Synthesis of Nanorod-Shaped Cobalt Hydroxycarbonate and … · The thermal gravity and differential...

Synthesis of Nanorod-Shaped Cobalt Hydroxycarbonate and Oxide with the Mediation ofEthylene Glycol

Xiaowei Xie,† Panju Shang,‡ Zhiquan Liu,*,‡ Yongge Lv,† Yong Li,† and Wenjie Shen*,†

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian116023, China, and Shenyang National Laboratory for Materials Science, Institute of Metal Research, ChineseAcademy of Sciences, Shenyang 110016, China

ReceiVed: NoVember 19, 2009; ReVised Manuscript ReceiVed: December 21, 2009

Cobalt hydroxycarbonate nanorods are prepared by precipitation of cobalt acetate with sodium carbonate inethylene glycol. Structural and chemical analyses of the intermediate phases during the precipitation andaging process revealed that amorphous cobalt hydroxide acetate is formed at the initial stage where ethyleneglycol acts as a simple solvent and a coordinating agent. With the slow addition of sodium carbonate, carbonateanions are gradually intercalated into the interlayers by replacing the acetate and hydroxyl anions. This anion-exchange process induces a dissolution-recrystallization process in which ethylene glycol serves as a rate-controlling agent, producing rod-like cobalt hydroxide carbonate. During the aging process, ethylene glycolgradually incorporates into the structure to replace the carbonate and acetate anions; the interlayer structureis collapsed, and the nanorod-shape turns into thin crimped sheets. Co3O4 nanorods with a diameter of about10 nm and a length of 200-300 nm are then obtained by calcination of the nanorod-shaped cobalthydroxycarbonate precursor. This spontaneous shape transformation from the precursor to the oxide is attributedto the unique thermal stability of the cobalt hydroxycarbonate nanorods with the presence of ethylene glycoland acetate anions in the interlayers. The Co3O4 nanorods show a much superior catalytic activity for COoxidation to the conventional spherical Co3O4 nanoparticles, clearly demonstrating the morphology-dependentnanocatalysis.

Introduction

Co3O4 nanomaterials are widely used in chemical sensors,1

lithium-ion batteries,1,2 optical and magnetic materials,3,4 andheterogeneous catalysts,5 where their properties are stronglydependent on their size and morphology. Accordingly, novelCo3O4 structures such as nanocubes,5 nanorods,6 nanowires,7

nanotubes,8 nanowalls,9 nanofibers,3 and nanoplatelets10 havebeen synthesized successfully. Even hierarchically flower-likenanomaterials,11-13 mircospheres,14,15 and hollow spheres16 havealso been fabricated. These nanostructures are usually synthe-sized through a two-step process. The precursors, cobalthydroxide salts expressed in a formula of Co(OH)2-x(An-)x/n ·mH2O (A represents the anions), are initially synthesized inliquid phase. These lamellar precursors consist of positivelycharged Co(OH)2-x layers with anions and water moleculesresiding in the interlayer to restore charge neutrality. Not onlyinorganic anions but also organic anions could be intercalatedinto the layers, and the interlayer spacing of the resultantcompounds is expanded, depending on the size of the specificanion.17 Co3O4 nanostructures are then obtained by thermaldecomposition of the precursors at elevated temperatures. Withsuch a procedure, the shape of the Co3O4 materials is largelydependent on the structure of the precursors. Therefore, controlof uniform morphology of cobalt hydroxide salts through propersynthetic strategy plays a central role in obtaining Co3O4

materials with novel morphologies.

Among Co3O4 nanomaterials, one-dimensional Co3O4 nano-rods have attracted special attention because of the uniqueoptical and magnetic properties4 and the potential applicationsin lithium-ion battery electrodes.2 Solvothermal synthesis,18

molten salt approach,19 and inverse microemulsion route6 havebeen employed to prepare Co3O4 nanorods with the presenceof organic surfactants/templates. The diameters of the obtainednanorods are usually in the range of tens or hundreds ofnanometers and the lengths are up to several or tens ofmicrometers. Recently, cobalt hydroxycarbonate nanorods withthe diameter in the range of 2-200 nm have been synthesizedonly using carbonate precipitation in water.20 The carbonateanion acts as a structure-directing agent, which selectivelydecreases the rates of crystal growth along both the [001] andthe [100] directions, resulting in the [010]-elongated nanorods.This structure-directing effect of carbonate anion causing theformation of rod-like cobalt hydroxycarbonates has been furtherdemonstrated under hydrothermal conditions.4,13,21,22 Unfortu-nately, these rod-like cobalt hydroxycarbonates have beentransformed into one-dimensional arrays of Co3O4 nanoparticlesor porous Co3O4 nanorods by removing the hydroxyl andcarbonate species during the calcination process at 300-500°C.4,13,20-22 Nevertheless, this synthesis of rod-like cobalthydroxycarbonate precursors using a simple precipitation inliquid phase should be particularly stressed due to the advantagesof simple apparatus, easy control, and large-scale fabrication.On the other hand, precipitation or hydrolysis of metal salts oralkoxides in polyols were found to affect strongly the mannerof crystallites aggregation in the formation of metal hydroxidesalts and to permit the generation of interconnected pores,resulting in enhanced surface area and well-defined porosity.23-27

* To whom correspondence should be addressed. (Z.L.) Tel/Fax: +86-24-8370826. E-mail: [email protected]. (W.S.) Tel: +86-411-84379085. Fax:+86-411-8469447. E-mail: [email protected].

† Dalian Institute of Chemical Physics, Chinese Academy of Sciences.‡ Institute of Metal Research, Chinese Academy of Sciences.

J. Phys. Chem. C 2010, 114, 2116–21232116

10.1021/jp911011g 2010 American Chemical SocietyPublished on Web 01/13/2010

We have recently communicated that Co3O4 nanorods thatpreferentially expose reactive {110} planes are extremely activeand stable for low-temperature CO oxidation compared to theconventional Co3O4 nanoparticles which have only a fewexposed active sites, clearly demonstrating a morphology-dependent effect.28,29 The nanorod-shaped Co3O4 was obtainedby calcination of cobalt hydroxycarbonate nanorods preparedby precipitation of cobalt acetate with sodium carbonate inethylene glycol. In this work, we have extended the study ofthe formation mechanism of the cobalt hydroxycarbonateprecursors and the Co3O4 nanorods. The fabrication process ofthe cobalt hydroxycarbonate nanorods was elaborated by varyingthe synthetic parameters, and the nanorod-shape transformationfrom the precursors to the oxides was investigated as well.

Experimental Section

Materials Preparation. In a typical synthesis, 4.98 g ofCo(OAc)2 ·4H2O was dissolved in 60 mL of ethylene glycoland heated to 80 or 160 °C to form a homogeneous solution. Atotal of 200 mL of 0.2 M Na2CO3 aqueous solution was thenadded to the mixture through a syringe pump with a flow rateof 1.11 mL/min. The precipitate was further aged in the motherliquid at that temperature for 1 h. The preparation course wasperformed under vigorous stirring and nitrogen flow. Theprecipitate was filtered and thoroughly washed with distilledwater and ethanol, following by drying at 50 °C overnight undervacuum. These precursors were denominated as Co-T, whereT refers to the temperature of precipitation. Further calcinationof the Co-T precursors at 450 °C for 4 h in air produced theCo3O4 nanomaterials, which were assigned as Co3O4-T.

In order to verify the formation mechanism of the Co-160nanorods, the precipitates at different stages (10 min, 30 min,45 min, 90 min, and 3 h) were extracted from the syntheticsolution and then filtered and washed only with ethanol toexclude the influence of water. The obtained solids were driedat 50 °C overnight under vacuum.

For examining the effect of the aging process, the precipitatesobtained at 160 °C under the typical precipitation conditionswere aged for a certain period ranging from 1 to 24 h. Thesamples were then filtrated and thoroughly washed with distilledwater and ethanol. The resultant solids were dried at 50 °Covernight under vacuum.

Characterization. The X-ray powder diffraction (XRD)patterns were recorded on a Rigaku D/MAX 2500 X-raydiffractometer using a Cu KR radiation source operated at 40kV and 250 mA. The mean crystallite sizes of Co3O4 werecalculated from the Scherrer equation using the most intensive(311) peak, where the particle shape factor was taken as 0.9.30

The transmission electron microscopy (TEM) images wererecorded on a Philips Tecanai GP2P20 microscope. The specimenswere prepared by ultrasonically suspending the sample in ethanol,and a drop of the solution was placed on the carbon-enhancedcopper grids and dried in air.

The Fourier transformed infrared (FTIR) spectra were col-lected on a Bruker Vector 22 spectrometer using KBr pellets.The pellet typically contained 5 wt % samples in KBr.

The thermal gravity and differential thermal analysis (TG-DTA) were recorded on a Pyris Diamond of Perkin-Elmerthermogravimetric analyzer. The samples were heated fromroom temperature to 800 °C at a rate of 10 °C/min with flowingair (40 mL/min).

The N2 adsorption-desorption isotherm was recorded at-196 °C using a Micrometics ASAP 2000 instrument. Beforethe measurement, the sample was degassed at 150 °C for 5 h.

The specific surface area was calculated by multipointBraunauer-Emmett-Teller (BET) analysis of the nitrogenadsorption isotherm.

Catalytic Evolution. CO oxidation reaction was conductedwith a fixed-bed quartz tubular reactor at atmospheric pressure.200 mg samples were loaded and pretreated with a 20% O2/Hemixture (50 mL/min) at 450 °C for 1 h. The catalyst samplewas then cooled to -77 °C by putting the reactor into a dry icetrap. The feed gas of 1% CO/2.5% O2/He (50 mL/min) wasintroduced through a mass-flow controller. Selective oxidationof CO in H2-rich gas (the so-called PROX reaction) was alsocarried out with the same reactor system at atmospheric pressure.After the pretreatment as mentioned above, the catalyst sample(200 mg) was cooled to the desired reaction temperature andthe feed gases of 0.5%CO/1%O2/60%H2/He were introducedwith a flow rate of 50 mL/min. Effluents from the reactor wereanalyzed by the online gas chromatograph (HP 6890) equippedwith a TCD and a flame ionization detector (FID). Theconversion of CO was calculated based on the amounts of COconsumption and CO2 formation. To determine the concentrationof carbon dioxide produced quantitatively, a nickel catalystconverter was placed before the FID and used for convertingCO2 into methane.

Results and Discussion

Co3O4 Nanorods. Figure 1 shows the TEM images of theCo-T precursors. When the precipitation was conducted at 80°C, short nanorods with a diameter of about 10 nm and a lengthof 50-100 nm were produced. With elevating the temperatureto 160 °C, the diameter of the nanorods maintained at about 10nm, but the length enlarged to 200-300 nm.

Figure 2 shows the XRD patterns of the Co-T samples. Thediffractions representing cobalt hydroxycarbonate are in goodaccordance with previous reports.13,20 The Co-160 sample hasstronger (020), (221), and (412) diffractions but weaker (040),(231), (340), and (450) diffractions compared to those of theCo-80 sample. Figure 3 shows the FTIR spectra of the Co-Tprecursors. The broadband at about 3500 cm-1 is attributed tothe stretching vibration of the O-H bond, υ(OH).20,31 The bandsat about 1074, 835, 736, and 687 cm-1 are assigned to thestretching vibrations of υ(CdO), δ(CO3), δ(OCO), and F(OCO)

Figure 1. TEM images of the Co-80 (a and b) and Co-160 (c and d)precursors.

Cobalt Hydroxycarbonate Nanorods J. Phys. Chem. C, Vol. 114, No. 5, 2010 2117

in the carbonate anion, respectively.20,32,33 The band at about942 cm-1 can be ascribed to the δ(Co-OH) bending vibration,whereas that at about 518 cm-1 represents the Fw(CoOH)vibration.20,32 This indicates that hydroxyl and carbonate anionsare the intercalating species in the Co-T precursors. The majordifference in the FTIR spectra becomes obvious in the range of1000-2000 cm-1. The bands at 1524 and 1419 cm-1 of theCo-80 precursor correspond to the stretching vibrations ofυ(OCO2) and υ(CO3) in the carbonate anion.20,32 Whereas theasymmetric stretching υas(COO-) (1596 cm-1) and the sym-metric stretching υs(COO-) (1392 cm-1) bands in the Co-160precursor with a wavenumber separation of 204 cm-1 confirmthe presence of acetate anion in the structure, probably actingas a unidentate ligand toward cobalt cations.34-36 Therefore, theCo-80 precursor is cobalt hydroxycarbonate, but the Co-160precursor is acetate-containing cobalt hydroxycarbonate.

Figure 4 illustrates the thermal decomposition of the Co-Tsamples. The mass loss at about 100 °C is attributed to theelimination of molecular water. The Co-80 sample has two masslosses at about 221 and 246 °C, indicating the sequentialdehydroxylation and decomposition of carbonate anions. Forthe Co-160 sample, the single mass loss at 232 °C represents

the simultaneous dehydroxylation and decomposition/combus-tion of carbonate and acetate anions, releasing water and carbondioxide. No further mass loss has been detected above 400 °C,and the precursors are converted to cobalt oxides.

Figure 5 shows the XRD patterns of the Co3O4-T samplesobtained by calcination of the Co-T precursors at 450 °C.Apparently, typical diffractions of spinel crystalline structureof Co3O4 (JCPDS: 42-1467) were observed in both cases. Thecrystalline size of Co3O4 is 16 nm in the Co3O4-80 sample witha surface area of 62 m2/g, whereas it decreases to 12 nm in theCo3O4-160 sample having a surface area of 163 m2/g. Figure 6shows the TEM images of the Co3O4-T samples. The Co3O4-80 sample contains irregular spherical particles with size of10-30 nm which connected disorderly. The Co3O4-160 sampleshows nanorod-shape with a diameter of about 10 nm and alength of 200-300 nm. Notably, the Co3O4-160 samplemaintained the nanorod-shape structure of the precursor, show-ing the high rigidity and thermal stability of the acetate-containing cobalt hydroxycarbonate. This result is quite differentfrom the general observation that calcination of cobalt hydroxy-carbonate nanorods often yields Co3O4 spherical nanoparticlesor porous Co3O4 nanorods, instead of Co3O4 nanorods, due tothe removal of hydroxyl and carbonate species.4,13,20-22

Morphology Evolution. The chemical composition and shapeevolution of the Co-160 nanorods were then investigated

Figure 2. XRD patterns of the Co-80 and Co-160 precursors.

Figure 3. FTIR spectra of the Co-80 and Co-160 precursors.

Figure 4. TG-DTG profiles of the Co-80 and Co-160 precursors.

Figure 5. XRD patterns of the Co3O4-80 and Co3O4-160 samples.

2118 J. Phys. Chem. C, Vol. 114, No. 5, 2010 Xie et al.

intensively. When the addition of sodium carbonate wasconducted for 10 min, the precipitate had irregular sheet-shape(Figure 7b). The low-angle reflection at 13.146 Å is assignedto the (003) diffraction of the hydrotalcite-like structure, andthe diffractions at 6.564 and 4.360 Å represent the (006) and(009) lines (Figure 8b). The d values of these diffractions, d003

≈ 2d006 ≈ 3d009, also confirm the hydrotalcite-like structure.33,37

FTIR profile of this sample further validated the XRD observa-tion. As shown in Figure 9b, the broadband at 3500 cm-1

represents the stretching vibration of the O-H group and the

C-H stretching vibrations appear at 2944 (asymmetric) and2869 cm-1 (symmetric).38,39 The bands at 1580 and 1400 cm-1

with a wavenumber separation of 180 cm-1 corresponds to theasymmetric stretching υas(COO-) and symmetric stretchingυs(COO-) vibrations of unidentate acetate species.35,36 Thecharacteristic bands of 1091 and 1052 cm-1 are attributed to aneutrally adsorbed ethylene glycol on the surface withoutobvious shift in wavenumbers, compared to pure ethyleneglycol.27,35 The bands at 1338 and 1024 cm-1 are assigned tothe stretching vibrations of δ(CH3) and Fr(CH3), respectively.27

The band at 663 cm-1 is attributed to Co-O stretchingvibration.33 Therefore, this initial precursor should be cobalthydroxide acetate with a layered structure. For comparison,hydrolysis of cobalt acetate in ethylene glycol was conductedfor 10 min under the same conditions with the addition of water,instead of sodium carbonate. Layered cobalt hydroxide acetatewas obtained with almost the same chemical composition andstructure (Figures 7-9a). This clearly demonstrates that hy-drolysis of cobalt acetate in ethylene glycol is the major reactionduring the initial stage of precipitation, instead of carbonateprecipitation. With the gradual addition of sodium carbonatefrom 0.5 to 3 h, the irregular sheet was converted to rod-likeshape (Figure 7c-f); the (003), (006), and (009) diffractions

Figure 6. TEM images of the Co3O4-80 (a-c) and Co3O4-160 (d-f) samples.

Figure 7. TEM images of the precipitates obtained in water (a) andwith the addition of aqueous Na2CO3 solution for 10 min (b), 30 min(c), 45 min (d), 90 min (e), and 3 h (f).

Figure 8. XRD patterns of the precipitates obtained in water (a) andwith the addition of aqueous Na2CO3 solution for 10 min (b), 30 min(c), 45 min (d), 90 min (e), and 3 h (f).


representing the hydrotalcite-like structure gradually disappearedwhereas the typical diffractions of cobalt hydroxycarbonateappeared. The representative bands of acetate anions at about1600 and 1400 cm-1 distorted gradually and the wavenumberseparation increased to more than 200 cm-1, indicating thereleasing of acetate anion from the structure. Meanwhile, thebands representing carbonate anion at 1074 and 835 cm-1

enhanced significantly, confirming that the carbonate anionsintercalated into the interlayer structure, replacing the acetateanions. When sodium carbonate was fully added at 3 h, theprecipitate showed a nanorod-shape with a diameter of about10 nm and a length of 200-300 nm where acetate and carbonateanions were presented in the structure. That is, acetate-containingcobalt hydroxycarbonate nanorods.

Aging Effect. The typical acetate-containing cobalt hydroxy-carbonate nanorods were obtained by precipitation for 3 h andfurther aging for 1 h at 160 °C. The morphology evolution andchemical composition of this precursor during the course ofaging were examined. As shown in Figure 10, when the agingtime was extended to 6 h, the sample had a rod-like structurewith a diameter of about 10 nm and a length of 200-300 nm,similar to the Co-160 precursor. But the (100) reflection at about10 Å (Figure 2) disappeared (Figure 11a) due to the collapseof the layers.31,40 The bands at 1063 and 993 cm-1 correspondingto υas(CO) and υs(CO) stretching vibrations enhanced consider-ably (Figure 12a), indicative of an increased amount of ethyleneglycol in the structure.14,27,38 When the aging lasted for 12 h,the sample is composed of sheets and crimped nanorods

(Figure 10c,d). Two sawtooth-shaped diffractions at 2θ ) 33°and 59° intensified largely (Figure 11b), evidencing the turbos-tratic disordered structure that is commonly observed inR-hydroxides.31 Meanwhile, the diffractions of cobalt hydroxidecarbonate further weakened and almost disappeared. In the FTIRspectrum (Figure 12b), bending vibration of interlayer watermolecule appeared with the typical band at 1638 cm-1.41 Thevibrations of carbonate and acetate anions (1600-1400 cm-1)decreased significantly but the vibrations of ethylene glycol(1070 and 997 cm-1) increased considerably, indicating theincorporation of ethylene glycol into the structure. Furtherextending the aging time to 24 h, the sample containedaggregates of thin crimped sheets without definite shape (Figure

Figure 9. FTIR spectra of the precipitates obtained in water (a) andwith the addition of aqueous Na2CO3 solution for 10 min (b), 30 min(c), 45 min (d), 90 min (e), and 3 h (f).

Figure 10. TEM images of the samples obtained by aging the typicalprecipitate for 6 h (a and b), 12 h (c and d), and 24 h (e and f).

Figure 11. XRD patterns of the samples obtained by aging the typicalprecipitate for 6 h (a), 12 h (b), and 24 h (c).


10e,f). In the XRD pattern (Figure 11c), only two sawtooth-shaped diffractions were detected, the low-angle reflection ofthe layered structure completely vanished. The interlayer watermolecule with a bending vibration at 1641 cm-1 was enhanced(Figure 12c).41 The vibration bands of carbonate and acetateanions completely disappeared and there were only stretchingvibrations of ethylene glycol. Apparently, acetate and carbonateanions are gradually replaced by ethylene glycol during the agingprocess and this anion-exchange phenomenon in double layeredhydroxides has been well decumented.38,42,43 For example,ethylene glycol gradually incorporated into the structure ofZn-Al-CO3 double layered hydroxide and finally replaced thecarbonate anions.38

Growth Mechanism. Based on the results described above,it seems that the formation of the acetate-containing cobalthydroxycarbonate nanorods follows a multistage process de-pending on the role of ethylene glycol.

When cobalt acetate is dissolved into ethylene glycol at160 °C, cobalt alkoxyacetate is formed.27,35 The formation ofthis intermediate was proved and structurally characterized bydissolving zinc acetate in ethylene glycol and diethyleneglycol.44,45 Here, ethylene glycol acts as a simple solvent and acoordinating agent to cationic cobalt species. With the additionof carbonate sodium, cobalt hydroxide acetate is formed initiallyby hydrogenolysis of the cobalt alkoxyacetate intermediate. Asproved by XRD (Figure 8) and FTIR (Figure 9) measurements,hydrolysis of cobalt alkoxyacetate to cobalt hydroxide acetateoccurs at the early 10 min, instead of carbonate precipitation.The factor governing the formation of cobalt hydroxide acetateis the hydrolysis ratio, defined as h ) nH2O/nCo2+, and thevalue should be more than 26 in the case of cobalt acetatedissolving in diethylene glycol at 160 °C.35 Under the currentconditions, after adding sodium carbonate for 10 min, thehydrogenolysis ratio is as high as 60, apparently favoring theformation cobalt hydroxide acetate. The amorphous structure(Figure 7b) is formed during this fast nucleation. When thesynthetic solution is turned to basic conditions with the gradualaddition of Na2CO3 aqueous solution, however, carbonate anionsare then gradually intercalated into the interlayers by replacing

acetate and hydroxyl anions due to the strong affinity to theinterlayers.46,47 This anion-exchange process is kinetically drivenand induces a dissolution-recrystallization of the initially formedphase, producing novel crystalline phases. Because the additionrate of sodium carbonate is very slow, the growth of crystalsproceeds very slowly, favoring an anisotropic growth owing tothe structure-directing effect of the carbonate anions. Meanwhile,ethylene glycol serves as a rate-controlling agent by separatingthe nucleation of cobalt hydroxide acetate and the crystal growthof rod-like cobalt hydroxide carbonate. Hence, the acetate-containing cobalt hydroxycarbonate nanorods are formed throughthe fast nucleation and the slow crystalline growth.

When the precipitate is further aged in the mother liquid,ethylene glycol incorporates into the layered structure to replacecarbonate and acetate anions. As confirmed by the FTIRmeasurements (Figure 12), ethylene glycol gradually replacesthe carbonate and acetate anions in the structure and finally itpresents as the sole anions. Together with this, the interlayerstructure is collapsed and the nanorod-shape turns into largeaggregates of thin crimped sheets.

The inherent shape transformation from the Co-160 precursorto the Co3O4-160 nanorod is quite different from that of theCo-80 precursor, where the nanorod-shape precursor wastransferred to nanoparticle (Figure 6). This verifies the thermalstability of the Co-160 nanorods because of the presence ofethylene glycol and acetate anions at a proper ratio in theinterlayers. Ethylene glycol strongly affects the manner that thecrystallites aggregate and promotes the generation of a networkof small interconnected pores.23-27 On the other hand, thepeculiar synthesis route results in large amounts of acetate anionsin the Co-160 precursor, whereas the Co-80 precursor onlycontains carbonate and hydroxyl anions. Carbonate anionsintercalate into the layers only as free species,43,47,48 butunidentate acetate anions in the Co-160 precursor bind to cobaltatom through a single oxygen atom, forming intermolecularH-bonding with nearby hydroxyl groups. This positive role ofH-bonding in the formation of self-assembled Co3O4 nanotubeshas been recently demonstrated.49 Upon calcination, cobaltcomplexes with a strong intermolecular hydrogen bondingformed one-dimensional Co3O4 nanotubes, but short Co3O4

nanotubes with defects and broken spheres were produced whencobalt complexes with a weak or no intermolecular H-bondingwere used. Therefore, acetate anions acting as bridges enhancethe connection of adjacent layers through intermolecular interac-tion during calcination, maintaining the rod-like structure of theCo-160 precursor.

Catalytic Performance. Figure 13 compares the reactionperformance of the Co3O4 nanoparticles and nanorods for low-temperature oxidation of CO. The nanoparticles gave initial COconversion of 30% and then rapidly stabilized at about 10%with time on-stream. Interestingly, the nanorods showed 100%CO conversion at the initial 6 h and then only slightly decreasedwith time on-stream. Even after reaction for 12 h, the conversionof CO was still maintained at 80%, clearly evidencing that thenanorods are more active and stable than the nanoparticles.Figure 14 shows the temperature dependence of CO oxidationin H2-rich gas over the Co3O4 nanoparticles and nanorods.Apparently, the nanorods exhibited remarkably higher CO andO2 conversions than the nanoparticles in the temperature rangetested. At 120 °C, the conversion of CO was 99% over thenanorods, whereas it was only 85% over the nanoparticles; thecorresponding O2 conversions were 50% and 32%, respectively.The conversion of CO reached 100% at 140 °C over thenanorods, but it was 96% over the nanoparticles. Meanwhile,

Figure 12. FTIR spectra of the samples obtained by aging the typicalprecipitate for 6 h (a), 12 h (b), and 24 h (c).


the conversion of O2 over the nanorods approached 83% owingto the oxidation of hydrogen in the feed stream, whereas it wasonly 50% on the nanoparticles.

These results clearly demonstrate that the nanorods are moreactive than the nanoparticles. As we have recently reported,28

the Co3O4 nanorods tend to preferentially expose the {110}planes that are rich in Co3+ species acting as active sites forCO oxidation. However, the Co3O4 nanoparticles usually exposethe less reactive {001} and {111} planes, which contain onlyinactive Co2+ sites, resulting in a lower activity.

Conclusions

Calcination of acetate-containing cobalt hydroxycarbonatenanorods, obtained by precipitation of cobalt acetate with sodiumcarbonate in ethylene glycol, yielded nanorod-shaped Co3O4

materials. During the precipitation process, amorphous cobalt

hydroxide acetate was initially formed, followed by the gradualintercalation of carbonate anions into the interlayers, replacingthe acetate and hydroxyl anions. This anion-exchange processinduced a dissolution–recrystallization in which carbonate anionsacted as a structure-directing agent and ethylene glycol servesas a rate-controlling agent, producing rod-like acetate-containingcobalt hydroxide carbonate. During the aging process, ethyleneglycol gradually incorporated into the structure to replacecarbonate and acetate anions and finally it presented as the soleanion. Because of this, the nanorod-shape turned into largeaggregates of thin crimped sheets.

Co3O4 nanorods with a diameter of about 10 nm and a lengthof 200-300 nm were obtained by calcination of the acetate-containing cobalt hydroxycarbonate nanorods. This spontaneousshape transformation from the precursor to the oxide is attributedto the presence of ethylene glycol and acetate anions in the

Figure 13. CO oxidation over the Co3O4 nanoparticles and nanorods. Reaction conditions: 1.0 vol % CO/2.5 vol % O2/He; 15 000 mL/(g h);-77 °C.

Figure 14. CO oxidation in H2-rich gas over the Co3O4 nanoparticles and nanorods. Reaction conditions: 0.5 vol % CO/1.0 vol % O2/60 vol %H2/He; 15 000 mL/(g h).


nanorod-shaped precursor. The Co3O4 nanorods showed a muchbetter catalytic activity for CO oxidation than the sphericalCo3O4 nanoparticles, confirming the concept of morphology-dependent nanocatalysis.

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Synthesis of Nanorod-Shaped Cobalt Hydroxycarbonate and … · The thermal gravity and differential...

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