Characterization of colloidal chromia particles obtained by forced hydrolysis

Materials Research Bulletin 38 (2003) 1329–1339

Characterization of colloidal chromia particles obtained by forced hydrolysis A.E. Onjia, S.K. Milonjic´*, Dj. Cˇokesˇa, M. Cˇomor, N. Miljevic´ The Vincˇa Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia Received 11 June 2002; received in revised form 8 April 2003; accepted 14 May 2003

Abstract Colloidal particles of chromia have been prepared by forced hydrolysis of an aqueous solution containing chromium chloride hexahydrate. At elevated temperature, a controlled addition of potassium hydroxide yielded colloidal chromia particles. After the sol coagulation, amorphous dried residues (I) were converted to crystalline form (II) by heating at 1073 K. The thermal treatment was also accompanied by a reduction in both surface area (from 75 to 9 m2/g) and point of zero charge (pHpzc; from 4.7 to 4.0). Chromia dissolution over the studied pH range (pH ¼ 2–12) exhibits a parabolic trend, with minimum solubility at pHpzc. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Sol-gel chemistry; D. Surface properties

1. Introduction Several procedures have been published for the preparation of well-defined hydrous metal oxide particles [1]. Among them, the forced hydrolysis method has been extensively used to obtain monodispersed inorganic colloids [2–5]. In this method, the hydrolysis of metal cations at elevated temperature in the presence of some anions, such as sulfate or phosphate, leads to the formation of a metal-hydroxide sol, which gives rise during aging to narrow-sized distribution particles. A common way for chromia sol synthesis comprises hydrolysis and aging of a chrome alum solution [6–9]. The other procedures are based on the precipitation [10–13], or sonochemical reduction of an aqueous solution containing ammonium dichromate [14]. In recent years, many researchers in the environmental and materials science, as well as in various process technologies, have taken interest in the possible use of sorption properties of chromia [15–18]. To be used as a preconcentrating agent, for applications in trace analysis, pollutant removal, or in catalysis, chromia is required to be of high purity and, in most instances, chemically inert. Accordingly, *

Corresponding author. Tel.: þ38-111-4445472; fax: þ38-111-4445472. E-mail address: [email protected] (S.K. Milonjic´). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0025-5408(03)00137-5

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a precursor solution containing minimum species is highly desirable. The mentioned phosphate and sulfate anions improve the particle morphology, but these anions favor an increased background signal in ordinary analytical measurements (Atomic Absorption/Emission Spectroscopy, Mass Spectrometry, Ion Chromatography) [19,20]. A tendency of chromia surface towards specific sorption of oxygen bearing anions, ejecting nitrate ions, was observed [21]. Hence, the preparation of chromium(III)-oxide particles by thermal hydrolysis of a chloride solution and the characterization of the obtained solids has been the subject of the present study.

2. Experimental Chromia sol was prepared by the following procedure: 0.1 M/dm3 potassium hydroxide solution was added to 1350 ml water, at room temperature, to adjust pH of water to 10.6. The temperature was then raised to the boiling (102 8C) under reflux, followed by the dropwise addition of 102 M solution of chromium chloride into 150 ml of water (pH 10.6). After completion of chromium chloride addition, the boiling solution was aged for additional 5 h. At different stages of the synthesis, the solution was monitored by spectrophotometry (UV-Vis). To purify the sol, ultrafiltration (UF) in a stirred cell with Amicon Diaflo1 YC05 type membrane was carried out. The coagulation of the obtained sol was achieved by addition of 0.1 M/dm3 potassium hydroxide solution up to pH 7.5 in order to separate the dispersed (solid) phase from the dispersion (liquid) medium. The precipitates were repeatedly washed with deionized water and dried in an air oven at 110 8C for 24 h (denoted as solid I). A portion of the solid I was further heated at 800 8C for 4 h (denoted as solid II). The third studied solid (III) was a commercially available chromium(III)-oxide (Fluka 27085). The solid particles (I, II, and III) were examined by flame and graphite furnace atomic absorption spectrometry (F-AAS, GF-AAS; Perkin-Elmer 5000, HGA-400), inductively coupled plasma atomic emission spectroscopy (ICP-AES; Perkin-Elmer ICP/6500), energy dispersive X-ray spectrometry (EDXRF) (Canberra SL30170), X-ray diffractometry (XRD; Siemens-D 500, Cu Ka), Fouriertransform infrared spectroscopy (FT-IR; Michelon MB 100), thermogravimetry (Perkin-Elmer TGS-2), pH-metry (Beckman f71), and BET surface area measurements (Stro¨ hlein). All instrumental measurements were carried out using standard procedures. Samples for chemical analysis by AAS were prepared by wet digestion of 100 mg of solids in 5.0 ml HNO3/H2O2 (3:1) solution. After dilution to 100 ml volume, final solutions were filtered through a 0.22 mm pore size nylon membrane and assayed for metal impurity. The oxides solubility measurements were carried out by transferring 100 mg of the solids into 200 cm3 of water at various pH. Aliquots of the suspensions, withdrawn after several hours of equilibration in a thermostat (25  0:5 8C), under nitrogen atmosphere, were filtered through a syringe membrane filter (0.22 mm) and the chromium content in the filtrate was determined by ICP-AES or GF-AAS. The points of zero charge (pHpzc) of solids were determined using the batch equilibration method described elsewhere [22].

3. Results and discussion Fig. 1 shows the absorption spectra (UV-Vis) of the CrCl3 precursor solution (A), aging solution after addition of CrCl3 (B), final sol (C), sol after UF (D), and the UF permeate (E). The sample D was

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0.8

Absorbance

0.6

C 0.4

D

0.2

B E A

0.0 200

300

400

500

600

700

λ , nm Fig. 1. UV-Vis spectra of the precursor (A), aging solution (B), the final sol (C), the sol after UF (D), and UF permeat (E).

obtained by dilution of UF retentate to the initial volume by using HCl solution of the same pH as the sol. No changes in the spectra (C) were noticed several months later. Two distinct maxima in the visible region can be seen with all samples, except D. Compared to A, the peaks for B, C, and E were shifted towards lower l, which means new species were created by the ligand exchange mechanism where OH substitutes Cl. Absorbance in the spectra D gradually decreases across the UV-Vis region without maxima. Also, the peaks present in C disappeared after the sol purification by UF (spectra D). The obtained sol shows pH 2.0, which is below the point of zero charge (pHpzc) of chromia. It means that the surface of colloidal particles is positively charged. The addition of 0.1 M/dm3 potassium hydroxide solution dropwise to the sol to increase pH up to 7.5 resulted in the formation of a light greenish blue precipitate. This solid residue was treated as described above (see Section 2). The potassium content in solid samples (I, II, and III), analyzed by AAS, was found at a level of (I) 0.90, (II) 0.08, and (III) 0.11. By using EDXRF, screening for other impurities resulted in aluminum and calcium detected, in all samples at their Ka1 ¼ 1486 and Ka1 ¼ 3690 lines, respectively. Fig. 2 presents radioisotope excited (109 Cd) EDXRF spectra of the studied solids. Although the sample I was extensively washed, in order to remove chlorine and potassium, huge peaks at 2621 and 3313 energies are evident. However, for samples II and III these peaks do not exist. As sample II originates from sample I, it seems that both dechlorination process and removal of potassium occur during the heat treatment. No more extraneous elements were detected by further examinations. Thermogravimetric analysis (TGA) was made at a 2-min heating rate and 20 ml/min flow rate of nitrogen (Fig. 3). It is evident that the mass losses of samples II and III, up to 800 8C, are very small (<1%). In the case of sample I, the mass loss up to 400 8C (23%) resulted from dehydration and dexydroxylation processes. At temperatures higher than 750 8C, a new significant mass loss was ascribed to the crystallization process, [23].

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Cr

K

Cr

Cl

Ca I

Al

II

III

2

4

6

8

10

12

14

16

18

20

E , keV Fig. 2. EDXRF spectra of (I, II, and III) chromia samples.

Fig. 4 shows the XRD patterns for the laboratory prepared (I and II) and commercial (III) samples. The X-ray diffraction pattern for the sample I shows only 1 weak, broad peak at 2y ¼ 23–358. This indicates that the prepared chromia sample (I) is amorphous, while samples II and III show microcrystalline structure. Apparent crystallite sizes of chromia II and III powders were calculated from the XRD (104) peak, according to Scherrer’s equation [24] D¼

0:89l b1=2 cosY

(1)

where l is the wavelength for Cu Ka (0.15405 nm) radiation and b1/2 is the width at the half-height of diffraction profile originating only from crystallite size. The calculated crystallite diameters are 30.4 and 43.7 nm for chromia (II) and (III) samples, respectively. As already reported, the pH of

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105

III

100

II Weight loss, %

95 90 85 80 75

I 70 65 60 0

100

200

300

400

500

600

700

800

o

T, C Fig. 3. Thermogravimetric analysis of (I, II, and III) chromia samples.

precipitation greatly affects the crystallinity of chromia [25], so that, for pH < 10, the obtained solid is always amorphous, while for pH > 10, it is usually crystalline. Since kinetics of precipitation is controlled by pH, at low pH Cr3þ ions tend to form polynuclear complexes, with very slow rate of formation. The heating of amorphous chromia under air atmosphere, at higher temperatures, leads to its crystallization, as reported [26,27]. Measurements of the solid surface areas by BET low temperature (77 K) nitrogen adsorption method indicate that the heating of chromia in an air atmosphere diminishes the specific surface areas Sp. The obtained Sp of solid (I), (II), and (III) are 75, 9, and 4 m2/g, respectively. As expected, Sp for the amorphous sample (I) is much higher than the corresponding values for the crystalline samples (II and III). Different Sp values for chromia reported in the literature are: 35 m2/g for sonochemically prepared (200 nm) monodispersed amorphous particles [14], 42 m2/g for crystalline chromia obtained by urea hydrolysis, and aging of the obtained gel at 500 8C [28], 44 m2/g for commercial a-Cr2O3 (Degussa), obtained by flame hydrolysis, and the same value (44 m2/g) for laboratory synthesized sample by thermal decomposition of ammonium dichromate followed by calcination at 500 8C [29], 30 m2/g for amorphous polydispersed particles (20–150 nm) obtained by hydrolysis of a chrom alum solution (KCr(SO4)212H2O) [30], 7.0 m2/g for spheric amorphous monodispersed particles (320 nm) obtained by aging of chrom alum solution at elevated temperature [8], and 316 m2/g for hydrated amorphous chromia (260 nm) [31]. According to the cited values, Sp for colloidal chromia particles ranges from 30 to 50 m2/g, except in the case of uniform particles (7.0 m2/g) synthesized by the Matijevic group [6–8,17]. Higher Sp was noticed with hydrated samples, but there is an undistinguished difference between the amorphous and the crystalline form. Fig. 5 presents the FTIR spectra of the chromia samples. The spectrum for solid I differs from the other spectra in the region from 400 to 1200 cm1 and consists of two broad absorption bands around

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Fig. 4. XRD pattern of (I, II, and III) chromia samples.

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Fig. 5. FTIR spectra of (I, II, and III) chromia samples.

560 and 940 cm1. A weak band at 560 cm1 for sample I, and two strong absorption bands around 560 and 640 cm1 for samples II and III correspond to Cr–O lattice vibrations [30]. A broad band centered at 3440 cm1 and a weak band at 1640 cm1 originate from the structural hydroxyl groups and the adsorbed molecular water present at the surfaces of samples I, II, and III [32,33]. A weak, broad, band at 940 cm1 for sample I probably characterizes oxygen species of the Cr–O–Cr type [33]. A band around 1390 cm1, for samples I and II, can be ascribed to the presence of some CO32 ions due to absorption of CO2 by the samples. Results from the determination of the point of zero charge (pHpzc) of chromia (samples I, II, and III) by batch equilibration method are shown in Fig. 6. The measured pHpzc of investigated (I), (II), and (III) samples are 4.7, 4.0, and 6.7, respectively. The pHpzc values for samples I and II are significantly lower than the pHpzc for sample III, and are different from the literature data for various chromia samples (Table 1). It should be noted that the pHpzc values determined by electrophoresis are in fact isoelectric points (pHiep). In the absence of specific adsorption of counter ions, pHpzc is equal to pHiep. The values of pHpzc given in Table 1 can be explained by different chromia preparation procedures. According to Fig. 6, sample II has lower pHpzc than sample I, as a consequence of sample dehydration. However, during the heat treatment, the sample I undergoes not only dehydration and dehydroxylation but also structural changes (from amorphous to crystalline), which can lead to different pHpzc values [36,37]. As also observed the pHpzc is constant at different ionic strengths (103–101 M KCl), indicating no specific affinity of this background electrolyte for chromia surface.

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10

pHf

8

III

I

6

4

II 2

2

4

6

8

10

12

pH i Fig. 6. Determination of pHpzc of (I, II, and III) chromia samples, in 0.1 M KCl solution (pHi, initial value; pHf, final value).

Table 1 Point of zero charge (pHpzc) of chromium(III) oxide and hydroxide Material

Characteristic

Method

Electrolyte

pHpzc

Reference

Cr(OH)3 (I) Cr2O3 (II) Cr2O3 (III) Cr(OH)3 (A) Cr(OH)3 (B) Cr(OH)3 Cr(OH)3 (A) Cr(OH)3 (B) a-Cr2O3 Cr(OH)3 (A-1) Cr(OH)3 (A-2) Cr(OH)3 (B) Cr(OH)3 (A-1(010)) Cr(OH)3 (A-1(037)) Cr(OH)3 (B-2) Cr(OH)3 Cr(OH)3 Cr(OH)3 a-Cr2O3 a-CrOOH

Dried from sol Heated from (I) Commercial SO4 impurities SO4 removed Amorphous SO4 impurities SO4 removed Hydrated urea, 500 8C Alkali washed Acid washed Dried 60 8C 0.1 M Na2SO4 resuspension 0.37 M Na2SO4 resuspension 60 8C, alkali washed Amorphous Amorphous Amorphous Commercial Synthetic

Batch method Batch method Batch method Potent. titration Potent. titration Electrophoresis Electrophoresis Electrophoresis Titration Titration Titration Titration Titration Titration Titration Electrophoresis Electrophoresis Electrophoresis Electrophoresis Electrophoresis

KCl KCl KCl NaClO4 NaClO4 KNO3 NaClO4 NaClO4 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 NaClO4 KCl KCl KCl

4.7 4.0 6.7 <4 8.3 9.2 8.9 8.7 6.35 8.40 7.90 5.50 6.75 6.25 6.30 9.2 8.4 8.3 7.9 9.1

This work This work This work [8] [8] [16] [17] [17] [28] [30] [30] [30] [30] [30] [30] [31] [34] [35] [35] [35]

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1337 1,0

I 5,0

0,8

0,6 3,0

II

2,0

III

0,4

[Cr], mg/g

[Cr], mg/g

4,0

0,2

1,0

0,0 2

4

6

8

10

0,0 12

pH Fig. 7. Fraction of the solid (I, II, and III) chromia samples, dissolved as a function of pH.

Fig. 7 shows the measured chromium contents in the solutions, [Cr], as a function of pH. It is evident that chromia disolution displays a parabolic trend, which is more pronounced in the case of solid I. Because of the absence of any complexing ligands, including carbonate, the influence of pH on [Cr] is attributed to the formation of Cr(III) hydroxo complexes. For all three solids, dissolution is more pronounced at lower and higher pHs. As expected, pH of minimum solubility coincides with the pHpzc values of each investigated chromia sample. Rai et al. [38], investigating the precipitation/dissolution of chromia, also reported parabolic solubility curves with two regions of high solubility pH < 6 and pH > 10. However, the interpretations of thermodynamic and/or kinetic data of the system are quite difficult, due to impossible identification of all species [39]. One should bear in mind that these data are valid in the case of absence of any oxidation species, which may induce different reactive dissolution [40].

4. Conclusions The forced hydrolysis of chromium caused by controlled addition of potassium hydroxide to an aqueous chromium chloride solution, at an elevated temperature, yields a stable dispersion of colloidal chromia particles. Investigation of the particles after coagulation of chromia sol indicates their amorphous hydrous structure, converted to crystalline form by calcination at higher temperature. The heat treatment of the solid removes chlorine and potassium present as impurities, reduces solid’s surface area, and shifts its point of zero charge towards lower pH value. Chromia dissolution takes place as a parabolic function of pH, with minimum solubility at the point of zero charge.

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Acknowledgements The authors are grateful to Slavica Zec, MSc, from the same Institute for the XRD analysis. The authors also thank unknown referee for his/her helpful comments and suggestions, which greatly improved the manuscript. The Ministry of Science, Technology and Development, Republic of Serbia, Project No. 1978, financially supported the research.

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