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Applied Catalysis A: General 206 (2001) 1318
Ozone decomposition, benzene and CO oxidation overNiMnO3-ilmenite and NiMn2O4-spinel catalysts
D. Mehandjiev, A. Naydenov, G. IvanovInstitute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Received 15 December 1999; received in revised form 20 March 2000; accepted 21 March 2000
Abstract
The catalytic activities of NiMnO3 andNiMn2O4 during heterogeneous catalyticdecomposition of ozoneand ozone-catalytic
oxidation(OZCO) of benzene at low temperatures (2080C) have been investigated. The sensitivity of thetwo oxides towards
strong catalytic poisons, such as nitrogen oxides, during the decomposition of ozone has also been estimated. On the basis ofthe experimental results obtained it is concluded that the NiMnO 3 and NiMn2O4 obtained have a high activity with respect
to the reactions of ozone decomposition and CO and CH oxidation in the presence of ozone at temperatures close to the
room temperature. The sample with an ilmenite structure shows, in all cases, a higher catalytic activity. The surface oxygen
of NiMnO3 is more reactive at room temperature than is the case of NiMn 2O4. The hypothesis according to which when the
two metal cations are in octahedral coordination the catalyst activity is higher and the stability towards catalytic poisons is
enhanced has proved to be correct. It should be noted that a catalyst has been synthesized which is able to decompose ozone at
room temperature andto activate theorganic molecule to a degree permitting catalyticoxidation by ozone at room temperature.
In addition, this catalyst shows a relatively high stability with respect to poisoning by nitrogen oxides. 2001 Published by
Elsevier Science B.V.
Keywords: Ozone decomposition; Ozone-catalytic oxidation; Catalytic poisons; Nickel-manganese oxides
1. Introduction
The interest in developing catalysts to be applied
to processes of low-temperature neutralization of or-
ganic pollutants in waste gases has increased consid-
erably during the past years. One of the promising
methods in this respect is the ozone-catalytic oxida-
tion (OZCO) where heterogeneous catalytic decom-
position of ozone is used to obtain highly reactive
atomic oxygen able to oxidize harmful organic com-
pounds at low temperatures including room tempera-
ture [1]. In previous studies [25] it has been shown
that the simple oxides of Ni, Co, Fe and Mn have a
Corresponding author.
high activity in the decomposition of ozone and in the
OZCO process of organic compounds. It is known that
mixed 3d-transition metal oxides are more active than
are simple oxides [611]. In [12] it has been shown
that in oxidation reactions NiMnO3 with an ilmenite
structure has a high activity which is comparable to
that of the spinel NiMn2O4. That activity may be as-
sociated with the position of the two cations in oc-
tahedral coordination. It was of interest to compare
the catalytic activities of these oxides during hetero-
geneous catalytic decomposition of ozone and OZCO
of benzene at low temperatures (2080C) and also
to estimate the sensitivity of the two oxides towardsthe nitrogen oxides, which are known [4] to be strong
catalytic poisons during the decomposition of ozone.
0926-860X/01/$ see front matter 2001 Published by Elsevier Science B.V.
PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 5 7 0 - 6
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Fig. 1. FTIR spectra of the samples investigated.
2. Experimental
2.1. Synthesis and characterization of the catalysts
Nickelmanganese mixed oxides were prepared
from carbonate precursors. They were synthesized
by adding, with constant stirring, a 0.5 M solution
of metal nitrates (in a ratio of 1:1 and 1:2) to a
1 M solution of sodium bicarbonate. The precipitateformed was filtered and dried at atmospheric pres-
sure. Ilmenite was obtained using the precursor with
Ni:Mn=1:1. The thermal treatment consisted of heat-
ing with a rate of 10C/min up to 450C, maintaining
this temperature for 5 h and, finally, rapid cooling to
room temperature. The spinel was prepared by ther-
mal treatment with a heating rate of 10C/min up to
750C, keeping the sample at this temperature for
5 h, then cooling it quickly to room temperature. The
precursor used in this case had a Ni:Mn ratio of 1:2.
For more details see [12,1416]. The atomic ratio
of the metals was controlled by atomic absorption
analysis and the results showed that the compositionsof the two samples corresponded to NiMnO3 and
NiMn2O4.
The structure of the samples obtained was char-
acterized by X-ray analysis with a DRON (Russia)
diffractometer using Cu K radiation. The magnetic
studies were carried out with a Faraday type mag-
netic balance. According to the chemical analysis
and the magnetic measurements the ion distribution
in the sublattices of the spinel was: Ni2+0.20Mn2+
0.80[Ni2+0.80Mn3+0.40Mn4+0.80]O4. According
to modern concepts [13], both ions are in octahedralcoordination in the ilmenite. The FTIR-spectra of the
samples are different in the region connected with the
MO bond (Fig. 1). The specific surface area of the
samples was determined by low-temperature adsorp-
tion of N2. The results are 44 m2/g for NiMnO3 and
8 m2/g for NiMn2O4.
2.2. Reaction parameters
The pre-treatment of the samples consisted in
a 20 min heating at 300C in an oxygen flow. A
0.20.4 mm fraction was used. Ozone was synthe-
sized from dried oxygen (gas flow rate 4 l/h), using anozone generator with silent discharge (68 kV). The
generator was equipped with glass coaxial electrodes.
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D. Mehandjiev et al. / Applied Catalysis A: General 206 (2001) 1318 15
The initial concentration of ozone ranged from 22.0
to 24.5g/m3. The ozone analysis was performed by
an on-line ozone analyser (Ozomat GM, Germany)
with an accuracy of0.1 g/m3. The residual ozone
was decomposed in a catalytic reactor filled with
an OCA-1 (Bulgaria) catalyst [17]. The experiments
were carried out with a circulation ratio of 1:70. The
reaction temperature varied between 19 and 80C and
was maintained with an accuracy of0.2C. Carbon
monoxide and benzene were dosed by an Ismatex
MS2/6 (Switzerland) pump. The initial concentrations
were varied within the limits of 0.51.5 vol.% for
CO and 0.010.03 vol.% for benzene. The carrier gas
was air and oxygen (99.8%). The rate of complete
oxidation was estimated by measuring the quantity of
CO2 (by Infralyt 2106, ex-GDR) formed during the
reaction. The experiments on the so-called depletive
oxidation [18] were performed in an integral reactor.
A CO flow in argon was passed through the catalyst
layer, i.e. this occurred in the absence of an oxidizingagent from the gas phase. The process was controlled
on the basis of the CO2 concentration at the reactor
outlet. The behaviour of the catalysts during ozone
decomposition in the presence of nitrogen oxides was
investigated using an integral pulse reactor. At its
inlet, 20 cm3 of nitrogen oxides with a concentration
of 68 vol.% were injected. The formation of nitrates
on the catalyst surface was controlled by an FTIR
(Brucker) spectrometer.
Fig. 2. Temperature dependencies of the conversion degrees of ozone, CO and benzene on the catalysts investigated.
3. Experimental results
Fig. 2 shows the temperature dependencies of the
conversion degrees of ozone, CO and benzene on
the two catalysts. Evidently, the ilmenite catalyst has
a higher catalytic activity at lower temperatures. It
should be noted that when molecular oxygen is used
on both catalysts oxidation of benzene and CO be-
gins only at 150200C. Hence during OZCO, the
temperature of catalyst efficiency is lower by 100
150C.
Table 1 shows the activation energies and rate con-
stants calculated per gram of catalyst and unit sur-
face. Obviously, ozone decomposition proceeds with a
relatively high activation energy. During oxidation of
benzene and carbon monoxide on an ilmenite catalyst,
the activation energy is lower than is the case of the
spinel catalyst. The rate constants per gram ilmenite
are higher than those per gram spinel and the high-
est rate constant corresponds to ozone decomposition.The rate constant per unit spinel surface area is higher
probably because of the higher concentration of active
sites. However, if we accept the activation energy as
a measure of the catalyst activity, then ilmenite is the
more active catalyst with respect to the reactions un-
der consideration. This is in agreement with the results
in a previous paper [12] dealing with catalytic oxida-
tion of benzene with molecular oxygen on analogous
catalysts, however, at high temperatures.
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16 D. Mehandjiev et al. / Applied Catalysis A: General 206 (2001) 1318
Table 1
Kinetic parameters of the reactions on the catalysts investigated
Catalysts Reaction
Ozone decomposition (30C) CO oxidation by O3 (40C) C6H6 oxidation by O3 (30
C)
NiMnO3Reaction ratea (mol/g s) 2.63106 (0.60107) 1.86108 (0.42109) 8.76108 (2.00109)
Activation energy (kJ/mol) 44 26 34
NiMn2O4Reaction ratea (mol/g s) 2.09106 (4.18107) 3.23109 (0.65109) 4.14108 (8.28109)
Activation energy (kJ/mol) 40 48 50
a Values in parenthesis are in mol/m2 s.
Fig. 3. Results on so-called depletive oxidation on the samples at 30C.
Fig. 3 presents the results of depletive oxidation of
CO over the two catalysts. It is evident that the active
oxygen content is higher in the case of ilmenite. Thecurves in Fig. 3 are of an overshot response type which
indicates that the regeneration of active sites on the
surface is the rate-controlling step of the reaction [19].
It is known [4] that when ozone is formed by air,
a definite amount of nitrogen oxides is present in the
gas phase and they block the ozone decomposition.
In the present investigations the ilmenite catalyst has
a higher resistivity with respect to nitrogen oxides in
the gas phase, as is shown in Fig. 4. Nitrate groups
poisoning the catalyst are formed on its surface. The
same has been observed with both catalysts (Fig. 5).
Bielanski and Haber [20] have divided the oxides
into three groups: (1) oxides on which oxygen is ad-sorbed mainly in the form of electron-rich species (ox-
ides of Ni, Mn and Co); (2) oxides on which oxygen
Fig. 4. Results from the experiments on pulse poisoning of the
catalysts by nitrogen oxides during the reaction of ozone decom-
position.
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D. Mehandjiev et al. / Applied Catalysis A: General 206 (2001) 1318 17
Fig. 5. IR spectra of the samples after the poisoning by nitrogen
oxides.
is adsorbed in the form of species less rich in elec-
trons, such as O2 (oxides of Zn and Ti) and oxides
which do not adsorb oxygen (oxides of Mo and W).
The first group of oxides, to which the oxides used in
the present work belong, are characterized by a high
concentration of electron-donor centres, due to which
the electron-rich species O and O2 are formed dur-
ing oxygen adsorption. It is supposed that the O
provides the complete oxidation [2023]. It is estab-
lished that O interacts with CO forming CO2 radi-
cals [24]. In our earlier investigation we observed that
when CO/O3/O2 is passed through CeO2, no radicals
originating from CO appeared, i.e. O3 is the only con-
stituent of the gas mixture that can react with the sur-
face. This means that the decomposition of ozone pre-
cedes the oxidation of CO, i.e. the EleyRideal mech-
anism is possible. The existence of O
has not beenobserved experimentally, but is logically suggested.
The general form of the reactions taking place is
O3 + Z ZO+O2 (1)
O3 + ZO Z + 2O2 (2)
Writing these reactions so as to present the changes
in oxidation state of the metal ions, one obtains
Mn+ +O3 Mn+1O +O2 (3)
Mn+1O + R Mn+ + RO (4)
Mn+1O +O3 Mn+ + 2O2 (5)
Mn+1O +O2 Mn+1O3
(6)
where R is the organic compound or CO and M rep-
resents the metal ion.
Comparison of the results shows that the decom-
position of ozone on the two catalysts leads to the
formation of active oxygen. According to our opinion
the O form is the most probable one and this leads
to a sharp drop in the temperature needed for com-
plete oxidation of benzene and its removal from the
gas mixture.
4. Conclusions
On the basis of the experimental results obtained
and their interpretation it can be concluded that
NiMnO3 and NiMn2O4 have a high activity with
respect to the reactions of ozone decomposition and
CO and C6H6 oxidation in the presence of ozone at
temperature close to the room temperatures, i.e they
are appropriate for the OZCO process.The sample with an ilmenite structure shows in all
cases a higher catalytic activity. The surface oxygen
of NiMnO3 is more reactive at room temperature than
in the case of NiMn2O4.
The initial hypothesis according to which with the
two metal cations in octahedral coordination the cat-
alyst activity is higher and the stability towards cat-
alytic poisons is enhanced has proved to be correct.
It should be noted that a catalyst (NiMnO3) has been
synthesized which is able to decompose ozone at room
temperature and to activate the organic molecule to
a degree permitting catalytic oxidation by ozone at
room temperature. In addition, this catalyst shows arelatively high stability with respect to the poison-
ing by nitrogen oxides during the reaction of ozone
decomposition.
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