Catalysis Communications 42 (2013) 6267

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Catalysis Communications

j ourna l homepage: www.e lsev ie r .com/ locate /catcomShort CommunicationPreparation and catalytic activity in ethanol combustion reaction ofcobaltiron spinel catalystsYasmina Hammiche-Bellal a,, Amel Benadda a, Laaldja Meddour-Boukhobza a, Siham Barama a,Amar Djadoun b, Akila Barama a

a Laboratoire des Matriaux Catalytiques et Catalyse en Chimie Organique, Facult de Chimie, USTHB, BP32 El Alia, Bab Ezzouar, 16111 Algiers, Algeriab Laboratoire de Gophysique, FSTGAT, USTHB, BP32 El Alia, Bab Ezzouar, 16111 Algiers, Algeria Corresponding author. Tel.: +213 662 97 57 56; fax:E-mail address: y.hammichebellal@gmail.com (Y. Ham

1566-7367/$ see front matter 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.catcom.2013.07.042a b s t r a c ta r t i c l e i n f oArticle history:Received 17 May 2013Received in revised form 13 July 2013Accepted 19 July 2013Available online 7 August 2013

Keywords:Cobalt ferriteSpinelCoprecipitationCatalytic combustionVOCsIroncobalt spinel catalysts were prepared via the coprecipitation method. The effect of different parameters ontextural, structural and catalytic properties, in ethanol combustion, was investigated. The CoFe2O4 phase wasobtained at calcination temperatures as low as 500 C and the usage of ammonia as precipitating agent, resultsin the formation of Fe2O3 in addition to the spinel phase. The catalyst prepared using nitrate salts, NaOH asprecipitation agent and calcined at 600 C had the best catalytic performance achieving ethanol completeoxidation at 271 C.

2013 Elsevier B.V. All rights reserved.1. Introduction

Transition metal ferrites present good catalytic performances forvarious reactions, due to surface oxygen mobility property, electricalconductivity and stability under oxidative conditions [1]. These proper-ties are linked to the possibility of the reconstructing of spinel surfacelayer, the presence of structural defects and electron exchange betweenneighboring ions. Hence, they have been studied in methane combus-tion [13], CO oxidation [4], n-butenes oxidative dehydrogenation [5]and FischerTropsch process [6]. In a recent study, the catalytic perfor-mance of MeFe2O4 (Me = Cu, Ni, Zn and Cd) in the volatile organiccompounds (VOCs) combustion was found to depend on the nature ofMe cation, indeed, multiple valence cations promote the creation of ox-ygen vacancies which are active sites for oxygen adsorption [7], there-fore, the cobalt may be expected to induce a good catalytic activity indeep oxidation reactions as it has variable valence (II, III).

The catalytic properties of catalysts are strongly affected by everystep of the preparation together with the starting materials' nature.Variousmethods have been developed to synthesize CoFe2O4, includinghydrothermal process [8], solgel technique [9], combustion method[10], microwave-assisted route [11], electrospinning method [12] andcoprecipitation [1316]. The last one is a relatively simple process+213 21 24 80 08.miche-Bellal).

ghts reserved.and is, therefore, suitable for mass production; the choice of thecoprecipitation conditions might have a great importance.

VOCs are considered as great contributors to the atmospheric pollu-tion [17,18] and catalytic combustion is recognized as a convenientwayfor VOCs emission prevention [19].

In the present paper, we aim to report the catalytic properties ofCoFe2O4 nanoparticles synthesized by coprecipitation method. Theeffect of precipitation agent, thermal treatment and precursor salts'nature, on the catalytic properties in the ethanol combustion, was care-fully investigated.2. Experimental

2.1. Preparation protocols

A mixture of chloride or nitrate salts aqueous solutions (Co2+/Fe3+ = 2) was precipitated with NaOH, NH4OH or H2C2O4, under con-stant stirring at 70 C. The precipitates were washed and dried at 80 Cfor 12 h. The as-prepared precursors were named CP-Cl-Na, CP-Cl-NHand CP-Cl-OX when chloride salts were used as starting materials,whereNa, NHandOXstand for theprecipitation agent: NaOH, ammoniaand oxalic acid respectively. The precursor prepared from the nitratesalts was named: CP-N-Na. The four precursors were calcined, for 5 hat 500, 600, 700 and 800 C in air. The calcined solids will be noted asfollows: CP-Cl-Na-X, CP-Cl-NH-X, CP-Cl-OX-X, CP-N-Na-X, where Xstands for the calcination temperature.

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63Y. Hammiche-Bellal et al. / Catalysis Communications 42 (2013) 62672.2. Characterization

The total cobalt and iron contents were determined by Atomic Ab-sorption Spectroscopy (AAS) using Analyst 700 Perkin-Elmer modeflame apparatus.

The XRD analysis was carried out using a Philips X-ray diffractome-ter and Cu-K radiation. The diffraction patterns were recorded in therange 2 = 1090 at 1.2 min1. Mean crystallite size of the detectedphases was determined from the line broadening of the attributed XRDlines using the WilliamsonHall method. The lattice parameter wasestimated from the Rietveld analysis of the powder XRD data.

Thermogravimetric analysis of the precursors was performed in aSETARAM TG-DTA12 system; samples were heated from 25 to 900 C,at a heating rate of 10 C/min, in flowing Argon.

The nitrogen adsorptiondesorption isotherms were recorded on aMicromeritics ASAP 2010.

2.3. Catalytic test

The catalytic activity of the solids was evaluated in total oxidation ofethanol using a glass fixed bed reactor. Prior to catalytic tests, solidswere activated, by heating in flowing air up to 300 C for 15 min, to re-move any eventual physisorbedwater molecules. Air was used as carri-er gas for ethanol vapor and oxidant reactant. The catalyst (0.2 g) wastested under a gas hourly space velocity (GHSV) of 30 m3h1 kg1 inthe temperature range 200400 C. The conversion of ethanol,X%, is de-fined as the percentage of ethanol feed that is reacted, the reportedvalues were calculated after that a stationary regime was reached. Thereactant mixture was analyzed using a Clarus 500 Perkin-Elmerchromatograph equipped with a FID detector and a 20% carbowax/Chromosorb packed column.

3. Results and discussion

3.1. Characterization of the precursors

a. XRDFig. 1a depicts the XRD patterns of the as-prepared precursors to-gether with the reference patterns. The CP-Cl-NH showed an amor-phous structure corresponding to a poorly crystallized mixture ofCo(OH)2 and ferrihydrite (Fe5O7OH4H2O) phases, similar observa-tions have been reported previously [20]. The CP-Cl-Na, CP-N-Naand CP-Cl-OX exhibited better crystallized structures, correspondingFig. 1. (a) XRD diffractograms of the catalysts precursors. (b) XRD diffrato CoFe2O4 phase for CP-Cl-Na and CP-N-Na and an oxalate metalsalts' mixture in the case of CP-Cl-OX. The formation of spinelphase before the calcination step has been, already, reported byWang et al. [16] and Tseung et al. [21].

b. TGAThe TGA curves of the precursors are shown in Fig. 2. In the CP-Cl-NHprofile, the first step (50200 C) is a dehydration process, corre-sponding to a 13% weight loss attributed to the elimination of twowater molecules per metal hydroxide formula unit. During the sec-ond step (200400 C), the anhydrous cobalt and iron hydroxidesdecomposed, giving rise to a 10% weight loss which is close to thecalculated weight loss (11.6%) assuming the formation of Fe2O3and CoO. Similar weight loss steps were observed for CP-Cl-Na, oc-curring in the same temperature ranges, which may indicate thatthe same phases are decomposing in each case, pointing out thatthe spinel phase, observed in XRD, is not the only phase present inthe CP-Cl-Na solid but also metal hydroxides are formed. Additionalsmall features, above 400 C, were noticed on the CP-Cl-Na profile,which could only be explained by the formation of a spinel phasewith high oxygen content that can be lost during heating processto form a stoichiometric spinel [22].The CP-Cl-OX TGA curve is characterized by two weight loss stepsoccurring in the 150200 C and the 200400 C temperatureranges and corresponding to 17% and 34%weight losses respectively,the first weight loss could be attributed to a dehydration process ofoxalate mixture based on the calculated value (19.9%) assumingthe elimination of two H2O molecules per formula unit. The secondstep was attributed to the decomposition of anhydrous oxalateinto CoOFe2O3 as the observed weight loss agrees well with thetheoretical one (35.2%).

3.2. Characterization of the calcined solids

a. AASThe results of chemical analysis, for four selected samples, are re-ported in Table 1, the measured metal contents are, quite, in goodagreement with the expected stoichiometry.

b. XRDThe XRD diffractograms of the calcined solids are depicted in theFig. 1b. The examination of these patterns demonstrated that usingNaOH and oxalic acid as precipitation agents led to the formationof pure CoFe2O4 spinel crystalline phase (JCPDF#221086), whileusing ammonia as precipitation agent resulted in the formation ofctograms of the catalysts calcined at 500 C. (0: CoFe2O4, *: Fe2O3).

Fig. 2. TGADTG curves of precursors: CP-Cl-NH (a), CP-Cl-Na (b), and CP-Cl-OX (c).

64 Y. Hammiche-Bellal et al. / Catalysis Communications 42 (2013) 6267traces of Fe2O3 in addition to the CoFe2O4 spinel phase, a cobalt sin-gle phase consisting in Co3O4 should, also, be formed as the Co2+/Fe3+ ratio is close to 2 but it couldn't be differentiated fromCoFe2O4 as they are isomorphic. Increasing calcination temperatureresulted in a better crystallization, as deduced from the XRD lineintensities and crystallite size (Table 2). The lattice parameterTable 1Chemical analysis results for the prepared solids calcined at 500 C.

Sample Calculated composition % Co % Fe

CP-Cl-Na-500 Co1.02Fe1,98O4 25.62 47.13CP-Cl-NH-500 Co0.97Fe2.03O4 24.37 48.32CP-Cl-OX-500 Co0.99Fe2.01O4 24.87 47.85CP-N-Na-500 Co1.09Fe1.91O4 27.38 45.47increased with calcination temperature, which is likely due to theconversion of an oxygen rich spinel (CoFe2O4 + ) into a stoichio-metric spinel (CoFe2O4), involving the reduction of cobalt and/oriron ions [14].It's noteworthy that the decomposition of oxalate mixture led to theformation of the spinel phase at temperatures as low as 500 C; thisresult is different from the findings of Gabal et al. [23], who showedthat the calcination of the oxalate mixed precursors didn't lead tothe crystallization of CoFe2O4 before 1000 C.

c. N2 adsorption/desorptionThe N2 adsorption/desorption isotherms and pore size distributioncurves are represented in Fig. 3.

The isotherms are related to type IV of the IUPAC classification, char-acteristic of mesoporous materials, except for the solids prepared fromoxalate precursors, which seem rather related to type II adsorption

image of Fig.2

Table 2Crystallite sizes, lattice parameters and BET specific surface area measurements for the prepared solids calcined at 500 and 600 C.

Sample CoFe2O4 CP-Cl-Na-500 CP-Cl-Na-600 CP-Cl-NH-500 CP-Cl-NH-600 CP-Cl-OX-500 CP-Cl-OX-600 CP-N-Na-500 CP-N-Na-600

Crystallite size (nm) 8.97 13.87 11.29 12.11 19.78 20.34 9.85 11.92Lattice parameter () 8.391 8.322 8.331 8.310 8.383 8.343 8.355 8.275 8.325BET area (m2/g) 69.1 27.9 32.8 33.2 42.7 16.1 62.2 49.6

65Y. Hammiche-Bellal et al. / Catalysis Communications 42 (2013) 6267isotherms indicating a nonporous character [24], the lowporosity of thissolid become more marked after calcination at higher temperature(600 C), as shown by the pore distribution curve.

The samples prepared via hydroxide coprecipitation presented a H1type hysteresis loop on the desorption isotherm[24] when the oxalatesynthesized powders showed a H3 type hysteresis loop and presenteda large pore distribution in comparison with solids prepared via thehydroxide route.

The BET area surfaces measurements are shown in Table 3, notice-able differences were observed. The highest surface area values wereobtained for the samples prepared by NaOH, this might be attributedto the formation of the spinel phase before the calcination step. The for-mation of Fe2O3 can explain the smaller surface area of the ammoniaprecipitated solids, assuming that the Fe2O3 particles are deposited onthe spinel oxide surface blocking some pores. For the oxalate preparedsolids, it has been reported that oxalate precursors formhigh size aggre-gates compared to the hydroxides, leading to smaller surface areas aftercalcination [25].3.3. Catalytic activity

The catalytic tests results, obtained in the ethanol combustion, overthe CP-Cl-Na annealed at different temperatures are shown in Fig. 4a.Fig. 3. N2 adsorption/desorption isotherms and pore size distribuThe calcination at 500 C led to the best catalytic performance with acomplete combustion temperature of 310 C.

Fig. 4b and c shows the effect of precipitation agent on the catalyticactivity of CoFe2O4 solids, for two calcination temperatures 500 and600 C, respectively, the CP-Cl-NH-X samples showed the lowest cata-lytic activity, when the oxalate and sodiumhydroxide precipitated sam-ples presented close activities.

The comparison of isoconversion temperatures leads to the conclu-sion that the use of NaOH as precipitation agent led, generally, to thebest catalytic results (Table 3).

Comparison of nitrate and chloride based solids (Fig. 4d) showedthat the latter presented lower performances which are characteristicsof a chlorine poisoning effect observed in many studies where chlorineprecursors were used [26].

For the nitrate based solids, the onset temperature was nearby200 C and a total conversion was achieved at about 275 C. This is aquite interesting result compared towhat has been reported in the liter-ature for nickel and copper ferrites where the onset temperature ofethanol combustion was about 250 C [7]. It is known that in semicon-ducting ferrites, the electrical conduction is determined by the electronexchange between Fe2+ and Fe3+. The formation of Fe2+ ions is en-hanced by themultiple valence cation and induces the occurrence of ox-ygen vacancies in the MFe2O4 structure [27]. Therefore, the activity ofCoFe2O4 could be attributed to the ability of Co2+ to transform intotion curves of CoFe2O4 powders calcined at 500 and 600 C.

image of Fig.3

Table 3Temperatures of 10, 50 and 90% isoconversion levels for the prepared solids calcined at 500 and 600 C.

Sample CP-Cl-Na-500 CP-Cl-NH-500 CP-Cl-OX-500 CP-Cl-Na-600 CP-Cl-NH-600 CP-Cl-OX-600 CP-N-Na-500 CP-N-Na-600

T10 (C) 200 306 233 236 312 271 218 218T50 (C) 286 378 270 296 405 316 257 258T90 (C) 310 398 310 320 423 422 271 271

66 Y. Hammiche-Bellal et al. / Catalysis Communications 42 (2013) 6267Co3+ which results in the formation of oxygen vacancies that are activesites for oxygen adsorption.

In an attempt to correlate the catalytic activity with textural andstructural properties of the solids, we have represented the T10 andT90 versus the particle size and BET surface (Fig. 5a, b and c). Onecan remark that there is no clear relation between crystallite sizeand catalytic performances; indeed, CP-Cl-OX-500 has a relativelygood activity despite its high particle size. Meanwhile, a good corre-lation between the BET surface areas and isoconversion tempera-tures could be noticed: for the solids prepared using chloride salts,NaOH and oxalic acid, the higher the surface area is, the lower arethe isoconversion temperatures. On the other hand, the solids pre-pared using ammonia presented the lowest performances in the eth-anol combustion although they showed medium BET surfaces, thisodd performance can only be related to the formation of Fe2O3phase. Catalytic activity of Fe2O3 solid prepared by precipitationfrom chloride precursor and NaOH was measured (Fig. 4e) and theFig. 4. Ethanol conversion as a function of reaction temperature (a) effect of calcination tempthe catalytic performances of chloride based solids, calcination temperature: 500 C, (c) sametemperature 500 & 600 C. (e) Catalytic activity of Fe2O3.results showed a very low catalytic activity for this solid as the etha-nol is totally converted, only, at 400 C.4. Conclusion

CoFe2O4 solids have beenprepared via copreciptation using differentprecipitation agents and metal precursors.

Using NaOH as precipitation agent led to the crystallization ofthe spinel phase even before the calcination step. A pure spinelphase was obtained when using NaOH and oxalic acid for the pre-cipitation when a mixture of spinel phase and Fe2O3 was obtainedwith ammonia.

The NaOH precipitated solids presented the best catalytic perfor-mances, especially, when nitrates were used as metal precursor.A good correlation between surface areas of single phase cobaltferrites and catalytic performances was demonstrated.erature on catalytic performances of CP-Cl-Na solids, (b) effect of precipitation agent onas (b) calcination temperature: 600 C, and (d) effect of metal source nature, calcination

image of Fig.4

Fig. 5. (a) The T10, T90 isoconversion temperatures versus crystallite size, (b, c) the T10, T90 isoconversion temperatures versus specific surface areas.

67Y. Hammiche-Bellal et al. / Catalysis Communications 42 (2013) 6267The formation of Fe2O3 within the solids prepared via ammonia pre-cipitation was responsible of the lower catalytic activities observed forthese solids.

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Preparation and catalytic activity in ethanol combustion reaction of cobaltiron spinel catalysts1. Introduction2. Experimental2.1. Preparation protocols2.2. Characterization2.3. Catalytic test

3. Results and discussion3.1. Characterization of the precursors3.2. Characterization of the calcined solids3.3. Catalytic activity

4. ConclusionReferences