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Chemical Engineering Journal 169 (2011) 282–289

Contents lists available at ScienceDirect

Chemical Engineering Journal

journa l homepage: www.e lsev ier .com/ locate /ce j

ffects of excess manganese in lanthanum manganite perovskite on loweringxidation light-off temperature for automotive exhaust gas pollutants

arvaneh Esmaeilnejad-Ahranjani, Abbasali Khodadadi, Hessam Ziaei-Azad, Yadollah Mortazavi ∗

atalysis and Nanostructured Materials Laboratory, School of Chemical Engineering, College of Engineering, University of Tehran, Enghelab Ave., P.O. Box 11365-4563, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 27 September 2010eceived in revised form 22 February 2011ccepted 24 February 2011

eywords:xhaust pollutionanthanum manganite perovskite

a b s t r a c t

The effects of excess manganese on enhancement of oxidation activity of LaMn1+xO3+ı perovskite forCO and propane removal from a synthetic automotive exhaust gas are reported. Various LaMn1+xO3+ı

(x = 0.05–0.4) perovskite-type nanocatalysts were prepared by a microwave-assisted “gel combustion”method and calcined at 700 ◦C in air. Scanning electron microscopy (SEM) of the samples demonstrateshighly porous and frothy particles. More than twice enhancement in the BET surface area of the cata-lyst samples is observed for the LaMn1.2O3+ı catalyst, as compared to LaMnO3. X-ray powder diffraction(XRD) analyses show only the perovskite structure for the catalysts. H2-, CO-, and C3H8- temperature

on-stoichiometryatalytic oxidation

programmed reductions (TPR) indicate that the excess manganese oxides in LaMn1+xO3+ı significantlyenhance the reducibility of the catalysts. The manganese oxides may be in the form of a separate phaseof amorphous and/or small crystallites. Redox titration test results show that, as the amount of Mn inthe structure increases, the average oxidation state of manganese reduces resulting in lowering cationvacancies. The light off temperature for CO and propane oxidation on LaMn1.2O3+ı is decreased by 57and 38 ◦C, respectively as compared to those of LaMnO3 perovskite. LaMn1.2O3+ı is stable under harsh

ts ox

conditions of the pollutan

. Introduction

Catalytic convertors are standard technology for the con-rol automotive emission, such as nitrogen oxides (NOx), carbon

onoxide (CO), and unburned hydrocarbons (HC). Conven-ional convertors containing expensive noble metals dispersedn ceramic-support materials are widely used. However, theiratalytic activity deteriorates because of the agglomeration androwth of metal particles at high temperatures during vehicle use1,2].

Perovskite oxides with general formula ABO3, where B is amaller cation responsible for catalytic activity surrounded by sixxygens in octahedral coordination and A is a large cation respon-ible for the thermal resistance coordinated by 12 oxygens, haveeen considered as promising alternatives to noble metals. Whatakes perovskites attractive with respect to noble metals is their

ower cost, high thermal stability, and high catalytic activity [3].

Lanthanum manganite oxide LaMnO3+ı exhibits non-

toichiometry on one or more sublattices because manganesen the B-site can have the valences Mn3+ or Mn4+. This meanshat the perovskite can be either oxidized or reduced as a function

∗ Corresponding author. Tel.: +98 21 6696 7793; fax: +98 21 6696 7793.E-mail address: mortazav@ut.ac.ir (Y. Mortazavi).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.02.062

idation.© 2011 Elsevier B.V. All rights reserved.

of temperature and oxygen partial pressure. Interstitial oxygenis almost unlikely, for the LaMnO3+ı structure consists of a closepacked LaO3 lattice with Mn in the O6 octahedral sites. Oxidativenon-stoichiometry is then possible through cation vacancies.Accordingly, the composition of LaMnO3+ı is better expressed asLa1−εMn1−εO3 with [4];

ε = ı

3 + ı

or

LaMn3+1−2ı

Mn4+2ı

O3+ı = 3 + ı

3La3/3+ı�ı/3+ıMN3+

(3−6ı)/3+ı

MN4+(6ı)/3+ı

�ı/3+ıO3.00

where ı is excess (interstitial) oxygen in the perovskite structureand � is cation vacancy.

It has been shown that this portion of Mn4+ in LaMnO3+ı can bevaried. By heating the sample in air at different temperatures rang-ing from 700 to 1000 ◦C this portion decreases from 32 to 22% [5]and by heating the sample in a He atmosphere at 700 ◦C, LaMnO3+ı

is obtained containing about 18% Mn4+ [6].Little attention has been paid to the non-stoichiometric per-

ovskites, especially on their oxidation activity performance. Insome literatures, solid solubility in air of the perovskite that is inequilibrium with La2O3 and MnOx has been determined. Hebert

ical Engineering Journal 169 (2011) 282–289 283

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P. Esmaeilnejad-Ahranjani et al. / Chem

t al. [7] prepared Mn-deficient single-phase LaMn1−yO3, with= 0.1. Arulraj et al. [8] prepared single-phase samples of compo-ition La1−xMnO3, with x = 0.2 and LaMn1−yO3 with y = 0.2 in air at50 ◦C. The composition of La1−xMnO3 in equilibrium with Mn3O4as determined to be x = 0.12 and x = 0.1 in air at 800 and 1300 ◦C,

espectively [9]. In a comprehensive study, Roosmalen et al. [10]llowed maximum solubility limits for both the La-rich and the Mn-ich phase boundaries of La1−xMn1−yO3±ı. They found an averagef x = 0.09 with no significant temperature dependence in equilib-ium with MnOx and y = 0.1 in equilibrium with La2O3 at 1200 ◦C,ecreasing to 0.462 cation % Mn at 1000 ◦C and 0.452 cation % Mnt 950 ◦C. LaMnO3 indeed is a perovskite that can accept struc-ural defects on the La- as well as on the Mn-sites up to a certainmount.

Structure of lanthanum manganite oxides and so the presencef cation and/or anion vacancies in A and B sublattices of the per-vskite structures La1−xMnO3+ı and LaMn1−x′ O3+ı (0 < x, x′ < 0.2)how as La or Mn non-stoichiometry increases, the oxygen con-ent and so Mn4+ decreases more with La-deficient compositionshan with Mn-deficient ones [11].

LaMnO3 perovskite with defects in Mn sites has exhibited theigh activity in the total oxidation of methane. In such a case theypes of vacancies introduced should be responsible for an easierctivation of the lattice �-oxygen. The non-stoichiometric per-vskite can easily release oxygen and hence become more activehan the stoichiometric one [12].

Compared to various methods used for synthesis of metal oxideanoparticles, combustion synthesis is simple in process, low inost and saving in time and energy consumption. This methodnables a good chemical homogeneity, a narrow size distributionnd a high surface area [13]. Fuel is the source of carbon and hydro-en, which on combustion forms COx and H2O and liberates heat.he fuel also forms complexes with the metal cations facilitatingomogeneous mixing of the cations in the solution, presumably

mpeding the preferential precipitation of ionic species [14]. Dif-erent types of fuels may be used in the combustion synthesis.rganic polymers such as polyethylene glycol (PEG), polyvinyl alco-ol (PVA), citric acid, glycine, urea, sorbitol, carbohydrazide are alsosed as the fuel [14–17].

In this study a very representative type of perovskite, namelyaMnO3, has been therefore chosen and the effects of excess Mnn the LaMn1+xO3+ı perovskite oxides on enhancing their catalyticctivity for abatement of the automotive exhaust gas pollutants arenvestigated.

. Experimental

.1. Catalyst preparation

LaMn1+xO3+ı (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) powders wererepared via a microwave-assisted “gel combustion” synthesisethod, using lanthanum and manganese nitrates (from Merck)

s precursors and oxidants together with sorbitol as the fuel. A sto-chiometric amount of sorbitol for its complete oxidation was used,o that LaMn1+xO3+ı could be formed from the solution combustioneactions without extra oxygen. The nitrate precursors and fuelere dissolved in deionized water, thoroughly stirred, and some

xcess water evaporated at 150 ◦C to form a viscous gel. The gel waseated and combusted for about 30 s in a microwave oven (850 W)t 100% power. After evaporation of excess water and heating up

o the combustion temperature, the sample was reacted vigorouslyith evolving a large amount of gases and left a dark brown porousass. The sample was ground and calcined at 700 ◦C in still air

or 5 h. This calcination temperature has been reported by otheresearch groups [5,11].

Fig. 1. XRD patterns of LaMnO3, and LaMn1.2O3+ı calcined at 700 ◦C (fresh samples)and LaMn1.2O3+ı after TPR at 600, 850 and 1000 ◦C in 7% H2/Ar mixture. The temper-ature was raised at 10 ◦C min−1 to the final reduction temperature and maintainedfor 30 min. Perovskite (�); La2O3 (♦); MnO (*).

The overall combustion reaction can be written as follows:

La(NO3)3 · 6H2O + (1 + x)Mn(NO3)2 · 4H2O + nC6H14O6 + (13n

−(12 + 4.5x))O2 → LaMnO3 + x

2MnO3 + 6nCO2 + 7nH2O

+(

52

+ x)

N2

Mn2O3 was prepared according to the method described above;using manganese nitrate and sorbitol followed by calcination at700 ◦C in still air for 5 h.

2.2. Catalyst characterization

The crystal structures of the catalysts were determined by X-raypowder diffraction (XRD) with a Bruker AXS D8 Advanced diffrac-tometer using Cu K� (� = 1.54056◦A) radiation in the 2� range of20–70◦ with a scanning speed of 4◦ min−1.

The mean crystallite size of the catalyst powders was deter-mined from XRD line-broadening measurement by Scherrer’sequation:

D = k�

ˇ cos �

where � is the wavelength, ˇ is the true half-peak width and � isthe diffraction angle.

The microscopic morphology as well as the elemental com-position of some catalysts was observed using scanning electronmicroscopy (SEM) and energy dispersive spectroscopy (EDS), usingPhilips XL30 apparatus.

BET and H2-temperature programmed reduction (TPR) testswere carried out using a Quantachrome CHEMBET-3000 apparatus.BET surface areas were measured at the liquid nitrogen tempera-ture by N2 adsorption. The TPR experiments were performed on0.10 g of perovskite samples by 10 sccm (cm3 (STP) min−1) of 7.0%

284 P. Esmaeilnejad-Ahranjani et al. / Chemical Engineering Journal 169 (2011) 282–289

Tab

le1

Mn

oxid

atio

nst

ate

and

spec

ific

surf

ace

area

ofLa

Mn

1+xO

3+ı

cata

lyst

s.

Cat

alys

tFo

rmu

laa

Mn

4+M

n3+

BET

m2g−1

Peak

1Pe

ak2

Peak

3Pe

ak4

Are

abTe

mp

erat

ure

(◦ C)

Are

abTe

mp

erat

ure

(◦ C)

Are

abTe

mp

erat

ure

(◦ C)

Are

abT 5

0C

OT 5

0H

CT 9

0C

OT 9

0H

C

LMLa

Mn

O3. 1

60.

330.

678

5400

420

2000

510

7300

187

232

227

314

LM1.

05La

Mn

1.05

O3. 2

30.

300.

759

1100

330

4700

415

2700

475

6600

187

233

229

305

LM1.

1La

Mn

1.1O

3. 2

80.

240.

8617

2000

280

4000

380

3100

450

6100

130

220

150

300

LM1.

2La

Mn

1.2O

3. 4

00.

201.

0119

3000

280

3800

380

3700

440

5700

130

194

140

280

LM1.

3La

Mn

1.3O

3. 5

40.

151.

1515

2700

290

4100

390

3000

440

6400

138

207

162

292

LM1.

4La

Mn

1.4O

3. 6

60.

101.

3011

2600

300

4200

405

2900

450

6700

147

211

170

297

aEv

alu

atio

nba

sed

onre

dox

titr

atio

nan

alys

is.

bC

alcu

late

dby

dec

onvo

luti

on,w

ith

Gau

ssia

nfi

t,of

the

TPR

curv

es.

Fig. 2. SEM micrographs of the calcined LaMnO3 (a), and LaMn1.2O.

H2 in Ar. Temperature was raised from 50 to 1000 ◦C at a heatingrate of 10 ◦C min−1.

CO-TPR of the catalysts was studied in a quartz tube reactor by50 sccm of 6.0% CO in N2. Also C3H8-TPR was studied by 10 sccmof 0.2% C3H8 in N2. In this experiment, 0.10 g of the catalysts wasemployed. The reactor temperature was increased by 10 ◦C/min ina tubular furnace and controlled by a temperature controller and aK-type thermocouple inside the catalyst bed. The reactor effluentswere instantaneously analysed by an online FTIR instrument. FTIRmeasurements were performed by a Bruker Vector22 spectrometerequipped with a 0.8 cm ID and 10 cm length gas cell with KBr win-dows. The FTIR was used in transmission mode with a resolution of5 cm−1 in the range of 4000–400 cm−1. Peak areas in the ranges of2400–2280, 2230–2030 and 3040–2830 cm−1 are used for CO2, COand C3H8, respectively.

To determine the concentrations of Mn3+ and Mn4+ ions, redoxtitration according to the reported procedure was used [9]. In thisexperiment, 0.010 g of the catalysts was employed. Upon treat-ment of tri- and tetra-valent manganese with hydrochloric acid,manganese ions were reduced to Mn (II), the evolved chlorine wasreduced in situ with potassium iodide. The liberated iodine wastitrated with sodium thiosulfate solution. Based on the molarityand consumed volume of the latter solution, the number of molesof Mn3+ and Mn4+ per unit mole of catalyst sample was estimated.In this procedure the following reactions occur:

2Mn(III) + 2Cl− → Cl2 + 2mn(II)

Mn(IV) + 2Cl− → Cl2 +mn(II)

P. Esmaeilnejad-Ahranjani et al. / Chemical Engineering Journal 169 (2011) 282–289 285

F

C

I

r

c1tfaitsT

2

Cdsccmilateeagp

ig. 3. H2-TPR profiles of as prepared LaMn1+xO3+ı perovskite and Mn2O3 catalysts.

l2 + 2I− → 2Cl− + I2

2 + 2S2O32− → S4O6

2− + 2I−

Amount of oxygen (ı) was calculated in a similar methodeported by Hammami et al. [11].

Thermal deactivation tests were carried out under the reactionondition (total flow = 50 sccm, 6.0 mol% CO and 0.2 mol% C3H8 with0% excess oxygen) over 0.10 g catalyst as the following; tempera-ure was first increased to 700 ◦C (20 ◦C min−1) and remained thereor 2 h. The temperature was then decreased to T50 of C3H8, defineds temperature of 50% conversion of propane on fresh catalyst thats different for each sample, and conversion was measured. Afterhat, temperature was decreased to T50 of CO to examine its conver-ion at this temperature. Again, temperature was raised to 700 ◦C.his sequence was repeated 20 times for each catalyst.

.3. Catalytic activity measurement

A synthetic exhaust gas containing 6.0 mol% CO and 0.2 mol%3H8 with 10% excess oxygen in N2 was used for evaluation of oxi-ation activity of the catalysts. A total flow rate of 50 sccm of theynthetic exhaust gas, using mass flow controllers, and 0.10 g of theatalysts were employed. The perovskite catalysts were pelleted,rushed and sieved to particle sizes of 0.125–0.297 mm (50–120esh size), and then retained on a small amount of quartz wool

n an 7.0 mm-ID quartz micro reactor. A K-type thermocouple wasocated in the middle of the catalyst bed, the reactor was placed intubular furnace, and a PID controller was used to adjust the reac-

or temperature. Concentration of CO, CO2 and C3H8 in the reactor

ffluents were analysed by Shimadzu GC-8A gas chromatographquipped with porapack Q column, a methanizer and an FID. Alsoset of valves allowed bypassing the reactor feed directly into theas chromatograph, which provided a direct measurement of theollutants’ concentrations in the feed.

Fig. 4. CO and C3H8-TPR profiles of as prepared LaMnO3 (dotted line) andLaMn1.2O3+ı (solid line) catalysts.

3. Results and discussion

3.1. Catalysts structure, morphology and redox properties

XRD patterns of fresh LaMnO3 (LMO) and LaMn1.2O3+ı (LM1.2O)catalysts, calcined at 700 ◦C for 5 h, are given in Fig. 1. The meancrystallite size of LaMnO3 and LaMn1.2O3+ı, determined by Scher-rer’s equation, was 34 and 22 nm, respectively. The patterns showonly the perovskite structure (JCPDS 32-0484) for both samples.However, at the moderate calcination temperature of 700 ◦C, all ofthe manganese of the samples may not be in the form of perovskiteoxide [18]. The absence of any peak characterizing manganeseoxide in XRD patterns of both samples may indicate that residualmanganese and possibly lanthanum oxides are in the amorphousand/or small crystallite forms highly dispersed in the perovskite.The main form of manganese oxide in this calcination temperatureis Mn2O3 [19]. XRD peaks of Mn2O3 at 2� = 32.4◦ may coincide withthat of the perovskite and other MnOx XRD peaks are small.

It has been reported that for LaNiO3 and LaCoO3, separate metaloxide in the form of amorphous or crystallite, is present even at highcalcination temperatures [18,20]. Despite the formation of crys-tallite manganese oxide alone at 550–800 ◦C, neither perovskitenor other oxide crystallite phases are observed in XRD pattern ofSmMnO3 [21]. However, the TPR profile of this sample shows peaksat low temperatures, which are attributed to reduction of metaloxide phases or even perovskite oxide with amorphous structure.The similar results for non-stoichiometric La–Mn perovskite havebeen reported with no XRD peaks for any metal oxide [11].

BET specific surface areas of LaMn1+xO3+ı perovskites arereported in Table 1. The BET area increases from 8 to 19 m2 g−1

with the excess manganese up to Mn/La atomic ratio of 1.2, thenreduces to 11 m2 g−1 at Mn/La of 1.4. For manganese oxide, the BETarea is 7 m2 g−1. Up to Mn/La of 1.2, excess Mn may form sepa-rate MnOx phases of smaller sizes dispersed in the perovskite bulk,resulting in an increase in the BET area. Further addition of excessMn (Mn/La = 1.4) may lead to sintering MnOx phase and lower theBET area. Berenov et al. [22] reported that increasing Mn/La ratioin LaMnO3 perovskite lead to a reduction of sintering temperatureto lower values.

SEM micrographs of the LaMnO3 and LaMn1.2O3+ı catalysts cal-

cined at 700 ◦C are presented in Fig. 2. Both samples exhibit frothyagglomerated particles with large voids in their structure. For-mation of these features is attributed to the escape of a largeamount of gas during the combustion of organic species used in

286 P. Esmaeilnejad-Ahranjani et al. / Chemical Engineering Journal 169 (2011) 282–289

ction

ttLfi

naoapTamtalh

3

s

Fig. 5. CO (a) and C3H8 (b) oxidation as a fun

he preparation process. Smaller agglomerates are observed inhe SEM micrograph of the catalyst including excess manganese,aMn1.2O3+ı with higher BET surface area. The EDS analysis con-rmed the nominal La/Mn atomic ratio of the catalyst samples.

Manganese oxidation state of perovskite catalysts after calci-ation at 700 ◦C was determined by iodometry method. The Mn4+

nd Mn3+ contents and general formula of non-stoichiometric per-vskites are reported in Table 1. LaMnO3 has 33% Mn4+ that is ingreement with the results reported by other researchers [11]. Theresence of Mn4+ has been confirmed with XPS analysis [23,24].he mean oxidation state and oxygen content linearly decrease asresult of increasing the excess manganese in the catalysts. Thisay indicate the formation of some oxygen deficiencies, although

he catalysts are prepared by gel combustion and calcination in airtmosphere to avoid the oxygen deficiency. The same results foranthanum manganite perovskites with deficient in lanthanum siteave been reported by Hammami et al. [11].

.2. Temperature-programmed reduction

H2-TPR profiles of LaMn1+xO3+ı and Mn2O3 samples are pre-ented in Fig. 3. The samples reduce in two distinct temperature

of temperature over LaMn1+xO3+ı catalysts.

ranges of 200–550 ◦C and >720 ◦C. The peak at >720 ◦C is ascribedto the reduction of Mn3+ to Mn2+. The samples, initially black,become gray/white after reduction at 1000 ◦C, suggesting that newphases, including Mn2+, have been formed [23]. XRD pattern forLaMn1.2O3+ı after TPR up to 1000 ◦C (Fig. 1) reveals the disap-pearance of the perovskite phase, with concurrent formation ofindividual La2O3 and MnO phases, indicating that Mn4+ and Mn3+

are reduced to only the 2+ oxidation state. However, completereduction of Mnn+ into Mn0 in the perovskite does not occur [25,26].XRD pattern of LaMn1.2O3+ı after TPR up to 850 ◦C (Fig. 1) exhibitsthat perovskite gradually reduces. Even at this high temperature,under hydrogen, perovskite phase does not completely disappear.

The reduction in the low-temperature range of 200–550 ◦C isquite complex with peak temperatures ranging from 280 to 510 ◦C.A major peak at 420 ◦C and a minor shoulder at about 510 ◦C areobserved for LaMnO3 catalyst. Shoulders are appeared for the per-ovskites containing excess Mn at about 280–330 ◦C. These three

peaks/shoulders temperatures and areas, measured after decon-volution, for various catalysts are summarized in Table 1. Thedata for fourth peak at >720 ◦C is also included in Table 1. Thepeaks/shoulders are denoted peaks 1, 2, 3, and 4 in order of increas-ing their temperature, in Table 1. The reduction temperature of

ical Engineering Journal 169 (2011) 282–289 287

stoaa

2poaiF

Lcempttivict

tpptaimtAioaLt

sTfifiph

CFttta

3

aMotcNtn

P. Esmaeilnejad-Ahranjani et al. / Chem

econd and third peaks decrease with the amount of excess Mn, upo Mn/La atomic ratio of 1.2 and then increase with further additionf excess Mn. The first and third areas increase up to Mn/La of 1.2nd then decrease with excess Mn, whereas the second peak showsreverse trend.

The catalysts reduction in the low-temperature range of00–550 ◦C may be attributed to reduction of Mn4+ to Mn3+ androbably amorphous and/or small crystallite forms of manganesexide dispersed in the perovskite. The XRD pattern of LaMn1.2O3+ı

fter TPR up to 600 ◦C shows that the perovskite structure remainsntact and the bulk structure does not significantly changes (seeig. 1), that is in agreement with the other reports [24].

After partial reduction by H2-TPR up to 400 ◦C of LaMnO3 andaMn1.2O3+ı, 78% and 100% of Mn4+ is removed, respectively. Con-entration of Mn4+ ions was determined by redox titration afterach pre-reduction and compared to that of fresh catalysts. Thisay infer that the second peak in TPR profiles of the catalysts can

artly be attributed to the reduction of Mn4+ to Mn3+ [27,28]. Theemperature of this peak decreases by addition of excess Mn, upo Mn/La atomic ratio of 1.2. Other reports [29] suggest that, withncreasing manganese non-stoichiometry in LaMn1+xO3+ı, cationacancy array is perturbed by oxygen vacancies, which trap Mn3+

ons, so the reduction of Mn4+ is easier. Further addition of Mn, mayause sintering of samples, so the second peak again shifts slightlyo higher temperature.

The first and third peaks of TPR may be ascribed to reduc-ion of the fine crystallites and/or amorphous manganese oxidehase. Manganese oxide (Mn2O3) first reduces to Mn3O4 (TPReak at 410 ◦C) and then to MnO (510 ◦C peak, Fig. 3). Fine crys-allites and amorphous manganese oxide species may be reducedt even lower temperatures [19]. Therefore, the minor shouldern TPR at lower temperatures of about 280–300 ◦C (see Fig. 3)

ay be ascribed to the reduction of amorphous and/or fine crys-allites of manganese oxide phase dispersed in the catalysts.ddition of the excess Mn into the stoichiometric perovskite has

ncreased the areas of the both first and third peak up to Mn/Laf 1.2 (Table 1). More than twice enhancement in the BET surfacerea of LaMn1.2O3+ı catalyst samples is observed, as compared toaMnO3. This may shift the third peak of TPR to lower tempera-ures.

Further addition of excess Mn to Mn/La of 1.4 may cause exces-ive sintering of MnOx particles, which in turn results in a shift of allPR peaks to higher temperatures. For Mn/La > 1.2, the area of therst and third peaks reduce because the amount of amorphous andne crystallite manganese oxide decreases. Also, area of the secondeak increases because the crystallite manganese oxide reduces atigh temperature.

TPR profiles of LaMnO3 and LaMn1.2O3+ı in presence of CO and3H8 as the synthetic exhaust gas constituents are presented inig. 4. Reduction of the catalysts by CO occurs at lower temperatureshan that by C3H8; however, both TPR spectra have two distinctemperature ranges, similar to H2-TPR. As compared to LaMnO3,he reduction onset temperatures of LaMn1.2O3+ı in presence of COnd C3H8 are decreased by 100 and 70 ◦C, respectively.

.3. Catalytic oxidation activity and thermal stability

The temperature dependence of CO and propane oxidationctivity of LaMn1+xO3+ı catalysts and that of blank, La2O3 andn2O3, are shown in Fig. 5a and b. Quartz chips are used, instead

f the catalyst, in the blank run. The test without catalyst shows

hat a minor thermal oxidation of CO occurs at 300 ◦C, at whichomplete oxidation of CO occurs even for the least active catalyst.o thermal oxidation is observed for propane, even at tempera-

ures for its complete catalytic oxidation. Lanthanum oxide showso activity for CO oxidation, however, it enhances propane oxida-

Fig. 6. Effect of pre-reduction treatments (partial H2-TPR up to 400 ◦C) on CO (a)and C3H8 (b) oxidation over LaMn1.2O3+ı catalyst.

tion at high temperatures. Compared to LaMnO3 perovskite, Mn2O3shows slightly higher activity for oxidation of both CO and propane.

T50 and T90, defined as the temperatures for 50% and 90% con-version, respectively, for both CO and propane are shown in insetsfigures in Fig. 5. The CO oxidation activity of the catalysts sharplyenhances as Mn/La atomic ratios in the catalyst increase to 1.1, andcorrespondingly, T50 and T90 decreases by 57 and 77 ◦C, respec-tively. Further addition of excess Mn up to Mn/La ≥ 1.3 results inslightly higher T50 and T90s. Propane oxidation activity of the cat-alysts gradually increases with excess Mn up to Mn/La of 1.2 andthen decreases, as shown in Fig. 5b and its inset figure. The T50 andT90 of propane oxidation on LaMn1.2O3+ı decrease by 38 and 34 ◦C,respectively.

It seems three interrelated factors of BET surface area, the sepa-rate manganese oxides, and Mn4+ affect the oxidation activity of thecatalysts. The dispersed MnOx amorphous and/or crystallite phasesresults in larger BET surface areas, low-temperature reducibilityand higher oxidation activity of the catalysts, up to x = 0.2 excessMn in LaMn1+xO3+ı. At Mn/La atomic ratio > 1.2, the surface areamay reduce as a result of sintering of the manganese oxide. Thehigher catalyst activity may be attributed to higher catalyst surfacearea being exposed to the reaction feed.

Moreover introducing excess Mn or La3+ vacancies, which allow

altering the bind of oxygen–metal, may activate surface oxygen(�-oxygen) [12]. According to the literature, it is known that ata lower temperature range, electrophilic oxygen species (O2− andO−) are the catalytically active species for the total oxidation. In this

288 P. Esmaeilnejad-Ahranjani et al. / Chemical Engineering Journal 169 (2011) 282–289

F rve (aT

rccamtp

brmi(tI(otavcttfitt

ig. 7. Thermal stability of LaMnO3, LaMn1.2O3+ı , and Mn2O3. Last point of each cu50 C3H8 = 194, 232 and 225◦C for LM1.2O, LMO and Mn2O3, respectively.

egard it has been suggested that the dispersed MnOx phases areatalytically more active than crystallite MnOx phases [30]. Theseatalysts with dispersed MnOx phases are easier to reduce and theyre characterized by a high defect concentration and high oxygenobility [18]. Thus, the presence of weakly bound oxygen related

o the anionic vacancies, observed mainly for non-stoichiometricerovskites, lead to enhancement of the catalytic activity.

For better explanation of oxidation properties of catalysts arief theory of oxidation reaction mechanism is useful. Oxidationeaction of CO and hydrocarbons on oxides are often described byeans of the Mars and van Krevelen model [5,31]. This mechanism

nvolves a two-step redox reaction: first, the oxidisable reactantCO and hydrocarbon) and the active oxygen react on the surfacehen the reduced surface site is reoxidized by gas phase oxygen.n the LaMn1+xO3+ı catalysts, the reaction sites are manganese ionsMn3+ and Mn4+). Therefore, at lower temperatures, as the diffusionf oxygen ions (O−) into the lattice is inhibited, the redox reactionakes place at these sites by consuming and regenerating weaklydsorbed oxygen. These species are accommodated in the oxygenacancies which concentration is increased by increasing vacan-ies in the La or Mn site cations. This mechanism is connected tohe presence of anionic vacancies on the surface. At high tempera-

ure, the oxidation process involves lattice oxygen as active speciesor substituted as well as unsubstituted samples. This mechanisms connected to the presence of lattice species in the layers nearo surface. Then, the evolution of these species, was attributed tohe reduction of the Mn site cations to lower oxidation state [5,31].

fter deactivation at 700 ◦C) give conversion values at T50 CO = 130, 187 and 180 ◦C;

According to this mechanism, two independent steps have to bedistinguished for CO and propane oxidation:

R + O–MnIV+ → P + –MnIII+surface

O2 + –MnIII+ → O–MnIV+

where, R and P are the reactants and products, respectively. Thismodel assumes that the gaseous oxygen reaction with the surfaceoccurs when a reduced site is available. This implies that the for-mation of manganese at high oxidation state i.e., Mn4+ becomes thelimiting factor in the kinetics.

Fig. 6 displays the light-off curves of LaMn1.2O3+ı catalyst carriedout after a partial H2-TPR up to 400 ◦C. The TPR experiments wereperformed by 10 sccm (cm3 (STP) min−1) of 7.0% H2 in Ar, followedby cooling of the sample in Ar. Then adsorption of oxygen by 0.5%O2 in Ar was allowed at room temperature. The catalyst oxidationactivity is sharply reduced as a result of the catalyst reduction. T50of CO and propane is increased by 70 and 80 ◦C, respectively. AlsoT90 of CO has increased by about 300 ◦C and propane does not reachthe T90 up to 500 ◦C. This can be due to reduction of Mn4+ that isthe important factor in redox reaction.

Fig. 7 presents the thermal stability of LaMn1.2O3+ı, LaMnO3 andMn2O3 catalysts under 50 sccm of the synthetic exhaust gas for 20temperature cycles of 700 ◦C for 2 h and T50 of propane and T50 ofCO, each for 30 min. No activity loss is observed for LaMn1.2O3+ı

and LaMnO3 catalysts, while the activity of Mn2O3 decreases after

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[29] J. Topfer, J.B. Goodenough, Charge transport and magnetic properties in per-ovskites of the system La–Mn–O, Solid State Ionics 101–103 (1997) 1215–1220.

P. Esmaeilnejad-Ahranjani et al. / Chem

he fourth cycle. LaMn1.2O3+ı and LaMnO3 catalysts preserve theirurface area. BET area of Mn2O3 decreases from 7.0 to 4.5 m2 g−1,ndicating particle sintering as the main cause of thermal deactiva-ion. Although in both oxidation of CO and propane, the catalyticctivity of Mn2O3 is higher than that of LaMnO3, but Mn2O3 stabilitys low.

. Conclusion

Non-stoichiometric LaMn1+xO3+ı (x = 0.05–0.4) catalysts for oxi-ation of pollutants in a synthetic automotive exhaust gas wasrepared via a microwave-assisted gel combustion synthesis. Thectivity of the catalysts significantly increases by incorporatingxcess manganese in the perovskites. T50 and T90 of CO and propanes the pollutants in the exhaust gas show sharp minima (maxima inxidation activity) at about 20% excess manganese (LaMn1.2O3+ı).educibility and BET surface area of the catalysts show maxima athe same amount of excess manganese. It seems that the formationf highly dispersed amorphous and/or fine crystallites of sepa-ate MnOx≥1.5 phases, including Mn4+, along with decreased cationacancies and increased oxygen vacancies are the main causes ofhe significant activity enhancements. In accordance partial reduc-ion of LaMn1.2O3+ı catalyst considerably decreases its oxidationctivity. The stability of LaMn1.2O3+ı, comprising MnOx≥1.5 sepa-ate phase, in the perovskite is much higher than MnOx≥1.5 phaselone.

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