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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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14 D. Mehandjiev et al. / Applied Catalysis A: General 206 (2001) 1318

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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