Phenol photodegradation

Figure 40: Spectra of phenol photodegradation and reduction in the intensity of 10 ppm phenol peak observed by UV-Vis spectrophotometry in the presence of 65 mg/50 mL of SnO2/La 0.6 wt. % Nps, under UV light irradiation, reaction time (2-3 h), sampling time, (12-13), sample volume (250.00 mL), pH of the reaction medium (5.7) and inlet air flow 4 L/min.

Since La was the best photoactive, its photocatalytic activity SnO2/La 0.6 wt. % was examined using phenol. The phenol typical absorption spectrum is characterized by a band at 269 nm attributed to absorption in the aromatic ring.

4.5.2.1 Under UV light irradiation

The UV-Vis spectra obtained for 10 phenol sample with 2 h at pH 5.7 and with SnO2/La, 0.6 wt. % under UV light irradiation. The adsorption ability of the samples was tested, and it was found that after 30 min in dark conditions, very minimal changes in phenol adsorption were observed. In Figure 40 the absorption spectra of a 10 ppm phenol solution during illumination in the presence of SnO2/La 0.6 wt. %. UV scans (240-320 nm) used also to investigate different phenol byproducts, also in the Figure the appearance of different phenol byproducts are emerged. Several studies have shown that phenol first undergone to BQ later the ring was opened to form other smaller byproducts such as carboxylic acids [262, 429]. A clear decrease from phenol absorbance band intensity was observed with increasing photoirradiation time within 15 min. An immediate increase at 290 nm absorbance presence of HQ band intensity and much more increase at 246 nm absorbance which is BQ band intensity. A decrease in the phenol absorbance band also noticed. This phenomenon can be associated with the degradation of phenol intermediates, HQ and BQ, respectively, within 15 min of photoirradiation.

After 30 min of light irradiation, the phenol absorbance band reduced, but a concurrent increase and broadening of the 290 nm band and a rising increment with the broadening of the 246 nm band was observed. After 60 min of irradiation, the characteristic peak at 290 nm reduced, and the intensity of 246 nm peak decreased with the disappearance of the phenol peak. After 90 min of irradiation, it was clear that all phenol was degraded and also the formed intermediates are being further degraded. The degradation as observed from the optical spectroscopy point of view is different from the information gained using chromatography, as the extinction coefficients play an important part in the final optical absorption of the byproducts. Considering BQ, it was found that it absorbs nearly 10 times more than phenol around 269 nm, which is the peak absorption of phenol. Thus the formation of byproducts would mask the degradation level of phenol if only optical spectroscopy was used. The analysis in UV-Vis spectroscopy would not render a complete picture of the phenol degradation process, thus most of the subsequent disscussions are much based on chromatograpic studies.

4.5 Photocatalytic degradation 155

Table 17: Comparison of phenol degradation after 2.5 h of irradiation with various dopants Catalyst-Sample % of Phenol

Table 17 compares the photocatalytic degradation of phenol in the presence of various catalysts, where phenol photodegradation was measured using UV-Vis spectroscopy. In a control experiment, the photodegradation of phenol was examined without adding the catalyst, there was no significant reduction in the phenol concentration was observed and only 3% reduction was detected after 2.5 h. It is noticed from the Table 17 that at 120°C the photodegradation rate of phenol was 24% for SnO2/I 0.01 wt. %, this was

higher than the photodegradation rate of the control 13% at 120°C, but raised for undoped SnO2 to 46% at 150°C, and reduced for SnO2/I 0.01 wt. % to 43%.

Under UV light irradiation technique, 86% of phenol photodegraded with SnO2/Ce 0.6 wt. % samples after 120 min irradiation, but with SnO2/Nd 0.6 wt.% it reached to 95%

and gave about 100% with SnO2/La 0.6 wt.%. Results of photocatalytic activity upon doping the catalyst with SnO2/Nd 0.2 wt. %, showed 57%, enhanced to 95% with SnO2/Nd 0.6 wt. % demonstrated an improved activity of about fourfold in Nd³⁺ doping.

4.5.2.2 Under solar light irradiation

Table 18: Comparison of phenol photodegradation after 2.5 h of solar irradiation

Catalyst Phenol photodegradation

Undoped 11%

SnO2/I 0.01 wt. % 8%

SnO2/I 0.2 wt. % 22%

SnO2/I 0.3 wt. % 24%

SnO2/I 0.4 wt. % 50%

SnO2/I 1.0 wt. % 61%

SnO2/Sb 0.6 wt. % 95%

Table 18 shows a comparison of two used catalysts for 10 ppm phenol aqueous solution, at pH 5.7 photodegradation under solar light irradiation and inlet air flow 4 L/min. The results showed that the reduction of about 61% within less than 2.5 h with SnO2/I 1.0 wt. %, but effectively 95% of phenol photodegradation reduction by doped SnO2/Sb 0.6 wt. %. All the natural sunlight experiments were conducted on full sunny days, when the incident power of the sunlight was measured to be between 80 to 90 kW/m² and the other parameters kept fixed.

Figure 41 showed that within 150 min of visible light irradiation the 10 ppm phenol solution in water was photodegraded by 96-99%, thus demonestrating that SnO2/Gd Nps are efficient visible light photocatalysts. Concerning with SnO2/Gd 0.4 wt.%, 93% of phenol photodegraded at 120 min. but > 94% and 96% of phenol photodegraded with SnO2/Gd 0.4 wt.% at 120 and 150 min or as it indicated in the Figure.

4.5 Photocatalytic degradation 157 To explore the photocatalytic activity of undoped and doped SnO2 Nps, the degradation of phenol under artificial UV, visible, and natural sunlight was investigated. The photocatalytic activity of SnO2 was evaluated through comparison with undoped SnO2

under the same experimental conditions. It is clearly observed from the previous Figures that the photocatalytic activity of the different doped SnO2 is higher than the undoped SnO2 under the UV,visible and sunlight irradiations.

SnO2/Gd Nps showed 99% degradation of phenol under visible light irradiation more than undoped.

Figure 41: Comparison of the decrease of phenol concentrations degradation with 65 mg/50 mL of SnO2/Gd 0.4 wt. % and SnO2/Gd 0.6 wt. % Nps, under visible light irradiation, reaction time (2-3 h), sampling time, (12-13), sample volume (250.00 mL), pH of the reaction medium (5.7) and inlet air flow 4 L/min

4.5.2.3 Under visible light irradiation

Under UV irradiations the maximum percent photodegradation of phenol for undoped SnO2 of 46% was observed after 150 min, whereas in the presence of 0.4 wt.% SnO2 /La showed complete 100% photodegradation of phenol within 120 min only. SnO2 /La was found to be the highest catalytic activity on phenol, the most photoactive, since 120 min of irradiation was needed to accomplish complete photodegradation of phenol.

The effectiveness of SnO2/Gd is clearly observed for the photodegradation of phenol under visible light. About 99% degradation of phenol 10 mg/L was achieved after 150 min when 0.6 wt. % SnO2/Gd was applied. Undoped SnO2 showed only 15% of the same concentration of phenol degradation after 150 min of visible light.