Catalytic activity of mesoporous Ni/CNT, Ni/SBA-15 and (Cu, Ca, Mg, Mn, Co)-Ni/SBA-15 catalysts for CO2 reforming of CH4

Article abstract of DOI:10.1039/c6ra07635a

The CO₂ reforming of CH₄ to H₂ over a catalyst is an effective method for renewable energy generation. In this study, SBA-15 and CNT were chosen as supports for the Ni-based catalysts prepared by the impregnation method. The FESEM images demonstrated that NiO particles were rhombic and well distributed on the SBA-15 surface. The XRD patterns showed that the chemical state of Ni changed after the reforming reaction; the main crystals on the fresh and spent Ni/SBA-15 were found to be NiO and Ni⁰. The catalytic performance of Ni/SBA-15 in CO₂/CH₄ reforming was found to be superior to that of Ni/CNT. The results of the TGA and BET analysis demonstrated that spent Ni/SBA-15 (at 600 °C) showed no catalytic decay as an insignificant amount of coke was deposited on the catalyst supports. Moreover, Ni-based bimetallic catalysts were studied for the reforming reaction, and the activity of the catalysts with respect to metals was observed to follow a particular order: Cu-Ni > Mg-Ni > Co-Ni > Ca-Ni > Mn-Ni. The Cu-Ni/SBA-15 catalyst exhibited higher catalytic activity at a reaction temperature of 650 °C as compared to the others; the H₂ yield (40%) was not decreased as the reaction time increased, and the conversion of CO₂ and CH₄ is 77% and 75%, respectively.

Full text of DOI:10.1039/c6ra07635a

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Catalytic activity of mesoporous Ni/CNT, Ni/SBA-15
and (Cu, Ca, Mg, Mn, Co)–Ni/SBA-15 catalysts for
CO2 reforming of CH4
Cite this: RSC Adv., 2016, 6, 73887
a
bc
and Chi-Jen Changc
*
Yong-Ming Dai, Chi-Yuan Lu
The CO2 reforming of CH4 to H2 over a catalyst is an effective method for renewable energy generation. In
this study, SBA-15 and CNT were chosen as supports for the Ni-based catalysts prepared by the
impregnation method. The FESEM images demonstrated that NiO particles were rhombic and well
distributed on the SBA-15 surface. The XRD patterns showed that the chemical state of Ni changed after
the reforming reaction; the main crystals on the fresh and spent Ni/SBA-15 were found to be NiO and
Ni0. The catalytic performance of Ni/SBA-15 in CO2/CH4 reforming was found to be superior to that of
Ni/CNT. The results of the TGA and BET analysis demonstrated that spent Ni/SBA-15 (at 600 ꢀC) showed
no catalytic decay as an insignificant amount of coke was deposited on the catalyst supports. Moreover,
Ni-based bimetallic catalysts were studied for the reforming reaction, and the activity of the catalysts
with respect to metals was observed to follow a particular order: Cu–Ni > Mg–Ni > Co–Ni > Ca–Ni >
Mn–Ni. The Cu–Ni/SBA-15 catalyst exhibited higher catalytic activity at a reaction temperature of 650 ꢀC
as compared to the others; the H2 yield (40%) was not decreased as the reaction time increased, and the
conversion of CO2 and CH4 is 77% and 75%, respectively.
Received 23rd March 2016
Accepted 24th June 2016
DOI: 10.1039/c6ra07635a
www.rsc.org/advances
strengthened the interaction between Ni and Al2O3, and
retarded the sintering. Laosiripojana et al.7 reported that Ce–
1. Introduction
ZrO2 with a Ce/Zr ratio of 3/1 exhibited the best catalytic
performance in terms of activity and stability. Further, Pompeo
et al.8 indicated that in systems containing ZrO2 the deactiva-
tion levels were observed to be the lowest for Ni and Pt sup-
ported on a-Al2O3–ZrO2 because these catalysts inhibited the
reactions that lead to carbon deposition.
These studies demonstrated that CO2 reforming efficiency is
enhanced when a good dispersion of the metallic active sites,
thermal stability and low carbon deposition is obtained in the
reaction. Superior reforming performance was observed when
Ni-based catalysts were used at high temperatures above 800 ꢀC.
In order to reduce the energy required for the reforming reac-
tion, the choice of the support is important. Earlier studies have
reported that the use of mesoporous materials as catalyst
support signicantly enhances catalytic activity by allowing
macromolecules to penetrate and be easily adsorbed onto the
surface of the catalyst.11,12 Recently, ordered mesoporous silica,
SBA-15, has been used for catalytic oxidation and adsorption at
low temperatures.13,14
In recent years, considerable attention has been paid to CO2
and natural gas because of their contribution to the global
greenhouse effect. In order to compensate for the effects of
natural gases, processes have been developed for producing
energy by the regeneration of CH4, H2, CH3OH, and synthesis
gases using CO2. For instance, CO2 reforming of CH4 using
catalysts has generally been considered as an effective method
for hydrogen energy production.
Noble metal and Ni-based catalysts have long been used in
the CO2 reforming reaction; however, these catalysts have some
drawbacks in the reaction, such as a high energy cost, low
thermal stability of metallic sites, and low selectivity and lead to
coke deposition on the catalysts.1 Thus, new types of reforming
catalysts have been studied to improve the CO2 reforming effi-
ciency, such as, Ni/AlPO4,2 Ca–Ni/a-Al2O3,3 Pd-CeOx/a-Al2O3,4
Rh/Al2O3,5 Ni/Mg/Al/Ce mixed oxides,6 Ni/Ce–ZrO2,7 Ni/Pt/a-
Al2O3–ZrO2,8 Ni/SiO2,9 and Pt/ZrO2/Al2O3.10 Hou et al.3 found
that small amounts of Ca increased the activity and stability of
Ni/a-Al2O3 and that Ca improved the dispersion of Ni,
Studies have been conducted on the use of SBA-15 as
a catalyst support as it exhibits uniform nano-pore channels,
a high specic surface area (500–800 m2 gꢁ1), and a large pore
volume (0.5–1.0 cm3 gꢁ1) and is chemical inert and thermally
and chemically stable.15,16 This study aims to employ the SBA-15
as a support for the Ni-based catalyst and evaluate the catalytic
activity for CO2–CH4 reforming at lower temperature. The
aDepartment of Science Education and Application, National Taichung University of
Education, Taichung 403, Taiwan
bDepartment of Public Health, Chung Shan Medical University, Taichung 402, Taiwan
cDepartment of Family and Community Medicine, Chung Shan Medical University
Hospital, Taichung 402, Taiwan. E-mail: cylu2280@csmu.edu.tw; Fax: +886-4-
24730022 ext. 12182; Tel: +886-4-23248179
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catalytic performance of Ni/SBA-15 in CO2–CH4 reforming is pore structure of SBA-15. SBA-15 was suspended in ethanol with
compared with that of Ni/carbon nanotube (CNT). Then, SBA-15 ultrasonic. Then, a droplet was deposited on a copper grid
supported Ni-based bimetallic catalyst were studied for CO2– supporting
a perforated carbon lm. Ni particle size,
CH4 reforming with 360 min reaction time.
morphologies, and dispersion of metallic active phases were
investigated using eld emission scanning electron microscopy
(FESEM; Model JSM-6700F, JEOL). The samples were immobi-
lized on the copper holder with the carbon glue. Before analysis,
the holder with samples was pretreated at 200 ꢀC. The Bru-
nauer–Emmett–Teller (BET) surface area of the supports and Ni
catalysts was measured at 77 K by gravimetric methods with
a BET-201-AEL apparatus. The BET surface area was calculated
from the adsorption isotherm, and the ratio of pore volume and
the average pore diameter was obtained from the pore size–
volume distribution. An STA 6000 simultaneous thermal
analyzer (PerkinElmer, USA), controlling thermogravimetric
analysis (TGA), was used identify the thermal stability of the
2. Experiment
2.1. Preparation of catalysts
Both of SBA-15 and CNT were used as supports in this study.
SBA-15 was obtained using a triblock copolymer-Pluronic P123
(EO20PO70EO20, Aldrich) and tetraethyl orthosilicate (98%,
Aldrich) under acidic conditions (HCl, 2 M) following the
hydrothermal method reported in our study (Chuang et al.,
2010). The CNTs were synthesized from the 3 wt% Fe/Al2O3
supported catalyst by the chemical vapor deposition (CVD)
ꢀ
method at 800 C with C2H2 (H2 : C2H2 ¼ 6 : 1) for 1 h study.
ꢀ
materials from room temperature to 900 C with the rate of 10
Then the CNT were soaked in the acidic solution for 24 h to
ꢀC minꢁ1 under air atmosphere.
ꢀ
remove Fe and followed by washed and dried at 100 C.
In this experiment, the nominal Ni loading weight on the
support was 20 wt% for Ni/CNT and Ni/SBA-15. For Ni-based
bimetallic catalysts, the loading weight of Ni and secondary
metal (Cu, Ca, Mg, Mn, Co) is 10 wt% and 10 wt%, respectively.
All the catalysts were introduced by excess-solution impregnation
using metal nitrate as precursor. During the impregnation, the
solutions (3.0 g SBA-15 and 3.0 g CNTs were added in the
precursor contained-distilled water, respectively) were heated at
70 ꢀC and constantly stirred until totally evaporated. Aerwards,
Ni/CNT, Ni/SBA-15, and (Cu, Ca, Mg, Mn, Co)–Ni/SBA-15 catalysts
were obtained with dried at 100 ꢀC and then calcined at 450 ꢀC.
3. Results and discussion
3.1. Porous structure of SBA-15
The SBA-15 structure was observed by FESEM and TEM. Fig. 1(a)
and (b) show the surface morphologies of SBA-15, as observed
from the FESEM images. SBA-15 was found to have a pillar-like
structure (1.5–4 mm). The FESEM images obtained at a high
magnication of ꢂ150 000 clearly showed uniform pore chan-
nels on the SBA-15 surface.
Fig. 2(a) and (b) show the TEM images of the SBA-15. These
images clearly show that the pores have a well-ordered hexag-
onal structure (two-dimensional, P6mm), and pore channels
have uniform diameters of 4–5 nm. The thickness of pore wall is
estimated to be 3–4 nm from the black-white contrast in the
2.2. CO2 reforming of CH4 test
Catalytic activity measurements for the CO2 reforming of CH4
were conducted at atmospheric pressure in a micro-catalytic
reactor of quartz tube under a steady-state condition. The
concentration ratio of CO2/CH4 was 1, and the reactions were
reformed from 600 to 700 ꢀC. Each catalyst was packed in
a straight-tube reactor. A thermocouple was placed in the center
of catalyst bed to record the reaction temperature and was also
used to control the furnace. The reactant ows were adjusted to
the volume hour space velocity (SV) of 5000–20 000 hꢁ1 under
atmospheric pressure. The reactants and products were moni-
tored by an online gas chromatograph/thermal conductivity
detector (Agilent GC 7890N). Thus, the conversion was calcu-
lated based on the inlet and outlet concentrations of the reac-
tants and products.
2.3. Characterization of catalyst
An X-ray powder diffractometer (XRD) (SIEMENS D5000) was
used to identify the crystalline species of Ni/SBA-15 and Ni/CNT
catalysts. The powdered samples were pressed onto suitable
holders. The scanning range of 2q was from 1ꢀ to 5ꢀand from
20ꢀ to 80ꢀ, respectively; and the scanning speed was 2ꢀ minꢁ1
.
XRD patterns were manually analyzed with the Joint Committee
of Powder Diffraction Standard card (JCPDS). Transmission
electron microscope (TEM) observations were made with a JEOL
microscope (JEM-1200CXII) operated at 120 keV to observe the Fig. 1 FESEM images of SBA-15 support.
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Fig. 3 FESEM images of Ni catalysts: (a) Ni/SBA-15, (b) Ni/CNT.
Fig. 2 TEM images of SBA-15 support.
carbon-based supports. This nding suggests that the shape the
Ni particles is inuenced by the type of the support in the
impregnation process. The size of Ni particles is in the range of
20–70 nm, and the average size of the Ni particles in Ni/CNT is
smaller than that in Ni/SBA-15.
TEM image (Fig. 2(b)). The observed well-arrayed pore channel
structure of SBA-15 is in agreement with previous studies.15,16
Table 1 lists the values of the BET surface area and the struc-
tural characteristics of SBA-15 and CNT. SBA-15 shows a larger
surface area and pore volume (532.3 m2 gꢁ1, 0.61 m3 gꢁ1) than
CNT (65.1 m2 gꢁ1, 0.44 m3 gꢁ1). On the other hand, CNT shows
In order to conrm the dispersion of the Ni active phase and
the variation in the catalyst support structure, a BET analysis of
fresh Ni catalysts was conducted; the results of this analysis are
given in Table 1. The analysis results demonstrate that Ni/SBA-
15 and Ni/CNT were all mesoporous catalysts (the mesoporous
volume is greater than 80% of the total volume). The results also
demonstrated that the BET surface area of SBA-15 is reduced
from 532.3 to 337.7 m2 gꢁ1 aer SBA-15 is loaded with 20 wt%
Ni. The BET surface area of CNT is also observed to slightly
reduce from 65.1 to 53.3 m2 gꢁ1 when loaded with the same. In
contrast, the total pore volume and pore diameter of SBA-15
increase from 0.61 to 0.66 m3 gꢁ1 and from 45.9 to 78.7 nm,
respectively. The average pore diameter of CNT catalyst
increases as well, from 76.4 to 119.3 nm. According to Agnihotri
et al.,17 this is because the BET surface analysis of the catalysts is
conducted over four areas: inside the pore channels; on the
interstitial channels between the supports; on the outer groove
˚
˚
a larger pore diameter (76.4 A) than SBA-15 (45.9 A).
3.2. Morphologies of Ni/SBA-15 and Ni/CNT
The surface morphologies of Ni/SBA-15 and Ni/CNT, speci-
cally, the distributions of the Ni particles on the catalyst
supports, observed by FESEM are shown in Fig. 3(a) and (b),
respectively. Interestingly, the Ni particles on the two supports
are observed to be of different types: the Ni particles supported
on SBA-15 have a rhombic shape while those supported on CNT
are spherical. In the case of Ni/SBA-15, Ni particles appear to be
inserted into SBA-15. Further, from Fig. 3(a) and 1(b), a few meal
phases are also found to be deposited along the pore channels
of SBA-15. Comparing Ni/CNT (Fig. 3(b)) with the earlier study,18
the spherical shapes of Ni particles were always appeared on the
Table 1 BET surface area and structural characteristics of the fresh and spent Ni catalystsa
Ratio of pore volume (%)
SBET (m2 gꢁ1
)
DBJH (A)
VBJH (m3 gꢁ1
)
Vmicro (<2 nm)
Vmeso (2–50 nm)
Vmacro (>50 nm)
˚
Samples
SBA-15
532.3
337.7
301.9
65.1
53.3
47.9
45.9
78.7
68.6
76.4
119.3
123.6
0.61
0.66
0.52
0.44
0.16
0.15
17
11
11
22
5
82
88
88
70
84
87
1
1
1
8
11
10
Ni/SBA-15-fresh
Ni/SBA-15-spent
CNT
Ni/CNT-fresh
Ni/CNT-spent
3
a
SBET ¼ BET surface area; DBJH ¼ BJH pore diameter and VBJH ¼ total BJH pore.
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sites where the two supports meet; and on the external surface width of the relevant diffraction peak and q, the angle of
of the supports. The deposition of Ni particles on the supports diffraction. The mean NiO particle size of the catalyst is thus
increases the pore diameter of the Ni catalysts. The Ni particles calculated as 20.3 nm. Next, small-angle XRD (2q ¼ 1–5ꢀ) was
were found to be well distributed on both the CNT and SBA-15 employed to analyze the structure of the SBA-15 supports; the
catalyst. The maintenance of ordered hexagonal arrangement of XRD results are shown in Fig. 4(b).
the SBA-15 frameworks upon Ni particles introduction is further
These results shows three well-dened peaks at 2q values of
supported by the N2-adsorption data. Although the incorpora- 1–3ꢀ; these peaks are indexed as (100), (110), and (200) reec-
tion of Ni particles into SBA-15 led to a continuous decrease of tions for the pores with a hexagonal straight pores P6mm
the specic surface area and the cumulative pore volume, high straight structure.19 The small-angle XRD patterns further show
surface areas and pore volumes as well as very narrow pore-size that the mesoporous structure of supports stabilizes aer
distributions are observed in SBA-15-supported materials with impregnation. Next, we analyzed the XRD patterns of the fresh
Ni particles. A similar decrease of the BET and pore volume has Ni/SBA-15 catalysts; the SBA-15 support was found to be ther-
already been reported by Berndt et al. for mesoporous V- mally stable aer calcination with active Ni at 450 ꢀC, as evi-
containing MCM materials.18 Thus, it can be concluded that denced by retention of the three well-dened peaks. The intense
˚
well-organized mesoporous SBA-15 catalysts with a high surface (100) peak reects a d spacing of 81.5 A, which corresponds to
˚
area could be obtained at all Ni loadings in the present study.
a large unit-cell parameter (a0 ¼ 94.1 A). The value of a0 can be
calculated by an equation devised in a previous study:20
dð100Þ ꢂ 2
pffiffi
3.3. XRD analysis of Ni/SBA-15 and Ni/CNT
a0 ¼
(2)
XRD analysis was conducted on both the catalysts at 2q ¼ 20–80ꢀ
to study the crystal phases of Ni. Fig. 4 shows the XRD patterns of
fresh and spent Ni catalysts. In the case of Ni/SBA-15, the main
crystal peaks of NiO were observed at 2q ¼ 37.3ꢀ (111), 43.4ꢀ
(200), 62.9ꢀ (220), 75.5ꢀ (311) aer Ni impregnation on SBA-15, as
shown in Fig. 4(a). The mean NiO particle size can be calculated
in terms of the grain diameter D (nm) from the NiO (111) peaks
(2q ¼ 37.3ꢀ) using the Scherrer equation written as:
3
˚
Next, the wall thickness was calculated as 48.2 A by the
following equation:
Wall thickness ¼ a0 ꢁ poresize
(3)
The calculated wall thickness of SBA-15 is found to be larger
than the observed wall thickness (Fig. 2). Further, from the
values listed in Table 1, the pore size of SBA-15 calculated using
the Barrett–Joyner–Halenda (BJH) method in the BET analysis is
signicantly smaller than the repeat distance determined by
XRD; this is because the latter value includes the thickness of
the pore wall. Since the wall thickness is large, the structure of
SBA-15 would not be affected or destroyed easily, the reported
that the use of mesoporous SBA-15 silica as a support can allow
the generation of a highly efficient VOx–SBA-15 catalyst exhib-
iting high selectivities (80%) to olens at high propane
conversions (42%) in the oxidative dehydrogenation of
propane.21 In the case of Ni/SBA-15, the pore volume and pore
diameter were not reduced aer impregnation and calcination,
as given in Table 1. The results demonstrate that the SBA-15
structure remained stable in spite of the impregnation of 20
wt% Ni. In the case of Ni/SBA-15, the pore volume and pore
diameter were not reduced aer impregnation and calcination,
as given in Table 1. The results demonstrate that the SBA-15
structure remained stable in spite of the impregnation of 20
wt% Ni.
Kl
D ¼
(1)
B1=2 cos q
where K is a constant with a value of 0.89 and l is the wave
function, 0.154056 nm (Cu/K-alpha1). B1/2 is the half-intensity
The XRD patterns of the Ni/CNT catalyst are shown in
Fig. 4(c). The peaks occurring at 2q ¼ 27.2ꢀ and 43.3ꢀ can be
attributed to the graphite structure of the CNT supports with
(002) and (100) graphitic planes. Characteristic peaks of Ni0 in
the Ni/CNT catalyst were observed at 2q ¼ 44.6ꢀ (111), 51.9ꢀ
(200), and 76.4ꢀ (220) aer catalyst preparation. The mean Ni0
particle size D can be calculated from the NiO (111) peak at 2q
¼ 44.6ꢀ. The Ni0 particle size of the catalyst is calculated as
14.0 nm. The particle size of the active phase over Ni/SBA-15 or
Ni/CNT calculated from the XRD analysis results (Fig. 4) is
Fig. 4 XRD patterns of Ni catalysts.
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found to be smaller than the particle size observed in the
FESEM images (Fig. 3). The XRD patterns of Ni/SBA-15 catalyst
suggest that two possible types of active phases appear on the
SBA-15 support; one is rhombic, and the other is deposited
along the pore channel, as shown in Fig. 4(a). Moreover, the
XRD patterns show that the main crystal phase of Ni/SBA-15
and Ni/CNT is NiO and Ni0, respectively. It is hence found
that the types of Ni crystal phase is related to the shape of
metal Ni (spherical and rhombic) on the support, as shown in
Fig. 3.
3.5. H2-TPR analysis of Ni/SBA-15 and Ni/CNT
H2-TPR is an efficient method to characterize the reducibility of
supported nickel-based catalysts.25,26 Fig. 6 shows the TPR
spectra for the two samples. It is obvious that the H2-TPR curves
of the Ni/SBA-15 and Ni/CNT. Only two peaks appeared: one
broad peak at high temperature and a shoulder peak at low
temperature. Therefore, different peaks could be assigned to
different interactions between the NiO and support: the rst
shoulder peak occurs at 250–300 ꢀC (ref. 27) that could most
likely be attributed to H2 consumption of bulk NiO particles
which has no or weak interaction with support. The second
broad hydrogen consumption peaks, locating at around 310–
3.4. XPS analysis of Ni/SBA-15 and Ni/CNT
ꢀ
400 C (ref. 28) might be ascribed to the reduction of NiO and
XPS is a useful surface chemical analysis technique that is used
to determine the species and chemical states of the elements on
the surfaces of the materials. Fig. 5 shows the XPS spectra for
the two samples. As our purpose is to analyze the valence state
and chemical environment of Ni element on the CNT and SBA-
15 support, we put our emphasis on the electron binding energy
range of 850–865 eV, which covers the 2p3/2 XPS spectrum of Ni
element. The typical Ni peaks at 851.7–853 eV,22 853.6–855.5 eV
(ref. 23) and 855.8–856 eV (ref. 24) from Ni, NiO and Ni2O3,
respectively. Comparing our results with these data, we could
nd that for the sample Ni element on the SBA-15 support, the
Ni element is still in the form of Ni phase. The Ni element on
the SBA-15, the NiO is formed as a result of the NiO on SBA-15
support, since the electron binding energy of NiO 2p3/2 shis to
857.5 eV. These results are consistent with the XRD results,
indicating that the main crystal phase of Ni/SBA-15 and Ni/CNT
is NiO and Ni, respectively.
the presence of Ni-species having a lower interaction with the
SBA-15 and CNT.
3.6. Comparison of the reforming reactions over Ni/SBA-15
and Ni/CNT
The CO2–CH4 reforming reactions over Ni/SBA-15 and Ni/CNT
catalysts were studied. Fig. 7 shows the CO2 reforming reactions
at 600 ꢀC over Ni/SBA-15 and Ni/CNT catalysts, which result in CO2
conversion and CO production. The inlet stream comprising 10
ml CO2 and 10 ml CH4 was fed at a space velocity of 5000 hꢁ1. The
CO yield is dened as the CO concentration in the outlet gas. The
Ni/SBA-15 catalyst exhibits a more effective catalyst reforming
activity (44%) than that of Ni/CNT. The primary and side reactions
of CO2–CH4 reforming are divided in four steps:29
Carbon dioxide reforming: CO2 + CH4 /
2CO + 2H2; DH298 ¼ 41 kJ mol (a)
Reverse water-gas shift: H2 + CO2 / H2O + CO;
(b)
Methane decomposition: CH4 /
C + 2H2; DH298 ¼ 75 kJ mol (c)
C + CO2; DH298 ¼ 171 kJ mol (d)
Boudouard reaction: 2CO /
For the generation of hydrogen energy, CO causes decay of
the Pt electrodes in proton-exchange membrane fuel cells
Fig. 5 The XPS spectra of (a) Ni/SBA-15, (b) Ni/CNT.
Fig. 6 H2-TPR analysis of Ni/SBA-15 and Ni/CNT.
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Fig. 8 FESEM images of Ni catalysts on CO2 reforming of CH4 reaction
at 600 ꢀC: (a) Ni/SBA-15, (b) Ni/CNT.
change aer the reaction. Further, a negligible amount of coke
was deposited on the SBA-15 or CNT catalyst support, as shown
in Fig. 8. The FESEM images also conrmed that no side reac-
tions occurred, which was obviously because of the negligible
coke deposition.
Fig. 7 CO2 conversion and CO production over Ni/SBA-15 and Ni/
CNT catalysts on reforming at 600 ꢀC.
Next, a thermogravimetric analysis of the fresh and spent Ni
catalysts was conducted. The analysis results, shown in Fig. 9,
demonstrate that Ni/SBA-15 and Ni/CNT provided good thermal
stabilization (100–800 ꢀC). As clearly seen in Fig. 8, the rst step
in the weight loss of the sample could be observed from 120 ꢀC to
(PEMFCs).30 The CO2 reforming reaction over Ni/SBA-15 and Ni/
CNT shows the concentration of produced CO is 19% and 10%,
respectively. The ratio of CO production to CO2 conversion is
low but acceptable for renewable energy since more than 40%
CO2 reacted with CH4.3,7,8 The results indicated that CO
concentration in the outlet gas is low, and the side reactions
(b)–(d) could not be observed since the amount of CO produꢀced
in them is lower than the amount of CO2 converted at 600 C.
ꢀ
180 C, which might be attributed to the elimination of active
carbonaceous species which was responsible for the formation
of synthesis gas.31 The curve initially experienced a slight rise in
the region from 300 ꢀC to 400 ꢀC, which is probably derived from
the oxidation of metallic Ni particles. The weight losses from 450
ꢀC to 700 ꢀC are caused by the oxidation of carbon.32,33 It could be
clearly observed that the weight loss of the total carbon amount
deposition over the used Ni/SBA-15 catalyst (5.4%) and Ni/CNT
catalyst (8.9%), in agreement with the results of XRD (Fig. 4).
3.7. Characterization of spent Ni catalysts
In CO2 reforming of CH4, coke deposition is the main factor for
catalyst decay during the reforming reaction at high tempera-
ture. The side reactions (b)–(d) are obviously not analyzed by gas
chromatography. Therefore, the spent catalysts were charac-
terized by FESEM. Fig. 8 shows the FESEM images of Ni/SBA-15
and Ni/CNT catalysts aer CO2 reforming of CH4 over these
catalysts at 600 ꢀC. These results indicate that the maximum CO
yield is 19.9% and 10.7% for Ni/SBA-15 and Ni/CNT, respec-
tively. As observed in Fig. 8(a), the images show that the Ni
particle in case of the spent Ni/SBA-15 catalyst is spherical. This
shape is different from that in case of the fresh Ni/SBA-15
catalyst (Fig. 3(a)) because it changes with the chemical state
aer the reaction. In case of CO2–CH4 reforming over Ni/SBA-15,
the main crystal peaks of Ni0 in the spent Ni/SBA-15 are
observed at 2q ¼ 44.6ꢀ (111), 51.9ꢀ (200), and 76.4ꢀ (220), as
shown in Fig. 4(a). The mean Ni0 particle size can be calculated
as 18.9 nm from the Ni (111) peak at 2q ¼ 44.6ꢀ. In contrast to
Fig. 9 TGA analysis of spent catalysts (CO2 reforming of CH4 reaction
the size, the chemical state and shape of the Ni particles did not at 600 ꢀC).
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The XRD patterns were clearly and sharply observed because of efficiency. Considering both the CO2 conversion and CH4
the disappearance of impurities. The negligible amount of coke conversion performances, Ni/SBA-15 exhibits optimum catalytic
deposited on the catalyst, as found in the TGA also conrms the activity at the space velocity of 10 000 hꢁ1 which corresponds to
high thermal stabilization. However, the BET surface area, pore 63% CO2 conversion and 59% CH4 conversion.
volume, and pore diameter of the spent catalysts (Ni/SBA-15 and
In the next experiment, an inlet stream comprising 20 ml
Ni/CNT) are not inuenced by the coke deposition or the CO2 and 20 ml CH4 was fed at a space velocity of 10 000 hꢁ1
.
blocking of the support pores, as shown in Table 1.
Fig. 11 shows the conversions of CO2 and CH4 in the reforming
reaction over Ni/SBA-15 at different reaction temperatures. The
CO2 and CH4 conversions did not increased with the reaction
temperature up till 650 ꢀC. When the temperature was
3.8. Effect of space velocity and reaction temperature on
reforming reaction over Ni/SBA-15
ꢀ
increased from 650 to 700 C, both CO2 and CH4 conversions
increased arrived at 80%. These results demonstrated that the
Ni/SBA-15 conversion efficiency increases with the temperature
increased and that Ni/SBA-15 provides good thermal stabiliza-
In this study, Ni/SBA-15 is found to have better catalytic activity
in a reforming reaction than Ni/CNT. Next, the optimum space
velocity and reaction temperature for CO2 reforming of CH4
over Ni/SBA-15 were studied. Fig. 10 shows the conversions ꢀof
CO2 and CH4 in a reforming reaction over Ni/SBA-15 at 600 C
and at different space velocities of the inlet feed stream from
5000 to 20 000 hꢁ1. It is found that both CO2 and CH4 conver-
sions increased as the space velocity increases to 10 000 hꢁ1. It
was considered that low space velocity inhibits the reforming
reaction because of the long retention time of the reactants.
However, as the space velocity increases to 20 000 hꢁ1, the
conversion efficiency of CO2 decreases, although it is still better
than the CO2 conversion efficiency at the space velocity of 5000
hꢁ1. On the other hand, CH4 conversion is lowest (55%) at the
space velocity of 20 000 hꢁ1. Under high space velocity, the
reactant gases had less time to adsorb and react with the cata-
lyst, thus leading to low efficiency of CO2 and CH4 conversion
ꢀ
tion at high temperature (700 C).
3.9. H2 production efficiency of reforming reaction over Ni/
SBA-15
Fig. 12 shows the H2 yield in the reforming reaction over Ni/SBA-
15 at different reaction temperatures and a space velocity of
10 000 hꢁ1. The H2 yield is dened as the H2 concentration in the
outlet gas. From the H2 yield values and results of the previous
experiment on the CO2 and CH4 conversion efficiencies, the H2
yield from the reforming reaction is found to be directly related
to the conversion of CO2 and CH4. The H2 yield is 43% at the
reaction temperature of 700 ꢀC and decreases as the reaction
temperature decreases from 700 ꢀC to 600 ꢀC. Daza et al.6 studied
the CO2 reforming of methane over Ni/Mg/Al/Ce mixed oxides
and found that at 700 ꢀC, the H2 yield is 15–29% whereas the CO2
and CH4 conversions are from 50–88%. Pompeo et al.8 studied
Fig. 10 The conversions of CO2 and CH4 over Ni/SBA-15 with Fig. 11 The conversions of CO2 and CH4 over Ni/SBA-15 with different
different space velocity on reforming at 600 ꢀC.
reaction temperature on reforming at a space velocity of 10 000 hꢁ1
.
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consumption, Ni-based bimetallic catalysts, Cu–Ni/SBA-15, Co–
Ni/SBA15, Mg–Ni/SBA15, Mn–Ni/SBA15, and Ca–Ni/SBA15, were
studied for CO2–CH4 reforming, as shown in Fig. 13. Fig. 13
shows the conversions of CO2 and CH4 in a reforming reaction
over Ni-based bimetallic catalysts at 650 ꢀC at a space velocity of
10 000 hꢁ1. The catalyst activity for CO2–CH4 reforming was in
the following order: Cu–Ni/SBA-15 > Mg–Ni/SBA-15 > Co–Ni/SBA-
15 ¼ Ni/SBA-15 > Mn–Ni/SBA-15 > Ca–Ni/SBA-15. These reactions
of CO2–CH4 reforming are steps (a), (c) and (d) are strong
endothermic reactions, and the C–H bond energy in methane is
as high as 435 kJ molꢁ1. The high temperature is favorable to the
three reactions (a), (c) and (d). Therefore, with the reaction
temperature increasing, the conversion of methane and carbon
dioxide increase correspondingly. It also should be noted that
Ni-based bimetallic catalysts exhibit better performance than
monometallic Ni catalysts of the supports investigated.29
Fig. 12 H2 yield over Ni/SBA-15 with different reaction temperature
on reforming at a space velocity of 10 000 hꢁ1
.
Comparing Fig. 11 with 13, Cu–Ni/SBA-15 and Mg–Ni/SBA-15
showed the better performance than that of Ni/SBA-15. The
good performances over Ni-based bimetallic catalysts were
suggested to be due to the electronic and geometric effects.33
Thus, the higher catalytic reactivity of the Cu–Ni/SBA-15 and
Mg–Ni/SBA-15 in better performance is due to the higher
number of active sites available on the Cu–Ni/SBA-15 and Mg–
Ni/SBA-15 than those on the Ni/SBA-15. This is attributed to the
higher dispersion and the smaller particle size in the Cu–Ni/
SBA-15 and Mg–Ni/SBA-15. Also, the smaller size and good
dispersion of Ni particles may be prepared over bimetallic
catalysts, as shown in TEM image of Fig. 14.
the CO2 reforming of CH4 at 700 ꢀC over Ni and Pt catalysts
supported on a-Al2O3 and ZrO2 and found an H2 yield of 10–40%
and CO2 and CH4 conversions of 40–90%. In comparison with
the H2 yield with previous studies,3,7,8 the H2 yield obtained in
this study using the Ni/SBA-15 catalyst is higher and the catalyst
preparation method is simpler and less expensive.
3.10. CO2 reforming of CH4 over (Cu, Ca, Mg, Mn, Co)–Ni/
SBA-15
In the studies above, the CO2 reforming of CH4 can be optimally
performed over Ni/SBA-15 at 650 ꢀC and for a 10 000 hꢁ1 space
velocity of the inlet feed stream. In order to reduce the energy
Fig. 13 CO2 conversion and CH4 conversion over Ni-based bimetallic
catalysts on CO2 reforming of CH4 reaction at 650 ꢀC at a space
velocity of 10 000 hꢁ1
.
Fig. 14 TEM images of five bi-metallic supported catalysts.
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4. Conclusions
Ni-Based catalysts were prepared for use in CO2 reforming of
CH4. The catalysts were analyzed, and it was found that Ni/SBA-
15 shows better conversion efficiency than of Ni/CNT. The
˚
thickness of the SBA-15 pore wall is 48.2 A (determined by XRD
and BET analyses); this thick porous wall is suitable for catalyst
preparation and the reforming reaction at high temperature.
The porous structure of Ni/SBA-15 facilitates the stabilization of
the reforming reaction. Moreover, (Cu, Ca, Mg, Mn, Co)–Ni/SBA-
15 catalysts were also studied for CO2–CH4 reforming. Cu–Ni/
SBA-15 catalyst exhibited higher catalytic activity at a reaction
ꢀ
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Acknowledgements
This work was performed under the auspices of the National
Science Council of the Republic of China, under Contract MOST
97-2218-E-040-003, to which the authors wish to express their
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