**2. Materials and methods**

#### **2.1. Chemicals and experimental set-up**

Acetone, toluene, p-xylene and formaldehyde used in our experiment was analytical reagent. The TiO2 photocatalyst was prepared with 100 % anatase using the sol-gel method, and immobilized as a film (thickness 0.2 mm) on glass springs. Ethanol, tetrabutyl orthotitanate, diethanolamine, N,N-dimethylformamide and polyethylene glycol used as raw materials for photocatalyst preparation were of analytical grade and utilized without further purification. AgNO3, Ce(NO3)3•6H2O were used as Ag or Ce source for modified TiO2 samples. Deionized water was used throughout the study.

342 Advanced Aspects of Spectroscopy

doping agents has not been discussed yet.

**Figure 1.** Decomposition path of VOCs with UV/TiO2.

reaction using GC-MS analysis was also performed.

**2.1. Chemicals and experimental set-up** 

**2. Materials and methods** 

Photocatalytic degradation of VOCs on UV-illuminated titanium dioxide (TiO2) is proposed as an alternative advanced oxidation process for the purification of water and air. Heterogeneous photo-catalysis using TiO2 has several attractions: TiO2 is relatively inexpensive, it dispenses with the use of other coadjutant reagents, it shows efficient destruction of toxic contaminants, it operates at ambient temperature and pressure, and the reaction products are usually CO2 and H2O, or HCl, in the case of chlorinated organic compounds. Decomposition path of VOCs with UV/TiO2 or UV/TiO2/doped ions is shown in Fig. 1. However, the formations of by-products, such as CO, carbonic acid and coke-like substances, were often observed. These by-product formations are due to low degradation rate of intermediate compounds that are formed by the partial oxidation of VOCs. In order to improve the VOC degradation rate, some authors reported on the enhancement of VOC degradation through the addition of anions (dopant = S, N, P, etc), cations (dopant = Pt, Cu, Mg, etc), polymers or co-doped with several ions on TiO2, while the difference between

In this chapter, toluene, p-xylene, acetone and formaldehyde were chosen as the model VOCs because they were regarded as representative indoor VOCs for determining the effectiveness and capacity of gas-phase air filtration equipment for indoor air applications, 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 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

Acetone, toluene, p-xylene and formaldehyde used in our experiment was analytical reagent. The TiO2 photocatalyst was prepared with 100 % anatase using the sol-gel method, A schematic diagram of the experimental system for photo-oxidation is shown in Fig. 2. The experiments were performed in a cylindrical photo-catalytic reactor with inner diameter 18.0 mm. A germicidal lamp (wavelength range 200-300 nm) with the maximum light intensity at 254 nm was installed in the open central region. The desired amount of representative sample, that is acetone, toluene, p-xylene or formaldehyde, was injected into the obturator. Then, the photo-catalytic degradation was performed by transporting the gas across the photo-catalyst continuously when UV lamp was turned on. Glass spring coated by a TiO2 thin film was filled around the lamp. In whole experiment, humidity was controlled and adjusted with vapour. In some experiments it was replaced with a 15 W black-light lamp with a maximum light intensity output at 365 nm. After a stabilized period of about 3 h, the pollutant concentrations in the outlet gas became the same as in the inlet gas, and the experiment was started by turning on the UV lamp. Relative humidity of the reactor was detected with humidity meter. Oxygen concentration was controlled with oxygen detector.

**Figure 2.** Schematic diagram of experimental set-up. 1- Minitype circulation pump; 2 - Germicidal lamp; 3 - Obturator (airproof tank, 125 L); 4 - Lacunaris clapboard; 5, 6 - Sampling spots; 7-10 – Inlet & Outlet; 11 - Temperature-humidity detector; 12 - Probe; 13 - Gas heated container; 14 - Humidity controller.

#### **2.2. Photo-catalyst preparation**

Fig. 3 shows the schematic flow-chart of the experimental procedure.TiO2 precursor sols were prepared by adding tetrabutylorthotitanate (400 mL) into ethanol (960 mL) at room temperature. Then diethanolamine (69.1 mL) was added, and the mixture stirred for 2 hr. Subsequently, ethanol (120 mL), deionized water (25.2 g), 5 wt% AgNO3 or Ce(NO3)3 were added dropwise to the solution. After stirring for 15 min, N,N-dimethylformamide (16.8 mL) was added. This reduced surface tension and made a smooth coating of the thin film on the carrier. The solution was then left to rest for 24 hr. Finally, polyethylene glycol (4.32 g) dissolved in ethanol (120 mL) at 50 °C was added dropwise to the solution. The final solution was left to sit for 12 hr, after which the TiO2 gel had formed. The prepared mixture could remain stable for months at ambient temperature. Thin film TiO2 photocatalyst was formed by dip-coating with a velocity of 5 cm/s. Glass springs were selected as the photocatalyst carrier due to their excellent transparency and long light diffusion distance. Fig. 4 was the sketch of glass spring structure. These were immersed in the TiO2 gel mixture, and then dried at room temperature. This was followed by calcination at 500 °C in a muffle furnace for 2 hr. The glass springs were coated repeatedly (total of five times) using this method to form a thin TiO2 photocatalyst film. The TiO2 film was very stable and durable, and no loss was observed during its application.

**Figure 3.** Flowchart of photo-catalyst preparation.

**Figure 4.** Sketch of glass spring structure.

## **2.3. Analytical methods**

344 Advanced Aspects of Spectroscopy

**2.2. Photo-catalyst preparation** 

and no loss was observed during its application.

**Figure 3.** Flowchart of photo-catalyst preparation.

Fig. 3 shows the schematic flow-chart of the experimental procedure.TiO2 precursor sols were prepared by adding tetrabutylorthotitanate (400 mL) into ethanol (960 mL) at room temperature. Then diethanolamine (69.1 mL) was added, and the mixture stirred for 2 hr. Subsequently, ethanol (120 mL), deionized water (25.2 g), 5 wt% AgNO3 or Ce(NO3)3 were added dropwise to the solution. After stirring for 15 min, N,N-dimethylformamide (16.8 mL) was added. This reduced surface tension and made a smooth coating of the thin film on the carrier. The solution was then left to rest for 24 hr. Finally, polyethylene glycol (4.32 g) dissolved in ethanol (120 mL) at 50 °C was added dropwise to the solution. The final solution was left to sit for 12 hr, after which the TiO2 gel had formed. The prepared mixture could remain stable for months at ambient temperature. Thin film TiO2 photocatalyst was formed by dip-coating with a velocity of 5 cm/s. Glass springs were selected as the photocatalyst carrier due to their excellent transparency and long light diffusion distance. Fig. 4 was the sketch of glass spring structure. These were immersed in the TiO2 gel mixture, and then dried at room temperature. This was followed by calcination at 500 °C in a muffle furnace for 2 hr. The glass springs were coated repeatedly (total of five times) using this method to form a thin TiO2 photocatalyst film. The TiO2 film was very stable and durable,

**tetrabutyl ethanol diethanolamine deionized water N,N-dimethylformamide** 

**polyethylene glycol**

**Mixing and Stirring**

**Laying for 12 h**

**Dip-coating**

**Drying**

**TiO2 Film**

**C for 2 h**

**Calcining at 500◦**

The concentrations of acetone, toluene, and p-xylene were analyzed by a gas chromatograph (Model GC-14C, Shimadzu, Japan) with a flame ionization detector (FID). The oven temperature was held at 60 °C and detector temperature maintained constant at 100 °C. The end products of the reaction were detected by GC-MS. GC-MS analysis was conducted using an HP 6890N GC and HP 5973i MSD. A HP-5 capillary column (30m×0.32mm ID) was used isothermally at 60 °C. The carrier gas (helium) flow-rate was 30 cm/s, and the injector and detector temperatures were 150°C and 280°C, respectively. Intermediate products analysis was done by EI mode and full scan. Formaldehyde concentration in gas stream was determined by acetylacetone spectrophotometric method. HCHO absorbed by deionized water in acetic acideammonium acetate solution would react with acetylacetone to form a steady yellow compound. HCHO concentration in the gas stream was then determined by measuring light absorbance at 413 nm with a spectrophotometer (UV/Vis 722). Temperature and humidity were measured with a temperature-humidity detector (Model LZB-10WB, Beijing Yijie Automatic Equipment Ltd., China).

The characteristics of the immobilized nano-structured TiO2 thin film were analyzed by field-emission scanning electron microscopy (FE-SEM, Model JSM 6700F, JEOL, Japan) and X-ray diffractometry (XRD, Rigaku, D-max-γA XRD with Cu Kα radiation, λ = 1.54178 Å). The surface area of the TiO2 film was also analyzed using gas adsorption principles (Detected by Micromeritics, American Quantachrome Co., NOVA 1000). The synthesized samples had a BET surface area of 56.3 m2/g, compared with Degussa P25 TiO2 with a surface area of 50.2 m2/g.

The degradation rates (%) of acetone, toluene, p-xylene and formaldehyde were calculated as follows:

$$\eta = \frac{C\_i - C\_o}{C\_i} \times 100\% \tag{1}$$

where *Ci* is the inlet concentration, and *Co* is the outlet concentration at steady state.
