Sulfur Doped TiO 2

Chapter 5 Photocatalytic Results

5.3 Sulfur Doped TiO 2

165

with the original substrates to adsorb onto the catalyst, considering that the intermediates may adsorb more strongly the original substrates will be displaced. The nitrogen doped catalysts offered better reactivity than the undoped catalyst, showing that nitrogen doping provides.

Overall the TiO₂:N 1:3 catalyst shows the best activity with the TiO₂:N 1:4 catalyst having only slightly better activity for degradation of SA. This is odd as the TiO2:N 1:4 catalyst has better physiochemical properties in all parameters that affect photocatalyst namely higher surface area (73 vs 65 m². g^-1), narrower band gap (2.7 vs 2.8 eV) and slower electron-hole recombination rate (Figure 4.22). Though for most of the degradations there is adsorption and desorption at various intervals making it difficult to determine if the final value for a given degradation is completely accurate. None of the catalysts synthesised showed better activity than P25 overall, TiO2:N 1:3 and TiO2:N 1:4 showed better activity only for the degradation of SA compared to P25.

166

as before irradiation. The TiO2:S 1:2 catalyst shows a linear decrease in the amount of caffeine until 100 min at which time caffeine begins to desorb from the surface of the catalyst until at 160 min the final value is 𝐶 𝐶⁄ ₀ =1. The TiO₂:S 1:3 catalyst has a similar trend as it can be seen that for the final two sample points after 140 and 160 min there is an increase in the amount of caffeine. The TiO₂:S 1:4 catalyst shows no activity for either adsorption or degradation of caffeine. P25 had better degradation efficacy for caffeine than any of the sulfur doped catalysts.

Figure 5.9: Photocatalytic degradation of caffeine with sulfur doped catalysts. Reaction mixture consisted of 5 ppm of each compound, 50 mg catalyst in 100 mL of solution. The reactions were monitored by HPLC at a wavelength of 210 nm.

5.3.2 Aspirin

Figure 5.10 shows the degradation reactions with various sulfur doped catalysts. The TiO2:S 1:2 and TiO₂:S 1:3 catalysts show some adsorption of aspirin after stirring in the dark. After initial irradiation there is a slight decrease in the amount of aspirin for the TiO2:S 1:2 catalyst

167

but a slight increase in the amount of aspirin for the TiO2:S 1:3 catalyst. The TiO2:S 1:2 catalysts shows a linear reduction in the amount of caffeine until the 100 min sample interval thereafter the amount of caffeine increases back to the initial concentration. The TiO2:S 1:3 catalyst also shows desorption towards the end of the reaction going from 0.79 to 0.85. The TiO2:S 1:4 catalyst shows no activity towards aspirin at all neither adsorbing the substrate nor degrading it. P25 performs better for the degradation of aspirin than any of the sulfur doped catalysts.

Figure 5.10: Photocatalytic degradation of aspirin with sulfur doped catalysts. Reaction mixture consisted of 5 ppm of each compound, 50 mg catalyst in 100 mL of solution. The reactions were monitored by HPLC at a wavelength of 210 nm.

5.3.3 Phenacetin

Figure 5.11 shows the photocatalytic degradation reactions of phenacetin using the various sulfur doped catalysts. Both the TiO2:S 1:2 and TiO2:S 1:3 catalysts show adsorption after dark stirring while the TiO2:S 1:4 catalyst shows no adsorption after dark stirring. The

168

TiO2:S 1:2 shows an increase in phenacetin concentration after initial irradiation while the TiO₂:S 1:3 shows a slight decrease in concentration. Once again the TiO₂:S 1:2 catalyst shows desorption of the substrate after 100 min to a final concentration, after 160 min, equal to the starting concentration. Unlike the reactions for aspirin and caffeine the TiO₂:S 1:3 catalyst does not show linear desorption after 120 min but rather fluctuation. The TiO2:S 1:4 catalyst shows no activity for adsorption or degradation of phenacetin over the course of the reaction.

Figure 5.11: Photocatalytic degradation of phenacetin with sulfur doped catalysts. Reaction mixture consisted of 5 ppm of each compound, 50 mg catalyst in 100 mL of solution. The reactions were monitored by HPLC at a wavelength of 210 nm.

5.3.4 Salicylic Acid

Figure 5.12 shows the photocatalytic degradation of SA with the various sulfur doped catalysts. All of the sulfur doped catalysts show linear degradation of SA. The TiO2:S 1:2 catalyst shows the lowest degradation of SA and TiO2:S 1:4 catalyst has the best degradation

169

rate of SA. Catalysts showed very strong adsorption of SA after stirring in the dark adsorbing between 30-40% of the initial concentration of SA. All sulfur doped catalysts had better activity for the degradation of SA than P25.

Figure 5.12: Photocatalytic degradation of SA with sulfur doped catalysts. Reaction mixture consisted of 5 ppm of each compound, 50 mg catalyst in 100 mL of solution.

The reactions were monitored by HPLC at a wavelength of 210 nm.

5.3.5 Final Comments on the Photocatalytic Activity of Sulfur doped TiO

2

Looking at Figures 5.9-5.11 all catalysts display a large amount of desorption this could be due to the former reasons proposed. The first reason proposed was competitive adsorption of substrate molecules. The second reason proposed was that degradation products would displace the original substrate molecules from the surface of the catalyst. This adsorption and desorption witnessed makes it difficult to accurately determine the activity of the catalysts for that particular molecule. That the highest amount of substrate degraded was again SA is predictable as previously stated being the smallest molecule should be the fastest to adsorb

170

and therefore degrade fastest. The TiO2:S 1:2 catalyst shows no activity for 3 of the substrates (caffeine, aspirin and phenacetin) though for SA almost completely linear degradation and degrades 50%. This still indicates good photocatalytic activity for the TiO₂:S 1:2 catalyst as it is better than that of P25 for the same compound. All of the sulfur doped catalysts showed better activity than the undoped catalyst meaning that doping with sulfur improves photocatalysis of TiO2. Overall the TiO2:S 1:3 catalyst seems better than the TiO₂:S 1:4 catalyst. However the desorption of caffeine, aspirin and phenacetin seen in Figures 5.8-5.11 place the actual amount of these substrates degraded into question. The final values taken as they are for all catalysts would suggest that the TiO₂:S 1:3 and TiO₂:S 1:4 have very similar activity; looking at the properties of each in chapter 4 this would make sense. The band gap for both catalysts is the same 2.4 eV, while the recombination rate for TiO₂:S 1:3 is low (Figure 4.33) the surface area for TiO₂:S 1:4 is higher (130 vs 69 m². g^-1).

Given that the copper doped catalysts all showed adsorption and desorption for all substrates, it was determined that these catalysts should not be used to optimize parameters. Despite some positive results from the copper doped catalysts the fact that the final concentration may not be as accurate as seen in the graphs may prove detrimental to optimization. This meant that the best catalyst to optimize with should be chosen based on degradation of SA as for the nitrogen and sulfur doped catalysts linear degradation was obtained. Based on these criteria the best catalyst to optimize with was determined to be the TiO2:S 1:4 catalyst as it had the highest degradation rate for SA.

171

5.4 Comparison of Obtained Results to Previous Results