Huo Guoyan, Song Dayong, Gao Yangai
(College of Chemistry and Environmental Science, Hebei University, Hebei Baoding, 071002 )
AbstractThe perovskite compound BiFeO was synthesized by sol-gel method. The compound was investigated by XRD and the results show that its crystal structure belongs to rhombohedral system with space group R3c. This compound was used as photocatalyst for the photocatalytic degradation of methyl orange. The influence of the acidity of solution, the amounts of photocatalyst, the concentration of methyl orange and irradiation time of sunlight on the efficiency of degradation have been studied. A mechanism related to the process of photocalysis is proposed. The results show that BiFeO possesses photocatalytic activity.
In recent years, many attentions have been focused on the semiconductor-based photocatalytic materials for degradating toxicants as they can be induced electron-hole pairs on their surface or inner of them by solar radiation[1-2]. The generated electrons and holes possess strong reduction and oxidation abilities, respectively, which both can react with HO molecules and OH ions adsorbed on the surface of semiconductor-based photocatalytic materials particles and finally generate hydroxyl radicals. In many photocatalytic materials, TiO powder is considered to be a very efficient catalyst that is non-toxic and stable. However, due to its wide bandgap (3.2eV), it is only suitable for work using ultraviolet light as the energy source, such as high pressure mercury lamp and less than 5% of the sunlight on the earth. Recently, perovskite-type compounds are employed for the degradation organic pollutants in the wastewater owing to their narrower bandgap which can be easily excited under visible light irradiation
Bismuth-containing oxides are of considerable importance as heterogeneous catalysts for a variety of processes including the oxidative coupling of methane, the ammoxidation of propylence, and the oxidation of hydrocarbons, carbohydrates and CO[6-9]. Based on the advantage of bismuth-containing oxides and perovskite-type compounds, the photocatalytic behavours which BiFeO decolours methyl orange have been studied.
2.1 Apparatus and reagents
Y-2000 X-ray diffractometer (Aolong X-ray Apparatus Ltd. Corporation, Dandong), UV-Vis8500 spectrometer (Tianmei Scientific Apparatus Ltd. Corporation, Shanghai), pH-3C acidimeter (Leici Analytic Apparatus Factory, Shanghai), DHG-9023A oven (Shanghai Exact Apparatus and Meter Ltd. Coporation, Shanghai)
Methyl orange (analytical reagent grade), Sodium hydroxy (analytical reagent grade), hydric sulphate (analytical reagent grade)
2.2 Synthesis of BiFeO
An amount of Bi(NO (99.9%) and Fe(NO (99.5%) was dissolved in a diluted HNO solution with stirring and following then citric acid was added to the mixture. Under stirring condition the above mixture solution was heated until it formed dry gel. BiFeO was prepared by hand-mixing dry gel in mortar. Then they were pressed into pellets under 10Mpa pressure for 1 min following then preheating at 500°C for 6h. The sample was annealed in air at 750°C for 16h and it was cooled to room temperature in an oven. Phase analysis and characterization were carried out by Y-2000 X-ray diffractometer.
2.3 Experimental processes and data collection
The maximum absorbance of the methyl orange was obtained by using UV-Vis8500 spectrometer from 180 to 1000nm (see Table 1). The maximum absorbance of methyl orange solution is obtained at 507nm when the pH value of initial methyl orange solution is less than 3.1 and the maximum absorbance of initial orange solution is measured at 464nm when the pH value of methyl methyl orange solution is larger than 4.4. An amount of BiFeO was added to a certain concentration methyl orange solution at a given pH value and following then it was exposed to solar light for several hours. Then, the photocatalyst was filtered. The degradation ratio was calculated based on formula: D = [(A C )/A]*100%, where A is the absorbance of original methyl orange solution and A is the absorbance of methyl orange solution measured after finishing photocatalyzed reaction.
Table 1 Maximum absorbable wavelength of methyl orange under different pH
|pH value of methyl orange solution||wavelength of maximun absorbance|
|pH ≥ 4.4
pH ≤ 3.1
3 RESULTS AND DISCUSSION
3.1 Crystal structure of BiFeO
The sample cooled from the sintering temperature was characterized by X-ray powder diffraction using Cu Kα radiation at room temperature. Fig. 1 shows the XRD pattern of BiFeO and the compound BiFeO can be attributed to rhombohedra system with lattice parameters a = 5.5752(8) and c = 13.8552(25) in a hexagonal cell setting and the space group R3c agreed with the literature reported. The observed reflections of BiFeO are in accord with those calculated using the CaRIne program.
Fig. 1 X-ray diffraction pattern of BiFeO compound
3.2 Effect of solution acidity on degradation ratio of methyl orange
Different pH values of methyl orange solution, 1mg/ dm, 100cm, was obtained by adding a 2.0mol/dm NaOH solution or by adding a 2.0mol/ dm HSO solution and the absorbance of each solution was measured. Then, photocatalyst, BiFeO, 50mg, was introduced into the solutions mentioned above. Following they were exposed to sunlight for 4 hours from 11:00 to 15:00 in summer. Subsequently, these solutions were filtered and the absorbances were measured by using UV-Vis8500 spectrometer. Table 2 represents the influence of original solution acidity on the degradation ratio of methyl orange. It can be seen from Table 2 that optimum acidity of original methyl orange is around 2.0 in pH value and in this pH value the decgradation ratio is up to 99.3%. This can be assigned to different structure of methyl orange in different acidity. The methyl orange structure transition with acidity of solution can be showed as the following:
pH ≤ 3.1, quinonoid structure pH ≥ 4.4, azo-type structure
Table 2 Effect of acidity of solution on degradation ratio of methyl orange
|Degradation ratio (%)||99.3||67.8||47.5||21.8||13.8|
It indicates that quinonoid methyl orange can easily be degradated by BiFeO photocatalysis.
3.3 Effect of methyl orange concentration on the degradation ratio of methyl orange
Photocatalyst, BiFeO, 4×100mg, was added to various concentrations of 100cm methyl orange solution of pH= 2 adjusted by 2mol/dm HSO, 1mg/dm, 5mg/dm, 10mg/dm and 15mg/dm, respectively and following then they were exposed to solar light 4 for hours from 11:00 to 15:00 in summer. Denpendence of degradation of methyl orange on the concentration of methyl orange is shown in Fig. 2. It can be seen from Fig. 2 that degradation ratio of methyl orange decrease with the concetration of methyl orange. When the concentration of methyl orange is less than 5mg/dm the degradation ratio decreases slowly and is more than 85%. However, when the concentration of methyl orange is higher than 5mg/dm the degradation ratio decreases rapidly. This can be assigned to that the degradation products of methyl orange are absorbed on the surfaces of photocatalyst and the absorbate runs off difficultly.
Fig.2 Denpendence of degradation of methyl orange on original concentration of methyl orange
3.4 Effect of amount of photocatalyst on the degradation ratio of methyl orange
Different amounts of BiFeO, 40mg, 60mg, 80mg and 100mg, were added to 100cm, 5mg/dm, methyl orange solution of pH » 2 adjusted by 2mol/dm HSO, respectively. Afterwards, they were exposed to sunlight for 4 hours from 11:00 to 15:00 in summer. Dependence of degradation of methyl orange on the amount of photocatalyst, BiFeO, is shown in Fig. 3. It can be seen from Fig. 3 that degradation ratio of methyl orange increases rapidly and that degradation ratio is up to 93% when the amount of photocatalyst is more than 80mg in 200cm methyl orange solution.
Fig.3 Effect of amount of BiFeO on degradation of methyl orange
3.5 Effect of irradiation time in sunlight on the degradation ratio of methyl orange
100mg photocatalyst, BiFeO, was added to 100cm, 5mg/dm, methyl orange solution of pH » 2 adjusted by 2mol/dm HSO. Subsequently, the solution was irradiated for different times in solar light. The dependence of degradation ratio of methyl orange on the irradiation time is presented in Fig. 4. It can be seen from Fig. 4 that the degradation ratio of methyl orange increases with irradiation time. From experimental result it can be derived that the degradation products are absorbed on the surfaces of photocatalyst, BiFeO, and the absorbate escapes slowly from photocatalyst. Therefore, escaping the products of degradation methyl orange from the surface of photocatalyst, BiFeO, is the determining-step in this reaction.
Fig. 4 Dependence of degradation ratio of methyl orange on the irradiation time
3.6 Reaction mechanism
The acidity-dependent degradation can mainly be attributed to the variations of surface charge properties of the photocatalyst. Afterward, this condition changes the absorbing behaviour of methyl orange on the photocatalyst surface. Since methyl orange has an anionic configuration its adsorption is preferred in an acidic solution. One hand, this increases methyl orange in close contact with the catalyst and enhances its oxidative degradation by positive holes or hydroxyl radicals upon photoexcitation. On the other hand, quinonoid structure of methyl orange is unstable. Based on the experimental results and analysis mentioned above, a reaction mechanism was proposed as follows:
where MO and MM represent methyl orange and micromolecular substance, respectively.
ACKNOWLEDGEMENTS This work was supported by Foundation for Doctorate, Education Department of Hebei Province (B2002102), China.