Catalytic Steam Reforming of Natural Gas over a New Ni Exsolved Ruddlesden-Popper Manganite in SOFC Anode Conditions

Article abstract of DOI:10.1002/cctc.201902306

The catalytic behavior of the new Ni exsolved Ruddlesden-Popper (RP) manganite (La_1.5Sr_1.5Mn_1.5Ni_0.5O₇) for the reforming reaction was studied. The material was synthesized by the Pechini method and reduced to induce the formation of two phases: n=1 RP structure LaSrMnO₄ decorated with Ni nanoparticles. Ni impregnation on (La,Sr)₂MnO₄ ceramic support of similar composition was also prepared for comparison. The catalytic measurements were carried out in a reduction-reaction process with low steam to carbon content (S/C=0.15) at 700, 800 and 850 °C. The exsolved material exhibits a better performance than the impregnated for the methane steam reforming reaction, especially at 850 °C with higher conversion and H₂ production rate. However, in light alkane gas mixtures (CH₄?C₂H₆ and CH₄?C₃H₈), the behavior is affected due to the competition between reactions and low available metallic active sites, without affecting the H₂ production. The exceptional overall results considering this new material as a promising anode material in a SOFC fed with Colombian natural gas.

Full text of DOI:10.1002/cctc.201902306

Accepted Article
Title: Catalytic steam reforming of natural gas over a new Ni exsolved
Ruddlesden-Popper manganite in SOFC anode conditions
Authors: Sebastián Vecino-Mantilla, Erika Quintero, Camilo Fonseca,
Gilles Gauthier, and Paola Gauthier-Maradei
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10.1002/cctc.201902306
ChemCatChem
FULL PAPER
Catalytic steam reforming of natural gas over a new Ni exsolved
Ruddlesden-Popper manganite in SOFC anode conditions
[a][b] [a] [a] [a]
Sebastián Vecino-Mantilla , Erika Quintero , Camilo Fonseca , Gilles H. Gauthier* , Paola Gauthier-
[a]
Maradei*
Abstract: The complete catalytic behavior for the reforming reaction
on the new Ni exsolved Ruddlesden-Popper (RP) manganite
(La1.5Sr1.5Mn1.5Ni0.5O7) was studied. The material was synthesized by
the Pechini method and reduced to induce the formation of two
phases: n= 1 RP structure LaSrMnO4 decorated with Ni nanoparticles.
Ni impregnation on (La,Sr)2MnO4 ceramic support of similar
composition was also prepared for comparison. The catalytic
measurements were carried out in a reduction-reaction process with
low steam to carbon content (S/C= 0.15) at 700, 800 and 850 °C. The
exsolved material exhibits a better performance than the impregnated
for the methane steam reforming reaction, especially at 850 °C with
higher conversion and H2 production rate. However, in light alkane
gas mixtures (CH4-C2H6 and CH4-C3H8), the behavior is affected due
to the competition between reactions and low available metallic active
sites, without affecting the H2 production. The exceptional overall
results considering this new material as a promising anode material
in a SOFC fed with Colombian natural gas.
The most interesting and widely studied fuel for SOFC is the
natural gas[3–5] consisting mainly of methane (CH4), e.g. around
82 mol% in the Cusiana gas available in Colombia (Table 1).[6]
Table 1 Colombian natural gas (Cusiana) composition.[6]
Cusiana gas (Colombia)
Compounds
Composition [%mol]
82.195
CH4
C2H6
10.841
C3H8
3.510
CO2
1.900
Minority hydrocarbons
1.035
N2
0.519
H2S
2-50 ppm
Nevertheless, the direct internal reforming at SOFC anode
using natural gas (or pure methane as main component) is still
facing a main issue: carbon deposition due to parasitic reactions
such as methane cracking.[3] There are four different ways to
consider the hydrogen (H2) production from hydrocarbon fuels
and avoid carbon deposition: dry reforming (with CO2), steam
reforming (with H2O), partial oxidation (with O2) and autothermal
reforming (with steam and O2).[7–12] Each process is achieved
under different operating conditions and any or all of them could
be selected to study the behavior of a specific anode material in
a catalytic way. However, according to the literature, water vapor
has been considered as a particularly relevant component to be
mixed to methane; steam to carbon ratios (S/C) higher than 1.0
are generally necessary to avoid the coke formation on the anode
surface, making steam reforming one of the most important
reaction taking place at the anode surface.
Introduction
Given the global energetic context, the solid oxide fuel cells
(SOFC) appears as an interesting “clean” technology to produce
electricity with high efficiency from different kind of hydrocarbon
fuels.[1] Instead of the direct use of hydrogen which is still
problematic in view of remaining issues related to its handling,
storage and distribution infrastructure, the direct internal steam
reforming of hydrocarbons at the SOFC anode side is relevant.[2]
This fuel flexibility in SOFC systems is possible due to high
operating temperatures (T > 600 °C) allowing the “in situ” H2
production by direct internal reforming at the anode side, which is
consumed simultaneously by the electrochemical oxidation
reaction, generating electricity and heat.[3]
Nevertheless, such high S/C ratios dilute the amount of fuel and
may lead to thermomechanical damages due to large
temperature gradients at the anode side: indeed, in comparison
to the electrochemical reactions, which is exothermic, methane
steam reforming (MSR) is a highly endothermic reaction (¡Error!
No se encuentra el origen de la referencia.)). In parallel to MSR
reaction, the excessive addition of H2O further converts CO to
CO2 by the slightly exothermic Water Gas Shift (WGS) reaction
represented as ¡Error! No se encuentra el origen de la
referencia.). However, the overall reaction is the Methane overall
steam reforming (¡Error! No se encuentra el origen de la
referencia.)) that is globally highly endothermic. In addition,
methane dry reforming (MDR, ¡Error! No se encuentra el origen
de la referencia.)) could also be present, which is also an
endothermic reaction.[13] A consequence of thermal gradients in
the cell due to presence of excess steam would be the
condensation process of unreacted products.
[a]
S. Vecino-Mantilla, E. Quintero, C. Fonseca, Dr. G. Gauthier, Dr. P.
Gauthier-Maradei
Grupo de investigación INTERFASE, escuela de ingeniería química,
Universidad Industrial de Santander, carrera 27 calle 9, ciudad
universitaria, Bucaramanga, Colombia
E-mail: mapaomar@uis.edu.co; gilgau@uis.edu.co
S. Vecino-Mantilla
[b]
Instituto de Tecnología Química (Universitat Politècnica de València
– Consejo Superior de Investigaciones Científicas), Avd. de los
Naranjos s/n, 46022 Valencia, Spain.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201902306
ChemCatChem
FULL PAPER
of nickel. The addition of metallic nanoparticles can be realized by
different methods and the exsolution technique leads generally to
better performance and less degradation than conventional
impregnation.[37–44]
o
ΔH = 206.1 kJ mol
-1
CH + H O ⟷ CO + 3H
4 2 2
(1)
(2)
(3)
(4)
o
-1
CO + H O ⟷ CO + H
2 2 2
ΔH = -41.2 kJ mol
o
-1
CH + 2H O ⟷ CO + 4H
4 2 2 2
ΔH = 165.0 kJ mol
o
ΔH = 247.3 kJ mol
-1
CH + CO ⟷ 2CO + 2H
4 2 2
Previously,[22] the synthesis and exsolution conditions of a new
RP material, La1.5Sr1.5Mn1.5Ni0.5O7±δ (LSMN n= 2), was reported,
with the aim to obtain, in reducing atmosphere, a biphasic material
made of electrocatalytic active Ni metallic nanoparticles
decorating a n= 1 RP LaSrMnO4±δ manganite using in situ
exsolution technique for short reduction times and high
temperatures (up to 850 °C). The present work deals with the
study of catalytic behavior for the steam reforming reaction using
Colombian natural gas composition (methane, ethane, and
propane) with low water content, comparing the behavior of the
exsolved material to the conventional Ni impregnated manganite
of the same RP structure.
Alternatively, the process should be fed with low S/C ratio,
considering the in situ production of steam stemming from the
electrochemical oxidation of the hydrogen, itself obtained from the
steam reforming reaction at the anode.[14,15] Such operation
scheme is called Gradual Internal Reforming - GIR (schematized
in ¡Error! No se encuentra el origen de la referencia.),
proposed by Vernoux et al. to avoid thermomechanical issues.[16]
GIR was experimentally and theoretically demonstrate, resulting
in better long-term stability of the cell operating without water
excess, but with still insufficient global performance.[17,18]
Therefore, the development of new materials with better specific
(electro-)catalytic properties not only for GIR but also for the
electro-oxidation of the produced hydrogen is necessary.
Results and Discussion
Catalysts study
As shown previously,[22] the LSMN n= 2 material exhibits a
Ruddlesden-Popper (RP) structure with 2 layers of octahedra in
the perovskite-like stack and tetragonal space group I4/mmm.
The calculated parameters by Rietveld refinement are a=b=
3.8543(2) Å, c= 20.1898(19) Å and Vol.= 299.93(4) Å3. On the
other hand, the La0.5Sr1.5MnO4±δ (LSMO n= 1) material was
selected to be synthesized and subsequently impregnated due to
its structural similarity with LaSrMnO4 (hereafter referred to as
LSM n= 1), i.e. the manganite that is obtained as support after Ni
exsolution from LSMN n= 2. The difference is that
La0.5Sr1.5MnO4±δ can be easily prepared in air. This material also
exhibits a RP structure with 1 layer of octahedra in the perovskite-
like stack and tetragonal space group I4/mmm. A total of 9 sets of
~3 g of corresponding material, were synthesized according to the
methodology. For each synthesis, the lattice parameters (a=b, c)
and the unit-cell volume were determined from the X-ray
diffraction (XRD) data using structure refinement based on the
Full Pattern Matching – Le Bail method. The comparison of the
results allowed the selection of the samples to be mixed (or
discarded), obtaining a final homogeneous stock of around 20 g
of pure phase material (Figure S1 and Table S1).
Figure 1 Diagram of the Gradual Internal Reforming process. Modified by
the author from.[10]
The conventional cermet based on nickel and yttria-stabilized
zirconia (Ni/YSZ) exhibits unsatisfactory performance during
operation with natural gas (methane).[19–21] As described
elsewhere,[22] one of the most promising alternatives is the
development of Mixed Ionic and Electronic Conducting (MIEC)
anode materials, in particular with Ruddlesden-Popper (RP)
structure (the structure of this materials, of composition
An+1BnO3n+1 or (AO)(ABO3)n, consists of n perovskite-like layers,
ABO3, separated by one single rock salt-like layers, AO).[23] The
interesting transport properties of those materials coupled to their
high thermal and mechanical stability not only in the air but in
reducing atmosphere, make them now a valuable option for an
application as SOFC electrode (cathode or anode).[24–32] However,
some authors recently pointed out their poor catalytical
performance for direct use of methane (e.g. La0.6Sr1.4MnO4).[33]
The catalytic behavior of a MIEC anode with respect to
reforming can be improved by the addition of metallic
nanoparticles. Most of the reported active catalysts are based on
expensive noble metals like Pt, Ru or Rh.[34–36] However,
compared to the latter, nickel-based catalysts are desirable for
SOFC industrial applications considering the availability and cost
The structure of the as-obtained LSMO n= 1 powder was
successfully refined using Rietveld method based on X-ray
diffraction data and pseudo-Voigt peak shape function with
reasonable reliability factors of Rp = 5.40 %, Rwp = 3.78 %, and
χ2 = 1.99 and very few residual intensity, as observed graphically
in Figure 2. The refined lattice parameters are a=b= 3.8605(9) Å,
c= 12.4143(30) Å and cell volume V=185.020(8) Å3; these results
are in satisfactory agreement with literature.[45–50] It is worth noting
a small diffraction peak around 2θ=25 º in the material diffraction
pattern, which was identified as SrCO3. Such impurity could have
been removed increasing the sintering temperature to 1300 ºC[45]
but considering it would possibly affect the microstructural
properties, particularly the surface area and grain size, we
decided to continue the study with a slightly impure LSMO n= 1
sample.
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10.1002/cctc.201902306
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FULL PAPER
those authors (a=b= 3.8378(3) Å, c= 12.7286(13) Å, Vol.=
187.48(3) Å3),[45] the trend remains, the small deviations being
probably related to the difference of reduction time (16 h at 850
ºC) and therefore to the sample reduction degree. In the case of
Ni, in all samples, the lattice parameters remain the same and
agree well with the literature values.[51] The amount of
impregnated Ni was confirmed by X-Ray Fluorescence
spectroscopy (XRF) using the sample reduced at 850 ºC for 4 h.
The measured Ni mass fraction is 0.059 ± 0.004 (Figure S2), in
reasonable agreement with the desired theoretical Ni
concentration (~0.05 mass fraction).[22]
To complete the XRD results, scanning electron microscopy
(FEG-SEM) images were recorded (Figure 5 A–C), which exhibit
the presence of spherical nanoparticles (Ni) with less than 50 nm
of average size that decorate the surface of the manganite oxide.
They confirm the successful impregnation of the support. In
addition, a transmission electron microscopy (TEM) image and
energy dispersive X-ray (EDX) concentration mapping of the main
elements for a characteristic zone of the Ni-impregnated LSMO
n= 1 powder after reduction at 850 °C for 4 h are shown in Figure
6. The corresponding elemental mapping also confirms the
composition for the manganite support, i.e. La, Sr, and Mn are
homogeneously distributed in the entire grain. The presence of Ni
is evidenced as a few isolated spots which consist of metallic Ni.
It is worth noting that nickel is deposited as hemispherical Ni
particles in only specific zones of the surface, suggesting that,
using the impregnation method, the active phase is not uniformly
distributed on the support, as has been reported in the
literature.[53–55] Such results differ from those obtained in the case
of exsolution, for which the surface is covered with anchored
(strong particle-oxide matrix interaction) small and uniformly
distributed Ni nanoparticles (less than 40 nm).[22] These
differences will be key factors that probably influence the
materials´ performance, activity, selectivity, and stability with
respect to steam reforming.
Figure 2 LSMO n= 1 Rietveld refinement using XRD data.
LSMO n= 1 powder was then impregnated according to the
methodology, then reduced at the selected temperature and time,
respectively T and tr. The corresponding XRD patterns are
presented in Figure 3. Extra peaks are evidenced at 2θ= 44.5 and
51.9° (highlighted as red dots), which are associated with the
presence of Ni.[51] Additionally, we can observe that the support
structure remains stable; so, in each case, a structure refinement
by Full Pattern Matching – Le Bail method was successfully
performed, using a tetragonal system with I4/mmm space group
for the main phase, and an FCC cubic cell with Fm-3m space
group for Ni.
ꢀ
Ni
850 ºC 8 h
850 ºC 4 h
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
800 ºC 8 h
800 ºC 4 h
750 ºC 8 h
ꢀ
A
B
ꢀ
3.854
3.852
3.850
3.848
3.846
3.844
12.630
12.615
12.600
12.585
12.570
♣
LSMO n =1
Ni (a=b=c)
ꢀ
ꢀ
Reported a value
♦
♣
♣
♣
♣
♦
♣
♦
10
20
30
40
50
60
70
♦
3.540
3.525
3.510
♦
2θ /°
Figure 3 Ni/LSMO n = 1 material obtained at different temperature (T) and
750ºC 8h 800ºC 4h 800ºC 8h 850ºC 4h 850ºC 8h
750ºC 8h 800ºC 4h 800ºC 8h 850ºC 4h 850ºC 8h
reduction time (tr).
C
187.25
186.90
186.55
186.20
185.85
185.50
The refined lattice parameters of both phases (LSMO n= 1 and
Ni) are presented in Figure 4 (A–C) with reasonable reliability
factors of Rp= 3.97 – 4.18 %, Rwp= 5.57 - 5.61 % and “goodness
of fit (GOF)” or χ2= 1.56 - 1.68 (χ2 values between 1.0 and 2.9 are
generally considered satisfactory[52]) also confirmed by the
graphical analysis of each refinement. By comparison with the
values obtained from the analysis of the oxidized phase LSMO n=
1, an expansion of the c-parameter and the unit-cell volume is
evidenced, in agreement with Sandoval et al.[45] Such effect is
related to the reduction of Mn4+ to Mn3+ and the generation of
oxygen vacancies (δ) in the oxide structure. However, although
the calculated values are not exactly the same as reported by
♠
♠
♠
♠
♠
750ºC 8h 800ºC 4h 800ºC 8h 850ºC 4h 850ºC 8h
Figure 4 Results of LeBail refinements using XRD data for impregnated LSMO
n= 1 reduced at 750, 800 and 850 °C during 4 and 8 h. A) a parameter of reduced
LSMO n= 1 and Ni B) c parameter of reduced LSMO n= 1 C) Unit-cell volume of
reduced LSMO n= 1.
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10.1002/cctc.201902306
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Figure 6 TEM image of Ni-impregnated LSMO n= 1 reduced at 850 °C for 4 h,
On top, zoom of a Ni nanoparticle (bright field). On the bottom, the elemental
grain mapping (dark field).
Figure 5 SEM image of reduced material at A) 750 °C for 8 h, B) 800 °C for 4 h
and C) 850 °C for 4 h.
Preliminary tests: conditions for intrinsic kinetics
The average Ni particle size (̾Ŭ ) was determined at different
ͤ
A preliminary test was carried out to guarantee that the
experiments will be carried out within a region of intrinsic kinetics,
and to define the adequate flow to be used at the reactor inlet.
The selected reagent flow rate was first fixed at the same value
as for the exploratory test presented previously, i.e. 128 mL (STP)
min-1 dry basis or 143.7 mL (STP) min-1 wet basis.[22] The other
two points to evaluate the kinetic regime were defined by
multiplying and dividing the flow rate by a factor 1.5. For these
tests, the volume hourly space velocity (VHSV) value was 172000
mL (STP) g-1 h-1. The corresponding results are illustrated in
Figure 7.
reduction conditions (T and tr) using the scanning electron
micrographs (SEM), based on about 100 particles in four selected
T – tr conditions to obtain characteristic frequency histograms.
The grouped particle size data were analyzed using as likelihood
fitting method a lognormal distribution as already described
previously for exsolved material;[22] the selected distributions are
shown in Figure S3. In the impregnated LSMO n= 1 material,
reduced at 850 °C during 4 h, the calculated ̾Ŭ value is 38.20 ±
ͤ
2.43 nm, similar to the value obtained for an 8 h treatment
(̾ =38.68 ± 2.13 nm); the material nanostructure seems not to
ͤ
Ŭ
be significative affected by the reduction treatment duration.
However, when the temperature decreases to 800 and 750 °C,
the calculated ̾ value is significatively smaller (34.84 ± 1.46 nm
20
-1 -1
Equilibrium
VHSV= 17200 mL (STP) g
h
Ŭ
ͤ
and 32.85 ± 1.62 nm, respectively). As a conclusion, the
temperature seems to have more influence on the particle size, in
agreement with what has been described by other authors;[53,56,57]
this is also coherent with the results presented and discussed
previously for Ni exsolution.[22]
15
10
5
1.5w
w
w
/1.5
1
1
1
F
F /1.5
1
1.5F
1
1
0
95.8
143.7
215.6
-1
Input flow rate /mL (STP) min
Figure 7 Influence of the input flow rate on CH4 conversion at constant
space velocity.
In the steady-state conditions, an average CH4 conversion of
9.30 ± 0.40 mol% was reached without significant variation
between the three evaluated points, indicating a kinetic regime,
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10.1002/cctc.201902306
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Without Ni (WN)
Impregnation
Exsolution
LSMO n=1
Ni/LSMO n=1
Ni/LSM n=1
20
15
10
20
15
10
5
20
15
10
5
Equilibrium
Equilibrium
Equilibrium
A
B
C
t = 8 h
r
t = 8 h
r
0.8
0.6
0.4
0.2
0.0
0
0
0
100
200
300
400
500
0
100
200
300
400
500
0
100
200
300
400
500
20
15
10
20
16
12
8
20
Equilibrium
Equilibrium
Equilibrium
D
F
E
t = 4 h
r
t = 4 h
r
t = 8 h
r
t = 8 h
r
15
10
5
0.8
0.6
0.4
0.2
0.0
4
0
0
0
100
200
300
400
500
0
100
200
300
400
500
0
100
200
300
400
500
20
15
10
20
15
10
5
20
15
10
5
Equilibrium
Equilibrium
Equilibrium
G
H
I
t = 4 h
r
t = 4 h
r
t = 8 h
r
t = 8 h
r
0.8
0.6
0.4
0.2
0.0
0
0
0
100
200
300
400
500
0
100
200
300
400
500
0
100
200
300
400
500
Time on stream /min
Figure 8 CH4 conversion at different reaction temperatures (750, 800 and 850 °C) during 4 and 8 h of exposure to reactive gas and employing LSMO n= 1 (support),
Ni/LSMO n= 1 (impregnation) and Ni/LSM n= 1 (exsolution) as catalysts.
with no limitations by external diffusion, in this range of operating
conditions and for the selected VHSV value. Based on these
results and considering the thermodynamic equilibrium,
of Ni-impregnated or exsolved catalysts. Tao et al.[58] also
reported, for the manganite oxide La0.75Sr0.25Cr0.5Mn0.5O3, an
insignificant catalytic behavior of the perovskite for the same
reaction, even at higher S/C ratio. Generally speaking, such
doped lanthanum manganites are not very active and are not
used as a pure phase for CH4 reforming reactions, but their
secondary role to promote nickel-based reforming catalysts has
been recognized, making them very good candidates as catalytic
support.[59]
a
volumetric flow rate of 143.7 mL (STP) min-1 wet basis was
chosen for the following catalytic study.
Methane catalytic steam reforming reaction
The methane steam reforming in low S/C ratio has been studied
as the most relevant reaction taking place at the very early stage
of operation (startup) of an SOFC system operating in GIR
conditions. A series of catalytic tests were performed with CH4 at
a total exposure time of 8 h. The performance of the 3 synthesized
materials, LSMO n= 1 (support), Ni/LSMO n= 1 (impregnation)
and Ni/LSM n=1 (exsolution), was evaluated in terms of CH4
molar conversion. The results are shown in Figure 8. A direct
comparison with other exsolved materials reported in the
literature is not possible because most of the studies have been
performed in different operating conditions (S/C>1, partial
pressure of CH4 and H2O, flow, etc); as a consequence, the
catalytic behavior will be compared between exsolved and
impregnated samples.
For all the Ni-containing catalysts, the catalytic behavior at
different exposure times is represented by two different zones: the
CH4 conversion is initially low but gradually increases until it
reaches
a constant value corresponding to steady-state
conditions. In a similar way as presented by other authors,[60–64]
such behavior is maybe related to surface modifications during
the steam reforming reaction. Therefore, the analysis of the
results was performed considering the average values from data
measured in this stable zone for conversion, i.e. conversion rate,
H2 formation rate, selectivities to formed C compounds and H2 to
CO ratio (Eq. (5) to (9)).
A reason to explain such catalytic behavior of the Ni-containing
materials (Figure 8) would be the partial reoxidation of the Ni
particle surface as reported in the literature.[65–67] Such
phenomenon could be attributed in this case to the increasing
oxygen partial pressure (PO2) due to the presence of steam in the
reactor, despite being in low concentration. This partial oxidation
First, low catalytic activity is evidenced for the support material,
which reaches less than 1 mol % of CH4 conversion values at
different temperatures (T= 750, 800 and 850 °C), being stable
during the time on stream (8 h). Such result indicates the reduced
influence of the supporting manganite on the catalytic properties
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10.1002/cctc.201902306
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is probably reversed while H2 is produced and H2O consumed.
Even though, it is slow at low temperature e.g. 750 and 800 °C, in
which the catalytic behavior did not reach a stable zone during the
8 h of the test.
The exsolved material exhibits better catalytic performance than
the catalyst prepared by Ni-impregnation in the working operating
conditions. It is worth mentioning the particular behavior observed
for the impregnated material at 850 °C. At this temperature, CH4
conversion is lower than the apparent value reached for the same
material at 800 °C. This behavior can be attributed to Ni particle
coarsening due to the temperature increase, reducing the number
of catalytic active sites, as suggested by various authors.[53,56,57]
To confirm such assumption, the impregnated materials
(Ni/LSMO n= 1 reduced during 4 h) were analyzed by Scanning
Electron Microscopy (SEM) after reaction at 800 and 850 °C and
compared to fresh materials; the results are presented in Figure
9 (Figure S4). A rise in the average particle size is clearly
In the steady-state conditions, impregnated and exsolved
materials show a CH4 conversion much higher than that obtained
for LSMO n= 1 (support) for all reaction temperatures (T= 750,
800 and 850 °C); however, the steady-state conditions are
achieved at different rates. At 750 °C, the conversion increases
slowly in both samples, reaching values below 5 mol% without a
visible steady-state condition after 8 h of exposure to reactive gas.
The results show that a stable value of CH4 conversion at this
temperature and using impregnated material is achieved faster
than that observed for exsolved material. The catalytic behavior
improves significantly at 800 °C compared with that obtained at
750 °C. The reaction is promoted by the temperature. However,
during the 8 h of exposure to reactive gas, a steady-state
condition is still not reached hindering an adequate comparison
between catalysts. The steady-state condition is obtained for both
materials at 850 °C, being faster for the exsolved material. On the
other hand, it is worth noting that the catalytic behavior observed
for both materials indicates that the reduction time used during
the materials pre-treatment (tr= 4 and 8 h) do not have a
significant effect on CH4 conversion at temperatures higher than
800 °C. Hence, a 4 h treatment for pre-reduction seems high
enough to get a stable performance of the catalyst.
ŭ
evidenced for a reaction temperature of 850 °C (̾ Ɣ38.20 ± 2.43
+
ŭ
nm vs ̾ Ɣ 62.97 ± 3.02 nm before and after reaction,
+
respectively) while at 800 °C, the Ni particle size does not
ŭ
+
ŭ
+
significantly change (̾ Ɣ34.86 ± 1.46 nm vs ̾ Ɣ35.62 ± 1.62
nm before and after reaction, respectively). In this way, the effect
of temperature on the coarsening of Ni nanoparticles is
particularly critical for the impregnated material, explaining the
decrease in the CH4 conversion of the sample operating at 850 °C.
At 850 °C and in steady-state conditions (Table 2), CH4
conversion reaches 9.71 mol% using exsolved material with a
conversion rate of 8.17 mmol min-1 g-1 and H2 formation rate of
23.5 mmol min-1 g-1, while the impregnated material allows a CH4
conversion of 7.89 mol% with a conversion rate of 6.65 mmol min-
1 g-1 and H2 formation rate of 19.6 mmol min-1 g-1.
Table
2 Results for catalytic steam reforming of methane at 850 °C:
impregnated vs exsolved material pre-treated at 850 °C during 4 and 8 h.
Reduction
Impregnated
Exsolved
time [h]
4
8
4
8
4
8
7.89 ± 0.20
7.40 ± 0.14
6.65 ± 0.11
6.24 ± 0.12
19.6 ± 0.51
18.3 ± 0.28
9.71 ± 0.50
9.19 ± 0.51
8.17 ± 0.40
7.90 ± 0.10
23.5 ± 1.0
CH4 conversion
[mol%]
CH4 conversion rate
[mmol min-1 g-1]
H2 production rate
[mmol min-1 g-1]
22.4 ± 0.29
Outlet dry gas composition [mol%]
Figure 9 Ni particle size distribution in Ni/LSMO n= 1 (impregnation) before
4
8
4
8
4
8
4
8
4
8
4
8
4
8
4
8
16.36 ± 0.20
16.77 ± 0.20
15.80 ± 0.13
14.86 ± 0.20
62.49 ± 0.16
63.31 ± 0.36
4.39 ± 0.08
4.09 ± 0.10
0.96 ± 0.02
0.97 ± 0.02
0.80 ± 0.01
0.79 ± 0.01
0.20 ± 0.01
0.21 ± 0.01
3.60 ± 0.04
3.63 ± 0.04
16.03 ± 0.10
16.09 ± 0.20
18.31 ± 0.20
17.61 ± 0.18
59.29 ± 0.30
60.22 ± 0.36
5.51 ± 0.10
5.23 ± 0.06
0.86 ± 0.02
0.86 ± 0.02
0.86 ± 0.01
0.86 ± 0.01
0.14 ± 0.01
0.14 ± 0.01
3.32 ± 0.03
3.36 ± 0.03
N2
and after the methane steam reforming reaction at 800 and 850 °C.
The improvement of CH4 conversion when the exsolved
material is used, by comparison to impregnation, can be attributed
to the strong nanoparticle-matrix oxide interaction given by the
exsolution mechanism. In an exsolved material, the metallic
active particles are embedded into the oxide support contrary to
the impregnated active phases in which the particles are just
deposited on the support surface.[40,57,68] Such characteristic
makes the exsolved particles less mobile on the surface and the
coalescence rate is considerably decreased. This phenomenon
can only occur when the particles are very close to each other.
Such phenomenon can be evidenced by SEM, in the case of
exsolved material pretreated at 850 °C during 4 h: there is no
H2
CH4
CO
CO2
S
CO
S
CO
2
H2/CO ratio
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10.1002/cctc.201902306
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statistically difference between both values of the average particle
200
150
100
50
ŭ
size, so the sintering effect is not appreciable (̾ = 39.81 ± 2.21
+
CH + H O ꢀ CO + 3H (MSR)
2
4
2
ŭ
nm vs ̾ = 41.83 ± 1.99 nm before and after reaction, respectively,
+
CO + H O ꢀ CO + H (WGS)
2 2 2
CH + CO ꢀ 2CO + 2H (MDR)
2
Figure S4). Consequently, the particle migration cannot be the
dominant mechanism due to the strong nanoparticle-matrix oxide
interaction with practically null sintering by crystallite migration.[69–
4
2
71]
However, as explained previously,[22] the particle coarsening
0
does not completely disappear, as another mechanism may take
place known as atomic migration or Ostwald ripening process.
Such mechanism is slower than the particle migration and
describes the growth of a larger particle by consuming a smaller
one without direct connection; clusters of atoms from a small
particle migrate through the oxide surface and merge into another
large particle to reach the equilibrium. This effect becomes much
serious when the temperature increases.[70,72]
-50
-100
-150
-200
100 200 300 400 500 600 700 800 900 1000
Concerning the selectivity towards C-containing products
present in the reaction (CO and CO2), the comparison for T=
850 °C and tr= 4 and 8 h is presented in Table 2. In the case of
CO, the obtained values for calculated average selectivity seems
to be higher than for CO2, in both cases of exsolved and
impregnated materials. It globally means that Ni metal exhibits a
high activity as reported previously[73–75] and that selectivity is not
affected by the reduction time (tr). These results also indicate that
the Methane Steam Reforming (MSR) reaction (Eq. (1))
dominates on Water Gas-Shift (WGS) reaction (Eq. (2)), in
agreement with the conditions of low S/C ratio (low S/C ratio will
increase the CO selectivity and the CO2 selectivity for higher
ratios).[76] To confirm such analysis, the H2/CO ratio was
calculated for both materials in the same operating conditions
(Table 2). Those values are higher than 3.00 (stoichiometric value
for MSR reaction), confirming that the WGS reaction is well
present, to a lesser extent, during the catalytic test, which
consumes CO and produces H2 as well as CO2, being particularly
the case for the impregnated material. In conclusion, although for
both materials the selectivity to MSR reaction is higher, the
impregnated material is significative less selective than the
exsolved material, the latter case being more interesting for a
SOFC anode application.
Temperature /°C
Figure 10 Variation with the temperature of the Gibbs free energy (∆G) of
all possible reactions during methane (CH4) reforming reactions: Methane
Steam Reforming (MSR), Water Gas Shift (WGS), Methane Dry Reforming
(MDR). (Data taken from literature).[82]
the MSR reaction. This is probably due to a lower propensity for
Ni-particles coarsening.
Catalytic steam reforming reaction for other main light
hydrocarbons present in Natural Gas
Considering the composition of Colombian natural gas in which
other minor hydrocarbon compounds such as ethane (C2H6) and
propane (C3H8) are also present (Table 1), additional experiments
were performed using CH4 – C2H6, and CH4 – C3H8 mixtures.
Steam deficient conditions were kept constant in agreement with
the GIR concept (S/C: 0.15). The tests were carried out at
different reaction temperatures (750, 800 and 850 °C) using
exsolved material after reduction at 850 °C during 4 h as the
catalyst. The gas mixtures that were used for these tests could
not be adjusted at the exact CH4 composition of Colombian
natural gas (80 mol%) due to system restrictions. Nevertheless,
the CH4 composition was always much greater than that of the
other light hydrocarbon compounds in the mixture. An additional
test, operated only with CH4 in a new composition (the lowest one
defined by the CH4 - C3H8 mixture, i.e. 43.26 mol%) and keeping
constant the other operating conditions, confirms that no
significant variations are observed in the conversion, selectivity or
H2/CO ratio when the CH4 composition in the feed stream is
modified (see Table S2); the only change is the molar distribution
of products in the outlet gas composition.
The results presented above are coherent with the
thermodynamic data of the most important reactions occurring
during methane reforming as shown by the behavior of Gibbs free
energies (∆G) as a function of temperature (Figure 10)¡Error! No
se encuentra el origen de la referencia.. At 850 °C and 1 atm,
MSR and MDR are the most preponderant reactions due to their
endothermicity, being spontaneous at temperatures higher than
620 and 641 °C, respectively. On the other hand, the WGS
reaction is only favored at high steam and low H2 content[77] and
thermodynamically promoted at low temperatures (lower than
823 °C), as it is moderately exothermic. Then, in the present
operating conditions, a low CO2 production is expected, indicating
the moderate participation of MDR; it results in a high CO
concentration and, thereby, a greater selectivity to the MSR
reaction with respect to other reactions.
As for methane steam reforming, the catalytic behavior for
hydrocarbon mixtures at different exposure time exhibits two
different zones: the hydrocarbon compound conversion is initially
low but gradually increases until it reaches a constant value
corresponding to steady-state conditions. In this region, the
average amount of products, selectivity, H2 production rate,
hydrocarbon conversion, and conversion rates are calculated in
order to make a comparison.
Then, as a partial conclusion, both catalysts are clearly active
for H2 production, but the exsolved material seems a better option
due to the higher CH4 conversion, H2 production, selectivity for
The first reactive dry gas composition tested was 10 mol% C2H6
– 49.92 mol% CH4 – balance N2 and the results are presented in
Table 3. H2 is formed as the major product with an important
production rate, similar to the results presented for methane
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10.1002/cctc.201902306
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catalytic steam reforming reaction (Table 2). Such behavior is of
referencia. However, it is worth noting that the CH4 production
great interest to the SOFC application directly fed with natural gas. during ethane reforming makes MSR reaction itself will be also
Other products such as CH4, CO and C2H6 are also present in
significant amounts whereas CO2 and C2H4 were quantified in
much less quantity.
present. On the other hand, C2H4 formation was confirmed by
additional experiments based on pure ethane steam reforming
reaction (Table S3).
Table 3 Cataytic behavior for the steam reforming using methane-ethane
mixture at different temperatures over exsolved material pretreated at 850 °C
during 4 h.
Table 4 Expected ethane and propane reforming reactions according to
literature.[78–81]
∆H°
Reactions
Name
Ethane steam
Temperature [°C]
[kJ mol-1]
750
800
850
C2H6+2H2O ꢀ
2CO+5H2
ESR
EDR
352.6
reforming
Ethane dry
reforming
C2H6 conversion
3.74 ± 0.05
5.27 ± 0.05
8.13 ± 0.03
rate [mmol min-1 g-1]
C2H6+2CO2 ꢀ
4CO+3H2
438.6
137.9
-83.8
214.6
497.9
620.9
126.9
84.7
CH4 conversion rate
3.73 ± 0.10
22.6 ± 0.10
3.05 ± 0.06
22.3 ± 0.20
1.21 ± 0.20
24.6 ± 0.20
[mmol min-1 g-1]
Ethane
C2H6 ꢀ C2H4+H2
C2H6+H2 ꢁ 2CH4
EDhy.
EHyd.
Et.SR
PSR
H2 production rate
dehydrogenation
Ethane
[mmol min-1 g-1]
hydrogenolysis
Ethylene steam
reforming
N2
H2
31.31 ± 0.16
17.18 ± 0.08
38.52 ± 0.18
6.42 ± 0.12
0.61 ± 0.05
0.73 ± 0.03
5.25 ± 0.03
31.46 ± 0.16
16.93 ± 0.13
39.21 ± 0.22
5.88 ± 0.09
0.67 ± 0.03
1.90 ± 0.01
4.015 ± 0.04
30.58 ± 0.28
18.25 ± 0.10
39.51 ± 0.29
6.11 ± 0.02
0.51 ± 0.02
3.17 ± 0.02
1.87 ± 0.01
C2H4+2H2O ꢀ
2CO+4H2
CH4
CO
C3H8+3H2O ꢀ
3CO+7H2
Propane steam
reforming
CO2
C2H4
C2H6
C3H8+3CO2 ꢀ
6CO+4H2
Propane dry
reforming
PDR
Propane
C3H8 ꢀ C3H6+H2
PDhy.
PCrk.
Pr.SR
PCrk.2
CO
CO2
C2H4
0.76 ± 7x10-3
0.07 ± 5x10-3
0.17 ± 2x10-3
0.57 ± 4x10-3
0.06 ± 2x10-3
0.37 ± 4x10-3
0.47 ±1x10-3
0.04 ±1x10-3
0.49 ±2x10-3
dehydrogenation
Propane cracking
1
C3H8 ꢀ C2H4+CH4
C3H6+3H2O ꢀ
3CO+6H2
Propylene steam
reforming
381.1
73.7
2C3H8 ꢀ
Propane cracking
2
C3H6+C2H6+CH4
The complexity of the products stream increase with the carbon
chain length of the compounds in the feedstock in agreement with
ethane and propane possible reforming reactions (Table 4).[78–81]
The value of Gibbs free energies (∆G) as a function of
temperature for the most important reactions occurring during
ethane reforming shows that ethylene (C2H4) and CH4 are
spontaneously produced in operating conditions (Figure 11, data
taken from literature).[82]¡Error! No se encuentra el origen de la
Finally, as indicated before, the WGS reaction is only promoted
at high steam and low H2 content[77] and thermodynamically
favored at low temperatures (lower than 823 °C, exothermically
moderate). Therefore, in dry operating conditions, CO2 production
is lower while CO is higher (see Table 3).
According to Figure 10 and Figure 11, MSR and ethane steam
reforming (ESR) are thermodynamically favored by temperatures
higher than 620 and 475 °C, respectively, and due to the
endothermicity of these reactions, the conversion should be
favored at a higher temperature. However, Figure 12 and Table 3
show that C2H6 conversion and its conversion rate increase with
the temperature, as expected, while the behavior for the CH4
conversion and its conversion rate follow an inverse trend
affecting CO selectivity, but not the H2 production rate (Table 2
and Table 3). Those results indicate that CH4 conversion is
significative affected by the presence of other hydrocarbon
compounds; e.g. at 850 °C, methane conversion changes from
9.67 mol% in CH4 feed stream (Table 2) to 2.21 mol% and in CH4
- C2H6 mixture feed stream (Figure 12). A competition between
reactions and low available metallic active sites (5 wt% metallic
Ni on the surface) can probably explain such behavior. Few
studies have tried to explain the reactivity of CH4 vs C2H6 during
steam reforming of gas mixtures;[83–85] in many cases,
contradictory results are described which are mainly focused on
the use of methane as the model reactant.[85]
300
C H + 2H O ꢀ 2CO + 5H (ESR)
2
2
6
2
C H + 2CO ꢀ 4CO + 3H (EDR)
2
6
2
2
200
100
0
C H ꢀ C H + H (EDhy.)
2
2
2
4
6
C H + H ꢁ 2CH (EHyd.)
2
6
2
4
C H + 2H O ꢀ 2CO + 4H (Et.SR)
2
4
2
2
-100
-200
-300
100 200 300 400 500 600 700 800 900 1000
Temperature /°C
Figure 11 Variation with the temperature of Gibbs free energy (∆G) of all
possible reactions during ethane (C2H6) reforming: Ethane Steam Reforming
(ESR), Ethane Dry Reforming (EDR), Ethane Dehydrogenation (EDhy.),
Ethane Hydrogenolysis (EHyd.), Ethylene Steam Reforming (Et.SR). (Data
taken from literature).[82]
Today, the most accepted reaction mechanism for MSR, still
under discussion, can be represented by 3 main steps: 1) CH4
decomposition on the metallic surface to chemisorbed carbon
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10.1002/cctc.201902306
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atoms (C) and hydrogen fragments 2) dissociative adsorption of
water to H and OH species and 3) OH or O species combine with
C to form CO. The first step requires the C-H activation on the
metal surface (e.g. Ni), which, according to Wei and Iglesia,[86] is
the limiting step in the mechanism of catalytic MSR. The results
indicate that the presence of C2H6 promotes a decrease in the
CH4 conversion rate that seems to be due to a greater difficulty in
the C-H activation of methane; at 850 °C, it changes from 8.17
mmol min-1 g-1 using a CH4 feed stream to 1.21 mmol min-1 g-1 in
a CH4 - C2H6 mixed feed stream (Table 2 and Table 3). In this vein,
when CH4 and C2H6 are fed together, the activation of C2H6 C-H
the H2 production by the reforming reactions, especially the
ethane reforming reactions (ESR, EDhy. and Et.SR).
In conclusion, it is possible to affirm that the ethane reforming
reactions are more favored in severe reforming conditions (steam
deficient conditions) than methane reforming when CH4 – C2H6
mixture is used. Besides, this trend is exacerbated at high
temperatures, for which CH4 and C2H4 production are promoted
as observed by the drop of CH4 conversion and increase of C2H4
composition and selectivity (Table 3).
The second tested dry gas composition was 4 mol% C3H8 –
43.26 mol% CH4 – balance N2. As for the other cases, the catalytic
behavior is split into two characteristic steps: the hydrocarbon
compounds conversion is initially low but gradually increases
(faster than for CH4 or CH4-C2H6 mixture) until it reaches a
constant value corresponding to steady-state conditions. At this
point, the average amounts of products, selectivity, H2 production
rate, hydrocarbon conversion, and conversion rates are
calculated. The H2 is formed as the major product with an
important production rate (Table 5); however, these values are
lower than those obtained in the case of CH4 and CH4 - C2H6
mixture. On the other hand, in a similar way as before, the low
CO2 production is expected due to the thermodynamically
unfavored WGS reaction in the current operating conditions.
As for the MSR and ESR, the propane steam reforming (PSR)
is also thermodynamically favored with a temperature higher than
441 °C (Figure 14) due to its endothermicity. However, and
similarly to the results obtained for the steam reforming of CH4 -
C2H6 mixture, the CH4 conversion, and conversion rate tends to
decrease with a rise of temperature, and even reaches negative
values at 850 °C, while the C3H8 conversion and conversion rate
increase with the temperature, as expected, the conversion
reaching almost 100 mol% (Figure 13). Such trend confirms that
CH4 conversion is significative affected by the presence of other
hydrocarbon compounds, as in the case of ethane mixture.
In agreement with some other authors,[89,90] high temperatures
result in enhanced production of CH4, C2H4, C2H6 and C3H6 due
to a significant thermal decomposition of C3H8, evidenced in the
present study by individual experiments concerning propane
steam reforming reaction (Table S4). In this case, the activation
of C3H8 first C-H bond is easier than for CH4 and even than C2H6
(C-H bond in CH3 group 417 kJ mol-1 and C-H bond in CH2 group
398 kJ mol-1);[91] the dehydrogenation of these two kinds of bond
is favored as well as the rupture of adsorbed C3 radicals due to
the weakness of the C-C bond (347 kJ mol-1),[91] which improves
the formation of methyl, ethyl, vinyl and propyl radicals occupying
metallic Ni active sites and promotes the C3H8 related propane
cracking and the reforming reactions e.g. ESR, Et.SR, Pt.SR. The
possible change in the H2/CO ratio depends on the degree of
competition between these reactions. In the current operating
conditions, no significant variation in the H2/CO (Figure 13) is
observed, remaining practically constant with an increase of the
reaction temperature. This behavior is also related to the system
deficient steam conditions, the C3H8 composition in the feed
stream and the inhibition of the WGS with the temperature, which
reduce the CO consumption and favor proportionally the H2
production by the reforming reactions.
[87]
bonds occurs more easily than that of CH4 and it is related to
the difference of energy bond strength (421.77 kJ mol-1 vs 438.89
kJ mol-1, respectively).[88] Thus, the formation of ethyl radicals (C2)
will occur to a large degree, occupying first the available metallic
active sites and hence promoting the C2H6 reforming reactions.
Temperature /°C
750
800
850
3.25
3.00
2.75
2.50
2.25
C H conversion
2
35.10 ± 0.47
49.81 ± 0.52
75.78 ± 0.83
6
/mol%
6.79 ± 0.24
5.58 ± 0.11
2.21 ± 0.35
CH conversion
4
/mol%
Figure 12 H2/CO ratio, C2H6 and CH4 conversion at different reaction
temperatures in the methane-ethane mixture steam reforming reaction over
exsolved material pretreated at 850 °C during 4 h.
C2H6 conversion is, thereby, higher than that of CH4 and is
promoted advantageously by an increase of temperature (Figure
12). Concerning the particular trend for CH4, the results must be
analyzed carefully, since the ESR is also producing CH4 (Table 4)
and, in this way, distorts the effect of the temperature on CH4
conversion rate. CH4 production is given by ethane
hydrogenolysis (EHyd.) and stems from the scission of the C-C
bond of an adsorbed C2 radical, whose energy bond is weaker
than C-H bond (376.66 kJ mol-1).[88] Thus, the additional CH4
formation leads to a decrease of CH4 conversion in all operating
conditions (Figure 12) when C2H6 is present in the reaction
mixture. This behavior becomes more important when the
temperature increases. Additionally, the calculated experimental
H2/CO mole ratio variation is presented in Figure 12 as a function
of the reaction temperature. This ratio increases proportionally
with the reaction temperature; such behavior is related to the
system deficient steam conditions and the inhibition of the WGS
with the temperature, which reduces CO consumption and favors
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10.1002/cctc.201902306
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temperature rises: CH4 is produced faster from higher
hydrocarbons than reformed, due to its lower reactivity.
Table 5 Catalytic behavior for the steam reforming using methane-propane
mixture at different temperatures over exsolved material pretreated at 850 °C
during 4 h.
Temperature [°C]
750
800
850
3.25
3.00
2.75
2.50
2.25
2.00
1.75
Temperature [°C]
750
800
850
C3H8 conversion
2.14 ± 0.02
3.17 ± 0.02
4.11 ± 0.02
rate [mmol min-1 g-1]
H2 production rate
15.7 ± 0.20
16.0 ± 0.20
17.8 ± 0.10
[mmol min-1 g-1]
N2
H2
43.13 ± 0.19
12.27 ± 0.06
36.76 ± 0.27
5.08 ± 0.05
0.60 ± 0.02
0.24 ± 3x10-3
42.94 ± 0.07
12.20 ± 0.12
37.44 ± 0.07
5.07 ± 0.04
0.56 ± 0.01
0.63 ± 0.01
41.70 ± 0.17
13.43 ± 0.08
37.40 ± 0.08
5.90 ± 0.04
0.27 ± 0.01
0.90 ± 0.02
CH4
CO
CO2
C2H4
0.15 ± 2x10-
C2H6
0.03 ± 2x10-4
0.10 ± 9x10-4
1.50
3
49.93 ± 0.57
73.52 ± 0.04
95.38 ± 0.09
C H conversion
3
C3H6
C3H8
0.23 ± 1x10-3
1.67 ±0.01
0.20 ± 1x10-3
0.87 ± 2x10-3
0.10± 2x10-3
0.15± 3x10-3
8
/mol%
CH
4
2.34 ± 0.01
0.56 ± 0.02
-0.26 ± 0.02
conversion
-1 -1
0.74 ± 5x10-3
0.09 ± 2x10-3
0.07 ± 7x10-4
0.01 ± 6x10-5
0.01 ± 6x10-5
0.74 ± 4x10-3
0.07 ± 2x10-3
0.14 ± 2x10-3
0.02 ± 3x10-4
0.03 ± 3x10-4
0.74±5x10-3
0.02± 2x10-3
0.18± 4x10-3
0.03± 5x10-4
0.03± 1x10-3
rate /mmol min
g
CO
Figure 13 H2/CO ratio at different reaction temperatures in the methane-
propane mixture steam reforming reaction over exsolved material pretreated at
850 °C during 4 h.
CO2
C2H4
C2H6
C3H6
Conclusions
The catalytic steam reforming of natural gas (and methane as its
main constituent) in low steam conditions is of special interest for
the development of SOFC systems. Therefore, new anode
materials with a high and stable catalytic behavior are requested
in order to design systems that would be directly fed with
hydrocarbon fuels. Ni-decorated Ruddlesden-Popper manganite
catalyst (prepared by exsolution or impregnation) are clearly
effective for H2 production through methane steam reforming
(MSR) reaction due to the presence of the metallic active phase
on the oxide support. However, the results obtained with the new
proposed material, i.e. La1.5Sr1.5Mn1.5Ni0.5O7 reduced to Ni-
exsolved LaSrMnO4, suggest that this catalyst is a better option
due to the high CH4 conversion, H2 production, selectivity for the
MSR reaction and stability during 8 h of reaction time. This
behavior is largely due to a lower propensity to Ni coarsening of
the exsolved manganite because the metallic particles are
embedded in the ceramic support; in comparison to the
impregnated material, the particles are only deposited on the
surface and makes them easily mobile.
450
300
150
0
C H + 3H O ꢁ 3CO + 7H (PSR)
3
8
2
2
C H + 3CO ꢁ 6CO + 4H (PDR)
3
6
2
2
C H ꢁ C H + H (PDhy.)
3
2
3
6
8
C H ꢁ C H + CH (P.Crk.)
3
8
2
4
4
C H + 3H O ꢁ 3CO + 6H (Pt.SR)
3
6
2
2
2C H ꢀ C H + C H + CH (PCrk.2)
4
8
3
2
6
3
6
-150
-300
-450
100 200 300 400 500 600 700 800 900 1000
Temperature /°C
Figure 14 Variation with the temperature of Gibbs free energy (∆G) of all
possible reactions during propane (C3H8) reforming: Propane Steam
Reforming (PSR), Propane Dry Reforming (PDR), Propane Dehydrogenation
(PDhy.), Propane Cracking 1 (P.Crk.), Propylene Steam Reforming (Pt.SR),
Propane Cracking 2(PCrk.2). (Data taken from the literature).[82]
In addition, it was proved that the reforming reactions in severe
reforming conditions of other light hydrocarbons such as ethane
and propane (minor constituents of natural gas) are favored with
respect to methane using feed gas mixtures. Their presence
promotes parallel reactions (PCrk., PCrk.2, EHyd, EDhy., etc),
which produce H2 and, in turn, an additional amount of CH4. The
methane steam reforming being not kinetically favored, a
consequence is the significative drop of CH4 conversion rate
when the temperature is raised: CH4 is produced faster from
higher hydrocarbons than reformed, due to its lower reactivity.
In conclusion, C3H8 will be significative more reactive than CH4
for the steam reforming reactions when the temperature
increases, promoting parallel reactions, including CH4 formation
(PCrk., PCrk.2, EHyd and EDhy.), which steam reforming will not
be kinetically favored (even if it is thermodynamically favored); it
explains the significant decrease of CH4 conversion rate when the
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10.1002/cctc.201902306
ChemCatChem
FULL PAPER
2θ = 2 - 70° for qualitative analysis and 2θ = 2 - 90° for Rietveld analysis
with a measurement step of 0.020353° (2θ). The X-ray diffraction data
were processed using JANA 2006 software package.[99]
It is worth noting that, although the target for the most recent
investigations in the field of SOFC is to operate at lower
temperatures (500-700 °C)[92–94], the so-called Intermediate
Temperature (IT) SOFC range, the results obtained in this study
are not far from the classical range of operation (600-800 °C).
However, considering the novelty of the material and the
exceptional catalytic properties reported in the present paper,
there is still a place to microstructure and material process
optimization, a decrease of the operating temperature to IT-SOFC
range being probably possible considering the present material.
As a conclusion, in view of its catalytic properties, the newly
developed material, based on the Ni exsolution process in which
Incipient wetness impregnation
As the objective was to compare the behavior of exsolved LSMN n= 2
material to the catalyst produced by a more classical method, LSMO n= 1
powder was impregnated with Ni using Incipient Wetness Impregnation
(IWI) technique, also known as pore volume impregnation. In the case of
Ni, an aqueous nickel nitrate solution is normally used, for which a smaller
Ni particle size has been reported in the literature, respect to other
salts.[100] In this case, the Ni content was the same as obtained by the
exsolution, i.e. ~0.05 mass fraction, as shown in previous works.[22] Into
details, a solution of Ni(NO3)2 was prepared using stoichiometric amounts
of NiCO3 (≥ 99.9 % Alfa Aesar) and concentrated nitric acid (HNO3, ≥
65 %vol Merck), diluted in an ethanol:deionized water solution (20:80
mol%) to decrease the surface tension of pure water and increase the
material´s wettability.[101] The LSMO n= 1 was impregnated with the correct
volume (defined previously), dried at 110 °C during 12 h, then heat-treated
in the air at 500 °C during 90 min[102–105] to obtain NiO/LSMO n= 1 material.
the remaining oxide is
a
Mixed Ionic and Electronic
Conductor[45,95,96] can be considered as a potentially interesting
SOFC anode in a system that would run with the main
hydrocarbon compounds present in natural gas. On the catalytical
point of view, additional issues to be considered with respect to
the application would be the sensitivity of the catalyst to the
formation of coke and the presence of sulfide impurities, issues
that will be addressed in a forthcoming study, in the same way as
the electrochemical properties as SOFC anode.
Reduction study
The operating temperatures (T) for the reduction study were selected
considering the conditions of exsolution of LSMN n= 2 (750, 800 and
850 °C) described previously.[22] Around 0.5 g of fresh NiO/LSMO n= 1
material was reduced in a tubular furnace CARBOLITE CTF 12/65/550
using a 3 mol% H2/N2gas mixture (Cryogas) with a flow of 55 mL (STP)
min-1 and the lowest reduction times to obtain the complete exsolution
were selected considering the lowest conditions to obtain the complete
exsolution (tr= 4 or 8 h).[22] The powder obtained at each temperature-
reduction time point (Ni/LSMO n= 1) was characterized by XRD at RT as
described in the characterization section. In addition, the material was
analyzed by X-Ray Fluorescence spectroscopy (XRF) using an S2 Ranger
Bruker spectrometer equipped with a Pd X-ray tube, to confirm the correct
Experimental Section
Synthesis
The manganite powders were synthesized by the Pechini or citrate
complexation route.[97] In the case of La1.5Sr1.5Mn1.5Ni0.5O7±δ (hereafter
referred to as LSMN n= 2), the procedure is described in previous works.[22]
For La0.5Sr1.5MnO4±δ (LSMO n= 1), stoichiometric amounts of La2O3 (≥
99.9 % Alfa Aesar), SrCO3 (≥ 99.9 % Sigma Aldrich) and MnCO3 (≥ 99.9 %
Sigma Aldrich) were used. Before weighing La2O3 and SrCO3, those
reagents were calcined in air at 1000 and 500 °C, respectively, for 1 hour,
to remove hydration products (and carbonation in the case of lanthanum
oxide). The precursors were initially dissolved in a solution made of
concentrated nitric acid (HNO3, ≥ 65 vol% Merck) in excess and citric acid
(CA, ≥99.5%Merck), added in the molar ratio CA:(metal ion)total= 3:1.
Under constant stirring, the resulting solution was slowly heated from room
temperature to 120 °C using a hot plate, in order to reduce the volume of
liquid. Polyethylene glycol (≥ 99 %, Panreac) was added as a polymerizing
agent (1.5 mL per gram of targeted product). Then, the resulting mixture
was stirred and heated at 150 °C just to form a viscous gel, which was
subsequently dried and heat-treated in air at 300 °C (2 h) and 500 °C (3 h)
to ensure total organic matter decomposition Finally, the product was
calcined in air at 1000 °C (6 h) and 1100 °C (6 h) with intermediate grinding
and pelletizing steps. In order to ensure a good homogeneity in the particle
size of the synthesized materials (LSMN n= 2 and LSMO n= 1), the
powders were individually ball-milled during 12 h at a speed of 50 rpm in
a dispersion made of acetone:powder:3 mm-sized zirconia balls in a 5:1:5
weight ratio. After the milling step, the ceramic balls were removed, and
the resulting mixtures were dried. Homogenous powders with a particle
size distribution between 125 and 200 µm were finally obtained by
adequate sieving, with the objective to eliminate intra-particle diffusion
effects during the catalytic reactions.[98]
amount of impregnated Ni.
A morphological characterization was
performed on the reduced powder by scanning electron microscopy (FEG-
SEM) in a FEG Quanta 650 microscope at high vacuum, acceleration
voltage of 25 kV, secondary electrons (SE) signal, Everhart Thomeley
Detector (ETD) and by transmission electron microscopy (TEM) in an FEI
TITAN Themis 300 equipped with a Super-X quad EDS for elemental
analysis. In this case, the powder was crushed and dropped in the form of
alcohol suspension on carbon-supported copper grids followed by
evaporation under ambient conditions.
Catalytic test
Experimental set-up: the measurements of the catalytic behavior for steam
reforming were carried out using the experimental set-up described
previously.[22] The composition of each effluent constituent was obtained
by a gas chromatograph (GC, SRI instruments 8610C) using He (grade
5.0, CRYOGAS) as mobile phase, equipped with a solenoid gas sampling
valve heated at 60 °C, two packed columns (molecular sieve 13X 6 in and
hayesep D 6 in), a thermal conductivity detector (TCD) heated at 150 °C
and controlled by PeakSimple 4.44 free software. Standard gas cylinders
with different gases compositions were employed for the outlet products
quantification.
Preliminary tests: preliminary tests were carried out to confirm that the
experiments correspond to the region of intrinsic kinetics, in which the heat
and mass transport resistances are negligible, and for which the changes
observed on molar concentrations are exclusively due to chemical
reactions taking place in the process. Therefore, to ensure a plug flow
pattern and minimize back mixing, the recommendations described in the
literature suggest, on one hand, a ratio between the catalyst bed length (L)
and the particle size (dp) upper than 50 (L/dp > 50) and on the other hand,
a ratio between the inner diameter of the reactor (Dr) and dp upper than
10 (Dr/dp > 10).[106–108]
Fresh materials characterization
Fresh LSMN n= 2 and LSMO n= 1 powders were characterized by powder
X-ray diffraction (XRD) at room temperature (RT) using a Bruker D8-
ADVANCE powder diffractometer operated in Bragg-Brentano geometry,
equipped with a Lineal Lynx Eye detector and a beam of CuKα1,2 radiation
(λ = 1.5418 Å). The diffractometer was operated over the angular range
This article is protected by copyright. All rights reserved.

10.1002/cctc.201902306
ChemCatChem
FULL PAPER
In
Out
LSMN n= 2 material was diluted with SiC grains (SiC:catalyst 10:1
weight ratio) to minimize heat-transfer effects[109] and introduced in the
reactor as fixed-bed between catalyst-free SiC and two pieces of quartz
wool. Considering the results obtained previously,[22] the amount of
catalyst was fixed to 50 mg to ensure both the CH4 conversion and the
composition of products being far from the thermodynamic equilibrium.
Finally, to avoid external diffusional limitations, i.e. interphase
concentration gradients between gas and solid surface, preliminary
experiments were performed, based on the principle that the reactant
conversion at any space velocity (e.g. volume hourly space velocity,
VHSV) should be independent of the linear velocity through the catalyst
bed when the external diffusional conditions are negligible. Therefore,
different tests were performed keeping the VHSV value as a constant,
while increasing both the input volumetric flow rate (volume of inlet gas per
unit time) and catalyst volume (or weight). During those tests, the reactant
conversion is expected to change while the interphase limitations (external
diffusion) are present.[98,110] The range of input volumetric flow rate that
avoids external diffusional limitation is thus obtained when the reactant
conversion does not change, which allows to determine correct operating
conditions, i.e. volumetric flow rate and catalyst amount. Considering that
CH4 is the most abundant hydrocarbon in Cusiana gas, Colombia, (Table
1), these tests were carried out using only a dry mixture of 82 mol% CH4
(N2 as balance) humidified to a steam to carbon ratio (S/C) of 0.15 for
different reactor inlet flow rates. It is worth noting that prior to this test, the
catalyst sample was in situ reduced using 55 mL (STP) min-1 of 3 mol%
H2/N2 mixture (Cryogas) during 4 h at T= 850 °C, as defined previously.[22]
Where, WCat is the catalyst weight (g),
n
and n
C H
n
are the individual
2n+2
C
H
n
2n+2
light hydrocarbons molar flow rate at the inlet and outlet of the reactor
Out
(mmol min-1) and nH is the H2 molar flow rate at the outlet of the reactor
2
(mmol min-1). Besides, the selectivity ƳSjƷ to CO, CO2, C2H4, C2H6, and
C3H6, were defined as the ratio between the molar flow rate of all carbon
atoms in the outlet of the selected j product and, that of the reacted light
hydrocarbons i employed as reactants as shown in Eq. (8), where φ is the
carbon number of the selected j product or i reactive and n the molar flow
rate.
Out
φ n
j
j
Sj=
dimensionless
(8)
N
In Out
φ Ƴn -n
∑
Ʒ
i=1
i
i
i
Finally, H2 to CO ratio was defined by Eq. (9).
Out
n
H
H
2
2
=
dimensionless
(9)
Out
CO
n
CO
It is worth noting that blank tests were performed at different temperatures
using only SiC, and no conversion was observed, neither for methane nor
the other light hydrocarbon compounds.
Acknowledgements
Operating conditions for catalytic tests: LSMN n= 2 catalyst was
preliminarily reduced (in situ conditions) using 55 mL (STP) min-1 of 3
mol% H2/N2 mixture (Cryogas) at previously selected temperatures of 750,
800 or 850 °C. At 750 °C, the reduction time was tr= 8 h, i.e. the minimum
condition to reach the complete Ni exsolution.[22] In the case of 800 and
850 °C, tr was fixed to 4 h that corresponds to the completion of the
exsolution process, and 8 h for better comparison with T=750 °C. The
catalytic behavior at each operating condition was measured during 8 h
using atmospheric pressure and the same reducing temperature as for the
catalytic test. The starting reaction mixture was fixed to 82 mol% CH4 (N2
as balance) humidified up to S/C=0.15 according to the SOFC anode
conditions suggested by the GIR concept.[16] The steam content was
adjusted by flowing the adequate dry CH4-N2 mixture throughout the
bubbler containing distilled water maintained at exactly 46 °C. The
composition in each effluent constituent was obtained at regular time
intervals (each 20 min) using online GC analysis. For the NiO/LSMO n= 1
(impregnated material) the reaction procedure was the same as described
for exsolution material (LSMN n= 2). In the case of LSMO n= 1 material,
the steam reforming reaction was started without a prior reduction step.
Considering the composition of Colombian natural gas in which ethane
(C2H6) and propane (C3H8) are also present with methane but in lower
concentration, additional tests were carried out at their respective molar
compositions (Table 1). Therefore, this study used mixtures of ethane – N2
(10 mol% C2H6), propane – N2 (4 mol% C3H8), CH4-C2H6 and CH4-C3H8.
The authors acknowledge the financial support of the Colombian
Administrative Department of Science, Technology and
Innovation
COLCIENCIAS
(Project
#
110265842833
“Symmetrical high temperature Fuel Cell operating with
Colombian natural gas” (contract # 038-2015) and S. Vecino-
Mantilla´s Ph.D. scholarship (call #647)) and of the Spanish
National Research Council CSIC (Project # COOPA20112). The
authors are also grateful to UIS’ X-Ray Laboratory (Parque
Tecnológico Guatiguará) for XRD measurements, Microscopy
Laboratory (Parque Tecnológico Guatiguará) for the FESEM
analysis, and finally to María Fabuel (UPV) and Marielle Huvé
(UCCS) for their contribution to materials synthesis and
characterization, respectively.
Keywords: Exsolution • Natural gas • Nickel • Ruddlesden-
Popper • Steam reforming
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10.1002/cctc.201902306
ChemCatChem
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The deep catalytic study over the Ni
S. Vecino-Mantilla, E. Quintero,
Fonseca, G. Gauthier*, P. Gauthier-
Maradei*
C
exsolved
Ruddlesden-Popper
manganite (Ni/LSM) is presented.
The exceptional catalytic behavior for
the steam reforming reaction of light
hydrocarbon constituents of natural
gas, even better than materials
obtained by classical impregnation,
converts this material in a potential
SOFC anode in a system that would
operate with such fuel.
Page No. – Page No.
Catalytic steam reforming of natural
gas over
Ruddlesden-Popper manganite in
SOFC anode conditions
a
new Ni exsolved
This article is protected by copyright. All rights reserved.