**3. Results and discussion**

## **3.1. SEM and XRD Images of the Photocatalyst**

FE-SEM analysis of the particle size and shape of the synthesized TiO2 sample showed it consisted of uniform nano-particles (Fig. 5). However, some cracks were found on the surface. A major contributor to these cracks could be the greater surface tension resulting from the small diameter (0.5 mm) of the glass springs and the high-temperature sintering process. In further experiments, we decreased the temperature from 500 °C to 450 °C. At the lower temperature, there were fewer cracks on the surface but they were not eliminated completely.

**Figure 5.** SEM photographs and macroscopic morphology of the TiO2 thin film coated on a glass spring.

The left and right photographs were taken at 1000× and 50 000× magnification, respectively.

According to Scherrer's equation (Eq. 2) and XRD patterns, the particle size of TiO2 (D) was calculated to be 35 nm.

$$D = k \lambda / \beta \cos \theta \tag{2}$$

The crystalline phase of the TiO2 catalyst was analyzed by XRD (Fig. 6). All the diffraction peaks in the XRD pattern could be assigned to tetragonal anatase TiO2, with lattice constants of a=0.3785 nm, b=0.3785 nm, and c=0.9514 nm.

**Figure 6.** XRD spectrum of anatase crystalline phase of the TiO2 catalyst

#### **3.2. Effect of doped Ag/TiO2 or Ce/TiO2 on decomposition of VOCs**

346 Advanced Aspects of Spectroscopy

completely.

calculated to be 35 nm.

of a=0.3785 nm, b=0.3785 nm, and c=0.9514 nm.

**Figure 6.** XRD spectrum of anatase crystalline phase of the TiO2 catalyst

**3. Results and discussion** 

**3.1. SEM and XRD Images of the Photocatalyst** 

FE-SEM analysis of the particle size and shape of the synthesized TiO2 sample showed it consisted of uniform nano-particles (Fig. 5). However, some cracks were found on the surface. A major contributor to these cracks could be the greater surface tension resulting from the small diameter (0.5 mm) of the glass springs and the high-temperature sintering process. In further experiments, we decreased the temperature from 500 °C to 450 °C. At the lower temperature, there were fewer cracks on the surface but they were not eliminated

**Figure 5.** SEM photographs and macroscopic morphology of the TiO2 thin film coated on a glass spring.

The left and right photographs were taken at 1000× and 50 000× magnification, respectively. According to Scherrer's equation (Eq. 2) and XRD patterns, the particle size of TiO2 (D) was

The crystalline phase of the TiO2 catalyst was analyzed by XRD (Fig. 6). All the diffraction peaks in the XRD pattern could be assigned to tetragonal anatase TiO2, with lattice constants

*D k* / cos (2)

The characters of catalyst are important for the degradation of VOCs. Fig. 7 illustrates the degradation rates of acetone, toluene, and p-xylene (ATP) as functions of irradiation time when pure TiO2, Ag-TiO2 and Ce–TiO2 were used. As controls, blank experiments in the absence of TiO2 had been studied. The results corresponded to the flow-rate of 1 L/min, initial concentration of 0.1 mol/m3 and relative humidity of 35%. It was found that all the conversions of ATP in the TiO2/UV, Ag-TiO2/UV and Ce-TiO2/UV processes were increased with irradiation time. Table 1 shows the degradation rates for both catalysts after 8-h photocatalytic reaction. It can be seen from Fig. 7 and Table 1 that the doping of silver or cerium ions could improve the photo-activity of TiO2 effectively. Furthermore, the degradation character of the photo-catalyst was in the order Ce-TiO2>Ag-TiO2>TiO2. Besides, the results of blank experiments in the absence of TiO2 showed that the removal efficiency of ATP was very low. For example, the removal efficiency of acetone was merely 6.3% after 8 h and lower than 46.5% for pure TiO2, which means that TiO2 plays an important role in photo-catalytic reaction.

(Ag-TiO2, 5wt%)

**Figure 7.** Effect of doped Ag/Ce/TiO2 on decomposition of ATP.


**Table 1.** ATP degradation rates for different catalysts after 8 hrs.

Fig. 8 illustrated the effect of doped Ag/Ce/TiO2 on decomposition of HCHO. The conditions were as follows: flow-rate of 3 L/min, initial concentration of 0.1 mg/m3, relative humidity of 35%. It was found that conversions of HCHO in the TiO2/UV, Ag-TiO2/UV and Ce-TiO2/UV processes were increased with irradiation time. It could be seen that the doping of silver or cerium ions could improve the photo-activity of TiO2 effectively. Furthermore, the degradation character of the photo-catalyst was in the order Ce-TiO2 > Ag-TiO2 > TiO2.

**Figure 8.** Effect of doped Ag/Ce/TiO2 on decomposition of HCHO.

The reason was as follows: Ag/Ce doping could narrow the band gap. The narrower band gap will facilitate excitation of an electron from the valence band to the conduction band in the doped TiO2, thus increasing the photo-catalytic activity of the material. At the same time, silver or cerium species could create a charge imbalance, vacancies and unsaturated chemical bonds on the catalyst surface. It will lead to the increase of chemisorbed oxygen on the surface. Surface chemisorbed oxygen has been reported to be the most active oxygen, and plays an important role in oxidation reaction. Herein, silver or cerium modified TiO2 might have better activity for the oxidation of VOCs. Furthermore, samples after Ag/Ce doping treatment showed a slight change of colour from white to yellowish.

The photo-catalytic activity of Ce-TiO2 in the oxidative degradation of VOCs being higher than that of Ag-TiO2 may be explained as follows: Compared to Ag, Ce doping serves as an electron trap in the reaction because of its varied valences and special 4f level. For Ce3+-TiO2, the Ce 4 f level plays an important role in interfacial charge transfer and elimination of electron-hole recombination. So, Ce doping could enhance the electron-hole separation and the decomposition rate of VOCs could be elevated. Moreover, the valence electrons of TiO2 catalyst are excited to the conduction band by UV light, and after various other events, electrons on the TiO2 particle surface are scavenged by the molecular oxygen to produce reactive oxygen radicals. Furthermore, redox reactions between the pollutant molecules and reactive oxygen radicals happened, VOC molecules were turned into harmless inorganic compounds, such as CO2 and H2O at the end.

## **3.3. Effect of Hydrogen Peroxide**

348 Advanced Aspects of Spectroscopy

(Ce-TiO2, 5wt%)

acetone toluene p-xylene

012345678 Irradiation time(h)

Fig. 8 illustrated the effect of doped Ag/Ce/TiO2 on decomposition of HCHO. The conditions were as follows: flow-rate of 3 L/min, initial concentration of 0.1 mg/m3, relative humidity of 35%. It was found that conversions of HCHO in the TiO2/UV, Ag-TiO2/UV and Ce-TiO2/UV processes were increased with irradiation time. It could be seen that the doping of silver or cerium ions could improve the photo-activity of TiO2 effectively. Furthermore, the degradation character of the photo-catalyst was in the order Ce-TiO2 > Ag-TiO2 > TiO2.

> 02468 Irridiation time(h)

TiO2 Ag/TiO2 Ce/TiO2

Catalyst TiO2 Ag-TiO2 Ce-TiO2 *η* (acetone, %) 46.5 55.5 82.0 *η*(toluene, %) 43.2 46.4 76.2 *η* (*p*-xylene, %) 29.8 31.2 77.8

**Figure 7.** Effect of doped Ag/Ce/TiO2 on decomposition of ATP.

*η* (%)

**Table 1.** ATP degradation rates for different catalysts after 8 hrs.

**Figure 8.** Effect of doped Ag/Ce/TiO2 on decomposition of HCHO.

0

20

40

*η* (%) 60

80

100

Hydrogen peroxide is considered to have two functions in the photo-catalytic degradation. It accepts a photo-generated conduction band electron, thus promoting the charge separation, and it also forms OH•. The addition of H2O2 increases the concentration of OH• radicals since it inhibits the electron-hole recombination.

Experiments were conducted to evaluate the effect of H2O2 on the toluene/p-xylene photodegradation. The conditions were as follows: flow rate of 1 L/min, initial concentration of 0.1 mol/m3, relative humidity of 35%, and photo-catalyst of pure TiO2. As shown in Fig. 9, the removal efficiency of toluene or p-xylene increased with reaction time.

In the first 3 h, the degradation rate of toluene or p-xylene without H2O2 was higher because of the competitive adsorption between toluene or p-xylene molecules and hydrogen peroxide. Then, more reactants and/or radical molecules were produced during the photochemistry course, which led to the improvement of toluene or p-xylene decomposition. The final degradation rates of toluene and p-xylene with H2O2 were up to 97.1 and 95.4% after 8 h, respectively.

The degradation of acetone was studied with and without hydrogen peroxide (Fig. 10). Overall, the acetone removal efficiency increased with reaction time. Initially, the degradation rate of without H2O2 was higher than that of with H2O2 because of competitive adsorption between acetone and hydrogen peroxide after hydrogen peroxide addition to the

(*P*-xylene)

**Figure 9.** Effect of toluene/p-xylene degradation on hydrogen peroxide.

sample chamber (10 mL per 30 min, 30 % H2O2, RH 35 %). As the reactants and/or byproducts accumulated on the catalyst, and there was no new super-oxidation supplied, the catalyst deactivated and the degradation rate increased slowly after 2 hr. Hydroxyl radicals were produced due to the presence of hydrogen peroxide (Eq. 3). This decreased recombination of electron-hole pairs, and consequently the final acetone degradation rate was up to 91.8 % after 8 hr. Consumption of hydroxyl radical likely played an important role in deactivation of the catalyst. An appropriate volume of hydrogen peroxide could enhance the degradation rate, while too much could decrease the degradation rate (Eq. 4).

Photo-Catalytic Degradation of Volatile Organic Compounds (VOCs) over Titanium Dioxide Thin Film 351

$$\text{HO} \succ \models \text{\color{red}{\ast\ast\ast\text{ OH}}} \text{OH} \mathsf{+OH}^{\ast} \tag{3}$$

$$\text{HxO} \star \text{OH} \rightarrow \text{HxO} \star \text{HO} \text{:}\tag{4}$$

**Figure 10.** Effect of acetone degradation on hydrogen peroxide.

During deactivation the catalyst color in process without H2O2 changed from light white to khaki, while with H2O2 was light khaki. This suggests catalyst deactivation in process without H2O2 was more extensive than in with H2O2. Moreover, the change in catalyst color after the reaction indicates that the reaction occurred in the surface of the catalyst. After sintering at 390 °C for 1 h the catalyst recovered its original color. This again suggests that catalyst deactivation was due to the accumulation of reactants and by-products on the catalyst surface, which impeded degradation reactions. To compare the performance of H2O2/UV, we evaluated the potential of acetone degradation by H2O2 alone. The concentration of acetone remained almost the same over 8 h. Consequently, we concluded that H2O2 alone could not remove the VOCs.

#### **3.4. Effect of initial concentration**

350 Advanced Aspects of Spectroscopy

*η* (%)

*η* (%)

(Toluene)

H2O2 none

012345678 Irradiation time(h)

(*P*-xylene)

012345678 Irradiation time(h)

sample chamber (10 mL per 30 min, 30 % H2O2, RH 35 %). As the reactants and/or byproducts accumulated on the catalyst, and there was no new super-oxidation supplied, the catalyst deactivated and the degradation rate increased slowly after 2 hr. Hydroxyl radicals were produced due to the presence of hydrogen peroxide (Eq. 3). This decreased recombination of electron-hole pairs, and consequently the final acetone degradation rate was up to 91.8 % after 8 hr. Consumption of hydroxyl radical likely played an important role in deactivation of the catalyst. An appropriate volume of hydrogen peroxide could enhance the degradation rate, while too much could decrease

**Figure 9.** Effect of toluene/p-xylene degradation on hydrogen peroxide.

H2O2 none

the degradation rate (Eq. 4).

In order to discuss the effect of VOCs initial concentration on photo-catalytic degradation rates, we investigated the removal efficiency of ATP and HCHO under different initial concentrations. The ATP concentrations in the experiment ranged between 0.05-0.3 mol/m3. The conditions were as follows: gas flow-rate of 1 L/min, relative humidity of 35%, Cedoped TiO2 as photo-catalyst, and irradiation time of 8 hr. The results showed that the photo-catalytic degradation rates decreased with increasing ATP initial concentration, just illustrated in Fig. 11. Based on the Langmuir-Hinshelwood equation, the degradation rate decreased with increasing initial concentration while the absolute amount of degraded pollutants may increase. At higher initial concentration, the UV light might be absorbed by gaseous pollutants rather than the TiO2 particles, which led to the reduction of the photodegradation efficiency. Moreover, at different initial concentrations, acetone was easiest to be destructed, while p-xylene was difficult to be removed among ATP from gas flow.

**Figure 11.** Effect of ATP initial concentration on the photo-catalysis of ATP by TiO2.

As a main indoor pollutant, the indoor formaldehyde concentration is usually below 0.5 ppmv. It is worth discussing whether the low level of indoor HCHO can be decreased to a value below 0.1 mg/m3 (specified in the indoor air quality standard of China). So in our experiment, the HCHO concentrations in the experiment ranged between 0.1-0.5 mg/m3. The conditions were as follows: relative humidity of 35%, Ce-doped TiO2 as photo-catalyst, and irradiation time of 120min. The results showed that the photo-catalytic degradation rates decreased with increasing HCHO initial concentration, just illustrated in Fig. 12.

**Figure 12.** Effect of initial HCHO concentration on HCHO degradation by TiO2.

In gas-phase photo-catalyst, collision frequency between radicals and HCHO affected the removal efficiency. When formaldehyde molecule reaches to the catalyst surface, the photo oxidation will occur. At higher initial concentration, the UV light might be absorbed by gaseous pollutants rather than the TiO2 particles, which led to the reduction of the photodegradation efficiency.
