This journal is c The Royal Society of Chemistry 2011 Catal. Sci. Technol., 2011, 1, 1383–1392 1383

Cite this: Catal. Sci. Technol., 2011, 1, 1383–1392

Catalytic properties of cobalt and nickel ferrites dispersed in mesoporous

silicon oxide for ethylbenzene dehydrogenation with CO2

Tiago Pinheiro Braga,aBarbara Maria Campos Sales,

aAntonio Narcisio Pinheiro,

a

W. T. Herrera,bE. Baggio-Saitovitch

band Antoninho Valentini*

a

Received 16th May 2011, Accepted 15th August 2011

DOI: 10.1039/c1cy00176k

A polymeric precursor method was applied to synthesize catalysts of the general formula MFe2O4

(M = Co and Ni) in order to contribute to studies on ethylbenzene dehydrogenation in the

presence of CO2. The catalysts were characterized by TG, H2-TPR, XRD, Mossbauer

spectroscopy (MS), N2 adsorption/desorption isotherms and TPD-CO2. Investigations using XRD

and MS revealed that the spinel structures of CoFe2O4 and NiFe2O4 phases were formed. The

catalytic results suggested that materials with a spinel structure are particularly interesting for

ethylbenzene dehydrogenation. Compared to the other catalysts synthesized the sample containing

cobalt ferrite showed higher conversion and good styrene selectivity. The analysis of the spent

catalyst (DRX) showed that the CoFe2O4 phase was stable under the reaction conditions and that

a coke (TG) deposit was more pronounced for the NiFeSi.

1. Introduction

Catalytic ethylbenzene dehydrogenation in the presence of

steam (reaction (1)) is a representative process for the produc-

tion of styrene, an important monomer for synthetic

polymers.1–4

C6H5–CH2CH3(g) " C6H5–CHQCH2(g) + H2(g) (1)

The catalysts used in the above process are usually based on

iron oxide, which can be further modified by the addition of

promoters, such as potassium, chromium, calcium and

cerium.5 Due to relatively fast catalytic deactivation, many

research groups have dedicated their efforts to synthesize

catalysts with better stability.6–8

Currently, researchers have studied the use of carbon

dioxide (CO2) as a co-feeder gas in ethylbenzene dehydrogen-

ization (reaction (2)) with the aim of introducing an alternative

route to the steam process.7–10

C6H5–CH2CH3(g) + CO2(g)

" C6H5–CHQCH2(g) + CO(g) + H2O(g) (2)

With this goal in mind, a large amount of catalysts with

diverse composition have been tested in ethylbenzene

dehydrogenization in the presence of CO2.7–11 Nevertheless,

the results observed point to the need for more research to

improve catalytic stability and thus to decrease the

process cost.

Various publications have shown that, for catalysts

containing hematite as the active phase, magnetite forms

during the dehydrogenation reaction, which subsequently

affects the conversion of ethylbenzene to styrene.4,5,9,12 On

the other hand, there are some publications pointing to

materials with spinel structures, such as ferrites, as presenting

remarkable catalytic activity and stability in this reaction.13,14

In addition to their attractive redox and acid/base properties,

spinel structures are versatile compounds offering structural

stability.15 Therefore, materials of the general type MFe2O4

can be used in heterogeneous catalysis.15–18 Specifically in the

ethylbenzene dehydrogenization reaction, the ferrites of Co

and/or Ni may be interesting due to their dehydrogenation

ability. Additionally, these ions (Co2+ or Ni2+) may occupy

different positions in the spinel structure, changing the

interactions of CO2 with the catalyst surface and the redox

properties of the Fe3+, which is the main site for ethylbenzene

adsorption.19

The synthesis of ferrites of Co and Ni with nanometric size

has already been described in the literature.20–22 Nevertheless,

in order to apply these materials (ferrites of Co and Ni with

nanometric size) in heterogeneous catalysis, it is necessary to

avoid the sintering process. In this context, we report the

synthesis and characterization of MFe2O4 (M = Ni or Co)

compounds embedded in SiO2 with the purpose of studying

their catalytic performance for ethylbenzene dehydrogenation

in the presence of carbon dioxide.

a Langmuir - Laboratorio de Adsorcao e Catalise, Departamento deQuımica Analıtica e Fısico-Quımica, Universidade Federal do Ceara,Campus do Pici, CEP 60455-970, Fortaleza CE, Brasil.E-mail: valent@ufc.br; Fax: +55 85 3366 9982;Tel: +55 85 3366 9951

bDepartamento de Fısica Experimental, Centro Brasileiro dePesquisas em Fısica, Rua Dr. Xavier Sigaud 150, CEP 22290-180,Rio de Janeiro RJ, Brasil

CatalysisScience & Technology

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1384 Catal. Sci. Technol., 2011, 1, 1383–1392 This journal is c The Royal Society of Chemistry 2011

2. Experimental

2.1. Catalyst preparation

The precursor polymeric method used in this report was based

on the chelation of cations (metals) by citric acid in aqueous

solution.23–25 Nickel nitrate hexahydrate {Ni(NO3)2�6H2O},

cobalt nitrate hexahydrate {Co(NO3)2�6H2O}, iron nitrate

nonahydrate {Fe(NO3)3�9H2O}, tetraethylorthosilicate

(TEOS), citric acid monohydrate (CA){C6H8O7�H2O} and

ethylene glycol (EG) {C2H6O2} were used as starting

materials. A CA/metal ratio of 2 : 1 (mol) was used for all

samples; the metal amount was the sum of Fe3+, Co2+ or

Ni2+ and Si4+, and the mass ratio of CA/EG was kept at 2/3.

The different samples were denoted by the general formula

MFeSi, where M was the transition metal (Co or Ni) used. In

addition, a sample containing only iron oxide and silicon oxide

was prepared and labeled FeSi.

For the synthesis of the CoFeSi sample, 0.0085 mol and

0.017 mol of Co and Fe precursors, respectively, were dissolved

in distilled water at room temperature. CA (0.151 mol) was

dissolved in ethanol at 50 1C followed by the addition of 0.05 mol

of a Si precursor (TEOS) with stirring. Next, the aqueous

solution was added to the CA-TEOS-ethanol solution and stirred

for 15 min at 50 1C. Subsequently, the EG was added and the

mixture was stirred at 100 1C until it became a viscous resin. The

resin was treated at 300 1C for 1 h under air, and the resulting

precursor composite was milled and treated at 700 1C under a

flow of air for 1 h.

2.2. Catalyst characterization

The calcination step was followed by thermogravimetric

analysis (TG) using a 10 1C min�1 heating rate under a flow

of air at 50 ml min�1 for 10 mg of a sample. The crystal

structure of the metal oxides was characterized by X-ray

diffraction analysis (XRD) with a CuKa irradiation source

(l = 1.5418 A, 30 kV and 20 mA). Transmission Mossbauer

spectra were recorded at room temperature and at 4.2 K in

constant acceleration mode with a 57Co (Rh) source. The

Mossbauer data were fitted to discrete Lorentzian functions

using the least-square fitting routine of the NORMOSs

software package. All isomer shift values (d) were quoted

relative to aFe.Specific surface area and pore volume of the catalysts were

determined by N2 adsorption/desorption isotherms at liquid

nitrogen temperature. Before N2 adsorption, the catalyst

sample (40 to 50 mg) was evacuated at 200 1C for 2 h. The

TPR analyses (temperature-programmed reduction) of the

catalysts were carried out in the range of 100–950 1C in a

quartz reactor by passing a 5% H2/N2 flow (30 ml min�1) over

the sample at a heating rate of 10 1C min�1. The H2

consumption was monitored by a thermal conductivity

detector (TCD) connected to the reactor outlet. The CO2

temperature-programmed desorption was carried out in the

range of 40–500 1C under a He flow (10 1C min�1, 30 ml min�1).

The catalysts (200 mg) were preheated under He flow

(30 ml min�1) at 550 1C for 1 h, after which the temperature

was decreased to 60 1C and the He flow was changed to pure

CO2 (30 ml min�1 for 0.5 h). The desorbed CO2 was detected

by an online thermal conductivity detector (TCD) after passing

through a trap (�20 1C) to remove any trace of water.

To assess the amount and the properties of coke deposited,

the spent catalysts were analyzed by thermogravimetric

analysis (TG) using a 10 1C min�1 heating rate under a flow

of air at 50 ml min�1 and Fourier Transform Infrared (FTIR)

in the range of 400 to 4000 cm�1 using KBr pellets containing

1.0 wt% of the sample.

2.3. Catalytic activity

Catalytic tests were carried out in a fixed bed stainless steel

microreactor using 150 mg of a sample at 550 1C and

atmospheric pressure with a CO2 to ethylbenzene molar ratio

of 30. The molar flow rate of ethylbenzene was controlled at

1.446 mmol h�1. Prior to reaction, the catalyst was heated

under a N2 flow (30 ml min�1) up to the reaction temperature,

and the reaction mixture (CO2, ethylbenzene and N2) was then

introduced at a flow rate of 30 ml min�1. The resulting

catalytic ethylbenzene conversion was analyzed by gas

chromatography using an instrument equipped with a FID

and a nonpolar capillary column (30 m � 0.25 mm � 0.25 mm,

similar to AT-1). The ethylbenzene conversion (CEt) was

calculated according to eqn (1), and the styrene selectivity

(SSt) was calculated according to eqn (2).

CEt = (ethylbenzenein� ethylbenzeneout)/ethylbenzenein (1)

SSt = (styreneout)/n(hydrocarbon products) (2)

3. Results and discussion

3.1. Thermal analysis

As in previous publications,23–25 the TG/DTG curves (not

shown) could be divided into two temperature ranges for

weight loss. The first range (40–300 1C) (near 10 wt%) was

related to the evaporation of adsorbed water and any remaining

ethylene glycol. The second stage (300–450 1C) was related to

the decomposition of the residual organic material of the

polymeric chains formed by polyesterification25,26 and

subsequent production of CO2 by the decomposition of

residual organic precursors (near 50 wt%). After that, no

weight loss was detected up to 700 1C, which suggested that

the decomposition of organic material had finished.

Employing organic precursors (CA and EG), which were

removed as volatile compounds during the calcination step in

the preparation of the catalysts, resulted in cavities and the

simultaneous rearrangement of the solid, which might have

been responsible for the morphological properties displayed

by the material (pore diameter and volume).

3.2. X-Ray diffraction

After the calcination process (700 1C under airflow), the

catalyst powders were analyzed by X-ray diffraction (XRD)

to gather information on the solid phase formed (Fig. 1). In

spite of the low crystalline organization, the XRD patterns of

the catalyst CoFeSi suggested the formation of a spinel

structured phase (CoFe2O4, JCPDS 02-1045). Likewise, the

sample NiFeSi also pointed to the spinel structure of nickel

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ferrite (NiFe2O4, JCPDS 44-1485) in addition to the presence

of nickel oxide (NiO, JCPDS 78-0423). The FeSi sample

showed a hematite pattern (a-Fe2O3, JCPDS 86-2368) and a

maghemite phase (g-Fe2O3, JCPDS 39-1346).

Due to the similarity in the diffraction profile of the nickel

ferrite and cobalt ferrite with the maghemite phase (g-Fe2O3),

the presence of a g-Fe2O3 phase could not be initially ruled out

for CoFeSi and NiFeSi samples.

From the results shown above, it seemed that nickel ferrite

(NiFe2O4) was easier to form than cobalt ferrite (CoFe2O4)

despite the fact that the ionic radii of Fe3+ (0.67 A) were

smaller than the radii of Ni2+ (0.83 A) and closer to

Co2+ (0.68 A). It therefore seemed reasonable to assume that

a replacement of the Fe3+ ions by the Co2+ ions in both the

tetrahedral A and octahedral B sites of the cation sublattices

with the corresponding decrease in the lattice parameters27

would be more favourable than by the larger Ni2+ ions.

To clarify the XRD results, the diagrams were analyzed by

means of the Rietveld refinement procedure28 from which the

crystallographic lattice parameters of the samples were

determined. To achieve this, the DBWS9807 program

described by Young et al.29 was applied. By this method it

was possible to quantify the different phases formed and the

resulting data are listed in Table 1. The crystallite sizes of

the samples were calculated from the XRD patterns using the

Debye–Scherrer formula.30 The calculated results illustrated in

Table 1 indicated that the catalysts were nanometre-sized

crystalline powders and probably with a high number of

crystalline defects.

3.3. Mossbauer spectroscopy

As observed in the XRD results, the small size of the particles

synthesized and the slight difference in the lattice parameters

of the maghemita (g-Fe2O3) and ferrite phases (CoFe2O4 and

NiFe2O4) made the XRD insufficient for characterization of

the powders. As shown in Fig. 2a, there were two sextets very

close to one another and a superparamagnetic central doublet

in the Mossbauer spectra of FeSi at room temperature. In

addition, one six-line subspectra with an isomer shift of d =

0.30 mm s�1 relative to aFe, a quadrupole shift of

e = �0.20 mm s�1, and a magnetic hyperfine field Bhf =

51.5 tesla were also observed. This type of pattern is typical of

hematite (a-Fe2O3). The other one six-line subspectra had d =

0.50 mm s�1, a quadrupole shift of e = B0.02 mm s�1, and

Bhf = 50 tesla, which are all characteristics of maghemite.31

Alternatively, the Mossbauer spectrum of the FeSi sample

was also acquired at a low temperature because the influence

of superparamagnetism could be counteracted by reducing the

sample temperature. The superparamagnetic relaxation was

suppressed at 4.2 K (Fig. 2a), but the presence of different iron

environments in the sample was evidenced by two six-line

subspectra. This result corroborated with XRD observations on

the presence of hematite and maghemite phases in the samples.

Similarly, as shown in Fig. 2b and c, the room temperature

Mossbauer spectra of the CoFeSi and NiFeSi samples

exhibited the superposition of a magnetic split sextet pattern

and a superparamagnetic doublet. The Mossbauer spectrum at

4.2 K for the sample CoFeSi was fitted with four subspectra.

Two six-line subspectra with isomer shifts of d = 0.25 and

0.38 mm s�1 (relative to aFe), quadrupole shifts of e = 0.02

and �0.08 mm s�1, and magnetic hyperfine fields of Bhf =

50.3 and 52.3 tesla were attributed to cobalt ferrite. These

hyperfine parameters corresponded to a distribution of Fe in

site A (tetrahedral) and site B (octahedral). Nevertheless, a

broad sextet with Bhf = 30–49 tesla was also observed, which

was probably due to cobalt ferrite with an extremely small

particle size. The broad sextet is typical for Fe particles in the

magnetic relaxation regime. Moreover, at 4.2 K, great deals of

magnetic moments were blocked, as evidenced by the appearance

of a broad and asymmetric magnetic sextet.32 In Fig. 2b

(at 4.2 K), we see that 90% of particles were blocked and

the remaining central doublet (10%) suggested that there were

very small superparamagnetic particles (less than 4 nm). These

results were in agreement with the mean diameter for a CoFeSi

sample (4 nm), as estimated by XRD analysis (Table 1).

The Mossbauer spectrum at 4.2 K for the NiFeSi sample

was fitted with three magnetic subspectra. The hyperfine field

values for sites A (tetrahedral) and B (octahedral) were 49.6

and 52.8 tesla, respectively. The corresponding isomer shift

values were 0.31 and 0.40 mm s�1 relative to aFe. In addition,

one broad sextet with Bhf = 30–51 tesla was observed, which

was probably due to nickel ferrite with a very small particle

size. In addition, the decrease of the magnetic hyperfine field

from the bulk value could be attributed to the very small

particle size.33 In the spectra of NiFeSi (Fig. 2c, 4.2 K), unlike

CoFeSi (Fig. 2b, 4.2 K), we saw that 100% of the particles

were blocked, suggesting that the size of the nanoparticles was

larger than 4 nm of diameter. Thus, the Mossbauer results

Fig. 1 X-Ray diffraction profile of the catalysts after the calcination

process.

Table 1 Relative mass percentage and grain size of the phasesobserved

Crystallinephase

Sample

CoFeSi NiFeSi FeSi

Massa

(%) Gs/nmMassa

(%) Gs/nmMassa

(%) Gs/nm

NiFe2O4 86.2 11.4CoFe2O4 100.0 4.5a-Fe2O3 88.5 13.2g-Fe3O4 11.5 4.1NiO 13.8 15.7

a Mass of crystalline phase (wt%). Gs = grain size (nm).

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were in agreement with the X-ray diffraction patterns, which

suggested a particle diameter of 11.4 nm (Table 1).

3.4. Temperature-programmed reduction

To acquire more information on the properties of the catalysts

and to clarify the chemical environment of the iron oxide,

which influences the reducibility of the catalyst, temperature-

programmed reduction (TPR) analyses were carried out

(Fig. 3). At first the TPR pattern of the FeSi catalyst suggested

four peaks of reduction. However, this pattern is somewhat

complex and was decomposed into five well-defined Gaussian-

shaped peaks and an incomplete one at high temperature

(agreement factor of 0.997).

The H2 consumption attained a maximum at 430 1C and

was related to the reduction of Fe3+ to Fe2+, probably due to

the formation of the magnetite phase (Fe3O4).9,12 In addition,

the peak at 575 1C, which overlapped with the first (430 1C)

and the other peaks at higher temperatures, might have been

due to the reduction of Fe2+ to Fe0. This low reduction

temperature (575 1C) might have been the result of the small

particle diameter, as indicated by XRD analysis (Table 1).

Furthermore, the H2 consumption peak at 575 1C was

probably due to Fe2+ reduction because of the amount of H2

consumption observed for the peaks at higher temperatures,

which was lower than the peak at 430 1C. This phenomenon

suggested either incomplete Fe2+ reduction or the formation

of FeO instead of the desired Fe3O4 phase at 430 1C. Further-

more, certainly the three H2 consumption peaks in the tem-

perature range of 600–820 1C (with a maximum at 645, 705

and 780 1C), as well as a peak at temperatures higher than

900 1C, were associated with Fe2+ reduction to produce

metallic iron.12,34

A possible explanation for the presence of four H2

consumption peaks at temperature ranges higher than 600 1C

for the FeSi sample might be drawn from the textural properties.

Metal oxide particles placed inside of porous materials are more

difficult to reduce compared to metal oxides positioned on

external surfaces.12,35 Therefore, the peak at 645 1C might

have been due to Fe2+ reduction outside of the porous

channel, while the peaks at 705 and 780 1C and higher than

900 1C might have been from the reduction of Fe2+ inside the

porous channel. Furthermore, the TPR profile of the FeSi

sample suggested (as did the XRD) a high proportion of iron

in the Fe3+ oxidation state, which was interesting for

dehydrogenation.4,9,12

The TPR profile of the NiFeSi catalyst had a starting H2

consumption process near 200 1C, and a shoulder at 520 1C

Fig. 2 57Fe Mossbauer spectra of the ferrites, at room temperature

(300 K) and at 4.2 K. FeSi (a), CoFeSi (b) and NiFeSi (c).

Fig. 3 Temperature-programmed reduction (TPR) profiles of the

samples.

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was clearly observed. The pattern was decomposed into five

well-defined Gaussian-shaped peaks and an incomplete one at

high temperature (agreement factor of 0.998). The peak at

414 1C could be attributed to the Ni2+ and Fe3+ (to Fe2+)

reduction processes. Similar to the FeSi sample, the peak at

426 1C could be attributed to the reduction of Fe3+ to Fe2+.

The Ni2+ reduction in a temperature range was typical for a

nickel oxide phase dispersed in SiO2,36 which was determined

to be present by XRD analysis (Fig. 1). Nevertheless, the

peak at 500 1C could be attributed to the reduction of Ni2+

in the spinel phase, which had an intense interaction with

the iron oxide and might have been localized in octahedral

or tetrahedral positions. The peaks at temperatures

higher than 600 1C were assumed to be the result of Fe2+

reduction.

The TPR profile of the CoFeSi catalyst presented a poorly

defined pattern, but it was successfully decomposed into seven

well-defined Gaussian-shaped peaks and an incomplete one at

high temperature (agreement factor of 0.998). The weak peak

at 345 1C was associated to the reduction of the Co3O4 phase

(Co3+ to Co2+). The low H2 consumption observed corrobo-

rated with the XRD analysis, which did not indicate the

presence of this phase. The presence of this phase might have

been missed, however, due to a smaller proportion. The peak

at 436 1C was attributed mainly to the reduction of Fe3+ (to

Fe2+). Similar to the NiFeSi sample, the peaks at 518 1C and

at 524 1C could be also attributed to the reduction of Fe3+ to

Fe2+ and the reduction of Co2+ to Co0, both in the spinel

phase and possibly localized in octahedral or tetrahedral

positions. The peaks for temperatures higher than 600 1C

might have been due to Fe2+ reduction influenced by the

cobalt presence and/or textural properties.

Different from the NiFeSi sample the CoFeSi catalyst

showed low H2 consumption peaks in the temperature range

of 600 1C to 700 1C. In addition, it is observed a well defined

H2 consumption peak at 840 1C, which may be due to the

CoFe2O4 phase reduction.

The H2 consumption peaks that appeared at temperatures

higher than 600 1C were particularly interesting for ethylben-

zene dehydrogenation because they provided evidence that the

reaction temperature (550 1C) did not completely reduce the

transitions metals.

Assuming that the iron oxide species (Fe3+) was the

main active site for ethylbenzene dehydrogenation,6 the

TPR profiles suggested that the addition of nickel additives

could not delay the deactivation process during the

catalytic test. However, and at the same level of importance,

the TPR patterns pointed to a mixture of different oxide

phases, as did XRD and Mossbauer analysis. In addition

to the low particle diameter (Table 1), this heterogeneity of

phases in the catalyst composition suggested that these

samples might present a significant number of defects in

the surface, an interesting phenomenon for the catalytic

process.

On the other hand, the H2 consumption profile in a

temperature range higher than 600 1C for the CoFeSi sample

suggests that cobalt addition could delay the catalyst

deactivation due to a reduction process during the catalytic

test carried out at 550 1C.

3.5. Specific surface area measurements

From the perspective of the catalysis and the catalytic char-

acterization, the surface area determination was an important

parameter for application. Thus, the N2 adsorption/

desorption isotherms were measured and the isotherm profile

and pore diameter distribution are presented in Fig. 4a and b.

The samples CoFeSi and NiFeSi showed similar isotherm

profiles that were characteristic of mesoporous materials (type

IV according to IUPAC classification and the H2 hysteresis

loop). This behavior indicated that the pore structure was

complex and consisted of interconnected networks of pores of

different sizes and shapes. On the other hand, the sample FeSi

showed isotherms of type II characteristics of mesoporous

powders (H3 hysteresis loop). A type H3 loop is usually

assigned to slit-shaped pores due to non-rigid aggregates of

particles.37

The profiles of pore diameter distribution are presented in

Fig. 4b (according to the BJH method) and exhibited a narrow

pore size distribution, with the majority of the diameters

located in the range of 16 to 238 A. Additionally, Table 2 lists

the surface area, total pore volume and average pore diameter

of the studied powders. In spite of the similarity in the average

pore diameter and pore diameter distribution (Fig. 4b) for the

samples, the addition of transition metals (Co and Ni) to the

catalyst promoted the synthesis of samples with better textural

properties. For instance, the catalyst NiFeSi possessed higher

specific surface area (49% higher) and total pore volume (40%

higher) in comparison with the FeSi sample, while the sample

CoFeSi showed a specific surface area 18% higher than the

FeSi sample.

Such textural effects in the porosity of the synthesized solids

might be related to the ratio of residual organic precursors, as

verified in the TG analysis. As such, samples with a spinel

structure (CoFeSi and NiFeSi) were seen to display a higher

Fig. 4 N2 adsorption/desorption isotherms of the catalysts (a). Pore

diameter distribution of the samples (b) (calculated using the BJH

model).

Table 2 Textural properties of the samples determined by N2

adsorption isotherms

Sample S/m2 g�1 Vp/cm3 g�1 Dp/A

CoFeSi 346 0.55 63NiFeSi 439 0.76 70FeSi 294 0.54 74

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ratio of residual organic substances. After calcination, this can

result in differences in the characteristics of the pores, which

are perceptible in the values for the surface area, pore volume,

pore diameter and changes in the hysteresis behaviour.

3.6. Temperature-programmed desorption

It is accepted that the basic sites play an important role in

ethylbenzene dehydrogenation.19 Therefore, it was interesting

to determine the effect of nickel and cobalt addition on the

basicity of the catalyst samples. To verify the relative basicity,

temperature-programmed desorption of CO2 (TPD-CO2)

experiments was carried out (Fig. 5). The data in Fig. 5 are

in arbitrary units, and the intensity of the deflection was not

quantified with regard to the number of moles of CO2. Never-

theless, the experimental conditions were rigorously the same

for all samples, and thus it was possible to compare the areas

of the TPD profiles.

The TPD patterns of NiFeSi and FeSi samples showed one

peak for CO2 desorption, while the catalyst CoFeSi presented

two peaks for desorption. The maximum desorption peak for

NiFeSi was observed around 90 1C, whereas the maximum for

FeSi was around 118 1C. On the other hand, the desorption

curves corresponding to the CoFeSi sample were around

97 and 368 1C, respectively. The patterns of TPD-CO2 were

decomposed for all samples into two or three well-defined

Gaussian-shaped peaks. Therefore, different types of basic

sites (or CO2 to metal oxides interactions) might be

distinguished.

The first CO2 desorption temperature range (53–200 1C) was

attributed to weak basicity. However, the second CO2

desorption temperature range (337–420 1C) was typical of a

medium basicity. Thus, the peak of CO2 desorption at 370 1C

suggested that the CoFeSi sample had a higher basic site

strength. Consequently, it was expected that the CoFeSi would

have the highest ethylbenzene conversion, followed by FeSi

and then NiFeSi. Considering the amount of CO2 desorbed

from the sample (the total areas of the TPD profiles), the

number of basic sites followed the order: FeSi > CoFeSi >

NiFeSi. NiFeSi stood out as a solid with relatively low

basicity, while FeSi had a large amount of basic sites.

In general, basic catalysts can improve the adsorption of

acidic CO2 to supply the surface with oxygen, suppressing the

deposition of coke.38 Additionally, the number and strength of

the basic sites of the catalyst had an effect on the catalytic

performance.39 Several research papers12,40 have shown that

the catalytic activity for ethylbenzene dehydrogenization is

increased with improved CO2 adsorption ability.

3.7. Catalytic performance

The ethylbenzene conversion and styrene selectivity for the

catalytic samples are depicted in Fig. 6a and b. All the samples

presented a similar activity and showed a reasonable selectivity

in ethylbenzene dehydrogenization. However, some important

differences were observed.

The CoFeSi sample demonstrated a higher ethylbenzene

conversion and styrene selectivity than either FeSi or NiFeSi

catalyst. Initially, the NiFeSi and FeSi catalysts presented a

similar ethylbenzene conversion (near 25%); however, the

catalytic deactivation was higher for the FeSi sample.

Analysis of the byproducts observed for the samples can

help in the interpretation of catalyst properties. Considering

benzene selectivity (Table 3), which is the main byproduct for

all the catalysts tested, it is important to point out that the

NiFeSi sample showed the highest benzene selectivity (near

23%) up to 120 min. At first, a superior benzene production

meant higher catalyst acidity and consequently, more signifi-

cant carbon deposition. Taking into account the TPD-CO2

measurements, the NiFeSi sample presented the lowest ability

for CO2 adsorption by mass of any catalyst tested, which

meant higher acidity relative to the FeSi and CoFeSi samples.

The deactivation of catalysts may occur due to the reduction

of the active phase and/or by blocking of the surface by

carbonaceous deposits.12 To investigate the cause of catalyst

deactivation during ethylbenzene dehydrogenization, studies

were conducted to verify possible changes in the chemical

properties of the catalysts, as well as determining the amount

and nature of coke deposits on the catalysts. For this, XRD

analysis after the catalytic tests were performed, and the

Fig. 5 Temperature-programmed desorption of CO2 (TPD-CO2) for

the catalysts after calcination.

Fig. 6 Catalytic performance as a function of time for the catalysts:

ethylbenzene conversion (a), styrene selectivity (b).

Table 3 Catalytic selectivity observed for by-products

Samples

Selectivitya (%)

Benzene Toluene Ethylene + ethane + methane

CoFeSi 2.5 0.8 1.1NiFeSi 23.5 1.3 0.8FeSi 8.6 — —

a Average values for reaction period up to 120 min.

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results are depicted in Fig. 7. These data made it possible to

verify some differences from the fresh catalysts (Fig. 2).

The pattern presented in Fig. 7 for the FeSi sample pointed

to Fe3+ reduction during the reaction process. In addition, the

diffraction pattern of the FeSi sample pointed to magnetite

phase formation (Fe3O4 JCPDS 26-1136) and the maghemite

phase was still detected (g-Fe2O3 JCPDS 39-1346).

Through careful XRD pattern (Fig. 7) analysis, refinement

based on the Rietveld method28 was carried out, and the

weight percentages for the different phases detected are listed

in Table 4. Interestingly, for the FeSi sample, maghemite was

the major phase (89 wt%) in the used sample but not the fresh

sample (Table 1), suggesting that the maghemite phase was

formed from hematite during the reaction process simultaneously

to magnetite phase formation. From the literature,41 however,

the hematite phase has a superior stability to maghemite. In

spite of the fact that experimental data presented here are not

enough to support this model, a possible explanation for the

observed result was the formation of maghemite particles within

a shell of hematite. These core–shell particles were then partially

reduced during ethylbenzene dehydrogenation, maintaining the

maghemite in the interior. If Fe2+ was not as active in

ethylbenzene dehydrogenation as Fe3+,13 the core–shell

particle formation theory might explain the observed decreases

in the conversion of ethylbenzene presented in Fig. 6a.

On the other hand, it is necessary to carry out analysis in

order to clarify if the reaction conditions (CO2 presence) are

favourable to maghemite formation.

The reaction process also promoted the partial reduction of

the NiFeSi sample, forming metallic nickel (Ni, JCPDS

01-1258). However, the NiFeSi pattern suggested that a ratio

of the structures of nickel ferrite (NiFe2O4, JCPDS 44-1485)

and nickel oxide (NiO, JCPDS 78-0423) was maintained after

24 h of reaction time. On the other hand, the relative mass

percentage (Table 4) calculated indicated a decrease in the

wt% of NiFe2O4 and an increase for NiO. These results

suggested that the Fe3+ from the spinel phase was reduced,

which then likely promoted magnetite phase formation without

Ni2+ reduction. This hypothesis was suggested because the

NiO wt% for the used sample (Table 4) was higher than for

the fresh sample (Table 1). This observation might be related

to a superior stability for the Ni2+ species relative to the Fe3+

or that a higher proportion of tetrahedral sites were occupied by

Ni2+. The disproportionate occupation would consequently

lead to less exposure to the reduction process. The metallic

nickel formed might have been from the reduction of the NiO

detected in the fresh sample. Furthermore, it is well known

that nickel in the metallic state (Ni0) is able to promote a C–H

bond disproportionation reaction,42 which for some reactions

is responsible for a high ratio of coke deposition.13,42 Thus, it

was expected that the NiFeSi sample would present a high

amount of coke deposition.

In contrast to the other samples, the CoFeSi catalyst seemed

to not undergo phase modification due to the reaction process.

The XRD pattern of CoFeSi after the reaction was very

similar to that for the fresh catalyst (Fig. 1), which indicated

a superior stability for the CoFe2O4 phase under the reaction

conditions. If the change did occur, it was in a very low

proportion. In addition, taking into account the by-product

selectivity (Table 3), the CoFeSi sample showed a lower

benzene selectivity, which suggested a low coke deposition.

Therefore, the XRD results strengthen the suggestion that

the H2 consumption peak at 840 1C showed by the CoFeSi

sample (Fig. 3) may be due to CoFe2O4 phase reduction.

In spite of the valuable information acquired from the

catalytic test expressed in Fig. 6, the reaction time used for

the FeSi sample was only of 6 h due to the high deactivation.

Therefore, catalytic tests with a period of 24 h were carried out

for the samples CoFeSi and NiFeSi. The respective ethylbenzene

conversions were 13% and 10%, which suggested the same rate

of deactivation for both samples over 24 h. Additionally, the

ethylbenzene conversion observed for the NiFeSi suggested

that high surface area alone was not a determining factor for

superior catalytic conversion. In spite of the higher surface area

presented by the NiFeSi catalyst (Table 2), the ethylbenzene

conversion values were lower than the CoFeSi sample.

Therefore, the CoFeSi catalyst was the most active solid

(conversion near 17% after 4 h) due to the combined effects

of basicity, surface area and reducibility. Despite the H2-TPR

profile of CoFeSi resembling that of NiFeSi, the XRD analysis

of the spent catalysts revealed that the spinel structure of the

CoFeSi sample was maintained, presenting a hard reducibility

under the ethylbenzene dehydrogenation conditions and the

temperature-programmed desorption of CO2 revealed a

medium basicity for the CoFeSi sample, justifications for its

better catalytic performance.

To find another reason for the decrease in ethylbenzene

conversion, the spent catalysts were analyzed by thermo-

gravimetric analysis (TG) (Fig. 8).

This thermogravimetric analysis of the spent catalysts was

carried out to estimate the amount of coke deposited, as

revealed by the reduction in weight due to coke oxidation in

Fig. 7 X-Ray diffraction profiles of the samples after the catalytic

tests.

Table 4 Relative mass percentage of the phases observed after thecatalytic process

Crystalline phase (%)

Samples

CoFeSi NiFeSi FeSi

NiFe2O4 — 27 —CoFe2O4 100 — —g-Fe2O3 — — 89Fe3O4 — 11NiO — 64 —Ni — 9 —

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air. Notice that the thermal analysis results (Fig. 8) pointed to

a lower coke deposition for FeSi, while the NiFeSi catalyst

presented a higher coke deposition. The thermograms of three

coked catalyst samples revealed coke contents of 45.0, 71.2,

and 3.2 wt% for the CoFeSi, NiFeSi and FeSi samples,

respectively. However, it is necessary to notice the time on

stream of each catalyst before the TG analysis.

The catalytic test data, TPD-CO2 measurements, X-ray

diffraction and thermal analysis of the spent catalysts were

in good agreement and showed that the NiFeSi sample

presented higher coke deposition, lower ethylbenzene conversion

and higher benzene selectivity. These properties were probably

due to the lower CO2 adsorption ability (Fig. 5) and the

formation of metallic nickel (Fig. 7) in this sample. Further-

more, these properties could explain the lower selectivity for

styrene of this solid.

The thermograms of coked samples exhibited a coke oxidation

temperature range of 300 to 450 1C for all the catalysts, although

the sample FeSi also showed coke elimination (burning) with an

inflection point at 540 1C (Fig. 8), which is related to amorphous

carbon.35 Such behaviour (a low temperature burning) was

characteristic of either a carbon deposit with a low molecular

weight or a low C/H ratio43–45 but also of carbonaceous

material containing oxygen in its structure.43,46

To study the molecular structure of the coke deposited on

the spent catalysts, the samples were dissolved in concentrated

HF and the material (coke) extracted was analyzed by Fourier

transformed infrared spectroscopy (FT-IR) (Fig. 9).

The spectroscopy results revealed the presence of aliphatic

C–H (2918 cm�1 with low intensity) and a band at 1600 cm�1

for frame vibrations of benzene.47 Additionally, a peak at

740 cm�1 was due to the C–H in-plane bending.48 All these

peaks illustrated that the precursors of the coke were mainly

saturated hydrocarbons.

Further inspecting the FTIR spectrum, a peak at 1400 cm�1

was due to the C–H deformation vibration of a vinyl group

bonded to oxygen.49 However, this peak could also be assigned

to the CQO group of a carboxylate or carbonate-type

species.50 The presence of a CQO group in the carbonaceous

material was reinforced by a peak with low intensity at

1705 cm�1 for FeSi. In addition, the bands at 1200, 1160

and 1110 cm�1 were assigned to OH, COC and CO groups.51

The presence of an OH group in the carbonaceous samples

was also suggested by a wide peak at 3418 cm�1; however, the

presence of residual water in the carbonaceous sample could

not be ruled out.

The coke precursors were mainly composed of aliphatic

hydrocarbons as well as oxygen-containing and polyaromatic

structures. The adsorption peaks of the FT-IR for aliphatic

hydrocarbons and oxygen-containing moieties were much

weaker than the peaks for the aromatic hydrocarbons. There-

fore, there was a larger amount of aromatic hydrocarbons in

the coke precursor. Accordingly, the results from TG and

FT-IR analysis suggested that the coke was generated from

ethylbenzene but also from CO2.

The observation from the data of TG and FTIR that the

coke can be deposited as much of the ethylbenzene as of the

carbon dioxide corroborates with the value of DG1 at 550 1C

(�7,9 kJ mol�1) for the reaction (3).

CO2 + 2H2 " 2H2O + C (3)

In Table 5 are given the relative values of the amount of

deposited coke and the ethylbenzene converted for each

catalyst, according to the period of reaction time. The data

in Table 5 show that the catalyst NiFeSi presents a high ratio

of coke formation. Similarly, the data obtained by TG indicate

a significant amount of coke in the sample CoFeSi, if

compared to the catalyst FeSi.

Conversely, if it is taken into account the reaction time that

each catalyst was submitted and the total amount of

ethylbenzene converted during this period, it is verified that

the ratio of coke deposition for the catalyst CoFeSi is similar

(or somewhat lower) to the FeSi sample.

Therefore, although the coke deposition was not suppressed,

the addition of Co can be interesting because it can be observed

Fig. 8 Thermogravimetric profile of the samples carried out under

synthetic air flow. NiFeSi and CoFeSi after the catalytic test of 24 h,

FeSi after 6 h of time on stream.

Fig. 9 Fourier-transform infrared (FTIR) spectroscopy of material

(coke) extracted from the samples, after the catalytic tests.

Table 5 Amount of coke deposited and ethylbenzene convertedduring the catalytic test

Property

Samples

CoFeSi NiFeSi FeSi

Coke/wt% 45.0 71.2 3.2Mass of coke/mg 122.72 370.83 4.95Reaction time/h 24 24 6Etbz conv./mg 569.86 417.53 77.27X 0.0089 0.0370 0.0106

Etbz conv. = total of ethylbenzene converted during the reaction test.

X=mass of coke (mg) divided by reaction time and by Etbz conv. (mg).

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a superior ratio of ethylbenzene conversion without the

depreciation of the selectivity for styrene.

Considering the X value from Table 5, the catalytic deactivation

(Fig. 6a) could not be attributed mainly to the coke deposition, in

spite of the fact that its presence can make difficult the active site

accessibility for ethylbenzene.

The catalytic deactivation observed, however, can be

attributed mainly to the partial reduction of iron oxide, as

suggested by the TPR profile (Fig. 3). As pointed out by Fig. 6a,

a higher ratio of catalytic deactivation takes place in the first

two hours of time on stream. This behaviour corroborates well

with the XRD suggestions, which pointed out that the catalyst

samples have a high number of crystalline defects. For the

catalytic process a surface with crystalline defects may be more

active, because generally a highest surface free energy is observed

in surfaces with lower coordination surface atoms. On the other

hand, it may present a superior tendency of deactivation.

Accordingly, the XRD profile of the CoFeSi sample after

the catalytic test did not pointed out a new phase formation

due to the low crystalline diameters or the amorphous

characteristic.

4. Conclusions

In this study, cobalt and nickel ferrites dispersed in mesoporous

silicon oxide were synthesized by a polymeric precursor method

and tested as catalysts for ethylbenzene dehydrogenation in the

presence of CO2. The CoFeSi catalyst presented higher

ethylbenzene conversion and good styrene selectivity, which

were ascribed to the poor reducibility under the reaction

conditions and the existence of medium strength basic sites.

Conversely, the NiFeSi sample showed higher coke deposition,

lower ethylbenzene conversion and high benzene selectivity,

probably as a result of the lower CO2 adsorption ability and

the formation of metallic nickel during the reaction process for

this material. Furthermore, the lower stability observed for the

solid FeSi may be correlated with the formation of Fe3O4

during the ethylbenzene reaction. Therefore, in spite of the high

coke deposition, the results suggested that materials with a

spinel structure, such as ferrites of cobalt, might be particularly

useful for ethylbenzene dehydrogenation. On the other hand,

as the results suggested that ethylbenzene and CO2 contribute

to coke deposition, it is necessary to carry out studies in order

to better understand the CO2 role in the ethylbenzene

dehydrogenation with CO2.

Acknowledgements

The authors acknowledge the ‘‘Universidade Federal do

Ceara’’ (UFC), CNPq/CT-PETRO, and Dr J.M. Sasaki

(Laboratorio de Raios X) for XRD measurements. T.P.

Braga, B.M.C. Sales and A.N. Pinheiro express gratitude for

scholarships from CNPq and CAPES.

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