Endogenous Nanoparticles Strain Perovskite Host Lattice Providing Oxygen Capacity and Driving Oxygen Exchange and CH4 Conversion to Syngas

Article abstract of DOI:10.1002/anie.201915140

Particles dispersed on the surface of oxide supports have enabled a wealth of applications in electrocatalysis, photocatalysis, and heterogeneous catalysis. Dispersing nanoparticles within the bulk of oxides is, however, synthetically much more challenging and therefore less explored, but could open new dimensions to control material properties analogous to substitutional doping of ions in crystal lattices. Here we demonstrate such a concept allowing extensive, controlled growth of metallic nanoparticles, at nanoscale proximity, within a perovskite oxide lattice as well as on its surface. By employing operando techniques, we show that in the emergent nanostructure, the endogenous nanoparticles and the perovskite lattice become reciprocally strained and seamlessly connected, enabling enhanced oxygen exchange. Additionally, even deeply embedded nanoparticles can reversibly exchange oxygen with a methane stream, driving its redox conversion to syngas with remarkable selectivity and long term cyclability while surface particles are present. These results not only exemplify the means to create extensive, self-strained nanoarchitectures with enhanced oxygen transport and storage capabilities, but also demonstrate that deeply submerged, redox-active nanoparticles could be entirely accessible to reaction environments, driving redox transformations and thus offering intriguing new alternatives to design materials underpinning several energy conversion technologies.

Full text of DOI:10.1002/anie.201915140

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Endogenous Nanoparticles Strain Perovskite Host Lattice
Providing Oxygen Capacity and Driving Oxygen Exchange and
CH4 Conversion to Syngas
Authors: Kalliopi Kousi, Dragos Neagu, Leonidas Bekris, Evangelos
Papaioannou, and Ian Saxley Metcalfe
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915140
Angew. Chem. 10.1002/ange.201915140
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10.1002/anie.201915140
Angewandte Chemie International Edition
RESEARCH ARTICLE
Endogenous Nanoparticles Strain Perovskite Host Lattice
Providing Oxygen Capacity and Driving Oxygen Exchange and
CH4 Conversion to Syngas
Kalliopi Kousi,*[a] Dragos Neagu,*[a] Leonidas Bekris,[a] Evangelos I. Papaioannou[a] and Ian S.
Metcalfe*[a]
Abstract: Particles dispersed on the surface of oxide supports have
enabled a wealth of applications in electro- photo- and heterogeneous
catalysis. Dispersing nanoparticles within the bulk of oxides is,
however, synthetically much more challenging and therefore less
explored, but could open new dimensions to control material
properties analogous to substitutional doping of ions in crystal lattices.
Here we demonstrate such a concept allowing extensive, controlled
growth of metallic nanoparticles, at nanoscale proximity, within a
perovskite oxide lattice as well as on its surface. By employing
operando techniques, we show that in the emergent nanostructure,
the endogenous nanoparticles and the perovskite lattice become
reciprocally strained and seamlessly connected, enabling enhanced
nanocomposite leads to strain within the perovskite oxide lattice,
enhancing oxide ion transport and oxygen reduction activity[1].
Similarly, embedding noble metal nanoparticles within other
inorganic crystal lattices enhances electronic transport in a
manner analogue to substitutional doping[2]. Therefore,
embedding metallic nanoparticles within non-metallic crystal
lattices could open new possibilities to tailor materials , in
particular through strain engineering, which has been shown to
control multiple properties including oxide ion, electron and
thermal transport[3–5]
properties[10]
,
catalytic reactivity[3,6–9] and magnetic
.
However, producing and controlling such nanocomposites
remains challenging. Currently, these are being prepared by
oxygen exchange. Additionally,
even deeply embedded
nanoparticles can reversibly exchange oxygen with a methane stream, assembling metallic nanoclusters together with the non-metallic
driving its redox conversion to syngas with remarkable selectivity and
long term cyclability while surface particles are present. These results
not only exemplify the means to create extensive, self-strained
nanoarchitectures with enhanced oxygen transport and storage
capabilities, but also demonstrate that deeply submerged, redox-
active nanoparticles could be entirely accessible to reaction
environments, driving redox transformations and thus offering
intriguing new alternatives to design materials underpinning several
energy conversion technologies.
host lattice. However, this approach is limited to using a
combination of metals (usually noble metals) and host lattices that
do not interact chemically during preparation[1,11] posing
fundamental limitations over the type of materials and
nanostructures that can be achieved. An alternative would be to
prepare such composites through a controlled disassembly
process instead. The atoms which are meant to form the
embedded phase would be initially dispersed as ions within the
host lattice, as a solid solution, and upon reduction precipitated
out of the lattice. Redox exsolution is one such process, employed
for dispersing confined, partly submerged metallic nanoparticles
on the surface of oxides. Exsolved nanoparticles display
improved resistance to carbon deposition and agglomeration and
enhanced activity due to strain[12,13], being extensively applied in
Introduction
Metallic nanoparticles dispersed on the surface of oxide supports
have proven to be instrumental in controlling and tailoring the
surface reactivity of materials for a wide variety of catalytic
applications, especially in energy conversion technologies.
However, what if nanoparticles could be dispersed within the bulk
of materials rather than on their surface? How would such
materials be made and what would be the impact on the material
properties? A recent report shows that by assembling gold
nanoparticles together with perovskite oxides (ABO3) in a
electrochemical and catalytic processes[14–21]
.
Here we employ the exsolution concept outside its conventional
use to homogenously grow metallic particles throughout the bulk
of perovskite oxides of 100-150 µm particle size (Fig. 1). We show
that the high dispersion achieved through this approach yields
nanoparticles within nanoscale proximity to each other, inducing
strain in both the metal and host lattices. In turn, this greatly
enhances oxygen exchange across the nanocomposite,
seamlessly connecting even the deepest embedded particles to
the gas phase environment. Moreover, we show that the
embedded nanoparticles can undergo cyclic, redox
transformation between metal and metal oxide state, acting as
readily accessible nanoscale reservoirs for oxygen storage,
maintaining nanocomposite integrity and remain protected
against agglomeration or other deactivation processes. Such
unique combination of high oxygen transport, high oxygen
storage capacity as well as redox robustness, makes these
materials appealing for energy conversion technologies that rely
[a]
Dr. K. Kousi, Dr. D. Neagu, B.Eng. L. Bekris, Dr. E. I. Papaioannou,
Prof. I. S. Metcalfe
School of Engineering
Newcastle University
Merz Court, NE1 7RU
E-mail: kalliopi.kousi@ncl.ac.uk, dragos.neagu@ncl.ac.uk,
ian.metcalfe@ncl.ac.uk
Supporting information for this article is given via a link at the end of
the document.
on cyclic oxygen storage such thermochemical solar to fuel[22]
,
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10.1002/anie.201915140
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RESEARCH ARTICLE
redox flow batteries[23] and chemical looping[24]. To illustrate the
applicability of our concept, we employ it towards the challenging
process of CH4 activation and conversion[25,26] via chemical
looping partial oxidation (CLPO). In this process, the oxygen
carrier material which reacts cyclically with a reducing and an
oxidizing stream also needs to fulfil the catalytic role of CH4
activation[27–30]. We exemplify this by using a titanate perovskite
oxide (La0.8Ce0.1Ni0.4Ti0.6O3), which is traditionally not ideal for
such applications due to low surface area and low oxygen
exchange capability[31]. However, upon the formation of surface
and bulk metallic nanoparticles (Fig. 2a), this becomes very active
for the process, producing synthesis gas with high selectivity and
minimal carbon deposition at lower temperatures than
formed due to the extensive degree of exsolution achieved in this
case (almost all the substituted Ni was exsolved, about 0.35 Ni
out of the 0.4 in La0.8Ce0.1Ni0.4Ti0.6O3, Supp. Note 1).
Close examination of this system revealed that the particles
exsolved on the surface and in the bulk display key fundamental
differences in terms of size, population, unit cell parameter and
experienced strain. First, the particles exsolved in the bulk are
much smaller as compared to the ones on the surface,
approximately 13 vs 45 nm. This is probably due to the strain
associated to particles growing within the bulk which, according
to theoretical calculations is expected to be easier to
accommodate for smaller particles[34,35]. Secondly, they are also
more numerous, 1000 vs 100 particles per µm2 respectively (Fig
2f, 2g, S4). It is worth noting that at these tremendously high
population densities, the bulk particles are in nanoscale proximity
of each other, maximising the strain they can exert on the host
perovskite lattice. Indeed, the interparticle distance is of the same
order of length size as the particles themselves (~13 and ~33 nm,
respectively), as derived from a Voronoi tessellation of their
spatial arrangement (Fig 2e, 2h, S4). Notably, as compared to
standard Ni metal (3.5168 Å), surface particles are expansively
strained by 0.29% (3.5272 Å), while the ones in the bulk are, not
surprisingly, even more strained, at 0.37% (3.5301 Å). The high
bulk particle population also induces microstrain in the remnant
perovskite matrix rABO3, roughly twice as high as compared to
the initial perovskite (see peak broadening in Fig 2b and strain
analysis in Fig. S5).
conventional materials[32]
.
Figure 1. Controlling particle immersion with different preparation methods.
Results and Discussion
A perovskite oxide with endogenous nanoparticles
This striking difference in size and population between surface
and bulk particles is also reflected in the shape of the Ni peak in
the X-ray synchrotron diffraction pattern (Fig. 2f). The peak
consists of a sharp tip (larger, surface particles) and broad base
(smaller, bulk particles). The size-peak correspondence was also
confirmed by chemically removing the particles at the surface,
which also removes the sharp component of the Ni peak (Fig. S6).
Quantification by Rietveld refinement from X-ray data revealed
that overall the system exsolved ~8 wt.% Ni metal (corresponding
to almost all the substituted Ni exsolving, 0.35 mol Ni per mol
rABO3), out of which 85% corresponds to the particles located
within the bulk (Fig. S5). Overall, this system can be visualised
as comprising of 1 wt.% of 45 nm Ni particles dispersed on the
surface and 7 wt.% of 13 nm Ni particles dispersed within the bulk.
This high ratio of bulk/surface exsolved particles is consistent with
our design principle of maximising particle formation within the
bulk as opposed to the surface.
In order to promote bulk as opposed to surface exsolution, we
introduce a set of design principles. We select a titanate
perovskite with relatively low cation transport capabilities[33] in
order to favour local particle nucleation as opposed to transport of
the exsolvable ions to the surface. We introduce slight A-site
deficiency in conjunction with relatively high B-site substitution
(0.4 Ni in La0.8Ce0.1Ni0.4Ti0.6O3 is equivalent to 10 wt.% Ni) to
promote exsolution and increase the likelihood of metal
nucleation throughout the perovskite lattice. Additionally, in order
to overcome the relatively high energy barriers associated with
nucleation and growth within an oxide lattice (arising from strain
and oxide lattice reconstruction around the particle)[34,35], we
increase the temperature and/or duration at which the exsolution
process is carried out.
The synthesized perovskite displays high degree of crystallinity
(illustrated by sharp peaks in X-ray diffraction patterns, Fig.2b and
S1) and a particle size of 80-160 µm (Fig. S2). Figures 2c, 2d, 2i,
S3 show that in such conditions the exsolution process still occurs
in a controlled manner, producing homogenously dispersed
particles on the surface and within the bulk. This is also reflected
in the XRD pattern which indicates the presence of a main,
remnant perovskite phase, henceforth referred to as rABO3, a Ni
metal phase peak and reflections associated with La2TiO5 which
Overall, the design employed here enables the creation of a self-
nanostructured, self-strained composite system at the
macroscale, which is expected to exhibit emergent properties[36,37]
including enhanced bulk oxygen transport and therefore enable
the connection between the bulk particles and the gas stream.
,
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10.1002/anie.201915140
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RESEARCH ARTICLE
a
c
d
e
f
g
h
i
b
Ni metal phase
Total Ni
Ni nanoparticle size
Interparticle distance
Bulk
(13 nm)
Bulk
(33 nm)
0)
11
1)
11
0)
20
(
Surface
(45 nm)
(
(
P
Bulk
(13 nm)
P
P
0.2
Surface
(45 nm)
Surface
(130 nm)
0.1
0.0
La2TiO5
Ni (111)
10
9.8
10.0
2 q (°)
10.2
0
20
40
60
0
50 100 150 200
Size (nm)
6
8
Size (nm)
2 q (°)
Figure 2. A system with submerged particles. (a) 3D model of a catalyst particle with nanoparticles dispersed on its surface and within its bulk. (b) Room temperature
XRD patterns of La0.8Ce0.1Ni0.4Ti0.6O3, in as-prepared state (black) and after exsolution at 1000 °C (10 h) in H2 (blue). Both diffractograms are normalised on a scale
from 0 to 1 (see Fig. S1 for details). (c)-(d) SEM micrographs showing: (c) an overview of the surface of the perovskite after exsolution. (d) Cross-section view after
exsolution revealing surface and bulk particles. (e) Detail of the particles exsolved in the bulk (statistically half of the particles are missing due to the fact that the bulk
was cleaved in order to acquire the image). The micrograph is superimposed with a corresponding Voronoi tessellation highlighting a virtual partitioning of the
perovskite bulk into nanodomains associated to each particle exsolved within it. (f) Ni metal peak from b deconvoluted into surface and bulk particles contribution
using Rietveld refinement. (g) Ni metal particle size distribution of d calculated by image analysis. (h) Domain size distribution calculated by image analysis on the
Voronoi diagram in e (see Fig. S4 for additional information). (i) TEM images of the perovskite lattice before (left) and after exsolution (right); superimposed are the
lattice models highlighting the expansion of the perovskite unit cell after reduction.
Controlling endogenous nanoparticles, strain and oxygen
exchange
In order to understand the effect of these parameters on oxygen
exchange as well as oxygen capacity, we measured the oxygen
uptake during oxidation of these samples. Since the
characteristics of the surface particles are similar across these
samples, it can be assumed that the surface exchange kinetics
are similar and therefore the oxygen uptake experiments will
reflect relative oxygen diffusivities within the bulk. As shown in Fig
3d, the sample only having surface particles displays a very low
oxygen exchange rate and low oxygen capacity characteristic of
a perovskite titanate system. As the embedded metal content is
increased, both the oxygen capacity and oxygen exchange rate
increase dramatically while the temperature at which oxygen
exchange initiates decreases.
By employing different temperatures and durations for the
reduction of the perovskite, the degree of the embedded metal
loading, but most importantly, the size and interparticle distance
of the embedded particles can be easily controlled. A series of
such samples, ranging from a sample with no embedded particles
(only surface particles) to samples with increasingly higher bulk
metal nanoparticles content are shown in Fig. 3. It appears that
the temperature controls the interparticle distance (smaller at
lower temperatures), while duration controls size (larger with
increasing time). From the phase and microstructure analysis
shown in Fig. 3a-c it is evident that bulk particles emerge even at
the lower reduction temperature of 900 °C, given enough time,
and overall their weight fraction always dominates the contribution
coming from the surface particles (compare NiS and NiB in Fig. 3f).
In the prepared range of samples, the size of the embedded
particles varies from ~3.5 to ~6.5 and to ~13 nm, while the
corresponding interparticle distance varies from ~20 to ~25 and
to ~33 nm, respectively (Fig. 3f, S7). At the same time, the particle
size and population at the surface of these samples are very
similar (Fig. S7).
In order to quantify these effects, we fit the oxygen uptake
measurement with a logistic law (characteristic of such oxidation
processes – Eq. 18, Fig. S8) and plot the corresponding oxygen
capacity (ζG, mol O per mol perovskite) and time constants (τ, h-1)
associated with oxygen exchange rate in Fig. 3f. It is apparent
from this figure that, as expected, the oxygen capacity scales
proportionally with the metal content (compare NiB and ζG in Fig.
3f). On the contrary, the rate of oxygen exchange does not seem
to correlate with the embedded metal content (compare NiB and τ
in Fig. 3f), but seems to correlate much more strongly with
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RESEARCH ARTICLE
a
b
c
Ni metal content
8%
1000°C (10 h)
1
0.4%
900°C (30 h)
7%
3.5%
900°C (10 h)
900°C (0 h)
0
f
10
8
10
8
0.5
0.4
0.3
0.2
0.1
0.0
10
8
0.4
0.3
0.2
0.1
0.0
35
30
25
20
15
15
10
5
9.8
9.9
10.0 10.1
2q (°)
5
10
2q (°)
15
101
100
d
e
600°C
450°C
0.83 eV
3
2
1
0
0.75 eV
0.57 eV
0.95 eV
0.4
0.3
0.2
0.1
0.0
6
6
6
4
4
4
10-1
10-2
2
2
2
0
NiS
wt.%
0
0
0
NiB
wt.%
s/d
d
s
zG
t
100
200
Time (min)
300
400
1.38
1.31
1.25
1.19
h-1
1/T (K-1)
nm
nm
Figure 3. Controlling bulk exsolution, strain and oxygen exchange. (a) Room temperature XRD of samples reduced at 900 °C for 0 h (dark grey), 10 h (dark blue)
and 30 h (blue) and at 1000 °C for 10 h (light blue). (b) Detail of the Ni peak from a together with corresponding total (surface and bulk) Ni content (wt.%) calculated
by Rietveld refinement. Schematic illustrations of samples without (left) and with (right) bulk particles. (c) SEM micrographs and calculated Voronoi tessellation of
the spatial arrangement of embedded particles contained in the samples denoted in a by the blue range of colours. (d) Oxygen weight gain and corresponding
oxygen capacity (mol O per mol of perovskite) as a function of time, up to a temperature of 600 °C, for the samples shown in a. (e) Arrhenius plot corresponding to
the data shown in d together with the calculated activation energy for oxygen transport. (f) Parallel axes plot for the samples shown in a for the Ni metal content at
the surface (NiS, wt.%) and in the bulk (NiB, wt.%) calculated from Rietveld refinement of the data from a, oxygen capacity (ζG, mol O per mol of perovskite), time
constant of oxygen exchange calculated from the data shown in d (τ, h-1, see Fig. S8), ratio of the average bulk particle diameter (s) and average distance between
neighbouring bulk particles (d), average bulk particle diameter (s, nm) and average distance between neighbouring bulk particles (d, nm, see Fig. S7).
parameters related to the size and interparticle distance of the
embedded particles, in particular, the ratio between these
(compare τ and s/d in Fig. 3f). This is probably not surprising
considering that such parameters are expected to be indicative of
the degree of strain the particles exert over the host lattice.
Nonetheless, an Arrhenius plot of the oxygen exchange data
revealed that the activation energies for the oxygen transport are
characteristic of perovskites, although generally much closer to
values reported for perovskites with higher oxide ion diffusivities
than titanates[38]. This indicates that while the perovskite is still
responsible for mediating oxygen exchange between the surface
and the embedded metal nanoparticles, its oxygen transport
capabilities have been strongly enhanced by the introduction of
strain via the embedded metal particles.
In order to better understand the process of oxygen re-
incorporation and further straining of the rABO3 matrix, we monitor
oxygen uptake by gas composition analysis as a function of
temperature and correlate this with real time structural information
via Rietveld refinement of synchrotron X-ray data (Fig. 4, S9).
From room temperature (RT) up to ~400 °C the system only
undergoes thermal expansion (Fig 4a-e). The thermal expansion
of Ni is consistent with literature values, 1.34 10-5 °C-1 while the
expansion of the rABO3 matrix, 0.94 10-5 °C-1, is characteristic of
similar perovskite systems[41],[42], indicating that up to this
temperature, they behave as individual phases.
Starting at 400 °C, the Ni phase begins to oxidise, as evidenced
by a decrease in its fraction from 8 to 4 wt.%. At around 550 °C,
the slope in the expansion of the unit cell of the perovskite
increases. This is, however, counterintuitive because the residual
(stoichiometric) perovskite lattice is expected to contract upon
oxidation[40,43]. Thus, it appears that the lattice expansion from Ni
to NiO accounts for the observed lattice expansion of the
perovskite, demonstrating that the two phases now behave as if
connected. Most notable, though, within the same temperature
window, the degree of microstrain in the rABO3 matrix increases
dramatically, ~6 fold (Fig. 4d, S9), whilst maintaining the crystal
and grain structure (Fig. S1, S10). The whole process is
completed by 600 °C, with further increase in temperature not
causing any additional structural changes. Analysis of the sample
Further increase in strain through reversible oxygen
incorporation
A way to further increase strain would be to take advantage of the
different redox expansion of the perovskite phase and the
embedded Ni phase[39]. That is, upon oxidation, the Ni lattice
expands by about 15% due to oxygen incorporation. On the
contrary, when oxygen is incorporated in the perovskite oxygen
vacancies, this has minimal effect on lattice dimensions, which
are, in turn, dominated by the increased charge of the B-site ions,
leading to a net contraction of the unit cell of <1%[40]
.
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RESEARCH ARTICLE
a
c
d
e
f
Oxidation&
Strain
5.0
4.5
4.0
Thermal
expansion
200
100
0
50
100
150
6
4
2
8.0
4.0
0.0
0
9.5
10.0
2q (°)
10.5
g
b
10.4
10.2
10.0
9.8
200
400
600
Temperature (°C)
200
400
600
Temperature (°C)
Figure 4. Increasing strain levels to unlock oxygen ion exchange. (a) XRD data acquired during the temperature programmed oxidation of a system with submerged
nanoparticles presented in 3D plot as a function of diffraction angle, time and scan number. (b) XRD data from a presented in a 2D plot as a function of diffraction
angle, time and corresponding temperature. (c) O2 (%) consumption during experiment a. (d)-(e) structural information calculated from a by Rietveld refinement
showing: (d) Ni metal content and rABO3 microstrain (relative to the initial, reduced state value) as a function of time and temperature and (e) Unit cell parameter of
the Ni metal and rABO3 phase as a function of time and temperature. (f) Cross-section SEM image of the sample after the experiment described in a. (g) TEM image
and close up detail of the sample after the experiment described in a, showing the crystallographic alignment between the perovskite and a NiO nanoparticle within
it. The dotted line highlights the interface between the particle and the perovskite.
after this experiment indicated that all the Ni metal had been
converted to NiO while still remaining socketed (Fig. 4d, 4f), the
La2TiO5 phase remained unchanged (Fig. S1), and the
crystallographic alignment between the bulk particles and the
perovskite was preserved (Fig. 4g). It is worth noting that this
dramatic increase in microstrain during the first oxidation
experienced by the rABO3 appears to be permanent, being
maintained during additional redox cycles (Fig. S11) and causes
a further 15% increase of the oxygen exchange time constant τ.
Overall, this denotes that we have created a system with very high
oxygen capacity which largely comes from the embedded Ni
nanoparticles, while the strain that these particles induce is
responsible for the fast oxygen exchange capabilities.
broader peak at 600 °C, selectively yielding syngas with a H2:CO
2:1 ratio, ideal for synthetic fuel production (Fig. 5a-b)[45]. The
reference sample displays a similar CH4 activation profile, starting
50 °C higher (in spite of the large surface area and thus
availability of active sites), while only the first sharp peak
corresponds to oxygenated gaseous products (albeit being less
selective, Fig. 5a-b). The second peak, spreading over a 100 °C
temperature window, corresponds exclusively to CH4 cracking
(Fig. S13). This indicates that in Ni/Al2O3, once the NiO has been
converted to Ni metal particles these become active sites for
carbon deposition[44]. On the contrary, in the exsolved system,
due to the confined and aligned nature of the metallic Ni particles,
this mechanism is not favoured, consistent with previous
observations whereby exsolved particles are able to supress coke
in hydrocarbon environments[13]
.
Using an endogenous nanoparticle system to activate
methane
Consequently, the Ni-rABO3 is almost exclusively selective
towards syngas (87%) while Ni/Al2O3 demonstrates overall 88%
selectivity towards carbon deposition.
To demonstrate the utility of this system, we apply it to the
challenging process of CH4 activation, by monitoring methane
conversion and selectivity as a function of temperature (Fig. 5).
We compare, on a weight basis, our low surface area system (~1
m2 g-1) having most particles enclosed in the bulk (Ni-rABO3),
against a high surface area (~100 m2 g-1) Ni/Al2O3 with a similar
loading of metal particles (10 wt.%, ~8 nm size, Fig. S12), which
are instead located on the surface (state-of-the-art for methane
conversion[44]).
The outstanding behaviour of the submerged particle system
under reaction conditions seems to stem from synergistic
contribution of its components. In order to deconvolute this, two
variations of our system were prepared and tested. Firstly, a
system where surface particles were removed, keeping the ones
in the bulk intact, showed no activity, (P1 in Fig. 5a), indicating
that the presence of confined surface particles is essential to
enable coke-free methane activation (Fig. 5d). Secondly, a
system only having surface particles was also inactive in this
The Ni-rABO3 system activates CH4 in a narrow temperature
window, with a sharp consumption peak at 550 °C, followed by a
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10.1002/anie.201915140
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temperature window (P2, Fig. 5a) indicating that in absence of
sufficient oxygen capacity, the surface particles alone are not
enhance oxygen exchange considerably[46]. Additionally, the
introduction of embedded nanoparticles also enhances the
oxygen vacancy content of the support oxide material which is
also expected to contribute towards enhanced oxygen exchange
(see Supplementary Note 2). Thus, it seems that our system
behaves kinetically as if all the NiO particles are on the surface,
yet because they are actually submerged in the perovskite matrix,
this allows for controlled release of the oxygen in order to
5
a
b
100
80
60
40
20
0
4
3
2
1
0
P1, P2
Ni/rABO3
selectively produce syngas[47]
.
Operando mechanistic insight of CH4 conversion with
endogenous particles
Ni/Al2O3
500 600
700
Temperature (°C)
CO CO2
C
2
Carbon products
In order to gain a better understanding of how the above-
mentioned components of the system operate during CH4
conversion, we monitor this process operando, combining gas
composition analysis with real time structural information via
Rietveld refinement of synchrotron X-ray data (Fig. 6, S14). Based
on this we identify five stages in this process (Fig. 6b, 6g). Firstly,
from RT to 500 °C the system only seems to undergo thermal
expansion, with seemingly uncorrelated thermal expansion
coefficients of the two phases (~1.06 10-5 and ~1.29 10-5 °C-1 for
c
d
Ni-rABO3
Reduced
Oxidized
Tested
NiO
Ni
10.0
10.5
2 q (°)
rABO3 and NiO, respectively)[42]
.
Figure 5. Using submerged nanoparticle system to activate methane. (a)
In the second stage, methane activation is initiated through a
sharp CH4 consumption peak at ~550 °C. This coincides with a
decrease in the NiO fraction from ~10.3 to 9 wt.% which
corresponds to the amount of NiO we have shown to be on the
surface (Fig. 3f). Therefore, it seems that the initial sharp CH4
peak is mostly due to the surface particles being reduced, which
is probably not surprising considering that the Ni/Al2O3 system
exhibits a similar initial peak. In this stage, the unit cell parameter
of the rABO3 matrix also rises sharply, indicating it is being
reduced by CH4. This process is probably facilitated by the newly
formed surface Ni metal particles which activate CH4, with the
perovskite supplying the O2- ions (Eq. 1). Consequently, this
leaves vacancies in the perovskite lattice (Eq. 2) which enable
hopping sites for oxygen to be transported and, at the same time,
the electrons released in the perovskite lattice will decrease the
oxidation state of the B-site ions (Eq. 3), expanding the unit cell.
Methane consumption as function of temperature for various systems
a
(150 mg) : Ni/Al2O3 (green) (SSA ~ 100 m2/g), a system with surface and bulk
exsolved particles, Ni-rABO3 (blue) (SSA ~ 1 m2/g) containing nanoparticles
exsolved at the surface and within the bulk, and P1 and P2 (black),
representative of a sample with only surface particles and one with only bulk
particles (all surface particles were etched), respectively. (b) Selectivity over
carbon-containing products corresponding to the experiment described in a. (c)
Room temperature XRD data for Ni-rABO3 at different stages: fresh (i.e.
reduced), after oxidation and after testing for CH4 activation in the experiment
described in a. (d) SEM image of the surface after the experiment described in
a.
enough to promote CH4 conversion. However, when both surface
and bulk particles are present (Ni-rABO3), they operate in tandem
and all the submerged NiO particles (located as deep as tens
of µm under the surface) are being used and converted to Ni
metal during the catalytic transformation of CH4 to syngas, as
evidenced by XRD (Fig. 5c). In fact, the phase change from NiO
to Ni accounts for 85% of the conversion to oxygenated products,
the remaining being due to the oxygen capacity of the rABO3
matrix. Last but not least, the rABO3 matrix is capable of efficiently
mediating O transfer between the embedded particles and the
surface-activated CH4 due to its strain-induced functionality. This
is also apparent from comparing the activity of systems with
various embedded nanoparticle content which shows that
conversion generally increases with increasing amount of
embedded nanoparticles and thus with potential bulk strain, as
given by the s/d parameter (see Fig. 3 and S7e). As shown above,
in reduced state, the metallic nanoparticles experience expansive
strain, while in oxidised state (required for methane conversion),
it is the perovskite that experiences expansive strain (the unit cell
parameter of NiO is larger than that of the perovskite by ~7%).
Expansive (or tensile) strain has been previously shown to
퐶퐻4 + 푂2− ⟶ 퐶푂 + 2퐻2 + 2ⅇ−
푂푂푥 ⟶ 푂2− + 푉푂∙∙
(Eq. 1)
(Eq. 2)
(Eq. 3)
ⅇ− + 퐵퐵푥 ⟶ 퐵퐵′
In the third stage, which spans a time/temperature window of 10
minutes/ 50 °C, respectively, the unit cell of the perovskite
remains relatively constant while the submerged NiO
nanoparticles are being gradually converted to Ni metal. During
this stage CH4 is almost exclusively converted to syngas (H2:CO
2:1, Fig. 6b). The fact that the rABO3 unit cell does not change
during this stage is rather intriguing considering that it should
increase due to further reduction or, most importantly, due to
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10.1002/anie.201915140
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continued thermal expansion. This seems to imply that the
dramatic lattice contraction that accompanies the transformation
of NiO to Ni (15%) greatly affects the perovskite lattice,
counterbalancing and negating its thermal expansion.
In the fourth stage, starting once all the bulk NiO has been used,
there is an increase in the slope of the rABO3 unit cell which is
larger than the one observed in the thermal expansion stage,
indicating that the perovskite is now being slightly reduced and
assisting with the conversion of a small, final amount of CH4. Once
this is completed, the slope of the unit cell of the perovskite reverts
to the value corresponding to thermal expansion.
Therefore, as opposed to the first stage dominated by seemingly
uncorrelated thermal expansion, under reaction conditions the
system works in a concerted manner.
Time (min)
g
a
d
80
90
100
110
10.5
10.2
9.9
10.5
10.2
9.9
9.6
9.6
1
2
4
e
6
4
2
3
5
b
c
5
4
3
2
1
0
5
CH4
CH4
O2
4
3
2
1
0
H2
0
3
6
4
2
H2
CO
2
1
CO2
CO
CO2
Methane activation
0
10
5
f
10
Thermal
expansion
5
0
0
500
550
600
650
30
40
50
60
70
Temperature (°C)
Time (min)
Figure 6. Operando mechanistic insight into CH4 conversion with submerged nanoparticles. (a) XRD data acquired during the temperature programmed reduction
under 5% CH4 of a system with submerged nanoparticles presented as 2D plot, as a function of diffraction angle, time and corresponding temperature. (b) CH4
consumed and corresponding H2, CO2, CO produced during the experiment described in a. (c) NiO content and perovskite unit cell parameter calculated from a by
Rietveld refinement and plotted as a function of time and temperature. (d) 2D plot of the XRD pattern of the material in operando while cycling the material between
redox feeds at 650 oC. (e) CH4, O2, H2, CO, CO2 gas composition at the outlet during experiment described in d and rABO3 microstrain during cycling calculated from
Rietveld refinement. (f) Ni and NiO content and perovskite unit cell parameter calculated from d by Rietveld refinement and plotted as a function of time. (g) Schematic
of methane conversion mechanism corresponding to the main stages identified from a.
In view of potential applicability, we monitor the catalytic, redox
performance and structure in operando during cycling (Fig. 6d-f).
We choose to do this at 650 °C, which is at the end of the activity
the phase analysis shown in Fig 6f indicates that the embedded
particles are being cycled between reduced and oxidised state
and do not redissolve into the perovskite upon oxidation because
temperature window identified by the experiment shown in Fig. 6b. the fraction of the NiO remains constant and equal to the starting
For the reduction (CH4) cycle we used a length of 10 minutes and
for the oxidation cycle (O2) 5 minutes. This experiment revealed
that the system reversibly operates in a coke-free, highly selective
manner, showing no signs of mechanical degradation (e.g.
cracks) that normally occur in composites under cycling (Fig. S15).
Based on the structural changes identified from the Rietveld
refinement of the data, the oxidation of the system is very fast,
occurring in less than 1 minute, while the reduction was slower,
typically being completed in about 6 minutes (Fig. 6f). Additionally,
value upon consecutive redox cycles. Moreover, throughout this
cyclic transformation the perovskite matrix remains strained to the
same levels induced by the first oxidation (Fig 4d, 6e).
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RESEARCH ARTICLE
650 °C 50 mL min-1
In order to test long term stability and cyclability, we use double
the amount of material (300 mg) than in the previous experiments
a
570 °C 30 mL min-1
650 °C 30 mL min-1
100
80
60
40
20
0
100
80
60
40
20
0
and we operate our system at 570 °C and 30 mL min-1 (NTP). In
these conditions, we observe that selectivity remains stable over
130 cycles at >80%, while the conversion starts at 70% and
stabilizes at 55% after 90 cycles (Fig. 7b-c), while carbon
depositions remains <1% (Fig. S16). At the same time, after this
extensive cycling, the overall microstructure of the composite is
retained while the average size of the embedded particles
increases slightly by about 10% and some of the smallest
nanoparticles (<5 nm) seem to have disappeared (see Fig. S16).
As compared to the literature, where similar mass to flow rate
ratios were used, our system achieves similar conversion and
selectivity, at a temperature on average of 300 °C lower, and,
generally, over a larger number of cycles demonstrating better
0
5
10
15
20
25
30
Cycle Number
b
100
80
60
40
20
0
100
80
60
40
20
0
overall stability[30,48,49]
.
0
30
60
90
120
Conclusion
Cycle number
The results presented above vividly illustrate the extensive,
homogenous and controlled endogenous growth of metallic
nanoparticles at nanoscale proximity within perovskite oxides.
The hierarchical nature of this system shares strong parallel with
substitutional doping of materials[11]. Just like doping has been
revolutionary in controlling an extensive array of material
properties, bulk nanocluster “doping” of oxides could provide an
entirely new dimension to tailor hierarchically nanostructured
materials and their emergent functionality. However, as opposed
to substitutional doping which maintains the continuity of the host
lattice, changing its local or extended chemical structure, the
nanocluster insertion causes both physical and chemical
disruption of the host lattice, inducing local strain and forming new
interfaces within the host lattice, offering new opportunities for
strain and interface engineering of materials.
Cycle25
Cycle75
Cycle130
c
8
4
0
H2
CH4
CO
CO2
0
2
4
6
Time (min)
Figure 7. Long term stability. (a)-(b) Conversion and selectivity as a function
of cycle number, during CH4 conversion to syngas using submerged
nanoparticle system (a) at different temperatures and flow rates, using 150 mg
of sample. (b) at 570 °C and 30 mL min-1 (NTP), using 300 mg of sample, over
130 cycles. (c) Outlet gas composition versus time in 3 representative cycles
from experiment b.
Application and long-term durability
By growing a high population of nanoparticles throughout the
oxide, we simultaneously induce strain throughout both phases,
greatly enhancing oxygen transport and exchange as well as
overall system reactivity. This is consistent with previous reports
showing that expansive strain assists with reversible oxygen
incorporation/release[3] while a degree of 1% strain (in our case
0.37% for the bulk particles) increases catalytic reactivity by up to
300%[6,9]. Such strain-enhanced oxygen transport could be
exploited in many fields including, but not limited to, oxide ion
transport membranes, electrolytes or electrodes.
In order to better understand the potential application of the
system, we employ chemical looping partial oxidation of CH4
(CLPO). We test the system under different conditions to identify
suitable operating conditions (Fig. 7a). We choose two
temperatures, 570 and 650 °C, corresponding to the middle and
end of the temperature activity window identified above (Fig. 6b).
We also change the contact time in order to monitor its effect on
the activity and selectivity of the system.
Since the selectivity is seemingly mediated by the embedded
particles and both temperatures are enough to enable oxygen
exchange to access them, CO selectivity varies from 80 to 90%,
proving that regardless of the operating conditions, our system
remains highly selective to the desired products. As expected, the
conversion seems to be higher at higher temperature and longer
contact times. In all cases, the selectivity towards carbon
deposition was lower than 1%.
The large fraction of embedded-metal phase means that the
system displays a very large oxygen storage capacity (about one
order of magnitude higher than conventional perovskites[48]),
which is important in fields such as chemical looping,
thermochemical solar to fuels conversion or three way catalysts
and the reversibility over such a large number of cycles
demonstrated here unlocks new ways of stabilising
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10.1002/anie.201915140
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RESEARCH ARTICLE
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RESEARCH ARTICLE
Controlled growth of metallic
nanoparticles at the surface and bulk
of perovskite oxides induce strain
and promote oxygen exchange with
a methane stream leading to syngas
production.
Kalliopi Kousi,* Dragos Neagu,*
Leonidas Bekris, Evangelos I.
Papaioannou and Ian S. Metcalfe
Page No. – Page No.
Endogenous Nanoparticles Strain
Perovskite Host Lattice Providing
Oxygen Capacity and Driving
Oxygen Exchange and CH4
Conversion to Syngas
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