PREPARATION OF COBALT TITANATE NANOPARTICLES AND THEIR PHOTOCATALYTIC ACTIVITY TOWARD METHYLENE BLUE

N. Fodil Cherifa,b*, N.A. Laoufic, R. Benrabaaa,d, A. Faresa, A. Baramaa

a Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, Faculté de Chimie, USTHB, BP32, El- Alia, 16111 Bab Ezzouar, Alger, Algérie.

b Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques CRAPC, BP 384, Bou-Ismail, 42004, Tipasa

c Laboratoire des Phénomènes de Transfert, Faculté de Génie Mécanique et de Génie des Procédés, Département de Cryogénie et de Génie Chimique USTHB, BP32, El- Alia, 16111 Bab Ezzouar, Alger, Algérie.

d Université 20 Août-Skikda, Faculté de Technologie, Département de Pétrochimie & Génie des Procédés, BP26, route Al-Hadaiek, 21000, Skikda, Algérie.

(*):

I.  Introduction

Cobalt titanates are known as useful materials because of the possibility of their technical application as green pigments, catalysts or in electric devices. Three cobalt titanates are known. They are the following: cobalt methatitanate, CoTiO3, having the ilmenite structure, cobalt orthotitanate, Co2TiO4, with inverse spinel structure, and pseudobrookite-type cobalt dititanate, CoTi2O5 [1]. There are known several methods for the preparation of cobalt titanates. The most popular is a solid-state reaction between titanium dioxide and cobalt oxide [2]. Although this method needs high temperature, about 1000oC, and long time, about 10 hours, small amounts of unreacted initial compounds can be expected. Therefore, the methods based on the synthesis in the solution phase, like precipitation [3], sol-gel [4], or Pechini process [5], providing suitable mixing of the reagents, have recently been studied intensively.

Photocatalytic degradation processes have been widely applied as techniques of destruction of organic pollutants in waste water [6]. With an appropriate light irradiation, the photocatalyst generates electron/hole pairs with free electrons produced in the empty conduction band and leaving positive holes in the valence band. These electron/hole pairs are capable of initiating a series of chemical reactions that eventually mineralize the pollutants [7].

This work reports on the preparation of cobalt titanate by the co-precipitation method, using solutions of Titanium tetrachloride (TiCl4) as titanium source, CoCl2 as cobalt source and KOH as a precipitating agent. The aim of our investigations was the use of cobalt titanate as photocatalyst for the degradation of organic pollutants.

II.  Materials and methods

5 ml of titanium chloride was transferred into 250 ml beaker and added with 100 ml of distilled water (DW) (named solution A). 2.2 g of Cobalt chloride was transferred into a 100 ml clean beaker and 20 ml of DW was added to it and the contents were stirred for 5 minutes till the solution was clear (named solution B). These two solutions(A+B) were stirred together for 30 minutes at 40°C. Then, required quantity of KOH was transferred dropwise by means of burette into the beaker containing titanium chloride and Cobalt chloride. The contents were continuously stirred with the help of magnetic stirrer for 2 hours after addition of KOH. After 2 hours, the contents were filtered using Wattman filter paper and precipitate was washed with hot DW for 10 to 15 times in order to eliminate chloride ion. After sufficient washing, the precipitate was dried in hot air oven for 8 hours at 90°C. The dried powder was annealed at 600°C and 900°C for 2 hours in a furnace.

Nitrogen adsorption isotherms allow determining the specific surface area by means of the BET method (Brunauer, Emmett and Teller). The specific surface area of the powders was recorded on a Quantachrome Novawin 2. The crystalline phase of the catalysts powders was analyzed by X-ray diffraction (XRD) patterns recorded on Xpert-pro diffractometer, with Cu Ka radiation in the 2θ range from 5 to 90° with 3° min−1. Raman spectroscopy was performed on Horiba Labram HR Evolution. Microstructural characterization was performed by scanning electron microscopy (SEM) in a Zeiss Supra mke apparatus. XPS spectra were recorded with a Thermo VG Scientific ESCALAB 250 spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV and 500 µm spot size). The specimens were pressed against insulating double-sided adhesive tapes on sample holders and pumped overnight in the fast entry lock.

The photocatalytic activity of as prepared catalysts was evaluated by degradation of aqueous solution of methylene blue (MB) under UV light irradiation. 50 mg of catalyst was added to 50 ml of 10 mg/L aqueous solution of MB. Before illumination, the mixture was stirred in dark for 30 min to achieve adsorption desorption equilibrium between catalyst and dye solution. The solution was exposed to UV light under stirring. At given time intervals, 5 ml of aliquots was withdrawn and centrifuged to remove catalyst. The concentration of methylene blue in aqueous solution was determined with the help of UV-Vis spectrophotometer.

III.  Results and discussion

The XRD patterns of the powders calcined at different temperatures (Fig.1); show that there is a clear difference in the peak position between the XRD patterns of powders calcined at 600 °C and 900 °C. Pure cobalt titanate (Co2TiO4) with spinel structure was obtained at 900 °C. The main lines of Co2TiO4 (JCPDS PDF 39-1410) were found at 2q(°) » 18.27 w (111), 30.056 m (220), 35.39 vs (311), m (222), 42.99 w (400), 53.43 w (422), 56.83 m (511), 62.40 m (441). The patterns also indicate that higher calcinations temperatures promote an increase in crystallite size. The values of crystallite size are listed in table 1. This result is confirmed by the average particle sizes calculated with Scherrer’s equation.

Fig.1. XRD patterns of powder obtained at 600 °C and 900 °C.

Table 1. The phase, crystallites grain sizes and surface BET of coprecipited samples.

Catalysts / Crystalline
phase / Phase
(%) / Cs (nm) XRD / SBET (m2/g) / Lattice parameter (Å)
600°C / TiO2
Co3O4 / 10
90 / 29
36 / 67 / a=b= 4.975; c= 2.95
a=b=c= 8.088
900°C / Co2TiO4 / 100 / 42 / 10 / a=b=c= 8.412

Corresponding to the surface specific area ( SSA ) data, measured using the BET method, powder calcined at 600 °C has the largest SSA whereas the powder heat treated at 900°C has the lowest SSA. The values are listed in table 1. This could be related to the temperature increases, suggesting particle grain growth. This result is in agreement with the results obtained for XRD.

The XPS samples surveys (Fig.3.) indicate that cobalt, titanium and oxygen are the major components on the surface of theses powders.

Table 2. Binding energies of Co 2p, Ti 2p, O 1s electron for powder cobalt titanate treated at 900°C

Temperature (°C) / Eb (eV) / Eb (eV) / Eb (eV)
Co 2p3/2 / Co 2p1/2 / Ti 2p3/2 / Ti 2p1/2 / O 1s
900 / 777.30 / 792.70 / 454.70 / 460.30 / 526.60

Fig. 3: XPS survey of powder calcined at 900°C.

Laser Raman spectra of the sample is shown on (Figure 4-a). Titanium oxide is characterized by three bands, located at 397, 514, and 640 cm-1. The bands at 397 and 640 cm-1 are assigned to B1g and E1g modes, respectively, while the band at 640 cm-1 is a doublet of A2g and B1g modes. TiO2 with 10 wt% in the composite Co3O4-TiO2 gives rise to a new absorption near 679 cm-1 and two shoulders, at 446 and 512 cm-1, assigned to the A1g, Eg, and F2g active Raman modes of the direct spinel Co3O4, respectively. FTIR results (figure 4-b) showed two strong absorption bands at 661 and 564 cm-1 which is an indication of the formation of Co3O4 with a spinel structure

Fig. 4: a-Raman spectra and b- IRTF spectra of powder calcined at 600°C.

SEM images (fig.5) of powders calcined at 600 and 900°C shows that the spinel samples are always agglomerated regardless of the calcination temperature.

Fig.5. SEM pictures of powders calcined at 600°C (left side) and 900°C (right picture).

The figure 6 shows that the concentration of MB did not obviously decrease in the presence of catalyst when the suspension was irradiated with UV radiation at wavelength λ = 365 nm. With this experiment we obtained approximatively 5 % of degradation after 3 hours of irradiation.

Fig. 6: UV–vis absorption spectra of methylene Blue vs time in the presence of catalyst calcined at 600°C at wavelength λ = 365 nm.

Conclusion

In this research, a study has been carried out on photocatalytic activity of cobalt titanate nanoparticles on MB. Their physical and chemical characterization was done by XRD, Raman, XPS and SEM. Photocatalytic effect of the nanoparticles were successfully studied in this work. The nature and structure of dyes were important parameter to choose a suitable photocatalyst. The degradation efficiency of is 40% for MB at during a period of 150 min under UV exposure for the catalyst obtained after heat treatment at 600°C.

References

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