Combined methane reforming over nano LaNiO3 catalyst with modified active surface

Article abstract of DOI:10.1007/s11164-017-3196-9

In this study, nano catalyst LaNiO₃ with perovskite structure was synthesized using the citrate sol–gel method in the combined methane reforming with CO₂ and O₂ (CRM). The effects of increasing the surface area of the LaNi

Full text of DOI:10.1007/s11164-017-3196-9

Res Chem Intermed
DOI 10.1007/s11164-017-3196-9
Combined methane reforming over nano LaNiO3
catalyst with modified active surface
Alireza Jahangiri1 Majid Saidi1,2 Farhad Salimi3
Abolfazl Mohammadi4
•
•
•
Received: 12 August 2017 / Accepted: 4 November 2017
Ó Springer Science+Business Media B.V., part of Springer Nature 2017
Abstract In this study, nano catalyst LaNiO3 with perovskite structure was syn-
thesized using the citrate sol–gel method in the combined methane reforming with
CO2 and O2 (CRM). The effects of increasing the surface area of the LaNiO3
perovskite on the catalytic activity were investigated by changing the method of
preparing and creating holes in the surface of the samples. Physical and chemical
properties of the samples, before and after the reactor test, were determined through
ICP, AA, XRD, TGA, TPR, BET, SEM, EDX and TEM techniques. The results of
XRD, ICP, AA, SEM, EDX and TEM tests indicated that the citrate sol–gel method
is a good way to prepare a homogeneous perovskite LaNiO3 sample on a scale of
nanometers. The results of the TPR test showed using etching in the citrate sol–gel
method can produce samples with high stability. The BET results indicated that the
surface area of the LaNiO3 sample tripled with the method suggested in this paper.
Changes in preparation method lead to induction time decreasing and temperature
increasing. Use of etching in the citrate sol gel method had no significant effect in
the results of activity tests versus time reaction at a temperature of 800 °C. TGA
curves revealed no production of coke over the process for the produced samples.
Keywords Citrate sol–gel method Á Nano perovskite catalyst Á Substitution Á
Surface area Á CRM process
&
Alireza Jahangiri
jahangiri@eng.sku.ac.ir
1
2
Faculty of Engineering, Shahrekord University, Shahrekord, Iran
School of Chemistry, College of Science, University of Tehran, PO Box 14155-6455, Tehran,
Iran
3
4
Department of Chemical Engineering, Faculty of Engineering, Kermanshah Branch, Islamic
Azad University, Kermanshah, Iran
Department of Chemical Engineering, University of Bojnord, Bojnord, Iran
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A. Jahangiri et al.
Introduction
Today, use of syngas, H2 and CO, has been recognized as an intermediate for
production of clean energy and valuable chemical materials. Production of liquid
hydrocarbon fuels with high octane numbers in Fischer–Tropsch process, methanol,
aldehydes and alcohols and production of electricity power in fuel cells are all
processes that derive from H2 and CO as the primary materials [1].
The most important process of synthetic gas production is the catalytic reforming
process. The primary material of this process is heavy hydrocarbon compounds, natural
gas or coal. Since all petroleum resources are reducing, it seems that natural gas is the
best choice for the production of syngas. Catalytic processes for converting CH4 into
syngas involve steam (SR), dry (DR) or partial oxidation (POR) reforming of methane
[2], individually or in combiination. In the DR process, the ratio of H2 to CO in the
product is approximately one, which is appropriate for Fischer–Tropsch reactions. But
this process is endothermic and requires high energy [3]. The POR process is
exothermic, with the disadvantage of creating hot spots in the catalyst bed, making
temperature control difficult. The combined DR and POR processes for syngas
productionoffer the advantages of both processes. In fact, the heat required for the DR
reaction is provided by heat released through the PR reaction, and also the possibilities
of hot spots and coke aggregation in the catalyst bed are minimized [4, 5]. Recently, use
of nano perovskite catalysts (ABO3) in the methane reforming processes has received
special attention due to their significant resistance to coking, high catalytic activity and
stability, and the doubly functional role of perovskite catalysts simultaneously as both
base and active phases after reduction [6, 7]. In ABO3 perovskite structures, the A site
cation is a lanthanide and/or alkaline earth and the B site cation is a transition metal
[8, 9]. Among the preparation methods of perovskite catalysts, the citrate sol–gel method
has been acknowledged by researchers as the most efficient and the simplest [10–12].
This method provides the possibility of attaining crystalline particles with nanometer
size and high thermal stability. Low active surface area is the greatest weakness of
perovskite structures prepared through the sol–gel citrate method. In previous works, we
reported the behavior of La1-xSmxNiO3-d [13], LaNi1-xFexO3 [14], LaNi1-xCoxO3 [15]
and LaNi1-xMgxO3-d [16] as catalysts in a CRM process in a conventional fixed-bed
system. However, all samples showed low surface area as an effective parameter in
catalytic evaluations. In this work, the active surface area was increased by making holes
in the LaNiO3 provskite structure. We investigated the catalytic behavior of the LaNiO3
before and after increasing in the surface area through characterization and reactor tests
in the CRM Process. For ease in naming, LaNiO3 perovskite catalyst is denoted by the
letters A and B in parentheses before and after the change in surface area, respectively.
Experimental
Synthesis of perovskite LaNiO3 (A, B) with citrate sol–gel method
In order to synthesize LaNiO3 (A), first, Citric acid (99% C6H8O7, Merck) and
ethylene glycol [99% (C2H4 (OH)2, Merck] were dissolved in distilled water under
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Combined methane reforming over nano LaNiO3 catalyst with…
vigorous stirring. Then, nickel nitrate salt [99% (Ni (NO3)2Á6H2O, Merck) was
added. The molar ratios of citric acid to Ni nitrate and ethylene glycol were 5 and 1,
respectively. When Ni nitrate was well dissolved in the solution, stoichiometric
quantity of lanthanum nitrate salt [99% La (NO3)2Á6H2O, Merck] was added to the
solution. The final solution was stirred slowly by magnetic stirrer until the solution
was transformed into a gel with high viscosity at an approximate temperature of
60 °C after 12–14 h. The obtained gel was annealed in air at 800 °C for 4 h in an
electric furnace for calcination. The heating rate was 1 °C min-1 up to 400 and
3 °C min-1 up to 800 °C [11–13].
Sample LaNiO3 (B) was prepared in accordance with the method presented by
Shu et al. [17]. In this method, zinc nitrate salt [Zn (NO3)2Á6H2O, 99%, Merck] was
used to create holes on the surface of the LaNiO3. Thus Zn (NO3)2 was introduced
into the precursor after the gel formation step. In these experiments, after
calcination, Zn (NO3)2 was transformed into ZnO and a perovskite structure.
Ammonium chloride (NH4Cl, 99%, Merck) with two molar concentrations under
vigorous agitation is required to dissolve ZnO and etch it from the LaNiO3
perovskite structure. After etching of ZnO for 2 h, the sample was washed twice
with deionized water and a vacuum pump and finally re-calcined at 800 °C for 2 h
for stabilization of the catalyst structure. During the etching process (Eq. 1), the
smell of ammonia indicates the correct procedure of the reaction.
2NH4Cl þ ZnO ! 2NH3 þ ZnCl2 þ H2O
ð1Þ
ZnO is used because of its high ionic radius, making hole in the perovskite
structure, and also generation of dissoluble ZnO and separation after calcination.
The stages producing catalyst LaNiO3 (A, B) are shown schematically in Fig. 1.
Characterization techniques
Samples were analyzed with different characterization techniques before and after
the reactor tests. Perovskite-structure, phase purity and particle size of the catalytic
samples were studied before and after the reactor tests using X-ray diffraction
˚
(XRD, Phillips PW 1840, k = 1.54056 A). Particle size was determined based on
the Scherer equation. Sample morphology was established using scanning electron
microscopy (SEM, Philips XL30) and Transmission electron microscopy (TEM,
PHILIPS CM200 FEG). Inductively coupled plasma (ICP) emission spectroscopy
(Perkin-Elmer ICP/5500) was used to determine the lanthanum and nickel metals.
Atomic absorption (AA) spectroscopy (AAS-009 model) was employed for
determination of Zn content in the sample. In this study, surface area of the
samples was evaluated by a Quantachrom CHEMBET-300 device based on the BET
method. To investigate the reduction properties of the samples, the temperature
programmed reduction test with TPD/TPR 29000 was done in the range of room
temperature up to 850 °C. Quantitative analysis of coke formed on the used
catalysts during stability tests were studied using thermal gravity analysis (TG, PL
Thermal Science STA 1500).
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A. Jahangiri et al.
Fig. 1 Steps of LaNiO3 (A, B) synthesis (steps shown witha dashed line have been added to enhance the
active area)
Catalytic activity evaluation
The feed gases (H2, CO2, CH4, O2 and N2, with high-purity of 99%) were entered
into a fixed bed quartz reactor with internal diameter of 14 mm and length of 40 cm.
The middle part of the reactor was considered as the catalyst loading place,
supported by a quartz wool bed. The mass flow controllers (Model 5850, Brooks
Instrument, MFC) and monitoring system (Readout, Brooks Instrument) were used
after calibration to determine the flow rate of gases entering the reactor. Flow rate of
exhaust gas from the reactor was determined using a soap flow meter. A tube
furnace was used to heat the reactor. To control the reactor temperature, a
temperature indicator (TI) was located in the middle of the catalytic bed. In order to
precisely control the catalyst bed, furnace temperature was controlled. For this
purpose, another TI was located in the catalyst bed at the furnace chamber and the
temperature of the catalytic bed was controlled by controlling the furnace
temperature. The temperatures of the furnace and catalyst bed were measured and
controlled with two thermocouples (Ni–Cr, K-type, and 0.5 mm diameter) and PID
thermo-controllers (Model Jumo iTRON08). A pressure gauge was placed at the top
of the reactor in the pathway of the feed gases to indicate the pressure of the system
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Combined methane reforming over nano LaNiO3 catalyst with…
while performing reactor tests. Operating pressure in this study was one atmosphere.
Higher pressure indicates choking in the reactor due to coke formation. The
reactants and products were analyzed online by means of a gas chromatograph
(Model 6890N, Agilent Technologies), provided with two detectors (FID and TCD)
which is capable of determining the amount of reactant and products simultane-
ously. Star software was used to analyze the data. The schematic of the reactor tests
setup is presented in Fig. 2.
Procedure of catalytic activity
In order to investigate the amount of conversion of feed gases (CH4, CO2 and O2) to
syngas and the concentration of products, a certain amount of catalyst (0.4 g) was
loaded into the reactor. Activity studies for all of the samples were performed under
a temperature treatment between 600 and 800 °C and atmospheric pressure. The
trend of reactor temperature is shown in Fig. 3.
In order to perform reduction operations, after loading of catalyst, the gas
mixture 20% H2/N2 was used, with the flow of 50 ml min-1 . For this purpose, the
reactor bed temperature, with temperature gradient of 5 °C min-1 , was increased to
700 °C and maintained for 2 h at 700 °C. Then, the reactor temperature was
returned to room temperature under N2 gas. To evaluage catalytic activity, the CH4,
CO2, O2 and N2 gases were introduced into the reactor with flow rates of 30/30/15/
25 ml min-1 , respectively. Water produced during the reaction was separated by a
silica gel bed. After vapor separation, gas exiting the reactor was entered online into
a gas chromatograph (GC). The results were recorded using a computer connected
to the GC. In this study, to evaluate the catalytic performance, the CH4 and CO2
H2
H2
conversions,
[15].
and
ratios and H2 and CO yields are defined as follows
CO
CH4ðConv:Þ
CH4;in À CH4;out
 100
CH4 conversion ð%Þ ¼
CO2 conversion ð%Þ ¼
H2
CH4;in
CO2;in À CO2;out
 100
CO2;in
H2;out
ratio ¼
CO
COout
H2
H2;out
À
Á
2 moles of CH4 in feed
ratio ¼
CH4ðConvÞ
CH4;in À CH4;out
moles of H2 produced  100
Yield of H2 ð%Þ ¼
moles of CO produced  100
moles of CH4 in feed þ moles of CO2 in feed
Yield of CO ð%Þ ¼
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A. Jahangiri et al.
Fig. 2 Schematic of reactor tests system
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Combined methane reforming over nano LaNiO3 catalyst with…
1200
1000
800
600
400
200
0
5
10
Time [hr]
15
20
Fig. 3 Thermal treatment used in the CRM process
Results and discussion
Characterization of LaNiO3 (A, B) samples before the catalytic test
To identify the phases in the samples, X-ray diffraction patterns were measured in
the range of 10°–80° and analyzed using High Score software. The diffraction
patterns of LaNiO3 (A) sample are presented in Fig. 4.
The obtained results showed that the LaNiO3 (A) sample has a perovskite crystal
phase without any other crystalline phases with relatively sharp peaks. The X-ray
patterns of this sample were consistent with the referenced X-ray pattern of the
sample LaNiO3 (reference code: 0633-088-01) with rhombohedra phase. These
results were in agreement with results presented by others [18, 19]. The LaNiO3
Counts
900
400
100
0
10000
2500
0
10
20
30
40
50
60
70
80
Position [°2Theta]
Fig. 4 Real and referenced XRD patterns of LaNiO3 (A) sample after calcination at 800 °C (reference
code of LaNiO3 sample: 0633-088-01)
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A. Jahangiri et al.
crystallite size was calculated to be 31 nm at 2h = 47.3°, by applying the Scherer
equation. As mentioned in the ‘‘Synthesis of perovskite LaNiO3 (A, B) with citrate
sol–gel method’’ section, in order to increase the active surface area of the LaNiO3
sample, zinc nitrate and etching operations were used in the final steps of sample
preparation. XRD patterns of the LaNiO3 sample before and after etching are shown
in the Fig. 5. As expected, tbe LaNiO3 sample before etching has been formed by
two phases of LaNiO3 perovskite and ZnO (reference code: 01-080-0074), although
a small amount of NiO with reference code 00-004-0835 is seen as impurity in the
sample. After the etching operation, the main phase of the sample was LaNiO3.
However, small amounts of zinc oxide and nickel oxide were detected in XRD
patterns. The X-ray diffraction pattern of the LaNiO3 (B) sample before and after
etching with reference patterns are shown in Fig. 5.
Results of BET, ICP and AA tests
The results obtained from the measured weight percentage of metal contents (ICP
and AA tests) and surface area (by BET method) of LaNiO3 perovskite catalyst are
presented in Table 1.
The ICP results showed the concentration of Ni and La was close to the nominal
values indicating that the citrate sol–gel method is an effective procedure for
preparation of the LaNiO3 (A) sample. However, the etching method led to
decreased weight percentage of Ni in the LaNiO3 (B) sample because of the
presence of ZnO compound in the LaNiO3 (B) structure. The Zn content in the
Counts
900 after etching
400
100
0
before etching
900
400
100
0
LaNiO3
10000
2500
0
ZnO
10000
2500
0
NiO
10000
2500
0
10
20
30
40
50
60
70
80
Position [°2Theta]
Fig. 5 XRD patterns of LaNiO3 (B) samples before and after etching process with reference pattern
(reference codes of LaNiO3, ZnO and NiO samples are 0633-088-01, 01-080-0074 and 00-004-0835,
respectively)
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Combined methane reforming over nano LaNiO3 catalyst with…
Table 1 Elemental analysis (by ICP and AA tests), BET surface areas and crystallite size of LaNiO3
sample before and after etching
LaNiO3 (A)
LaNiO3 (B)
La (%)
67.5 (70.3)a
32.5 (29.7)
–
68.8 (70.3)
26.4 (29.7)
4.5 (0)
8.6
Ni (%)
Zn (%)
surface area (m2 g-1
Crystal sizeb (nm)
)
3.15
19.6
10.6
aNominal values of metal contents are presented in parentheses
bCrystal size of the samples calculated by Scherer equation
sample LaNiO3 (B) versus weight percent was determined by AA test and is shown
in Table 1. The content of Zn was significant and showed that dissolving ZnO and
etching it from the LaNiO3 perovskite structure did not occur correctly, in according
with the XRD results.
The specific surface areas of the LaNiO3 (A) sample were low, which not
unexpected, since the LaNiO3 (A) sample prepared by the citrate sol–gel method
and long exposure of the sample with high temperature led to synthesis of low
surface area solids. This is consistent with the results reported by other researchers
[20]. The LaNiO3 (B) sample has a surface area of approximately three times more
than the LaNiO3 (A) sample. It seems that presence of Zn with effective ionic radius
˚
of 0.75 A in the etching process could lead to artificial voids in the final structure of
LaNiO3 (B) that increase active surfaces of the sample. This result is in agreement
with the findings of Shu et al. [17] on the perovskite structure of LaCoO3, although
they could synthesize this sample with much higher surface area (30 m2 g-1).
Results of SEM and TEM tests
In order to estimate the homogeneity of the synthesized solids, LaNiO3 (A, B)
samples were examined by SEM and TEM tests, presented in Fig. 6.
The SEM images showed that LaNiO3 (A) sample has a uniform nanostructure
with approximately spherical particles. According to TEM images of this sample,
the particles are in agglomerated and fine form with variable size in the range of
20–40 nm. The particle sizes obtained from the images verified the crystallite sizes
that we calculated via the Scherer equation in ‘‘Characterization of LaNiO3 (A, B)
samples before the catalytic test’’ section.
The SEM image of the LaNiO3 (B) sample was in agreement with XRD results. It
seems that big particles in the images were related to ZnO compound as a
contaminated phase. The TEM image showed that LaNiO3 (B) was not spherical in
form. However, the particles sizes were in the range of nanometers.
The homogeneity of the system was confirmed by EDX measurements at two
points on the samples. The results are presented in Fig. 7.
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A. Jahangiri et al.
Fig. 6 SEM and TEM images of the calcined LaNiO3 (A, B) perovskites
Since the preparation method does not include any separation step (crystalliza-
tion, filtration, etc.) [21], the LaNiO3 (A, B) samples are in close agreement with
those obtained by elemental analysis (‘‘Results of BET, ICP and AA tests’’ section)
and theoretical calculations.
Study on the reducibility of the LaNiO3 (A, B)
In order to study the reducibility of LaNiO3 (A, B) samples, TPR experiments were
carried out; results are shown in Fig. 8.
The TPR profile of LaNiO3 (A) showed that this sample is reduced in two main
peaks. The first one occurs in the temperature range between 300 and 450 °C, which
represents reduction of Ni3? to Ni2?, corresponding to La2Ni2O5 formation [22, 23].
The second one occurs at temperatures higher than 450 °C, due to reduction of Ni2?
to Ni0. The ratio of the second peak to the first peak was 1.9. This ratio should
theoretically be equal to 2 [24]. These results were in good agreement with the
results of XRD. The following equations show the reduction theory of perovskite
LaNiO3 with H2 in two stages [25].
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Combined methane reforming over nano LaNiO3 catalyst with…
(A)
(B)
Fig. 7 La, Ni, and Zn distributions observed at two points along the LaNiO3 (A, B) samples by EDS and
the theoretical amounts for calcined at 800 °C
Fig. 8 TPR curves of the calcined LaNiO3 (A, B) samples
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A. Jahangiri et al.
2LaNiO3 þ H2 ! La2Ni2O5 þ H2O
ð2Þ
ð3Þ
La2Ni2O5 þ 2H2 ! La2O3 þ 2Ni þ 2H2O
The LaNiO3 (B) sample is also reduced in two areas. In this case, the first and
second reduction peaks occur at higher temperatures. Since the B sample contains
ZnO and NiO impurities, the amount of H2 consumption in the first peak decreases.
In this peak, Ni3? decreases into Ni2?. The second peak of reduction was far
broader than before. This can be attributed to the presence ZnO and NiO metal
oxide and reduction of the Ni2? and Zn2? to Ni0 and Zn0. This confirmed the
stabilizing effect of changes in synthesis method on the perovskite structures at
higher temperatures.
Activity test
The activity tests of the LaNiO3 (A, B) samples as a function of reaction
temperature were performed in the CRM process. The CH4 and CO2 conversions
and product yields are shown in Figs. 9 and 10, respectively.
According to Figs. 9 and 10, for both samples, the CH4 and CO2 conversions
increased with the increase of the reaction temperature. CO2 conversion was always
less than CH4. Also, water was produced all of the reaction. It also can be seen that
by increasing temperature, the amount of H2 and CO yields increased and the CO
(A)
(B)
Fig. 9 CH4 (a) and CO2 (b) conversion as a function of the reaction temperature for LaNiO3 (A, B)
samples [CH4/CO2/O2 = 1/1/0.5, WHSV = 15L/(h g)]
123

Combined methane reforming over nano LaNiO3 catalyst with…
(A)
(B)
Fig. 10 H2 (a) and CO (b) yields as a function of the reaction temperature for LaNiO3 (A, B) [CH4/CO2/
O2 = 1/1/0.5, WHSV = 15L/(h g)]
yield was consistently higher than H2. Tomishige et al. [26] applied Ni/Al2O3 and
Pt/Al2O3 and Rezaie et al. [27] used noble metal catalysts in this process and
reported similar results.
These behaviors could be explained by the fact that exothermic CH4 combustion
reactions (Eqs. 4 and 5) at low temperatures and endothermic dry reforming
reactions (Eq. 6) and reverse water–gas shift (Eq. 7) at high temperatures were
dominant reactions in the CRM process.
3
CH4 þ O2 ! CO þ 2H2O; DH298 ¼ À 520 kJ molÀ1
ð4Þ
2
CH4 þ 2O2 ! CO2 þ 2H2O; DH298 ¼ À 802:3 kJ molÀ1
CH4 þ CO2 $ 2CO þ 2H2; DH298 ¼ 247 kJ molÀ1; DR
CO þ H2O $ CO2 þ H2; DH298 ¼ À 41 kJ molÀ1; WGS
ð5Þ
ð6Þ
ð7Þ
On the other hand, CH4 combustion reactions at low temperatures produced
water, CO2 and CO, which increased the yield of CO in comparison to H2. From the
catalytic perspective, it was observed that induction time (the time required for the
precursor perovskite to start showing activity [28]) of the LaNiO3 (B) sample
decreased at temperatures below 800 °C, due higher surface area and stability with
123

A. Jahangiri et al.
respect to LaNiO3 (A). In other words, the presence of Zn with high effective ionic
radius in this synthesis method could lead to creation of artificial voids in the final
structure of LaNiO3 (B) that increase active surface area and stability of the sample
at high temperatures in accordance with BET and TPR results, respectively.
However, when approaching the temperature of 800 °C, both samples showed
similar catalytic behavior. This showed that the effect of surface area decreased
compared to other variables with increasing in reaction temperature.
Catalytic test results of LaNiO3 (A, B) samples (CH4 and CO2 conversions, H2
H2
CO
H2
CH4ðConv:Þ
and CO yields and
and
ratios) in terms of reaction time in temperature
of 800 °C in the CRM process are presented in Figs. 11, 12 and 13.
As illustrated in these figures, LaNiO3 (A,B) samples show a similar behavior at
a temperature of 800 °C, and even the sample with lower active surface area,
[LaNiO3 (A)], shows better catalytic behavior, due to uniform distribution of active
phase (Ni0) in the crystal lattice of perovskite. According to the results of XRD, ICP
and AA tests, the Zn element in a small amount was substituted by Ni in the
perovskite structure of the LaNiO3 (B) sample and reduced the uniformity of the
sample (B), as seen in the SEM and TEM images.
In fact, from the catalytic point of view, a contrast existed between the crystalline
structure and surface area and stability in this study. Since change in the surface area
and stability of the LaNiO3 were low due to modifications in preparation of the
(A)
(B)
Fig. 11 CH4 (a) and CO2 (b) convection as a function of the reaction time for LaNiO3 (A, B) [CH4/CO2/
O2 = 1/1/0.5, WHSV = 15L/(h g)]
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Combined methane reforming over nano LaNiO3 catalyst with…
(A)
(B)
Fig. 12 H2 (a) and CO (b) yields as a function of the reaction time for LaNiO3 (A, B) [CH4/CO2/
O2 = 1/1/0.5WHSV = 15L/(h g)]
catalyst, thus these variables had fewer effects compared to the distribution crystal
structure at 800 °C.
Regarding the feed ratio, the ratios of
H2
CO
H2
CH4ðConv:Þ
and
were, respectively, less
and more than one. This shows that reactions (6) and (8) were carried out as the
main reactions in the temperature of 800 °C.
CO2 þ H2 $ CO þ H2O; DH298 ¼ 41 kJ molÀ1; RWGS
ð8Þ
Physical and chemical characterization of the samples after the activity test
XRD tests and the Scherer equation were used in order to study the crystal structure
of LaNiO3 (A, B) samples and determine the crystal size of nickel. The obtained
results are shown in Fig. 14 and Table 2.
The crystal structure of LaNiO3 (A, B) samples, after the reactor test, were
completely different from those of the fresh samples. Crystal structure of LaNiO3
was lost to form of the La2O2CO3 (reference code No: 01-084-1963), La (OH)
3
(reference code No: 01-075-1900) and Ni0 (reference code No: 01-087-0712)
phases. The La2O2CO3 structure, in accordance with the following equation (Eq. 9),
plays an important role in the removal of coke produced during the CRM process
[25].
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A. Jahangiri et al.
(A)
(B)
H2
CO
H2
CH4ðConv:Þ
Fig. 13
(a) and
(b) ratios as a function of the reaction time for LaNiO3 (A, B) [CH4/
CO2/O2 = 1/1/0.5, WHSV = 15L/(h g)]
La2O3 þ CO2 ! La2O2CO3 þ CH4 ! La2O3 þ 2H2 þ 2CO
ð9Þ
As mentioned, from the catalytic point of view, the main factors for conversion
of CH4 and CO2 gases in the reaction were in the structure of LaNiO3, Ni0 and
La2O3 phases, after reduction and during the CRM process. On the other hand, the
main factor of coke production at high temperatures in CRM process was
endothermic reaction of CH4 decomposition (Eq. 10). Therefore, according to
Eq. 9, presence of La2O2CO3 in the catalyst structure not only reflects ease of CH4
conversion, but also prevents coke formation and reproduces La2O3 compound to
the reaction of CO2 with it. La (OH)3 phases have been produced due to hydrolysis
of La2O3 with water during the reaction and exposure of the used catalyst with
atmospheric moisture. The presence of the metallic active phase (Ni0) was a reason
for appropriate catalytic behavior in all samples.
CH4 $ C þ 2H2 DH298 ¼ 75 kJ molÀ1
ð10Þ
On the other hand, presence of O2 in the feed of CRM, according to Eq. 11,
resulted in burning of coke produced during the process, such that no peaks were
observed related to coke in the XRD results.
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Combined methane reforming over nano LaNiO3 catalyst with…
Counts
LaNiO3 (B)
900
HH
H
H
400
100
0
C
CC
C
LaNiO3 (A)
C
C
C
C
C
CC
C
400
100
0
C
C C
M
C
C
CC
CM
H
C C C C C
C
20
30
40
50
Position [°2Theta]
60
70
80
Fig. 14 XRD patterns of LaNiO3 (A, B) samples after CRM process [CH4/CO2/O2 = 1/1/0.5,
WHSV = 15L/(h g), reaction temperature = 800 °C] La2O2CO3 (C), La (OH)3 (H), Ni0 (M)
Table 2 Results of nickel particle size and coke formation rate for LaNiO3 (A, B) after CRM process
Catalyst
Rate of coke formation
Size of nickel particle (nm)
Reaction time (h)
LaNiO3 (A)
LaNiO3 (B)
0.005
0.009
24
22
16.95
11.95
The unit of coke formation rate: [mol of carbon mol-1 of (CH4 ? CO2)converted h-1
]
C þ O2 ! CO2 DH ¼ À 393:7 kJ molÀ1
ð11Þ
The crystal structure of LaNiO3 (B) was very similar to the LaNiO3 (A) sample
after the activity test, with a difference in the peaks of the La(OH)3 phase, which
were sharper than the peaks in sample (A). The lower percentage of CO2 conversion
in this sample can be attributed to the presence of La (OH) , among others. By
3
comparing the sizes of nickel metallic crystals in these two samples, it was found
that both structures have the same size, although it seems that nickel has been less
agglomerated in the LaNiO3 (B) sample in comparison with XRD results of the
‘‘Characterization of LaNiO3 (A, B) samples before the catalytic test’’ section.
The TGA test was used to study the coke produced in the CRM process (Fig. 15).
The coke-calculated theoretical values are presented in the Table 2. For LaNiO3 (A,
B) samples, TGA curves approximately did not represent coke formation.
123

A. Jahangiri et al.
Fig. 15 TGA curves of the LaNiO3 (A, B) catalysts after the stability test (feed ratio CH4/CO2/O2 = 1/1/
0.5, reaction temperature = 800 °C)
These results confirmed the results obtained from the coke theory amounts.
According to the TGA curves, it was not possible to study in detail the relationship
between method of preparation and the amount of coke deposition because of the
very small amount of coke produced. However, due to the oxidation of the metallic
active sites (weight increase) overlapping with oxidation of carbon deposits (weight
loss), the quantification of the deposited carbon was not always accurate. The results
of TGA were consistent with XRD analysis after the activity test.
Conclusions
The results of characterization and reactor tests of the catalysts in this study can be
summarized as follows:
1. ICP and AA tests and EDX results showed that nickel percentage decreased in
the structure of the LaNiO3 sample when an etching method was used in
preparation.
2. Results of XRD patterns indicated that both LaNiO3 (A, B) samples had
approximately the same crystalline structure after calcinations.
3. The BET test results showed surface area of LaNiO3 sample tripled after
etching.
4. SEM and TEM images revealed that the LaNiO3 (A) sample has an
approximately spherical uniform nanostructure. The particles were in compact
shape with variable size in the range of 20–40 nm. SEM images of the LaNiO3
(B) sample showed the perovskite structure also has a contaminating phase. The
TEM image of this sample also showed that the shape of the particles, despite
the nanostructure, was not necessarily spherical.
5. According to the TPR analysis results, two LaNiO3 (A, B) perovskite samples
were reduced in two areas. The first and second reduction stages for the LaNiO3
(B) sample occurred at higher temperatures and made the metal reduction more
difficult.
6. Changes in preparation method to enhance the surface area led to induction time
decreasing with temperature increasing. However, use of etching in the citrate
123

Combined methane reforming over nano LaNiO3 catalyst with…
sol-gel method had no significant effect on results of activity tests versus time
reaction at a temperature of 800 °C.
Acknowledgements The authors gratefully acknowledge the financial support of this work by Iranian
National Science Foundation. Funding was provided by Iran National Science Foundation (Grant No.
93038553).
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