Methane combustion and CO oxidation on zirconia-supported La, Mn oxides and LaMnO3 perovskite

Article abstract of DOI:10.1006/jcat.2001.3441

La, Mn oxides dispersed on high surface area tetragonal ZrO₂ were prepared, characterized, and their catalytic activity for methane combustion at 650-1050 K, and CO oxidation at 350-800 K were investigated. XRD of zirconia-supported samples showed only tetragonal zirconia phase with traces of monoclinic zirconia up to 6% loading. X-ray absorption spectroscopy showed the formation of LaMnO₃, dispersed on zirconia at high La-Mn loading. At low loading, the presence of disordered oxide species strongly interacted with the support. TPR showed the presence of LaMnO₃ at high La-Mn loading, and La and Mn species at low loading. Supported catalysts with La-Mn content ≥ 4% activated methane combustion at 723-753 K, and gave complete CH₄ conversion at 923-973 K. All catalysts with La-Mn loading ≥ 4% gave 100% CO₂ selectivity. Both tetragonal ZrO₂ and ZLaMn-2 produced CO, giving CO₂ selectivity of 15% at 723 K, which increased to ≤ 50% at 873-923 K. The conversion to CO₂ was 100% at ≤ 800 K for all catalysts, except pure zirconia support. For samples with ≥ 4% loading and LaMnO₃, the conversion reached 100% at < 550 K.

Full text of DOI:10.1006/jcat.2001.3441

Journal of Catalysis 205, 309–317 (2002)
doi:10.1006/jcat.2001.3441, available online at http://www.idealibrary.com on
Methane Combustion and CO Oxidation on Zirconia-Supported
La, Mn Oxides and LaMnO3 Perovskite
,1
S. Cimino, S. Colonna,† S. De Rossi,‡ M. Faticanti,‡ L. Lisi,§ I. Pettiti,‡ and P. Porta‡
Dipartimento d’Ingegneria Chimica, Universita` “Federico II,” Napoli, Italy; †Istituto di Struttura della Materia, CNR, Via Fosso del Cavaliere 100,
00133 Roma, Italy; ‡Centro di Studio “Struttura e Attivita` Catalitica di Sistemi di Ossidi,” CNR, c/o Dipartimento di Chimica, Universita` degli Studi di
Roma “La Sapienza,” Piazzale Aldo Moro 5, 00185 Roma, Italy; and §Istituto di Ricerche sulla Combustione, CNR, c/o Dipartimento d’Ingegneria
Chimica, Piazzale V. Tecchio 80, 80125 Napoli, Italy
Received June 8, 2001; revised September 24, 2001; accepted September 24, 2001
A great variety of metal ions can be introduced in the
perovskite structure provided that their ionic radii fit the
ZrO2-supported La, Mn oxide catalysts with different La, Mn
loading (0.7, 2, 4, 6, 12, and 16 wt% as LaMnO3) were prepared
by impregnation of tetragonal ZrO2 with equimolar amounts of La
and Mn citrate precursors and calcination at 1073 K. The catalysts
were characterized by X-ray diffraction (XRD), X-ray absorption
spectroscopy (XAS), and BET specific surface area determination.
The redox properties were tested by temperature-programmed re-
duction (TPR), and the catalytic tests were carried out for methane
combustion at 650–1050 K and for CO oxidation at 350–800 K.
XRD revealed the presence of tetragonal zirconia with traces of the
monoclinic phase. LaMnO3 perovskite was also detected for loading
higher than 6%. XAS and TPR experiments suggested that at high
loading small crystallites of LaMnO3, not uniformly spread on the
zirconia surface, were formed; while at low loading, La, Mn oxide
species interacting with the support, and hard to be structurally
defined, prevailed. The catalysis study indicated that the presence
of a perovskite-like structure is necessary for the development of
highly active sites. Dilute catalysts were in fact poorly active even
when considering the activity per gram of La, Mn perovskite-like
composition. For methane combustion and CO oxidation, similar
trends of the activity as a function of the loading point to a similar-
ity of the active sites for the two reactions on the examined catalytic
sizes of the 12 coordinated A and octahedral B sites, e.g.,
a
a
rA > 0.90 A and rB > 0.51 A. Moreover, various oxidation
states of the A and B cations are allowed in the perovskite
structure as well as anionic or cationic defectivity (5–8).
Therefore, the ABO3 perovskite can be considered an ideal
host matrix for many transition or nontransition metal ion
solid solutions.
Oneofthemaindrawbacksoftheperovskitesistheirvery
low surface area (some square meters per gram) when they
are prepared by solid-state reaction of the oxide precursors
at high temperature (5, 8). Although a method based on
citrates precursors (9) allows the production of perovskites
even with a few tens of square meters per gram of specific
surface area after calcination at 1073 K (10–13), rapid sin-
tering at temperatures such as 1173 K cannot be avoided.
Therefore, dispersion of perovskites on suitable high sur-
face area refractory supports could increase the specific ac-
tivity and thermal resistance of these catalysts. Among the
proposed supports, -Al2O3 is the most widely investigated
(14–18), due to its high surface area and thermal stability
when doped with superficial La2O3. Nevertheless, interac-
tions occurring between active phase and support result
in the formation of mixed perovskites or in the inclusion of
transitionmetalionsinthealuminalattice, whichreducethe
activity of the catalyst (15–18). Interactions between Mg2+
ions and LaMnO3 have also been reported when magne-
sia is used as a support (18). ZrO2 could represent a good
support, since it should give moderate interactions with per-
ovskite. Fujii et al. and Mizuno et al. (19, 20) found that the
specific activity of supported LaCoO3 and La0.8Sr0.2CoO3
perovskites in the oxidation of propane was significantly en-
hanced only through the dispersion of a few layers on ZrO2.
This paper reports the results concerning the prepara-
tion, characterization, and catalytic activity for methane
combustion of La, Mn oxides dispersed on high surface area
tetragonal ZrO2. In fact, LaMnO3 perovskite was found to
be one of the most active catalysts for the total oxidation
system.
c
2002 Elsevier Science
Key Words: zirconia-supported La, Mn perovskites; redox prop-
erties; methane combustion; CO oxidation.
1. INTRODUCTION
Oxides with a perovskite structure (ABO3) and con-
taining transition metal ions have attracted considerable
interest for many years due to their activity and thermal
stability in the catalytic total oxidation of hydrocarbons
(1–3). In particular, the catalytic combustion of methane
represents a promising way for power generation to avoid
the formation of harmful nitrogen oxides (NO), which are
x
otherwise produced at the high temperatures of the flame
combustion (4, 5).
1 To whom correspondence should be addressed. Fax: 39 06 490324.
E-mail: piero.porta@uniroma1.it.
309
0021-9517/02 $35.00
2002 Elsevier Science
All rights reserved.
c

310
CIMINO ET AL.
a
of methane (5, 10, 12). CO oxidation was also investigated were calculated in the range 2.5 < k < 11.5 A 1 by using a
to have information on the catalytic properties toward ox- Kaiser window.
idation in a lower temperature range compared to that in
which methane oxidation takes place (13).
Temperature-programmed reduction (TPR) experi-
ments were performed as reported in (10) using a Micro-
meriticsTPD/TPR2900analyzerequippedwithaTCdetec-
tor and coupled with a Hiden HPR 20 mass spectrometer.
Samples (0.1 g) were preheated in flowing air at 1073 K for
2 h and then, after cooling at room temperature, reduced
with a 2% H2/Ar mixture (25 Ncm3 min 1), and heated at
10 K min up to 1073 K. Water produced by the sample
reduction was condensed in a cold trap before reaching the
detectors.
2. EXPERIMENTAL
La, Mn oxides/ZrO2 catalysts with different La–Mn load-
ing (hereafter designated as ZLaMn with the number after
the label indicating the nominal perovskite content: 0.7, 2, 4,
6, 12, and16wt%)werepreparedbyimpregnationoftetrag-
onal ZrO2 with equimolar amounts of citrate precursors.
Tetragonal ZrO2 with a large surface area was prepared
by the method of Chuah et al. (21). An aqueous solution
of 0.075 M ZrOCl2 was added, dropwise (1 mL min 1) and
under magnetic stirring, to a 5 M NH4OH solution. The
hydrous zirconia, after digestion in its mother solution at
373 K for 48 h, was filtered, washed with water until the
AgNO3 test for Cl detection gave no opalescence in the
washing solution, dried in a furnace overnight at 383 K, and
finally calcined at 1073 K for 5 h (raising the temperature
slowly by 1 K min 1).
1
Methane catalytic combustion experiments were per-
formed in the experimental apparatus already described
elsewhere (13, 18). The space velocity was 40,000 Ncm3
1
g
1 h in all tests (0.4 g catalyst powder, 150–250 m),
with a feed gas composition of 0.4% CH4, 10% O2, and N2
balance. Product concentrations were continuously mea-
sured after passing a CaCl2 water trap by five on-line-
independent nondispersive infrared analyzers for CH4
(high and low conc.), CO, CO2, and NO. The reproducibi-
lity of results was verified carrying out each test twice and
loading fresh catalysts every time.
The dispersion of La, Mn oxides on the ZrO2 support
was performed by the citrate method (9, 15). Two solutions
were added to a weighted amount of ZrO2: an aqueous
solution of citric acid was added first, and subsequently a
solution containing La(NO3)3 and Mn(NO3)2 in 1 : 1 pro-
portions was added. The molar ratio between the citric acid
and the overall metal nitrates was fixed at 1. The resulting
solution was kept at 383 K until dryness, and the samples,
after grinding, were calcined at 1073 K for 5 h. For compari-
son, unsupported LaMnO3 was also prepared by the citrate
method as reported in (11).
CO oxidation with O2 was studied following the exper-
imental procedure already described in (13), using 0.5 g
of catalyst supported on a silica-fritted disk inside a silica
reactor vertically positioned in a programmable electrical
heater. The compositions of the reactants were adjusted to
1% CO, 20% O2, and He balance, and the space velocity
was 12,000 Ncm3 g 1 h 1. Effluent from the reactor was
sampled and analyzed by gas chromatography; mass bal-
ance with respect to carbon was 100% 2%.
Kinetic parameters for both reactions were evaluated un-
der the assumption of isothermal plug flow conditions, on
the basis of a first-order rate expression with respect to CH4
or CO and a zeroth pseudo-order with respect to oxygen
because it was present in large excess.
The BET surface area of samples (SA, m2 g 1) was eval-
uated by nitrogen adsorption at 77 K in a vacuum glass
apparatus.
Phase analysis was performed by X-ray powder diffrac-
tion using a Philips PW 1029 diffractometer with Ni-filtered
CuK radiation. Spectra were recorded in a 2ϑ range from
20 to 60 .
X-ray absorption spectroscopy (XAS) measurements of
some ZLaMn materials (2, 6, and 12% loading) and of
3. RESULTS AND DISCUSSION
3.1. Structural Properties
Phase analysis, performed by X-ray powder diffraction,
pure LaMnO3 were performed at the ESRF Italian beam- showed that with up to 6% loading no other phase but
line GILDA (Grenoble, France). Mn and La K-edges were tetragonal zirconia (with only traces of the monoclinic
measured at 77 K in a fluorescence detection mode using phase) was revealed. Figure 1 shows that in the ZLaMn-12
a high purity Ge detector for the ZLaMn samples and in and ZLaMn-16 samples, in addition to the ZrO2 X-ray
a transmission mode for LaMnO3. Powder samples were diffraction (XRD) pattern, small peaks are visible. Their in-
deposited on Millipore membranes or mixed to an appro- tensity increases with the increase of La–Mn loading, their
priate amount of boron nitride (BN) and pressed into pel- position corresponding to that of the strongest peaks of
lets. The beamline monochromator was equipped with two LaMnO3 perovskite.
Si (311) crystals for Mn K-edge measurements and two Si
The preparation method leads to the stabilization of
(511) crystals for La K-edge measurements. The EXAFS ZrO2 in its tetragonal phase characterized by a BET surface
analysis was performed by using the complete FEFF8 pack- area of 82 m2 g 1 even after 5 h calcination at 1073 K, mak-
age (22). Fourier transforms (FTs) for the signal k3 (k) ing it suitable as a catalyst support. The dispersion of active

METHANE COMBUSTION AND CO OXIDATION
311
fitting procedure of the catalysts two La–O contributions
were used to reproduce the near-neighbors coordination.
The second shell structures were fitted using both La–Mn
and La–La contributions. Debye–Waller factors and E0 and
S02 parameters were fixed to the value obtained from the fit
of the pure perovskite. The coordination number (N) and
bond distances (R) were left to vary in the fit. The Debye–
Waller factors were fixed to avoid the strong correlation
with the coordination numbers. The fit results are reported
in Table 1, and the fit profiles (dots) are shown in Fig. 2.
From these results it is evident that perovskite is the most
abundant phase in the ZLaMn-12 sample. In fact, the dis-
a
tances (found equal to 3.39 and 3.92 A for the La–Mn and
La–La contributions, respectively) agree well with the ex-
a
pected distances for LaMnO3 (3.33–3.39 and 3.89 A, res-
pectively). The reduced coordination numbers indicate a
disorder on the surface of the support and/or the presence
of small crystallites. For the ZLaMn-2 sample, the FT ex-
hibits a different shape indicating a peculiar kind of species
presentonthesurface. Thefitresultsshowareductionofthe
La–Mn and La–La coordination numbers and a shift of the
coordination distances, the La–Mn distance being greater
and the La–La distance shorter than those in the perovskite
structure. These results should imply the formation of
FIG. 1. XRD patterns of ZrO2, LaMnO3, and ZLaMn catalysts. As-
terisks indicate the most intense X-ray lines of LaMnO3.
components and the following calcination step yielded final
1
surface areas of ZLaMn samples in the range 50–67 m2 g
.
The variations of surface area values can be related to the
uncontrolled small amounts of monoclinic zirconia present
on the various samples.
The structural investigation on the nature of the active
phase of the zirconia-supported samples was carried out
by X-ray absorption spectroscopy (XAS) at the La K-edge
(for ZLaMn-2, ZLaMn-6, and ZLaMn-12) and at the Mn
K-edge (for ZLaMn-6 and ZLaMn-12).
Figure 2 shows the FTs of the k3 (k) signals, measured
at the La K-edge, of the previously described catalysts and
of the unsupported LaMnO3 perovskite. For LaMnO3 the
FT shows two low intensity first shell peaks due to different
a
La–O bond distances [2.48 and 2.75 A, respectively, from
crystallographic data (11)] and a very intense second shell
a
peak at about 2.8 A representing the further shells. The FTs
for the ZLaMn catalysts show the presence of a broad first
shell peak in the range 1.0–2.5 A, with an intensity which
decreases with the increase of La content, and second shell
structures in the range 2.5–4.4 A. These second shell fea-
tures for the ZLaMn-2 and ZLaMn-6 samples are shifted
to higher R values and are less intense, while ZLaMn-12 re-
a
a
sembles pure LaMnO3 with a strong second shell structure
a
at about 2.8 A.
The FT of pure perovskite was fitted by using the crys-
tallographic bond distances data (11), leaving free the E0
FIG. 2. La K-edge: Fourier transforms of the k3 (k) signals for the
ZLaMn-2, ZLaMn-6, and ZLaMn-12 samples compared to pure LaMnO3.
The continuous line represents the experimental data, and the dots are the
shift, the S02 term, and the Debye–Waller factors. In the fit profiles.

312
CIMINO ET AL.
a
TABLE 1
data (1.96 and 3.37–3.39 A, respectively). Only the Mn–Mn
distance results are longer than those expected from the
a
Fitting Results Obtained from the EXAFS Analysis at the La
and Mn K-Edgesa
LaMnO3 crystal structure (3.88 A), but this is probably due
to the fact that multiple scattering contributions, particu-
larly those in the planar Mn–O–O, were neglected in this
fit. As for the ZLaMn-6 sample, the FT shows a lower in-
tensity second shell structure and a Mn–La distance close
to that of pure perovskite. This suggests, in agreement with
the results drawn from the La K-edge measurements, that
in ZLaMn-6 the oxide dispersed on ZrO2 is a perovskite-
like structure, although not well organized as indicated by
the low coordination number around Mn.
La K-edge
La–O
La–O
La–Mn La–La La–Mn La–La
ZLaMn-2
0.8
R(A) 0.02
N
3.5
2.44
4.8
1.6
2.3
3.64
—
—
—
—
˚
2.60
3.48
ZLaMn-6
N
0.8
4.3
2.48
1.9
1.4
2.0
3.67
2.0
4.64
1.2
4.65
˚
R(A) 0.02
2.63
3.51
ZLaMn-12
3.2. Redox Properties
N
0.8
3.2
2.42
3.3
2.60
4.6
3.39
3.8
3.92
—
—
—
—
˚
R(A) 0.02
The TPR profiles of the ZLaMn-supported catalysts are
shown in Fig. 4, where the patterns of bulk LaMnO3, MnO,
Mn3O4, Mn2O3, MnO2, and ZrO2 are reported for compar-
ison. The total H2 uptake estimated from the experiments
carried out up to 1073 K and that related to the first peak
evaluated up to the minimum baseline value reached in the
823–923 K temperature range are reported in Table 2.
MnO displays a very small peak at 740 K probably due
to the reduction of some surface manganese oxide species
with a Mn oxidation state higher than 2+. The reduc-
tion temperature of the other manganese oxide reference
Mn K-edge
Mn–O Mn–La Mn–Mn
ZLaMn-6
N
0.8
6.2
1.92
0.8
3.42
0.9
3.87
˚
R(A) 0.03
ZLaMn-12
N
0.8
6.0
1.94
2.2
3.39
2.0
4.01
˚
R(A) 0.03
a N is the coordination number, and R is the bond distance for the
different coordination shells.
extremely disordered and dispersed metal oxides on the
support surface. By increasing the La–Mn loading up to 6%
some differences in the FT can be observed, in particular
the presence of a double peak in the second shell structure.
The best fit of ZLaMn-6 was obtained using two La–La and
two La–Mn contributions, as reported in Table 1. This fit re-
veals an increase in the total La-La and La-Mn coordination
numbers pointing to a greater aggregation of the supported
oxide species with respect to ZLaMn-2. The found La-La
and La-Mn bond distances are different from those of the
perovskite structure suggesting that a completely organized
perovskite is formed only at high loading.
Figure 3 shows the FTs of the k3 (k) signals, measured at
the Mn K–edge, of the ZLaMn-6 and ZLaMn-12 samples
and of pure LaMnO3. An intense first shell peak in the
a
R = 0.8–2.0 A range, which is evidently broadened for the
ZLaMn-6 sample, is observed, due to Mn–O coordination.
a
The structures in the 3–4 A range are due to the further
shells.
Fits of these spectra were performed using three differ-
ent coordination shells: Mn–O, Mn–La, and Mn–Mn. Also
in this case the Debye–Waller factors were fixed to the val-
ues obtained from the fit of the pure perovskite. The re-
sults are reported in Table 1, and fit profiles are shown in
Fig. 3 (dots). These results confirm that the 12% sample
exhibits a perovskite-type structure. In fact, the Mn–O and
FIG. 3. Mn K-edge: Fourier transforms of the k3 (k) signals for the
ZLaMn-6 and ZLaMn-12 samples compared to pure LaMnO3. The contin-
Mn–La distances are in agreement with the crystallographic uous line represents the experimental data and the dots are the fit profiles.

METHANE COMBUSTION AND CO OXIDATION
313
FIG. 4. TPR profiles of LaMnO3-supported catalysts and reference compounds. (a) Per gram of catalyst. (b) Per mole of Mn. Bars in (a) and (b)
1
1
1
correspond to 0.1 molH
g
K
and 1 mmolH molMn
K
1, respectively.
2
2
phases is, as expected, in the order of MnO2 (two steps) < with a maximum in the 670–700 K range. This signal has
Mn2O3 <Mn3O4.
been attributed to the reduction of Mn4+ (present as 35%
ZrO2 shows a quite sharp signal peaked at about 490 K fraction in bulk LaMnO3) to Mn3+ in analogy to the unsup-
and a broader signal peaked at about 700 K. The first peak ported catalyst (10, 12). In addition to this peak, a shoulder
was assigned, by simultaneous mass spectrometry analysis, appears at lower temperature (maximum at about 533 K),
to desorption of CO2 present as an impurity in the air used more evident for diluted catalysts, as shown in Fig. 4b where
for the sample pretreatment. The second peak indicates the H2 uptake has been referred to the weight of the active
that some reduction of ZrO2 occurs at high temperature. phase (considered as LaMnO3 in the calculation). The ra-
Both signals undergo a strong decrease in the TPR profile tio between the shoulder and the signals decreases with in-
of the ZLaMn-2 catalyst, suggesting that they should be creasing La–Mn loading, thus suggesting that the first peak
associated to phenomena occurring on the support surface could be related to a more easily reducible manganese ox-
which are inhibited by La–Mn oxides dispersion.
ide species. For ZLaMn-2, a further signal appears in the
The TPR profiles of the supported ZLaMn catalysts show 850–1000 K temperature range, as shown in Fig. 4b. This
more clearly, with the increase of La–Mn content, the typi- signal, which cannot be associated to the perovskite phase,
cal features of bulk LaMnO3 giving rise to a multiple signal indicates that at low loading very stable manganese oxide
species are formed on the zirconia surface and strongly in-
teract with the support.
TABLE 2
At about 973 K another reduction phenomenon, whose
extension increases with perovskite loading, starts for sup-
Results from TPR Experimentsa
ported samples with a perovskite content 12%. In order
to better indicate this reduction reaction, a TPR experi-
ment up to 1273 K has been carried out for ZLaMn-16 (not
shown). The same reduction process was detected for bulk
LaMnO3 and attributed to the further reduction of man-
ganese from a +3 to a +2 oxidation state (10, 12) with the
destruction of the perovskite structure. This confirms the
formation of perovskite crystallites at higher La–Mn load-
ing, also detected by XRD and inferred by XAS analysis,
showing a redox behavior similar to that of unsupported
perovskite.
Catalyst
H2–1073
H2–850
T1
T2
ZLaMn-2
ZLaMn-4
ZLaMn-6
ZLaMn-12
ZLaMn-16
LaMnO3
0.43
0.44
0.63
0.47
0.47
0.59
—
—
536
538
534
533
—
1000
670
671
690
698
702
0.40
0.54
0.38
0.33
0.21
aHydrogen consumption (molH molM1n) in the range RT-1073 K (H2-
1073), and RT-850 K (H2-850). Temperatures (K) of the absolute maxima
in the range RT-850 (T1), and in the range 850–1073 (T2). RT, room tem-
perature.
2

314
CIMINO ET AL.
FIG. 5. Methane combustion: (a) CH4 conversion as a function of the reaction temperature, (b) Arrhenius plot.
Values of the total H2/Mn ratio estimated for supported reactions. At 973 K 8% of CH4 is converted with 70% CO2
La–Mn samples (Table 2) are lower, except for the ZLaMn- selectivity, whereas at 1073 K methane conversion increases
6 sample, than those evaluated for bulk LaMnO3, indicating up to 45% with about 15% CO2 selectivity.
that the pure perovskite undergoes a deeper reduction up to
The reported reaction rates were not normalized for the
1073 K. In all the ZLaMn samples but ZLaMn-6 this result surface areas of the samples, because we think that the re-
could be due to the presence of some Mn2+ species or to a ported surface area values are mainly attributed to the sup-
partial inhibition of the high temperature reduction process port, where the supported species are not uniformly spread.
when the active phase is dispersed on the support. On the
The results of catalytic activity tests of ZLaMn sam-
other hand, the H2 uptake related to the first peak and ples and zirconia are reported in Fig. 5a. Supported cata-
associated to the reduction of Mn4+, both in the perovskite lysts with a La–Mn content 4% activate methane com-
and in the oxides, is higher for supported catalysts. This bustion at 723–753 K and give complete CH4 conversion
may suggest that the interaction with the zirconia support within 923–973 K, whereas ZLaMn-2 activates the reac-
promotes the formation of Mn4+ whose fraction appears to tion at about 150 K higher temperature, showing poor cata-
be higher for the ZLaMn-6 catalyst.
lytic performances very close to that of tetragonal ZrO2.
All catalysts with a La–Mn loading 4% give 100%
CO2 selectivity. On the other hand, tetragonal ZrO2 and
ZLaMn-2 also produce CO both giving a CO2 selec-
tivity of 15% at 723 K, which increases up to about
50% at 873–923 K. At higher temperatures pure zirco-
nia maintains this selectivity, whereas for ZLaMn-2 a
3.3. Methane Combustion
Blank tests in the absence of catalyst have been per-
formed under the same reaction conditions of catalytic
tests in order to evaluate the contribution of homogeneous
TABLE 3
Kinetic Parameters for Methane Catalytic Combustion
Reaction ratea
RRperov
(mmol gp
a
CH4 order
(773K)
1
7
1
1
1
1
1
1
Catalyst
Ea (kJ mol
)
A
10 (L g
h
)
T50 (K)
(mmol g
h
)
h
)
ZrO2
102.0
99.0
100.9
93.8
88.8
85.2
97.5
1.14
1.25
8.79
4.29
2.47
1.57
22.7
1000
981
874
855
844
839
784
0.09
—
8.0
ZLaMn-2
ZLaMn-4
ZLaMn-6
ZLaMn-12
ZLaMn-16
LaMnO3
0.16
0.83
1.23
1.56
1.72
3.69
0.79
0.82
0.83
20.7
20.5
13.0
10.8
3.7
aAt 773 K, YCH = 0.4%, YO = 10%. RR, reaction rate.
4
2

METHANE COMBUSTION AND CO OXIDATION
315
of perovskite (last column of Table 3, circles in Fig. 6). In this
case the best activity is shown by ZLaMn-4 and ZLaMn-6
which give a value more than five times higher than that of
the unsupported perovskite. For La–Mn loading at 12%
the reaction rate starts to decrease slowly, but still remains
higher than that of LaMnO3.
In Fig. 5b the Arrhenius plots obtained from the catalytic
activity data are reported for pure zirconia, all supported
catalysts, and pure LaMnO3. Unsupported perovskite and
supported catalysts with a La–Mn content 4% give rise
to plots showing a unique slope, thus suggesting the occur-
rence of a single kinetic regime in the investigated range of
temperature. Onthecontrary, ZLaMn-2showsthesamebe-
havior of pure zirconia giving rise to Arrhenius plots which
can be interpolated by two lines with a different slope.
The one at T > 973 K is probably related to the occur-
rence of homogeneous reactions whose contribution be-
comes non negligible at this temperature for poorly active
catalysts. Values of the activation energy (Ea) and preexpo-
nential factor (A), estimated from the plots shown in Fig. 5b,
are reported in Table 3. All ZLaMn catalysts give rise to
Arrhenius plots with a slope quite close to that of LaMnO3
which result in values of Ea ranging from 85 to 101 kJ
mol 1. Thus, the interaction with ZrO2 does not signifi-
cantly modify the nature of active sites for methane acti-
vation, on the contrary to what is frequently reported for
alumina-supported perovskites because of the formation
of poorly active mixed oxides (16–18). Moreover, catalytic
activity measurements at fixed temperature and CH4 con-
centrations variable in the range 0.1–1% give an apparent
methane reaction order for the supported catalyst very sim-
ilar to that of bulk LaMnO3 (n 0.8, Table 3), probably
caused by an inhibition effect related to product adsorp-
tion on the active perovskite phase. A weak effect of oxygen
FIG. 6. Methane combustion: reaction rate per gram of catalyst
(lower curve) and per gram of perovskite (upper curve) as a function
of perovskite loading at 773 K.
decrease of this value to about 15% selectivity at 1043 K is
observed.
LaMnO3 apparently has the highest activity, as is also
shown by the temperature necessary to give 50% conver-
sion (fourth column of Table 3) and by the value of the
reaction rate as evaluated at 773 K and referred to as the
total catalyst amount (sixth column of Table 3, triangles in
Fig. 6). Nevertheless, based on the reasonable assumption
that the active site in the supported samples is in the per-
ovskite or in its active Mn component, we computed the
activity of the supported samples and of LaMnO3 per gram
FIG. 7. CO oxidation: (a) CO conversion as a function of the reaction temperature, (b) Arrhenius plot.

316
CIMINO ET AL.
TABLE 4
Surface Area (SA) and Kinetic Parameters for CO Catalytic Oxidation
1
1
7
1
1
a
1
1
Catalyst
SA (m2 g
)
Ea (kJ mol
)
A
10 (L g
h
)
T50 (K)
Reaction ratea
RRperov (mmol/gp
h
)
1
1
(mmol g
h )
ZrO2
83
55
67
50
55
53
63
15
84.5
62.9
61.9
64.6
68.4
62.3
67.1
62.5
1.51
0.22
0.88
10.6
25.1
10.9
38.3
8.85
749
643
556
491
495
472
470
477
1.8e-3
0.06
0.33
2.00
1.80
3.66
3.82
2.85
—
ZLaMn-0.7
ZLaMn-2
ZLaMn-4
ZLaMn-6
ZLaMn-12
ZLaMn-16
LaMnO3
9.13
16.5
50.0
30.1
30.5
23.9
2.85
a At 473 K, YCO = 1%, YO = 20%.
2
concentration on the reaction rate is observed, correspond- 3.4. CO Oxidation
ing to an apparent reaction order of 0.15 (O2 in the range
The conversion to CO2 was 100% at T
800 K for all
4–21%, CH4 0.4%), in agreement with data reported for
similar catalysts based on LaMnO3 supported on alumina
(18). Such results are in line with a redox mechanism on
perovskite-based catalysts, characterized by an intrinsically
fast reoxidation step.
In conclusion, the effect of dispersion in ZLaMn-2 leads
to the formation of a disordered oxide species with a low
activity toward total CH4 oxidation. For 4–6% loading a
maximum dispersion of the active component is reached,
resulting in the higher activity per gram of perovskite (cir-
cles in Fig. 6). A further loading increase (>6%) results in
a particle aggregation of La–Mn oxides with an ensuing de-
crease of catalytic activity per gram of the perovskite active
phase.
catalysts but the pure zirconia support. For the samples
with 4% loading and for LaMnO3 the conversion reached
100% below 550 K (Fig. 7a).
The apparent activation energy (Ea) for pure zirconia
was higher (84.5 kJ mol 1) than that for ZLaMn samples
and pure LaMnO3, all showing very similar values in the
range from 62 to 68 kJ mol 1 (Fig. 7b). The activity of pure
zirconia at 573 K on both a weight and a surface area ba-
sis was one order of magnitude lower than the most dilute
supported catalyst (ZLaMn-0.7). Therefore ZrO2 is sup-
posed to marginally contribute to the activity of the sup-
ported samples, while even very small amounts of the active
phase produce a remarkable shift of the conversion values
to lower temperatures (Fig. 7a).
In analogy to the results found for methane oxidation,
Table 4 and Fig. 8 clearly show that the CO oxidation activ-
ities per gram of perovskite of ZLaMn-0.7 and ZLaMn-2
are lower than those for all the other ZLaMn samples, but
the pure LaMnO3. This result confirms that in the ZLaMn-
0.7 and ZLaMn-2 samples the perovskite structure is not
present and metal oxides prevail, whereas in ZLaMn-4 and
ZLaMn-6 a perovskite-like phase starts to organize. We
suggest that the high oxidation activity is a feature of the
perovskite structure, possibly related to the ease of both
oxygen adsorption/desorption and the local change of the
cation oxidation state.
4. CONCLUSIONS
XRD analysis for the zirconia-supported samples shows
only the tetragonal zirconia phase with traces of monoclinic
zirconia up to 6% loading. At a higher loading XRD also
shows the presence of the perovskite LaMnO3 phase.
XAS analysis confirms the formation of LaMnO3 dis-
persed on zirconia at high La–Mn loading, whereas at low
loading the presence of disordered oxide species strongly
interacting with the support is suggested.
FIG. 8. CO oxidation: reaction rate per gram of catalyst (lower curve)
and per gram of perovskite (upper curve) as a function of perovskite load-
ing at 473 K.

METHANE COMBUSTION AND CO OXIDATION
317
TPR experiments indicate the presence of LaMnO3 at
high La–Mn loading and La and Mn oxide species at low
loading.
The activity trends for methane combustion and CO oxi-
dation appear rather similar. At low La–Mn loading, disor-
dered oxide species strongly interacting with support pre-
vail, and the activity is low, even when calculated at per
gram of perovskite. The higher activity of the catalysts with
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