Influence of transition metal electronegativity on the oxygen storage capacity of perovskite oxides

Article abstract of DOI:10.1039/c6cc01997h

The selection of highly efficient oxygen carriers (OCs) is a key step necessary for the practical development of chemical looping combustion (CLC). In this study, a series of ABO₃ perovskites, where A = La, Ba, Sr, Ca and B = Cr, Mn, Fe, Co, Ni, Cu, are synthesized and tested in a fixed bed reactor for reactivity and stability as OCs with CH₄ as the fuel. We find that the electronegativity of the transition metal on the B-site (λ_B), is a convenient descriptor for oxygen storage capacity (OSC) of our perovskite samples. By plotting OSC for total methane oxidation against λ_B, we observe an inverted volcano plot relationship. These results could provide useful guidelines for perovskite OC design and their other energy related applications.

Full text of DOI:10.1039/c6cc01997h

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Received 00th January 20xx,
Influence of transition metal electronegativity on the
oxygen storage capacity of perovskite oxides†
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
b
a
*a,c
Lu Liu, Daniel D. Taylor, Efrain E. Rodriguez and Michael R. Zachariah
www.rsc.org/chemcomm
The selection of highly efficient oxygen carriers (OCs) is a key step and are natural candidates for OC's due to their high oxygen
conductivity, thermal stability, ease of synthesis, and relatively
necessary for the practical development of chemical looping
low cost. In the perovskite structure, the
alkali, alkaline earth, or lanthanide metal, and the
transition metal. Nevertheless, given the myriad of potential
combinations of the and cations, choosing the optimal
A
cation is typically an
3
combustion (CLC). In this study, a series of ABO perovskites, where
B
cation a
A = La, Ba, Sr, Ca and B = Cr, Mn, Fe, Co, Ni, Cu, are synthesized and
tested in a fixed bed reactor for reactivity and stability as OCs with
A
B
6
CH
metal on the B-site (λ
capacity (OSC) of our perovskite samples. By plotting OSC for total The role of the transition metal
4
as the fuel. We find that the electronegativity of the transition perovskite for CLC applications is daunting.
B
), is a convenient descriptor for oxygen storage
B
is more important than that of
considering that it is the redox active metal that participates
in the CLC reactions. Therefore, we have conducted a
systematic study of 9 different La1ꢀxCa compositions where
the site was varied between Cr to Cu. Furthermore, we
(BSCF) since it is a commonly
A
methane oxidation against λB, we observe an inverted volcano plot
relationship. These results could provide useful guidelines for
perovskite OC design and their other energy related applications.
x 3
BO
B
included Ba0.5Sr0.5Co0.8Fe0.2O
3
used material in oxygen evolution catalysis. In this
communication we report trends of particular materials
descriptors such as electronegativity versus our independent
Chemical looping combustion (CLC) is a novel approach
towards the combustion of fuels that utilizes a twoꢀstep redox
process to achieve carbonaceous combustion, offering a smaller
4
variable oxygen storage capacity (OSC) with CH as the fuel.
1
,2 Our goal is to find various descriptors that could be utilized by
future studies to select the optimal oxides for various CLC
applications.
carbon footprint with no thermodynamic energy penalty.
Instead of directly burning hydrocarbon in air, metal oxides are
used as oxygen carriers (OC’s) in the fuel reactor where they
are reduced while carbonaceous fuel is converted to carbon
dioxide and water. The reduced OC's are regenerated in an air All samples were prepared using an aerosolꢀassisted spray
reactor, and through cycling these oxides,
flameless pyrolysis method as explained in more detail in section S1 of
combustion is achieved with the temperature in air reactors the ESI. Briefly, precursor nitrates were dissolved in water,
a
o
often lower than 1000 C, thus avoiding NO
generated from traditional combustion with air (~80% N
formation that is atomized, then decomposed into the resultant perovskite metal
x
3
oxides in a furnace. This technique ensures homogeneous
mixing of elements; and large surface area and low crystallinity
of resulting powders. Nanometer scale particles with a spherical
morphology were formed for all sample. Representative SEM
2
).
The resultant process under ideal conditions generates a pure
CO stream, after steam condensation in the fuel reactor.
2
3
images of LaCrO are shown in Figure S1 of the ESI.
One of the significant roadblocks in implementation of CLC is
finding the right OC, and the multiple considerations include
high phase stability, redox activity, low cost, and being Powder Xꢀray Diffraction (XRD) experiments, performed on a
environmentally benign. Potential candidates studied as OC’s Bruker D8 diffractometer with Cu Kα radiation, were used to
4
include binary oxides of first row transition metals, such as characterize the asꢀsynthesized materials – shown in Figure S2
iron, copper, nickel, manganese, cobalt, and their mixed metal of the ESI. Table S1, provided in the ESI, presents the refined
oxides. Perovskites have also been explored as novel OC’s space group, crystallite size, and lattice parameters resulting
considering their high redox cyclic stabilities and superior from Le Bail fitting for each pattern. All refinements were
5
7
oxygen transport capacities. Perovskites are generally performed with the TOPAS 4.2 software. While most samples
2
–
formularized as AB
O
3
, with
A
and
B
cations and the O anion could be adequately fit in the cubic space group and therefore
should be considered pseudocubic, final refinements were
performed in either cubic (Pmꢀ3
m), orthorhombic (Pbnm), or
rhombohedral ( ꢀ3 ) space groups depending on the presence
R
c
of peak splitting and satellite peaks. Each sample had a
crystallite size smaller than 35 nm, determined by the Scherrer
equation, as expected with this synthesis technique. The
exceptions being LaMn0.5Ni0.5
O
3
and Ba0.5Sr0.5Co0.8Fe0.2O
3
which had crystallite sizes of 77.8(5) and 66.3(4) nm,
respectively. Each sample was phase pure except for
LaMn0.5Cu0.5
3 3 3 3
O , LaCoO , and La0.5Ca0.5CoO . LaCoO and
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La0.5Ca0.5CoO3 both had
a weak and broad peak at of oxygen available for reaction with methane (i.e. partial
approximately 28.5° 2θ (approximately 3.0 Å). This peak could reduction or methane cracking).
be attributed to La , however, it is not possible to definitively
identify this phase as there are no other impurity peaks present
at higher angles in the XRD patterns. For LaMn0.5Cu0.5 there
are impurity peaks present at approximately 29.5 and 35.5° 2θ
approximately 3.0 and 2.5 Å, respectively). The peak at 29.5°
2
O
3
O
3
(
2
3
θ can possibly be attributed to La
5.5° 2θ to CuO. As with the previous samples, the lack of
2 3 2
O or Cu O and the peak at
additional impurity peaks makes the definitive identification of
these phases impossible. Other phases considered but ruled out
include calcium oxide and carbonate, binary metal oxides, the
metallic elements, and all oxygen deficient perovskite based
phases present in the ICDD PDFꢀ2 database.
Powder XRD patterns were also collected on the post CLC
reactor study samples. Each of these patterns is provided in
Figure S4 of the ESI. Le Bailing fitting of each pattern showed
that each sample was phase pure with all peaks being indexed
in either the cubic
rhombohedral ( ꢀ3 ) space groups. Representative Le Bail
refinements for La0.5Ca0.5CoO , and LaNiO
3
(Pmꢀ3m), orthorhombic (Pbnm), or
R
c
3
, LaCu0.5Mn0.5
O
3
are provided in Figures S5ꢀS7 of the ESI. Table S2, provided in
the ESI, presents the refined space group, crystallite size, and
lattice parameters resulting from the Le Bail fitting of each
pattern. Considering impurity phases are no longer present in the
post fixed bed experiment samples and that the OSC of these
samples is steady over the 50 cycles tested, these impurity phases
appear to have a minimal effect on the performance of these samples
as oxygen carriers.
3 3
All but two samples, LaMn0.5Ni0.5O and Ba0.5Sr0.5Co0.8Fe0.2O ,
experienced an increase in crystallite size, as determined by the
Scherrer equation. This suggests that the powders are sintering
as they are held at 750 °C during the fixed bed experiments.
This sintering can be seen as the diffraction peaks sharpening
and the appearance of clear satellite peaks. Further evidence for
this sintering is provided by TEM and SEM experiments shown
in Figure S3 of the ESI. Here, (a) TEM, (b) HRTEM, and (c)
SEM images, along with a (d) selected area electron diffraction
4 2
Fig. 1 (a) OSC (δ) and (b) CH selectivity to CO of tested
o
perovskites in 50 cycles with methane at 750 C in a fixedꢀbed
reactor.
In this study, we found that
A cation substitution has a
negligible effect on OSC, but there exists an inverted volcano
dependence for transferable oxygen during total methane
(
SAED) image are provided for La0.5Ca0.5CoO . The particle
3
morphology became less spherical and agglomerates are
beginning to form. Interestingly, this sintering does not appear
to affect the OSC for each sample as the delta value remains
fairly consistent across 50 cycles (Figure 1a) consistent with the
oxidation with the
B cation's electronegativity (Pauling scale).
The global reaction between perovskite and methane can be
represented as,
8
, 9
observations of others.
4
AB
Thus we can define oxygen storage capacity (OSC) in CLC as
δ, which was measured by quantitative analysis of CO in the
3 4 2 2
O + δ CH ꢀ 4 ABO3ꢀδ + δ CO + 2δ H O
TGAꢀMS data in Figure S8, of the ESI, implies that our
perovskites did not spontaneously release O up to 750 °C
under an Argon flow, thus suggesting the reaction between the
OC and CH follows a gasꢀsolid reaction mechanism.
2
2
fuel step. Figure 1 (a) shows the OSC (δ) over 50 CLC cycles
in the fixed bed reactor and clearly indicates that all perovskites
are stable OCs. The amount of CO produced was quantified
2
4
from the effluent in the fuel step, and carbon coking generated
from methane thermal decomposition, the major side reaction,
To determine the ability of each of these samples to act as OCs
for the combustion of CH , approximately 200 mg of sample
4
is measured by the CO species in the air step. Shown in Figure
x
1
0
were loaded into a quartz flow reactor within a tube furnace.
CLC reactivity tests were then performed by alternating the gas
flow between 11% CH /Ar for 2 minutes (combustion) and
0% O /Ar for 5 minutes (regeneration) while the sample was
1
(b), we define CO
2
selectivity as:
4
ꢅꢁꢂꢃ
ꢀꢁꢂꢃ
ꢄ
ꢆ
2
2
ꢅꢁꢆꢇꢆꢅ
ꢁꢂ
ꢃ
held isothermally at 750 °C. Full experimental details are
provided in section S1 of the ESI. This experiment was
designed to prevent the formation of CO, as is often determined
by the reduction extent of the metal oxide. Therefore, it is
important to note that the OSC reported here is the oxygen
available for the combustion of methane, not the total amount
where
and
perovskites expressed γ
n
CO2 are the number of moles detected in the fuel step
the number of moles detected in the air step. All the
values higher than 95%, indicating
n
C
CO2
1
1
their low coking ratio.
2
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In an attempt to provide some microscopic structural context to electron affinity. Therefore, the degree of hybridization
these results we show in Figure 2a the relationship between between the B dꢀstates and the O 2 ꢀstates would be dependent
p
1
2
OSC and λ
For the mixed
for λ
Since the OSC’s of the samples with mixed La and Ca
occupancy on the site showed negligible difference, we
concluded that substituting Ca for La on the site has a
negligible effect of the OSC, which agrees with previous
B
(
B
site electronegativity) on the Pauling Scale.
on electronegativity.
B
site samples, a simple molar average was used
B
. The resulting plot shows an inverted volcano behavior.
A
A
1
3, 14,15,16,17
publications.
the metal is the redox active center of the perovskite. For
comparison, we also plotted the average occupancy of the soꢀ
called orbital for the transition metal, as determined by
Suntivich et al.
This also makes sense considering that
B
e
g
1
4
,
h
versus OSC. Although true O symmetry is
not present in all of these materials, the structural distortions
are too minor to invert the basic crystal field splitting energies
(i.e. make t2gꢀderived states higher in energy than the egꢀderived
ones). Therefore, we use only as a convenient label across
e
g
various perovskites in searching for broad trends in properties
such as catalytic activity or oxygen storage capacity. In cases
where two transition metals are present, only the more
catalytically active element was considered. Here, the trend
appears less obvious than for λ , although a volcanoꢀtype
B
relationship does seem plausible. Overall,
e
g
occupancy does
not appear to be as useful
electronegativity.
a
descriptor as ꢀsite
B
Utilizing a materials descriptor such as electronegativity
provides a general understanding of OC design with ABO3
perovskites given that the underlying mechanisms are still
unknown. Past studies have found that oxygen transport and
mobility in perovskite structures are highly relevant to their
other applications, thus considerable attention has been focused
on partial substitutions for the
combinations.
A
and
Recently, Imanieh et al. explored the effect
of substituted atom size on the reactivity of hydrocarbons with
AB (CaMnO ) perovskites in order to identify the optimal OC
for CLC applications. At high reaction temperature (>800 C)
favoring O release, CaMnO perovskite with ~10% Sr and Fe
doped in the and sites, respectively, helps improve their
B sites to find optimal
1
8,19
O
3
3
2
0
o
2
3
A
B
oxygen storage capacity (OSC) due to higher crystalline
distortion. X. Dai et al. found that compared to Nd, Eu
substitution for La on the
A site maintains a relatively high Fig. 2 The relation of OSC (δ) and (a) the λB (B site
catalytic 1activity and structural stability during redox electronegativity) in methane total oxidation and (b) the eg
2
reactions.
electron occupancy.
Since Bockris and Otagawa’s work in 1980s, descriptors While we have not found a direct correlation between
correlating 5ꢀcoordinate surface ions’ electronic properties of electronegativity and oxide conductivity yet, other workers
perovskite oxides and their activities towards oxygen have found oxygen vacancy formation as a useful descriptor for
1
3
electrocatalysis have attracted a lot of interest. It has been electrochemical applications of perovskite oxides. Kitchin et
reported that the filling of manifold of the octahedral crystal al. used density function theory (DFT) calculations to correlate
field of the cations, mainly related to the oxygen binding to the decrease in the oxygen vacancy formation energy to
the perovskite surface , can be used as an efficient descriptor increasing atomic numbers from Cr to Cu in La
for reactivity in oxygen evolution reaction (OER) in fuel cell which were explored as anode materials in SOFC's. Through
e
g
B
1
3
B
3
O and SrBO3,
22
1
4
applications employing perovskites
and waterꢀsplitting DFT calculations, Carter et al. demonstrated that ion diffusion
1
5
reactions
.
Thus
e
g
in perovskite, contributed to occupied in the La1ꢀxSrx
valence band states, are responsible for the reactive states near
B
O family ( = Cr, Mn, Fe, Co) and Sr Fe
B
3
2
2ꢀ
x
Mo O depends on two processes: oxygen vacancy formation
x
6
Fermi level, could be used as an effective descriptor, consistent and vacancy mediated oxygen migration, and found that
with Norskov’s dꢀband theory. From our own results, however, oxygen vacancy formation energy could be used as a descriptor
1
8
it appears that
driven redox reactions.
g
e occupancy is not as relevant for temperatureꢀ for oxide ion transport properties’ evaluation.
Yang et al. utilized Xꢀray Absorption Near Edge Structure
The role of Bꢀsite electronegativity could be related to that of (XANES) and fluorescence O Kꢀedge to study the effects of the
the B—O bond strength. Electronegativity, as defined by
B
and A site, respectively, on the electronic structure and found
Mulliken, is the average of the ionization potential and the that changes in
A
site substitution have less of an effect relative
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to the ꢀsite, and increasing the
ChemComm
B
B
site electron count can Notes and references
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d
and
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site could result in 2.
1
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.
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1
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trend in the oxygen storage capacity includes
B cations with
9
1
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.
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8
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1
18.
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2
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ꢀband model (the
ꢀband
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d
2
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d
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2
2
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4.
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1
4
Ba0.5Sr0.5Co0.8Fe0.2
O
3
in OER , we also prepared this
perovskite to validate the inverted volcano plot with no
lanthanum/calcium in site. As indicated in Figure 2a, the
fits reasonably well in the
inverted volcano plot, indicating that this behavior is more
2
6.
A
2
2
2
7.
8.
9.
OSC of Ba0.5Sr0.5Co0.8Fe0.2O
3
universal and applies to other
transition metals on the site.
A
site OC perovskites with 3d
B
Our study concludes that
3d transition metal B site
electronegativity could be used as a useful descriptor in
choosing perovskite oxygen carriers for combustion of CH
We found an inverted volcano plot relationship for the
lanthanum/calcium perovskite series. While the choice for the
site (either La or Ca) has a negligible effect on the OSC, the
4
.
A
choice of the
optimal OC’s.
B site is more relevant towards the design of
We acknowledge the Army Research Office and the
Department of Commerce/NIST award 70NANB12H238 for
support.
4
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