Preparation and Photocatalytic Properties of TiO 2-Al 2 O 3 Composite Loaded Catalysts

This paper presents an experimental approach to study catalytic effects of Fe modified nanometer titanium dioxide (TiO 2 ) loaded on aluminium oxide (Al 2 O 3 ). Sol-gel method was used to prepare modified TiO 2 loaded on carrier. Purification tests were conducted in a self-developed instrument to study catalytic effects of TiO 2 loaded on Al 2 O 3 with different contents through degradation rate. The modification mechanism was studied by scanning electron microscope (SEM). Results showed that loading on Al 2 O 3 improved photocatalytic effect of TiO 2 modified with Fe. The best photocatalytic effect was achieved under catalytic action of Al 2 O 3 loaded with 10% TiO 2 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 TiO 2 . SEM results of four catalysts showed that nanometer TiO 2 was coated in form of grain on the surface of Al 2 O 3 .The optimal loading content was 10% at which the nanometer TiO 2 grains were coated on the surface of Al 2 O 3 uniformly.


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].TiO 2 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 TiO 2 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 TiO 2 particle fixed on montmorillonite showed better photocatalytic property, because specific surface area of the nanometer TiO 2 became bigger when loaded on carriers such as diatom class [7,8], montmorillonite [9,10], and kaolinite [11][12][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 TiO 2 particles, and this interaction was helpful for separation of the electronic hole of nanometer TiO 2 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 TiO 2 loaded on Al 2 O 3 for major kinds of gases in tail gas and (2) study modification mechanism of the composite materials.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.

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.

Test Method.
Photocatalytic materials produced by solgel 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, CO 2 , and NO X were measured and used to calculate concentration rate with the following equation: where  was the degradation rate;  0 was initial concentration;  1 was concentration in the end of tests.reversible chemical reactions in which CO and CO 2 acted as reactant and products for each other;

Results and Discussion
(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 HNO 3 and H 2 O under the action of catalyst.
The quantitative impacts of Fe 3+ on the catalytic effects of TiO 2 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.
(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 Al 2 O 3 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.

Catalysis of the Incandescent Light.
The optimal dosage of carrier was obtained at present.Therefore TiO 2 loaded on the optimal dosage of 10% Al 2 O 3 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, CO 2 , 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, CO 2 , 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.
To observe quantitatively the impact of Al 2 O 3 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 TiO 2 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 Al 2 O 3 weakened light source sensitivity of TiO 2 .

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.
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.TiO 2 nanoparticles exist on A1 2 O 3 in the form of particles.Doped Al 2 O 3 inhibits the growth of TiO 2 grains.The specific surface area of TiO 2 nanoparticles increased, so both adsorption capacity and catalytic ability are enhanced.The average particle size of TiO 2 nanoparticles prepared is 15.2 nm and nano-TiO 2 prepared is anatase phase, so it has higher catalytic efficiency.

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 TiO 2 crystal structure and lattice [15], TiO 2 particle size [16], and electronic defects and hole capture agent had a decisive It could be seen from the scanning electron microscope pictures that there was obvious synergistic reaction between nanometer TiO 2 and active Al 2 O 3 grains.Nanometer TiO 2 were coated in the form of grain with particle size ranging from 10 to 20 nm on the surface of Al 2 O 3 [18].Figure 16(a) shows the SEM of Al 2 O 3 calcining sample; it could be found that surfaces of Al 2 O 3 were smooth.Figure 16(d) shows the SEM picture of Al 2 O 3 loaded with 5% TiO 2 .In this figure, the surfaces of the Al 2 O 3 were only coated with a handful of TiO 2 grains, because content of TiO 2 particles was insufficient to cover the surface of Al 2 O 3 .Figure 16(c) shows the SEM picture of Al 2 O 3 loaded with 10% TiO 2 .In this picture, the TiO 2 particles were uniformly coated on the surfaces of the Al 2 O 3 crystals and the TiO 2 particles kept well hexagonal shapes.These characteristics would accelerate the air pollutants moving to the surfaces of TiO 2 and make a good contribution to the degradation of the contaminants.
At the same time, the catabolite would leave the TiO 2 particles quickly.In this way, the catalysis of the TiO 2 particles was ameliorated.Figure 16(b) showed the magnified patterns of the Al 2 O 3 loaded with TiO 2 (15%) calcining sample.It could be seen that TiO 2 particles were coated on the surface of Al 2 O 3 .However, the content of TiO 2 was too large to cause obvious cracks on the surface of Al 2 O 3 ; photogenerated electrons and holes were covered, resulting in negative influence on the catalysis [19,20].

Conclusions
(1) Loading on Al 2 O 3 improved photocatalytic effect of TiO 2 modified with Fe 3+ , and the best purification effect was achieved when the loaded content of TiO 2 was 10%.The degradation rates of CO, CO 2 , 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, CO 2 , 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 TiO 2 and active Al 2 O 3 grains.Nanometer TiO 2 was coated in form of grain on the surface of Al 2 O 3 .The optimal loading content was 10%; at this condition, nanometer TiO 2 grains were coated on the surface of Al 2 O 3 uniformly.

2. 1 .
Preparation of Photocatalytic Materials.Modified nanometer TiO 2 was prepared by sol-gel method.(1) Prepare TiO 2 sol doped with 0.1% Fe 3+ , used as control sample; (2) prepare Al 2 O 3 saturated solution with a certain amount of alumina and deionized water; (3) the wet gel composed of TiO 2 sol and Al 2 O 3 saturated solution was prepared by a 30minute violent mixing; (4) dry gel was obtained by drying the wet gel in vacuum at 80 ∘ C. (5) TiO 2 loaded on Al 2 O 3 was obtained by calcination at 500 ∘ C in a muffle furnace for 2 hours.

Figure 2 :
Figure 2: Sample board for purification test.

Figure 3 :Figure 4 :
Figure 3: Degradation rate of CO under catalytic action of different catalysts.

Figure 5 :Figure 6 :
Figure 5: Degradation rate of NO X under catalytic action of different catalysts.

Figure 7 :
Figure 7: Degradation rates of CO, CO 2 , NO X , and HC under catalytic action of different catalysts.

Figure 8 :Figure 9 :
Figure 8: Concentration of CO versus time under catalytic effect of catalyst 1 and catalyst 3, respectively.

Table 1 :
Four catalysts prepared in this study.
The initial concentrations of CO, CO 2 , NO X , and HC were recorded after concentration in reaction tank was steady.Test data were measured every 10 min; concentrations of CO, CO 2 , NO X , and HC versus time during 60 min were plotted in Figures3-6.Figures 3-6 represent concentration change of the four gases under catalytic action of unmodified TiO 2 and TiO 2 loaded on Al 2 O 3 with different contents.The differences shown in each figure represent catalytic action of TiO 2 loaded on Al 2 O 3.1.Catalysis of Ultraviolet Light. 3 .It was found that(1) change of CO and CO 2 concentration was not apparent, it was mainly because a series of complex REDOX reactions happened under the effect of catalyst between CO and CO 2 , and these reactions were