Cobalt nanoparticles embedded in a porous carbon matrix as an efficient catalyst for ammonia decomposition

Article abstract of DOI:10.1039/c7cy00086c

A metallic Co nanoparticle catalyst embedded in a carbon matrix was synthesized through a facile solvothermal method and subsequent thermal treatment. The synthesized Co-C precursors were calcined in N₂ and limited air atmosphere respectively, which were applied for ammonia decomposition. It was found that different thermal treatments have a significant effect on the catalyst microstructure, particle size, components and catalytic performance. Compared to the Co@C-700-N sample obtained in N₂ flow, the CoO_x@C-700-A catalyst obtained in limited air atmosphere showed better activity with a ～55% NH₃ conversion (500°C and GHSV = 15 000 h^-1). Moreover, the structure sensitivity of NH₃ decomposition over Co catalysts is proposed over a series of CoO_x@C-A catalysts. With an increase of Co particle size, the catalytic activity showed an obvious decrease.

Full text of DOI:10.1039/c7cy00086c

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ARTICLE
Cobalt nanoparticles embedded in porous carbon matrix as an
efficient catalyst for ammonia decomposition
Lei Li, *a Ruiyu Jiang,a Wei Chu,b,c Hui Cang,a Huawei Chen a and Jinlong Yan*b
Received 00th January 20xx,
Accepted 00th January 20xx
Metallic Co nanoparticles embedded in carbon matrix catalyst was synthesized through a facile solvothermal method and
subsequent thermal treatment. The synthesized Co-C precursors were calcined in N2 and limited air atmospheres
respectively, which were applied for ammonia decomposition. It was found that different thermal treatment had a
significant effect on the catalyst microstructure, particle size, components and catalytic performance. Comparing to that
Co@C-700-N sample obtained in N2 flow, the CoOx@C-700-A catalyst obtained in limited air atmosphere showed better
activity with ~55% NH3 conversion (500 °C and GHSV = 15,000 h-1). Moreover, a structure sensitivity of NH3 decomposition
over Co catalysts is proposed over a series of CoOx@C-A catalysts. With an increase of Co particle size, the catalytic activity
showed an obvious decrease.
DOI: 10.1039/x0xx00000x
www.rsc.org/
the nature and structure of the support materials. Among
these investigated supports, carbon materials with their high
thermal stability and electrical conductivity are advantageous
1. Introduction
In recent years, energy and environment issues have
become increasingly prominent. How to obtain clean energy
and eliminate pollutions has attracted extensive research
attention. Liquid ammonia has recently been considered as a
potential source for clean and carbon oxide free hydrogen.1–3
Generating pure hydrogen via ammonia decomposition has
gained much attention because of the low cost of this
technology and ease of operation compared to the complex
systems required for reforming methanol or hydrocarbons.
In the past decade, many different catalysts have been
developed for ammonia decomposition. Many efforts have
been devoted on the study of Ru, 4–8 Ni, 9–13 Fe14–17 and Co 18–21
based catalysts for ammonia decomposition. It has been
demonstrated that Ru catalysts showed the highest catalytic
activity among the common metal catalysts. However, Ru-
based catalysts have the obvious disadvantages of high cost
and limited availability for large scale applications. Catalysts
based on transition metals, such as Fe, Ni and Co, also show
good activity for NH3 decomposition, though they usually work
at higher temperatures than Ru-based catalysts. Therefore, it
is necessary to develop new catalysts with high activity at low
temperature.
in high temperature reactions and are thought to be able to
transfer electrons from the support to the active species, thus
facilitating the recombinative desorption of surface nitrogen
atoms.4,6,26,27 Commercial carbon nanotubes (CNTs) supported
residual Fe, Co or Fe-Co alloy nanoparticles were highly active
for NH3 decomposition while the microstructure of CNTs
14,29,30
remained unchanged.28,25 Subsequently, Duan et al.
prepared Fe or Ni nanoparticles on the top of carbon
nanofibers (CNFs) for ammonia decomposition by the catalytic
chemical vapor decomposition method. The investigated
catalysts showed highly active and stable at high temperature
over 25 h. Besides, different thermal pre-treatment were used
to improve the interaction between Co nanoparticles and
carbon support. It revealed that metal dispersion, metal
particle size, support porosity and the electronic modification
of the active sites of cobalt/carbon catalysts have great effects
on ammonia decomposition activity. 19,21
Based on the recognization of the advantage of carbon
materials, in this paper, Co nanoparticles embedded in carbon
matrix were facile prepared as catalysts for ammonia
decomposition. The aim is to study the impact of thermal
calcination of the synthesized Co-C precursors in N2 and
limited air atmosphere on the properties and the catalyst
performance in the reaction of NH3 decomposition. Various
techniques including XRD, BET, SEM, TEM, ICP, XPS, TG-DTA
and H2-TPR were used to characterize the catalysts to
understand the structure performance relationship.
Various supports (activated carbon, MgO, Al2O3, ZrO2,
carbon nanofibers, and carbon nanotubes) have been
employed to load base metal nanoparticles for ammonia
decomposition.12–15,22–25Through these studies, it is well-
accepted that the activity of the catalysts is closely related to
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binding energies were referenced to the C1 s line at 284.8 eV
from adventitious carbon.
The Raman spectra were collected on
a
Raman
spectrometer (JY, HR 800) using a 514 nm laser. The metal
loadings on porous carbon were determined by inductively
coupled plasma-atomic emission spectrometry (ICP-AES, PE
OPTIMA 2100DV).
Thermogravimetric analysis (TGA) was simultaneously
performed on a DTG-60 instrument (Shimadzu, Japan) at a
heating rate of 10 °C/min under N2 and air atmospheres
respectively.
H2-TPR was performed in a conventional setup equipped
with a thermoconductive detector. In typical runs, 30 mg of
sample was heated to 120 °C at 10°C/min under Ar flow of 50
mL/min and kept at this temperature for 1 h to remove
adsorbed water. After the sample cooled down to RT and was
switched to a 8% H2/Ar (v/v, 40 mL/min) mixture, the sample
temperature was programmed to 800 °C at 10°C/min.
Fig. 1. Scheme of the catalyst preparation.
2. Experimental
2.3 Catalyst activity
2.1 Catalysts preparation
The embedded Co@C catalysts were prepared by a simple
solvothermal method31 and subsequence thermal treatment
The activities of the catalysts were evaluated in a
continuous flow quartz reactor with pure NH3 (99.999%) feed
at atmospheric pressure. Prior to the reaction, 25mg powder
sample was activated with 25% H2/Ar at 350 °C for 1h. After
the catalyst activation, pure NH3 was introduced into the
reactor. Then the pure NH3 was introduced into the reactor
until it reached a steady status. This period usually takes 0.5 h.
For temperature-dependent conversion measurements of NH3,
the temperature was varied between 400 and 500 °C in 25°
steps with a special space velocity of 15000 h-1. For each
temperature, four values were recorded within 1h under
steady-state conditions using a GC-9860. The GC is equipped
with a thermal conductive detector (TCD) and a 3 m Poropak Q
packed column and using H2 as carrier gas. NH3 fractional
conversion was calculated by a standardized method on
nitrogen atom basis.14 Ammonia decomposition rate
(RateNH3:molNH3molCo-1h-1) was calculated by multiplying the
space velocity with the ammonia conversion and dividing the
metal molar quantities. Blank tests indicated that the
ammonia conversion in a blank reactor or over the supports
was less than 1.0 % at 600 °C.
(Fig. 1). Typical synthesis process as follows: 0.6
g of
Co(AC)2•4H2O was first resolved in 60 mL of a mixed H2O and
EG solution (v/v = 1:1). Then, 1.0 mL of furfural was
introduced. After stirring for 0.5h, the solution was transferred
into a 100 mL Teflon-lined stainless steel autoclave and kept at
180 °C for 15 h. The resulting products were collected from the
autoclave, centrifuged and washed with deionized water and
ethanol for three times, and finally dried at 70 °C for 10 h. The
calcination of as-synthesized sample was carried out through
two approaches. One approach is that the sample was heated
to 700 °C at a rate of 2 °C/min and kept for 3 h in the N2 flow.
The corresponding sample was denoted as Co@C-700-N
catalyst. The other approach is that the sample was
transferred into a crucible with a lid, which was put in a muffle
furnace. Then the temperature was elevated to desired
temperature (i.e., 700, 800, and 900 °C) at a rate of 10 °C/min
and maintained for 1 h. The obtained samples were denoted
as CoOx@C-700-A, CoOx@C-800-A and CoOx@C-900-A catalyst,
respectively.
2.2 Catalyst characterization
N2 adsorption analysis was performed at 77 K using a
Micromeritics ASAP 2020 to access the surface areas and pore
size distributions. All the samples were outgassed at 200 °C for
6 h.
3. Results and discussion
3.1. Structural characterization of embedded Co@C catalysts
Powder X-ray diffraction (XRD) patterns were recorded on
a Philips X’Pert MPD Pro X-ray diffractometer with graphite-
monochromatized Cu Kα radiation at a scanning rate of 2°/min
in the 2θ range from 20 ° to 80 °.
TEM images were taken over
instrument operated at 200 kV. The SEM images were taken
on Hitachi S-4800 scanning electron microscope to
a JEOL JEM-2000EX
a
determine the catalyst morphology and microstructure.
XPS data were obtained with an ESCALab220i-XL electron
spectrometer from VG Scientific using 300 W Al-K radiations.
The base pressure was approximately 3 × 10−9 mbar. The
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100
80
B
ꢀ
Co3O4
CoO
∗
Co
∗
60
∗
ꢀ
ꢀ
b
a
d
a
∗
40
∗
ꢀ
ꢀ
20
0
0
20
30
40
50
θ /
60
70
80
100 200 300 400 500 600 700
2
°
Temperature (°C)
Fig. 2. TGA curves of Co-C precursors calcined at a heating rate Fig. 3. (A) and (B) XRD patterns of different samples. a =
CoOx@C-700-A; b = CoOx@C-800-A, c = CoOx@C-900-A and d =
Co@C-700-N.
of 10
respectively.
°
C/min until 700
°C in an air (a) and N2 atmosphere (b)
All of the synthesized Co-C precursors need to be
thermally treated before reaction. To have
understanding on the carbonization
The XRD patterns of Co nanoparticles embedded in
a
better carbon matrix samples calcined in N2 and limited air
process, atmosphere are shown in Fig. 3. No diffraction peaks of cobalt
thermogravimetric analysis (TGA) was employed in this study. compounds are detected in the synthesized Co-C precursor,
The TGA curves of synthesized Co-C precursors measured in air indicating that the size of cobalt species is too small and below
and N2 flow respectively are observed in Fig. 2. It can be the detection limits of XRD (Fig. 3A). When the synthesized Co-
observed that the carbonization/gasification of as-synthesized C precursors were calcined at 700 °C or above temperature in
Co-C precursors occurred from 200 to 700 °C. And in both of limited air atmosphere, the characteristic peaks of CoOx
the cases the weight loss below 200 °C is generally attributed species appear, which are corresponding to the mixture of Co,
to evaporation of adsorbed water. Under an air atmosphere, a Co3O4 and CoO species. The diffraction peaks located at 31.2,
significant weight loss is observed at temperatures ranging 59.3 and 65.2° correspond to crystalline facets (220), (511) and
from 250 to 400 °C, as a result of complete combustion of (440) of Co3O4 crystalline phase (JCPDS 01-078-1970). The
carbon matrix. No apparent weight loss is detected when the diffraction peaks (2θ = 44.2, 51.5 and 75.8°) are assigned as
temperature is higher than 400 °C. When the measurement the (111), (200) and (220) reflections of Co crystal phase
was carried out under a N2 atmosphere, the weight of Co-C (JCPDS 01-089-7093). And the peaks located at 36.6, 42.3,
precursors decreased gradually as the temperature was 61.4, 73.6 and 77.8° are attributed to the crystalline facets
increased from 250 to 700 °C, mainly due to organics and (110), (-111), (-202), (-112) and (220) of CoO crystalline phase
carbon lost or burned in this heating process.32 The total (JCPDS 01-072-1474). Seen from Fig.3A, the diffraction peaks
weight loss is ca. 65 wt % at 700 °C under a N2 atmosphere, of CoOx become sharp with an increase in calcination
which is much less than the weight loss (ca. 85 wt %) under an temperature for a series of CoOx@C-A samples. This means that
air atmosphere. It suggests that more carbon lost or burned the average crystallite size of the CoOx particles increases due
off in air atmosphere than in N2 atmosphere, which results in to thermal treatment. Based on the reflection peak at 2θ =
generating more porosity, ensuring better accessibility of the 36.6°, the mean size of CoOx nanoparticles in CoOx@C-700-A
encapsulated Co to NH3 molecules. This is consistent with the sample is calculated to be ca. 6 nm according to Scherrer’s
BET results. When the Co-C precursors were calcined in limited equation. The corresponding mean sizes of CoOx particles are
air atmosphere, more mesopores generated in this heating ca. 26 nm and ca. 38 nm for CoOx@C-800-A and CoOx@C-900-
process.
A sample (Table 1). The results are also supported by TEM
measurement (Fig. 4). In addition, it is also observed that the
reflection peak of graphite carbon (002) becomes obvious and
sharp for a series of CoOx@C-A samples, indicating of high
graphitization degree with an increase of calcination
temperature.
A
#
Co3O4
Co
graphite
CoO
ꢀ
∗
#
∗
ꢀ
ꢀ
∗
ꢀ
ꢀ
ꢀ
∗
Fig. 3B presents the comparative XRD patterns of
CoOx@C-700-A and Co@C-700-N sample. Compared with
CoOx@C-700-A sample, only Co crystallite phase is observed
for Co@C-700-N sample. The effect of autoreduction of nickel
and cobalt oxide over carbon supports has been observed
c
b
a
uncalcined
60 70 80
33
34
earlier by Gandía and Montes,
Sun
and Coville 35. This
20
30
40
50
θ /
suggests that carbon support works as reducing agent during
calcination of Co-C precursors with formation of partially
reduced Co species. Owing to an existence of limited oxygen
2
°
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during calcination process, both of cobalt oxides and Co atmosphere, the particles size of CoOx nanoparticles grew to a
crystallite phase occurred together for CoOx@C-700-A sample. mean size of 24.2 nm (Fig. 4c). This means that partial of CoOx
nanoparticles gathered/sintered together at higher calcination
The morphology and particle size were further
temperature. And the obvious (111) crystallographic plane of
characterized by TEM. As shown in Fig. 4a, the spherical CoOx
Co3O4 is confirmed (Fig. 4f). Compared with the CoOx@C-700-A
nanoparticles are homogeneously distributed on the carbon
sample, the Co particles embedded in carbon matrix show a
matrix. The size distribution of these spherical CoOx
similar mean size of 3.5 nm for Co@C-700-N sample (Fig. 4b),
nanoparticles ranges from 2.5 to 7 nm with a mean size of 4.5
which might be attributed to N2 protection during heating
nm, and the corresponding d-spacing obtained from the lattice
process. The particle size obtained from the TEM
fringes were 0.205, 0.213 and 0.245 nm, which were
characterization is consistent with that from the XRD
consistent with the d-spacing value of the (111)
measurements. In addition, only the (111) and (200)
crystallographic planes of the metallic Co and the (110), (-111)
crystallographic planes of the metallic Co are found from
crystallographic planes of the metallic CoO (Fig. 6d). When the
HRTEM image (Fig. 4e).
Co-C precursors were calcined at 800 °C in limited air
Fig. 4. TEM images and particle size distribution of CoOx@C-700-A sample (a), Co@C-700-N sample (b), and CoOx@C-800-A (c).
And their corresponding HRTEM images (d-f).
Table 1 Physical properties of different CoOx@C nanocomposites.
b
b
b
b
b
Sample
Co content a
(wt %)
18.73
SBET
Sex
Vmi
Vtot
Dp
Particle size Particle size
(m2 g-1)
545.2
430.1
225.1
525.2
(m2 g-1)
112.1
131.5
152.9
27.9
(cm3 g-1)
0.18
0.12
0.03
0.19
(cm3 g-1)
0.31
0.33
0.27
0.23
(nm)
2.28
3.10
4.80
1.77
c (nm)
6.0
d (nm)
4.5
CoOx@C-700-A
CoOx@C-800-A
CoOx@C-900-A
Co@C-700-N
34.71
37.31
27.11
26.4
38.1
-
24.2
-
3.5
a Determined by ICP-AES analysis.
b SBET: BET surface area; Sex: external surface area; Vmi: micropore volume; Vtot: total volume; Dp: average pore diameter.
c Calculated by Scherrer equation. The mean crystallite size of cobalt oxide was calculated based on the reflection peak at 2θ =
36.6 °.
d Calculated by TEM images.
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The key design strategy for our prepared embedded
catalyst is that the carbon matrix should have a highly porous
structure, which facilitates NH3 molecules diffusion and
adsorption. When the Co-C precursors were calcined in limited
air or N2 atmosphere at high temperature, the pore structure
and surface areas of different samples were further
investigated by N2 sorption technique. N2 sorption isothermals
and pore size distributions of all embedded Co@C samples are
illustrated in Fig. 5A. Typical of type-IV nitrogen sorption
isothermals with H4-type hysteresis loops are observed over all
samples, suggesting the existence of irregular pore structure.
Whether the Co-C precursors were calcined in limited air or N2
atmosphere at high temperature, an obvious capillary
condensation step occurs at a medium relative pressure of
(P/P0) 0.45–0.95, implying a narrow mesopore diameter
distribution (Fig. 4B and Table 1). Besides, the uptake of
nitrogen rises noticeably at relative pressures of less than 0.1,
indicating that more micropores are presented in these
samples. It has been widely reported that physical and
chemical activation of carbon materials at high temperature
helps to open and widen their microporosity even with a
conversion to mesopores.36
B
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
c
b
a
d
100
1
10
Pore Diameter (nm)
Fig. 5. (A) and (B) BET adsorption–desorption isotherm and
pore diameter, respectively. a = CoOx@C-700-A; b = CoOx@C-
800-A, c = CoOx@C-900-A and d = Co@C-700-N.
Fig. 6 shows SEM photograph and corresponding EDS
elemental mapping of typical CoOx@C-700-A sample. It can be
seen that the microsphere exhibits a nanosheets-shaped
structure. The EDS elemental maps confirm that C, O and Co
elements are highly dispersed in CoOx@C-700-A sample. The
relative atomic content of these element are also listed in
Table 2. The mass ratio of Co is 13.7%, which is approximate to
the results obtained by ICP measurement (Table 2).
The calculated BET surface areas, pore volumes and pore
sizes are presented in Table 1. It can be seen that the BET
surface area of CoOx@C-700-A exhibits the largest specific
surface area with a value of 545.2 m2g-1, and its corresponding
micropore volume is 0.18 m3g-1. With an increase of
calcination temperature, the SBET values increase and SEX
values decrease, which indicates that more meso/macro pores
generated during calcination process, and calcination
temperature remarkably affect the pore structure of these
composites. In comparison with CoOx@C-700-A sample, the
Co@C-700-N sample also exhibits
a comparable specific
surface area with a value of 525.2 m2g-1, and its corresponding
micropore volume is 0.19 m3 g-1. However, its external surface
is very low with a value of 27.9 m2g-1, indicating the existence
of a large number of micropores. This means that heating
treatment in limited air is contributed to the formation of
developed porosity (mesopores/macropores).
Fig. 6. SEM photograph and elemental mapping images of
CoOx@C-700-A for C, O and Co with color superposition (C =
blue; O = green; and Co = red).
A
Table 2 The EDS results of the catalyst of CoOx@C-700-A.
Element
C K
Mass ratio
71.33
Atomic ratio
83.44
a
b
O K
15.20
13.35
Co K
Total
13.47
3.21
c
100.00
100.00
d
0.0
0.2
0.4
0.6
0.8
1.0
3.2. Nature of cobalt species supported on carbon matrix and
their reducibility
Relative pressure (P/P0)
X-ray photoelectron spectroscopy (XPS) was employed to
study the chemical state of Co in the embedded Co@C
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catalysts. XPS survey spectrum reveals the presence of cobalt,
carbon and oxygen (Fig. 7a). Co 2p XPS peaks show more
complex patterns (Fig. 7b). In the CoOx@C-700-A sample,
fitting of the Co 2p envelope leads to the identification of
three patterns: cobalt oxide species (Co 2p3/2 at 780.2, 782.4
eV) and the corresponding shakeup satellite signals (789.0 eV
and 804.6 eV). 37,38 Moreover, no zero-valent metal species (Co
2p1/2 or 2p3/2) were observed, except the shakeup satellite
CoOX
b)
CoOX
CoOX
Cosat
Cosat
CoOx@C-700-A
CoO
Co0
CoOX
Co0
XCosat
39
signals of Co0 (785.2 eV and 801.0 eV). It indicates that a
CoOX
Cosat
large number of cobalt species remain in the form of cobalt
oxide in CoOx@C-700-A sample, which is consistent with the
XRD results. As for the Co@C-700-N sample, peaks
characteristic of metallic Co are clearly observed with the
typical binding energies of 793.3 and 778.4 eV, which are
assigned to Co 2p1/2 and Co 2p3/2 electrons of Co metal
respectively. In the cobalt region, it is also seen that the peaks
Co@C-700-N
810 805 800 795 790 785 780 775 770
Binding energy (eV)
Fig. 7. (a) XPS survey spectrum of carbon supported Co-based
characteristic of cobalt oxide species and the shakeup satellite catalyst (CoOx@C-700-A and Co@C-700-N). (b) Fitting of Co 2p
signals of Co0 and CoOx. It further demonstrates that the
envelopes.
autoreduction phenomenon of cobalt oxides occurred during
In order to study the reducibility of CoOx nanoparticles
the calcination process of synthesized precursors.
and the possible cobalt-support interaction, the H2-TPR
The surface structure of Co species, as well as the carbon
experiments were carried out. H2-TPR profiles of various Co-
matrix, were also studied by Raman spectroscopy. Fig. 8
based samples are presented in Fig. 9. It can be seen that there
illustrates the Raman spectra of different samples calcined in
exist two obvious low-temperature reduction peaks at 250-450
limited air atmosphere at temperatures ranging from 700 to
°C for other three samples, except for Co@C-700-N sample.
900 °C. Vibration modes of CoO can be observed at ca. 670 cm-
40 The obvious peak between 2500 and 2800 cm-1, related to
Another reduction peak occurs at above 500 °C. Previous
1
.
studies indicated that the reduction peaks occurred below 450
°C, which could be assigned to the continuous reduction of
Co3O4 to CoO and metallic cobalt. And high temperature
reduction peaks above 500 °C, which were attributed to the
gasification of carbon support.44 These results are also
consistent with above XRD and XPS analysis. A series of Co@C-
A samples show obvious crystallite phases of cobalt oxide,
while there are very little cobalt oxide species except zero-
valent Co species over the Co@C-700-N sample. It indicated
that cobalt oxide species were easily reduced by carbon during
the thermal treatment in nitrogen flow at higher
temperature.19,45 Note that the reduction peaks of CoOx
species over the CoOx@C-700-A sample were shifted toward
lower reduction temperature in comparison with other two
samples. It suggests that smaller CoOx particles would be easily
reduced at lower temperature. As for the possible cobalt-
carbon interaction, Zhang et al. 21 and Xiong et al. 46 also found
a similar phenomenon over MWCNTs supported Co catalysts
respectively. The reduction peaks of cobalt oxides shifted to
lower temperature when the interaction of cobalt with carbon
increased. Therefore, it can be speculated that this interaction
will favor the electron transfer between cobalt oxide and
carbon surface. In addition, the intensities and areas of
reduction peaks (especially for carbon peaks) also changed
greatly, which can be correlated with the Co contents in
various samples.
the 2D band, is only detected over the CoOx@C-900-A sample.
Besides, two Raman bands center are also observed at 1335
and 1580 cm-1 respectively. The former band (D-band) is
associated with a double-resonance effect and the latter band
(G-band) corresponds to graphite in-plane vibrations with E2g
41,42
symmetry.
The intensity ratios of the D-band to the G-
band (ID/IG) (ratio of amorphous to graphitic carbon) over
different samples were calculated to be 0.932, 0.909 and 0.881
respectively, indicating the amorphous carbon structure with a
high content of lattice edges or defects in plane terminations
of disordered graphite of carbon materials. With the aid of Co
catalysts, porous graphite carbon could be prepared at
relatively low temperature (<1000 °C).43 Combining these
results and those from XRD analysis and HRTEM images, it can
also be speculated that the degree of graphitization is
relatively low in the carbon layers around CoOx nanoparticles.
4x104
3x104
2x104
a)
C1s
C(KVV)
Co2p
O1s
CoOx@C-700-A
Co@C-700-N
1x104
0
1400 1200 1000 800
600
400
200
0
Binding energy (eV)
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140
120
100
(B)
G
D
D*
80
60
40
20
0
c
Co@C-700-N
CoOx@C-A
b
a
0
5
10
15
20
25
30
35
40
Particle size (nm)
500
1000
1500
2000
2500
3000
3500
Raman shifts (cm-1)
Fig. 10. (A) Ammonia decomposition reaction activity test over
different catalysts: CoOx@C-700-A, CoOx@C-800-A, CoOx@C-
900-A and Co@C-700-N. (B) Reaction rate for NH3
decomposition at 500 °C and 15,000 h-1 as a function of Co
particle size.
Fig. 8. Raman spectra of Co-C precursors calcined at different
temperature under limited air atmosphere, (a) CoOx@C-700-A,
(b) CoOx@C-800-A and (c) CoOx@C-900-A.
Fig. 10A illustrates the catalytic activities of the
embedded Co@C catalysts for NH3 decomposition as
a
function of reaction temperature. The results show a similar
behavior that the ammonia conversions increased with an
increase of the catalytic reaction temperature among these
catalysts, indicating faster decomposition at higher
temperature. Among these investigated catalysts, the
CoOx@C-700-A catalyst shows the highest activity with a ~55%
NH3 conversion under the reaction conditions (500 °C and
a
b
c
15,000 h-1). With an increase of calcination temperature,
a
d
series of CoOx@C-A catalysts show an obvious decrease of NH3
conversion. The rates of NH3 decomposition over different
catalysts are also presented in Fig. 10B. The rates of NH3
decomposition decrease obviously with an increase of cobalt
particle size, showing a remarkable size-dependent activity.
This correlation between particle size and NH3 conversion
were also observed on the surface of Ru, Ni and Fe-based
catalysts.6,10,14,15,47 The ammonia decomposition activities
were greatly influenced by particle size of active components.
100 200 300 400 500 600 700 800 900
Temperature (°C)
Fig. 9. H2-TPR profiles of various Co-based samples, (a)
CoOx@C-700-A, (b) CoOx@C-800-A, (c) CoOx@C-900-A and (d)
Co@C-700-N.
3.3 Catalytic performance
3.3.1. NH3-T dependence
It can be concluded that ammonia decomposition is
structure-sensitive reaction over Co-based catalysts.
a
As contrast, the catalytic activity of Co@C-700-N catalyst
is lower than that of CoOx@C-700-A catalyst, although the
former has a higher Co contents in carbon matrix. On the basis
of above characterizations, different heating treatment greatly
affects the microstructure and components of investigated
catalysts. Note that the particle size of these two types of
catalyst is similar to each other. Therefore, we propose that
the particle size difference is probably not a crucial cause for
different activities. According to N2 sorption results, the
CoOx@C-700-A catalyst obtained via a thermal treatment in
limited air exhibits more mesopores rather than micropores,
which are beneficial for NH3 molecules diffusion/adsorption
and activation. We have previously found that the porosity of
supports had an significant effect on catalytic activity.48 In
present study, this higher catalytic performance should be
attributed to the more developed porosity of CoOx@C-700-A
70
(A)
60
Co@C-700-N
CoO
CoO
CoO
x
@C-700-A
@C-800-A
@C-900-A
50
40
30
20
10
0
x
x
400
425
450
475
500
Temperature (°C)
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catalyst. Some studies also pointed out that the fraction of B5
sites over catalyst surface were assigned to be more active for
6,10,12
ammonia decomposition.
In fact, the morphology and
structure of Co particles over comparison catalysts are also
influenced by different thermal treatments, which in turn
affects the concentration of probable B5-like sites. The
maximum activity per gram Co of the CoOx@C-700-A catalyst
should be much higher than that of Co@C-700-N catalyst.
For comparison, Table
3 summarizes the catalytic
performance of representative carbon supported base metal
catalysts reported previously for ammonia decomposition. The
NH3 conversion of CoOx@C-700-A catalyst in this work is
superior to CNF or CNT supported Co, Fe or Ni-based catalysts,
although the metal contents in catalyst are different. The
observed high catalytic activity of CoOx@C-700-A catalyst
should be attributed to the following reasons. On one hand,
cobalt particles with relatively smaller sizes provide a higher
concentration of active sites. On the other hand, the unique
electron reservoirs capacity of graphitic carbon, which would
effectively transfer electrons from cobalt surface to support
and promote recombinative desorption of intermediate
nitrogen atoms.25,49 Besides, the high catalytic performance
can be also influenced by the cobalt-carbon interaction. Chen
et al. found that, due to confinement effects, iron particles
inside CNTs showed higher catalytic activity.50 And the
improved redox properties of the confined iron catalyst may
play a more important role during reaction process. Because
most Co particles are embedded in carbon matrix for our
investigated catalysts, it is reasonable to assume that this
confinement effect contributes to high NH3 conversion.
Fig. 11. NH3 conversion at T = 500 °C and GHSV = 15,000 h-1 as
a function of time for NH3 decomposition over CoOx@C-700-A
catalyst.
3.3.2. Stability
As reviewed in the introduction section, these types of
carbon supports, such as CNF, CNTs and meso-carbon, have
received enough attention during ammonia decomposition
reaction. Interestingly, no methane, which could be a possible
byproduct from methanation of the carbon support, was
detected by GC in any run. In our study, no methane was
detected in the evaluated temperature range. The durability of
the CoOx@C-700-A catalyst in ammonia decomposition was
tested for the catalytic decomposition of ammonia at 500 °C
for a period of 20 h (Fig.11). At the beginning of the catalytic
test, the NH3 conversion shows a low activity. After half an
hour, a steady catalyst activity is achieved within 20 h. This
superior performance should be attributed to the special
structure of catalyst. Owing to spatial restriction, Co particles
are partially retarded inside graphitic carbon matrix, showing a
sufficient stability under harsh reaction condition.
Table 3 Catalytic activity of various carbon supported base
metal catalysts for ammonia decomposition.
Catalyst
Metal
content
(wt%)
3.5
7.2
4.1
T
(°C)
GHSV
(h−1)
NH3
conversion
(%)
Fe/CNFs [14]
Ni/CNFs [29]
Co/CNTs [25]
600
650
600
(500)
700
600
500
500
500
6500
36,000
5000
51.3
90.5
76.0
(~ 8)
75.1
~ 45
74.6
~ 10
Conclusions
The catalytic performance was strongly affected by the
history of catalyst preparation. The CoOx@C-700-A catalyst
calcined in limited air atmosphere exhibited higher catalytic
activity for NH3 decomposition comparing to that Co@C-700-N
sample calcined in N2 flow, although they had an approximate
CoOx particle size (ca. 4 nm). When the Co-C precursors were
calcined in limited air atmosphere, which resulted in
generating more porosity, ensuring better accessibility of the
encapsulated Co to NH3 molecules. A series of CoOx@C-A
catalysts, calcined in limited air ranging from 700 to 900 °C,
Fe/CNTs [25]
Fe/MWCNTs [28]
Co/MWCNTs [21]
Ni/MWCNTs [49]
CoOx@C-700-A
2.8
5
10
2.9
18.7
5000
36,000
6000
30,000
15,000
~ 55
showed
a clear size-dependence activity for ammonia
decomposition. Moreover, owing to these unique embedded
CoOx@C nanocomposites, they can be applied in many
important areas, such as catalysis, magnetism and Li-ion
batteries.
Acknowledgements
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