# Preparation and Photocatalytic Properties of TiO2-Al2O3 Composite Loaded Catalysts

Authors: Jianzhong Pei; Weisi Ma; Rui Li; Yanwei Li; Hongzhao Du
Journal: Journal of Chemistry
 (2015)

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## 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.


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## Body

## 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 (NOX), 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 (
1
) research catalytic efficiency of nanometer TiO2 loaded on Al2O3 for major kinds of gases in tail gas and (
2
) study modification mechanism of the composite materials.


## 2. Experimental

### 2.1. Preparation of Photocatalytic Materials

Modified nanometer TiO2 was prepared by sol-gel method. (
1
) Prepare TiO2 sol doped with 0.1% Fe3+, used as control sample; (
2
) prepare Al2O3 saturated solution with a certain amount of alumina and deionized water; (
3
) the wet gel composed of TiO2 sol and Al2O3 saturated solution was prepared by a 30-minute violent mixing; (
4
) dry gel was obtained by drying the wet gel in vacuum at 80°C. (
5
) 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 Table1.Table 1
Four catalysts prepared in this study.
Catalyst type
Notes
Catalyst 1
TiO2 doped with 0.1% Fe3+, as the control
Catalyst 2
Al2O3 loaded with 5% catalyst 1
Catalyst 3
Al2O3 loaded with 10% catalyst 1
Catalyst 4
Al2O3 loaded with 15% catalyst 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 Figure1). 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.Table 2
Parameters of AVL-4000.
Gas type
HC (ppm)
CO (%)
CO2 (%)
NO
X (ppm)
Testing range
0~20000
0~10
0~20
0~5000
Accuracy
1
0.01
0.1
1Figure 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 Figure2). 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 NOX were measured and used to calculate concentration rate with the following equation:
(1)
η
=
C
0
-
C
1
C
0
*
100
%
,
where η was the degradation rate; C
0 was initial concentration; C
1 was concentration in the end of tests.Figure 2
Sample board for purification test.


## 2.1. Preparation of Photocatalytic Materials

Modified nanometer TiO2 was prepared by sol-gel method. (
1
) Prepare TiO2 sol doped with 0.1% Fe3+, used as control sample; (
2
) prepare Al2O3 saturated solution with a certain amount of alumina and deionized water; (
3
) the wet gel composed of TiO2 sol and Al2O3 saturated solution was prepared by a 30-minute violent mixing; (
4
) dry gel was obtained by drying the wet gel in vacuum at 80°C. (
5
) 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 Table1.Table 1
Four catalysts prepared in this study.
Catalyst type
Notes
Catalyst 1
TiO2 doped with 0.1% Fe3+, as the control
Catalyst 2
Al2O3 loaded with 5% catalyst 1
Catalyst 3
Al2O3 loaded with 10% catalyst 1
Catalyst 4
Al2O3 loaded with 15% catalyst 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 Figure1). 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.Table 2
Parameters of AVL-4000.
Gas type
HC (ppm)
CO (%)
CO2 (%)
NO
X (ppm)
Testing range
0~20000
0~10
0~20
0~5000
Accuracy
1
0.01
0.1
1Figure 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 Figure2). 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 NOX were measured and used to calculate concentration rate with the following equation:
(1)
η
=
C
0
-
C
1
C
0
*
100
%
,
where η was the degradation rate; C
0 was initial concentration; C
1 was concentration in the end of tests.Figure 2
Sample board for purification test.


## 2.2.1. Test Instruments

A set of test equipment for exhaust gas purification was developed to conduct purification experiments (see Figure1). 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.Table 2
Parameters of AVL-4000.
Gas type
HC (ppm)
CO (%)
CO2 (%)
NO
X (ppm)
Testing range
0~20000
0~10
0~20
0~5000
Accuracy
1
0.01
0.1
1Figure 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 Figure2). 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 NOX were measured and used to calculate concentration rate with the following equation:
(1)
η
=
C
0
-
C
1
C
0
*
100
%
,
where η was the degradation rate; C
0 was initial concentration; C
1 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, NOX, and HC were recorded after concentration in reaction tank was steady. Test data were measured every 10 min; concentrations of CO, CO2, NOX, 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 NOX especially in the first 30 minutes. This is mainly because NOX 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 NOX 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 NOX, 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, NOX, 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, NOX, 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 NOX 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 NOX 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 (
1
) 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 (
2
) 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 NOX 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 Figures13 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 Figure15. 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].


## 3.1. Catalysis of Ultraviolet Light

The initial concentrations of CO, CO2, NOX, and HC were recorded after concentration in reaction tank was steady. Test data were measured every 10 min; concentrations of CO, CO2, NOX, 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 NOX especially in the first 30 minutes. This is mainly because NOX 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 NOX 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 NOX, 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, NOX, 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, NOX, 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 NOX 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 NOX 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 (
1
) 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 (
2
) 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 NOX 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 Figures13 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 Figure15. 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 NOX 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 NOX 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.



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*Source: 806568-2015-02-19.xml*