Influence of method of preparation on the activity of La-Ni-Ce mixed oxide catalysts for dry reforming of methane

Article abstract of DOI:10.1039/c4ra07098d

La-Ni_x-Ce_1-x mixed oxide catalysts were prepared by a sol-gel method varying the Ni composition (0 ≤ x ≤ 1). The catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), BET surface area, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), H₂ chemisorption and Fourier transform infrared spectroscopy (FT-IR) techniques. CO₂ reforming of methane was carried out at atmospheric pressure and 800 °C, maintaining a reactant CO₂/CH₄/N₂ ratio of 80/80/80 (total flow rate = 240 ml min^-1, GHSV of 28800 h^-1). The catalysts offered higher activity even at lower Ni compositions. LaNi_0.4Ce_0.6O₃. showed the highest conversion of CH₄ and CO₂. The H₂/CO ratio in the syngas was stable at 0.85 ± 0.02. The performance of the sol-gel catalysts was compared with that of the hydrothermally prepared catalysts, reported earlier. High surface area and better Ni dispersion were found to be the reasons for superior activity of the sol-gel catalysts.

Full text of DOI:10.1039/c4ra07098d

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PAPER
Influence of method of preparation on the activity
of La–Ni–Ce mixed oxide catalysts for dry
reforming of methane†
Cite this: RSC Adv., 2014, 4, 50226
a
a
b
a
a
a
T. V. Sagar, N. Sreelatha, G. Hanmant, M. Surendar, N. Lingaiah, K. S. Rama Rao,
b
c
a
C. V. V. Satyanarayana, I. A. K. Reddy and P. S. Sai Prasad*
La–Ni
1). The catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma optical
emission spectroscopy (ICP-OES), BET surface area, X-ray photoelectron spectroscopy (XPS),
temperature-programmed reduction (TPR), chemisorption and Fourier transform infrared
x
–Ce1ꢀx mixed oxide catalysts were prepared by a sol–gel method varying the Ni composition (0 # x
#
H
2
spectroscopy (FT-IR) techniques. CO
2
reforming of methane was carried out at atmospheric pressure
ꢁ
ꢀ1
and 800 C, maintaining a reactant CO
/CH /N
2 4 2
ratio of 80/80/80 (total flow rate ¼ 240 ml min
,
ꢀ
1
GHSV of 28 800
h
). The catalysts offered higher activity even at lower Ni compositions.
and CO . The H /CO ratio in the syngas was
Received 14th July 2014
Accepted 29th September 2014
LaNi0.4Ce0.6 . showed the highest conversion of CH
O
3
4
2
2
stable at 0.85 ꢂ 0.02. The performance of the sol–gel catalysts was compared with that of the
hydrothermally prepared catalysts, reported earlier. High surface area and better Ni dispersion were
found to be the reasons for superior activity of the sol–gel catalysts.
DOI: 10.1039/c4ra07098d
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are technically and economically viable for the reforming
1
. Introduction
4
reaction. In order to prevent Ni from deactivation due to coke
5
As methane and carbon dioxide are recognized as the major formation and sintering, its insertion in a denite chemical
contributors to global warming, their emissions into the structure like the ABO
atmosphere have to be restricted. For carbon dioxide emission perovskite LaNiO is generally preferred as La affords thermal
control, the carbon capture and sequestration (CCS) technology stability and Ni provides catalytic activity. Modications to this
type perovskite is oen adopted. The
3
3
6
is vigorously pursued. However, carbon dioxide is nowadays perovskite in the form of LaNi1ꢀx
M
x
O
3
(M ¼ Mg, Cu, Rh, Ru, Co,
7
8,9
viewed as a cheap raw material for chemical synthesis rather Fe and Al) and LaNi
than its expensive capture and storage. Catalytic carbon dioxide with LaNi0.3Al0.7
reforming or dry reforming of methane is found to be an
important method for this purpose not only for regulating the have been extensively studied by many researchers. The
gas emissions but also to generate useful syngas that can enhancement in catalytic behavior by presence of CeO was
attributed to its unique structural and redox properties. In the
x
M
1ꢀx
O
3
(M ¼ Al, Fe) are also reported
8
3
O suggested as the best system.
The advantages of inclusion of Ce in the mixed oxide system
2
1
10
produce oxygenated products via Fisher–Tropsch synthesis.
Dry reforming is a highly endothermic reaction necessitating reforming reaction, the rate is controlled by the interaction of
high reaction temperatures at which coke formation and sin- adsorbed methane or surface hydrocarbon species with lattice
tering of catalysts become serious problems for smooth trans- oxygen in CeO
formation of the reactants into products. Noble metal catalysts lowed by a rapid gas–solid reaction between reactant CO
like Pt, Pd, Rh and Ir have been explored for this purpose as they the reduced support Ce to regenerate the lattice oxygen.
show better coke and sulfur resistance. However, their limited Besides, due to its high oxygen storage capacity CeO could
and as a
commercial applications. In this aspect, nickel based catalysts promoter increases Ni dispersion, which enables decrease in
2
, as it is the slow reaction step. This step is fol-
2
and
O
2 3
2,3
2
11,12
availability and high cost make them less attractive for enhance the activity of the perovskite catalysts
13
x 3
the coke formation on the Ni surface. La1ꢀxCe NiO prepared
1
4
by substitution of Ce in place of La was studied by Qi et al.
a
Inorganic and Physical Chemistry Division, CSIR – Indian Institute of Chemical leaving the Ni composition constant. In a recent publication,15
Technology, Uppal Road, Hyderabad – 500 607, India. E-mail: saiprasad@iict.res.
we have reported that catalysts prepared by hydrothermal
method and varying Ni and Ce composition, keeping La
constant, offer better activity. Recently, Ni–Ce oxide solid solu-
tion has shown much promise in its methane partial oxidation
in; Fax: +91 40 27160921; Tel: +91-40-2719 3163
b
CSIR – National Chemical Laboratory, Pune-411008, India
c
National Institute of Technology, Warangal-506004, India
†
Electronic supplementary information (ESI) available: XPS gures of Ni 2p, O 1s
16
17
activity. Xu et al. also studied autothermal reforming of
and C 1s of La–Ni –Ce1ꢀx mixed oxide catalysts. See DOI: 10.1039/c4ra07098d
x
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methane on Ce–Ni oxide catalysts. Dry reforming of methane line (285 eV) was used. Charge neutralization of 2 eV was used to
has not been reported on Ni–Ce oxide solid solution or the solid balance the charge up of the sample. The spectra were de-
solution containing mixed oxide catalysts.
convoluted using Sun Solaris based Vision-2 curve resolver. The
The aim of the present work is to examine the inuence of location and the full width at half maximum (FWHM) value for
method of preparation on the activity of the catalyst during the species were rst determined using the spectrum of pure
x
3
methane dry reforming. LaNi Ce1ꢀxO catalysts were prepared sample. The location and FWHM of products, which were not
by sol–gel method and their activity was compared with those obtained as pure species were adjusted until the best t was
prepared by hydrothermal method. The advantages of the sol– obtained. Symmetric Gaussian shapes were used in all cases.
gel method over the hydrothermal method were brought Binding energies for identical samples were, in general repro-
forward.
ducible within ꢂ0.1 eV. H Chemisorptions studies were per-
2
formed using an Autosorb iQ apparatus. Catalyst samples (30
mg), taken in a quartz reactor, were rst reduced in H gas at a
2
ꢁ
2
. Experimental
ꢀ1
ꢀ1

ow rate of 60 ml min and with a heating rate of 10 C min
ꢁ
2
.1 Catalyst preparation
The sol–gel method similar to that reported in the literature
was adopted for the synthesis of the La–Ni –Ce oxide cata-
up to 600 C. They were then cooled in He ow to room
temperature. Hydrogen gas was then introduced in pulses and
the adsorption uptake was analyzed using a TCD. The coke
content of the used catalysts was determined in CHNS analyzer
9
x
1ꢀx
lysts with 0 # x # 1. Required amounts of La(NO ) $6H O
3
3
2
(ElementaV, Germany).
(Fluka), Ni(NO ) $6H O (SD ne) and Ce(NO ) $9H O (SD ne)
3 2 2 3 3 2
were separately dissolved in known quantities of hot propionic
acid. Aer mixing and stirring under reux for 24 h, the resin
like material obtained was dried at reduced pressure and
2
.3 Catalytic activity test
The evaluation of catalysts was performed in a xed-bed quartz
micro reactor (i.d. 12 mm) under atmospheric pressure by
passing a mixture of CO
ꢁ
ꢁ
ꢀ1
calcined at 800 C with a temperature ramping of 2 C min
.
The quantities of nitrate salts were selected in such a way that
2
, CH
4
and N
2
at a CO
2
/CH
4
/N
2
ratio of
the nished catalysts contained their Ni content with x ¼ 0.2,
ꢀ
1
ꢀ1
8
0/80/80 (total ow rate ¼ 240 ml min , GHSV of 28 800 h ).
0
.3, 0.4, 0.6 and 0.8. A sample with x ¼ 1.0 (i.e., La–Ni oxide
3
The activity tests were carried out using 0.5 cm (ꢃ0.8 g) of
sample without Ce) was also prepared for the sake of
comparison.
catalyst (18/25 BSS mesh) diluted with 50% of the ceramic
beads. Prior to the activity measurements, the samples were
reduced in situ under 60% H
ꢁ
2
–N
2
gas mixtures at 600 C for 6 h.
2
.2 Characterization studies
The data were analyzed online on a Chemito 8610 gas chro-
matograph equipped with a carbosphere column using Argon
as a carrier gas and a TCD detector. Aer attaining the required
temperature, the reaction was allowed to attain steady-state for
a period of 1 h. Subsequently, the product analysis was carried
out three times with an interval of 30 min. The values provided
here are the average values. The accuracy was within ꢂ5%.
The XRD patterns of the catalysts were obtained on an Ultima-IV
diffractometer (M s . Rigaku Corporation, Japan) using nickel-
ꢀ
1
˚

ltered Cu Ka radiation (l ¼ 1.54 A). The measurements were
ꢁ
recorded in steps of 0.045 with count time of 0.5 s in 2q range
of 10 to 80 . Identication of the crystalline phases was made
with the help of JCPDS les. BET surface areas were determined
by N adsorption on a SMART SORB 92/93 instrument (M s
ꢁ
ꢀ1
.
2
SMART Instruments, India). Prior to BET measurement, the
samples were dried at 150 C for 2 h. The chemical analysis of
the samples was carried out by inductively coupled plasma
optical emission spectroscopy (ICP-OES) using a Varian 725ES
instrument. TPR studies were performed using a home-made
apparatus. Catalyst samples (50 mg), taken in a quartz reactor,
3
. Results and discussion
ꢁ
3.1 Surface area & X-ray diffraction
Table 1 illustrates the specic surface areas of the catalysts. The
surface area is seen to increase initially in catalysts up to x ¼ 0.3
followed by a decreasing trend. The decrease is more percep-
tible beyond x ¼ 0.6. The specic surface area values are much
2
were reduced with 10% H –Ar gas mixture owing at a ow rate
ꢀ
1
ꢁ
ꢀ1
ꢁ
of 30 ml min and with heating rate of 5 C min up to 900 C.
Before the TPR run, the catalysts were pretreated in argon at 300
ꢁ
C for 2 h. The hydrogen consumption was monitored using Table 1 Specific surface area (SSA) and FT-IR bands of catalyst
thermal conductivity detector of a gas chromatograph (Varian, systems
8
301). FT-IR spectra were obtained on a Perkin Elmer (Spectrum
ꢀ
1
FT-IR (cm
(CH
)
GX, USA) instrument, using KBr pellet method. XPS measure-
ments were made on a KRATOS AXIS 165 instrument. The non-
monochromatized Al-Ka X-ray source (hn ¼ 1486.6 eV) was
operated at 12.5 kV and 16 mA. Before acquisition of the data
the sample was out-gassed for about 3 h at 100 C under a
pressure of 1.0 x 10 torr to minimize surface contamination.
2
ꢀ1
Catalyst
SSA (m g
)
d
s
3
)
d
as (CH
3
) & n
3
(NO)
3
n (NO)
LaNi0.8Ce0.2
LaNi0.6Ce0.4
LaNi0.4Ce0.6
LaNi0.3Ce0.7
O
O
O
O
3
3
3
3
1.39
9.29
10.16
11.67
6.91
1464
1464
1471
1471
—
1379
1379
1369
1369
1376
—
ꢁ
850
853
850
858
ꢀ2
The XPS instrument was calibrated using Au as a standard LaNi0.2Ce0.8O3
material. For energy calibration, the carbon 1s photoelectron
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higher than the catalysts prepared by the hydrothermal method
2
ꢀ1
(
which varied between 0.4 to 4.1 m g ). Table 2 provides the
experimental and estimated compositions of the sol–gel cata-
lysts, which agreed very closely. The XRD patterns of the cata-
lysts, including LaNiO
x values, peaks due to La
only are found. A coherent 2q shi towards lower angle is evi-
denced with increase of x for the LaNi Ce1ꢀx mixed oxides. This
observation is due to the dissolution of Ni ions in the La
3
perovskite, are displayed in Fig. 1. At low
ꢁ
2
O
3
(2q ¼ 27.5, 31.9, 45.6 and 54.08 )
x
2
+
2 3
O
lattice forming the LaNiO perovskite, as seen in catalysts with
3
ꢁ
higher values of x. The peaks at 23.6, 32.9, 47.3 and 58.9 (2q)
clearly reveal the presence of the typical rhombohedral phase of
LaNiO (JCPDS 34-1028) with higher intensity, particularly in
3
catalysts with x ¼ 0.8 and 1.0. A drastic decrease in the surface
area (Table 1) in these samples also indicates the formation of
crystalline perovskite. The NiO shows much diffused peaks in
catalysts with low Ni content probably due to the presence of
smaller sized particles, not being detected by XRD. Similarly, no
peaks due to crystalline CeO are observed, excepting its weak
2
signals in catalysts with x ¼ 0.3 and 0.4. The present XRD data,
18
Fig. 1 XRD patterns of La–Ni
x
–Ce1ꢀx mixed oxide catalysts. (a) x ¼ 1;
when compared with those of the results of Lima et al., reveal
interesting points. In the case of catalysts reported by Lima
et al., with the substitution of La by Ce, there was the formation
(
(
b) x ¼ 0.8; (c) x ¼ 0.6; d) x ¼ 0.4; (e) x ¼ 0.3; and (f) x ¼ 0.2. (*) La
2 3
O ,
O) LaNiO
3
, (-) CeO
2
, (^) NiO.
3 2
of LaNiO and CeO existed in a discrete crystalline phase.
On the other hand, in the present work when Ce is
species are seen only at higher compositions in the sol–gel
catalysts. The Ni dispersion, as calculated by H chemisorptions
substituted for Ni, leaving lanthana constant, CeO becomes
2
2
amorphous and La O crystallizes out. The appearance of La O
2 3
2
3
also reveals higher values (18.65, 21.79 and 32.74%) for sol–gel
samples with x ¼ 0.2, 0.3 and 0.4, respectively compared to
as a separate phase indicates greater interaction between Ni and
Ce increasing the possibility of formation of Ni–Ce solid solu-
(10.34, 13.05 and 31.13%) for the hydrothermal catalysts of the
19
tion at low values of x, as reported in the literature, though it is
not clearly visible in the XRD patterns of the present samples. In
order to verify the possibility of formation of the solid solution,
same composition.
samples containing Ce and Ni, with the same compositions as 3.2
that of the mixed oxides were prepared separately and were
examined by XRD analysis. These samples have indicated the
H
2
-temperature programmed reduction
The TPR patterns of the LaNi Ce1ꢀx oxide catalysts are shown in
Fig. 2. Usually, LaNiO (inset) exhibits two peaks corresponding
x
3
2+
formation of Ni–Ce oxide solid solution as evidenced by the
3+
2+
0
to Ni / Ni and Ni / Ni reduction with the area ratio of
second/rst equaling 2, as reported in the literature. It is
decrease in the lattice parameter of CeO
Fig. S1, ESI†). This lends credence to the hypothesis of the
2
from 5.430 to 5.416 A˚
20
(
interesting to note from the TPR proles of the catalysts that the
peak shapes and their temperature maxima have considerably
varied with the Ni composition. At low x values, the catalysts
essentially contain La
reducible. On the other hand, Ni and Ce oxides are the reducible
species in the mixed oxides. In the case of Ni containing CeO
presence of Ni–Ce solid solution in the mixed oxide. Only at
higher loadings crystalline Ni is noticed as a result of agglom-
eration. A comparison of the morphology of the present series
of catalysts with those prepared by the hydrothermal method
reveals an interesting observation. Even at low composition of
Ni (x ¼ 0.2 and 0.3), the hydrothermally prepared catalysts show
2 3
O as a discrete phase, which is not
2
20
catalysts, Yonggang et al. assigned the TPR peaks into a, b, g
and d types. The a peak noticed at around 330 C is due to the
the existence of LaNiO
3
in their XRD patterns; whereas such
ꢁ
Table 2 Elemental analysis of La–Ni
x
–Ce1ꢀx catalytic systems
Ni
Ce
La
Catalysts
Theoretical
Experimental
Theoretical
Experimental
Theoretical
Experimental
LaNi0.8Ce0.2
LaNi0.6Ce0.4
LaNi0.4Ce0.6
LaNi0.3Ce0.7
LaNi0.2Ce0.8
O
O
O
O
O
3
3
3
3
3
17.929
12.659
7.972
5.818
3.77
17.59
11.36
8.10
5.79
3.73
10.70
20.14
28.55
32.41
36.07
10.74
20.54
28.1
33.2
35.27
53.041
49.436
47.174
45.905
44.702
53.78
52.17
48.8
46.01
45.27
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Fig. 2 TPR Profiles of La–Ni
x ¼ 0.8; (c) x ¼ 0.6; (d) x ¼ 0.4; (e) x ¼ 0.3; and (f) x ¼ 0.2.
x
–Ce1ꢀx mixed oxide catalysts. (a) x ¼ 1; (b)
reduction of the adsorbed oxygen on the Ni–Ce mixed oxide.
ꢁ
The b peak appearing at 400 C is specically related to the
reduction of NiO dispersed on ceria and the g peak, normally
ꢁ
observes at around 550 C, and represents the reduction of NiO
2
intimately in contact with CeO . The d peak, on the other hand,
is a result of the reduction of CeO with its characteristic peak
2
ꢁ
displayed beyond 800 C (not seen in the present patterns). In
the case of the present catalysts, the low temperature reduction
ꢁ
peak (at temperatures <400 C) is seen at higher Ni contents,
whereas the b peak dominates in the catalysts indicating the
availability of dispersed Ni species. The merging of g peak with
b can be due to agglomeration of NiO. The appearance of
distinct peaks due to NiO in the XRD patterns of the catalysts
with x ¼ 0.8 and 0.6 also supports this phenomenon. It is also
reported that the Ni species in Ni–Ce oxide solid solution are
19
not easily reducible.
Fig. 3 La 3d core level XPS spectra of La–Ni
catalysts. (a) x ¼ 1; (b) x ¼ 0.6; (c) x ¼ 0.4; (d) x ¼ 0.3; (e) x ¼ 0.2.
x
–Ce1ꢀx mixed oxide
Therefore, the peaks representing the reduction of Ni in the
catalyst appear at higher temperature regions. This peak is
distinguishable in catalysts with high Ni compositions. Thus, in
support of the XRD results, the TPR information also conrms
The Ce 3d XP spectra of LaNi Ce1ꢀx catalysts are shown in
x
3
the presence of the perovskite (LaNiO ) in the catalysts with
Fig. 4. As can be seen from the gure, the Ce 3d XP spectra are
complex and appear in the range of 880–920 eV due to the
hybridization of the O 2p valence band with the Ce 4f level in the
high Ni loading and the existence of NiO in two forms, the
agglomerated and the well dispersed species. Particularly, the
catalyst with x ¼ 0.4 displays combined characteristics of both
the low and high Ni containing catalysts.
23,24

nal state of photoionization.
The Ce 3d5/2 peaks appear to
be more intense in comparison to Ce 3d peaks, suggesting
3
/2
4
+
that cerium exists mainly in Ce oxidation state.
3.3 X-ray photoelectron spectroscopy
On the other hand, the stabilization of certain amount of
3
+
The core-level XPS proles of La 3d of La–Ni
x
–Ce1ꢀx mixed Ce oxidation state is also observed in samples with low Ce-
oxides are shown in Fig. 3. Due to the close similarity of binding content, where the intensity of the satellite peak around 916.6
energies of Ni 2p3/2 and La 3d3/2, it is difficult to distinguish eV decreases with simultaneous increase in the characteristic
2
1
3+
between these two species. On the other hand, the binding peaks of Ce (885.7 and 903.9 eV). These results suggest that at
3
+
energies recorded at 853.7 and 836.8 eV tted well with La 3d3/2 low concentrations of Ce (x ¼ 0.8, 0.6 and 0.4) it exists as Ce
and 3d5/2 states, respectively. The most intense 853.7 eV peak ion, whereas the excess cerium in higher concentrations exists
overlaps with that of the Ni 2p peak due to Ni in NiO (Fig. S2, as Ce . These results evidently support the observations made
2
+
4+
16,22
ESI†).
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+3 state, possibly the Ni existing in the LaNiO
3
perovskite form.
The lower B.E. of 853.7 eV for the sol–gel catalysts suggests that
the Ni exists in its +2 state.
3.4 Fourier transform infrared spectroscopy
The results of characterization performed using FT-IR spec-
troscopy are shown in Fig. 5. With the usage of nitrate precur-
sors in catalyst preparation, the residual nitrates can be present
in the coordination sphere of the corresponding metallic
cation, in turn surrounded by a propionic acid molecule (bands
ꢀ1
at ꢃ853, 1379 and 1464–1471 cm ). The observed IR band in
ꢀ
1
the range of 3000–3600 cm correspond to –OH stretching of
structural hydroxyl groups and physisorbed and interlayer
2
6,27
water.
3
The IR spectrum of LaNiO catalyst does not show any
28,29
characteristic peaks due to its low resistivity.
show bands in the region of 546–489 cm , in agreement with
the vibrational stretching frequencies of the metal–oxygen
All catalysts
ꢀ
1
15,30
bonds.
3.5 Catalytic activity test
The activity of the La–Ni –Ce
the methane dry reforming was evaluated at 800 C and atmo-
mixed oxide catalysts during
x
1ꢀx
ꢁ
spheric pressure. It can be observed that the activity of the La–
Ni
x
–Ce1ꢀx catalysts increases upto x ¼ 0.4 and then on
decreases continuously. LaNi0.4Ce0.6
O
3
has exhibited the high-
est activity among all the catalysts reaching the conversion
levels of more than 90%. The LaNiO has shown low conver-
3
sions (less than 20%) for both methane and carbon dioxide
under the conditions of evaluation. This interesting observation
clearly suggests the signicance of the Ce in the dry reforming
of methane. The initial increase in the conversions of methane
and carbon dioxide can be attributed to the availability of well
dispersed nickel. Fig. 6 reveals the variations in dispersion and
also catalytic activity with the change in Ni content. The
Fig. 4 Ce 3d core level spectra of La–Ni –Ce1ꢀx mixed oxide catalysts.
x
(
a) x ¼ 0.6; (b) x ¼ 0.4; (c) x ¼ 0.3; and (d) x ¼ 0.2.
from the TPR and XRD analyses, where the Ce–Ni interaction
prevails in the low Ni containing catalysts.
Two O 1s peaks are noticed in the ranges 529.2–531.9 eV and
529.9–532.3 eV for all samples which can be attributed to
presence of hydroxyl and carbonate groups and adsorbed water
25
(
Fig. S3, ESI†). The obtained C 1s spectra show two main peaks
at ꢃ284.6 and 285.9 eV which might be due to hydrocarbon
contamination (C–C/C–H and C–O bonds) (Fig. S4, ESI†).
Besides, the small peak observed in the region of 288.9–289.3 eV
25,26
3+
represents the carbonate species
when exposed to atmosphere.
which reacted with La
A comparison of the XPS data of the sol–gel catalysts, with
those prepared by the hydrothermal method reveals that when x
¼
0.2 and 0.3, the hydrothermal catalysts have their XPS peaks
with B.E. at 855.3 eV, about 2 eV higher than those of the cor-
Fig. 5 FT-IR patterns of La–Ni
x
–Ce1ꢀx mixed oxide catalysts. (a) x ¼ 1;
responding sol–gel catalysts. The higher B.E. of Ni represents its
(b) x ¼ 0.8; (c) x ¼ 0.6; (d) x ¼ 0.4; (e) x ¼ 0.3; and (f) x ¼ 0.2.
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conversions for both methane (91.6%) and carbon dioxide
(93.5%) can be observed. The catalyst also retained its activity to
a larger extent during the 9 h on time, showing a reasonable
stability. Similarly, the syngas ratio also attained a steady value
in the region of 0.87–0.84.
A comparison of the XRD results of the sol–gel catalysts
before and aer reaction reveals that there is a decrease in the
intensity of LaNiO signal in the used catalysts (Fig. S5, ESI†).
3
This may be due to the reduction of catalysts in hydrogen ow
prior to the activity run. The catalysts showed sintering
tendency, though not severe. This is observed by the H chem-
2
isorptions studies of the fresh and used (x ¼ 0.4) catalysts where
˚
only a slight increase in the particle size from 32.1 to 38.5 A is
recorded.
Table 3 presents the coke content of the catalysts measured
aer 9 h of evaluation. In all the cases the sol–gel catalysts
recorded lower values, than the corresponding hydrothermally
prepared catalysts. Thus, the sol–gel catalysts are found to be
less susceptible to coking.
A comparison of the activity patterns of the LaNi0.4Ce0.6O3
sol–gel and hydrothermally prepared catalysts is shown in
Fig. 8. Over a period of 9 h, the sol–gel catalyst has exhibited
Fig.
La–Ni
6
Variation of activity and Ni dispersion with x, in the
–Ce1ꢀx mixed oxide (0 < x < 1) sol–gel catalysts.
x
dispersion and activity curves varied in a similar fashion,
implying that activity is proportional to dispersion. The high
activity of catalysts even at low x values can be understood as a
result of higher dispersion. The hydrothermally prepared cata-
lysts, reported in our previous publication showed low disper-
sion as described above. The decrease in conversions could be a
result of Ni agglomeration.
higher conversions of both CO
superiority.
2 4
and CH revealing its
0
It is a well-established fact in the literature that Ni is the
8,31
active site for dry reforming of methane. LaNiO ultimately
3
leads to the formation of well-dispersed Ni on La O . When Ce
2
3
18
is substituted for La, Lima et al. reported the possibility of
formation of Ni/La–Ce–O. By keeping La constant and varying
Ce and Ni content, in the present catalysts, we propose that
apart from the formation of the perovskite, the additional nickel
existed in a well dispersed form. At low values of x the formation
of Ni–Ce oxide solid solution appears to be feasible. As the x
value increases its composition decreases. In the case of
LaNi0.4Ce0.6O3 the active Ni species might have reached its
optimum level, making it the best catalyst. The existence of
CeO
activity.
solution improves the stability of the catalysts by modifying the
2
in the proximity of Ni helps in further increasing the
32,33
It was reported that the formation of Ni–Ce solid
17
2
chemical environment around Ni. CeO is reduced either by
the hydrogen formed during the reaction or by the carbon
deposits formed during the decomposition of methane on Ni
3
Fig. 7 Time on stream behaviour of LaNi0.4Ce0.6O catalyst during the
ꢁ
dry reforming of methane at 800 C.
sites. CO2 is decomposed on reduced ceria (Ce O ), in turn
2
3
2
oxidizing it to CeO and forming CO. Thus, the variation of
Ce–Ni ratio in La–Ce–Ni mixed oxide (rather than La–Ce ratio)
appears to have a positive effect in getting higher conversions
Table
3
Coke content of the catalysts measured after 9 h of
(
more than 90% for both the gases). Therefore, the superior evaluation
catalytic performance of the LaNi0.4Ce0.6 can be explained by
the formation of highly dispersed Ni on CeO or La or their
O
3
Carbon (%)
2
2 3
O
mixed oxide, as proposed by Lima et al., The formation of Ni–Ce
Catalyst
Sol–gel
Hydrothermal
solid solution stabilizes the dispersed Ni. The higher conver-
LaNi0.8Ce0.2
LaNi0.6Ce0.4
LaNi0.4Ce0.6
LaNi0.3Ce0.7
LaNi0.2Ce0.8
O
O
O
O
O
3
3
3
3
3
0.34
0.18
0.15
0.2
0.55
0.39
0.34
0.38
0.32
sion of CO
reaction, as reported previously.
The results of time-on-stream analysis performed on
catalyst are shown in Fig. 7. Promising
2
might be due to the facile reverse water gas shi
34
LaNi0.4Ce0.6O
3
0.25
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Fig. 8 A comparison of the activity profiles of the LaNi0.4Ce0.6O sol–
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. Conclusions
The sol–gel method of preparation of the catalysts is found to be
advantageous than the hydrothermal method, in terms of
higher activity even at lower Ni compositions. The catalysts
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less susceptible to coking. LaNi0.4Ce0.6O3 catalyst shows the
maximum activity. It shows reasonable stability during the
reaction.
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