Abstract

This paper presents an experimental approach to study catalytic effects of Fe³⁺ modified nanometer titanium dioxide (TiO₂) loaded on aluminium oxide (Al₂O₃). Sol-gel method was used to prepare modified TiO₂ loaded on carrier. Purification tests were conducted in a self-developed instrument to study catalytic effects of TiO₂ loaded on Al₂O₃ with different contents through degradation rate. The modification mechanism was studied by scanning electron microscope (SEM). Results showed that loading on Al₂O₃ improved photocatalytic effect of TiO₂ modified with Fe³⁺. The best photocatalytic effect was achieved under catalytic action of Al₂O₃ loaded with 10% TiO₂ composite; the degradation rates were 6.9%, 13.8%, 21.4%, and 49.2%, respectively, 0.7%, 3.9%, 1.3%, and 15.1% larger than unloaded TiO₂. SEM results of four catalysts showed that nanometer TiO₂ was coated in form of grain on the surface of Al₂O₃. The optimal loading content was 10% at which the nanometer TiO₂ grains were coated on the surface of Al₂O₃ uniformly.

1. Introduction

There was a lot of automobile exhaust in the highway tunnels. This exhaust contained a lot of gas phase carbon monoxide (CO), nitrogen oxide (NO_X), hydrocarbons, and other pollutants [1]. Meanwhile, the harmful gas inside tunnels was hard to be expelled, especially in long tunnel encountering long time traffic jams, which would seriously influence driving safety and personnel health [2]. Therefore, it was necessary to reduce harmful gas in tunnels, especially in long tunnel. Semiconductor optical catalyst which emerged in the 1970s was a new oxidation technology, showing good function for degradation of organic pollutants [3]. TiO₂ powders as a kind of photocatalytic materials had been successfully applied in pollutant disposal, especially in sewage treatment and indoor waste gas purification, and certain achievements have been made [4, 5]. However, disadvantages were also shown in actual application. For example, the TiO₂ was difficult to precipitate and recover due to its tiny particles, during which the active ingredient was lost by a large percentage [6].

In order to solve the problems above, a considerable number of research efforts have been made. At present, photocatalytic materials fixed to a carrier had become a current trend. Nanometer TiO₂ particle fixed on montmorillonite showed better photocatalytic property, because specific surface area of the nanometer TiO₂ became bigger when loaded on carriers such as diatom class [7, 8], montmorillonite [9, 10], and kaolinite [11–13], so as to increase utilization rate of the light and absorption ability of pollutants. In addition, a few carriers such as metal chemicals could interact with nanometer TiO₂ particles, and this interaction was helpful for separation of the electronic hole of nanometer TiO₂ particles, which could improve the catalytic effect [14]. However, these researches were mainly used for waste water treatment; few studies were conducted in the aspect of gas purification in tunnels. Therefore, the goal of this paper was to research catalytic efficiency of nanometer TiO₂ loaded on Al₂O₃ for major kinds of gases in tail gas and study modification mechanism of the composite materials.

2. Experimental

2.1. Preparation of Photocatalytic Materials

Modified nanometer TiO₂ was prepared by sol-gel method. Prepare TiO₂ sol doped with 0.1% Fe³⁺, used as control sample; prepare Al₂O₃ saturated solution with a certain amount of alumina and deionized water; the wet gel composed of TiO₂ sol and Al₂O₃ saturated solution was prepared by a 30-minute violent mixing; dry gel was obtained by drying the wet gel in vacuum at 80°C. TiO₂ loaded on Al₂O₃ was obtained by calcination at 500°C in a muffle furnace for 2 hours.

Four catalysts were prepared by the above procedures, namely, catalyst 1, catalyst 2, catalyst 3, and catalyst 4, respectively. The details of the catalysts are shown in Table 1.

2.2. Test Instruments and Methods

2.2.1. Test Instruments

A set of test equipment for exhaust gas purification was developed to conduct purification experiments (see Figure 1). The test equipment was composed of motorcycle engine, filter and reaction tank, and an AVL-4000 exhaust gas analyzer (parameters are shown in Table 2). There were inlet control valves in the lower part of reaction tank, and the exhaust port was wetted in the upper portion of the test equipment. To simulate the natural wind in tunnel, a fan was located in the lower side. Sample bracket for test board coated with photocatalytic materials was located in the bottom. A filter filled with material was installed to prevent dust flowing into the reaction tank. Two kinds of light sources, respectively, ultraviolet (UV) light with a wavelength of 287.5 nm and incandescent light similar to the light in tunnels with a wavelength of 500~700 nm were used in this text.

Figure 1 

Equipment for exhaust gas purification.

2.2.2. Test Method

Photocatalytic materials produced by sol-gel method were evenly daubed on an organic glass board. The board was 300 mm × 300 mm in length (shown in Figure 2). The board daubed with photocatalytic materials was put into the homemade exhaust catalytic reaction equipment. Purification tests were conducted under the two kinds of light sources when the initial concentration was steady. In this text, degradation rate was calculated and used as evaluation index for catalytic property. Concentrations of HC, CO, CO₂, and NO_X were measured and used to calculate concentration rate with the following equation: where was the degradation rate; was initial concentration; was concentration in the end of tests.

Figure 2 

Sample board for purification test.

3. Results and Discussion

3.1. Catalysis of Ultraviolet Light

The initial concentrations of CO, CO₂, NO_X, and HC were recorded after concentration in reaction tank was steady. Test data were measured every 10 min; concentrations of CO, CO₂, NO_X, and HC versus time during 60 min were plotted in Figures 3–6. Figures 3–6 represent concentration change of the four gases under catalytic action of unmodified TiO₂ and TiO₂ loaded on Al₂O₃ with different contents. The differences shown in each figure represent catalytic action of TiO₂ loaded on Al₂O₃. It was found that(1)change of CO and CO₂ concentration was not apparent, it was mainly because a series of complex REDOX reactions happened under the effect of catalyst between CO and CO₂, and these reactions were reversible chemical reactions in which CO and CO₂ acted as reactant and products for each other;(2)concentration of HC declined with the increase of action time and it was mainly because HC was decomposed into water and carbon oxide;(3)there was notable decline in concentration of NO_X especially in the first 30 minutes. This is mainly because NO_X was oxidized into HNO₃ and H₂O under the action of catalyst.

Figure 3 

Degradation rate of CO under catalytic action of different catalysts.

Figure 4 

Degradation rate of CO₂ under catalytic action of different catalysts.

Figure 5 

Degradation rate of NO_X under catalytic action of different catalysts.

Figure 6 

Degradation rate of HC under catalytic action of different catalysts.

The quantitative impacts of Fe³⁺ on the catalytic effects of TiO₂ are plotted in Figure 7. Degradation rates of four gases were calculated according to data obtained from purification tests and (1). The following findings were observed.(1)In general, doping Fe³⁺ improved the photocatalysis efficiency of nano-TiO₂.(2)Catalytic effect of catalyst 3 was the best, with degradation rate of CO, CO₂, HC, and NO_X, respectively, 6.9%, 13.8%, 21.4%, and 49.2%, increased by 0.7%, 3.9%, 1.3%, and 15.1% compared to TiO₂ unloaded on Al₂O₃.(3)Compared with the catalysis of catalyst 1, the degradation rates of the four gases under the catalysis of catalyst 2 and catalyst 4 were reduced. Therefore, catalyst loaded on Al₂O₃ had a significant influence on the catalysis effect. Too high or too low content would reduce the effect of the catalyst. The optimal dose was suggested to be controlled at about 10% in this paper.

Figure 7 

Degradation rates of CO, CO₂, NO_X, and HC under catalytic action of different catalysts.

3.2. Catalysis of the Incandescent Light

The optimal dosage of carrier was obtained at present. Therefore TiO₂ loaded on the optimal dosage of 10% Al₂O₃ was used to study catalytic effect under natural light in tunnels. To this end, purification experiments were performed under incandescent lamp light, and concentration changes of CO, CO₂, NO_X, and HC are, respectively, plotted in Figures 8–11. Each curve in Figure 8 represents concentration change of CO under catalytic action of catalyst 1 and catalyst 3 in natural light condition. It could found that, under incandescent lamp light condition, concentration of CO, CO₂, HC, and NO_X decreased as testing time increased, especially in the action of catalyst 3. Degradation rates of four components of exhaust were, respectively, 1.9%, 2.3%, 5.1%, and 27.8% under the catalytic action of catalyst 3, 0.6%, 0.6%, 2.3%, and 8.2% larger compared to degradation rate under catalyst 1.

Figure 8 

Concentration of CO versus time under catalytic effect of catalyst 1 and catalyst 3, respectively.

Figure 9 

Concentration of CO₂ versus time under catalytic effect of catalyst 1 and catalyst 3, respectively.

Figure 10 

Concentration of NO_X versus time under catalytic effect of catalysts 1 and 3, respectively.

Figure 11 

Concentration of HC versus time under catalytic effect of catalysts 1 and 3, respectively.

To observe quantitatively the impact of Al₂O₃ on the purification, degradation rates of four components of exhaust were calculated according to (1) and plotted in Figure 12. It could be found that with catalytic action of catalyst 1, compared to catalytic effect under UV light, degradation rates of four gases were decreased by 8.0%, 10.8%, 6.9%, and 20.1%, mainly because the wavelength range of UV is easier to activate TiO₂ and with catalytic action of catalyst 3, compared to catalytic effect under UV light, degradation rates of four gases were decreased by 6.1%, 8.5%, 1.8%, and 0%, respectively. It could be concluded that Al₂O₃ weakened light source sensitivity of TiO₂.

Figure 12 

Degradation rates of CO, CO₂, HC, and NO_X under photocatalytic action of catalyst 4 and catalyst 2, UV light and ordinary light condition, respectively.

3.3. Analysis of Catalytic Process Using GCMS and XRD

Carrene was used to collect exhaust for 20 minutes. Then the collected exhaust was concentrated by the nitrogen concentration instrument. This first group is the liquor before being catalyzed. As a contrast test, exhaust was collected and then was put in the equipment for exhaust gas purification, and, after 20 minutes, the liquor was taken out and was concentrated by the nitrogen concentration instrument. This second group is the liquor after being catalyzed. Then the first group and second group were tested by GC.

Total ionic chromatogram of automobile exhaust before and after using catalyst is shown in Figures 13 and 14, respectively. According to Figures 13 and 14, it can be found that most of automobile exhaust concentration has dropped after using catalyst, which can explain that using catalyst can work on reducing automobile exhaust.

Figure 13 

Total ionic chromatogram of automobile exhaust.

Figure 14 

Total ionic chromatogram of automobile exhaust after using catalyst.

X-ray diffraction (XRD) of catalysts 2 and 4 is shown in Figure 15. According to Figure 15, it can be found that XRD of two kinds of catalysts is similar. The diffraction peak intensity and width of two kinds of catalysts are different. TiO₂ nanoparticles exist on A1₂O₃ in the form of particles. Doped Al₂O₃ inhibits the growth of TiO₂ grains. The specific surface area of TiO₂ nanoparticles increased, so both adsorption capacity and catalytic ability are enhanced. The average particle size of TiO₂ nanoparticles prepared is 15.2 nm and nano-TiO₂ prepared is anatase phase, so it has higher catalytic efficiency.

Figure 15 

XRD of catalysts 2 and 4, respectively.

3.4. Analysis of Modification Mechanism

Photocatalytic reaction rate was closely related to many factors such as catalyst characteristics, system composition, and reactant type. Generally speaking, catalyst features such as TiO₂ crystal structure and lattice [15], TiO₂ particle size [16], and electronic defects and hole capture agent had a decisive influence on the light-catalyzed role of TiO₂ [17]. Quanta 200 scanning electron microscope instrument made by FEI was used to make crystal analysis of TiO₂-Al₂O₃ composite, in purpose of analyzing modification mechanism of TiO₂ composite loaded on Al₂O₃. Parameters of scanning electron microscope instrument were set as follows. Speed up voltage was 20 Kv, and magnification was 10000 times. SEM of modified and unmodified TiO₂ is plotted in Figure 16.

Figure 16 

The SEM picture of TiO₂-Al₂O₃ composite.

It could be seen from the scanning electron microscope pictures that there was obvious synergistic reaction between nanometer TiO₂ and active Al₂O₃ grains. Nanometer TiO₂ were coated in the form of grain with particle size ranging from 10 to 20 nm on the surface of Al₂O₃ [18]. Figure 16(a) shows the SEM of Al₂O₃ calcining sample; it could be found that surfaces of Al₂O₃ were smooth. Figure 16(d) shows the SEM picture of Al₂O₃ loaded with 5% TiO₂. In this figure, the surfaces of the Al₂O₃ were only coated with a handful of TiO₂ grains, because content of TiO₂ particles was insufficient to cover the surface of Al₂O₃. Figure 16(c) shows the SEM picture of Al₂O₃ loaded with 10% TiO₂. In this picture, the TiO₂ particles were uniformly coated on the surfaces of the Al₂O₃ crystals and the TiO₂ particles kept well hexagonal shapes. These characteristics would accelerate the air pollutants moving to the surfaces of TiO₂ and make a good contribution to the degradation of the contaminants. At the same time, the catabolite would leave the TiO₂ particles quickly. In this way, the catalysis of the TiO₂ particles was ameliorated. Figure 16(b) showed the magnified patterns of the Al₂O₃ loaded with TiO₂ (15%) calcining sample. It could be seen that TiO₂ particles were coated on the surface of Al₂O₃. However, the content of TiO₂ was too large to cause obvious cracks on the surface of Al₂O₃; photogenerated electrons and holes were covered, resulting in negative influence on the catalysis [19, 20].

4. Conclusions

(1)Loading on Al₂O₃ improved photocatalytic effect of TiO₂ modified with Fe³⁺, and the best purification effect was achieved when the loaded content of TiO₂ was 10%. The degradation rates of CO, CO₂, HC, and NO_X were 6.9%, 13.8%, 21.4%, and 49.2%, respectively, under UV light condition, 0.7%, 3.9%, 1.3%, and 15.1% larger compared to unloaded condition. Under ordinary light condition, degradation rates of the CO, CO₂, HC, and NO_X were 1.1%, 5.6%, 5.6%, and 49.2%, respectively, almost the same as the results in the ultraviolet light condition.(2)It could be seen from the scanning electron microscope pictures that there was obvious synergistic reaction between nanometer TiO₂ and active Al₂O₃ grains. Nanometer TiO₂ was coated in form of grain on the surface of Al₂O₃. The optimal loading content was 10%; at this condition, nanometer TiO₂ grains were coated on the surface of Al₂O₃ uniformly.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.