228990289 co ii co iii hydrotalcite like compounds

8/10/2019 228990289 Co II Co III Hydrotalcite Like Compounds

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International Journal of Inorganic Materials 3 (2001) 2329

Co(II)Co(III) hydrotalcite-like compounds

a,b b c c , a b*B. Zapata , P. Bosch , G. Fetter , M.A. Valenzuela , J. Navarrete , V.H. Laraa

Gerencia de Catalisis, Instituto Mexicano del Petroleo, 07730 Mexico, D.F., Mexicob

Departamento de Qumica, Universidad Auton oma Metropolitana-I, 09340 Mexico, D.F., Mexicoc

Laboratorio de Catalisis y Materiales, IPN-ESIQIE, 07738 Mexico, D.F., Mexico

Received 7 June 2000; accepted 12 July 2000

Abstract

The synthesis of monometallic hydrotalcite-like compounds (MHLC) type Co(II)Co(III) materials was achieved by precipitation of asolution of cobalt nitrate(II) followed by microwave irradiation during the hydrothermal treatment step. The effects of irradiation time,

precipitant agent and atmosphere were studied in order to establish the preparation conditions and the stability of these materials. As

expected, these materials required careful preparation conditions, mainly a very slow addition of the precursors. NH OH had to be used as4

precipitant agent in an air atmosphere. Short microwave irradiation times during the hydrothermal treatment provided a better

crystallization of MHLC. The low stability of these compounds (|2008C) was explained by the presence of several Co-complexes

between the layers of the laminar structure. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal

analysis and thermogravimetric analysis (DTA/TGA), and thermoprogrammed oxidation (TPO) were used as characterization techniques.

2001 Published by Elsevier Science Ltd.

Keywords: Clays; Hydrotalcite-like compounds; Cobalt; Microwave irradiation; Microporous materials; Anionic exchangers

1. Introduction tested also as supports of transition metals in selectivehydrogenation of hydrocarbons and steam reforming [6].

Anionic clays or double layered hydroxides or hydro- Monometallic hydrotalcite-like compounds i.e., one

talcite-like compounds are represented by the general transition metal used with both valences, M(II) and M(III),n 2

formula: [M(II) M(III) (OH) ][A ] ?mH O where in the same network have not been often studied [7,8]. The12x x 2 x/n 2

n 2

M(II) and M(III) are metal cations and A denotes early work devoted to these compounds was reported by

anions. Their structure consists of brucite-like layers, Taylor [9] in 1980. He studied the formation and properties21 31

Mg(OH) , with a partial M(II) substitution by M(III) of Fe Fe hydroxycarbonates and compared them with2resulting in a net positive charge balanced by interlayer the natural compounds encountered in soil. He proposed

21 31anions associated with variable amounts of water mole- that Fe Fe hydroxycarbonates maybe the meta-stablecules [1]. The main applications of anionic clays are as precipitates rather than a hydro-magnetite phase previouslycatalysts, catalyst supports, flame retardants, molecular postulated. Zeng et al. in 1998 [7] synthesized

21 21 31

sieves, ion exchangers, antiacids, antipeptins, PVC stabi- Mg Co Co hydroxynitrates to use them as environ-lizers and to purify waste waters [2] among others. As their mental catalysts for the decomposition of greenhouse gas,21composition and their chemical properties may be varied, N O. They found that a fraction of the Co cations was

231

the hydrotalcite-like compounds have attracted attention in partially oxidized to Co forming the non-typical hydro-21 21 31 12 12catalysis [3 5]. Indeed, mixed metal oxides obtained by talcite-like compound Mg Co Co (OH) (NO )0.3 0.6 0.2 2 3 0.2

thermal decomposition of the hydrotalcites can promote H O. In cobalt hydroxide compounds the same authors [8]2

base-catalyzed reactions such as polymerization, condensa- reported the synthesis through interconversion of brucite-tion or alkylation. Moreover, these materials have been like and hydrotalcite-like phases. These compounds were

obtained by means of a complex synthesis in which the

effect of addition time, aging time and preparative atmos-*Corresponding author.

phere on phase transformations was determined. AccordingE-mail address: [email protected] (M.A. Valen-21 31

zuela). to these results the hydrotalcite-like phase Co Co is

1466-6049/01/$ see front matter 2001 Published by Elsevier Science Ltd.

P II: S 1 4 6 6 -6 0 4 9 (0 0 )0 0 0 9 7 -0


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24 B. Zapata et al. /International Journal of Inorganic Materials3 (2001) 2329

only formed from the b-Co(OH) precursor using air as a tation, samples were irradiated for variable time intervals2

reaction medium. (110 min) in a domestic microwave oven (Philco)

According to the work of Xu and Zeng [8] and operating at 90 W and a frequency of 4.75 MHz. For

references included therein, if NaOH is used as a precipi- comparison purposes, an additional sample was prepared

tant agent, the reaction with Co(II) precursor favors the without microwave irradiation, only aging it for 2 h. After

formation of brucite structures Co(OH) . In contrast if the precipitation reaction, a sample portion was washed2

NH OH is used, as it is a weak base it favors the formation with 1 l of deionized water and the solids were recovered4

2

of a hydrotalcite-like structure, due to the OH deficiency. by centrifugation. The other sample portion was keptThe chemistry of Co compounds in aqueous solution tends without any treatment. All samples were dried in a vacuum

31to favor many Co complexes [10] such as [Co(NH ) ] , oven at room temperature for 18 h. Hereafter, the prepared

3 621 31 31 samples will take the following nomenclature: C5[Co(NH ) ] , [Co(NH ) (H O)] , [Co(H O) ] and

3 6 3 5 2 2 621 conventional preparation, MW-nW5microwaved sample[Co(NH ) (NO )] which could be the cobalt precursors

3 5 2

for n minutes and washed; MW-nWW5microwaved sam-(after thermal decomposition) of hydrotalcite or bruciteple irradiated for n minutes and non-washed (WW5structures. Most of them are highly soluble in water andwithout washing).frequently, after washing, are withdrawn leading to a metal

deficiency.2.2. Characterization techniquesKomarneni et al. [1113] have shown that microwave

irradiation under hydrothermal conditions leads to a veryX-ray diffraction (XRD) patterns were obtained in arapid synthesis of anhydrous ceramic oxides and hydroxy-

Siemens D-500 diffractometer coupled to a copper anodelated phases including hydrotalcite-like materials. The useX-ray tube. The Ka radiation was selected with a dif-of microwave irradiation reduces significantly the longfracted beam monochromator. The vibrational spectracrystallization time and the resulting crystallite size is(FTIR) were obtained in a Nicolet 170sx, equipped with asmaller if compared to the conventionally synthesizedPyrex cell, using KBr pellets. Spectra were obtained atmaterials. Furthermore, the microwave technique has beenroom temperature after 32 scans and with a resolution of 4applied to the synthesis of the M(II) M(III) hydrotalcite-

21cm . The samples were heated in situ at 50, 100, 150,like compounds [14]. Although this technique is expected200 and 2508C. DTA and TGA analysis were performed into diminish preparation time, these compounds present lowa Setaram-92 apparatus using a heating rate of 108/ min instability. Furthermore, the hydrotalcite properties dependAr atmosphere. Temperature-programmed oxidation (TPO)on the precipitant agent, the precursors and the synthesisprofiles were obtained in a conventional TPD/TPR-2900atmosphere. In this work we studied the Co(II) Co(III)Micromeritics apparatus, without any pretreatment. A 15hydrotalcite synthesis varying systematically each one ofml/min flow of 5% O /He was fed to the samples at athese factors. As the resulting compounds may differ in 2

heating rate of 58C/min from room temperature to 5008C.their thermal behaviour from those conventionally syn-The detection was carried out with a thermal conductivitythesized, the thermal study is also presented.detector.

2. Experimental 3. Results and discussion

2.1. Synthesis 3.1. Effect of irradiation time

Several experimental conditions were used in order to Table 1 correlates the microwave irradiation time with

establish the effect of microwave irradiation time, washing, the XRD identified compounds. In the MW-1W sample a

synthesis atmosphere and precipitant agent. The samples poorly crystallized hydrotalcite was detected. The differ-

were synthesized in a three necked round-bottom flask

equipped with magnetic stirring and pH meter. The cobaltTable 1

aprecursor solution (1 M) was prepared using Co(NO ) ?3 2 Code samples and synthesis conditions

6H O (Baker, 99% purity). Initially, a NaOH solution2 Sample Obtained

(Baker, 0.5 M) or an ammoniacal solution (Baker, 0.5 M)compound

was poured into the flask. A stream of air or nitrogen (40C Co O

3 4ml/min) was continuously fed. The metallic solution wasMW-1W Hydrotalcite

added dropwise by means of a micro-valve (clinical use) to MW-3W Amorphous solidhave a flow of 20 drops/min, typically all the Co solution MW-5W Amorphous solid

MW-10W Amorphous solidreacted in 4851 min.aAll samples were synthesized at room temperature and All samples were synthesized in air atmosphere, NH OH was used as

4

pH was monitored during the precipitation. After precipi- precipitant agent and they were washed after the precipitation.


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B. Zapata et al. /International Journal of Inorganic Materials3 (2001) 2329 25

Fig. 1. X-ray diffraction patterns of fresh conventional sample (C ), fresh microwave irradiated sample (MW-1W) and fresh microwave irradiated sample

without washing (MW-1WW).

ence between this sample washed or not and C synthesis is (006) and ( 009) reflections is low. The broadness of thediffraction peak points towards a low crystallite size.clearly shown in Fig. 1, in sample C only Co O is

3 4

These small crystallites of CoMHLC are present with aobserved. If the irradiation time was increased, the hydro-large amount of non-identified Co-amorphous compounds.talcite network was destroyed and an amorphous solid wasThe chemistry of Co compounds in aqueous solution tendsformed as only a short irradiation time of 1 min seems to

31to form many Co complexes such as [Co(NH ) ] ,preserve the two oxidation states of cobalt required to 3 6

21 31 31[Co(NH ) ] , [Co(NH ) (H O)] , [Co(H O) ] andsynthesize the hydrotalcite-like compound. 3 6 3 5 2 2 6

21[Co(NH ) (NO )] which could inhibit the formation of

3 5 2

3.2. Effect of precipitant agent the CoMHLC.

Table 2 shows the FTIR wavenumber of the samples

When 0.5 M NaOH aqueous solution was used as MW-1WW and MW-1W. The bands were assigned fromprecipitant agent, at the same preparation conditions of the values reported in the literature [7]. Comparing the

2

sample MW-1, a microcrystalline brucite solid was ob- results for OH, NO and CoO vibration bands, and as3

tained. As NaOH is a strong base, the oxidation of the shown in Fig. 2, no differences between MW-1W and

required amount of Co(II) cations to Co(III) cations MW-1WW are observed, these results agree with the XRD

becomes difficult and the main formed compound is diffractograms, Fig. 1. The three bands at 1032, 1095 and21

Co(OH) . If NH OH was used, the pH variation was 10.6 1223 cm in sample MW-1WW could correspond to the2 4

at the beginning of the synthesis up to 8.3 at the end. presence of Co-complexes.

Instead, using NaOH the pH range varied from 13 to 11.5.

These differences in the pH variation between the two 3.4. Temperature effect

preparations could explain the formation of hydrotalcite

with ammonia solution and brucite with NaOH. Therefore, Fig. 3 displays the FTIR spectra of the MW-1WW

the number of OH radicals, during the synthesis is the oneTable 2of the determining variables to be considered to form a 2FTIR band assignments in terms of OH, NO , CoO species and

3

hydrotalcite-like structure. Co-complexes2

Sample OH NO CoO Co-complexes33.3. Formation of ammonium-complexes

MW-1WW 3417 1383 827 1095

1616 641 1032Fig. 1 shows the X-ray diffraction patterns of the non-

555 1223washed samples prepared in air (MW-1WW) and the same 506sample washed once, (MW-1W). In both preparations MW-1W 3417 1383 827

1616 641Comonometallic hydrotalcite-like compound (Co555MHLC) was obtained, although the intensity of the main506peaks at d58.03, 4.22 and 2.63 A corresponding to (003),


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Fig. 2. FTIR spectra of fresh microwave irradiated washed sample (MW-1W) and fresh microwave irradiated sample without washing (MW-1WW).

sample taken at several temperatures. As expected, the 50 to 1508C the intensity increased slightly and at 20021

intensity of the OH bands (3421 and 1608 cm ) de- 2508C the band was clearly resolved.

creased significantly from 50 up to 1508C, and at 200 to TGA and DTA results of the MW-1W sample are shown

2508C both bands disappear indicating the CoMHLC in Fig. 4. There are three temperature intervals where2 21

network destruction. The NO band intensity (1383 cm ) weight is lost. Firstly, from 25 to 1568C, secondly from3

and those assigned to the Co-complexes (1032 to 1223 156 to 2008C and the last up to 3748C. The first zone is21

cm ) remain in the whole heating range (502508C). attributed to the loss of interlaminar and surface water. The

Hence, the Co-complexes are more stable than the Co second zone was interpreted as the leaching out decompo-2

MHLC, and the main precursor of Co O is the Co sition of NO interlaminar anions of CoMHLC. The last3 4 3

MHLC. The CoO bands had a different behavior. From zone could correspond to the destruction of the laminar

Fig. 3. FTIR spectra of microwave irradiated without washing sample (MW-1WW) at different treatment temperatures.


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Fig. 4. TGA and DTA profiles of the fresh microwave irradiated washed sample (MW-1W).

structure, i.e. a strong dehydroxylation and phase trans- 1808C. As it has already been described, during the

formation to Co O . The DTA results agree with the TGA synthesis of our CoMHLC probably many ammonia3 4

observations. For instance, two endothermic peaks and one complexes are formed. These complexes are difficult to

exothermic peak are clearly resolved. Both endothermic identify, however some specific IR bands could be as-

peaks at 110 and 1788C correspond to the weight loss signed to the presence of small amounts of these com-2

attributed to water and NO , and the last exothermic peak pounds between the structure of the HT. For example, the3

at 3428C is typical of a phase transformation. peak that appeared at 1508C in sample MW-1WW could be

Fig. 5 compares the Temperature-Programmed Oxida- clear evidence of partial decomposition of these complex-

tion (TPO) profiles of cobalt nitrate hexahydrated to the es. The peak at 1808C was interpreted as the oxidation of21 31 21

profiles MW-1WW and MW-1W samples as obtained, in Co to Co , NO Co could be bonded to the surface.3

order to check the behavior of surface oxidation. The Sample MW-1W presents a shoulder at 1708C and a clear

reference material (cobalt nitrate) used as chemical pre- peak at 1808C. In sample MW-1W, the peak at 1708C iscursor in the preparation of CoMHLC presented only one shifted towards 1508C as some of the was Co-complex

peak at 2558C which was assigned to the oxidation of a removed during washings. Nevertheless, the peak at 1808C21 31

fraction of Co to Co . indicates a higher O consumption compared with sample2

21Sample MW-1WW presented two peaks at 150 and MM-1WW which indicates that the Co species are easier

Fig. 5. TPO profiles of a cobalt nitrate, of fresh microwave irradiated washed sample (MW-1W) and fresh microwave irradiated sample without washing

(MW-1WW).


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Fig. 6. X-ray diffraction patterns of fresh microwave irradiated washed samples prepared in nitrogen atmosphere and changing the irradiation time

(MW-1N to MW-10N).

31 21to transform to Co if compared with NO Co species. decomposition is too large compared with sample MW-

3

As shown by the FTIR results, a clear diminution of the 1WW which would mean a poor oxidation process of the21

characteristic band of NO Co was observed. precursor to obtain Co O oxide.3 3 4

3.5. Atmosphere effect

4. ConclusionFig. 6 displays the XRD patterns of samples synthesized

in nitrogen atmosphere. The preparation was similar to Monometallic Co hydrotalcite-like structure (MHLC)those reported in Table1 only the atmosphere changed. In may be synthesized using NH OH as precipitant agent,

4

all samples, only a brucite-like structure was observed. If microwave irradiation and with a long time of cobaltthe microwave irradiation time (110 min, samples MW- nitrate(II) addition. The microcrystalline solid was formed

1N to MW-10N) was increased, there was no modification in an amorphous matrix of Co-complexes identified byof the structure. FTIR. The CoMHLC was stable until 1508C and was

Fig. 7 shows the TPO profiles of one sample prepared in transformed to Co O . The amorphous Co-complex was3 4

nitrogen atmosphere (MW-1N) and compared with a the dominant compound and it was stable up to 2508C. Itsimilar one prepared in air atmosphere (MW-1WW). In the seems that the main precursor of the Co O is the Co

3 4

first case the main compound determined by X-ray diffrac- MHLC and the Co-complexes inhibit the transition, at lowtion was the Co(OH) -like brucite. In sample MW-1N the temperatures, of Co O . The MHLC is easily oxidized at2 3 4shoulder at about 1758C assigned to ammonia complexes low temperature (1508C) compared with Co(NO )?6H O

3 2

(2508C).

Acknowledgements

This work was partially sponsored by the Project IPN-

981046. B. Zapata acknowledges the economical support

of CONACYT.

References

[1] Allmann R. Acta Crystallogr 1968;B24:972.

[2] Vaccari A. Catal Today 1998;41:53, and references therein.

Fig. 7. TPO profiles which compare the effect of preparation atmosphere, [3] Vaccari A. Appl. Clay Sci. 1995; In: Synthesis Applications

nitrogen (MW-1N) and air (MW-1WW). Anionics Clays: 10.


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[4] Ramos E, Lopez T, Bosch P, Asomoza M, Gomez R. J SolGel [9] Taylor RM. Clay Miner 1980;15:369.

SciTech 1997;8:437. [10] Greenwood NN, Earnshaw A. Chemistry of the elements, Pergamon

[5] Lopez T, Bosch P, Asomoza M, Gomez R, Ramos E. Mater Lett Press, 1989.

1997;31:311. [11] Komarneni S, Roy R, Li QH. Mater Res Bull 1992;27:1393.

[6] Alzamora LE, Ross JRH, Kruissink EC,Van Reijen LL. J Chem Soc [12] Komarneni S, Li QH, Roy R. J Mater Chem 1994;4:1903.

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228990289 co ii co iii hydrotalcite like compounds

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