Surface properties, textural features and catalytic performance for

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Surface properties, textural features and catalytic performance for

NO + CO abatement of spinels MAl2O4 (M = Mg, Co and Zn)

developed by reverse and bicontinuous microemulsion method

A.E. Giannakas a,*, A.K. Ladavos b, G.S. Armatas a, P.J. Pomonis a

a Department of Chemistry, University of Ioannina, Ioannina 45110, Greeceb Department of Farm Organization and Management, University of Ioannina, Agrinio 30100, Greece

Received 4 September 2006; received in revised form 29 January 2007; accepted 6 February 2007

Available online 17 February 2007

Abstract

Three typical spinels of general formula MAl2O4 (M = Mg, Co and Zn) have been successfully prepared via a microemulsion method both in

the reverse and bicontinuous state. The final solids were characterized by X-ray diffraction followed by Rietveld analysis, N2 adsorption–

desorption porosimetry and SEM. Pore connectivity (c) was also calculated with Seaton’s method. The analysis of all these properties shown that

spinels prepared via reverse microemulsion route have better surface and textural properties than bicontinuous ones. The spinels were tested for

NO + CO reaction and reverse spinels shown better catalytic activity than bicontinuous ones while the full sequence of catalytic activity is:

ZnAl2O4-r > ZnAl2O4-b > MgAl2O4-r > MgAl2O4-b > CoAl2O4-r > CoAl2O4-b. The Rietveld analysis helped us to give an explanation about

the catalytic activity and shown that the configuration of inverse spinel phase is the critical factor for the catalytic behavior of final solids. The

reactants NO and CO react in a 2:1 ratio at low temperature but they convert in a 1:1 ratio at high temperatures. From the kinetic analysis the heats

of NO adsorption are estimated and are in full agreement with the results of catalytic activity.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Microemulsion; Spinels; Catalysis; NO + CO

1. Introduction

Spinels are placed among the most widely used of all solid

precursors or catalyst supports and have a general formula of

AB2O4. In the unit cell there are 32 oxygen ions with 8 of the 64

tetrahedral and 16 of the 32 octahedral sites occupied. When the

A ions are all housed by the tetrahedral sites and all the B ions

are in the octahedral ones we talk about ‘‘normal spinels’’. The

spinels MAl2O4 (M = Mg, Fe, Co, Cu, Ni, Mn and Zn) have this

structure. But in certain other spinels, the eight tetrahedral

positions are occupied, not by the A ions, but by one-half of B

ions, the rest of which, together with the A ions, are arranged

randomly in the 16 octahedral positions. These ‘‘inverse

spinels’’ are therefore formulated as B(AB)O4. Many workers

have prepared spinels with the general formula of MAl2O4

(M = Mg, Co, Zn, Cu and Ni) in the last few years [1–7] and

they have given emphasis to preparing solids with high surface

area. Here are some of the most interesting reports about spinels

in the last few years. CoAl2O4 spinels have been prepared by

hydrolysis of mixed metal alkoxides [1] with specific surface

area Sp = 235 m2g�1 and by a low temperature combustion

method [2] with Sp = 77 m2g�1. Also MgAl2O4 aluminates

have been prepared from a colloidal solution using filter

expansion aerosol generator [3] and achieve surface area of

259 m2g�1. Nanocrystalline metal aluminates MAl2O4

(M = Mn, Cu and Zn) have been prepared using the combustion

method [5] with surface area varying from 40–180 m2g�1.

Finally a zinc aluminate with Sp = 350 m2g�1 and micro-

porosity with narrow pore size distribution prepared using a

hydrothermal treatment method [6] at low temperatures.

In order to achieve final solids with high surface area, which

is an interesting property from catalytic point of view,

microemulsion method has also been used [8–11]. The main

advantages of this method are the high surface area and the

small particle size of resulting materials in the range of 5–

10 nm. Additionally by using this method the same final solid

www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 6969–6979

* Corresponding author. Tel.: +32 6510 98361; fax: +32 6510 98795.

E-mail address: [email protected] (A.E. Giannakas).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.02.031

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can be prepared in two different microemulsion states, reverse

and bicontinuous, with different properties as previously

reported [10]. There are two different types of microemulsions:

the oil in water (o/w) where the aqueous phase includes oily

nanodroplets and the water in oil (w/o) where the oily phase

includes water nanodroplets. In w/o microemulsions when the

concentration of water is low, the system is called reverse

microemulsion. As the water concentration increases the

nanodroplets of microemulsion increase in size and eventually

form a cluster, which is considered infinite. Recently the

microemulsion method has been used for preparing materials

with interesting catalytic applications [12–18]. These studies

show the increased interest for microemulsion method in

catalysis field. One of the most usual reactions where solids as

perovskites, supported metals or spinels and d-oxides are used

is the reduction of NO where CO used as the reducing gas

because of the worldwide increasing interesting to control NO

and CO emissions especially from motor vehicles and because

NO and CO generally are believed to be the precursors of acid

rain and photochemical smog [19–21].

The present study reports the successful use of the

microemulsion method for preparing high surface area

aluminates MAl2O4 with varying A site cations (M = Mg, Co,

and Zn) at final calcination temperature of 800 8C. All spinels

have been prepared using the reverse and bicontinuous

microemulsion routes. The resulting spinels were characterized

by X-Ray diffraction followed by Rietveld analysis, N2

adsorption–desorption porosimetry and scanning electron

microscopy (SEM) while the connectivity (c) of the samples

was determined using Seaton’s equations. Catalytic activity tests

on the prepared materials for the NO + CO reaction were carried

out and studied in order to investigatewhich is the combination of

ions M–Al (M = Mg, Co, and Zn) with higher catalytic activity.

2. Experimental

2.1. Microemulsion used and preparation of samples

The microemulsion A (see Table 1) used for the preparation

of the solids was based on the CTAB/1-butanol/n-octane/

Al(NO3)3 + B(NO3)x system, where B = Mg, Co, Cu or Zn. In

this microemulsion A a second microemulsion B based on

CTAB/1-butanol/n-octane/NH3 was added under stirring at

room temperature. After several conductivity tests [10,18] we

reached the conclusion that the compositions of microemulsion

shown in Table 1 were worthwhile for the synthesis of the

corresponding spinels and their subsequent comparison. More

information about the conductivity experiments has been

reported in a previous work [10]. The selected compositions for

every solid are shown in Table 1 and also in Fig. 1 in the form of

triangular phase diagrams.

Precipitation took place by adding the microemulsion A to

B, where B contains NH3 in excess for the precipitation of the

corresponding hydroxides.

The prepared precursor phases were then filtered, dried at

room temperature, heated at 800 8C for 4 h under atmospheric

conditions, and ground in an agate motor in order to obtain the

final solids. The final calcination temperature (800 8C) was

selected after thermogravimetric experiments. Above this

temperature no mass loss was observed.

The samples obtained and characterized will be designated as

MAl2O4-r (M = Mg, Co, and Zn) meaning the samples prepared

by reverse microemulsion and MAl2O4-b (M = Mg, Co, and Zn)

meaning the samples prepared by bicontinuous microemulsion.

2.2. XRD analysis—Rietvield analysis

In order to check the development of the crystal phases,

XRD patterns of the prepared materials were obtained after

heating at 800 8C for 4 h. The equipment used was a Bruker

Advance D8 system employing Cu Ka radiation

(l = 1.5418 A). The samples were placed in quartz sample

holders and the step scans were taken over the range of 2u

angles from 108 to 808 in steps of 0.028 (2u). The XRD patterns

were assigned using the Joint Committee on Powder Diffraction

Standards (JCPDS) database and then were analyzed with

Rietvield refinement using an applicable computer program.

2.3. Surface area and porosity

The specific surface area Sp (m2g�1) was estimated by N2

adsorption–desorption porosimetry at 77 K via the BET method.

The instrument employed was a Fisons 1900 Sorptomatic

system. Prior to the measurement, the sample (�250 mg) was

degassed overnight at 200 8C under pressure P of 10�2 bar.

2.4. Pore connectivity of spinels ZnAl2O4

From the data of adsorption–desorption isotherms, the pore

connectivity c of spinels was estimated according to the

methodology proposed initially by Seaton [22–24] and applied

in various similar studies [25,26]. Briefly, the method of

Table 1

The composition of microemulsions employed to the preparation of spinels MAl2O4 (M = Mg, Co and Zn)

CTAB 1-butanol n-Octane M(NO3)20.4 M, Al(NO3)30.8 M NH38N

Reverse micelle for spinel MAl2O4

Reverse microemulsion A 0.63 M 2.1 M 4.2 M 0.048 M, 0.096 M –

Reverse microemulsion B 0.51 M 1.7 M 4.6 M – 0.83 M

Bicontinuous micelle for spinel MAl2O4

Bicontinuous microemulsion A 0.54 M 1.8 M 3.6 M 0.1 M, 0.2 M –

Bicontinuous microemulsion B 0.42 M 1.4 M 3.8 M – 2.1 M

A.E. Giannakas et al. / Applied Surface Science 253 (2007) 6969–69796970

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probability f was obtained as a function of percolation

probability F from the adsorption isotherms using the pore

size distribution obtained as follows:

f ¼R r¼1

r� nrdrR r¼10

nrdr(1)

f

F¼ V flat max � Vdes

V flat max � Vads

(2)

Where r is the pore radius, nr the corresponding psd using the

BJH method for cylindrical pores, Vflat max is the part of the

desorption curve before the start of desorption and Vdes, Vads are

the corresponding volumes in desorption and adsorption curves.

Then the c and L values are obtained by fitting the results for

f and F, obtained as described above, to the following

generalized scaling relation (Eq. (3)) proposed by Kirkpatrick

[25]:

Lb=vcF ¼ G

��c f � 3

2

�L1=v

�(3)

where c, connectivity; L, pore length and b, v,critical exponents

with values 0.41 and 0.88, respectively.

We must underline the fact that the estimation of

connectivity using the above procedure is possible only in

cases where there is some hysteresis in the N2 adsorption–

desorption data. The determined values of c and L have been

calculated using the experimental data of desorption isotherms

which corresponds to hysterisis branch. We also mention that

this methodology is based on the percolation theory and on

percolation effects which are related to connectivity of the pore

system and its relation to the adsorption–desorption hysteresis

loop. For a detailed description one may refer to the original

papers by Seaton and co-workers [22–24]. Typical fitting

results according to Eq. (3) are shown in Fig. 4 for the various

spinels.

2.5. SEM images

The obtained solids after calcination at 800 8C were

observed by scanning electron microscopy using a JEOL

JSM 5600 instrument.

Fig. 1. Phase diagrams for the systems n-octane—CTAB/1-butanol 1.5% w/w-Mg(NO3)2 0.4M + Al(NO3)3 0.8 M (upper left); n-octane—CTAB/1-butanol 1.5% w/

w-Co(NO3)2 0.4M + Al(NO3)3 0.8 M (upper right); n-octane—CTAB/1-butanol 1.5% w/w-Zn(NO3)2 0.4M + Al(NO3)3 0.8 M (down left); n-octane—CTAB/1-

butanol 1.5% w/w-NH3 8 N (down right). The points A1, A2 correspond to the chemical composition of microemulsion A employed (also in Table 1) for the synthesis

of the spinels MgAl2O4, CoAl2O4 and ZnAl2O4 using reverse and bicontinuous microemulsion correspondingly. The points A01, A02 corresponds to the chemical

composition of microemulsion B (NH3 8 N) employed for synthesis.

A.E. Giannakas et al. / Applied Surface Science 253 (2007) 6969–6979 6971

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2.6. Catalytic activity

The catalytic activity tests of the solids for the NO + CO

reaction were carried out in a bench scale tubular plug flow

reactor (PFR) under atmospheric pressure. A mixture of

reactants NO/CO/He = 2/2/96 at a total flow rate of

100 cm3 min�1 was passed through the catalyst bed containing

0.20 g of the catalyst. The system was heated externally via a

tubular furnace, regulated by a SUR BERLIN controller, within

�1 8C, via a thermocouple in contact with the catalyst bed.

Analyses of the reactants and products were carried out using a

Carlo Erba GC equipped with a TCD, with He as carrier gas and

connected to a PC for data acquisition. The carrier gas flow was

kept at 30 ml min�1. A 10-port valve controlled via the PC

enables sampling of 1 ml of reactants or products for analysis

using a two-column system, Porapac Q and Molecular Sieve

13X, similar to that described in [27]. The catalysts were tested

in the temperature region of 160–580 8C and from the data

obtained the degrees of conversion and reaction rates were

calculated.

3. Results

3.1. XRD analysis—Rietvield analysis

The X-ray diffractograms obtained from the solids MAl2O4-

r and MAl2O4-b (M = Mg, Co, and Zn) include a normal spinel

crystal phase and an inverse spinel crystal phase for all solids.

Using as starting model these two kinds of phases for all solids,

a Rietveld refinement of the obtained XRD data was made

based on the methodology developed in [27] and using a

relevant computer program for quantitative phase analysis of

the multicomponent mixtures. The refinement parameters

accordingly to all the above phases include scale factors,

background coefficients and cell dimensions. The results are

shown in Fig. 2 and in Table 2.

We observe that the quantity of the inverse phase of spinel is

higher in the case of Zn containing spinels, followed by Mg

containing spinels and Co containing ones. Also spinels

prepared via reverse microemulsion route have higher quantity

of inverse phase than spinels prepared via bicontinuous

Fig. 2. Rietveld analysis of XRD data for the obtained spinels.


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microemulsion route. We can also see that for all materials the

cell parameters a, b, c (Table 2) are always lower than the

corresponding values of the phase in Data Bank.

3.2. Surface and porosity

Taking into account the adsorption desorption isotherms,

shown in Fig. 3, and the calculated values of both the specific

surface area (Sp) and the pore diameter (Dmax), listed in Table 3, it

could be said that all prepared spinels are typical mesoporous

materials. The N2 adsorption– desorption isotherms (Fig. 3) show

the characteristic hysterisis loop of H1 type for mesoporous

materials. A first observation is that aluminates prepared via

microemulsion method have high specific surface area in the

range of 150–250 m2g�1. Same typical values of ssa for identical

spinels prepared via a sol–gel method have been previously

reported (Ref. [10]) and are shown in Table 3 for comparison. In

addition the values of ssa and Dmax from solids obtained by

reverse method are different than those of bicontinuous obtained

solids. We can see from Fig. 3 and Table 3 that all aluminates

which have been prepared via the reverse microemulsion route

have higher values of specific surface area than aluminates which

have been prepared via the bicontinuous microemulsion route. A

similar differentiation between reverse and bicontinuous solids is

observed for the values of pore diameter (Dmax).

3.3. Pore connectivity

Pore connectivity c values are around 5–6 for the most of the

resulting spinels (Table 3, Fig. 4) except for CoAl2O4-r where

the parameter c takes the value of 12.1. This means that each

irregular pore is connected to 5–6 different outlets on the

average. These values are average ones and it is understood that

there are pores with larger number of outlets while some others

posses less connections. Trying to compare the values of pore

connectivity c between examined reverse and bicontinuous

aluminates, it is observed that in the case of MgAl2O4 and

ZnAl2O4 spinels the values are quite similar while in the case of

CoAl2O4 spinels a big difference in the values of c is observed.

So CoAl2O4-r has c = 12.1 which is three times higher than

CoAl2O4-b with c = 4.7.

3.4. SEM images

In Fig. 5 some typical SEM images of the examined

aluminates are shown. In the left hand part of row images are

shown the ‘‘reverse spinels’’, while in the right hand the

‘‘bicontinuous’’ ones for comparison. From all images it is

concluded that microemulsion method leads to final solids with

small particle size. All examined spinels show particle size

smaller than 100 nm except for CoAl2O4 which shows size of

Table 2

Results of Rietveld refinement for all studied solids

Code of sample Observed phases (%) (%) a b c

MgAlOr Mg Al2 O4 Fd-3m 58.9 8.09745 8.09745 8.09745

(Mg0.77Al0..23)(Al0.885Mg0.115)2O4 Fd-3m 41.1 8.07272 8.07272 8.07272

MgAlOb Mg Al2 O4 Fd-3m 64.3 8.10791 8.10791 8.10791

(Mg0.77Al0..23)(Al0.885Mg0.115)2O4 Fd-3m 35.7 8.06741 8.06741 8.06741

CoAlOr Co Al2 O4 Fd-3m 68.9 8.09871 8.09871 8.09871

(Co0.94 Al0.06)(Co0.06 Al1.94)O4 Fd-3m 31.1 8.08188 8.08188 8.08188

CoAlOb CoAl2O4 Fd-3m 75.2 8.09463 8.09463 8.09463

(Co0.94 Al0.06)(Co0.06 Al1.94)O4 Fd-3m 24.8 8.07515 8.07515 8.07515

ZnAlOr ZnAl2O4 Fd-3m 18.7 8.07356 8.07356 8.07356

(Zn0.3 Al0.7)Al1.7O4 Fd-3m 81.3 8.01641 8.01641 8.01641

ZnAlOb Zn Al2 O4 Fd-3m 30.3 8.08854 8.08854 8.08854

(Zn0.3 Al0.7)Al1.7O4 Fd-3m 69.7 8.04425 8.04425 8.04425

Phases from Data Bank a b c

Mg Al2 O4 Fd-3m 8.131 8.131 8.131

(Mg0.77Al0.23)(Al0.885Mg0.115)2O4 Fd-3m 8.125 8.125 8.125

CoAl2O4 Fd-3m 8.107 8.107 8.107

(Co0.94 Al0.06)(Co0.06 Al1.94)O4 Fd-3m 8.105 8.105 8.105

ZnAl2O4 Fd-3m 8.086 8.086 8.086

(Zn0.3 Al0.7)Al1.7O4 Fd-3m 8.000 8.000 8.000

Table 3

Spinels studied and some of their surface properties in comparison with the surface properties values of same spinels prepared via sol–gel method

Material Sp (m2g�1) Sp (m2g�1) by sol–gel method Vp (cm3g�1) Dmax (nm) c L

MgAl2O4-r 242 139 1.10 10.8 5.3 1.14 � 0.06

MgAl2O4-b 156 0.70 12.8 5.5 1.03 � 0.03

CoAl2O4-r 150 65 0.49 8.8 12.1 0.90 � 0.01

CoAl2O4-b 120 0.32 7.0 4.7 1.23 � 0.02

ZnAl2O4-r 144 144 0.23 4.8 5.1 1.00 � 0.02

ZnAl2O4-b 126 0.20 4.2 5.1 0.99 � 0.01


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some particles in the range of 100–150 nm. In a previous work

some images about ZnAl2O4-r and ZnAl2O4-b were reported and

we put emphasis on the differentiation of the structure between

reverse and bicontinuous solids. In the present study we tried to

underline the differentiation in the resulting particle size between

reverse and bicontinuous aluminates. Reverse microemulsion oil

phase includes smaller aquatic nanodroplets than those included

in bicontinuous microemulsion as noticed in introduction [10,18]

Fig. 3. N2 adsorption–desorption isotherms at 77 K of the examined spinels.

Fig. 4. Typical fitting results of Eq. (3) for the indicated solids and the corresponding c and L values.


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and this differentiation is expected to characterize the particle

size of the examined solids. This differentiation is clearly

depicted in the case of CoAl2O4-r and CoAl2O4-b spinels while it

is less clear in the cases of the other couples of spinels.

3.5. Catalytic activity

In Fig. 6 the temperature profiles of the conversion of the

reactants NO and CO over the tested catalysts are shown.

The aluminates containing Mg, Co and Zn species showed

significant activity in the range of 350–600 8C. A comparative

graph T50%NO and T50%CO is shown in Fig. 7 for all studied

materials.

From Figs. 6 and 7 is observed that aluminates containing Zn

species have the bigger activity followed by the Mg containing

ones and Co containing ones. Also reverse spinels achieve a

little higher catalytic activity than bicontinuous ones in all

cases. These two catalytic results will be discussed next.

Fig. 8 shows the temperature profiles of the ratio of CO2

production to that of N-containing species, i.e. CO2/

(N2 + N2O).

The ratio CO2/(N2 + N2O) takes the value of about unity at

low temperatures and increases towards the value of two as the

reaction temperature increases. This behavior will be discussed

later.

Finally in Fig. 9 we have drawn Arrhenius-type plots of the

form ln RNO = f (1000/T) as well as ln RCO = f (1000/T) where

R is the observed integral rate of conversion in mmol g�1 s�1.

As derived from the Arrhenius plots, there are two straight line

parts one at low temperatures and another at high temperatures.

Fig. 5. SEM images for the obtained aluminates. In the left part of row are the reverse aluminates: (a) MgAl2O4-r, (c) CoAl2O4-r and (e) ZnAl2O4-r. In the right part of

row are the bicontinuous aluminates: (b) MgAl2O4-b, (d) CoAl2O4-b and (f) ZnAl2O4-b.


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The apparent activation energies Eapp (NO) and Eapp(CO) for low

and high temperatures can be estimated (see Table 4) from their

slopes.

4. Discussion

From all the above noticed information about crystal phases,

specific surface area (ssa), pore diameter (Dmax) and pore

connectivity (c) it is clear that spinels prepared by reverse

microemulsion technique are obviously different from spinels

prepared by bicontinuous microemulsion technique. Reverse

spinels shown better surface and textural properties than

bicontinuous ones. So MgAl2O4-r, CoAl2O4-r and ZnAl2O4-r

spinels have higher values of specific surface area than

MgAl2O4-b, CoAl2O4-b and ZnAl2O4-b correspondingly. Also

CoAl2O4-r has much higher value of pore connectivity c than

CoAl2O4-b (Table 2). In addition spinels prepared via reverse

microemulsion route have smaller particles size than bicontin-

uous ones as it is shown by SEM images. This differentiation in

conjunction with the better properties of reverse solids might be

the reason for the higher catalytic activity of them compared to

the bicontinuous solids. But this issue about the differentiation

in catalytic activity and textural properties of reverse and

bicontinuous solids was the main subject of previous reports

[10,18] and is not going to be discussed extensively here. This

differentiation caused by the different size of aquatic

nanodroplets included in reverse and bicontinuous microemul-

sion states, is depicted in final size and in the properties of

resulted solids [10,18].

So the main subject of this work will be the reasons—

properties which differentiate the catalytic activity of the tested

aluminates. Looking carefully into Figs. 6 and 7 we reach the

conclusion that the catalytic activity of the samples follows the

sequence ZnAl2O4-r> ZnAl2O4-b> MgAl2O4-r> MgAl2O4-

b> CoAl2O4-r> CoAl2O4-b. Taking into account all the

properties analyzed and discussed above we can observe that

the catalytic activity can be corelated with the presence of

inverse crystal phase of spinel in solids. In the inverse crystal

phase of spinels some of the A ions (AB2O4) take the place of B

ions in the spinel formula and as a result we have a final solid

which support the mechanism of the catalytic reaction. Looking

carefully in the results of refinement of spinels in Table 3 it is

observed that the sequence of the % percentage of inverse phase

is the same with the sequence of catalytic activity of the

samples: ZnAl2O4-r> ZnAl2O4-b> MgAl2O4-r> MgAl2O4-

b> CoAl2O4-r> CoAl2O4-b. In other words in the formation of

inverse spinel phase some of the B ions (AB2O4), which in

normal spinels phase are in octahedral positions with oxygen

atoms, changing their positions with some of the A ions, which

in normal spinels phase take the tetrahedral sites with oxygen

atoms. So it seems that the inverse spinel phase support better

the mechanism of catalytic reaction. This probably caused by

the changing in the interatomic range between A–O and B–O

bonds in the inverse spinel phase. These interatomic distances

between A, B and O ions may be important for the mechanism

of catalytic reaction according to Laberty study [28] who

proposes that the mechanism of catalytic reaction in such solids

takes place with the participation of the bulk oxygen. This result

is ascribed better in Fig. 10 where we have plotted the ratio of

normal to inverse spinel phase as a function of the temperature

Fig. 6. Temperature profiles of NO (XNO) and CO (XCO) conversion of the

examined catalysts.

Fig. 7. Comparative histograms of T50%NO and T50%CO for all examined

catalysts.

Table 4

Materials studied and the calculated values of Eapp NO and CO in low (LT) and high (HT) temperature region

Material Eapp (NO, LT) (kJ/mol) Eapp (CO, LT) (kJ/mol) Eapp (NO, HT) (kJ/mol) Eapp (CO, HT) (kJ/mol) Transition temperature (8C)

MgAl2O4-r 56.16 60.85 14.73 19.54 514

MgAl2O4-b 58.12 61.84 17.67 21.41 527

CoAl2O4-r 61.16 65.13 7.31 11.78 533

CoAl2O4-b 65.91 66.41 15.08 16.99 550

ZnAl2O4-r 47.64 50.79 10.76 13.02 460

ZnAl2O4-b 54.62 57.08 5.94 9.35 470


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pywhere the reduction of NO reaches the 50%. It is observed that

as the value of the ratio of normal spinel crystal phase to inverse

spinel crystal phase increases, the value where solids achieve

the temperature of 50% conversion of NO, decreases. So we

reach the conclusion that the % percentage of inverse phase of

spinel in final solid is the reason for the alternation of catalytic

activity in final solids.

Except from the reason which differentiates the catalytic

activity of tested solids it must be underlined the fact that solids

with the formula of spinel (MAl2O4, M = Co, Mg, Zn) do not

usually used as single catalysts. Usually such solids are used as

support catalysts in combination with d-oxides or metal

particles (Pt, Pd). So a remarkable result is that with

microemulsion method we achieve to prepare spinel solids

Fig. 8. Temperature profiles of the products ratio CO2/(N2 + N2O) (left hand axis) and the (%) yield of N2 (*) and N2O (~) (right hand axis) for the examined

catalysts.

Fig. 9. Arrhenius plots for the NO (*) and CO (*) conversion.


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which can be used as single catalyst due the inverse spinels

phases are formed with this method.

In accordance with previous works [29–33] the reaction

between NO and CO proceeds via two possible main routes:

2NO þ 2CO ! N2þ 2CO2 route1

where the ratio of the products CO2 and N2 is 2:1

2NO þ CO ! N2O þ CO2andN2O ! N2þ 1=2O2 route2

where the ratio of C-containing products (CO2) to that of N-

containing ones (N2 + N2O) equals unity.

Fig. 8 shows the temperature profiles of the ratio of CO2

production to that of N-containing species. It is observed that at

low reaction temperatures the ratio of CO2/(N2 + N2O) obtains

values of about unity and increases gradually towards the value

of two as the reaction temperature increases. So, it seems that

for all tested catalysts which contains Mg+2, Co+2 and Zn+2 as A

cation the examined reaction follows route 2 at low

temperatures up 200–350 8C. In the temperature range 400–

580 8C a gradual change of mechanism to route 1 occurs and

above 460–570 8C temperature range route 1 predominates for

tested solids.

In order to calculate the apparent activation energies for the

NO reduction and CO oxidation as well as to check the

transition point from one reaction route to the other, we have

plotted the relevant Arrhenius lines in the form: ln R = f (1000/

T). The plots have been drawn separately for the low (LT) and

high temperature (HT) regions. Between those two regions

there is a typical saddle point corresponding to the transition

region of the rate. The exact transition temperatures for every

solid were calculated and are listed in Table 4 for comparison. It

is observed that the value of transition temperature for every

spinel is in full agreement with catalytic results. So the lowest

value of transition temperature is for Zn species containing

spinels followed by Mg and finally Co. Above this transition

temperature the conversion of both NO and CO is always higher

than 70% because in this range the mechanism of reaction

follows route 2 were NO is fully reduced directly to N2 without

the intermediate step of reduction of NO to N2O

(N2O! N2 + 1/2O2). From the corresponding Arrhenius plots,

shown in Fig. 9, the apparent activation energies Eapp for NO

and CO conversions at low and high reaction temperatures were

calculated and are listed in Table 4.

From the results reported in Table 4 it can be seen that Eapp is

in every case higher in LT as compared to HT for both reactants

and Eapp for NO conversion is systematically lower compared

to that of CO. The Arrhenius plots provide more reliable values

of Eapp for NO and CO for the low temperature region and this is

where the Eapp values of both NO and CO (Table 4) for the

reverse spinels are lower than those of the bicontinuous ones.

This fact validating the results of the catalytic experiments

(Fig. 6) which were discussed above. Furthermore, as it is

shown in Table 4 the values of Eapp(NO), Eapp(CO) in high

temperature region are very low. This is mainly due to the

diffusion phenomena which appear in high temperature region

because of the fact that our aluminates are all porous materials.

For this reason these values are not used for comparison or other

calculations.

5. Conclusions

The microemulsion method has been applied successfully

for the preparation of high surface area spinels MgAl2O4,

CoAl2O4 and ZnAl2O4 both prepared in reverse and

bicontinuous microemulsion state. The solids after final

calcinations at 800 8C were characterized by X-Ray analysis

and Rietvield refinement, N2 adsorption–desorption porosim-

ety in 77K and calculation of the pore connectivity c was

calculated and SEM. The aluminates prepared via reverse

microemulsion method show different surface and textural

properties than aluminates prepared via bicontinuous route. All

final spinels were examined for their catalytic activity in the

reaction of reduction NO by CO. Catalysts prepared via reverse

route show a higher catalytic activity than bicontinuous ones

because of their better surface and textural properties. The

catalyst containing Zn species achieved the highest catalytic

activity followed by the Mg and Co containing species and this

behavior was in agreement with the increment of the %

percentage of inverse crystal phase of spinel. The decomposi-

tion of NO and CO is differentiated at the low and high

temperature regions exhibiting very low Eapp at high

temperatures a fact which was explained by diffusion

phenomena appearing in porous solids, but higher Eapp at

low temperatures. The Eapp for NO conversion is always lower

than Eapp for CO oxidation, a result which is compatible with

the hysteresis observed for the CO conversion as compared to

NO conversion. Finally the values of Eapp (NO, LT) and Eapp (CO,

LT) are in a full agreement with the catalytic behavior of the

tested spinels a fact which validates our catalytic results.

Acknowledgements

Part of this work was carried out with financial assistance

from a GROWTH project GSRT-CT-2000-00317 of the E.U.

Fig. 10. Schematic presentation of the ratio of normal to inverse spinel phase of

all studied solid as a function of T50%NO.


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The authors are grateful to the Ring of the Laboratory Units and

Centers of the University of Ioannina for the XRD mesur-

amentes, and SEM images.

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