Selective and efficient catalytic and photocatalytic oxidation of diphenyl sulphide to sulfoxide and sulfone: the role of hydrogen peroxide and TiO2 polymorph

In this paper, we describe the role of anatase and rutile crystal phases on diphenyl sulphide (Ph2S) catalytic and photocatalytic oxidation. The highly selective and efficient synthesis of diphenyl sulfoxide (Ph2SO) and diphenyl sulfone (Ph2SO2) at titanium dioxide was demonstrated. Ph2S oxidation in the presence of hydrogen peroxide at anatase-TiO2 can take place both as a catalytic and photocatalytic reaction, while at rutile-TiO2 only photocatalytic oxidation is possible. The reaction at anatase leads mainly to Ph2SO2, whereas, in the presence of rutile a complete conversion to Ph2SO is achieved after only 15 min (nearly 100% selectivity). Studies on the mechanistic details revealed a dual role of H2O2. It acts as a substrate in the reaction catalysed only by anatase, but it also plays a key role in alternative photocatalytic oxidation pathways. The presented study shows the applicability of photocatalysis in efficient and selective sulfoxide and sulfone production.


Introduction
Conventional routes of synthesis of many important organic chemicals oen require harsh operating conditions, such as high temperature and pressure. Moreover, sometimes the presence of toxic oxidation or reduction agents is required, far from the principles of green chemistry. The development of synthesis, which offers efficient organic compound production under mild reaction conditions, is highly desirable. Consequently, a photocatalytic organic synthesis, which utilizes economically sustainable and clean solar energy, has achieved tremendous attention in recent years. 1 However, photocatalysis has been mostly considered as a non-selective process, involving activation of small molecules to initiate a chemical reaction (especially in water). These types of photocatalytic reactions include mainly processes governed by reactive oxygen species such as oxidation of alcohols, 2 oxidation of aromatics 3,4 or oxidation of side chains. 5,6 Nevertheless, a carefully selected photocatalyst and synthesis conditions in a non-aqueous solvent can lead to highly selective photocatalytic ne product formation.
Oxidation of organic sulphides (diphenyl sulphide, dimethyl sulphide) is one of the most important processes in organic chemistry. The organic sulfoxides and sulfones, which are the products of this reaction, were found to be very important substances for pharmacy, medicine, or substrates in drug synthesis. They can be used for the production of antibacterial, antifungal, 7 antihypertensive agents, 8 and vasodilators. 9 Many catalytic systems, containing, e.g., V, Re, Ti, Mo, Te, W, Se, Fe elements, have been studied in a sulphide to sulfoxide oxidation. [10][11][12][13][14] Titanium dioxide is used as a catalyst due to its desired physicochemical properties, high chemical stability, low cost of production, and lack of toxicity. Additionally, TiO 2 doped with vanadium was found to be an active catalyst of Ph 2 S oxidation giving nearly 100% conversion of Ph 2 S with high selectivity to Ph 2 SO 2 . 15 SiO 2 -TiO 2 mesoporous xerogel catalyses oxidation to the nal product, diphenyl sulfone, with the selectivity of 85%. 16 Furthermore, it was conrmed that titanosilicate zeolites (inter alia, Ti-beta, 17 Ti-MWW, 18 Ti-FER, MTS-1 (ref. 19) and Ti-ITQ-6 (ref. 20)) present a good catalytic activity in this process. NaIO 4 , trichloroisocyanuric acid, sodium perborate, KMnO 4 , can be successfully applied as oxidizing agents. 21 However, hydrogen peroxide, H 2 O 2 , seems to be the most suited oxidizer due to its low environmental impact (H 2 O is the only side product), nontoxicity, and high content of active oxygen. [22][23][24] Aromatic sulphides can also be transformed using photocatalysts such as wide bandgap oxide semiconductors (TiO 2 , ZnO, etc.). Although, they undergo deep oxidation in the aqueous media, sulfoxides, and sulfones are the main reaction products in the presence of organic solvents. 25 To the best of our knowledge, the air is the most common source of oxygen in the photocatalytic transformation of sulphides described in the literature. In this system, even when selectivity is high, the concentration of the nal product is rather low. However, we showed recently that hydrogen peroxide is a crucial oxidation agent in the photocatalytic oxidation of Ph 2 S. 26 No Ph 2 S conversion aer 3 hours of the reaction in both catalytic and photocatalytic tests conducted in the absence of H 2 O 2 over bare and doped TiO 2 materials (concentrations of Ph 2 S oxidation products below the detection limit) was observed. Nevertheless, in the presence of hydrogen peroxide, the reaction can occur at P25 (anatase and rutile mixture) both on the catalytic and photocatalytic way, with signicantly higher conversion of Ph 2 S under irradiation compared to the dark process. In our previous paper, we studied the inuence of TiO 2 modication (doping with vanadium, zinc, or tin) on the Ph 2 S oxidation. We suggested that the crystal phase composition has a higher inuence on both catalytic and photocatalytic activity than the surface metal modications. 26 The goal of this work is to understand the difference between anatase and rutile in catalytic and photocatalytic Ph 2 S oxidation, describe the role of particular reactive oxygen species in these reactions, and learn how to control the reaction selectivity to different products.

Crystal structure, morphology and phase composition
The phase compositions of materials were studied by the powder X-ray diffraction (XRD) using a Rigaku MiniFlex 600 Xray diffractometer (Cu K a radiation, 0.3 mm Ni lter) in 2q range from 10 to 90 , at speed 3 min À1 and 0.05 step. The crystal size was calculated based on the Scherrer equation. To calculate the crystal size the most intense peaks were used, i.e., (101) and (110) for anatase and rutile, respectively. Materials morphology was examined by scanning electron microscopy (SEM) Tescan VEGA 3 with an LaB 6 emitter. The measurements were performed on a carbon sheet.
The specic surface area (SSA) of studied materials was determined from nitrogen adsorption-desorption isotherms at 77 K using a Quantachrome Autosorb iQ-MP-AG-AG instrument. The Brunauer-Emmett-Teller model was applied.

Studies on reaction conversion and product selectivity
Catalytic and photocatalytic oxidation of diphenyl sulphide (Ph 2 S, 98%, Sigma Aldrich) to diphenyl sulfoxide (Ph 2 SO, 96%, Sigma Aldrich) and sulfone (Ph 2 SO 2 , 97%, Sigma Aldrich), in the presence of hydrogen peroxide (30%, Sigma Aldrich) was investigated. The reaction mixture consisted of 0.4 mmol dm À3 of diphenyl sulphide, 10 cm 3 of acetonitrile (99.9%, Sigma Aldrich) and 0.1 mmol dm À3 of bromobenzene ($99.5%, Sigma Aldrich) used as the internal standard. In a typical experiment, 5 mg of the tested material powder was sonicated for 2 min in 10 cm 3 of the reaction mixture, then 2 mmol dm À3 of 30% hydrogen peroxide was added (suspension concentration of 0.5 g dm À3 ). The prepared suspension was placed in a round quartz cuvette (5 cm dia., 1 cm optical path) and irradiated for 3 hours with a xenon lamp (XBO-150, Instytut Fotonowy) equipped with 10 cm 0.1 mol dm À3 CuSO 4 aqueous solution lter and a 320 nm cut-off lter. To determine the apparent quantum efficiency the photon ux in the range of 300 to 400 nm was measured with the StellarNet spectroradiometer. In the case of the catalytic studies, the reaction tests were performed in the dark in order to avoid a light-assisted conversion of Ph 2 S. The progress of the reactions was monitored by HPLC analysis of the reaction mixture using a mixture of acetonitrile/water in the volume ratio of 70 : 30 as the eluent. The samples of the reaction mixtures were ltered using a 0.22 mm nylon membrane lter and analysed by the PerkinElmer Flexar chromatograph equipped with the COL-Analytical C18 column (150 mm Â 4.6 mm i.d., 5 mm pore size). The column was maintained at 25 C during the analysis and the UV detector was set at 254 nm.

Electrochemical measurements
The electrochemical measurements were carried out in a threeelectrode system with Ag/Ag + electrode [AgNO 3 (10 mmol dm À3 ) in 0.1 mol dm À3 Bu 4 NClO 4 in acetonitrile], platinum wire, and carbon electrode as the reference, counter, and working electrodes, respectively. As the electrolyte 0.1 mol dm À3 LiClO 4 solution in acetonitrile was used. The potential of the reference electrode relative to the standard hydrogen electrode (SHE) was set at +0.62 V based on the cyclic voltammetry measurement for ferrocene (scan speed 10 mV s À1 ) and the literature value for Fc + /Fc couple. The electrode potential was controlled by an electrochemical analyser (Bio-Logic, SP-150). The cyclic voltammetry (CV) was collected at room temperature, with a scan rate of 60 mV s À1 , from À0.1 V to 2.0 V vs. Ag/Ag + .

Hydroxyl radicals generation
The hydroxyl radicals generation by studied materials was examined in the reaction of terephthalic acid (TA) hydroxylation. TiO 2 (0.5 g dm À3 ) suspended in 16 cm 3 of the TA solution (Aldrich, 98%; 3 Â 10 À3 mol dm À3 dissolved in 0.01 mol dm À3 NaOH, pH ¼ 7.6) was irradiated with a xenon lamp (XBO-150, Instytut Fotonowy). To avoid excitation of TA a 320 nm cut-off lter was used as well as a near infra-red and IR lter (10 cm optical path, 0.1 mol dm À3 solution of CuSO 4 ). Samples of 1.5 cm 3 were collected during irradiation, then centrifuged to separate the photocatalyst powder. In the reaction of non-uorescent TA with hydroxyl radicals, hydroxyterephthalic acid (TAOH) is formed. TAOH was monitored by emission spectroscopy. TAOH shows a broad emission band at l max ¼ 425 nm (when excited at l exc ¼ 315 nm). Moreover, TAOH generation in the presence of hydrogen peroxide (2 mmol dm À3 ) was investigated.

Morphology and phase composition
Materials in the form of white powders were used for the catalytic tests. Scanning electron microscopy revealed that anatase and rutile materials form aggregates in size ca. 1 mm and below 0.5 mm, respectively (Fig. 1). The crystal structure of the materials was conrmed using X-ray diffraction (XRD) (Fig. S1 †). The crystal size of anatase and rutile materials is ca. 20 and 50 nm, respectively. In XRD of all samples only one polymorph is present, only CR-EL material contains a small admixture of anatase, up to 1%. 27 Porosimetry measurements showed that the anatase samples have a much higher specic surface area than the rutile materials (Table 1).
Ph 2 SO 2 was the main product of the photocatalytic Ph 2 S oxidation at anatase materials. In the case of N100 material 100% conversion of Ph 2 S was achieved aer 45 minutes of irradiation (Fig. 2a). In the case of the reaction conducted at TRX_A material, total Ph 2 S oxidation was reached aer 1 hour of irradiation ( Fig. 2b). A slightly better selectivity to Ph 2 SO 2 was observed in the case of N100 (100% aer 90 minutes of irradiation) than for TRX_A material. Diphenyl sulphide oxidation at anatase materials in the dark was tested as well. The reaction progress was observed for both anatase samples. Additionally, the reaction products were not observed in the absence of TiO 2 in the reaction mixture, even when the mixture was heated to 40 C. Nevertheless, aer 3 hours of irradiation in the same reaction mixture (without TiO 2 , with H 2 O 2 ) a slight conversion (ca. 7%) of Ph 2 S to Ph 2 SO as the nal product took place ( Fig. S2a †). We suggest that the residual oxidation of diphenyl sulphide is possible due to H 2 O 2 photolysis, although 320 nm cut-off lter was used. The absorbance of the initial reaction mixture is negligible above 320 nm ( Fig. S2b †), however, direct photolysis of H 2 O 2 initiated at any wavelength lower than 380 nm was reported by Cataldo. 28 These results clearly show that the oxidation of Ph 2 S in the presence of anatase materials is possible both as catalytic and photocatalytic processes. However, a signicantly higher conversion of Ph 2 S for photocatalytic conditions compared to the dark processes was observed ( Fig. S3a and b †). A similar positive effect of light was observed when Ph 2 SO as a substrate was used (Fig. S10 †). Both photocatalytic and catalytic oxidation of Ph 2 S at anatase materials leads mainly to Ph 2 SO 2 production with the selectivity of almost 100% aer 3 h of tests (Fig. 2a, b and S3c, d †). Nevertheless, for both anatase materials, during the rst hour more sulfoxide is formed upon irradiation than under purely catalytic conditions.
Results of photocatalytic oxidation of Ph 2 S at rutile materials are depicted in Fig. 2c and d. Photocatalytic conversion of Ph 2 S at rutile materials was signicantly faster compared to both photocatalytic and catalytic conditions applied to anatase materials. The total conversion of Ph 2 S was observed aer 15 and 30 min of irradiation for TRX_R and CR-EL, respectively ( Fig. 2c and d). Based on these data and the measured photon ux the apparent quantum efficiency of Ph 2 S to Ph 2 SO conversion within the rst 15 min of irradiation was $2%. Oxidation of Ph 2 S at rutile materials led to Ph 2 SO as the main product. High selectivity to Ph 2 SO at the beginning of the photocatalytic reaction was noticed (ca. 100%). During the reaction course the selectivity to Ph 2 SO was decreasing, while the production of Ph 2 SO 2 became the dominant process. However, upon further irradiation, the production of Ph 2 SO increased again, hence diphenyl sulfoxide was the main product of the long term photocatalytic oxidation at rutile (Fig. 2c and d). Contrary to anatase, rutile induced no conversion in the absence of light, neither for Ph 2 S nor Ph 2 SO used as a substrate ( Fig. S4 and S11a †).
Interestingly, the total concentration of analyzed reactants (Ph 2 S, Ph 2 SO, Ph 2 SO 2 ) decreased by ca. 5% during the experiments with rutile material under illumination. Moreover, in the case of photocatalytic reaction over CR-EL material, at chromatograms the appearance of low amounts of a new compound, not detected for any other material described above was observed. To elucidate the role of H 2 O 2 in this reaction a new portion of H 2 O 2 was added aer 90 min of irradiation (Fig. S5 †). No increase of the selectivity to Ph 2 SO 2 was observed. Furthermore, in our previous work we showed that aer almost   29 It is worth comparing the presented results with our earlier work where we tested the activity of doped titanium dioxide.
Here we conclude, that rutile is not acting as a catalyst (i.e., it is not active in the dark), however, we previously reported that materials with higher amounts of this polymorph show a higher activity in the oxidation of Ph 2 S. Therefore, it can be concluded that the presence of rutile has an indirect effect on the catalytic processthe material itself is not active, but the coexistence of rutile and anatase enhances the catalytic activity of the materials. Such anatase/rutile composites appeared also very active in photocatalytic transformations, both of Ph 2 S and Ph 2 SO, due to a synergistic effect of these phases reported in numerous publications on P25 and similar materials. 30-34 N100 and TRX_A appear more active catalytically, therefore, in a similar time window the second oxidation product, Ph 2 SO 2 , becomes the main product both in the dark and upon irradiation. The pure rutile phase, despite of its catalytic inactivity, appears the most active photomaterial. In this case a mixture of Ph 2 SO and Ph 2 SO 2 is obtained.

Reaction mechanism
Research studies on sulphides oxidation describe both catalytic and photocatalytic processes. The mechanism of catalytic oxidation involves H 2 O 2 activation on the catalyst surface. Al-Maksound et al. studied catalytic oxidation of various sulphides using H 2 O 2 and self-synthesized TiO 2 (75-80% of anatase). 22 The authors showed high selectivity to sulfones. Rodríguez-Padrón et al. studied microwave-assisted catalytic oxidation of sulphides using H 2 O 2 and commercial anatase, as well as self-synthesized protein-templated anatase TiO 2 in ethanol solution. 35 The authors showed that Ph 2 S oxidation over reference anatase leads to sulfone production, while at proteintemplated TiO 2 mainly sulfoxide was produced. Radko et al. studied catalytic oxidation of sulphides in the presence of titanosilicates and found that the dimethyl sulphide and diphenyl sulphide conversion increased with the increasing titanium content in the zeolite structure. 20 In our current study catalytic Ph 2 S oxidation with H 2 O 2 at anatase TiO 2 to Ph 2 SO 2 was observed. Contrary to that, no catalytic oxidation of Ph 2 S at rutile materials was observe. We suggest that the opposite results can be attributed to the various activation paths of hydrogen peroxide (and maybe Ph 2 S) on the anatase/rutile surfaces. However, the differences can also originate from a low specic surface area of rutile materials, and therefore a low number of active sites. In order to elucidate the effect of specic surface area, the catalytic test with self-synthesized rutile material (SSA of ca. 25 m 2 g À1 ) was performed. Although the specic surface area was 3-5 times higher in this case, no catalytic oxidation of Ph 2 S was observed as well.
Published articles describing photocatalytic oxidation of sulphides postulate that the photocatalytic oxidation is initiated by the formation of a surface bound radical cation generated as a result of oxidation with the photogenerated hole. 29 Also, superoxide radical anion (O 2 c À ) plays a key role in the reaction. [36][37][38] Zhang et al. suggested that O 2 c À reacts with generated organosulfur radical. 39 Lang et al. studied Ph 2 S visible light induced oxidation in the presence of a dye (alizarin red S) and TEMPO as the redox mediator to increase the dye stability. [40][41][42][43][44] The authors reported Ph 2 S conversion to Ph 2 SO, pointing at O 2 c À importance in the reaction mechanism. Li et al. conrmed signicance of superoxide radical anions and sulphide cation radicals by experiments involving radical scavengers and 18 O 2 . 45 Vosooghian et al. showed that oxidation of diphenyl sulphide was effective only in the oxygen rich atmosphere (detectable amounts of products were neither observed in the presence of atmospheric oxygen nor under argon ux). 29 In the presence of photosensitizers able to produce singlet oxygen, it is postulated that 1 O 2 may participate in the Ph 2 S oxidation. [46][47][48][49] The mechanism described in the literature (illustrated in Fig. 3, blue color) cannot fully explain our results, which show that no oxidation products are formed in the absence of hydrogen peroxide. In other words, the mechanism involving Ph 2 Sc + formation and its reaction with either O 2 c À or 1 O 2 should lead to the products (Ph 2 SO, Ph 2 SO 2 ), which have not been detected.
According to our knowledge the role of H 2 O 2 in the mechanism and its inuence on the efficiency of photocatalytic Ph 2 S oxidation have not been reported in the literature. The presence of H 2 O 2 dramatically inuences the generation of HOc and O 2 c À , both at rutile and anatase material (Fig. 3, red color). Photocatalytic production of hydroxyl radicals at rutile is inef-cient, however, the generation of this reactive species is strongly enhanced in the presence of H 2 O 2 , as the result of its reduction. On the other hand, a similar effect of H 2 O 2 on anatase photoactivity is not observed. Addition of H 2 O 2 leads to increasing O 2 c À production at anatase, as the result of H 2 O 2 oxidation with holes. 50 Generally, rutile has better reduction properties compared to anatase, meanwhile, anatase is a better oxidant. 31 Using various TiO 2 phase compositions in the combination with H 2 O 2 can be an effective way to control the selectivity of the reaction in the system, in which a ne product formation is highly dependent on the generated ROS type. The HOc generation was monitored in the reaction of terephthalic acid oxidation. Hydroxyl radicals are generated as the result of a hole reaction with surface hydroxyl groups or adsorbed water molecules. In the reaction of TA with hydroxyl radicals, highly-uorescent hydroxyterephthalic acid (TAOH) is formed. TAOH generation in the presence of hydrogen peroxide (2 mmol dm À3 ) was also investigated. The TAOH formation is depicted in Fig. S6. † In the absence of hydrogen peroxide anatase shows a more efficient HOc generation compared to the rutile (TAOH concentration amounted 57 mmol dm À3 and 29 mmol dm À 3 respectively). However, aer addition of H 2 O 2 to the system containing anatase, TAOH generation decreased (10 mmol dm À3 ), while it signicantly increased for rutile material (452 mmol dm À3 ). Undoubtedly, combination of hydrogen peroxide and rutile signicantly enhances HOc generation.
The presence of hydrogen peroxide can lead to the increased concentration of photogenerated O 2 c À and/or HOc radicals, whereas only the inuence of O 2 c À on the oxidation of Ph 2 S is known. In order to determine the possible role of hydroxyl radicals in the reaction, the Fenton process was induced in the presence of Ph 2 S. In the Fenton reaction (production of HOc in the reaction of H 2 O 2 with iron(II) cations) the conversion of Ph 2 S and efficient production of Ph 2 SO were observed (Fig. 4a). Slight oxidation of Ph 2 SO (Ph 2 SO 2 formation) in the Fenton process was also noticed. However, an inefficient Ph 2 SO 2 generation might originate from the transformation of other species that contaminate the original Ph 2 SO sample (Fig. 4b). This shows that the hydroxyl radical and/or hydrogen peroxide (the reactant in this reaction) is able to oxidize Ph 2 S to Ph 2 SO. Recently, we described that the addition of tert-butyl alcohol (hydroxyl radical scavenger) signicantly diminishes the Ph 2 S conversion. 26 Furthermore, isotopic oxygen (H 2 18 O) experiments, reported by Li et al. revealed that the oxygen atoms of sulfoxide (obtained photocatalytically from phenyl sulphide in water containing mixture, in the absence of H 2 O 2 ) originated mainly from water. 51 We concluded that hydroxyl radicals play a signicant role in the rst step of diphenyl sulphide oxidation, yielding Ph 2 SO. Similarly, in our actual system HOc is involved in the oxidation of Ph 2 S to Ph 2 SO, but the further oxidation to Ph 2 SO 2 is not possible. In order to explain why the Fenton reaction leads only to Ph 2 SO generation, cyclic voltammetry measurements were performed (Fig. 5). In the case of Ph 2 S, two oxidation peaks ðE 0 a;Ph2S ¼ 1:83 V vs: SHE; E 00 a;Ph2S ¼ 2:07 V vs: SHEÞ and one reduction peak ðE 00 c;Ph2S ¼ 2:01 V vs: SHEÞ were observed in the cyclic voltammogram. In the case of Ph 2 SO only one oxidation peak was detected ðE 0 a;Ph2SO ¼ 2:43 V vs: SHEÞ. Finally, the CV of Ph 2 SO 2 has shown neither oxidation nor reduction peaks in the explored potential range. In the case of Ph 2 S cyclic voltammogram, the rst oxidation at 1.83 V vs. SHE can presumably be attributed to Ph 2 Sc + formation. Furthermore, based on the CV measurements the formation of Ph 2 SO as an irreversible process was found. The second oxidation process at 2.07 V vs. c;Ph2S ¼ 2:01 V vs: SHEÞ with oxidation and reduction halfwaves separated by 0.06 V, pointing at one electron reversible process. Based on these considerations we conclude that oxidation of Ph 2 S to Ph 2 SO can be achieved by HOc (HOc/H 2 O ¼ 2.27 V vs. SHE, pH ¼ 7 (ref. 52)). CV of Ph 2 SO reveals that the potential of sulfoxide oxidation is too high to enable its oxidation by hydroxyl radicals. Nevertheless, Ph 2 SO can be still directly oxidized to Ph 2 SOc + by TiO 2 VB holes (step III, Fig. 3). 53 To summarize, the electrochemical and Fenton experiments showed that Ph 2 S can be easily oxidized by HOc to Ph 2 Sc + . However, holes are required for further oxidation of Ph 2 SO to the Ph 2 SOc + radical, which explains why the Fenton reaction led only to Ph 2 SO. The Fenton process does not generate O 2 c À , however, Ph 2 SO as a product of the Fenton reaction was effortlessly detected. It is noteworthy that in the Fenton reaction the concentration of hydrogen peroxide was the same that in the photocatalytic system. The results suggest that H 2 O 2 is involved in the Ph 2 Sc + to Ph 2 SO conversion (Fig. 3). Our  hypothesized mechanism explains the results of photocatalytic oxidation of Ph 2 S at anatase and rutile materials. In the case of rutile, a very fast conversion of Ph 2 S was observed, due to a highly efficient H 2 O 2 reduction, and thus effective HOc formation (Fig. S6 †). These radicals oxidize Ph 2 S within rst few minutes of the reaction. Our study on HOc generation shows that in the presence of rutile and H 2 O 2 ca. 400 mmol dm À3 of TAOH is generated aer 15 min of irradiation (in aqueous solution). This concentration of hydroxyl radicals trapped as TAOH is equal to the initial concentration of Ph 2 S in the tested reaction. These results clearly show that a fast conversion of Ph 2 S at rutile CR-EL is a purely photocatalytic process, being a result of an efficient H 2 O 2 reduction. At anatase, which preferably oxidizes H 2 O 2 , HOc generation is much less effective, therefore, the oxidation rate is lower.
The presented results indicate that hydrogen peroxide and superoxide radical anion play a similar role, oxidizing the radical cations, Ph 2 Sc + and Ph 2 SOc + , to sulfoxide and sulfone, respectively. However, to unequivocally conrm these conclusions, the inuence of oxygen on the reaction was tested. The studied reaction mixtures were purged with argon or oxygen before (15 min) and during the experiments. For the dark reaction at TRX_A, the observed conversion and selectivity were independent on the presence of oxygen (Fig. S7 †). This result suggests that catalytic oxidation of Ph 2 S involves only the activation of hydrogen peroxide activation, without any contribution of O 2 (alternatively, the contribution of oxygen is low, with a negligible inuence on the products formation). However, when TRX_A was used as a photocatalyst (under irradiation), the inuence of O 2 on the reaction progress was signicant. A decrease of the Ph 2 S conversion from ca. 60% under ambient conditions to ca. 35% under argon atmosphere and increased conversion to ca. 75% under the oxygen rich atmosphere was observed (Fig. S8a †). Under the oxygen free conditions, the Ph 2 S conversion decreased, but photocatalytic oxidation of Ph 2 S to Ph 2 SO was still observed. Moreover, changes in the selectivity to Ph 2 SO 2 at TRX_A were observed. The selectivity to Ph 2 SO 2 increased in the following order: under argon, ambient, and oxygen-rich conditions (Fig. S8b †). In the case of the photocatalytic reaction at CR-EL, no signicant changes in the Ph 2 S conversion under various conditions were observed (Fig. S9a †).
Nevertheless, remarkable changes in the selectivity to Ph 2 SO were observed. Under oxygen-rich conditions a decrease in the production of Ph 2 SO, and an increase in the Ph 2 SO 2 production aer rst 15 min of the reaction, were observed (Fig. S9b †). Moreover, the point of the second increase of the Ph 2 SO production appeared earlier (aer 30 min of irradiation) for oxygen-rich conditions.
The rst stable oxidation product, Ph 2 SO, can be further oxidized by holes from the valence band of TiO 2 to Ph 2 SOc + . This intermediate is further transformed to Ph 2 SO 2 in the reaction with superoxide radical (O 2 c À ) formed as the result of either oxygen reduction or H 2 O 2 oxidation. To conrm this path a possibility of Ph 2 SO oxidation, both in the dark and upon irradiation in the presence of hydrogen peroxide and TiO 2 , was studied. In the case of anatase material, an efficient conversion of Ph 2 SO to Ph 2 SO 2 under both catalytic and photocatalytic conditions was observed, with a much better efficiency upon irradiation (Fig. S10 †). In the case of rutile, no conversion was observed in the dark, however, irradiation induced this reaction (Fig. S11 †). Clearly, the higher rate of the photocatalytic reaction in the case of anatase material is the result of a more efficient O 2 c À production at anatase than on the rutile in the presence of H 2 O 2 . Furthermore, the possibility of Ph 2 SO oxidation upon irradiation in the presence of TiO 2 , but in the absence of H 2 O 2 , was tested (Fig. S12 †). Low conversion of Ph 2 SO under such conditions was observedca. 13% and 9% aer 3 hours of irradiation in the presence of anatase and rutile, respectively. In a similar test, performed under anaerobic conditions (Arsaturated solution and addition of Fe 3+ as an electron acceptor), the conversion of Ph 2 SO to Ph 2 SO 2 was not observed. Abovementioned results clearly show that O 2 c À radicals can easily react with Ph 2 SOc + , yielding Ph 2 SO 2 . We suggest that the formation of Ph 2 SO 2 is possible in the presence of O 2 c À and Ph 2 SO + (hole oxidized Ph 2 SO), however, such reaction has a low effectivity. In order to enhance the production of Ph 2 SO 2 , the presence of hydrogen peroxide is necessary. The full mechanism of the photocatalytic Ph 2 S oxidation in the presence of hydrogen peroxide is shown in Fig. 3.

Conclusions
In summary, we demonstrated that Ph 2 S oxidation in the presence of hydrogen peroxide at anatase-TiO 2 can take place both as a catalytic and photocatalytic reaction. In contrast, at rutile materials only photocatalytic oxidation is possible, and no reaction progress was observed in the dark. The reaction at anatase leads mainly to Ph 2 SO 2 as the product, whereas, the same reaction in the presence of rutile leads mainly to Ph 2 SO formation. In the case of rutile the total conversion of Ph 2 S was observed already aer 15 min (compared to at least 1 h required for a catalytic conversion) with nearly 100% selectivity towards Ph 2 SO. It is the rst reported photocatalytic conversion of Ph 2 S using H 2 O 2 in the presence of pure rutile. Moreover, the efficiency of this reaction under the tested conditions is also much higher compared to the systems described in literature so far (apparent quantum yield of ca. 2%, concentration of products in the absence of H 2 O 2 below the detection limit). The conversion level and the reaction selectivity can be controlled by the choice between rutile and anatase polymorphs, as well as by irradiation time (in the case of rutile).
Catalytic oxidation of Ph 2 S involves only the activation of hydrogen peroxide at the catalyst surface, without any contribution of dioxygen. Whereas, the presence of O 2 in the photocatalytic reaction increases the Ph 2 S conversion due to generation of O 2 c À . Studies on the mechanistic details revealed the dual role of H 2 O 2 (Fig. 3). At the same time, it plays the role of a substrate in the reaction catalysed only by anatase, but it also plays a key role in alternative oxidation pathways available through photocatalysis. Hydrogen peroxide can be either oxidized or reduced photocatalytically, resulting in the generation of O 2 c À and HOc radicals, respectively. Hydroxyl and superoxide radicals play an important role in various reaction steps. Hydroxyl radicals can oxidize Ph 2 S to Ph 2 Sc + , however, Ph 2 SO oxidation to Ph 2 SOc + requires a stronger oxidant, i.e. holes. The cation radical intermediates, Ph 2 Sc + and Ph 2 SOc + , react with superoxide anions or directly with H 2 O 2 yielding stable sulfoxide (Ph 2 SO) and sulfone (Ph 2 SO 2 ).
The presented studies prove the applicability of photocatalysis in an efficient and selective synthesis of sulfoxide and sulfone through oxidation of organic sulphides.

Conflicts of interest
There are no conicts to declare.