A type of iron-doped titania thin film was prepared by means of sol-gel method to degrade indoor formaldehyde (HCHO), ammonia (NH3), and benzene (C6H6) under sunlight. The photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis spectroscopy, and energy dispersive spetra (EDS). The results showed that the iron was doped in the TiO2 photocatalyst successfully. The absorption edge of doped TiO2 had red shifts and the doped TiO2 had a stronger absorption than the pure TiO2 in the visible region. Fe-doped TiO2 thin film prepared with the optimal preparation condition could remove indoor HCHO, NH3 and C6H6 effectively under solar light irradiation. The removal percentage of HCHO, NH3 or C6H6 after 9 h photocatalytic reaction under solar light reached 55%, 53.1%, and 37.5%, respectively, when they existed in the air individually. When the three pollutants were mixed in the air, the removal percentage decreased to 33.3%, 28.3%, and 28%. The degradation reaction of the three pollutants followed the pseudo first-order kinetics, which reflects that the photocatalytic reaction was controlled by the surface chemical reaction and the reaction rate was controlled by concentration of reactants.
People are paying more and more attention to the pollution caused by indoor decoration with the improvement of living standard and indoor decoration gradually becoming popular [
Photocatalysis technology has been proved to be potentially advantageous for indoor air purification [
Doping is one of the important methods of improving visible light photocatalytic activity of titania [
In this paper, Fe-doped TiO2 photocatalysts were attempted to degrade three typical indoor air pollutants: formaldehyde, ammonia, and benzene under daylight illumination. Usually, formaldehyde, ammonia, and TVOC exist in the indoor air at the same time, but there are few reports on the photocatalytic degradation of mixed indoor air pollutants. So, we not only studied the photocatalytic degradation of formaldehyde, ammonia, and benzene when they exist in indoor air separately, but also studied photocatalytic degradation of the mixed formaldehyde, ammonia, and benzene in the indoor air. To avoid TiO2 powder suspending in the indoor air, the Fe-doped TiO2 thin films were prepared with flat glass as carrier. The research finding provides a feasible way for enhancement of indoor air quality.
Pure and Fe-doped TiO2 thin films were prepared by sol-gel technique. The sol was prepared with tetrabutyl titanate as precursor, anhydrous ethanol as solvent, deionized water or Fe(NO3)3 solution as reactive substance, and glacial acetic acid as stabilizer. 7 mL of tetrabutyl titanate was dissolved in 50 mL of anhydrous ethanol. The mixed solution was sealed with plastic wrap and stirred for 0.5 h at room temperature to form solution 1. 14 mL glacial acetic acid was dropped into solution 1 with constant stirring for 0.5 h in air to form solution 2. Then 6 mL deionized water or Fe(NO3)3 solution of different concentrations was added dropwise in solution 2 within 60 min under vigorous stirring at room temperature to form the sol, followed by aging for 24 h to prepare the gel. The plate glass was inserted vertically into the gel. After keeping 10 seconds, it was pulled out of the gel with the speed of 12 cm/min. Thus, the gel film was formed on the glass surface. Then the glass coated with the sol was dried for 20 min at 100°C. At last, the glass was calcined for 2 h at 500°C (20°C/min).
The crystal structure of pure and Fe-doped TiO2 was determined by a D-max-2500/PC X-ray diffractometer (XRD) equipped with Cu-Ka radiation. Morphology of Fe-doped TiO2 thin film was examined by HITACHI (Japan) S-4800 scanning electron microscopy (SEM). The ultraviolet-visible (UV-Vis) absorption spectra of pure and Fe-doped TiO2 thin films were recorded on a Shimadzu (Japan) UV-2550 spectrophotometer. The energy dispersive spectra of the Fe-doped TiO2 thin film were recorded on JEM-2010 transmission electron microscopy (TEM).
The degradation test was performed on a self-designed, cuboid, airtight, glass reactor (60 × 60 × 20 cm) with a hole, which was connected to sampling tube of KC-6D air sampler. An electric fan was installed on the bracket of the airtight reactor in order to circulate the mixture of pollutants (individual formaldehyde, ammonia, benzene, or the mixture of three pollutants) and air. Eight pieces of coated glass were placed vertically and evenly in the reactor. A predetermined amount of formaldehyde was injected in vessel under dark. The photoreactor was then kept in darkness for 2 h to allow the adsorption of formaldehyde into photocatalyst to approach the state of equilibrium. The photoreactor was then exposed to sunlight to activate the photocatalytic degradation. The experiments were carried out in good sunny days of summer (June to July) with a maximum temperature of 30°C (between 8 a.m. to 5 p.m.). The intensity of indoor sunlight was in the range between 500 and 1000 lux. The concentration of formaldehyde, ammonia, and benzene was periodically measured every 60 min. The formaldehyde concentration was measured with the 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) spectrophotometric method. The ammonia concentration was measured with Nessler’s reagent spectrophotometric method. The benzene concentration was measured by the benzene detector.
Figure
XRD spectra of TiO2 catalysts.
Figure
SEM photograph of Fe-doped TiO2 thin film.
Figure
UV-Vis absorption spectra.
Figure
EDS spectra.
Table
Degradation ratios of individual formaldehyde, ammonia, and benzene.
Iron dopant amount (mole fraction) % | Degradation ratio of HCHO (%) | Degradation ratio of NH3 (%) | Degradation ratio of C6H6 (%) |
---|---|---|---|
0 | 35.2 | 39.5 | 26 |
0.25 | 37.1 | 41.0 | 28.6 |
0.75 | 40.4 | 50.3 | 35.2 |
1.00 | 55.0 | 53.1 | 37.5 |
1.25 | 38.9 | 46.2 | 34.2 |
Since the photocatalytic activity of Fe-doped TiO2 thin film with the doping amount of iron was 1.00% which was the highest, the photocatalytic degradation experiment of mixed formaldehyde, ammonia, and benzene was conducted with this optimal Fe-doped TiO2 thin film. The results showed that although the three pollutants were mixed in the air, they were still removed effectively after 9 h photocatalytic reaction under solar light. The degradation ratios of formaldehyde, ammonia, and benzene in gas mixture were 33.3%, 28.3%, and 28% separately, but they were lower than the degradation ratios of formaldehyde, ammonia, and benzene existing in air separately (55.0%, 53.1%, and 37.5%). This is due to the fact that gas molecules occupy fully the active sites of the photocatalyst, different molecules compete with each other, and the intermediate product of different pollutant accumulating on the photocatalyst surface prevents the effective contact of gas molecules and photocatalyst.
Formaldehyde, ammonia, and benzene exist in the indoor air at the same time usually, so the kinetics of photocatalytic reaction was studied with the experimental data of mixed pollutants over the optimal designed Fe-doped TiO2 thin film; that is, Fe(III) dopant amount was 1%. Langmuir-Hinshelwood (L-H) equation has been widely used to describe the process of photocatalytic reaction. At low reactant concentration, which is a reasonable assumption for most indoor air pollution problems, the L-H model is simplified to a pseudo first-order expression:
Kinetics of degradation reaction of the mixed pollutants.
From Figure
The Fe-doped TiO2 thin film was prepared by means of sol-gel method. The solar light catalytic activity of TiO2 was improved significantly by doping with iron (III). Using the optimal Fe-doped TiO2 photocatalyst, the formaldehyde, ammonia, and benzene could be removed effectively whether existing separately or mixedly in the air. But the degradation ratios of formaldehyde, ammonia, and benzene in gas mixture after 9 h photocatalytic reaction under solar light (33.3%, 28.3%, and 28%) were lower than those of formaldehyde, ammonia, and benzene existing in air individually (55%, 53.1%, and 37.5%). So the photocatalytic degradation of indoor air pollutants using solar light is very promising. The photocatalytic degradation reaction kinetic of three pollutants followed the pseudo first-order kinetic model, the photocatalytic reaction was controlled by the surface chemical reaction, and the reaction rate was controlled by concentration of reactants.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors greatly acknowledge the financial support from the Tianjin North China Geological Exploration Bureau Research Project and the Hebei Postdoctoral Science Foundation.