Effectiveness of photocatalytic degradation of phenol in aqueous solution using semiconductor oxides (SO) prepared by a sol-gel method was examined. The physical and chemical properties of synthesized catalysts were investigated by X-ray diffraction (XRD), diffuse reflectance UV-Vis spectroscopy (DRS), and N2-adsorption measurements. The optimal conditions of the photocatalytic degradation of phenol using prepared titanium dioxide sample were defined.
Heterogeneous photocatalysis on the semiconductors allows achieving complete mineralization of the various classes toxic and biorefractory organic substances [
Among SO photocatalysts (PC) high activity have Fe2O3, WO3, ZnO and TiO2. Iron oxide polymorphs of hematite (
Energy band gap of investigated semiconductor oxides.
The activity of semiconductor oxides prepared by the sol-gel methods was investigated under the same conditions for searching of the most effective system in the reaction of the phenol photodegradation. The optimal parameters of the phenol photodegradation on the synthesized PC were defined.
All solvents and chemicals used in this work were of analytical grade and were used without further purification. Inorganic (Fe(NO3)3·9H2O, Zn(NO3)2, Na2WO4·2H2O), and organic (Ti(
Synthesis of PC was carried out by sol-gel method which allows obtaining nanosized metal oxide particles with desired structural and morphological properties [
Fe2O3 sample was synthesized similar to the method [
WO3 sample was prepared by thermal decomposition of tungstic acid obtained by the sol-gel method [
Synthesis of nanocrystalline ZnO powder was performed similar to [
Nanocrystalline TiO2 particles were synthesized by the hydrolysis of titanium isopropoxide [
As a reference sample used a titanium dioxide Degussa P25 (TiO2 P25), obtained by high-temperature gas-phase oxidation of titanium tetrachloride vapors [
In order to characterize the powders instrument measurements were performed with X-ray diffraction (diffractometer DRON 3 M generating CoK
Photocatalytic degradation of phenol was carried out in a 0.5 L quartz reactor with a jacket under air bubbling (velocity 50 mL·min−1) and temperature from 293 K to 323 K. The reaction mixture was agitated with a magnetic stirrer (800 rpm·min−1). The concentrations of phenol and catalyst were 0.532 mM and 1 g·L−1, respectively. Low-pressure mercury lamp DRB-8 submerged in a quartz casing used as a UV-radiation source with a maximum emission output at 254 nm. Reaction time was 3 hours. The separation of reaction mixture was performed by centrifugation. The effectiveness of photocatalytic process was evaluated relative to the photolysis carried out under similar conditions without catalyst usage. The phenol conversion was determined by the aromatic content recorded by the absorbance of the solution at 270 nm (
Figure
X-ray diffraction patterns of the semiconductor oxides: 1—ZnO; 2—
The XRD pattern of Fe2O3 sample shows that all basal reflections in the range of Bragg angles (
Investigation of the crystal structure of WO3 confirms the presence of hexagonal phase (R6/mmm), JCPDS No. 33-1387). Low peak-height indicates the weakly crystallized structure.
The X-ray diffraction pattern of the prepared TiO2 sample is presented basal reflections (at around 2Θ 25.4, 44.2, and 56.4) corresponding to the titanium dioxide anatase phase [
As is known, after calcination for 2 h TiO2 P25 is a mixture of anatase and rutile phase (82 and 18%, resp.) [
XRD analysis of the synthesized ZnO shows strong and high peaks indicating the high purity and crystallinity. Location of the basal reflections confirms hexagonal structure of ZnO (JCPDS No. 80-0075).
Diffuse reflectance spectroscopy allows obtaining information about light absorption range and band gap of the semiconductor [
Diffuse reflectance spectra of semiconductor oxides: 1—
The optical absorption spectrum of the ZnO sample is represented by a broad and intense band and characterized by a sharp increase of absorption at 400 nm and a slight decrease at shorter wavelengths.
In the DRS of the TiO2 P25 and synthesized TiO2 powder the drastic increasing of the light absorption at
The surface area and micropore volume of the synthesized materials are defined with nitrogen adsorption-desorption isotherm. Surface parameters and the energy band gap of the SO are shown in Table
Physical and chemical characteristics of the investigated semiconductor oxides.
Sample |
|
|
|
|
---|---|---|---|---|
|
25,7 | 0,18 | 2,0 | 2000 |
WO3 | 50 | 0,005 | 2,55 | 20 |
ZnO | 41 | —d | 3,23 | 3000 |
TiO2 | 45,3 | 0,1 | 3,43 | 5–7 |
TiO2 P25b | 52 | 0,18 | 3,23 | 28 |
—: not determined.
The results of photolysis and photocatalytic degradation study of phenol using prepared semiconductor oxides and TiO2 P25 samples are shown in Figure
Effectiveness of investigated semiconductor oxides in photocatalytic phenol degradation. Experimental conditions: initial phenol concentration = 0.536 mM; catalyst concentration = 0.1 g·L−1; pH = 5.9;
During the photolysis of phenol under UV-C irradiation the appearance of light brown color and increase of the optical density of analyzed solution are observed, which can be explained by the formation of colored intermediates: benzoquinone, hydroquinone, and catechol [
The study of phenol conversion dependence on the SO nature found that the least active are the
Table
Degree of substrate conversion (
Sample |
|
|
|
|
---|---|---|---|---|
|
— | 14 | 0,7 | 0,3 |
WO3 | — | 26 | 1,3 | 2,6 |
ZnO | 24 | 52 | 2,6 | 6,3 |
TiO2 | 79 | 85 | 4,2 | 9,2 |
TiO2 P25 | 74 | 91 | 4,5 | 8,6 |
—: the reduction of optical density does not occur.
The use of
Investigation of the degradation process of phenol was followed by pH measuring of the reaction mixture. In all cases the pH decrease is associated with the formation of short-chain fatty acids [
It is found that the specific activity of SO in the reaction of phenol photocatalytic degradation changes in a number of
The influence of the catalyst concentration on the phenol conversion in water (Figure
Effect of TiO2 concentration on phenol conversion. Experimental conditions: initial phenol concentration = 0.536 mM; pH = 5.9;
Effect of initial phenol concentration on its conversion in water is shown on Figure
Effect of initial phenol concentration on its conversion. Experimental conditions: TiO2 concentration = 0.1 g·L−1; pH = 5.9;
Phenol photodegradation efficiency largely depends on the pH value. Studies showed that aromatic content degradation of phenol and reduction of TOC in an acidic medium (pH = 3) are significantly low (37% and 34%, resp.) in comparison with in alkaline medium (pH = 8) (72% and 56%, resp.). Effect of pH on the photodegradation degree of phenol can be caused by changing in the surface charge of semiconductor, phenol chemical transformations in the solution, and carbonate ions formation which are effective scavengers of OH• radicals [
The study of photocatalytic phenol conversion dependence on reaction temperature was carried out in range from 293 to 323 K. Maximum conversion (96% of aromaticity and 86% of TOC conversion) in temperature range 303–313 K was observed.
The efficiency evaluation of the synthesized TiO2 sample in the real conditions was carried out in Kiev tap water [
The photocatalytic conversion of phenol in tap water using synthesized TiO2 samples.
The specific activity of investigated semiconductor oxides in the reaction of phenol photocatalytic degradation changes in a number of
The authors declared that there is no conflict of interests.