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Effect of Ce Substituted Hydrotalcite-derived Mixed Oxides on Total Catalytic Oxidation of Air Pollutant
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 3 (2016) 277 – 281
Advances in Functional Materials (Conference 2015), AFM 2015
Effect of Ce substituted hydrotalcite-derived mixed oxides on total catalytic oxidation of air pollutant. Genty E.a, Brunet J.a, Pequeux R.a, Capelle S.a, Siffert S.a, Cousin R.a* a
ULCO, UCEIV (EA-4492), F-59140 Dunkerque, France
Abstract
Co6Al2-yCeyHT hydrotalcite like compounds were synthesized by co-precipitation method. Hydrotalcite structure as well as the mixed oxides obtained after calcination were studied by several physic-chemical techniques: N2-Sorption, XRD, H2-TPR. The mixed oxides were also tested for the toluene total oxidation. The physico-chemical studies revealed modifications in the structural characteristics (surface area, morphology) as well as in reducibility properties of the formed mixed oxides. The solid containing the higher cerium content was the most active in this reaction. Furthermore, relation between the reducibility and T50 for the toluene oxidation was demonstrated, suggesting a Mars Van Krevelen mechanism for the toluene total oxidation with Co-Al mixed oxide. The key factor of these solids is the dispersion of the ceria in the solid. Copyright © 2014 Elsevier Ltd. All rights reserved. © 2016 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of Advances in Functional Materials Selection and peer-review under responsibility of Conference Committee Members of Advances in Functional Materials (Conference 2015). (Conference 2015). Keywords: Mixed oxides, Hydrotalcite, Cerium, Cobalt, Toluene, Catalytic oxidation
1. Introduction Catalytic oxidation represents an efficient method for the removal of the Volatile Organic Compounds (VOCs) and have a several advantages compared to the traditional flame combustion. Supported noble metals are generally considered as efficient catalysts for VOCs oxidation [1, 2]. However, due to the high cost of the noble metals, many researchers devoted to the development of suitable catalysts containing only transition metal oxides [2]. Hydrotalcitelike compounds are presented as precursors of mixed oxides, with an enormous potential for the generation of well
*
Corresponding author. Tel.: (+33) 03 28 65 82 76; fax: (+33) 03 28 65 82 39 E-mail address: [email protected]
2214-7853 © 2016 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of Advances in Functional Materials (Conference 2015). doi:10.1016/j.matpr.2016.01.069
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E. Genty et al. / Materials Today: Proceedings 3 (2016) 277 – 281
dispersed, active and very stable catalysts. This class of layered double hydroxides consists of positively charge metal hydroxide layers separated from each other by anions and water molecules. A wide range of possible cations and anions that could be incorporated in the Hydrotalcite (HT) structure gives rise to several materials [3]. Indeed after calcination treatment mixed oxides are formed and possess unique properties like high surface area and porosity, good thermal stability, good mixed oxides homogeneity, basic properties and undergo high metal dispersion [4]. The mixed oxides containing Co and Al issued from the hydrotalcite precursor show a good catalytic activity for the VOC oxidation [3, 5, 6]. The key factors in activity of the cobalt oxides solids are the nature and distribution of these species on the solid surface for the supported catalysts [7-8]. The use of cobalt oxide promoted by cerium oxide can lead to an improvement in the oxygen storage capacity, which evidently enhances the oxidation process. The most important ceria property is the oxygen reservoir corresponding to the storage and the release of the oxygen via a redox shift between Ce4+ and Ce3+ [3]. The aim of this work is to study the catalytic contribution of cerium in the mixed oxide Co6Al2HT500 for the toluene total oxidation. For this, a part of aluminium (Al3+) of the hydrotalcite structure is replaced by cerium (Ce3+). The hydrotalcite precursors and mixed oxides were characterized by powder XRD, N2Sorption, Hydrogen Temperature Programmed Reduction (H 2-TPR) and catalytic oxidation test. 2. Experimental 2.1. Preparation of Catalysts The Hydrotalcite like compounds Co6Al2-xCexHT with theoretical molar ratios M2+/M3+ equal to 3 is prepared by coprecipitation method. An aqueous solution containing appropriate amounts of Co 2+, Al3+ and Ce3+ ions (solution A) was added, whilst stirred, dropwise into another solution containing Na2CO3 (solution B). During the synthesis, the temperature and the pH were maintained respectively at ambient temperature (20°C) and 10.5 with NaOH solution. The solution was stirred at ambient temperature during 18 h. Then, the precipitate was filtered, washed several times with hot deionized water (50°C) and dried at 60°C for 64 h. The samples were called Co 6Al2-xCexHT (where HT corresponds to hydrotalcite structure and 0 < x < 0.8). The thermal treatment was performed under a flow of air (4 L.h–1, 1 °C.min–1, 4 hours at 500 °C). The catalysts were named Co 6Al2-xCexHT500 (with 0 < x < 0.8). 2.2. Characterization Techniques and catalytic tests Pore size data and specific surface area measurements of the catalysts calcined at 500°C were determined from nitrogen adsorption isotherms obtained by Sorptomatic 1990 series apparatus (Thermo Finnigan) after evacuation under vacuum at 400°C. Crystallinity of solids was determined at room temperature by X-Ray Diffraction (XRD) technique with a Bruker D8 Advance diffractometer equipped with a copper anode (λ = 1.5406 Å) and a LynxEye Detector. The scattering intensities were measured over an angular range of 10° ≤ 2θ ≤ 80° for all samples with a stepsize of Δ(2θ) = 0.02° and a count time of 4s per step. Temperature-Programmed Reduction (H2-TPR) experiments were carried out in an Altamira AMI-200 apparatus. Prior to the TPR experiment, 30 mg samples were active under argon at 150°C for 1 hour. The samples were then heated from ambient temperature to 900°C under H 2 flow (5 vol.% in Ar, 30mL.min-1) at a heating rate of 5 °C.min-1. The activity for toluene total oxidation of the catalysts (100 mg) was measured in a continuous flow system in a fixed bed reactor at atmospheric pressure. After reaching a stable flow (100 mL.min-1 with 1000 ppm of C7H8, 20 vol%O2 in He), the reactants passed through the catalyst bed and the temperature was increased from room temperature to 500°C (1°C.min -1). The feed and the reactor outflow gases were analysed on line by a micro-gas chromatograph (Agilent 490 Micro gas chromatography). 3. Results and discussion 3.1. Characterization of Dried Samples X-rays diffraction patterns of the dried solids are reported on the Fig. 1(a). The patterns reveal the typical diffractograms of hydrotalcite like compounds. In fact, cobalt-aluminium hydrotalcite phase (ICDD-JCPDS files 510045 noted #) is observed for all the samples. Moreover, for the sample containing cerium, a phase corresponds to the CeO2 phase is observed (ICDD-JCPDS files 34-0394 noted o). The intensity of this patterns increases when the cerium content increases. The presence of CeO2 phase is explained by the Ce3+ (1.01 Å) possesses a relatively larger ionic radius than the Al3+ (0.5 Å). Therefore the Ce3+cations are difficult to insert in the hydrotalcite structure.
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S #
2
#
#2
#
2
##
o
S
S
oS
o
S
S
o
Co6Al1,2Ce0,8HT
Co6Al1,2Ce0,8HT500
Co6Al1,6Ce0,4HT
Intensity (a.u.)
Intensity (a.u.)
Co6Al1,6Ce0,4HT500
Co6Al1,7Ce0,3HT Co6Al1,8Ce0,2HT Co6Al1,9Ce0,1HT
Co6Al1,7Ce0,3HT500
Co6Al1,8Ce0,2HT500
Co6Al1,9Ce0,1HT500
Co6Al2HT Co6Al2HT500 10
20
30
40
50
60
70
2 theta (°)
80
(a)
20
30
40
50
60
2 theta (°)
70
80
(b)
Fig. 1. XRD patterns for (a) the dried solids (b) the mixed oxides (#: Co6Al2(OH)16CO3, 4H2O, S: Co3O4, CoAl2O4 or Co2AlO4, o: CeO2)
The cell parameter of Co-Al hydrotalcite phase was calculated assuming a 3R polytype with a hexagonal cell. The values of “a” and “c” parameters were reported in Table 1. The lattice parameter “a” (characteristic of cation – cation distance in the brucite layer) increases when the cerium is added to the synthesis of hydrotalcite. The larger ionic radius of Ce3+ caused this expansion of the cation-cation distance. The lattice parameter “a” reaches a maximum equal to 3.083 Å which can be explained by a maximum amount of cerium inserted into the hydrotalcite structure. Concerning the “c” lattice parameter, an increasing of this parameter is observed when the cerium content increases. This can be explained by the low polarizing ability of Ce 3+ compared to Al3+. This difference affects the electrostatic interaction between the interlayer anions and the positively charged hydroxide layers [9, 10]. The specific surface areas, measured by BET method, for the dried samples, are reported in the Table 1. The specific surface areas increase slightly when the cerium is incorporated in the solid. Table 1. Summary of the physicochemical properties for the hydrotalcite like compounds. Dried solid Co6Al2HT Co6Al1.9Ce0.1HT Co6Al1.8Ce0.2HT Co6Al1.7Ce0.3HT Co6Al1.6Ce0.4HT Co6Al1.2Ce0.8HT
Lattice parameters a (Å) c (Å) 3.075 22.78 3.078 22.82 3.083 23.12 3.083 23.02 3.083 22.91 3.084 23.03
Specific surface area (m².g-1) 111 115 121 123 128 138
Co: Al: Ce molar ratio Theorical Experimental 6:2 6 : 2.1 6 : 1.8 : 0.2 6 : 1.9 : 0.12 6 : 1.8 : 0.2 6 : 1.78 : 0.23 6 : 1.8 : 0.2 6 : 1.69 : 0.29 6 : 1.6 : 0.4 6 : 1.61 : 0.38 6 : 1.2 : 0.8 6 : 1.22 : 0.73
3.2. Characterization of Calcined Samples The dried samples are calcined at 500°C during 4h under a dried air flow. The elementary analysis reported in Table 1 shows that the Co:Al:Ce ratios are respected for all the samples. To investigate the structure of the samples, X-Rays Diffraction analyses are performed. The diffractograms are shown in Fig. 1 (b). Several crystalline phases are observed. The first corresponds to three spinels phases: cobalt – cobalt Co3O4 (ICDD-JCPDS files 42-1467) or cobalt - aluminium: CoAl2O4 (ICDD-JCPDS files 44-0160) and Co2AlO4 (ICDD-JCPDS files 38-0814). These phases cannot be distinguished because their characteristic diffraction peaks are close in intensity and position [6, 11]. The Co-Al mixed oxide leads to the normal spinel, i.e., CoAl 2O4 corresponding to the Co2+ in the tetrahedral positions and the Al3+ in the octahedral position. A fourth phase is observable for solids containing cerium: the cerianite CeO 2 (ICDDJCPDS files 34-0394). The isotherms presented in Fig. 2 (a) for Co6Al2-xCexHT500 solids correspond to type IV for mesoporous materials. The average pore diameters of each sample as well as their specific surface area are shown in Table 2. The average pore diameters of all samples are between 3 and 8 nm, no significant modification in the solid porosity are observed. Concerning the surface area measurement, a small increase is observed for solids containing a small amount of cerium (x İ 0.3 Co6Al2-xCexHT500). When the proportion of cerium increases, this trend is reversed, with a significant decrease in the specific surface area. This decrease can be explained by a higher crystallisation of
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the cerianite phase. After performing the structural analysis of various solids, a study of the redox properties of the solid is undertaken by H2-RTP.
Co6Al1.2Ce0.8HT500
Co6Al1.6Ce0.4HT500 TCD Signal (a.u.)
Volume (a.u.)
Co6Al1.2Ce0.8HT500
Co6Al1.7Ce0.3HT500 Co6Al1.8Ce0.2HT500
Co6Al1.6Ce0.4HT500 Co6Al1.7Ce0.3HT500 Co6Al1.8Ce0.2HT500
Co6Al1.9Ce0.1HT500 Co6Al1.9Ce0.1HT500 Co6Al2HT500
Co6Al2HT500 0,0
0,1
CeO2500
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
Pression relative (P/P0)
1,0
100
(a)
200
300
400
500
600
700
800
Temperature (°C)
(b)
Fig 2. (a) N2 adsorption and desorption isotherms and (b) H2-TPR profiles of the mixed oxides
The analysis by Temperature-Programmed Reduction (H2-TPR) was performed and reported on Fig. 2 (b). Regarding the mixed oxide Co6Al2HT500, two reduction zones are observed: one for less than 400 °C and another at higher temperature (T > 400 °C). The first zone (330°C) corresponds to the reduction of Co 3O4 species into metal Cobalt (Co0) while the second (693 °C) corresponds to the reduction of cobalt-aluminium spinel species (CoAl2O4 or Co2AlO4) cobalt metal (Co0) [11]. The solids containing cerium have also two reduction zones (T <400 °C and T> 400 °C). However, a gradual decrease in the temperature of these reductions is observed when the proportion of cerium increases in the solid. Furthermore, for both the solids containing more cerium (y = 0.4 to 0.8), a signal change on the two reduction zones is observed. The first step is composed of three stages of reduction corresponding to the reduction of Co3O4 into Co0 and species CeO2 surface species into Ce2O3. The second domain for these two solids has a modification of the signal with the presence of three peaks corresponding to reductions of the spinel phases (CoAl 2O4 and Co2AlO4) and ceria present in the catalyst bulk (bulk CeO 2). Hydrogen consumptions in the temperature range corresponding to the catalytic activity (T <400 °C) and for the overall H2-TPR analysis are reported in Table 2. For the solids Co6Al2-yCeyHT500, a slight increase in the total H2 consumption is observed when the cerium proportion increases. In addition, the values of the hydrogen consumption before 400 °C, show a slight increase which is proportional to the cerium amount, which corresponds to the reduction of Ce 4+ species present on the surface of the solid. The better reducibility is then observed in case of Co6Al1.2Ce0.8HT500 solid. Table 2. Summary of the physic-chemical properties for the mixed oxides and T50 for the toluene total oxidation Pore H2 Consumption (µmol.g-1) SBET Mixed oxides diameter -1 (m².g ) T < 400°C Total Temperature corresponding to the first reduction peak (nm)
T50 (°C)
Co6Al2HT500
123
7.5
2656
11715
335
287
Co6Al1.9Ce0.1HT500
130
4.9
2665
11775
322
282
Co6Al1.8Ce0.2HT500
126
6.2
2670
11865
302
274
Co6Al1.7Ce0.3HT500
128
6.7
2685
11937
286
269
Co6Al1.6Ce0.4HT500
118
4.5
2825
12013
224
261
Co6Al1.2Ce0.8HT500
108
3.7
2993
12393
195
248
The mixed oxides and ceria are tested as catalysts in the total oxidation of toluene (Fig. 3(a)). When the conversion is complete, H2O and CO2 are the only products observed. However at beginning of toluene conversion for all samples, few ppm of benzene are detected. Different reaction paths have been already recognized [12]. The T50 temperature (temperature for 50% toluene conversion) is chosen as a measure of catalytic activity of the mixed oxides (Table 3). The catalytic activity can be established by the following order: Co6Al1.2Ce0.8HT500 > Co6Al1.6Ce0.4HT500 > Co6Al1.7Ce0.3HT500 > Co6Al1.8Ce0.2HT500 > CeO2 > Co6Al1.9Ce0.1HT500 > Co6Al2HT500
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E. Genty et al. / Materials Today: Proceedings 3 (2016) 277 – 281
310
100 CeO2 90
70
300
Co6Al1.6Ce0.4HT500 290
Co6Al1.7Ce0.3HT500 Co6Al1.8Ce0.2HT500
280
Co6Al1.9Ce0.1HT500 60
Co6Al2HT500 270
T50 (°C)
Toluene conversion (%)
80
Co6Al1.2Ce0.8HT500
50 40
260
30
250
20
240
10
Ce proportion
230
0 100
150
200
250
Temperature (°C)
300
220
350
180
(a)
200
220
240
260
280
300
320
340
Temperature corresponding at the first reduction peak in H 2-TPR(°C)
(b)
Fig 3. (a) Conversion of toluene (%) on mixed oxides vs. reaction temperature (°C) and (b) relation between T 50 and the temperature corresponding at the first reduction peak in the H2-TPR (°C)
An increase in the catalytic activity for the toluene oxidation with the incorporation of cerium into the solids is observed. The most active solid contains the largest proportion of cerium. This correlation suggests an increase in the oxygen mobility in the solid with the cerium proportion. This hypothesis is correlated with the results obtained during the H2-TPR. Indeed, the solid showing a better reducibility at low temperature (<400 °C) also has the best activity against toluene oxidation. This relationship has been demonstrated by plotting the T 50 according to the temperature of the first reduction peak observed in H 2-TPR (Fig. 3 (b)). This linearity obtained between the reducibility and the conversion of toluene suggests a Mars Van Krevelen mechanism (redox mechanism). Moreover, the adding of cerium into the mixed oxide shows a better activity for toluene total oxidation than CeO2 and CoAl mixed oxide. A synergistic effect between ceria and Co-Al oxide is an important condition for having a cobalt cerium-based catalyst with a good activity at low temperature. Indeed, this result suggests that the cerium oxide allows better oxygen mobility on the solid surface, promoting the oxygen exchange with cobalt oxide. In fact, the cobalt oxide could be reduced during the toluene oxidation and re-oxidised with oxygen issued from the ceria. This synergistic effect between cerium and cobalt is a key factor for the good activity of these solids. 4. Conclusion CoAlCe hydrotalcite-like compounds are prepared by conventional co-precipitation. The several physico-chemical characterizations indicated that all solids Co6Al2-xCexHT prepared by the hydrotalcite route contain cerianite and hydrotalcite phases. The calcination at 500 °C resulted in the formation of mixture of spinel and cerianite phases. These mixed oxides are also tested in the toluene total oxidation and showed an increase of the reactivity when the content of cerium increases. Relationship between reducibility and reactivity of these solids is revealed and a synergetic effect between Co and Ce is evidenced. Thus, the Co6Al1.2Ce0.8 mixed oxide prepared via hydrotalcite route seems to be a promising catalyst for the VOC total oxidation. 5. Acknowledgements Authors want to acknowledge the ADEME as well as Nord-Pas-de-Calais region for the funding of the work. Authors also acknowledge the "Centre Commun de Mesure" of the University Littoral Côte d’Opale. References [1] L.F. Liotta, Appl. Catal. B Environ. 100 (2010) 403. [2] T. Barakat, J.C. Rooke, E. Genty, R. Cousin, S. Siffert, B.-L. Su, Energy Environ. Sci. 6 (2013) 371. [3] L.F. Liotta, H. Wu, G. Pantaleo, A.M. Venezia, Catal. Sci. Technol. 3 (2013) 3085. [4] G. Fan, F. Li, D.G. Evans, X. Duan, Chem. Soc. Rev. 43 (2014) 7040. [5] E. Genty, R. Cousin, S. Capelle, C. Gennequin, S. Siffert, Eur. J. Inorg. Chem. 2012 (2012) 2802. [6] A. Pérez, R. Molina, S. Moreno, Appl. Catal. A Gen. 477 (2014) 109. [7] R. Bechara, D. Balloy, D. Vanhove, Appl. Catal. A Gen. 207 (2001) 343. [8] D. Song, J. Li, J. Mol. Catal. A Chem. 247 (2006) 206. [9] J. Das, D. Das, K.M. Parida, J. Colloid Interface Sci. 301 (2006) 569. [10] O.R. Neto, N.F.P. Ribeiro, C.A.C. Perez, M. Schmal, M.M.V.M. Souza, Appl. Clay Sci. 48 (2010) 542. [11] E. Genty, J. Brunet, C. Poupin, S. Casale, S. Capelle, P. Massiani, S. Siffert, R. Cousin, Catalysts 5 (2015) 851. [12] S. Lars, T. Andersson, J Catal, 98 (1986) 138.
ScienceDirect Materials Today: Proceedings 3 (2016) 277 – 281
Advances in Functional Materials (Conference 2015), AFM 2015
Effect of Ce substituted hydrotalcite-derived mixed oxides on total catalytic oxidation of air pollutant. Genty E.a, Brunet J.a, Pequeux R.a, Capelle S.a, Siffert S.a, Cousin R.a* a
ULCO, UCEIV (EA-4492), F-59140 Dunkerque, France
Abstract
Co6Al2-yCeyHT hydrotalcite like compounds were synthesized by co-precipitation method. Hydrotalcite structure as well as the mixed oxides obtained after calcination were studied by several physic-chemical techniques: N2-Sorption, XRD, H2-TPR. The mixed oxides were also tested for the toluene total oxidation. The physico-chemical studies revealed modifications in the structural characteristics (surface area, morphology) as well as in reducibility properties of the formed mixed oxides. The solid containing the higher cerium content was the most active in this reaction. Furthermore, relation between the reducibility and T50 for the toluene oxidation was demonstrated, suggesting a Mars Van Krevelen mechanism for the toluene total oxidation with Co-Al mixed oxide. The key factor of these solids is the dispersion of the ceria in the solid. Copyright © 2014 Elsevier Ltd. All rights reserved. © 2016 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of Advances in Functional Materials Selection and peer-review under responsibility of Conference Committee Members of Advances in Functional Materials (Conference 2015). (Conference 2015). Keywords: Mixed oxides, Hydrotalcite, Cerium, Cobalt, Toluene, Catalytic oxidation
1. Introduction Catalytic oxidation represents an efficient method for the removal of the Volatile Organic Compounds (VOCs) and have a several advantages compared to the traditional flame combustion. Supported noble metals are generally considered as efficient catalysts for VOCs oxidation [1, 2]. However, due to the high cost of the noble metals, many researchers devoted to the development of suitable catalysts containing only transition metal oxides [2]. Hydrotalcitelike compounds are presented as precursors of mixed oxides, with an enormous potential for the generation of well
*
Corresponding author. Tel.: (+33) 03 28 65 82 76; fax: (+33) 03 28 65 82 39 E-mail address: [email protected]
2214-7853 © 2016 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of Advances in Functional Materials (Conference 2015). doi:10.1016/j.matpr.2016.01.069
278
E. Genty et al. / Materials Today: Proceedings 3 (2016) 277 – 281
dispersed, active and very stable catalysts. This class of layered double hydroxides consists of positively charge metal hydroxide layers separated from each other by anions and water molecules. A wide range of possible cations and anions that could be incorporated in the Hydrotalcite (HT) structure gives rise to several materials [3]. Indeed after calcination treatment mixed oxides are formed and possess unique properties like high surface area and porosity, good thermal stability, good mixed oxides homogeneity, basic properties and undergo high metal dispersion [4]. The mixed oxides containing Co and Al issued from the hydrotalcite precursor show a good catalytic activity for the VOC oxidation [3, 5, 6]. The key factors in activity of the cobalt oxides solids are the nature and distribution of these species on the solid surface for the supported catalysts [7-8]. The use of cobalt oxide promoted by cerium oxide can lead to an improvement in the oxygen storage capacity, which evidently enhances the oxidation process. The most important ceria property is the oxygen reservoir corresponding to the storage and the release of the oxygen via a redox shift between Ce4+ and Ce3+ [3]. The aim of this work is to study the catalytic contribution of cerium in the mixed oxide Co6Al2HT500 for the toluene total oxidation. For this, a part of aluminium (Al3+) of the hydrotalcite structure is replaced by cerium (Ce3+). The hydrotalcite precursors and mixed oxides were characterized by powder XRD, N2Sorption, Hydrogen Temperature Programmed Reduction (H 2-TPR) and catalytic oxidation test. 2. Experimental 2.1. Preparation of Catalysts The Hydrotalcite like compounds Co6Al2-xCexHT with theoretical molar ratios M2+/M3+ equal to 3 is prepared by coprecipitation method. An aqueous solution containing appropriate amounts of Co 2+, Al3+ and Ce3+ ions (solution A) was added, whilst stirred, dropwise into another solution containing Na2CO3 (solution B). During the synthesis, the temperature and the pH were maintained respectively at ambient temperature (20°C) and 10.5 with NaOH solution. The solution was stirred at ambient temperature during 18 h. Then, the precipitate was filtered, washed several times with hot deionized water (50°C) and dried at 60°C for 64 h. The samples were called Co 6Al2-xCexHT (where HT corresponds to hydrotalcite structure and 0 < x < 0.8). The thermal treatment was performed under a flow of air (4 L.h–1, 1 °C.min–1, 4 hours at 500 °C). The catalysts were named Co 6Al2-xCexHT500 (with 0 < x < 0.8). 2.2. Characterization Techniques and catalytic tests Pore size data and specific surface area measurements of the catalysts calcined at 500°C were determined from nitrogen adsorption isotherms obtained by Sorptomatic 1990 series apparatus (Thermo Finnigan) after evacuation under vacuum at 400°C. Crystallinity of solids was determined at room temperature by X-Ray Diffraction (XRD) technique with a Bruker D8 Advance diffractometer equipped with a copper anode (λ = 1.5406 Å) and a LynxEye Detector. The scattering intensities were measured over an angular range of 10° ≤ 2θ ≤ 80° for all samples with a stepsize of Δ(2θ) = 0.02° and a count time of 4s per step. Temperature-Programmed Reduction (H2-TPR) experiments were carried out in an Altamira AMI-200 apparatus. Prior to the TPR experiment, 30 mg samples were active under argon at 150°C for 1 hour. The samples were then heated from ambient temperature to 900°C under H 2 flow (5 vol.% in Ar, 30mL.min-1) at a heating rate of 5 °C.min-1. The activity for toluene total oxidation of the catalysts (100 mg) was measured in a continuous flow system in a fixed bed reactor at atmospheric pressure. After reaching a stable flow (100 mL.min-1 with 1000 ppm of C7H8, 20 vol%O2 in He), the reactants passed through the catalyst bed and the temperature was increased from room temperature to 500°C (1°C.min -1). The feed and the reactor outflow gases were analysed on line by a micro-gas chromatograph (Agilent 490 Micro gas chromatography). 3. Results and discussion 3.1. Characterization of Dried Samples X-rays diffraction patterns of the dried solids are reported on the Fig. 1(a). The patterns reveal the typical diffractograms of hydrotalcite like compounds. In fact, cobalt-aluminium hydrotalcite phase (ICDD-JCPDS files 510045 noted #) is observed for all the samples. Moreover, for the sample containing cerium, a phase corresponds to the CeO2 phase is observed (ICDD-JCPDS files 34-0394 noted o). The intensity of this patterns increases when the cerium content increases. The presence of CeO2 phase is explained by the Ce3+ (1.01 Å) possesses a relatively larger ionic radius than the Al3+ (0.5 Å). Therefore the Ce3+cations are difficult to insert in the hydrotalcite structure.
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E. Genty et al. / Materials Today: Proceedings 3 (2016) 277 – 281 #
S #
2
#
#2
#
2
##
o
S
S
oS
o
S
S
o
Co6Al1,2Ce0,8HT
Co6Al1,2Ce0,8HT500
Co6Al1,6Ce0,4HT
Intensity (a.u.)
Intensity (a.u.)
Co6Al1,6Ce0,4HT500
Co6Al1,7Ce0,3HT Co6Al1,8Ce0,2HT Co6Al1,9Ce0,1HT
Co6Al1,7Ce0,3HT500
Co6Al1,8Ce0,2HT500
Co6Al1,9Ce0,1HT500
Co6Al2HT Co6Al2HT500 10
20
30
40
50
60
70
2 theta (°)
80
(a)
20
30
40
50
60
2 theta (°)
70
80
(b)
Fig. 1. XRD patterns for (a) the dried solids (b) the mixed oxides (#: Co6Al2(OH)16CO3, 4H2O, S: Co3O4, CoAl2O4 or Co2AlO4, o: CeO2)
The cell parameter of Co-Al hydrotalcite phase was calculated assuming a 3R polytype with a hexagonal cell. The values of “a” and “c” parameters were reported in Table 1. The lattice parameter “a” (characteristic of cation – cation distance in the brucite layer) increases when the cerium is added to the synthesis of hydrotalcite. The larger ionic radius of Ce3+ caused this expansion of the cation-cation distance. The lattice parameter “a” reaches a maximum equal to 3.083 Å which can be explained by a maximum amount of cerium inserted into the hydrotalcite structure. Concerning the “c” lattice parameter, an increasing of this parameter is observed when the cerium content increases. This can be explained by the low polarizing ability of Ce 3+ compared to Al3+. This difference affects the electrostatic interaction between the interlayer anions and the positively charged hydroxide layers [9, 10]. The specific surface areas, measured by BET method, for the dried samples, are reported in the Table 1. The specific surface areas increase slightly when the cerium is incorporated in the solid. Table 1. Summary of the physicochemical properties for the hydrotalcite like compounds. Dried solid Co6Al2HT Co6Al1.9Ce0.1HT Co6Al1.8Ce0.2HT Co6Al1.7Ce0.3HT Co6Al1.6Ce0.4HT Co6Al1.2Ce0.8HT
Lattice parameters a (Å) c (Å) 3.075 22.78 3.078 22.82 3.083 23.12 3.083 23.02 3.083 22.91 3.084 23.03
Specific surface area (m².g-1) 111 115 121 123 128 138
Co: Al: Ce molar ratio Theorical Experimental 6:2 6 : 2.1 6 : 1.8 : 0.2 6 : 1.9 : 0.12 6 : 1.8 : 0.2 6 : 1.78 : 0.23 6 : 1.8 : 0.2 6 : 1.69 : 0.29 6 : 1.6 : 0.4 6 : 1.61 : 0.38 6 : 1.2 : 0.8 6 : 1.22 : 0.73
3.2. Characterization of Calcined Samples The dried samples are calcined at 500°C during 4h under a dried air flow. The elementary analysis reported in Table 1 shows that the Co:Al:Ce ratios are respected for all the samples. To investigate the structure of the samples, X-Rays Diffraction analyses are performed. The diffractograms are shown in Fig. 1 (b). Several crystalline phases are observed. The first corresponds to three spinels phases: cobalt – cobalt Co3O4 (ICDD-JCPDS files 42-1467) or cobalt - aluminium: CoAl2O4 (ICDD-JCPDS files 44-0160) and Co2AlO4 (ICDD-JCPDS files 38-0814). These phases cannot be distinguished because their characteristic diffraction peaks are close in intensity and position [6, 11]. The Co-Al mixed oxide leads to the normal spinel, i.e., CoAl 2O4 corresponding to the Co2+ in the tetrahedral positions and the Al3+ in the octahedral position. A fourth phase is observable for solids containing cerium: the cerianite CeO 2 (ICDDJCPDS files 34-0394). The isotherms presented in Fig. 2 (a) for Co6Al2-xCexHT500 solids correspond to type IV for mesoporous materials. The average pore diameters of each sample as well as their specific surface area are shown in Table 2. The average pore diameters of all samples are between 3 and 8 nm, no significant modification in the solid porosity are observed. Concerning the surface area measurement, a small increase is observed for solids containing a small amount of cerium (x İ 0.3 Co6Al2-xCexHT500). When the proportion of cerium increases, this trend is reversed, with a significant decrease in the specific surface area. This decrease can be explained by a higher crystallisation of
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the cerianite phase. After performing the structural analysis of various solids, a study of the redox properties of the solid is undertaken by H2-RTP.
Co6Al1.2Ce0.8HT500
Co6Al1.6Ce0.4HT500 TCD Signal (a.u.)
Volume (a.u.)
Co6Al1.2Ce0.8HT500
Co6Al1.7Ce0.3HT500 Co6Al1.8Ce0.2HT500
Co6Al1.6Ce0.4HT500 Co6Al1.7Ce0.3HT500 Co6Al1.8Ce0.2HT500
Co6Al1.9Ce0.1HT500 Co6Al1.9Ce0.1HT500 Co6Al2HT500
Co6Al2HT500 0,0
0,1
CeO2500
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
Pression relative (P/P0)
1,0
100
(a)
200
300
400
500
600
700
800
Temperature (°C)
(b)
Fig 2. (a) N2 adsorption and desorption isotherms and (b) H2-TPR profiles of the mixed oxides
The analysis by Temperature-Programmed Reduction (H2-TPR) was performed and reported on Fig. 2 (b). Regarding the mixed oxide Co6Al2HT500, two reduction zones are observed: one for less than 400 °C and another at higher temperature (T > 400 °C). The first zone (330°C) corresponds to the reduction of Co 3O4 species into metal Cobalt (Co0) while the second (693 °C) corresponds to the reduction of cobalt-aluminium spinel species (CoAl2O4 or Co2AlO4) cobalt metal (Co0) [11]. The solids containing cerium have also two reduction zones (T <400 °C and T> 400 °C). However, a gradual decrease in the temperature of these reductions is observed when the proportion of cerium increases in the solid. Furthermore, for both the solids containing more cerium (y = 0.4 to 0.8), a signal change on the two reduction zones is observed. The first step is composed of three stages of reduction corresponding to the reduction of Co3O4 into Co0 and species CeO2 surface species into Ce2O3. The second domain for these two solids has a modification of the signal with the presence of three peaks corresponding to reductions of the spinel phases (CoAl 2O4 and Co2AlO4) and ceria present in the catalyst bulk (bulk CeO 2). Hydrogen consumptions in the temperature range corresponding to the catalytic activity (T <400 °C) and for the overall H2-TPR analysis are reported in Table 2. For the solids Co6Al2-yCeyHT500, a slight increase in the total H2 consumption is observed when the cerium proportion increases. In addition, the values of the hydrogen consumption before 400 °C, show a slight increase which is proportional to the cerium amount, which corresponds to the reduction of Ce 4+ species present on the surface of the solid. The better reducibility is then observed in case of Co6Al1.2Ce0.8HT500 solid. Table 2. Summary of the physic-chemical properties for the mixed oxides and T50 for the toluene total oxidation Pore H2 Consumption (µmol.g-1) SBET Mixed oxides diameter -1 (m².g ) T < 400°C Total Temperature corresponding to the first reduction peak (nm)
T50 (°C)
Co6Al2HT500
123
7.5
2656
11715
335
287
Co6Al1.9Ce0.1HT500
130
4.9
2665
11775
322
282
Co6Al1.8Ce0.2HT500
126
6.2
2670
11865
302
274
Co6Al1.7Ce0.3HT500
128
6.7
2685
11937
286
269
Co6Al1.6Ce0.4HT500
118
4.5
2825
12013
224
261
Co6Al1.2Ce0.8HT500
108
3.7
2993
12393
195
248
The mixed oxides and ceria are tested as catalysts in the total oxidation of toluene (Fig. 3(a)). When the conversion is complete, H2O and CO2 are the only products observed. However at beginning of toluene conversion for all samples, few ppm of benzene are detected. Different reaction paths have been already recognized [12]. The T50 temperature (temperature for 50% toluene conversion) is chosen as a measure of catalytic activity of the mixed oxides (Table 3). The catalytic activity can be established by the following order: Co6Al1.2Ce0.8HT500 > Co6Al1.6Ce0.4HT500 > Co6Al1.7Ce0.3HT500 > Co6Al1.8Ce0.2HT500 > CeO2 > Co6Al1.9Ce0.1HT500 > Co6Al2HT500
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310
100 CeO2 90
70
300
Co6Al1.6Ce0.4HT500 290
Co6Al1.7Ce0.3HT500 Co6Al1.8Ce0.2HT500
280
Co6Al1.9Ce0.1HT500 60
Co6Al2HT500 270
T50 (°C)
Toluene conversion (%)
80
Co6Al1.2Ce0.8HT500
50 40
260
30
250
20
240
10
Ce proportion
230
0 100
150
200
250
Temperature (°C)
300
220
350
180
(a)
200
220
240
260
280
300
320
340
Temperature corresponding at the first reduction peak in H 2-TPR(°C)
(b)
Fig 3. (a) Conversion of toluene (%) on mixed oxides vs. reaction temperature (°C) and (b) relation between T 50 and the temperature corresponding at the first reduction peak in the H2-TPR (°C)
An increase in the catalytic activity for the toluene oxidation with the incorporation of cerium into the solids is observed. The most active solid contains the largest proportion of cerium. This correlation suggests an increase in the oxygen mobility in the solid with the cerium proportion. This hypothesis is correlated with the results obtained during the H2-TPR. Indeed, the solid showing a better reducibility at low temperature (<400 °C) also has the best activity against toluene oxidation. This relationship has been demonstrated by plotting the T 50 according to the temperature of the first reduction peak observed in H 2-TPR (Fig. 3 (b)). This linearity obtained between the reducibility and the conversion of toluene suggests a Mars Van Krevelen mechanism (redox mechanism). Moreover, the adding of cerium into the mixed oxide shows a better activity for toluene total oxidation than CeO2 and CoAl mixed oxide. A synergistic effect between ceria and Co-Al oxide is an important condition for having a cobalt cerium-based catalyst with a good activity at low temperature. Indeed, this result suggests that the cerium oxide allows better oxygen mobility on the solid surface, promoting the oxygen exchange with cobalt oxide. In fact, the cobalt oxide could be reduced during the toluene oxidation and re-oxidised with oxygen issued from the ceria. This synergistic effect between cerium and cobalt is a key factor for the good activity of these solids. 4. Conclusion CoAlCe hydrotalcite-like compounds are prepared by conventional co-precipitation. The several physico-chemical characterizations indicated that all solids Co6Al2-xCexHT prepared by the hydrotalcite route contain cerianite and hydrotalcite phases. The calcination at 500 °C resulted in the formation of mixture of spinel and cerianite phases. These mixed oxides are also tested in the toluene total oxidation and showed an increase of the reactivity when the content of cerium increases. Relationship between reducibility and reactivity of these solids is revealed and a synergetic effect between Co and Ce is evidenced. Thus, the Co6Al1.2Ce0.8 mixed oxide prepared via hydrotalcite route seems to be a promising catalyst for the VOC total oxidation. 5. Acknowledgements Authors want to acknowledge the ADEME as well as Nord-Pas-de-Calais region for the funding of the work. Authors also acknowledge the "Centre Commun de Mesure" of the University Littoral Côte d’Opale. References [1] L.F. Liotta, Appl. Catal. B Environ. 100 (2010) 403. [2] T. Barakat, J.C. Rooke, E. Genty, R. Cousin, S. Siffert, B.-L. Su, Energy Environ. Sci. 6 (2013) 371. [3] L.F. Liotta, H. Wu, G. Pantaleo, A.M. Venezia, Catal. Sci. Technol. 3 (2013) 3085. [4] G. Fan, F. Li, D.G. Evans, X. Duan, Chem. Soc. Rev. 43 (2014) 7040. [5] E. Genty, R. Cousin, S. Capelle, C. Gennequin, S. Siffert, Eur. J. Inorg. Chem. 2012 (2012) 2802. [6] A. Pérez, R. Molina, S. Moreno, Appl. Catal. A Gen. 477 (2014) 109. [7] R. Bechara, D. Balloy, D. Vanhove, Appl. Catal. A Gen. 207 (2001) 343. [8] D. Song, J. Li, J. Mol. Catal. A Chem. 247 (2006) 206. [9] J. Das, D. Das, K.M. Parida, J. Colloid Interface Sci. 301 (2006) 569. [10] O.R. Neto, N.F.P. Ribeiro, C.A.C. Perez, M. Schmal, M.M.V.M. Souza, Appl. Clay Sci. 48 (2010) 542. [11] E. Genty, J. Brunet, C. Poupin, S. Casale, S. Capelle, P. Massiani, S. Siffert, R. Cousin, Catalysts 5 (2015) 851. [12] S. Lars, T. Andersson, J Catal, 98 (1986) 138.