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Palladium catalyst supported on stair-like microstructural CeO2 provides enhanced activity and stability for low-concentration methane oxidation
Applied Catalysis A: General 524 (2016) 237–242
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Palladium catalyst supported on stair-like microstructural CeO2 provides enhanced activity and stability for low-concentration methane oxidation Tianyu Guo a , Jianping Du b , Jinting Wu b , Jinping Li a,∗ a b
Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, China College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 30 March 2016 Received in revised form 29 June 2016 Accepted 30 June 2016 Available online 1 July 2016 Keywords: Stair-like structural CeO2 Self-assembly Methane oxidation Catalytic properties
a b s t r a c t Octahedral CeO2 microparticles possessing a stair-like microstructure comprising self-assembled nanosized rectangular blocks were synthesized by a hydrothermal synthetic route. The samples were characterized by X-ray diffraction, scanning/transmission electron microscopy, and nitrogen adsorption/desorption techniques. The structural functions of the as-synthesized CeO2 as support for the Pd catalyst were investigated by the catalytic oxidation of low-concentration methane. The results showed that the unique stair-like structure of CeO2 improved the stability of the Pd catalyst for continuous conversion of low-concentration methane, which suggests that the as-synthesized CeO2 has potential application in the removal of low-concentration methane from ventilation air and other oxidations. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Global warming has become a serious environmental problem, and greenhouse gases such as CO2 and CH4 are primarily responsible for it [1]. However, in comparison to CO2 , the warming potential of methane is 21 times higher than that of carbon dioxide [2]. Obviously, tackling climate change requires us to reduce the emission of methane. As we know, a large amount of methane comes from coal mine ventilation air all over the world, especially in China. Methane accounts for approximately 1vol.% of ventilation air (VA), and massive amounts of VA is emitted into the atmosphere per year at a very fast flow rate. Therefore, it is a challenge to reduce or limit methane emission into the atmosphere. At present, the feasible methods include adsorption separation and catalytic oxidation [3,4]. It is confirmed that catalytic oxidation of methane is an efficient method for capturing low-concentration methane [5,6]. However, the low stability of catalysts limits their effective application in the catalytic oxidation of methane. It is therefore important to develop a novel catalytic material. Cerium oxide (CeO2 ) is one of the most important rare earth oxides due to its strong storage–release oxygen capacity related to Ce4+ /Ce3+ redox cycles [7]. It has wide applications in catalysis
∗ Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.apcata.2016.06.040 0926-860X/© 2016 Elsevier B.V. All rights reserved.
[8–10], fuel cells [11], optics [12], sensors [13], and sorbents [14]. Especially, CeO2 can be used as catalyst support materials [15–17]. Presently, CeO2 of various morphologies such as nanoparticles [18], nanowires [19], nanotubes [20], nanorods [21], nanocubes [22], and octahedrons [23] have been synthesized using different methods. The unique morphologies endow CeO2 with many functions. For example, one-dimensional CeO2 nanotubes were used as a photocatalyst for pollutant degradation [20], and CeO2 nanorods suppressed TiO2 photocatalysis [24]. Obviously, the morphologydependent properties have attracted great interest [25]. Therefore, it is essential to synthesize CeO2 possessing unique morphologies and structures in order to improve the properties of Pd-based catalysts for methane oxidation. Herein, octahedral CeO2 (OCO) possessing a stair-like microstructure constructed by the self-assembly of nanosize rectangular blocks was synthesized by a hydrothermal synthetic route and characterized by X-ray diffraction (XRD), scanning/transmission electron microscopy (SEM), and N2 adsorption/desorption techniques. The properties of the Pd/OCO catalyst were studied by catalytic oxidation of low-concentration methane by simulating the components of ventilation air, and the results showed that the as-synthesized CeO2 was an effective support material.
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microscopy (SU8000) and transmission electron microscopy equipped with EDX (TEM, Hitachi H-800, Japan), respectively. The Brunauer–Emmett–Teller (BET) surface area and pore-size distribution were determined by N2 adsorption–desorption measurements using a Micromeritics Tristar II 3020 analyzer. The pore-size distribution was obtained from desorption isotherms by the Barrett–Joyner–Halenda equation. 2.4. Catalyst test
Fig. 1. XRD pattern of the as-synthesized CeO2 .
2. Experimental 2.1. Self-assembly of octahedral CeO2 with stair-like structures OCO consisting of nano-sized rectangular blocks was synthesized as follows. Cerium (III) nitrate hexahydrate (5.83 mmol) was dissolved in a mixed solution of deionized water (48 mL) and anhydrous ethanol (16 mL) under vigorous stirring. 2.74 mmol of hexadecyltrimethylammonium bromide (CTAB) was then added, and the solution was continually stirred for 30 min. Finally, the mixture was transferred into a 100 mL Teflon-lined steel autoclave and kept at 150 ◦ C for 24 h. The products were obtained by centrifugal separation. 2.2. Preparation of the Pd/OCO catalysts The as-synthesized CeO2 was used as a support material. The Pd catalysts were prepared as follows, and Pd contents were 0.5, 1, and 1.5 wt.%. First, the OCO samples were impregnated with an aqueous solution of palladium nitrate for 24 h. Then, the products were dried naturally and calcined at 450 ◦ C for 2 h in a muffle furnace. For comparison, Pd/CCO (CCO: commercial CeO2 ) and Pd/CONW (CONW: CeO2 nanowires; the synthesis process was described in the Supporting information) catalysts were also prepared according to the same process. 2.3. Characterization of samples The as-synthesized CeO2 was characterized by X-ray diffractometry using Cu K␣ radiation ( = 0.15418 nm). The morphologies and structures were analyzed by scanning electron
The Pd/OCO catalysts were evaluated in a fixed-bed microreactor (inner diameter of 6 mm) at atmospheric pressure. The catalyst (150 mg, 20–40 mesh) was fixed in the center of the quartz reactor. The composition of the feed gas was 1.0% CH4 , 20% O2 , and 79% N2 , and the total flow rate of gas was set to 40 mL min−1 . The gas hourly space velocity was 16,000 mL g−1 h−1 . Prior to each reaction, the catalyst was activated in N2 gas (30 mL min−1 ) from room temperature to 200 ◦ C at a rate of 10 ◦ C min−1 and maintained at that temperature for 1 h, followed by reduction in H2 at 450 ◦ C for 2 h. The temperature was then reduced to 200 ◦ C and the mixed gases were introduced into reactor. After CH4 was oxidized for an hour, the conversion rates were recorded under steady-state conditions. The temperature was then increased from 200 to 600 ◦ C and data were collected by the same method at various temperatures. The temperature was controlled with a K-type thermocouple inserted at the center of the catalyst bed. The reaction temperature was increased from 200 to 600 ◦ C at a rate of 10 ◦ C min−1 in 50 ◦ C increments. The effluent gases were analyzed online with a gas chromatograph (ZHONGKEHUIFEN GC-6890A) equipped with a TDX–01 column and a thermal conductivity detector. The activities were evaluated by temperatures (T10% and T90% ), which represented the temperatures at 10 and 90% methane conversions, respectively. For comparison, commercial CeO2 and CeO2 nanowires were also investigated. 3. Results and discussion The XRD pattern of the as-synthesized CeO2 is shown in Fig. 1. Typical diffraction peaks at 28.5◦ , 33.1◦ , 47.5◦ , 56.4◦ , 59.0◦ , 69.4◦ , 76.7◦ , and 79.1◦ correspond to the diffraction of the (111), (200), (220), (311), (222), (400), (331), and (420) planes of CeO2 , respectively (JCPDS No. 34-0394), which indicates that the as-synthesized CeO2 (OCO) had a cubic fluorite structure [26]. No other peaks are observed, revealing the high purity of OCO. The structure of OCO was further analyzed by the N2 adsorption/desorption measurements. Fig. 2 shows the isotherm and the pore-size distribution of the as-synthesized CeO2 . A type IV isotherm with an H3 hysteresis loop in the relative pressure (P/P0 )
Fig. 2. (a) N2 adsorption/desorption isotherm and (b) pore-size distribution of as-synthesized CeO2 .
T. Guo et al. / Applied Catalysis A: General 524 (2016) 237–242
Scheme 1. Self-assembly of CeO2 and its function in the formation of stair-like structures.
range of 0.2–1.0 was observed (Fig. 2a), indicating a mesoporous structure characteristic of as-synthesized CeO2 . However, the low volume-adsorbed values showed that OCO had very few mesopores or micropores, as evidenced by the low specific surface area (Table S1, Supporting information). At relative pressures higher than 0.8, the volume-adsorbed values increased quickly, implying that OCO had macroporous characteristics, which may be ascribed to the accumulated macropores. The pore-size distribution had a wide range, and the pore diameter was more than 2 nm [Fig. 2b and Table S1 (Supporting information)]. The SEM images show that the as-synthesized CeO2 had a uniform size (approximately 2 m) and dispersion (Fig. 3a). The magnified image in Fig. 3b shows the octahedral morphology. The red rectangular area was further magnified and is displayed in Fig. 3c. The octahedral contour is clearly visible, and a single particle indicates that the octahedral structure of CeO2 was self-assembled by rectangular blocks 100–150 nm in size (Fig. 3d). These blocks were piled up in an orderly manner, forming a stairlike microstructure at different facets. During the growth process, the CTAB surfactant acted as a structure-directing agent [27] and induced growth of the CeO2 stair-like microstructure, which was formed by the orderly arrangement of nano-sized blocks in certain directions (Scheme 1). In order to elaborate the role of CTAB in the
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synthesis of OCO, CeO2 was synthesized in the absence of CTAB and its SEM images are shown in Fig. S1 in the Supporting information. It was found that the obtained CeO2 was octahedral. The images show accumulated irregular morphologies of non-uniform size, implying that CTAB played a key role in the self-assembly of the rectangular blocks, resulting in the stair-like microstructure. The TEM images of octahedral CeO2 and the supported-Pd catalyst are shown in Fig. 4. It is clear that octahedral CeO2 was composed of nano-sized blocks (Fig. 4a) with regular edges, similar to cubic blocks, and the lattice fringes are clearly visible (Fig. 4b). The selected area was analyzed by EDX. Cerium and oxygen peaks were observed, as well as copper peaks due to the Cu grid in the TEM test (inset in Fig. 4a). The results indicate that pure cerium oxide was synthesized. The stair-like microstructure of octahedral CeO2 may be effective in inhibiting the aggregation of metal nanoparticles, and so we prepared an octahedral CeO2 -supported Pd catalyst (Pd/OCO). The TEM images show that the Pd nanoparticles were dispersed on the different terraces of the stair-like structure (Fig. 4c and d), corresponding to the gradient surfaces shown in Fig. 3d. The average particle size of the Pd was approximately 10 nm. The fine structures of Pd are also observed in the high-resolution TEM image (inset in Fig. 4d). No diffraction peaks of metallic Pd were found in the XRD patterns (Fig. S2a, Supporting information), implying there were no large particles. In all tests, only CO2 could be detected, and CO was not found in the effluent gases, implying the complete conversion of methane to carbon dioxide. The performance of the Pd/OCO catalysts (Pd loadings were 0.5, 1.0, and 1.5 wt.%) for methane oxidation is shown in Fig. 5. It is clear that methane conversion over the Pd/OCO catalysts increased with increasing temperature from 200 to 600 ◦ C. The conversion also increased with increasing Pd loading at constant temperature, whereas there was low conversion of methane over the pure OCO material (Fig. 5a). In particular, the catalyst maintained high stability from 400 to 600 ◦ C, suggesting that the Pd nanoparticles anchored on the surfaces of the stair-like OCO exposed more active sites and that the stair-like structures may suppress migration of nanoparticles between different terraces.
Fig. 3. SEM images of as-synthesized CeO2 with stair-like structure.
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Fig. 4. TEM images of (a, b) as-synthesized CeO2 and the (c, d) Pd/OCO catalyst.
Fig. 5. Catalytic properties of the Pd/OCO catalysts in methane oxidation: (a) CH4 conversion over catalysts with various Pd loadings at temperatures from 200 to 600 ◦ C. (b) Stabilities of the Pd/OCO catalysts for 0.5, 1.0, and 1.5 wt.% loadings at 600, 500, and 400 ◦ C, respectively. (c) Stabilities of the Pd/OCO catalysts at 600 ◦ C. (d) Activities of the Pd/OCO catalysts at 200, 300, 400, 500, and 600 ◦ C.
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Fig. 6. (a) CH4 conversions over Pd/OCO, Pd/CCO, and Pd/CONW with the same Pd loading at various temperatures. (b) Stabilities of different catalysts with 0.5 wt.% loading at 600 ◦ C.
Fig. 5b shows the stabilities of the Pd/OCO catalysts with various Pd loadings at different temperatures during the reaction over 20 h. The catalysts with 0.5, 1.0, and 1.5 wt.% Pd loading exhibited considerable stability at 600, 500, and 400 ◦ C, respectively. When the temperature was further increased to 600 ◦ C, all the catalysts exhibited high stability over 20 h and the methane conversions reached 100% (Fig. 5c), suggesting that the OCO material may suppress sintering of Pd nanoparticles at high temperatures, which was confirmed by the XRD results of the Pd/OCO catalysts after the reaction. Pd diffraction peaks were not found in the XRD patterns (Fig. S2b, Supporting Information), implying that Pd nanoparticles were not sintered on the OCO surface, which may have provided more efficient active sites for methane conversion. From the turnover frequency (TOF) results, it is clear that the activity decreased with increasing Pd content from 0.5 to 1.5 wt.% between 400 and 600 ◦ C (Fig. 5d). However, we found that the TOF values for the 1.5 wt.% loading were higher than for the 1.0 wt.% loading at 300 and 350 ◦ C, and the corresponding values only decreased by 6%–35% at temperatures over 400 ◦ C (Table S2, Supporting information). The results also showed that the stair-like structure of ceria played a critical role in inhibiting the migration of nanoparticles between different terraces, especially for the catalysts with high loading. In order to further confirm the structural function of OCO, a comparison with commercial CeO2 (CCO) and CeO2 nanowires (CONW) was performed. Commercial CeO2 consists of micrometer particles (Fig. S3a and b, Supporting information) and possesses mesoporous structures (Fig. S4, Supporting information). CeO2 nanowires are cage shaped (Fig. S3c and d, Supporting information), with microporous structures and high BET surface areas (Fig. S5 and Table S1, Supporting information). The properties of the Pd/OCO, Pd/CCO, and Pd/CONW catalysts with 0.5 wt.% Pd contents are shown in Fig. 6. The CH4 conversion over Pd/OCO is close to that over Pd/CONW and higher than that over Pd/CCO at temperatures below 350 ◦ C, whereas the CH4 conversion on the Pd/OCO catalyst was higher than those over the Pd/CCO and Pd/CONW catalysts at temperatures between 350 and 600 ◦ C (Fig. 6a). More importantly, the Pd/OCO catalyst maintained a higher stability at 600 ◦ C during methane oxidation as compared to the Pd/CCO and Pd/CONW catalysts (Fig. 6b). The results showed that the Pd/CONW catalyst exhibited low activity and the Pd/CCO catalyst exhibited low stability at high temperatures. In comparison, the OCO/Pd catalyst exhibited high activity and high stability. In general, the activity and stability of a catalyst are related to the specific surface area and pore structure of the support material [28,29]. In this work, the as-synthesized OCO had neither an extensive pore structure nor a high specific surface area (Table S1). However, the interesting results were that the Pd/OCO catalyst exhibited good activity at low temperatures and high stability
at high temperatures. Obviously, the excellent performance of the Pd/OCO catalyst is not attributed to the pore structure and BET specific surface area, but rather to the unique stair-like structures of OCO. The possible reason for these results is that the stair-like microstructure formed by nano-sized blocks can inhibit the migration of nanoparticles on the gradient surface of CeO2 (Scheme 1), which effectively prevents nanoparticle sintering at high temperatures, thus retaining more active sites on the Pd/OCO catalyst. Therefore, octahedral CeO2 possessing a stair-like microstructure will have potential application in catalysis. 4. Conclusion Octahedral CeO2 microparticles (OCO) possessing a stairlike microstructure was self-assembed by nano-sized rectangular blocks. The catalytic properties of the OCO-supported Pd catalyst were studied by the catalytic oxidation of low-concentration methane. The results showed that the stair-like microstructure of OCO played a key role in the use of a Pd catalyst for the conversion of low-concentration methane. As compared to commercial and nanosized CeO2 , OCO is a preferred support. The results showed that the Pd/OCO catalyst exhibited enhanced activity and stability for low-concentration methane oxidation, suggesting that OCO is an effective support material, and thus it has potential application in catalysis. Acknowledgement The authors express thanks for partial financial support by National Natural Science Foundation of China (21136007 and 51572185), Natural Science Foundation of Shanxi Province (2014011016-4), and Coal-Based Scientific and Technological Key Project of Shanxi Province (MQ2014-10). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.06. 040. References [1] H. Yang, P. Xie, L. Ni, R.J. Flower, Environ. Sci. Technol. 45 (2011) 4203–4204. ˜ [2] J. Fernández, P. Marín, F.V. Díez, S. Ordónez, Fuel Process. Technol. 133 (2015) 202–209. [3] B. Yuan, X. Wu, Y. Chen, J. Huang, H. Luo, S. Deng, Environ. Sci. Technol. 47 (2013) 5474–5480. [4] A. Urd˘a, I. Popescu, T. Cacciaguerra, N. Tanchoux, D. Tichit, I.-C. Marcu, Appl. Catal. A: Gen. 464–465 (2013) 20–27.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Palladium catalyst supported on stair-like microstructural CeO2 provides enhanced activity and stability for low-concentration methane oxidation Tianyu Guo a , Jianping Du b , Jinting Wu b , Jinping Li a,∗ a b
Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, China College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 30 March 2016 Received in revised form 29 June 2016 Accepted 30 June 2016 Available online 1 July 2016 Keywords: Stair-like structural CeO2 Self-assembly Methane oxidation Catalytic properties
a b s t r a c t Octahedral CeO2 microparticles possessing a stair-like microstructure comprising self-assembled nanosized rectangular blocks were synthesized by a hydrothermal synthetic route. The samples were characterized by X-ray diffraction, scanning/transmission electron microscopy, and nitrogen adsorption/desorption techniques. The structural functions of the as-synthesized CeO2 as support for the Pd catalyst were investigated by the catalytic oxidation of low-concentration methane. The results showed that the unique stair-like structure of CeO2 improved the stability of the Pd catalyst for continuous conversion of low-concentration methane, which suggests that the as-synthesized CeO2 has potential application in the removal of low-concentration methane from ventilation air and other oxidations. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Global warming has become a serious environmental problem, and greenhouse gases such as CO2 and CH4 are primarily responsible for it [1]. However, in comparison to CO2 , the warming potential of methane is 21 times higher than that of carbon dioxide [2]. Obviously, tackling climate change requires us to reduce the emission of methane. As we know, a large amount of methane comes from coal mine ventilation air all over the world, especially in China. Methane accounts for approximately 1vol.% of ventilation air (VA), and massive amounts of VA is emitted into the atmosphere per year at a very fast flow rate. Therefore, it is a challenge to reduce or limit methane emission into the atmosphere. At present, the feasible methods include adsorption separation and catalytic oxidation [3,4]. It is confirmed that catalytic oxidation of methane is an efficient method for capturing low-concentration methane [5,6]. However, the low stability of catalysts limits their effective application in the catalytic oxidation of methane. It is therefore important to develop a novel catalytic material. Cerium oxide (CeO2 ) is one of the most important rare earth oxides due to its strong storage–release oxygen capacity related to Ce4+ /Ce3+ redox cycles [7]. It has wide applications in catalysis
∗ Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.apcata.2016.06.040 0926-860X/© 2016 Elsevier B.V. All rights reserved.
[8–10], fuel cells [11], optics [12], sensors [13], and sorbents [14]. Especially, CeO2 can be used as catalyst support materials [15–17]. Presently, CeO2 of various morphologies such as nanoparticles [18], nanowires [19], nanotubes [20], nanorods [21], nanocubes [22], and octahedrons [23] have been synthesized using different methods. The unique morphologies endow CeO2 with many functions. For example, one-dimensional CeO2 nanotubes were used as a photocatalyst for pollutant degradation [20], and CeO2 nanorods suppressed TiO2 photocatalysis [24]. Obviously, the morphologydependent properties have attracted great interest [25]. Therefore, it is essential to synthesize CeO2 possessing unique morphologies and structures in order to improve the properties of Pd-based catalysts for methane oxidation. Herein, octahedral CeO2 (OCO) possessing a stair-like microstructure constructed by the self-assembly of nanosize rectangular blocks was synthesized by a hydrothermal synthetic route and characterized by X-ray diffraction (XRD), scanning/transmission electron microscopy (SEM), and N2 adsorption/desorption techniques. The properties of the Pd/OCO catalyst were studied by catalytic oxidation of low-concentration methane by simulating the components of ventilation air, and the results showed that the as-synthesized CeO2 was an effective support material.
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microscopy (SU8000) and transmission electron microscopy equipped with EDX (TEM, Hitachi H-800, Japan), respectively. The Brunauer–Emmett–Teller (BET) surface area and pore-size distribution were determined by N2 adsorption–desorption measurements using a Micromeritics Tristar II 3020 analyzer. The pore-size distribution was obtained from desorption isotherms by the Barrett–Joyner–Halenda equation. 2.4. Catalyst test
Fig. 1. XRD pattern of the as-synthesized CeO2 .
2. Experimental 2.1. Self-assembly of octahedral CeO2 with stair-like structures OCO consisting of nano-sized rectangular blocks was synthesized as follows. Cerium (III) nitrate hexahydrate (5.83 mmol) was dissolved in a mixed solution of deionized water (48 mL) and anhydrous ethanol (16 mL) under vigorous stirring. 2.74 mmol of hexadecyltrimethylammonium bromide (CTAB) was then added, and the solution was continually stirred for 30 min. Finally, the mixture was transferred into a 100 mL Teflon-lined steel autoclave and kept at 150 ◦ C for 24 h. The products were obtained by centrifugal separation. 2.2. Preparation of the Pd/OCO catalysts The as-synthesized CeO2 was used as a support material. The Pd catalysts were prepared as follows, and Pd contents were 0.5, 1, and 1.5 wt.%. First, the OCO samples were impregnated with an aqueous solution of palladium nitrate for 24 h. Then, the products were dried naturally and calcined at 450 ◦ C for 2 h in a muffle furnace. For comparison, Pd/CCO (CCO: commercial CeO2 ) and Pd/CONW (CONW: CeO2 nanowires; the synthesis process was described in the Supporting information) catalysts were also prepared according to the same process. 2.3. Characterization of samples The as-synthesized CeO2 was characterized by X-ray diffractometry using Cu K␣ radiation ( = 0.15418 nm). The morphologies and structures were analyzed by scanning electron
The Pd/OCO catalysts were evaluated in a fixed-bed microreactor (inner diameter of 6 mm) at atmospheric pressure. The catalyst (150 mg, 20–40 mesh) was fixed in the center of the quartz reactor. The composition of the feed gas was 1.0% CH4 , 20% O2 , and 79% N2 , and the total flow rate of gas was set to 40 mL min−1 . The gas hourly space velocity was 16,000 mL g−1 h−1 . Prior to each reaction, the catalyst was activated in N2 gas (30 mL min−1 ) from room temperature to 200 ◦ C at a rate of 10 ◦ C min−1 and maintained at that temperature for 1 h, followed by reduction in H2 at 450 ◦ C for 2 h. The temperature was then reduced to 200 ◦ C and the mixed gases were introduced into reactor. After CH4 was oxidized for an hour, the conversion rates were recorded under steady-state conditions. The temperature was then increased from 200 to 600 ◦ C and data were collected by the same method at various temperatures. The temperature was controlled with a K-type thermocouple inserted at the center of the catalyst bed. The reaction temperature was increased from 200 to 600 ◦ C at a rate of 10 ◦ C min−1 in 50 ◦ C increments. The effluent gases were analyzed online with a gas chromatograph (ZHONGKEHUIFEN GC-6890A) equipped with a TDX–01 column and a thermal conductivity detector. The activities were evaluated by temperatures (T10% and T90% ), which represented the temperatures at 10 and 90% methane conversions, respectively. For comparison, commercial CeO2 and CeO2 nanowires were also investigated. 3. Results and discussion The XRD pattern of the as-synthesized CeO2 is shown in Fig. 1. Typical diffraction peaks at 28.5◦ , 33.1◦ , 47.5◦ , 56.4◦ , 59.0◦ , 69.4◦ , 76.7◦ , and 79.1◦ correspond to the diffraction of the (111), (200), (220), (311), (222), (400), (331), and (420) planes of CeO2 , respectively (JCPDS No. 34-0394), which indicates that the as-synthesized CeO2 (OCO) had a cubic fluorite structure [26]. No other peaks are observed, revealing the high purity of OCO. The structure of OCO was further analyzed by the N2 adsorption/desorption measurements. Fig. 2 shows the isotherm and the pore-size distribution of the as-synthesized CeO2 . A type IV isotherm with an H3 hysteresis loop in the relative pressure (P/P0 )
Fig. 2. (a) N2 adsorption/desorption isotherm and (b) pore-size distribution of as-synthesized CeO2 .
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Scheme 1. Self-assembly of CeO2 and its function in the formation of stair-like structures.
range of 0.2–1.0 was observed (Fig. 2a), indicating a mesoporous structure characteristic of as-synthesized CeO2 . However, the low volume-adsorbed values showed that OCO had very few mesopores or micropores, as evidenced by the low specific surface area (Table S1, Supporting information). At relative pressures higher than 0.8, the volume-adsorbed values increased quickly, implying that OCO had macroporous characteristics, which may be ascribed to the accumulated macropores. The pore-size distribution had a wide range, and the pore diameter was more than 2 nm [Fig. 2b and Table S1 (Supporting information)]. The SEM images show that the as-synthesized CeO2 had a uniform size (approximately 2 m) and dispersion (Fig. 3a). The magnified image in Fig. 3b shows the octahedral morphology. The red rectangular area was further magnified and is displayed in Fig. 3c. The octahedral contour is clearly visible, and a single particle indicates that the octahedral structure of CeO2 was self-assembled by rectangular blocks 100–150 nm in size (Fig. 3d). These blocks were piled up in an orderly manner, forming a stairlike microstructure at different facets. During the growth process, the CTAB surfactant acted as a structure-directing agent [27] and induced growth of the CeO2 stair-like microstructure, which was formed by the orderly arrangement of nano-sized blocks in certain directions (Scheme 1). In order to elaborate the role of CTAB in the
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synthesis of OCO, CeO2 was synthesized in the absence of CTAB and its SEM images are shown in Fig. S1 in the Supporting information. It was found that the obtained CeO2 was octahedral. The images show accumulated irregular morphologies of non-uniform size, implying that CTAB played a key role in the self-assembly of the rectangular blocks, resulting in the stair-like microstructure. The TEM images of octahedral CeO2 and the supported-Pd catalyst are shown in Fig. 4. It is clear that octahedral CeO2 was composed of nano-sized blocks (Fig. 4a) with regular edges, similar to cubic blocks, and the lattice fringes are clearly visible (Fig. 4b). The selected area was analyzed by EDX. Cerium and oxygen peaks were observed, as well as copper peaks due to the Cu grid in the TEM test (inset in Fig. 4a). The results indicate that pure cerium oxide was synthesized. The stair-like microstructure of octahedral CeO2 may be effective in inhibiting the aggregation of metal nanoparticles, and so we prepared an octahedral CeO2 -supported Pd catalyst (Pd/OCO). The TEM images show that the Pd nanoparticles were dispersed on the different terraces of the stair-like structure (Fig. 4c and d), corresponding to the gradient surfaces shown in Fig. 3d. The average particle size of the Pd was approximately 10 nm. The fine structures of Pd are also observed in the high-resolution TEM image (inset in Fig. 4d). No diffraction peaks of metallic Pd were found in the XRD patterns (Fig. S2a, Supporting information), implying there were no large particles. In all tests, only CO2 could be detected, and CO was not found in the effluent gases, implying the complete conversion of methane to carbon dioxide. The performance of the Pd/OCO catalysts (Pd loadings were 0.5, 1.0, and 1.5 wt.%) for methane oxidation is shown in Fig. 5. It is clear that methane conversion over the Pd/OCO catalysts increased with increasing temperature from 200 to 600 ◦ C. The conversion also increased with increasing Pd loading at constant temperature, whereas there was low conversion of methane over the pure OCO material (Fig. 5a). In particular, the catalyst maintained high stability from 400 to 600 ◦ C, suggesting that the Pd nanoparticles anchored on the surfaces of the stair-like OCO exposed more active sites and that the stair-like structures may suppress migration of nanoparticles between different terraces.
Fig. 3. SEM images of as-synthesized CeO2 with stair-like structure.
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Fig. 4. TEM images of (a, b) as-synthesized CeO2 and the (c, d) Pd/OCO catalyst.
Fig. 5. Catalytic properties of the Pd/OCO catalysts in methane oxidation: (a) CH4 conversion over catalysts with various Pd loadings at temperatures from 200 to 600 ◦ C. (b) Stabilities of the Pd/OCO catalysts for 0.5, 1.0, and 1.5 wt.% loadings at 600, 500, and 400 ◦ C, respectively. (c) Stabilities of the Pd/OCO catalysts at 600 ◦ C. (d) Activities of the Pd/OCO catalysts at 200, 300, 400, 500, and 600 ◦ C.
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Fig. 6. (a) CH4 conversions over Pd/OCO, Pd/CCO, and Pd/CONW with the same Pd loading at various temperatures. (b) Stabilities of different catalysts with 0.5 wt.% loading at 600 ◦ C.
Fig. 5b shows the stabilities of the Pd/OCO catalysts with various Pd loadings at different temperatures during the reaction over 20 h. The catalysts with 0.5, 1.0, and 1.5 wt.% Pd loading exhibited considerable stability at 600, 500, and 400 ◦ C, respectively. When the temperature was further increased to 600 ◦ C, all the catalysts exhibited high stability over 20 h and the methane conversions reached 100% (Fig. 5c), suggesting that the OCO material may suppress sintering of Pd nanoparticles at high temperatures, which was confirmed by the XRD results of the Pd/OCO catalysts after the reaction. Pd diffraction peaks were not found in the XRD patterns (Fig. S2b, Supporting Information), implying that Pd nanoparticles were not sintered on the OCO surface, which may have provided more efficient active sites for methane conversion. From the turnover frequency (TOF) results, it is clear that the activity decreased with increasing Pd content from 0.5 to 1.5 wt.% between 400 and 600 ◦ C (Fig. 5d). However, we found that the TOF values for the 1.5 wt.% loading were higher than for the 1.0 wt.% loading at 300 and 350 ◦ C, and the corresponding values only decreased by 6%–35% at temperatures over 400 ◦ C (Table S2, Supporting information). The results also showed that the stair-like structure of ceria played a critical role in inhibiting the migration of nanoparticles between different terraces, especially for the catalysts with high loading. In order to further confirm the structural function of OCO, a comparison with commercial CeO2 (CCO) and CeO2 nanowires (CONW) was performed. Commercial CeO2 consists of micrometer particles (Fig. S3a and b, Supporting information) and possesses mesoporous structures (Fig. S4, Supporting information). CeO2 nanowires are cage shaped (Fig. S3c and d, Supporting information), with microporous structures and high BET surface areas (Fig. S5 and Table S1, Supporting information). The properties of the Pd/OCO, Pd/CCO, and Pd/CONW catalysts with 0.5 wt.% Pd contents are shown in Fig. 6. The CH4 conversion over Pd/OCO is close to that over Pd/CONW and higher than that over Pd/CCO at temperatures below 350 ◦ C, whereas the CH4 conversion on the Pd/OCO catalyst was higher than those over the Pd/CCO and Pd/CONW catalysts at temperatures between 350 and 600 ◦ C (Fig. 6a). More importantly, the Pd/OCO catalyst maintained a higher stability at 600 ◦ C during methane oxidation as compared to the Pd/CCO and Pd/CONW catalysts (Fig. 6b). The results showed that the Pd/CONW catalyst exhibited low activity and the Pd/CCO catalyst exhibited low stability at high temperatures. In comparison, the OCO/Pd catalyst exhibited high activity and high stability. In general, the activity and stability of a catalyst are related to the specific surface area and pore structure of the support material [28,29]. In this work, the as-synthesized OCO had neither an extensive pore structure nor a high specific surface area (Table S1). However, the interesting results were that the Pd/OCO catalyst exhibited good activity at low temperatures and high stability
at high temperatures. Obviously, the excellent performance of the Pd/OCO catalyst is not attributed to the pore structure and BET specific surface area, but rather to the unique stair-like structures of OCO. The possible reason for these results is that the stair-like microstructure formed by nano-sized blocks can inhibit the migration of nanoparticles on the gradient surface of CeO2 (Scheme 1), which effectively prevents nanoparticle sintering at high temperatures, thus retaining more active sites on the Pd/OCO catalyst. Therefore, octahedral CeO2 possessing a stair-like microstructure will have potential application in catalysis. 4. Conclusion Octahedral CeO2 microparticles (OCO) possessing a stairlike microstructure was self-assembed by nano-sized rectangular blocks. The catalytic properties of the OCO-supported Pd catalyst were studied by the catalytic oxidation of low-concentration methane. The results showed that the stair-like microstructure of OCO played a key role in the use of a Pd catalyst for the conversion of low-concentration methane. As compared to commercial and nanosized CeO2 , OCO is a preferred support. The results showed that the Pd/OCO catalyst exhibited enhanced activity and stability for low-concentration methane oxidation, suggesting that OCO is an effective support material, and thus it has potential application in catalysis. Acknowledgement The authors express thanks for partial financial support by National Natural Science Foundation of China (21136007 and 51572185), Natural Science Foundation of Shanxi Province (2014011016-4), and Coal-Based Scientific and Technological Key Project of Shanxi Province (MQ2014-10). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.06. 040. References [1] H. Yang, P. Xie, L. Ni, R.J. Flower, Environ. Sci. Technol. 45 (2011) 4203–4204. ˜ [2] J. Fernández, P. Marín, F.V. Díez, S. Ordónez, Fuel Process. Technol. 133 (2015) 202–209. [3] B. Yuan, X. Wu, Y. Chen, J. Huang, H. Luo, S. Deng, Environ. Sci. Technol. 47 (2013) 5474–5480. [4] A. Urd˘a, I. Popescu, T. Cacciaguerra, N. Tanchoux, D. Tichit, I.-C. Marcu, Appl. Catal. A: Gen. 464–465 (2013) 20–27.
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