Pt–TiO2–Nb2O5 heterojunction as effective photocatalyst for the degradation of diclofenac and ketoprofen

Materials Science in Semiconductor Processing 107 (2020) 104839

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Pt–TiO2–Nb2O5 heterojunction as effective photocatalyst for the degradation of diclofenac and ketoprofen �ndez-Laverde b, H. Rojas b, J.A. Navío c, M. O. Sacco a, J.J. Murcia b, A.E. Lara b, M. Herna c a, * C. Hidalgo , V. Vaiano a

Department of Industrial Engineering, University of Salerno, via Giovanni Paolo II, 132, 84084, Fisciano, SA, Italy Grupo de Cat� alisis, Escuela de Ciencias Químicas, Universidad Pedag� ogica y Tecnol� ogica de Colombia UPTC, Avenida Central del Norte, Tunja, Boyac� a, Colombia Instituto de Ciencia de Materiales de Sevilla (ICMS), Consejo Superior de Investigaciones Científicas CSIC–Universidad de Sevilla, Am�erico Vespucio 49, 41092, Seville, Spain b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Photocatalysis Pt–TiO2–Nb2O5 Heterojunction Diclofenac Ketoprofen

Pt–TiO2–Nb2O5 heterojunction was synthetized and studied for the photocatalytic removal of diclofenac (DCF) and ketoprofen (KTF) under UV light irradiation. The physical-chemical properties of the prepared catalysts were analysed by different characterization techniques revealing that the lowest platinum nanoparticle size and the better metal distribution was observed in Pt–TiO2–Nb2O5 sample. The Pt–TiO2–Nb2O5 heterojunction possessed the best photocatalytic activity toward both the photodegradation and mineralization of the two selected pol­ lutants. The optimal photocatalyst showed a DCF and KTF mineralization rate of 0.0555 and 0.0746 min 1, respectively, which were higher than those of Pt–TiO2 (0.0321 min 1 for DCF and 0.0597 min 1 for KTF). The experiments driven to analyse the effects of free radical capture showed that ⋅OH, ⋅O2 and hþ have a primary role in reactive during the photocatalytic reaction. The improved photocatalytic performances of the Pt–TiO2–Nb2O5 heterojunction could be argue by a direct Z-scheme mechanism in which the Pt0 nanoparticles could act as a bridge between TiO2 and Nb2O5, improving the electron-hole separation and, ultimately, enhancing the pho­ tocatalytic removal rate of both DCF and KTF.

1. Introduction Pharmaceuticals constitute a wide and extraordinarily class of organic compounds, generally designed to be both highly active and stable, with the aim to efficiently execute a specific physiological action in humans and animals [1,2]. However, the use of pharmaceuticals in everyday life represents a relevant source of contamination of the aquatic environment, since several of these compounds are not completely metabolized by the lived organisms. They are, therefore, eliminated by excrements and then transported into the wastewater treatment plants (WWTPs) [3]. The most common worldwide used WWTPs are mainly based on the activated sludge technique and are not designed to treat water polluted with pharmaceuticals even if present at low concentrations. Moreover, biological processes based on the acti­ vated sludge in WWTPs, as well as the other following chemical or physical treatments, may convert pharmaceuticals into other products that in some case can be more toxic than the original waste compounds. Therefore the applied treatments are ineffective in their removal [4,5].

Due to their daily use and consequently input into the aquatic envi­ ronment, pharmaceuticals behave as persistent organic pollutants in the aquatic environment [6]. Although, normally pharmaceuticals do not present acute toxic ef­ fects on aquatic organisms due to their presence in trace levels (in the range from ng/L up to μg/L), several worries have been raised for chronic exposure, due to their continuous discharge into aquatic envi­ ronment [7]. Until now the health effects of consuming pharmaceutical waste contaminated water is not well documented, even if some authors [8] reports that fishes and some other aquatic organisms in heavily polluted waters exhibit both male and female sex features. The concern to effectively remove these pollutants from wastewater before their input into water bodies has culminated in the development of other wastewater treatment technologies such as membrane separa­ tion [9], chemical oxidation [10], electrochemical separation [11], biodegradation [12], and advanced oxidation processes (AOPs) [13–17]. AOPs are suggested when water pollutants have a high chemical stability. In fact, they are able to totally mineralize the

* Corresponding author. E-mail address: [email protected] (V. Vaiano). https://doi.org/10.1016/j.mssp.2019.104839 Received 20 May 2019; Received in revised form 17 October 2019; Accepted 11 November 2019 Available online 21 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.

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contaminants into carbon dioxide, water and inorganic compounds or, at least, they allow the partial oxidation of pollutants into more biode­ gradable and/or less toxic substances [14]. Among AOPs, heterogeneous photocatalysis has become an attractive treatment technology to reme­ diate aquatic environmental contamination [18–20]. An ideal photo­ catalyst is a semiconductor chemically and biologically inert, photoactive, stable, inexpensive, non-toxic and active both under visible and UV light [21–24]. Although there are different photocatalysts, the choice is often limited to titanium dioxide (TiO2) [25,26]. Since TiO2 has been the semiconductor most widely employed in photocatalytic pro­ cesses focused in environmental remediation, many efforts of the sci­ entific community has been oriented to improve the photocatalytic efficiency of this oxide [27]. In order to achieve this objective, different alternatives are employed. For example, sulfation is considered a suit­ able method to increase the specific surface area of sol-gel prepared TiO2 [28]. On the other hand, platinum (Pt) photodeposition leads to expand the activity of TiO2 in the visible light region of the electromagnetic spectrum; this phenomena happen because oxidized states of the plat­ inum seem to promote the visible response by photoexciting surface plasmons in the metal atom [29–32]. Pt addition on the TiO2 surface is also able to avoid the recombination of the photogenerated charges because metal nanoparticles act as electron sink [29,33]. Moreover, several studies reports that for further improving the performances of the photocatalytic process, one way could be the coupling two semi­ conductors material to form a heterojunction [34,35]. Typically, in the semiconductors heterojunction, under light irradiation, the photo­ generated electrons can thermodynamically migrate from the conduc­ tion band (CB) with lower potential of one semiconductor to the CB band of the second semiconductor with higher potential. Instead, the migra­ tion of electrons from the valence band (VB) of the first semiconductor to the second one with lower VB potential to occupy the holes generated there, induces an excess of positive charge in the VB of the first semi­ conductor. Consequently, the number of available charges increases, allowing a higher velocity of generation of reactive oxidant species (ROS) that can oxidize the pollutants [36]. Over the past decade, TiO2-based heterojunctions, such as ZnO/TiO2 [37], WO3/TiO2 [38], CdS/TiO2 [39] and g-C3N4/BiOBr [40] have been proved to be effective to increase the photocatalytic or photoelectrocatalytic activity. Another suitable semiconductor to be coupled with TiO2 could be Nb2O5. The latter is n-type transition metal oxide semiconductor widely used in heterogeneous photocatalysis [41–43]. Therefore, Nb2O5–TiO2 system can be considered as promising photocatalyst for application in the water and wastewater treatment [44]. On this basis, the further addition of electron transfer mediator like Pt [45], embedded in the Nb2O5–TiO2 interface can strength the charge transfer and consequently result in a vectorial electron transfer via a Z-scheme system [46], which can consequently increase the photocatalytic activity. To the best of our knowledge, the TiO2–Nb2O5 heterojunction photocatalyst with Pt nanoparticles deposited on its surface has not been studied before for the photocatalytic degradation of pharmaceuticals. For this reason, the aim of this work is to study the effect of Pt addition on TiO2–Nb2O5 com­ posite towards the photodegradation of two different pharmaceuticals compounds. In particular, diclofenac (DCF) and ketoprofen (KTF) were used as target pharmaceuticals contaminants, since they are frequently detected in aquatic environment such as surface water, and drinking water [47].

the photocatalytic activity of the oxides under study, the synthesis of different series of photocatalysts was carried out. Firstly, the commercial oxides were modified by sulfation. Then, the preparation of a mixture between TiO2 and Nb2O5 oxides was carried out and finally these ma­ terials were modified by platinum addition. The explanation of these procedures is included below. The mixture between commercial titania and niobia (1:1 Nb:Ti weight ratio) was realized by adding the two oxides under stirring for 1 h in isopropanol suspension (100 mL). After that, the solvent was evapo­ rated at 82 � C. The final sample was labelled as TiO2–Nb2O5. The sulfated and platinized materials were prepared by following the methodology previously reported by Iervolino et al. [28]. Briefly, the commercial materials were sulfated by immersion in a 1 M H2SO4 so­ lution and stirred for 1 h. The precipitate of sulfated TiO2 or Nb2O5 was recovered by filtration, dried and calcined at 650 � C for 2 h, with a heating ramp of 4 � C min 1. Then the platinized oxides were achieved by photodeposition method starting from hexachloroplatinic acid (H2PtCl6, Aldrich 99.9%) as Pt precursor. The nominal amount of Pt was equal to 0.5 wt %. The metal content was chosen since in our previously works, the addition of 0.5 wt% Pt on TiO2 has demonstrated to be suitable to obtain a good nanoparticles distribution on TiO2 surface and significantly improving of photocatalytic activity of this oxide [29,50,51]. The 0.5 wt% of Pt photodeposition on semiconductor surface was realized starting from a suspension of the sulfated oxides in distilled water containing iso­ propanol (Merck 99.8%) and the appropriate amount of platinum pre­ cursor was prepared. The suspension, under N2 atmosphere, was subsequently irradiated for 120 min with an Osram Ultra-Vitalux lamp (300 W) which possesses a sun-like radiation spectrum with a main emission line in the UVA range at 365 nm. Light intensity on the sus­ pensions, used for the photodeposition of platinum, was 60 W m 2. After the Pt photodeposition, the powders were recovered by filtration and dried at 110 � C for 12 h. All the prepared photocatalysts are reported in Table 1. 2.2. Photocatalytic materials characterization Structural, morphological and optical properties of the photo­ catalysts were evaluated by different characterization techniques. N2 physisorption: Specific surface area (SBET) measurements were carried out at low-temperature N2 adsorption by using a Micromeritics ASAP 2010 instrument. Before analysis a degasification of the samples was performed at 150 � C. UV–Vis spectrophotometry – Diffuse reflectance (UV–Vis DRS): This technique was employed for the evaluation of the light absorption properties in the photocatalysts. The spectra were collected by using a T90þ UV–Vis PG Instruments Ltd. (Thermovision TH) spectrophotom­ eter equipped with an integrating sphere and using BaSO4 as reference. Band-gaps values were calculated from the corresponding Kubel­ ka–Munk functions, F(R∞), which are proportional to the absorption of radiation by plotting (F(R∞) � hυ)1/2 against hυ. X-ray diffraction (XRD): XRD patterns were obtained on a PW1700 Table 1 Summary of the characterization results for the tested photocatalysts. Photocatalyst

2. Experimental

TiO2 Nb2O5 S–TiO2 Pt–TiO2 Pt–Nb2O5 Pt–S–TiO2 Pt–S–Nb2O5 Pt–TiO2–Nb2O5 Pt–S–TiO2–Nb2O5

2.1. Photocatalytic materials preparation Commercial titania (TiO2) and niobia (Nb2O5) (Aldrich) was employed as reference materials and used as received. It has been extensively reported that sulfation and platinization are effective methods to improve the photocatalytic properties of TiO2 [28, 29,33,48,49]. So, in order to evaluate the effect of these treatments over 2

SBET (m2/g) 9.10 3.33 8.18 6.74 2.60 8.80 2.20 5.50 5.82

Band gap (eV) 3.33 3.40 3.28 3.17 3.30 3.20 3.30 3.25 3.24

Binding energy (eV) Ti 2p

O 1s

458.5 458.5 458.7 – 458.5 458.5 458.7

529.8 529,9 529.8 529.8 529,9 529.8 529.8 529.8 529.9

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Philips diffractometer equipped with Ni filter and graphite mono­ chromator, the Cu Kα radiation was employed; all analyses were carried out between 10 and 80� 2θ. Peaks were fitted by using a Voigt function. Transmission Electron Microscopy (TEM): Morphology of the samples and platinum particles sizes were evaluated using a Philips CM200 instrument. Before the analysis, the samples were putting on ultrasound and dispersed in ethanol. After that, a drop of the samples was setting on a carbon grid. X-ray photoelectron spectroscopy (XPS): The XPS analyses were performed by using a Leybold–Heraeus LHS-10 spectrometer. Constant pass energy of 50 eV was employed. The main chamber, works at a pressure lower than 2 � 10 9 Torr. This equipment includes an EA200MCD hemispherical electron analyser with a dual X-ray source working with Al Kα (hυ ¼ 1486.6 eV) at 120 W and 30 mA. C 1s signal located at 284.6 eV was used as internal energy reference. In order to remove chemisorbed water, it was necessary to degas the samples in the prechamber of the instrument. These assays were carried out at 150 � C and a pressure lower than 2 � 10 8 Torr. Fourier transformed infrared spectroscopy (FT-IR): A Thermo Scientific-Nicolet iS10 equipment was employed for these analyses. The samples were analysed in ATR mode; the spectra were collected at wavenumber between 4000 and 400 cm 1, with 2 cm 1 as resolution value.

of the oxides analysed was observed in the platinized materials. From the UV–Vis DRS spectra shown in Fig. 1, it is possible to note that neither TiO2 nor Nb2O5 (Fig. 1a) present absorption in the visible region of the electromagnetic spectrum [49]. However, as expected, after platinum addition all materials exhibit absorption in the visible region (Fig. 1b). In addition, with respect to Pt–TiO2 and Pt–S–TiO2, the absorption edges of Pt–TiO2–Nb2O5 and Pt–S–TiO2–Nb2O5 samples appear at higher wavelength (in the region of 410–415 nm) but lower than the absorption edges observed for Pt–Nb2O5 and Pt–S–Nb2O5, in agreement with literature data [44,49]. It is worthwhile to note that for the mixed oxides samples (Pt–TiO2–Nb2O5 and Pt–S–TiO2–Nb2O5), no significantly decrease in the light absorption in UV region can be detectable, indi­ cating the possible absence of a direct interface charge transfer between TiO2 and Nb2O5 [49] and suggesting that Pt could have a key role in the charge transfer interaction mechanism in the heterojunction. The XRD patterns for all the photocatalysts are presented in Fig. 2. As it can be seen, titania in anatase phase is detected in Pt–TiO2 sample, identified by its main diffraction peak located at about 25� [55]. The

2.3. Photocatalytic tests The photocatalytic tests were made in a pyrex photoreactor having a cylindrical geometry (ID ¼ 2.6 cm, LTOT ¼ 41 cm and VTOT ¼ 200 mL). UV-LEDs strip was provided by LEDlightinghut (nominal power: 12 W/m; light intensity: 48 mW/cm2; main wavelength emission: 365 nm). The UV-LEDs strip was positioned around and in contact with the external body of the photoreactor to uniformly irradiate the volume of the so­ lution. The suspension inside the reactor was continuously recirculated thanks to an external peristaltic pump (Watson Marlow). The total volume of solution was 100 mL with an initial concentration of DCF and KTF at 12.5 mg L 1 while the typical photocatalyst dosage was 0.5 g L 1. Before the irradiation, the suspension was left in dark conditions for 120 min to achieve the adsorption/desorption equilibrium of the pol­ lutants on the photocatalyst surface and after, the photocatalytic test was began under UV light irradiation up to 60 min. At regular times, about 2 mL of the suspension was withdrawn and filtered in order to remove photocatalyst powders. The photodegradation of the chosen pharmaceuticals was monitored by measuring the maximum absorbance values of the organic molecules using a UV–Vis spectrophotometer (Evolution 201). In particular, the maximum absorption peak for CEF and KTF was 275 nm [52] and 260 nm [53], respectively. The Total Organic Carbon (TOC) was measured by a catalytic (Pt–Al2O3) combustion method in a tubular flow reactor operated at 680 � C. The solution was injected in the reactor fed with air to oxidize the organic carbon into CO2, whose concentration in gas–­ phase was monitored by a continuous analyser (Uras 14, ABB) [54]. 3. Results and discussion 3.1. Photocatalytic materials characterization A summary of the characterization results is presented in Table 1. Firstly, it can be seen that Pt–Nb2O5 sample presents a surface area lower than Pt–TiO2. After sulfation, the specific surface area of TiO2 slightly increases. On the contrary, after sulfation and platinization the surface area of Nb2O5 decreases. The materials containing mixed oxides present an intermediate SBET value compared with other samples. Band gap values were calculated by UV–Vis DRS analyses. These data are included in Table 1, being in the range 3.17–3.30 eV. It seems that sulfation, platinization or oxides combination did not have a significant effect over this parameter. Just a slight decreasing of the band gap value

Fig. 1. UV–Vis DRS spectra of (a) bare oxides and (b) platinized materials. 3

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main peak due to the (1 0 0) plane corresponding to crystalline niobia in hexagonal phase was identified in the samples containing this oxide at 28.58� [6]. It appears that sulfation pre-treatment did not modify the crystalline structure of TiO2 or Nb2O5. In the samples prepared with mixed oxides, the characteristic peaks of both niobia and titania were detected (25 and 28.58� , respectively). No signals of platinum were observed in the samples [56], probably due to the low content of this metal which is under the detection limit of this technique. Fig. 3 shows selected TEM images of the photocatalysts. In general, platinum nanoparticles are heterogeneously distributed on supports surface (Fig. 3a, b, 3c and 3f). It is also noted that there are zones with high agglomeration of Pt particles. The lowest platinum nanoparticles size was observed in Pt–TiO2–Nb2O5 sample (Fig. 3f). It appears that the simultaneous presence of both oxides favours the more homogeneous photodeposition of Pt nanoparticles over each oxide. It is also appearing that in this sample (Pt–TiO2–Nb2O5) the agglomeration of metallic particles is lower than the observed in other analysed samples. XPS analyses were also carried out (Fig. 4). A signal located in O1s region at 529�1 eV was observed in all the photocatalysts due to the oxygen atoms in the TiO2 and/or Nb2O5 lattice [3,6]. In the Ti2p region of the TiO2 based photocatalysts (Pt–TiO2, Pt–S–TiO2–Nb2O5 and Pt–TiO2–Nb2O5), the main peak is located at 458�1eV. This signal is assigned to Ti4þ ions in TiO2 lattice [3]. For the Nb2O5 based samples, the main binding energy values in the Nb 3d region were observed at

about 206 and 209 eV, which are consistent with literature about XPS of Nb2O5/TiO2 heterojunctions [49]. Any significant difference in the in­ tensity or position of the signals previously identified was observed for all the analysed samples. The software UNIFIT 2009 [7] was employed to make the decon­ volution of the Pt 4f region and selected spectra are presented in Fig. 5. From these analyses, it was possible to identify the characteristic doublet of platinum (4f7/2 and 4f5/2). Moreover, in all the samples, it was observed the presence of reduced and oxidized Pt species. In fact, the main peaks of metallic (Pt0) and partially oxidized platinum species (Ptδþ) are centred at binding energies of 70.5 and 75.5 eV, respectively [3]. From the comparison of the obtained spectra, it is possible to observe that sulfation or the presence of the mixed oxide did not modify the position of the signals identified or the platinum oxidation states. Fig. S1 (included in supplementary material) shows the survey spectra obtained from selected photocatalysts analysed, while Table S1 includes the atomic composition of the photocatalytic materials measured by XPS. A qualitative FTIR study of all the photocatalysts was also performed. Fig. 6 shows selected FTIR spectra obtained for the analysed samples in the range between 4000 and 2400 cm 1. In the TiO2 based samples, it is possible to identify a band located at 3698 cm 1 corresponding to iso­ lated hydroxyl groups (Ti–OH). Two additional clear bands at 3393 and 3214 cm 1 were also identified. These bands are characteristic of the

Fig. 2. XRD patterns for the analysed photocatalysts. 4

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Fig. 3. TEM images of (a) Pt–TiO2, (b) Pt–Nb2O5, (c) Pt–S–Nb2O5, (d) Pt–S–TiO2, (e) Pt–S–TiO2–Nb2O5 and (f) Pt–TiO2–Nb2O5.

5

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the experiment results in terms of DCF degradation as a function of irradiation time. In the absence of the photocatalyst (photolysis), the DCF concentration did not change under UV irradiation for 60 min, indicating that the photolysis of DCF can be neglected. Different trend was observed in the presence of all the photocatalysts. In fact under UV irradiation, the DCF relative concentration progressively decreased. In particular, after 60 min of UV irradiation, the DCF degradation was equal to 17 and 95% using TiO2 and S–TiO2, respectively. The obtained results could be interpret taking into account the improvement of the acid properties of sulfated TiO2 that, typically, increases the photo­ catalytic performances towards the degradation of organic pollutants [59]. For further improving the photocatalytic activity, Pt was deposited on the surface of TiO2 and S–TiO2. As it is possible to observe from the data reported in Fig. 7a, the Pt–TiO2 sample demonstrated photoactivity higher than TiO2 and S–TiO2, leading to the almost total DCF degrada­ tion in 30 min of UV irradiation. The raise of the performances can be ascribe to the Pt particles that work as electrons wells, thus inhibiting the recombination of photogenerated charge carriers [28]. In addition, as it was observed by XPS analyses (Fig. 5), all platinized samples pre­ sent platinum oxidized (Ptδþ) and metallic species (Pt0). These species, mainly Ptδþ, can act as adsorption sites for the strongly electronegative – O groups; a better drug adsorption (such as DCF molecule) by OH or C– on semiconductor surface improve the efficiency in the pollutant pho­ todegradation [29]. With respect to Pt–TiO2, a lower DCF degradation was observed with Pt–S–TiO2 sample (84% after 30 min of UV irradia­ tion), clearly demonstrating that, in the case of DCF, the metallization of the sulfated TiO2 led to a decrease of photocatalytic activity. This last result it can be related to the decreasing in the absorption of Pt–S–TiO2 material in the visible region of the electromagnetic spectrum, as it was observed by UV–Vis DRS (Fig. 1b). Moreover, the activity of TiO2 was also compared with the activity of the Nb2O5, which until now is not well explored as photocatalyst for the DCF degradation. Surprising, at the same irradiation time (60 min), the DCF degradation was higher using Nb2O5 than TiO2, being 26 and 17% for Nb2O5 and TiO2, respectively. The best performance observed by using niobia as photo­ catalyst may be favoured by an acid-base interaction, since Nb2O5 is a strong acid solid [60] and DCF has cationic basic group (-NH) in its molecular structure. Similar results were obtained in the methylene blue degradation [44]. A different effect was observed for S–Nb2O5 and Pt–Nb2O5. In contrast with literature dealing with the photocatalytic hydrogen pro­ duction [61], in our case, the presence of Pt on Nb2O5 surface did not induce a significant enhancement of photocatalytic activity. The inter­ action of TiO2 and Nb2O5 in presence of Pt particles (Pt–TiO2–Nb2O5) improves the photocatalytic performances in term of DCF degradation, allowing to achieve the complete DCF degradation after 20 min of UV irradiation time. As it can be seen, the preparation method employed is effective enough to achieve the coupling of titania and niobia and therefore to obtain the best properties of each oxide in the same mate­ rial. Fig. 7b reports the experimental results in terms of KTF degradation as a function of irradiation time. The results showed that under only UV irradiation (photolysis), the KTF concentration gradually decreases [62, 63], until to reach the degradation value equal to 80% after 60 min. It is important to underline that this value did not change after 120 and 180 min of irradiation (data not reported). The photodegradation rate increased when the studied photocatalysts are used. In particular, the best results were observed using Pt–TiO2, Pt–S–TiO2 and Pt–TiO2–Nb2O5 photocatalysts, which allowed to achieve the total KTF degradation after 30 min of UV irradiation. In addition, it is important to underline that also in case of KTF, the Nb2O5 photocatalyst showed an unexpected high photocatalytic degradation performances (100% after 60 min of UV irradiation), slightly higher than TiO2 (90% after 60 min of UV irradiation). In addition, in order to characterize the mineralization performance of the tested photocatalysts, TOC measurements were also carried out after 60 min of UV irradiation (Table 2). In particular, in the case of DCF,

Fig. 4. XPS analyses for Ti2p, O1s and Nb3d regions for all the photocatalysts.

interaction between water and surface titanium atoms (Ti–OH2) through hydrogen bonds [8]. The spectra of Nb2O5 based photocatalysts also show hydroxyl groups signals in the FTIR spectra [57,58]. Sulfation, platinum or niobia addition did not significantly alter the TiO2 spectrum. 3.2. Diclofenac and ketoprofen photodegradation 3.2.1. Screening of the prepared photocatalysts The photocatalytic degradation of DCF and KTF under UV light irradiation was investigated using all the synthetized photocatalysts (Fig. 7). The photocatalytic degradation was carried out by using 100 mL of DCF or KTF solution with an initial pollutant concentration of 12.5 mg L 1 and a catalyst dosage of 0.5 g L 1. In detail, Fig. 7a reports 6

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Fig. 5. XPS spectra deconvoluted in the Pt 4f region for (a) Pt–Nb2O5, (b) Pt–S–TiO2, (c) Pt–S–TiO2–Nb2O5 and (d) Pt–S–TiO2–Nb2O5.

Fig. 6. FTIR spectra for the photocatalysts analysed.

70 and 54% TOC removal were achieved using Pt–TiO2 and Pt–S–TiO2, respectively, while the almost total TOC removal was obtained with Pt–TiO2–Nb2O5. Rizzo et al. [64] observed a similar result after 120 min of UV irradiation in presence of TiO2. In contrast to the DCF results, in the case of KTF, Pt–TiO2, Pt–S–TiO2 and Pt–Nb2O5–TiO2 showed a similar TOC removal being, in all cases, higher than 90%.

the pseudo-first-order kinetics [63,64]. The photodegradation rate (r) depends on the pollutant concentration or TOC in liquid phase in agreement with the following equation Eq. (1): r ¼ k⋅y

(1)

where y is the concentration of DCF or KTF (in mg⋅L 1) or TOC (in mgc L 1) and k is the kinetic constant (in min 1). Considering the mass balance (Eq. (2)) and integrating it between initial time (t ¼ 0) and a generic irradiation time t, it was obtained the equation Eq. (3)

3.2.2. Kinetics evaluation of pollutants degradation and mineralization The apparent kinetic constant for pharmaceuticals degradation and mineralization was evaluated (Table 2) in order to understand the in­ fluence of Nb2O5 presence in the platinized samples. For this purpose it was considered that the DCF and KTF photodegradation reaction follows 7

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dy ¼ dt ln

k⋅y

(2)

� � y ¼ k⋅t y0

(3)

The value of the kinetic constant k can be calculated by the slope of � � the straight line obtained by plotting ln yy0 versus the irradiation time (t). The obtained k values for all the investigated photocatalysts are reported in Table 2. Based on the calculated pseudo-first-order rate constant k, the halflife time (t1/2, min) for the photodegradation and mineralization of the tested pollutants was determined according to Eq. (4). t1=2 ¼

Table 2 Kinetic constant (k) and half-life time (t1/2) values for degradation and miner­ alization process with together TOC removal after 60 min of UV irradiation. Catalysts

Degradation

Mineralization

k, min

1

t1/2, min

TOC, %

k, min

1

t1/2, min

DCF

Pt–TiO2 Pt–S–TiO2 Pt–TiO2–Nb2O5

0.481 0.077 0.446

1.45 9.00 1.52

70 54 97

0.0321 0.0123 0.0555

21.59 56.35 12.48

KTF

Pt–TiO2 Pt–S–TiO2 Pt–TiO2–Nb2O5

0.143 0.136 0.174

4.85 5.09 3.98

96 93 98

0.0597 0.0392 0.0746

11.61 17.68 9.29

(4)

The obtained values for t1/2 are also reported in Table 2. It is possible to observe that, for both pollutants, the highest k and t1/2 values in terms of degradation and mineralization were obtained with Pt–TiO2–Nb2O5 photocatalyst. According to the obtained values, it can be deduced that the obtained t1/2 is sensibly lower than that one reported in the literature concerning the removal of pharmaceuticals by means of heterogeneous photocatalysis [65,66], confirming that the Pt–TiO2–Nb2O5 photo­ catalyst showed a photocatalytic activity higher than TiO2 based photocatalysts. Furthermore, it is important to observe that, in terms of minerali­ zation, the DCF kinetic constants is always lower than KTF probably due to chloride ions, generated during the degradation process that nega­ tively affected the photocatalytic removal efficiency [67]. However, it is worthwhile to note that, for both pollutants, the almost complete TOC removal has been observed using Pt–TiO2–Nb2O5 photocatalyst. As it was observed in the results previously presented in section 3.1, the mix of the oxides have a positive effect over physicochemical properties such as surface area and leads to decrease the band gap value. It can be also qualitatively observed by TEM analyses (Fig. 3) that platinum nano­ particles are more homogeneously distributed on TiO2–Nb2O5 than in other catalysts analysed. It is also observed that Pt nanoparticles present lower agglomeration in this sample. In summary, it is possible to argue that, for both tested pharma­ ceuticals, the best photodegradation and mineralization performances were obtained using Pt–TiO2–Nb2O5 photocatalyst, probably due to the combination of both oxides in the same material. In this way it was possible to obtain the better properties of each oxide, thus favouring the photocatalytic activity in the mineralization of the pharmaceuticals under study. Similar results have been previously reported in the degradation or organic dyes by using TiO2–Nb2O5 composites [44,68]. For this reason, Pt–TiO2–Nb2O5 sample was chosen to investigate the influence of photocatalyst dosage and pollutants initial concentration on photocatalytic performances.

Fig. 7. Photocatalytic degradation under UV light irradtiation of (a) DCF and (b) KTF using the different prepared samples.

Pollutant

ln 2 k

3.2.3. Influence of Pt–TiO2–Nb2O5 dosage To guarantee efficiency absorption of photons, the optimum dosage of Pt–TiO2–Nb2O5 photocatalyst was investigated. The experiments were carried out using different catalyst amounts (in the range 0.1–0.75 g L 1), while the initial DCF and KTF concentration was kept constant at 12.5 mg L 1. The results, as a function of irradiation time, in terms of DCF and KTF degradation were reported in Fig. 8. It can be noted that the photocatalytic activity of Pt–TiO2–Nb2O5 gradually improved with the increase of its dosage from 0.1 to 0.5 g L 1 both for DCF (Fig. 8a) and KTF (Fig. 8b), while at dosage higher than 0.5 g L 1 (0.75 g L 1), the photocatalytic performances decrease in case of DCF and did not change for KTF, reaching the complete degradation after 20 min of UV light irradiation.

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3.3. Possible photocatalytic mechanisms 3.3.1. The roles of reactive oxygen species (ROS) To undersrand the role of the ROS during the photocatalytic process, the effects of free radical capture were investigate to analyse the possible role of ∙OH, ∙O2 and hþ in the DCF and KTF photodegradation using Pt–TiO2–Nb2O5 photocatalysts with a dosage of 0.5 g L 1 and initial concentration of both pollutants equal to 12.5 mg L 1. The corre­ sponding scavenger probe molecules were: (AI) isopropanol (1 mmol L 1) for ∙OH [70], (BQ) benzoquinone (0.5 μmol L 1) for ∙O2 [71] and disodium ethylenediaminetetra-acetate (EDTA) (0.17 mmol L 1) for hþ. Fig. 9 shows the effects of the scavengers for the DCF and KTF photo­ degradation. In details, the addition of AI, BQ, and EDTA decreased both the DCF (Fig. 9a) and KTF (Fig. 9b) degradation rate, indicating that ⋅OH, ⋅O2 and hþ were all responsible for ROS during the photocatalytic degradation process. In particular, in the case of DCF, the inhibition of degradation rate due to EDTA addition was greater than BQ and AI, indicating that hþ played the most important role in the photocatalytic DCF degradation mechanism. For KTF, the photocatalytic performance was inhibited after the addition of the scavenger probe molecules,

Fig. 8. Photocatalytic degradation under UV light irradiation of (a) DCF and (b) KTF using different dosages of Pt–TiO2–Nb2O5 sample.

3.2.4. Influence of pollutants initial concentration The effect of DCF and KTF initial concentration, in the range 12.5–25 mg L 1, has been investigated by using 0.5 g L 1 of Pt–TiO2–Nb2O5 dosage. In particular, Fig. S2 (included in supplemen­ tary material) shows the results in terms of DCF and KTF degradation as a function of irradiation time. With regard to the photocatalytic DCF degradation (Fig. S2a, supplementary material), the activity of the Pt–TiO2–Nb2O5 photocatalyst remained unchanged from 6.5 to 12.5 mg L 1, reaching the complete degradation after 20 min of UV light irra­ diation, while for the further increase of initial concentration (25 mg L 1), the activity decreased until to reach the total degradation after 60 min. In agreement with literature [69], the declining of the photo­ catalytic activity at 25 mg L 1 initial concentration could be ascribed to the formation of ∙OH that our case was not sufficient to effectively remove DCF molecules from aqueous solution. Fig. S2b (included in supplementary material) shows the results in terms of KTF degradation. It is possible to observe that the activity of the Pt–TiO2–Nb2O5 photo­ catalyst decreased with the increase of the initial pollutant concentra­ tion from 6.5 up to 25 mg L 1, reaching the complete degradation after 20 and 60 min of UV light irradiation, respectively. Fig. 9. Effects of different scavengers on (a) DCF and (b) KTF photodegradation using Pt–TiO2–Nb2O5 under UV light irradiation. 9

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showing, in all cases, a similar degradation rate. This result clearly ev­ idences that ⋅OH, ⋅O2 and hþ are responsible of the KTF degradation mechanism at the same extent.

The CB edge potential (ECB) can be determined using the following relationship:

3.3.2. Proposed mechanism Considering the results from quenching studies (OH⋅, ⋅O2 and hþ were all responsible for ROS during the photocatalytic processes) and optical band gap measurements, the primary mechanism of Pt–TiO2–Nb2O5 photocatalyst was investigated. Generally, the capacity of a photocatalysts to move excited electrons to organic species previ­ ously adsorbed on its surface is controlled by the electronegativity of the conduction band (CB) and valence band (VB), together with the redox potentials of the adsorbate [72]. The electronegativity of CB and VB of a photocatalyst can be empirically calculated by Eq. (5) and Eq. (6) [73]:

Therefore, using the Ebg values for the tested photocatalysts (Table 1), it is necessary to evaluated X defined as the geometric mean of the electronegativities of the constituent atoms using the concept of the semiconductor electronegativity (Eq. (7)): qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi XðSÞ ¼ N X n1 X s2 ⋅⋅⋅X pn 1 X qn (7)

EVB ¼ X

Ee þ 0:5Ebg

ECB ¼ EVB

Ebg

(6)

where Xn, n and N are the electronegativity of the constituent atom, the number of species, and the total number of atoms in the compound, respectively [74,75].Using Eq. (7), the X value of TiO2 resulted to be equal to 5.8 eV, while the X value for Nb2O5 was 6.09 eV. Considering the calculated X and Ebg values, it was possible to calculate the EVB and the ECB potentials vs. NHE for TiO2 and Nb2O5 nanoparticles. In the case of TiO2, the obtained values were EVB ¼ 2.95 eV and ECB ¼ 0.35 eV vs. NHE, while for Nb2O5, it was found EVB ¼ 3.29 eV and ECB ¼ 0.1 eV vs. NHE. During the light irradiation there are two types of electron sepa­ ration processes for the photogenerated electron-hole: one is a double-transfer mechanism and the second one is a Z-scheme

(5)

where: X is the electronegativity of the semiconductor Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV) Ebg is the optical band gap energy of the semiconductor (calculated by the method described above).

Fig. 10. Energy band diagram scheme of the Pt–TiO2–Nb2O5 system: (a) double-transfer mechanism and (b) Z-scheme mechanism. 10

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mechanism (Fig. 10) [76]. The double-transfer mechanism (which is a common separation process for a large number of composite photo­ catalysts [77,78]) is shown in Fig. 10a. The photogenerated electrons in the CB of TiO2 during the UV irradiation could transfer to CB of the Nb2O5, and the holes accumulate on the VB of TiO2. However, in this case, as the CB of the Nb2O5 ( 0.1 eV vs. NHE) is less negative than the potentials of the O2/⋅O2 ( 0.33 eV vs. NHE) [79], the electrons pho­ togenerated on Nb2O5 are unable to reduce O2 to yield ⋅O2 , in contrast with the obtained experimental data (Fig. 9). On the contrary, the VB of TiO2 (þ2.95 eV vs. NHE) and Nb2O5 (þ3.29 eV vs. NHE) potentials are more positive than ⋅OH/OH (þ2.38 eV vs. NHE) and ⋅OH/H2O (þ2.72 eV vs. NHE) [80], and therefore could react with the OH and H2O to form ⋅OH radicals. According to the TEM analysis (Fig. 3) the Pt nano­ particles are heterogeneously distributed on both TiO2 and Nb2O5 sur­ face. Generally, the presence of Pt0 nanoparticles on semiconductor surface may become the center for trapping photoelectrons, enhancing the photocatalytic activity [29,81]. However, this hypothesis is in contrast with the reactive species-trapping experiments reported in Fig. 9. So, in the case of Pt–TiO2–Nb2O5, the double-transfer mechanism played a minor role. As a consequence, the electron transfer might follow a direct Z-scheme on the photocatalyst (Fig. 10b). The Pt0 nanoparticles could act as a bridge between TiO2 and Nb2O5 [82,83]. The Pt0 had a relatively low Fermi level (EF ¼ 0.5 V vs. NHE), which could serve as the electron acceptor for the photoexcited electrons. During the light irradiation, the electrons formed in the CB of Nb2O5 could be transferred to the VB of TiO2 through the Pt0 nanoparticles that have the role of promoters for the electron-hole separation [70,84]. At the same time, the holes in the VB of the TiO2 could transfer to the Pt0 nanoparticles and combine with the electrons in the CB of Nb2O5. In summary, the presence of Pt0, with a function of charge transmission bridge, could improve the electron-hole separation [85], allowing the electrons and holes to stay on the CB of the TiO2 and VB of Nb2O5, respectively. Moreover, with the hypothesis of the direct Z-scheme, the electrons in the CB of TiO2 could reduce the O2 to yield ⋅O2 , since CB of TiO2 ( 0.35 eV vs. NHE) was more negative than the potentials of O2/⋅O2 ( 0.33 eV vs. NHE). Furthermore, the holes on the VB of Nb2O5 could react with the OH or H2O to form ⋅OH radicals, since the VB of Nb2O5 (þ3.29 eV vs. NHE) was more positive than the potentials of the ⋅OH/OH (þ2.38 eV vs. NHE) and the ⋅OH/H2O (þ2.72 eV vs. NHE). In this case, the hypothesis is in good agreement with the experimental results on the role of ROS (Fig. 9), which evidenced that OH⋅, ⋅O2 and hþ have a role in the degradation of the DCF and KTF.

considering that the Pt–TiO2–Nb2O5 heterojunction follows a direct Zscheme mechanism in which the Pt0 nanoparticles could act as a bridge between TiO2 and Nb2O5, improving the electron-hole separation and, therefore, enhancing the photocatalytic removal rate of both DCF and KTF. Acknowledgements �n de Boyaca � for the A.E. Lara would like to thank Gobernacio concession of a PhD researcher grant. This work was financed with re­ sources from the Fondo Nacional de Financiamiento para la Ciencia, la �n (FCTel) del Sistema general de regalías (Con­ Tecnología e Innovacio vocatory No. 733 for the training of high level human resources of the � and Colciencias) and from Universidad Ped­ department of Boyaca �gica y Tecnolo �gica de Colombia. Moreover, a special thank is given ago to Eng. Paola Cortese for the support given in the photocatalytic tests. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104839. References [1] B. Halling-Sørensen, S. Nors Nielsen, P.F. Lanzky, F. Ingerslev, H.C. Holten Lützhøft, S.E. Jørgensen, Occurrence, fate and effects of pharmaceutical substances in the environment- A review, Chemosphere 36 (1998) 357–393. [2] K. Kümmerer, The presence of pharmaceuticals in the environment due to human use – present knowledge and future challenges, J. Environ. Manag. 90 (2009) 2354–2366. [3] N.H. Tran, M. Reinhard, K.Y.-H. Gin, Occurrence and fate of emerging contaminants in municipal wastewater treatment plants from different geographical regions-a review, Water Res. 133 (2018) 182–207. [4] J.H.O.S. Pereira, V.J.P. Vilar, M.T. Borges, O. Gonz� alez, S. Esplugas, R.A. R. Boaventura, Photocatalytic degradation of oxytetracycline using TiO2 under natural and simulated solar radiation, Sol. Energy 85 (2011) 2732–2740. [5] V.L. Cunningham, S.P. Binks, M.J. Olson, Human health risk assessment from the presence of human pharmaceuticals in the aquatic environment, Regul. Toxicol. Pharmacol. 53 (2009) 39–45. [6] V.J. Pereira, H.S. Weinberg, K.G. Linden, P.C. Singer, UV degradation kinetics and modeling of pharmaceutical compounds in laboratory grade and surface water via direct and indirect photolysis at 254 nm, Environ. Sci. Technol. 41 (2007) 1682–1688. [7] N.P. Xekoukoulotakis, C. Drosou, C. Brebou, E. Chatzisymeon, E. Hapeshi, D. FattaKassinos, D. Mantzavinos, Kinetics of UV-A/TiO2 photocatalytic degradation and mineralization of the antibiotic sulfamethoxazole in aqueous matrices, Catal. Today 161 (2011) 163–168. [8] J.D. Woodling, E.M. Lopez, T.A. Maldonado, D.O. Norris, A.M. Vajda, Intersex and other reproductive disruption of fish in wastewater effluent dominated Colorado streams, Comp. Biochem. Physiol. C Toxicol. Pharmacol. 144 (2006) 10–15. [9] D. Dolar, K. Ko�suti�c, Chapter 10 - removal of pharmaceuticals by ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), in: M. Petrovic, D. Barcelo, S. P� erez (Eds.), Comprehensive Analytical Chemistry, Elsevier, 2013, pp. 319–344. [10] F.J. Real, F.J. Benitez, J.L. Acero, J.J.P. Sagasti, F. Casas, Kinetics of the chemical oxidation of the pharmaceuticals primidone, ketoprofen, and diatrizoate in ultrapure and natural waters, Ind. Eng. Chem. Res. 48 (2009) 3380–3388. [11] B. Ramesh Babu, P. Venkatesan, R.K. Kanimozhi, C. Ahmed Basha, Removal of Pharmaceuticals from Wastewater by Electrochemical Oxidation Using Cylindrical Flow Reactor and Optimization of Treatment Conditions, 2009. [12] A. Joss, S. Zabczynski, A. G€ obel, B. Hoffmann, D. L€ offler, C.S. McArdell, T. A. Ternes, A. Thomsen, H. Siegrist, Biological degradation of pharmaceuticals in municipal wastewater treatment: proposing a classification scheme, Water Res. 40 (2006) 1686–1696. [13] Y. He, N.B. Sutton, H.H.H. Rijnaarts, A.A.M. Langenhoff, Degradation of pharmaceuticals in wastewater using immobilized TiO2 photocatalysis under simulated solar irradiation, Appl. Catal. B Environ. 182 (2016) 132–141. [14] M. Klavarioti, D. Mantzavinos, D. Kassinos, Removal of residual pharmaceuticals from aqueous systems by advanced oxidation processes, Environ. Int. 35 (2009) 402–417. [15] B.S.M. Al Balushi, F. Al Marzouqi, B. Al Wahaibi, A.T. Kuvarega, S.M.Z. Al Kindy, Y. Kim, R. Selvaraj, Hydrothermal synthesis of CdS sub-microspheres for photocatalytic degradation of pharmaceuticals, Appl. Surf. Sci. 457 (2018) 559–565. [16] X. Wang, W. Bi, P. Zhai, X. Wang, H. Li, G. Mailhot, W. Dong, Adsorption and photocatalytic degradation of pharmaceuticals by BiOClxIy nanospheres in aqueous solution, Appl. Surf. Sci. 360 (2016) 240–251. [17] R. Fagan, D.E. McCormack, D.D. Dionysiou, S.C. Pillai, A review of solar and visible light active TiO2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern, Mater. Sci. Semicond. Process. 42 (2016) 2–14.

4. Conclusions The effect of Pt addition on TiO2–Nb2O5 mixed oxide was assessed towards the photocatalytic degradation of diclofenac (DCF) and keto­ profen (KTF) under UV light irradiation. The physicochemical properties of all the samples were analysed by different characterization tech­ niques. In particular, XRD results showed that, for the samples prepared with mixed oxides, the characteristic peaks of both Nb2O5 and TiO2 were detected, while no signals of platinum were observed in the platinized samples. Moreover, the presence of reduced and oxidized Pt species was detected from XPS analysis. Finally, the TEM analysis evidenced that the lowest platinum nanoparticles size was observed in Pt–TiO2–Nb2O5 sample and that the simultaneous presence of both oxides induces a more homogeneous photodeposition of Pt nanoparticles over both ox­ ides. Photocatalytic activity results evidenced that the heterojunction Pt–TiO2–Nb2O5 showed the best photocatalytic activity toward both the photodegradation and mineralization of DCF and KTF. Using this cata­ lyst, the half-life time values for the mineralization of the two pollutants were equal to 12.48 min for DCF and 9.29 min for KTF, which were higher than those of Pt–TiO2 photocatalyst (21.59 min for DCF and 11.61 min for KTF). The experiments driven to analyse the effects of free radical capture showed that ⋅OH, ⋅O2 and hþ have a primary role for the photocatalytic degradation mechanism. These results can be explained 11

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