In operando XAS investigation of reduction and oxidation processes in cobalt and iron mixed spinels during the chemical loop reforming of ethanol

Article abstract of DOI:10.1039/c7ta06785b

FeCo₂O₄ and CoFe₂O₄ nanoparticles have been studied as oxygen carriers for the Chemical Loop Reforming (CLR) of ethanol. By using in operando X-ray absorption spectroscopy we have followed in real time the chemical and structural changes that take place on the materials as a function of temperature and reactive atmosphere (i.e. ethanol/water streams). During the first step of CLR for both oxides the most active chemical species are the cations in the tetrahedral sites, irrespective of their chemical nature. Quite rapidly the spinel structure is transformed into a mix of wüstite-type oxide and metal alloys, but the formation of a metal phase is easier in the case of cobalt, while iron shows a marked preference to form wüstite type oxide. Despite the good reducibility of FeCo₂O₄ imparted by the high amount of cobalt, its performance in the production of hydrogen is quite poor due to an inefficient oxidation by water steam, which is able to oxidize only the outer shell of the nanoparticles. In contrast, CoFe₂O₄ due to the residual presence of a reducible wüstite phase shows a higher hydrogen yield. Moreover, by combining the structural information provided by X-ray absorption spectroscopy with the analysis of the byproducts of ethanol decomposition we could infer that FeCo₂O₄ is more selective than CoFe₂O₄ for the selective dehydrogenation of ethanol to acetaldehyde because of the higher amount of Fe(iii) ions in tetrahedral sites.

Full text of DOI:10.1039/c7ta06785b

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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ARTICLE
In operando XAS investigation of reduction and oxidation
processes in cobalt and iron mixed spinels during the chemical
loop reforming of ethanol
Received 00th January 20xx,
Accepted 00th January 20xx
a,†
b,†
a
c
d
b
,a
DOI: 10.1039/x0xx00000x
F. Carraro, O. Vozniuk, L. Calvillo, L. Nodari, C. La Fontaine, F. Cavani, S. Agnoli*
www.rsc.org/
2 4 2 4
FeCo O and CoFe O nanoparticles have been studied as oxygen carriers for the Chemical Loop Reforming (CLR) of
ethanol. By using in operando x-ray absorption spectroscopy we have followed in real time the chemical and structural
changes that take place on the materials as a function of temperature and reactive atmosphere (i.e. ethanol/ water
streams). During the first step of CLR for both oxides the most active chemical species are the cations in the tetrahedral
sites, disregarding of their chemical nature. Quite rapidly the spinel structure is transformed into a mix of wüstite-type
oxide and metal alloy, but the formation of a metal phase is easier in the case of cobalt, while iron shows a marked
preference to form wüstite type oxide. Despite the good reducibility of FeCo
performance in the production of hydrogen is quite poor due to an inefficient oxidation by water steam, which is able to
oxidize only the outer shell of the nanoparticles. On the contrary CoFe due to the residual presence of a reducible
wüstite phase shows a higher hydrogen yield. Moreover, by connecting the structural information provided by x-ray
absorption spectroscopy with the analysis of the byproducts of ethanol decomposition we could infer that FeCo is more
for the selective dehydrogenation of ethanol to acetaldehyde because of the higher amount of
2 4
O imparted by the high amount of cobalt, its
2 4
O
2
O
4
selective than CoFe
2 4
O
Fe(III)
ions
in
tetrahedral
sites.
7
,8
structural flexibility.
Introduction
Spinels have general formula AB O and are constituted by a
2 4
close packed lattice of face centered cubic oxygen anions,
where the resulting tetrahedral and octahedral holes can be
variably filled by metal cations in such a way that the overall
Metal oxides have always held a strong position in catalysis
given their wide use as supports, promoters, and active
1
,2
phases.
Quite recently though, the interest for these
9
electroneutrality of the system is maintained. In their simplest
materials have increased even more because they perfectly
satisfy the stringent requirements enforced by the new
and most typical form, spinels are formed by divalent and
trivalent cations in a 1:2 ratio, however almost any type of
cation combination, encompassing also monovalent and
tetravalent ions in the right stoichiometry, can be fitted into
the structure. This gives access to the preparation of a wide
range of oxide materials that exhibit a large gamut of different
physicochemical properties. Nonetheless, the huge potential
of spinels, inherent to their structural and chemical versatility,
has been only partially exploited in practical applications
because of a limited understanding of very fundamental
processes such as their oxidation/reduction or phase
transitions and their dynamic behavior in operando
3
standards of sustainability and environmental “greenness” of
4
modern chemistry. This has stimulated the scientific
community to look for new oxides with special structural and
chemical properties that are optimized for specific catalytic
5
applications. In this context, complex oxides i.e. compounds
containing several types of metal cations in different structural
environments, represent ideal candidates for the development
of advanced materials with easily tunable multifunctional
2
,6
properties. Among these, a central spot is occupied by the
large family of spinels, which are crystalline oxides
characterized by excellent stability associated with high
1
0
conditions.
From the chemical point of view, spinels exhibit a quite
fascinating acid-base and redox activity, which play a key role
in several catalytic reactions involving oxygen species and in
2
,8
electrocatalysis. As a matter of fact, these oxides may
present Brønsted or Lewis acid (i.e. undercoordinated surface
cations) and basic sites (i.e. surface hydroxyls and oxygen
species). Moreover, according to the nature of the cations,
they can host a variety of redox couples (e.g. Fe(II)/Fe(III) or
Co(II)/Co(III)).
Unfortunately, such versatility and flexibility come at a price of
high complexity, so a high level of empiricism characterizes
this field. A substantial gap of knowledge still prevents a
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rational design of spinel materials for specific catalytic used as solid oxygen carriers and novel catalysts for the
applications.
selective oxidation of alcohols.
Recently however, due to the development of advanced in
operando techniques, it has been possible to obtain an
Results and Discussion
exceptionally
detailed
understanding
about
the
structure/activity correlation in these materials. As an
example, it has been shown by X-ray absorption spectroscopy
In this work, we have focused our attention on the two end
members of the family of mixed Fe-Co spinel oxides: CoFe
2 4
O
(XAS), that the high chemical reactivity observed in Mn based
2 4
(cobalt ferrite) and FeCo O (iron cobaltite) that were prepared
spinel catalysts in the oxygen evolution reaction and oxygen
evolution reaction can be traced back to Mn in octahedral
by a simple precipitation route from aqueous solution (see SI
for details about the synthesis).
1
1
sites. On the other hand, in the field of gas-phase
heterogeneous catalysis, it has been demonstrated by in situ
Diffuse Reflectance Infrared Fourier Transform Spectroscopy
Characterization of as-prepared catalysts
.
Combining X-ray Diffraction (XRD), Raman spectroscopy,
Energy Dispersive X-ray spectroscopy (EDX), Mössbauer and X-
ray Absorption Spectroscopy (XAS), we have determined the
structure and the chemical composition of the two samples.
Fig. 1b shows the XRD data for the two Co-Fe oxides. The
(DRIFTS) combined with x-ray absorption spectroscopy (XAS)
and Raman spectroscopy, that octahedrally coordinated Co(II)
sites are more easily oxidized to Co(III) in comparison with
Co(II) in tetrahedral site, and Co(III) species are the active
pattern of CoFe
Fe ), as reported in our previous papers Similarly, the
diffraction data of FeCo indicate the formation of a spinel
No other phases are observed, confirming that a
homogeneous and highly crystalline FeCo and CoFe
phases are formed at the synthesis temperatures (i.e. 450°C
for CoFe and 900ºC FeCo ). As a consequence of the
elevated calcination temperature, the iron cobaltite exhibits
2 4
O shows the typical reflections of magnetite
1
2
2
7
centers for the oxidation of organic volatile compounds.
(
3 4
O
These results highlight the great interest for understanding the
interplay between geometrical (e.g. tetrahedral vs octahedral
occupancy) and chemical factors (e.g. oxidation state) to
determine the final reactivity of materials.
2 4
O
2
8,29
structure.
2
O
4
2 4
O
In the present work, we decided to investigate two exemplary
2
O
4
2 4
O
mixed Co-Fe spinels, the cobalt rich iron cobaltite (FeCo
and the iron rich cobalt ferrite (CoFe ). This latter has
demonstrated to be an extremely interesting material for
2 4
O )
2
-1
2 4
O
lower specific surface area (4 m g ) and larger crystallite size
2
32 nm) than cobalt ferrites (12 nm crystallite size, 69 m g
-1
(
1
3,14
15,16
electrocatalysis,
oxidation reactions,
and as oxygen or
surface area) (see Table S1). The spinel structure of both
samples is also confirmed by the Raman spectra reported in
Fig. 1a. Vibrational spectroscopy is a powerful technique for
the structural analysis of spinel oxides, since their
spectroscopic fingerprint is highly specific, and strongly
affected by stoichiometry, cation distribution, and defects. The
Raman spectra of spinels are generally complex and show
more Raman-active modes than those predicted by group
1
electron carriers for a two steps Chemical Loop Reforming
7
1
8,19,20,21
(
CLR).
FeCo
untapped.
On the contrary, very few works are focused on
2 4
O
and its potential in catalysis still is largely
2,23,24
2
CLR represents a potentially carbon neutral process for the
production of high purity hydrogen. It is based on a dual
thermal redox cycle. At first the oxide material is exposed to
reducing atmosphere to obtain highly reduced phases and
then to steam for its reoxidation and consequent formation of
theory:
g 1g
F2g(1), E , F2g(2), F2g(3) and A . The additional
vibrations may appear because of local distortions of the
crystal lattice. These defects do not affect the long range order
of the system, and often cannot be directly detected by XRD.
2
H .
2
5
We have focused our attention on the use of ethanol as
2
6
reducing agent. This choice is quite appealing because
ethanol is a not toxic, low cost, fuel that can be easily obtained
from renewable sources and on the other hand Co-Fe mixed
2 4
The strong intensity and sharpness of the bands in the FeCo O
spectrum confirm the high crystallinity of the sample. Five
main vibration modes are observed at 192, 479, 507, 598 and
-
1
-1
73 cm , the latter showing a shoulder at about 645 cm . The
spinel are abundant and cheap materials with
a low
6
environmental impact. CLR of ethanol on spinels therefore has
a great potential for the sustainable production of hydrogen.
most intense vibration modes are related to MO (the peak at
4
−
73 cm and its shoulder) and MO
1
-1
6
6
stretching (at 192 cm ).
By
a combination of different in situ characterization
2 4
The CoFe O spectrum presents six vibration modes, as already
3
0
techniques and catalytic measurements we have investigated
the structural and chemical properties of these mixed Co-Fe
spinels during each step of the CLR of ethanol. For the first
time, we were able to follow in real time, the reduction
process of mixed spinels, monitoring the formation of a series
of reduced phases. We were also able to identify which are the
most chemically active cations inside the spinel structure and
what are the parameters that control the chemical activity
toward ethanol.
reported in the literature. The five first order Raman modes
can be observed at about 182, 306, 476, 562, 636 and 690
−
1
−1
cm . The intense vibration modes at 636 and 690 cm are the
1g(2) and A1g(1), respectively, which can be assigned to the
MO stretching vibrations and to the disorder effect of Co(II)
and Fe(III) over the T
The oxidation state and site occupancy of the metal species in
A
4
3
0,31
d
h
and O sites.
3
2,33,34
the spinel structure were determined by ex-situ XAS.
In
transition metals K-edges, XANES transitions involve the
excitation of a 1s photoelectron into a mainly 4p derived
continuum of states. In general, when the valence of the
absorbing metal atom is increased (i.e the metal is present in a
The fundamental knowledge acquired in this study may be a
stepping stone for the design of new improved materials to be
2
| J. Name., 2012, 00, 1-3
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edge peak intensity of cobaltite is higher than that of ferrite,
indicating that the cobaltite contains a higher amount of
tetrahedrally coordinated Co ions. Conversely, the Fe K-edge
pre-peak intensity is higher in the CoFe
2 4
O sample. The cation
distribution was determined by EXAFS data of the starting
2,34
3
materials.
The crystallographic site occupancy in spinels is
defined by the inversion parameter γ, i.e. the fraction of
9
tetrahedral sites occupied by majority ions. The best fits of
the FT EXAFS data are reported in Fig. S1 and S2 and the
corresponding parameters are reported in Tables S3 and S4. At
Fe K-edge, the first peak at about 2 Å (not phase-corrected) is
due to the contribution of two Fe-O single scattering paths:
FeTetra-O and FeOcta-O, which correspond to the interatomic
distances between the two first coordination shells of oxygen
ions, respectively. In the region between 2.0 and 3.5 Å, two
contributions are observed, one at around 2.5 Å is related to
FeOcta-MOcta (M=Fe, Co) scattering paths and another one at
about 3.5 Å to FeTetra-MTetra or FeTetra-MOcta interactions. In fact,
due to the similar cross sections, from EXAFS spectra it is not
possible to distinguish between Fe and Co as backscattering
atom. The EXAFS spectra at Co K-edge can be interpreted
along the same line as well. An inversion degree of 0.8 and 0.6
was calculated for CoFe O and FeCo O , respectively. These
2
4
2 4
28
values are in agreement with previously reported data.
Therefore, the cation distribution can be summarized as:
Tet
Oct
[Co(II)0.4Fe(III)0.6Co(III) ] O4
Oct
FeCo O : [Co(II)0.6Fe(III)0.4
2
]
Tet
4
1
CoFe O : [Co(II)0.2Fe(III)0.8
2
] [Co(II)0.8Fe(III)1.2] O4
4
The results obtained by EXAFS were further verified by
Mössbauer spectroscopy measurements, which were taken
both at RT and 80 K.
Concerning FeCo O , RT spectra shows a broad magnetic
2
4
pattern (Fig. 2a) as results of the magnetic relaxation due to
2
8
high ordering temperature. On the contrary, CoFe O , shows
2
4
a fully magnetically ordered RT spectrum. (Fig. 2b). No traces
of superparamagnetic components are detectable.
Fig. 1 Raman spectra (a) and XRD patterns of as prepared materials; XANES spectra at
A
Fe(c,e) and Co (d,f) K-edge for as prepared CoFe
2 4 2 4
O (c,d), FeCo O (e,f) and reference
preliminary fit excluded the presence of pure Fe(III) oxides,
such as hematite. After cooling down the FeCo O sample to
compounds.
2
4
8
0 K, the spectrum transforms into a well-defined magnetic
higher oxidation state), a shift of the absorption edge toward
higher energy is observed. XANES and Fourier Transform (FT)
EXAFS spectra for the Co-Fe spinels are shown in Fig. 1 c,d,e,f.
Comparing the energy position of the absorption edge in the
XANES spectra with that of the reference samples, it is possible
to determine with great accuracy the oxidation state of each
pattern (Fig. 2c), typical of Fe species in a spinel structure. As
explained in the Experimental Section, the spectrum was fitted
by using only two broad sextets. The so-obtained parameters
were attributable to Fe(III) in tetrahedral and octahedral
environment, whose relative areas are 44 and 56 respectively.
Therefore, cation distribution within the spinel structure is
similar to that suggested by EXAFS data, and the cobaltite can
element in the spinel structure (Fig. 1 c,d,e,f). In CoFe
2 4
O , the
oxidation state of Fe and Co is close to +3 and +2, respectively,
3
Tet
Oct
2
be written as: [Co0.56Fe(III)0.44
Concerning the CoFe sample, the spectrum acquired at 80 K
Fig. 2d) was firstly fitted by using two sextets, relative to the
] [Co1.44Fe(III)0.56] O
4.
2 4
as expected; whereas in FeCo O , the oxidation state for Fe
2 4
O
and Co are +3 and +2/+3, respectively. Moreover, the fine
structure of the pre-edge can provide further information
(
3
2,33,34
octahedral and tetrahedral site. The tetrahedral and
octahedral populations were estimated respectively in 37 and
about the structure of spinels.
edge peaks, which correspond to 1s
p mixing), may occur at about 15-20 eV before the white line.
At K-edges in fact, pre-
→
3d transitions (with 3d-
63 of the total Fe. Once again, assuming γ= 0.37, the
stoichiometry of the spinels is in good agreement with the
4
An increase of the intensity of these features corresponds to
the removal of the inversion symmetry in the environment of
the absorbing atom. Therefore, the pre-edge feature can
provide a direct indication whether a metal center is located in
a tetrahedral or octahedral site. At the Co K-edge, the pre-
EXAFS data, in fact the proposed formula is
Tet
Co0.26Fe(III)0.74
Oct
4
] [Co0.74Fe(III)1.26] O . The small discrepancy in
the so calculated stoichiometry can be ascribed either to small
errors in the area calculation or to small differences in the
recoilless fraction of the two different sites.
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Fig. 3 Oxidation states calculated from the edge position in the XANES spectra collected
during the annealing treatment in reductive atmosphere (0.69 % ethanol in N ).
2
outlines a marked difference of reducibility in the two spinels.
FeCo is stable up to 350°C, above this temperature it starts
2 4
O
2 4 2 4
Fig. 2 Mössbauer spectra of FeCo O and Co FeO , collected at RT, a) and b), and at
8
0 K, c) and d). Black dots represent the experimental data, black lines the calculated
to reduce quickly, and at 425°C the material is mainly made up
by the corresponding body centered cubic (bcc) metal alloy
phases, as suggested by the spectral fingerprint of FT EXAFS
spectra (Fig. S6) and by the XRD patterns of the reduced
spectra, red line the Fe(III) tetrahedral site and blue lines the sub-components of the
octahedral site.
In both cases, the proposed fitting highlights the linewidth
broadening due to the effect of a multiplicity of hyperfine
magnetic fields on the Fe(III) nuclei. If the contribution of the
Near Next Neighbors (NNN) on the tetrahedral site can be
3
6
samples (Fig. S7). Moreover, several works indicate that
alloys are the most stable phases formed under reducing
3
7,38
condition for mixed ferrites.
2 4
CoFe O shows a similar
3
5
behavior, although the reduction is more gradual: Fe starts to
reduce at 150°C and becomes metal Fe at 500ºC, whereas Co
is stable up to 250 ºC and then it is gradually reduced to metal
Co. It has to be noted however, that the higher reducibility of
neglected, on the contrary on the octahedral sites it needs to
be taken in account.
Therefore, by using the binomial distribution described in the
Experimental Section, the proper number of subcomponents
and their relative areas can be obtained. Considering negligible
the areas smaller than 5% of the total iron, the binomial
distribution on the octahedral sites in iron cobaltite predicts
the presence of four sextets, whose relative areas are given by
P(4):P(3):P(2):P(1)=18:30:29:16. Also in cobalt ferrite, the
binomial distribution predicts four sextets, with an intensity
ratio of P(4):P(3):P(2):P(1)=11:25:33:22. The intensity of the
sextets were recalculated on the basis of the total area of the
octahedral site and fixed during the fitting procedure. The
parameters are reported in table S5, supplementary materials.
2 4
CoFe O can be also due to the higher surface area and
defectivity of this material. In order to investigate the effect of
cation site occupancy in the two spinels, we carefully analyzed
the intensity trends of the EXAFS peaks (Fig. 4) and of the LCF
2 4 2 4
of XANES spectra (Fig. 5) in CoFe O and FeCo O during the
thermal treatment in presence of ethanol. From these data a
general behavior of the reduction mechanism is deduced,
which appears to be largely controlled by the type of cation
These differences in NNN configuration reflect the different γ
in the two spinels, that is the different statistic distributions of
Co atoms in the cobaltite and ferrite structures.
In operando XAS: annealing in reductive atmosphere
2 4 2 4
Freshly prepared FeCo O and CoFe O spinel oxides were
2
-
evaluated as electrons/O carrier materials for hydrogen
production via CLR of ethanol. The changes of the materials
during the redox cycle were followed in real time by XAS
measurements in order to determine the reactivity of the two
metal ions in the different geometrical-sites. As first step, we
investigated the reaction of the materials toward ethanol as a
function of temperature. In operando XAS was performed
during the annealing of the samples from RT to 500°C in
reducing atmosphere (0.69% vol of ethanol in N
2
). Fig.
3
reports the variation of the mean oxidation state of Fe and Co
determined from the edge position as a function of
temperature. The change of the average oxidation state of the
cations was calculated by XANES spectra (Fig. S3), whereas
modifications in the geometric site were deduced by the
EXAFS data (Fig. 4). The body of our experimental data
Fig. 4 Fourier transform EXAFS spectra at Co K(a,c) and Fe K (b,d) edge for CoFe
2 4
O
(
a,b), FeCo (c,d) during the annealing treatment in reductive atmosphere (0.69 %
2 4
O
4
| J. Name., 2012, 00, 1-3
ethanol in N2).
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to wüstite type oxides. Comparing the two oxides, FeCo
2 4
O
2 4
seems to be more stable toward reduction than CoFe O , as
evidenced by the higher temperature needed to reduce the
signals coming from the spinel. However, the higher stability
may be due to the larger size and low surface area of iron
cobaltite nanoparticles that prevent a fast surface reaction. On
the other hand, CoFe
conversion to the wüstite phases.
significant role in FeCo , as a matter of fact, the LCF data of
Fig. 5c indicate the presence of a relevant fraction of FeO
about 10%), which remains stable up to 350°C. Such wüstite
2
O
4
shows an almost immediate
Defectivity plays a
2 4
O
(
phase is likely the product of a low temperature reaction
channel of ethanol with extrinsic defects. This alternative
reaction path is already active at room temperature but is
limited by the number of defects, and once they are
consumed, the reaction does not proceed further.
At high temperatures (> 350°C), the wüstite phases and the
residual ferrite/cobaltite are mostly converted to metal
phases. From the LCF analysis, it is clear that the formation of
the metal phases starts only when a relatively large amount of
Fig. 5 LCFs (% of composition vs temperature) of Fe(a,c) and Co (b,d) K-edge for
CoFe
2 4 2 4
O (a,b) and FeCo O (c,d) during the annealing treatment in reducing
(
Co, Fe) mixed wüstite is formed; therefore, a direct
atmosphere (0.69 % ethanol in N
2
)
decomposition of the spinel to the metal seems unlikely. In
coordination. The cations in tetrahedral sites (A) are more general, the reduction of ferrites follows a path that is strongly
prone to reduction than those in octahedral sites (B). This is dependent on the type of reducing agent (H , CO, alcohols),
2
4
temperature, but also morphological factors. In the case of
2
evidenced by the decrease of the peak corresponding to MOcta
-
4
3
M
Tetra bonds until its complete suppression at about 350°C, the Fe O it has been determined by thermodynamic data,
3
4
whereas the MOcta-MOcta peak intensity decreases only slightly. that a triple point exist at 1121 K where Fe O , FeO, Fe coexist,
3
4
Combining the EXAFS data with the LCF analysis at the Fe K whereas for lower temperature Fe O is directly converted to
3
4
and Co K edges (Fig. 5), it can be concluded that the metal Fe. This prediction has been later verified experimentally
4
4
disappearance of the MOcta-MTetra peak is related to the in the experimental work of Bohn et al. On other hand, for
formation of M(II) oxide phases, probably a mixed (Fe,Co)O1-x
several spinel ferrites it has been reported an easy and direct
wüstite (as suggested also by the XRD patterns acquired on the formation of metal phases: for example, NiFe O is directly
,
2
4
reduced samples, Fig. S7b). In the case of FeCo
2
O4, the reduced at 850°C by simulated biomass pyrolysis gas to a
4
5
maximum conversion to M(II) oxides is reached at 400°C, metal Nickel phase and Fe O4.
Similarly, accurate
whereas in the case of CoFe O at 375°C. These M(II) oxides experimental and theoretical investigations suggest that
2 4
3
are highly disordered, also in the short range order, since no during the reduction with syngas, only in the limit of very low
clear EXAFS peaks can be identified. Only in the case of (<0.03) CO/CO and H /H O ratios, CoO and NiO are formed
2
2
2
majority cations of the spinels (B cations, i.e. Fe in ferrite and from Co,Ni mixed ferrites, otherwise a direct decomposition to
2
0
Co in cobaltite), the growth of a shoulder related to M-O metal Ni and Co and iron enriched spinel is observed. More
bonds typical of wüstite structures is identified at 1.7 Å, recently, in the case of the reduction of cobalt ferrites, at
starting from 375-400°C. The higher activity of tetrahedral 600°C by a mixture of syngas and CO no trace of wüstite
2
sites is somehow in contrast with a previous report about the phase was observed by in situ XRD, but only a direct
1
8
reactivity of mixed ferrites toward methanol, where by ex situ conversion of mixed spinel to a Co-Fe metal alloy. Anyway all
characterization it was suggested higher activity of the reports in the literature suggest that the introduction of
octahedral sites, however it is in agreement with other Co, Ni, and other first row transition metal cations facilitates
a
3
9
4
0,41
29,46
investigations on the reaction mechanism of spinel.
As a the reduction of both the parent spinel Co O and Fe O .
3 4 3 4
matter of fact, Mössbauer experiments suggest that during the Our in operando results therefore outline a different scenario
thermal or chemical reduction of ferrites, the removal of that clearly points to the formation of a large fraction of
oxygen induces a change in the oxidation state of cations that intermediate wüstite phases even at very modest
therefore adjust their site occupancy, by populating interstitial temperature. It is also interesting to note that the presence of
4
1
sites: i.e. sites that are not normally occupied in the parent metal Fe phase in reduced FeCo O is observed only above
2
4
structures, however the structural details of such processes 400°C, which is a much higher temperature than that required
are strongly dependent on the specific oxide structures. Our for the formation of metal Co in the same material or metal Fe
XAS data therefore, for the first time capture time resolved in CoFe O . As a matter of fact, the strength of Co-O bonds is
2
4
snapshots of tetrahedral cations diffusion into normally smaller than Fe-O and one of the lowest among first row
4
0
7
unoccupied octahedral sites forming cation excessive spinels, transition metal oxides, and in general the introduction of Co
which at the limit of high reduction are eventually converted in iron spinel has the effect to promote reducibility. This is in
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ARTICLE
Journal Name
agreement with previous TPR measurements and reactivity the prevailing reactions, which result in the oxide reduction,
2
9
studies toward methanol and suggest that not only the are the total and partial ethanol oxidation. Other by-products
chemical composition, but also structural factors are very as acetaldehyde, acetone, ethyl acetate, ethylene and ethane
important to understand the reactivity of mixed oxides. Finally, were detected mainly at the beginning of the reduction. This
the analysis of the M-O peak intensity (1.2-1.6 Å) indicates that means that the formation of these species is strongly
the A cations are fully reduced at temperature lower than that connected to the presence of the spinel phases, whereas as
for the B cations in both samples (Fe in FeCo
CoFe ). Notably, the LCF analysis suggests that single metal suppressed. This has been confirmed by in operando
spinels (Co or Fe ) are not formed in any of the materials investigation by XAS (see below).
2 4
O and Co in soon as the mixed wüstite is formed their amount is strongly
2 4
O
3
O
4
3
O
4
\
st
during the annealing in ethanol.
After the 1 step with ethanol, the reduced material was then
o
re-oxidized by water steam at 450 C during the same time on
CLR of ethanol: catalytic tests
nd
Bearing in mind these results, we tested the redox and stream of 20 min, which corresponds to the 2 step of the
catalytic properties of these materials with respect to the two- conventional two-step CLR process (Fig. S12, 13 and 14). Mass
step CLR of ethanol at two different temperatures: at 350°C, spectroscopy data indicate a significant difference in the
because this is the threshold for the reduction of the catalysts; amount of produced H
and at 450°C, because at this temperature both spinels are is proportional to the reduction extent of the solid together
mostly reduced to a metal phase. Previously, the catalytic with its ability of being re-oxidized by means of H O. The
properties of M Fe spinels as catalyst for CLR at 450°C were higher hydrogen yield (~20 %) is obtained on CoFe , whereas
investigated by Vozniuk et al. In the CLR process, the first on FeCo its amount is 3-4 times lower (~5%). Beside the
step is the anaerobic oxidation of ethanol, whereas the second hydrogen production, the common feature for all the samples
one is the reoxidation of the catalyst by water vapor exposure. is the formation of CO (CO and CO) during the re-oxidation
2 x
and CO gases over two spinels, which
2
x
y
O
4
2 4
O
2
7
2
O
4
x
2
Considering that the anaerobic oxidation is a stoichiometric step (Fig. S13 and S14), because of the gasification of
process, the reduction state of the material is pivotal in carbonaceous residues, previously formed during the
determining the selectivity of the reaction path. Therefore, the reduction step (see CHNS elemental analysis in Table S2
st
temperature has a major role in this reaction. The amount of performed after the 1 step with ethanol).
each reaction product is affected by the extent of the metal CLR of ethanol: in operando XAS.
oxide reduction, which is a general statement valid for all In order to have an atomic scale insight into the origin of the
2
1
oxygen carrier materials.
different chemical activity of the two spinels, in operando XAS
o
During the reduction step performed at 350 C for 20 min, the measurements have been carried out under Chemical-Loop
ethanol conversion and the selectivity to the main products conditions. In particular, we have monitored the Fe K and Co K-
were determined by gas chromatography (on-line micro-GC). edges during ethanol (20 minutes, 5.29% in nitrogen) or water
The ethanol conversion over FeCo
Fig. S9) is 18% and 20%, respectively. The main products working temperature (450°C) was reached in inert atmosphere
formed during the reaction were identified as follows: at 5°C/min, and XAS measurements were also acquired during
acetaldehyde, ethyl acetate, acetone, H O, CO , H , ethylene the heating treatment. Fig. 6 shows the XANES and EXAFS
and ethane. Acetaldehyde is the predominant product for spectra after each process step for both samples at the Fe K
FeCo with an initial selectivity of about 69 %, (which and Co K edges. After the thermal treatment, the oxidation
decreases to 57% at the end of the cycle). On the other hand, states of Fe and Co in FeCo are almost unchanged, whereas
acetaldehyde selectivity over CoFe is significantly lower and a slight reduction of CoFe is evidenced by the decrease of
2
O
4
(Fig. S8) and CoFe
2
O
4
(20 minutes, 8.31% in nitrogen) exposure at 450°C. The
(
2
2
2
2 4
O
2 4
O
2
O
4
2 4
O
with an opposite trend as a function of time, ranging from 17 the white line intensity at both edges, as well as by the shift of
to 28% at the beginning and after 20 min of ethanol exposure, Fe K-edge energy toward lower values. This behavior could
respectively. The formation of acetaldehyde is likely due to the have two explanations: (i) cobaltite was synthesized at higher
ethanol dehydrogenation/oxydehydrogenation reactions. Ethyl temperature than ferrite and, therefore, it is expected to be
acetate represents the second most important reaction more stable; (ii) the particle size of cobaltite is larger than that
product, and the two spinels show similar selectivity (19% in of ferrite, therefore, cobaltite is less reactive. Moreover, the
cobaltite and 28% in ferrite at the end of the cycle). Notably, EXAFS peaks of both materials after annealing in inert
the selectivity to acetone over CoFe
2
O
4
is in the range from 17 atmosphere are less intense due to an increase of the disorder
is much lower (<2%). of the structures (high cation mobility) and to the noise
to 11%, while its selectivity over FeCo
2
O
4
Fig. S10 and S11 show the conversion and the selectivity to the induced by temperature and gas flow. EXAFS data suggest that
main products, obtained during the reduction step performed the metal cations in tetrahedral environment are the most
o
at 450 C for 20 min. Already at the beginning of the reaction, reactive species under these conditions, since the Mocta-Mtetra
the ethanol conversion is quantitative and is stable during the interactions suffer the largest drop in intensity. From the LCF
reduction time for both oxides. One of the main products is analysis (Fig. S4, time=0 min), we can conclude that only a
hydrogen, with an initial selectivity of about 30% (FeCo
and 51% (CoFe ), which increases up to 73 and 78%, 5%) are converted to a wüstite phase after the annealing in
respectively, at the end of the cycle. The trend observed for inert atmosphere.
2
O
4
)
2 4 2 4
small fraction of CoFe O (about 20%) and of FeCo O (about
2 4
O
hydrogen very similar to that of CO
x
and H
2
O, suggesting that
6
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3
Fig. 6 XANES spectra at Co K-edge (a,c) and Fe K-edge (b,d) and Fourier transforms of k χ(k) at Co K-edge (e,g) and Fe K-edge (f,h) of FeCo
O
2 4
(a,b,e,f) and CoFe
O
2 4
(c,d,g,h ) after
three successive steps: annealing in inert atmosphere at 450°C, 20 minutes of ethanol dosing (0.08 ml/h) and 20 minutes of water dosing (0.04 ml/h).
2 4
In operando quick XAS spectra acquired during the following other hand, CoFe O is reduced to a mixed wüstite in first
reduction and re-oxidation cycles (Fig. 6), confirm that the two place and, subsequently, to a Co-Fe alloy. After 2 minutes in
spinels exhibit a rather different reactivity. After the first ethanol atmosphere, about 80% of Fe and 45% of Co are
reduction cycle in ethanol atmosphere, FeCo
2 4
O is almost converted to a wüstite phase (Fig. S4). Interestingly, Co prefers
completely reduced: both the Fe K and Co K-edges XANES (Fig. to form a metal phase rather than a reduced Co(II) oxide, as
6a and 6b, and the corresponding LCF in Fig. S4) and EXAFS demonstrated by the almost concurrent formation of CoO and
(Fig. 6e and 6f) data confirm the formation of metal phases. Co metal. Fe, instead, is quickly and almost quantitatively
The peak corresponding to M-O bonds disappears, whereas transformed into wüstite, which is only partially reduced to
that corresponding to M-M in the metal phase appears. About metal. This quantitative conversion of Fe is in contrast with the
5
0 % of the iron cobaltite is rapidly converted (2 min) to a behavior of Fe in FeCo
wüstite phase. This phase, however, has only a transient both to metallic and wüstite phases. At the end of the cycle on
existence due to the fast reduction to the metal phase. The CoFe , the reduction of Fe to metal is only 60% to be
formation of metal Fe and Co starts as soon as some mixed compared with 90% for cobalt. It has been previously reported
wüstite phase is formed, and after 10 minutes reaches a that the addition of Co to -Fe , forming non-stoichiometric
plateau that corresponds to the conversion of about 85% of Co spinel, has a strong influence on its reduction properties. Pure
2 4
O , where was gradually converted
2 4
O
γ
2 3
O
4
7
and 75% of Fe to metal alloy phases, . Both cations are easily iron oxide and spinels are reduced only to FeO, whereas the
18
reduced; however, this tendency is higher for Co than for Fe, cobalt doped ferrite can be reduced up to Fe-Co alloys
which in general requires a higher temperature and is more During the subsequent oxidation process, Co is marginally
efficiently stabilized in the wüstite phase. Overall, the time oxidized by water (Fig. 6c and 6g); whereas Fe is clearly
evolution of the XANES spectra indicates that FeCo
unstable under reducing conditions and high temperature. 6d). The easier oxidation of iron is responsible for the higher
Nonetheless, during the successive oxidation cycle with water yield of hydrogen on CoFe with respect to FeCo . It
2 4
O is rather oxidized to a final oxidation state slightly higher than 2+ (Fig.
O
2 4
2 4
O
vapor, the EXFAS spectra remain unchanged, indicating that should be highlighted that we have not found evidence
the reduced phases are quite difficult to re-oxidize (Fig. 6a and (neither from EXAFS spectra nor from LCF of XANES data) of
6
b). This limited oxidation explains the low hydrogen yield the formation of single metal spinels (Co
detected during catalytic measurements for this material. Combining XAS and catalytic tests: main results.
Previous in situ X-ray Photoemission Spectroscopy (XPS) study Overall, the body of our experimental data indicates that at
3 4 3 4
O or Fe O ).
2
7
on Co-Mn mixed spinel ferrites on the other hand, indicate high temperature (450°C) both spinels are highly active for the
that the surface of these materials can be efficiently oxidized total decomposition of ethanol at 450°C, producing H and CO
by water steam. This apparent contradiction can be solved as main products, whereas at 350°C the overall conversion
considering the low surface area of FeCo , and that XAS is a decreases and other reaction paths, especially oxidative
2
x
2 4
O
bulk technique. Moreover, it has been previously reported for dehydrogenation, becomes relevant. The different Co/Fe ratio
other mixed spinels that the rate determining step of water in the oxides is a key parameter for controlling their chemical
oxidation is the diffusion of oxygen through the newly formed activity. The cobalt rich spinel can be easily reduced, but it is
4
8
oxide layer. Therefore, we can assume that a thin oxide very difficult to be re-oxidized by a mild oxidant as water. In
passivation layer that can be hardly detected by XAS is formed contrast, a higher iron fraction makes the system more
on the metal surface and prevents further oxidation. On the
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reversible, because in this case an intermediate iron wüstite FeCo metal alloys. Conversely, considering the ratio between
4
9
phase, which is a better oxygen buffer, can be stably formed.
Concerning the different selectivity in the anaerobic ethanol of the cation plays an important role. In fact, the total
oxidation FeCo strongly favors the oxidative reduction of Co to Co(0) is faster with respect to that of Fe.
dehydrogenation to acetaldehyde with respect to CoFe 4. The This is reflected in the oxidation step of CLR, where the re-
reduced phases (wüstite and metal alloy), the chemical nature
2 4
O
2
O
very high acetaldehyde yield observed at the beginning of the oxidization of cobaltite by a mild oxidant such as water is
reaction on iron cobaltite can be traced back to very active practically ineffective. This different behavior influences the
tetrahedral species that are almost immediately converted to ethanol decomposition selectivity and yield. At mild
less reactive octahedral divalent species. CoFe
2 4 2 4
O on the other temperature (350°C) FeCo O strongly favors the oxidative
2 4
hand, shows a less pronounced selectivity toward dehydrogenation to acetaldehyde compared to CoFe O .
acetaldehyde. In fact, it was suggested that a decrease of the Moreover, the very high acetaldehyde yield observed at the
amount of Fe(III) in tetrahedral sites in Co/Fe mixed spinel beginning of the reaction can be traced back to very active
5
0
promotes dehydrogenation rather than dehydration. As a tetrahedral species that are almost immediately converted to
matter of fact, our data indicate that the ratio of Fe(III) in less reactive octahedral divalent species. At higher
tetrahedral sites in the two spinels is inversely proportional to temperature (450°C), both spinels are almost totally reduced
acetaldehyde selectivity. Another important parameter that to metal phases and are highly active for the total
2 x
controls the selectivity of the reaction is the type of majority decomposition of ethanol, producing H and CO as main
5
0,51,52
carriers in the material.
theory of catalysis in semiconductors, n-type semiconductors In conclusion, our study indicate that CoFe
can easily provide electrons and favor dehydration reactions. promising material than FeCo for CLR due to a more
Conversely, the presence of mobile holes favors oxidative reversible chemistry connected to the better stability of the
dehydrogenation. Therefore, p-type FeCo is a better wüstite phase. Nonetheless, the fundamental knowledge
catalyst for the oxidative dehydrogenation of ethanol to acquired in this study may be a stepping stone for the design
acetaldehyde, compared to CoFe 4, which is an n-type of new improved materials to be used as solid oxygen carriers
However upon reduction, CoFe is and novel catalysts for the selective oxidation of alcohols.
wüstite-type oxide that is p-type
According to the electronic products.
5
3
2
4
O is a more
2 4
O
2 4
O
2
O
49
semiconductor.
converted to
2 4
O
a
a
semiconductor. This change may explain why as a function of
time the acetaldehyde selectivity increases for the cobalt
ferrite, see Fig. S9.
Experimental
Synthesis of spinel oxides
2 4
CoFe O was synthesized using the co-precipitation method
2
7
reported by Vozniuk et al. Briefly, a solution containing metal
nitrate precursors in the desired molar ratios was drop-by-
drop added into the reaction vessel containing 0.5 L of 2 M
NaOH (Sigma-Aldrich) aqueous solution at 50°C under vigorous
stirring. The suspension was stirred for 2 h at 50°C keeping the
pH above 13 by adding a 3M NaOH solution if necessary. The
precipitate was separated by vacuum filtration, washed with
distilled water and dried at 120°C. Finally, the solid material
Conclusion
By combining different characterization techniques (XRD,
Raman and Mössbauer Spectroscopies), in operando Quick-
EXAFS and catalysis measurements, we have investigated the
structural and chemical properties of mixed Co-Fe spinels
2 4 2 4
(FeCo O and CoFe O ) during each step of the CLR of ethanol.
Firstly, we have studied the freshly prepared materials and we
have determined the oxidation state of Fe and Co and the
inversion parameter γ in the two structures (0.8 and 0.6 for
was calcined in static air at 450°C for 8h with the heating rate
-1
of 10°C min . FeCo
2
O
4
was synthesized following a recipe
28
CoFe
2
O
4
and FeCo
2
O
4
, respectively). Then, by means of in
previously reported in literature. Briefly, iron chloride
FeCl ·6H O, Sigma-Aldrich) and cobalt chloride (CoCl ·6H O,
operando Quick-EXAFS measurements, we have followed in
real time the solid state redox processes of these mixed spinels
as a response to temperature and reactive atmosphere. In
particular, we monitored the reduction of the catalysts during
(
3
2
2
2
Sigma-Aldrich), with a Co:Fe molar ratio of 2:1 , were dissolved
in distilled water and poured into a boiling 2M KOH solution
under vigorous stirring at 80°C for 1 hour. After repeated
filtering and washing with boiling distilled water, the resulting
power was dried at 120°C and then calcined at 900°C for 17h
2
the first step of CLR of ethanol and the oxidation during H O
steam exposure. These structural data were combined with
the analysis of the reaction products in order to identify which
are the most chemically active cations inside the spinel
structure and what parameters control the chemical activity
toward ethanol decomposition. These experiments have
highlighted the influence of both coordination and chemical
nature of the cations on the catalytic properties. The cations in
tetrahedral sites of the spinel structure show a higher
reducibility with the respect of octahedral ones, independently
from their chemical nature. The reduction of the spinel
structure leads to the formation of a wüstite phase and of
-1
with a heating rate of 5°C min .
Materials characterization
In operando Quick EXAFS measurements were carried out in
transmission mode at the Fe K and Co K edges at the ROCK
beamline (SOLEIL synchrotron, France). The Quick-EXAFS
monochromator (equipped with a Si(111) channel-cut crystal)
operated at an oscillation velocity of 2 Hz. The detectors were
ionization chambers. The samples in powder form were
diluted with boron nitride (mass ratio=1:1.2) and inserted into
8
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ARTICLE
quartz capillaries (diameter=1.5 mm, wall thickness=0.03 mm).
The capillaries were centered perpendicularly to the X-ray
beam and connected to a heated gas line. The samples were
heated at the desired temperature (from RT to 500°C) by a
heating gun. During the annealing in reductive atmosphere
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syringe pump and nitrogen as carrier gas (0.69% of ethanol in
N2). During chemical loop reforming (CLR) of ethanol, the
samples were heated at the desired temperature (350 or
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dosed alternatively (20 or 40 minutes steps) using syringe
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continuously acquired during these processes. The
corresponding metal foils were measured simultaneously and
used to calibrate and align the spectra. Pellets of the various
oxide standards were measured in transmission mode at RT.
Powders were characterized by P-XRD in a Philips X’Pert
X’Celerator, with Cu-Kα radiation in a 2θ range between 5-80°
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(
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the pelletized sample (with particles diameter ≈0.25 to 0.6
mm) in the fixed-bed quartz flow reactor with an internal
diameter of 12 mm and a length of 30 cm. The products were
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Conflicts of interest
There are no conflicts to declare.
23 Zhou X., Huang J., Zhang F.M., Zhao Y., Zhang Y., Ding Y..
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The authors wish to acknowledge the award of beamtime on
the ROCK beamline at Synchrotron SOLEIL under proposal
number 20150492. The work on ROCK was supported by a
public grant overseen by the French National Research Agency
2
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(ANR) as a part of the “Investissements d’Avenir” program ref:
ANR-10-EQPX-45. This work was co-funded through
a
SINCHEM Grant. SINCHEM is a Joint Doctorate programme
selected under the Erasmus Mundus Action 1 Programme (FPA
2013-0037). Financial support by the University of Padova
1 O. Vozniuk, C. Bazzo, S. Albonetti, N. Tanchoux, J.-M. M Millet,
F. Bosselet, F. Di Renzo, F. Cavani ChemCatChem 2017, DOI:
through the grant "Attrezzature scientifiche finalizzate alla
ricerca - Bando 2012" is acknowledged.
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0.1002/cctc.201601605R1
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Notes
and
references
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Journal Name
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA06785B
FeCo O and CoFe O nanoparticles have been studied as oxygen carriers for the Chemical Loop
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Reforming of ethanol by using in operando x-ray absorption spectroscopy