Controllable Assembly of Hierarchical MacroporousâMesoporous

Article pubs.acs.org/Langmuir

Controllable Assembly of Hierarchical Macroporous−Mesoporous LnFeO3 and Their Catalytic Performance in the CO + NO Reaction Zhen-Xing Li,†,‡ Fu-Bo Shi,† and Chun-Hua Yan*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ Institute of New Energy, China University of PetroleumBeijing, Beijing 102249, China

Downloaded by UNIV OF PENNSYLVANIA on August 22, 2015 | http://pubs.acs.org Publication Date (Web): July 31, 2015 | doi: 10.1021/acs.langmuir.5b01519

S Supporting Information *

ABSTRACT: A new synthesis strategy to prepare hierarchical macroporous−mesoporous materials employing poly(ethylene oxide)-poly(phenylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) as a single template and an acid adjusting agent was reported. There is a hierarchical structure including macropores with a size of 50−100 nm and mesopores in the macroporous walls with a size of 3−5 nm. The macroporous walls are composed of rare earth orthoferrite nanoparticles with a size of 5−10 nm. These hierarchically porous materials show high catalytic activities for the CO + NO reaction, and NO can be fully converted to N2 at temperatures as low as 350 °C, indicating their potential in the catalytic conversion of automotive exhaust gas and other catalysis-related fields. This synthesis strategy is a facile method for the preparation of hierarchical porous materials and may give us a guideline for the synthesis of functional materials with further catalytic applications.

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INTRODUCTION With the development of the automobile industry, automobile exhaust gas has become one of the major sources of air pollution. The control of automobile exhaust pollution is particularly significant for reducing air pollution. Three-way catalyst is widely applied in the treatment of automobile exhaust,1−9 and the noble metals (Pt, Pd, and Rh) are the main active component of the catalyst.10−15 Recently, more and more efforts have been undertaken to replace the precious metal with low-cost transition-metal catalysts.16−19 Because a high oxygen vacancy is formed and migrated, rare earth orthoferrite materials (LnFeO3) have proven to be promising candidates. In 1992, mesoporous silica materials were discovered,20,21 and more and more researchers have swarmed this research field of mesoporous materials because these materials are promising as catalyst carriers and in other fields in chemistry.22 There are lots of advantages in mesoporous materials such as highly uniform channels, large surface areas, narrow pore size distributions, adjustable pore sizes over a considerably large range, and so on.23−25 Hierarchical macroporous−mesoporous materials, especially those providing interconnected mesopores © 2015 American Chemical Society

in macroporous walls, have attracted significant attention for combining the excellent performance of mass transport in macropores with the strong product confinement and increased surface area in mesopores,26−31 which can improve the diffusion rate of reactant molecules on the inorganic network composed of pores and channels. Similarly, for applications in catalysis, the hierarchical macroporous−mesoporous materials with textural mesopores and interconnected macropores can provide superior accessibility and chemical activities. The template method is a highly effective technique for preparing porous materials.32−37 The conventional preparation of hierarchically macroporous−mesoporous materials usually resorts to a dual template, using surfactant as mesoporous templates and colloidal crystals as macroporous templates.38−40 In this paper, we describe a new synthesis strategy to prepare hierarchical macroporous−mesoporous materials employing poly(ethylene oxide)-poly(phenylene oxide)-poly(ethylene Received: April 29, 2015 Revised: July 19, 2015 Published: July 21, 2015 8672

DOI: 10.1021/acs.langmuir.5b01519 Langmuir 2015, 31, 8672−8679

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60 mesh) molecular sieves to separate CO, N2, and NO, and column 2 uses Porapak Q for the separation of N2O and CO2. An infrared detector is used to detect the low concentrations of NO, CO, N2O, and CO2. The test temperature is in the range of 50−400 °C.

oxide) (PEO-PPO-PEO) as a single template and an acid adjusting agent. The amphiphilic triblock copolymer with long hydrophobic segments is a promising template in the preparation of hierarchically macroporous−mesoporous materials. The hierarchical macroporous−mesoporous rare earth orthoferrite materials and nanocrystalline frameworks are prepared by the approach of a simple one-step self-assembly process, and amphiphilic triblock copolymer Pluronic F127 (EO106PO70EO106) is the only template. Nitric acid is used to control the rate of hydrolysis. In the fabrication of macroporous structures, no swelling agent was added. The hierarchically porous structure includes macropores with a size of 50−100 nm and mesopores in the macroporous walls with a size of 3−5 nm. The macroporous walls are composed of rare earth orthoferrite nanoparticles with a size of 5−10 nm. The hierarchical macroporous−mesoporous rare earth orthoferrite materials show high catalytic activities toward the CO + NO reaction, and NO can be completely converted to N2 at temperatures as low as 350 °C, indicating that these materials are promising candidates for automobile exhaust control and other catalytic realms.

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RESULTS AND DISCUSSION Compared to mesoporous materials, hierarchically macroporous−mesoporous materials on multiple length scales have lots of novel properties and provide potential applications in practical and industrial processes, for instance, adsorption, catalysis, chemical sensing, storage and transportation of gases and fluids, separation, some biological applications, and other application,41−45 as macropores provide better guest molecule accessibility, while the mesopores have large pore volumes and high surface areas. Evidence for the formation of macroporous and mesoporous structures is provided by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1 exhibits the SEM and TEM images and a

EXPERIMENTAL SECTION

Preparation of Hierarchical Macroporous−Mesoporous LnFeO3. At room temperature, 0.9 g of Pluronic F127 (Mav = 12 600, EO106PO70EO106) was dissolved in 20 mL of propanol. Then 1.04 g of nitric acid, 2.5 mmol of ferric nitrate, and 2.5 mmol of rare earth nitrate (rare earth = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) were added to this solution, and the solution was vigorously stirred. After being covered with PE film, the solution was stirred at room temperature for about 12 h and was then placed into a 60 °C drying oven for 48 h for solvent evaporation. After 48 h of aging at 60 °C, the sample was dried at 180 °C for 30 min and then at 100 °C for 24 h. The calcination temperature was from room temperature to 400 °C, and the ramping rate was 1 °C min−1. Calcination requires 4 h under air. Calcination under higher temperature was carried out under air for 1 h, and the temperature ramp was 10 °C min−1. Characterization. X-ray powder diffraction (XRD) patterns of the samples were recorded by a Rigaku D/max-2000 X-ray powder diffractometer (Japan) using Cu Kα (λ = 1.5405 Å) radiation. Scanning electron microscopy (SEM) was performed using a DB-235 focused ion beam (FIB) system, and the acceleration voltage was 15 kV. Transmission electronic microscopy (TEM) was used with a JEOL-2100 (Japan), which was operated at 200 kV. High-resolution transmission electronic microscopy (HRTEM) and selected-area electron diffraction (SAED) were carried out on a JEOL-2100F (Japan), which was operated at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) was carried out using the EDS detector of a JEOL-2100F (Japan). The N2 adsorption and desorption isotherms were taken using the ASAP 2010 analyzer (Micromeritics Co. Ltd.) at 78.3 K. The sample was outgassed at 393 K under vacuum, and the residual pressure is better than 10−3 Torr. Using the Barrett−Joyner− Halenda (BJH) model, the pore volumes and size distributions were computed with the desorption branches of the N2 adsorption and desorption isotherms. The inductively coupled plasma-atomic emission spectrometer (ICP-AES) measurements were carried out on a Leeman Profile SPEC. Dynamic light scattering (DLS) was recorded using a HORIBA SZ-100 nanoparticle analyzer. CO + NO Reaction Test. A flow reactor system containing a reaction tube was used to test the catalytic properties of samples. In a typical CO + NO reaction, the amount of sample is 50 mg (40−60 mesh), and the sample was pretreated at 100 °C in a He atmosphere for 1 h. The gas consisted of 0.67% NO, 1.33% CO, and 98% He, and the carrier gas was He. The gas flow rate is 10 mL min−1. The gas composition was monitored online on a Shimadzu GC-14C equipped with a TCD detector and a Bruker Tensor 27 FT-IR. The gas chromatograph (GC) is dual-column system: column 1 uses 5A (40−

Figure 1. Morphology and structure of LaFeO3. (a) SEM image of the overall morphology of LaFeO3. (b) High-magnification TEM image of LaFeO3. (c) HRTEM image of a typical macroporous wall. (The inset in c is the corresponding SAED pattern.) (d) Schematic illustration that demonstrates the microstructure of hierarchically macroporous− mesoporous LaFeO3 comprising closely packed nanoparticles.

schematic diagram exhibiting the structure of hierarchically macroporous−mesoporous LaFeO3. The SEM image (Figure 1a) confirms that LaFeO3 materials are macroporous in structure, with an average macropore diameter of 50 to 100 nm. The magnified TEM image of LaFeO3 materials is shown in Figure 1b, revealing that the mesoporous structure is on the macroporous wall, and the average diameter of the mesopores is 3−5 nm. The high-resolution TEM (HRTEM) image shows that the mesoporous walls are highly crystalline, and the lattice spacing is 0.29 nm, which agrees well with the value of 0.28 nm for the (200) crystal planes of LaFeO3 in Figure 1c, and the LaFeO3 nanoparticles composing the walls are 5−10 nm in size. The inset of Figure 1c is the selected-area electron diffraction (SAED) pattern of the mesostructure domains, which further confirms that the mesoporous wall is highly crystalline. In Figure 1d, the geometrical structure of the hierarchically macroporous−mesoporous LaFeO3 is further schematically illustrated to show the mesoporous structure composed of aggregated nanosized crystalline particles. The elemental analysis of the hierarchically macroporous−mesoporous 8673


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Figure 2. EDS point scanning of the as-obtained hierarchically macroporous−mesoporous LaFeO3.

LaFeO3 is measured by energy-dispersive spectroscopy (EDS) of La and Fe (Figure 2). The Cr and Cu element peaks are from the TEM holder and copper mesh of the sample. The results of the EDS point-scanning experiments at arbitrary points on the samples show that the atomic ratio of La to Fe is 1:1, which agrees well with the original molar ratio of the preparation of hierarchically macroporous−mesoporous LaFeO3. The analysis of an inductively coupled plasma-atomic emission spectrometer (ICP-AES) further confirmed this result. This suggests that La 3+ and Fe3+ are distributed homogeneously in the framework of the hierarchically macroporous− mesoporous structure, further confirming the observed results from EDS point-scanning experiments. The wide-angle XRD pattern of as-prepared hierarchically macroporous−mesoporous LaFeO3 is indexed to the orthorhombic phase (JCPDS card no. 74-2203) in Figure 3, which agrees well with the above HRTEM result. After annealing at 500 °C, no impurities of the crystallized precursor are detected,

and the broadening of the reflection peaks in the diffraction distinctly suggests the formation of nanocrystalline LaFeO3.With increasing annealing temperature, the diffraction peaks become narrower, which indicates that the LaFeO3 nanoparticles grow larger. After annealing at a higher temperature of 600 °C, the wide-angle XRD pattern shows a weak diffraction peak at 29°, which can be indexed to the (011) reflection of hexagonal La2O3 (JCPDS card no. 83-1344), indicating a trace of La2O3 in LaFeO3. Figure 4 exhibits TEM images of as-obtained hierarchically macroporous−mesoporous

Figure 3. Wide-angle XRD patterns of as-obtained hierarchically macroporous−mesoporous LaFeO3 and LaFeO3 after annealing at different temperatures.

Figure 4. TEM images of as-obtained hierarchically macroporous− mesoporous LaFeO3 after annealing at different temperatures: (a) asobtained LaFeO3, (b) 500 °C, (c) 600 °C, and (d) 700 °C. 8674


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Figure 5. Nitrogen gas adsorption−desorption (a) and isotherm pore size distribution curves (b) of as-obtained hierarchical macroporous− mesoporous LaFeO3 and LaFeO3 after annealing at different temperatures.

temperature reaches 700 °C, the mesoporous structure in the macroporous walls is completely destroyed (Figure 5b). Since the hierarchical macroporous−mesoporous structure is very efficient in catalysis, the successful synthesis of hierarchical macroporous−mesoporous LaFeO3 inspires us to extend this method to the facile preparation of other rare earth orthoferrite materials. To demonstrate the feasibility of this new strategy, we present the synthesis of hierarchical macroporous− mesoporous LnFeO3 (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) due to its importance in catalytic applications. TEM and XRD results indicate that our approach could meet these requirements. TEM images (Figure 6) reveal that the hierarchically macroporous−mesoporous structure is still preserved. The macroporous diameter is 50 to 100 nm. Nitrogen adsorption−desorption analysis (Figure S1 in the Supporting Information) shows a narrow pore size distribution, and the mesoporous diameter is 2−5 nm. The BET surface areas are 44.6, 45.6, 45.0, 48.2, 62.0, 56.7, 54.5, 60.5, 62.1, 64.9, and 69.4, respectively (Table S1 in the Supporting Information). Figure S2 shows wide-angle XRD patterns of hierarchical macroporous−mesoporous LnFeO3. With the increase in atomic number in LnFeO3, the diffraction peaks are broadened and the degree of crystallization gradually deteriorated. It is known that as the Ln3+ ionic radius decreases, the crystal structure is distorted because of the mismatched size between small Fe3+ and large Ln3+.47,48 The distortion in LnFeO3 is markedly presented, and the tolerance factor t is at a lower limit of perovskites. Therefore, some LnFeO3 phases (Ln = Tb, Dy, Ho, Er, Tm, Yb) exhibited traces of Ln2O3 phases around the diffraction peak of 29° at a high annealing temperature of 600 °C.49 On the basis of the above SEM and TEM results and BET analyses, it is noted that the hierarchically macroporous− mesoporous LnFeO3 with a nanocrystalline wall can be easily synthesized using a simple sol−gel route, and amphiphilic triblock copolymer F127 is the only template. In order to clarify the effect of amphiphilic triblock copolymer F127 in the formation of macroporous structure, the propanol solution containing amphiphilic triblock copolymer F127, Ln3+, and Fe3+ in the preparation process was analyzed by dynamic light scattering (DLS), and the DLS diameter is ca. 40 nm (Figure 7). Therefore, it is speculated that triblock copolymer F127 in this solution forms larger micelles, named supermicelles, than in the synthesis of the mesoporous materials. It suggests a

LaFeO3 after annealing at different temperatures. Although the LaFeO3 nanoparticles grow with increasing annealing temperature, which agrees with the above the XRD pattern, the hierarchically porous structure remains. When the annealing temperature is up to 700 °C, the mesoporous structure in the macroporous walls is destroyed because of the grain growth of LaFeO3. The porous property of the hierarchically macroporous− mesoporous LaFeO3 was analyzed through the nitrogen adsorption−desorption isotherms. Figure 5 shows the nitrogen adsorption−desorption isotherms of hierarchically macroporous−mesoporous LaFeO3. In accordance with the IUPAC classification, the nitrogen adsorption−desorption isotherms show a type IV isotherm and a type H2 hysteresis loop,46 which indicates a uniform cylindrical mesoporous structure in hierarchically macroporous−mesoporous LaFeO3. The pronounced hysteresis loop is present in the nitrogen adsorption− desorption isotherm curve, which indicates that the porous structure is an intersection network. The Barrett−Joyner− Halenda (BJH) analysis exhibits a high BET surface area of 43.6 m2 g−1 and a large pore volume of 0.20 cm3 g−1. The pore size distribution estimated using the classical BJH model via the analysis of the desorption branch of the nitrogen adsorption− desorption isotherms fits very well with this value. The main pore size is 15 nm, and the extremely small part of the pore size is 3.6 nm (Table 1). As the annealing temperature increases, the BET surface areas of the hierarchically macroporous− mesoporous LaFeO3 decrease from 43.6 to 37.4 m2 g−1. It is believed that the increased annealing temperature partially destroys the mesoporous structure in the macroporous walls, which further confirms the TEM result. When the annealing Table 1. Surface Areas, Pore Size, and Pore Volume of the As-Obtained Hierarchically Macroporous−Mesoporous LaFeO3 and LaFeO3 after Annealing at Different Temperatures temperature (°C)

surface area (m2 g−1)

pore size (nm)

pore volume (cm3 g−1)

400 500 600 700

43.6 40.5 37.4 14.8

3.6, 15 3.5, 14 3.5, 18

0.20 0.19 0.20 0.13 8675


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Figure 6. TEM images of as-obtained hierarchical macroporous−mesoporous LnFeO3 species: (a) PrFeO3, (b) NdFeO3, (c) SmFeO3, (d) EuFeO3, (e) GdFeO3, (f) TbFeO3, (g) DyFeO3, (h) HoFeO3, (i) ErFeO3, (j) TmFeO3, and (k) YbFeO3.

the Supporting Information shows TEM images of as-obtained LaFeO3 using different amounts of Pluronic F127. When the amount of Pluronic F127 is 0.5 g, the macroporous structure does not form. When the amount increases to 0.7 g, some macroporous structure can be observed in the sample. However, when the amount is more than 0.9 g, the macroporous structure starts to collapse, with Pluronic F127 serving only as a mesoporous template. Hence, the amount of

cooperative self-assembly mechanism in which amphiphilic triblock copolymer F127 acts as a dual templating agent. Amphiphilic triblock copolymer F127 not only directs the formation process of the mesostructure among Ln and Fe species but also templates the aggregation of the mesophase through the formation of supermicelles.50−54 We also studied the effect of the added amount of Pluronic F127 on the hierarchical macroporous−mesoporous structure. Figure S3 in 8676


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mesoporous structures not only can provide high surface area and large pore volumes but also can efficiently provide guest molecules with rapid diffusion to the catalytically active sites in the mesopores.54,57 The hierarchical macroporous−mesoporous structures have been studied in diffusion-limited processes theoretically, and it was found that this porous structure was able to inprove the catalytic efficiency effectively.58,59

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CONCLUSIONS We described a new synthesis strategy to prepare hierarchical macroporous−mesoporous materials containing poly(ethylene oxide)-poly(phenylene oxide)-poly(ethylene oxide) as a single template and an acid adjusting agent, and the behavior of the triblock copolymer is studied in propanol solution. There is a hierarchical structure including macropores with a size of 50− 100 nm and mesopores in the macroporous walls with a size of 3−5 nm. The macroporous walls are composed of rare earth orthoferrite nanoparticles with a size of 5−10 nm. These porous materials show high catalytic activities for the CO + NO reaction, and NO can be completely converted to N2 at a temperature of as low as 350 °C, indicating that they can be used in the catalytic conversion of automotive exhaust gas and other catalytically related fields. The high catalytic activity of these materials can be attributed to the high oxygen vacancy formation and migration, small particle size, high surface area, and large pore volumes associated with the accessible hierarchical mesoporous−macroporous structures. We believe that this synthesis strategy is a facile method for the preparation of hierarchical porous materials and may give us a guideline for the synthesis of functional materials with further catalytic applications.


Figure 7. Dynamic light scattering (DLS) diameter of the propanol solution containing triblock copolymer F127, Ln3+, and Fe3+ in the preparation process.

Pluronic F127 plays a key role in forming a hierarchical macroporous−mesoporous structure. The CO + NO reaction is of fundamental importance in the treatment of automotive exhaust gas. Ikeda et al. reported a SiO2-coated Rh catalyst for the NO + CO reaction, and the 50% NO conversion temperature was 280−340 °C.55 After annealing at 900 °C for 12 h, the 50% NO conversion temperature increased to 300−420 °C.55 Hecker et al. reported the reduction of NO on a silica-supported rhodium catalyst, and the 100% NO conversion temperature is higher than 523 K.60 The catalytic activity profiles of hierarchically macroporous−mesoporous LaFeO3 for the CO + NO reaction are shown in Figure 8. It can be seen that the NO conversion

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01519. Nitrogen gas adsorption−desorption, pore size and pore volume, surface areas, isotherms, and pore size distribution curves of the as-obtained hierarchically macroporous−mesoporous LnFeO3. Wide-angle XRD patterns of the as-obtained hierarchically macroporous− mesoporous LnFeO3 and after annealing at 700 °C. CO + NO reaction as a function of temperature for the asobtained hierarchically macroporous−mesoporous LaFeO3 and LaFeO3 after annealing at 700 °C. (PDF)

Figure 8. CO + NO reaction as a function of temperature for the asobtained hierarchical macroporous−mesoporous LaFeO3.

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increases with the increase in reaction temperature, a 100% conversion is obtained at temperature of as low as 350 °C, and the selectivity of N2 is 100%, indicating high catalytic activities and selectivities for the CO + NO reaction. After annealing at 700 °C, the 100% conversion temperature increases and the selectivity of N 2 decreases (Figure S4 in Supporting Information) because the porous structure is severely damaged and the surface area decreases, thus reducing catalytically active sites. The high catalytic activities of hierarchically macroporous−mesoporous LaFeO3 materials can be attributed to the high oxygen vacancy formation and migration, small particle size, high surface area, and large pore volumes associated with the accessible hierarchical mesoporous−macroporous structures. The high oxygen vacancy formation in this system plays a key role in the observed excellent catalytic activity.49,56 It is interesting and noteworthy that the hierarchical macroporous−

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-6275-4179. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (nos. 21461162001, 21425101, 21321001, 21371011, and 21331001).

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