Oxidation of carbon monoxide over MLaOx perovskites supported on mesoporous zirconia

Article abstract of DOI:10.1002/cctc.201400013

The oxidation of CO was studied on a series of MLaO₃ perovskites (M=Co, Fe, Ni) supported on mesoporous zirconia that are synthesized by using a glycine-mediated method and characterized by using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy. The activity of the catalysts in the CO oxidation reaction follows the order Co>Ni>Fe.

Full text of DOI:10.1002/cctc.201400013

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DOI: 10.1002/cctc.201400013
Oxidation of Carbon Monoxide over MLaOx Perovskites
Supported on Mesoporous Zirconia
Nikolai A. Davshan,[a] Alexander L. Kustov,[a] Olga P. Tkachenko,[a] Leonid M. Kustov,*[a, b] and
Chang Hwan Kim[c]
The oxidation of CO was studied on a series of MLaO3 perov-
skites (M=Co, Fe, Ni) supported on mesoporous zirconia that
are synthesized by using a glycine-mediated method and char-
acterized by using X-ray photoelectron spectroscopy, X-ray ab-
sorption spectroscopy, and diffuse reflectance infrared Fourier
transform spectroscopy. The activity of the catalysts in the CO
oxidation reaction follows the order Co>Ni>Fe.
Introduction
Controlling exhaust gas emissions is a problem of utmost con-
cern.[1] Mixed metal oxides with a composition ABO3 known as
perovskites[2,3] demonstrate high thermal stability and en-
hanced catalytic activity in CO and hydrocarbon oxidation. The
perovskite-type materials are less expensive than the support-
ed noble metals. The major drawback of perovskite catalysts is
the low specific surface area (a few m2 gꢀ1). The materials can
be significantly improved by supporting the perovskites on
porous carriers with a developed surface area.[4] Mesoporous
zirconia is of special interest as a durable carrier. The porous
structure of ZrO2 helps in the formation and stabilization of
nanoparticles of the active component by preventing their mi-
gration, coalescence, and sintering in the course of high-tem-
perature processes occurring in catalytic processes.
The use of mixed perovskite membranes for simultaneous
in situ oxygen separation and catalytic oxidation has been re-
ported.[7] Ca and Sr-substituted lanthanum ferrite perovskites
(La1ꢀxAxFeO3, A=Ca, Sr and x=0.1–0.2) were prepared by
means of a conventional method using citric acid. The oxida-
tion processes over these catalytic membranes were per-
formed at 1000–1333 K, and the catalysts demonstrated stable
performance for several days on stream.
The perovskite-type LaSrCoO4 mixed oxides were prepared
by using the gelatin, polyglycol gel, and polyacrylamide gel
methods, and these oxides were used for CO and C3H8 oxida-
tion.[8] The catalytic activity of LaSrCoO4 prepared by using the
polyacrylamide gel method was the highest, which was ex-
plained in terms of the higher mobility of oxygen vacancies
and lattice oxygen, smaller particle size, and larger BET surface
areas.
The ZrO2-supported perovskites were synthesized and stud-
ied herein as CO oxidation catalysts. Pechini supported perov-
skites LaBO3 (B=Mn, Fe, Co, Ni, Cu) on a cordierite honeycomb
support[5] and succeeded in the preparation of supported cata-
lysts that are resistant to high temperatures[6] Cordierite-sup-
ported lanthanum manganite and cobaltite are shown to be
quite active in the oxidation reactions, especially if supported
on a secondary sublayer (Ln2O3, ZrO2, and LaBO3).
LaMn1ꢀxCuxO3ꢁd perovskite oxides (x=0–1) were prepared
by using the Pechini and sol–gel methods.[9] The catalysts dem-
onstrated excellent catalytic activity in the oxidation of meth-
ane and CO. The Pechini samples were more homogeneous
and contained smaller particles (higher specific surface area).
In CO oxidation, the oxides with x=0.2 and 0.4 were the most
active. The Pechini samples demonstrated higher activity and
stability than the sol–gel samples.
The perovskite-type catalysts with LaCoO3 and La0.9Ba0.1CoO3
compositions have been prepared by using the sol–gel
method, and their catalytic activity was studied in CO oxida-
tion.[10] The La0.8Ba0.1CoO3 catalyst demonstrated enhanced cat-
alytic activity as compared with the LaCoO3 catalyst. Barium
substitution appears to be responsible for the low-temperature
activity of the catalyst by affecting the redox and oxygen de-
sorption properties.
[a] N. A. Davshan, A. L. Kustov, O. P. Tkachenko, Prof. L. M. Kustov
N.D. Zelinsky Institute of Organic Chemistry
Russian Academy of Sciences
47 Leninsky prospect, Moscow 119991 (Russian Federation)
Fax: (+7)4991372935
E-mail: LMK@ioc.ac.ru
[b] Prof. L. M. Kustov
Chemistry Department
Moscow State University
1 Leninskie Gory, Bldg 3, Moscow119992 (Russian Federation)
Modified perovskite-type oxides synthesized through copre-
cipitation and a conventional citrate method[11] (LaCoO3, La-
Co0.8Cu0.2O3, La0.8Sr0.2Co0.8Cu0.2O3, and La0.8M0.2FeO3, M=Ce and
Sr) demonstrated high catalytic activity in the low-temperature
CO oxidation, and the samples manifested good stability up to
6008C. La0.8Sr0.2Co0.8Cu0.2O3 showed CO conversions close to
[c] C. H. Kim
General Motors Global Research and Development
Chemical Sciences & Materials Systems Laboratory
30500 Mound Rd., Warren, MI 48090 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201400013.
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100% at 3558C. Of the other four synthesized perovskite for-
mulations, the LaCoO3 sample was the best performer in the
whole range of CO conversions.
etc.) or layered double hydroxides (clays and hydrotalcites), or
hierarchical zeolites deserve attention as carriers for the oxida-
tion-type catalysts.[18,19]
The perovskite-type catalysts La0.7Sr0.3Cr1ꢀxRuxO3 (0.025<x<
0.100) were synthesized by annealing a mixture of metal
oxides and carbonates up to 10008C in air.[12] The CO oxidation
activity increases with the increase in the degree of substitu-
tion (x). The concentration of Ru4+ on the surface and its sta-
bility are the determining factors for the CO oxidation activity.
The important conclusions drawn herein are that Ru perov-
skites have high thermal stability and high CO oxidation activi-
ty.
The goal of this article was to prepare, characterize, and test
in CO oxidation a series of MLaO3 perovskites (M=Co, Fe, Ni)
supported on mesoporous zirconia acting as a robust and du-
rable carrier.
Results and Discussion
Characterization of the supported perovskites
SEM and surface area
The unsupported and supported LaRuO3-type lanthanum
ruthenate perovskites[13] were synthesized through freeze
drying, in situ synthesis, coprecipitation, and deposition precip-
itation. These innovative synthesis methods lead to the forma-
tion of LaRuO3 with improved catalytic properties. LaRuO3 also
demonstrates a reasonably high thermal stability and can be
a potential candidate for many catalytic reactions even at ele-
vated temperatures.
The SEM data reveal that the perovskites LaMO3 (M=Co, Ni,
Fe) supported on mesoporous zirconia are amorphous materi-
als (see the Supporting Information). The BET surface areas of
the three catalysts measured by using nitrogen adsorption
differ insignificantly: 155, 143, and 135 m2 gꢀ1 for LaCoO3,
LaNiO3, and LaFeO3. The dispersion of the perovskites support-
ed on mesoporous zirconia is high and uniform with the size
of nanoparticles below 2–3 nm, as confirmed by the absence
of any reflections in the XRD patterns (region of coherent scat-
tering).
The ZrO2-supported La and Mn oxide catalysts with different
La and Mn loadings (0.7–16 wt% as LaMnO3) were prepared[14]
through the impregnation of tetragonal ZrO2 with La and Mn
citrates. The presence of a perovskite-like structure is necessary
for the development of highly active sites for CO oxidation at
350–800 K.
Diffuse reflectance infrared Fourier transform spectroscopy
The perovskite-type oxides La1ꢀxCexCoO3 (x=0–0.1) were
prepared and evaluated in the selective CO oxidation in hydro-
gen.[15] The citrate synthesis methodology favored the forma-
The diffuse reflectance infrared Fourier transform (DRIFT) spec-
tra of the three samples prepared by supporting LaCoOx,
LaFeOx, and LaNiOx on mesoporous zirconia and calcined at
6008C are shown in Figure 1. The hydroxyl groups are charac-
terized by the stretching frequencies of 3673, 3678, and
3680 cmꢀ1 attributed to the vibrations of terminal ZrꢀOH.[26]
Notably, no distinct bands attributable to the supported perov-
skites encapsulated in the mesopores of ZrO2 are observed in
the spectra.
tion of perovskites with
a low specific surface area
(<14 m2 gꢀ1). The catalytic test results in the CO oxidation reac-
tion revealed a 100% CO conversion at 2008C. LaCoO3 and
La0.95Ce0.05CoO3 catalysts demonstrated good stability.
Ba(Ce,Pd)O3 perovskite demonstrated an unusual behavior[16]
in CO oxidation by oxygen, in spite of the low surface area. If
O2 is present in excess, the kinetics show CO inhibition consis-
tent with a Langmuir–Hinshelwood mechanism in which both
reactants compete for the same adsorption sites. The Arrhe-
nius activation energy for this reaction is surprisingly low, 7.8ꢁ
0.3 kcalmolꢀ1. This low value is due to the weak adsorption of
CO on Pd2+ ionic sites located at the catalyst surface. If the
concentration of O2 is limited, the reaction orders for both CO
and O2 show a strong dependence on the PCO/PO2 ratio. The
catalytic activity of Pd-substituted BaCeO3 is attributed to the
increased bulk oxygen mobility in the presence of square
planar Pd2+ ions that are located at the perovskite B sites,
each adjacent to an oxygen vacancy.
The presence of Lewis acid sites on the surface of these cat-
alysts, as well as their relative strengths, can be studied by
using the spectra of adsorbed CO and CD3CN. The high-fre-
quency shifts for CO and CD3CN relative to the gas-phase fre-
quencies of these molecules are observed owing to the inter-
Jin et al. generalized the results of long-term efforts aimed
at research and development of industrial oxide catalysts for
oxidation processes.[17] The bonding strength of the surface
oxygen determined from the surface atomic structure appears
to be the most important factor. The existing approaches to
the synthesis of mixed oxide systems such as perovskites with
good stability in the high-temperature processes are analyzed.
Thus, the number of papers devoted to perovskites support-
ed on appropriate mesoporous matrices is limited, though di-
verse mesoporous materials, such as silicates (MCM-41, SBA-15,
Figure 1. IR spectra of OH groups for the LaCoOx/ZrO2, LaFeOx/ZrO2, and
LaNiOx/ZrO2 samples.
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actions with metal cations (Lewis acid sites). The spectra of the
samples after the adsorption of CO and CD3CN are shown in
Figures 2 and 3, respectively.
dinated Fe ions Fe2+ and/or Fe3+ [28]
. The change in the oxida-
tion state of Fe caused by the change in the oxidation state of
Fe is in agreement with the literature data on the loss of a part
of mobile oxygen by perovskites upon high-temperature an-
nealing,[29] presumably owing to the LaFe3+O3!LaFe2+O2.5
transition. The band at 2086 cmꢀ1 may be assigned to the
monocarbonyl species Fe2+ꢀCO.[28,29] The bands at 2058 and
1997 cmꢀ1 are most likely characteristic of the binuclear car-
bonyl group [Fe2+ꢀ(CO)ꢀFe2+].
The IR spectrum of the LaNiOx/ZrO2 sample demonstrates
absorption bands at 2086, 2005, and 1915 cmꢀ1 (main bands)
and shoulders at 1970 and 1890 cmꢀ1 after CO adsorption. The
position of the first absorption band indicates that this band
most likely belongs to CO complexes with low-coordinated Ni
ions Ni2+, which is in agreement with the literature data show-
ing, in addition, that low-coordinated Ni ions Ni3+ do not form
carbonyl species.[28] These data are in agreement with the liter-
ature data on the loss of a part of mobile oxygen by perov-
skites upon high-temperature annealing,[29] presumably owing
to the LaNi3+O3!LaNi2+O2.5 transition. The doublet bands at
2005, 1970 cmꢀ1 and 1915, 1890 cmꢀ1 may be assigned to the
asymmetric and symmetric bands of the bridged dicarbonyl
species Ni(CO)2 formed by the interaction of CO with Ni2+
ions.[28]
Figure 2. IR spectra of CO adsorbed on the LaCoOx/ZrO2, LaFeOx/ZrO2, and
LaNiOx/ZrO2 samples.
The oxidation state of low-coordinated ions in LaCoOx/ZrO2,
LaFeOx/ZrO2, and LaNiOx/ZrO2 was also studied through the ad-
sorption of deuteroacetonitrile used as a probe. The DRIFT
spectra of adsorbed CD3CN are shown in Figure 3. The bands
of the stretching vibrations of the CꢂN group at 2281 and
2266 cmꢀ1 in the spectrum of LaCoOx/ZrO2 can be attributed
to CD3CN complexes, with perovskite nanoparticles attributed
to the interaction with coordinatively unsaturated Co3+
(2281 cmꢀ1) and Co2+ (2266 cmꢀ1) ions, that is, Lewis acid sites
of the moderate and low strength, respectively. The blueshift
of the CꢂN stretching vibration is 28 and 13 cmꢀ1, which is
similar to that of gas-phase acetonitrile.[30] This conclusion
agrees fairly well with the data obtained in studying CO ad-
sorption on these samples. The differences in the relative in-
tensities of the bands corresponding to the complexes with
Co3+ and Co2+ ions observed in the spectra of CO and CD3CN
may be due to the differences in the extinction coefficients of
these molecules. The extinction coefficient of adsorbed CO in-
creases significantly with the increase in the frequency shift
toward higher frequencies, whereas for acetonitrile the
changes in the extinction coefficient are much less signifi-
cant.[31] Notably, the spectra of acetonitrile adsorbed on Lewis
acid sites of ZrO2 described in the literature[27] contain only
one band at 2300–2295 cmꢀ1, which coincides with a shoulder
observed in the spectra of the LaCoOx/ZrO2 sample.
Figure 3. IR spectra of CD3CN adsorbed on the LaCoOx/ZrO2, LaFeOx/ZrO2,
and LaNiOx/ZrO2 samples.
The spectrum of the LaCoOx/ZrO2 sample demonstrates ab-
sorption bands at 2137 cmꢀ1 (main band) and low-intensity
bands at 2058 and 1981 cmꢀ1 after CO adsorption (Figure 2).
The band at 2137 cmꢀ1 attributed to its low frequency most
likely belongs to CO complexes with low-coordinated Co
ions—Co2+ in agreement with the literature data.[27] The pres-
ence of two oxidation states of Co is in good agreement with
the literature data on the loss of a part of mobile oxygen by
perovskites upon high-temperature annealing,[28] owing to the
LaCo+3O3!LaCo+2O2.5+1= O2 transition. The doublet bands at
2
2058 and 1981 cmꢀ1 may be assigned to the di- and tricarbon-
yl species Co(CO)2 and Co(CO)3 formed by the interaction of
CO with Co2+ and Co3+ ions, respectively.[29]
The broad absorption band of the stretching vibration of
the CꢂN group at 2266 is observed in the spectra of LaFeOx/
ZrO2 and LaNiOx/ZrO2, which can be ascribed to CD3CN com-
plexes with low-coordinated metal ions, as well as the band at
2106 cmꢀ1 corresponding to the stretching vibration of the
CꢀD group. The band at 2266 cmꢀ1 observed in the spectra of
the LaFeOx/ZrO2 and LaNiOx/ZrO2 samples can be attributed to
acetonitrile complexes with coordinatively unsaturated Fe2+
The IR spectrum of the LaFeOx/ZrO2 sample demonstrates
the low-intensity absorption band at 2171 cmꢀ1, main bands at
2086 and 2058 cmꢀ1, and shoulders at 1997 cmꢀ1 after CO ad-
sorption. The position of the first absorption band shows that
this band most likely belongs to CO complexes with low-coor-
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and Ni2+ ions, that is, Lewis acid sites of low strength. The
blueshift of the CꢂN stretching vibration is 13 cmꢀ1, which is
similar to that of gas-phase acetonitrile.[30] This conclusion
agrees fairly well with the data obtained in studying CO ad-
sorption on the same samples.
Table 1. XPS data of the LaCoOx/ZrO2, LaFeOx/ZrO2, and LaNiOx/ZrO2 sam-
ples.
Perovskite Binding energy [eV]
Atomic ratio
O1s
Zr3d5/2 M[a] 2p3/2 La3d5/2 M[a]/Zr La/Zr M[a]/La
In addition, an intense band at 2173 cmꢀ1 is observed in the
spectra of both LaFeOx/ZrO2 and LaNiOx/ZrO2 samples. This
band can be associated with the stretching vibration of the C=
C=N skeleton in a ketene imine molecule that is a tautomer of
CD3ꢀCꢂN.[32,33]
LaCoOx
LaFeOx
LaNiOx
530.5 182.4
530.1 182.3
530.9 182.3
780.9
711.1
854.5
834.4
834.7
834.6
0.206
0.139
0.409
0.344 0.599
0.350 0.397
0.397 0.259
[a] M=Co, Fe, and Ni.
Thus, by using DRIFT spectroscopy with adsorbed probe
molecules, it was found that coordinatively unsaturated Co, Fe,
and Ni ions are present on the surface of the systems repre-
senting mesoporous zirconia with supported LaCoOx, LaFeOx,
and LaNiOx nanoparticles.
Fe in the surface layers of the LaFeOx/ZrO2 sample, most likely
in two oxidation states (Fe2+ and Fe3+). The BE of the Ni2p3/2
component, 855.0 eV, derived by the deconvolution of the
La3d+Ni2p3/2 electron spectrum shows that Ni in the surface
layers of the LaNiOx/ZrO2 sample also exists, most likely in two
oxidation states (Ni2+ and Ni3+).
X-ray photoelectron spectroscopy
The data derived from XPS analysis about the electronic
states of Co, Fe, and Ni are in good agreement with the data
derived from DRIFTS analysis.
The X-ray photoelectron spectroscopy (XPS) survey spectra of
the surface and subsurface layers (ꢃ30 ꢁ) are shown in
Figure 4. C1s, O1s, La3d, La4d, Zr3d, Zr3p, and Zr3s photo-
The surface atomic ratio M/La (M=Co, Fe or Ni) differs from
unity, which indicates the migration of La to the surface layers
or conversely the migration of Co, Fe, and Ni to the bulk of
supported perovskite microcrystals.
The BE of the La3d5/2 component, 834.4–834.7 eV (Table 1),
is characteristic of the La3+ oxidation state, and the BE of the
Zr3d5/2 component, 182.3–182.4 eV, is specific for the Zr4+ oxi-
dation state in supported perovskite samples.
X-ray absorption spectroscopy
The Co K-edge X-ray absorption near edge structure (XANES)
spectra of the LaCoOx/ZrO2 sample and reference compounds
are shown in Figure 5a, which demonstrates that the spectrum
of the LaCoOx/ZrO2 sample differs from those of the Co foil
and CoO references and resembles the spectrum of a Co3O4
spinel reference. At the same time, the energetic position of
the Co K-edge absorption spectrum of the LaCoOx/ZrO2
sample is higher than that of the spectrum of Co3O4. This ob-
servation indicates that the main part of Co in these samples
exists in the Co3+ oxidation state. These data agree with the
XPS data that indicate the presence of both Co2+ and Co3+
cations. The Co K-edge extended X-ray absorption fine struc-
ture (EXAFS) spectra of the LaCoOx/ZrO2 sample and reference
compounds are shown in Figure 5b, which demonstrates that
the EXAFS spectrum of the LaCoOx/ZrO2 sample differs from
the spectra of Co foil, CoO, and Co3O4 spinel references. The
first peak at an uncorrected distance of approximately 1.5 ꢁ is
observed in the spectrum of the LaCoOx/ZrO2 sample. The po-
sition of this peak is close to that in the spectrum of the spinel
and is lower than that in the spectrum of CoO. This peak corre-
sponds to oxygen atoms as nearest neighbors to the central
Co atom. The second peak is located at the same distance as
the second peak in the spectrum of Co3O4 and corresponds to
the next CoꢀO atomic pair. The third peak locates at a distance
higher than that of the first peak in the Co foil spectrum, the
second peak in the CoO spectrum, and the third peak in the
Figure 4. XPS survey spectra of the LaCoOx/ZrO2, LaFeOx/ZrO2, and LaNiOx/
ZrO2 samples.
electron lines and C KLL, O KVV, and La MNN Auger electron
lines are observed in the spectra of supported perovskites. In
addition, Co2p photoelectron and Co LMM Auger electron
lines are observed in the spectrum of LaCoOx/ZrO2; the Fe2p
photoelectron line in the spectrum of LaFeOx/ZrO2; and the
Ni2p photoelectron line overlapped with the La3d photoelec-
tron line and Ni LMM Auger electron lines in the spectrum of
LaNiOx/ZrO2.
The binding energies (BEs) of photoelectron lines and sur-
face atomic ratios for LaCoOx/ZrO2, LaFeOx/ZrO2, and LaCoNix/
ZrO2 samples are given in Table 1.
The XPS data indicate the BE of the Co2p3/2 doublet compo-
nent to be 780.9 eV as well as the presence of satellites charac-
teristic of Co cations. From these data, one can conclude that
Co in the LaCoOx/ZrO2 sample exists in surface layers, most
likely in both 2+ and 3+ oxidation states. The BE of the
Fe2p3/2 component, 711.1 eV, is indicative of the existence of
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Figure 6. Normalized Ni K-edge a) XANES and b) EXAFS spectra of the
LaNiOx/ZrO2 sample and reference compounds.
Figure 5. Normalized Co K-edge a) XANES and b) EXAFS spectra of the
LaCoOx/ZrO2 sample and reference compounds.
Notably, the local structure of Co atoms in the LaCoOx/ZrO2
sample differs from that of the Co atom in unsupported
LaCoO3, the crystallographic data of which were used for the
FEFF file.
Co3O4 spectrum. Therefore, this peak reflects the presence (as
a third neighbor) of atoms heavier than Co atoms, for example,
La atoms. The average local structure of Co atoms in the
LaCoOx/ZrO2 sample obtained from the model fit of EXAFS
data is presented in Table 2. The processing of the EXAFS data
of the LaCoOx/ZrO2 sample demonstrates that the nearest
neighbors of the central Co atom are an average number of
3.2ꢁ0.3 oxygen atoms at an average real distance of 1.93ꢁ
0.01 ꢁ. The second shell contains an average number of 1.4ꢁ
0.5 oxygen atoms at an average real distance of 2.87ꢁ0.03 ꢁ.
The third coordination shell contains an average number of
2.2ꢁ0.3 La atoms at an average real distance of 3.39ꢁ0.01 ꢁ.
The Ni K-edge XANES spectra of the LaNiOx/ZrO2 sample and
reference compounds are shown in Figure 6a, which demon-
strates that the spectrum of this sample differs from that of
the Ni foil reference and resembles that of an NiO reference.
At the same time, the energetic position of the Ni K-edge ab-
sorption spectrum of the LaNiOx/ZrO2 sample is higher and the
intensity of the white line is lower than that for NiO. This find-
ing indicates that the electronic state and local structure of Ni
atoms in the LaNiOx/ZrO2 sample differ from those of NiO.
These data agree with the XPS data that indicate the presence
of both Ni2+ and Ni3+ cations.
The Ni K-edge EXAFS spectra of the LaNiOx/ZrO2 sample and
reference compounds are shown in Figure 6b, which demon-
strates that the EXAFS spectrum of the LaNiOx/ZrO2 sample dif-
fers from that of the Ni foil and resembles the spectrum of
NiO. The first peak is situated at an uncorrected distance of ap-
proximately 1.5 ꢁ in the spectrum of the LaNiOx/ZrO2 sample.
The distance of this peak in the spectrum of the supported
perovskite is slightly shorter than that in the spectrum of NiO.
This peak corresponds to oxygen atoms as nearest neighbors
to the central Ni atom, and its intensity is lower in the spec-
trum of the supported perovskite. The second peak is situated
Table 2. EXAFS data of the LaCoOx/ZrO2 and LaNiOx/ZrO2 samples.
Sample
Atomic
pair
r
[ꢁ]
Coordination
number
l2
[10ꢀ3 ꢁ2] [eV]
DE
LaCoOx/
ZrO2
CoꢀO
1.93ꢁ0.01 3.2ꢁ0.3
4ꢁ1
16ꢁ1
CoꢀO
CoꢀLa
NiꢀO
2.87ꢁ0.03 1.4ꢁ0.5
3.39ꢁ0.01 2.2ꢁ0.3
2.09ꢁ0.01 5.2ꢁ0.5
1ꢁ1
9ꢁ1
15ꢁ1
29ꢁ2
24ꢁ1
LaNiOx/
ZrO2
12ꢁ2
NiꢀNi
NiꢀLa
2.92ꢁ0.01 3.9ꢁ0.7
3.16ꢁ0.01 6.9ꢁ0.9
7ꢁ1
8ꢁ1
10ꢁ1
29ꢁ1
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at roughly the same distance as the second peak in the spec-
trum of NiO and corresponds to the NiꢀNi atomic pair. The
average local structure of Ni atoms in the LaNiO3/ZrO2 sample
obtained from the model fit of EXAFS data is presented in
Table 2. The calculation of the EXAFS data of the LaNiOx/ZrO2
sample demonstrates that the nearest neighbors of the central
Ni atom are an average number of 5.2ꢁ0.5 oxygen atoms at
an average real distance of 2.09ꢁ0.01 ꢁ. The second shell con-
tains an average number of 3.9ꢁ0.7 Ni atoms at an average
real distance of 2.92ꢁ0.01 ꢁ. The third coordination shell con-
tains an average number of 6.9ꢁ0.9 La atoms at an average
real distance of 3.16ꢁ0.01 ꢁ. Thus, the local structure of Ni
atoms in perovskites supported on ZrO2 differs from that of
the Ni atom in unsupported LaNiO3, the crystallographic data
of which were used for the FEFF file.
the 10% conversion follow a different trend: LaCoO3/ZrO2
(1658C)>LaNiO3/ZrO2 (1808C)>LaFeO3/ZrO2 (2108C).
Conclusions
The nanoparticles of perovskites with different transition metal
ions (Ni, Co, Fe) based on La were supported on mesoporous
zirconia and demonstrated high activity in CO oxidation be-
cause LaCoO3 was the most active of all the perovskites stud-
ied. In complete oxidation, the use of non-noble metal cata-
lysts with activities comparable to or even lower than those of
the conventional Pd or Pt catalysts is quite challenging be-
cause it enables one to dramatically reduce the cost of the cat-
alysts for environmental applications without sacrificing the
performance (activity) of the materials.
Catalytic activity
Experimental Section
The prepared catalysts were tested in the reaction of oxidation
of a gas mixture containing CO and oxygen.
Preparation of LaMO3/mesoporous ZrO2
The synthesis of mesoporous zirconia was performed according to
the previously developed and modified method.[20] A 15% solution
of cetyltrimethylammonium bromide (CTMABr) was prepared by
dissolving CTMABr (19.7 g) in acidified water (110 mL, pH 2.0) at
408C and mixing the solution for 3 h. Then, a 70% solution of zir-
conium isopropoxide in 2-propanol (76 g) was added to the
CTMABr solution under rigorous stirring (CTMABr/Zr molar ratio=
0.33:1.0). The mixture was stirred for 1 h at 408C and then placed
in an autoclave with Teflon lining and heated at 608C for 48 h. The
precipitate was filtered off, washed multiple times with ethanol,
and dried in air at 758C for 48 h. The obtained zirconium dioxide
(ꢃ10 g; yield: ꢃ50%) was calcined in flowing air at 6008C.
The change in CO conversion with the increase in tempera-
ture over the Co, Ni, and Fe perovskites supported on mesopo-
rous zirconia is shown in Figure 7. The turnover frequencies for
the three studied perovskites are given in Table 3.
A glycine complex of La and Co/Fe/Ni was prepared by the recipe
described in Refs. [21and22]. For this purpose, La and Co/Fe/Ni ni-
trates taken in equimolar amounts ([La]=0.005 mol; [M]=
0.005 mol) were dissolved in distilled water (50 mL). After the com-
plete dissolution of the salts, an aqueous glycine solution (50 mL;
0.05 mol of glycine) was added. The molar ratio of glycine to the
sum of cations was 5:1.
The samples of supported La–metal perovskites (LaMOx, M=Co,
Ni, Fe) were prepared through the impregnation of the precalcined
mesoporous carrier (ZrO2) with a solution of the prepared LaꢀCo/
Fe/Ni glycine complex. Some amount (1.5 g) of the air-dried sup-
port was impregnated with the aqueous solution of the precursor
(10 mL), which corresponds to the supported perovskite
(ꢃ10 wt%). Then, the sample was dried at 60–658C and calcined in
air at 400 or 6008C for 8 h. The sample prepared on the basis of
the carrier precalcined at 6008C was annealed at 6008C.
Figure 7. CO oxidation over the zirconia-supported perovskites.
Table 3. Turnover frequencies for CO oxidation at 50% conversion.
Sample
T50% conv. [8C]
Turnover frequency [hꢀ1
]
LaNiOx/ZrO2
LaCoOx/ZrO2
LaFeOx/ZrO2
300
200
300
34.1
30.9
37.2
Characterization of the samples
The chemical analysis of the samples for the contents of the
metals (Zr, Co, Ni, Fe, and La) was performed by using atomic emis-
sion spectroscopy. The sample loading was dissolved in a hot mix-
ture of concentrated acids HF, HNO3, and HCl. Furthermore, the so-
lution was evaporated and the solid was dissolved in HCl. After re-
peating this method twice, the obtained solution was analyzed for
La, Co, Ni, Fe, and Zr within the accuracy of ꢁ3 rel.%.
The following order of the activity in CO oxidation, based on
the temperature of the 50% conversion, was found in the tem-
perature range 50–4008C: LaCoO3/ZrO2 (2008C)>LaFeO3/ZrO2
(2758C)>LaNiO3/ZrO2 (3008C).
However, the Ni-containing catalyst demonstrates a higher
activity in CO oxidation in the low-temperature range as com-
pared with the Fe-containing catalyst and the temperatures of
The DRIFT spectra were recorded at RT in the frequency range of
6000–400 cmꢀ1 with a resolution of 4 cmꢀ1 on a NICOLET Protege
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460 spectrometer equipped with a diffuse reflectance attachment
developed earlier at the N.D. Zelinsky Institute of Organic Chemis-
try, Moscow.[23] To have a satisfactory signal-to-noise ratio, 500
spectra were collected. CaF2 powder was used as a reference.
Before the spectroscopic measurements, the samples were evacu-
ated at 4008C for 2 h to remove physically adsorbed water. The fol-
lowing probe molecules were used to test surface sites of different
nature: CO as a probe for Lewis acid sites and low-coordinated
metal ions, and CD3CN as a probe for both Lewis and Brønsted (if
present) acid sites. The probe molecules were adsorbed at RT and
an equilibrium pressure of 5 Torr (1 Torr=133.3 Pa) for CO and of
96 Torr for CD3CN (saturated pressures).
ity tests were performed at different temperatures, ranging from
50 to 5008C. The temperature in the reactor was ramped with
a step size of 508C until a 100% CO conversion was achieved.
After that, the temperature was decreased stepwise to return to
the starting point of the conversion cure at which zero conversion
was observed. The feed gas mixture consisted of 4.5 vol% CO,
22.5 vol% O2, and He balance. The total feed flow rate was held
constant at 10 cm3 minꢀ1, with a volume hourly space velocity of
6000 hꢀ1. The turnover frequency was calculated as the number of
moles of CO converted per mole of the supported peroxide per
hour.
The effluent gas mixture from the reactor was analyzed with a gas
chromatograph equipped with a thermal conductivity detector
and a molecular sieve 5A column to determine CO conversion.
The phase composition of the samples and the particle size of the
supported metal were determined from XRD analysis. The XRD pat-
terns were recorded with a DRON-2 diffractometer with Ni-filtered
CuKa radiation (l=0.1542 nm) in a step-scanning mode, with
a step size of 0.028 and a counting time of 0.6 sstepꢀ1 at 2q=10–
858. All major reflections were covered in this scan range. The
phases were identified by comparing the position and intensity of
the lines with the data from the files of the International Center for
Diffraction Data. To estimate precisely the average particle size of
the phases, the diffraction patterns were recorded at 2q=35–508,
with a step size of 0.028 and a counting time of 1.2 sstepꢀ1. The
crystal sizes of the nanoparticles were determined from X-ray line
broadening analysis.
Acknowledgements
We acknowledge support from HASYLAB (DESY, Germany) that
made it possible to perform X-ray absorption spectroscopy stud-
ies, project I-20060226 EC, and the Ministry of Science and Educa-
tion of the Russian Federation (project nos. 8441 and
16.513.11.3024). The use of a unique setup with microwave acti-
vation for catalyst preparation is acknowledged.
The X-ray absorption (XANES and EXAFS) spectra (Co K edge at
7709 eV and Ni K edge at 8333 eV) were measured at the Hasylab
X1 station (Hamburg, Germany) with a Si(111) double crystal mon-
ochromator. The spectra were recorded in the transmission mode
at ꢀ1968C. The spectrum of the Co or Ni foil was registered simul-
taneously between the second and third ionization chambers for
energy calibration. The EXAFS data analysis was performed with
the VIPER software.[24] The reference spectra were recorded by
using standard reference compounds: CoO, Co3O4, Co foil, NiO,
and Ni foil. The required scattering amplitudes and phase shifts
were calculated with the ab initio FEFF8.10 code.[25] Fitting was per-
formed in the k and r spaces.
Keywords: carbon monoxide
lanthanum · oxidation · perovskites
· heterogeneous catalysis ·
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Published online on May 28, 2014
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