Exsolution of Fe-Ni alloy nanoparticles from (La,Sr)(Cr,Fe,Ni)O3 perovskites as potential oxygen transport membrane catalysts for methane reforming

Article abstract of DOI:10.1039/c9ta03711j

(La_0.75Sr_0.25)(Cr_0.5Fe_0.5-xNi_x)O₃ perovskites were investigated as potential catalysts for oxygen transport membranes for methane reforming applications. XRD was performed to study the structural changes that took place when Fe was partially substituted by Ni, in both oxidising and reducing environments. TGA measurements demonstrated that the oxygen deficiency of these perovskites under reducing conditions was enhanced by increasing the level of Ni doping, due to the reduction of the Fe³⁺/Fe⁴⁺ and Ni²⁺ species to lower cation valences. SEM and TEM-EDX analyses showed that upon reduction exsolution of bimetallic Fe-Ni alloy nanoparticles took place on the surface of the perovskite, which was beneficial for the catalyst's activity. The optimum stoichiometry was the (La_0.75Sr_0.25)(Cr_0.5Fe_0.35Ni_0.15)O₃ perovskite, which during catalytic testing, demonstrated 72% CH₄ conversion, which was 20 times higher than that of the initial perovskite.

Full text of DOI:10.1039/c9ta03711j

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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DOI: 10.1039/C9TA03711J
ARTICLE
Exsolution of Fe-Ni alloy nanoparticles from (La,Sr)(Cr,Fe,Ni)O3
perovskites as potential Oxygen Transport Membrane catalysts
for methane reforming
Received 00th January 20xx,
Accepted 00th January 20xx
Despoina Papargyriou, a David Miller a and John Thomas Sirr Irvine* a
DOI: 10.1039/x0xx00000x
(La0.75Sr0.25)(Cr0.5Fe0.5-xNix)O3 perovskites were investigated as potential catalysts for Oxygen Transport Membranes for
methane reforming applications. XRD was performed to study the structural changes that take place when Fe was partially
substituted by Ni, in both oxidising and reducing environments. TGA measurements demonstrated the oxygen deficiency
of these perovskites in reducing conditions was enhanced by increasing the level of Ni doping, due the reduction of the
Fe3+/Fe4+ and Ni2+ species to lower cation valences. SEM and TEM-EDX analyses showed upon reduction exsolution of
bimetallic Fe-Ni alloy nanoparticles took place on the surface of the perovskite, which was beneficial for the catalyst’s
activity. The optimum stoichiometry was the (La0.75Sr0.25)(Cr0.5Fe0.35Ni0.15)O3 perovskite, which during catalytic testing,
demonstrated 72% CH4 conversion, that was 20 times higher than that of the initial perovskite.
OTM.11–13 This process is similar to the solid oxide fuel cell
operation and with the addition of a metal catalyst,
perovskites are very promising candidate materials as catalysts
Introduction
Oxygen Transport Membranes (OTMs) have been the subject
of research in recent years as they can efficiently separate
oxygen from air and reduce the cost of processes that require
pure oxygen.1–3 The application of OTMs for syngas production
from natural gas and other hydrocarbon sources has attracted
a lot of attention. The use of dense ceramic membranes with
mixed oxygen-ionic and electronic conductivity, makes it
possible to combine oxygen separation from air, partial
oxidation of methane and reforming, in a single reactor.4
for OTMs.10
Here we investigate (La0.75Sr0.25)(Cr0.5Fe0.5)O3 perovskites as
potential materials for the catalytic layer of OTMs. These
materials demonstrate good chemical and mechanical stability
under membrane’s operating conditions, compatibility with
the interconnect materials of the membrane, high electronic
and ionic conductivity and good catalytic activity for methane
reforming reactions.14,15
To further improve the catalytic activity of these perovskites
and consequently the OTM’s performance, the incorporation
of metal nanoparticles is investigated.16 Generally, several
deposition techniques such as physical vapor deposition and
chemical impregnation have been used to form metal
nanoparticles on oxide supports for catalytic applications.17,18
However, these techniques are difficult to perform in the OTM
system, as the catalytic layer is deeply buried in the OTM
structure.19 Moreover, the nanoparticles formed by deposition
techniques often suffer from agglomerations due to weak
interaction with the oxide support.17,20
To overcome these issues, in-situ nanocatalyst exsolution has
emerged recently as an alternative procedure. This method
allows the growth of metal nanoparticles from the perovskite
lattice on its surface through a single reduction step.21 A good
control of the nanoparticle size and distribution20 and the
regeneration of the nanostructure via a redox cycle22,23 can be
achieved by this technique. Moreover, these exsolved
nanoparticles demonstrate very good thermal stability and
good anti-coking behaviour, due to their strong interaction
with the parent perovskite.20 Finally, several studies have
shown the exsolution of metal nanoparticles from the B-site of
One important aspect of membrane material development is
the enhancement of their catalytic performance. For this
purpose, a lot of scientists have investigated the influence of
the addition of a catalyst, as an exchange layer in the fuel side
of the OTMs, in order to improve the surface exchange kinetics
and thus, the oxygen flux. In general, the most commonly used
catalysts for methane reforming in OTMs are the Ni supported
catalysts.5–8 Ni has the ability to easily break the C-H bond and
activate the CH4 molecule for partial oxidation reactions.
Moreover, when applied for OTM processes, Ni helps maintain
the oxygen chemical potential driving force across the
membrane by consuming the oxygen that is transported.9,10
Furthermore, a lot of attention has been drawn to perovskites,
since they can accelerate the oxygen reduction and therefore
improve the incorporation of oxygen ions on the surface of the
a.
School of Chemistry, University of St Andrews, Fife, KY16 9ST, UK
Email: jtsi@st-andrews.ac.uk
Electronic Supplementary Information (ESI) available: [Additional graphs of the
thermodynamic equilibrium and blank test for the catalytic testing, TGA data
during the redox cycle, additional SEM and TEM analyses of the exsolved samples].
See DOI: 10.1039/x0xx00000x
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the perovskites offers better electrocatalytic properties, when
tested in fuel cells, electrolysis cells and for automotive
emission control.24–26
Table 1 Stoichiometry of the synthesised perovskites and their abbreviations
View Article Online
DOI: 10.1039/C9TA03711J
Compound Stoichiometry
Abbreviation
(La0.75Sr0.25)(Cr0.5Fe0.5)O3
(La0.75Sr0.25)(Cr0.5Fe0.47Ni0.03)O3
(La0.75Sr0.25)(Cr0.5Fe0.45Ni0.05)O3
(La0.75Sr0.25)(Cr0.5Fe0.40Ni0.10)O3
(La0.75Sr0.25)(Cr0.5Fe0.35Ni0.15)O3
(La0.75Sr0.25)(Cr0.5Fe0.30Ni0.20)O3
LSCF
In this study,
a series of (La0.75Sr0.25)(Cr0.5Fe0.5-xNix)O3
perovskites is investigated as potential materials for the
catalytic layer of OTMs. The influence of nanoparticles
exsolution from this perovskite on its catalytic activity is
evaluated and the most promising stoichiometry for the
catalytic layer of the OTM is identified.
LSCFNi3
LSCFNi5
LSCFNi10
LSCFNi15
LSCFNi20
Experimental section
Catalytic Testing
Synthesis of the materials
For the catalytic testing of the synthesised materials, a
premixed gas of 12.9% CH4, 25.8% CO2 and 61.3% H2 was used.
This gas was humidified through a water bubbler at room
temperature, to obtain 3% of steam in the gas stream. This gas
composition was selected to replicate the gas feed of an OTM
used in industry. It corresponds to the exhaust of a coal
gasification unit.3 Typically, this exhaust gas is fed to the OTM
to generate syngas from the unreacted hydrocarbons.3
Figure 1 shows the experimental set-up used for testing the
catalytic activity of the synthesised materials. The flow of the
premixed gas was controlled with an Aalborg mass flow
controller that was calibrated for the humidified gas mixture in
the range of 0-50 ml/min. The reactor was placed in a
microfurnace, which had a heating capacity of up to 950 °C. An
internal thermocouple was placed close to the catalyst bed to
monitor the exact temperature of each experiment. The outlet
of the reactor was channelled to a water trap with a peltier
cooler to condense any water from the gas stream. Then, the
outlet gas was fed to a Gas Chromatograph (GC) for analysis.
The GC was equipped with a HP-PLOT carbon plot column and
was used to separate the H2, CO, CO2 and CH4, with He as a
carrier gas.
The (La0.75Sr0.25)(Cr0.5Fe0.5-xNix)O3 perovskites were synthesised
via a modified combustion synthesis with citric acid and
ethylene glycol.27 The precursors used for this synthesis were
La(NO3)3·6H2O, Sr(NO3)2, Cr(NO3)3·9H2O, Fe(NO3)3·9H2O and
Ni(NO3)2·6H2O. These metal nitrates were diluted in distilled
water in appropriate stoichiometric ratios. Citric acid and
ethylene glycol were mixed with the metal nitrate solution
under magnetic stirring at room temperature. Once total
dissolution was achieved, the temperature was increased to
90-100 °C. The solution was stirred, until a swelling gel formed.
Then, the temperature was gradually increased to 300 °C and
the gel combusted in air forming a dark brown powder. The
obtained powders were calcined in air at 1300 °C for 4 h, to
obtain a single-phase perovskite structure. The samples were
reduced in a tube furnace under 5% H2/Ar at 900 °C for 20 h,
to exsolve the metal nanoparticles on the surface of the
perovskites. Table 1 presents the stoichiometry of the
perovskites synthesised in this work and their corresponding
abbreviations.
Characterisation techniques
Room temperature powder X-ray diffraction (XRD) was
performed on a PANalytical Empyrean diffractometer operated
in reflection mode using Cu-Kα1 radiation. The obtained XRD
patterns were analysed with STOE WinXPOW software to
determine the phase purity, crystal structure and cell
parameters of the perovskites after different treatments.
Thermogravimetric analysis (TGA) was conducted using a
Netzsch STA 449C instrument equipped with Proteus thermal
analysis software. The morphology of the samples surface was
analysed using a JEOL JSM-6700 field emission 74 scanning
electron microscope (FEG-SEM). Higher resolution imaging of
the exsolved nanoparticles was conducted with a FEI Titan
Themis Transmission Electron Microscope (TEM) fitted with an
Oxford ISIS EDX system for elemental analysis.
To maintain a reducing environment during heating and
cooling, a 5% H2/Ar gas mixture was used. When the desirable
temperature was achieved, the premixed gas was introduced
into the reactor and the evolution of the reaction products
was monitored by the GC.
The configuration of the fixed-bed reactor used in these
experiments is also illustrated in Figure 1. A tubular quartz
microreactor, with internal diameter of 8 mm, external
diameter of 10 mm and length 30 cm was used. For each
experiment, 300 mg of catalyst was used, and the height of the
catalyst bed was approximately 5 mm. All the samples were
tested as powders and were supported between quartz wool
plugs, with porosity between 5 to 30 μm. The space velocity
used for these experiments was approximately 6000 h-1 and
the temperature was maintained at 900 °C. This temperature
falls within the range of the operating temperature of the
OTMs (900 -1000 °C).
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Results and Discussion
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DOI: 10.1039/C9TA03711J
Crystal structure of the as-prepared and reduced perovskites
As-prepared perovskites. The partial substitution of Fe by Ni
in the (La0.75Sr0.25)(Cr0.5Fe0.5)O3 perovskite was investigated and
the solubility limit of Ni on the B-site of the perovskite was
determined by XRD analysis. The XRD patterns of the
synthesised materials after calcination at 1300 °C for 4 h are
presented in Figure 2. According to the literature, LSCF exhibits
an orthorhombic structure with Pnma space group.15 After
partially substituting Fe with 3-5 mol% of Ni on the B-site of
LSCF, the perovskite retained its orthorhombic structure.
However, when the Ni doping was increased to 10-20 mol%,
the symmetry of the perovskite changed to rhombohedral,
with R-3c space group. Moreover, as shown in Figure 2a, for Ni
doping of 20 mol%, a peak corresponding to unreacted NiO
was observed, indicating that the solubility limit of Ni on the B-
site of LSCF was between 15 and 20 mol%.
Figure 1 Flow diagram of the set up used for the catalytic experiments
The main reactions expected to take place at the OTM’s
operating temperate are methane steam reforming (Eq. 1),
methane dry reforming (Eq. 2) and the reverse water-gas shift
reaction (Eq. 3).
A magnified section of the XRD pattern (32-33 o) is given in
Figure 2b. This shows the (020) and (112) reflections which
correspond to the orthorhombic structure and the (110) and
(104) reflections which correspond to the rhombohedral
structure and illustrates the phase transition between 5 and 10
mol% of Ni doping on LSCF. In addition to that, these
reflections showed a gradual shift to higher 2θ angles, when
the Ni doping was increased. This suggests that the unit cell
volume of the perovskite reduced when Ni partially
substituted Fe.
CΗ4 + H2Ο → CO + 3H2
CΗ4 + CO2 → 2CO + 2H2
CO2 + H2 ↔ CO + H2O
(1)
(2)
(3)
The performance of the catalysts used in this work was
evaluated based on the CH4 and CO2 conversion to gaseous
products such as CO and H2. These conversions were calculated
according to the following equations based on the gas
concentrations from the inlet and outlet of the reactor.
Assuming the oxygen content is maintained, this cell
contraction was attributed to a change in the oxidation states
of Fe in LSCF when it was partially substituted by Ni. More
specifically, in LSCF the oxidation state of Fe is a mixture of
both Fe3+ (0.645Å) and Fe4+ (0.585Å).15,28,29 When Ni2+ (0.69 Å)
was incorporated into the structure, considering its ionic
radius which is larger than that of Fe3+ and Fe4+,29 an increase
in the unit cell volume was expected. However, to maintain
the charge balance, when Ni2+ is incorporated into the
structure, Fe tends to obtain the +4 oxidation state, causing a
decrease in the cell volume. Similar tendencies have been
CH4
4
CCH4
CCO2
=
=
in−CH out ⋅ 100
(4)
CH4in
CO2
in−CO out ⋅ 100
(5)
2
CO2in
The thermodynamic equilibrium for the used gas composition
was calculated with the HSC Chemistry software at 900 °C and
1 atm. According to the thermodynamic equilibrium, full CH4
conversion and 90% CO2 conversion are expected (Figure S1a).
Finally, a blank catalytic test was performed at 900 °C with
6000 h-1 space velocity. When no catalyst was present (Figure
S1b), the CH4 concentration remained constant, indicating that
CH4 did not participate in any reaction. However, the CO2 and
H2 concentrations decreased, while the CO and H2O increased.
This indicated that the reverse water-gas shift reaction took
place in the gas phase to a small extent, according to Eq. 3.
Therefore, to reach the thermodynamic equilibrium, the
addition of a catalyst was necessary.
reported
in
the
literature
before
for
the
(La0.75Sr0.25)(Cr0.5Mn0.5)O3 perovskite by Jardiel et al. The
authors have observed a decrease in the unit cell volume of
the perovskite when increasing the Ni doping, due to changes
in the oxidation state of Mn on the B-site.30
The unit cell parameters of the synthesised LSCF compounds
are presented in Table 2. The pseudocubic cell volume was
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(10D4)OI: 10.1039/C9TA03711J
a)
b)
* XRD holder
NiO reflection
(110)
*
42 43 44 45 46
2θ (ο)
LSCFNi20
LSCFNi15
LSCFNi10
(112)
(020)
LSCFNi5
LSCFNi3
LSCF
10
20
30
40
50
2θ (ο)
60
70
80
90
32
32.4 32.8 33.2
2θ (ο)
Figure 2 a) Room temperature XRD patterns of the LSCFNi perovskites b) Magnified section of the room temperature XRD patterns
Table 2 Unit cell parameters of the as-prepared LSCFNi perovskites
LSCF
LSCFNi3
Pnma
LSCFNi5
Pnma
LSCFNi10
R-3c
LSCFNi15
R-3c
LSCFNi20
R-3c
Space Group
a (Å)
Pnma
5.4925(4)
5.5326(3)
7.7645(6)
5.4890(8)
5.5319(6)
7.7647(12)
5.4866(13)
5.5301(7)
7.7621(14)
5.5196(4)
-
5.5148(3)
-
5.5105(3)
-
b (Å)
c (Å)
13.3600(6)
13.3465(4)
13.3383(6)
Volume (Å3)
Va (Å3)
235.95(2)
58.95
235.77(7)
58.94
235.51(5)
58.88
352.50(5)
58.75
351.53(3)
58.59
350.77(4)
58.46
calculated to compare the change in the unit cell size for the Popper phase (A2BO4) similar to (La,Sr)m+1(Cr,Fe)mO3m+1. This
LSCF with the different Ni doping. This confirms that the unit observed decomposition of the perovskite to Ruddlesden
cell of the perovskite contracted when the Ni doping on the B- Popper phases in reducing environments is not unusual and
site of LSCF increased.
has been reported for complex perovskite compositions with
Reduction of the perovskites. Room temperature XRD double A-site and B-site substitution.31–33 According to the
patterns were collected on the LSCFNi compounds after literature, the coexistence of an ABO3, A2BO4 and A2O3 or BO
reduction in 5% H2/Ar for 20 h, to investigate any changes in phases is thermodynamically favourable.31 In the case of these
the crystal structure of the perovskites after this treatment. perovskites, the XRD patterns showed no indication of any
The post-reduction XRD patterns are presented in Figure 3. other phase, such as A2O3 or BO to balance the formation of
LSCF has previously shown to undergo a phase transition from the A2BO4 phase. However, there is evidence in the literature
an orthorhombic to rhombohedral structure upon reduction.15 that in LSCF the exsolution of Fe can take place.15,34 So, if any
Similar to LSCF, for LSCFNi3 and LSCFNi5 the crystal structure Fe2O3 phase forms, it can be reduced to Fe0. However, this
changed from orthorhombic to rhombohedral with R-3c space would be below the detection limit of the XRD, due to the very
group. However, the LSCF samples with Ni doping greater than low phase fraction.
10 mol%, which exhibited an initial rhombohedral structure, Furthermore, the intensity of the peak corresponding to this
went through a phase transition to a cubic system, with Pm- A2BO4 phase decreased when the Ni doping increased, as
3m space group.
presented in Figure 3. As explained above, when the Ni doping
Although the main perovskite structure was retained after increased on the B-site of LSCF the Fe4+/Fe3+ ratio increased,
reduction, the formation of additional reflections in the XRD since Fe tends to obtain the +4 oxidation state, in order to
patterns were observed. These correspond to a Ruddlesden compensate
the
charge
balance.
Thus,
during
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(110)
▪
a)
b)
▪ A2BO4
30 32 34
2θ(ο)
LSCFNi20
(110,104)
LSCFNi15
LSCFNi10
LSCFNi5
LSCFNi3
LSCF
10
20
30
40
50
2θ(ο)
60
70
80
90
32
32.5
2θ(ο)
33
Figure 3 a) Post-reduction room temperature XRD of the LSCFNi perovskites b) Magnified section of the room temperature XRD patterns
Table 3 Post-reduction unit cell parameters of the LSCFNi perovskites
LSCF
R-3c
LSCFNi3
R-3c
LSCFNi5
R-3c
LSCFNi10
Pm-3m
3.8949(3)
-
LSCFNi15
Pm-3m
3.8909(1)
-
LSCFNi20
Pm-3m
3.8866(1)
-
Space Group
a (Å)
5.5224(17)
13.485(4)
356.16(21)
59.36
5.5178(15)
13.483(4)
355.50(18)
59.25
5.5186(10)
13.483(4)
355.60(8)
59.26
c (Å)
Volume (Å3)
Va (Å3)
59.088(9)
59.09
58.906(3)
58.91
58.712(3)
58.71
reduction, Fe must undergo more transition steps from Fe4+ to between 5 and 10 mol% doping, as explained earlier. In
Fe0. As a result, less Fe2+/Fe0 species will form, and the addition, these reflections show a shift to higher 2θ angles
formation of the A2BO4 phase will be less favourable.
when the Ni doping increased. Therefore, even after reduction
Similar trends, but to the opposite direction, have been the cells with the higher Ni doping exhibited a smaller unit cell.
reported in the literature for the (La0.75Sr0.25)(Cr0.5Mn0.5)O3 The cell parameters of the LSCFNi compounds after reduction
perovskite. More specifically, when La was substituted by Pr are presented in Table 3. The pseudocubic cell volume was
on the A-site, Mn obtained the Mn3+ oxidation state on the B- calculated, to compare the unit cell volume of the samples
site and during reduction the decomposition to A2BO4 phase that exhibited different structures. As with the as-prepared
was more favourable, because the reduction of Mn3+ to Mn2+ samples, the unit cell volume decreased, when the level of Ni
was easier.35
doping increased.
Figure 3b presents the magnified area of the XRD patterns at The XRD analysis of the (La0.75Sr0.25)(Cr0.5Fe0.5-xNix)O3
o
32-33 . The (110,114) reflection of the samples that exhibited compounds, as listed in Table 2 and Table 3, is summarised in
the rhombohedral structure and the (110) reflection of the Figure 4. It illustrates the relationship between the Ni doping
samples with the cubic structure are illustrated. The change in and the pseudocubic cell volume before (black points) and
the peak shape shows the change in the symmetry of the cell after reduction (red points). As explained above, the unit cell
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volume of LSCF decreased, when the Ni doping increased, with
similar trends observed before and after reduction. However,
the relationship between the Ni doping and the pseudocubic
cell volume was not linear. A change in the gradient of the
trend line was observed, which is related to a change in the
cell symmetry, from orthorhombic to rhombohedral for the as-
prepared samples and from rhombohedral to cubic for the
reduced samples. Therefore, by extrapolating the trendlines, it
can be estimated that the change in the cell symmetry
occurred with Ni doping of 7 mol%, approximately. Finally, this
plot shows that for all the samples the cell volume increased
after the reduction treatment. As explained above, during
reduction Cr3+ remained stable and the Fe3+/Fe4+ redox couple
reduced to lower cation valences.31,34 Since the ionic radius of
Fe4+ (0.585Å) is smaller than that of Fe3+(0.645Å) and Fe2+
(0.78Å), the cell expands during reduction.29
100.2
99.8
99.4
99
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DOI: 10.1039/C9TA03711J
a)
LSCF
98.6
98.2
97.8
LSCFNi3
LSCFNi5
LSCFNi10
LSCFNi15
0
100 200 300 400 500 600 700 800 900
Temperature (oC)
550
500
450
400
350
300
0.26
b)
0.24
0.22
0.2
Thermal analysis
Thermogravimetric analysis in reducing environment
.
Thermogravimetric analysis was carried out for the LSCFNi
compounds in 5% H2/Ar up to 900 °C, to investigate the oxygen
deficiency of the perovskites during reduction and the
influence of the Ni doping on it. Figure 5a presents the weight
change of the samples while heating up to 900 °C in 5% H2/Ar.
All the materials showed similar behaviour under reducing
conditions. More specifically, weight loss took place in one
main step between 380-500 °C (Figure S2), which corresponds
to the loss of oxygen from the perovskite due to the reduction
of the material. Based on TGA studies for similar perovskite
structures, it is expected that these are the temperatures
where phase transitions take place during the reduction of the
perovskite.34
0.18
0.16
0.14
0.12
0.1
0
5
10
15
Ni doping (mol %)
Figure 5 a) Thermogravimetric Analysis for the LSCF compounds in 5% H2/Ar b)
Relation between the Ni doping on LSCF and the reduction temperature and
oxygen deficiency
on the B-site of LSCF showed greater mass losses on reduction.
More specifically, LSCF lost 0.96% of its initial weight, which
corresponds to 0.138 oxygen per formula unit. When the Ni
doping increased up to 15 mol%, the perovskite lost 1.71% of
its initial weight, which corresponds to 0.244 oxygen per
formula unit. Additionally, when Ni was incorporated in LSCF
the reduction temperature shifted to lower values, indicating
that the Ni doping in LSCF increased the reducibility of the
perovskite. Thus, the reduction temperature for LSCF was 500
°C and for LSCFNi15 decreased to 380 °C.
According to Figure 5a, the samples with more Ni incorporated
The TGA from this experiment is summarised in Figure 5b. It
illustrates the relationship between the Ni doping in LSCF and
the reduction temperature, as well as the oxygen deficiency of
the perovskite during reduction. Obviously, by increasing the
Ni doping in LSCF the oxygen deficiency of the perovskite
during reduction gradually increased, while the reduction
temperature decreased. However, this relationship was not
linear. Two different regimes were evident, as observed with
the XRD analysis (Figure 4) and can be attributed to the change
in the symmetry of the cell from orthorhombic to
rhombohedral.
According to the literature, when the reduction takes place in
LSCF, Fe3+ and Fe4+ reduce to lower cation valences.15,34 Cr3+ on
the B-site of LSCF is stable during reduction and occupies the
6-fold coordination of the perovskites lattice.15 In this study,
when
Figure 4 Relationship between Ni doping in the LSCFNi perovskites and the
pseudocubic cell volume of the as-prepared (black points) and pre-reduced (red
points) samples
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Ni was incorporated in the B-site of LSCF, Ni2+ can also reduce
to Ni0. The more Ni was incorporated into the lattice the more
oxygen the perovskite lost and at lower temperatures, due to
further reduction of the Ni2+ species. This can explain the
higher oxygen deficiency of the perovskite, when the Ni doping
in LSCF increased.
View Article Online
DOI: 10.1039/C9TA03711J
Assuming the difference in the observed weight loss of the
LSCFNi compounds is attributed to the reduction of the extra
Ni that was doped in the lattice, the amount of Ni that was
reduced from the LSCFNi perovskites can be estimated. More
specifically, LSCF lost 0.138 moles of oxygen, due to the
reduction of Fe3+/Fe4+ to lower cation valences. LSCFNi15 lost
0.244 moles of oxygen due to the reduction of Fe3+/Fe4+ and
the reduction of Ni2+. The difference between these samples
was 0.106 moles of oxygen. This is equivalent to the moles of
Ni that reduced from Ni2+ to Ni0, since every O2- that the
perovskite lost corresponds to one Ni2+ that reduced to Ni0.
Therefore, the moles of Ni2+ reduced to Ni0 can be calculated.
Table 4 summarises the % weight loss, the calculated moles of
oxygen (δ) that each perovskite lost during reduction and the
estimated moles of Ni reduced from each perovskite. As
described in Figure 5, the relationship between the oxygen
deficiency and the Ni doping was not linear, due to the change
in the cell symmetry. Therefore, LSCFNi3 and LSCFNi5, which
exhibited an orthorhombic structure, only 50% of the Ni of the
perovskite was reduced to Ni0. On the other hand, LSCFNi10
and LSCFNi15 that exhibited a rhombohedral structure, 70% of
the perovskite Ni was reduced to Ni0. This indicates that since
the rhombohedral structure exhibits a higher symmetry than
the orthorhombic, it will further facilitate the formation of
oxygen vacancies in the perovskite upon reduction.36
Figure 6 Calculated oxygen atoms per formula unit of the LSCFNi compounds
during the TGA redox cycling experiments at 900 °C
According to this plot, all the tested compositions were very
stable during redox cycling, and the achievable oxygen
deficiency was reversible. Therefore, they demonstrated high
redox tolerance, since they can reversibly lose and
reincorporate oxygen in their structure.
Nanoparticles exsolution
The post-reduction SEM micrograph of LSCFNi15 is given in
Figure 7a. This shows after annealing the sample in 5% H2/Ar
at 900 °C for 20 hr, the exsolution of metal nanoparticles took
place on the surface of the perovskite. These nanoparticles
were uniformly dispersed on the surface of LSCFNi15 and their
average diameter was approximately 31 nm (Figure 7b). It is
worth pointing out that no nanoparticle decoration was
observed on the single phase LSCFNi15 perovskite before
reduction (Figure S4).
Thermogravimetric analysis in redox environment. The redox
stability of these samples was investigated at 900 °C, to study
the reversibility of the oxygen deficiency in the perovskite
lattice that took place during reduction. The samples were
heated up to 900 °C in air and during an isotherm at this
temperature 5% H2/Ar (R) and air (O) were introduced
alternately. The materials were equilibrated for 2 h at each
step and the process was repeated three times. The
reoxidation process appeared to be faster than the reduction
process and the perovskite recovered the initial oxygen
content during the first few minutes of the reoxidation step
(Figure S2). This suggests the oxygen diffusion was more
favourable in the already reduced perovskite, due to the
presence of oxygen vacancies in the structure. Figure 6
presents the calculated atomic oxygen of the LSCF samples
during the redox cycling experiments at 900 °C.
Figure 7c shows the TEM micrograph of an exsolved metal
nanoparticle on the surface of LSCFNi15. This nanoparticle was
well-anchored on the surface of the perovskite by one third of
its diameter. Similar observations have been reported in the
literature for exsolved Ni nanoparticles from strontium
titanates, suggesting that this strong interaction between the
parent perovskite and the metal nanoparticle leads to
enhanced thermal stability and coking resistance.20
EDX analysis performed suggests the exsolved nanoparticle
from LSCFNi15 was an Fe-Ni alloy and the Fe:Ni ratio was 20:80
(Figure 7d). It has been reported in the literature before the
exsolution of Fe from the LSCF perovskite, due to reduction of
the Fe3+/Fe4+ redox couple to Fe0.15,28 However, when Ni was
incorporated in the structure, both Fe and Ni reduced
simultaneously and formed an Fe-Ni solid solution. The
exsolution of Fe-Ni alloy nanoparticles with various ratios from
different perovskite structures has been reported in the
literature before, confirming that the two elements exsolve as
an alloy and not as two separate metals.32,37,38 Therefore, in
the case of the LSCFNi compounds the formation of bimetallic
Fe-Ni nanocatalysts took place.
Table 4 Summary of the TGA analysis in 5% H2/Ar
Ni doping (mol)
Weight Loss (%)
Moles of
oxygen, δ
0.138
Ni reduced
(mol)
0
0.96
1.06
1.12
1.45
1.71
-
The exsolution of Fe-Ni alloy nanoparticles was observed for
all the LSCFNi samples (Figure S5a, b). The average particle
sizes were similar for the different perovskites and varied
between 31-35 nm (Figure S5c, d). However, the EDX analysis
suggests that for lower Ni doping the exsolved nanoparticles
0.03
0.05
0.10
0.15
0.152
0.161
0.207
0.244
0.014
0.023
0.069
0.106
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micrograph for Ni f) corresponding elemental mapping of the TEM micrograph
had lower Ni content and the Fe:Ni ratio was 35:65 (Figure
S5g). Overall, for all the LSCFNi compounds the exsolved alloy
nanoparticles were richer in Ni than Fe. This was because the
Gibbs free energy of reduction of the NiO to Ni is lower than
that of Fe2O3 to Fe, as it has been calculated by Neagu et al.21
Therefore it was easier for the reduction of Ni2+ to Ni0 to take
place, than the reduction of Fe3+ to Fe0.
Figure 7e, f presents the elemental mapping of the exsolved
nanoparticle from the LSCFNi15. The distribution of Fe (light
blue) and Ni (purple) was uniform in the area of the
nanoparticle, confirming that they formed as an alloy. No
oxygen was detected around the nanoparticle, indicating that
both metals were fully reduced. Moreover, the signal for Ni
was higher than the signal of Fe, since the alloy contained
more Ni than Fe, as discussed above. The rest of the elements
were uniformly distributed in the support area, which was the
reduced LSCFNi15 perovskite.
View Article Online
for Fe
DOI: 10.1039/C9TA03711J
exsolution of Fe-Ni alloy nanoparticles was accompanied by
the formation of a Ruddlesden Popper phase. These two
phases formed simultaneously, when the reduction of
Fe3+/Fe4+ and Ni2+ from the B-site of the perovskite took place.
The Cr3+ species on the B-site of LSCF were stable. Therefore
the expected phase was similar (La,Sr)m+1(Cr,Fe)mO3m+1
.
A general schematic diagram of this reaction for an ABO3
perovskite is illustrated in Figure 8a. According to previous
works,21,26 when the exsolution from the B-site of a
stoichiometric ABO3 perovskite takes place, it is usually
accompanied by the formation of A-site oxides. These A-site
oxides (AO) in combination with the ABO3 perovskite can form
Am+1BmO3m+1 Ruddlesden Popper phases. This composition can
be also described as an A-site excess perovskite. Therefore, the
perovskite balances the exsolution of nanoparticles from the
B-site by the formation of these phases.
Similar phase transformations have been reported in the
literature for double perovskites during exsolution. Du et al.
studied the double perovskite Sr2FeMo0.65Ni0.35O6−δ, which
decomposed on reduction to the Ruddlesden−Popper phase
Sr3FeMoO7−δ, the Sr(FeMo)O3−δ perovskite, and the FeNi3
bimetallic alloy nanoparticle catalyst.32 Liu et al. developed a
(Pr0.4Sr0.6)3(Fe0.85Mo0.15)2O7 Ruddlesden Popper phase with in-
situ exsolution of Co-Fe alloy nanoparticles for CO2, electrolysis
by reducing the (Pr0.4Sr0.6)(Co0.2Fe0.7Mo0.1)O3-δ perovskite in
Reduction mechanism
To summarise this analysis, a possible mechanism can be
proposed for the reduction and exsolution of metal
nanoparticles
from
the
(La0.75Sr0.25)(Cr0.5Fe0.5-xNix)O3
perovskites. During the reduction of the perovskite the
Average diameter: 31 nm
a)
b)
40
35
30
25
20
15
10
5
~ 100 particles
o
5%H2/N2 at 850 C.39 In general, the coexistence of ABO3 and
Am+1BmO3m+1 can be observed in complex perovskite systems
(A1-xA′x)(B1-y-zB′yB′′z)O3, which undergo phase transformations
in reducing environments.34
A schematic representation of the exsolution of Fe-Ni alloy
nanoparticles
from
the
(La0.75Sr0.25)(Cr0.5Fe0.5-xNix)O3
0
100 nm
perovskites is presented in Figure 8b. As illustrated, the
starting material is a stoichiometric perovskite with the metal
cations incorporated in the structure. During reduction, the
exsolution of Fe-Ni alloy nanoparticles from the B-site takes
place, leading to the formation of a Ruddlesden Popper phase
similar to (La,Sr)m+1(Cr,Fe)mO3m+1. The exsolution starts from
areas close to
0
10 20 30 40 50 60 70 80
Particle size (nm)
La
Cr
c)
d)
1
2
Sr
La
Fe
1
Au
Au
Sr
Ni
2
B0+O2
BO2
a)
1
3
5
7
9
11 13 15
Am+1BmO3m+1
X-ray Energy (keV)
m=1→A2BO4
m=2→A3B2O7
e)
f)
nABO3
AO
m=3→A4B3O10
mABO3
.....
(n-1)ABO3
(n-1-m)ABO3
b)
Ni
Fe
Reduction
Figure
7
Exsolution of Fe-Ni alloy nanoparticles from LSCFNi15 a) SEM
micrograph b) particle size histogram for the exsolved particles in a c) TEM
micrograph of exsolved particle d) EDX spectra of the parent perovskite (1) and
the exsolved particle (2) e) corresponding elemental mapping of the TEM
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Figure
8 Illustration models for a) reaction mechanism and b) surface
enhancing the performance by a factor of 20. However, the
CO2 conversion of LSCF was 73% and of LSCFNi15 was 81%,
which
View Article Online
DOI: 10.1039/C9TA03711J
morphology evolution during the reduction of an ABO3 perovskite, where the
exsolution of metal nanoparticles is accompanied by the formation of
Ruddlesden Popper phase
a
the surface and as the perovskite keeps reducing the cation
diffusion takes place from areas deeper from the structure.18
Therefore, the (La,Sr)m+1(Cr,Fe)mO3m+1 phase forms a shell
around the perovskite particles.
100
a)
Initial
LSCF
LSCFNi5
80
60
40
20
0
LSCFNi10
LSCFNi15
Catalytic experiments
Figure 9a presents the gas composition from the outlet of the
reactor after 20 h of testing with the catalysts, in comparison
with the gas composition from the inlet of the reactor.
According to the data, when LSCF was used as catalyst the CH4
concentration did not change significantly. However, the CO2
and H2 concentrations decreased, while the CO and H2O
increased, indicating that the reverse-water gas shift reaction
(Eq. 3) took place. By increasing the Ni content and
incorporating the Fe-Ni alloy nanoparticles on the surface of
LSCF, the CH4 concentration gradually decreased. Moreover,
by increasing the Ni doping, the CO2, H2O concentrations
decreased, while the H2 and CO increased, with a H2/CO ratio
between 2.3-2.5. This suggests that in parallel with the
reverse-water gas shift reaction the methane steam reforming
reaction also took place according to Eq. 1 and to a smaller
extent, the methane dry reforming reaction (Eq. 2).
H₂
CH₄
CO₂
CO
H₂O
100
80
60
40
20
0
b)
LSCF
LSCFNi3
LSCFNi5
LSCFNi10 LSCFNi15
Figure 9b presents the evolution of the CH4 conversion with
time during 20 h of catalytic testing at 900 C. LSCF exhibited a
o
very low CH4 conversion, which was stable with time. By
increasing the Ni doping the CH4 conversion increased,
indicating that the Fe-Ni alloy nanoparticles are the active sites
for the CH4 consumption. Thus, by incorporating Ni in the
structure of LSCF and with the exsolution of the Fe-Ni alloy
nanoparticles, the activity of the perovskite increased
regarding CH4 conversion from 3.5% for LSCF to 72% for
LSCFNi15. Moreover, after an initial equilibration time, all the
materials demonstrated very good stability during the 20 h of
testing, as no decrease in the CH4 conversion was observed.
Figure 9c presents the evolution of the CO2 conversion with
time for all the catalysts. This diagram indicates that there was
a small enhancement in the CO2 conversion, when the Ni
content increased. More specifically, LSCF demonstrated a CO2
conversion of 72% and LSCFNi15 of 81%. This suggests that the
Fe-Ni alloy nanoparticles did not contribute significantly to the
CO2 conversion, which took place from the reverse water-gas
shift reaction. The additional CO2 conversion for the Ni doped
samples was due to the methane dry reforming reaction.
Figure 10 summarises the data collected during the catalytic
testing of the LSCFNi catalysts and demonstrates the influence
of Ni doping on CH4 and CO2 conversion. The values used for
this graph were obtained at the end of the 20 h catalytic
testing, when the materials were stabilised. According to this
graph, by increasing the Ni doping on the LSCF perovskite the
catalyst was more active regarding both CH4 and CO2
conversion, as discussed previously. However, the influence of
Ni doping on the CH4 conversion was more significant than on
the CO2 conversion. More specifically, LSCF demonstrated a
CH4 conversion of 3.5% while LSCFNi15 demonstrated 72%,
0
5
10
Time (h)
15
20
100
80
60
40
20
0
c)
LSCF
LSCFNi3
LSCFNi5
LSCFNi10
LSCFNi15
0
5
10
Time (h)
15
20
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Figure 9 Catalytic testing for the LSCFNi perovskites at 900 °C a) Gas composition from
the outlet of the reactor after 20 h testing b) Evolution of CH4 conversion with time c)
Evolution CO2 conversion with time
exsolution and the catalytic activity of the material regarding
View Article Online
CH4 conversion. Overall, the improved catDaOlyIs:t1s0.p10e3r9fo/Cr9mTaA0n3c7e11iJs
a synergistic effect between the oxygen deficiency of the
perovskite and the exsolution of Fe-Ni alloy nanoparticles
during reduction.
Post-test catalyst characterisation
Post-test microstructure characterisation was performed on
LSCFNi15. Figure 11a shows the SEM micrograph of LSCFNi15.
After 20 h testing at 900 °C, the limited formation of carbon
fibres around some of the exsolved nanoparticles was
observed. These carbon fibres were short and grew in parallel
with the perovskite surface, without uplifting the nanoparticle.
This is the so-called “base growth” mechanism for the carbon
fibre formation. It is typically observed for the exsolved
nanoparticles, suggesting the interaction between the
nanoparticle and the parent perovskite is strong.20 Therefore,
this did not affect the catalyst performance.
Figure 11b presents the particle size distribution of the
exsolved nanoparticles after testing. The average particle size
has increased from 31 (Figure 7b) to 46 nm. Moreover, the
EDX line scan (Figure 11d) on the TEM micrograph of Figure
11c shows the formation of an oxygen layer around the
nanoparticle of approximately 5 nm thickness. This can be also
observed by the elemental mapping performed on this particle
(Figure S6c). However, the nanoparticle remained well-
anchored on the parent perovskite after 20 h testing. Finally,
the EDX analysis on the nanoparticle showed a change on the
Fe-Ni alloy composition from 20:80 to 10:90 (Figure S6b).
These results suggest that when the nanoparticles were
exposed to the reaction gas mixture, morphological and
compositional modifications took place. The increase of the
particle size was attributed to a combination of particle
oxidation during catalytic testing and rearrangement of the
perovskite elements. Overall, due to the strong interaction
between the exsolved nanoparticles and the parent
perovskite, the nanoparticles can undergo transformations,
but remain pinned on the surface.43 Thus, these changes were
not detrimental for the catalysts activity, which were stable
Figure 10 Relation between Ni doping on the LSCFNi perovskites and CH4 and CO2
conversion after 20 hr of testing at 900 °C
indicates just an enhancement of a factor of 1.12. These data
strongly confirm that the active sites for the methane steam
and dry reforming reactions are the Fe-Ni alloy nanoparticles,
while the reverse water-gas shift reaction occurs at the
perovskite backbone. In accordance to previous works,40–42 the
higher catalytic activity in the substituted perovskites can
happen from the CO2 adsorption on the perovskite backbone
and the CH4 dissociation on the metallic particles.
This increase in the performance of the perovskite can be
explained based on the characterisation of the materials. More
specifically, it was found that by increasing the Ni doping on
the B-site of LSCF, the reducibility of the perovskite increased,
as more Fe and Ni will reduce and the exsolution of metal
nanoparticles will be further facilitated. Moreover, TEM-EDX
analysis showed by increasing the level of Ni doping on the
LSCF perovskite, the Fe:Ni ratio of the alloy nanoparticles
changed from 35:65 to 20:80. This led to the greater catalytic
activity of the LSCFNi perovskite regarding CH4 conversion.
In addition, Figure 10 shows the relationship between the Ni
doping and CH4 conversion was not linear and demonstrated
an interesting trend, which was similar to the trends presented
in Figure 4 and Figure 5b for the cell volume and the oxygen
deficiency of the LSCF perovskite during reduction. A change in
the trend occurred for Ni doping of approximately 7 mol%,
which was where the phase transition from the orthorhombic
to rhombohedral system took place. As discussed before, by
increasing the Ni doping on the B-site of LSCF, the oxygen
deficiency of the perovskite during reduction increased and
the exsolution of the Fe-Ni alloy nanoparticles was enhanced.
This led to the improved activity of the perovskites regarding
CH4 conversion. Therefore, the different trends in the
catalyst’s performance in relation to the Ni doping, indicate
that there was a strong correlation between the perovskite
structure, the extent of the Fe-Ni alloy nanoparticles
over 20 h catalytic testing at 900 °C.
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Conclusions
In summary, the partial substitution of Fe by Ni in the
(La0.75Sr0.25)(Cr0.5Fe0.5-xNix)O3 perovskite was investigated and
these materials were tested as potential catalysts for OTMs.
The exsolution of Fe-Ni alloy nanoparticles was observed when
the materials were exposed to a reducing environment. These
nanoparticles significantly enhanced the catalytic activity of
the parent perovskite for methane reforming reactions and
were stable over 20 h testing. The optimum composition was
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
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The authors would like to thank Dr Jonathan Lane for helpful
discussions and Praxair Inc for funding. We would also like to
acknowledge support from EPSRC for Platform Grant
EP/K015540/1.
Notes and references
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DOI: 10.1039/C9TA03711J
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