2.1 Photocatalysts preparation

TiO₂ sol-gel was obtained by controlled hydrolysis of titanium tetraisopropoxide (Aldrich, 97%) in an isopropanol solution (volume ratio isopropanol/water 1:1). The powder material was filtered and dried at 120°C for 12 hours. Afterwards, a portion of this material was calcinated at 650°C for 2 hours using a heating ramp of 4°C/min¹⁷. Commercial TiO₂ Sigma Aldrich was used as the reference material.

Fluoridation was performed by immersing the uncalcined TiO₂ powder in a 10 mM solution of NaF. The pH value selected for the fluoridation process was 3.0, facilitating the anchoring of fluorine ions on the titania surface. The solution used to control pH was 1M HCl. This suspension was left under continuous stirring for 1 hour, then filtered, dried, and calcinated at 650°C for 2 hours using a heating ramp of 4°C/min to obtain F-TiO₂^17,18.

Sulfonation was obtained by suspending uncalcined TiO₂ sol-gel powder in a 1M H₂SO₄ solution. This suspension was left under continuous stirring for 1 h. After this, the solid was filtrated, dried, and calcinated at 650°C for 2 hours using a heating ramp of 4°C/min to obtain S-TiO₂^17,18.

The platinum addition on bare, sulfated, and fluorinated TiO₂ was carried out using photochemical reduction. A suspension of these materials in a solution of the metal precursor was prepared (5g of each photocatalyst per liter of H₂PtCl₆.6H₂O). 0.5% of the total weight of TiO₂ was the nominal amount of Pt selected for this work. In this process, a solution of 0.3 M isopropanol was employed as an electron donor.

The Pt photodeposition was performed in a batch Pyrex reactor wrapped in aluminum foil under illumination for 2 hours with an intensity of 60 W/m^2, which was measured using a Delta OHM HD 2102.2 photoradiometer. A continuous flow of N₂ was employed to maintain an inert atmosphere inside the reactor. Finally, the photocatalysts were filtered and dried at 120°C for 12 hours. The materials synthesized were labeled as Pt-F-TiO₂ and Pt-S-TiO₂^17,18.

2.2 Photocatalysts characterization

A complete characterization of the photocatalysts prepared was performed, and the analysis for each instrumental technique was previously reported in different studies^17-21. A general description of the equipment employed for this analysis is presented as follows:

The crystalline phase composition of the prepared materials was analyzed using a Philips PW1700 diffractometer.

Measurements of the Specific surface area (S_BET) were performed in a Micromeritics ASAP 2010 instrument.

The UV-Vis DRS was recorded on a Varian spectrophotometer (model Cary 100).

X-Ray Fluorescence (XRF) spectrometry was used to find the chemical composition of the solids in a Panalytic Axios sequential spectrometer.

Morphology analysis and distribution of particles within the materials were carried out in a Philips CM200 microscope.

The Infrared spectroscopy analysis was performed using a Thermo Scientific Nicolet iS50 spectrometer.

X-ray photoelectron spectroscopy (XPS) analysis was performed with a Leybold-Heraeus LHS-10 spectrometer.

Chromium (VI) chemical photoreduction tests

Each of the synthesized photocatalysts was tested in the reaction of photochemical reduction of chromium. The reaction conditions employed in this study are described as follows:

To determine the actual effectiveness of the photocatalytic process, two blank reactions were carried out:

To ensure the repeatability of the photocatalytic treatment, duplicate trials were performed for each reaction, which found an error percentage of 0.1%.

Determination of chromium (VI) concentration: The variation in Cr (VI) concentration was determined by the 1,5-diphenylcarbazide (DPC) method at 542 nm²². For the spectrophotometric evaluations, a Thermo Scientific Evolution 300 UV-Vis spectrophotometer was used. Prior to the UV-Vis analysis, a pre-treatment was carried out on the samples. A colorimetric method that consisted of the addition of 1,5-diphenylcarbazide in an acid medium was used. This compound bound to Cr (VI) produces a violet color in the solution. The initial and final concentrations of each sample were taken into account to determine the percentage of chromium (VI) removal during the 2 hours of reaction.

Determination of total chromium content: The samples taken from the liquid phase during the reactions were examined before and after photocatalytic evaluation by using two methods described in the Standard Methods for Examination of Water and Wastewater: SM 3111B and 3113B. By following these methodologies, the Atomic Absorption Spectrometry techniques were determined in a Shimadzu AA-7000 spectrometer²³. For this analysis, the flame atomization from air-acetylene combustion and a gas flow of 2.8 L/min were used. The light source used was a hollow cathode Cr/Ni model BGC-D2 lamp, and the wavelength selected for Cr determination was 357.9 nm.

Finally, to determine the real photochemical reduction or adsorption of chromium on the surface of the photocatalytic material, each of them was analyzed by XPS. From this technique, it was possible to identify the changes in the oxidation state of this chemical element. XPS analysis was performed using aluminum anodes as an X-ray source, using a constant step energy of 100 eV. The scale was corrected using the binding energy (BE) of the spectra by reference to the C- (C, H) component at 284.8 eV of the C 1s peak.

Photocatalysts characterization

In general, the characterization results of the photocatalysts analyzed have been previously reported by our research group^17-21. To compare the data in the current study, we included a summary of the main results as follows:

XRD:Figure 1 shows the XRD patterns of all photocatalysts; it was observed that crystalline phases Anatase and Rutile coexist only in TiO₂ Sol-gel identified by their characteristic peaks located at the 2 theta positions of 25.23 and 27.33, respectively, the modified photocatalysts seem to be exclusive to the crystalline phase Anatase, possibly owing to the protective effect of F^- and SO₄ ^-2 on the surface of TiO₂ during the calcination process^17,21. The photodeposition of Pt nanoparticles during the process of preparing materials did not modify the composition of the crystalline phases of TiO₂²⁴.

In Table 1, it was noted that the values of the anatase crystallite sizes increased after the sulfation and fluoridation treatments. It has been reported that the presence of F^- ions improves the crystallinity of the titania and allows the anatase crystals to be larger¹⁹.

Figure 1 XRD patterns for all photocatalysts 

S _BET : The lowest values in specific surface area corresponded to the TiO₂ sol-gel due to the particles sintering in the calcination stage to 650°C (Table 1). In the S-TiO₂ and F-TiO₂ materials, the surface area value significantly increases, which agrees with the protective nature of the sulfate and fluorine species on the TiO₂ surface during calcination, inhibiting the agglomeration of particles²⁵. It was also observed that after platinization, the surface area decreases, which may be caused by the obstruction of the TiO₂ pores by Pt nanoparticles¹⁷.

UV-Vis DRS: Band gap values of the photocatalysts calculated from UV-Vis DRS analysis show small decreases in these values after sulfation, fluoridation, and platinization, as observed in Table 1. UV-Vis DR spectra of all photocatalysts are presented in Figure 2; the characteristic absorption edge of TiO₂ at around 360-380 nm can be seen for all samples. A slight absorption shift towards the visible region of the electromagnetic spectrum can also be observed for fluorinated and platinized photocatalysts; this is due to the presence of fluoride ions (F^-) on the TiO₂ surface²⁶, and in the case of platinized materials due to the gray coloration of the powders attributed to the presence of Pt⁰ species on the surface.

Figure 2 UV-Vis DR spectra of all photocatalysts 

XRF: From the XRF analysis, it was possible to determine that the real amount of platinum present in the samples was 0.25 wt.% for Pt-F-TiO₂ and 0.25 wt.% for Pt-S-TiO₂. These values are lower than the nominal 0.5 wt.%. Traces of chloride ions that came from the metal precursor were also found (Table 1)

TEM: In Figure 3, TEM micrographs of the platinized samples are shown, where it is possible to observe small dark and spherical Pt particles, which are placed on the anatase surface. The histograms in Figure 3 represent the Pt particle size distribution. As seen in the Pt-F-TiO₂ (Figure. 3a) sample, the Pt particles are distributed in a more heterogeneous way compared with Pt-S-TiO₂ (Figure. 3b). The average metal particle size was > 6 nm for fluorinated photocatalysts and between 6-7 nm for sulfated photocatalysts. The highest dispersion of platinum on the sulfated photocatalyst could be due to its surface area, which is higher than that observed for Pt-F-TiO₂ (Table 1). The presence of oxygen vacancies in the sulfated solid can also contribute to the homogeneous distribution of Pt⁹.

Table 1 Main photocatalysts characterization results 

FTIR: Infrared spectroscopy analysis allowed the identification of OH-isolated groups, H-bonded hydroxyl groups, Ti-OH interactions, and Ti-OH₂ interactions coming from water adsorbed on the TiO₂ surface²⁷.

XPS:Figure 4a shows the XPS analysis in the O 1s region with a signal located at 529.8 eV corresponding to the surface O^2- species present in all photocatalysts as reported in Table 1. As it can be seen, the TiO₂ sol-gel does not present a shoulder; however, when the sulfation and fluoridation process was performed, an evident shoulder located at 532 eV was observed, thus demonstrating the existence of OH^- groups on the surface. The highest intensity of this shoulder observed in F-TiO₂ and S-TiO₂ photocatalysts may indicate a higher degree of hydroxylation in these materials, mainly due to the presence of oxygen vacancies. After platinum addition, the shoulder disappears: it is because the platinum on the surfaces attracts these electronegative groups to generate the photochemical reduction, thus changing the chemical environment and decreasing the hydroxylation on the surface for the Pt-F-TiO₂ and Pt-S-TiO₂.

The fluorinated photocatalysts were analyzed by XPS, and in the F 1s region, the main signal located at 684 eV in Figure 4b corresponds to fluoride ions present on the surface as ≡Ti-F species. In the fluorinated samples, an exchange of ligands between the F^- and OH^- on the TiO₂ surface (≡Ti-OH) ⁽²⁸ exists. A decrease in the intensity of the fluorine signal was also observed after platinization: this is because the highly electronegative fluoride groups contribute to the photochemical reduction of Pt²⁺ to Pt⁰ so that the degree of surface fluoridation decreases in the Pt-F-TiO_2. The atomic content of fluorine was 1.83% for F-TiO₂ and 1.12% for Pt-F-TiO₂.

As can be seen in Figure 4c, the Ti 2p_3/2 region in TiO₂ is very similar for all samples, with peaks centered at binding energies of 458.5±0.2 eV. This value is assigned to Ti⁴⁺ species present in the lattice of TiO₂ as the main component: this indicates that the chemical environment of titanium was unchanged after modification by sulfation, fluoridation, or platinization.

Table 1 reports that the O/Ti atomic ratio estimated for the samples analyzed is lower than the stoichiometric value O/Ti=2, proving the presence of certain oxygen vacancies on the surface of this oxide. S-TiO₂ showed the lowest O/Ti ratio of 1.70. This result agrees with what has been found in previous studies, where it is indicated that the pre-treatment of TiO₂ with sulfuric acid and subsequent calcination at high temperature (> 600 ºC) results in a surface rich in oxygen vacancies²⁹. In the case of Pt-S-TiO₂ photocatalysts, the ratio O/Ti increases to 1.88, suggesting that oxygen vacancies were partially removed during the photodeposition process.

Figure 5 shows the spectra obtained for Pt 4f_7/2 and Pt 4f_5/2 in the Pt-S-TiO₂ photocatalyst. From these spectra, it is possible to observe that in the platinized materials, species of oxidized and reduced Pt coexist. This can be seen by the characteristic doublets of these species located at 71 eV and 72.4 eV for Pt⁰ and Pt²⁺, respectively. These results indicate that platinum was not completely reduced on the TiO₂ surface, as evidenced by the XRF analysis presented above.

The Pt-S-TiO₂ sample was selected for XPS analysis before (Figure 5a) and after (Figure 5b) chromium reduction reactions. From this analysis, it was possible to determine that this material presents high stability after photocatalytic reaction, with no modifications in the Pt 4f region being observed.

Figure 3 TEM micrographs of platinized materials. (a) Pt-F-TiO₂ and b) Pt-S-TiO₂ 

Photochemical reduction of chromium (VI)

The main results obtained in the photochemical reduction of chromium (VI) reaction are represented in Figure 6. Firstly, a blank reaction under UV-Vis radiation and without a photocatalyst was carried out. During this test, a decrease in the chromium (VI) concentration (i.e., 4.83 %) was observed, which could be caused by a possible reduction of this metal on the glass reactor walls since they changed color after the reaction.

Then, a model reaction in the presence of the photocatalyst without irradiation was also performed. For this test, the material showing the best behavior in the Cr photocatalytic reduction reaction (Pt-S-TiO₂) was selected. This demonstrated that the Cr adsorption on the solid can take place³⁰, especially because after 5 hours under continuous stirring, the concentration of chromium in the liquid phase decreased by 21.5%.

Likewise, as reported in section 2.2, the adsorption-desorption balance was performed at the start of each reaction with the different materials. The Cr(VI) percentage removal from the fluid phase was very similar for all the photocatalysts, close to 11%, thus indicating similar surface adsorption properties for all materials.

During the photocatalytic reaction, it was possible to determine that when both UV-Vis radiation and photocatalyst are in the reaction medium, the reduction of chromium significantly increases. Thus, as seen in Figure. 6, the percentage of Cr (VI) removal is higher after reacting with all the photocatalysts tested.

As observed in Figure 6, during the first 20 minutes of the photocatalytic reaction, there is an important reduction in the concentration of chromium. However, after 30 min, the concentration of this metal had a slight tendency to increase. This behavior can be explained by the possible re-oxidation of chromium. Re-oxidation is mostly due to the competition of H₂O for the holes and hydroxyl radicals such that the reduction of the metal is unfavorable, and the initial oxidation state of the metal species occurs, thus resulting in the desorption of the metal from the TiO₂ surface^30-33.

Figure 4 XPS spectra a) O 1s region, b) F 1s region, c) Ti 2p region 

Figure 5 XPS spectrum of Platinum 4f region for Pt-S-TiO₂. (a) before reaction and (b) after reaction 

In general, it was observed that the processes of sulfation, fluoridation, and subsequent addition of platinum improved the photocatalytic activity of the TiO₂ sol-gel, thus leading to an increase in the effectiveness of this oxide in the elimination of Cr (VI) in the reaction medium.

The best performance in the reduction of Cr was observed by using Pt-S-TiO₂ as a photocatalyst after 2 hours of reaction. With this material, the concentration of chromium (VI) decreased up to 82,13%. At the end of the reaction, the Cr (VI) content in the fluid phase was 8.2x10^-6 mM, equivalent to 0.42 mg/L of Cr (VI), which is lower than the maximum permissible limits given by the Colombian regulation for wastewater and surface water discharges (i.e., 0.50 mg/L) ^(34,35. As seen in Figure 7, only photocatalytic treatment with Pt-S-TiO₂ compliance was achieved by Colombian regulation; however, the second-best performance was achieved with Pt-F-TiO₂ (i.e., 0,52 mg/L Cr (VI))

Figure 6 Percentage of Cr (VI) removal, determined by UV-Vis spectrophotometry. a) all reactions. b) Y-axis enlargement 

Figure 7 Concentration of Cr (VI) in mg/L after 2 hours of photocatalytic treatment compared Colombian regulation 

The top activity observed with Pt-S-TiO₂ in the photoreduction of Cr can be attributed to the physicochemical properties of this material: (i) this material presents a higher surface area than fluoridated material. (ii) Sulfation and Pt addition led to a small reduction in the band gap value compared to TiO₂ sol-gel, thus leading to a decrease in the distance for the promotion of electrons from the valence band to the conduction band in the solid, resulting in a higher reaction efficiency. (iii) Platinized material exhibits absorption in the visible region of the electromagnetic spectrum, thus leading to a better use of the energy. (iv) As observed by TEM analysis, the distribution of Pt nanoparticles was the best and resulted in most of the particles on the surface being able to act effectively as a sink for the photogenerated electrons and increase the electron-hole half-time. (v) The TiO₂ sulfation inhibits the rutile phase formation, thus allowing the anatase phase, which is considered more active in photocatalytic processes, to be favored¹⁷.

As previously indicated in Section 2, the Cr (VI) content was evaluated by UV-Vis spectrophotometry, while the total chromium content in the fluid phase after 2 hours of photocatalytic treatment was studied using atomic absorption spectrometry. Figure 8 represents the concentration of total chromium in the liquid phase at the end of the photocatalytic treatment. As shown in this figure, the final concentration of this metal was 4x10^-3 mM (2,08mg/L) using Pt-S-TiO₂ and 4.5x10^-3 mM (2,34mg/L) with Pt-F-TiO₂.

Figure 8 Total chromium concentration measured by atomic absorption spectroscopy in the samples after 2 hours of photocatalytic reaction 

The overall trend of photocatalytic was TiO₂ sol-gel < S-TiO₂ < TiO₂ (Sigma)

< F-TiO₂ < Pt-F-TiO₂ <Pt-S-TiO₂: this is in good agreement with that also found by UV-Vis analysis. In order to determine the oxidation state of chromium deposited on the surface external of photocatalysts analyzed, TiO₂ Sol-gel and Pt-S-TiO₂ were selected to be studied by XPS. These solids were recovered after 2 hours of photocatalytic reaction.

Figure 9 presents the XPS spectra obtained for the Cr 2p_3/2 region; signals are not very clear due to the low chromium content; however, by comparison with theoretical spectra²⁶, the presence of characteristic signals of Cr⁶⁺, Cr³⁺, and Cr⁰ oxidation states was evidenced; these signals are located at 580, 576 and 574 eV, correspondingly. The results obtained show Cr⁰ after the reaction, species due to the photocatalytic treatment performed, allowing the Cr photoreduction on the photocatalyst surface. It has been widely reported in the literature that metals can present photochemical reduction, confirming the metallic spices of Cr⁰ on the surface of the solid^29,36.

Best photocatalyst recycling: The stability of the photocatalyst Pt-S-TiO₂ was evaluated by reusing it in four consecutive cycles. The reaction conditions included a light intensity of 120 W/m², the presence of nitrogen as an inert gas, constant stirring, and a maximum time of 2 hours. The solid was recovered by filtration and drying at 100°C for 4 hours for use in the next cycle. As evidenced in Figure 10, after the 4th cycle, there was a 30% activity loss compared with the first cycle. This was probably due to the saturation on the surface of the photocatalyst by the reduced chromium species in each of the cycles.

Evaluation of the impact of reaction parameters on photochemical reduction of chromium

Since the Pt-S-TiO₂ had the best performance in the photoreduction of Cr, this photocatalyst was selected to test the impact of different reaction parameters on the overall effectiveness of this material in the removal of Cr from the liquid phase. Table 2 describes the evaluated reaction conditions.

Light intensity effect: As shown in Figure 11, it was found that an increase in light intensity from 30 to 120W/m² led to a higher effectiveness in the removal of Cr. This is mainly due to a higher photonic efficiency, which increases the energy available for the photocatalytic process.

Reaction time effect: It was also found that an increase in reaction time from 2 to 5 hours led to an increase in the concentration of chromium in the fluid phase. This is because, in the reaction medium, the Cr (III) species can compete with H₂O for hydroxyl radicals and holes. As a result, the photoreduction of the metal is at a disadvantage, and the initial oxidation state of chromium is regenerated, as seen in Equation 1^37,38; then, a desorption of the Cr from the outside of TiO₂ can occur.

Reaction atmosphere effect: It was found that with the reaction parameters used in this study, the reaction atmosphere (nitrogen or oxygen) did not significantly modify the reduction of Cr.