INVESTIGATION OF THE PHOTOCATALYTIC ACTIVITY OF TITANIUM DIOXIDE DOPED WITH S

Titanium dioxide (TiO 2 ) is widely used as a photocatalyst for the purification of industrial wastes. Scientists have attempted to enhance its photocatalytic activity by doping it with metallic and non-metallic elements. Metal dopants can shift TiO 2 absorption from ultra violet (UV) to visible light but may reduce photocatalytic efficiency due to recombination centres. Doping TiO 2 with non-metals like N, C, S, and F can narrow the energy band gap and improve photocatalytic efficiency. In this work, we studied S-doped TiO 2 , which combines high photocatalytic activity and structural stability. Titanium dioxide was synthesized via the sol-gel method using a precursor solution containing the titanium aqua complex [Ti(OH 2 ) 6 ] 3+ ·3Cl¯. To prepare TiO 2 doped with 2

Titanium dioxide (TiO 2 ) is widely used as a photocatalyst for the purification of industrial wastes.Scientists have attempted to enhance its photocatalytic activity by doping it with metallic and non-metallic elements.Metal dopants can shift TiO 2 absorption from ultra violet (UV) to visible light but may reduce photocatalytic efficiency due to recombination centres.Doping TiO 2 with non-metals like N, C, S, and F can narrow the energy band gap and improve photocatalytic efficiency.In this work, we studied S-doped TiO 2 , which combines high photocatalytic activity and structural stability.
Titanium dioxide was synthesized via the sol-gel method using a precursor solution containing the titanium aqua complex [Ti(OH 2 ) 6 ] 3+ •3Cl¯.To prepare TiO 2 doped with 2%, 4%, and 8% S, corresponding amount of sodium sulfate (Na 2 SO 4 , 10% (w/w) solution) was added.For mesoporous TiO 2 , the same process was conducted without the addition of salt.Treating the dispersion gradually with a 10% sodium hydroxide (NaOH) solution caused the dispersion to thicken at a pH around 5. adsorbed Na + and Cl -ions.Following purification, the hydrogel was dried for 4 hours at 140 °C [1].
The pH at the point of zero charge (pH PZC ) characterizes the surface charge of the sorbent.The pH PZC of an adsorbent denotes the pH value at which its surface charge becomes neutral.Above the pH PZC , the surface charge is negative that facilitates cation adsorption.Conversely, below the pH PZC , the surface acquires a positive charge, leading to anion attraction.To determine the pH PZC values of the samples TiO 2 , 2% S-TiO 2 , 4% S-TiO 2 , and 8% S-TiO 2 , the drift method was employed.The final pH was plotted against the initial pH, as depicted in Fig. 1.The pH PZC values are as follows: 4.02 for TiO 2 , 4.11 for 2% S-TiO 2 , 4.15 for 4% S-TiO 2 , and 2.08 for 8% S-TiO 2 .Adsorption from a neutral pH solution shifts the pH PZC of these samples towards the acidic side (pH > pH PZC ), suggesting a negative surface charge on S-doped titanium dioxide [2].
The photocatalytic properties of the samples were checked using Congo red (CR) as a model compound.The photodegradation was carried out in a batch photocatalytic reactor equipped with a 40W UV lamp and a magnetic stirrer, as shown in Fig. 2. Fig. 2. Photodegradation experiment setup [3] The catalytic photodegradation of Congo red was monitored using its The fraction of photodecomposed compound was calculated using Eq. 1 as its concentration is proportional to the absorbance.
where A 0 is the absorbance of dye solution before the photo-irradiation, A t is the absorbance of solutions in suspension after photo-irradiation for certain time t [4].
The kinetics of the photocatalytic removal of Congo red dye using TiO 2 and TiO 2 doped with S can provide information about the reaction mechanism.When irradiating the CR solution containing the sorbent with UV light, a decrease in the concentration of the dye was observed with a half-reaction time of approximately 1 hour (Fig. 3a).This process was evident through the reduction in light absorption at a wavelength of 500 nm, which corresponds to the peak absorption of dye.
To quantitatively describe the process, a pseudo-first-order kinetics model was employed.This model assumes that the reaction rate is directly proportional to the concentration of the dye.The pseudo-first-order kinetic model for the photocatalytic removal of Congo red is presented in Eq. 2.

𝑙𝑛 𝐶
where C 0 and A 0 are the initial concentration and absorbance of Congo red, while C t and A t correspond to time t.
The rate constant for the catalytic photoremoval of Congo red in the presence of TiO 2 was calculated from the slope of the straight line obtained from the linear dependence of ln(A/A 0 ) on time (t) (Fig. 3b).The rate constant (k) for the photocatalytic removal of Congo red increases from 6.22 × 10 -3 min -1 to 9.31 × 10 -3 min -1 with increasing sulfur content in the sample.The linear regression coefficient (R 2 ) is close to unity in all cases (Table 1 0,00622 ± 0,00014 0,99069 2% S-TiO 2 0,00688 ± 0,00022 0,97862 4% S-TiO 2 0,00752 ± 0,00013 0,99371 8% S-TiO 2 0,00931 ± 0,00018 0,99285 The photocatalytic decomposition of Congo red using a UV radiation source is satisfactory in all systems analyzed.A one-hour irradiation of Congo red solution with UV light allows for the removal of 50 to 70% of the dye, depending on the sulfur content of the photocatalyst.
It was found that the 4% S-TiO 2 and 8% S-TiO 2 samples were effective catalysts for the heterogeneous photodegradation of Congo red dye and showed activity 1.2 and 1.5 times higher than unmodified TiO 2 , respectively.
3 due to gel formation.The formation of globular TiO 2 particles formed through the condensation of Ti(OH) 4 •2H 2 O molecules in the reaction medium.The resulting hydrogel, with a pH of approximately 7.0, was washed with distilled water to remove

Fig. 3 .
Fig. 3. Kinetics of the catalytic photodegradation of Congo red in the presence of TiO 2 supplemented with different amounts of sulfur (0, 2, 4, 8%): a) dependence of the remaining fraction of Congo red on irradiation time, calculated as the percentage of absorbance decrease at the maximum of the dye; b) logarithm of the remaining fraction of Congo red as a function of irradiation time and regression line

Table 1 .
). Reaction rate constants for the photodecomposition of Congo red