H as a
hydrogen storage material.
Acknowledgements
This work was nancially supported by the Cultivating pro-
gramme of Middle-aged backbone teachers (HG0092), the
Cultivating programme for Excellent Innovation Team of
Chengdu University of Technology (HY0084) and Innovative
Experimental Items for College Students of Sichuan Province
Fig. 10 Hysteresis loop and magnetic properties of 9.86 wt% Fe–B/MWCNTs.
(SZH1106CX04).
The 9.86 wt% Fe–B/MWCNTs catalyst lifetime provided
1
14 480 total turnovers (TTON) in N H decomposition over 30
2 4
Notes and references
ꢀ1
h. The average turnover frequency (ATOF) value was 1.06 s
,
which is 94.2% of its initial TOF (Table 2), illustrating that it is a
highly stable catalyst. During cycling slight deactivation occurs.
ICP analysis reveals that no Fe leaching occurs, which rules out
the loss of active phases aer repetitive use at room tempera-
ture. Moreover, TEM, SAED and XRD results indicate that no
changes occur in the particle size distribution, crystallization or
structure change for Fe–B/MWCNTs aer repetitive use at room
temperature (Fig. 9 and S16†). It appears that the B oxidation on
the active surface of Fe–B is responsible for the deactivation, as
shown by EDAX (Fig. S16†) and XPS (Fig. S17†). In particular, the
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