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palygorskite
Journal of Colloid and Interface Science 377 (2012) 277–283
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Visible light induced CO2 reduction and Rh B decolorization over electrostatic-assembled AgBr/palygorskite Xiaojie Zhang a,b, Jinli Li a,c, Xin Lu b, Changqing Tang b,⇑, Gongxuan Lu a,⇑ a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Department of Metallurgical and Chemical Engineering, Jiyuan Vocational and Technical Collage, Jiyuan 454650, Henan Province, PR China c Graduate University of the Chinese Academy of Sciences, Beijing 100101, PR China b
a r t i c l e
i n f o
Article history: Received 9 November 2011 Accepted 3 February 2012 Available online 23 March 2012 Keywords: AgBr/palygorskite Visible light CO2 reduction Rh B decolorization
a b s t r a c t AgBr/palygorskite composite was prepared by an in situ electrostatic adsorption–deposition–precipitation method and characterized by field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), UV–Vis diffuse reflection, and BET surface measurements techniques. The layer negative charge and larger specific surface area of palygorskite, along with the poor cation-exchange ability of tetra-n-butyl ammonium cation (NðCH2 CH2 CH2 CH3 Þþ 4 ) due to its larger ion radius, could mainly account for high dispersity of AgBr on the surface of fibrous palygorskite. The rate of Rh B decolorization and CO2 reduction with H2 as a proton donor and reductant over AgBr/palygorskite was about three and two times faster than that of the corresponding bare AgBr, respectively. The strategy reported in this work can be easily extended to synthesize other palygorskite-based heterostructure catalysts. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Recycling of CO2 to chemicals has been an increasingly important research topic due to its desirable role in alleviating fossil fuel depletion and the global warming problem. Nevertheless, CO2 reduction is an energy intensive and thermodynamically unfavorable process because of the inert properties of CO2. If a renewable energy source (e.g., solar energy and hydrogen energy) can be used to convert CO2 in a friendly manner, CO2 reduction such as photocatalytic reduction should be a promising technical and environmentally friendly solution [1–11]. Dimitrijevic et al. investigated systematically the mechanism of photocatalytic CO2 reduction to methane with H2O over Titania nanoparticles [10]. However, many stable photocatalysts exploited usually, such as TiO2 and zeolitebased catalysts for photocatalytic CO2 reduction, are active only in the ultraviolet-light region that only accounts for less than 5% of the solar spectrum. Therefore, the low efficiency of solar energy utilization hinders significantly a practical application of photocatalysis. Consequently, many attempts (e.g., doping) have been made to engineer large-band-gap photocatalysts to extend their absorption edge to the visible light, even near infrared-light region [12,13]. With copper and/or Pt nanoparticles loaded nitrogendoped TiO2 nanotube arrays as catalysts, Varghese et al. investi⇑ Corresponding authors. Fax: +86 931 4968178. E-mail addresses: [email protected] (X. Zhang), [email protected] (J. Li), [email protected] (X. Lu), [email protected] (C. Tang), [email protected] (G. Lu). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.02.070
gated photocatalytic CO2 reduction to hydrocarbon fuels with water over under visible light irradiation [13]. Since plasmonic photocatalysis (i.e., photocatalysis assisted by the surface plasmon resonance of noble metal nanoparticles (e.g., Au and Ag)) was proposed by Awazu et al. [14], the development of highly efficient plasmonic photocatalysts has been attracting comprehensive attentions [15–20]. Hou et al. investigated mechanismically the plasmonic enhancement of photocatalytic CO2 reduction to hydrocarbon fuel with water over Au/TiO2 under visible light irradiation [20]. Recently, Ag/AgX (X = Cl, Br, or I) have proved to be a class of promising visible light-driven photocatalyst [21–29]. By an ion-exchange followed by photoreduction method, Wang et al. prepared Ag/AgX (X = Cl, Br, and/or I) plasmonic photocatalysts and employed them as catalysts to degrade organic pollutants under visible light irradiation [21,22]. Generally, the size of Ag@AgX (X = Cl, Br, or I) synthesized is usually larger, so the corresponding specific surface area is low. It has been demonstrated that, however, higher specific surface area could enhance photocatalytic performance by providing more adsorption and desorption sites for reactants and products, respectively [30,31]. One good strategy to increase specific surface area is to employ high surface area supports for active photocatalysts [32]. With high surface area Al2O3 as a matrix, Ag/AgX (X = Br or I)/Al2O3 composite was prepared by a deposition–precipitation followed by photoreduction method and used to inactivate Escherichia coli and degrade organic pollutants under visible or simulated solar light irradiation [33,34]. Palygorskite with a zeolite-like channel structure is a high surface area, porous, and fibrous hydrated magnesium aluminum
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silicate. For there is considerable substitution of aluminum by magnesium in the octahedral layer, palygorskite has a moderately high layer negative charge. This layer charge, the channel, and high surface area endow palygorskite with moderate cation exchange and high adsorption capacity. Because the negatively charged surface of palygorskite particles could exert electrostatic interactions to adsorb cations and/or repel anions by electrostatic interactions, accordingly, it has been widely used to remove cationic dyes [35] and promote the separation of charged intermediates produced in the organic dye-sensitized reduction of water to hydrogen process [36]. Since the point of zero charge (pzc) of palygorskite is at about 4.0–4.5 [37], consequently, one could anticipate that negatively charged palygorskite particles could exert electrostatic interactions to facilitate the adsorption of Ag+ ion as well as to inhibit the diffusion of Br ion and moreover mediate the precipitation reaction between Ag+ ion and Br ion on palygorskite surfaces in neutral or basic solutions. This, along with its high surface area, could enhance the dispersity of AgBr on the surface of palygorskite. Additionally, the zeolite-like channel could also act as solid nanoreactors to mediate the preparation of AgBr [33]. Based on the above concepts, we prepared high surface area AgBr/palygorskite composite photocatalysts with palygorskite as a matrix by an in situ ectrostatic adsorption–deposition–precipitation method. As pointed out in the literature, the presence of proton donors (e.g., H2O and H2) while the binding of CO2 to the catalyst surface can diminish the redox potential by almost an order of magnitude [10,38]. With H2O as a proton donor, Abou Asi et al. synthesized AgBr/TiO2 by a deposition–precipitation method and investigated the performance of photocatalytical CO2 reduction to hydrocarbons under visible light irradiation [39]. Nevertheless, with H2O as a proton donor, the reduction of H2O could compete with the reduction of CO2. With H2 as a reductant, Grätzel et al. reported the selective conversion of CO2 to CH4 over highly dispersed Ru/ RuOx loaded TiO2 at room temperature under UV irradiation [1]. The photocatalytic activity of the composites was evaluated by the reduction of CO2 to CH4 with H2 as a proton donor and reductant under visible light irradiation. It is well-known that the adsorption and accumulation of reactants on the surface of catalysts is vital in the photocatalytic process. Palygorskite could adsorb cationic dye selectively due to its layer negative charge. In addition to the application to solid–gas photocatalysis, the photocatalytic activity of the composites was also evaluated by the decolorization of the cationic Rhodamine B (Rh B) dye. In the current study, the effects of bromine precursor and palygorskite on photocatalytic performance were investigated systematically. In addition, the probable mechanism was also discussed. 2. Materials and methods 2.1. Materials Commercially palygorskite (Xuyi County of Jiangsu Province, China) and high purity reactant gases were used in this study as received. Other chemicals used were of analytic reagent grade without further purification.
Fig. 1. Schematic illustration of Ag/AgBr assembled on the surface of palygorskite.
magnetically for 24 h. Subsequently, 36 ml 5 105 mol L1 tetra-n-butyl ammonium bromide (N(CH2CH2CH2CH3)4Br, TBAB) or NaBr aqueous solution was added to the above suspension drop by drop and stirred magnetically for 2 h. Then, the resulting white yellow precipitate was centrifuged, washed with deionized water, and dried at 383 K in air. For comparison, AgBr was also prepared without the addition of palygorskite with tetra-n-butyl ammonium bromide (N(CH2CH2CH2CH3)4Br) as a bromine precursor according to the same procedure. All the above processes were carried out in the dark. For convenience, the AgBr/palygorskite composites synthesized with NaBr and TBAB as a bromine precursor are denoted as AgBr/palygorskite (NaBr) and AgBr/palygorskite (TBAB), respectively. AgBr/palygorskite (NaBr) was used only when the effect of bromine precursor on the performance of CH4 evolution was investigated. The pH value of the solution determined on a Markson model 6200 pH meter was adjusted by the addition of hydrochloric acid or sodium hydroxide. 2.3. Characterization of catalysts UV–Vis diffuse reflection spectra (UV–Vis DRS) of samples were obtained using a U-3010 UV–Vis spectrometer and converted from reflectance to absorbance by the Kubelka–Munk method. The contents of Ag were determined using a TAS-990 graphite furnace atomic absorption spectrometer (GF AAS) equipped with a deuterium lamp as a background correction system, a standard transversely heated graphite furnace, and an Ag hollow cathode lamp. X-ray diffraction (XRD) patterns of samples were obtained on a PANalytical X’Pert PRO X-ray diffractometer using Cu Ka radiation operated at 40 kV and 40 mA. Scanning electron microscopic (SEM) images were obtained on a JSM-6701 F field emission scanning electron microscope (FE-SEM) at 5 kV under high vacuum mode. The BET surface measurements were obtained by measuring N2 adsorption isotherms at 76.2 K using an ASAP 2010 surface analyzer, with a pretreatment temperature of 383 K. The typical range of thickness chosen for t-plot measurements was 3.5–5 Å. 2.4. Evaluation of photocatalytic performance
2.2. Preparation of catalysts The AgBr/palygorskite composites were prepared by an in situ electrostatic adsorption–deposition–precipitation method. The strategy for the fabrication of AgBr/palygorskite heterostructure is shown schematically in Fig. 1. Briefly, 1 g palygorskite was added to 1000 mL distilled water at pH 7 while stirring, followed by sonicating for 10 min. Then, 30 ml 5 105 mol L1 AgNO3 aqueous solution was added quickly to the above suspension and stirred
2.4.1. Reduction of CO2 The reaction of photocatalytic CO2 reduction to CH4 with H2 as a proton donor and reductant was carried out in a Pyrex flask of ca. 195 mL with a ca. 20 cm2 flat window at 315 ± 2 K. A 450 W xenon lamp, equipped with a 400 nm cutoff filter, was used as light source. In a typical photocatalytic experiment, the catalyst was deposited onto the inner surface of the flat window according to the reported procedure for preparing ITO-based electrodes [40].
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Briefly, 100 mg AgBr/palygorskite or 20 mg AgBr was suspended in 10 mL water in the Pyrex flask and sonicated for 5 min unless otherwise indicated. The flask was heated slowly under an infrared lamp until the suspension was dried and a layer of film was formed on the inner surface of the flat window and then dried overnight at 383 K. Prior to the introduction of the reactant gases (i.e., CO2 and H2), high purity N2 was bubbled through the flask for 30 min to purge oxygen. Subsequently, 3 mL high purity 12CO2 or 13CO2 and 12 mL H2 were introduced into the photoreduction system. In our experiments, only methane was detected as the gas product by gas chromatography and gas chromatography–mass spectrometry (GC/MS). The performance of photocatalytic CO2 reduction to CH4 was evaluated by measuring the amount of CH4 evolved using a GC112A gas chromatography equipped with a capillary AT.PoraQ column (30 m 0.53 mm 10 lm) and a flame ionization (FID) detector using N2 as gas carrier. The combined standard uncertainty in the measurement of CH4 yields was found to be about 5%. In order to check the reproducibility of photocatalytic behavior including photocatalytic CO2 reduction and Rh B decolorization, each sample was tested in three runs. 2.4.2. Decolorization of Rh B A 300 W tungsten halogen lamp, equipped with a 400 nm cutoff filter, was used as light source. The photocatalytic reaction was carried out in a Pyrex flask of ca. 150 mL with a ca. 12 cm2 flat window. The reaction mixtures inside the flask were maintained in suspension by means of a magnetic stirrer. In a typical photocatalytic experiment, 100 mg AgBr/palygorskite or 20 mg AgBr was suspended in 100 mL 8 106 mol L1 Rh B aqueous solution at pH 7 unless otherwise stated. The concentration of Rh B was monitored spectrophotometrically at 554 nm. UV–Vis absorbance spectra of the samples were recorded on a HP8453 spectrophotometer. Prior to irradiation, the suspensions were stirred magnetically in the dark for 60 min in order to reach an adsorption–desorption equilibrium of Rh B on the catalysts in the dark. During the decolorization reaction, aliquots of 2 mL of the suspension were taken out from the reaction system at intervals and centrifuged at 6000 rpm for 8 min to settle particles. Additionally, to rule out further the effect of particles on absorbance, the above mixture was filtrated through a dialysis membrane (Nylon 6, pore diameter 0.2 lm). 3. Results and discussion 3.1. Characterization of catalysts The characterization of atomic absorption spectroscopy indicates that the mass ratio of AgBr to palygorskite is ca. 1/4 for the AgBr/palygorskite samples. As shown in Fig. 2, the X-ray diffraction (XRD) pattern of either pure AgBr or AgBr/palygorskite shows several peaks at 26.9°, 31.1°, 44.4°, 55.2°, 64.6°, 73.4°, and 81.7°, which are assigned to diffraction from the (1 1 1), (2 0 0), (2 2 0), (2 2 2), (4 0 0), (4 2 0), and (4 2 2) facet of the cubic phase AgBr crystals (PCPDF No. 06-0438) respectively, indicating that the AgBr samples are of high crystallinity. In comparison with the XRD patterns of AgBr/palygorskite before and after reaction, they are nearly identical except that the diffraction peaks (i.e., 38.2° and 77.8°, shown in the insert of Fig. 2) corresponding to cubic metallic Ag (JCPDS No. 79-0148) appeared after reaction, which implies that AgBr is surprisingly stable and metallic Ag occurs under visible light irradiation. Fig. 3 gives the UV–Vis diffuse reflection spectra of palygorskite, the fresh and used AgBr/palygorskite. Compared with pure palygorskite, either the as-prepared or used AgBr/palygorskite has strong absorption in the both UV and visible light region.
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Fig. 2. XRD patterns of samples (a) the as-prepared AgBr, (b) the as-prepared AgBr/ palygorskite (NaBr), (c) the as-prepared AgBr/palygorskite (TBAB), and (d) the used AgBr/palygorskite. The insert is partial magnification of the XRD pattern of the used AgBr/palygorskite.
Fig. 3. UV–Vis diffuse reflection spectra of samples (a) palygorskite, (b) the asprepared AgBr/palygorskite (NaBr), (c) AgBr/palygorskite (TBAB), and (d) the used AgBr/palygorskite.
Especially, the optical absorption of the used AgBr/palygorskite in the visible light region is almost as strong as in the UV region, which could be ascribed to the localized surface plasmon resonance (LSPR) of silver nanoparticles deposited on AgBr particles. As pointed out in the literature [22,25,26], when an electromagnetic field oscillates, the weakly bound electrons of metallic silver nanoparticle respond collectively, which can produce a plasmon state. If the frequency of the incident light matches that of the plasmon oscillation, light should be absorbed, namely LSPR absorption. Since metallic Ag nanoparticles deposited on AgBr have various shapes and diameters, the frequency of plasmon oscillation covers a wide range, and hence, Ag/AgBr can absorb in a wide range of visible light. The LSPR absorption further confirms the existence of metallic Ag particle on the surface of AgBr under light irradiation. The enhancement of optical absorption helps to enhance the efficiency of solar energy utilization and photocatalytic performance. Fig. 4 displays the SEM images of pure palygorskite and various AgBr samples. As shown in Fig. 4 b, the as-prepared AgBr displays irregular morphology. Liu et al. investigated the reaction between cetyltrimethyl ammonium bromide (CTAB) and AgNO3 and postulated that the cationic surfactant could form AgBr/CTAB complex with a metastable layer structure through their quaternary ammonium groups. Furthermore, the layered metastable AgBr/CTAB complex converted readily into anisotropic AgBr particles when
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Fig. 4. SEM images of palygorskite and the as-prepared AgBr/palygorskite (a) palygorskite, (b) the as-prepared AgBr, (c) the as-prepared AgBr/palygorskite (NaBr), and (d) the as-prepared AgBr/palygorskite (TBAB). The insert (b) is partial magnification of the SEM image of the as-prepared AgBr.
heated [41]. In considering of the comparability of the structure, we postulated that similar phenomena should be observed when TBAB reacting with AgNO3 instead of CTAB or furthermore heated. The irregular AgBr particles are easy to connect with fibrous palygorskite surfaces, which is advantageous to subsequent photocatalytic reactions. Additionally, it is clearly seen that irregular AgBr particles are highly dispersed on the surface of fibrous palygorskite in the AgBr/palygorskite (TBAB) sample (Fig. 4d), while in the AgBr/ palygorskite (NaBr) sample, near spherical AgBr particles are severely aggregated and loosely connected with the surface of fibrous palygorskite (Fig. 4c). 3.2. Effect of palygorskite Fig. 5a compares the rate of CH4 evolution over AgBr (20 mg) and AgBr/palygorskite (100 mg) under visible light irradiation. It can be obviously seen that palygorskite enhances the rate of CH4 evolution by a factor of 2, about 4.8 lmol h1 (g AgBr)1 for AgBr/palygorskite. Similarly, we also observed a beneficial effect during Rh B decolorization. As shown in Fig. 5b, the decomposition of Rh B followed the pseudofirst-order kinetics, the corresponding constant (k) being 0.027 and 0.109 min1 for AgBr and AgBr/palygorskite, respectively. The promotion effect of palygorskite could be attributed to the following several factors. Firstly, palygorskite can supply enough large interfaces due to its high specific surface area, so that photocatalytic centers AgBr particles could be highly dispersed, as shown in Fig. 4d. The BET specific surface area of AgBr, palygorskite, and AgBr/palygorskite was found to be about 1.3, 115.2, and 55.9 m2 g1, respectively. Secondly, the size of AgBr particles synthesized in the presence of palygorskite is smaller compared with that of bare AgBr particles, as shown in Fig. 4b and d, which could facilitate charge transfer and moreover suppress the recombination of electron–hole pair [42,43] as well as
to improve the redox ability of AgBr particle due to quantum size effects. In neutral or basic solution, the surface of the palygorskite particle is negatively charged, and accordingly, a diffuse electrical double layer is produced in the vicinity of the solid interface. Therefore, palygorskites particles could exert electrostatic interactions to mediate the diffusion of Br ion and moreover slow down the precipitation reaction between Ag+ ion and Br ion. In addition, the zeolite-like channel could act as solid nanoreactors [33] when Ag+ ion reacts with Br ion to form AgBr, which could inhibit AgBr particles from agglomerating. Thirdly, the porous structure of palygorskite, verified by BET measurements, could facilitate the access of reactants to get to the active sites, reduce the reflection of light, and thus enhance the efficiency [44,45]. Especially, palygorskite could facilitate the adsorption and accumulation cationic Rh B dye on the AgBr surface due to electrostatic interactions in Rh B decolorization experiments, which could enhance the photocatalytic performance. Additionally, the unique Mg-containing zeolite-like structure of palygorskite could facilitate the adsorption of H2 and CO2 [46,47]. In addition to efficiency, the stability of photocatalyst is also very important for its practical application. Consequently, the photocatalytic stability of the as-prepared composite was further investigated. As shown in Fig. 6, the photocatalytic activity of the obtained AgBr/palygorskite photocatalyst unobviously decreased under visible light irradiation, even after three and six cycles for CO2 reduction and Rh B decolorization, respectively. The results demonstrate that the obtained photocatalyst is stable under visible light irradiation. Kobayashi et al. found that AgBr nanowires encapsulated within the inner space of single-wall carbon nanotubes (SWCNTs) displayed surprising stability under UV light irradiation. They inferred that the encapsulation could prevent the reduction of AgBr nanowire by impeding the diffusion of Br atoms [48]. The result of t-plot analysis indicated that AgBr depositing on the surface
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Fig. 5. Effect of palygorskite on photocatalytic performance (a) CO2 reduction and (b) Rh B decolorization. C0 and C represent the concentration of Rh B at irradiation time 0 and t, respectively. Error bars represent standard deviations of triplicate measurements.
of palygorskite results in a significant decline in the microporosity of palygorskite. Specifically, the micropore area and the micropore volume decreased from about 42.8 m2 g1 and 0.019 cm3 g1 for pure palygorskite to 8.6 m2 g1 and 0.003 cm3 g1 for the composite AgBr/palygorskite, which reveals that AgBr could enter into the channels of palygorskite. Accordingly, we postulate that the channel of palygorskite could help to enhance the stability of AgBr. 3.3. Effect of bromine precursor Fig. 7a compares the rate of CH4 evolution over 100 mg AgBr/ palygorskite (NaBr) and 100 mg AgBr/palygorskite (TBAB) under visible light irradiation. Although there are striking similarities in both the XRD pattern and UV–Vis diffuse reflection spectrum of between AgBr/palygorskite (TBAB) and AgBr/palygorskite (NaBr), as shown in Fig. 2 and Fig. 3, it can be obviously seen that there is obvious difference in the average rate of CH4 evolution, namely 4.8 and 2.6 lmol h1 (g AgBr)1 for AgBr/palygorskite (TBAB) and AgBr/palygorskite (NaBr), respectively. Similar phenomena were also observed in the process of Rh B decolorization (Fig. 7b). Obviously, the pseudofirst-order kinetics constant (k) is 0.038 and 0.109 min1 for AgBr/palygorskite (NaBr) and AgBr/palygorskite (TBAB), respectively. The remarkable difference could be mainly ascribed to the difference in the dispersity of AgBr on palygorskite surfaces, as shown in Fig. 4c and d. Na+ ion could be easy to exchange Ag+ ion adsorbed due to its smaller radius compared with tetrabutyl ammonium cation (NðCH2 CH2 CH2 CH3 Þþ 4 ), and thereby the cation-exchange effect could impair the mediating function of palygorskite when Ag+
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Fig. 6. Consecutive recycling dynamic curves over AgBr/palygorskite (TBAB) (a) CO2 reduction and (b) Rh B decolorization. C0 and C represent the concentration of Rh B at irradiation time 0 and t, respectively. Error bars represent standard deviations of triplicate measurements.
ion reacting with Br ion, which results in the poor dispersity of AgBr on palygorskite surfaces for AgBr/palygorskite (NaBr). In neutral or basic solutions, Ag+ ion could adsorb onto the surface of palygorskite by electrostatic interactions, which results in a low concentration of free Ag+ ion and subsequently retards the following precipitation reaction between Ag+ ion and Br ion. Moreover, the moderate layer negative charge of palygorskite could facilitate and mediate Ag+ ion adsorption. Meanwhile, the cation-exchange effect competes with the electrostatic interaction. The smaller the radius of the cation, the stronger the cation-exchange effect, and more severely the function of palygorskite should be impaired. Secondly, quantum size effects and the suppressed recombination of electron–hole pair could partly account for the enhancement. Compared with AgBr/palygorskite (NaBr), the size of AgBr particles in AgBr/palygorskite (TBAB) is smaller, as shown in Fig. 4c and d, which could facilitate charge transfer [42,43] and improve the redox ability of AgBr particle due to quantum size effects. Liu et al. postulated that the complex of Ag+ ion and CTAB could inhibit AgBr colloid growth when AgNO3 reacting with CTAB [41]. Accordingly, we inferred that Ag+ ion could coordinate with TBAB, which could also prevent AgBr particles from agglomerating. Additionally, TBAB could also inhibit AgBr colloid growth through steric hindrance. 3.4. Inference of photocatalytic mechanism Control tests display that CH4 was not detected in the dark or in the presence of palygorskite alone under otherwise identical
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Abundance
Scan 64 (0.692 min): MS10358.D 550 CH4 500 17 450 400 14 350 300 16 250 200 150 15 100 50 0 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19
m/z--> Fig. 8. GC–MS chromatograms of products obtained under SIM mode.
(ii) Production of Br and Ag
e þ Agþ ! Ag þ
Ag þ ht ! h þ e þ
h þ Br ! Br (iii) Production of H
Br þ H2 ðAds:Þ ! H þ HBr þ
h þ H2 ðAds:Þ ! H þ Hþ (iv) Reduction and hydrogenation of CO2 as well as regeneration of Br
H þ e þ CO2 ðAds:Þ ! HCOO
HCOO þ HBr ! HCOOHðAds:Þ þ Br HCOOHðAds:Þ þ H þ e ! H2 CðOHÞO H2 CðOHÞO þ HBr ! HCHOðAds:Þ þ H2 O þ Br Fig. 7. Effect of bromine precursor on photocatalytic performance (a) CO2 reduction and (b) Rh B decolorization. C0 and C represent the concentration of Rh B at irradiation time 0 and t, respectively. Error bars represent standard deviations of triplicate measurements.
or H2 CðOHÞO þ HBr ! H2 CðOHÞ2 ðAds:Þ þ Br HCHOðAds:Þ þ H þ e ! CH3 O H2 CðOHÞ2 ðAds:Þ þ H ! CH2 þ H2 O CH3 O þ HBr ! CH3 OHðAds:Þ þ Br
conditions, indicating that visible light and AgBr are essential in the photocatalytic reduction of CO2 process. To verify whether the carbon atom of the product CH4 stemmed from the reactant gas, high purity isotopically labeled 13CO2 was introduced instead of 12CO2 into the photoreduction system. The products were analyzed by Gas chromatography–mass spectrometry (GC/MS), and a selected ion monitoring (SIM) mode was adopted to increase the sensitivity of the GC/MS detection. Ions with m/z = 14, 15, 16, and 17 were used for CH4 detection. GC/ MS spectrometry analysis was performed on an Agilent 7890A/ 5975C GC/MS spectrometry (Agilent 7890A GC equipped with a capillary HP-5 column and flame ionization (FID) detector, with N2 as gas carrier; Agilent MS 5975C quadrupole mass spectrometer). As shown in Fig. 8, the labeled product 13CH4 was detected. Additionally, control tests indicated that CH4 was not detected without the introduction of CO2 after reaction for 24 h under otherwise identical condition. This further confirms that the carbon atom of CH4 indeed originates from CO2. On the basis of measurements and observations described above, we propose a probable mechanism for the reduction of CO2 to CH4 with H2 as a proton donor and reductant over AgBr/ palygorskite under visible light irradiation. The various steps involved in the mechanism are as follows: (i) Excitation of AgBr þ
AgBr þ ht ! h þ e
CH3 OHðAds:Þ þ H ! CH3 þ H2 O (v) Evolution of CH4
CH2 þ H ! CH4
CH2 þ H2 ! CH3 þ H
CH3 þ H2 ! CH4 þ H CH3 þ H ! CH4
As for the decolorization of Rh B, the mechanism of the degradation of Rh B is similar to the reported [22,25,26]. That is, the photogenerated hole cannot only oxidize the Rh B dye directly, but also oxidize a Br ion to a Br radical, and subsequently decolorize Rh B dye. On the other hand, the photogenerated electron could not only reduce the Rh B dye directly, but also react with O2 to form active oxygen species (i.e., O 2 and OH), and subsequently decolorize Rh B dye. 4. Conclusion In summary, AgBr nanoparticles were assembled electrostatically on fibrous palygorskite surface by an in situ adsorption–deposition–precipitation method in virtue of the negatively charged surface of palygorskite. Due to the effect of cation exchange, bromine precursor has a significant effect on the dispersity of AgBr on palygorskite surface and moreover on the photocatalytic performance of AgBr/palygorskite. Compared with the corresponding bare AgBr particle, the assembled material exhibits excellent photocatalytic activity for the decolorization of cationic Rh B dye and
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the reduction of CO2 to CH4 under visible light irradiation. The porous structure and larger specific surface area of palygorskite could mainly account for the excellent photocatalytic activity of AgBr/ palygorskite. In addition, negative charge of palygorskite particles could facilitate the adsorption and accumulation of cationic Rh B dye on the AgBr surface due to electrostatic interactions, which could enhance the photocatalytic performance. The channel of palygorskite could enhance the stability of AgBr/palygorskite by inhibiting the loss of Br atom. This study will promote and broaden practical applications of plasmonic photocatalysts in environmental and energy issues. Acknowledgments The financial supports from the National Science Foundation of China (21173242), the National Basic Research Program of China (Nos. 2007CB613305 and 2009CB220003), Henan Educational Committee (No. 2011C150002), and Jiyuan Vocational and Technical Collage (JZXY-2011-110) are gratefully acknowledged. References [1] K.R. Thampi, J. Kiwi, M. Grätzel, Nature 327 (1987) 506. [2] Y. Kohno, T. Tanaka, T. Funabiki, S. Yoshida, Phys. Chem. Chem. Phys. 2 (2000) 2635. [3] I.H. Tseng, W.C. Chang, J.C.S. Wu, Appl. Catal. B: Environ. 37 (2002) 37. [4] J.S. Hwang, J.S. Chang, S.E. Park, K. Ikeue, M. Anpo, Top. Catal. 35 (2005) 311. [5] V.P. Indrakanti, J.D. Kubicki, H.H. Schobert, Energy Environ. Sci. 2 (2009) 745. [6] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, ACS Nano 4 (2010) 1259. [7] S.C. Yan, S.X. Ouyang, J. Gao, M. Yang, J.Y. Feng, X.X. Fan, L.J. Wan, Z.S. Li, J.H. Ye, Y. Zhou, Z.G. Zou, Angew. Chem. Int. Ed. 49 (2010) 6400. [8] C.C. Yang, Y.H. Yu, J. Am. Chem. Soc. 1328 (2010) 8398. [9] H. Yoshida, Heterogeneous Photocatalytic Conversion of Carbon Dioxide, in: L. Zang (Ed.), Springer-Verlag London Limited, London, 2011, p. 531. [10] N.M. Dimitrijevic, B.K. Vijayan, O.G. Poluektov, T. Rajh, K.A. Gray, H.Y. He, P. Zapol, J. Am. Chem. Soc. 133 (2011) 3964. [11] M.R. Hoffmann, J.A. Moss, M.M. Baum, Dalton Trans. 40 (2011) 5151. [12] K. Xie, N. Umezawa, N. Zhang, P. Reunchan, Y.J. Zhang, J.H. Ye, Energy Environ. Sci. 4 (2011) 4211. [13] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes, Nano Lett. 9 (2009) 731. [14] K. Awazu, M. Fujimaki, C. Rockstuhl, J. Tominaga, H. Murakami, Y. Ohki, N. Yoshida, T. Watanabe, J. Am. Chem. Soc. 130 (2008) 1676. [15] T. Hirakawa, P.V. Kamat, Langmuir 20 (2004) 5645. [16] Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 127 (2005) 7632. [17] E. Kowalska, O.O.P. Mahaney, R. Abe, B. Ohtani, Phys. Chem. Chem. Phys. 12 (2010) 2344.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Visible light induced CO2 reduction and Rh B decolorization over electrostatic-assembled AgBr/palygorskite Xiaojie Zhang a,b, Jinli Li a,c, Xin Lu b, Changqing Tang b,⇑, Gongxuan Lu a,⇑ a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Department of Metallurgical and Chemical Engineering, Jiyuan Vocational and Technical Collage, Jiyuan 454650, Henan Province, PR China c Graduate University of the Chinese Academy of Sciences, Beijing 100101, PR China b
a r t i c l e
i n f o
Article history: Received 9 November 2011 Accepted 3 February 2012 Available online 23 March 2012 Keywords: AgBr/palygorskite Visible light CO2 reduction Rh B decolorization
a b s t r a c t AgBr/palygorskite composite was prepared by an in situ electrostatic adsorption–deposition–precipitation method and characterized by field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), UV–Vis diffuse reflection, and BET surface measurements techniques. The layer negative charge and larger specific surface area of palygorskite, along with the poor cation-exchange ability of tetra-n-butyl ammonium cation (NðCH2 CH2 CH2 CH3 Þþ 4 ) due to its larger ion radius, could mainly account for high dispersity of AgBr on the surface of fibrous palygorskite. The rate of Rh B decolorization and CO2 reduction with H2 as a proton donor and reductant over AgBr/palygorskite was about three and two times faster than that of the corresponding bare AgBr, respectively. The strategy reported in this work can be easily extended to synthesize other palygorskite-based heterostructure catalysts. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Recycling of CO2 to chemicals has been an increasingly important research topic due to its desirable role in alleviating fossil fuel depletion and the global warming problem. Nevertheless, CO2 reduction is an energy intensive and thermodynamically unfavorable process because of the inert properties of CO2. If a renewable energy source (e.g., solar energy and hydrogen energy) can be used to convert CO2 in a friendly manner, CO2 reduction such as photocatalytic reduction should be a promising technical and environmentally friendly solution [1–11]. Dimitrijevic et al. investigated systematically the mechanism of photocatalytic CO2 reduction to methane with H2O over Titania nanoparticles [10]. However, many stable photocatalysts exploited usually, such as TiO2 and zeolitebased catalysts for photocatalytic CO2 reduction, are active only in the ultraviolet-light region that only accounts for less than 5% of the solar spectrum. Therefore, the low efficiency of solar energy utilization hinders significantly a practical application of photocatalysis. Consequently, many attempts (e.g., doping) have been made to engineer large-band-gap photocatalysts to extend their absorption edge to the visible light, even near infrared-light region [12,13]. With copper and/or Pt nanoparticles loaded nitrogendoped TiO2 nanotube arrays as catalysts, Varghese et al. investi⇑ Corresponding authors. Fax: +86 931 4968178. E-mail addresses: [email protected] (X. Zhang), [email protected] (J. Li), [email protected] (X. Lu), [email protected] (C. Tang), [email protected] (G. Lu). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.02.070
gated photocatalytic CO2 reduction to hydrocarbon fuels with water over under visible light irradiation [13]. Since plasmonic photocatalysis (i.e., photocatalysis assisted by the surface plasmon resonance of noble metal nanoparticles (e.g., Au and Ag)) was proposed by Awazu et al. [14], the development of highly efficient plasmonic photocatalysts has been attracting comprehensive attentions [15–20]. Hou et al. investigated mechanismically the plasmonic enhancement of photocatalytic CO2 reduction to hydrocarbon fuel with water over Au/TiO2 under visible light irradiation [20]. Recently, Ag/AgX (X = Cl, Br, or I) have proved to be a class of promising visible light-driven photocatalyst [21–29]. By an ion-exchange followed by photoreduction method, Wang et al. prepared Ag/AgX (X = Cl, Br, and/or I) plasmonic photocatalysts and employed them as catalysts to degrade organic pollutants under visible light irradiation [21,22]. Generally, the size of Ag@AgX (X = Cl, Br, or I) synthesized is usually larger, so the corresponding specific surface area is low. It has been demonstrated that, however, higher specific surface area could enhance photocatalytic performance by providing more adsorption and desorption sites for reactants and products, respectively [30,31]. One good strategy to increase specific surface area is to employ high surface area supports for active photocatalysts [32]. With high surface area Al2O3 as a matrix, Ag/AgX (X = Br or I)/Al2O3 composite was prepared by a deposition–precipitation followed by photoreduction method and used to inactivate Escherichia coli and degrade organic pollutants under visible or simulated solar light irradiation [33,34]. Palygorskite with a zeolite-like channel structure is a high surface area, porous, and fibrous hydrated magnesium aluminum
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silicate. For there is considerable substitution of aluminum by magnesium in the octahedral layer, palygorskite has a moderately high layer negative charge. This layer charge, the channel, and high surface area endow palygorskite with moderate cation exchange and high adsorption capacity. Because the negatively charged surface of palygorskite particles could exert electrostatic interactions to adsorb cations and/or repel anions by electrostatic interactions, accordingly, it has been widely used to remove cationic dyes [35] and promote the separation of charged intermediates produced in the organic dye-sensitized reduction of water to hydrogen process [36]. Since the point of zero charge (pzc) of palygorskite is at about 4.0–4.5 [37], consequently, one could anticipate that negatively charged palygorskite particles could exert electrostatic interactions to facilitate the adsorption of Ag+ ion as well as to inhibit the diffusion of Br ion and moreover mediate the precipitation reaction between Ag+ ion and Br ion on palygorskite surfaces in neutral or basic solutions. This, along with its high surface area, could enhance the dispersity of AgBr on the surface of palygorskite. Additionally, the zeolite-like channel could also act as solid nanoreactors to mediate the preparation of AgBr [33]. Based on the above concepts, we prepared high surface area AgBr/palygorskite composite photocatalysts with palygorskite as a matrix by an in situ ectrostatic adsorption–deposition–precipitation method. As pointed out in the literature, the presence of proton donors (e.g., H2O and H2) while the binding of CO2 to the catalyst surface can diminish the redox potential by almost an order of magnitude [10,38]. With H2O as a proton donor, Abou Asi et al. synthesized AgBr/TiO2 by a deposition–precipitation method and investigated the performance of photocatalytical CO2 reduction to hydrocarbons under visible light irradiation [39]. Nevertheless, with H2O as a proton donor, the reduction of H2O could compete with the reduction of CO2. With H2 as a reductant, Grätzel et al. reported the selective conversion of CO2 to CH4 over highly dispersed Ru/ RuOx loaded TiO2 at room temperature under UV irradiation [1]. The photocatalytic activity of the composites was evaluated by the reduction of CO2 to CH4 with H2 as a proton donor and reductant under visible light irradiation. It is well-known that the adsorption and accumulation of reactants on the surface of catalysts is vital in the photocatalytic process. Palygorskite could adsorb cationic dye selectively due to its layer negative charge. In addition to the application to solid–gas photocatalysis, the photocatalytic activity of the composites was also evaluated by the decolorization of the cationic Rhodamine B (Rh B) dye. In the current study, the effects of bromine precursor and palygorskite on photocatalytic performance were investigated systematically. In addition, the probable mechanism was also discussed. 2. Materials and methods 2.1. Materials Commercially palygorskite (Xuyi County of Jiangsu Province, China) and high purity reactant gases were used in this study as received. Other chemicals used were of analytic reagent grade without further purification.
Fig. 1. Schematic illustration of Ag/AgBr assembled on the surface of palygorskite.
magnetically for 24 h. Subsequently, 36 ml 5 105 mol L1 tetra-n-butyl ammonium bromide (N(CH2CH2CH2CH3)4Br, TBAB) or NaBr aqueous solution was added to the above suspension drop by drop and stirred magnetically for 2 h. Then, the resulting white yellow precipitate was centrifuged, washed with deionized water, and dried at 383 K in air. For comparison, AgBr was also prepared without the addition of palygorskite with tetra-n-butyl ammonium bromide (N(CH2CH2CH2CH3)4Br) as a bromine precursor according to the same procedure. All the above processes were carried out in the dark. For convenience, the AgBr/palygorskite composites synthesized with NaBr and TBAB as a bromine precursor are denoted as AgBr/palygorskite (NaBr) and AgBr/palygorskite (TBAB), respectively. AgBr/palygorskite (NaBr) was used only when the effect of bromine precursor on the performance of CH4 evolution was investigated. The pH value of the solution determined on a Markson model 6200 pH meter was adjusted by the addition of hydrochloric acid or sodium hydroxide. 2.3. Characterization of catalysts UV–Vis diffuse reflection spectra (UV–Vis DRS) of samples were obtained using a U-3010 UV–Vis spectrometer and converted from reflectance to absorbance by the Kubelka–Munk method. The contents of Ag were determined using a TAS-990 graphite furnace atomic absorption spectrometer (GF AAS) equipped with a deuterium lamp as a background correction system, a standard transversely heated graphite furnace, and an Ag hollow cathode lamp. X-ray diffraction (XRD) patterns of samples were obtained on a PANalytical X’Pert PRO X-ray diffractometer using Cu Ka radiation operated at 40 kV and 40 mA. Scanning electron microscopic (SEM) images were obtained on a JSM-6701 F field emission scanning electron microscope (FE-SEM) at 5 kV under high vacuum mode. The BET surface measurements were obtained by measuring N2 adsorption isotherms at 76.2 K using an ASAP 2010 surface analyzer, with a pretreatment temperature of 383 K. The typical range of thickness chosen for t-plot measurements was 3.5–5 Å. 2.4. Evaluation of photocatalytic performance
2.2. Preparation of catalysts The AgBr/palygorskite composites were prepared by an in situ electrostatic adsorption–deposition–precipitation method. The strategy for the fabrication of AgBr/palygorskite heterostructure is shown schematically in Fig. 1. Briefly, 1 g palygorskite was added to 1000 mL distilled water at pH 7 while stirring, followed by sonicating for 10 min. Then, 30 ml 5 105 mol L1 AgNO3 aqueous solution was added quickly to the above suspension and stirred
2.4.1. Reduction of CO2 The reaction of photocatalytic CO2 reduction to CH4 with H2 as a proton donor and reductant was carried out in a Pyrex flask of ca. 195 mL with a ca. 20 cm2 flat window at 315 ± 2 K. A 450 W xenon lamp, equipped with a 400 nm cutoff filter, was used as light source. In a typical photocatalytic experiment, the catalyst was deposited onto the inner surface of the flat window according to the reported procedure for preparing ITO-based electrodes [40].
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Briefly, 100 mg AgBr/palygorskite or 20 mg AgBr was suspended in 10 mL water in the Pyrex flask and sonicated for 5 min unless otherwise indicated. The flask was heated slowly under an infrared lamp until the suspension was dried and a layer of film was formed on the inner surface of the flat window and then dried overnight at 383 K. Prior to the introduction of the reactant gases (i.e., CO2 and H2), high purity N2 was bubbled through the flask for 30 min to purge oxygen. Subsequently, 3 mL high purity 12CO2 or 13CO2 and 12 mL H2 were introduced into the photoreduction system. In our experiments, only methane was detected as the gas product by gas chromatography and gas chromatography–mass spectrometry (GC/MS). The performance of photocatalytic CO2 reduction to CH4 was evaluated by measuring the amount of CH4 evolved using a GC112A gas chromatography equipped with a capillary AT.PoraQ column (30 m 0.53 mm 10 lm) and a flame ionization (FID) detector using N2 as gas carrier. The combined standard uncertainty in the measurement of CH4 yields was found to be about 5%. In order to check the reproducibility of photocatalytic behavior including photocatalytic CO2 reduction and Rh B decolorization, each sample was tested in three runs. 2.4.2. Decolorization of Rh B A 300 W tungsten halogen lamp, equipped with a 400 nm cutoff filter, was used as light source. The photocatalytic reaction was carried out in a Pyrex flask of ca. 150 mL with a ca. 12 cm2 flat window. The reaction mixtures inside the flask were maintained in suspension by means of a magnetic stirrer. In a typical photocatalytic experiment, 100 mg AgBr/palygorskite or 20 mg AgBr was suspended in 100 mL 8 106 mol L1 Rh B aqueous solution at pH 7 unless otherwise stated. The concentration of Rh B was monitored spectrophotometrically at 554 nm. UV–Vis absorbance spectra of the samples were recorded on a HP8453 spectrophotometer. Prior to irradiation, the suspensions were stirred magnetically in the dark for 60 min in order to reach an adsorption–desorption equilibrium of Rh B on the catalysts in the dark. During the decolorization reaction, aliquots of 2 mL of the suspension were taken out from the reaction system at intervals and centrifuged at 6000 rpm for 8 min to settle particles. Additionally, to rule out further the effect of particles on absorbance, the above mixture was filtrated through a dialysis membrane (Nylon 6, pore diameter 0.2 lm). 3. Results and discussion 3.1. Characterization of catalysts The characterization of atomic absorption spectroscopy indicates that the mass ratio of AgBr to palygorskite is ca. 1/4 for the AgBr/palygorskite samples. As shown in Fig. 2, the X-ray diffraction (XRD) pattern of either pure AgBr or AgBr/palygorskite shows several peaks at 26.9°, 31.1°, 44.4°, 55.2°, 64.6°, 73.4°, and 81.7°, which are assigned to diffraction from the (1 1 1), (2 0 0), (2 2 0), (2 2 2), (4 0 0), (4 2 0), and (4 2 2) facet of the cubic phase AgBr crystals (PCPDF No. 06-0438) respectively, indicating that the AgBr samples are of high crystallinity. In comparison with the XRD patterns of AgBr/palygorskite before and after reaction, they are nearly identical except that the diffraction peaks (i.e., 38.2° and 77.8°, shown in the insert of Fig. 2) corresponding to cubic metallic Ag (JCPDS No. 79-0148) appeared after reaction, which implies that AgBr is surprisingly stable and metallic Ag occurs under visible light irradiation. Fig. 3 gives the UV–Vis diffuse reflection spectra of palygorskite, the fresh and used AgBr/palygorskite. Compared with pure palygorskite, either the as-prepared or used AgBr/palygorskite has strong absorption in the both UV and visible light region.
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Fig. 2. XRD patterns of samples (a) the as-prepared AgBr, (b) the as-prepared AgBr/ palygorskite (NaBr), (c) the as-prepared AgBr/palygorskite (TBAB), and (d) the used AgBr/palygorskite. The insert is partial magnification of the XRD pattern of the used AgBr/palygorskite.
Fig. 3. UV–Vis diffuse reflection spectra of samples (a) palygorskite, (b) the asprepared AgBr/palygorskite (NaBr), (c) AgBr/palygorskite (TBAB), and (d) the used AgBr/palygorskite.
Especially, the optical absorption of the used AgBr/palygorskite in the visible light region is almost as strong as in the UV region, which could be ascribed to the localized surface plasmon resonance (LSPR) of silver nanoparticles deposited on AgBr particles. As pointed out in the literature [22,25,26], when an electromagnetic field oscillates, the weakly bound electrons of metallic silver nanoparticle respond collectively, which can produce a plasmon state. If the frequency of the incident light matches that of the plasmon oscillation, light should be absorbed, namely LSPR absorption. Since metallic Ag nanoparticles deposited on AgBr have various shapes and diameters, the frequency of plasmon oscillation covers a wide range, and hence, Ag/AgBr can absorb in a wide range of visible light. The LSPR absorption further confirms the existence of metallic Ag particle on the surface of AgBr under light irradiation. The enhancement of optical absorption helps to enhance the efficiency of solar energy utilization and photocatalytic performance. Fig. 4 displays the SEM images of pure palygorskite and various AgBr samples. As shown in Fig. 4 b, the as-prepared AgBr displays irregular morphology. Liu et al. investigated the reaction between cetyltrimethyl ammonium bromide (CTAB) and AgNO3 and postulated that the cationic surfactant could form AgBr/CTAB complex with a metastable layer structure through their quaternary ammonium groups. Furthermore, the layered metastable AgBr/CTAB complex converted readily into anisotropic AgBr particles when
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Fig. 4. SEM images of palygorskite and the as-prepared AgBr/palygorskite (a) palygorskite, (b) the as-prepared AgBr, (c) the as-prepared AgBr/palygorskite (NaBr), and (d) the as-prepared AgBr/palygorskite (TBAB). The insert (b) is partial magnification of the SEM image of the as-prepared AgBr.
heated [41]. In considering of the comparability of the structure, we postulated that similar phenomena should be observed when TBAB reacting with AgNO3 instead of CTAB or furthermore heated. The irregular AgBr particles are easy to connect with fibrous palygorskite surfaces, which is advantageous to subsequent photocatalytic reactions. Additionally, it is clearly seen that irregular AgBr particles are highly dispersed on the surface of fibrous palygorskite in the AgBr/palygorskite (TBAB) sample (Fig. 4d), while in the AgBr/ palygorskite (NaBr) sample, near spherical AgBr particles are severely aggregated and loosely connected with the surface of fibrous palygorskite (Fig. 4c). 3.2. Effect of palygorskite Fig. 5a compares the rate of CH4 evolution over AgBr (20 mg) and AgBr/palygorskite (100 mg) under visible light irradiation. It can be obviously seen that palygorskite enhances the rate of CH4 evolution by a factor of 2, about 4.8 lmol h1 (g AgBr)1 for AgBr/palygorskite. Similarly, we also observed a beneficial effect during Rh B decolorization. As shown in Fig. 5b, the decomposition of Rh B followed the pseudofirst-order kinetics, the corresponding constant (k) being 0.027 and 0.109 min1 for AgBr and AgBr/palygorskite, respectively. The promotion effect of palygorskite could be attributed to the following several factors. Firstly, palygorskite can supply enough large interfaces due to its high specific surface area, so that photocatalytic centers AgBr particles could be highly dispersed, as shown in Fig. 4d. The BET specific surface area of AgBr, palygorskite, and AgBr/palygorskite was found to be about 1.3, 115.2, and 55.9 m2 g1, respectively. Secondly, the size of AgBr particles synthesized in the presence of palygorskite is smaller compared with that of bare AgBr particles, as shown in Fig. 4b and d, which could facilitate charge transfer and moreover suppress the recombination of electron–hole pair [42,43] as well as
to improve the redox ability of AgBr particle due to quantum size effects. In neutral or basic solution, the surface of the palygorskite particle is negatively charged, and accordingly, a diffuse electrical double layer is produced in the vicinity of the solid interface. Therefore, palygorskites particles could exert electrostatic interactions to mediate the diffusion of Br ion and moreover slow down the precipitation reaction between Ag+ ion and Br ion. In addition, the zeolite-like channel could act as solid nanoreactors [33] when Ag+ ion reacts with Br ion to form AgBr, which could inhibit AgBr particles from agglomerating. Thirdly, the porous structure of palygorskite, verified by BET measurements, could facilitate the access of reactants to get to the active sites, reduce the reflection of light, and thus enhance the efficiency [44,45]. Especially, palygorskite could facilitate the adsorption and accumulation cationic Rh B dye on the AgBr surface due to electrostatic interactions in Rh B decolorization experiments, which could enhance the photocatalytic performance. Additionally, the unique Mg-containing zeolite-like structure of palygorskite could facilitate the adsorption of H2 and CO2 [46,47]. In addition to efficiency, the stability of photocatalyst is also very important for its practical application. Consequently, the photocatalytic stability of the as-prepared composite was further investigated. As shown in Fig. 6, the photocatalytic activity of the obtained AgBr/palygorskite photocatalyst unobviously decreased under visible light irradiation, even after three and six cycles for CO2 reduction and Rh B decolorization, respectively. The results demonstrate that the obtained photocatalyst is stable under visible light irradiation. Kobayashi et al. found that AgBr nanowires encapsulated within the inner space of single-wall carbon nanotubes (SWCNTs) displayed surprising stability under UV light irradiation. They inferred that the encapsulation could prevent the reduction of AgBr nanowire by impeding the diffusion of Br atoms [48]. The result of t-plot analysis indicated that AgBr depositing on the surface
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Fig. 5. Effect of palygorskite on photocatalytic performance (a) CO2 reduction and (b) Rh B decolorization. C0 and C represent the concentration of Rh B at irradiation time 0 and t, respectively. Error bars represent standard deviations of triplicate measurements.
of palygorskite results in a significant decline in the microporosity of palygorskite. Specifically, the micropore area and the micropore volume decreased from about 42.8 m2 g1 and 0.019 cm3 g1 for pure palygorskite to 8.6 m2 g1 and 0.003 cm3 g1 for the composite AgBr/palygorskite, which reveals that AgBr could enter into the channels of palygorskite. Accordingly, we postulate that the channel of palygorskite could help to enhance the stability of AgBr. 3.3. Effect of bromine precursor Fig. 7a compares the rate of CH4 evolution over 100 mg AgBr/ palygorskite (NaBr) and 100 mg AgBr/palygorskite (TBAB) under visible light irradiation. Although there are striking similarities in both the XRD pattern and UV–Vis diffuse reflection spectrum of between AgBr/palygorskite (TBAB) and AgBr/palygorskite (NaBr), as shown in Fig. 2 and Fig. 3, it can be obviously seen that there is obvious difference in the average rate of CH4 evolution, namely 4.8 and 2.6 lmol h1 (g AgBr)1 for AgBr/palygorskite (TBAB) and AgBr/palygorskite (NaBr), respectively. Similar phenomena were also observed in the process of Rh B decolorization (Fig. 7b). Obviously, the pseudofirst-order kinetics constant (k) is 0.038 and 0.109 min1 for AgBr/palygorskite (NaBr) and AgBr/palygorskite (TBAB), respectively. The remarkable difference could be mainly ascribed to the difference in the dispersity of AgBr on palygorskite surfaces, as shown in Fig. 4c and d. Na+ ion could be easy to exchange Ag+ ion adsorbed due to its smaller radius compared with tetrabutyl ammonium cation (NðCH2 CH2 CH2 CH3 Þþ 4 ), and thereby the cation-exchange effect could impair the mediating function of palygorskite when Ag+
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Fig. 6. Consecutive recycling dynamic curves over AgBr/palygorskite (TBAB) (a) CO2 reduction and (b) Rh B decolorization. C0 and C represent the concentration of Rh B at irradiation time 0 and t, respectively. Error bars represent standard deviations of triplicate measurements.
ion reacting with Br ion, which results in the poor dispersity of AgBr on palygorskite surfaces for AgBr/palygorskite (NaBr). In neutral or basic solutions, Ag+ ion could adsorb onto the surface of palygorskite by electrostatic interactions, which results in a low concentration of free Ag+ ion and subsequently retards the following precipitation reaction between Ag+ ion and Br ion. Moreover, the moderate layer negative charge of palygorskite could facilitate and mediate Ag+ ion adsorption. Meanwhile, the cation-exchange effect competes with the electrostatic interaction. The smaller the radius of the cation, the stronger the cation-exchange effect, and more severely the function of palygorskite should be impaired. Secondly, quantum size effects and the suppressed recombination of electron–hole pair could partly account for the enhancement. Compared with AgBr/palygorskite (NaBr), the size of AgBr particles in AgBr/palygorskite (TBAB) is smaller, as shown in Fig. 4c and d, which could facilitate charge transfer [42,43] and improve the redox ability of AgBr particle due to quantum size effects. Liu et al. postulated that the complex of Ag+ ion and CTAB could inhibit AgBr colloid growth when AgNO3 reacting with CTAB [41]. Accordingly, we inferred that Ag+ ion could coordinate with TBAB, which could also prevent AgBr particles from agglomerating. Additionally, TBAB could also inhibit AgBr colloid growth through steric hindrance. 3.4. Inference of photocatalytic mechanism Control tests display that CH4 was not detected in the dark or in the presence of palygorskite alone under otherwise identical
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Abundance
Scan 64 (0.692 min): MS10358.D 550 CH4 500 17 450 400 14 350 300 16 250 200 150 15 100 50 0 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19
m/z--> Fig. 8. GC–MS chromatograms of products obtained under SIM mode.
(ii) Production of Br and Ag
e þ Agþ ! Ag þ
Ag þ ht ! h þ e þ
h þ Br ! Br (iii) Production of H
Br þ H2 ðAds:Þ ! H þ HBr þ
h þ H2 ðAds:Þ ! H þ Hþ (iv) Reduction and hydrogenation of CO2 as well as regeneration of Br
H þ e þ CO2 ðAds:Þ ! HCOO
HCOO þ HBr ! HCOOHðAds:Þ þ Br HCOOHðAds:Þ þ H þ e ! H2 CðOHÞO H2 CðOHÞO þ HBr ! HCHOðAds:Þ þ H2 O þ Br Fig. 7. Effect of bromine precursor on photocatalytic performance (a) CO2 reduction and (b) Rh B decolorization. C0 and C represent the concentration of Rh B at irradiation time 0 and t, respectively. Error bars represent standard deviations of triplicate measurements.
or H2 CðOHÞO þ HBr ! H2 CðOHÞ2 ðAds:Þ þ Br HCHOðAds:Þ þ H þ e ! CH3 O H2 CðOHÞ2 ðAds:Þ þ H ! CH2 þ H2 O CH3 O þ HBr ! CH3 OHðAds:Þ þ Br
conditions, indicating that visible light and AgBr are essential in the photocatalytic reduction of CO2 process. To verify whether the carbon atom of the product CH4 stemmed from the reactant gas, high purity isotopically labeled 13CO2 was introduced instead of 12CO2 into the photoreduction system. The products were analyzed by Gas chromatography–mass spectrometry (GC/MS), and a selected ion monitoring (SIM) mode was adopted to increase the sensitivity of the GC/MS detection. Ions with m/z = 14, 15, 16, and 17 were used for CH4 detection. GC/ MS spectrometry analysis was performed on an Agilent 7890A/ 5975C GC/MS spectrometry (Agilent 7890A GC equipped with a capillary HP-5 column and flame ionization (FID) detector, with N2 as gas carrier; Agilent MS 5975C quadrupole mass spectrometer). As shown in Fig. 8, the labeled product 13CH4 was detected. Additionally, control tests indicated that CH4 was not detected without the introduction of CO2 after reaction for 24 h under otherwise identical condition. This further confirms that the carbon atom of CH4 indeed originates from CO2. On the basis of measurements and observations described above, we propose a probable mechanism for the reduction of CO2 to CH4 with H2 as a proton donor and reductant over AgBr/ palygorskite under visible light irradiation. The various steps involved in the mechanism are as follows: (i) Excitation of AgBr þ
AgBr þ ht ! h þ e
CH3 OHðAds:Þ þ H ! CH3 þ H2 O (v) Evolution of CH4
CH2 þ H ! CH4
CH2 þ H2 ! CH3 þ H
CH3 þ H2 ! CH4 þ H CH3 þ H ! CH4
As for the decolorization of Rh B, the mechanism of the degradation of Rh B is similar to the reported [22,25,26]. That is, the photogenerated hole cannot only oxidize the Rh B dye directly, but also oxidize a Br ion to a Br radical, and subsequently decolorize Rh B dye. On the other hand, the photogenerated electron could not only reduce the Rh B dye directly, but also react with O2 to form active oxygen species (i.e., O 2 and OH), and subsequently decolorize Rh B dye. 4. Conclusion In summary, AgBr nanoparticles were assembled electrostatically on fibrous palygorskite surface by an in situ adsorption–deposition–precipitation method in virtue of the negatively charged surface of palygorskite. Due to the effect of cation exchange, bromine precursor has a significant effect on the dispersity of AgBr on palygorskite surface and moreover on the photocatalytic performance of AgBr/palygorskite. Compared with the corresponding bare AgBr particle, the assembled material exhibits excellent photocatalytic activity for the decolorization of cationic Rh B dye and
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