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Mesoporous manganese oxide catalysts for formaldehyde removal: influence of the cerium incorporation
10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.
Mesoporous manganese oxide catalysts for formaldehyde removal: influence of the cerium incorporation Jhon Quiroz -Torres, a,b,c Rémy Averlant, a,b,c Jean-Marc Giraudon,a,b,c Jean-François Lamoniera,b,c a
Univ Lille Nord de France, F-59000 Lille, France CNRS, UMR8181, France c USTL, Unité de Catalyse et de Chimie du Solide F-59652 Villeneuve d’Ascq, France b
Abstract Manganese oxide mesoporous materials were prepared by using template assisted method followed by an acidic treatment. The cerium addition to manganese oxide mesoporous structure was also studied. All the solids were characterized by XRD and specific surface area and pore size distributions were calculated from nitrogen sorption studies. XRD results suggested the formation of the MnOx-CeO2 solid solution with the fluorite-type structure. The BET surface area values and the pore size distributions allowed to conclude the important role of the surfactant by the creation of narrow mesopores in the material. Keywords: Manganese and cerium oxides, mesoporous materials, VOC
1. Introduction Formaldehyde (HCOH) is regarded as the major indoor air pollutant emitted from widely used building and decorative materials. Long-term exposure to indoor air containing even few ppb of HCOH may cause adverse effects on human health. Catalytic oxidation is one of the most promising technologies for controlling HCOH pollutant. For instance, noble metals supported catalysts have been reported to possess high activity for the complete oxidation of hundreds of ppm of HCOH into CO2 and H2O [C. Zhang and H. He]. However, the concentration of indoor HCOH emission is much lower (<1 ppm) and the corresponding catalytic treatment is energy consuming at this condition. Consequently, for this application it is crucial to develop a material combining high capacity adsorption and catalytic performances. Mixed-valent octahedral molecular sieves (OMS) of manganese oxides, which can have applications in energy storage, in acid catalysis and in ion-exchange processes, are extensively reported. The generation of mixed-valent manganese oxide mesoporous materials might lead to versatile system for oxidation catalysis [S.L. Suib]. Recently, high surface area manganese mesoporous material has been obtained through a surfactant-assisted wet-chemistry route [K. Sinha]. This mesoporous oxide material is able to eliminate VOCs at low temperature. With the possibility of multiple valencies for Mn species and the high efficient of redox couple (Ce4+/Ce3+) in oxidation reactions, the mixed MnOx-CeO2 samples are very interesting materials for further detailed investigations. In this work we present the strategies of incorporating cerium ions into the ordered phase of mesoporous manganese oxide catalysts using the surfactant assisted wet-chemistry route.
518
J.Q. Torres et al.
2. Experimental 2.1. Sample preparation A precipitate formed by mixing an aqueous solution of Mn(NO3)·6H2O (16.7 g in 150 mL of H2O) with an aqueous solution of NaOH (4.8 g NaOH in 50 mL of H2O) was added to an aqueous solution of cetyltrimethylammonium bromide (67 g in 150 mL of H2O). The resulting mixture was heated to 75 °C and then stirred for 1 h. The final gel obtained in a sealed beaker was transferred to an oven and heated for 12 h at 75 °C. The solid residue was filtered, washed with water, dried in air and finally calcined at 500°C for 6 h (1°C min -1). The calcined sample (6.0 g) was treated with an aqueous solution of H2SO4 (120 mL - 10 mol L-1) by stirring in a beaker for 1 h. The final product was filtered, and the residue was washed with water and dried at 105 °C. Pure manganese oxide has been also prepared without surfactant. Three binary oxides (n)MnOx- (1-n)CeO2 (n = 0.25, 0.50, 0.75) were prepared by co-precipitation from aqueous solution of cerium and manganese nitrates using the same procedure as below.
2.2. Characterization X-ray diffraction (XRD) measurements were made at room temperature (λ =1.5418 Å). The diffraction patterns have been indexed by comparison with the Joint Committee on Powder Diffraction Standards (JCPDS) files. Nitrogen adsorption/desorption isotherms were obtained at -77K. BET and BJH analyses were used to determine the total specific surface area, pore volume and pore size distribution of the samples.
3. Results and discussion 3.1. XRD analyses a)
(3)
b)
(3)
(2)
(2)
(1)
0
10
20
30
40 2θ (°)
50
60
70
80
(1)
0
10
20
30
40
50
60
70
80
2θ (°)
Fig. 1. XRD patterns of pure manganese oxides a) with surfactant b) without surfactant (1) as made, (2) calcined and (3) acid treated.
The wide angle powder XRD patterns of the solids after each step of the synthesis, (1) solid dried in air, (2) calcined and (3) treated with sulfuric acid are shown in Fig. 1. The brown solid obtained (dried in air) showed diffraction peaks which could be attributed to the crystalline phase Mn3O4 (Hausmanite). After calcination the solid turned black and showed diffraction peaks due to the hausmanite phase and also peaks of lower intensity ascribed to the Mn5O8 phase. After acidic treatment the peaks due to the hausmanite phase disappeared and only peaks due to the monoclinic phase Mn5O8 were observed. In brief, the samples underwent the following phase transitions: Mn3O4 (dried in air), Mn3O4 + Mn5O8 (calcined) and Mn5O8 (H2SO4 treated). During the calcination step, the Mn3+ of the Mn3O4 was partially oxidized into Mn4+ to give Mn5O8
Mesoporous manganese oxide catalyst for formaldehyde removal
519
which stabilizes Mn2+ species from the Mn3O4 phase [B. Gillot]. After the acidic treatment, the hausmanite phase was totally oxidized to generate exclusively the Mn5O8 monoclinic phase. The XRD patterns obtained after each synthesis step were the same for both preparations but a higher cristallinity of the pure manganese oxides was observed when surfactant was employed (Fig. 1a and 1b).
a)
b) n= 0 n= 0
n= 0.25
n= 0.25
n= 0.50
n= 0.50
n= 0.75
n= 0.75 46
n= 1 0
10
20
30
40
50
60
70
48
50
2θ (°)
80
2θ (°)
Fig. 2. Powder diffraction patterns of (n)MnOx-(1-n)-CeO2 after calcination at 500°C a) wide angle and b) expanded view of the (220) peaks.
The cerium incorporation to the MnOx was also studied by XRD analysis (Fig. 2). For samples with n values between 0 and 0.5 the main peaks in the patterns were those of a fluorite structure similar to that of pure ceria. But for samples with n = 0.25 and n = 0.50 the diffraction lines were broader. The diffraction profiles of samples with n≥0.75 showed the crystallization of Mn3O4 together with fluorite structure. Figure 2b (zoom of Fig. 2a) showed the incremental shift to higher angles of the (220) diffraction peak for the fluorite phase related to the formation of Ce-Mn-O solid solutions. The substitution of Ce4+ by Mn3+ in the fluorite structure seems to be possible when considering their structural similarity, the incremental shift of each diffraction line to higher angles is associated with the smallest ionic radius of Mn3+ (0.66 nm) in comparison with that of Ce4+ (0.94 nm) [M. Machida]. Considering the 2θ maximal value of the (220) diffraction peak (Fig. 2b), it seems that the solubility limit of manganese ions into the ceria structure is reached for the solid with n = 0.5.
3.2. Porous Properties Table 1. BET surface areas of (n)MnOx-(1-n)CeO2. n
Calcined samples (m2 g-1)
Acid treated samples (m2 g-1)
Surface area deviation (%)
1
20
82
76
0.75
91
160
43
0.50
84
98
8
0.25
130
137
5
1*
31
121
74
* Synthesized without surfactant
520
J.Q. Torres et al.
According to the highest full width at half maximum of the diffraction peaks of calcined sample with n = 0.25, the surface specific area (SSA) of the corresponding solid was the highest (Table 1). The decrease in the SSA value related to the n increase could be due to the manganese oxide formation having low BET surface area. The acidic treatment had important effect on the SSA of pure manganese oxides (increase of 75%). However, this effect was strongly attenuated with the increase in cerium concentration in the sample (Table 1). Figure 3 shows the pore size distribution of the different oxides before and after the acidic treatment. For the calcined samples the pore size distribution was broader and centered between 10 and 20 nm. After the acidic treatment and for n = 1 and n = 0.75, a narrow mesoporous distribution centered at 3 nm and 2 nm, respectively, was observed. Based on Sinha works [K. Sinha], this result could be explained by the formation of interwoven nanofibrous aggregates with wormhole-like mesoporosity, however the formation of these narrow mesopores took place in a minor proportion for n = 0.75. Finally for n = 0.50 and 0.25, no significant effect of the H2SO4 treatment was observed. In the binary oxides, the mechanism involved in the mesoporosity creation is clearly related to the manganese oxide presence. Moreover the interaction of manganese species with surfactant at the beginning of the synthesis is crucial to generate narrow mesopores. Indeed without surfactant, pure manganese oxide after acidic treatment didn’t reveal such mesoporosity. n= 1
10
100 Pore Radius (A)
10
100 Pore Radius (A)
n= 0.25
n= 0.50
n= 0.75
10
100 Pore Radius (A)
10
100 Pore Radius (A)
Figure 3. Pore size distribution of (n)MnOx-(1-n)-CeO2 calcined (circle symbol) and acid treated (cross symbol).
4. Conclusion We have successfully synthesized pure and mixed manganese and cerium oxides using the surfactant assisted wet-chemistry route followed by acidic treatment. The XRD analyses of calcined samples reveal the substitution of Ce4+ by Mn species in the fluorite structure forming a solid solution with a solubility limit of 50% Mn. For all the solids, the acidic treatment has important effect in the BET surface area. For the binary oxides a narrow pore size distribution was created after H2SO4 treatment, with a major extension for solids having high Mn content.
References H. He, 2005, Perfect catalytic oxidation of formaldehyde over a Pt/TiO2 catalyst at room
temperature, 6, 211-214. S.L. Suib, 1997, Manganese Oxide Mesoporous Structures: Mixed-Valent Semiconducting Catalyst, Science, 276, 926-930. A.K. Sinha, 2008, Preparation and Characterization of Mesostructured δ-Manganese Oxide and Its Application to VOCs Elimination, J. Phys. Chem. C, 112, 16028-16035. B. Gillot, 2001, Particle size effects on the oxidation-reduction behavior of Mn3O4 hausmannite, Materials Chemistry and Physics, 70, 54-60. M. Machida, 2000, MnOx-CeO2 Binary Oxides for Catalytic NOx Sorption at Low Temperatures. Sorptive Removal of NOx, 12, 3158-3164.
Mesoporous manganese oxide catalysts for formaldehyde removal: influence of the cerium incorporation Jhon Quiroz -Torres, a,b,c Rémy Averlant, a,b,c Jean-Marc Giraudon,a,b,c Jean-François Lamoniera,b,c a
Univ Lille Nord de France, F-59000 Lille, France CNRS, UMR8181, France c USTL, Unité de Catalyse et de Chimie du Solide F-59652 Villeneuve d’Ascq, France b
Abstract Manganese oxide mesoporous materials were prepared by using template assisted method followed by an acidic treatment. The cerium addition to manganese oxide mesoporous structure was also studied. All the solids were characterized by XRD and specific surface area and pore size distributions were calculated from nitrogen sorption studies. XRD results suggested the formation of the MnOx-CeO2 solid solution with the fluorite-type structure. The BET surface area values and the pore size distributions allowed to conclude the important role of the surfactant by the creation of narrow mesopores in the material. Keywords: Manganese and cerium oxides, mesoporous materials, VOC
1. Introduction Formaldehyde (HCOH) is regarded as the major indoor air pollutant emitted from widely used building and decorative materials. Long-term exposure to indoor air containing even few ppb of HCOH may cause adverse effects on human health. Catalytic oxidation is one of the most promising technologies for controlling HCOH pollutant. For instance, noble metals supported catalysts have been reported to possess high activity for the complete oxidation of hundreds of ppm of HCOH into CO2 and H2O [C. Zhang and H. He]. However, the concentration of indoor HCOH emission is much lower (<1 ppm) and the corresponding catalytic treatment is energy consuming at this condition. Consequently, for this application it is crucial to develop a material combining high capacity adsorption and catalytic performances. Mixed-valent octahedral molecular sieves (OMS) of manganese oxides, which can have applications in energy storage, in acid catalysis and in ion-exchange processes, are extensively reported. The generation of mixed-valent manganese oxide mesoporous materials might lead to versatile system for oxidation catalysis [S.L. Suib]. Recently, high surface area manganese mesoporous material has been obtained through a surfactant-assisted wet-chemistry route [K. Sinha]. This mesoporous oxide material is able to eliminate VOCs at low temperature. With the possibility of multiple valencies for Mn species and the high efficient of redox couple (Ce4+/Ce3+) in oxidation reactions, the mixed MnOx-CeO2 samples are very interesting materials for further detailed investigations. In this work we present the strategies of incorporating cerium ions into the ordered phase of mesoporous manganese oxide catalysts using the surfactant assisted wet-chemistry route.
518
J.Q. Torres et al.
2. Experimental 2.1. Sample preparation A precipitate formed by mixing an aqueous solution of Mn(NO3)·6H2O (16.7 g in 150 mL of H2O) with an aqueous solution of NaOH (4.8 g NaOH in 50 mL of H2O) was added to an aqueous solution of cetyltrimethylammonium bromide (67 g in 150 mL of H2O). The resulting mixture was heated to 75 °C and then stirred for 1 h. The final gel obtained in a sealed beaker was transferred to an oven and heated for 12 h at 75 °C. The solid residue was filtered, washed with water, dried in air and finally calcined at 500°C for 6 h (1°C min -1). The calcined sample (6.0 g) was treated with an aqueous solution of H2SO4 (120 mL - 10 mol L-1) by stirring in a beaker for 1 h. The final product was filtered, and the residue was washed with water and dried at 105 °C. Pure manganese oxide has been also prepared without surfactant. Three binary oxides (n)MnOx- (1-n)CeO2 (n = 0.25, 0.50, 0.75) were prepared by co-precipitation from aqueous solution of cerium and manganese nitrates using the same procedure as below.
2.2. Characterization X-ray diffraction (XRD) measurements were made at room temperature (λ =1.5418 Å). The diffraction patterns have been indexed by comparison with the Joint Committee on Powder Diffraction Standards (JCPDS) files. Nitrogen adsorption/desorption isotherms were obtained at -77K. BET and BJH analyses were used to determine the total specific surface area, pore volume and pore size distribution of the samples.
3. Results and discussion 3.1. XRD analyses a)
(3)
b)
(3)
(2)
(2)
(1)
0
10
20
30
40 2θ (°)
50
60
70
80
(1)
0
10
20
30
40
50
60
70
80
2θ (°)
Fig. 1. XRD patterns of pure manganese oxides a) with surfactant b) without surfactant (1) as made, (2) calcined and (3) acid treated.
The wide angle powder XRD patterns of the solids after each step of the synthesis, (1) solid dried in air, (2) calcined and (3) treated with sulfuric acid are shown in Fig. 1. The brown solid obtained (dried in air) showed diffraction peaks which could be attributed to the crystalline phase Mn3O4 (Hausmanite). After calcination the solid turned black and showed diffraction peaks due to the hausmanite phase and also peaks of lower intensity ascribed to the Mn5O8 phase. After acidic treatment the peaks due to the hausmanite phase disappeared and only peaks due to the monoclinic phase Mn5O8 were observed. In brief, the samples underwent the following phase transitions: Mn3O4 (dried in air), Mn3O4 + Mn5O8 (calcined) and Mn5O8 (H2SO4 treated). During the calcination step, the Mn3+ of the Mn3O4 was partially oxidized into Mn4+ to give Mn5O8
Mesoporous manganese oxide catalyst for formaldehyde removal
519
which stabilizes Mn2+ species from the Mn3O4 phase [B. Gillot]. After the acidic treatment, the hausmanite phase was totally oxidized to generate exclusively the Mn5O8 monoclinic phase. The XRD patterns obtained after each synthesis step were the same for both preparations but a higher cristallinity of the pure manganese oxides was observed when surfactant was employed (Fig. 1a and 1b).
a)
b) n= 0 n= 0
n= 0.25
n= 0.25
n= 0.50
n= 0.50
n= 0.75
n= 0.75 46
n= 1 0
10
20
30
40
50
60
70
48
50
2θ (°)
80
2θ (°)
Fig. 2. Powder diffraction patterns of (n)MnOx-(1-n)-CeO2 after calcination at 500°C a) wide angle and b) expanded view of the (220) peaks.
The cerium incorporation to the MnOx was also studied by XRD analysis (Fig. 2). For samples with n values between 0 and 0.5 the main peaks in the patterns were those of a fluorite structure similar to that of pure ceria. But for samples with n = 0.25 and n = 0.50 the diffraction lines were broader. The diffraction profiles of samples with n≥0.75 showed the crystallization of Mn3O4 together with fluorite structure. Figure 2b (zoom of Fig. 2a) showed the incremental shift to higher angles of the (220) diffraction peak for the fluorite phase related to the formation of Ce-Mn-O solid solutions. The substitution of Ce4+ by Mn3+ in the fluorite structure seems to be possible when considering their structural similarity, the incremental shift of each diffraction line to higher angles is associated with the smallest ionic radius of Mn3+ (0.66 nm) in comparison with that of Ce4+ (0.94 nm) [M. Machida]. Considering the 2θ maximal value of the (220) diffraction peak (Fig. 2b), it seems that the solubility limit of manganese ions into the ceria structure is reached for the solid with n = 0.5.
3.2. Porous Properties Table 1. BET surface areas of (n)MnOx-(1-n)CeO2. n
Calcined samples (m2 g-1)
Acid treated samples (m2 g-1)
Surface area deviation (%)
1
20
82
76
0.75
91
160
43
0.50
84
98
8
0.25
130
137
5
1*
31
121
74
* Synthesized without surfactant
520
J.Q. Torres et al.
According to the highest full width at half maximum of the diffraction peaks of calcined sample with n = 0.25, the surface specific area (SSA) of the corresponding solid was the highest (Table 1). The decrease in the SSA value related to the n increase could be due to the manganese oxide formation having low BET surface area. The acidic treatment had important effect on the SSA of pure manganese oxides (increase of 75%). However, this effect was strongly attenuated with the increase in cerium concentration in the sample (Table 1). Figure 3 shows the pore size distribution of the different oxides before and after the acidic treatment. For the calcined samples the pore size distribution was broader and centered between 10 and 20 nm. After the acidic treatment and for n = 1 and n = 0.75, a narrow mesoporous distribution centered at 3 nm and 2 nm, respectively, was observed. Based on Sinha works [K. Sinha], this result could be explained by the formation of interwoven nanofibrous aggregates with wormhole-like mesoporosity, however the formation of these narrow mesopores took place in a minor proportion for n = 0.75. Finally for n = 0.50 and 0.25, no significant effect of the H2SO4 treatment was observed. In the binary oxides, the mechanism involved in the mesoporosity creation is clearly related to the manganese oxide presence. Moreover the interaction of manganese species with surfactant at the beginning of the synthesis is crucial to generate narrow mesopores. Indeed without surfactant, pure manganese oxide after acidic treatment didn’t reveal such mesoporosity. n= 1
10
100 Pore Radius (A)
10
100 Pore Radius (A)
n= 0.25
n= 0.50
n= 0.75
10
100 Pore Radius (A)
10
100 Pore Radius (A)
Figure 3. Pore size distribution of (n)MnOx-(1-n)-CeO2 calcined (circle symbol) and acid treated (cross symbol).
4. Conclusion We have successfully synthesized pure and mixed manganese and cerium oxides using the surfactant assisted wet-chemistry route followed by acidic treatment. The XRD analyses of calcined samples reveal the substitution of Ce4+ by Mn species in the fluorite structure forming a solid solution with a solubility limit of 50% Mn. For all the solids, the acidic treatment has important effect in the BET surface area. For the binary oxides a narrow pore size distribution was created after H2SO4 treatment, with a major extension for solids having high Mn content.
References H. He, 2005, Perfect catalytic oxidation of formaldehyde over a Pt/TiO2 catalyst at room
temperature, 6, 211-214. S.L. Suib, 1997, Manganese Oxide Mesoporous Structures: Mixed-Valent Semiconducting Catalyst, Science, 276, 926-930. A.K. Sinha, 2008, Preparation and Characterization of Mesostructured δ-Manganese Oxide and Its Application to VOCs Elimination, J. Phys. Chem. C, 112, 16028-16035. B. Gillot, 2001, Particle size effects on the oxidation-reduction behavior of Mn3O4 hausmannite, Materials Chemistry and Physics, 70, 54-60. M. Machida, 2000, MnOx-CeO2 Binary Oxides for Catalytic NOx Sorption at Low Temperatures. Sorptive Removal of NOx, 12, 3158-3164.