Impressive low reduction temperature of synthesized mesoporous ceria via nanocasting

Materials Letters 130 (2014) 218–222

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Impressive low reduction temperature of synthesized mesoporous ceria via nanocasting Chonnikarn Deeprasertkul, Rujirat Longloilert, Thanyalak Chaisuwan, Sujitra Wongkasemjit n The Petroleum and Petrochemical College and Center of Excellence for Petrochemical and Materials Technology, Chulalongkorn University, Bangkok, Thailand

art ic l e i nf o

a b s t r a c t

Article history: Received 7 February 2014 Accepted 17 May 2014 Available online 27 May 2014

Ceria or cerium oxide, with high surface area and ordered structure, was prepared by the nanocasting method using MCM-48 porous material as a hard template. Optimal conditions were investigated to obtain ordered mesoporous ceria having high surface areas of 224.7 m2/g and ordered structure with 50% weight ceria using 30 min stirring time at 100 1C evaporation temperature of solvent. The mesoporous ceria was characterized using various techniques. The Temperature-programmed reduction results provided only surface reduction temperatures at 4001–600 1C. & 2014 Elsevier B.V. All rights reserved.

Keywords: Mesoporous ceria MCM-48 template Nanocasting process TPR

1. Introduction Cerium oxide or ceria (CeO2) has also been widely used for oxygen storage capacity [1] and environmental catalysis [2]. The most important application of ceria is as a three-way catalysis promoter in catalytic converter, for the elimination of toxic autoexhaust gases [3,4–6]. Ceria has two characteristics [4] appropriate for use in three-way catalysts: (1) an oxidation state between Ce3 þ and Ce4 þ , CeO2/Ce2O3, under oxidizing and reducing conditions and (2) oxygen storage and release properties. The catalytic performance of cerium oxide can be increased by its structural properties, such as surface area and crystal shape. Ceria with mesoporous structure and high surface areas has been synthesized by nanocasting method with various templates, both soft and hard [7,8]. The hard templates have shown many advantages over the soft ones, especially for producing highly crystalline walls, predictability and controllability [4,7]. Moreover, the hard templates can provide well-ordered structure of frameworks, leading to high surface areas of replica. In this study, the ordered mesoporous (MSP) ceria materials were synthesized via nanocasting process using a hard template MCM-48 directly synthesized from home-made silatrane and the structure directing agent CTAB at 140 1C for 16 h [9]. Optimal conditions of the nanocasting method were investigated to obtain ordered MSP ceria having high surface areas. Physical and reduction properties were characterized using X-ray diffraction (XRD), X-ray fluorescence

spectrophotometer (XRF), N2 adsorption/desorption analysis, Transmission electron microscope (TEM), Field emission scanning electron microscope (FE-SEM), X-ray fluorescence spectrophotometer (XRF), and Temperature programmed reduction (TPR).

2. Experimental The MCM-48 and inorganic cerium nitrate (50%, 60%, 70%, and 80% weight of ceria) were dissolved in 5 ml of ethanol. After stirring (30 min, 1, 2, and 4 h), the ethanol in the mixture was removed by evaporation in an oven (501, 100 1C) or at ambient temperature. The process was repeated to get the two and three filling cycles of ceria. The obtained powder was heated in a ceramic crucible at 550 1C for 6 h to decompose the nitrate species. The silica hard template was removed by using 2 M NaOH at 50 1C three times, and the mixture was centrifuged to obtain the product. The product was washed by deionized water and centrifuged until the washing was neutral and dried at 100 1C. The products were characterized by XRD. The morphology of the products was characterized using SEM and TEM. Specific surface area, pore volume, and average pore size were determined using the Brunauer–Emmett–Teller (BET) method on a Quantasorb JR instrument. The element contents in products were analyzed by XRF. The reducibility of products was analyzed by TPR.

3. Results and discussion n

Corresponding author. Tel.: þ 66 2 218 4133; fax: þ 66 2 215 4459. E-mail address: [email protected] (S. Wongkasemjit).

http://dx.doi.org/10.1016/j.matlet.2014.05.124 0167-577X/& 2014 Elsevier B.V. All rights reserved.

Nanocasting process:(Effect of cerium oxide percentage by weight).The ordered MSP ceria at 50–70% weight of cerium oxide

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Fig. 1. XRD patterns of (a) MCM-48 and the ordered MSP ceria resulted from various.(b) ceria percentages by weight, (c) stirring times, (d) evaporation temperatures of solvent, and (e) filling cycles.

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(Fig. 1(b)), which was the negative replica of the MCM-48 template, showed the same characteristic diffraction peaks at {211} and {220} as the silica template, whereas the 80% weight showed only one peak at {211}. These results suggested that the ceria replicas still retained some order from their template. The XRD pattern of the 50% weight of ceria showed higher intensity and sharper peaks than the others probably due to agglomeration of ceria in the pore channel of MCM-48 from the higher ceria loading during the casting process [10]. Moreover, the intensity of

Table 1 BET of ordered MSP ceria at different evaporation temperatures and stirring times. Ordered mesoporous ceria

Surface area (m2 /g)

Stirring time 30 min 1h 2h 4h

246.6 241.4 213.9 214.0

Evaporated temperature RT 50 1C 100 1C

238.4 211.8 254.0

the ordered MSP ceria exhibited lower intensity than the MCM-48, indicating less order of the replica than the template. Thus, the best ceria percentage to obtain a high surface area with maintaining the MCM-48 structure was 50%. Nanocasting process: (Effect of stirring time).The obtained XRD patterns of all ordered MSP ceria, shown in Fig. 1(c), exhibited the MCM-48 pattern. The intensity of the peak at {211} decreased with stirring time due to more precursors that went inside the pore and agglomerated, resulting in the less ordered structure at 4 h stirring time [10,11]. These results indicate that the ordered MSP ceria at 30 min stirring time had the most ordered structure. Specific surface area results, see Table 1, also decreased with increasing stirring time. The highest surface area (246.6 m2/g) was obtained at 30 min stirring time. Thus, the appropriate stirring time was 30 min. Nanocasting process: (Effect of evaporated temperature).The precursor inside the pore void was expected to migrate and impregnate inside the pore during the evaporation of solvent [12]. The XRD patterns of the ordered MSP ceria are shown in Fig. 1(d), also maintaining the MCM-48 pattern although the sample obtained at ambient temperature showed less ordered than the others. It can be concluded that the structure of the retained MCM-48 and the studied range of the temperature had no influence on the migration of the precursor inside the pore. The highest surface area (254 m2/g) in Table 1 was obtained from the sample evaporated at

Fig. 2. N2 adsorption–desorption isotherms and pore size distribution (inset) of (a) MCM-48. and the ordered MSP ceria resulted from (b) one, (c) two, and (d) three filling cycles.

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Table 2 BET and XRF analyses of MCM-48, ceria powder and ordered MSP ceria at different filling cycles. Sample

MCM-48 Ceria powder Ordered MSP ceria (1 filling cycle) Ordered MSP ceria (2 filling cycles) Ordered MSP ceria (3 filling cycles) a b n

Element contentsn(%) Ce

O

Si

– – 54.526 58.791 53.415

– – 43.541 38.764 44.395

– – 1.933 2.445 2.190

BET surface area (m2/g)

Pore volume (cm3/g)

Pore size (nm)

ɑ0a (nm)

d211 (nm)

Wall thicknessb (nm)

1614 77.1 224.7 207.9 192.0

1.1 – 0.6 0.4 0.2

2.5 – 4.8 4.5 4.5

8.60 – – – –

3.51 – – – –

1.53 – – – –

ɑ0 ¼d211(6)1/2. Wall thickness¼ɑ0/3.0919  pore diameter/2. Data were obtained from XRF.

Fig. 3. SEM images of (a) ceria, and ordered MSP ceria resulted from (b) one, (c) two.and (d) three filling cycles, and (e) and (f) TEM images of the synthesized ceria.

100 1C probably due to the fact that the evaporation time was the shortest (around 1 h). At the longer evaporation time, the precursor migrating inside the pore at the ambient temperature

caused the pore blocking and the distortion of the MSP structure, resulting in the less-ordered XRD pattern [13]. The suitable evaporation temperature in this study, then, was at 100 1C.

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ordered after removal of the silica template. Comparing the TEM image of the ceria product with that of MCM-48 (not shown), They are similar to each other, indicating that the mesoporous ceria has the same ordered structure as MCM-48 Temperature programmed reduction (TPR):TPR profiles of the ceria powder and the ordered MSP ceria are shown in Fig. 4. Two reduction peaks were observed at 3501–650 1C and 7001–850 1C, relating to the reduction of the surface-capping oxygen of CeO2 and the bulk-phase lattice oxygen [3,5]. The highest intensity of the low-temperature peak (Fig. 4(a)) was observed on the ordered MSP ceria obtained at one filling cycle. The large area peak obtained implies that the obtained MSP ceria from one filling cycle and having the highest surface area consumed the highest H2. This is its advantage for use as a catalyst in three-way catalyst to convert toxic auto-exhaust gases to less harmful gases [17].

4. Conclusions

Fig. 4. TPR profiles of the synthesized ceria resulted from (a) one, (b) two, and (c) three filling.cycles (d) ceria powder.

Nanocasting process: (Effect of filling cycle).The XRD results (Fig. 1(e)) also exhibited the diffraction peaks of the MCM-48 structure. The XRD peaks obtained from three filling cycles (Fig. 1 (e)) exhibited the sharpest diffraction peaks at {211} and {220} when compared to the others, suggesting that it better duplicated the structure of the template than the others. In other words, it has a more ordered structure. This result confirmed that a repeated filling cycle can effectively increase the filling degree [14]. The N2 adsorption–desorption isotherms of the MCM-48 and ordered MSP ceria are shown in Fig. 2. It was found that the IUPAC classification of the synthesized ceria was a type IV isotherm with H3 hysteresis loop, corresponding to MSP materials with slit-like pores. The isotherms show two steps at P/Po ¼ 0.5–0.7 and 0.9–1.0, reflecting the capillary condensation and the interparticle porosity, respectively [15]. The pore size distribution curve (inset) shows the sharp peak with the pore diameter around 6 nm which confirms the ordered mesoporous ceria contains uniform pores. The structural parameters of various samples are shown in Table 2. The increased filling cycle decreased the BET specific surface area and the pore volume, meaning that ceria infiltrated the template pore [15]. The pore size of the ordered MSP ceria was in a range of 4.5–4.8 nm which was larger than the wall thickness of MCM-48 (1.53 nm). This could indicate that the fillings of the pores overlapped in some region of the template [16]. The highest surface area of the obtained ceria was 224.7 m2/g at one filling cycle. The other ceria products were still larger than the ceria powder directly calcined from the precursor at 550 1C for 6 h. SEM images of ceria and ordered MSP ceria are shown in Fig. 3. The ordered MSP ceria possessed spherical morphology, comprised of small, round crystalline structures of ceria. Interestingly, additional filling cycles resulted in a denser spherical morphology of the ordered MSP ceria (Fig. 3 (b)–(d)). The SEM results can be well related to the surface area results that the denser spherical morphology gave the lower surface area. To confirm the order of the mesoporous ceria structure, TEM of the ceria product was analyzed, as shown in Fig. 3(e), (f)). The mesoporous ceria was well

The ordered MSP ceria was successfully synthesized using MSP MCM-48 silica as hard template via the nanocasting method. The optimum conditions achieved were by using 50% weight of ceria, 30 min stirring time, 100 1C evaporation temperature, and one filling cycle. The ordered MSP ceria exhibited a much larger surface area and also showed a larger area under the TPR peak, referring to the strong reduction at a lower temperature.

Acknowledgments The work is supported by the Thailand Research Fund, the Ratchadapisake Sompote Endowment Fund, and the Center of Excellence for Petrochemical and Materials Technology, Chulalongkorn University, Thailand. The authors are also thankful to Professor Ram Seshadri for kindly sponsoring a Ph.D. student to experience different research techniques at the Materials Department, University of California, Santa Barbara, USA, and Mr. John M. Jackson for English proof-reading.

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