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Ag-loaded ZnO materials for photocatalytic water treatment
Accepted Manuscript Ag-loaded ZnO materials for photocatalytic water treatment Maria J. Sampaio, Maria J. Lima, Daniel L. Baptista, Adrián M.T. Silva, Cláudia G. Silva, Joaquim L. Faria PII: DOI: Reference:
S1385-8947(16)30753-7 http://dx.doi.org/10.1016/j.cej.2016.05.105 CEJ 15265
To appear in:
Chemical Engineering Journal
Please cite this article as: M.J. Sampaio, M.J. Lima, D.L. Baptista, A.M.T. Silva, C.G. Silva, J.L. Faria, Ag-loaded ZnO materials for photocatalytic water treatment, Chemical Engineering Journal (2016), doi: http://dx.doi.org/ 10.1016/j.cej.2016.05.105
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Ag-loaded ZnO materials for photocatalytic water treatment Maria J. Sampaio 1, Maria J. Lima1, Daniel L. Baptista2, Adrián M.T. Silva1, Cláudia G. Silva1, Joaquim L. Faria1* 1
Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and
Materials (LSRE–LCM), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal 2
Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre,
91501-970, Brazil *Corresponding author: [email protected]
Abstract ZnO materials with different morphologies were obtained by hydrothermal and solid state techniques. The ZnO materials were loaded with different amounts of silver nanoparticles (atomic percentages ranging from 0.25 to 1) by a liquid impregnation method. The materials were characterized by physical adsorption of nitrogen, scanning electron microscopy (SEM), UV-Vis diffuse reflectance spectroscopy (DRUV-Vis), Xray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR). The photocatalytic efficiency of the materials was assessed in the degradation of phenol using simulated solar light, by means of the pseudo-first-order kinetic constant (kapp) for phenol conversion and corresponding degree of mineralization. The photocatalyst made of ZnO prepared by thermal decomposition of zinc acetate (ZnO-t) loaded with a 0.25 atomic percentage of Ag (0.25%Ag/ZnO-t) was the most efficient, reaching a 58% mineralization after 60 min irradiation (kapp = 7.7 × 10 -2 min-1). Trapping of photogenerated holes and radicals by selective scavengers showed that photogenerated holes played the main role on phenol oxidation. The study 1
was extended to a complex mixture containing phenol, resorcinol, 4-methoxyphenol and 4-chorophenol using bare ZnO-t and 0.25%Ag/ZnO-t photocatalysts, either in powder form or immobilized on glass rings (packed in a continuous photocatalytic reactor). The conversion of the phenolic compounds was faster when Ag particles were deposited on the photocatalyst. Keywords: Photocatalysis; Zinc oxide; Silver nanoparticles loading; Water treatment; Phenolic compounds
1.
Introduction
In recent years, water issues have been on the focus of increasing international concern. The intensification of agricultural and industrial activities has led to the contamination of groundwater by fertilizers and other chemicals [1, 2]. Phenol and its derivatives can be found in a wide variety of industrial effluents (e.g., olive oil mills, petrochemical, pulp and paper industries [3-6]) and are known as toxic and refractory organic compounds, therefore hard to degrade using conventional biological treatment processes. The development of effective processes capable of removing persistent organic pollutants from waste waters has been considered an important environmental challenge. The application of heterogeneous photocatalysis technology has gained increasing attention in the last decades due to its effectiveness in degrading a wide variety of recalcitrant organic water pollutants [7, 8], and also because they meet the requirement of sustainable process development when solar energy is used. Semiconductors such as TiO2 and ZnO have been extensively studied as important catalysts for photocatalytic water treatment [9-11]. ZnO, which has a band gap similar to TiO2 (ca. 3.2 eV), has been used as an interesting alternative material for photocatalytic applications due to its
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physical and chemical stability, high oxidative capacity, low cost and availability [12, 13]. Owing to the fact that less than 5% of the solar spectrum at earth’s surface consists of UV light, the natural band gap of semiconductors like TiO2 and ZnO represents a drawback when considering the use of solar-driven photocatalytic processes. Several authors have reported a positive effect of loading Ag onto ZnO materials in the photocatalytic degradation of organic pollutants in air and aqueous media [13-15], and also in water disinfection and microbial control [16]. An improvement in the performance of these semiconductors being attributed to the presence of Ag nanoparticles acting as electron sinks, trapping the photoexcited electrons and therefore increasing the yield and lifetime of the charge separation state. In the present work, ZnO materials with distinct morphologies were loaded with different amounts of Ag nanoparticles aiming at an increase in the efficiency of the photocatalysts. Materials were tested under simulated solar light and using phenol as a model compound, since it is a standard molecule largely used for photocatalysts’ screening [17, 18]. The reaction mechanism of phenol photocatalytic degradation was evaluated using selective radical and hole scavengers. Taking into account water resource contamination either by accident, or in purpose as military or terrorist targeting, the silver properties were tested aiming rapid photocatalytic applications. Moreover, and considering the advantages of using photocatalysts in fixed supports, the most efficiency photocatalyst found for phenol degradation was immobilized on glass Raschig rings and tested in the oxidation treatment of a solution containing a mixture of phenolic compounds (which can model several typical hazardous compounds), namely phenol, resorcinol, 4-methoxyphenol and 4-chorophenol.
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2.
Experimental
2.1 Materials preparation Needle-like ZnO (ZnO-n) was prepared by a hydrothermal procedure, as described elsewhere [19, 20]: an aqueous solution of 4 M NaOH (Aldrich > 97%) was heated to 70 ºC with strong stirring, and after that an aqueous solution of 2 M Zn(NO3)2·6H2O (Aldrich > 99%) was added dropwise and left stirring for 1 h. The resulting precipitate was collected, washed with deionized water and dried in an oven at 110 ºC. Another material, ZnO-t was prepared by a solid-state thermal process [19, 21]. Briefly, a porcelain capsule containing zinc acetate dehydrate (Aldrich > 99.5%) was placed in a horizontal oven and heated until 600 ºC under air flow for 2 h. A commercial ZnO material from Evonik-Degussa GmbH (ZnO-ev; AdNano VP 20, aggregated nanoparticles of hydrophilic ZnO) was used as received. The liquid impregnation method was used to prepare the Ag/ZnO samples [22] The amounts of Ag loaded were 0.25, 0.5 and 1.0 at.% (atomic percentage). Briefly, 1 g of ZnO material was added to 250 mL of MiliQ water with different amounts of silver precursor (AgNO3, Alfa Aesar > 99.9 %). The mixture was stirred for 24 h and then evaporated until dry at boiling temperature. The final catalysts were labeled as xAg/ZnO-y, were x and y correspond to the percentage of Ag loaded and to the type of ZnO, respectively. The catalysts were then calcined under a 50 mL min-1 N2 flow at 400 ºC for 2 h. Selected materials were also immobilized on Raschig rings using the dip-coating technique, as described elsewhere [23].
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2.2 Materials characterization The specific surface area (SBET) of the powder materials was determined by multipoint analysis of N2 adsorption isotherms at -196 ºC using the BET method in a Quantachrome NOVA 4200e apparatus. The morphology of the ZnO materials was analyzed by scanning electron microscopy (SEM) using a FEI Quanta 400 FEG ESEM/EDAX Genesis X4M (15 keV) instrument. High-resolution transmission electron microscopy (HRTEM) of Ag/ZnO samples was performed using a Cs-corrected FEI Titan 80/300 microscope at INMETRO. Diffuse reflectance UV–Vis spectra (DRUV-Vis) of the materials were measured on a JASCO V-560 UV-Vis spectrophotometer equipped with an integrating sphere. The spectra were recorded in diffuse reflectance mode and transformed to equivalent absorption Kubelka–Munk units. The band gap of the photocatalysts was determined using the measured absorption edge as function of the energy. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos AXIS Ultra HSA, with VISION software for data acquisition and CASAXPS software for data analysis. The analysis was carried out with a monochromatic Al Kα X-ray source (1486.7 eV), operating at 15kV (90 W), in FAT (Fixed Analyser Transmission) mode, with a pass energy of 40 eV for regions ROI and 80 eV for survey. Temperature programmed reduction (TPR) experiments were carried out using an AMI-200 Catalyst Characterization Instrument (Altamira Instruments) equipped with a quadrupole mass spectrometer (Ametek, Mod. Dymaxion). The sample (150 mg) was placed in a U-shaped quartz tube and heated at 5 ºC min-1 up to the desired temperature under a flow of 5% (v/v) H2 diluted in Ar (total flow rate of 30 cm3 (STP) min-1). The H2 consumption was followed using a thermal conductivity detector (TCD).
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2.3 Photocatalytic experiments The photocatalytic experiments using ZnO-based materials were performed in a 50 mL pyrex batch reactor (7.0 cm height and 4.5 diameter) loaded with a 20 mg L-1 phenol solution (PH; Aldrich 99%) under simulated solar light irradiation for 60 min, using a Solarbox 1500e system (CO.FO.MEGRA). The reactor was placed in the bottom center of the solar simulator and the suspensions were irritated from the top. The catalyst load was fixed at 1 g L-1 and the suspensions were magnetically stirred and continuously saturated with an air flow. The irradiation source consisted of a 1500 W xenon lamp equipped with a cut-off soda-lime glass UV filter (for λ < 290 nm) with infrared reflection coating, to simulate outdoor exposure (irradiance equal to 30.9 mW cm-2, determined with an Ocean Optics spectroradiometer in the range 300-650 nm). The same procedure was implemented for the experiments using a mixture of phenolic compounds: phenol (PH), 4-methoxyphenol (MP; Fluka > 98%), resorcinol (RC; Aldrich > 99%) and 4-chlorophenol (CP; Aldrich > 99 %) with a concentration of 20 mg L−1 each. In this case the irradiation was carried out for 2 h. Some materials were immobilized on Raschig rings. In this case a glass cylindrical reactor (19 mm of internal diameter and 33 mm length) with 8.2 mL of useful volume was packed with nine TiO2-coated rings. In a typical experiment, the mixture of phenolic compounds was introduced into the reactor using a peristaltic pump at constant flow rate (Q = 17 mL min-1) from the reservoir containing 50 mL of phenolic mixture. The solution was continuous recycled during the photocatalytic experiments. The irradiation procedure was the same as described for the phenol solutions, except for the length of exposition, which was extended to 8 h. Reactions in the absence of catalyst were performed as blank experiments in order to characterize the pure photochemical regime of the phenolic compounds. Each
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experiment was preceded by a dark period of 30 min before switching the lamp on to establish the adsorption-desorption equilibrium. The concentration of the phenolic compounds was evaluated by high performance liquid chromatography (HPLC) with Hitachi Elite LaChrom apparatus equipped with Lichrocart Purospher Star RP-18 endcapped column (250 mm×4.6 mm, 5 µm particles), using a mobile phase of a mixture water: methanol in a gradient step. The total organic carbon (TOC) content of the initial and final samples was determined using a Shimadzu TOC-5000A apparatus. The photocatalytic pathway of phenol degradation was studied using 1.0 mM solutions of EDTA (292.2 mg L-1) and tert-butanol (t-BuOH, 74.1 mg L-1) as hole and radical scavengers, respectively [24-26].
3.
Results and Discussion
3.1 Materials characterization The specific surface area (SBET) of the bare ZnO materials, determined by N2 adsorption at -196 ºC, were 2, 6 and 26 m2 g−1 for ZnO-n, ZnO-t and ZnO-ev, respectively. Tryba et al. have reported that the deposition of silver on semiconductors may enhance the adsorption capacity of the photocatalyst by 10 to 30% [27]. On the other hand, others studies show that, during the preparation method, the Ag nanoparticles can aggregate forming small clusters, thus reducing the surface area of the resulting material [13]. In this study, no change on the S BET of the ZnO materials was observed after Ag loading, which may be attributed to the small amount of Ag deposited on the ZnO materials. Fig. 1 shows representative SEM micrographs of the different ZnO materials. These images clearly reveal the differences in the particle size and shape of the ZnO catalysts. The SEM micrograph of ZnO-n (Fig. 1a) shows needle-like particles of about 40-50 nm
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in diameter and 600 nm in length. The ZnO-t material (Fig. 1c) is constituted by spherical particles with diameters between 100-250 nm and 1D needle-like structure with uneven diameter distribution along their longitudinal direction among 500-900 nm. Commercial ZnO-ev (not show) consists mainly on small (50–100 nm) cylindrical and spherical particles. Fig. 1b and 1d give representative HRTEM images of Ag spheroidal particles deposited on ZnO-n and ZnO-t, respectively. The HRTEM images suggest a good distribution of Ag particles with similar size (5 nm) over the ZnO materials.
a
b
Ag
500 nm
c
d
5 nm
Ag 500 nm
Figure 1 - SEM micrographs of ZnO-n (a) and ZnO-t (c) and HRTEM micrographs of 0.25%Ag/ZnO-n (b) and 0.25%Ag/ZnO-t (d), respectively.
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The diffuse reflectance UV-Vis spectra of the photocatalysts are shown in Fig. 2. The ZnO materials have a characteristic intense band absorption in the UV spectrum range (λ < 400 nm) corresponding to the band gap transition of the semiconductor. The values of the band gap energy obtained were 3.20, 3.22 and 3.24 eV for ZnO-n, ZnO-t and ZnO-ev, respectively (Fig. 2d). The presence of Ag nanoparticles on ZnO materials leads to the increase of the absorption in the visible light range between 400 and 600 nm, due to the Ag surface plasmon band [28, 29]. It was also observed that for the catalysts with low amounts of silver (0.25 at.%) the band gap energy is similar to those of bare ZnO materials. However, a slight decrease in the band gap was detected for 1.0%Ag/ZnO-t and 1.0%Ag/ZnO-n samples, 3.21 eV and 3.16 eV, respectively. This observation was more pronounced for the Ag/ZnO-ev catalysts, decreasing from 3.24 eV to 3.12 eV from bare ZnO-ev to 1.0%Ag/ZnO-ev, respectively. This decrease in the band bap on Ag/ZnO-ev materials suggests a better distribution of Ag nanoparticles on the ZnO-ev surface, which hold the highest S BET (26 m2 g−1) in comparison with ZnO-n and ZnO-t, resulting in an increase of the absorption in the entire visible spectrum range (Fig. 2c).
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Figure 2 – Diffuse reflectance UV-Vis spectra of Ag/ZnO-n (a), Ag/ZnO-t (b) Ag/ZnO-ev (c) materials, and plot the Kubelka-Munk (KM) units as function of the light energy of the neat ZnO materials (d): ZnO-n (i); ZnO-t (ii); ZnO-ev (iii).
Regarding the position of the surface plasmon resonance band for Ag/ZnO-n and Ag/ZnO-t materials (Fig. 2a and 2b inset, respectively), the absorption maxima occur at 11
454 nm, while for Ag/ZnO-ev a red-shifted band with a maximum peaking at 476 nm is observed (Fig. 2b inset). This shift may be attributed to the presence of Ag nanoparticles with different size and shape [30]. As expected, the absorption in the visible spectral range is directly proportional to the amount of Ag loaded on the different ZnO materials. The XPS profiles of the Ag/ZnO materials are shown in Figs. 3(a-c). A similar spectrum was observed for all the materials, being detected two peaks at binding energy (BE) of 368.1 eV and 374.1 eV corresponding to Ag3d3/2 and Ag3d3/2 with a spin-orbital splitting photoelectrons at 6.0 eV [31, 32]. If the peaks in the 3d level are shifted to lower BE (< 0.6 eV), Ag could be at different states (Ag0, Ag2O and AgO). The deconvolution of the XPS profiles showed two peaks in the 3 d5/2 level, at 367.8 and 368.2 eV and for 3 d 3/2 level, at 373.9 and 374.2 eV , corresponding to oxidized Ag (AgO) and to Ag0 state, respectively. It has been reported that AgNO3, used as silver precursor, can be easily decomposed at low temperatures to give Ag2O [33]. However, Ag2O is very unstable above ~120 ºC, being converted to AgO at temperatures between 100 and 200 ºC, while AgO starts to decompose into Ag above 400 ºC. The TPR analyzes were carried out in order to identify the decomposition temperature of Ag species in 1%Ag/ZnO-t (Fig. 3b), before and after the calcination treatment at 400 ºC. The TPR profile of bare ZnO-t was also evaluated as control. Two peaks in the 100-220 ºC range were observed for the non-calcined material, which could be assigned to the decomposition of Ag2O to form AgO and also to the formation of NO2 resulting from the AgNO3 decomposition [33]. After the thermal treatment at 400 ºC, the peaks corresponding to the decomposition of Ag2O and NO2 are absent indicating that this temperature is adequate to guarantee the
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elimination of NO2 from the silver precursor. From TPR analysis, no signal was detected for ZnO confirming the stability of the material among the temperatures tested.
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Figure 3 – XPS spectra of Ag/ZnO-t (a), Ag/ZnO-n (b) and Ag/ZnO-ev (c) and TPR analysis of Ag/ZnO materials (d).*n.c.: not calcined
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3.2. Photocatalytic experiments 3.2.1. Photocatalytic activity of ZnO and Ag/ZnO materials The effect of silver deposition on the efficiency of ZnO materials was studied for the photocatalytic oxidation of phenol in water under simulated solar light. The photochemical degradation of phenol was carried out at first, in the absence of catalyst. Under these conditions, it was observed a decrease in phenol concentration of 5% after 60 min irradiation. It was also observed that a maximum of 5% decrease in phenol concentration after the dark adsorption period was obtained for both ZnO and Ag/ZnO materials. Results show that kinetics of photocatalytic phenol degradation follows a pseudo-first order rate law for all the tested catalysts. The efficiency of the materials was evaluated in terms of the apparent first order constant (kapp), determined by non-linear curve fitting to the experimental data (Fig. 4). Different behaviors were observed depending on the ZnO materials used. Among the bare ZnO materials, ZnO-n was the less active catalyst, while ZnO-ev was the most efficient, with kapp of 9.8×10 -3 and 6.5×10 -2 min-1, respectively. Nevertheless, when silver was loaded on ZnO-ev, a significant decrease in kapp was observed being more pronounced for the sample containing 1.0 at.% (kapp = 3.3×10-2 min-1). In fact, some authors have shown that the deposition of noble metals not always improves the photoactivity of semiconductors. Depending on the structure of the photocatalyst, the Ag deposition may result in the blockage of the active centers of the metal oxide semiconductor, thus reducing its photocatalytic activity [22, 30]. In the case of ZnO-t materials, the addition of Ag leads to an increase in the photocatalytic activity. It was found that the lowest amount of silver (0.25 at. %) led to the highest phenol degradation efficiency (kapp = 7.7×10-2 min-1). However, the 15
photoefficiency of the ZnO-t materials declined when the amount of Ag increased. This may suggest that the effective quantity of holes and electrons generated when 0.25 at.% Ag is added to ZnO is adequate to degrade an initial concentration of 20 ppm of phenol. An increase in the silver load may lead to a surplus charge separation which is not efficiently contributing to the reaction. For Ag/ZnO-n materials, an increase in kapp with increasing Ag load was observed, with a kapp of 3.34¯10 -2 min-1 being obtained using the 1.0%Ag/ZnO-n catalyst, corresponding to a 3.4 fold increase compared with bare ZnO-n. A sample with 1.4 at.% Ag was also tested (not shown), but a decrease in the efficiency for phenol degradation was obtained (kapp =2.61¯10-2 min-1).
Figure 4 – First order apparent rate constants (kapp) for the photocatalytic reactions using neat and Ag-loaded ZnO materials.
Total organic carbon (TOC) measurements were performed for evaluating the degree of mineralization achieved by the photocatalytic treatment of the phenol containing
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solutions. In general, an increase in the TOC removal was obtained when Ag nanoparticles were incorporated in the ZnO materials (Table 1). Table 1 – TOC removal of the ZnO and Ag/ZnO photocatalyst at the end of 60 min reaction. XTOC (%) Catalyst no Ag 0.25%Ag 0.50%Ag 1.0%Ag ZnO-n 1.5 26 29 48 ZnO-ev 46 69 69 24 ZnO-t 43 58 45 39
The mineralization efficiency was quantified by using a synergy factor (RTOC), defined as XTOC, 60 min (Ag/ZnO)/XTOC, 60 min (ZnO), where XTOC corresponds to the TOC conversion. The highest RTOC was obtained for 1.0%Ag/ZnO-n (RTOC = 33.8). For 0.25%Ag/ZnO-ev and 0.25%Ag/ZnO-t, RTOC values of 1.5 and 1.4 were obtained, indicating 33% and 26% mineralization increases in comparison with the respective bare ZnO materials. These results underline the positive effect of the incorporation of Ag in the ZnO materials.
3.2.2 Photocatalytic degradation pathway Loading Ag onto ZnO induces the formation of a Schottky barrier at the metalsemiconductor interphase surface [20, 34]. Due to the higher Fermi level energy of ZnO compared with Ag, the electrons can easily flow from the photoexcited metal oxide to Ag. The transfer of electrons to the Ag nanoparticles is normally faster than the electron-hole (e-/h+) recombination process occurring in the ZnO phase, and thus high amounts of electrons (e-) from ZnO can be stored in the Ag nanoparticles. Ag particles act as electron sinks where adsorbed oxygen may be reduced yielding superoxide radical (O2•−). As a result, more holes (h+), which are strongly oxidizing species, are available to drive oxidation reactions, namely the formation of hydroxyl radicals (HOy) from hydroxyl anions and/or also direct oxidation of phenol.
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The main routes occurring during the photocatalytic phenol degradation were investigated by using EDTA and t-BuOH, which act as scavengers for holes and radicals, respectively [24]. Fig. 5 shows the evolution of phenol concentration during the photocatalytic experiments using bare ZnO-t (Fig. 5a) and 0.25%Ag/ZnO-t (Fig. 5b) catalysts in the presence of electron/hole scavengers. It was observed for both materials that the addition of EDTA or t-BuOH leads to a decrease in the phenol degradation rate, which indicates that both radical species and photogenerated holes are involved in the phenol degradation process. For ZnO-t the presence of t-BuOH led to a decrease in kapp, from 5.1¯10-2 to 3.7×10 -2 min-1. This decrease was even more pronounced in the presence of EDTA (kapp = 1.0¯10-2 min-1). A similar behaviour was observed for 0.25%Ag/ZnO-t (Fig. 5b), where the presence of t-BuOH and EDTA led to a decrease in the kapp from 7.7¯10-2 min-1 to 4.3¯10 -2 min-1 and 1.8¯10 -2 min−1, respectively. The results indicate that, although reactive species, such as hydroxyl (HO•), superoxide (O2•−) and/or hydroperoxyl (HOO•) radicals, are responsible for part of the photocatalytic phenol degradation, photogenerated holes on the catalyst surface (h+) seem to play the main role, since kapp was much more reduced when EDTA was used as hole scavenger.
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Figure 5. Effect of hole/radical scavengers (1 mM EDTA/t-BuOH) on the photocatalytic degradation of phenol using ZnO-t (a) and 0.25%Ag/ZnO-t (b).
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Yet, in the presence of 0.25%Ag/ZnO-t the contribution of radical species in the phenol 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
degradation reaction is more pronounced comparing with bare ZnO-t, since 45% and 28% decrease in kapp was observed when t-BuOH was added to 0.25%Ag/ZnO-t and ZnO-t suspensions, respectively. These results may be related to the more efficient charge separation occurring upon photoexcitation of 0.25%Ag/ZnO-t, leading to a higher availability of surface electrons and holes and the formation of higher amounts of radical species. 3.2.3 Reutilization tests It is known that depending of the photocatalytic reaction conditions, the photocorrosion of ZnO may occur [11, 35]. Thus, photostability of ZnO and 0.25%Ag/ZnO-t was investigated by reusing these materials in consecutive phenol degradation reactions at natural pH conditions (pH = 6.1) under simulated solar light. Before each run the catalyst was washed with deionized water and dried at 110 ºC. Fig. 6 shows the phenol concentration evolution in photocatalytic reactions using ZnO and 0.25%Ag/ZnO-t in four utilization cycles.
Figure 6 – Reusability assessment for ZnO-t (¢) and 0.25%Ag/ZnO-t () for phenol degradation. 20
The above results show that ZnO-t maintains its activity during four cycles of utilization, suggesting a good stability of this material within the reaction conditions used. For the experiments using 0.25%Ag/ZnO-t, a slight improvement in the kinetics of phenol degradation was observed for the second run. This may be attributed to partial photoreduction of Ag species during the previous photocatalytic reaction. The presence of Ag species in the oxidized state was noticed by XPS analysis, which may change to its reduced form upon irradiation. In fact, several studies reported the use of the photo-impregnation method to deposit and reduce Ag nanoparticles on catalysts increasing their activity and stability [22, 36, 37]. In terms of mineralization, a slight decrease (18% and 10% for ZnO and 0.25%Ag/ZnO-t, respectively) in the TOC removal capacity was observed after four uses.
3.2.4 Photocatalytic degradation of a mixture of phenolic compounds Taking into account the photocatalytic efficiency observed for ZnO-t and 0.25%Ag/ZnO-t in the degradation of phenol, the photocatalytic treatment of a mixture containing four phenolic compounds was studied. Phenol (PH), resorcinol (RC), 4methoxyphenol (MP) and 4-chlorophenol (CP) were selected as model compounds. The separation of catalyst powders from the treated water requires separation steps that should be taken into account for technological applications. Thus, a set of experiments were also performed using the catalysts immobilized on glass Raschig rings. A dark period for each experiment was carried out for 30 min before irradiation. No significant decrease in the initial concentration for the different compounds was observed during the dark period. The experimental results indicate that the degradation of the individual
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compounds in the mixture obeys a pseudo-first-order rate law. Using the information from the kinetic curves (see supplementary data, SD2) it was determined the half life time (t1/2) for each compound. The relationship between the t1/2 and the individual compounds for the experiments using ZnO-t and 0.25%Ag/ZnO-t in the form of powder and immobilized on glass Raschig rings are shown in Fig. 7.
Figure 7 – Half life time (t1/2) degradation as a function of each phenolic compound using powder (_p) and films (_f) of bare ZnO-t and 0.25%Ag/ZnO-t.
The above results show that, in general the deposition of Ag improves the photocatalytic efficiency of the resulting material. Shorter half life time is needed for each compound be converted in 50% when 0.25%Ag/ZnO-t was used, either as a powder or film. It was also observed that the individual compounds present in the mixture are converted simultaneously, indicating that no competition occurs between each compound during the photocatalytic reaction. As expected, the use of a catalyst in the form of powder leads to a faster conversion, i.e., a shorter half-life compared to that of the immobilized 22
photocatalyst. This is obviously expected since the contact surface is larger in the suspended material, due to the larger amount of catalyst per unit of volume. Nevertheless, several practical aspects arise from the use of a catalyst powder form. Those include the need for post-separation techniques, as well as the difficulty to recover the catalyst in continuous flow reaction systems. This recovery is more important as the cost of the catalyst increases with the deposition of noble metals. Thus a compromise must be established between the desired conversion and the need of recycling the catalyst. In terms of TOC removal, the presence of Ag revealed to be advantageous, being of 6% and 27% using ZnO, and 14% and 42% using 0.25%Ag/ZnO-t with the photocatalyst immobilized and in powder form, respectively. Globally, the results show that the deposition of Ag nanoparticles on ZnO materials may improve their photocatalytic efficiency by increasing the photocatalytic degradation rate as well as the mineralization capacity. Moreover, when applied to near to real field conditions (i.e., mixture of compounds that may be encountered in waste waters and the possibility of using the catalyst immobilized on physical support to be employed in continuous systems) the use of Ag/ZnO photocatalysts could be of major relevance for solar applications.
Conclusions Photocatalysts of ZnO loaded with Ag are more efficient than the neat synthesized materials, with relation to the conversion and mineralization of phenol and some of its derivatives. The highest efficiency was obtained when a small amount of Ag was deposited on ZnO-t. The most active material consisted of 0.25 at.%Ag over ZnO-t. The increase in
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the activity was ascribed to the action of Ag nanoparticles as electrons sinks, thus increasing the efficiency and lifetime of charge separation in the semiconductor. The important decrease in kapp observed when EDTA was used as hole scavenger suggests that the excited holes play a major role in the mechanism of phenol degradation. For the reactions using 0.25%Ag/ZnO-t, the contribution of radicals to the phenol degradation mechanism was more evident than for bare ZnO-t, as a result of the higher amount of electrons and holes generated at the surface of the metal-loaded catalyst. The reutilization tests proved that both ZnO and 0.25%Ag/ZnO-t materials have good stability under the reaction conditions tested. In addition, both ZnO and 0.25%Ag/ZnO-t performed efficiently (the latter being superior) in the photocatalytic treatment of a mixture of phenol, resorcinol, 4-methoxyphenol and 4-chorophenol, using the photocatalyst suspended or immobilized on glass rings. The latter form is of particular interest for technological applications.
Acknoledgements This work was financially supported by Project POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM funded by FEDER funds through COMPETE2020 Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT - Fundação para a Ciência e a Tecnologia. Additional funding by project NORTE-07-0162-FEDER-000050, financed by QREN, ON2 and FEDER and Convénio - FCT/CAPES 2014/2015 – Brasil. MJL and MJS thank FCT for the research grants PD/BD/52623/2014 and SFRH/BD/79878/2011, respectively. AMTS and CGS acknowledge the FCT Investigator Programme (IF/01501/2013 and IF/00514/2014,
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respectively) with financing from the European Social Fund and the Human Potential Operational Programme.
References [1] S. Malato, P. Fernández-Ibáñez, M.I. Maldonado, J. Blanco, W. Gernjak, Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends, Catal. Today, 147 (2009) 1-59. [2] G. Odukkathil, N. Vasudevan, Toxicity and bioremediation of pesticides in agricultural soil, Rev. Environ. Sci. Biotechnol., 12 (2013) 421-444. [3] D. Mantzavinos, N. Kalogerakis, Treatment of olive mill effluents: Part I. Organic matter degradation by chemical and biological processes—an overview, Environ. Inter., 31 (2005) 289-295. [4] A.M.A. Pintor, V.J.P. Vilar, R.A.R. Boaventura, Decontamination of cork wastewaters by solar-photo-Fenton process using cork bleaching wastewater as H2O2 source, Sol. Energy, 85 (2011) 579-587. [5] J. Saien, H. Nejati, Enhanced photocatalytic degradation of pollutants in petroleum refinery wastewater under mild conditions, J. Hazard. Mater., 148 (2007) 491-495. [6] T. Zhang, X. Wang, X. Zhang, Recent Progress in TiO2-Mediated Solar Photocatalysis for Industrial Wastewater Treatment, Inter. J. Photoenergy, 2014 (2014) 12. [7] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: A review, Water Res., 44 (2010) 2997-3027. [8] A. Pal, K.Y.-H. Gin, A.Y.-C. Lin, M. Reinhard, Impacts of emerging organic contaminants on freshwater resources: Review of recent occurrences, sources, fate and effects, Sci. Total Environ., 408 (2010) 6062-6069.
25
[9] S.-Y. Lee, S.-J. Park, TiO2 photocatalyst for water treatment applications, J. Ind. Eng. Chem., 19 (2013) 1761-1769. [10] J.-M. Herrmann, Titania-based true heterogeneous photocatalysis, Environ. Sci. Pollut. Res., 19 (2012) 3655-3665. [11] S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, Strategies of making TiO2 and ZnO visible light active, J. Hazard. Mater., 170 (2009) 560-569. [12] J. Anderson, G.V.d.W. Chris, Fundamentals of zinc oxide as a semiconductor, Rep. Prog. Phys., 72 (2009) 126501. [13] R. Georgekutty, M.K. Seery, S.C. Pillai, A Highly Efficient Ag-ZnO Photocatalyst: Synthesis, Properties, and Mechanism, J. Phys. Chem. B, 112 (2008) 13563-13570. [14] K. Rajeshwar, M.E. Osugi, W. Chanmanee, C.R. Chenthamarakshan, M.V.B. Zanoni, P. Kajitvichyanukul, R. Krishnan-Ayer, Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media, J. Photochem. Photobiol. C: Photochem. Rev., 9 (2008) 171-192. [15] Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei, J. Zhu, Photocatalytic Activity of Ag/ZnO Heterostructure Nanocatalyst: Correlation between Structure and Property, J. Phys. Chem. C, 112 (2008) 10773-10777. [16] S. Das, S. Sinha, M. Suar, S.-I. Yun, A. Mishra, S.K. Tripathy, Solar-photocatalytic disinfection of Vibrio cholerae by using Ag@ZnO core–shell structure nanocomposites, J. Photochem. Photobiol. B: Biol., 142 (2015) 68-76. [17] H.Z. Malayeri, B. Ayati, H. Ganjidoust, Photocatalytic phenol degradation by immobilized nano ZnO: intermediates & key operating parameters, Water environment research : a research publication of the Water Environment Federation, 86 (2014) 771-778.
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[18] A. Di Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater., 211–212 (2012) 3-29. [19] C.G. Silva, M.J. Sampaio, S.A.C. Carabineiro, J.W.L. Oliveira, D.L. Baptista, R. Bacsa, B.F. Machado, P. Serp, J.L. Figueiredo, A.M.T. Silva, J.L. Faria, Developing highly active photocatalysts: Gold-loaded ZnO for solar phenol oxidation, J. Catal., 316 (2014) 182-190. [20] Y. Dong, S. Zhan, P. Wang, A facile synthesis of Ag Modified ZnO nanocrystals with enhanced photocatalytic activity, J. Wuhan Univ. Technol. Mat. Sci. Edit., 27 (2012) 615-620. [21] C. Tian, Q. Zhang, A. Wu, M. Jiang, Z. Liang, B. Jiang, H. Fu, Cost-effective large-scale synthesis of ZnO photocatalyst with excellent performance for dye photodegradation, Chem. Commun., 48 (2012) 2858-2860. [22] E. Pulido Melián, O. González Díaz, J.M. Doña Rodríguez, G. Colón, J.A. Navío, M. Macías, J. Pérez Peña, Effect of deposition of silver on structural characteristics and photoactivity of TiO2-based photocatalysts, Appl. Catal. B: Environ., 127 (2012) 112-120. [23] M.J. Sampaio, C.G. Silva, A.M.T. Silva, J.L. Faria, Kinetic modelling for the photocatalytic degradation of phenol by using TiO2-coated glass raschig rings under simulated solar light, J. Chem. Technol. Biotechnol., 91 (2016) 346–352 [24] L.M. Pastrana-Martínez, J.L. Faria, J.M. Doña-Rodríguez, C. FernándezRodríguez, A.M.T. Silva, Degradation of diphenhydramine pharmaceutical in aqueous solutions by using two highly active TiO2 photocatalysts: Operating parameters and photocatalytic mechanism, Appl. Catal. B: Environ., 113–114 (2012) 221-227. [25] N. Serpone, I. Texier, A.V. Emeline, P. Pichat, H. Hidaka, J. Zhao, Post-irradiation effect and reductive dechlorination of chlorophenols at oxygen-free TiO2/water
27
interfaces in the presence of prominent hole scavengers, J. Photochem. Photobiol. A: Chem., 136 (2000) 145-155. [26] C. Minero, G. Mariella, V. Maurino, D. Vione, E. Pelizzetti, Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Ions. 2. Competitive Reactions of Phenol and Alcohols on a Titanium Dioxide−Fluoride System, Langmuir, 16 (2000) 8964-8972. [27] B. Tryba, M. Piszcz, A.W. Morawski, Photocatalytic and self-cleaning properties of Ag-doped TiO2, Open Mat. Sci. J., 4 (2010) 5-8. [28] Z. Xuming, C. Yu Lim, L. Ru-Shi, T. Din Ping, Plasmonic photocatalysis, Rep. Prog. Phys., 76 (2013) 046401. [29] P. Christopher, H. Xin, S. Linic, Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures, Nat. Chem., 3 (2011) 467-472. [30] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications, Chem. Rev., 111 (2011) 3669-3712. [31] T. Liu, B. Li, Y. Hao, F. Han, L. Zhang, L. Hu, A general method to diverse silver/mesoporous–metal–oxide nanocomposites with plasmon-enhanced photocatalytic activity, Appl. Catal. B: Environ., 165 (2015) 378-388. [32] R.K. Sahu, K. Ganguly, T. Mishra, M. Mishra, R.S. Ningthoujam, S.K. Roy, L.C. Pathak, Stabilization of intrinsic defects at high temperatures in ZnO nanoparticles by Ag modification, J. Colloid Inter. Sci., 366 (2012) 8-15. [33] K.H. Stern, High Temperature Properties and Decomposition of Inorganic Salts Part 3, Nitrates and Nitrites, J. Phys. Chem. Reference Data, 1 (1972) 747-772.
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[34] N. Sobana, M. Muruganadham, M. Swaminathan, Nano-Ag particles doped TiO2 for efficient photodegradation of Direct azo dyes, J. Mol. Catal. A: Chem., 258 (2006) 124-132. [35] M. Farrokhi, J.-K. Yang, S.-M. Lee, M. Shirzad-Siboni, Effect of organic matter on cyanide removal by illuminated titanium dioxide or zinc oxide nanoparticles, J. Environ. Health Sci. Eng., 11 (2013) 23. [36] M.A. Behnajady, N. Modirshahla, M. Shokri, A. Zeininezhad, H.A. Zamani, Enhancement photocatalytic activity of ZnO nanoparticles by silver doping with optimization of photodeposition method parameters, J. Environ. Sci. Health, Part A, 44 (2009) 666-672. [37] C. Ren, B. Yang, M. Wu, J. Xu, Z. Fu, Y. lv, T. Guo, Y. Zhao, C. Zhu, Synthesis of Ag/ZnO nanorods array with enhanced photocatalytic performance, J. Hazard. Mater., 182 (2010) 123-129.
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Highlights
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Ag-loaded ZnO are efficient photocatalysts for phenolic compounds degradation
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Phenol degradation efficiency is related to the ZnO type and Ag load
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ZnO-t and 0.25%Ag/ZnO-t materials show good stability
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0.25%Ag/ZnO-t coated Raschig rings are efficient for phenolics photodegradation
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S1385-8947(16)30753-7 http://dx.doi.org/10.1016/j.cej.2016.05.105 CEJ 15265
To appear in:
Chemical Engineering Journal
Please cite this article as: M.J. Sampaio, M.J. Lima, D.L. Baptista, A.M.T. Silva, C.G. Silva, J.L. Faria, Ag-loaded ZnO materials for photocatalytic water treatment, Chemical Engineering Journal (2016), doi: http://dx.doi.org/ 10.1016/j.cej.2016.05.105
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Ag-loaded ZnO materials for photocatalytic water treatment Maria J. Sampaio 1, Maria J. Lima1, Daniel L. Baptista2, Adrián M.T. Silva1, Cláudia G. Silva1, Joaquim L. Faria1* 1
Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and
Materials (LSRE–LCM), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal 2
Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre,
91501-970, Brazil *Corresponding author: [email protected]
Abstract ZnO materials with different morphologies were obtained by hydrothermal and solid state techniques. The ZnO materials were loaded with different amounts of silver nanoparticles (atomic percentages ranging from 0.25 to 1) by a liquid impregnation method. The materials were characterized by physical adsorption of nitrogen, scanning electron microscopy (SEM), UV-Vis diffuse reflectance spectroscopy (DRUV-Vis), Xray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR). The photocatalytic efficiency of the materials was assessed in the degradation of phenol using simulated solar light, by means of the pseudo-first-order kinetic constant (kapp) for phenol conversion and corresponding degree of mineralization. The photocatalyst made of ZnO prepared by thermal decomposition of zinc acetate (ZnO-t) loaded with a 0.25 atomic percentage of Ag (0.25%Ag/ZnO-t) was the most efficient, reaching a 58% mineralization after 60 min irradiation (kapp = 7.7 × 10 -2 min-1). Trapping of photogenerated holes and radicals by selective scavengers showed that photogenerated holes played the main role on phenol oxidation. The study 1
was extended to a complex mixture containing phenol, resorcinol, 4-methoxyphenol and 4-chorophenol using bare ZnO-t and 0.25%Ag/ZnO-t photocatalysts, either in powder form or immobilized on glass rings (packed in a continuous photocatalytic reactor). The conversion of the phenolic compounds was faster when Ag particles were deposited on the photocatalyst. Keywords: Photocatalysis; Zinc oxide; Silver nanoparticles loading; Water treatment; Phenolic compounds
1.
Introduction
In recent years, water issues have been on the focus of increasing international concern. The intensification of agricultural and industrial activities has led to the contamination of groundwater by fertilizers and other chemicals [1, 2]. Phenol and its derivatives can be found in a wide variety of industrial effluents (e.g., olive oil mills, petrochemical, pulp and paper industries [3-6]) and are known as toxic and refractory organic compounds, therefore hard to degrade using conventional biological treatment processes. The development of effective processes capable of removing persistent organic pollutants from waste waters has been considered an important environmental challenge. The application of heterogeneous photocatalysis technology has gained increasing attention in the last decades due to its effectiveness in degrading a wide variety of recalcitrant organic water pollutants [7, 8], and also because they meet the requirement of sustainable process development when solar energy is used. Semiconductors such as TiO2 and ZnO have been extensively studied as important catalysts for photocatalytic water treatment [9-11]. ZnO, which has a band gap similar to TiO2 (ca. 3.2 eV), has been used as an interesting alternative material for photocatalytic applications due to its
2
physical and chemical stability, high oxidative capacity, low cost and availability [12, 13]. Owing to the fact that less than 5% of the solar spectrum at earth’s surface consists of UV light, the natural band gap of semiconductors like TiO2 and ZnO represents a drawback when considering the use of solar-driven photocatalytic processes. Several authors have reported a positive effect of loading Ag onto ZnO materials in the photocatalytic degradation of organic pollutants in air and aqueous media [13-15], and also in water disinfection and microbial control [16]. An improvement in the performance of these semiconductors being attributed to the presence of Ag nanoparticles acting as electron sinks, trapping the photoexcited electrons and therefore increasing the yield and lifetime of the charge separation state. In the present work, ZnO materials with distinct morphologies were loaded with different amounts of Ag nanoparticles aiming at an increase in the efficiency of the photocatalysts. Materials were tested under simulated solar light and using phenol as a model compound, since it is a standard molecule largely used for photocatalysts’ screening [17, 18]. The reaction mechanism of phenol photocatalytic degradation was evaluated using selective radical and hole scavengers. Taking into account water resource contamination either by accident, or in purpose as military or terrorist targeting, the silver properties were tested aiming rapid photocatalytic applications. Moreover, and considering the advantages of using photocatalysts in fixed supports, the most efficiency photocatalyst found for phenol degradation was immobilized on glass Raschig rings and tested in the oxidation treatment of a solution containing a mixture of phenolic compounds (which can model several typical hazardous compounds), namely phenol, resorcinol, 4-methoxyphenol and 4-chorophenol.
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2.
Experimental
2.1 Materials preparation Needle-like ZnO (ZnO-n) was prepared by a hydrothermal procedure, as described elsewhere [19, 20]: an aqueous solution of 4 M NaOH (Aldrich > 97%) was heated to 70 ºC with strong stirring, and after that an aqueous solution of 2 M Zn(NO3)2·6H2O (Aldrich > 99%) was added dropwise and left stirring for 1 h. The resulting precipitate was collected, washed with deionized water and dried in an oven at 110 ºC. Another material, ZnO-t was prepared by a solid-state thermal process [19, 21]. Briefly, a porcelain capsule containing zinc acetate dehydrate (Aldrich > 99.5%) was placed in a horizontal oven and heated until 600 ºC under air flow for 2 h. A commercial ZnO material from Evonik-Degussa GmbH (ZnO-ev; AdNano VP 20, aggregated nanoparticles of hydrophilic ZnO) was used as received. The liquid impregnation method was used to prepare the Ag/ZnO samples [22] The amounts of Ag loaded were 0.25, 0.5 and 1.0 at.% (atomic percentage). Briefly, 1 g of ZnO material was added to 250 mL of MiliQ water with different amounts of silver precursor (AgNO3, Alfa Aesar > 99.9 %). The mixture was stirred for 24 h and then evaporated until dry at boiling temperature. The final catalysts were labeled as xAg/ZnO-y, were x and y correspond to the percentage of Ag loaded and to the type of ZnO, respectively. The catalysts were then calcined under a 50 mL min-1 N2 flow at 400 ºC for 2 h. Selected materials were also immobilized on Raschig rings using the dip-coating technique, as described elsewhere [23].
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2.2 Materials characterization The specific surface area (SBET) of the powder materials was determined by multipoint analysis of N2 adsorption isotherms at -196 ºC using the BET method in a Quantachrome NOVA 4200e apparatus. The morphology of the ZnO materials was analyzed by scanning electron microscopy (SEM) using a FEI Quanta 400 FEG ESEM/EDAX Genesis X4M (15 keV) instrument. High-resolution transmission electron microscopy (HRTEM) of Ag/ZnO samples was performed using a Cs-corrected FEI Titan 80/300 microscope at INMETRO. Diffuse reflectance UV–Vis spectra (DRUV-Vis) of the materials were measured on a JASCO V-560 UV-Vis spectrophotometer equipped with an integrating sphere. The spectra were recorded in diffuse reflectance mode and transformed to equivalent absorption Kubelka–Munk units. The band gap of the photocatalysts was determined using the measured absorption edge as function of the energy. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos AXIS Ultra HSA, with VISION software for data acquisition and CASAXPS software for data analysis. The analysis was carried out with a monochromatic Al Kα X-ray source (1486.7 eV), operating at 15kV (90 W), in FAT (Fixed Analyser Transmission) mode, with a pass energy of 40 eV for regions ROI and 80 eV for survey. Temperature programmed reduction (TPR) experiments were carried out using an AMI-200 Catalyst Characterization Instrument (Altamira Instruments) equipped with a quadrupole mass spectrometer (Ametek, Mod. Dymaxion). The sample (150 mg) was placed in a U-shaped quartz tube and heated at 5 ºC min-1 up to the desired temperature under a flow of 5% (v/v) H2 diluted in Ar (total flow rate of 30 cm3 (STP) min-1). The H2 consumption was followed using a thermal conductivity detector (TCD).
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2.3 Photocatalytic experiments The photocatalytic experiments using ZnO-based materials were performed in a 50 mL pyrex batch reactor (7.0 cm height and 4.5 diameter) loaded with a 20 mg L-1 phenol solution (PH; Aldrich 99%) under simulated solar light irradiation for 60 min, using a Solarbox 1500e system (CO.FO.MEGRA). The reactor was placed in the bottom center of the solar simulator and the suspensions were irritated from the top. The catalyst load was fixed at 1 g L-1 and the suspensions were magnetically stirred and continuously saturated with an air flow. The irradiation source consisted of a 1500 W xenon lamp equipped with a cut-off soda-lime glass UV filter (for λ < 290 nm) with infrared reflection coating, to simulate outdoor exposure (irradiance equal to 30.9 mW cm-2, determined with an Ocean Optics spectroradiometer in the range 300-650 nm). The same procedure was implemented for the experiments using a mixture of phenolic compounds: phenol (PH), 4-methoxyphenol (MP; Fluka > 98%), resorcinol (RC; Aldrich > 99%) and 4-chlorophenol (CP; Aldrich > 99 %) with a concentration of 20 mg L−1 each. In this case the irradiation was carried out for 2 h. Some materials were immobilized on Raschig rings. In this case a glass cylindrical reactor (19 mm of internal diameter and 33 mm length) with 8.2 mL of useful volume was packed with nine TiO2-coated rings. In a typical experiment, the mixture of phenolic compounds was introduced into the reactor using a peristaltic pump at constant flow rate (Q = 17 mL min-1) from the reservoir containing 50 mL of phenolic mixture. The solution was continuous recycled during the photocatalytic experiments. The irradiation procedure was the same as described for the phenol solutions, except for the length of exposition, which was extended to 8 h. Reactions in the absence of catalyst were performed as blank experiments in order to characterize the pure photochemical regime of the phenolic compounds. Each
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experiment was preceded by a dark period of 30 min before switching the lamp on to establish the adsorption-desorption equilibrium. The concentration of the phenolic compounds was evaluated by high performance liquid chromatography (HPLC) with Hitachi Elite LaChrom apparatus equipped with Lichrocart Purospher Star RP-18 endcapped column (250 mm×4.6 mm, 5 µm particles), using a mobile phase of a mixture water: methanol in a gradient step. The total organic carbon (TOC) content of the initial and final samples was determined using a Shimadzu TOC-5000A apparatus. The photocatalytic pathway of phenol degradation was studied using 1.0 mM solutions of EDTA (292.2 mg L-1) and tert-butanol (t-BuOH, 74.1 mg L-1) as hole and radical scavengers, respectively [24-26].
3.
Results and Discussion
3.1 Materials characterization The specific surface area (SBET) of the bare ZnO materials, determined by N2 adsorption at -196 ºC, were 2, 6 and 26 m2 g−1 for ZnO-n, ZnO-t and ZnO-ev, respectively. Tryba et al. have reported that the deposition of silver on semiconductors may enhance the adsorption capacity of the photocatalyst by 10 to 30% [27]. On the other hand, others studies show that, during the preparation method, the Ag nanoparticles can aggregate forming small clusters, thus reducing the surface area of the resulting material [13]. In this study, no change on the S BET of the ZnO materials was observed after Ag loading, which may be attributed to the small amount of Ag deposited on the ZnO materials. Fig. 1 shows representative SEM micrographs of the different ZnO materials. These images clearly reveal the differences in the particle size and shape of the ZnO catalysts. The SEM micrograph of ZnO-n (Fig. 1a) shows needle-like particles of about 40-50 nm
7
in diameter and 600 nm in length. The ZnO-t material (Fig. 1c) is constituted by spherical particles with diameters between 100-250 nm and 1D needle-like structure with uneven diameter distribution along their longitudinal direction among 500-900 nm. Commercial ZnO-ev (not show) consists mainly on small (50–100 nm) cylindrical and spherical particles. Fig. 1b and 1d give representative HRTEM images of Ag spheroidal particles deposited on ZnO-n and ZnO-t, respectively. The HRTEM images suggest a good distribution of Ag particles with similar size (5 nm) over the ZnO materials.
a
b
Ag
500 nm
c
d
5 nm
Ag 500 nm
Figure 1 - SEM micrographs of ZnO-n (a) and ZnO-t (c) and HRTEM micrographs of 0.25%Ag/ZnO-n (b) and 0.25%Ag/ZnO-t (d), respectively.
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The diffuse reflectance UV-Vis spectra of the photocatalysts are shown in Fig. 2. The ZnO materials have a characteristic intense band absorption in the UV spectrum range (λ < 400 nm) corresponding to the band gap transition of the semiconductor. The values of the band gap energy obtained were 3.20, 3.22 and 3.24 eV for ZnO-n, ZnO-t and ZnO-ev, respectively (Fig. 2d). The presence of Ag nanoparticles on ZnO materials leads to the increase of the absorption in the visible light range between 400 and 600 nm, due to the Ag surface plasmon band [28, 29]. It was also observed that for the catalysts with low amounts of silver (0.25 at.%) the band gap energy is similar to those of bare ZnO materials. However, a slight decrease in the band gap was detected for 1.0%Ag/ZnO-t and 1.0%Ag/ZnO-n samples, 3.21 eV and 3.16 eV, respectively. This observation was more pronounced for the Ag/ZnO-ev catalysts, decreasing from 3.24 eV to 3.12 eV from bare ZnO-ev to 1.0%Ag/ZnO-ev, respectively. This decrease in the band bap on Ag/ZnO-ev materials suggests a better distribution of Ag nanoparticles on the ZnO-ev surface, which hold the highest S BET (26 m2 g−1) in comparison with ZnO-n and ZnO-t, resulting in an increase of the absorption in the entire visible spectrum range (Fig. 2c).
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Figure 2 – Diffuse reflectance UV-Vis spectra of Ag/ZnO-n (a), Ag/ZnO-t (b) Ag/ZnO-ev (c) materials, and plot the Kubelka-Munk (KM) units as function of the light energy of the neat ZnO materials (d): ZnO-n (i); ZnO-t (ii); ZnO-ev (iii).
Regarding the position of the surface plasmon resonance band for Ag/ZnO-n and Ag/ZnO-t materials (Fig. 2a and 2b inset, respectively), the absorption maxima occur at 11
454 nm, while for Ag/ZnO-ev a red-shifted band with a maximum peaking at 476 nm is observed (Fig. 2b inset). This shift may be attributed to the presence of Ag nanoparticles with different size and shape [30]. As expected, the absorption in the visible spectral range is directly proportional to the amount of Ag loaded on the different ZnO materials. The XPS profiles of the Ag/ZnO materials are shown in Figs. 3(a-c). A similar spectrum was observed for all the materials, being detected two peaks at binding energy (BE) of 368.1 eV and 374.1 eV corresponding to Ag3d3/2 and Ag3d3/2 with a spin-orbital splitting photoelectrons at 6.0 eV [31, 32]. If the peaks in the 3d level are shifted to lower BE (< 0.6 eV), Ag could be at different states (Ag0, Ag2O and AgO). The deconvolution of the XPS profiles showed two peaks in the 3 d5/2 level, at 367.8 and 368.2 eV and for 3 d 3/2 level, at 373.9 and 374.2 eV , corresponding to oxidized Ag (AgO) and to Ag0 state, respectively. It has been reported that AgNO3, used as silver precursor, can be easily decomposed at low temperatures to give Ag2O [33]. However, Ag2O is very unstable above ~120 ºC, being converted to AgO at temperatures between 100 and 200 ºC, while AgO starts to decompose into Ag above 400 ºC. The TPR analyzes were carried out in order to identify the decomposition temperature of Ag species in 1%Ag/ZnO-t (Fig. 3b), before and after the calcination treatment at 400 ºC. The TPR profile of bare ZnO-t was also evaluated as control. Two peaks in the 100-220 ºC range were observed for the non-calcined material, which could be assigned to the decomposition of Ag2O to form AgO and also to the formation of NO2 resulting from the AgNO3 decomposition [33]. After the thermal treatment at 400 ºC, the peaks corresponding to the decomposition of Ag2O and NO2 are absent indicating that this temperature is adequate to guarantee the
12
elimination of NO2 from the silver precursor. From TPR analysis, no signal was detected for ZnO confirming the stability of the material among the temperatures tested.
13
Figure 3 – XPS spectra of Ag/ZnO-t (a), Ag/ZnO-n (b) and Ag/ZnO-ev (c) and TPR analysis of Ag/ZnO materials (d).*n.c.: not calcined
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3.2. Photocatalytic experiments 3.2.1. Photocatalytic activity of ZnO and Ag/ZnO materials The effect of silver deposition on the efficiency of ZnO materials was studied for the photocatalytic oxidation of phenol in water under simulated solar light. The photochemical degradation of phenol was carried out at first, in the absence of catalyst. Under these conditions, it was observed a decrease in phenol concentration of 5% after 60 min irradiation. It was also observed that a maximum of 5% decrease in phenol concentration after the dark adsorption period was obtained for both ZnO and Ag/ZnO materials. Results show that kinetics of photocatalytic phenol degradation follows a pseudo-first order rate law for all the tested catalysts. The efficiency of the materials was evaluated in terms of the apparent first order constant (kapp), determined by non-linear curve fitting to the experimental data (Fig. 4). Different behaviors were observed depending on the ZnO materials used. Among the bare ZnO materials, ZnO-n was the less active catalyst, while ZnO-ev was the most efficient, with kapp of 9.8×10 -3 and 6.5×10 -2 min-1, respectively. Nevertheless, when silver was loaded on ZnO-ev, a significant decrease in kapp was observed being more pronounced for the sample containing 1.0 at.% (kapp = 3.3×10-2 min-1). In fact, some authors have shown that the deposition of noble metals not always improves the photoactivity of semiconductors. Depending on the structure of the photocatalyst, the Ag deposition may result in the blockage of the active centers of the metal oxide semiconductor, thus reducing its photocatalytic activity [22, 30]. In the case of ZnO-t materials, the addition of Ag leads to an increase in the photocatalytic activity. It was found that the lowest amount of silver (0.25 at. %) led to the highest phenol degradation efficiency (kapp = 7.7×10-2 min-1). However, the 15
photoefficiency of the ZnO-t materials declined when the amount of Ag increased. This may suggest that the effective quantity of holes and electrons generated when 0.25 at.% Ag is added to ZnO is adequate to degrade an initial concentration of 20 ppm of phenol. An increase in the silver load may lead to a surplus charge separation which is not efficiently contributing to the reaction. For Ag/ZnO-n materials, an increase in kapp with increasing Ag load was observed, with a kapp of 3.34¯10 -2 min-1 being obtained using the 1.0%Ag/ZnO-n catalyst, corresponding to a 3.4 fold increase compared with bare ZnO-n. A sample with 1.4 at.% Ag was also tested (not shown), but a decrease in the efficiency for phenol degradation was obtained (kapp =2.61¯10-2 min-1).
Figure 4 – First order apparent rate constants (kapp) for the photocatalytic reactions using neat and Ag-loaded ZnO materials.
Total organic carbon (TOC) measurements were performed for evaluating the degree of mineralization achieved by the photocatalytic treatment of the phenol containing
16
solutions. In general, an increase in the TOC removal was obtained when Ag nanoparticles were incorporated in the ZnO materials (Table 1). Table 1 – TOC removal of the ZnO and Ag/ZnO photocatalyst at the end of 60 min reaction. XTOC (%) Catalyst no Ag 0.25%Ag 0.50%Ag 1.0%Ag ZnO-n 1.5 26 29 48 ZnO-ev 46 69 69 24 ZnO-t 43 58 45 39
The mineralization efficiency was quantified by using a synergy factor (RTOC), defined as XTOC, 60 min (Ag/ZnO)/XTOC, 60 min (ZnO), where XTOC corresponds to the TOC conversion. The highest RTOC was obtained for 1.0%Ag/ZnO-n (RTOC = 33.8). For 0.25%Ag/ZnO-ev and 0.25%Ag/ZnO-t, RTOC values of 1.5 and 1.4 were obtained, indicating 33% and 26% mineralization increases in comparison with the respective bare ZnO materials. These results underline the positive effect of the incorporation of Ag in the ZnO materials.
3.2.2 Photocatalytic degradation pathway Loading Ag onto ZnO induces the formation of a Schottky barrier at the metalsemiconductor interphase surface [20, 34]. Due to the higher Fermi level energy of ZnO compared with Ag, the electrons can easily flow from the photoexcited metal oxide to Ag. The transfer of electrons to the Ag nanoparticles is normally faster than the electron-hole (e-/h+) recombination process occurring in the ZnO phase, and thus high amounts of electrons (e-) from ZnO can be stored in the Ag nanoparticles. Ag particles act as electron sinks where adsorbed oxygen may be reduced yielding superoxide radical (O2•−). As a result, more holes (h+), which are strongly oxidizing species, are available to drive oxidation reactions, namely the formation of hydroxyl radicals (HOy) from hydroxyl anions and/or also direct oxidation of phenol.
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The main routes occurring during the photocatalytic phenol degradation were investigated by using EDTA and t-BuOH, which act as scavengers for holes and radicals, respectively [24]. Fig. 5 shows the evolution of phenol concentration during the photocatalytic experiments using bare ZnO-t (Fig. 5a) and 0.25%Ag/ZnO-t (Fig. 5b) catalysts in the presence of electron/hole scavengers. It was observed for both materials that the addition of EDTA or t-BuOH leads to a decrease in the phenol degradation rate, which indicates that both radical species and photogenerated holes are involved in the phenol degradation process. For ZnO-t the presence of t-BuOH led to a decrease in kapp, from 5.1¯10-2 to 3.7×10 -2 min-1. This decrease was even more pronounced in the presence of EDTA (kapp = 1.0¯10-2 min-1). A similar behaviour was observed for 0.25%Ag/ZnO-t (Fig. 5b), where the presence of t-BuOH and EDTA led to a decrease in the kapp from 7.7¯10-2 min-1 to 4.3¯10 -2 min-1 and 1.8¯10 -2 min−1, respectively. The results indicate that, although reactive species, such as hydroxyl (HO•), superoxide (O2•−) and/or hydroperoxyl (HOO•) radicals, are responsible for part of the photocatalytic phenol degradation, photogenerated holes on the catalyst surface (h+) seem to play the main role, since kapp was much more reduced when EDTA was used as hole scavenger.
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Figure 5. Effect of hole/radical scavengers (1 mM EDTA/t-BuOH) on the photocatalytic degradation of phenol using ZnO-t (a) and 0.25%Ag/ZnO-t (b).
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Yet, in the presence of 0.25%Ag/ZnO-t the contribution of radical species in the phenol 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
degradation reaction is more pronounced comparing with bare ZnO-t, since 45% and 28% decrease in kapp was observed when t-BuOH was added to 0.25%Ag/ZnO-t and ZnO-t suspensions, respectively. These results may be related to the more efficient charge separation occurring upon photoexcitation of 0.25%Ag/ZnO-t, leading to a higher availability of surface electrons and holes and the formation of higher amounts of radical species. 3.2.3 Reutilization tests It is known that depending of the photocatalytic reaction conditions, the photocorrosion of ZnO may occur [11, 35]. Thus, photostability of ZnO and 0.25%Ag/ZnO-t was investigated by reusing these materials in consecutive phenol degradation reactions at natural pH conditions (pH = 6.1) under simulated solar light. Before each run the catalyst was washed with deionized water and dried at 110 ºC. Fig. 6 shows the phenol concentration evolution in photocatalytic reactions using ZnO and 0.25%Ag/ZnO-t in four utilization cycles.
Figure 6 – Reusability assessment for ZnO-t (¢) and 0.25%Ag/ZnO-t () for phenol degradation. 20
The above results show that ZnO-t maintains its activity during four cycles of utilization, suggesting a good stability of this material within the reaction conditions used. For the experiments using 0.25%Ag/ZnO-t, a slight improvement in the kinetics of phenol degradation was observed for the second run. This may be attributed to partial photoreduction of Ag species during the previous photocatalytic reaction. The presence of Ag species in the oxidized state was noticed by XPS analysis, which may change to its reduced form upon irradiation. In fact, several studies reported the use of the photo-impregnation method to deposit and reduce Ag nanoparticles on catalysts increasing their activity and stability [22, 36, 37]. In terms of mineralization, a slight decrease (18% and 10% for ZnO and 0.25%Ag/ZnO-t, respectively) in the TOC removal capacity was observed after four uses.
3.2.4 Photocatalytic degradation of a mixture of phenolic compounds Taking into account the photocatalytic efficiency observed for ZnO-t and 0.25%Ag/ZnO-t in the degradation of phenol, the photocatalytic treatment of a mixture containing four phenolic compounds was studied. Phenol (PH), resorcinol (RC), 4methoxyphenol (MP) and 4-chlorophenol (CP) were selected as model compounds. The separation of catalyst powders from the treated water requires separation steps that should be taken into account for technological applications. Thus, a set of experiments were also performed using the catalysts immobilized on glass Raschig rings. A dark period for each experiment was carried out for 30 min before irradiation. No significant decrease in the initial concentration for the different compounds was observed during the dark period. The experimental results indicate that the degradation of the individual
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compounds in the mixture obeys a pseudo-first-order rate law. Using the information from the kinetic curves (see supplementary data, SD2) it was determined the half life time (t1/2) for each compound. The relationship between the t1/2 and the individual compounds for the experiments using ZnO-t and 0.25%Ag/ZnO-t in the form of powder and immobilized on glass Raschig rings are shown in Fig. 7.
Figure 7 – Half life time (t1/2) degradation as a function of each phenolic compound using powder (_p) and films (_f) of bare ZnO-t and 0.25%Ag/ZnO-t.
The above results show that, in general the deposition of Ag improves the photocatalytic efficiency of the resulting material. Shorter half life time is needed for each compound be converted in 50% when 0.25%Ag/ZnO-t was used, either as a powder or film. It was also observed that the individual compounds present in the mixture are converted simultaneously, indicating that no competition occurs between each compound during the photocatalytic reaction. As expected, the use of a catalyst in the form of powder leads to a faster conversion, i.e., a shorter half-life compared to that of the immobilized 22
photocatalyst. This is obviously expected since the contact surface is larger in the suspended material, due to the larger amount of catalyst per unit of volume. Nevertheless, several practical aspects arise from the use of a catalyst powder form. Those include the need for post-separation techniques, as well as the difficulty to recover the catalyst in continuous flow reaction systems. This recovery is more important as the cost of the catalyst increases with the deposition of noble metals. Thus a compromise must be established between the desired conversion and the need of recycling the catalyst. In terms of TOC removal, the presence of Ag revealed to be advantageous, being of 6% and 27% using ZnO, and 14% and 42% using 0.25%Ag/ZnO-t with the photocatalyst immobilized and in powder form, respectively. Globally, the results show that the deposition of Ag nanoparticles on ZnO materials may improve their photocatalytic efficiency by increasing the photocatalytic degradation rate as well as the mineralization capacity. Moreover, when applied to near to real field conditions (i.e., mixture of compounds that may be encountered in waste waters and the possibility of using the catalyst immobilized on physical support to be employed in continuous systems) the use of Ag/ZnO photocatalysts could be of major relevance for solar applications.
Conclusions Photocatalysts of ZnO loaded with Ag are more efficient than the neat synthesized materials, with relation to the conversion and mineralization of phenol and some of its derivatives. The highest efficiency was obtained when a small amount of Ag was deposited on ZnO-t. The most active material consisted of 0.25 at.%Ag over ZnO-t. The increase in
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the activity was ascribed to the action of Ag nanoparticles as electrons sinks, thus increasing the efficiency and lifetime of charge separation in the semiconductor. The important decrease in kapp observed when EDTA was used as hole scavenger suggests that the excited holes play a major role in the mechanism of phenol degradation. For the reactions using 0.25%Ag/ZnO-t, the contribution of radicals to the phenol degradation mechanism was more evident than for bare ZnO-t, as a result of the higher amount of electrons and holes generated at the surface of the metal-loaded catalyst. The reutilization tests proved that both ZnO and 0.25%Ag/ZnO-t materials have good stability under the reaction conditions tested. In addition, both ZnO and 0.25%Ag/ZnO-t performed efficiently (the latter being superior) in the photocatalytic treatment of a mixture of phenol, resorcinol, 4-methoxyphenol and 4-chorophenol, using the photocatalyst suspended or immobilized on glass rings. The latter form is of particular interest for technological applications.
Acknoledgements This work was financially supported by Project POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM funded by FEDER funds through COMPETE2020 Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT - Fundação para a Ciência e a Tecnologia. Additional funding by project NORTE-07-0162-FEDER-000050, financed by QREN, ON2 and FEDER and Convénio - FCT/CAPES 2014/2015 – Brasil. MJL and MJS thank FCT for the research grants PD/BD/52623/2014 and SFRH/BD/79878/2011, respectively. AMTS and CGS acknowledge the FCT Investigator Programme (IF/01501/2013 and IF/00514/2014,
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respectively) with financing from the European Social Fund and the Human Potential Operational Programme.
References [1] S. Malato, P. Fernández-Ibáñez, M.I. Maldonado, J. Blanco, W. Gernjak, Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends, Catal. Today, 147 (2009) 1-59. [2] G. Odukkathil, N. Vasudevan, Toxicity and bioremediation of pesticides in agricultural soil, Rev. Environ. Sci. Biotechnol., 12 (2013) 421-444. [3] D. Mantzavinos, N. Kalogerakis, Treatment of olive mill effluents: Part I. Organic matter degradation by chemical and biological processes—an overview, Environ. Inter., 31 (2005) 289-295. [4] A.M.A. Pintor, V.J.P. Vilar, R.A.R. Boaventura, Decontamination of cork wastewaters by solar-photo-Fenton process using cork bleaching wastewater as H2O2 source, Sol. Energy, 85 (2011) 579-587. [5] J. Saien, H. Nejati, Enhanced photocatalytic degradation of pollutants in petroleum refinery wastewater under mild conditions, J. Hazard. Mater., 148 (2007) 491-495. [6] T. Zhang, X. Wang, X. Zhang, Recent Progress in TiO2-Mediated Solar Photocatalysis for Industrial Wastewater Treatment, Inter. J. Photoenergy, 2014 (2014) 12. [7] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: A review, Water Res., 44 (2010) 2997-3027. [8] A. Pal, K.Y.-H. Gin, A.Y.-C. Lin, M. Reinhard, Impacts of emerging organic contaminants on freshwater resources: Review of recent occurrences, sources, fate and effects, Sci. Total Environ., 408 (2010) 6062-6069.
25
[9] S.-Y. Lee, S.-J. Park, TiO2 photocatalyst for water treatment applications, J. Ind. Eng. Chem., 19 (2013) 1761-1769. [10] J.-M. Herrmann, Titania-based true heterogeneous photocatalysis, Environ. Sci. Pollut. Res., 19 (2012) 3655-3665. [11] S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, Strategies of making TiO2 and ZnO visible light active, J. Hazard. Mater., 170 (2009) 560-569. [12] J. Anderson, G.V.d.W. Chris, Fundamentals of zinc oxide as a semiconductor, Rep. Prog. Phys., 72 (2009) 126501. [13] R. Georgekutty, M.K. Seery, S.C. Pillai, A Highly Efficient Ag-ZnO Photocatalyst: Synthesis, Properties, and Mechanism, J. Phys. Chem. B, 112 (2008) 13563-13570. [14] K. Rajeshwar, M.E. Osugi, W. Chanmanee, C.R. Chenthamarakshan, M.V.B. Zanoni, P. Kajitvichyanukul, R. Krishnan-Ayer, Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media, J. Photochem. Photobiol. C: Photochem. Rev., 9 (2008) 171-192. [15] Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei, J. Zhu, Photocatalytic Activity of Ag/ZnO Heterostructure Nanocatalyst: Correlation between Structure and Property, J. Phys. Chem. C, 112 (2008) 10773-10777. [16] S. Das, S. Sinha, M. Suar, S.-I. Yun, A. Mishra, S.K. Tripathy, Solar-photocatalytic disinfection of Vibrio cholerae by using Ag@ZnO core–shell structure nanocomposites, J. Photochem. Photobiol. B: Biol., 142 (2015) 68-76. [17] H.Z. Malayeri, B. Ayati, H. Ganjidoust, Photocatalytic phenol degradation by immobilized nano ZnO: intermediates & key operating parameters, Water environment research : a research publication of the Water Environment Federation, 86 (2014) 771-778.
26
[18] A. Di Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater., 211–212 (2012) 3-29. [19] C.G. Silva, M.J. Sampaio, S.A.C. Carabineiro, J.W.L. Oliveira, D.L. Baptista, R. Bacsa, B.F. Machado, P. Serp, J.L. Figueiredo, A.M.T. Silva, J.L. Faria, Developing highly active photocatalysts: Gold-loaded ZnO for solar phenol oxidation, J. Catal., 316 (2014) 182-190. [20] Y. Dong, S. Zhan, P. Wang, A facile synthesis of Ag Modified ZnO nanocrystals with enhanced photocatalytic activity, J. Wuhan Univ. Technol. Mat. Sci. Edit., 27 (2012) 615-620. [21] C. Tian, Q. Zhang, A. Wu, M. Jiang, Z. Liang, B. Jiang, H. Fu, Cost-effective large-scale synthesis of ZnO photocatalyst with excellent performance for dye photodegradation, Chem. Commun., 48 (2012) 2858-2860. [22] E. Pulido Melián, O. González Díaz, J.M. Doña Rodríguez, G. Colón, J.A. Navío, M. Macías, J. Pérez Peña, Effect of deposition of silver on structural characteristics and photoactivity of TiO2-based photocatalysts, Appl. Catal. B: Environ., 127 (2012) 112-120. [23] M.J. Sampaio, C.G. Silva, A.M.T. Silva, J.L. Faria, Kinetic modelling for the photocatalytic degradation of phenol by using TiO2-coated glass raschig rings under simulated solar light, J. Chem. Technol. Biotechnol., 91 (2016) 346–352 [24] L.M. Pastrana-Martínez, J.L. Faria, J.M. Doña-Rodríguez, C. FernándezRodríguez, A.M.T. Silva, Degradation of diphenhydramine pharmaceutical in aqueous solutions by using two highly active TiO2 photocatalysts: Operating parameters and photocatalytic mechanism, Appl. Catal. B: Environ., 113–114 (2012) 221-227. [25] N. Serpone, I. Texier, A.V. Emeline, P. Pichat, H. Hidaka, J. Zhao, Post-irradiation effect and reductive dechlorination of chlorophenols at oxygen-free TiO2/water
27
interfaces in the presence of prominent hole scavengers, J. Photochem. Photobiol. A: Chem., 136 (2000) 145-155. [26] C. Minero, G. Mariella, V. Maurino, D. Vione, E. Pelizzetti, Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Ions. 2. Competitive Reactions of Phenol and Alcohols on a Titanium Dioxide−Fluoride System, Langmuir, 16 (2000) 8964-8972. [27] B. Tryba, M. Piszcz, A.W. Morawski, Photocatalytic and self-cleaning properties of Ag-doped TiO2, Open Mat. Sci. J., 4 (2010) 5-8. [28] Z. Xuming, C. Yu Lim, L. Ru-Shi, T. Din Ping, Plasmonic photocatalysis, Rep. Prog. Phys., 76 (2013) 046401. [29] P. Christopher, H. Xin, S. Linic, Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures, Nat. Chem., 3 (2011) 467-472. [30] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications, Chem. Rev., 111 (2011) 3669-3712. [31] T. Liu, B. Li, Y. Hao, F. Han, L. Zhang, L. Hu, A general method to diverse silver/mesoporous–metal–oxide nanocomposites with plasmon-enhanced photocatalytic activity, Appl. Catal. B: Environ., 165 (2015) 378-388. [32] R.K. Sahu, K. Ganguly, T. Mishra, M. Mishra, R.S. Ningthoujam, S.K. Roy, L.C. Pathak, Stabilization of intrinsic defects at high temperatures in ZnO nanoparticles by Ag modification, J. Colloid Inter. Sci., 366 (2012) 8-15. [33] K.H. Stern, High Temperature Properties and Decomposition of Inorganic Salts Part 3, Nitrates and Nitrites, J. Phys. Chem. Reference Data, 1 (1972) 747-772.
28
[34] N. Sobana, M. Muruganadham, M. Swaminathan, Nano-Ag particles doped TiO2 for efficient photodegradation of Direct azo dyes, J. Mol. Catal. A: Chem., 258 (2006) 124-132. [35] M. Farrokhi, J.-K. Yang, S.-M. Lee, M. Shirzad-Siboni, Effect of organic matter on cyanide removal by illuminated titanium dioxide or zinc oxide nanoparticles, J. Environ. Health Sci. Eng., 11 (2013) 23. [36] M.A. Behnajady, N. Modirshahla, M. Shokri, A. Zeininezhad, H.A. Zamani, Enhancement photocatalytic activity of ZnO nanoparticles by silver doping with optimization of photodeposition method parameters, J. Environ. Sci. Health, Part A, 44 (2009) 666-672. [37] C. Ren, B. Yang, M. Wu, J. Xu, Z. Fu, Y. lv, T. Guo, Y. Zhao, C. Zhu, Synthesis of Ag/ZnO nanorods array with enhanced photocatalytic performance, J. Hazard. Mater., 182 (2010) 123-129.
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Highlights
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Ag-loaded ZnO are efficient photocatalysts for phenolic compounds degradation
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Phenol degradation efficiency is related to the ZnO type and Ag load
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ZnO-t and 0.25%Ag/ZnO-t materials show good stability
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0.25%Ag/ZnO-t coated Raschig rings are efficient for phenolics photodegradation
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