Highly stable, mesoporous mixed lanthanum– cerium oxides with tailored structure and reducibility S. Liang, Esteban Broitman, Y. Wang, A. Cao and G. Veser Linköping University Post Print N.B.: When citing this work, cite the original article. The original publication is available at www.springerlink.com: S. Liang, Esteban Broitman, Y. Wang, A. Cao and G. Veser, Highly stable, mesoporous mixed lanthanum–cerium oxides with tailored structure and reducibility, 2011, Journal of Materials Science, (46), 9, 2928-2937. http://dx.doi.org/10.1007/s10853-010-5168-y Copyright: Springer Verlag (Germany) http://www.springerlink.com/?MUD=MP Postprint available at: Linköping University Electronic Press http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-88524 CREATED USING THE RSC ARTICLE TEMPLATE (VER. 3.1) - SEE WWW.RSC.ORG/ELECTRONICFILES FOR DETAILS ARTICLE TYPE www.rsc.org/xxxxxx | XXXXXXXX Highly stable, mesoporous mixed lanthanum-cerium oxides with tailored structure and reducibility Shuang Liang ab , Esteban Broitman c, Yanan Wang ab , Anmin Cao ab and Götz Veser *ab Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X 5 First published on the web Xth XXXXXXXXX 200X DOI: 10.1039/b000000x Abstract : Pure and mixed lanthanum and cerium oxides were synthesized via a reverse microemulsion-templated route. This approach yields highly homogeneous and phase-stable mixed oxides with high surface areas across the entire range of La:Ce ratios from pure lanthana to pure 10 ceria. Surprisingly, all mixed oxides show the fluorite crystal structure of ceria, even for lanthanum contents as high as 90%. Varying the La:Ce ratio not only allows tailoring of the oxide morphology (lattice parameter, pore structure, particle size, and surface area), but also results in a fine-tuning of the reducibility of the oxide which can be explained by the creation of oxygen vacancies in the ceria lattice upon La addition. The described approach should be applicable to a 15 broad range of other mixed oxides, and hence opens the path towards functional tailoring of oxide materials, such as rational catalyst design via fine-tuning of redox activity. 14, 17, 19-21 . Introduction Here, we report the synthesis of nanostructured mixed La/Ce-oxides via a straightforward reverse microemulsion- Functional materials based on rare earth elements are of broad 55 templated approach, and the systematic investigation of the technological interest, and the synthesis of nanostructured rare La:Ce-ratio on their reducibility. Reverse microemulsion- 20 earth oxides in particular, such as nanostructured lanthanum templated syntheses offer exceptional control over synthesis and cerium oxides, has found increasing attention in recent 22, 23 conditions . Confining the materials synthesis into the years.1-4 For example, nanostructured lanthana has been 5, 6 nanosized micelles not only allows regulation of particle size, shown to be an outstanding absorbent for H 2S (a widely 60 and hence yields high surface area materials, but it also exerts regulated air pollutant and a strong poison for typical metal 7, 8 control over the hydrolysis rate of different precursors for 25 catalysts ), while ceria is widely investigated as catalyst mixed oxides and hence allows for finely controlled synthesis support for noble metals in a number of reactions 9-12 . Beyond of highly homogeneous mixed oxides with excellent thermal the pure oxides, growing interest is focusing on mixed cerium 24, 25 stability . Due to these well-controlled synthesis and other rare earth oxides 4, 13-15 . Doped lanthana 65 conditions, we were able to synthesize homogeneous mixed nanoparticles are of interest as chemically stable photo- 16 La/Ce-oxides over the entire range of La:Ce-ratios. The 30 luminescent materials , and doped ceria shows strongly resulting oxides are characterized by high surface areas and, improved performance as catalyst support and oxygen storage more importantly, exceptional stability with no phase component in applications such as automotive exhaust control o separation up to temperatures in excess of 1000 C. The full 4, 17-19 . The latter is due to the fact that-unlike many traditional 70 control of the composition furthermore enables fine tailoring support materials-cerium oxide plays an important, active role of the reducibility of the materials. 35 as oxygen storage and oxygen transfer material in redox 4+ reactions based on the facile conversion between Ce and Experimental section Ce 3+ on the oxide surface. Doping ceria has been shown to further improve this performance due to increased reducibility Materials and syntheses and oxygen storage capacity. For example, computational Chemical reagents including lanthanum isopropoxide (Alfa- 40 studies have shown that doping ceria with Zr or Th cations 75 aesar, La 40%), cerium isopropoxide (Alfa-aesar, Ce 37- lowers the reduction energy and increases the oxygen 3 45%), poly(ethylene glycol)-block -poly(propylene glycol)- mobility . More recent results indicate that La dopings are block -poly(ethylene glycol) (PEPP, Aldrich, Mn=2000), even more effective in favoring the Ce 4+ /Ce 3+ reduction isooctane (Aldrich, 99.8%), 1-pentanol (Aldrich, 99+%), process than other trivalent dopants such as Sc, Mn, Y, Gd, 4 anhydrous 2-propanol (Aldrich, 99.5%) were used as received 45 and that this effect is enhanced with increasing La content . 80 without further purification. Pure lanthanum oxide (La 2O3), Despite this interest in mixed La 2O3-CeO 2 in particular, the cerium oxide (CeO 2), and mixed lanthanum cerium oxides synthesis of stable, homogeneous mixed oxides still poses a (La xCe 1-xO2-0.5x ) were all synthesized by a reverse major challenge. Previous studies reported phase separation of 17, 18 microemulsion method, as adapted previously in our mixed La 2O3-CeO 2 at elevated temperature , and the 26, 27 laboratory . For example, to prepare La 0.5 Ce 0.5 O1.75 , 10.97 50 solubility limit for La 2O3-CeO 2 solid solutions prepared by 85 g de-ionized water, 38.57 g isooctane and 12.85 g PEPP, 150 various methods is typically reported to be between 40%-70% g 1-pentanol were used to obtain a reverse microemulsion. 0.5 This journal is © The Royal Society of Chemistry [year] Journal Name , [year], [vol] , 00–00 | 1 g lanthanum isopropoxide and 0.59 g cerium isopropoxide of the oxide, created by the agglomeration of dense were dissolved in 108 ml anhydrous 2-propanol by stirring, nanoparticles about 10-20 nm in size. The mesoporous and refluxed at 95°C for 2 h. The solution was then introduced character of the materials is confirmed by BET, as seen in the to the microemulsion at room temperature. After ageing for 72 typical type IV isotherms with H2 hysteresis shown in Figure 5 h, the water and oil phases were separated by temperature- 60 2 (top, a and b). The surface area of the lanthana is close to induced phase separation (TIPS). The product phase was 100 m2/g after calcination at 450 °C (see Table 1), well in washed several times with acetone, and remaining volatile excess of previously reported surface areas of less than 30 residues were removed via freeze drying. Finally, the dried m2/g 30, 31 . XRD shows that the sample is largely amorphous powder was calcined at 450 °C or 750 °C for 2 h in air. after calcination at 450 °C (Figure 2 top, c), in agreement with 65 the known high crystallization temperature of La 2O3 of ~700 10 Characterization oC5, 32, 33 . The specific surface area was determined via nitrogen Cerium oxide synthesized in the same way shows a similar sorption in a Micromeritics ASAP 2020 gas adsorption morphology and structure (Figure 1 c and d, and Figure 2, analyzer using the BET method. Prior to the measurement, the bottom, a and b) and similarly high surface area of ~80 m 2/g o samples were degassed for 2 h at 200 C under high vacuum. 70 (Table 1), which can be attributed to the small particle size 15 The compositions of mixed oxides were determined by templated by the micelles in the microemulsion. Unlike Energy-dispersive x-ray analysis (EDX) equipped on Philips La 2O3, however, CeO 2 is already highly crystalline after XL-30 field emission scanning electron microscope (SEM). calcination at 450 oC, as confirmed by XRD (Figure 2 c, The X-ray diffraction (XRD) measurements were performed bottom). It is worthwhile to note that, compared to La 2O3, the o with a high-resolution powder X-ray diffractometer (Phillips 75 isotherm of CeO 2 calcined at 450 C (Figure 2, bottom, a,) 20 PW1830) in line focus mode employing Cu-Kα radiation ( λ = exhibits a different type H2 hysteresis loop with a linear 1.5418 Å). Crystal phases were identified based on JCPDS adsorption isotherm, indicating a highly disordered cards. Particle sizes were calculated from the Debye-Scherrer mesoporous structure with a wide pore size distribution 34, 35 . equation. Sample morphology was determined by After high-temperature calcination at 750 oC, the hysteresis transmission electron microscopy (JEOL-2000FX). 80 loop changes to a type H1 hysteresis (Figure 2, bottom, b,) 25 Temperature-programmed reduction by hydrogen (H 2-TPR) attributed to agglomerates of spheres with roughly uniform was conducted using a Micromeritics Chemisorb 2750 system size, resulting in narrow pore size distributions 34 . equipped with a thermal conductivity detector. During the Table 1 Surface area and particle size of La 2O3 and CeO 2 TPR analysis, the samples were first oxidized in 5% O 2/He at 450 oC for 2 h, and then TPR was performed by heating the 450 °C 750 °C Surface Particle Surface Particle 30 sample (100 mg) at 5 °C/min to 900 °C in a 10% H 2/Ar flow area (m 2/g) Size (nm) area (m 2/g) Size (nm) (30 ml/min).