**4. Conclusion**

366 Advanced Aspects of Spectroscopy

**Figure 27.** Kinetics of HCHO degradation.

0

0.1

0.2

0.3

Co/Ci

0.4

0.5

0.6

HCHO OH HCOOH <sup>H</sup> (11)

HCHO <sup>H</sup> H2 CHO (12)

CHO <sup>H</sup> H2 CO (13)

CHO O2 CO2 OH (14)

CHO OH H2O CO (15)

CHO HO2 OH <sup>H</sup> CO2 (16)

CHO <sup>O</sup> OH CO (17)

CHO <sup>O</sup> H CO2 (18)

HCOOH OH H2O <sup>H</sup> CO2 (19)

CO <sup>O</sup> CO2 (20)

CO OH CO2 <sup>H</sup> (21)

TiO2 Ag/TiO2 Ce/TiO2

As mentioned in equation 7, k was the basic kinetic parameter for VOCs photo-catalytic

12345678 Irradiation time (h)

activity. Fig. 27 showed the first order kinetic equation fitting the experimental data.

In this chapter, nano-structured TiO2, Ag-TiO2 and Ce-TiO2 thin films coated on glass springs were prepared by sol-gel method at room temperature. Toluene, p-xylene, acetone and formaldehyde were chosen as the model VOCs, the photo-catalytic degradation characters of them by TiO2/UV, TiO2/doped Ag/UV and TiO2/doped Ce/UV was tested and compared. The effects of doped Ag/Ce ions, hydrogen peroxide, initial concentration, gas temperature, relative humidity of air stream, oxygen concentration, gas flow rate, UV light wavelength and photo-catalyst amount on decomposition of the pollutants by TiO2/UV were analyzed simultaneously. Furthermore, the mechanism of titania-assisted photo-catalytic degradation was analyzed, and the end product of the reaction using GC-MS analysis was also performed.

Results were as follows: (1) Characterization of this film by SEM and XRD showed it consisted of nanoparticles, and the crystalline phase was anatase. (2) Doped Ag or Ce ions could enhance the photo-catalyst ability. The degradation character of the photo-catalyst was in the order Ce-TiO2>Ag-TiO2>TiO2. (3) Hydrogen peroxide could promote the activation of catalyst, and toluene & p-xylene degradation rate with hydrogen peroxide was higher than that without it. The final degradation rates of toluene and p-xylene using H2O2 were up to 97.1 and 95.4% after 8 h, respectively. (4) The photo-catalytic degradation rates decreased with increasing VOCs initial concentration. Acetone was easiest to be destructed, while p-xylene was difficult to remove from gas flow. (5) The degradation efficiency gradually increased with gas temperature and 45 °C had the best removal efficiency. (6) 35% was the optimal humidity for photo-catalyst process under the experimental conditions. (7) Higher concentration of oxygen was better for HCHO removal. (8) The flow rate greatly influenced the degradation rate. For acetone and toluene, the degradation rate was highest with a flow rate of 3 L/min. For p-xylene, the degradation rate was highest when the flow rate was 7 L/min. The highest degradation rates for acetone, toluene and p-xylene were 77.7 %, 61.9 % and 55 %, respectively. (9) Illumination using a 254 nm light source was better than 365 nm. (10) The photo-catalytic degradation efficiency increased with increasing the amount of TiO2 when TiO2 amount was lower than 70mg. (11) In the gas mixture, acetone and p-xylene had much lower degradation rates than for their pure counterparts. The opposite trend was observed for toluene. Among acetone, toluene and p-xylene, the removal efficiency of acetone was highest both when pure and as a part of the gas mixture. (12) The photo-catalytic process used in pollutant degradation involved the adsorption of pollutants on the surface sites, and chemical reactions of converting pollutant into CO2 and H2O at the end. By-products of toluene or p-xylene were detected by GC-MS analysis, and involved phenol, benzaldehyde, aldehydes, alcohols, etc. The reaction rate constant (k) of ATP was sequenced kAcetone>kToluene>kP-xylene, meaning that the decomposition capability of acetone was the best, probably due to molecular structure and molecular weight. Formic acid was the main byproduct during the decomposition of HCHO. The reaction rate constant (k) of TiO2、Ag/TiO2、Ce/TiO2 was sequenced kCe/TiO2>kAg/TiO2>kTiO2, meaning that Ce/TiO2 had the best photo-catalytic abilities among the catalysts.

## **Author details**

Wenjun Liang, Jian Li and Hong He *College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China* 
