Microemulsion-Based Synthesis of CeO 2 Powders with High Surface Area and High-Temperature...

Microemulsion-Based Synthesis of CeO2 Powders withHigh 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 orCe(NO3)3 stabilized by AOT (sodium bis(2-ethylhexyl) sulfosuccinate), DDAB (di-n-didodecyldimethyl-ammoniumbromide), orDDAB + Brij35surfactantmixtures.MicellarDTAB(n-dodecyltrimethylammoniumbromide) and vesicular DDAB systems were also used as media for generating CeO2. Characterization ofthe powders by X-ray powder diffractometry, laser-Raman spectroscopy, and Fourier transform infraredspectroscopy 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-sizedcrystallites were formed, comparing with the 85-nm-sized crystallites when cubic CeO2 was created viathermal decomposition of cerium oxalate. Surface characterization, by X-ray photoelectron spectroscopy,N2 sorptiometry, and high-resolution electron microscopy showed AOT- or (DDAB + Brij 35)-stabilizedmicroemulsions to assist in formation of crystallites exposing surfaces of large specific areas (up to ca. 250m2/g) but of low stability to high-temperature calcination (28-13 m2/g at 800 °C). In contrast, the double-chained 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, thelatter cerias could be considered as appropriate components for total oxidation (combustion) catalysts.

1. Introduction

Ceria (CeO2) and related oxides, namely, Ce-Zr-O andCe-Pr-O, enjoy a breadth of surface-driven applicationsin heterogeneous catalysis and in the fabrication of fuelcells, microelectronics, gas sensors, and polishing materi-als. Relevant details of these, and other applications, haverecently been reviewed by Trovarelli.1 Though they haveonly lately been found to be independent,2,3 the surfacespecific area (m2/g) and oxygen storage capacity (OSC)are crucial properties for most of these applications. Forinstance, the potential of ceria in the catalysts developedfor the treatment of automotive exhaust gases stems fromits ability to store oxygen under lean fuel conditions andto release it when the oxygen concentration becomesvirtuallynil (rich fuel conditions).4 This perfectly reversibleredox behavior,5 which facilitates economic production ofH2 when water vapor is used as an oxidant for CeO2-x,5as well as acido-basic surface properties,6 means thatceria is an important ingredient in the chemical composi-tion of catalysts employed in fluid catalytic cracking, SOxremoval, ethylbenzene dehydrogenation, and the water-gas shift reaction.1,7

Accordingly, synthesis of ceria with large surface specificareas and oxygen storage capacities has been at the focusof numerous researchers.1 Whereas remarkable advanceshave been scored toward synthesis of cerias of thermallystable, large OSCs, using zirconia1-3,8 and praseodia9

additives, obtaining pure ceria of thermally stable, high-area surfaces has not hitherto been accomplished with-out stabilizing additives of zirconia.10-15 This latter un-accomplished objective has hindered the promising ap-plications of ceria in combustion catalysts for natural gasturbines.16 In natural gas combustion processes catalystsmust withstand high temperatures (800-900 °C).16 Al-though endeavors to synthesize high-area ceria powdershave profited from recent developments of methods toprepare nanosized solid particles,17 such as the sol-gel,1,18

complexation,1,13,19,20 and hydrothermal1,10,15,21,22 and mi-croemulsion1,11,12,14,23,24 methods, the accomplishmentshitherto achieved are confined to yielding pure cerias of

* Corresponding author. Permanent address: Faculty of Science,Minia University, El-Minia 61519, Egypt. E-mail: [email protected]: 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 120-155.

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Surf. 2001, 190, 261.(7) Trovarelli, A.; de Leitenburg, C.; Boaro, M.; Dolcetti, G. Catal.

Today 1999, 50, 353.

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(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,

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11223Langmuir 2004, 20, 11223-11233

10.1021/la040079b CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 11/11/2004

specific surface areas amounting to 150-200 m2/g.25 Whensubjected to high-temperature treatments at 600-800 °C,nano-cerias have been found to suffer considerable particlegrowth, leading to catastrophic reduction in the surfaceaccessibility.25 At >1000 °C, an effective sintering resultsin ceramic-grade, dense-bulk cerias of poor surface areas(<5 m2/g).15,19,25

The present paper presents and describes endeavors toobtain ceria powders exposing surfaces, not only of highspecific areas but also of high thermal stability. Amicroemulsion preparation method was adopted, imple-menting well-characterized surfactants (and some of theirmixtures) that are not customarily employed in thepreparation of ceria: AOT, DDAB, DTAB, and Brij 35(Chart 1). For control purposes, (i) a precipitation wasalso carried out in the absence of n-heptane only (i.e., insurfactant-containing aqueous media) or in the absenceof surfactant(s) as well (i.e., in aqueous media solely) and(ii) a reference cubic CeO2 was obtained by thermaldecomposition of a homemade cerium oxalate.

The rationale for using these surfactants is (1) the phasebehavior and structure of microemulsion systems therebystabilized are well characterized,26-31 (2) DDAB + Brij 35microemulsion systems have a large single-phase region

as compared with that of pure DDAB, and this allowsstudies away from the lower “haze” phase boundary wherethe system is known to exhibit attractive interactionsleading to nonspherical aggregates,29 and (3) the watersolubilization of reactants is enhanced upon partiallyreplacing DDAB by Brij 35,31 thus increasing the yield ofprecipitates.

The resulting ceria products were subjected to a set ofbulk and surface analytical techniques in order to (i) verifythe ceria bulk composition and structure and (ii) thesurface chemical composition, structure, and specific area.These properties were also determined after calcinationat 400-800 °C. It is worth noting, that the present workis part of a comprehensive investigation, taking place inthis laboratory, in hopes of synthesizing simple, andcomposite, metal oxides suitable as components forcombustion catalysts.

2. Experimental Section

2.1. Chemicals and Surfactants. Cerous nitrate, Ce(NO3)3‚6H2O, and chloride, CeCl3‚7H2O, were the ceria precursorcompounds used; they were 99 and 99.9% pure Aldrich products,respectively. The precipitants were 25% aqueous NH3 (Riedel-de Haen) and 40% aqueous (C4H9)4NOH (TBAH) (Aldrich). Thehydrocarbon medium was provided by 99.3% pure n-heptane,C7H16 (Merck). The surfactants were (i) AOT, (sodium bis(2-ethylhexyl) sulfosuccinate, Sigma, 99% pure), (ii) DDAB, (di-n-didodecyldimethylammonium bromide, homemade), (iii) DTAB,(n-dodecyltrimethylammonium bromide, Aldrich, 99% pure), and(iv) Brij 35 (polyoxyethylene 23 monododecyl ether, C12H25(OCH2-CH2)nOH, Sigma), where n ) 23 on average. The chemicalstructure of the surfactants is schematized in Chart 1.

The cationic surfactant DDAB was prepared by following therecipe detailed elsewhere32 using N,N-dimethyldodecylamine(99.5%, Aldrich) and n-dodecyl bromide (97%, Aldrich) in themolar ratio of 1:1.1 with ethanol and refluxing at 80 °C for 4 h.The ethanol was then rotary-evaporated, and the product thusobtained was dried at 50 °C overnight in a vacuum oven. Laterit was washed thoroughly with Na-dried diethyl ether (>99.5%,Riedel deHaen), 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 1HNMR (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, dropwiseaddition of 25% aqueous ammonia solution to a continuouslystirred 0.1 M aqueous solution of cerium(III) nitrate (200 mL).Initially a light yellow precipitate was formed, which turnedpurple 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 distilledwater, and dried at 100 °C overnight. The precipitate, whichturned yellow on drying, was ground, sealed in vials, and storedover silica gel in a desiccator until further use.

CeOx(AOT) was synthesized by mixing two heptane-continuousAOT microemulsions, one of which contained aqueous cerium(III)nitrate solution of 0.05 M, and the other contained 10% ammoniasolution. The AOT concentration was 0.1 M, and the amount ofaqueous solution was chosen so that the system was just belowits emulsification failure boundary.26-28 The mixing was carriedout by dropwise addition of the ammonia microemulsion to theother, over 2-3 days with continuous stirring. The stablesemitransparent suspension thus formed was stirred for 1 h andthen kept overnight. Solid particles were separated by centrifu-gation (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 driedat 100 °C overnight. The dried solid, which was yellow, was groundby pestle and mortar and stored.

(23) Martinez-Arias, A.; Fernandez-Garcia, M.; Ballesteros, V.;Salamanca, L. N.; Conesa, J. C.; Otero, C.; Soria, J. Langmuir 1999,15, 5, 4796.

(24) Rodriguez, J. A.; Hanson, J. C.; Kim, J.-Y.; Liu, G.; Iglesias-Juez, A.; Fernandez-Garcia, M. J. Phys. Chem. B 2003, 107, 3535.

(25) Bernal, S.; Calvino, J. J.; Gatica, J. M.; Cartes, C. L.; Pintado,J. M. In ref 1, pp 85-153.

(26) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1979, 70, 577.(27) Robinson, B. H.; Toprakcioglu, C.; Dore, J. C.; Chieux, P. J.

Chem. Soc., Faraday Trans. 1 1984, 80, 13.(28) Eastoe, J.; Robinson, B. H.; Steytler, D. C.; Thorn-Lesson, D. J.

Colloid Interface Sci. 1991, 36, 1.(29) Bumajdad, A. Ph.D. Thesis, Bristol University, U.K., 2000.(30) Bumajdad, A.; Eastoe, J.; Nave, S.; Steytler, D. C.; Heenan, R.

K.; Lu, J. R.; Timmins, P. Langmuir 2003, 19, 2560.(31) Bumajdad, A.; Eastoe, J.; Steytler, D. C.; Heenan, R. K.; Timmins,

P. Langmuir 2003, 19, 2560. (32) Zana, R. J. Colloid Interface Sci. 1980, 78, 330.

Chart 1. Schematic Chemical Structures of theSurfactants

11224 Langmuir, Vol. 20, No. 25, 2004 Bumajdad et al.

CeOx(DAB) was produced at room temperature by a slowdropwise addition, with continuous stirring, of 25% ammoniasolution to a heptane-continuous DDAB microemulsion of 0.1 Mcerium(III) nitrate as the aqueous core. The total amount ofaqueous cerous nitrate + ammonia solution was maintained atless than the maximum solubility limit of the microemulsion,which can be easily determined visually by following the turbidityand/or the aqueous solution precipitation.29-31 The stirring wascontinued for 1 h following mixing. Particle separation andcleanup were the same as for CeOx(AOT) above.

CeOx(DAB5) was obtained by dissolving a weighed amount ofcerium(III) chloride salt in a 0.05 M solution of DDAB in 100 mLof water, to eventually reach a salt concentration of 0.05 M. Themixture was stirred vigorously, and then a 45.45-mL aliquot of25% ammonia solution was slowly added dropwise, over 1 h, andstirred at room temperature for 1 h further. The purple solutionthus obtained was sealed in a tube and stirred continuously for5 days in a water bath adjusted at 80 °C (the suffix “5” identifiesthese oxides in the paper). The solid residue, which suffered acolor change from purple to yellow, was separated by filtrationthrough Whatman 42 paper, washed thoroughly with acetone,dried at 100 °C overnight, ground by pestle and mortar, andstored. It is worth mentioning that at this concentration of DDAB,the preferred aggregate structure would be vesicles (the usedconcentration is more than the critical vesicle concentration, ofDDAB ) 9 × 10-4 M27 but less than the concentration requiredto form lamellar liquid crystals, which is 0.086 M).34

CeOx(TAB5) was produced following the same procedureapplied to obtain CeOx(DAB5), except for using DTAB insteadof DDAB. Here the aqueous system contains micelles (criticalmicelle concentration of DTAB ) 0.0133 M).35

CeOx(BDN) was obtained by rapid mixing of two microemul-sions stabilized by a 0.01 M Brij 35 + 0.09 M DDAB surfactantmixture, one containing 0.05 M solution of cerium(III) nitrateand the other 10% ammonia solution (the suffix N refers to NH3as a precipitant), with continuous stirring for 1 h. The mixturewas kept overnight, and then the resulting precipitate wasseparated by centrifugation, washed thoroughly by ultrasoni-cation in acetone, dried at 100 °C overnight, ground, and stored.

CeOx(BDT) was prepared following the same procedure appliedto yield CeOx(BDN), except that the precipitant was a 10% TBAH.The addition of the TBAH microemulsion was carried outdropwise over 2 days with continuous stirring.

For convenience, the designations and preparation precursorsof the above cerias are further summarized in Table 1. It is worthnoting that (i) the stirring was magnetic, (ii) an ultrasonic bathwas used, (iii) the centrifuge employed was a Beckman J-2 MC,(iv) volumes of the precipitants added (NH3 or TBAH) weredecided on basis of the Wmax of the microemulsion, which is themolar ratio of the aqueous phase and the surfactant(s) content(i.e., Wmax ) [aqueous]/[surfactant(s)] above which the Winsor IIsystem is obtained (i.e., water-in-oil microemulsion in equilibriumwith excess lower water phase),36 (v) the surfactant(s) concentra-tion was held constant at 0.1 M for microemulsion systems andat 0.05 M for aqueous systems, and the oil-continuous phase was

always n-heptane, and (vi) the above indicated cerias werecalcined (heated in air) at 400, 600, and 800 °C for 2 h, using aThermolyne 6000 electronically controlled muffle furnace. Forsimplicity, the calcination products are discerned from theuncalcined parent by suffixing a number symbolizing thecalcination 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 calcinationproducts at 600 and 800 °C, respectively.

2.3. Characterization Methods and Techniques. Thecrystalline bulk structure, average crystallite size, and unit cellparameters were determined by X-ray powder diffractometry(XRD), whereas the chemical composition of both crystalline andnoncrystalline domains of the bulk was identified by Fouriertransforminfrared (FT-IR)and laserRaman(LRa)spectroscopies.The bulk thermal stability was probed by means of thermo-gravimetry (TG) and differential thermal analyses (DTA). Surfacechemical composition and specific area were determined usingX-ray photoelectron spectrometry (XPS) and BET analysis of N2sorptiometry (SBET), respectively. Surface microstructure andmorphology were examined by high-resolution transmissionelectron microscopy (HRTEM) and scanning electron microscopy(SEM). It is worth noting that the test samples were not groundbefore analysis, except for those subjected to IR spectroscopy.

XRD was carried out (at 2θ ) 10-80° and RT) using a modelD5000 Siemens diffractometer (Germany) equipped with Ni-filtered Cu KR radiation (λ ) 0.15406 nm). The diffractometerwas operated with 1° diverging and receiving slits at 50 kV and40 mA and a continuous scan was carried out with a step sizeof 0.014° and a step time of 0.2 s. An on-line automatic searchsystem (PDF Data Base) facilitated an observed data match withJCPDS standards.37 Crystallite sizing was realized by adoptingthe line-broadening technique and Sherrer formula,38 whereasunit cell lattice parameters were determined by means of aninstalled Win-Metric software (v.2, Siemens Corp.). To warranta 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 (<1wt %) over the frequency range of 4000-400 cm-1 and at aresolution of 4 cm-1, using a model 2000 Perkin-Elmer FTspectrometer (U.K.). An on-line data station facilitated spectraacquisition and handling. On the other hand, LRa spectra weretaken of test materials, which were lightly compacted, at 200-3600 cm-1 and a resolution of 0.2 cm-1, employing a Perkin-Elmer system 2000 Raman spectrometer (USA), equipped witha near-infrared diode pumped Nd:YAG laser (1.064 µm). Thelaser power at the test sample was in the range of 50-100 mW.

TG and DTA analyses were carried out on heating, typicallya 10-15-mg portion of test materials at 10 °C/min and 50 cm3

of air/min, using a Shimadzu thermal system, model 50, (Japan)equipped with a work station. Highly sintered R-Al2O3 (ShimadzuCorp.) was the thermally inert reference for DTA measurements.

XPS spectra were recorded on a model VG Scientific 200spectrometer (U.K.) spectrometer using Al KR radiation (1486.6eV) operating at 13 kV and 23 mA. The spectra acquisition andhandling were carried out by means of an on-line Eclipse datasystem (U.K.). The test materials were compacted onto the sample(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.

(37) International Center for Diffraction Data, 12 Campus Boulevard,Newton Square, PA.

(38) Matyi, R. J.; Schwartz, L. H.; Butt, J. B. Catal. Rev.-Sci. Eng.1987, 29, 41.

Table 1. Designation and Preparation Precursors of Uncalcined Ceriaa

uncalcined ceria preparation precursors

CeOx(NIT) NH3 (25%)aqV + Ce(NO3)3 (0.1 M)aqCeOx(AOT) {AOT (0.1 M) + heptane + NH3 (10%)aq}V + {AOT (0.1 M) + heptane + Ce(NO3)3 (0.05 M)aq}CeOx(DAB) NH3 (25%)aqV + {DDAB (0.1 M) + heptane + Ce(NO3)3 (0.1 M)aq}CeOx(DAB5) NH3 (25%)aqV + [DDAB (0.05 M) + CeCl3 (0.05 M)] w 5 daysCeOx(TAB5) NH3 (25%)aqV + [DTAB (0.05 M) + CeCl3 (0.05 M)] w 5 daysCeOx(BDN) {(Brij + DDAB) (0.1 M) + heptane + NH3 (10%)aq} + {(Brij + DDAB) (0.1 M) + heptane +

Ce(NO3)3 (0.05 M)aq}, fast mixingCeOx(BDT) {(Brij + DDAB) (0.1 M) + heptane + TBAH (10%)aq}V {(Brij + DDAB) (0.1 M) + heptane +

Ce(NO3)3 (0.05 M)aq}, slow mixinga Key: V ) added from a buret; { } ) microemulsion; aq ) [ ] ) aqueous medium; w stirred continuously for.

Synthesis of CeO2 Powders Langmuir, Vol. 20, No. 25, 2004 11225

holder (8 mm in diameter) in an ambient atmosphere, mountedand stored in the introduction chamber until a vacuum of 10-9

to 10-10 Torr (1 Torr ) 133.3 Pa) was reached, and thentransferred into the analysis chamber for data acquisition (0.2eV step; 250 ms dwell time; 0.7 eV resolution; up to 10 scans).All binding energy values were determined with respect to theC(1s) line (284.6 eV) originating from adventitious carbon, andthe standard deviation of the peak position was estimated to be(0.5 eV. The surface atomic percentage (%) of the elementsobserved was calculated from the peak areas (in counts eV/s)with integral subtraction of the background.39

N2 adsorption isotherms were determined on test samples atliquid nitrogen temperature (-195 °C), using an automatic ASAP2010 Micromeritics sorptiometer (USA) equipped with a de-gassing platform and an on-line data acquisition and handlingsystem loaded with BET-based40 analytical software for the sur-face area (SBET/m2 g-1) determination. The N2 gas was a 99.999%pure product of KOAC (Kuwait), and the test materials (500 (2 mg) were pre-degassed at 110 °C and 10-5 Torr for 3 h.

HRTEM was performed on a JSM-3010 JEOL ultra-high-resolution electron microscope (Japan) operated at 300 kV and119 µA. A few particles of test sample dispersed in 2-propanolwere picked up on a copper microgrid, dried at ambienttemperature, examined for an even distribution using a stereomicroscope, and then admitted into the microscope for analysis.SEM was conducted using a JSM-630 JEOL scanning electronmicroscope (Japan) operated at 30 kV. Fine particles of the testsample were sprinkled uniformly over double adhesive tape andsputter coated with gold to a thickness of 5 nm by means of aBalzer SCD 050 sputter coater (USA) prior to admission into themicroscope.

3. Results and Discussion3.1. Material Bulk Characteristics. Crystalline

structure of the solid products of the various synthetic

routes followed (Table 1), before and after calcination,was determined by XRD. The powder diffractogramsobtained displayed characteristic diffraction patterns, atypical exampleofwhich (exhibitedbyCeOx(NIT)) is shownin Figure 1, along with diffraction patterns observed fora reference cubic CeO2 (600 °C calcination product ofhomemade cerium oxalate) and the RhPt sample holder.These background RhPt peaks are labeled with an asteriskin Figure 1. The comparison reveals that the diffractogramof CeOx(NIT) is very similar to that displayed for thereference CeO2. The diffraction peaks monitored in thelatter diffractogram are almost identical to those filed forcubic CeO2 in JCPDS card no. 04-0593.37 Accordingly, thethreestrongestdiffractionpeaks (at3.067,1.890,and1.616Å) of the test ceria correspond to the cubic ceria crystalfacets (111), (220), and (311), respectively. It is worthnoting, however, that these three strongest peaks cor-respond to significantly different d spacings than thosefiled for Ce(OH)3 in JCPDS cards 74-0665 (at 5.6196 (98%),3.2445 (73%), and 3.1513 Å (100%))37 and 19-0284 (at5.6200 (35%), 3.1600 (100%), and 1.8610 Å (40%)).31 Thisproves that Ce(OH)3, though expected, was not formed atany detectable level in the microemulsions used.

Figure 1 reveals, moreover, that the diffraction peaksof CeOx(NIT) are broader and weaker than those of thereference CeO2, thus accounting for a smaller averagecrystallite size and lower crystallinity, respectively. Figure2, which compares the diffractograms obtained over the

(39) Zaki, M. I.; Kappenstein, C. Z. Phys. Chem. 1992, 176, 97.(40) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938,

60, 309.

Figure 1. X-ray powder diffractograms observed for test(CeOx(NIT)) and reference (CeO2(Ref.)) cerias and the sampleholder (PhPt) of the diffractometer. Peaks are indicated by thecorresponding d spacing (in Å) and reflecting crystal facet.

Figure 2. X-ray powder diffractograms observed for the testsamples indicated at 2θ ) 20-37°.


2θ range 20-37° for all of the test cerias synthesized,discloses that CeOx(NIT) still assumes the largest averagecrystallite size and strongest crystallinity among thesesystems. After calcination at 400-800 °C, the diffractionpeaks observed for all of the synthesized cerias wereconsiderably sharpened and strengthened, consistent witha considerable, though varying, advance of crystallizationand crystal growth. A representative set of diffractogramsis shown in Figure 3 for CeOx(BDN) and its calcinationproducts. It is worth noting that calcination of all of thesynthesized cerias to the highest temperature applied (800°C) still indicated the only presence of cubic CeO2.

Averagecrystallite sizes derived from thediffractogramsare compiled in Table 2. They indicate, indeed, that theuncalcined cerias are organized in much smaller crys-tallites (L ) 21-6 nm) than the reference CeO2 (L ) 85nm). They also indicate that CeOx(NIT) has relatively thelargest average crystallite size (L ) 21 nm) and CeOx-(BDN) the smallest (L ) 6 nm). What is obvious, however,is that apart from CeOx(NIT), which was prepared withoutsurfactants, cerias synthesized using surfactants are allorganized in nanosized crystallites (L e 13 nm). Amongthese nanosized cerias, CeOx(AOT), CeOx(DAB), CeOx-(DAB5), and CeOx(TAB5) are shown to resist particlegrowth after calcination at up to 600 °C, with a slight sizeincrease at 800 °C (Table 2). The common factor in thesesyntheses is the use of only single surfactants, whetherAOT, DDAB, or DTAB (Chart 1).

Using Win-Metric software, the unit cell lattice dimen-sions were determined for the reference and syntheticcerias. Essentially, equal values were obtained for the

cell parameters (a ) b ) c) as well as the angles (R ) â) γ ) 90°). The common unit cell parameter (a) values()5.4087-5.4106 Å) obtained for the uncalcined cerias,

Figure 3. X-ray powder diffractograms observed for CeOx-(BDN) and calcination products at 400-800 °C, as indicated.

Table 2. Unit Cell Parameter (a/(0.0003 Åa), AverageCrystallite Size (L/(1 nma), L-Derived Surface Area

(SL/(2 m2 g-1)b), and N2-Determined Surface Area (SBET/(2 m2 g-1)c) of the Test Cerias versus CeO2(Ref.)

ceria a/ÅL/nm

SL/m2 g-1

SBET/m2 g-1 f ()SBET/SL)d)

CeOx(NIT) 5.4106 21 40 119 3.0400 °C 5.4091 21 40 100 2.5600 °C 5.4087 19 44 66 1.5800 °C 5.4099 43 19 20 1.0

CeOx(AOT) 5.4104 8 105 250 2.4400 °C 5.4113 9 93 118 1.3600 °C 5.4085 16 53 26 0.5800 °C 5.4096 34 25 13 0.5

CeOx(DAB) 5.4105 9 93 144 1.5400 °C 5.4094 13 65 142 2.2600 °C 5.4092 19 44 70 1.6800 °C 5.4096 34 24 45 1.9

CeOx(DAB5) 5.4097 13 65 125 1.9400 °C 5.4098 17 49 84 1.7600 °C 5.4098 28 30 83 2.8800 °C 5.4104 29 29 55 1.9

CeOx(TAB5) 5.4087 13 65 184 2.8400 °C 5.4093 17 49 118 2.4600 °C 5.4102 17 49 110 2.3800 °C 5.4090 34 24 36 1.5

CeOx(BDN) 5.4092 6 140 239 1.7400 °C 5.4093 11 76 187 2.5600 °C 5.4115 16 53 103 1.9800 °C 5.4110 43 19 28 1.5

CeOx(BDT) 5.4089 10 84 173 2.0400 °C 5.4098 12 70 146 2.1600 °C 5.4105 19 44 77 1.8800 °C 5.4100 57 14 26 1.8

CeO2(Ref.) 5.4136 85 10 11 1.1a XRD determined. b Calculated adopting an idealized cubic

model and the theoretical density of CeO2 (F ) 7.134 g/cm3),19

implementing the formula SL ) 6/FL.19,41 c Determined by BETanalysis of N2 adsorption isotherms measured at liquid nitrogentemperature (-195 °C). d Rounded to the nearest decimal.

Figure 4. LRa spectra of the ceria samples indicated.


which are also listed in Table 2, are rather close not onlyto that ()5.4136 Å) determined for the reference CeO2but also to that ()5.411 Å) filled in the JCPDS card (no.

04-0593)37 of cubic CeO2. Table 2 shows, moreover, thatthis observation extends to the calcined cerias. Accord-ingly, one can justifiably suggest that the unit cell ofcrystalline cerias is free of measurable tetragonal distor-tion (i.e., c/a > 1).23

Figure 5. TG curves obtained for CeOx(BDN) (A) and CeOx-(NIT) (B). The insets display corresponding DTA curves.

Figure 6. FT-IR spectra of the ceria samples indicated.

Figure 7. Deconvoluted XPS peaks monitored over the C 1s(A), O 1s (B), and Ce 3d (C) photoelectron emission ranges forthe indicated ceria samples.


There is a general consensus that slight departures fromthe internal cubic symmetry of cubic CeO2 is ratherdifficult to distinguish using XRD, because of the smallatomic scattering factor of oxygen in comparison to thatof Ce.23 Hence a better assessment of subtle cubic latticedistortions may be realized by complementing the XRDdata with LRa spectroscopic data.9,23 In the case of a purecubic fluorite structure, only one band would be Ramanactive at 460-465 cm-1,9 which is due to T2g vibrationmode of the Ce-O bond in the fluorite structure. LRaspectra taken of the uncalcined and calcined ceriasdisplayed only a band at 460-463 cm-1. Two representa-tive spectra, which were shown by CeOx(DAB) andCeOx(NIT), are compared to that obtained for the referenceCeO2 in Figure 4. These spectra indicate that the largerthe crystallite size, the narrower and stronger thediagnostic cubic CeO2 peak is. The absence of a weak peakat 570-575 cm-1, assignable to space defects includingO2- vacancies,9,23 excludes with certainty the presence ofbulk anionic vacancies. Accordingly, the LRa resultscomplement the XRD results, indicating that the micro-emulsion preparation method yielded cerias of distortion-free and anionic vacancy-free cubic fluorite structure.Importantly, using surfactants, whether single or mixed,restricts the structure growth into nanosized crystallites.

3.2. Thermal Behavior. TG and DTA curves obtainedfor the as-synthesized cerias have helped discern the twotypes of thermal behavior (I and II), manifested in Figure5. Behavior I, which is exhibited by cerias (CeOx(BDT),CeOx(DAB), CeOx(AOT) and CeOx(BDN)) having crystal-lite sizes e10 nm (Table 2), is characterized by a largetotal mass loss (>18%) occurring via a slow, endothermicmass-loss step (e4%; Tmax ) 50-100 °C) and a subsequentrapid, 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 crys-tallites of relatively larger sizes (>10 nm), involves asmaller total mass loss (<10%) occurring throughout twosubsequent, slow mass-loss steps, the earlier is endo-thermic (<4%; Tmax ) 50-100 °C) and the latter is veryweakly exothermic (<6%; Tmax ) 200-250 °C), followedby a high-temperature, mass-invariant endothermic pro-cess (Tmax >930 °C). In both behaviors, the early, endo-thermic mass-loss step is due, most likely, to removal ofweakly bound species (e.g., physisorbed water molecules),whereas the subsequent exothermic mass-loss step is

assignable, most probably, to an oxidative removal oforganic moieties (e.g., sticking surfactant species). Thehigh-temperature, endothermic mass-invariant process,distinctive of behavior II, may be considered as entropy-driven, i.e., a structure-disordering process.

Geometric surface areas derived from the crystallitesizes, and compiled in Table 2, reveal expectedly, thatcerias exhibiting thermal behavior-I (Figure 5) assumelarger specific areas (93-140 m2/g) than those (65-84m2/g) derived for cerias giving rise to behavior II. Thismay, obviously, explain their larger uptakes (i.e., largertotal mass loss) of organic moieties. Moreover, theirsmaller crystallite sizes may, according to previousreports,23,24 sustain the bulk structure against high-temperature modifications (Figure 5). IR spectra weretaken of the uncalcined and calcined cerias, a set of whichcomparing spectra taken of behavior I and II cerias withthat taken of the reference CeO2 is displayed in Figure 6.Whether calcined or uncalcined, these spectra clearlyresemble the reference cubic CeO2 in exhibiting theblackout absorption of the Ce-O lattice vibrations at <650cm-1.42 They also show that CeOx(BDN), a behavior I ceria(L > 10 nm), exposes contaminants of a different nature(organic) than the natural (inorganic) contaminantspresent in behavior II CeOx(NIT) (L < 10 nm). The organicnature of the contaminants of CeOx(BDN), obviously downto the surfactants used, is evidenced by the ν(C-C)/ν(C-O)/δ(C-H) absorptions at 1558-948 cm-1,43 whereasthe inorganic nature of the contaminants of CeOx(NIT) isevidence by the ν(OdCsO) (at 1693, 1612, and 1546cm-1)44 and ν(NO3

-) (at 1384 cm-1)45 absorptions. More-over, CeOx(BDN) is shown to retain a larger water content(ν(OH) and δ(HOH) at 3313 and 1641 cm-1, respectively,45

than CeOx(NIT). Upon calcination, much of the IRabsorptions of both type of contaminants and retainedwater is shown to be largely eliminated. At 800 °Ccontaminants are hardly detectable in CeOx(NIT) and arejust seen in the spectrum of CeOx(BDN).

(41) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity;Academic Press: London, 1967; p 2.

(42) Zaki, M. I.; Sheppard, N. J. Catal. 1983, 80, 114.(43) Bellamy, L. J. The Infrared Spectra of Complex Molecules;

Chapman and Hall: London, 1975.(44) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89.(45) Degen, I. A. Tables of Characteristic Group Frequencies for the

Interpretation of Infrared and Raman Spectra; Acolyte Publications:Harrow, 1997.

Table 3. Surface Chemical Composition and Corresponding XPS-Derived Characteristics of 400 °C Calcined Cerias

TAT% (DR)a

C 1sb O 1sc Ce 3dd

I II III I II I II III IV V VI VII VIII IX

BE/(0.5 eV 284 286 288 529 532 882 885 888 898 900 903 906 910 916speciese CsC/

CsHCsO OdCsO/

OdCsHO2- OH/H2O Ce4+ Ce3+ Ce4+/Ce3+ Ce3+/Ce2+ Ce4+ Ce3+ Ce3+ Ce4+ Ce4+

CeOx(BDN) 25.7 (I ) 51; II ) 23; III ) 26%) 38.2 (I ) 43; II ) 43%) 36.1 (I ) 16; II ) 15; III ) 8; IV ) 13; V ) 12; VI ) 16; VII ) 10;VIII ) 3; IX ) 7%)

CeOx(BDN)-4 2.2 (I ) 54; II ) 15; III ) 27%) 4.7 (I ) 80; II ) 20%) 93.1 (I ) 16; II ) 4; III ) 16; IV ) 18; V ) 15; VI ) 5; VII ) 3;VIII ) 0; IX ) 12%)

CeOx(TAB5)-4 1.9 6.1 92.0CeOx(DAB)-4 1.0 3.5 95.5CeOx(DAB5)-4 0.7 3.0 96.3CeOx(BDT)-4 3.1 11.9 85.0CeOx(NIT)-4 1.2 3.7 95.1CeOx(AOT)-4 0.7 2.4 96.9

a TAT% ) total atomic percentage of C, O, and Ce surface sites; DR ) deconvolution results. b An additional weak peak was resolvedat 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, andCeOx(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 resolvedat 533 ( 0.5 eV (O 1s peak of SO4

2-, CO32-, and/or H2O species) for CeOx(DAB)-4. d Fundamental and satellite (shake-up) peaks at <898

( 0.5 eV are due to Ce 3d5/2 emissions, whereas those occurring at g898 ( 0.5 eV are due to Ce 3d3/2 emissions. e Assignments suggestedare based on data reported in refs 18 and 39.


3.3. Surface Composition. The surface chemicalcomposition was determined using XPS for uncalcinedand 400 °C calcined cerias. Spectra obtained for CeOx-(BDN) and CeOx(BDN)-4 are compared in Figure 7,whereas values derived from the whole set of spectra aresummarized inTable3. Inall cases,photoelectronemissionpeaks were displayed over the C 1s, O 1s, and Ce 3d bindingenergy (BE/eV) ranges only. Therefore, Figure 7 is confinedto these ranges only. It is obvious from Figure 7 that thepeaks displayed are composed of discrete bands. Conse-quently, the peaks were deconvoluted by means of astandard PeakFit software (v.4; product of AISN SoftwareInc., USA) in order to resolve the separate constituents(e.g., Figure 7). These are set out in Table 3, each indicatedby a Roman numeral and the corresponding BE values,in each range of electron emission (C 1s, O 1s, and Ce 3d).Appropriate assignments are also given correspondingly.Table 3 also includes the total atomic percentage (TAT%)

for each element (C, O, and Ce), as derived from integratedpeak areas and element sensitivity factors.39 For CeOx-(BDN) and CeOx(BDN)-4 only, there is also a breakdownof each TAT% into component contributions.

First, TAT% of surface C of uncalcined cerias isconsiderably high and significantly reduced in magnitudeupon calcination, e.g., 25.7 (CeOx(BDN)) f 2.2% (CeOx-(BDN)-4). Most of the initial surface-C is due to surfactantmolecules (C-O species II). This elimination is, thus, mostlikely associated with the exothermic mass loss observedin thermal behavior I (Figure 5). The result is to leavebehind on the calcination product (e.g., CeOx(BDN)-4)small amounts of adventitious carbon (species I) andcarbonato/organic oxygenate species. Second, TAT% ofsurface O of uncalcined cerias is uncommonly high. Uponcalcination, it is considerably reduced to TAT% corre-sponding, for most test samples, to an atomic O/Ce ratio<2, e.g., 38.2 (CeOx(BDN) f 4.7% (CeOx(BDN)-4). The

Figure 8. SEM (A), TEM (B), and HRTEM (C, D) micrographs obtained at the indicated magnifications for CeOx(BDN).


initially high oxygen level is shown to be largely contrib-uted to by surface water, which is considerably removedupon heating (Figure 5). Third, the TAT% of surface Ce,shown to constitute Ce4+ and Ce3+, is increased consider-ably upon calcination, with an obvious increase in theCe4+/Ce3+ atomic ratio. This increase in surface Ce isprobably a result of the heat-induced elimination ofsurfactant and water species, whereas the elevation inthe Ce4+/Ce3+ ratio may be related to surface oxidation.Detection of surface Ce3+ may, in the first place, be ascribedto the preparation path (i.e., the usage of Ce(III) precursorcompound) and/or surface deoxygenation under the X-raybeam. This latter reason could still be responsible for thedetection of Ce3+ in the surface layer of the calcinationproduct CeOx(BDN)-4. It is worth recalling here that LRaspectra have not been identified for bulk oxygen vacancies.Thus, oxygen vacancies associated with the XPS-detectedCe3+ are not attributed to bulk sites.

3.4. Surface Microstructure. Figure 8 exhibits elec-tron micrographs obtained for CeOx(BDN) at increasingmagnification. The SEM (Figure 8A) images show particleaggregates of irregular shapes and large sizes (200-100µm). When 100 nm thick sections of the aggregates were

examined by TEM, the micrograph obtained (Figure 8B)reveals that the material particles are nanosized, almostuniformly shaped, and only slightly agglomerated. Acloseup of single material agglomerate, using HRTEM,resulted in a micrograph (Figure 8C) corresponding to anapproximate size of 60 nm. At a still higher magnificationthe HRTEM micrograph obtained (Figure 8D) resolvesnonagglomerated and slightly agglomerated nanoparticlesof sizes in the range 10-5 nm. It also shows that theparticles have polyhedral shapes. These observations aregenerally applicable to all of the uncalcined cerias. Thesole difference between the various test samples lies inthe breadth of the average particle size range, which aresatisfactorily close to the XRD-determined crystallite sizes(Table 2). The evident stability of nanoparticles againstagglomeration may probably be attributed to someremaining surfactant molecules (Figure 6).

A HRTEM micrograph obtained for particles of CeOx-(BDN) at a 5-nm resolution (Figure 9) showed fringes ofd spacings ()0.275 nm) very close to that (0.2706 nm)filed for the crystal (200) facet of cubic CeO2 (JCPDS 04-0593).37 By recording a large amount of HRTEM imagesfrom unselected areas and identifying the lattice fringes

Figure 9. HRTEM micrograph obtained at the indicated magnification for CeOx(BDN).


and particle orientation, the facets of the particle surfaceshave been found to be dominated by (111) and (200) planes,which were observed when the particles were orientedalong the (110) direction. Thus, the present results are inline with previous findings showing low-indexed facets tobe energetically favorably exposed.47 A thorough particleshape analysis of ceria, carried out by Wang and Feng,22

revealed two types: (i) octahedra enclosed with (111)planes, and (ii) truncated octahedra enclosed by (111) and(100) planes (i.e., more spherical-like).

3.5. Surface Area. N2-determined specific surface area(SBET/ m2 g-1) values for uncalcined and calcined ceriasare given in Table 2. The surface area is shown to decreaseas the crystallite size increases. This is graphicallypresented for the uncalcined cerias in Figure 10, in whichthe results obtained for the reference CeO2 are inset forcomparison purposes. The derived f factor ()SBET/SL)assumes values in the range 1.5-3.0 for uncalcined cerias,which accounts for slight agglomeration between theirnanosized particles, most probably via formation of jointlow-index crystal facets.19 This is expected behavior ofnanosized particles, to reduce the initially high surfaceenergy.19 Therefore, it is expected to find the heat-inducedparticle growth (i.e., reduction of total surface energy) tobe accompanied by a decrease in the f factor (Table 2),apart from a few exceptions with the 400 °C calcinationproducts. Such exceptions are usually attributed toevolution of N2-accessible particle porosity.19

Considering the specific surface areas determined forthe surfactant-prepared uncalcined cerias, the cerias canbe grouped into three classes: (i) class I, 100-150 m2/g,CeOx(DAB) and CeOx(DAB5); (ii) class II, 150-200 m2/g,CeOx(BDT) and CeOx(TAB5); and (iii) class III, 200-250

m2/g, CeOx(AOT) and CeOx(BDN). It is evident from Table2 that all of test cerias have higher surface areas than thesurfactant-free CeOx(NIT), though the latter still has amuch higher surfaces area (119 m2/g) than that (11 m2/g)determined for the reference CeO2 which was preparedby thermal decomposition of cerium oxalate. These resultsindicate that thesurfactant-aidedpreparationyieldsceriasof higher surface area than of that produced via thethermal decomposition method. Furthermore, cerias ofstill higher surface areas (up to 250 m2/g) can be preparedin microemulsions. Among the surfactant-aided prepara-tions however, cerias produced using microemulsionstabilized by the single surfactant AOT and by the mixedsurfactants (DDAB + Brij 35), i.e., those having doublehydrocarbon chains, exhibit the highest specific surfaceareas. In fact the surface areas exhibited by these lattermaterials, i.e., CeOx(AOT) and CeOx(BDN), represent thehighest ever reported.1,3,13,14,18,19

However, since the prime objective of the presentinvestigation was not only to produce high-area ceriasbut also to improve stability against high-temperaturetreatments (up to 800 °C), the surface area is graphicallypresented as a function of the calcination temperature inFigure 11. It is obvious from the results that lossesconceded by the surface area of class III cerias (of theinitially highest surface accessibility) as a function ofcalcination temperature are much higher than class I andII cerias. Relatively speaking, however, class I cerias,which have the lowest initial surface area, show the mostthermally stable surface areas. The 800 °C calcinationproducts, CeOx(DAB)-8 and CeOx(DAB5)-8, exhibit thehighest specific surface area (ca. 50 m2/g) among the 800°C calcination products of these systems. It is worth

(46) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.;Powell, C. J.; Rumble Jr., J. R. NIST X-ray Photoelectron SpectroscopyDatabase; Web Version: v.3; NIST, 2004.

(47) Farragher, A. L. Adv. Colloid Interface Sci. 1979, 11, 3.

Figure 10. A histogram relating the specific surface area (SBET/m2 g-1) and average crystallite size (L/nm) of the indicatedcerias.

Figure 11. Plots of the specific surface area as a function ofthe calcination temperature of the cerias indicated.


mentioning that only zirconia-modified cerias that havebeen reported10,12 to lead upon high-temperature calcina-tion at 500-900 °C to products of comparably high surfaceareas (75-40 m2/g).

4. ConclusionsThe above results indicate the following:(1) The microemulsion method of preparation, as applied

in the present investigation, yields cubic CeO2 of onlyslightly agglomerated nanosized particles and anionicvacancy-free bulk structure, exposing higher specificsurface areas than cerias obtained via the traditionalthermal decomposition method.

(2) When surfactants are used, the surface areas aresignificantly enlarged, and cerias of surface areas amount-ing to 250 m2/g are generated.

(3) Application of high calcination temperatures helpsburn off surfactant residues leading eventually to almostclean surfaces, without imposing any detectable bulkcrystalline structural modifications.

(4) Employing surfactant-stabilized microemulsions(AOT, DDAB, or DDAB + Brij 35) helps produce ceriaparticles of higher surface areas than using micellar(aqueous DTAB) or vesicle (aqueous DDAB) systems. OnlyDDAB microemulsions could produce high-temperaturestable, high-area cerias of promising application in thechemical makeup of combustion catalysts. The reason forthat is not fully understood, and further investigation isplanned to clarify the role of droplet mixing kinetics andthe reaction-domain size on properties of these ceriaparticles.

Acknowledgment. The financial support of KuwaitUniversity Research Administration Grant No. SC02/01is highly appreciated. Also the excellent technical as-sistance found at the XRD (GS03/01), EMU (GS03/01),and other analytical units of SAF (GS01/01) of KuwaitUniversity Faculty of Science is profoundly acknowledged.

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