**3.5. Effect of UV Light Wavelength**

352 Advanced Aspects of Spectroscopy

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.

*η* (%)

decreased with increasing HCHO initial concentration, just illustrated in Fig. 12.

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

*η* (%)

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

> 0.1 0.2 0.3 0.4 0.5 Cin (mg/m3 )

0.05 0.1 0.15 0.2 0.25 0.3 Initial concentration (mol/m3

)

Acetone Toluene P-xylene In order to investigate the influence of the UV intensity on the photo-catalytic efficiency, the experiments were performed using two lamp configurations (254 and 365 nm). The effect of UV light wavelength on the efficiency of HCHO degradation is shown in Fig. 13. Just shown in Fig.13, 254 nm UV light provided more effective HCHO photo-degradation than 365 nm UV light.

**Figure 13.** Effect of UV light wavelength on HCHO degradation.

The effect of UV light wavelength on the efficiency of ATP degradation is shown in Fig.14. 254 nm UV light provided more effective ATP photodegradation than 365 nm UV light. Degradation of ATP in the UV/TiO2 process followed the same trend.

**Figure 14.** Effect of UV wavelength on degradation of acetone, toluene, and p-xylene by TiO2/UV processes.

The different results obtained with 254 and 365 nm UV lamps were mainly due to the stronger UV irradiation from the 254 nm lamp (about 58 W/m2 on its surface) than that from the 365 nm lamp (30 W/m2 on its surface). This illustrates that the 254 nm UV lamp irradiated photons with higher energy, which led to more efficient degradation with TiO2/UV.

#### **3.6. Effect of gas flow rate**

354 Advanced Aspects of Spectroscopy

> > 0

10

20

30

*η*(%)

40

254nm 365nm

50

*η*(%)

*η* (%) 254nm 365nm

254nm 365nm

processes.

(a) Acetone

2468 Irridiation time(h)

(b) Toluene

2468 Irridiation time(h)

(c) P-xylene

2468 Irradiation time(h)

**Figure 14.** Effect of UV wavelength on degradation of acetone, toluene, and p-xylene by TiO2/UV

The effect of gas flow rate on ATP degradation was studied at an initial concentration of 0.1 mol/m3 and relative humidity of 35 %, just as shown in Fig. 15. When the flow rate was increased from 3–9 L/min, degradation of toluene and acetone decreased. With a flow rate >3 L/min the reactants have shorter residence time on the photocatalyst surface and consequently do not bind to the active sites. In general, an increase in gas flow rate results in two antagonistic effects. These are a decrease in residence time within the photocatalytic reactor, and an increase in the mass transfer rate. In our opinion, the decrease in degradation with increasing gas flow rate showed that the residence time of pollutant molecules with TiO2 is an important factor. However, the degradation rate at 1 L/min was the lowest. This was due to adsorption of active species on the catalyst, which led to a decrease in the reaction between pollutant molecules and active species. For p-xylene, the degradation rate was the highest when the flow rate was 7 L/min. From these results it can be concluded that gas flow rate remarkably influences the degradation rate. While both toluene and p-xylene are aromatic hydrocarbons, toluene is an unsymmetrical molecule and p-xylene is symmetrical. Consequently, the adsorption and degradation of toluene were greater than for p-xylene under the same flow rate. The highest degradation rates for acetone, toluene, and p-xylene were 77.7, 61.9, and 55 %, respectively.

(a) Acetone

(c) P-xylene

**Figure 15.** Effect of flow rate on the degradation of acetone, toluene, and p-xylene by TiO2/UV processes.

The Langmuir–Hinshelwood (L–H) rate expression has been widely used to describe the gas–solid phase reaction for heterogeneous photocatalysis. Assuming that mass transfer is not the limiting step, and that the effect of intermediate products is negligible, then the reaction rate in a plug-flow reactor can be expressed as:

$$r = -\frac{d\mathbb{C}\_{VOC}}{dt} = \frac{k\text{KC}}{1 + \text{KC}}\tag{5}$$

where *k* and *K* are the L–H reaction rate constant and the L–H adsorption equilibrium constant, respectively; and t is the time taken for ATP molecules to pass through the reactor. After integration of Equation (5) the following linear expression can be obtained:

356 Advanced Aspects of Spectroscopy

0

0

10

20

30

η(%)

40

50

60

10

20

30

40

η(%)

50

60

1L/min 3L/min 5L/min 7L/min 9L/min

> 1L/min 3L/min 5L/min 7L/min 9L/min

70

processes.

(b) Toluene

12345678 Irradiation time(h)

(c) P-xylene

12345678 Irradiation time(h)

The Langmuir–Hinshelwood (L–H) rate expression has been widely used to describe the gas–solid phase reaction for heterogeneous photocatalysis. Assuming that mass transfer is not the limiting step, and that the effect of intermediate products is negligible, then the

> 1 *VOC dC kKC*

*dt KC* (5)

**Figure 15.** Effect of flow rate on the degradation of acetone, toluene, and p-xylene by TiO2/UV

reaction rate in a plug-flow reactor can be expressed as:

*r*

$$\frac{\ln\left(\mathbf{C}\_{in} \,/\,\mathbf{C}\_{out}\right)}{\left(\mathbf{C}\_{in} - \mathbf{C}\_{out}\right)} = \frac{kKT}{\left(\mathbf{C}\_{in} - \mathbf{C}\_{out}\right)} - K \tag{6}$$

where *Cin* and *Cout* are the inlet and outlet concentrations of ATP, respectively; and *T* is the recurrent time of VOCs in the reactor.

If the L–H model is valid, a plot of *ln(Cin/Cout)*/*(Cin-Cout)* versus 1/*(Cin-Cout)* should be linear. This was the case with our data (Fig. 16), and the linearity correlation coefficients of acetone, toluene and p-xylene were 0.9989, 0.9995 and 0.9992, respectively. This result suggests that the reaction occurs on the photocatalyst surface through an L–H mechanism and not in the gas phase.

(c) P-xylene

**Figure 16.** Plot of ln(Cin/Cout)/(Cin-Cout) and 1/(Cin-Cout).

#### **3.7. Degradation of Pure Individual VOCs and Their Mixture**

Gaseous-phase photo-degradation for pure individual VOCs (acetone, toluene, and pxylene) and their mixture was carried out in the continuous flow reactor system. The gas stream passed through the reactor at a flow rate of 5 L/min and contained 0.1 mol/m3 pure acetone, toluene, or p-xylene, or 0.3 mol/m3 of their mixture. The gas residence time was 72 s in the reactor. The experiment was run for 8 hr, and samples were collected at hourly intervals.

Both acetone and p-xylene in the mixed gas degraded at much lower rates than their pure individual gases under the same conditions, just as shown in Fig. 17. However, the opposite trend was observed for toluene. Toluene has an unsymmetrical structure, which leads to instability and promotes adsorption and degradation of pollutant molecules on the catalyst surface according to the L-H mechanism. In addition, the byproducts of acetone and pxylene produced in the reaction could promote toluene degradation. In contrast, degradation of acetone and p-xylene in the mixed gas was reduced by competitive adsorption and catalysis of toluene. Among the pure gases and the mixture, acetone had the highest degradation efficiency. Furthermore, the efficiency of pure toluene degradation was lower than that of pure p-xylene degradation due to structural stability.

#### **3.8. Effect of gas temperature**

The effect of gas temperature on photo-catalytic degradation of gaseous toluene was investigated in the range of 25-50 °C (Fig. 18). The conditions were as follows: gas flow-rate of 1 L/min, relative humidity of 35%, irradiation time of 8 h, photo-catalyst of Ce-doped TiO2, and initial concentration of 0.1 mol/m3. Degradation efficiency of toluene gradually

intervals.

**3.8. Effect of gas temperature** 

(c) P-xylene

40 60 80 100 120 1/(*Ci* -*Co* )

y = 0.7013x - 19.667 R = 0.9992

Gaseous-phase photo-degradation for pure individual VOCs (acetone, toluene, and pxylene) and their mixture was carried out in the continuous flow reactor system. The gas stream passed through the reactor at a flow rate of 5 L/min and contained 0.1 mol/m3 pure acetone, toluene, or p-xylene, or 0.3 mol/m3 of their mixture. The gas residence time was 72 s in the reactor. The experiment was run for 8 hr, and samples were collected at hourly

Both acetone and p-xylene in the mixed gas degraded at much lower rates than their pure individual gases under the same conditions, just as shown in Fig. 17. However, the opposite trend was observed for toluene. Toluene has an unsymmetrical structure, which leads to instability and promotes adsorption and degradation of pollutant molecules on the catalyst surface according to the L-H mechanism. In addition, the byproducts of acetone and pxylene produced in the reaction could promote toluene degradation. In contrast, degradation of acetone and p-xylene in the mixed gas was reduced by competitive adsorption and catalysis of toluene. Among the pure gases and the mixture, acetone had the highest degradation efficiency. Furthermore, the efficiency of pure toluene degradation was

The effect of gas temperature on photo-catalytic degradation of gaseous toluene was investigated in the range of 25-50 °C (Fig. 18). The conditions were as follows: gas flow-rate of 1 L/min, relative humidity of 35%, irradiation time of 8 h, photo-catalyst of Ce-doped TiO2, and initial concentration of 0.1 mol/m3. Degradation efficiency of toluene gradually

**Figure 16.** Plot of ln(Cin/Cout)/(Cin-Cout) and 1/(Cin-Cout).

0

10

20

ln(*Ci* /*Co* )/(*Ci* -*Co* )

30

40

50

60

70

**3.7. Degradation of Pure Individual VOCs and Their Mixture** 

p-xylene

lower than that of pure p-xylene degradation due to structural stability.

**Figure 17.** Degradation with H2O2 of pure acetone, toluene, p-xylene, and their mixture.

increased when gas temperature was below 45 °C, but decreased at >45 °C. The increase in temperature would lead to the production of free radicals that could effectively collide with toluene molecules. Moreover, higher temperature may increase the oxidation rate of toluene at the interface. However, with increasing temperature, the adsorptive capacities of toluene on catalyst decreased, which led to the reduction of toluene removal efficiency.

**Figure 18.** Effect of gas temperature on the photo-catalysis of toluene.

#### **3.9. Effect of photo-catalyst amount**

In photo-catalytic degradation of organic compounds, the optimal TiO2 concentration depends mainly on both the nature of the compounds and the reactor geometry. In this work, the influence of TiO2 amount on HCHO photo degradation was investigated. A set of gaseous experiments with different amount of TiO2 from 0 to 100mg was carried out at the RH of 35% and the initial HCHO concentration of 0.1mg/m3. The degradation rates of HCHO for different amount TiO2 were presented in Fig. 19. The photo-catalytic degradation efficiency increased with increasing the amount of TiO2 when TiO2 amount was lower than 70mg. When the TiO2 amount was more than 70mg, the photo-catalytic degradation efficiency was decreased. So 70mg of TiO2 amount was the optional amount in our experiment. And the thickness of 70mg of TiO2 amount was about 0.2mm.

**Figure 19.** Effect of TiO2 amount on HCHO degradation

At the same time, in our investigation, the effect of photo-catalyst concentration on the degradation of acetone in the gas flow was also analyzed in order to optimize the amount of TiO2. Different concentrations (15-105 mg/L) of TiO2 precursor sols were prepared by using different amounts of tetrabutyl orthotitanate. The conditions of the experiment were as follows: gas flow-rate of 1 L/min, relative humidity of 35%, Ce-doped TiO2 as photo-catalyst, and irradiation time of 8 h. BET surface area of the synthesized samples was tested (see Table 2). The results showed that BET surface area increased with increasing photo-catalyst amount.


**Table 2.** BET surface area for synthesized photo-catalyst.

360 Advanced Aspects of Spectroscopy

increased when gas temperature was below 45 °C, but decreased at >45 °C. The increase in temperature would lead to the production of free radicals that could effectively collide with toluene molecules. Moreover, higher temperature may increase the oxidation rate of toluene at the interface. However, with increasing temperature, the adsorptive capacities of toluene

In photo-catalytic degradation of organic compounds, the optimal TiO2 concentration depends mainly on both the nature of the compounds and the reactor geometry. In this work, the influence of TiO2 amount on HCHO photo degradation was investigated. A set of gaseous experiments with different amount of TiO2 from 0 to 100mg was carried out at the RH of 35% and the initial HCHO concentration of 0.1mg/m3. The degradation rates of HCHO for different amount TiO2 were presented in Fig. 19. The photo-catalytic degradation efficiency increased with increasing the amount of TiO2 when TiO2 amount was lower than 70mg. When the TiO2 amount was more than 70mg, the photo-catalytic degradation efficiency was decreased. So 70mg of TiO2 amount was the optional amount in our

> 0 20 40 60 80 100 TiO2 (mg)

20 25 30 35 40 45 50 55 Temperature(℃)

on catalyst decreased, which led to the reduction of toluene removal efficiency.

**Figure 18.** Effect of gas temperature on the photo-catalysis of toluene.

experiment. And the thickness of 70mg of TiO2 amount was about 0.2mm.

**3.9. Effect of photo-catalyst amount** 

60

70

80

*η* (%) 90

100

**Figure 19.** Effect of TiO2 amount on HCHO degradation

0

20

40

*η* (%) 60

80

100

Fig. 20 showed that the photo-catalytic degradation efficiency increased with increasing the amount of TiO2. It was suggested that increasing efficiency was due to the increase of the surface area. It could be observed that the degradation efficiency increased with increasing the amount of the catalyst until it reached a plateau at 90-105 mg/L of TiO2. This indicated that when the amount of TiO2 was overdosed, the surface area was saturated, and then the intensity of UV was attenuated because of decreased light penetration and increased light scattering.

**Figure 20.** Effect of TiO2 amount on the photo-catalysis of acetone.

#### **3.10. Effect of relative humidity of air stream**

The effect of relative humidity (0-60% RH) of air stream on HCHO decomposition was examined by adding water vapor to a fixed concentration of HCHO. TiO2 photocatalyst was used in this experiment. Fig. 21 showed the experimental results at different relative humidity. The degradation rate increased with increasing relative humidity up to 35% and then started to decrease, which meant that 35% was the optimal humidity for photo-catalyst process under the experimental conditions. When the reaction time was 120min, the highest removal efficiency of HCHO was 60.2% when RH was 35%.

**Figure 21.** Effect of RH on decomposition of HCHO.

The enhancement of photo-catalytic reaction rate is frequently found in the presence of water vapor because hydroxyl groups or water molecules can behave as hole traps to form surface-adsorbed hydroxyl radicals. In photo-catalyst process, the hydroxyl radicals formed on the illuminated TiO2 can not only directly attack HCHO molecules, but also suppress the electron-hole recombination. However, higher RH can be attributed to the competition for adsorption between HCHO and hydroxyl radicals, thus decrease the removal efficiency of HCHO.

#### **3.11. Effect of oxygen concentration**

The effect of oxygen concentration on HCHO degradation was presented in Fig. 22.

**Figure 22.** Effect of oxygen concentration on HCHO degradation.

The results corresponded to the initial concentration of 0.1mg/m3, relative humidity of 35% and reaction time of 120min. It is obvious that oxidation rates for HCHO increased with increasing O2 concentration under fixed conditions. As mentioned above, hydroxyl radical is an important factor to the HCHO photo-catalyst. At the same time, oxygen radical is also key factor for HCHO removal, which can react with HCHO on the TiO2 surface and turn HCHO into CO2 and H2O.

#### **3.12. Mechanism of photo-catalytic degradation of VOCs**

362 Advanced Aspects of Spectroscopy

HCHO.

process under the experimental conditions. When the reaction time was 120min, the highest

The enhancement of photo-catalytic reaction rate is frequently found in the presence of water vapor because hydroxyl groups or water molecules can behave as hole traps to form surface-adsorbed hydroxyl radicals. In photo-catalyst process, the hydroxyl radicals formed on the illuminated TiO2 can not only directly attack HCHO molecules, but also suppress the electron-hole recombination. However, higher RH can be attributed to the competition for adsorption between HCHO and hydroxyl radicals, thus decrease the removal efficiency of

10 20 30 40 50 60 relative humidity (%)

The effect of oxygen concentration on HCHO degradation was presented in Fig. 22.

0 5 10 15 20 V/V (O2/Air,%)

removal efficiency of HCHO was 60.2% when RH was 35%.

**Figure 21.** Effect of RH on decomposition of HCHO.

50

52

54

56

*η* (%) 58

60

62

**3.11. Effect of oxygen concentration** 

**Figure 22.** Effect of oxygen concentration on HCHO degradation.

*η* (%) The heterogeneous photo-catalytic process used in pollutant degradation involved the adsorption of pollutants on the surface sites, and the chemical reactions of converting pollutants into carbon dioxide and water. Activation of TiO2 is achieved through the absorption of a photon (hv) with ultra-band energy from UV irradiation source. This results in the promotion of an electron (e−) from the valence band to the conduction band, with the generation of highly reactive positive holes (h+) in the valence band. This caused aggressive oxidation of the surface-adsorbed toxic organic pollutants and converts them into CO2 and water.

In the degradation of toluene or p-xylene, the OH• radicals attack the phenyl ring of toluene or p-xylene, and some products, such as phenol, benzaldehyde or benzoic acid, may be produced during the reaction, and they were converted into CO2 and H2O at the end (Fig. 23). We could also observe that acetone was easily destructed to CO2 and H2O by photocatalysts. By-products of toluene or p-xylene were detected by GC-MS, and involved phenol, benzaldehyde, aldehydes, alcohols, etc.

**Figure 23.** Suggested pathway for the photo-catalytic destruction of ATP.

The reaction rate constant (*k*) was chosen as the basic kinetic parameter for ATP since it was important in determination of VOCs photo-catalytic activity. The first order kinetic equation:

$$\ln\left(\frac{C\_i}{C\_o}\right) = k \times t + b \tag{7}$$

was used to fit experimental data in Fig. 24

where *Co* is the concentration of ATP remaining in the solution at t, and *Ci* is the initial concentration at t = 0.

The variations in ln(Ci/Co) as a function of irradiation time are given in Fig. 25. Reaction rate constant (k), linearity correlation coefficient (R) and intercept (b) data for the photo-catalytic destruction of ATP are exhibited in Table 3. The k of ATP could be ordered as follows: kAcetone>kToluene>kP-xylene, meaning that the decomposition capability of acetone was the best. The reason was probably due to molecular structure and molecular weight.

**Figure 25.** Relation between ln(Ci/Co) and irradiation time, and linear fits for ATP.


**Table 3.** Values of k, R and b for the photo-catalytic destruction of ATP.

concentration at t = 0.

was used to fit experimental data in Fig. 24

**Figure 24.** Kinetics of ATP degradation.

0.2

0.3

0.4

0.5

0.6

*Co/Ci*

0.7

0.8

0.9

1

ln *<sup>i</sup> o*

*C* 

*<sup>C</sup> ktb*

where *Co* is the concentration of ATP remaining in the solution at t, and *Ci* is the initial

The variations in ln(Ci/Co) as a function of irradiation time are given in Fig. 25. Reaction rate constant (k), linearity correlation coefficient (R) and intercept (b) data for the photo-catalytic destruction of ATP are exhibited in Table 3. The k of ATP could be ordered as follows: kAcetone>kToluene>kP-xylene, meaning that the decomposition capability of acetone was the best.

012345678 Irradiation time(h)

The reason was probably due to molecular structure and molecular weight.

(7)

Toluene P-xylene Acetone

> During the HCHO decomposition by photo-catalytic processing, formic acid was identified as the intermediate from the photo-degradation of formaldehyde. In our experiment, ion chromatography (IC) was used to determine the byproducts by sampling the gas products into distilled water. The result in this study showed that formic acid was also found. The probably pathway of HCHO destruction was shown in Fig. 26. The related reactions of HCHO destruction were shown with equations (8)-(21).

$$\begin{array}{c} \mathsf{c}\mathsf{c}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e} \mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e} \mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e} \mathsf{e} \mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e} \mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e}\mathsf{e} \mathsf{e} \mathsf{e} \mathsf{e} \mathsf{e}\mathsf{e}$$

X : TiO2, Ag/TiO2 or Ce/TiO2

**Figure 26.** Suggested pathway for the photo-catalytic destruction of HCHO

$$\text{CHCHO} \uparrow \text{ } \text{c} \begin{array}{c} \longrightarrow \text{H} \bullet \text{ } \text{CHO} \text{} \\ \end{array} \tag{8}$$

HCHO <sup>O</sup> OH CHO (9)

HCHO OH H2O CHO (10)

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

$$\text{HCHO} \star \text{H}\bullet \xrightarrow{\text{H}\_2\star} \text{H}\_2\star \text{CHO}\bullet \tag{12}$$

$$\text{CHO} \bullet \text{H} \bullet \xrightarrow{\text{H}\_{2}} \text{H}\_{2} \bullet \text{CO} \tag{13}$$

$$\bullet \text{CHO} \bullet \bullet \text{O}\_2 \xrightarrow{\bullet \text{-} \text{CO}\_2 \bullet \text{OH} \bullet} \tag{14}$$

$$\bullet \text{CHO} \bullet \bullet \text{OH} \bullet \xrightarrow{\bullet \text{-} \rightarrow \text{H}\_2\text{O} \bullet \text{CO}} \tag{15}$$

$$\text{CHO}\bullet\text{HO}\_2\bullet\xrightarrow{\text{H}\_2\text{O}\bullet\text{H}\bullet\text{H}\bullet\text{+CO}\_2}\tag{16}$$

$$\bullet \text{CHO} \bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \tag{17}$$

$$\bullet \text{CHO} \bullet \bullet \bullet \longrightarrow \bullet \bullet \star \text{CO}\_2 \tag{18}$$

$$\text{HCOOH} \bullet \text{OH} \bullet \xrightarrow{\text{H}\_2\text{O} \bullet \text{H} \bullet \text{CO}\_2} \tag{19}$$

$$\mathbf{\color{red}{CO}\bullet\color{red}{0}\bullet\color{red}{S}\bullet\color{red}{O}}\tag{20}$$

$$\text{CO} + \text{OH} \bullet \xrightarrow{\text{H}} \text{CO}\_2 + \text{H} \bullet \tag{21}$$

As mentioned in equation 7, k was the basic kinetic parameter for VOCs photo-catalytic activity. Fig. 27 showed the first order kinetic equation fitting the experimental data.

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

The variations in ln(Ci/Co) as a function of reaction time were given in Fig. 28. The reaction rate constant for TiO2、Ag/TiO2、Ce/TiO2 were 0.1871、0.2302、0.2724, respectively, which meant that Ce/TiO2 had the best photo-catalytic abilities among the catalysts.

**Figure 28.** Relationship between ln(Ci/Co) and reaction time.
