Langmuir 2004, 20, 11223-11233
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Microemulsion-Based Synthesis of CeO2 Powders with High Surface Area and High-Temperature Stabilities Ali Bumajdad, Mohamed I. Zaki,* Julian Eastoe,† and Lata Pasupulety Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, Safat, 13060 Kuwait Received June 1, 2004. In Final Form: September 10, 2004 Pure ceria powders, CeO2, were synthesized in heptane-microemulsified aqueous solutions of CeCl3 or Ce(NO3)3 stabilized by AOT (sodium bis(2-ethylhexyl) sulfosuccinate), DDAB (di-n-didodecyldimethylammonium bromide), or DDAB + Brij 35 surfactant mixtures. Micellar DTAB (n-dodecyltrimethylammonium bromide) and vesicular DDAB systems were also used as media for generating CeO2. Characterization of the powders by X-ray powder diffractometry, laser-Raman spectroscopy, and Fourier transform infrared spectroscopy revealed that in the presence of surfactants almost-agglomerate-free nanosized crystallites (6-13 nm) of anionic vacancy-free cubic CeO2 were produced. In the absence of surfactants 21-nm-sized crystallites were formed, comparing with the 85-nm-sized crystallites when cubic CeO2 was created via thermal decomposition of cerium oxalate. Surface characterization, by X-ray photoelectron spectroscopy, N2 sorptiometry, and high-resolution electron microscopy showed AOT- or (DDAB + Brij 35)-stabilized microemulsions to assist in formation of crystallites exposing surfaces of large specific areas (up to ca. 250 m2/g) but of low stability to high-temperature calcination (28-13 m2/g at 800 °C). In contrast, the doublechained DDAB was found to generate cubic CeO2 crystallites of lower initial surface areas (144 (microemulsion) to 125 (vesicles) m2/g)) but of higher thermal stability (55-45 m2/g at 800 °C). Hence, the latter cerias could be considered as appropriate components for total oxidation (combustion) catalysts.
1. Introduction Ceria (CeO2) and related oxides, namely, Ce-Zr-O and Ce-Pr-O, enjoy a breadth of surface-driven applications in heterogeneous catalysis and in the fabrication of fuel cells, microelectronics, gas sensors, and polishing materials. Relevant details of these, and other applications, have recently been reviewed by Trovarelli.1 Though they have only lately been found to be independent,2,3 the surface specific area (m2/g) and oxygen storage capacity (OSC) are crucial properties for most of these applications. For instance, the potential of ceria in the catalysts developed for the treatment of automotive exhaust gases stems from its ability to store oxygen under lean fuel conditions and to release it when the oxygen concentration becomes virtually nil (rich fuel conditions).4 This perfectly reversible redox behavior,5 which facilitates economic production of H2 when water vapor is used as an oxidant for CeO2-x,5 as well as acido-basic surface properties,6 means that ceria is an important ingredient in the chemical composition of catalysts employed in fluid catalytic cracking, SOx removal, ethylbenzene dehydrogenation, and the watergas shift reaction.1,7 * Corresponding author. Permanent address: Faculty of Science, Minia University, El-Minia 61519, Egypt. E-mail:
[email protected]. Fax: 0020862360888. † Permanent address: School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. (1) Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; Imperial College Press: London, 2002. (2) Mamontov, E.; Brenzy, R.; Koranne, M.; Egami, T. J. Phys. Chem. 2003, 107, 13007. (3) Schulz, H.; Stark, W. J.; Maciejewski, M.; Pratsinis, S. E.; Baiker, A. Mater. Chem. 2003, 13, 2979. (4) Taylor, K. C. In Catalysis-Science and Engineering; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 5, pp 120155. (5) Ostuka, K.; Hatano, M.; Morikawa, A. J. Catal. 1983, 80, 114. (6) Zaki, M. I.; Hasan, M. A.; Al-Sagheer, F. A.; Pasupulety, L. Colloids Surf. 2001, 190, 261. (7) Trovarelli, A.; de Leitenburg, C.; Boaro, M.; Dolcetti, G. Catal. Today 1999, 50, 353.
Accordingly, synthesis of ceria with large surface specific areas and oxygen storage capacities has been at the focus of numerous researchers.1 Whereas remarkable advances have been scored toward synthesis of cerias of thermally stable, large OSCs, using zirconia1-3,8 and praseodia9 additives, obtaining pure ceria of thermally stable, higharea surfaces has not hitherto been accomplished without stabilizing additives of zirconia.10-15 This latter unaccomplished objective has hindered the promising applications of ceria in combustion catalysts for natural gas turbines.16 In natural gas combustion processes catalysts must withstand high temperatures (800-900 °C).16 Although endeavors to synthesize high-area ceria powders have profited from recent developments of methods to prepare nanosized solid particles,17 such as the sol-gel,1,18 complexation,1,13,19,20 and hydrothermal1,10,15,21,22 and microemulsion1,11,12,14,23,24 methods, the accomplishments hitherto achieved are confined to yielding pure cerias of (8) Mamontov, E.; Egami, T.; Brezny, R.; Koranne, M.; Tyagi, S. J. Phys. Chem. B 2000, 104, 11110. (9) Rossignol, S.; Gerard, F.; Mesnard, D.; Kappenstein, C.; Duprez, D. Mater. Chem. 2003, 13, 3017. (10) Si, R.; Zhang, Y.-W.; Xiao, C.-X.; Li, S.-J.; Kou, Y.; Yan, C.-H. Phys. Chem. Chem. Phys. 2004, 6, 1056. (11) Masui, T.; Fujiwara, K.; Peng, Y.; Sakata, T.; Machida, K.-I.; Mori, H.; Adachi, G.-Y. J. Alloys Compd. 1998, 269, 116. (12) Terribile, D.; Trovarelli, A.; Llorca, J.; de Leitenburg, C.; Dolcetti, G. Catal. Today 1998, 43, 79. (13) Zhang, F.; Jin, Q.; Chan, S.-W. J. Appl. Phys. 2004, 95, 4319. (14) He, Y.; Yang, B.; Cheng, G. Mater. Lett. 2003, 57, 1880. (15) Ahniyaz, A.; Fujiwara, T.; Fujino, T.; Yoshimura, M. J. Nanosci. Nanotechnol. 2004, 4, 233. (16) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Appl. Catal., A 2002, 234, 1. (17) Campanati, M.; Fornasari, G.; Vaccari, A. Catal. Today 2003, 77, 299. (18) Alifanti, M.; Baps, B.; Blangenois, N.; Naud, J.; Grange, P.; Delmon, B. Chem. Mater. 2003, 15, 395. (19) Duran, P.; Capel, F.; Gutierrez, D.; Tartaj, J.; Moure, C. J. Eur. Ceram. Soc. 2002, 22, 1711. (20) Rocha, R. A.; Muccillo, E. N. S. Mater. Res. Bull. 2003, 38, 1979. (21) Zhou, X.-D.; Huebner, W.; Anderson, H. U. Chem. Mater. 2003, 15, 378. (22) Wang, Z. L.; Feng, X. J. Phys. Chem. B 2003, 107, 13563.
10.1021/la040079b CCC: $27.50 © 2004 American Chemical Society Published on Web 11/11/2004
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Chart 1. Schematic Chemical Structures of the Surfactants
Bumajdad et al.
as compared with that of pure DDAB, and this allows studies away from the lower “haze” phase boundary where the system is known to exhibit attractive interactions leading to nonspherical aggregates,29 and (3) the water solubilization of reactants is enhanced upon partially replacing DDAB by Brij 35,31 thus increasing the yield of precipitates. The resulting ceria products were subjected to a set of bulk and surface analytical techniques in order to (i) verify the ceria bulk composition and structure and (ii) the surface chemical composition, structure, and specific area. These properties were also determined after calcination at 400-800 °C. It is worth noting, that the present work is part of a comprehensive investigation, taking place in this laboratory, in hopes of synthesizing simple, and composite, metal oxides suitable as components for combustion catalysts. 2. Experimental Section
specific surface areas amounting to 150-200 m2/g.25 When subjected to high-temperature treatments at 600-800 °C, nano-cerias have been found to suffer considerable particle growth, leading to catastrophic reduction in the surface accessibility.25 At >1000 °C, an effective sintering results in ceramic-grade, dense-bulk cerias of poor surface areas (99.5%, Riedel deHae¨n), recrystallized twice from ethyl acetate (99.5%, Aldrich), and then vacuum-dried again at 50 °C overnight, crushed, and saved in bottles sealed with parafilm in a desiccator. The structure and high purity of the product were verified by 1H NMR (a Bruker Spectrospin Evance, DPX), elemental analysis (a CHNS LECO 932), and the maximum water solubility (Wmax) of heptane continuous microemulsion measurements. 2.2. Test Cerias. CeOx(NIT) was prepared by a slow, dropwise addition of 25% aqueous ammonia solution to a continuously stirred 0.1 M aqueous solution of cerium(III) nitrate (200 mL). Initially a light yellow precipitate was formed, which turned purple on stirring. After all the ammonia solution was added, the precipitate was stirred further in the mother liquor for 1 h, filtered through Wattman 42 paper, washed with double distilled water, and dried at 100 °C overnight. The precipitate, which turned yellow on drying, was ground, sealed in vials, and stored over silica gel in a desiccator until further use. CeOx(AOT) was synthesized by mixing two heptane-continuous AOT microemulsions, one of which contained aqueous cerium(III) nitrate solution of 0.05 M, and the other contained 10% ammonia solution. The AOT concentration was 0.1 M, and the amount of aqueous solution was chosen so that the system was just below its emulsification failure boundary.26-28 The mixing was carried out by dropwise addition of the ammonia microemulsion to the other, over 2-3 days with continuous stirring. The stable semitransparent suspension thus formed was stirred for 1 h and then kept overnight. Solid particles were separated by centrifugation (10 000 rpm, 1 h), ultrasonicated in acetone several times (each time was followed by centrifuging at 10 000 rpm for 0.5 h) in order to completely remove the surfactant molecules, and dried at 100 °C overnight. The dried solid, which was yellow, was ground by pestle and mortar and stored. (32) Zana, R. J. Colloid Interface Sci. 1980, 78, 330.
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Table 1. Designation and Preparation Precursors of Uncalcined Ceriaa uncalcined ceria
preparation precursors
CeOx(NIT) CeOx(AOT) CeOx(DAB) CeOx(DAB5) CeOx(TAB5) CeOx(BDN)
NH3 (25%)aqV + Ce(NO3)3 (0.1 M)aq {AOT (0.1 M) + heptane + NH3 (10%)aq}V + {AOT (0.1 M) + heptane + Ce(NO3)3 (0.05 M)aq} NH3 (25%)aqV + {DDAB (0.1 M) + heptane + Ce(NO3)3 (0.1 M)aq} NH3 (25%)aqV + [DDAB (0.05 M) + CeCl3 (0.05 M)] w 5 days NH3 (25%)aqV + [DTAB (0.05 M) + CeCl3 (0.05 M)] w 5 days {(Brij + DDAB) (0.1 M) + heptane + NH3 (10%)aq} + {(Brij + DDAB) (0.1 M) + heptane + Ce(NO3)3 (0.05 M)aq}, fast mixing {(Brij + DDAB) (0.1 M) + heptane + TBAH (10%)aq}V {(Brij + DDAB) (0.1 M) + heptane + Ce(NO3)3 (0.05 M)aq}, slow mixing
CeOx(BDT) a
Key: V ) added from a buret; { } ) microemulsion; aq ) [ ] ) aqueous medium; w stirred continuously for.
CeOx(DAB) was produced at room temperature by a slow dropwise addition, with continuous stirring, of 25% ammonia solution to a heptane-continuous DDAB microemulsion of 0.1 M cerium(III) nitrate as the aqueous core. The total amount of aqueous cerous nitrate + ammonia solution was maintained at less than the maximum solubility limit of the microemulsion, which can be easily determined visually by following the turbidity and/or the aqueous solution precipitation.29-31 The stirring was continued for 1 h following mixing. Particle separation and cleanup were the same as for CeOx(AOT) above. CeOx(DAB5) was obtained by dissolving a weighed amount of cerium(III) chloride salt in a 0.05 M solution of DDAB in 100 mL of water, to eventually reach a salt concentration of 0.05 M. The mixture was stirred vigorously, and then a 45.45-mL aliquot of 25% ammonia solution was slowly added dropwise, over 1 h, and stirred at room temperature for 1 h further. The purple solution thus obtained was sealed in a tube and stirred continuously for 5 days in a water bath adjusted at 80 °C (the suffix “5” identifies these oxides in the paper). The solid residue, which suffered a color change from purple to yellow, was separated by filtration through Whatman 42 paper, washed thoroughly with acetone, dried at 100 °C overnight, ground by pestle and mortar, and stored. It is worth mentioning that at this concentration of DDAB, the preferred aggregate structure would be vesicles (the used concentration is more than the critical vesicle concentration, of DDAB ) 9 × 10-4 M27 but less than the concentration required to form lamellar liquid crystals, which is 0.086 M).34 CeOx(TAB5) was produced following the same procedure applied to obtain CeOx(DAB5), except for using DTAB instead of DDAB. Here the aqueous system contains micelles (critical micelle concentration of DTAB ) 0.0133 M).35 CeOx(BDN) was obtained by rapid mixing of two microemulsions stabilized by a 0.01 M Brij 35 + 0.09 M DDAB surfactant mixture, one containing 0.05 M solution of cerium(III) nitrate and the other 10% ammonia solution (the suffix N refers to NH3 as a precipitant), with continuous stirring for 1 h. The mixture was kept overnight, and then the resulting precipitate was separated by centrifugation, washed thoroughly by ultrasonication in acetone, dried at 100 °C overnight, ground, and stored. CeOx(BDT) was prepared following the same procedure applied to yield CeOx(BDN), except that the precipitant was a 10% TBAH. The addition of the TBAH microemulsion was carried out dropwise over 2 days with continuous stirring. For convenience, the designations and preparation precursors of the above cerias are further summarized in Table 1. It is worth noting that (i) the stirring was magnetic, (ii) an ultrasonic bath was used, (iii) the centrifuge employed was a Beckman J-2 MC, (iv) volumes of the precipitants added (NH3 or TBAH) were decided on basis of the Wmax of the microemulsion, which is the molar ratio of the aqueous phase and the surfactant(s) content (i.e., Wmax ) [aqueous]/[surfactant(s)] above which the Winsor II system is obtained (i.e., water-in-oil microemulsion in equilibrium with excess lower water phase),36 (v) the surfactant(s) concentration was held constant at 0.1 M for microemulsion systems and at 0.05 M for aqueous systems, and the oil-continuous phase was (33) Fontana, A.; De Maria, P.; Siani, G.; Robinson, B. H. Colloid Surf., B 2003, 32, 365. (34) Kang, C.; Khan, A. J. Colloid Interface Sci. 1993, 156, 218. (35) van Os, N. M.; Haak, J. R.; Rupert, L. A. M. In Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993. (36) Winsor, P. A. Trans. Faraday Soc. 1948, 44, 376.
always n-heptane, and (vi) the above indicated cerias were calcined (heated in air) at 400, 600, and 800 °C for 2 h, using a Thermolyne 6000 electronically controlled muffle furnace. For simplicity, the calcination products are discerned from the uncalcined parent by suffixing a number symbolizing the calcination temperature to the parent designation. Hence, CeOx(NIT)-4 is the 400 °C calcination product of CeOx(NIT), whereas CeOx(NIT)-6 and CeOx(NIT)-8 are its calcination products at 600 and 800 °C, respectively. 2.3. Characterization Methods and Techniques. The crystalline bulk structure, average crystallite size, and unit cell parameters were determined by X-ray powder diffractometry (XRD), whereas the chemical composition of both crystalline and noncrystalline domains of the bulk was identified by Fourier transform infrared (FT-IR) and laser Raman (LRa) spectroscopies. The bulk thermal stability was probed by means of thermogravimetry (TG) and differential thermal analyses (DTA). Surface chemical composition and specific area were determined using X-ray photoelectron spectrometry (XPS) and BET analysis of N2 sorptiometry (SBET), respectively. Surface microstructure and morphology were examined by high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). It is worth noting that the test samples were not ground before analysis, except for those subjected to IR spectroscopy. XRD was carried out (at 2θ ) 10-80° and RT) using a model D5000 Siemens diffractometer (Germany) equipped with Nifiltered Cu KR radiation (λ ) 0.15406 nm). The diffractometer was operated with 1° diverging and receiving slits at 50 kV and 40 mA and a continuous scan was carried out with a step size of 0.014° and a step time of 0.2 s. An on-line automatic search system (PDF Data Base) facilitated an observed data match with JCPDS standards.37 Crystallite sizing was realized by adopting the line-broadening technique and Sherrer formula,38 whereas unit cell lattice parameters were determined by means of an installed Win-Metric software (v.2, Siemens Corp.). To warrant a credible comparison between the results, comparable amounts (20-25 mg) of the test samples were used for analysis. IR spectra were taken from KBr-supported test materials ( 1).23
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Table 3. Surface Chemical Composition and Corresponding XPS-Derived Characteristics of 400 °C Calcined Cerias TAT% (DR)a C I BE/(0.5 eV speciese
II
1sb
O III
I 529 O2-
1sc
Ce 3dd II
CeOx(BDN)
284 286 288 CsC/ CsO OdCsO/ CsH OdCsH 25.7 (I ) 51; II ) 23; III ) 26%)
532 OH/H2O
38.2 (I ) 43; II ) 43%)
CeOx(BDN)-4
2.2 (I ) 54; II ) 15; III ) 27%)
4.7 (I ) 80; II ) 20%)
CeOx(TAB5)-4 CeOx(DAB)-4 CeOx(DAB5)-4 CeOx(BDT)-4 CeOx(NIT)-4 CeOx(AOT)-4
1.9 1.0 0.7 3.1 1.2 0.7
6.1 3.5 3.0 11.9 3.7 2.4
I
II
III
IV
V
VI
VII
VIII
IX
882 885 888 898 900 903 906 910 916 Ce4+ Ce3+ Ce4+/Ce3+ Ce3+/Ce2+ Ce4+ Ce3+ Ce3+ Ce4+ Ce4+ 36.1 (I ) 16; II ) 15; III ) 8; IV ) 13; V ) 12; VI ) 16; VII ) 10; VIII ) 3; IX ) 7%) 93.1 (I ) 16; II ) 4; III ) 16; IV ) 18; V ) 15; VI ) 5; VII ) 3; VIII ) 0; IX ) 12%) 92.0 95.5 96.3 85.0 95.1 96.9
a TAT% ) total atomic percentage of C, O, and Ce surface sites; DR ) deconvolution results. b An additional weak peak was resolved at 290 ( 0.5 eV (C 1s satellite peak of OdCsN and/or OdCsH species) for CeOx(NIT)-4, CeOx(TAB5)-4, CeOx(DAB5)-4, CeOx(BDT)-4, and CeOx(BDN)-4 and 287 ( 0.5 eV (C 1s peak of OdCsN and/or OdCsH species) for CeOx(DAB)-4. c An additional weak peak was resolved at 533 ( 0.5 eV (O 1s peak of SO42-, CO32-, and/or H2O species) for CeOx(DAB)-4. d Fundamental and satellite (shake-up) peaks at 18%) occurring via a slow, endothermic mass-loss step (e4%; Tmax ) 50-100 °C) and a subsequent rapid, exothermic mass-loss step (>14%; Tmax ) 240-300 °C). Behavior II, which is characteristic of cerias (CeOx(NIT), CeOx(DAB5), and CeOx(TAB5)) composed in crystallites of relatively larger sizes (>10 nm), involves a smaller total mass loss (
