Hydrogen production by methane decomposition over Ni-Cu-SiO2 catalysts: Effect of temperature on catalyst deactivation

Article abstract of DOI:10.1039/c6ra05782a

Catalytic decomposition of methane (CDM) is a simple process for the production of high-purity, CO_X-free (CO or CO₂) hydrogen. The CDM is a moderately endothermic reaction, and high temperatures are thermodynamically favorable for achieving high methane conversion. However, Ni-Cu catalysts easily lose their activities at high temperature. To study the effect of temperature on the deactivation of Ni-Cu catalysts, a 65% Ni-15% Cu-SiO₂ catalyst was prepared by the heterophase sol-gel method. A series of kinetic experiments (routes I, II, III) were designed to test the catalytic performance and generate by-product carbon structures. The effects of reaction temperature and methane dissociation rate on catalyst deactivation were studied. The phase transition temperature was estimated. Based on the kinetic experiments, TEM images, XRD data, TGA-DSC curves, and TEM-EDX data, a thoroughly deactivation study of the 65% Ni-15% Cu-SiO₂ catalyst was carried out. The results of this study proved that high degree of graphitization was the key factor contributing to the deactivation of Ni-Cu catalysts. Fragmentation and phase separation at high temperature were both responsible for carbon atom enrichment and a high degree of graphitization, which in turn caused the 65% Ni-10% Cu-25% SiO₂ catalyst to lose activity at high temperature.

Full text of DOI:10.1039/c6ra05782a

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Page 1 of 21
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Hydrogen production by methane decomposition over Ni-Cu-SiO2
catalysts: Effect of temperature on catalyst deactivation
Jiamao Li, Chao Xiao, Liangping Xiong, Xiaojun Chen, Linjie Zhao, Liang Dong,
Yang Du, Yong Yang, Heyi Wang*, Shuming Peng*
Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics,
Mianyang 621900, Sichuan, PR China
Catalytic decomposition of methane (CDM) is a simple process for the production of
highꢀpurity, COXꢀfree (CO or CO2) hydrogen. The CDM is a moderately endothermic
reaction, and high temperature are thermodynamically favorable for achieving high
methane conversion. However, NiꢀCu catalysts easily lose their activities at high
temperature. To study the effect of temperature on the deactivation of NiꢀCu catalysts,
65%Niꢀ15%CuꢀSiO2 catalyst was prepared by the heterophase solꢀgel method. A
series of kinetic experiments (routes I, II, III) were designed to test the catalytic
performance and generate byꢀproducts carbon structures. The effects of reaction
temperature and methane dissociation rate on catalyst deactivation were studied. The
phase transition temperature was estimated. Based on the kinetic experiments, TEM
images, XRD data, TGAꢀDSC curves, and TEMꢀEDX data, a thoroughly deactivation
study of the 65%Niꢀ15%CuꢀSiO2 catalyst was carried out. The results of this study
proved that high degree of graphitization was the key factor contributing to the
deactivation of NiꢀCu catalysts. Fragmentation and phase separation at high
temperature were both responsible for carbon atom enrichment and high degree of
graphitization, which in turn cause the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst to lose
activity at high temperature.
Keywords: NiꢀCu alloy particle, Catalyst deactivation, Methane decomposition,
Kinetic study, Hydrogen production
1.Introduction
Recently, hydrogen is increasingly being considered as a promising and
1
environmentally friendly energy source , due to the rapid depletion of fossilꢀfuel
resources and increased environmental pollution resulting from greenhouse gas
emission. The largeꢀscale industrial hydrogen production process is currently based
on the wellꢀknown steam reforming of methane 2, 3. However, this process and the
partial oxidation of methane involve the formation of a certain amount of CO2 and
CO as byꢀproducts, which contribute to global warming 4, 5. Moreover, for the
effective use of hydrogenꢀderived energy, people are focusing on the use of fuel cells
to generate electricity, as these cells can directly convert chemical energy into
electricity 6ꢀ10. However, even low concentrations (a few ppm) of CO may act as
poison for proton exchange membrane fuel cells (PEMFCs) 9. Moreover, CO2 (at ppm
10
levels) is detrimental to alkaline fuel cells (AFCs) . Hence, critical issues must be
overcame for the successful application of hydrogen in fuel cells. Consequently, the
* pengshuming@caep.cn; hywang@caep.cn

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aforementioned hydrogen production processes must include an additional
purification step to remove COX, which in turn increases process cost. As opposed to
the steam reforming of methane and partial oxidation of methane, catalytic
decomposition of methane (CDM) is a direct means of hydrogen production, without
the concomitant formation of COX 11. In addition to generating COxꢀfree hydrogen,
CDM also affords highly valuable carbon structures, such as nanotubes (CNTs) and
nanofibers (CNFs). These byꢀproducts, CNTs or CNFs, have significant potential due
to their superior surface properties, high tenacity, high electron conductivity, as well
as metallic and semiconductor characteristics 12.
The CDM reaction is as follows 13:
CH4 → C + 2H2, ∆= 75.52 kJ/mol
(1)
Traditionally, Niꢀbased catalysts have been widely used in the CDM, because of
their partially filled 3dꢀorbitals, which can facilitate the dissociation of hydrocarbon
molecules through partial acceptance of electrons 14. Moreover, the CDM is a
moderately endothermic reaction. Therefore, high temperature are thermodynamically
15
favorable for achieving high methane conversion . Although supported Ni catalysts
16
show superior performance during the CDM at temperatures below 600 °C
,
Niꢀbased catalysts are deactivated immediately at temperatures above 600 °C 17, 18. In
order to enhance the catalytic stability at high temperature, copper is introduced into
Niꢀbased catalysts 19, 20. Previous reports state that the NiꢀCu catalyst exhibits better
performance than the Ni catalyst at high temperature for the CDM, i.e., the carbon
yields (gC/gNi) of the 75%Niꢀ15%CuꢀAl2O3 catalyst was 700 at 625 °C and the
65%Niꢀ25%Cu/Nb2O5 catalyst resulted in the maximum carbon yield of 743 gC/gNi
at 600 °C 21, 22. In general, the doping of Cu can have several effects on Niꢀbased
catalysts. First, introduction of Cu can decrease the reduction temperature of Ni, since
Cu has a lower reduction temperature than Ni 15. The reduced Cu at low temperatures
dissociates hydrogen molecules into hydrogen atoms. The hydrogen molecules can
spread to the surface of the neighboring NiO species and facilitate the reduction of Ni
23
species in the mixed oxide . Second, in several reactions it has been reported the
electronic interactions between Ni and a second metal can remarkably enhanced
catalytic activity 25-27. Therefore, due to Cu is richer in d electrons, it can exert an
24
electronic effect on the Ni species . Third, Cu affects the affinity of the Niꢀmetal
surface toward graphite 28, 29, i.e., Cu addition to Ni improves the desired resistance of
the catalyst to carbon deposition by decreasing the amount of coke formed 30.
Although doping of a Cu component can effectively improve the catalytic
performance of Niꢀbased catalysts, the methane conversion decreases remarkably at
temperatures beyond 750 °C 31, 32. Some authors 15, 32ꢀ34 have reported that Ni catalysts
easily aggregated into large clusters at high temperature during thermal treatment, and
23
the enlargement of the metal particles results in poor catalytic performance
.
Moreover, when the reaction temperature reaches a certain level, NiꢀCu alloy particles
enter the soꢀcalled quasiꢀliquid state and lose their activities during the CDM reaction
32, 35. In addition, when the NiꢀCu particles stay in the quasiꢀliquid state, bambooꢀlike
2

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carbon nanotubes (BLCNTs) will be formed 36. The growth of BLCNTs causes the
NiꢀCu alloy particles break down into several pieces, and the balance between the
methane dissociation rates and diffusion rates of carbon is disturbed with the change
in the NiꢀCu particle size 15, 37. As a result, the particles are encapsulated by carbon
atoms in a very short time, leading to deactivation of the NiꢀCu catalyst.
15, 38
Lots of reporters
investigated the deactivation mechanism of NiꢀCu alloy
particles. According to them, the NiꢀCu particles enter a quasiꢀliquid state, and
fragmentation of these particles during BLCNT growth at high temperature may be a
39
key factor for catalyst deactivation . While others 33 stated the excessive methane
dissociation rate could lead to the formation of a large amount of carbon atoms on the
surface of NiꢀCu particles, which was responsible for the notable loss of catalytic
activity at high temperature. Although the structural damage of NiꢀCu particles and
the high carbon production rate had been reported to make a significant contribution
to catalyst deactivation at high temperature, some processes of them had not been
studied in detail yet. Moreover, it was not clearly understand the effect of
graphitization about catalyst deactivation. As a continuation of the authors’ research
on the properties and those wellꢀknown deactivation mechanisms of NiꢀCu alloy
catalysts, we focused on the effects of temperature on NiꢀCuꢀSiO2 catalysts during the
CDM in the temperature range 600 °Cꢀ780 °C. A series of more precise reaction
routes had been designed to test the performance of the catalysts, and we also
analyzed the reaction kinetics and solved the aboveꢀmentioned issues. In addition, the
morphology of the carbon structures and catalyst had been investigated, which could
help us understand the detailed process of catalyst deactivation. Finally, the findings
would try to be detailed in elaborating the deactivation mechanism of the NiꢀCu
particles at high temperature.
2. Experimental
2.1. Catalyst preparation
65%Niꢀ10%Cuꢀ25%SiO2 catalyst (mass fraction) was prepared using the
heterophase solꢀgel method 40, 41, by mixing the active components (mixture of NiO
and CuO), with alcosolꢀcontaining silica in certain proportions. Suspensions of active
components and alcosol were dried under a flow of air at room temperature and
calcined at 650 °C for 3 h. The alcosol was prepared by mixing 50 mL of
tetraethylorthosilicate (TEOS), 40 mL of ethanol, 2 mL of water, and 0.5 mL of 40%
HCl. The silica content in the alcosol was 0.142 g/mL, and the alcosol could be
diluted with ethanol. Mixtures of NiO and CuO were obtained from the mixtures of
Ni(NO3)2 and Cu(NO3)2 calcined at 450°C, and the Ni(NO3)2 and Cu(NO3)2 mixtures
were obtained by evaporating the mixed solution of Ni(NO3)2 and Cu(NO3)2.
2.2. Catalyst performance tests
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The catalyst performance was evaluated during the CDM in a fixedꢀbed quartz
reactor (10 mm i.d.) under atmospheric pressure. A Kꢀtype thermocouple was placed
in the catalyst bed to detect the temperature of the bed. Metallic NiꢀCu particles (15
mg) were placed in the quartz reactor, and hydrogen reduction reaction commenced
when increasing the furnace temperature to 600 °C under a flow of N2 at 20 cm3/min.
Reduction treatment of the catalysts was continued for 1 h under 50% H2 in N2 at a
flow rate of 20 cm3/min. After the reduction process, three routes were considered for
the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst performance tests.
Route I was a stepꢀwise heating model for the CDM. The test was commenced
at 600 °C, following the reduction process, and purified methane (99.99%) was
passed through the reactor. After 20 min, the temperature was increased at a rate of
4 °C/min for 5 min (total of 20 °C), maintained at this level for 15 min, and then
further increased to the next high preset temperature. The stepꢀwise heating was
continued until the temperature reached a certain level (600 °C, 620 °C, 640 °C,
660 °C, 680 °C, 700 °C, 720 °C, 740 °C, 760 °C, 780 °C), and this the temperature
was maintained until a CDM reaction time of 200 min. In this route, the flow rate of
methane (13 cm3/min) was constant during the CDM.
Route II was somewhat similar to route I. This test was commenced at 600 °C
with purified methane at a flow rate of 13 cm3/min, and the temperature change was
similar to the pattern in the stepꢀwise heating model. When the furnace temperature
reached the desired level, 640 °C, 660 °C and 680 °C, these temperatures were
maintained until the CDM was complete, i.e., for 200 min. In addition, when the
reaction time reached 100 min, the flow rate of methane would be changed from 13
cm3/min to 33 cm3/min, 25 cm3/min, and 20 cm3/min, corresponding to 640 °C,
660 °C, and 680 °C, respectively. After an additional 80 min, the methane flow rates
would be reduced to 13 cm3/min and maintained at this level for 20 min.
Route III commenced from 600 °C, and temperature increase was similar to the
pattern used in the stepꢀwise heating model, under the same flow conditions. When
the temperature was increased up to 640 °C, the methane flow was converted to a
flow of Ar or H2 at the rate of 13 cm3/min. Under Ar or H2 flow, the temperature was
increased to 720 °C at the rate of 10 °C/min and then maintained at this level until the
reaction time reached 180 min. Then, the temperature was decreased from 720°C to
640 °C and the Ar (H2) flow was changed to methane flow at the rate of 13 cm3/min.
This temperature and methane flow were maintained for 300 min and 400 min,
respectively.
In addition, when the catalyst was subjected to different thermal treatment
processes, especially at high temperatures, the particle size changed as the result of
aggregation. Moreover, the structure of the catalysts changed due to selfꢀorganizing
42ꢀ44
which may regulate the size of particles to adapt to the reaction conditions
.
Therefore, for a more precise and detailed study of the catalyst deactivation and
catalyst performance, the particle size and shape of the catalyst should be preserved
during each experiment. In addition, it was clear that during the CDM over
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NiꢀCuꢀSiO2 catalysts, the carbon filament obeys the “tipꢀgrowth” model, and the
metal particle is at the tip of the carbon filament 15, 32. Because of this growth model,
the carbon filament byꢀproduct can play as a the role of a living support and aid in the
dispersion of the metal particles. Therefore, the catalyst particles can maintain their
size and shape to a certain extent during the carbon filament growth even at high
temperatures. Thus, if the initial methane conversion in the presence of a
65%Niꢀ10%Cuꢀ25%SiO2 catalyst at 600 °C can be controlled in the range of 22.5%
–23.5%, the catalysts could be considered to have very similar size and shape in each
experiment of using route X (X=I, II, III). Therefore, compared with traditional
method of CDM reaction, the series of reaction routes can obtain more precise data
from the test of catalytic performance.
The formation of gaseous reaction products was monitored using an onꢀline gas
VARIAN CPꢀ3800 chromatograph (GC) combined with a TDXꢀ01 column, which
was equipped with a thermal conductivity detector (TCD) for H2 and CH4 analysis.
The concentrations of methane and hydrogen at the fixed bed outlet were determined,
which were based on these calibrated data.
Fig. 1. Methane conversions using stepꢀwise heating model
2.3. Catalyst characterization
Powder Xꢀray diffraction (XRD) patterns were recorded on an Xꢀray instrument
(X’ pert PRO) using CuKα (λ= 1.5406˚A) radiation at 40 kV and 40 mA, in a
continuous scan mode. The XRD patterns were recorded in the scan range of 2θ =
10 ~ 90°, at a scan rate of 10 °/min. The mean crystallite sizes of the NiꢀCu particles
were calculated from the DebyeꢀScherrer formula, where the particle shape factor was
taken as 0.9 45.
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The detailed structure of the deposited carbon was observed using a Tecnai G2
F20 SꢀTwin transmission electron microscope (TEM) operated at 200 kV. Samples for
TEM analysis were prepared by drying the catalyst particles in ethanol on
molybdenum grids. TEMꢀEDX (energy dispersive Xꢀray) observations was carried
out for elemental analysis of the specimen surface.
Raman spectra were recorded on a Renishaw inVia confocal Raman microscope
equipped with a 532 nm laser as the excitation source laser.
Thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and
differential scanning calorimetry (DSC) were used to characterize the thermal
properties of carbonaceous materials such as graphite and CNTs. TGA and DTA
experiments were performed between 25 °C and 900 °C with an SDT Q600 system, in
air (80 cm3/min) at a heating rate of 20 °C/min. The mass of each sample was 5 mg.
3.Results and discussion
3.1. Catalytic performance
-3.4
680 o
C
In rCH4= ꢀ8.559(1/T)+5.387
720 o
C
700 o
C
-3.6
-3.8
-4.0
-4.2
-4.4
-4.6
R2=0.9995
C
660 o
640 o
C
620 o
C
600 o
C
1.00
1.02
1.04
1.06
1.08
1.10
1.12
1.14
1.16
1/T*103, 1/K
Fig. 2. Arrhenius plots for the investigated catalysts
Fig. 1 shows the methane conversion rates as a function of reaction time
obtained in route I for a series of nearly equal masses of 65%Niꢀ10%Cuꢀ25%SiO2
catalysts at different reaction temperatures. Because the initial methane conversion
rate was specified, the theoretical equilibrium methane conversion rates obtained
during each CDM experiment in route I was consistent at the same temperature. It
indirectly indicated that Ni–Cu particles in each experiment of route I had the same
states and similar structures at the same reaction temperature.
For the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst, due to the CDM is an endothermic
reaction, an increase in reaction temperature greatly improved the methane conversion
rate. However, within the temperature range of 600–780 °C, especially at 720 °C, the
catalysts showed the highest activity during the stepꢀwise heating process, and the
methane conversion rate was increased by 65%. With the further increase in reaction
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temperature, the methane conversion rates started to decrease. When the temperature
was increased to 780 °C, the methane conversion rate was lower than 15%. The
65%Niꢀ10%Cuꢀ25%SiO2 catalyst exhibited similar properties as the unsupported Ni–
Cu–Co ternary catalysts 37. In addition, compared to the earlier reports on similar
catalyst systems, the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst exhibited higher H2 formation
rate at in the temperature range 600 °Cꢀ700 °C. The highest H2 formation rate of the
65%Niꢀ10%Cuꢀ25%SiO2 catalyst reached to 34 mmol/(min gNiCu), which far
exceeded the H2 formation rate of the Ni78Cu22ꢀCNT catalysts (13.5 mmol/(min
g
NiCu))15 and the NiꢀCuꢀcatalysts (8 mmol/(min gNiCu)) 24.
b
a
c
d
e
f
Fig.3.TEM images of carbon deposits on NiꢀCu particles in route I at different temperatures:
(a: 640 °C, b: 660 °C, c: 680 °C, d: 700 °C, e: 720 °C, f: 740 °C.)
3.2. Quasi-liquid state
In previous literature 35, 36, researchers proposed that catalysts become unstable
in a quasiꢀliquid state, and this state caused the catalysts to generate BLCNTs.
Because of the growth of BLCNTs, the catalysts might undergo fragmentation. The
fragmentation process could change the size of the particles, which was
responsible for deactivation 15. Therefore, it is meaningful to determine a way to
effectively obtain the phase transition temperature of the quasiꢀliquid state for Ni–Cu
catalysts. To effectively obtain the phase transition temperature, the Arrhenius
formula was taken into consideration and several temperatures (600–720 °C in route I)
were selected. The Arrhenius plots (rCH4) of the Ni–Cu alloy particles are represented
in Fig. 2, where rCH4 is the average methane conversion rate (mol/min) in the initial
stage divided by the catalyst weight. In addition, the calculation method of Arrhenius
plots references to the work of Wang et al and Reshetenko et al 46, 47. It should be
noted that when the temperature increased to higher than 680 °C, the slope of the
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Arrhenius curve started to decline. The decreased slope of the Arrhenius curve might
imply that a few Ni–Cu particles started to enter the quasiꢀliquid state at a temperature
of approximately 680 °C.
Using Route III
♦
•
Ni&Cu
♦
C
♦
crystallite size of Ni-Cu
(111)
(200)
760OC
740OC
720OC
♦
•
•
22.7 nm
22.8 nm
23.2nm
♦
•
700OC
680OC
660OC
26.4 nm
31.2 nm
34.8 nm
32.5 nm
640OC
620OC
33.6 nm
20
40
60
80
2θ
(degree)
Fig. 4. XRD patterns of spent catalysts at different reaction temperatures in route I
(The crystal sizes of the NiꢀCu alloys were calculated from the peak due to the (2 0 0) plane at around 51°)
In order to validate this conjecture, the carbon structures deposited on the Ni–Cu
particles were investigated by TEM. The micrographs of the carbon structures and
Ni–Cu particles are shown in Fig. 3. In addition, the different states of NiꢀCu particles
cause themselves generation of different carbon structures. For example, the NiꢀCu
particle forms full CNTs and OLCNTs in crystalline state, but the NiꢀCu particle
generates the BLCNTs in quasiꢀliquid state. Therefore, the state of NiꢀCu particle can
be indirectly determined by detecting the carbon structure. Moreover, due to superior
properties of those kinds of CNTs, the structure of them can be maintained after CDM
reaction. A careful investigation of the carbon structures deposited on the
65%Niꢀ10%Cuꢀ25%SiO2 catalyst in route I at different temperatures, it revealed that
BLCNTs were only observed at 700 °C, 720 °C, and 740 °C. However, at temperature
below 700 °C, the NiꢀCu particles only formed full CNTs, hollow CNTs, and
octopusꢀlike carbon filaments (OLCNTs) 48. As noted above, the formation of
BLCNTs could prove that the catalyst particles were in a quasiꢀliquid state, thus, a
certain amount of Ni–Cu particles started to transform into the quasiꢀliquid state at
approximately 680 °C.
As shown in Fig. 4, the size of the Ni–Cu particles decreased with the increase in
temperature. When the reaction temperature was below 700 °C, the particle size was
approximately 33 nm. However, the particle size of the Ni–Cu catalysts started to
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decrease as the temperature increased and reached a minimum of 22.4 nm at 760 °C
in route I. The phenomenon (NiꢀCu particle size decreased during the CDM reaction)
was the so called fragmentation process, which often occurred when NiꢀCu alloy was
in quasiꢀliquid state. Due to the NiꢀCu particle suffer fragmentation process, this
result might also prove that NiꢀCu particles started to transform into the quasiꢀliquid
state at approximately 680 °C. Therefore, those data combining the stepꢀwise heating
model with the Arrhenius equation could effectively be used to obtain the phase
transition temperature with a reasonable degree of accuracy.
3.3. Effects of structural damage
Fig.5.TEM images of carbon deposits on NiꢀCu particles in route I at different temperatures after fragmentation: (a:
640 °C, b and C: 700 °C, d: 720 °C, e and f: 740 °C, g and h: 760 °C)
From the previously reported data, it is clear that the mechanism of deactivation
of Niꢀbased catalysts is very complicated at high temperature, because of the change
in the states of catalysts and the growth of the carbon structures 23. However, in
previous studies, the key factors that caused the deactivation of catalysts in the
quasiꢀliquid state were found to be fragmentation and phase separation during the
growth process 15, 31. In addition, particle size, composition, and metalꢀsupported
interactions were responsible for Ni–Cu particles attaining a quasiꢀliquid state.
According to some reports, when Ni–Cu particles attained a quasiꢀliquid state,
BLCNTs started to generate and the Ni–Cu particles suffered periodical jumping 35, 49
.
In this growth model, a particle in the quasiꢀliquid state was sucked into the hollow of
the CNTs through a capillary force at the first stage 50. Then, nucleation of
bambooꢀknot graphite sheets occurred at the graphene wallꢀcatalyst junction 49. Thus,
the inner wall around the bottom of the Ni–Cu catalyst particles started to grow, and
this process added to the growth of the inner wall edge through diffusion along the
graphene sheet 51. When the surface tension of this particle became larger than the
capillary force, the Ni–Cu particle jumped out of the hollow and BLCNTs was formed
in these circles 23. However, as Cu affects the affinity of the Niꢀmetal surface with
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graphite, the inner walls of the BLCNTs might become rough and form an irregular
carbon layer, as shown in Fig. 5c and 5g. Moreover, the rough inner wall and high
growth rate of the inner carbon layer could contribute to the generation of a tail,
located at the bottom of a Ni–Cu particle during the BLCNT growth, as shown in Fig.
5d and 5f (red circle). During the periodical jumping of Ni–Cu alloy particles, the
tails might be blocked in the interior of the BLCNT structure, and some fragmented
Ni–Cu particles could be observed, as shown in Fig. 5bꢀ5h. As the Ni–Cu particles
suffered fragmentation during the growth of BLCNTs, the balance between the
methane dissociation rate and the carbon diffusion rate was disturbed with the change
in the size of the Ni–Cu particles. Smaller particles led to a higher methane
dissociation rate and a lower carbon diffusion rate 52; this would cause carbon
enrichment on the surface of the Ni–Cu particles. The carbon enrichment could lead
to the coverage of the NiꢀCu particle surface, resulting in deactivation. This
phenomenon was one of the wellꢀknown factors for the deactivation of Ni–Cu–SiO2
catalysts at high temperature.
Table 1.
Concentration of Ni and Cu over NiꢀCuꢀSiO2 catalyst at different temperatures, as detected by EDX.
Number
1
Ni Content (%)
84.4
83.6
32.2
74.8
86.5
51.8
56.8
66.6
81.8
1.6
Cu Content (%)
15.6
2
16.4
3
67.8
4
25.2
5
13.5
6
48.2
7
43.2
8
33.4
9
18.2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
98.4
83.1
70.3
73.4
76.8
70.7
8.5
16.9
29.7
26.6
23.2
29.2
91.5
87.7
62.6
48.2
54.3
26.7
32.7
74.6
12.3
37.4
51.8
45.7
73.3
67.3
25.4
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To investigated in detailed the mechanism of fragmentation process and phase
separation, TEMꢀEDX was used to evaluate the concentrations of Ni and Cu in the
fragmented Ni–Cu alloy particles, as shown in Fig. 5. In addition, the concentrations
are marked by red arrows numbered 1 to 23. The concentrations of Ni and Cu were
also calculated by TEM imaging and an analysis software, and the results are shown
in Table 1. As shown in Fig. 5a, when the NiꢀCu particles were in crystalline state, the
NiꢀCu particles did not suffer fragmentation process and also did not have this
phenomenon of phase separation. Though fragments of the catalyst were sucked into
the same BLCNT, they had different Ni and Cu contents. This is called the phase
separation phenomenon 15, 37. Moreover, upon careful study the Ni content increased
from the bottom of the BLCNT to the tip, while the Cu content decreased. By
combining the details of the fragmentation process and the data shown in Table 1, the
phase separation phenomenon can be described in detailed as follows: when the Ni–
Cu particle suffered structural damage and generated a tail, a part of the head was
enriched with Ni and the tail was enriched with Cu; Moreover, if the tail that was
enriched with Cu was sucked into the interior of the BLCNT, the Ni content in the rest
of the particle increased, as shown in Fig. 5d and 5f. In addition, a study 46 proposed
that Cu had negligible ability to crack methane and diffuse carbon atoms, which
caused the selective diffusion of carbon atoms through the Ni particles. As a result,
the rapid selective diffusion of carbon atoms might lead to fragmentation of the NiꢀCu
particles. Based on this theory and the fact that the tail of the Ni–Cu particle has a
higher Cu content and a lower Ni content, a Ni–Cu particle could undergo more
serious structural damage through the selective diffusion of carbon atoms into the Ni–
Cu structure.
Through above analysis, the rough inner wall of CNTs and a tail formed in
NiꢀCu particles might be the key factors for fragmentation process of Ni–Cu–SiO2
catalysts at high temperature. Moreover, the process of Cu content was enrichment in
the tail not only made contribution to fragmentation process, but also caused the
phase separation. Therefore, those factors acted on and influenced each other to cause
the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst to lose activity at high temperature.
3.4. Effect of excessive carbon formation
100
b
route II(640oC)
a
12
11
10
9
Route II(640oC)
Route II(660oC)
Route II(680oC)
Route I (720oC)
route II(660o
route II(680o
route I(720o
C)
C)
C)
90
80
70
60
50
40
30
20
zone 3
zone 1
zone 2
zone 3
720O
C
700O
C
13ml/min
680O
C
20ml/min
660O
C
25ml/min
33ml/min
640O
C
620O
C
8
600O
C
7
0
50
100
Reaction Time(min)
150
200
100
110
120
130
140
150
160
170
180
190
Reaction Time(min)
Fig. 6. Methane conversion in route II and route I at different temperatures
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In addition to NiꢀCu structural damage, the excessive carbon generation is
known to contribute significantly to the deactivation of the NiꢀCu catalysts 38. The
deactivation mechanism can be described like this: the high carbon production rate
with the relatively low carbon diffusion rate causes the carbon enriched on the surface
of particles, after that, the carbon atoms form carbon layers, which make the catalysts
lose activity. In this work, the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst showed superior
methane conversion of about 65% at 720 °C; therefore, excessive methane
dissociation could lead to the generation of a large amount of carbon atoms on the
surface of the NiꢀCu particles, which might responsible for the notable loss of
catalytic activity at high temperature.
To clearly investigate and validate whether the effect of excessive carbon
generation play an important role in catalyst deactivation at high temperature, the
maximum methane conversion at 720 °C in route I was taken as the reference to
design the route II experiment. In this route, several temperatures (640 °C, 660 °C and
680 °C) were selected, and it was found that the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst did
not undergo fragmentation or phase separation. As shown in Fig. 6a, the carbon
generation rate decreased with increasing reaction temperature for the reaction time of
100ꢀ200 min (zone 3). Although lower reaction temperatures caused the generation of
more carbon atoms in route II, the methane conversion showed no notable decline
after the methane flow rate was reverted to 13 cm3/min, as shown in Fig. 6b. For
inꢀdepth analysis of these data, the decreased methane conversion was used to
evaluate the extent of activity reduction on the NiꢀCu particles (Table 2). As the
temperature was increased from 640 °C to 680 °C in route II, the ratios of activity
reduction increased from 0.5% to 13.0%.
Table 2.
Methane conversion over 65%Niꢀ10%Cuꢀ25%SiO2 catalyst via route I and route II at different temperatures
samples
average methane
conversion (Zone 1)
(X)
average methane
conversion (Zone 2)
(Y)
Ratios of activity
reduction
((XꢀY)/X)
0.5%
Route II (640 °C)
Route II (660 °C)
Route II (680 °C)
Route I (720 °C)
34.5%
34.3%
43.6%
42.3%
3.0%
54.5%
47.4%
13.0%
65.5%
53.2%
18.8%
Clearly, the maximum methane flow rate and carbon generation rate at the lower
reaction temperature of 640 °C had almost no effect on the catalytic activity. This
implies that the methane dissociation rate might not exceed the carbon diffusion and
carbon deposition rates, or the enrichment of carbon atoms might not lead to complete
coverage of the surface of the NiꢀCu particles. Because the structures of the NiꢀCu
particles are stable at in the temperature range 640 °Cꢀ680 °C, only temperature
would contribute to catalyst deactivation. Therefore, the deactivation mechanism of
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NiꢀCu particles at high temperature can't be explained by simple excessive carbon
generation. However, the ratios of activity reduction increased with increasing of
temperature, so it may need to take into account the other effect which bases on
temperature. In addition, fragmentation process and phase separation might cause less
than 5.8% of activity reduction of the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst to lose their
ability to cause cracking of methane in route I at 720 °C.
3.5. Effect of graphitization
600OC
640OC
G band=1582
680OC
D band=1338
720OC
760OC
ID/IG=1.19
ID/IG=1.56
ID/IG=1.73
ID/IG=1.81
ID/IG=1.89
1100
1200
1300
1400
1500
1600
1700
1800
Raman shift (cm-1)
Fig. 7. Raman spectra of carbon filaments obtained over 65%Niꢀ10%Cuꢀ25%SiO2 catalyst via
route I.
From the above mentioned observations, it is clear that temperature plays an
important role in the deactivation of the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst by
excessive carbon formation, and some other factors may make contribution to catalyst
deactivation at high temperature. Some reports state 30 that Cu addition to a Niꢀbased
catalyst improved its desired resistance to carbon structure deposition by lowering the
amount of coke formed. Moreover, since Cu affects the affinity of the Niꢀmetal
surface toward graphite, it can inhibit the formation of a graphite layer on the nickel
surface 29. In addition, the graphite layer was very stable and could not be removed by
reacting with hydrogen 53. Since Cu inhibited the formation of the graphite layer, a
large amount of disordered carbon was generated by the 65%Niꢀ10%Cuꢀ25%SiO2
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catalyst in route I, as shown in Fig. 7. In addition, Two distinct peaks appear in the
spectrum of each sample 54, the Gꢀband at 1582 cmꢀ1, assigned to the formed
graphitized CNTs and the Dꢀband at 1338 cmꢀ1, related to the existence of the
disordered carbon such as amorphous carbon or defects in graphite layers 55. The peak
intensity ratio ID/IG 55 is used as a direct measure for sample quality due to its relative
response of graphite carbon to defective carbon. Therefore, from the date of Fig. 7,
the Raman spectra showed that the disordered carbon content reduced with an
increase in the temperature from 620 °C to 760 °C, as was evident from the change in
the ID/IG ratio from 1.89 to 1.19, implying that the degree of graphitization is
enhanced with increasing reaction temperature.
To clearly investigate the changes of the degree of graphitization, the TGAꢀDTA
analysis had been taken consideration. Generally, the TGAꢀDTA analysis employed to
study the graphitization degree, yield, thermal stability, and purity of accumulated
CNTs over the current catalysts. In addition, the weight loss is attributed to the
combustion of carbon in oxygen, and the oxidation temperature (inflection
temperature) of the carbon structures can be used to analysis the graphitization degree
56, 57. The TGAꢀDTA results in Fig. 8 revealed that the oxidation temperature of the
carbon structures generated by the NiꢀCu particles in route I shifted to higher values
with an increase in the CDM temperature. This indicates that high temperature
enhanced the degree of graphitization.
200
180
160
140
120
100
80
2
Inflection Temperature
658OC
653OC
639OC
660OC
668OC
1
0
Route I
600OC
60
640OC
680OC
720OC
760OC
-1
-2
40
20
0
100
200
300
400
500
600
700
800
900
Temperature (OC)
Fig.8. TGA and DTA analysis of used 65%Niꢀ10%Cuꢀ25%SiO2 catalysts.
Previous studies showed that methane dissociation occurred at the active sites of
the NiꢀCu (100 and 110) plane, while carbon deposition and carbon filament
formation occurred on the precipitation planes (111) 14. Because of the generation of
excessive carbon atoms and the weak resistance of Cu to carbon structure deposition
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at 720 °C in route I, some of carbon atoms might not diffuse to the precipitation
planes and were enriched on the NiꢀCu (100 and 110) planes. High temperature
enhanced the degree of graphitization; structural damage can cause a further increase
in the degree of graphitization as phase separation can decrease the Cu content.
Therefore, the carbon atoms could form graphite structures on the NiꢀCu (100 and 110)
planes and cover the active sites. However, although the NiꢀCu particles generated a
greater number of carbon atoms in route II at 640 °C than in route I at 720 °C and the
carbon atoms might be enriched on the (100 and 110) planes, the low degree of
graphitization might destabilize the carbon structure, so that the carbons were
58
removed by diffusion or reacting with hydrogen
.
Therefore, the
65%Niꢀ10%Cuꢀ25%SiO2 catalyst could retain its activity even after excessive carbon
formation for 100 min, as shown in Fig. 6.
Taking into account the fragmentation of the NiꢀCu particles and phase
separation, the detailed deactivation mechanism of the 65%Niꢀ10%Cuꢀ25%SiO2
catalyst at high temperature could be explained as following. First, since the CDM is
an endothermic reaction, high temperature contributes to excessive carbon formation,
resulting in the formation of numerous carbon atoms on the surface of the NiꢀCu
particles. Carbon atoms diffusion and methane atoms dissociation in copper are
negligible, therefore they selectively diffuse into the Ni particles, causing
fragmentation of the catalyst particles. Second, high temperature causes the NiꢀCu
particles entering a quasiꢀliquid state, and NiꢀCu catalysts beginning to form BLCNTs.
The growth of BLCNTs causes fragmentation and phase separation on NiꢀCu particles,
both of which disturb the balance between methane dissociation and carbon diffusion.
Hence, the carbon atoms are further enriched on the surface of the NiꢀCu particles.
Third, high temperature and lower Cu content enhance the degree of graphitization,
which promotes the formation of graphite layers from the enriched carbon atoms;
these layers covered the active sites of the NiꢀCu particles. Therefore, the
performance of the 65%Niꢀ10%Cuꢀ25%SiO2 catalyst decreased markedly at high
temperature during the CDM. By comparing these factors, although the structure
damage and excessive carbon formation could make contribution to the activity
reduction of NiꢀCu particles, this phenomenon of graphitization might be a key factor
for the deactivation of Ni–Cu–SiO2 catalysts at high temperature. Moreover, these
factors interacted with one another and made a significant contribution to catalyst
deactivation together.
3.6. Effect of carbon migration
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50
45
40
35
30
25
20
15
route III(Ar)
route III(H2)
zone 4
640oC
640oC
640oC
620oC
600oC
0
100
200
300
400
Reaction Time(min)
Fig. 9. Methane conversions in route III under different conditions
(temperature=720 C in zone 4 )
°
The 65%Niꢀ10%Cuꢀ25%SiO2 catalyst forms a large amount of disordered
carbon structures, some of which might have migration ability 59. Therefore, these
carbon structures may act as sources of carbon and contribute to the catalyst
deactivation at high temperature. In order to verify this conjecture, route III was taken
into consideration. Fig. 9 shows the kinetic curves for the CDM over
65%Niꢀ10%Cuꢀ25%SiO2 catalyst at 640 °C. In this route, the carbon source was
removed (not pass thought methane) to confirm that the CDM does not occur at a high
temperature of 720 °C. Since the catalysts were located at the tip and the reaction
condition was unchanged after reꢀpassed though methane, the effect of selfꢀorganizing
of the catalyst particles on the particle structure could be ignored. Hydrogen could
react with carbon to form methane, the amount of methane formed was very small and
hence may not contribute to the reduced activity of the 65%Niꢀ10%Cuꢀ25%SiO2
catalyst at high temperature under H2 flow. From Fig. 9, when the catalysts were
subjected to a high temperature of 720 °C under H2 flow at 13 cm3/min for 120 min,
the methane conversion increased from the lowest value of 34.5% to the highest value
of 36.3%, which was closer to the initial methane conversion of 36.8%. This result
confirms the above mentioned assumptions.
Under no matter what conditions, there existed a trend of covering catalytic
activity process, which means that the methane conversion increased with reaction
time after reꢀpassed though methane. This might be attributed to the reaction between
the generated hydrogen and the carbon atoms (not graphite carbon) covering the
active sites. Due to hydrogen may inhibit the carbon atoms migration and prevent the
formation of encapsulating coke 60, 61, a few of carbon atoms migrated to the surfaces
of catalysts. Therefore, 65%Niꢀ10%Cuꢀ25%SiO2 catalyst spent a short time on
recovering activity. In contrast, argon does not react with the carbon atoms, and hence,
16

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high temperatures allowed more carbon atoms to migrate to the surfaces of the NiꢀCu
catalysts; therefore, the catalyst takes a long time to recover its activity. However,
under Ar flow at 13 cm3/min for 120 min, in route III, the methane conversion
increased from the lowest value of 15.6% to the highest value of 32.9%, which was
much lower than the initial methane conversion of 35.8%. This result might be
attributed to the formation of graphite layers from the carbon atoms migrated to the
surface of the NiꢀCu particles.
4.Conclusions
A series of kinetic experiments (routes I, II, III) were designed to test the
catalytic performance of NiꢀCu catalysts and to analyze the byꢀproducts, carbon
structures. The effects of reaction temperature and methane dissociation rate on
catalyst deactivation were studied. Based on such data as kinetic experiments,
Arrhenius curve, XRD data, TEM images and so on, some results were obtained
about the deactivation mechanism of 65%Niꢀ10%Cuꢀ25%SiO2 catalyst deactivation.
First, it was found that the NiꢀCu particles were in the quasiꢀliquid state around
680 °C in route I, and when the NiꢀCu particles were in the quasiꢀliquid state,
catalytic activity decreased remarkably with increasing temperature. Second, the
detailed mechanism of fragmentation process and phase separation had been
elaborated, moreover, a tail formed in NiꢀCu particles and enriched copper content
might be the key factor for fragmentation process and phase separation. Third,
although fragmentation process, phase separation, and graphitization together make a
significant contribution to catalyst deactivation at high temperature, the high degree
of graphitization might play the most important role in the deactivation of NiꢀCu
catalysts at high temperature. In addition, because there existed a trend of covering
catalytic activity process during route III, it was conjectured that some of these carbon
structures might migrate to the surface of the NiꢀCu particles.
Acknowledgements
The authors are grateful for the financial support by the Science and Technology
Development Foundation (No.2013B0301036) and the Programs of Laboratory of
Advanced Nuclear Energy (LANE), managed by the Institute of Nuclear and
Chemistry (No. R2014ꢀ0303ꢀ03).
Notes and references
1.
2.
T. R. Karl and K. E. Trenberth, Science, 2003, 302, 1719-1723.
N. Muradov, F. Smith, C. Huang and A. T-Raissi, Catalysis Today, 2006, 116
281-288.
,
3.
4.
J. Tong and Y. Matsumura, Catalysis Today, 2006, 111, 147-152.
T. J. Crowley, Science, 2000, 289, 270-277.
17

RSC Advances
Page 18 of 21
View Article Online
DOI: 10.1039/C6RA05782A
5.
R. M. Navarro, M. A. Pena and J. L. Fierro, Chemical reviews, 2007, 107
3952-3991.
,
6.
M. K. Debe, Nature, 2012, 486, 43-51.
7.
T. Niknam, A. Kavousi Fard and A. Baziar, Energy, 2012, 42, 563-573.
S. S. Penner, Energy, 2006, 31, 33-43.
8.
9.
B. C. Steele and A. Heinzel, Nature, 2001, 414, 345-352.
J.-H. Wee and K.-Y. Lee, Journal of Power Sources, 2006, 157, 128-135.
10.
11.
L. Dong, Y. Du, J. Li, H. Wang, Y. Yang, S. Li and Z. Tan, International Journal of
Hydrogen Energy, 2015, 40, 9670-9676.
12.
J. N. Coleman, U. Khan, W. J. Blau and Y. K. Gun’ko, Carbon, 2006, 44
,
1624-1652.
13.
14.
15.
U. P. M. Ashik and W. M. A. Wan Daud, RSC Adv., 2015, 5, 46735-46748.
A. Dupuis, Progress in Materials Science, 2005, 50, 929-961.
A. C. Lua and H. Y. Wang, Applied Catalysis B: Environmental, 2013, 132-133
469-478.
,
16.
17.
Y. Shen and A. C. Lua, RSC Adv., 2014, 4, 42159-42167.
H. Ogihara, S. Takenaka, I. Yamanaka, E. Tanabe, A. Genseki and K. Otsuka,
Journal of Catalysis, 2006, 238, 353-360.
18.
19.
20.
S. Takenaka, Journal of Catalysis, 2003, DOI: 10.1016/s0021-9517(02)00185-9.
Y. Li, J. Chen, Y. Qin and L. Chang, Energy & Fuels, 2000, 14, 1188-1194.
Y. Echegoyen, I. Suelves, M. J. Lázaro, R. Moliner and J. M. Palacios, Journal of
Power Sources, 2007, 169, 150-157.
21.
22.
K. Li, G. Lu, W. Wang, J. Li and J. Liu, Chemistry Letters, 2005, 34, 48-49.
A. R. Naghash, Z. Xu and T. H. Etsell, Chemistry of Materials, 2005, 17
815-821.
,
23.
24.
25.
Y. Li, D. Li and G. Wang, Catalysis Today, 2011, 162, 1-48.
Y. Shen and A. C. Lua, Applied Catalysis B: Environmental, 2015, 164, 61-69.
H. Konnerth, J. Zhang, D. Ma, M. H.G.Prechtl and N. Yan, Chemical
Engineering Science, 2015, 123, 155-163.
26.
27.
28.
J.G Zhang, H. Asakura, J. V. Rijn, J. Yang, P. Duchesne, B. Zhang, X. Chen, P.
Zhang, M. Saeys and a. N. Yan, Green Chemistry, 2014, 16, 2432-2437.
J. G. Zhang, J. Teo, X. Chen, H. Asakura, T. Tanaka, K. Teramura and N. Yan, ACS
Catalysis, 2014, 4, 1574-1583.
J. Li, G. Lu, K. Li and W. Wang, Journal of Molecular Catalysis A: Chemical
2004, 221, 105-112.
,
29.
30.
N. Rodriguez, Journal of Catalysis, 1993, 140, 16-29.
A. Wolfbeisser, B. Klötzer, L. Mayr, R. Rameshan, D. Zemlyanov, J. Bernardi, K.
Föttinger and G. Rupprechter, Catal. Sci. Technol., 2015, , 967-978.
5
31.
32.
J. Chen, Y. Li, Z. Li and X. Zhang, Applied Catalysis A: General, 2004, 269,
179-186.
S. K. Saraswat and K. K. Pant, International Journal of Hydrogen Energy, 2011,
36, 13352-13360.
18

Page 19 of 21
RSC Advances
View Article Online
DOI: 10.1039/C6RA05782A
33.
34.
Y. Pei, G. Zhou, N. Luan, B. Zong, M. Qiao and F. Tao, Chemical Society
Reviews, 2012, 41, 8140.
H. Liu, Y. Li, H. Wu, T. Miyake and D. He, International Journal of Hydrogen
Energy, 2013, 38, 15200-15209.
35.
36.
J. Chen, Y. Li, Y. Ma, Y. Qin and L. Chang, Carbon, 2001, 39, 1467-1475.
D. Li, J. Chen and Y. Li, International Journal of Hydrogen Energy, 2009, 34
,
,
299-307.
37.
38.
39.
A. C. Lua and H. Y. Wang, Applied Catalysis B: Environmental, 2014, 156-157
84-93.
I. Suelves, M. Lazaro, R. Moliner, B. Corbella and J. Palacios, International
Journal of Hydrogen Energy, 2005, 30, 1555-1567.
L. B. Avdeeva, O. V. Goncharova, D. I. Kochubey, V. I. Zaikovskii, L. M. Plyasova,
B. N. Novgorodov and S. K. Shaikhutdinov, Applied Catalysis A: General, 1996,
141, 117-129.
40.
W. Wang, H. Wang, Y. Yang and S. Jiang, International Journal of Hydrogen
Energy, 2012, 37, 9058-9066.
41.
42.
M. Ermakova, Catalysis Today, 2002, 77, 225-235.
J. C. De Jesús, I. González, M. García and C. Urbina, Journal of Vacuum Science
& Technology A: Vacuum, Surfaces, and Films, 2008, 26, 913.
43.
44.
45.
H. Y. Wang and A. C. Lua, The Journal of Physical Chemistry C, 2012, 116,
26765-26775.
M. A. Ermakova, D. Y. Ermakov, L. M. Plyasova and G. G. Kuvshinov, Catalysis
Letters, 1999, 62, 93-97.
H. G. Jiang, M. Rühle and E. J. Lavernia, Journal of Materials Research, 2011,
14, 549-559.
46.
47.
H. Y. Wang and A. C. Lua, Chemical Engineering Journal, 2014, 243, 79-91.
T. V. Reshetenko, L. B. Avdeeva, Z. R. Ismagilov, A. L. Chuvilin and V. A.
Ushakov, Applied Catalysis A: General, 2003, 247, 51-63.
M. A. Ermakova, D. Y. Ermakov, A. L. Chuvilin and G. G. Kuvshinov, Journal of
Catalysis, 2001, 201, 183-197.
48.
49.
50.
51.
M. Lin, J. P. Tan, C. Boothroyd, K. P. Loh, E. S. Tok and Y. L. Foo, Nano letters
2007, , 2234-2238.
,
7
X. X. Zhang, Z. Q. Li, G. H. Wen, K. K. Fung, J. Chen and Y. Li, Chemical Physics
Letters, 2001, 333, 509-514.
S. Helveg, C. Lopez-Cartes, J. Sehested, P. L. Hansen, B. S. Clausen, J. R.
Rostrup-Nielsen, F. Abild-Pedersen and J. K. Norskov, Nature, 2004, 427
,
426-429.
52.
53.
D. Chen, K. Christensen, E. Ochoafernandez, Z. Yu, B. Totdal, N. Latorre, A.
Monzon and A. Holmen, Journal of Catalysis, 2005, 229, 82-96.
K. Otsuka, S. Kobayashi and S. Takenaka, Applied Catalysis A: General, 2000,
190, 261-268.
19

RSC Advances
Page 20 of 21
View Article Online
DOI: 10.1039/C6RA05782A
54.
55.
56.
57.
58.
59.
60.
61.
M. Michalska-Domańska, M. Norek, P. Jóźwik, B. Jankiewicz, W. J. Stępniowski
and Z. Bojar, Applied Surface Science, 2014, 293, 169-176.
J. Cheng, X. Zhang, Z. Luo, F. Liu, Y. Ye, W. Yin, W. Liu and Y. Han, Materials
Chemistry and Physics, 2006, 95, 5-11.
A. E. Awadallah, A. A. Aboul-Enein and A. K. Aboul-Gheit, Renewable Energy
,
2013, 57, 671-678.
K. Rajavel, M. Dinesh, R. Saranya and R. T. R. Kumar, RSC Adv., 2015,
5
,
20479-20485.
S. Fukada, N. Nakamura, J. Monden and M. Nishikawa, Journal of Nuclear
Materials, 2004, 329-333, 1365-1369.
G. Onyestyák, J. Valyon, K. Hernádi, I. Kiricsi and L. V. C. Rees, Carbon, 2003,
41, 1241-1248.
J. I. Villacampa, C. Royo, E. Romeo, J. A. Montoya, P. Del Angel and A. Monzón,
Applied Catalysis A: General, 2003, 252, 363-383.
C.-M. Chen, Y.-M. Dai, J. G. Huang and J.-M. Jehng, Carbon, 2006, 44
,
1808-1820.
20

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The activities of 65%Ni-10%Cu-SiO2 catalysts under different reaction temperatures using route I and route II