Photocatalytic properties of Zr-doped titania in the degradation of the pharmaceutical ibuprofen

Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 108–116

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Photocatalytic properties of Zr-doped titania in the degradation of the pharmaceutical ibuprofen J. Choina a , Ch. Fischer b , G.-U. Flechsig c , H. Kosslick a,b,∗ , V.A. Tuan d , N.D. Tuyen d , N.A. Tuyen d , A. Schulz a,b a

University of Rostock, Institute of Chemistry, Department of Inorganic Chemistry, Albert-Einstein-Str. 3a, Rostock, Germany Leibniz Institute of Catalysis e.V. (LIKAT), Albert-Einstein-Str. 29a, Rostock, Germany c University of Rostock, Institute of Chemistry, Department of Analytical, Technical and Environmental Chemistry, Dr.-Lorenz-Weg 1, Rostock, Germany d Vietnam Academy of Science and Technology (VAST), Institute of Chemistry, 18 Hoang Quoc Viet, CauGiay Distr., Hanoi, Viet Nam b

a r t i c l e

i n f o

Article history: Received 4 June 2013 Received in revised form 14 August 2013 Accepted 24 August 2013 Available online 2 September 2013 Keywords: Photocatalytic abatement Ibuprofen Zr-doped titania Reaction intermediates Polymerization

a b s t r a c t Zr-doped titania of anatase structure type has been prepared by a combined sol–gel and chemical vapour deposition (CVD) process. The material has been characterized by XRD, TEM and chemical analysis. The photocatalytic performance has been investigated in the oxidative decomposition of the pharmaceutical ibuprofen (IBP) down to low ppm concentrations. The change of the composition of the reaction solution after photocatalytic treatment was followed by UV–Vis spectroscopy and GC/MS. Formation of reaction intermediates was studied by HPLC coupled electrospray ionization time-of-flight mass spectrometry ESITOF-MS. The catalytic performance has been studied by varying the catalyst and substrate concentration as well as changing the catalyst-to-substrate ratio. The influence of pH, adsorption and re-use of the catalyst has been tested. The results confirm the high catalytic activity of Zr-doped titania compared to pure titania at low catalyst loading. The material shows the improved textural mesoporosity. However, more reaction intermediates were formed, leading to faster deactivation of the photocatalyst. ESI-TOF-MS measurements point to the formation of a couple of reaction intermediates, which poison the catalyst. © 2013 Published by Elsevier B.V.

1. Introduction The depollution of water by degradation of hazardous pollutants is of increasing interest. Pollution with hazardous compounds as pharmaceuticals, pesticides or endocrine disruptive compounds is a widely emerging problem. Even in very low concentrations present in water, they may have a severe and non-predictable impact on human health and environment. A large number of pharmaceuticals, especially non-steroidal anti-inflammatory drugs (NSAIDs), commonly available and used as a non-prescription medicaments are widely present in surface water, introduced by effluents from wastewater treatment plants (WWTPs) (Table 1). Among them, ibuprofen is a common pollutant. These contaminations are highly problematic because the surface waters are a main resources used for drinking water production. The treatment of such polluted waters requires the application of Advanced Oxidation Processes (AOPs). Among AOPs, photocatalysis is a very important and attractive process [2,3]. Heterogeneous photocatalytic water treatment is of special interest, since it does not require the use of any additional chemicals or soluble

∗ Corresponding author. Tel.: +49 0381 498 6384; fax: +49 0381 498 6382. E-mail address: [email protected] (H. Kosslick). 1010-6030/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jphotochem.2013.08.018

homogeneous catalysts and can be operated in principle with sunlight. The catalyst can be recovered and re-used. Many available studies deal with dyes common, toxic pollutants mainly coming from the textile industry [4–7]. Only a part of these studies deal with the formation of by-products and of remaining organics during the course of photocatalytic degradation at high concentration [8]. However, investigations of the photocatalytic degradation of pharmaceuticals, especially with low concentration, are still very limited [9–11]. In some cases high catalyst-to-substrate ratios and pollution concentrations are used in order to proof photocatalytic activity, to study the formation of toxic intermediates [12,13] or to perform toxicity tests [14]. Concentrations of the pharmaceutical pollutants and photocatalysts varied from 20 to 200 mg/L and 0.2 to 1.5 g/L, respectively [15,16]. A variety of semiconductors have been examined for their photocatalytic properties (TiO2 , SnO2 , ZnO, ZnS, WO3 ) and used as photocatalyst [17,18]. Among these materials, titania is highly attractive due to its unique properties. TiO2 is a non-hazardous compound and offers the possibility to be recovered and re-used. In principle, the eco-friendly titania photocatalyst can be used in water treatment processes. Many attempts have been undertaken to further improve the catalytic efficiency by doping with various transition metals (e.g. Cu, Co, Ni, Cr, Mn, V, W) and nonmetals (B, N, C, P, S) [19,20]. Different synthesis and doping methods have been applied [21–29]. Besides, Zr-doped TiO2 material have

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Table 1 Environmental occurrence of commonly used drugs in Germany [1]. Pharmaceuticals

Usage

Chemical struructure

Environmental occurrence Loading of drug (g/day)

Maximal concentration of drug in effluents (ppb)

Maximal concentration of drug in surface waters (ppb)

Ibuprofen

Analgesic, anti-inflammatory

200

3.4

0.53

Diclofenac

Analgesic, anti-inflammatory

100

2.1

1.2

Carbamazepine

Analgesic, antiepileptic

100

6.3

1.1

Naproxen

Analgesic, anti-inflammatory

50

0.52

0.39

Aminophenazone (or aminopyrine)

Analgesic, anti-inflammatory

50

1.0

0.34

Gemfibrozil

Blood lipid and cholesterol-regulator medicine

50

1.5

0.3

Indomethacin

Analgesic, anti-inflammatory

10

0.6

0.2

been prepared. Chang et al. [30] synthesized highly crystalline Zrdoped TiO2 using a nonhydrolytic sol–gel method by condensation of metal halides with alkoxides in anhydrous trioctylphosphine oxide (TOPO) at 320–400 ◦ C. Luo et al. [31] prepared Zr-doped titania by using the sol–gel method with mixed templates containing polyethylene glycol (PEG) and cetytrimethylammonium bromide (CTAB) at low temperature. Schiller et al. [32] synthesized TiO2 doped with zirconium (0–50 mol% Zr) by combining the sol–gel process with the inverse micro emulsion technique. Reported tests (at high catalyst-to-substrate ratio) point to high photocatalytic activity of the Zr-doped TiO2 . This makes this photocatalytic material interesting for wastewater treatment [33–35]. The objective of this work was to study the performance of Zrdoped titania photocatalyst in the photocatalytic decomposition of a pharmaceutical in more detail in order to improve the present knowledge regarding the influence of the catalyst-to-substrate ratio, the catalytic performance at lower catalyst and pollutant concentration as well as of appearing reaction intermediates. The abundant recalcitrant pharmaceutical ibuprofen has been used as a model compound. 2. Experimental 2.1. Materials Zr-doped titania photocatalyst was prepared by a sol–gel synthesis using TiCl4 (Aldrich reagent grade 99.9%) and chemical vapor

deposition using ZrOCl2 (Aldrich, reagent grade 98%) based on a modified procedure [30]. The commercial titania P25 photocatalyst used for comparison in the adsorption experiment was supplied by Evonik (Germany). Ibuprofen sodium salt (shortened to IBP) was supplied by Sigma–Aldrich (reagent grade > 98%). All photocatalytic solutions investigated were prepared using ultrapure water (>18 M TOC < 2 ppb). Additionally, 37 wt.% aqueous solution of hydrochloric acid (Merck, reagent grade 36.5–38.0%), and sodium hydroxide pellets (J.T Baker, reagent grade ≥97%) were used for the pH regulation of the photocatalytic reaction mixture. 2.2. Characterization and analyses XRD measurements were carried out on the X-ray diffractome˚ ter STADI-P (STOE) using Ni-filtered CuK␣ radiation ( = 1.5418 A). The catalyst was investigated regarding texture, particle shaped and size by TEM analysis. Prior to TEM measurements, a powdered Zr-TiO2 sample was dispersed in ethanol and deposited on copper grids. Electron microscopy experiments were done on a LIBRA 120 at 120 kV. Images were recorded with a digital camera with 2000 × 2000 pixels. The chemical composition of the catalyst was determined by atomic absorption flame emission spectroscopy (Shimadzu AA-6400F). The progress of the oxidative abatement of ibuprofen was monitored by UV–Vis spectroscopy (Varian, Cary WinUV spectrometer), based on a calibration plot followed by the disappearance of the absorption band belonging to the benzene ring of IBP at 222 nm. All data were obtained at room temperature.

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The formation of reaction intermediates and products of ibuprofen decomposition during photocatalytic treatment was controlled by HPLC coupled electrospray ionization time-of-flight mass spectrometry using the system 1200/ESI-TOF-MS 6210 (Agilent). The mass-to-charge range was scanned between m/z 50–1000. The analysis was carried out in negative electrospray ionization. An aqueous solution containing 10 vol.% of methanol (MeOH for HPLC, gradient grade, ≥99.8%) and 0.1 vol.% of formic acid (HCOOH) was used as the mobile phase. The flow rate was 1.0 mL/min. During the photocatalytic run, aliquots of the reaction solution were taken for analysis of residual ibuprofen by UV–Vis and ESI-TOF-MS. Before use, the reaction mixture was cleared using a 0.45 ␮m PTFE filter. 2.3. Photocatalysis The photocatalytic abatement of ibuprofen was carried out in a 250 mL bath reactor. Different amounts of photocatalyst powder were suspended in the aqueous solution of ibuprofen. The reactor was equipped with 4 UV–Vis solarium lamps (15 W, Philips) with a total power of 60 W irradiating the solution from the top. The distance between the applied lamps and the surface of ibuprofen solution was 15 cm. The photocatalytic reactor was placed in an aluminum box. A magnetic stirrer was arranged below. The aqueous solution was stirred during the experiment. The pH value of the solution was adjusted between 2 and 9 by addition of HCl(aq) and NaOH(aq) . The pH values of the ibuprofen solution were measured using a pH meter (WTW pH-330, Germany). Re-use experiments were carried out by adding aliquots of new IBP solution supplementing the photocatalytic-abated ibuprofen. The catalyst was not reactivated.

Fig. 1. Scheme of Zr-doping of TiO2 by chemical vapour deposition (CVD) method.

P25 photocatalysts are shown in Fig. 2. This nanocrystalline Zrdoped material contains small agglomerated particles of 10–15 nm size with regular shapes. Titania particles are somewhat larger (10–30 nm). The textural properties of Zr-TiO2 and titania have been studied by the nitrogen adsorption measurements. The BET surface area amounts to 100 and 55 m2 /g for Zr-TiO2 and titania, respectively. The nitrogen adsorption-desorption isotherms of Zr-TiO2 photocatalyst are shown in Fig. 3a. They belong to the isotherm type 4. In contrast to titania, the adsorption isotherm of Zr-TiO2 shows an additional adsorption step with a slight hysteresis at relative nitrogen pressure (P/Po) of 0.7–0.9 due to the presence of mesopores.

3. Results and discussion 3.1. Catalyst preparation and characterization A new Zr-doped catalyst was prepared from a low crystalline titania gel precursor as follows. A certain amount of TiCl4 was added drop wise into i-propanol under stirring. The resulting solution was introduced into distilled water in an ice-water bath with vigorously magnetic stirring. After that, the pH value of this acidic solution was adjusted to 7 by adding 10% ammonia solution. The resulting white gel was aged for 24 h at room temperature under stirring. The obtained white precipitate was filtered and washed repeatedly with distilled water until complete removal of chloride ions. Thereafter the precipitate was well-dispersed by treating in an ultrasonic bath. Then, 30 wt.% aqueous solution of hydrogen peroxide was added drop wise into this mixture under stirring. The resulting yellow transparent solution was poured into an autoclave and heated at 100 ◦ C for 20 h. After this hydrothermal aging procedure the precipitate was washed and dried at 100 ◦ C to obtain the powdered titania precursor. Zr-doping was carried out by chemical vapour deposition (CVD) using zirconyl chloride (ZrOCl2 ) solution. The titania precursor was placed into a quartz tube reactor of internal diameter of 2 cm and of length of 25 cm size. Prior to heating, the reaction system was purged with dry nitrogen. Then the titania precursor was contacted with a nitrogen stream flowing through the vessel containing the ZrOCl2 solution and heated to 350 ◦ C. After completing deposition, the material was heated to 500 ◦ C. The resulting Zr-doped TiO2 catalyst was washed and calcined in air before photocatalytic investigations. The experimental set up is shown in Fig. 1. The obtained Zr-doped titania catalyst contained ca. 5 mol% of zirconium. XRD results confirmed that the prepared 5% Zr-doped titania adopts a anatase structure, which confirms literature results [33]. TEM images of the prepared Zr-doped titania and titania

Fig. 2. TEM images of (a) Zr-TiO2 and (b) titania P25 photocatalysts.

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Fig. 4. Adsorption of ibuprofen on titania P25 and Zr-TiO2 photocatalysts in the dark, IBP conc. = 10 ppm, cat conc. 100 mg/L.

3.2. Photocatalytic decomposition and formation of intermediates

Fig. 3. (a) Nitrogen adsorption–desorption isotherms of Zr-TiO2 photocatalysts, (b) pore size (D) distribution of Zr-doped TiO2 photocatalyst determined by nitrogen adsorption–desorption measurement, based on (top) specific surface pore area (o) dA/d log (D), and (bottom) specific pore volume (+) dV/d log (D).

The BJH desorption analysis shows uni-modal mesopores with relative narrow size distribution having the pore size maximum at ca. 8–9 nm (Fig. 3b). The particles are compact as shown by TEM images. The mesopores are located between the agglomerated Zr-TiO2 crystals. The mesopore volume of Zr-TiO2 amounts to 0.25 cm3 /g. In contrast, the titania sample shows no mesoporosity. The influence of the Zr-doping on the surface properties of the catalyst has been checked by adsorption of ibuprofen. They show significant enhanced adsorption of IBP on Zr-doped TiO2 compared to undoped TiO2 . Already at short contact time the IBP is adsorbed on Zr-TiO2 (Fig. 4). ca. 20% abatement of ibuprofen is achieved by adsorption from a low concentrated solution containing 10 ppm IBP with 50 mg/L of catalyst. This improved adsorption is maintained at long contact time of 180 min in aqueous solution. In contrast, adsorption on titania is comparably low. This finding is explained by an increased hydrophobic nature of the Zr-doped catalyst surface, which favors adsorption of less polar ibuprofen molecules from water. Additionally, the presence of mesoporosity, as shown in nitrogen adsorption experiment, can contribute to the enhanced adsorption properties. Generally high adsorption affinity, and hence, increased concentration of the pollutant at the catalyst surface enhances the photocatalytic activity. The pollutant comes in close contact to the photocatalyst and released reactive oxygen species. The increase of the specific surface area with zirconium doping, compare to un-doped titania, cannot explain the markedly improved adsorption.

The photocatalytic decomposition behavior of IBP was followed by UV–Vis spectroscopy based on the intensity of the 222 nm absorbance of the aromatic ring of IBP. It is found, that this band decreases rapidly with photocatalytic treatment, indicating fast decomposition of IBP and oxidative ring opening. Interestingly, a new absorbance appears at 262 nm just after starting the photocatalytic treatment. This band is assigned to reaction intermediates. After an initial increase, the band at 262 nm decreases and disappears again. In the following, this 262 nm band was used as a measure for the formation and decomposition of reaction intermediates. UV radiation alone has no markedly effect on the IBP decomposition under the reaction conditions used. Additionally, ESI-TOF-MS measurements were carried out to follow the decomposition process (Fig. 5). In Fig. 5a is shown the negative ESI-TOF-MS spectrum of the used starting IBP material. Beside the m/z 205 peak related to the ibuprofen anion, higher mass-to-charge peaks are observed at m/z 433, 661 and 889 These peaks indicate the presence of agglomerated IBP species. The mass increment of 228 between the signals at m/z 205 and m/z 433, 661 and 889 indicates ibuprofen sodium salt units. Some impurities, e.g. at m/z 273 and corresponding agglomerated species with ibuprofen sodium salt as indicated by peaks arising at m/z 501 and 729 are also observed in ESI-TOF-MS spectrum. For the case of chemical bonding by C C coupling of IBP (formation of polymers), the mass increments between the peaks would lower due to the release of hydrogen atoms. Therefore, these species (Fig. 5a) should be aggregates of IBP and the impurity (m/z 273) held together by ionic interactions between the negatively charged carboxylic groups and the sodium ions and by non-polar interaction between the hydrophobic tails of the ibuprofen molecules. These aggregates are decomposed by photocatalytic treatment (Fig. 5b). After 30 min of photocatalytic treatment, the m/z peaks of agglomerated species decrease markedly. IBP is still present as indicated by the peak of the ibuprofen anion at m/z 205 [M−H]− (Fig. 5b). Additional peaks appear at higher and lower m/z values. Obviously, they belong to photocatalytic oxidation and decomposition products. Stability and intermediate oxidation products of IBP have been studied before [14,36–38]. They show formation of higher mass products compared to IBP due to side chain hydroxylation and carboxylation as initial oxidation processes. Lower mass products are formed by demethylation and decarboxylation combined with hydroxylation during oxidation. Accordingly, the observed higher mass m/z [M−H]− peaks at 221, 251 and higher are assigned to hydroxy ibuprofen (221), carboxylated hydroxyl ibuprofen (251) and related different side chain oxidation products (Fig. 6). Such

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Fig. 5. Negative ESI-TOF-MS of ibuprofen, the peak intensity corresponding to ibuprofen peak [M−H]− m/z 205 after photocatalytic decomposition over Zr-TiO2 (a) starting aqueous solution of ibuprofen 20 ppm, time of photocatalytic treatment (b) 30 min and (c) 240 min.

oxygenated species decompose readily under photocatalytic conditions. It is concluded that the observed higher mass molecule fragments (m/z > 300) point to the formation of polymeric species. They can be formed from phenols, alcohols or aldehydes present

in the reaction solution giving rise to lower mass peaks m/z < 205. They are known to be intermediates of the photocatalytic oxidation [8,10]. For example, the [M−H]− peak at (133) could be due to formation of 4-ethylbenzaldehyde or 2-methyl-1-phenylpropane,

Fig. 6. Possible formation of side-chain products, its deep oxidation products and open ring intermediates by decomposition of ibuprofen during photocatalysis.

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Fig. 7. Formation and decomposition of intermediates during the course of photocatalytic treatment of ibuprofen under UV–Vis radiation over titania and Zr-doped TiO2 photocatalysts: cat. conc. 20 mg/L, IBP conc. 10 ppm.

(149) to 4-isobutylphenol, (163) to 4-acetylbenzoic acid, (175) to 4isobutylacetophenon or 4-(hydroxyisobutyl)ethyl benzene, (177) to 1-(isobutylphenyl)-1-ethanol and others [14,36,38,39]. Even after an extended treatment time of 240 min (Fig. 5c), ibuprofen is not fully converted as indicated by the presence of the ibuprofen anion peak at m/z 205. A couple of intermediate decomposition products are still present in the reaction solution, which were not mineralized. They give rise to a variety of signals in the ESITOF-MS spectrum of the photocatalytic treated solution (Fig. 5c). The lower mass peaks at m/z < 205 are expected to belong to oxidation products like phenols or ketones and others as discussed above [14,40,41]. These peaks show the presence of some aromatics and reaction intermediates, giving rise to absorbance bands at 222 nm and 262 nm, respectively. Additionally, new signals appear after photocatalytic treatment at higher m/z values of 277, 291, 305 and 319 (Fig. 5c). The observed mass differences, with constant increments of 14, implies that they likely represent methyl substitutes of the compound giving rise to the m/z 277 signal. It is proposed that they belong to trimeric chinoid compounds as shown schematically in Eq. (1).

(1) This proposal is supported by recent studies. They show the possibility of the polymerization of aromatics, e.g. phenols, in low concentrated aqueous solution catalyzed by peroxidase enzyme [42–44]. The observation of oligomeric species during photocatalytic treatment of ibuprofen is new and has been found with titania photocatalyst, recently for the first time [45]. The comparison shows that reaction intermediates formed over titania are almost successively decomposed during the course of photocatalytic treatment (Fig. 7). Finally, mainly polymeric species remain in the reaction solution. In contrast, formed reaction intermediates are more difficult to decompose over Zr-TiO2 . 3.3. Course of photocatalytic abatement 3.3.1. Influence of catalyst-to-substrate ratio The influence of the substrate (IBP) concentration on the photocatalytic abatement over Zr-TiO2 is shown in Fig. 8. The decomposition process proceeds rapidly at the beginning of photocatalytic treatment, within the first 5 min. The onset activity reaches 0.36 mg IBP/mg cat. or 0.072 mg/mg per min at the substrate concentration of 60 ppm, which is distinguished higher than

113

Fig. 8. Abatement of ibuprofen in water over Zr-TiO2 catalyst under UV–Vis radiation, influence of IBP concentration: 20, 40 and 60 ppm, cat. conc. 10 mg/L.

in case of titania (compare Table 2). The enhancement of the onset activity is due to the increased specific surface area as well as improved adsorption properties of Zr-TiO2 . The onset activity is nearly linear related to the substrate concentration of the solution. It increases in the order: 20 ppm < 40 ppm < 60 ppm. Thereafter, the abatement of the substrate proceeds comparatively slow. At prolonged time of photocatalytic treatment, the rate of conversion decreases markedly with increasing substrate concentration (from 20 to 60 ppm), meaning decreasing catalyst-to-substrate ratio from 0.5 to 0.16. The conversion is nearly stopped beyond 60 min of reaction at high substrate concentration. Obviously, the catalyst is blocked or poisoned under these conditions. A similar behavior is observed with the formation of reaction intermediates of IBP during photocatalytic treatment. At the onset of reaction, the concentration of intermediates increases rapidly (Fig. 9). The increase is closely related to the substrate concentration of the solution. Thereafter, formation of intermediates proceeds more slowly. Interestingly, the concentration of intermediates is not decreased after long treatment time at the high catalyst loading with the substrate of up to 60 ppm IBP per 10 mg/L of Zr-TiO2 . It is assumed that reaction products of rapidly formed intermediates like chinoid polymers may poison the catalyst. The two-stage abatement, characterized by rapid decomposition of IBP at the onset of reaction followed by slow decomposition at longer reaction time, points to the contribution of different reaction mechanisms. It is concluded that the faster abatement is due to oxidation of IBP at highly reactive electron-hole pairs on the surface. The electron holes have a high oxidation potential. The slow conversion in the second stage is due to oxidation by radical oxygen species. The high abatement at the onset of the photocatalytic treatment is also facilitated by the enhanced adsorption property of Zr-doped titania. This makes the catalyst useful especially for the photocatalytic removal of low concentration hazardous pollutants. Increased catalyst-to-substrate ratio from 1:1 to 4:1 enhances Table 2 Abatement of ibuprofen in aqueouse solution during photocatalytic treatment over titania P25 and Zr-TiO2 (photocatalyst conc. 10 mg/L). The onset activity coresponds to the achived abatement in the first 5 min of treatment. Abatement (A) and corresponding experimantal effective first order reaction rates (r). Initial IBP concentration (ppm)

20 40 60

Titania P25

Zr-TiO2

A (mg)

r (min−1 )

A (mg)

r (min−1 )

1.25 1.4 1.5

0.025 0.028 0.030

1.4 2.9 3.6

0.028 0.058 0.072

A, mg of IBP (for titania P25 estimated average data after 5 min) [45]; r, mg of IBP per mg of catalyst and minute.

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Fig. 9. Formation of reaction intermediates of ibuprofen based on UV–Vis absorbance by max 262 nm, during photocatalytic abatement over Zr-TiO2 catalyst under UV–Vis radiation, influence of IBP concentration: 20, 40 and 60 ppm, cat conc. 10 mg/L.

Fig. 10. Abatement of ibuprofen over Zr-TiO2 catalyst under UV–Vis radiation, influence of catalyst concentration: 10, 20 and 40 mg/L, IBP conc. 10 ppm.

the conversion of ibuprofen (Fig. 10). Obviously, this is due to the diminished influence of catalyst poisoning by using catalyst in excess. At the same time, formation of intermediates is also increased (Fig. 11). Only with high catalyst-to-substrate ratios (catalyst in excess) doses the concentration of intermediates decrease after prolonged photocatalytic treatment. However, intermediates did not fully disappear even after 180 min (Fig. 11). Fig. 12 shows the corresponding UV–Vis spectra of reaction solutions after different reaction times.

Fig. 11. Formation of reaction intermediates of ibuprofen based on UV–Vis absorbance by max 262 nm, during photocatalytic abatement over Zr-TiO2 catalyst under UV–Vis radiation, influence of catalyst concentration: 10, 20 and 40 mg/L, IBP conc. 10 ppm.

Fig. 12. Photocatalytic abatement of IBP over Zr-TiO2 based on the absorbance at 222 nm and 262 nm; different cat-to-IBP ratio (a) 4:1; (b) 2:1; (c) 1:1, 1, starting conc of IBP 10 ppm; treatment time, 2–5 min, 3–30 min, 4–60 min, 5–120 min, 6–180 min.

At the beginning of the treatment, IBP is decomposed as indicated by the decrease of the 222 nm absorbance of IBP. The decrease of the 222 nm band is related to the catalyst-to-substrate ratio in the order 1:1 < 2:1 < 4:1. At the same time, the formation of intermediates is increased. The intensity of the corresponding absorbance at 262 nm increases with growing catalyst-to-substrate ratio during photocatalytic treatment. However, after 180 min of reaction the concentration of intermediates is decreased due to the excess of catalyst. At high catalyst-to-substrate ratio, as often used, Zr-doped titania is very active and a superior photocatalyst. 3.3.2. Influence of pH value The photocatalytic abatement of IBP over Zr-TiO2 is significantly improved with decreasing pH value. Rapid photocatalytic abatement is observed at the very beginning of the treatment (5 min). It increases by a factor of 6 with simultaneous decreasing the pH value from 9 to 2. Thereafter, degradation of ibuprofen proceeds again, but slower as already noted (Fig. 13). The noticeable improvement of the photocatalytic activity is explained by the changed surface properties of titania after Zrdoping. Obviously, the material behaves more hydrophobically. Therefore, non-polar molecules are preferentially adsorbed. The

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4. Conclusion Zr-doped titania has been prepared by an improved chemical vapor deposition procedure using a freshly prepared sol–gel titania precursor. The obtained photocatalytic material shows a high initial activity and exhibits improved adsorption properties in aqueous solution. Both of these properties make the catalyst interesting for the decomposition of hazardous compounds especially in low concentrated aqueous solution. ESI-TOF-MS measurements confirm rapid degradation of the ibuprofen over Zr-doped titania via formation of temporary reaction intermediates and indicate partial formation of polymeric compounds. Acknowledgments Fig. 13. Abatement of ibuprofen over Zr-TiO2 catalyst under UV–Vis radiation, influence of the pH: IBP conc. = 10 ppm, cat conc.=20 mg/L.

dissociation of the carboxylic acid group of ibuprofen is diminished with decreasing pH value, because ibuprofen is a weak acid (pKa = 4.4). This will favor the interaction of the IBP with the catalyst surface. Alignment of IBP molecules on the catalyst surface and in solution can be improved by hydrogen bonding between non-dissociated carboxyl groups. This effect can markedly enhance the adsorption/enrichment of the substrate at the catalyst surface. The self-agglomeration tendency of IBP is also shown by the formation of agglomerated species in the starting reaction solution. The polar hydrophilic intermediates, that are formed, are readily released from the hydrophobic catalyst surface into solution. 3.3.3. Re-use of catalyst The catalyst re-usability was checked by cycling experiments at high catalyst loading (10 mg/L catalyst per 10 ppm IBP) under neutral reaction conditions. Decomposed IBP was replaced by fresh IBP solution. The achieved abatement of ibuprofen decreased from 38% in the first run to 16% and 12% in the second and third run, respectively. The catalyst deactivation of 58% is highest after the first run due to the blocking of the catalyst, compared to 25% after the second run. The final abatement achieved with titania is higher with 85%, 75% and 58% in the 1st, 2nd and 3rd run, respectively. The deactivation behavior of titania and Zr-TiO2 is similar, although more intermediates are formed over the last one and accumulated in reaction solution (Fig. 14).

Fig. 14. Formation and accumulation of reaction intermediates of ibuprofen based on the absorbance at max 262 nm, during photocatalytic treatment over Zr-TiO2 catalyst under UV–Vis radiation, 3 cycles of photocatalytic treatment, IBP conc. 10 ppm, cat conc. 10 mg/L.

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