Fe-assisted Ru clusters supported on porous and graphitic carbon for ammonia decomposition to CO:X free hydrogen

Article abstract of DOI:10.1039/c6ra21211e

Porous and graphitic carbon supported Fe-assisted Ru cluster catalysts were prepared by a simple method. The textural and structural properties of the as-synthesized catalyst (Fe-C), the nearly pure carbon supported Ru catalyst (Ru/C^#) and the Fe-assisted Ru catalyst (Ru/Fe-C) were characterized by X-ray diffraction, high transmission electron microscopy, Raman spectra, X-ray photoelectron spectroscopy, inductively coupled plasma-atomic emission spectrometer and N₂ sorption techniques. The analysis results revealed a micro/mesoporous carbon system with high surface area, uniform pore-size distribution and high graphitization degree. The Fe-assisted Ru clusters catalysts showed significantly improved performance for NH₃ decomposition compared to that of Fe-C and Ru/C^# catalysts. It was concluded that Fe species not only played as a graphitization catalysts, but also promoted Ru catalysis, generating a synergistic effects during catalytic process.

Full text of DOI:10.1039/c6ra21211e

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Fe-assisted Ru clusters supported on porous and
graphitic carbon for ammonia decomposition to
Cite this: RSC Adv., 2016, 6, 102336
COx free hydrogen†
a
L. Li, F. Chen,b Y. Dai,a J. Wu,a J. L. Shaoa and H. Y. Lia
*
Porous and graphitic carbon supported Fe-assisted Ru cluster catalysts were prepared by a simple method.
The textural and structural properties of the as-synthesized catalyst (Fe–C), the nearly pure carbon
supported Ru catalyst (Ru/C#) and the Fe-assisted Ru catalyst (Ru/Fe–C) were characterized by X-ray
diffraction, high transmission electron microscopy, Raman spectra, X-ray photoelectron spectroscopy,
inductively coupled plasma-atomic emission spectrometer and N2 sorption techniques. The analysis
results revealed
a micro/mesoporous carbon system with high surface area, uniform pore-size
Received 23rd August 2016
Accepted 18th October 2016
distribution and high graphitization degree. The Fe-assisted Ru clusters catalysts showed significantly
improved performance for NH3 decomposition compared to that of Fe–C and Ru/C# catalysts. It was
concluded that Fe species not only played as a graphitization catalysts, but also promoted Ru catalysis,
generating a synergistic effects during catalytic process.
DOI: 10.1039/c6ra21211e
www.rsc.org/advances
experimental and theoretical study and determined that the
catalytic activity of Ru was dependent not only on the Ru
1. Introduction
particle size, but also on the Ru particle shape. Moreover, the
catalytic performance of catalyst is also inuenced signicantly
with various supports. Yin et al.19 found that carbon nanotubes
(CNTs) supported Ru catalyst showed the highest catalytic
performance, which is mainly related to the high Ru dispersion
on CNTs, and to the high graphitization and high purity of CNT
materials. Li and coworkers20 further demonstrated that the
higher graphitic degree of carbon used as supports showed
better catalytic performance. Combining Ru nanoparticles with
two-dimensional graphene nanosheets that forms a novel
composite catalyst showed excellent ammonia decomposition
activity.26 Meanwhile, the interaction between Ru particles and
the corresponding support, as well as the presence of the
remaining surface functional groups on the support, can also
inuence the electronic structure of Ru atoms, which is vital to
the reactivity of the corresponding Ru particles as well.21,22
Through these studies, it is well-accepted that the activity is
closely related to the nature and structure of the carbon mate-
rials supports.
In the present study, we report a facile method used to
prepare porous and graphitic carbon supported Ru-based
catalyst for ammonia decomposition (Scheme 1). During the
preparation process, the soluble NaCl was used as a porous
structure-directing template, and Fe species were employed as
carbon graphitization catalyst. We further compared the activity
of three different catalysts: Fe–C, Ru/C# and Ru/Fe–C catalysts.
The Ru/Fe–C catalyst showed the best catalytic activity among
these catalysts. The results from the present study show that
With the development of proton-exchange membrane fuel cells
(PEMFC), the on-site generation of hydrogen has become an
interesting alternative. In comparison to other carbonaceous H2
precursors such as methane or methanol, NH3 offers a higher
H2 storage capacity (17.7 wt%) and a higher energy density
(3000 W h kgꢀ1). In addition, it can be easily stored and trans-
ported under mild conditions compared with hydrogen. And
hydrogen source without COx are generated through catalytic
decomposition of ammonia, which can be directly applied for
fuel cell.1–4
In the past decade, many different catalysts have been
developed for ammonia decomposition. Many efforts have been
devoted on the study of Ru,5–9 Ni,10–13 Fe14–17 based catalysts for
ammonia decomposition. It has been demonstrated that Ru
catalysts showed the highest catalytic activity among the
common metal catalysts. Consequently, Ru-based catalysts have
attracted particular attention for hydrogen production from
ammonia because of their excellent catalytic performance.
Experimental observations have revealed that ammonia
decomposition is a structurally sensitive reaction on the surface
of Ru particles, and there is a close relationship between cata-
lyst particle size and activity.6,18,19 Karim et al.6 conducted an
aSchool of Chemistry and Chemical Engineering, Yancheng Institute of Technology,
Yancheng 224051, China. E-mail: lee_ycit@yahoo.com
bSchool of Chemical Biology and Materials Engineering, Suzhou University of Science
and Technology, Jiangsu, 215009, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra21211e
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distribution (PSD) plot was recorded by the DFT model. The
micropore volume (Vmicro) was estimated by using the t-plot
method.
Powder X-ray diffraction (XRD) patterns were recorded on
a Philips X'Pert MPD Pro X-ray diffractometer with graphite-
monochromatized Cu Ka radiation at a scanning rate of 2ꢁ
per min in the 2q range from 10ꢁ to 80ꢁ.
The morphology and microstructure of the HPC materials
were studied by a FEI Tecnai G2 F20 transmission electron
microscope (TEM) using an accelerating voltage of 200 kV.
X-ray photoelectron spectroscopy (XPS) analyses were carried
out using a JPS-90MS photoelectron spectrometer (JEOL, Japan)
with Mg Ka radiation (hm ¼ 1253.6 eV) in a vacuum of <10ꢀ7 Pa.
The C–C peak position was set to 284.8 eV and taken as an
internal standard.
Scheme 1 Illustration of preparation process for the Fe–C, Ru/Fe–C
and Ru/C# catalysts, respectively.
The Inductively Coupled Plasma-Atomic Emission Spec-
trometer (ICP-AES) with a type of ELAN 9000 was used to
determine the Ru contents.
this novel composite can be used as an efficient and promising
catalyst for ammonia decomposition or synthesis.
Raman spectra were collected from 200 to 2000 cmꢀ1 on
a Renishaw 2000 Confocal Raman Microprobe (Rhenishaw
Instruments, England) using a 514.5 nm argon ion laser.
2. Experimental section
Catalyst preparation
Catalytic activity
The synthesis procedure for several compared catalysts were
shown in Scheme 1 according to reported method.23 Typically,
carbon precursor C6H12O6 (0.1 mol), metal precursor
Fe(NO3)3$9H2O (2 mmol), and soluble-template sodium chlo-
ride (8.766 g) were sequentially dissolved in deionized water.
The resulting mixed solution was dried in drying oven and then
ground by agate mortar to obtain ne composite powders. For
the carbonization, the composite powders were heated at 850 ꢁC
for 2 h in a tube furnace under Ar (50 ml minꢀ1). Once cooled to
room temperature, the obtained powder was rst treated with
deionized water to dissolve sodium chloride, and the treated
sample was denoted as Fe–C catalyst. The Ru/Fe–C sample was
obtained by wetness incipient impregnation method using the
Fe–C sample and RuCl3$xH2O precursor with acetone as
solvent, followed by drying at 328 K for 5 h and calcination at
823 K for 2 h in Ar ow. The nearly pure porous carbon mate-
rials (named C#) were obtained by treating Fe–C sample with
hydrochloric acid to eliminate Fe-oxidic nanoparticles. The
Fe–C sample was dispersed in 50 wt% HCl solution and reuxed
at 90 ꢁC for 6 h. And the Ru/C# sample was obtained by wetness
incipient impregnation method using the pure carbon mate-
rials and RuCl3$xH2O precursor with acetone as solvent, fol-
lowed by drying at 328 K for 5 h and calcination at 823 K for 2 h
in Ar ow.
The activities of the catalysts were evaluated in a continuous ow
quartz reactor with pure NH3 (99.999%) feed at atmospheric
pressure. The amounts of N2 and unconverted NH3 in the
effluent were analyzed at room temperature by an online gas
chromatography equipped with a thermal conductive detector
(TCD) and a 3 m Poropak Q packed column and using H2 as
carrier gas. Blank tests indicated that the ammonia conversion in
a blank reactor or over the supports was less than 1.0% at 600 ꢁC.
3. Results and discussion
XRD patterns of graphitized carbon and their supported cata-
lysts are presented in Fig. 1. It can be seen that all patterns
exhibit two well-resolved diffraction peaks at about 2q ¼ 26ꢁ and
Catalyst characterization
N2 adsorption analysis was performed at 77 K using a Micro-
meritics ASAP 2020 to access the surface areas and pore size
distributions. All the samples were outgassed at 200 ꢁC for 12 h.
Then adsorption–desorption processes were conducted
between the relative pressure (P/P0) range from 10ꢀ6 to 1. The
specic surface area was calculated by the conventional Bru-
Fig. 1 XRD patterns of varied catalysts. (a) Fe–C (as synthesized), (b)
nauere–Emmette–Teller (BET) method. The pore size Ru/Fe–C (after H2 reduction), (c) Ru/C# (after H2 reduction) and (d) C#.
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43ꢁ, which can be indexed as the (002) and (100) reections of porosity. Ru clusters were homogeneously deposited onto the
the graphitic carbon, respectively. Aer through water washing surface of graphitic carbon via a wet-impregnation method
and HCl treatment, no NaCl and Fe phase were detected over (Fig. 2A and C). And their particle size distributions in the
carbon materials (C#) (Fig. 1d). As for Fe–C catalysts, only composite are presented in Fig. 2B and D. For Ru/Fe–C catalyst,
treated with water washing, show Fe3O4 crystallized phase. The the size distribution of deposited Ru clusters is 2.21 ꢂ 0.7 nm,
peaks are located at 30ꢁ, 36ꢁ, 43ꢁ, 54ꢁ, 57ꢁ and 63ꢁ, corre- except a few large particles. It might be speculated that Fe-oxidic
sponding to (100), (200), (310), (211), (301), (411) and (521) of species exist in the form of large particles by indirect correlation
Fe3O4 (JCPDS 01-089-2355), respectively. Subsequently, Ru with the Ru/C# catalyst. Through the comparison of Fig. 2C, it
clusters were introduced over carbon materials by a simple wet was found that there were no large particles except smaller Ru
impregnation method. Contrast to Fe–C catalysts, it is found clusters. During catalyst preparation procedure, the same
that Ru phase emerges aer reducing in H2 atmosphere. The procedure was adopted. It can be speculated that Fe-oxidic
diffraction peaks at about 2q ¼ 38ꢁ, 42ꢁ, 44ꢁ and 58ꢁ are particles exhibit a large size. Aer treated with HCl solution,
assigned as the (100), (002), (101) and (102) reections of Ru there remained large cave in carbon materials, also indirectly be
crystal phase (JCPDS 01-089-4903). In addition, the peak related with pore volume of BET characterization. In addition,
appearing in 2q ¼ 32ꢁ can be indexed to a-Fe2O3 (JCPDS 00-039- XRD and XPS characterization also veried the existence of Fe-
1346). Although all diffraction peaks are indexed, it is still oxidic particles. While the Ru/C# catalyst has an almost the
difficult to distinguish the overlapping peaks at around 43ꢁ, same size distribution with a value of 2.25 ꢂ 0.7 nm. Note that
which might be attributed to carbon, Ru and Fe3O4 species.
more Ru clusters over Ru/C# catalyst have a smaller size (<2 nm)
Fig. 2 shows (HR)TEM images of Ru/Fe–C and Ru/C# cata- compared with Ru/Fe–C catalyst. The nature of carbon support
lysts and their size distribution. Clearly, it is observed that treated with HCl solution would further inuence the particle
carbon support exhibits foam like structures with a disordered size distribution of Ru clusters. Smaller Ru nanoparticles would
provide more exposure of the active surface, which are readily
accessible for guest molecules in catalysis. Previous studies also
indicated that Ru crystallites with mean particle size of about
2.2 nm are extremely active for ammonia decomposition.24
However, the over-dispersed catalyst may result in too small
catalyst particles on the support, which may not provide enough
space to accommodate the recombination of N atoms to form
N2 molecules.25
HRTEM images (Fig. 2E and F) further provide detailed
information about the structure of prepared catalysts. Fig. 2E
demonstrates three different d-spacing patterns, 0.214, 0.234,
and 0.341 nm, which are corresponding to Ru (002), Ru (100)
and graphitic carbon species respectively. It is reported that the
measured d-spacing of hexagonal Ru crystals with different
crystallographic planes was 0.214, 0.205, 0.135, and 0.143 nm,
which was consistent with the d-spacing values of (002), (101),
(110), and (112) crystallographic planes of Ru crystals obtained
by XRD.26 The d-spacing value of Ru crystals can be assigned by
the XRD diffraction. A d-spacing value of 0.341 nm can indexed
as (002) reection of the graphitic carbon. As well-known that
the graphitic carbon is mainly produced from the amorphous
carbon which is in contact with the metal catalytic nano-
particles,27 so the shape of all graphitic sheets exhibits cycle-like
structure with different curvature. In addition, it also notes that
lattice fringe of graphitic carbon exhibits discontinuous state,
implying the prepared carbon is partly graphitized, which is
also veried by Raman characterization.
N2 sorption isothermals and pore size distributions of all
porous carbon supported catalysts are illustrated in Fig. 3. All
samples are typical of type-IV nitrogen sorption isothermals
with H4-type hysteresis loops, suggesting the existence of
irregular pore structure. An obvious capillary condensation step
at a medium relative pressure of (P/P0) 0.45–0.95, implying
a narrow mesopore diameter distribution. In addition, another
Fig. 2 (A, B and E) TEM images, the corresponding histograms of Ru
particle size distribution and high-resolution TEM image of Ru/Fe–C
catalysts. (C, D and F) TEM images, the corresponding histograms of Ru
particle size distribution and high-resolution TEM image of Ru/C#
catalysts.
N2 sorption is observed at a relatively high pressure of 0.95–1.0,
which can be attributed to the presence of macropores. The
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Fig. 4 Raman spectra of typical prepared carbon materials (C#).
band, associated with vibrations of carbon atoms with dangling
bonds in plane terminations of the disordered graphite and
related to the defects and disorders of structures in carbon
materials.20 The intensity ratios of the D-band to the G-band (ID/
IG) (ratio of amorphous to graphitic carbon) was calculated to be
0.936, indicating that the prepared carbon is partly graphitized.
This is consistent with the above XRD and TEM analysis. In fact,
the soluble NaCl was used as a porous structure-directing
template, while Fe species were employed as graphitization
catalyst during catalyst preparation process. Previous studies
also demonstrated that porous graphitic carbons could be also
ꢁ
Fig. 3 (A) N2 sorption isotherms and (B) pore-size distribution curves
for different samples (Fe–C, Ru/Fe–C and Ru/C#).
prepared at relatively low temperature (<1000 C) by means of
heterogeneous graphitization, which was carried out with the
aid of catalysts (i.e. Fe, Co, Ni, etc.).27,29–32
The electronic state of the Ru, O and Fe elements in the
catalyst were analyzed by XPS. Fig. 5a shows a survey of the XPS
spectra of Ru/Fe–C and Ru/C# catalyst. Both samples show
obvious peaks centered at the binding energies of around 533.1
and 287.4 eV, which are assigned to O 1s and C 1s XPS signals,
respectively. Additional peaks centered at the binding energies
of 484.08 and 461.08 eV are observed, which are assigned to Ru
3p1/2 and Ru 3p3/2 peaks, respectively. Weak Fe 2p peak is also
observed at the binding energies of around 713.08 eV. Seen
from Fig. 5b and c, in spite of the interference of the adventi-
tious carbon, the XPS spectrum can be resolved fairly well with
two spin–orbit-split doublets from two chemically different Ru
entities and the C 1s peak. Although the catalysts were prepared
from RuCl3 precursor even by H2 reduction at 500 ꢁC, it is
difficult to remove chlorine. This is in agreement with the
results of Mieth and Schwarz.37 As shown in Fig. 5b and c,
calculated BET surface areas, pore volumes and pore sizes are
shown in Table 1. It can be seen that the surface areas are range
from 300 to 387 m2 gꢀ1 over different samples, and the corre-
sponding total volumes vary from 0.258 to 0.402 m3 gꢀ1. By
means of micropore volumes data of different samples, it
demonstrates that mesopore volume signicantly reduces by
removing Fe species from carbon materials.
Raman spectroscopy is an effective tool to study the prop-
erties of the different carbon materials, including crystalline,
nanocrystalline and amorphous carbons. To further check the
graphitic character, the pure carbon materials were character-
ized by Raman spectroscopy, as shown in Fig. 4. It is observed
that two Raman bands center at ꢃ1335 and ꢃ1580 cmꢀ1
respectively. The peak at 1580 cmꢀ1 is the G band, characteristic
feature of ordered graphite carbon, attributed to the vibration of
sp2-bonded carbon atoms. Another peak at 1340 cmꢀ1 is the D
Table 1 Physical properties of varied catalysts
Samples
Ru contenta (wt%)
Fe contenta (wt%)
Fe/Ru molar ratio
SBET (m2 gꢀ1
)
Vmicro (cm3 gꢀ1
)
Vtotal (cm3 gꢀ1
)
Dp (nm)
Fe–C
—
0.97
0.91
1.16
1.15
0.57
—
2.14
1.13
366
387
300
0.086
0.093
0.081
0.378
0.402
0.258
4.12
4.15
3.44
Ru/Fe–C
Ru/C#
a
Determined by ICP-AES analysis.
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In order to further clarify the iron species over the surface of
prepared catalyst, the Fe and O part of the spectra are measured
and exploited. In Fig. S2,† the 2p Fe spectra are dominated by
higher oxidation states (mainly Fe3+). And the number of
Fe-oxidic species in Ru/Fe–C sample is obviously more than that
in Ru/C# sample. While O 1s peaks at 531.95 eV and 530.7 eV are
also attributed to the oxygen atoms in Fe2O3 and Fe3O4,
respectively (Fig. S3†). These ndings are consistent with above
XRD results. Moreover, the peak integration of the 2p Fe and 3d
Ru features are also illustrated in Table S1.† It can be seen that
there exist almost identical Fe/Ru ratios over the surface
composition of comparison samples. However, this is different
from the Fe/Ru ratios over the bulk composition (Table 1). Note
that the Ru/C# sample still has a Fe content of 0.ꢁ57 wt%, though
it was treated with 50 wt% HCl solution at 90 C for 6 h. This
means that the very little Fe-oxidic species embedded deeply in
carbon matrix cannot be directly dissolved, which were also
hard to be reduced by H2. Similar nding was also veried by Lu
et al.39 The graphitic carbon-shell protected cobalt nano-
particles had been achieved via a pyrolysis process, which
showed very high stability under strong acidic and basic
conditions.
The catalytic activities of the prepared porous carbon sup-
ported Ru-based catalysts for NH3 decomposition as a function
of reaction temperature are presented in Fig. 6. Interestingly, it
can be seen that there is basically no activity for Fe/C catalyst. It
might be attributed to less iron species or unreduced Fe-oxidic
species (Table 1). Once introducing Ru clusters into Fe–C
samples, it shows better catalytic activity than the Ru/C# cata-
lyst. It seems that the possible synergistic effect between Fe
nanoparticles and Ru clusters appears to be benecial for
activity. In order to clarify the synergic effects between Fe
species and Ru on the surface of carbon support, XPS charac-
terization was applied for study. In fact, the electron density of
Ru on Ru/Fe–C is slight lower than that on Ru/C# catalyst, which
do not contribute to the improvement of the catalytic activity.
However, more Fe-oxidic species could be reduced by abundant
generating H2 at a high reaction temperature, forming Fe0 as
single or dual active sites, which benets to rising activity. On
the other hand, the surface area and pore structure of the
Fig. 5 (a) Survey XPS spectra of Ru/Fe–C and Ru/C# catalyst after
reduction at 500 ꢁC. (b) High-resolution Ru 3d XPS spectra of Ru/Fe–
C. (c) High-resolution Ru 3d XPS spectra of Ru/C#.
different Ru species were observed. A signal at 280.2 eV is
attributed to Ru0, but not all Ru ions are reduced. The positions
of the Ru 3d peak at 280.6 and 281.0 eV show the presence of
RuO2 and RuCl3 species, respectively. As for the presence of Ru
oxide, it is possible that Ru nanoparticles were passivated upon
exposure to air.33 Note that Ru0/Rud+ ratio in Ru/Fe–C catalyst is
higher than that in Ru/C# catalyst as a comparison. Meanwhile,
it is also observed that there is 0.3 eV difference over the electron
binding energy of Ru 3p3/2 (Fig. S1†). In other words, the electron
density of Ru on Ru/Fe–C is slight lower than that on Ru/C#.
Fig. 6 Ammonia decomposition reaction activity test with Fe–C (-),
Ru/Fe–C (:) and Ru/C# (C) catalysts.
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Table 2 Properties and activities of comparison catalysts
Dispersionb (%)
Conversionc (%)
H2 formation ratec (mmol minꢀ1 gcat
)
TOFc (sꢀ1
)
Ead (kJ molꢀ1
)
ꢀ1
Samples
dRua (nm)
Ru/Fe–C
2.21
2.25
59.7
58.7
97.5
78.2
21.7
17.4
3.3
2.7
85.6
91.4
Ru/C#
a
c
Calculated on the basis of TEM images. b Calculated from the TꢀE1M results by employing the spherical model D ¼ 1.32/d.8,28 Obtained from the
ꢀ1
d
catalytic activities of Ru catalysts at 600 ꢁC. GHSV ¼ 20 000 ml h gcat
.
Activation energy was measured under pure NH3 ow at 375–450 ꢁC.
catalyst may be another major factor in this reaction. The Ru easy to large-scale production. Moreover, the durability of the
catalysts supported on the Fe-doped carbons have a higher Ru/Fe–C catalyst in ammonia decomposition was tested for the
ꢁ
surface area than that supported on the pure carbons. Thus the catalytic decomposition of ammonia at 550 C for a period of
Ru catalyst particles were well dispersed into the channel of the 50 h. As shown in Fig. S4,† ammonia conversion is almost
mesoporous carbon, which enhanced the adsorption of constant, indicating a very stable catalytic performance at high
ammonia. Interestingly, although the Ru/Fe–C catalyst has temperature.
a higher surface area and more mesopores, it still has a relative
In addition, the turnover frequency (TOF) of NH3 and the
lower catalyst dispersion (larger Ru particle size) evidenced by apparent activation energies (Ea) are presented in Table 2. The
TEM characterization. Previous studies indicated that a rela- turnover frequency (TOF) of NH3 can be calculated from the
tively larger catalyst particle can increase the catalytic amount of converted NH3 molecules per exposed surface Ru
activity.20,38 As mentioned in the beginning of this Section, N–H atom in unit time.8,28 Specically, the number of reacted mole-
bond cleavage and recombinative desorption of surface cules is obtained by the conversion and ow rate of ammonia at
nitrogen atoms instead of the NH3 adsorption is the rate- the corresponding temperature, while the number of the
limiting steps in ammonia decomposition. Over-dispersed available surface Ru atoms of each catalyst is calculated from
catalyst may result in too small catalyst particles on the the dispersion of Ru atoms measured by TEM, as shown in
support, which may not provide enough space to accommodate Table 1. With the help of Fe assistance, the Ru/Fe–C catalyst
the N–H split and the recombination of N atoms to form N2 exhibits a higher TOF value (3.3 sꢀ1) than that of Ru/C# catalyst
molecules.
In addition, there would be a correlation between the crys-
(2.7 sꢀ1).
The apparent activation energies (Ea) were obtained from the
tallites of the carbon support and activity, which was observed Arrhenius relationship between the rate constant (k) and the
by Zhu's studies.20 A lower graphitic degree results in the poor temperature (T), which can be described by the equation: ln(k)
electron conductivity of the carbon support, thus resulting in ¼ ꢀEa/RT + constant, and the results are listed in Table 2. It can
the lower activity of the carbon supported catalyst. As be seen that the activation energy of Ru/Fe–C catalyst obtained
mentioned before, both cleavage of N–H bond and recombi- from the Arrhenius plots (Fig. 7) is 85.6 kJ molꢀ1, which is lower
native desorption of surface nitrogen atoms may be the rate- than that of Ru/C catalyst (91.4 kJ molꢀ1). It indicates that Fe
limiting steps in ammonia decomposition on well-dispersed additives are facilitate the molecular activation during reaction
Ru catalyst.34 Support or additive which can facilitate electron process. Moreover, the Ea values in our studies are similar to the
feedback to the antibonding orbital of the transient metal– reported Ea value of Ru/CNTs catalyst.36
nitrogen bond should be benecial for these steps.35 In terms of
CNTs, it is a superior support for ammonia decomposition
because of its high metal dispersion and good degree of
graphitization, which is benecial for the electron transfer
between support and Ru.1 In our studies, porous carbon support
with a certain degree of graphitization is also benet for
improving catalytic performance of Ru-based catalyst.
For comparison, Table S2† summarizes the catalytic perfor-
mance of Ru/Fe–C and other typical Ru-based catalysts reported
previously for ammonia decomposition. No doubt that Ru
supported by CNTs (Ru/CNTs) was considered to be a high-
performance catalyst for ammonia decomposition. However,
Ru loading over CNTs has an important effect on activity. 5 wt%
Ru loading catalyst shows better catalytic performance than that
with around 1 wt% Ru loading. Note that our prepared Ru/Fe–C
catalyst showed a comparable activity with 1 wt% Ru/CNTs
Fig. 7 Arrhenius plots of Ru/C# (C) and Ru/Fe–C catalyst (-). The
catalyst prepared by Chang et al.36 Although the activity of Ru/
temperature range (375–450 ꢁC) used for the determination of
Fe–C catalyst is lower than that of graphene or CTF supported
Ru-based catalyst, the prepared process is relatively simple and
apparent activation energies was based on NH3 conversion values far
away from the equilibrium values.
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3
is 3.3 sꢀ1. The activation energy of Ru/Fe–C catalyst is 85.6 kJ
molꢀ1, which is lower than that of Ru/C# catalyst. It was
concluded that Fe species not only played as a graphitization
catalysts, but also promoted Ru catalysis, generating a synergistic
effects during catalytic reaction process.
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Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (21503184, 21407111), Talent introduction project
of Yancheng institute of technology (xj201507). And the research
fund of Collaborative Innovation Center for Ecological Building.
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