Nickel Exsolution-Driven Phase Transformation from an n=2 to an n=1 Ruddlesden-Popper Manganite for Methane Steam Reforming Reaction in SOFC Conditions

Article abstract of DOI:10.1002/cctc.201901002

An original way to perform the exsolution of Ni nanoparticles on a ceramic support was explored for the development of methane steam reforming catalyst in SOFC anode conditions. The n=2 Ruddlesden-Popper (RP) phase La_1.5Sr_1.5Mn₁

Full text of DOI:10.1002/cctc.201901002

Accepted Article
Title: Nickel exsolution-driven phase transformation from an n = 2
to an n= 1 Ruddlesden-Popper manganite for methane steam
reforming reaction in SOFC conditions
Authors: Sebastián Vecino-Mantilla, Paola Gauthier-Maradei, Marielle
Huvé, José Manuel Serra, Pascal Roussel, and Gilles H.
Gauthier
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Nickel exsolution-driven phase transformation from an n = 2 to an
n= 1 Ruddlesden-Popper manganite for methane steam reforming
reaction in SOFC conditions
Sebastián Vecino-Mantilla[a][b], Paola Gauthier-Maradei[a], Marielle Huvé[c], José Manuel Serra[b], Pascal
Roussel[c], Gilles H. Gauthier*[a]
Abstract: An original way to perform the exsolution of Ni
nanoparticles on a ceramic support was explored for the development
of methane steam reforming catalyst in SOFC anode conditions. The
n=2 Ruddlesden-Popper (RP) phase La1.5Sr1.5Mn1.5Ni0.5O7 has been
synthesized by the Pechini method and subsequently reduced with an
H2-N2 mixture at different temperatures and reduction times to induce
the formation of two phases: LaSrMnO4 (n=1 RP) decorated with
metallic Ni nanoparticles. Preliminary measurements of catalytic
behavior for the steam reforming have been carried out in a reduction-
reaction process with a mixture of 82 mol%CH4, 18 mol%N2 and low
steam to carbon ratio (S/C=0.15). The catalyst exhibits a selectivity
for CO production (0.97), 14.60 mol% CH4 conversion and around
24.19 mol% H2 production. Such catalytic behavior was maintained
for more than 4 h, with a constant rate of hydrogen production and
CH4 conversion rate.
directly the chemical energy of an oxidant gas and a large range
of fuels (fuel flexibility), without combustion as intermediate
step.[1,2] Nevertheless, the massive commercialization of SOFCs
is blocked mainly by cost and durability issues, such as stability
at high temperature (600 – 1000 °C), compatibility with the other
components of the cell, electrochemical activity at low
temperature and direct operation with hydrocarbon fuels without
coking or poisoning (catalytic behavior).[3,4] In particular, the
conventional cermet based on nickel and yttria-stabilized zirconia
(Ni/YSZ) still presents many issues as SOFC anode material.[5,6]
Therefore, one of the current interests in this field is the search for
new materials that can solve these problems in the cells and allow
its proper operation.
One of the most promising alternatives is the development of
Mixed Ionic and Electronic Conducting (MIEC) anodes. Apart
from the perovskite mixed oxides, the Ruddlesden-Popper (RP)
phases have demonstrated such kind of behavior, and generated
great expectation, historically as SOFC cathodes.[7,8] Due to their
varied and exceptional transport and structural properties,
together with their high thermal and mechanical stabilities, those
materials are considered as an interesting option in a variety of
catalytic and electrocatalytic processes like those occurring at
both electrodes of an SOFC.[9–18]
RP phases are materials whose structure results from the
intergrowth of one or several perovskite-type blocks (ABO3)
separated by one rock-salt structure layer (AO). Their general
formula is An+1BnO3n+1 or (AO)(ABO3)n, where A represents alkali,
alkaline earth, or rare earth metal cations located in the perovskite
and rock salt slabs, B refers to transition metal cations occupying
the anionic octahedral coordination of the perovskite block, and n
is the number of octahedra's layers in the perovskite block.[19]
Their possible use as anode material has been only recently
described and studied[20,21] and the resulting manganite family
could be a viable option. Several compositions have shown an
exceptional potential as an electrode (both anode and cathode) in
symmetrical SOFCs due to their stability in oxidizing and reducing
atmospheres, thermo-mechanical compatibility with common
electrolytes e.g. gadolinium-doped ceria (CGO), and acceptable
electrochemical properties, high temperature resistance, redox
stability and favorable total conductivity.[22,23] Notwithstanding,
some authors suggest that some compositions, e.g.
La0.6Sr1.4MnO4, are poor catalysts for direct use of methane.[24]
Introduction
Given the current environmental problems and the global
energetic context, it is necessary to develop and implement new,
clean, safe and reliable energetic alternatives, which ensure a
high and sustainable quality of life and, at the same time, avoid a
possible crisis. Presently, the Solid Oxide Fuel Cells (SOFCs) are
studied as potential devices to produce electricity and heat in a
clean and efficient way (chemical to electrical yield > 60 %), better
than conventional thermomechanical processes, converting
[a]
S. Vecino-Mantilla, 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: gilgau@uis.edu.co
[b]
[c]
S. Vecino-Mantilla, Dr. J.M. Serra
Instituto de Tecnología Química (Universitat Politècnica de València
– Consejo Superior de Investigaciones Científicas), Avd. de los
Naranjos s/n, 46022 Valencia, Spain.
M. Huvé, Dr. P. Roussel
Université de Lille, CNRS, Centrale Lille, ENSCL, Université
d´Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du
Solide, F-59000 Lille, France
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201901002
ChemCatChem
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[39]
It is however well known that electrochemical characteristics
and catalytic behavior of a mixed oxide anode can be improved
by the addition of small metallic nanoparticles (Ni, Ru, Rh, others)
on the surface of the MIEC using, for example, the impregnation
technique. Nevertheless, the use of impregnated material
followed by a heat-treatment at high temperature (> 700 °C) does
not guarantee a homogeneous distribution, size and a strong
metal-support interaction (nanoparticles anchorage)[25,26] to avoid
sintering problems in operando. As an alternative method to
create heterogeneous surface systems, it is possible to dissolve
active metals in their oxidized form within the electrode material
phase during the synthesis step. Then, under reducing
atmosphere and high temperature (driven force), nanosized
metallic particles can precipitate from the bulk to the surface. This
phenomenon is called in situ growth or exsolution[8,25,27–29] and, as
is previously presented, it could be considered as the basis for the
design and development of more sophisticated oxide materials
with advanced functionality.[30] Even though, such mechanism has
been applied mainly for perovskite structural oxides[31–34] and until
now few examples of exsolution in RP compounds have been
reported[27,29]
La1.5Sr1.5Mn1.25Ni0.75O6.67 synthesized at a higher temperature
(1450 ºC). In fact, the small differences are especially associated
with the stoichiometric La/Sr and/or Mn/Ni ratio, the synthesis
method, or the sintering temperature. As often the case of
transition metal oxides, the substitution at the A or B-sites of the
structure is correlated with
a
mechanism of charges
compensation, which implies changes in the unit cell size. Finally,
this material was analyzed by X-ray fluorescence (XRF) with a
result of 0.057 ± 0.002 weight fraction, confirming the adequate
stoichiometric Ni amount in the solid-state solution (see
Supplementary Information; Figure S2).
Experimental
Calculated
Difference
This study describes the preparation of a new RP material,
La1.5Sr1.5Mn1.5Ni0.5O7 (LSMN n= 2), which has been designed
using a retrosynthetic approach similar to organic chemistry,[35,36]
in order to be able to form, in reducing conditions, a biphasic
material made of electrocatalytic active metallic nanoparticles on
a n= 1 RP manganite of the type LaxSr2-xMnO4. The synthesis,
proof of exsolution and its catalytic behavior for the methane
steam reforming reaction using Colombian natural gas
composition in low water content is presented.
10
20
30
40
50
60
70
80
90
2 / º
Figure 1. LSMN n= 2 Rietveld refinement.
100
Results and Discussion
4+
3+
476 °C
Mn /Mn
99
98
97
96
LSMN n= 2 study
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 (No. 139). A total of 24 sets
of ~3 g of corresponding material, were synthesized as described
in the Experimental Section. For each synthesis, the lattice
parameters (a=b, c) and the unit-cell volume were determined
from the 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 40 g of pure phase
material (see Supplementary Information; Figure S1 and Table
S1).
2+
Ni /Ni
811 °C
0
100 200 300 400 500 600 700 800 900 1000
Temperature / °C
Figure 2. LSMN n= 2 thermogravimetric analysis in reducing atmosphere.
The structure of the as-obtained LSMN n= 2 powder was
successfully refined as a single phase using Rietveld method
based on X-ray diffraction data and pseudo-Voigt peak shape
Thermogravimetric analysis (TGA) performed from RT until
1000 °C in diluted H2 is presented in Figure 2. Between room
temperature and aprox. 850 °C, two weight losses are evidenced
in distinct temperature range, that must be attributed to the
reduction of Mn and/or Ni. In similar compositions of perovskite
structure type, stabilized in addition by Cr at the B site, Ni2+ and
mixed Mn3+/4+ oxidation states have been clearly established.[40]
We assume the same kind of defect equilibrium is also present in
function with reasonable reliability factors of Rp= 4.35 %, Rwp
=
5.77 %, and χ2= 1.51 and very few residual intensity, as observed
graphically in Figure 1, suggesting an effective structural analysis.
The refined lattice parameters are shown in Table 1 and those
results agree well with those reported for parent compounds such
as La1.4Sr1.6Mn2O7 obtained at 1450 °C,[37] LaSr2Mn1.6Ni0.4O7
prepared at 1500 °C[38] and, to
a
greater extent,
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the LSMN n=2 material. Indeed, the first weight loss is observed
between around 400 and 600 °C; this is the same temperature
range for which Mn4+ is reduced to exactly Mn3+ in the
analysis.[6,41–43] If such weight loss assignment is correct, Ni
exsolution should be evidenced for temperature treatment in
reducing conditions between T=750 °C and T=850 °C. According
to these results, the operating temperatures (T) for reduction (and
subsequent catalytic tests) were selected as T= 750, 800 and
850 °C, together with the following reduction times: tr= 1, 4, 8, 16,
24 and 48 h, as described in the Experimental Section.
(La,Sr)2MnO4+ series synthesized in air and treated in the same
[22]
conditions as the present work.
As a consequence, the first
weight loss can be assigned to the reduction of Mn4+ to Mn3+, with
consecutive formation of oxygen vacancies, in agreement with the
material charge balance. The second weight loss in reducing
atmosphere is observed between 750 and 850 °C and is
associated to the reduction of Ni2+ to Ni0. Such attribution is based
on the fact that the reduction of Mn3+ to Mn2+ in (La,Sr)2MnO4+
Exsolution study
The XRD patterns of LSMN n= 2 after reduction at 750 °C in 3
mol% H2/N2 are shown in Figure 3 as a function of the reduction
time tr. By heating LSMN n= 2 oxide in reducing atmosphere at
750 °C and short reduction times, a mixture of phases is obtained.
During the first hour the LSMN n= 2 phase changes and, although
it is still visible in a large proportion, the formation of a new phase
in a smaller quantity is also observed. At 4 h of reduction, it is
evident that ratio is slowly reversed, decreasing the amount of the
RP n= 2 phase. However, after 8 h of treatment, the sample
exhibits two well-defined phases, completely different than the
starting one, which can be indexed as a reduced Ruddlesden-
Popper phase of the LaSrMnO4 type (hereafter referred to as LSM
n= 1) and metallic Ni (the characteristic peaks of Ni phase at
around 44.5 and 51.9°, as shown as red dots in Figure 3).[44]
The formation of LSM n= 1 seems reasonable since the
synthesis conditions of LaxSr2-xMnO4 (x ≥ 1) materials have been
reported under reducing conditions.[45–47] Thus, the reduction
products can be proposed according to the Eq. (1).
parent compounds has been observed only for temperatures
[22]
higher than 900 °C in similar conditions.
Indeed, this second
weight loss measured between 730 and 860 °C corresponds to
m/m=-1.43%. This is in very close agreement with the theoretical
weight loss (m/m=-1.42%) calculated from the following
equation, considering a fixed 3+ oxidation state for Mn [Eq. (1)]:
2+
La1.5Sr1.5Mn1.5푁푖0.5
O
6.5
+ H2 → 1.5 LaSr푀푛O4 + 0.5 Ni + H2O
(1)
It is worth noting that the reduction to metal of Ni cations only is
confirmed by thermodynamic calculations for the reduction of
binary metal oxides of corresponding constituting elements (see
details in Supplementary Information; Figure S3).
Table 1. LSMN n= 2 structure parameters calculated by Rietveld refinement
using XRD data

Ni
La1.5Sr1.5Mn1.5Ni0.5O7±δ (LSMN n= 2)
This study
LaSrMnO4
Rp [%]
Rwp[%]
χ2
3.80
4.92
750 °C 48 h




750 °C 24 h
750 °C 16 h
1.29




a [Å]
3.8546(2)
20.1911(14)
300.01(3)
750 °C 8 h
750 °C 4 h
c [Å]
750 °C 1 h
V [Å3]
La1.5Sr1.5Mn1.5Ni0.5O7
La1.4Sr1.6Mn2O7
LaSr2Mn1.6Ni0.4O7
La1.5Sr1.5Mn1.25Ni0.75O6.67
[37]
[38]
[39]
10
20
30
40
50
2 / °
60
70
80
90
Rp
Rwp
9.53
12.36
[N.R.]
[N.R.]
10.6
12.9
Figure 3. XRD patterns of LSMN n= 2 reduced at 750 °C during different times
(tr)
χ2
1.4
[N.R.]
[N.R.]
a [Å]
c [Å]
V [Å3]
3.8686(6)
20.238(4)
302.88(3)
3.8502(1)
20.1134(6)
298.17(2)
3.847(2)
20.137(1)
298.02
The reduced samples in which the two LSM n= 1 and Ni phases
are clearly evidenced (tr from 8 to 48 h at 750 °C) are apparently
identical, indicating the process and material stability upon
reduction. A complete quantitative Rietveld refinement using XRD
data was carried out considering a K2NiF4-type structure with a
tetragonal unit cell and I4/mmm space group for the LSM n= 1
structure and FCC cubic cell with Fm-3m space group for Ni. The
results of lattice parameters of both phases and their weight
fraction are shown in Figure 4 (A–E). The refinements present
correct reliability factors, Rp= 4.26-5.12 %, Rwp= 5.34-6.67 % and
“goodness of fit (GOF)” or χ2= 1.28-1.93 (value between 1.0 and
2.9 is generally considered satisfactory),[48] confirmed by the
[N.R.]: Not-reported
As expected, metal oxides such as La2O3, SrO and MnO show
positive Gibbs free energy (ΔG) in the selected temperature
range; consequently, and in agreement with chemical knowledge
of the corresponding elements, these oxides are very difficult to
reduce using H2. On the other hand, the three other manganese
oxides i.e. Mn3O4, Mn2O3 and MnO2, as well as NiO shall be the
only easily reducible oxides, because of their calculated negative
ΔG, what is in concordance with the thermogravimetric
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graphical analysis of each refinement. Very slight evolution of the
lattice parameters can be evidenced along with the reduction
duration with an apparent a decrease while c parameter increases
for the LSM n= 1 up to tr= 24 h. The phenomenon seems to
stabilize for larger reduction times. Nonetheless, neither the
refined Ni lattice parameter nor the Ni weight fraction exhibits any
change along with the reduction duration, what means the cell
parameter changes for the n= 1 Ruddlesden-Popper phase is
possibly related to the presence of defects formed during Ni
exsolution that disappear within the crystal structure or the further
slow reduction of the same manganite with time. It is worth
nothing that for tr= 24 h the refined values for the LSM n= 1 lattice
parameters are in a good correspondence with the calculated for
the same material synthesized by other authors.[49–52]
visible as hemispherical nanoparticles on the surface of the
manganite (Figure 4 F).
The XRD patterns of LSMN n= 2 after reduction in 3 mol% H2/N2
at 800 and 850 °C using the same values of tr are shown in
Supplementary Figures S4 and S5, respectively. In these cases,
the intermediate mixture of phases is only visible for 1 h of
reduction and, for tr= 4 h, the LSM n= 1 and Ni phases are already
formed with an increasing crystallinity when T and/or tr raises.
This result can be associated to faster kinetics of exsolution at
higher temperature due to the fact that such kind of process
requires solid state diffusion within the crystal framework.[54]
The same conclusions can be obtained as in the previous case
(T= 750 °C), i.e. two phases are observed for reduction times
between 4 and 48 h, which are almost identical along the series,
except the slight change in the lattice parameters for the LSM n=
1 phase, already described in the low temperature exsolution
conditions. For all reduction times, no difference can be
evidenced for metallic Ni lattice parameter and weight fraction, as
shown in Supplementary Figures S6 (A–E) and S7 (A–E),
suggesting the complete Ni reduction and exsolution from the
crystal structure and LSM n= 1 formation, as can be also
confirmed in SEM results (Figure S6 F and Figure S7 F). Globally,
in any reducing conditions (t ≥ 4 h), all the lattice parameters (a, c
and cell volume) for LSM n= 1 phase, calculated by Rietveld
refinement, remain coherent with the values reported by other
authors;[49–52,55–59] the same can be concluded for Ni a lattice
parameter and weight fraction.
B
A
C
E
3.806
3.804
3.802
3.800
3.798
3.796
13.111
13.104
13.097
13.090
13.083
13.076
13.069


.
.


.
.
8 h
16 h
24 h
48 h
8 h
16 h
24 h
48 h
D
Reported value a
189.54
189.41
189.28
189.15
189.02
188.89
188.76
3.5441
3.5358
3.5275
3.5192
3.5109
3.5026




TEM and STEM images of the exsolved nanoparticles and its
perovskite matrix associated to energy dispersive spectroscopy
(EDS) mapping of the main elements for a characteristic zone of
LSMN n= 2 powder after reduction at 850 ºC for 4 h are shown in
Figure 5. The elemental mapping (Figure 5 C) confirm the grain
composition, i.e. La, Sr, and Mn are homogeneously distributed
in the entire grain and few tenths of nanometer-sized highly
concentered spots (in red) confirm the chemical nature of the
nanoparticles, which consists in pure and dense metallic Ni. Only
trace of Ni is detected in the area without Ni segregation, which
suggest that most of the Ni is exsolved on the surface. The
complete morphology of the reduced sample can be observed in
Figure 5 D: the LSM n= 1 surface is decorated with some
anchored small and uniformly distributed crystallized Ni
hemispherical nanoparticles with strong particle - oxide matrix
interaction.[54,60,61] The spacing between the lattice fringe are
consistent with the (111) interreticular distance of Ni and the [001]
zone axis of the perovskite. Such kind of morphological results
(nanoparticles embedded on the surface) are similar and
coherent with those reported for Ni exsolution from perovskites
8 h
16 h
24 h
48 h
8 h
16 h
24 h
48 h
F
0.100
0.075
0.050
0.025
0.000
8 h
16 h
24 h
48 h
Ni
LSM n= 1
Figure 4. XRD Rietveld refinement of LSMN n= 2 reduced at 750 °C during 8,
16, 24 and 48 h. A) a parameter of reduced LaSrMnO4. B) c parameter of
reduced LaSrMnO4. C) Vol. of reduced LaSrMnO4 D) a parameter of Ni
compared with the reported one E) Weight fraction of Ni F) SEM image of LSMN
n= 2 reduced at 750 °C and 4 h.
The refined a lattice parameter calculated for metallic nickel in
all four studied points match very well with the reported values (a=
3.5236(3) Å),[53] confirming that exsolved nanoparticles are
probably pure Ni, what makes sense on a chemical point of view.
In addition, the refined weight fraction of 0.050 for metallic Ni,
identical for all studied points at the precision level of the
refinement (at 3 times the standard deviation), is in a very good
agreement with the maximum stoichiometric and theoretical
values calculated using the above-mentioned Eq. (2) (0.946 LSM
n= 1 and 0.054 Ni) as well in accordance with the amount of
metallic Ni in LSMN n= 2, i.e. suggesting that all Ni2+ was
completely reduced and exsolved from the crystal structure and
[33]
structures such as La0.9Mn0.8Ni0.2O3,[62] La0.4Sr0.4Sc0.9Ni0.1O3
and La0.5Sr0.5Ti0.75Ni0.25O3.[32]
If it has been established that the particle size distribution and
average particle size values can be associated to many extrinsic
(oxygen partial pressure PO of the reduction gas mixture, etc) or
2
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A)
B)
C)
D1)
D2)
10 nm
D)
Figure 5. LSMN n= 2 powder reduced at 850 °C and 4 h A) TEM image B) HAADF images showing nanoparticles on the surface C) Corresponding EDS mapping
identify them as Ni D) HR TEM image of the exsolved nanoparticle and its perovskite matrix. The lattice fringe spacings are D1) close to 2 Å which correspond to
d111 of Ni and D2) close to 3.8 Å x 3.8 Å which match well with the [001] zone axis of a RP manganite.
intrinsic parameters (surface features,[63] mechanical stresses
2
and strains,[64] wetting angles,[65] defects such as vacancies and
w
dislocations[66,67]), affecting the formation (nucleation) and growth
of the Ni particles,[33] it was considered in this study only the main
effects of reduction temperature (T) and time (tr).
2
̅
Dp=D0e
(3)
(4)
2
2w2
The influence of T and tr on the Ni particle size was examined
√
e
σln=D0
-ew
using
a precise image analysis of the scanning electron
micrographs (FE-SEM) based on about 100 particles to obtain a
characteristic frequency histogram in each selected (T, tr)
conditions. The grouped particle size data were analyzed using
as likelihood fitting method a lognormal distribution [Eq. 2] as
reported in the literature[68–70] and treated in OriginPro8® with the
iteration algorithm named Orthogonal Distance Regression.
The results of the 14 lognormal distributions are shown in
Supplementary Figure S8 and they were used to determine the
statistical significance of the different studied variables (T and tr)
̅
on the average Ni particle size (Dp). A statistical ANOVA analysis
was performed through the Generalized Linear Models available
in the STATGRAPHICS Centurion XVII package with
a
2
D
ln(
)
⁄
D
2
0
confidence level of 95 % and a Square Sum Type III analysis,
allowing the measurement of the contribution of each variable.
The results are presented in Table 2, showing that p-value was
lower than 0.05 for both cases, which means that tr and T have a
statistically significant influence on the average Ni particle size in
perfect agreement with Gao et al.[33], Lai et al.[54] and Oh et al.[64]
-
A
2w
(
)
f D = (2π)0.5wD e
(2)
In this case, A is the area of the size distribution, w the scale
parameter defining the width of the size distribution (multiplicative
standard deviation) and D0 the median diameter. The average
̅
The Dp results obtained from the distributions were used to
̅
particle size (Dp) of the lognormal distribution and its standard
generate the contour plot by a physical interpolating method (Thin
Plate Spline, TPS). Figure 6 graphically shows the T and tr effect
deviation (σln) were determined using the following equations:
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on the average Ni particle size and confirms the results obtained
with ANOVA (Table 2), in which both variables have a high
influence on the particle size. Nevertheless, it is worth noting that
the temperature effect is apparently stronger than the reduction
time, i.e. high temperatures induce the rapid exsolution of Ni
particles with a wide distribution. During the exsolution process,
the self-grown nanoparticles are “anchored” and highly dispersed
on the oxide surface, displaying a higher tendency to avoid their
agglomeration and coarsening during the reduction/reaction
steps; the resulting material consists in an excellent option to
improve the catalytic behavior, prevent the sulfur poisoning and
carbon formation when hydrocarbons are used as fuels.[71]
Particle size / nm
60.00
48
44
40
36
32
28
24
20
16
12
8
56.00
52.00
48.00
44.00
40.00
36.00
32.00
28.00
24.00
̅
Table 2. Temperature and reduction time effect on 푫풑 (ANOVA result)
20.00
850
4
750
775
800
825
Sum of
squares
724.435
Mean
Variable
D.F.[*]
F-test
p-value
square
724.435
520.277
21.0855
Temperature / °C
Temperature (T)
1
1
11
13
34.36
24.67
0.0001
0.0004
Reduction time (tr) 520.277
Figure 6. Influence of the reduction temperature (T) and time (tr) on the Ni
Residual
Total (Corrected)
231.94
1366.44
exsolved particle size.
[*] D.F.: Degree of freedom
Despite the possible presence of Ni particle sintering, the
average particle size values and distributions obtained in the
present study are similar (< 100 nm) to what has been described
by other authors using exsolution of Ni or other metallic elements
from an oxide matrix.[5,33,54,67,71,79,80]
The variation in the particle size and its slow increase with time
at any studied reduction temperature could be associated with the
fact that the metal exsolution equilibria has not been reached.
Nevertheless, such hypothesis must be discarded, as it was
already demonstrated with the complete Ni exsolution according
to Eq. 1.
Preliminary catalytic study
Preliminary catalytic properties of the material were studied in
only one selected operating condition for exsolution e.g.: T=
850 °C and tr= 16 h. The reaction was performed at the same
temperature as for the reduction, T= 850 °C, using only small
amount of water for the methane reforming, i.e. a low steam to
carbon ratio (S/C) (GIR concept).[81] According to the literature,
S/C ratios higher than 1 are necessary to avoid the coke
formation/deposition on the anode surface. Nevertheless, such
high S/C ratios dilute the fuel content and may lead to
thermomechanical damages due to large temperature gradients
in the anode side (reforming reaction strongly endothermic, while
electrochemical reactions are exothermic) and the requirements
to produce steam in excess and condense the unreacted products
are energetically unfavorable. Therefore, with low S/C ratios, the
process needs a small amount of steam at the inlet and the
excess is produced in situ by the electrochemical oxidation of the
hydrogen obtained during the steam reforming reaction in the
anode side;[81,82] this requires the development of specific
materials.
During 8 h of reaction, CH4 molar conversion was calculated
(Figure 7 A) and the outlet gas compositions (H2, CO, and CO2)
was analyzed (Figure 7 B). In those figures, two main zones are
clearly evidenced with a first region in which a low conversion and
low products composition are measured but gradually increase
until they reach a second region of stable behavior. The latter,
which corresponds to steady-state operation and starts after
around 200 min, was used to determine the average values of
conversion, gas compositions and selectivity, in a similar way as
presented by other authors.[83]
Therefore, two possible mechanisms are possible to explain the
evolution of particles size due to sintering: particle migration
followed by coalescence or Ostwald-ripening.[72] The first
hypothesis is coalescence that only occurs between very close
particles,[54,60,61] a process strongly T dependent[72–74] mainly when
the Tamman temperature is exceeded (0.5Tmelting [K], Ni= 864
K).[75] When the nanoparticle-matrix oxide interaction is poor, the
particle exhibits a spherical shape, generally observed e.g. using
a typical metallic impregnation method. This case is more
propitious to particle/crystallite migration and coalescence.
However, it has been demonstrated that a strong interaction
between the particles and the matrix turns the particle shape to
hemispherical,[54,60,61] explained, in this case, by the fact that the
exsolved phase is embedded into the oxide matrix.[64,71] Such
characteristic makes them less mobile on the surface and the
coalescence rate is considerably decreased, occurring only when
the particles are very close to each other. This is the case of
exsolved nanoparticles, as mentioned before and therefore
cannot be the dominant mechanism.[54,76,77] As a consequence,
the only mechanism that could explain the observed increase of
nanoparticle size is probably Ostwald-ripening process. This
mechanism describes the growth of
a larger particle by
consuming a smaller one without direct connection, here, clusters
of atoms from a small particle migrate through the oxide surface
and merge into another large particles just to reach the
equilibrium; this effect becomes much serious when the
temperature increases.[76,78] As indeed for the coalescence
process, the driving force is the minimization of the total surface
energy of the system.
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10.1002/cctc.201901002
ChemCatChem
FULL PAPER
and Ni/YSZ) because they are on the thermodynamic equilibrium.
However, this preliminary result shows the high catalytic potential
of the exsolved LSMN n= 2 for the steam reforming reaction in the
selected operating conditions and therefore to be considered as
anode materials for SOFC; in a forthcoming study, the influence
of the exsolution conditions on the catalytic behavior and stability
over longer times (coke formation/deposition) will be presented.
A
24
21
18
15
12
9
LSM n=1 + Ni
Comercial Ni/YSZ
Equilibrium
La0.5Sr1.5MnO4
Table 3. Catalytic results of exsolved LSMN n = 2
S
SCO
H2 formation rate
[mol min-1 g-1]
CH4 conversion
rate [mol min-1 g-1]
H2/CO
ratio
CO2
16.7x10-3
±
6.15x10-3
±
0.03 ±
5x10-3
0.97 ±
5 x10-3
2.80 ±
0.03
6
1.45x10-3
0.42x10-3
3
CO+H2O⟷CO2+H2 ΔH°=-41.2 kJ mol-1
CH4+H2O⟷CO+3H2 ΔH°=206.1 kJ mol-1
(5)
0
0
100
200
300
400
500
Time / min
(6)
B
35
30
25
20
15
10
5
H2
H2 equil.
In addition, the presented results, together with the low CO2
production (0.26 mol%) and the calculated H2/CO ratio (2.80)
measured in the effluent stream suggest a practically null
contribution of Water-Gas Shift reaction [Eq. (5)][88] and the total
selectivity towards the steam reforming reaction [Eq. (6)][88] using
the exsolved material as catalyst (see Table 3). The catalytic
behavior was maintained for more than 4 h, with a stable CH4
conversion and H2 formation rate, a much better behavior as other
catalysts studied in similar S/C and temperature conditions, e.g.
Ce0.9Gd0.1O2-x (4.74x10-5 mol min-1 g-1)[89] or Ir/Ce0.9Gd0.1O1.95
(4.31 x10-4 mol min-1 g-1)[90] (Table 3). This apparent stability could
CO
CO2
CO equil.
CO2 equil.
be
a
sign of
a
potentially good resistance to coke
formation/deposition in severe operating conditions (S/C= 0.15),
which could be result of the fine metal dispersion and the strong
basicity of the La/Sr-containing support grant less sensitivity to
coking.[32,91]. Future studies with longer reaction time will be
carried out to confirm such hypothesis.
0
0
100
200
300
400
500
Time / min
Figure 7. Catalytic behavior of LSMN n= 2 reduced at T=850 °C for 16 h A)
CH4 molar conversion at 850 °C (theoretical equilibrium as dashed line),
compared with La0.5Sr1.5MnO4 B) H2, CO and CO2 molar composition in
comparison to theoretical equilibrium (dashed lines).
Conclusions
The direct influence of the Ni particles on the catalytic behavior
is evident by the comparison with a pure LSM material (similar in
composition to the support of the exsolved material), the latter
having very poor catalytic activity. The resulting LSM n= 1 + Ni
exhibits an exceptional activity: 24.19 mol% H2, 0.26 mol % CO2,
8.65 mol% CO corresponding to a stable CH4 conversion (14.60
mol%). The obtained values are very close to the maximum CH4
conversion, H2, CO2 and CO content that should be obtained with
the selected operating conditions (equilibrium) or those obtained
using Ni/YSZ material (Figure 7 A). The latter, comparable only
from a purely catalytic behavior point of view and during few hours,
because in previous works have been demonstrated that the
major difficulty of a direct natural gas (methane) operation over
Ni/YSZ is the possible formation/deposition of carbon species on
the Ni surface due to cracking of hydrocarbons resulting in the
loss of cell performance and low durability.[84–87]
In conclusion, the new material LSMN with n= 2 Ruddlesden-
Popper structure was successfully synthesized by Pechini
method as a single phase in air. In reducing atmosphere (H2), at
high temperature (750, 800 and 850°C) and different reduction
times (4, 8 16, 24 and 48 h), an exsolution reaction occurs,
resulting in the formation of LSM n= 1 together with embedded
metallic Ni nanoparticles without any other impurity, as
demonstrated by XRD analysis, TEM, and SEM, and with a direct
influence on the particle size distributions and their average
values. According to the preliminary test, and based on the
catalytic behavior, the exsolved LSMN n= 2 material appears to
be an interesting option to be considered as anode materials for
SOFC fed with natural gas (methane) in low steam conditions,
due to its high CH4 conversion, H2 production, selectivity to steam
reforming reaction and a constant behavior once the steady state
is reached. These catalytic properties could be an important proof
to believe in the resistance of the new materials to coke
Notwithstanding these results, it is not possible a complete and
direct comparison between both materials reduced LSMN n= 2
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901002
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FULL PAPER
Experimental set up: Steam reforming reaction with low steam content was
studied on the LSMN n= 2 powder for selected reduction conditions to
explore its potential as catalyst for this reaction. In further research this
topic will be studied into details. The measurements of catalytic behavior
for the reforming of methane were carried out in an experimental set-up. It
is divided into 4 main sections: the first one is the gases zone in which N2
(grade 5.0, CRYOGAS), 3 mol% H2/N2 mixture (Cryogas) and CH4 (grade
4.0, Cryogas) cylinders are available. In addition, this zone presents two
analog manometers (Bourdon-Haenni® 0-100 psi and Ashcroft® 0-100
psi), two Cole-Parmer® 150 mm correlated and calibrated flowmeters with
high-resolution valves and a Cole-Parmer® digital mass flowmeter (0-256
mL (STP) min-1) for the precise control flow that passes to the next zones.
The second zone is the gases saturation zone (humidification), which
consists of a stainless-steel bubbler with a capacity of 300 mL, equipped
with a type J immersion thermocouple and a clamp-on heater connected
to its own control system. Additionally, a 50 cm heating cord, also with its
own control system, wraps the bubbler outlet line to prevent the steam
condensation before entering the reactor. The third zone is the
reduction/reaction zone integrated by a tubular quartz reactor (øI: 9 mm,
øE: 12 mm and L: 300 mm) heated by a tubular furnace (CARBOLITE MTF
10/15/130) and a cold trap to condense the steam excess after the reaction
and before entering the gas chromatograph. According to the need
(reduction step, reaction step or reagents analysis step) the bypass, line A
or B will be used. Finally, the last zone is the analysis one, integrated by a
gas chromatograph (GC, SRI instruments 8610C) using He (grade 5.0,
CRYOGAS) as mobile phase, equipped with two packed columns
(molecular sieve 13X 6 in and hayesep D 6 in), a thermal conductivity
detector (TCD) and controlled by PeakSimple 4.44 free software.
formation/deposition, but this is only a hypothesis for which a
deeper and adequate study, is now required to support such
conclusion.
Experimental Section
Synthesis
Powder of La1.5Sr1.5Mn1.5Ni0.5O7±δ composition (hereafter referred to as
LSMN n= 2) powder was synthesized by a Pechini o citrate complexation
route[92] using stoichiometric amounts of La2O3 (≥ 99.9 % Alfa Aesar),
SrCO3 (≥ 99.9 % Sigma Aldrich), MnCO3 (≥ 99.9 % Sigma Aldrich) and
NiCO3 (≥ 99.9 % Alfa Aesar). Before using La2O3 and SrCO3, they were
calcined in air at 1000 and 500 °C, respectively, for 1 hour. Such pre-
treatments were performed to remove hydration products (and carbonation
in the case of lanthanum oxide) in order to facilitate the weighing of the
precursors in correct proportions. The precursors were initially dissolved
in a solution made of concentrated nitric acid (HNO3, ≥65 % 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 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 subsequently sintered in air at 1000 °C (6 h), 1100 °C (6
h) and 1300 °C (12 h), with intermediate grinding and pelletizing steps. In
order to assure a good homogeneity in the particle size of LSMN n= 2
material, the powder was ball-milled in an acetone:powder:3 mm-sized
zirconia balls dispersion (5:1:5 weight ratio) during 12 h at a speed of 50
rpm. After milling step, the balls were separated, and the resulting mixture
was dried. Homogenous powders with particle size distribution between
125 and 200 m, were re-obtained by sieving through steel sieves of the
referred mesh size.
Operating conditions: Around 100 mg of catalyst (LSMN n=2) was diluted
in SiC (SiC:LSMN n= 2 10:1 weight ratio) and introduced in the reactor as
fixed-bed between catalyst-free SiC and two pieces of quartz wool. Prior
to the behavior test, the catalyst sample was reduced in situ (55 mL (STP)
min-1 of 3 mol% H2/N2 mixture, Cryogas) according to the results obtained
in the reduction study. The catalytic behavior was measured during 8 h at
the same reducing temperature under atmospheric pressure. The reaction
mixture corresponds to 82 mol% CH4 (N2 as balance) humidified in a steam
to carbon ratio (S/C) of 0.15 according to the SOFC anode conditions
suggested by the Gradual Internal Reforming concept or GIR.[81] The
steam content was adjusted by flowing the adequate dry CH4-N2 mixture
throughout the bubbler containing distilled water maintained exactly 46 °C.
The total dry flow rate was 128 mL (STP) min-1 and the system was
operating at a volume hourly space velocity (VHSV) of 86200 mL (STP)
min-1 gcat-1. The composition in each effluent constituent (CH4, CO, H2, CO2,
and N2) was obtained at regular time intervals (each 20 min) using online
GC analysis. The CH4 conversion (XCH4), CH4 conversion rate and H2
formation rate were calculated using Eq.(7), (8) and (9) respectively, where
Characterization
Each synthesized powder (~3 g per synthesis) was characterized by X-ray
diffraction (XRD) at room temperature (RT) using a BrukerD8-ADVANCE
powder diffractometer operated in Bragg-Brentano geometry, equipped
with a Lineal LynxEye detector and a beam of CuKα1,2 radiation (λ=
1.5418 Å). The diffractometer was operated over the angular range 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.[93] The elemental analysis
was confirmed by X-Ray Fluorescence spectroscopy (XRF) in an S2
Ranger Bruker spectrometer equipped with a Pd X-ray tube.
In
Out
CH
n
and n
are the CH4 molar flow rate at the inlet and outlet of the
CH
4
4
reactor (mol min-1), Wcat is the catalyst weight (g), and nHOut is the H2 molar
2
flow rate at the outlet of the reactor (mol min-1). Blank tests were performed
at different temperatures using SiC only, and no conversion was observed
for methane.
Exsolution study
Thermogravimetric measurement in reducing atmosphere was carried out
to understand the LSMN n= 2 reduction behavior in this atmosphere. The
test was performed using a gravimetric analyser Hiden-Isochema model
IGA-003, applying the first cycle in N2 at 5 °C min-1 that aims to suppress
any further influence of moisture and/or adsorbed species and then, then
a second cycle one at 2 °C min-1 in 3 mol% H2/N2; both from RT to 1000 °C.
Reduction study: The operating temperatures (T) for the reduction study
were selected considering the results obtained by TGA. Around 0.5 g of
fresh LSMN n= 2 powder was reduced in a tubular furnace CARBOLITE
CTF 12/65/550 using a gas mixture 3 mol% H2/N2 (Cryogas) with a flow of
55 mL (STP) min-1 and different reduction times (tr= 1, 4, 8, 16, 24 and 48
h). The powder obtained at each temperature-reduction time point was
characterized by XRD at RT as previously described. An additional
microstructural characterization was performed on the reduced powder by
In
Out
-n
CH
n
CH
4
XCH
=
4 ×100 mol%
(7)
In
4
n
CH
4
In
CH
Out
-n
CH
CH4 conversion rate=n
mol min-1g-1
(8)
(9)
4
4
W
cat
Out
2
H2 conversion rate=Wn
mol min-1g-1
H
cat
The CO and CO2 selectivities (SCO and SCO2) were defined as the molar
flow rate ratios of the specified component in the outlet (nCO or nCO2) to the
total produced CO and CO2 molar flow rate (nCO+nCO2) [Eq. (10) and (11)].
The H2 to CO ratio is defined as is shown in Eq.(12).
field-emission scanning electron microscopy (FE-SEM) in
a ZEISS
microscope model ULTRA 55 and transmission electron microscopy in the
TEM FEI TITAN Themis 300 equipped with a Super-X quad EDS for
elemental analysis. The powder was crushed and dropped in the form of
alcohol suspension on carbon supported copper grids followed by
evaporation under ambient condition.
n
CO
(n +n
SCO
SCO
=
dimensionless
dimensionless
(10)
(11)
)
CO CO
2
n
CO
2
=
2
(n +n
CO CO
)
2
Catalytic test
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10.1002/cctc.201901002
ChemCatChem
FULL PAPER
n
H
H
2
2
=
dimensionless
(12)
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The authors acknowledge the financial support of the Colombian
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This article is protected by copyright. All rights reserved.

10.1002/cctc.201901002
ChemCatChem
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An original way to perform the
exsolution of Ni nanoparticles on a
ceramic support is explored for the
development of SOFC anode material.
At high temperature and reducing
atmosphere, the n=2 Ruddlesden-
Popper phase La1.5Sr1.5Mn1.5Ni0.5O7
transforms into the n=1 LaSrMnO4 with
S. Vecino-Mantilla, P. Gauthier-Maradei,
M. Huvé J.M. Serra, P. Roussel, G.
Gauthier*
Exsolution
Page No. – Page No.
Nickel
exsolution-driven
phase
transformation from an n = 2 to an n=
1 Ruddlesden-Popper manganite for
methane steam reforming reaction
La1.5Sr1.5Mn1.5Ni0.5O7
RP n=2
complete
exsolution
of
Ni
nanoparticles, converting the material
into an interesting catalyst for the
methane steam reforming reaction.
LaSrMnO4
+
Ni
RP n=1
This article is protected by copyright. All rights reserved.