Solar photocatalytic activity of TiO2 modified with WO3 on the degradation of an organophosphorus pesticide

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Solar photocatalytic activity of TiO2 modified with WO3 on the degradation of an organophosphorus pesticide ˜ a , L. Hinojosa-Reyes a , N.A. Ramos-Delgado a , M.A. Gracia-Pinilla b,c , L. Maya-Trevino a a,∗ J.L. Guzman-Mar , A. Hernández-Ramírez a

Universidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, San Nicolás de los Garza, N.L., Mexico1 Universidad Autónoma de Nuevo León, Facultad de Ciencias Físico-Matemáticas, Av. Universidad, Cd. Universitaria, San Nicolás de los Garza, N.L., Mexico c Universidad Autónoma de Nuevo León, Centro de Investigación e Innovación en Desarrollo de Ingeniería y Tecnología, PIIT Km 6, Carretera al Aeropuerto, Apodaca, N.L., Mexico b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• TiO2 and WO3 /TiO2 (2 and 5%) were tested in the photocatalytic malathion degradation. • The use of solar radiation in the photocatalytic degradation process was evaluated. • Modified catalyst showed greater photocatalytic activity than pure TiO2 . • The mineralization rate was improved when WO3 content on TiO2 was 2%.

a r t i c l e

i n f o

Article history: Received 31 December 2012 Received in revised form 24 June 2013 Accepted 26 July 2013 Available online xxx Keywords: WO3 –TiO2 Long-term stability Photocatalytic activity

a b s t r a c t In this study, the solar photocatalytic activity (SPA) of WO3 /TiO2 photocatalysts synthesized by the sol–gel method with two different percentages of WO3 (2 and 5%wt) was evaluated using malathion as a model contaminant. For comparative purpose bare TiO2 was also prepared by sol–gel process. The powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, diffuse reflectance UV–vis spectroscopy (DRUV–vis), specific surface area by the BET method (SSABET ), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy with a high annular angle dark field detector (STEM-HAADF). The XRD, Raman, HRTEM and STEM-HAADF analyses indicated that WO3 was present as a monoclinic crystalline phase with nanometric cluster sizes (1.1 ± 0.1 nm for 2% WO3 /TiO2 and 1.35 ± 0.3 nm for 5% WO3 /TiO2 ) and uniformly dispersed on the surface of TiO2 . The particle size of the materials was 19.4 ± 3.3 nm and 25.6 ± 3 nm for 2% and 5% WO3 /TiO2 , respectively. The SPA was evaluated on the degradation of commercial malathion pesticide using natural solar light. The 2% WO3 /TiO2 photocatalyst exhibited the best photocatalytic activity achieving 76% of total organic carbon (TOC) abatement after 300 min compared to the 5% WO3 /TiO2 and bare TiO2 photocatalysts, which achieved 28 and 47% mineralization, respectively. Finally, experiments were performed to assess 2% WO3 /TiO2 catalyst activity on repeated uses; after several successive cycles its photocatalytic activity was retained showing long-term stability. © 2013 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +52 8183294000x3438. E-mail addresses: [email protected], [email protected] (A. Hernández-Ramírez). 1 http://www.uanl.mx. 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.07.058

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1. Introduction Organophosphorus pesticides are the most widely used group of insecticides for crop pest management, among these pesticides, malathion is still extensively used for agricultural applications, and its extensive use has increased the quantities of malathion present in agricultural drain water. The photocatalytic oxidation process using TiO2 as a semiconductor catalyst under UV-light radiation has been proposed as an attractive and efficient method for the elimination of pesticides [1–7]. The photocatalytic process is characterized by the production of • OH radicals which are able to oxidize and mineralize almost any organic molecule [4]. However, a serious limitation for the application of TiO2 semiconductor in heterogeneous photocatalysis is the requirement of UV light to promote the electron transference process due to its wide bandgap (ca. 3.2 eV). A major issue governing the efficiency of photocatalytic degradation is minimizing electron–hole recombination by maximizing the rate of interfacial electron transfer to capture the photogenerated electron and/or hole. Due to limitations related to ensuring effective photoactivation, there has been increasing interest in searching for routes to go beyond the edge wavelength of 388 nm, which corresponds to the bandgap of TiO2 , the most used catalyst in the photocatalytic process. The primary methods to improve the photocatalytic activity involve modifying the catalyst by doping, metal coating, surface sensitization, increasing the surface area or by designing and developing secondary mixed oxides. Additionally, many studies have focused on mixed oxide semiconductors because an efficient charge separation can be obtained by coupling two semiconductor particles that have different energy levels [4,7]. Among these strategies, WO3 coupling has been studied to improve the photocatalytic activity of TiO2 because WO3 (Eg = 2.8 eV) can function as an electron accepting species, through a type II heterojunction [8]. However, the activity of WO3 /TiO2 has primarily been evaluated on the degradation of dyes using lamps as irradiation sources [9–16] whereas Tong et al. investigated the use of WO3 /TiO2 in water-splitting for producing O2 [17]. The photocatalytic activity of a semiconductor depends on its preparation method because it affects the physicochemical properties of the solid surface. In this context, the sol–gel method, which uses an aqueous or alcoholic mixture of inorganic salts or metallic alkoxides as precursors, allows components to be mixed at an atomic scale to prepare nanosized materials. The sol–gel synthesis occurs in a solution at room temperature through hydrolysis and condensation reactions, which give rise the sol that leads to the formation of a wet gel that still contains water and solvents that must be evaporated to produce a dry gel [18]. Heating gels to temperatures of several hundred degrees produces powders that have special characteristics. The incorporation of some active compound into the sol during the gelation stage allows the mixed elements to have a direct interaction with the semiconductor support, which therefore affects the surface properties, the particle size and consequently, the catalytic properties. In this study, TiO2 modified with two different amounts of WO3 (2 and 5%wt) was synthesized using the sol–gel method under acidic conditions with tetrabutyl orthotitanate (TBT) and ammonium p-tungstate (ApT) as precursors. The obtained materials were characterized by X-Ray diffraction (XRD), Raman spectroscopy, UV–vis spectroscopy with the diffuse reflectance mode (DRUV–vis), specific surface area by the Brunauer–Emmett–Teller (SSABET ) method, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The effect of the quantity of WO3 coupled to TiO2 was evaluated on the degradation of malathion in which the pH of the solution was adjusted to 7 and a catalyst loading of 1 g/L was used. The photocatalytic experiments were conducted outdoors using natural solar

radiation. The oxidation reaction was evaluated by monitoring the degradation of the pesticide using reversed-phase HPLC with UV detection and the percentage of mineralization was assessed by monitoring the total organic carbon (TOC) content in the aqueous solution. 2. Methodology 2.1. Synthesis and characterization of WO3 /TiO2 photocatalysts Tetrabutyl orthotitanate (TBT) from Fluka and ammonium ptungstate (ApT) from Sigma–Aldrich were used to synthesize the photocatalysts. All doping concentrations mentioned in this work are the nominal concentrations expressed as a percentage by weight, which were based on the assumed quantitative fraction of WO3 . 2-Butanol (Sigma–Aldrich) and glacial acetic acid (J.T. Baker) were HPLC grade. All chemicals used in this work were of analytical grade, and double-distilled water was used for preparing solutions. WO3 /TiO2 catalysts were prepared using the sol–gel process at a pH of 3. To obtain 10 g of TiO2 modified with WO3 , a mixture of 2-butanol (100 mL) and TBT (28.6 mL) was prepared; the pH of the solution was adjusted with glacial acetic acid, and a pre-hydrolysis was then performed using a mixture of 2-butanol and water in a volume ratio 20:1. After one hour, an aqueous solution of ApT (178 mg for 2% WO3 /TiO2 or 465 mg for 5% WO3 /TiO2 ) was added dropwise to the reaction media. The obtained gel was aged for 48 h and dried in an oven at 70 ◦ C. The resulted powder was washed with hot water (≈80 ◦ C) in order to remove residual precursors from the synthesis, dried again and then finely powdered in an agate mortar. The material was finally calcined at 500 ◦ C for 5 h. Similarly, the bare TiO2 catalyst was prepared according to the above procedure excluding the addition of ApT. The samples were labeled as 0%wt for bare TiO2 , 2%wt for 2% WO3 /TiO2 and 5%wt for 5% WO3 / TiO2 . The crystalline phase composition and crystallite size of the prepared samples were determined by X-ray diffraction measurements (XRD Siemens D500). The Scherrer formula (Eq. (1)) was used to estimate the average crystallite size of the synthesized particles: D=

k ˇ cos 

(1)

where D is the crystallite size,  is the X-ray wavelength, ˇ is the full-width at half maximum of the (1 0 1) diffraction peak of anatase titania and k = 0.89 is a coefficient [13]. The Raman spectra of the samples were recorded using a LabRam HR micro-Raman instrument coupled to a micro Olympus BX41 HORIBA Jobin Yvon at room temperature using an excitation wavelength of 632 nm. The optical absorption of the photocatalysts was determined using a DRUV–vis spectrophotometer Thermo Fisher Scientific-Evolution 300 instrument equipped with a TFS-Praying Mantis integrating sphere. A sample of BaSO4 (Spectralon) was analyzed as a reference. The values of the band gap energy (Eg) were calculated using Eq. (2): ˛(h) = A(h − Eg)

1/2

(2)

where ˛, , Eg, and A are the absorption coefficient, light frequency, band gap and absorption constant, respectively. The Eg values were determined by extrapolating the linear region of the plot to h = 0 [19,20]. The specific surface area (SSABET ) of the samples was calculated by BET method using the N2 adsorption isotherm measured in a Quantachrome Autosorb-1 instrument. All samples were degassed at 200 ◦ C before analysis. The morphologies, sizes and structures of the nanoparticles were characterized using FESEM (FEI-Nova nanosem 200), TEM and STEM. For the TEM and

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Table 1 Characterization of 0%wt, 2%wt and 5%wt photocatalysts. Catalyst

DRX crystalline phase

Eg (mV)

SSABET (m2 /g)

Scherer crystallite size (nm)

TEM particle size (nm)

STEM WO3 clusters sizes (nm)

0%wt 2%wt 5%wt

Anatase Anatase Anatase/Monoclinic WO3

3.17 3.11 3.00

56 99 72

28 23 24

– 19.4 (±3.3) 25.6 (±3.0)

– 1.1 (±0.2) 1.35 (±0.3)

STEM analyses, the sample was dropped (after being ultrasonicated) onto a carbon-coated copper grid, dried at room temperature and then analyzed using a FEI TITAN G2 80-300 instrument operated at 300 kV. 2.2. Evaluation of the solar photocatalytic activity (SPA) The SPA of the 2%wt and 5%wt photocatalysts were evaluated on the solar degradation of malathion. The catalyst (250 mg) was added to the planar reactor (borosilicate glass beaker Pyrex 600 mL, height: 12.5 cm, diameter: 9 cm) containing 250 mL of an aqueous solution of malathion (12 mg/L) operated in batch mode. The pH of the solution was adjusted to 7 using 1% (v/v) NH4 OH. For comparative purposes, the photocatalytic activity of the 0%wt was also evaluated. Before the photodegradation measurements, the suspension was magnetically stirred in the dark for 30 min to allow the adsorption–desorption equilibrium between the catalyst surface and the pollutant solution. After equilibrium, the photocatalytic experiments were conducted outdoors using natural solar radiation. During irradiation, stirring was maintained to keep the mixture in suspension and the photocatalytic experiments were performed in triplicate. At regular intervals, samples were withdrawn and filtered through a 0.45 ␮m nylon syringe filter to remove the photocatalyst. The degradation of malathion was performed for 180 min, and was evaluated by reversed-phase HPLC (PerkinElmer HPLC) with UV detection (202 nm). The chromatographic separation of malathion was conducted on a C18 Phenomenex column (250 mm × 4.60 mm, 5 ␮m) using acetonitrile:water [70:30] as the mobile phase at a flow rate of 1 mL/min. The mineralization of malathion was quantified by combustion catalytic oxidation using a total organic carbon analyzer (TOC-V CSH Shimadzu). To compare the photocatalytic experiments, the solar irradiation intensity was measured every 15 min using the Daystar Meter (Daystar, Inc) radiometer, whereas a UV Light Meter (290–340 nm UVA-UVB) Mannix 340 was used for the measurement of UV radiation. All experiments were done on sunny days (≈1030 W/m2 ) between 11:00 and 16:00 h when the solar intensity fluctuations were minimal. The solar fraction of UV radiation was approximately 4%. The accumulated solar UV radiation (kJ/m2 ) that impinged onto the photoreactor was calculated from the light irradiance and the irradiation time. 2.2.1. Stability of 2%wt catalyst The amount of WO3 released into the solution was estimated during the photocatalytic experiments to evaluate the chemical stability of the catalyst. A spectrophotometric method (UV–vis spectrophotometer Varian Corp., Model Cary 50) was used to determine the concentration of dissolved tungsten in the filtered solution samples every hour during the process. The analytical method was based on the reduction of W(VI) to W(V) by tin(II) chloride (SnCl2 ) in 3 M hydrochloric acid, then a green W(V)thiocyanate complex was formed with an absorption maximum at 400 nm [21]. The working range of the calibration curve was from 0.1 to 5 mg/L and the detection limit was 0.04 mg/L of tungsten. On the other hand, the feasibility of reusing 2%wt semiconductor after solar photocatalytic degradation was tested. The degradation and mineralization percentages of malathion pesticide were

estimated during 4 consecutives cycles of reutilization of 2%wt semiconductor. The experiments were carried out at the same experimental conditions described to evaluate its solar photocatalytic activity (250 mL of malathion C0 = 12 mg/L, 2%wt = 1 g/L, pH = 7). Before the following cycle, the catalyst was recovered, washed with hot water and dried at 60 ◦ C.

3. Results 3.1. Characterization of WO3 /TiO2 photocatalysts 3.1.1. Structural, optical and textural analyses The XRD patterns of the 0%wt, 2%wt and 5%wt catalysts are shown in Fig. 1. The presence of the primary diffraction peaks of TiO2 at 2␪ = 25.35◦ , 37.95◦ , 48.05◦ , 54.05◦ , 55.10◦ and 62.85◦ were observed, which can be, respectively indexed as the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) planes of anatase phase of titanium oxide (JCPDS no. 21-1272). Two weak signals corresponding to the primary reflections of monoclinic WO3 can only be observed in the 5%wt sample at 2 = 23.72◦ and 33.65◦ . These signals can be associated with the (2 0 0) and (2 0 2) planes of the monoclinic phase of WO3 (JCPDS No. 036-0101), which were also observed in previous works [22,23]. The crystallite sizes of the 0%wt, 2%wt and 5%wt catalysts were calculated using the Scherrer formula from X-ray diffraction pattern, obtaining average values of 28, 23 and 24 nm, respectively (Table 1). Because tungsten oxide reflections were not observed in the diffraction pattern for the 2%wt sample, Raman analyses were conducted to identify the vibrational modes of WO3 incorporated in the TiO2 semiconductor. The Raman spectra for the WO3 /TiO2 and bare TiO2 photocatalyst are shown in Fig. 2. The vibrational bands of the photocatalysts are observed as intense signals at 397.8, 517.7 and 637.5 cm−1 . These bands correspond to typical Raman peaks of the anatase phase assigned to the B1g , A1g and Eg vibrational modes of TiO2 [24,25]. Very weak bands observed in the Raman spectra of the 2%wt and 5%wt catalysts at 304, 795 and 965 cm−1 are related to the presence of WO3 . Sajjad et al. reported the presence of two weak bands at 292.6 and 805.4 cm−1 in a 4% WO3 /TiO2 sample, which were related to the presence of WO3 [14]. The peak closer to 795 cm−1 can be assigned to O–W–O stretching vibrations [26]. Pagnier et al. reported that the Raman spectrum of WO3 with a very small grain size, less than 5 nm, exhibits a weak band at 960 cm−1 . This band can be the result of a very short W-adsorbed O bond, which suggests the presence of a double bond [27]. Several authors have reported that this band (960–985 cm−1 ) can shift to different wavenumbers when the tungsten loading was increased. However, the authors agreed that this band corresponds to the symmetrical stretching mode of W O terminal bonds [16,28,29]. On the other hand, it was recently reported that WO3 was incorporated as an amorphous form depending on the different W/Ti molar ratio composition when TiO2 /WO3 was prepared using ultrasonic spray pyrolysis, suggesting that evolution of the WO3 crystalline phase occurred with the increasing content of WO3 [23]. In our study, Raman and HRTEM analyses confirmed that the sol–gel method allowed the incorporation of WO3 in the crystalline phase with a nanometric particle size using a low percentage of WO3 . The Raman

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a) 0% WT b) 2% WT c) 5% WT

au

a)

b)

Raman scattering intensity (a.u.)

5000

c) 5% WT b) 2% WT a) 0% WT

4000

3000

2000 304 795

1000

965

0 200

300

400

500

600

700

800

900

1000

Wavenumber (cm -1) Fig. 2. Raman spectra of catalysts (a) 0%wt, (b) 2%wt and (c) 5%wt.

c) 20

30

40

50

60

70

80

90

2θ

au

5% WT

23.72° 33.65°

20

22

24

26

28

30

32

the transfer of photogenerated holes from WO3 to TiO2 , and the electron transfer in the opposite direction [8]. Regarding the textural analysis, the adsorption–desorption isotherms of 0%wt, 2%wt and 5%wt were found to be type V (Results are shown in SI, Fig. S2), which is characteristic of mesoporous materials according to the Brunauer–Demming–Demming–Teller (BDDT) classification [30]. Specific surface areas (SSABET ) were calculated using the multipoint BET method, those were 56, 99 and 72 m2 /g for 0%wt, 2%wt and 5%wt, respectively (Table 1). The difference in crystallite size calculated by Scherrer formula and SSABET among the samples can be explained in terms of the synthesis parameters in the sol–gel method; it is well known that the extent of polymerization is largely dependent on the number of moles of water added to the alkoxide. An increase in the H2 O/alkoxide ratio results in an increased BET surface area, which is related to the increase of cross-linking due to the degree of polymerization. In the case of the 0%wt, the amount of water added was the stoichiometrically required. ApT, on the other hand, has a low solubility in water, so the quantity of water necessary to synthesize 2%wt and 5%wt was twice as much. This means that larger H2 O/alkoxide ratios were used, resulting in smaller crystallite sizes and a higher BET specific surface area [31].

34

2θ Fig. 1. XRD pattern of samples (a) 0%wt, (b) 2%wt and (c) 5%wt. The image below depicts a magnification of XRD pattern of 5%wt.

spectra confirmed the presence of WO3 in agreement with a XRD data; this result indicated that WO3 was present as a separated crystalline phase [14]. Regarding to the UV–vis spectra recorded data, the absorption edge of 5%wt catalyst was closer to the visible light region than that of 0%wt and 2%wt. The results are shown in Fig. S1 (see Supporting Information). The incorporation of WO3 extended the ability of TiO2 to respond to higher wavelengths. Concerning to the band gap energy (Eg) values, there was a visible-shift in the absorption edge for 5%wt (Eg = 3.00 eV) compared to the 2%wt (Eg = 3.11 eV) and 0%wt (Eg = 3.17 eV) (Table 1). These results were in agreement with previous reports [9,10,13,14] that show that the visible shift in the absorption edge of the WO3 /TiO2 catalyst depends on the synthesis method, the nature of the precursors, and the calcination temperature of the samples [22]. It was observed the decrease of the Eg values in the mixed oxides as a consequence of a type II heterojunction between WO3 and TiO2 structures, which enables

3.1.2. SEM and TEM analyses Fig. 3(a) and (b) present typical images of the high-resolution scanning electron microscopy of anatase TiO2 loaded with 2% and 5% of WO3 , respectively (Results of SEM-EDS analyses are shown in SI, Table S1). These images clearly reveal that the particle size of both samples range in the nanoscale; however, it can be observed that the 2%wt catalyst exhibited the lowest size with an average particle size of 20 nm, whereas the 5%wt catalyst exhibited an average particle size of 26 nm. Fig. 3(c) and (d) present the HRTEM images of these catalysts. In these figures, the crystal lattice was resolved for both systems. The 2%wt and 5%wt catalysts exhibit anatase TiO2 nanoparticles with high crystallinity, which was also evidenced by the resolved lattice fringes, in which the d-spacings of 0.362 and 0.36 nm correspond to the (1 0 1) lattice plane of the anatase phase for both systems. These images were used to determine the particle size of the 2%wt and 5%wt materials, which were 19.4 ± 3.3 nm and 25.6 ± 3 nm, respectively (The histograms of particle size are shown in SI, Fig. S3). These results were similar with the XRD analysis in which the average size of the crystallite was 23 and 24 nm for the 2%wt and 5%wt catalysts, respectively. Therefore, we suggest that increasing the loading of WO3 over anatase TiO2 increases the nanoparticle size. We observed different shapes of

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Fig. 3. HR-SEM Images of mixed oxide catalysts: (a) 2%wt, (b) 5%wt, and HR-TEM images of (c) 2%wt, (d) 5%wt, the red arrow indicates clusters of WO3 . High magnification images of HAADF-STEM showing the WO3 clusters supported on anatase (e) 2%wt, and (f) 5%wt. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

nanocrystals in both samples, mainly rectangular-like, hexagonallike and others such as square-like and rhomboid-like. Finally, Fig. 3 (e) and (f) present STEM-HAADF images that can be used to identify the size and distribution of the WO3 clusters over the anatase TiO2 nanoparticles. Recent studies [32] have revealed that in many oxide-on-oxide catalysts, the catalytic performance depends on the

precise surface structure of the active oxide component. Zhou et al. used the term “wetting interactions” to qualitatively describe the dispersion of an active oxide on a support surface [32]. The sizes of the WO3 clusters in the 2%wt and 5%wt catalysts, which are shown in Fig. 3(e) and (f), ranged from 0.8 to 1.6 nm and 1 to 2 nm, respectively (The histograms of WO3 cluster size are shown in SI,

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0.9 0.8

0.8

ln

C

= kapp t

0%WT

0.7

5%WT

0.6

2%WT

0.5 0.4 0.3 0.2 0.1 0

0

30

60

120

90

150

180

Time (min)

0.4

0% WT 2% WT 5% WT

0.0

(3)

where C0 is the equilibrium concentration after adsorption and C is the concentration at time t. The plot of ln (C0 /C) versus time presents a straight line, and the slope of this line, obtained by linear regression, equals the pseudo first-order rate constant, kapp . The estimated values of the kinetic parameters for the degradation and mineralization rates are summarized in Table 2, which show correlation coefficients >0.9733. Generally first-order kinetic is appropriate for the entire concentration range of ␮g/L or few mg/L.

0

50

100

150

200

250 2

Accumulated UV radiation (kJ/m ) Fig. 4. Solar photocatalytic degradation of malathion (12 mg/L) using 0%wt, 2% wt and 5%wt. The experiments were performed in triplicate; variability in solar experiments is shown in the inset.

The apparent rate constants (kapp ) obtained during the degradation of malathion were 1.81 × 10−2 , 3.64 × 10−2 and 3.86 × 10−2 min−1 for the 0%wt, 2%wt and 5%wt catalysts, respectively. The results were consistent with the initial observation that the photocatalytic degradation of malathion was faster using the modified TiO2 catalysts. TOC measurements were conducted in parallel with HPLC determinations to evaluate the photocatalytic ability of the catalysts (Fig. 5). As confirmed by the data obtained in this study, malathion mineralization was enhanced using the 2%wt catalyst at an extended irradiation time. In the presence of the 2%wt catalyst, 76% of TOC was mineralized after 300 min of irradiation (approximately 400 kJ/m2 ), whereas only 47 and 28% of TOC was transformed into CO2 in the case of the 0%wt and 5%wt catalysts, respectively. The observed rate constants for the mineralization of malathion using the 0%wt, 2%wt and 5%wt catalysts are illustrated in Fig. 6. It was observed that the mineralization rate of malathion using the 2%wt catalyst was increased compared to the

1.1 0% WT

1.0

2% WT 5% WT

0.9 0.8

TOC (C/C0)

The photocatalytic degradation efficiency using 0%wt, 2%wt and 5%wt catalysts was evaluated under natural solar light irradiation based on the percent of degradation and mineralization of an initial concentration of 12 mg/L malathion in an aqueous solution at pH 7 using a catalyst loading of 1 g/L. The solution pH of 7 was chosen for the photocatalytic experiments based on previous studies that reported that the degradation of malathion occurred rapidly under basic and neutral conditions and decreased under acidic conditions [35,36]. The first malathion degradation experiments were conducted under solar irradiation to evaluate direct photolysis without the addition of any catalyst. The concentration of malathion remained nearly constant after 180 min of solar irradiation. Thus, no obvious degradation of malathion was observed in this time period. Zhao et al. also observed that sunlight for itself was not able to hydrolyze and degrade malathion in river water [37]. In addition, a dark control experiment was conducted, indicating that the adsorption of malathion onto the surface of the catalyst in the absence of solar radiation was negligible. Although the adsorption process is influenced by the surface properties of the catalyst, such as its surface area, pore volume, and surface active sites, the structure of the organic molecule, its pKa , polarity, molecular weight, and functional groups determines its affinity to the catalyst surface [38]. For solar photocatalysis, a higher degradation percentage (∼99%) was obtained for the evaluated semiconductors when the accumulated UV radiation reached 180 kJ/m2 (≈120 min of reaction) (Fig. 4). The results indicated that the photocatalytic process was very effective in the removal of malathion and it was observed that the degradation of malathion was enhanced using the WO3 /TiO2 catalyst. No significant difference on the degradation percentage was observed for both modified catalysts (2 and 5%wt). Generally, the experimental results of the TiO2 photocatalytic oxidation of several organic contaminants revealed that the corresponding data fit to the Langmuir–Hinshelwood (L-H) kinetic model that can be simplified to a pseudo-first-order kinetic equation as follows for diluted solutions (concentration < 10−3 M) [14,15,19,39,40]: 0

0.6

0.2

3.2. Evaluation of the solar photocatalytic activity (SPA)

C 

1

1.0 Malathion (C/Co)

Fig. S3). An and Somorjai, who synthetized nanoparticles of Pt that were 0.3–5 nm in size, described the dependency of the particle size and selectivity for particles with sizes less than 5 nm. The authors observed that a small sized cluster corresponds to a considerable surface/volume ratio, which indicates that the percentage of surface atoms is a function of the cluster size. This surface/volume ratio abruptly decreased with slight increases in the size of the cluster, which indicates that catalytic reactions undergo dramatic changes in cluster sizes that are less than 5 nm [33]. In our work, the size of the WO3 clusters ranged from 0.8–1.6 nm (2%wt) to 1–2 nm (5%wt), indicating that the difference in cluster size can play a relevant role in the photocatalytic degradation of malathion. These WO3 clusters over anatase TiO2 nanoparticles and cuboidal-like anatase nanocrystals with {1 0 1}-type planes can be responsible for increases the photoactivity of WO3 /TiO2 [34].

Malathion C/C0

6

0.7 0.6 0.5 0.4 0.3 0.2 0

100

200

300

400 2

Accumulated UV radiation (kJ/m ) Fig. 5. TOC Abatement during the solar degradation of malathion using 0%wt, 2%wt and 5%wt photocatalysts.

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Table 2 Apparent first-order rate constants obtained for the solar photodegradation and mineralization of malathion, catalyzed by 0%wt, 2%wt and 5%wt. Catalyst

0%wt 2%wt 5%wt

Malathion degradation

TOC abatement

kapp (min−1 )

t1/2 (min)

r

kapp (min−1 )

t1/2 (min)

r

1.81 × 10−2 3.64 × 10−2 3.86 × 10−2

55 19 18

0.9923 0.9733 0.9837

2.3 × 10−3 4.6 × 10−3 1.2 × 10−3

301 151 578

0.9945 0.9802 0.9893

Fig. 6. Kinetics of the photocatalytic mineralization of malathion over (a) 0%wt; (b) 2%wt and (c) 5%wt under solar radiation.

use of the 0%wt and 5%wt catalysts (see Table 2). With the 2%wt catalyst, the rate constant of 4.6 × 10−3 min−1 was approximately two-times greater than the rate constant of unmodified TiO2 , whereas using the 5%wt catalyst, the rate constant was 1.2 × 10−3 min−1 , which was less than that using the unmodified TiO2 . The modified catalysts had similar activity during the degradation process; however, in the case of the mineralization, there was a notable difference when the content of WO3 in TiO2 was 5%. Although while using this catalyst the pollutant degradation was achieved, the subsequent decomposition of intermediates was lower. When the content of WO3 in TiO2 is in excess, the center of electron shifting on WO3 can be transformed into the center of electron compounding, which lowers the separation efficiency of electrons and holes [41], decreasing the formation of hydroxyl radicals responsible for the attack on the pollutant molecule. This process mainly affects the mineralization which involves the breakdown of the intermediates formed during the reaction [35]. It has been reported that, in general, the presence of dopants impedes the recombination process, which in turn prolongs the charge carriers lifetime. However, as a side effect a decrease of interfacial charge transfer rate takes place in the presence of impurities [42], therefore, it is very important to determine the optimal amount of dopants to avoid their action as recombination centers. On the other hand, the 2%wt catalyst had an increased specific surface area (99 m2 /g) and finer particle sizes than the 0%wt and 5%wt catalysts that might help to improve the contact area between the catalyst particles and the pollutant solution. Note that although both mixed oxide catalysts had WO3 present as cluster (<2 nm) particles on the surface of TiO2 , the 2%wt catalyst possessed smaller clusters than the 5%wt catalyst. The difference in cluster sizes also originated differences in surface area in both mixed semiconductors, which means that optimal amount of doping agent not only depends on the nature of the dopant, but also on the size of the clusters [43]. In this context, although 2%wt (0.7 mol% WO3 ) was

the optimum tungsten loading for the degradation of malathion, the optimum tungsten loading for degradation of other compounds could differ. Saepurahman et al. reported that the highest activity for WO3 /TiO2 prepared using the impregnation method for the degradation of methylene blue under visible lamp irradiation was 1 mol% WO3 [16]. The optimum tungsten loading for the degradation of methylene blue under visible light irradiation has been reported to be 3 mol% when the catalyst was synthetized using the sol–gel method [10], whereas the maximum improvement in the degradation of methyl orange and 2,4-dichlorophenol using UV and visible light irradiation with flame-made WO3 /TiO2 has been reported at 5 mol% [13]. Thus, in this study 2%wt tungsten-loaded TiO2 prepared by sol–gel process was more efficient than 5%wt and 0%wt for the decomposition of malathion under solar irradiation. The increase of photocatalytic activity of 2%wt semiconductor can be related to the enhanced charge separation which reduces the electron–hole pair recombination [11,34,44]. On the other hand, several authors have used mathematical approaches to relate the optical properties of semiconductors with the reactor geometry for the evaluation of the efficiency on the photon absorption. Those models are used to design and scale up photoreactors during wastewater treatment [45–50]. An important parameter used in those models is the optical thickness ( max ), which is a dimensionless parameter related to the scattering and absorption that occur when light travels through the entire depth of the reactor [50], and it was calculated using the following equation:

max = ˇ ∗ Ccat ∗ dPR

(4)

where ˇ is the dimensionless extinction coefficient, Ccat (1 g/L) is the catalyst loading and dPR (4.5 cm) is the distance traveled by light in the reactor [49]. It was calculated the  max of 2%wt in a planar reactor operated in batch-mode. The extinction coefficient (ˇ = 61.27 m2 /kg) of 2%wt was obtained by absorption measurements at 320 nm of the catalyst suspension (loading from 0.01 to 1 g/L). We assumed that the incident radiation reached by the reactor was perpendicular to the top solution during the degradation experiments and the depth was kept uniform and constant in the reactor. The calculated  max value of 2%wt for a planar reactor was 2.76. This value is related with the efficiency of the catalyst to absorb radiation, and therefore with the ability to degrade various organic compounds in a specific reactor geometry. Regarding to this parameter, Li Puma reported the optimum values of  max to be in the range 1.8–3.4 for thin-film slurry photocatalytic reactors, depending on flow condition and reaction kinetics [46]. Our calculated  max value falls within the reported range and this is the first approach for evaluating the optical thickness for 2%wt catalyst in a planar reactor operated in batch-mode. 3.2.1. Stability of 2%wt catalyst Dissolved tungsten during solar photocatalytic degradation of malathion using 2%wt and 5%wt catalysts was measured every hour in the filtered solution during the photocatalytic experiment (data not shown). The amount of dissolution of tungsten leached from both modified catalyst at neutral pH was below of the detection limit (0.04 mg/L of tungsten) of the spectrophotometric method

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Fig. 7. Results of the reuse experiments of 2%wt catalyst under solar radiation. (250 mL of malathion C0 = 12 mg/L, 2%wt = 1 g/L, pH = 7). (a) Degradation of malathion pesticide and (b) Mineralization efficiency.

[21], indicating the stability of the synthetized catalysts against photocorrosion. Another important feature of the catalyst to reduce the cost of the process is its ability to be regenerated and reused [51,52]. To evaluate the 2%wt catalyst activity on repeated uses, the catalyst was recovered and reused four times. The results of degradation and mineralization are shown in Fig. 7. The TOC removal efficiency was determined after 5 h for each experiment. The degradation and mineralization efficiency of the mixed oxide catalyst for consecutive cycles was in the range 94–99% and 74–83%, respectively; the 2%wt sample had no significant change in the activity after four cycles. This means that the catalyst possesses high activity and stability during the photocatalytic decomposition of malathion. Hence, the solar photocatalytic degradation using 2% WO3 /TiO2 is a good alternative method for treating water containing recalcitrant pollutants.

TiO2 semiconductor loaded with 2% and 5% of WO3 exhibited better solar photocatalytic behavior for the degradation of the malathion pesticide than bare TiO2 ; however, the content of WO3 in TiO2 in excess (5%), lowered the separation efficiency of electrons and holes. Thus, the mineralization rate and the percentage of TOC removal were improved when the content of WO3 was 2% due to the formation of smaller clusters and a higher surface area, which reduces the recombination process and results in better contact area between the catalyst particles and the pollutant, improving the photocatalytic reactivity and the destruction efficiency against the pesticide molecule. The 2%WO3 /TiO2 photocatalyst also showed good stability against the photocorrosion and it could be recycled several times without significant loss of its activity.

Acknowledgments 4. Conclusions The WO3 /TiO2 catalysts prepared by sol–gel method were stable during photocatalytic experiments under solar radiation. The

Financial Support from PAICYT-UANL and CONACYT-193883 is ˜ gratefully acknowledged. N.A. Ramos-Delgado and L. Maya-Trevino thanks to CONACyT for their PhD Grant.

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