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Plasmonic Ag-pillared rectorite as catalyst for degradation of 2,4-DCP in the H2O2-containing system under visible light irradiation
Accepted Manuscript Title: Plasmonic Ag-pillared rectorite as catalyst for Degradation of 2,4-DCP in the H2 O2 -containing system under visible light irradiation Author: Yunfang Chen Jianzhang Fang Shaoyou Lu Yan Wu Dazhi Chen Liyan Huang Cong Cheng Lu Ren Ximiao Zhu Zhanqiang Fang PII: DOI: Reference:
S0304-3894(15)00418-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.05.022 HAZMAT 16824
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
19-12-2014 12-5-2015 14-5-2015
Please cite this article as: Yunfang Chen, Jianzhang Fang, Shaoyou Lu, Yan Wu, Dazhi Chen, Liyan Huang, Cong Cheng, Lu Ren, Ximiao Zhu, Zhanqiang Fang, Plasmonic Ag-pillared rectorite as catalyst for Degradation of 2,4-DCP in the H2O2containing system under visible light irradiation, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.05.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Plasmonic Ag-pillared rectorite as catalyst for Degradation of 2,4-DCP in the H2O2-containing system under visible light irradiation Yunfang Chena, Jianzhang Fanga,c,*, Shaoyou Lud, Yan Wub, Dazhi Chenb, Liyan Huangb, Cong Chenga, Lu Rena, Ximiao Zhu a, Zhanqiang Fanga,c
a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, Guangdong, China
b
Institute of Engineering Technology of Guangdong Province, Key Laboratory of Water Environmental Pollution Control of Guangdong Province, Guangzhou 510440 c
Guangdong Technology Research Center for Ecological Management and Remediation of Urban Water System, Guangzhou 510006, China
d
Shenzhen Center for Disease Control and Prevention, Shenzhen 518055, China * Corresponding author. Tel: +8620 39310250, Fax: +8620 39310187 Email: [email protected]
The Ag-R catelyst was synthesized via a novel thermal decomposition method.
Ag-R catalyst possessed the synergistic effects of SPR and adsorption capacity.
The degradation of 2,4-DCP was evaluated in Ag-R/H2O2/visible light system.
1
Graphical abstract
Abstract This study aims at photocatalytic degradation of 2,4-DCP with the assistance of H2O2 in aqueous solution by a composite catalyst of Ag-rectorite. The catalysts were prepared via a novel thermal decomposition method followed after the 2
cation-exchange process. The synthesized nano-materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS)，Brunauer–Emmett– Teller (BET) surface analyzer, ultraviolet–visible light (UV–vis) absorption spectra, Field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The different mechanisms of degradation process with or without visible light irradiation were discussed, respectively. Moreover, the degradation efficiency of 2,4-DCP wastewater under Ag-rectorite/H2O2/visible light system was investigated by series of experiments, concerning on effects of major operation factors, such as H2O2 dosage and the initial pH value. The highest degradation rate was observed when adding 0.18 mL H2O2 into 50 mL 2,4-DCP solution, and the optimal pH value was 4 for the reaction. Afterwards, total organic carbon (TOC) experiments were carried out to evaluate the mineralization ratio of 2,4-DCP.
Keywords: Ag-rectorite; H2O2; Thermal decomposition; Photocatalysis; 2,4-DCP
1. Introduction With the increasingly urgent environmental standards and regulations regarding the wastewater discharge, it is a focus of international concern on how to eliminate hazardous materials from wastewater [1-5]. During the past 20 years, advanced 3
oxidation processes (AOPs) have been demonstrated to be one of the most effective approaches to eliminate organic pollutants [6-9]. As an environmental expected oxidant, H2O2 enable to provide hydroxyl radicals (·OH) in order to trigger advanced oxidation processes, such as O3/H2O2 [10], UV/H2O2 [11], photo-Fenton [12] and semiconductor photocatalysis processes. In addition, the catalytic wet hydrogen peroxide oxidation of organic contaminants can be carried out when using a noble metal as the catalyst. For example, according to the report of Nirupam Khanikar [13], both Cu(II)-kaolinite and Cu(II)-montmorillonite were used as catalysts for oxidizing three chlorinated derivatives of phenol in water in presence of H2O2. However, as a noble metal, metallic Ag has rarely been reported as the catalyst to promote hydrogen peroxide oxidation of wastewater. Moreover, nano-sized Ag particles exhibit a photoresponse in the visible light region due to their surface plasmon resonance (SPR) effect, which is produced by the collective oscillations of surface electrons [14-18]. It is assumed that visible light irradiation assistance may be beneficial for Ag/H2O2 oxidation process. In the Ag/H2O2/visible light system, hydrogen peroxide may act as an electron acceptor, instead of the most common oxygen. To the best of our knowledge, this is the first report on studying Ag/H2O2/visible light system. However, Ag nanoparticles are hardly usable since they are poor at dispersion stability and tend to aggregate in the liquid medium. To overcome the agglomeration, grafting Ag nanoparticles on clay compounds, which are interlamellar structures, should be one of the most effective solutions [19]. Rectorite is an interstratified 4
layered silicate mineral regarded as a regular (1:1) stacking of dioctahedral mica-like layers and dioctahedral smectite-like layers [20-23]. The cations of Na+, K+, and Ca2+ lie in the interlayer region, while the exchangeable hydrated cations reside in the latter, so theoretically Ag+- rectorite could be formed through exchange process. Then how to construct metallic Ag onto rectorite? On the basis of previous investigations, it seems logical to obtain metallic Ag nanoparticles via chemical reduction or photoreduction [24-26]. However, we found a novel route to obtain metallic Ag from the decomposition of Ag2C2O4 precursor during thermal method [27] after exchange process. In this way, part of Ag2C2O4 molecules appeared in the interlayers of rectorite firstly, and transferred to form Ag-pillared rectorite, which could possess a larger specific surface area. In this work, the synthesis of a novel composite catalyst of Ag-rectorite was achieved through a facile cation-exchange process followed by a thermal decomposition method, and its promotion in H2 O2 oxidation process towards the degradation of 2,4-DCP under visible light was investigated in details. 2. Experimental 2.1. Materials Rectorite was purchased from Rectorite Deposit of Zhongxiang, Hubei, P.R. China. AgNO3 was proved by Xilong Chemical Co., Ltd. H2C2O4 was supplied by Tianjinzhiyuan Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH) was purchased from Tianjin Kermel Chemical reagent Co. Ltd. 2, 4-DCP was purchased from Aladdin Reagent Co. Ltd. Other reagents were of analytical purity and were 5
received from Guangzhou Chemical Reagent Factory. Distilled water was used throughout this study. 2.2. Preparation of Ag-rectorite The Ag-rectorite compound (mentioned as Ag-R) was synthesized by a method involved ultrasonic-assisted ion-exchange. The schematic illustration of the synthesis of Ag-R could be identified during the process，as shown in Fig. 1. Two-tenths of a gram of rectorite was added into 50 mL of deionized water, and dispersed under ultrasonic assistance for 30 min. Then 4 mL AgNO3 solution (0.4 mol/L) was added dropwise into above solution with vigorously stirring to obtain a ratio of 8 mmol of Ag/g of rectorite. The mixed suspension was exposed to ultrasonic irradiation for 1 h. Subsequently, the as-formed wet products were separated through centrifugation, and washed with absolute ethanol and distilled water respectively. The obtained product was redispersed into 40 mL H2C2O4 solution (0.04 mol/L) and then stirring treatment was conducted after adjusting the pH to 5 using NaOH solution. After one night, the precipitate was separated by centrifuging and washed with deionized water. Finally, the precipitate was dried at 80 ℃ and then calcined at 450 ℃ in air for 2 h, respectively. Ag-R-1 was also prepared by the same procedure mentioned previously without the presence of H2C2O4. 2.3. Characterization of catalysts X-ray powder diffraction (XRD) patterns of as-prepared products were performed using a Bruker D8 ADVANCE X-ray diffractometer equipped with Cu kα radiation under operation conditions of 40 kV and 40 mA. X-ray photoelectron spectroscopy 6
(XPS) analysis was conducted on a Krato Axis ultra DLD photoelectron spectroscopy with monochromatic Al kα radiation. The binding energy (BE) was calibrated to C 1s line at 284.6 eV. The morphologies of the products were measured using Fieldemission scanning electron microscope (FE-SEM) images procured by a ZEISS Ultra 55 instrument operated at accelerating voltages between 20 and 30 kV. Highresolution transmission electron microscope (HRTEM, JEM2100HR, JEOL, Japan) was carried out with an accelerating voltage of 200 kV. Raman spectra were recorded on a Thermo Scientific DXR Raman spectrometer at room temperature. UV-vis diffuse reflectance spectra were collected from the UV-vis spectrophotometer (U3010, HITACHI, Japan) using BaSO4 as the reflectance sample. The specific surface areas of products were measured by the Brunauer-Emmett-Teller analysis (BET, ASAP 2020, Micromeritics, USA) on the basis of nitrogen adsorption isotherms. 2.4. Photocatalytic Activity Test The photocatalytic activities of the as-prepared catalysts were measured by the degradation of 2,4-DCP aqueous solution under visible light irradiation. Photocatalytic reactions under visible light irradiation were conducted in a 100 mL cylindrical quartz reactor equipped with a water circulation facility. A 1000 W Xe arc lamp with 420 nm cutoff filter as the light source was used to cause the photocatalytic reaction. In all experiments, the catalyst (0.05g) and a certain amount of H2O2 (3%) were added to 50 ml 2,4-DCP aqueous solution (30 mg/L). All these experiments were operated at room temperature. About 3 mL of the suspension was collected from the reaction cell at given time intervals, and then filtered through 0.45 μm membrane 7
filters for analysis. The determination of 2,4-DCP was carried out on high performance liquid chromatography (HPLC, Shimadzu) equipped with a UV detector (SPD-10AV) and C18 column (250 mm×4.6 mm). The mobile phase was a mixture of 60/40(v/v) ethanol-water mixture. The eluent was delivered at a rate of 0.8 mL·min-1 and the wavelength for detection was 284 nm. The degradation rate (η) was calculated as below:
C 0 C1 100% C0
(1)
Where C0 is the initial concentration and C1 is the concentration at time t. TOC concentration was determined by a TOC-TN vcp Shimadzu analyzer. 3. Results and discussion 3.1. Characterization of as-prepared materials 3.1.1. XRD analysis The X-ray diffraction (XRD) patterns of pristine rectorite, precursor Ag2C2O4-R, Ag-R-1 and Ag-R are shown in Fig. 2. The basal (001) and (002) diffraction peaks of the samples could be indexed to the rectorite, which are essential for rectorite with a highly ordered and oriented silicate layer structure. Obviously, the (002) peak at
2 7.978 of the rectorite shifted to a higher angle ( 2 8.895 ) as seen in the XRD pattern of the Ag-R catalyst, which were attributed to the smectite-like layers dehydration phenomena [28] in the process of fabricating the composites. The diffraction peaks attributed to Ag2C2O4 (JCPDS card No.22-1335) was observed in the XRD patterns of precursor Ag2C2O4-R. Four diffraction peaks at 2 38.114 , 44.319° and 64.423° of the Ag-R catalyst were indexed to the (111), (200) and (220) 8
reflections of metallic Ag (JCPDS 87-0597), which indicated that the Ag nanoparticles were formed in the interlayers of rectorite and/or on the surface of rectorite. This suggested that Ag2C2O4 can transform into metallic Ag via calcination, on the basis of the following decomposition reaction [29, 30]: Ag2C2O4→2Ag + 2CO2
(2)
Moreover, the X-ray diffraction (XRD) patterns of Ag-R-1 were similar with that of Ag-R, indicating that the metallic Ag could be obtained directly without H2C2O4. 3.1.2 XPS analysis Fig. 3 shows the XPS spectra of the nanocomposite of Ag-R. Strong peaks of Ag 3d were observed in the XPS spectra of Ag-R. Two typical peaks of Ag 3d located at about 368.1 eV and 373.9 eV can be attributed to the Ag 3d5/2 and Ag 3d3/2 binding energies, respectively. These peaks were attributed to metal Ag0 hence confirming the existence of Ag0. 3.1.3. Morphology and microstructure Fig. 4 shows the SEM images of pristine rectorite and Ag-R, respectively. The lamellar structure could be easily found in the Ag-R (Fig. 4b), in line with the structure of pristine rectorite (Fig. 4a). As shown in Fig. 4b, some similar size nanoparticles considered as metallic Ag nanoparticles (signed in Fig. 4a), were dispersed in the lamellar structure or on the surface of rectorite. This result could directly indicate that it was effective and dispersive to obtain metallic Ag nanoparticles via decomposition. However, Fig. 4b demonstrated partially exfoliation happened to the lamellar structures, in accordance with the results of XRD analysis. 9
Fig. 5 displays the TEM and HRTEM images of the as-synthesized Ag-R. As shown in Fig. 4a, the dark dots with small size in the catalyst purported the creation of metallic Ag particles. And its particle sizes were mostly around 10~20 nm. The corresponding HRTEM image (Fig. 5b) states the lattice spacings of the catalyst. The lattice spacings were found about d=0.236 nm and 0.204 nm, corresponding to the values for Ag (111) and Ag (200) phase, respectively. 3.1.4. BET analysis Table 1 displays the pore volume, pore size and BET surface area of synthesized Ag-R, Ag-R-1 and pristine rectorite, respectively. The BET surface area of Ag-R composite was 35 m2g-1, about 3.5 times more than that of pristine rectorite (11 m2g1
). The surface area of Ag-R was increased compared with that of Ag-R-1, which
reflected a better adsorption performance of Ag-R prepared through thermal method from Ag2C2O4. It was attributed to the precursor Ag2C2O4 “pillars” fixed in the interlayer region, which could open the layered structure of the rectorite and increase the specific surface area of the Ag-R catalyst. This result turned out the crucial advantage of the novel method. It is well known that a larger specific surface area means more active reaction or adsorption accessible for organic pollutants. The N2 adsorption-desorption isotherm of the Ag-R is shown in Fig. 6. According to the classification of International Union of Pure and Applied Chemistry (IUPAC), the obtained isotherm can be considered as type
with the type H3 hysteresis loop,
displaying that the sample happen to the typical characteristic of mesoporous materials. The inset in Fig. 6 is the pore size distribution curve calculated from 10
desorption branch of a nitrogen isotherm by the BJH method, further proving the presence of the mesoporous structure. 3.1.5. UV-vis DRS analysis Fig. 7 displays the UV–vis diffuse reflectance spectra of rectorite and Ag-R, respectively. As demonstrated in Fig. 7, Ag-R had an obvious absorption in visible light (λ>400 nm) region. Simultaneously, the absorption edge of Ag-R was blueshifted in comparison with that of rectorite, indicating its excellent ability of visible light absorption. Importantly, aside from the photoabsorption of pristine rectorite, the Ag-R compound displayed another wide weak absorption band around 550–700 nm, corresponding to the surface plasmon absorption of metallic Ag. As these results, the new synthetic composite was completely able to degrade organic contaminants under visible light illumination. 3.2. Degradation of 2,4-DCP under different H2O2-containing systems The degradation of 2,4-DCP was firstly investigated using Ag-R catalyst in the presence of H2O2, with or without visible light irradiation. The direct photolysis of 2, 4-DCP upon visible light irradiation was negligible under our experimental conditions. As shown in Fig. 8, it was observed that a faint decay of 2,4-DCP was observed in the system of Ag-R/H2O2 without visible light irradiation, which might be resulted from the catalytic ability of Ag nanoparticles to decompose H2O2 to produce ·OH radicals. It has been reported [31] that the degradation of 2,4-DCP can be accomplished in the presence of H2O2-pretreated Ag-R in the dark, which was 11
ascribed to the formation of a stable oxidant Ag-R, the process might be followed as Haber-Weiss (Eqs. (4) to (6)). Ag + H2O2 → AgOHsurf + ·OH
(3)
AgOHsurf + H2O2 → Ag + ·HO2 + H2O ·OH + H2O2 →·HO2 + H2O
(4) (5)
AgOHsurf + ·HO2 → Ag + H2O + O2
(6)
Meanwhile, the support rectorite absorbed the organic pollutants because of its adsorption capacity. However, just a small quantity of 2,4-DCP was degraded in the Ag-R/H2O2 system in the dark. This result might suggest that ·OH radicals were slightly in this way, which could not enough to promote the degradation process of 2,4-DCP. In order to promote the process, visible light irradiation was introduced to make use of the SPR of Ag nanoparticles. Clearly, in the system of Ag-R/H2O2 under visible light (420 nm), more than 95% decay of 2,4-DCP was observed, much more efficient than the degradation without visible light (in the dark). Because of the specific property of Ag nanoparticles under visible light irradiation, the possible pathway of degradation process was put forward. Silver particles could be excited by visible light and generated electrons due to surface plasmon resonance. While, H2O2 molecules in the solution, absorbed on the surface of the catalyst, then reacted with electrons to form ·OH radicals, which were the active species to degrade 2,4-DCP. In this way, the ·OH radicals could be continuous produced with irradiation. Moreover, no 2,4DCP degradation was achieved in the system of H2O2/visible light because direct 12
dissociation of H2O2 to •OH can be attained only through absorbing UV light (λ < 320 nm). 3.3. Photocatalytic performance In photocatalytic oxidation, oxygen is the most common electron acceptor and it is relative efficient. However, several studies [8, 9] have investigated the role of hydrogen peroxide as a better electron acceptor, because the potential for H2O2 reduction (0.72 V) is higher than that for oxygen (-0.13 V) reduction. To investigate the catalytic activity of Ag-R compounds in the Ag-R/H2O2/visible light system, the removal of 2, 4-DCP was carried out under visible light irradiation. In this section, the degradation of 2, 4-DCP was investigated under various conditions including different H2O2 concentrations and initial pH values. 3.3.1. Effect of H2O2 concentration The degradation efficiencies of 2, 4-DCP by Ag-R with different H2O2 dosages were illustrated in Fig. 9. In these series of experiments, H2O2 at various initial concentrations ([H2O2]0 = 0, 0.03, 0.09, 0.18, 0.36 mL/50 mL) were tested. Firstly, under irradiation, Silver particles could be excited by visible light and generated electrons due to surface plasmon resonance. As shown in Fig. 9, when the photocatalytic degradation was happened in the absence of H2O2, it was well known that the activation of oxidation occure via an electron transfer known as the “oxygen reduction reaction”, ORR [8].·O2- was produced by the reaction of dissolved O2 with a first photoinduced electron, then H+ helped to react with a second electron to form H2O2 (eqs (7) and (8)). H2O2 could then further be activated to·OH by accepting a 13
third photoinduced electron (eq (9)). The ·OH radical was reactive enough to promote the complete mineralization of 2,4-DCP. O2 + e- → ·O2-
(7)
·O2- + 2H+ + e- → H2O2
(8)
H2O2 + e- →·OH + OH-
(9)
It was reasonable suggested that the first two photoreduction steps of oxygen (eqs (7) and (8)) could be “short-cutted” by adding a certain amount of H2O2. As expected, when H2O2 dosages set at 0-0.18 mL/50 mL, the removal efficiency was significant enhanced. More than 95% 2,4-DCP was removed when 0.18 mL H2O2 was added. According to the report [7] of S. Adishkumar, the enhancement of degradation by the addition of H2O2 might be resulted from the following ways. Silver particles generated electrons due to surface plasmon resonance. Firstly, Oxygen acted as the primary acceptor of electrons in order to form super oxide radical anion (Eq. (10)). Meanwhile, H2O2 could trap the photogenerated electrons and producing hydroxyl radicals as shown by the (Eq. (11)). Secondly, H2O2 also reacted with superoxide anion to form ·OH radical (Eq. (12)). e- + O2 → ·O2-
(10)
e- + H2O2 → ·OH + OH-
(11)
H2O2 + ·O2- → ·OH + H+ + O2
(12)
However, when the dosage of H2O2 was increased continuously, the degradation activity was decreased. This situation was related to the fact that excessive H2O2 act as a self-scavenger for ·OH, following Eqs. (13) and (14). 14
H2O2 + ·OH → ·OOH + H2O
(13)
·OOH + ·OH → H2O + O2
(14)
Meanwhile, the support rectorite acted as two roles about absorbing the organic pollutants and facilitating the separate of photo electrons and holes. In addition, to quantitatively understand the photocatalytic reaction kinetics of the RhB degradation in experiments, the photocatalytic process of 2,4-DCP can be expressed as Eq. (15)
ln(
C0 ) kappt C
(15)
where C0 and C represent 2,4-DCP concentrations at time zero and t (mg/L), respectively, and kapp is the apparent pseudo-first-order rate constant (min-1). The experimental results obviously revealed that the apparent rate constant k was 0.44788 h-1 when 0.18 mL/50 mL H2O2 was added in the solution (inset of Fig. 9), which was much higher than others. For the sake of further studying photocatalytic mechanism over Ag-R/H2O2/visible light system, the corresponding experiment for 2,4-DCP degradation with 0.18ml H2O2 were put into practice. Meanwhile, tert-butyl alcohol (t-BuOH) as the scavenger of hydroxyl radical was used effectively. With the addition of t-BuOH, about 90% 2,4-DCP was removed after irradiation (shown in Fig.9), which was a quite closely data combined with the degradation efficiency without t-BuOH. Hence, hydroxyl radical could be the major active species in the degradation of 2,4-DCP under AgR/H2O2/visible light system. 3.2.2. Effect of initial pH 15
The effect of initial pH on 2,4-DCP degradation with the addition of an optimal dosage of H2O2 was then studied, as illustrated in Fig. 10. In general, the degradation rate gradually increased with decreasing of the initial pH level, and an optimal pH value was found at 4. While, in the Ag-R/H2O2/visible light system, we confirmed that ·OH radicals were mainly produced from the reaction shown by Eq. (4). Obviously, in acidic solution, it was easily found that H+ ions would promote the production of ·OH radicals, while OH- ions could depress the production of ·OH radicals in basic solution. That meant in an alkaline medium, H2O2 could rapidly decay into water and oxygen. Moreover, the pseudo-first-order kinetics of 2,4-DCP degradation on Ag-R catalysts, which were displayed inset of Fig. 10, demonstrated results accordance with the degradation rates. 3.4. Mineralization investigation Total organic carbon (TOC) experiments were carried out to evaluate the mineralization ratio of 2,4-DCP by Ag-R catalyst in the presence of H2O2 under the optimum condition (Fig. 11). The removal of TOC of 2,4-DCP in the Ag-R/H2O2 system was about 80% after 5 h of visible light irradiation. However, in the H2O2-free system, 30% of 2,4-DCP was degraded but with a mineralization ratio of only about 10% after being irradiated for 5 h, indicating that most of 2,4-DCP was translated to intermediate products instead of being mineralized. The acceleration of 2,4-DCP mineralization to CO2 could be therefore attributed to the generation of the more active oxidation agent ·OH by the activation of H2O2 with Ag-R catalyst (C6H3Cl2(OH) + 12H2O2 → 6CO2 + 13H2O + 2HCl). Meanwhile, visible light 16
irradiation could promote to produce more ·OH to mineralize 2,4-DCP. 3.5. Stability evaluation Degradation experiments were repeated three times after re-isolation of the catalyst, with no apparent decrease in photocatalytic activity (Fig. 12). This result demonstrated that the Ag-R catalyst was stable under the experimental reaction conditions employed here. 4. Conclusions In summary, Ag-R catalyst was prepared via a facile cation-exchange process followed by a thermal decomposition method. The photocatalytic performance was evaluated by examining the degradation of 2,4-DCP in Ag-R/H2O2/visible light system. The obtained Ag-R could activate H2O2 under visible light irradiation to generate strong hydroxyl radicals, which were able to degrade most organics in wastewater. In Ag-R/H2O2/visible light system, the highest degradation efficiency was observed when added 0.18 mL H2O2 into 50 mL solution. Meanwhile, the optimal pH value was 4 for the photocatalysis reaction. The study proved a new method to synthesize Ag-R composite, and it suggested that the great potential of Ag-R composite material for environmental applications. However, the mechanism of degradation process should be further investigated in depth in our next work. Acknowledgements This work was supported by Guangzhou science and technology plan projects 2014J4100007. References 17
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[22] Y. Zhang, Y. Guo, G. Zhang, Y. Gao, Stable TiO2/rectorite: Preparation, characterization and photocatalytic activity, Applied Clay Science, 51 (2011) 335-340. [23] Y. Guo, G. Zhang, H. Gan, Synthesis, characterization and visible light photocatalytic properties of Bi2WO6/rectorite composites, Journal of colloid and interface science, 369 (2012) 323-329. [24] S. Peng, L. Li, H. Tan, Y. Wu, R. Cai, H. Yu, X. Huang, P. Zhu, S. Ramakrishna, M. Srinivasan, Q. Yan, Monodispersed Ag nanoparticles loaded on the PVP-assisted synthetic Bi2O2CO3 microspheres with enhanced photocatalytic and supercapacitive performances, Journal of Materials Chemistry A, 1 (2013) 7630. [25] A. Prytuliak, E. Godlewska, K. Mars, D. Berthebaud, Synchrotron Study of AgDoped Mg2Si: Correlation Between Properties and Structure, Journal of Electronic Materials, (2014). [26] N.S. Nikolaeva, V.V. Ivanov, A.A. Shubin, Synthesis of Ag/ZnO mixture for powdered contact materials, Russian Journal of Applied Chemistry, 87 (2014) 405411. [27] V.V. Boldyrev, Thermal decomposition of silver oxalate, Thermochimica A cta, 388 (2002) 63-90. [28] T. Nishiyama, S. Shimoda, Ca-bearing rectorite from Tooho mine, Japan, Clays and Clay Minerals, 29 (1981) 236-240. [29] F. Delogu, Ag nanoparticles from the mechanochemical decomposition of Ag oxalate, Langmuir : the ACS journal of surfaces and colloids, 28 (2012) 10898-10904. [30] F. Delogu, The Decomposition of Ag Oxalate in Ball Drop, Rod Drop, and Ball 21
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Figure Captions Table1 BET surface area, pore volume and pore size of as-prepared Ag-R, Ag-R-1 and pristine rectorite. Fig. 1 Schematic illustration of the synthesized Ag NPs in the interlayer space of rectorite suspension.
22
Fig. 2 XRD pattern of pristine restorite, precursor Ag2C2O4-R, Ag-R-1 and Ag-R, respectively.
23
Fig.3 XPS peaks of Ag 3d for Ag-R catalyst.
Fig. 4 SEM images of (a) pristine restorite and (b) Ag-R catalyst.
24
Fig. 5 TEM image (a) and HRTEM image (b) of Ag-R catalyst.
25
Fig. 6 Nitrogen adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) pore size distribution plot (inset) of Ag-R product.
26
Fig. 7 UV-vis diffuse reflectance spectra of pristine restorite and Ag-R catalysts.
Fig. 8 Effect of different processes Ag-R/H2O2 (in dark), visible light/H2O2 and visible light/Ag-R/H2O2 on 2,4-DCP. 27
Fig. 9 The effects of different dosages of H2O2 added on the photocatalytic degradation of 2,4-DCP.
Fig. 10 Effect of initial pH on 2,4-DCP removal rate in the present of H2O2. 28
Fig. 11 Conversion of 2,4-DCP and TOC concentration during the mineralization of 2,4-DCP under different systems.
Fig. 12 The cycling runs of the degradation of 2,4-DCP in the Ag-R/H2O2 system 29
under visible light irradiation.
Table1 BET surface area, pore volume and pore size of as-prepared Ag-R, Ag-R-1 and pristine rectorite. Sample Surface area Pore Volume Pore Size 2 3 (m /g) (cm /g) (nm) Ag -R Ag-R-1 rectorite
35 26 11
0.061 0.050 3.0
30
7.0 7.8 11
S0304-3894(15)00418-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.05.022 HAZMAT 16824
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
19-12-2014 12-5-2015 14-5-2015
Please cite this article as: Yunfang Chen, Jianzhang Fang, Shaoyou Lu, Yan Wu, Dazhi Chen, Liyan Huang, Cong Cheng, Lu Ren, Ximiao Zhu, Zhanqiang Fang, Plasmonic Ag-pillared rectorite as catalyst for Degradation of 2,4-DCP in the H2O2containing system under visible light irradiation, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.05.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Plasmonic Ag-pillared rectorite as catalyst for Degradation of 2,4-DCP in the H2O2-containing system under visible light irradiation Yunfang Chena, Jianzhang Fanga,c,*, Shaoyou Lud, Yan Wub, Dazhi Chenb, Liyan Huangb, Cong Chenga, Lu Rena, Ximiao Zhu a, Zhanqiang Fanga,c
a
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, Guangdong, China
b
Institute of Engineering Technology of Guangdong Province, Key Laboratory of Water Environmental Pollution Control of Guangdong Province, Guangzhou 510440 c
Guangdong Technology Research Center for Ecological Management and Remediation of Urban Water System, Guangzhou 510006, China
d
Shenzhen Center for Disease Control and Prevention, Shenzhen 518055, China * Corresponding author. Tel: +8620 39310250, Fax: +8620 39310187 Email: [email protected]
The Ag-R catelyst was synthesized via a novel thermal decomposition method.
Ag-R catalyst possessed the synergistic effects of SPR and adsorption capacity.
The degradation of 2,4-DCP was evaluated in Ag-R/H2O2/visible light system.
1
Graphical abstract
Abstract This study aims at photocatalytic degradation of 2,4-DCP with the assistance of H2O2 in aqueous solution by a composite catalyst of Ag-rectorite. The catalysts were prepared via a novel thermal decomposition method followed after the 2
cation-exchange process. The synthesized nano-materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS)，Brunauer–Emmett– Teller (BET) surface analyzer, ultraviolet–visible light (UV–vis) absorption spectra, Field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The different mechanisms of degradation process with or without visible light irradiation were discussed, respectively. Moreover, the degradation efficiency of 2,4-DCP wastewater under Ag-rectorite/H2O2/visible light system was investigated by series of experiments, concerning on effects of major operation factors, such as H2O2 dosage and the initial pH value. The highest degradation rate was observed when adding 0.18 mL H2O2 into 50 mL 2,4-DCP solution, and the optimal pH value was 4 for the reaction. Afterwards, total organic carbon (TOC) experiments were carried out to evaluate the mineralization ratio of 2,4-DCP.
Keywords: Ag-rectorite; H2O2; Thermal decomposition; Photocatalysis; 2,4-DCP
1. Introduction With the increasingly urgent environmental standards and regulations regarding the wastewater discharge, it is a focus of international concern on how to eliminate hazardous materials from wastewater [1-5]. During the past 20 years, advanced 3
oxidation processes (AOPs) have been demonstrated to be one of the most effective approaches to eliminate organic pollutants [6-9]. As an environmental expected oxidant, H2O2 enable to provide hydroxyl radicals (·OH) in order to trigger advanced oxidation processes, such as O3/H2O2 [10], UV/H2O2 [11], photo-Fenton [12] and semiconductor photocatalysis processes. In addition, the catalytic wet hydrogen peroxide oxidation of organic contaminants can be carried out when using a noble metal as the catalyst. For example, according to the report of Nirupam Khanikar [13], both Cu(II)-kaolinite and Cu(II)-montmorillonite were used as catalysts for oxidizing three chlorinated derivatives of phenol in water in presence of H2O2. However, as a noble metal, metallic Ag has rarely been reported as the catalyst to promote hydrogen peroxide oxidation of wastewater. Moreover, nano-sized Ag particles exhibit a photoresponse in the visible light region due to their surface plasmon resonance (SPR) effect, which is produced by the collective oscillations of surface electrons [14-18]. It is assumed that visible light irradiation assistance may be beneficial for Ag/H2O2 oxidation process. In the Ag/H2O2/visible light system, hydrogen peroxide may act as an electron acceptor, instead of the most common oxygen. To the best of our knowledge, this is the first report on studying Ag/H2O2/visible light system. However, Ag nanoparticles are hardly usable since they are poor at dispersion stability and tend to aggregate in the liquid medium. To overcome the agglomeration, grafting Ag nanoparticles on clay compounds, which are interlamellar structures, should be one of the most effective solutions [19]. Rectorite is an interstratified 4
layered silicate mineral regarded as a regular (1:1) stacking of dioctahedral mica-like layers and dioctahedral smectite-like layers [20-23]. The cations of Na+, K+, and Ca2+ lie in the interlayer region, while the exchangeable hydrated cations reside in the latter, so theoretically Ag+- rectorite could be formed through exchange process. Then how to construct metallic Ag onto rectorite? On the basis of previous investigations, it seems logical to obtain metallic Ag nanoparticles via chemical reduction or photoreduction [24-26]. However, we found a novel route to obtain metallic Ag from the decomposition of Ag2C2O4 precursor during thermal method [27] after exchange process. In this way, part of Ag2C2O4 molecules appeared in the interlayers of rectorite firstly, and transferred to form Ag-pillared rectorite, which could possess a larger specific surface area. In this work, the synthesis of a novel composite catalyst of Ag-rectorite was achieved through a facile cation-exchange process followed by a thermal decomposition method, and its promotion in H2 O2 oxidation process towards the degradation of 2,4-DCP under visible light was investigated in details. 2. Experimental 2.1. Materials Rectorite was purchased from Rectorite Deposit of Zhongxiang, Hubei, P.R. China. AgNO3 was proved by Xilong Chemical Co., Ltd. H2C2O4 was supplied by Tianjinzhiyuan Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH) was purchased from Tianjin Kermel Chemical reagent Co. Ltd. 2, 4-DCP was purchased from Aladdin Reagent Co. Ltd. Other reagents were of analytical purity and were 5
received from Guangzhou Chemical Reagent Factory. Distilled water was used throughout this study. 2.2. Preparation of Ag-rectorite The Ag-rectorite compound (mentioned as Ag-R) was synthesized by a method involved ultrasonic-assisted ion-exchange. The schematic illustration of the synthesis of Ag-R could be identified during the process，as shown in Fig. 1. Two-tenths of a gram of rectorite was added into 50 mL of deionized water, and dispersed under ultrasonic assistance for 30 min. Then 4 mL AgNO3 solution (0.4 mol/L) was added dropwise into above solution with vigorously stirring to obtain a ratio of 8 mmol of Ag/g of rectorite. The mixed suspension was exposed to ultrasonic irradiation for 1 h. Subsequently, the as-formed wet products were separated through centrifugation, and washed with absolute ethanol and distilled water respectively. The obtained product was redispersed into 40 mL H2C2O4 solution (0.04 mol/L) and then stirring treatment was conducted after adjusting the pH to 5 using NaOH solution. After one night, the precipitate was separated by centrifuging and washed with deionized water. Finally, the precipitate was dried at 80 ℃ and then calcined at 450 ℃ in air for 2 h, respectively. Ag-R-1 was also prepared by the same procedure mentioned previously without the presence of H2C2O4. 2.3. Characterization of catalysts X-ray powder diffraction (XRD) patterns of as-prepared products were performed using a Bruker D8 ADVANCE X-ray diffractometer equipped with Cu kα radiation under operation conditions of 40 kV and 40 mA. X-ray photoelectron spectroscopy 6
(XPS) analysis was conducted on a Krato Axis ultra DLD photoelectron spectroscopy with monochromatic Al kα radiation. The binding energy (BE) was calibrated to C 1s line at 284.6 eV. The morphologies of the products were measured using Fieldemission scanning electron microscope (FE-SEM) images procured by a ZEISS Ultra 55 instrument operated at accelerating voltages between 20 and 30 kV. Highresolution transmission electron microscope (HRTEM, JEM2100HR, JEOL, Japan) was carried out with an accelerating voltage of 200 kV. Raman spectra were recorded on a Thermo Scientific DXR Raman spectrometer at room temperature. UV-vis diffuse reflectance spectra were collected from the UV-vis spectrophotometer (U3010, HITACHI, Japan) using BaSO4 as the reflectance sample. The specific surface areas of products were measured by the Brunauer-Emmett-Teller analysis (BET, ASAP 2020, Micromeritics, USA) on the basis of nitrogen adsorption isotherms. 2.4. Photocatalytic Activity Test The photocatalytic activities of the as-prepared catalysts were measured by the degradation of 2,4-DCP aqueous solution under visible light irradiation. Photocatalytic reactions under visible light irradiation were conducted in a 100 mL cylindrical quartz reactor equipped with a water circulation facility. A 1000 W Xe arc lamp with 420 nm cutoff filter as the light source was used to cause the photocatalytic reaction. In all experiments, the catalyst (0.05g) and a certain amount of H2O2 (3%) were added to 50 ml 2,4-DCP aqueous solution (30 mg/L). All these experiments were operated at room temperature. About 3 mL of the suspension was collected from the reaction cell at given time intervals, and then filtered through 0.45 μm membrane 7
filters for analysis. The determination of 2,4-DCP was carried out on high performance liquid chromatography (HPLC, Shimadzu) equipped with a UV detector (SPD-10AV) and C18 column (250 mm×4.6 mm). The mobile phase was a mixture of 60/40(v/v) ethanol-water mixture. The eluent was delivered at a rate of 0.8 mL·min-1 and the wavelength for detection was 284 nm. The degradation rate (η) was calculated as below:
C 0 C1 100% C0
(1)
Where C0 is the initial concentration and C1 is the concentration at time t. TOC concentration was determined by a TOC-TN vcp Shimadzu analyzer. 3. Results and discussion 3.1. Characterization of as-prepared materials 3.1.1. XRD analysis The X-ray diffraction (XRD) patterns of pristine rectorite, precursor Ag2C2O4-R, Ag-R-1 and Ag-R are shown in Fig. 2. The basal (001) and (002) diffraction peaks of the samples could be indexed to the rectorite, which are essential for rectorite with a highly ordered and oriented silicate layer structure. Obviously, the (002) peak at
2 7.978 of the rectorite shifted to a higher angle ( 2 8.895 ) as seen in the XRD pattern of the Ag-R catalyst, which were attributed to the smectite-like layers dehydration phenomena [28] in the process of fabricating the composites. The diffraction peaks attributed to Ag2C2O4 (JCPDS card No.22-1335) was observed in the XRD patterns of precursor Ag2C2O4-R. Four diffraction peaks at 2 38.114 , 44.319° and 64.423° of the Ag-R catalyst were indexed to the (111), (200) and (220) 8
reflections of metallic Ag (JCPDS 87-0597), which indicated that the Ag nanoparticles were formed in the interlayers of rectorite and/or on the surface of rectorite. This suggested that Ag2C2O4 can transform into metallic Ag via calcination, on the basis of the following decomposition reaction [29, 30]: Ag2C2O4→2Ag + 2CO2
(2)
Moreover, the X-ray diffraction (XRD) patterns of Ag-R-1 were similar with that of Ag-R, indicating that the metallic Ag could be obtained directly without H2C2O4. 3.1.2 XPS analysis Fig. 3 shows the XPS spectra of the nanocomposite of Ag-R. Strong peaks of Ag 3d were observed in the XPS spectra of Ag-R. Two typical peaks of Ag 3d located at about 368.1 eV and 373.9 eV can be attributed to the Ag 3d5/2 and Ag 3d3/2 binding energies, respectively. These peaks were attributed to metal Ag0 hence confirming the existence of Ag0. 3.1.3. Morphology and microstructure Fig. 4 shows the SEM images of pristine rectorite and Ag-R, respectively. The lamellar structure could be easily found in the Ag-R (Fig. 4b), in line with the structure of pristine rectorite (Fig. 4a). As shown in Fig. 4b, some similar size nanoparticles considered as metallic Ag nanoparticles (signed in Fig. 4a), were dispersed in the lamellar structure or on the surface of rectorite. This result could directly indicate that it was effective and dispersive to obtain metallic Ag nanoparticles via decomposition. However, Fig. 4b demonstrated partially exfoliation happened to the lamellar structures, in accordance with the results of XRD analysis. 9
Fig. 5 displays the TEM and HRTEM images of the as-synthesized Ag-R. As shown in Fig. 4a, the dark dots with small size in the catalyst purported the creation of metallic Ag particles. And its particle sizes were mostly around 10~20 nm. The corresponding HRTEM image (Fig. 5b) states the lattice spacings of the catalyst. The lattice spacings were found about d=0.236 nm and 0.204 nm, corresponding to the values for Ag (111) and Ag (200) phase, respectively. 3.1.4. BET analysis Table 1 displays the pore volume, pore size and BET surface area of synthesized Ag-R, Ag-R-1 and pristine rectorite, respectively. The BET surface area of Ag-R composite was 35 m2g-1, about 3.5 times more than that of pristine rectorite (11 m2g1
). The surface area of Ag-R was increased compared with that of Ag-R-1, which
reflected a better adsorption performance of Ag-R prepared through thermal method from Ag2C2O4. It was attributed to the precursor Ag2C2O4 “pillars” fixed in the interlayer region, which could open the layered structure of the rectorite and increase the specific surface area of the Ag-R catalyst. This result turned out the crucial advantage of the novel method. It is well known that a larger specific surface area means more active reaction or adsorption accessible for organic pollutants. The N2 adsorption-desorption isotherm of the Ag-R is shown in Fig. 6. According to the classification of International Union of Pure and Applied Chemistry (IUPAC), the obtained isotherm can be considered as type
with the type H3 hysteresis loop,
displaying that the sample happen to the typical characteristic of mesoporous materials. The inset in Fig. 6 is the pore size distribution curve calculated from 10
desorption branch of a nitrogen isotherm by the BJH method, further proving the presence of the mesoporous structure. 3.1.5. UV-vis DRS analysis Fig. 7 displays the UV–vis diffuse reflectance spectra of rectorite and Ag-R, respectively. As demonstrated in Fig. 7, Ag-R had an obvious absorption in visible light (λ>400 nm) region. Simultaneously, the absorption edge of Ag-R was blueshifted in comparison with that of rectorite, indicating its excellent ability of visible light absorption. Importantly, aside from the photoabsorption of pristine rectorite, the Ag-R compound displayed another wide weak absorption band around 550–700 nm, corresponding to the surface plasmon absorption of metallic Ag. As these results, the new synthetic composite was completely able to degrade organic contaminants under visible light illumination. 3.2. Degradation of 2,4-DCP under different H2O2-containing systems The degradation of 2,4-DCP was firstly investigated using Ag-R catalyst in the presence of H2O2, with or without visible light irradiation. The direct photolysis of 2, 4-DCP upon visible light irradiation was negligible under our experimental conditions. As shown in Fig. 8, it was observed that a faint decay of 2,4-DCP was observed in the system of Ag-R/H2O2 without visible light irradiation, which might be resulted from the catalytic ability of Ag nanoparticles to decompose H2O2 to produce ·OH radicals. It has been reported [31] that the degradation of 2,4-DCP can be accomplished in the presence of H2O2-pretreated Ag-R in the dark, which was 11
ascribed to the formation of a stable oxidant Ag-R, the process might be followed as Haber-Weiss (Eqs. (4) to (6)). Ag + H2O2 → AgOHsurf + ·OH
(3)
AgOHsurf + H2O2 → Ag + ·HO2 + H2O ·OH + H2O2 →·HO2 + H2O
(4) (5)
AgOHsurf + ·HO2 → Ag + H2O + O2
(6)
Meanwhile, the support rectorite absorbed the organic pollutants because of its adsorption capacity. However, just a small quantity of 2,4-DCP was degraded in the Ag-R/H2O2 system in the dark. This result might suggest that ·OH radicals were slightly in this way, which could not enough to promote the degradation process of 2,4-DCP. In order to promote the process, visible light irradiation was introduced to make use of the SPR of Ag nanoparticles. Clearly, in the system of Ag-R/H2O2 under visible light (420 nm), more than 95% decay of 2,4-DCP was observed, much more efficient than the degradation without visible light (in the dark). Because of the specific property of Ag nanoparticles under visible light irradiation, the possible pathway of degradation process was put forward. Silver particles could be excited by visible light and generated electrons due to surface plasmon resonance. While, H2O2 molecules in the solution, absorbed on the surface of the catalyst, then reacted with electrons to form ·OH radicals, which were the active species to degrade 2,4-DCP. In this way, the ·OH radicals could be continuous produced with irradiation. Moreover, no 2,4DCP degradation was achieved in the system of H2O2/visible light because direct 12
dissociation of H2O2 to •OH can be attained only through absorbing UV light (λ < 320 nm). 3.3. Photocatalytic performance In photocatalytic oxidation, oxygen is the most common electron acceptor and it is relative efficient. However, several studies [8, 9] have investigated the role of hydrogen peroxide as a better electron acceptor, because the potential for H2O2 reduction (0.72 V) is higher than that for oxygen (-0.13 V) reduction. To investigate the catalytic activity of Ag-R compounds in the Ag-R/H2O2/visible light system, the removal of 2, 4-DCP was carried out under visible light irradiation. In this section, the degradation of 2, 4-DCP was investigated under various conditions including different H2O2 concentrations and initial pH values. 3.3.1. Effect of H2O2 concentration The degradation efficiencies of 2, 4-DCP by Ag-R with different H2O2 dosages were illustrated in Fig. 9. In these series of experiments, H2O2 at various initial concentrations ([H2O2]0 = 0, 0.03, 0.09, 0.18, 0.36 mL/50 mL) were tested. Firstly, under irradiation, Silver particles could be excited by visible light and generated electrons due to surface plasmon resonance. As shown in Fig. 9, when the photocatalytic degradation was happened in the absence of H2O2, it was well known that the activation of oxidation occure via an electron transfer known as the “oxygen reduction reaction”, ORR [8].·O2- was produced by the reaction of dissolved O2 with a first photoinduced electron, then H+ helped to react with a second electron to form H2O2 (eqs (7) and (8)). H2O2 could then further be activated to·OH by accepting a 13
third photoinduced electron (eq (9)). The ·OH radical was reactive enough to promote the complete mineralization of 2,4-DCP. O2 + e- → ·O2-
(7)
·O2- + 2H+ + e- → H2O2
(8)
H2O2 + e- →·OH + OH-
(9)
It was reasonable suggested that the first two photoreduction steps of oxygen (eqs (7) and (8)) could be “short-cutted” by adding a certain amount of H2O2. As expected, when H2O2 dosages set at 0-0.18 mL/50 mL, the removal efficiency was significant enhanced. More than 95% 2,4-DCP was removed when 0.18 mL H2O2 was added. According to the report [7] of S. Adishkumar, the enhancement of degradation by the addition of H2O2 might be resulted from the following ways. Silver particles generated electrons due to surface plasmon resonance. Firstly, Oxygen acted as the primary acceptor of electrons in order to form super oxide radical anion (Eq. (10)). Meanwhile, H2O2 could trap the photogenerated electrons and producing hydroxyl radicals as shown by the (Eq. (11)). Secondly, H2O2 also reacted with superoxide anion to form ·OH radical (Eq. (12)). e- + O2 → ·O2-
(10)
e- + H2O2 → ·OH + OH-
(11)
H2O2 + ·O2- → ·OH + H+ + O2
(12)
However, when the dosage of H2O2 was increased continuously, the degradation activity was decreased. This situation was related to the fact that excessive H2O2 act as a self-scavenger for ·OH, following Eqs. (13) and (14). 14
H2O2 + ·OH → ·OOH + H2O
(13)
·OOH + ·OH → H2O + O2
(14)
Meanwhile, the support rectorite acted as two roles about absorbing the organic pollutants and facilitating the separate of photo electrons and holes. In addition, to quantitatively understand the photocatalytic reaction kinetics of the RhB degradation in experiments, the photocatalytic process of 2,4-DCP can be expressed as Eq. (15)
ln(
C0 ) kappt C
(15)
where C0 and C represent 2,4-DCP concentrations at time zero and t (mg/L), respectively, and kapp is the apparent pseudo-first-order rate constant (min-1). The experimental results obviously revealed that the apparent rate constant k was 0.44788 h-1 when 0.18 mL/50 mL H2O2 was added in the solution (inset of Fig. 9), which was much higher than others. For the sake of further studying photocatalytic mechanism over Ag-R/H2O2/visible light system, the corresponding experiment for 2,4-DCP degradation with 0.18ml H2O2 were put into practice. Meanwhile, tert-butyl alcohol (t-BuOH) as the scavenger of hydroxyl radical was used effectively. With the addition of t-BuOH, about 90% 2,4-DCP was removed after irradiation (shown in Fig.9), which was a quite closely data combined with the degradation efficiency without t-BuOH. Hence, hydroxyl radical could be the major active species in the degradation of 2,4-DCP under AgR/H2O2/visible light system. 3.2.2. Effect of initial pH 15
The effect of initial pH on 2,4-DCP degradation with the addition of an optimal dosage of H2O2 was then studied, as illustrated in Fig. 10. In general, the degradation rate gradually increased with decreasing of the initial pH level, and an optimal pH value was found at 4. While, in the Ag-R/H2O2/visible light system, we confirmed that ·OH radicals were mainly produced from the reaction shown by Eq. (4). Obviously, in acidic solution, it was easily found that H+ ions would promote the production of ·OH radicals, while OH- ions could depress the production of ·OH radicals in basic solution. That meant in an alkaline medium, H2O2 could rapidly decay into water and oxygen. Moreover, the pseudo-first-order kinetics of 2,4-DCP degradation on Ag-R catalysts, which were displayed inset of Fig. 10, demonstrated results accordance with the degradation rates. 3.4. Mineralization investigation Total organic carbon (TOC) experiments were carried out to evaluate the mineralization ratio of 2,4-DCP by Ag-R catalyst in the presence of H2O2 under the optimum condition (Fig. 11). The removal of TOC of 2,4-DCP in the Ag-R/H2O2 system was about 80% after 5 h of visible light irradiation. However, in the H2O2-free system, 30% of 2,4-DCP was degraded but with a mineralization ratio of only about 10% after being irradiated for 5 h, indicating that most of 2,4-DCP was translated to intermediate products instead of being mineralized. The acceleration of 2,4-DCP mineralization to CO2 could be therefore attributed to the generation of the more active oxidation agent ·OH by the activation of H2O2 with Ag-R catalyst (C6H3Cl2(OH) + 12H2O2 → 6CO2 + 13H2O + 2HCl). Meanwhile, visible light 16
irradiation could promote to produce more ·OH to mineralize 2,4-DCP. 3.5. Stability evaluation Degradation experiments were repeated three times after re-isolation of the catalyst, with no apparent decrease in photocatalytic activity (Fig. 12). This result demonstrated that the Ag-R catalyst was stable under the experimental reaction conditions employed here. 4. Conclusions In summary, Ag-R catalyst was prepared via a facile cation-exchange process followed by a thermal decomposition method. The photocatalytic performance was evaluated by examining the degradation of 2,4-DCP in Ag-R/H2O2/visible light system. The obtained Ag-R could activate H2O2 under visible light irradiation to generate strong hydroxyl radicals, which were able to degrade most organics in wastewater. In Ag-R/H2O2/visible light system, the highest degradation efficiency was observed when added 0.18 mL H2O2 into 50 mL solution. Meanwhile, the optimal pH value was 4 for the photocatalysis reaction. The study proved a new method to synthesize Ag-R composite, and it suggested that the great potential of Ag-R composite material for environmental applications. However, the mechanism of degradation process should be further investigated in depth in our next work. Acknowledgements This work was supported by Guangzhou science and technology plan projects 2014J4100007. References 17
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Figure Captions Table1 BET surface area, pore volume and pore size of as-prepared Ag-R, Ag-R-1 and pristine rectorite. Fig. 1 Schematic illustration of the synthesized Ag NPs in the interlayer space of rectorite suspension.
22
Fig. 2 XRD pattern of pristine restorite, precursor Ag2C2O4-R, Ag-R-1 and Ag-R, respectively.
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Fig.3 XPS peaks of Ag 3d for Ag-R catalyst.
Fig. 4 SEM images of (a) pristine restorite and (b) Ag-R catalyst.
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Fig. 5 TEM image (a) and HRTEM image (b) of Ag-R catalyst.
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Fig. 6 Nitrogen adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) pore size distribution plot (inset) of Ag-R product.
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Fig. 7 UV-vis diffuse reflectance spectra of pristine restorite and Ag-R catalysts.
Fig. 8 Effect of different processes Ag-R/H2O2 (in dark), visible light/H2O2 and visible light/Ag-R/H2O2 on 2,4-DCP. 27
Fig. 9 The effects of different dosages of H2O2 added on the photocatalytic degradation of 2,4-DCP.
Fig. 10 Effect of initial pH on 2,4-DCP removal rate in the present of H2O2. 28
Fig. 11 Conversion of 2,4-DCP and TOC concentration during the mineralization of 2,4-DCP under different systems.
Fig. 12 The cycling runs of the degradation of 2,4-DCP in the Ag-R/H2O2 system 29
under visible light irradiation.
Table1 BET surface area, pore volume and pore size of as-prepared Ag-R, Ag-R-1 and pristine rectorite. Sample Surface area Pore Volume Pore Size 2 3 (m /g) (cm /g) (nm) Ag -R Ag-R-1 rectorite
35 26 11
0.061 0.050 3.0
30
7.0 7.8 11