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Transition metal oxides (TMOs) supported on ordered mesoporous Ce0.1Mn0.9Oδ as high-efficient catalysts for toluene combustion
Materials Letters 263 (2020) 127230
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Transition metal oxides (TMOs) supported on ordered mesoporous Ce0.1Mn0.9Od as high-efficient catalysts for toluene combustion Jie Meng, Nengjie Feng, Fan Fang, Hui Wan ⇑, Guofeng Guan ⇑ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 8 September 2019 Received in revised form 2 November 2019 Accepted 21 December 2019 Available online 23 December 2019 Keywords: Toluene Catalytic combustion KIT-6 Ce0.1Mn0.9Od CuOx
a b s t r a c t Series of ordered mesoporous CexMn1-xOd (x = 0, 0.1, 0.3, 0.5) were successfully prepared with a hard template method using KIT-6. Among the mesoporous CexMn1-xOd catalysts, the rutile b-Ce0.1Mn0.9Od supports exhibited excellent catalytic activity for toluene oxidation. On basis of these, the effects of MOx (M = Co, Cu, Fe) on Ce0.1Mn0.9Od catalysts were investigated in the catalytic combustion of toluene. Among the as-prepared catalysts, the CuOx/Ce0.1Mn0.9Od exhibits the best performance of toluene catalytic combustion with the T90% of 254 °C and good stability. Ó 2019 Published by Elsevier B.V.
1. Introduction Volatile organic compounds (VOCs) may be the chemical products of a variety of human activities, which may result in serious photochemical smogs and causing considerable amount of damage on the environment and human health [1], such as toluene. In recent years, the catalytic combustion of VOCs has been regarded as one of the most economical and effective method, and the catalysts play an important role in the ultimate catalytic performance for toluene removal. Considering the abundant sources, relatively low price and robustness, the non-precious metal catalyst has attracted great interest in toluene catalytic combustion. For example, over the past few decades, manganese oxide (MnOx) based catalysts have been found to be a suitbale candidate for volatile organic compounds (VOCs) [2]. To further improve the catalytic activity, appropriate amount of ceria oxide is taken into consideration to dope in MnOx for accelerating the oxygen release rate, considering its high oxygen-storage capability [3]. The method is based on the fact that stable Ce3+ and Ce4+ can freely shift between CeO2 and CeO2–x [4]. In addition, the structure of catalytic materials also has a great influence on the catalytic activity. Among the materials with wellfined pore structure, ordered mesoporous materials have been
⇑ Corresponding authors. E-mail addresses: [email protected] (H. Wan), [email protected] (G. Guan). https://doi.org/10.1016/j.matlet.2019.127230 0167-577X/Ó 2019 Published by Elsevier B.V.
assumed as suitable catalysts because mesopores provide large specific surface area, which can contribute to enlaring the exposure of active sites. Besides, MnOx composited with other transition metal oxides (such as NiOx, FeOx, CoOx) exhibited higher catalytic activity for combustion reactions, which can be ascribed to the capability of manganese to form oxides with different oxidation states as well as its high oxygen storage capacity [5]. As a result, certain transition metal oxides can be loaded on the ordered mesoporous CexMn1-xOd catalysts to improve the intrinsic activity and make full use of the superior structure. In this work, a series of ordered mesoporous CexMn1-xOd(x = 0, 0.1, 0.3, 0.5) were successfully prepared with a hard template method using KIT-6. Then, the transition metal oxides (CoOx, CuOx, FeOx) loading Ce0.1Mn0.9Od catalysts were synthesized to investigate the interaction effect of TMOs-Ce0.1Mn0.9Od. The catalytic performance of as-prepared catalysts for toluene removal was evaluated by a gas chromatography instrument using a fixed bed.
2. Experimental section 2.1. Preparation of ordered mesoporous CexMn1-xOd (x = 0, 0.1, 0.3, 0.5) catalysts and transition MOx (M = Co, Cu, Fe) modified Ce0.1Mn0.9Od catalysts Meso-CexMn1-xOd was prepared as follows: ceria nitrate and manganese nitrate were dissolved into the ethanol after stirring at room temperature. The mesoporous silica template (KIT-6)
2
J. Meng et al. / Materials Letters 263 (2020) 127230
was added after the corresponding nitrates were completely dissolved. The mixture was dried and calcined at 400 °C for 5 h, then treated with 2 M hot NaOH solution to remove the silica template, followed by water and ethanol washing. Subsequently, the transition metal (Co, Cu, Fe) nitrate solution was added in the above solution. The mixture was stirred at 80 °C for 4 h. Then, the obtained sample was heated at 400 °C for 4 h to get the final catalyst.
silica sand to dissipate heat to prevent overheating. The total flow rate was kept at 65 mLmin 1, which was corresponding to a gas hourly space velocity (GHSV) that was 39,000 mL/gh 1. The feed gas mixture contained 2 g/m3 toluene.
2.2. Catalyst characterizations
Fig. 1 shows the XRD patterns of MOx/CexMn1-xOd (M = Cu, Co, Fe) catalysts. From Fig. 1a, it can be seen that all the catalysts show significant diffraction peaks at 28.7, 37.3, 42.8, 56.7, 59.4, 64.8 and 72.3°, which is corresponding to the rutile structure of b-MnO2 (JCPDS NO. 24-0735). Some obvious diffraction peaks at 28.5, 33.0, 47.5 and 56.4° appear, are in accordance with the cubic fluorite structure of CeO2 (34-0394) [6]. This phenomenon indicates that the crystal structure of MnOx is significantly changed after a certain amount of Ce atoms are incorporated. Meanwhile, the phenomenon that the diffraction peaks of MOx (M = Cu, Co, Fe) are not apparent may be due to the low loading on the catalyst surface of Ce0.1Mn0.9Od. From the small-angle XRD pattern (Fig. 1b), the diffraction peak of the (2 1 1) crystal surface is around 1°, indicating that the ordered mesoporous cubic symmetric structure (ia3d) of CexMn1-xOd exist with KIT-6 as a hard template. The N2 adsorption–desorption and pore size distribution curves of the catalysts are shown in Fig. 1c. All N2 adsorption–desorption curves of the catalyst samples are typical type IV isotherms with H2-type hysteresis loops, attributed to the reservation of the ordered mesoporous cubic symmetric structure (ia3d) of KIT-6. Moreover, after the loading of transition metal oxides, the pore size distribution is at 3.3–3.7 nm, which is slightly lower than pure
XRD patterns of the samples were recorded on a Bruker Axsd8 Advanlex automated powder X-ray diffraction meter using Cu Ka (k = 0.1541 nm) radiation. TEM experiments were measured on a JEM-2100 high-resolution transmission electron microscope operating at an accelerating voltage of 200 kV. The specific surface area and the mean pore diameter of the catalysts were determined by nitrogen adsorption in accordance with a Micromeritics ASAP 2020 physical adsorption-type instrument at 196 °C. The Brunauer-Emmett-Teller (BET) method was applied to calculate the specific surface areas of the samples. H2-temperatureprogrammed reduction (H2-TPR) measurements were carried out on a Micromeritics AutoChem II2920 automatic chemical adsorption analyzer. 2.3. Measurements of catalytic performance Catalytic activity tests were performed in a continuous flow fixed-bed reactor. A glass tube with an inner diameter of 6 mm was chosen as the reactor tube. About 100 mg catalyst with the average diameter of 40–60 mesh was placed into the tube, using
3. Results and discussion 3.1. Catalyst characterization
Fig. 1. (a and b) XRD patterns of MOX/Ce0.1 Mn0.9 Od (M=Cu, Co, Fe); (c) Pore size distribution and N2 adsorption-desorption isotherms (inset of c) of MOx/Ce0.1 Mn0.9 Od (M=Cu, Co, Fe).
3
J. Meng et al. / Materials Letters 263 (2020) 127230
Fig. 2. TEM images of MOx/Ce0.1Mn0.9Od (M = Cu, Co, Fe): (A1,A2)-Cu/ Ce0.1Mn0.9Od; (B1,B2)–Co/Ce0.1Mn0.9Od; (C1,C2)-Fe/Ce0.1Mn0.9Od.
1.0
b
a
1.0
0.8
Toluene Co nversion
Ce0.1Mn0.9O Ce0.3Mn0.7O
0.6
Ce0.5Mn0.5O CuOx/Ce0.1Mn0.9O
0.4
CoOx/Ce0.1Mn0.9O FeOx/Ce0.1Mn0.9O
0.2 0.0
Toluene Conversion
Ce0Mn1O
0.8
0.6
0.4
0.2
CuOx/Ce0.1Mn0.9O
None 0.0
50
100
150
200
250
300 350 o Temperature ( C)
400
450
0
10
20
30
40
Time (h)
50
60
70
80
Fig. 3. The activity of CexMn1-xOd and MOx/Ce0.1Mn0.9Od (M = Cu, Co, Fe) (a) and the stability of CuOx/Ce0.1Mn0.9Od (b).
Table 1 The T50 and T90 of CexMn1-xOd and MOx/Ce0.1Mn0.9Od. Catalysts
Ce0Mn1Od Ce0.3Mn0.7Od Ce0.5Mn0.5Od Ce0.1Mn0.9Od CuOx/Ce0.1Mn0.9Od CoOx/Ce0.1Mn0.9Od FeOx/Ce0.1Mn0.9Od
Catalytic activity T50 (oC)
T90 (oC)
191 184 260 137 130 194 220
310 298 330 275 254 297 314
Ce0.1Mn0.9Od. This indicates that only a little quantity of such oxides enter the internal structure of catalyst, resulting in a slight change in pore size and specific surface area. H2-TPR measurements were carried out to investigate the redox properties and phase compositions of MOx/Ce0.1Mn0.9Od (M = Cu, Fe, Co) in Fig. 1d. As it is shown, MnOx shows two reduction peaks (a and b) at 262 and 376 °C, corresponding to the reduction peaks of Mn4+?Mn3+ and Mn3+?Mn2+ [7]. Obviously, with the addition of the MOx (M = Co, Fe) component, the reduction temperature was higher than that of Ce0.1Mn0.9Od. When CuOx is loaded, the intensity of the reduction peak of the CuOx/Ce0.1Mn0.9Od decreased
4
J. Meng et al. / Materials Letters 263 (2020) 127230
and the reduction temperature was significantly lower than that of Ce0.1Mn0.9Od. This may be explained by the fact that CuOx nanoparticle on the surface of Ce0.1Mn0.9Od helps to increase the electron transfer rate, which can improve the catalytic activity [8]. TEM images of MOx/Ce0.1Mn0.9Od (M = Cu, Fe, Co) are respectively shown in Fig. 2. It can be seen from the Fig. 2 that all the samples possess regular and well-ordered mesoporous structure. Meanwhile, from Fig. 2, it can be clearly seen that there are almost no oxide agglomeration indicating the relatively good dispersion of MOx (M = Cu, Fe, Co) on Ce0.1Mn0.9Od surface.
successfully prepared in nano-replication method with KIT-6 as a hard template. The additions of Cu, Fe and Co oxides on Ce0.1Mn0.9Od also play a key role on the catalytic activity of toluene removal. Among them, CuOx/Ce0.1Mn0.9Od exhibits the best toluene catalytic efficiency when T90% is at 254 °C and the gas hourly space velocity is at 39,000 mL/g∙h. Moreover, a good stability is also observed in the results which may be attributed to the surface CuOx species that enhances the electron transfer on the catalyst surface, leading to the increase of structural defects and oxygen vacancies. Declaration of Competing Interest
3.2. Catalytic activity test The toluene catalytic oxidation activity of meso-CexMn1-xOd and MOx/Ce0.1Mn0.9Od (M = Cu, Fe, Co) are shown in Fig. 3a and Table 1. A partial introduction of Ceria can provide more oxygen vacancies and free oxygen for the catalyst, accelerating the redox cycle of the catalyst during the reaction, and effectively improving the catalytic activity. When the molar ratio of Ce and Mn is 1:9, the catalytic activity is optimized. T50 and T90 are respectively at the temperatures of 137 and 275 °C, with a gas hourly space velocity (GHSV) at 39000 mL/gh. Meanwhile, the introduction of CuOx can further increase the activity of the catalyst, T90 dropped further to 254 °C from 275 °C. This can be explained that the electron transfer property of CuOx is better than that of CoOx and FeOx for the reaction of toluene catalytic combustion [8]. Combined with H2-TPR, the introduction of CuOx can also improve the reduction performance of the catalyst and accelerate the redox cycle of the catalyst during the reaction. For a heterogeneous-catalyzed reaction, the catalyst durability and recycling play a key role in the practical application. The reaction stability for toluene oxidation with the time over CuOx/Ce0.1Mn0.9Od catalyst was also investigated and the result was shown in Fig. 3b. The CuOx/Ce0.1Mn0.9Od catalyst exhibited excellent stability for toluene oxidation in 72 h with the conversion of toluene over 92%. 4. Conclusion Ordered mesoporous CexMn1-xOd (x = 0, 0.1, 0.3, 0.5) and transition MOx (M = Co, Cu, Fe) modified Ce0.1Mn0.9Od catalysts were
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work was supported by the National Key Research and Development Program of China (2016YFC0204301), the Key Research and Development Plan of Jiangsu Province (BE2019118), the Jiangsu Postdoctoral Foundation (No. 1701014A), and the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (ZK201712). References [1] R. Malik, V.K. Tomer, T. Dankwort, Y.K. Mishra, L. Kienle, J. Mater. Chem. A 6 (2018) 10718–10730. [2] S. Behar, N.-A. Gómez-Mendoza, M.Á. Gómez-García, D. S´wierczyn´ski, F. Quignard, N. Tanchoux, Appl. Catal. A: Gen. 504 (2015) 203–210. [3] J. Qian, Y. Xue, Y. Ao, P. Wang, C. Wang, Chinese J. Catal. 39 (2018) 682–692. [4] N. Feng, J. Meng, Y. Wu, C. Chen, L. Wang, L. Gao, H. Wan, G. Guan, Catal. Sci. Technol. 6 (2016) 2930–2941. [5] C. Zhang, J. Wang, S. Yang, H. Liang, Y. Men, J. Colloid Interf. Sci. 539 (2019) 65– 75. [6] N. Feng, C. Chen, J. Meng, G. Liu, F. Fang, L. Wang, H. Wan, G. Guan, Appl. Surf. Sci. 399 (2017) 114–122. [7] T. Lin, L. Yu, M. Sun, G. Cheng, B. Lan, Z. Fu, Chem. Eng. J. 286 (2016) 114–121. [8] S. Hosseini, A. Niaei, D. Salari, M.C. Alvarez-Galvan, J. Fierro, Ceram. Inter. 40 (2014) 6157–6163.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Transition metal oxides (TMOs) supported on ordered mesoporous Ce0.1Mn0.9Od as high-efficient catalysts for toluene combustion Jie Meng, Nengjie Feng, Fan Fang, Hui Wan ⇑, Guofeng Guan ⇑ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 8 September 2019 Received in revised form 2 November 2019 Accepted 21 December 2019 Available online 23 December 2019 Keywords: Toluene Catalytic combustion KIT-6 Ce0.1Mn0.9Od CuOx
a b s t r a c t Series of ordered mesoporous CexMn1-xOd (x = 0, 0.1, 0.3, 0.5) were successfully prepared with a hard template method using KIT-6. Among the mesoporous CexMn1-xOd catalysts, the rutile b-Ce0.1Mn0.9Od supports exhibited excellent catalytic activity for toluene oxidation. On basis of these, the effects of MOx (M = Co, Cu, Fe) on Ce0.1Mn0.9Od catalysts were investigated in the catalytic combustion of toluene. Among the as-prepared catalysts, the CuOx/Ce0.1Mn0.9Od exhibits the best performance of toluene catalytic combustion with the T90% of 254 °C and good stability. Ó 2019 Published by Elsevier B.V.
1. Introduction Volatile organic compounds (VOCs) may be the chemical products of a variety of human activities, which may result in serious photochemical smogs and causing considerable amount of damage on the environment and human health [1], such as toluene. In recent years, the catalytic combustion of VOCs has been regarded as one of the most economical and effective method, and the catalysts play an important role in the ultimate catalytic performance for toluene removal. Considering the abundant sources, relatively low price and robustness, the non-precious metal catalyst has attracted great interest in toluene catalytic combustion. For example, over the past few decades, manganese oxide (MnOx) based catalysts have been found to be a suitbale candidate for volatile organic compounds (VOCs) [2]. To further improve the catalytic activity, appropriate amount of ceria oxide is taken into consideration to dope in MnOx for accelerating the oxygen release rate, considering its high oxygen-storage capability [3]. The method is based on the fact that stable Ce3+ and Ce4+ can freely shift between CeO2 and CeO2–x [4]. In addition, the structure of catalytic materials also has a great influence on the catalytic activity. Among the materials with wellfined pore structure, ordered mesoporous materials have been
⇑ Corresponding authors. E-mail addresses: [email protected] (H. Wan), [email protected] (G. Guan). https://doi.org/10.1016/j.matlet.2019.127230 0167-577X/Ó 2019 Published by Elsevier B.V.
assumed as suitable catalysts because mesopores provide large specific surface area, which can contribute to enlaring the exposure of active sites. Besides, MnOx composited with other transition metal oxides (such as NiOx, FeOx, CoOx) exhibited higher catalytic activity for combustion reactions, which can be ascribed to the capability of manganese to form oxides with different oxidation states as well as its high oxygen storage capacity [5]. As a result, certain transition metal oxides can be loaded on the ordered mesoporous CexMn1-xOd catalysts to improve the intrinsic activity and make full use of the superior structure. In this work, a series of ordered mesoporous CexMn1-xOd(x = 0, 0.1, 0.3, 0.5) were successfully prepared with a hard template method using KIT-6. Then, the transition metal oxides (CoOx, CuOx, FeOx) loading Ce0.1Mn0.9Od catalysts were synthesized to investigate the interaction effect of TMOs-Ce0.1Mn0.9Od. The catalytic performance of as-prepared catalysts for toluene removal was evaluated by a gas chromatography instrument using a fixed bed.
2. Experimental section 2.1. Preparation of ordered mesoporous CexMn1-xOd (x = 0, 0.1, 0.3, 0.5) catalysts and transition MOx (M = Co, Cu, Fe) modified Ce0.1Mn0.9Od catalysts Meso-CexMn1-xOd was prepared as follows: ceria nitrate and manganese nitrate were dissolved into the ethanol after stirring at room temperature. The mesoporous silica template (KIT-6)
2
J. Meng et al. / Materials Letters 263 (2020) 127230
was added after the corresponding nitrates were completely dissolved. The mixture was dried and calcined at 400 °C for 5 h, then treated with 2 M hot NaOH solution to remove the silica template, followed by water and ethanol washing. Subsequently, the transition metal (Co, Cu, Fe) nitrate solution was added in the above solution. The mixture was stirred at 80 °C for 4 h. Then, the obtained sample was heated at 400 °C for 4 h to get the final catalyst.
silica sand to dissipate heat to prevent overheating. The total flow rate was kept at 65 mLmin 1, which was corresponding to a gas hourly space velocity (GHSV) that was 39,000 mL/gh 1. The feed gas mixture contained 2 g/m3 toluene.
2.2. Catalyst characterizations
Fig. 1 shows the XRD patterns of MOx/CexMn1-xOd (M = Cu, Co, Fe) catalysts. From Fig. 1a, it can be seen that all the catalysts show significant diffraction peaks at 28.7, 37.3, 42.8, 56.7, 59.4, 64.8 and 72.3°, which is corresponding to the rutile structure of b-MnO2 (JCPDS NO. 24-0735). Some obvious diffraction peaks at 28.5, 33.0, 47.5 and 56.4° appear, are in accordance with the cubic fluorite structure of CeO2 (34-0394) [6]. This phenomenon indicates that the crystal structure of MnOx is significantly changed after a certain amount of Ce atoms are incorporated. Meanwhile, the phenomenon that the diffraction peaks of MOx (M = Cu, Co, Fe) are not apparent may be due to the low loading on the catalyst surface of Ce0.1Mn0.9Od. From the small-angle XRD pattern (Fig. 1b), the diffraction peak of the (2 1 1) crystal surface is around 1°, indicating that the ordered mesoporous cubic symmetric structure (ia3d) of CexMn1-xOd exist with KIT-6 as a hard template. The N2 adsorption–desorption and pore size distribution curves of the catalysts are shown in Fig. 1c. All N2 adsorption–desorption curves of the catalyst samples are typical type IV isotherms with H2-type hysteresis loops, attributed to the reservation of the ordered mesoporous cubic symmetric structure (ia3d) of KIT-6. Moreover, after the loading of transition metal oxides, the pore size distribution is at 3.3–3.7 nm, which is slightly lower than pure
XRD patterns of the samples were recorded on a Bruker Axsd8 Advanlex automated powder X-ray diffraction meter using Cu Ka (k = 0.1541 nm) radiation. TEM experiments were measured on a JEM-2100 high-resolution transmission electron microscope operating at an accelerating voltage of 200 kV. The specific surface area and the mean pore diameter of the catalysts were determined by nitrogen adsorption in accordance with a Micromeritics ASAP 2020 physical adsorption-type instrument at 196 °C. The Brunauer-Emmett-Teller (BET) method was applied to calculate the specific surface areas of the samples. H2-temperatureprogrammed reduction (H2-TPR) measurements were carried out on a Micromeritics AutoChem II2920 automatic chemical adsorption analyzer. 2.3. Measurements of catalytic performance Catalytic activity tests were performed in a continuous flow fixed-bed reactor. A glass tube with an inner diameter of 6 mm was chosen as the reactor tube. About 100 mg catalyst with the average diameter of 40–60 mesh was placed into the tube, using
3. Results and discussion 3.1. Catalyst characterization
Fig. 1. (a and b) XRD patterns of MOX/Ce0.1 Mn0.9 Od (M=Cu, Co, Fe); (c) Pore size distribution and N2 adsorption-desorption isotherms (inset of c) of MOx/Ce0.1 Mn0.9 Od (M=Cu, Co, Fe).
3
J. Meng et al. / Materials Letters 263 (2020) 127230
Fig. 2. TEM images of MOx/Ce0.1Mn0.9Od (M = Cu, Co, Fe): (A1,A2)-Cu/ Ce0.1Mn0.9Od; (B1,B2)–Co/Ce0.1Mn0.9Od; (C1,C2)-Fe/Ce0.1Mn0.9Od.
1.0
b
a
1.0
0.8
Toluene Co nversion
Ce0.1Mn0.9O Ce0.3Mn0.7O
0.6
Ce0.5Mn0.5O CuOx/Ce0.1Mn0.9O
0.4
CoOx/Ce0.1Mn0.9O FeOx/Ce0.1Mn0.9O
0.2 0.0
Toluene Conversion
Ce0Mn1O
0.8
0.6
0.4
0.2
CuOx/Ce0.1Mn0.9O
None 0.0
50
100
150
200
250
300 350 o Temperature ( C)
400
450
0
10
20
30
40
Time (h)
50
60
70
80
Fig. 3. The activity of CexMn1-xOd and MOx/Ce0.1Mn0.9Od (M = Cu, Co, Fe) (a) and the stability of CuOx/Ce0.1Mn0.9Od (b).
Table 1 The T50 and T90 of CexMn1-xOd and MOx/Ce0.1Mn0.9Od. Catalysts
Ce0Mn1Od Ce0.3Mn0.7Od Ce0.5Mn0.5Od Ce0.1Mn0.9Od CuOx/Ce0.1Mn0.9Od CoOx/Ce0.1Mn0.9Od FeOx/Ce0.1Mn0.9Od
Catalytic activity T50 (oC)
T90 (oC)
191 184 260 137 130 194 220
310 298 330 275 254 297 314
Ce0.1Mn0.9Od. This indicates that only a little quantity of such oxides enter the internal structure of catalyst, resulting in a slight change in pore size and specific surface area. H2-TPR measurements were carried out to investigate the redox properties and phase compositions of MOx/Ce0.1Mn0.9Od (M = Cu, Fe, Co) in Fig. 1d. As it is shown, MnOx shows two reduction peaks (a and b) at 262 and 376 °C, corresponding to the reduction peaks of Mn4+?Mn3+ and Mn3+?Mn2+ [7]. Obviously, with the addition of the MOx (M = Co, Fe) component, the reduction temperature was higher than that of Ce0.1Mn0.9Od. When CuOx is loaded, the intensity of the reduction peak of the CuOx/Ce0.1Mn0.9Od decreased
4
J. Meng et al. / Materials Letters 263 (2020) 127230
and the reduction temperature was significantly lower than that of Ce0.1Mn0.9Od. This may be explained by the fact that CuOx nanoparticle on the surface of Ce0.1Mn0.9Od helps to increase the electron transfer rate, which can improve the catalytic activity [8]. TEM images of MOx/Ce0.1Mn0.9Od (M = Cu, Fe, Co) are respectively shown in Fig. 2. It can be seen from the Fig. 2 that all the samples possess regular and well-ordered mesoporous structure. Meanwhile, from Fig. 2, it can be clearly seen that there are almost no oxide agglomeration indicating the relatively good dispersion of MOx (M = Cu, Fe, Co) on Ce0.1Mn0.9Od surface.
successfully prepared in nano-replication method with KIT-6 as a hard template. The additions of Cu, Fe and Co oxides on Ce0.1Mn0.9Od also play a key role on the catalytic activity of toluene removal. Among them, CuOx/Ce0.1Mn0.9Od exhibits the best toluene catalytic efficiency when T90% is at 254 °C and the gas hourly space velocity is at 39,000 mL/g∙h. Moreover, a good stability is also observed in the results which may be attributed to the surface CuOx species that enhances the electron transfer on the catalyst surface, leading to the increase of structural defects and oxygen vacancies. Declaration of Competing Interest
3.2. Catalytic activity test The toluene catalytic oxidation activity of meso-CexMn1-xOd and MOx/Ce0.1Mn0.9Od (M = Cu, Fe, Co) are shown in Fig. 3a and Table 1. A partial introduction of Ceria can provide more oxygen vacancies and free oxygen for the catalyst, accelerating the redox cycle of the catalyst during the reaction, and effectively improving the catalytic activity. When the molar ratio of Ce and Mn is 1:9, the catalytic activity is optimized. T50 and T90 are respectively at the temperatures of 137 and 275 °C, with a gas hourly space velocity (GHSV) at 39000 mL/gh. Meanwhile, the introduction of CuOx can further increase the activity of the catalyst, T90 dropped further to 254 °C from 275 °C. This can be explained that the electron transfer property of CuOx is better than that of CoOx and FeOx for the reaction of toluene catalytic combustion [8]. Combined with H2-TPR, the introduction of CuOx can also improve the reduction performance of the catalyst and accelerate the redox cycle of the catalyst during the reaction. For a heterogeneous-catalyzed reaction, the catalyst durability and recycling play a key role in the practical application. The reaction stability for toluene oxidation with the time over CuOx/Ce0.1Mn0.9Od catalyst was also investigated and the result was shown in Fig. 3b. The CuOx/Ce0.1Mn0.9Od catalyst exhibited excellent stability for toluene oxidation in 72 h with the conversion of toluene over 92%. 4. Conclusion Ordered mesoporous CexMn1-xOd (x = 0, 0.1, 0.3, 0.5) and transition MOx (M = Co, Cu, Fe) modified Ce0.1Mn0.9Od catalysts were
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work was supported by the National Key Research and Development Program of China (2016YFC0204301), the Key Research and Development Plan of Jiangsu Province (BE2019118), the Jiangsu Postdoctoral Foundation (No. 1701014A), and the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (ZK201712). References [1] R. Malik, V.K. Tomer, T. Dankwort, Y.K. Mishra, L. Kienle, J. Mater. Chem. A 6 (2018) 10718–10730. [2] S. Behar, N.-A. Gómez-Mendoza, M.Á. Gómez-García, D. S´wierczyn´ski, F. Quignard, N. Tanchoux, Appl. Catal. A: Gen. 504 (2015) 203–210. [3] J. Qian, Y. Xue, Y. Ao, P. Wang, C. Wang, Chinese J. Catal. 39 (2018) 682–692. [4] N. Feng, J. Meng, Y. Wu, C. Chen, L. Wang, L. Gao, H. Wan, G. Guan, Catal. Sci. Technol. 6 (2016) 2930–2941. [5] C. Zhang, J. Wang, S. Yang, H. Liang, Y. Men, J. Colloid Interf. Sci. 539 (2019) 65– 75. [6] N. Feng, C. Chen, J. Meng, G. Liu, F. Fang, L. Wang, H. Wan, G. Guan, Appl. Surf. Sci. 399 (2017) 114–122. [7] T. Lin, L. Yu, M. Sun, G. Cheng, B. Lan, Z. Fu, Chem. Eng. J. 286 (2016) 114–121. [8] S. Hosseini, A. Niaei, D. Salari, M.C. Alvarez-Galvan, J. Fierro, Ceram. Inter. 40 (2014) 6157–6163.