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Latin American applied research

versión impresa ISSN 0327-0793versión On-line ISSN 1851-8796

Lat. Am. appl. res. vol.42 no.1 Bahía Blanca ene. 2012

 

Evaluation of niobium oxide doped with metals in photocatalytic degradation of leather dye

G. C. Collazzo†*, D. S. Paz, S. L. Jahn, N. L. V. Carreño, E. L. Folleto

Chemical Eng. Department, Federal University of Santa Maria, Santa Maria-RS, 97105-900, Brazil.
Institute of Chemistry and Geoscience, Federal University of Pelotas, Pelotas- RS, 96010-90, Brazil.
*gabicollazzo@yahoo.com.br

Abstract— The aim of the present work was to verify the influence of the type of metal used in the presence of niobium pentoxide on the photocatalytic degradation of an industrial leather dye. Niobium oxide alone showed low photocatalytic activity, but with the impregnation of small quantities of metal, this material showed highly activity. It was observed that the type of metal significantly influenced the catalytic activity of niobium oxide. The results indicate that metal/Nb2O5 could be employed for the removal of dyes from wastewater. The order of photocatalytic activity of metal-doped Nb2O5 was as follows: (La, Zn, Ti) > (Sr, Sn, Ag) > (Co, V, Ga) > (Ni, Y, Cu). The metals Ce, Mn and Mo showed not efficient catalytic activity in degrading the leather dye.

Keywords— Niobium Oxide; Photocatalytic Activity; Metals; Doping.

I. INTRODUCTION

Nb2O5 is widely used as catalyst in a myriad of reactions such as dehydration (Carniti et al., 2006), hydration (Li et al., 2006), etherification (Peters et al., 2006), hydrolysis (Okuhara et al., 1998), dehydrogenation (Paiva et al., 2006), and mainly, oxidation reactions (Braga et al., 2007). Niobium oxide has also been used as oxide support for metals such as Pt (Brown and Kemball, 1996), Ni (Ko et al., 1983), Co (Silva et al., 1993) and Mo (Wachs et al., 1996). When used in this manner, the properties of the niobia are improved by the addition of these elements, while their high selectivity is still maintained.

The function of niobium compounds in catalysis may be that of promoter or active phase, support, solid acid catalyst, or redox material (Nowak and Ziolek, 1999). Application of Nb2O5 in the photodegradation of contaminants has not been studied sufficiently (Kominami et al., 2001; Esteves et al., 2008). Thus, the use of Nb2O5 as a catalyst may be a new alternative for the photodegradation of contaminants.

This study aimed to evaluate niobium oxide doped with different metals in the degradation of azodye Direct Black 38

II. METHODS

A. Catalyst Preparation

To verify the influence of metals on the photocatalytic degradation of a dye, Nb2O5 doped with various metals (2 wt% metal) was prepared. A commercial niobium oxide (CBMM, Brazil), Nb2O5.nH2O, was used as starting material. The following metals were impregnated on niobium oxide: La, Ga, V, Co, Cu, Zn, Y, Ag, Ti, Mo, Sn, Ce, Sr, Ni and Mn; the precursor salts were La(NO3)3.6H2O, Ga2(SO4)3, NH4VO3, CoCl2.6H2O, CuSO4.5H2O, C4H6O4Zn.2H2O, Y(NO3)3. 6H2O,AgNO3, (NH4)2TiF6, (NH4)6Mo7O24.4H2O, SnCl4. 5H2O, Ce(NO3)3.6H2O, Sr(NO3)2, Ni(NO3)2.6H2O and MnSO4.H2O, respectively. The methodology used to prepare the Nb2O5 doped with metals was wet impregnation. The salt was dissolved in deionized water, and subsequently, the oxide was added to this solution, which remained under constant stirring for 3 h at 80 °C. Thereafter, the material obtained was dried at 100 °C for 10 h, calcined at 500 °C for 3 h and the temperature rate was 5°C/min.

B. Characterization methods

The powder was characterized by X-ray diffraction and nitrogen adsorption by BET. X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance diffractometer; the X-ray source was Cu-Kα radiation (40 kV e 40mA). Data were collected over the 2θ range 20-80° with a step size of 0.05° and a count time of 35 s.

The Brunauer-Emmett-Teller (BET) surface area measurements were carried out by N2 adsorption-desorption at 77 K using Quantachrome Autosorb Automated Gas Sorption instrument, in the range of relative pressure (P/Po) of 0 to 0.99.

C. Photocatalytic Tests

Direct Black 38, an azo dye extensively used in the leather industry, was used as the model compound. The chemical structure of the dye is given in Fig. 1.


Fig. 1.Chemical structure of Direct Black 38 dye.

The reactor was batch-type, consisting of a glass tube (internal diameter of 7.5 cm and 18.0 cm in height) with a mercury vapor lamp (80 W) fixed at the center and protected by a quartz bulb. The outside of the reactor was covered with aluminum foil to protect from external UV radiation, so that the rays were reflected into the reactor. Furthermore, the reactor was equipped with an aluminum cylinder to keep the temperature constant.

Four hundred milliliters of a 100 mg.L-1 Direct Black 38 dye solution was mixed with 0.4 g of the catalyst. The effect of the pH on the photodegradation was studied, which was adjusted by addition of HCl or NaOH. Prior to photoirradiation, the suspensions underwent ultrasound for 10 min. The suspension was magnetically stirred for 60 min to establish the adsorption equilibrium. An initial sample was taken for analysis at the end of the adsorption period, in order to determine the bulk concentration. This was considered the initial concentration for the photocatalytic experiments. The solution was then illuminated with the mercury-vapor lamp (irradiation intensity of 12 J.cm-2) and the solution stirred (100 rpm) using magnetic stirrer and aliquots were taken at certain periods of time. These samples were centrifuged and filtered with a PVDF membrane (0.22 µm) to completely remove catalyst particles. The dye degradation was followed on a Shimadzu 1650C UV spectrophotometer. The photocatalytic degradation performance of the process was defined as % Degradation =[(A0 - A)/A0]x 100, where A0 is the initial absorbance and A is the final absorbance, at λmax. = 590 nm.

II. RESULTS AND DISCUSSION

A. Characterization of samples

The XRD pattern for samples of niobium oxide and niobium oxide doped with some metals such as Ti, Zn and La is shown in Fig. 2. All samples are summarized in the amorphous phase. In general, calcination at more than 773 K is required for crystallization from amorphous niobic powder to Nb2O5 powder (Amini and Sacks, 1991). The absence of intensity of the reflection corresponding to Ti, Zn and La may be due to their lower concentration in the prepared samples.


Fig. 2. XRD of Nb2O5 and metal/Nb2O5 samples calcined at 500 oC.

Table 1 shows the BET surface areas of metal/Nb2O5 samples. The metal-supported catalysts had a surface area of about 26 m2.g-1, and is similar to that obtained for niobium pentoxide commercially calcined at 500 °C for 3 hours (26.70 m2.g-1). This difference is negligible due to the low amount of metal impregnated on niobium oxide. Silva et al. (2002) observed that the area of niobium oxide did not change after impregnation of 2 wt% Ag and subsequent calcination at 500 °C for 5 h and 24 m2.g-1.

Table 1. BET surface area, and total pore volume of Nb2O5 doped.

B. Photocatalytic reaction

The effect of pH values on the photodegradation of the dye was studied in the range 2.5-9.5 (Fig. 3). The degradation was higher in acid medium (pH 2.5). Nb2O5 surface is fully protonated below pH 4 and is deprotonated for pH values higher than 5.5 (Prado et al., 2008). Considering the Direct Black 38 structure, the positive charge excess in the metal/Nb2O5 surface promotes a strong interaction with SO3- groups of the dye, promoting surface reaction with the valence band holes (h+). Valence band holes (h+) of Nb2O5 could have played an important role in the dye degradation in acidic pH.


Fig. 3. Influence of pH on the degradation of dye for the Nb2O5 (1.0 g.L-1) at 25 oC.

The evolution of the dye degradation with irradiated time for all photocatalysts is shown in Fig. 4. UV-light alone was not effective for the degradation of the dye molecules which was only observed with the simultaneous presence of catalyst and UV-light.

Fig. 4. Photocatalytic degradation of DB38 as a function of time irradiation. Experimental conditions: Co = 100 mg.L-1, Ccatalyst = 1 g.L-1, pH = 2.5, temperature = 25°C.

We first studied the effect of degradation of the Direct Black 38 dye with niobium oxide without the presence of metals. We observed about 24% of degradation of the dye at an irradiation time of 100 min. The results for niobium oxide doped with metals showed that the metals presenting the best results were La, Zn and Ti, with 80, 78 and 75 % degradation, respectively

It can be said that these elements, substantially increases the photodegradation activity than that obtained with niobium oxide alone. Liqiang et al. (2004) observed that La doped TiO2 nanoparticles exhibited higher photocatalytic degradation of phenol. Qin et al. (2007) investigated the effect of La doping on the photocatalytic property of CoO/SrTiO3, and the results showed that a suitable concentration of La doping greatly improved the photocatalytic activity for water decomposition to hydrogen. Chakrabarti and Dutta (2006) studied the photocatalytic activity of ZnO in the degradation of Methylene Blue and Eosin Y, and found 58 and 39 % degradation, respectively.

The metals Sr, Sn and Ag also showed a good photocatalytic activity, with degradation results ranging from 73-70%. The catalysts containing these elements such as Sr2Sb2O7 (Lin et al., 2006), SnO2/TiO2 (Lin et al., 2008) and Ag/Nb2O5 (Silva et al., 2002) were reported to be efficient in material degradation of organic compounds. Co, V and Ga presented results of about 50% degradation, while the metals Ni, Y and Cu resulted in about 40% degradation. Ce and Mn presented values of about 30%, almost equal of the Nb2O5 alone, while Mo did not increase photocatalytic activity.

The mechanisms involved in the doping of metal on oxides are complex and it is difficult to predict the exact effect of dopants in oxide matrix. The performance of catalyst in the degradation of dyes is a complex function involving many parameters such as morphology of the particle surface, crystallite size, surface charge density, dye adsorption capacity of the material, which is related to the BET surface area, pore size distribution, nature and concentration of the metal ion, ionic size and the oxidation states of the dopants, capacity to surface adsorb water and hydroxyl groups. Esteves et al. (2008) observed that most of the materials with doping metals show a higher percentage of degradation than that obtained with niobium oxide alone. These modifications (resulting from the doping) are responsible for producing materials with lower band-gap, diminishing the energy of the electron/hole pair formation and consequently increasing catalytic activity. Devi et al. (2009) observed that surface adsorbed water and hydroxyl groups are crucial for the photocatalysis. The concentration of adsorbed water and hydroxyl groups on the photocatalyst surface is directly related to the surface acid base properties of the sample.

III. CONCLUSIONS

Metal doping of niobium oxide significantly improved its photocatalytic activity in the degradation of Direct Black 38 dye.

The type of metal significantly influenced the catalytic activity of niobium oxide. Generally speaking, the order of photocatalytic activity of metal-doped Nb2O5 was as follows: (La, Zn, Ti) > (Sr, Sn, Ag) > (Co, V, Ga) > (Ni, Y, Cu). The dopants Ce, Mn and Mo showed not effective for the dye degradation in aqueous solution. The results indicate that Nb2O5 impregnated with metals could be employed for the removal of dyes from wastewater.

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Received: September 20, 2010.
Accepted: April 1, 2011.
Recommended by Subject Editor Orlando Alfano.

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