Comparison of N-doped TiO 2 photocatalysis with commercial photocatalysts

The extent of decomposition of toluene vapor in a continuous-flow reactor with a residence time of 2.35 mins was evaluated to explore other application fields such as air cleaners. The comparison results of photocatalytic activity between the N-doped TiO₂ produced in this study and the commercial photocatalysts of P25 and ST01 are shown in Figure 4.11. One can see that the toluene removal efficiency for N-doped TiO₂ gradually increased with the time in the continuous flow reactor, and it then reached a plateau of around 7.5%±0.5%. On the other hand, the toluene removal efficiencies of both ST01 and P25 were less than that of the N-doped TiO₂. Although the toluene

removal efficiency of ST01 was slightly higher than that of P25, the difference in the removal efficiency was within 2%. With the relative reactivity of P25 photocatalyst (0.125 mole/hr), defined by the accumulated amount of toluene removal over 60 mins of operation, as the reference base (relative reactivity = 1), it was calculated that the photoactivity of ST01 photocatalyst was approximately 3.2 times higher than that of P25 photocatalyst. The relative activity for the N-doped photocatalyst prepared in this study, in comparison, was notably greater than both commercial photocatalysts, with reactivity of 11.2 and 3.5 times higher than those of P25 and ST01 photocatalysts, respectively.

Few studies literature data have compared the visible-light induced photocatalytic activity of the N-doped TiO₂ with those of the commercial photocatalysts. Li et al. (2005a, 2005b) prepared the N-doped and N-F-codoped TiO₂ photocatalysts via spray pyrolysis and demonstrated that the photocatalytic activities of N-doped and N-F-codoped TiO₂were about 2 and 7 times, respectively, higher than that of the P25 photocatalyst in terms of the initial removal rate of acetaldehyde using a 150 W Xe lamp visible light source with 420 nm cut filter. Chen et al. (2005) prepared nanocolloid N-doped photocatalysts via a liquid phase method and found that the photocatalytic activity of the nanocolloid N-doped photocatalyst was 7 times higher than that of the P25 photocatalyst in terms of the methylene blue decomposition with a 540 nm visible light source. Our result on comparing the visible light-driven photocatalytic activity of the N-doped photocatalyst and the commercial P25 was qualitatively similar to those by Li et al. (2005b) and Chen et al. (2005) prepared via other

processes. In addition, the N 1s spectra of N-doped TiO₂reported in this study was also very similar to those prepared by Chen et al. (2005), both exhibiting peaks at around 400-402 eV.

The Kubelka-Munk absorption spectra (Francisco et al., 2000) of N-doped TiO₂, ST01 and P25 were shown in Figure 4.12. The UV-visible spectra of N-doped TiO₂, ST01 and P25 were consistent with their toluene removal efficiency. As observed that the N-doped TiO₂ prepared in this study has a clear red shift into the visible light absorption range, with two absorption edges at around 400 and 520 nm. On the other hand, there was only one absorption edge at around 395 and 405 nm, respectively, for the ST01 and the P25 photocatalysts. Also, although the wavelength of the absorption edge of P25 was slightly larger than that of ST01, their removal efficiencies were very similar (<2%) in the visible light range as demonstrated in Figure 4.11. This might be due to that the photocatalytic activity was also influenced by many other factors such as crystalline size, surface and bulk defects, porosity, surface area, impurities and active sites such as Ti³⁺ and OH sites.

4.2.4. Summary

Effective removal of odorous VOCs in both batch and continuous flow photocatalytic reactors under visible and UV light sources were demonstrated as well. The results showed that the N-doped TiO2 photocatalyst was more effective in removing IPA under both UV and visible light sources than the un-doped TiO2photocatalyst. Furthermore, the N-doped TiO2photocatalyst was superior in toluene removal than the commercial ST01 and P25 TiO2 photocatalysts under visible light

irradiation. Considering that UV light accounts for only 3~5 % of the solar light intensity, and that indoor lighting is predominantly in the visible light range, the results presented in this study strongly suggest that the N-doped TiO₂ photocatalysts prepared via the APPENS process has a potential application in the arena of indoor and outdoor air pollution control.

0 10 20 30 Reaction Time (min)

0 40 80 120 160

C o n ce n tr a ti o n (p p m )

IPA, with N-doped TiO₂ Acetone, with N-doped TiO₂ IPA, with TiO₂

Acetone, with TiO₂

Figure 4.5a The decomposition of IPA and the formation of acetone in a batch photocatalytic reactor under UV light (10 W, peak at 364.2 nm). Error bars indicated the error range of

repeated experimental data.

0 20 40 60 80 Reaction Time (min)

0 40 80 120 160

C o n c e n tr a ti o n (p p m )

IPA, with N-doped TiO₂ Acetone, with N-doped TiO2

IPA, with TiO₂ Acetone, with TiO₂

Figure 4.5b The decomposition of IPA and the formation of acetone in a batch photocatalytic reactor under visible light (10 W, peaks at 435, 488, 545, 587 and 611 nm). Error bars

indicated the error range of repeated experimental data.

0 10 20 30 40 50

Reaction Time (min)

0 20 40 60 80 100

R e m o v a l E ff ic ie n c y (% )

N-doped TiO

, UV-light N-doped TiO

, VIS light TiO

, UV light

TiO

, VIS light

Figure 4.6 The removal efficiency of toluene as a function of time in a batch photocatalytic reactor under UV (10 W, peak at 364.2 nm) and visible light (10 W, peaks at 435, 488, 545, 587 and 611 nm) sources. Error bars indicated the error range of repeated experimental data.

0 100 200 300 400

Figure 4.7 Concentration variation of NO and NO₂by photo-catalytic reaction under visible light irradiation. The inlet concentrations of NO and NO2was 16.7 ppm and 1.1 ppm,

respectively. The carrier gas composition was 20%N₂+80%O₂.

0 100 200 Time (min)

0 5 10 15 20

C o n c e n tr a ti o n (p p m )

0 2 4 6



NO2 

NO

Light on

75 230

Figure 4.8 Concentration variations of NO and NO₂by photo-catalytic reaction under UV light irradiation. The inlet concentrations of NO and NO₂was 17.4 ppm and 0.9 ppm,

respectively. The carrier gas composition was 20%N₂+80%O₂.

0 50 100 150 200 250 Time (min)

0 4 8 12

C o n c e n tr a ti o n (p p m )

0 2 4 6



NO2 

NO

Light on

75 165

Figure 4.9 Concentration variations of NO and NO₂by photo-catalytic reaction under UV light irradiation. The inlet concentrations of NO and NO2was 10 ppm and 0.9 ppm,

respectively. The carrier gas composition was 20%N₂+80%O₂.

0 1 2 3 4 5 6 7 8

R em o v a l R a te (u m o l/ h r)

NO removal NOx removal

10 ppm 17.4 ppm

Inlet Concentration

Figure 4.10 Removal rates of NO and NOxat different inlet NO concentrations under UV light irradiation

0 20 40 60 80 100

Operation Time (min)

0 3 6 9 12

T o lu e n e r e m o v a l e ff ic ie n c y (% ) N-Doped TiO2 ST01

P 25

Figure. 4.11 Comparison of the toluene decomposition between P25, ST01 and the N-doped TiO₂(TiO_2-xN_x) photocatalysts tested in a continuous flow reactor under visible light (10W)

illumination. The residence time in the reactor was 2.35 minutes.

400 500 600 700 Wavelength (nm)

K u b e lk a -M u n k a b so rp ti o n u n it (a .u .)

N-Doped TiO

P25

ST01

Figure 4.12 Kubelka-Munk absorption spectra of the commercial photocatalysts (P25 and ST01) and the N-doped TiO2(TiO2-xNx) photocatalyst prepared in this study.