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Porous Ag-ZnO microspheres as efficient photocatalyst for methane and ethylene oxidation: Insight into the role of Ag particles
Applied Surface Science 456 (2018) 493–500
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Full Length Article
Porous Ag-ZnO microspheres as efficient photocatalyst for methane and ethylene oxidation: Insight into the role of Ag particles ⁎
Xianglin Zhua,1, Xizhuang Lianga,1, Peng Wanga, , Ying Daib, Baibiao Huanga, a b
T
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State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China School of Physics, Shandong University, Jinan 250100, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag-ZnO Porous structure Photocatalytic Methane and ethylene oxidation
In this paper, porous microsphere Ag-ZnO photocatalyst was prepared and used in photocatalytic degrading of methane and ethylene at room temperature. The Ag-ZnO photocatalyst showed good stability and high efficiency under the simulated sunlight. Both methane and ethylene can be oxidized into CO2. In the photocatalytic reactions, the decorating of Ag nanoparticles plays key roles in promoting the separation of photo-generated carriers, promoting the oxygen reduction process and realizing the complete mineralization of gas molecules.
1. Introduction As two kinds of common hydrocarbons, methane (CH4) and ethylene (C2H4) have shown important applications in energy field and industrial production. Methane is one of the most widely used fuels and ethylene is widely used for the industrial production. However, the existences of low concentration methane and ethylene play negative influences on some certain fields. Methane is one kind of harmful greenhouse gas, which is over twenty times than that of equivalent mass carbon dioxide and has contributed about 20% global warming during the past decades [1–4]. Ethylene is harmful in the fresh-keeping industry at low concentration [5]. Reducing low concentration of CH4 and C2H4 is meaningful for the environment and economy. The best advisable way is conversing CH4 and C2H4 to CO2. Photocatalyst has been used to decompose organic pollutant into carbon dioxide and water only with solar energy, which is low cost, efficient and no secondary pollution. But it is still a challenge to break methane and ethylene because high CeH bond energies under normal reaction conditions [6–8]. Photocatalyst has attracted a lot of attention in energy conversion and pollution purification recent years [9–22]. Among the various photocatalysts, semiconductor photocatalysts is one of the best choices because of their good stabilities and adjustable band gaps and high activity. ZnO with a band gap of 3.2 eV, has been extensively studied in photocatalysis and shows high photo-oxidation ability. Most of the traditional studies about ZnO’s photocatalytic are carried out in water solution [23–29]. However, ZnO is not so stable when used in water solution because of the photochemical corrosion. It is reported that
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1
Corresponding authors. E-mail addresses: [email protected] (P. Wang), [email protected] (B. Huang). These authors contributed equally to this work.
https://doi.org/10.1016/j.apsusc.2018.06.127 Received 4 May 2018; Received in revised form 11 June 2018; Accepted 14 June 2018 Available online 21 June 2018 0169-4332/ © 2018 Published by Elsevier B.V.
some of O2− in ZnO can be oxidized into O2 gradually by photogenerated holes during the photodegradation process, which then induces the dissolving of Zn2+ in water irreversibly. People have tried to enhance the stability of ZnO by enhancing the separation efficiency of the carriers trough constructing surface heterojunctions or loading noble metal on the surface etc [28,29], while it is still unstable for ZnO used in water phase. In the gas phase photocatalytic reaction carried out in air conduction, there is much slower humidity compared to that in the water phase which can avoid the dissolving the Zn2+. Thus, ZnO is more suitable for the photocatalytic application in gas conduction. The decorating of Ag nanoparticles on semiconductor photocatalysts can enhance the photocatalytic activity and has been one of the research hotspots in photocatalytic field [30–37]. Study finds that the loading of a small amount of silver will significantly enhance the photocatalytic activity through promoting the separation efficiency of carrier and extending light absorption [25,56–59]. Herein, we synthesized ZnO porous microflowers by the thermal decomposition of the microflowers precursor. Then Ag-ZnO microspheres were prepared through a simple photodepositing of Ag particles progress. The prepared Ag-ZnO photocatalyst has excellent activity in methane and ethylene photocatalytic degradation benefited to the porous structure, higher carriers’ separation efficiency, and more active species.
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2. Experimental section
BaSO4 was used as a reference. The nitrogen adsorption/desorption isotherms of samples were measured by a Kubo-X1000 apparatus to investigate the Brunauer-Emmett-Teller (BET) surface area of the photocatalysts. A 300 W Xe lamp (320–2500 nm) was used as light source (PLS-SXE 300, Beijing Perfectlight Technology Co. Ltd.). The concentration of gases was determined with Shimadzu GC-2014C and CEAULIGHT GC-7920.
2.1. Materials and reagents All the reagents used in the experiment, including Zn(NO3)2·6H2O, CO(NH2)2, ethanol, methanol, AgNO3 and TiO2 were purchased from Sinopharm Chemical Reagent and without further purification. 2.2. Preparation of ZnO with porous microflowers
2.5. Photocatalytic experiments
Firstly, Zn4(CO3)(OH)6·H2O microflowers precursor was prepared through a simple one-step hydrothermal method. Typically, 30 mmol Zn(NO3)2·6H2O and 60 mmol urea were dissolved in 80 ml deionized water and then transferred into 100 ml Teflon-lined autoclaves, which were then sealed at 120 °C for 2 h. After cooling down to room temperature, precursor was filtered and washed with absolute ethylalcohol and deionized water three times and then dried at 60 °C for 5 h. ZnO with porous microflowers was got by heat treating the Zn4(CO3) (OH)6·H2O sample directly at 400 °C for 2 h.
The photocatalytic activities of as-prepared samples were evaluated through CH4 and C2H4 degradation under UV–visible light irradiation. Typically, 0.5 g photocatalyst was dispersed at the bottom of quartz glass reactor (Volume: 400 ml), and then CH4 or C2H4 was injected. The system was stirred and kept in dark for two hours to attain adsorption saturation. Then, photocatalytic experiment was carried out under irradiation of 300 W Xe lamp. The reaction temperature of ethylene and methane in the reactor was maintained at 5 °C and 25 °C by a circulating cooling system, respectively. 0.5 ml of gas was taken out every 15 or 30 min and injected into gas chromatography to detect the concentration of gases. Simultaneous determination of ethylene or methane concentrations were performed with an online gas chromatograph (Shimadzu GC-2014C and CEAULIGHT GC-7920) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Photoelectrochemical characterizations were evaluated in a quartz cell through three-electrode system based on CHI660C electrochemical work station. A 300 W Xe lamp with an AM 1.5G filter (100 mW/cm2) was used as the light source. The scan rate for the linear sweep voltammetry was 10 mV/s. A Pt plate and a commercially available Ag/ AgCl electrode were used as counter electrode and reference electrode. 0.05 g photocatalyst was spin coated on the FTO glass, which was served as working electrode. 0.2 M Na2SO4 solution was used as the electrolyte.
2.3. Preparation of Ag-ZnO with porous microspheres 0.6 g ZnO microflower was dispersed in 10% methanol aqueous solution and certain amount of AgNO3 (0.01 M) was added under stirring. All these processes were conducted in dark. The mixed solution was put under the irradiating of UV–vis light for half an hour. The precipitate was then filtered and dried, finally heated at 300 °C for 30 min. The as-prepared Ag-ZnO composites with different weight ratios of Ag species and ZnO were denoted as X Ag-ZnO (X = 0.1%, 0.5%, 1.0%, 1.3%, 1.5%, 1.7%, 2.0%). The theoretical mass ratio (Ag/ Zn = 1.5%) in the samples shows the best performance than others, which will be referred to as 1.5% Ag-ZnO in the following. In order to compare the performance of our synthetic photocatalysts, we have also synthesized other representative materials. 1.5% Ag-TiO2 photocatalysts were also prepared according to the above method, besides that ZnO was replaced with commercial anatase titanium dioxide. 1.5% Ag-ZnO (Ref) photocatalysts were also prepared according to a previous literature [35].
2.6. Electrochemical experiments Electrochemical oxygen reduction reaction (ORR) measurements were carried out in a three-electrode glass cell with Pt plate and Ag/ AgCl in saturated KCl solution used as the counter electrode and the reference electrode, respectively. Linear sweep voltammograms were recorded by exposing photoelectrodes in the dark at a scan rate of 10 mV/s. The electrolyte used in all measurements was an aqueous solution containing 0.2 M Na2SO4 at a pH value of 6.8. Before experiment, Argon (99.999%) was bubbled for 0.5 h to remove the dissolved O2 to obtain Ar saturated solution, and oxygen (99.999%) was bubbled for 0.5 h to obtain O2 saturated solution.
2.4. Materials characterization XRD patterns of the samples were carried out on a Bruker D8 Advance X-ray diffractometer with Cu Kα irradiation (λ = 0.154056 nm). The morphologies of as-prepared samples were characterized using scanning electron microscopy (SEM, Hitachi S4800) with an Energy Dispersive Spectrometer (EDS). UV–vis diffuse reflectance spectra (DRS) were characterized using a Shimadzu UV 2550 spectrophotometer equipped with an integrating sphere, and
Fig. 1. XRD patterns of Zn4(CO3)(OH)6·H2O, ZnO and Ag-ZnO samples with depositing different proportions of Ag. 494
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Fig. 2. SEM and TEM images of the as-prepared samples: (a), (d) Zn4(CO3)(OH)6·H2O. (b), (e) ZnO. (c), (f) and (g-i) 1.5% Ag-ZnO. Scale bars: (a-c) 2 μm, (d-g) 100 nm, (h-i) 5 nm.
3. Results and discussion
magnification morphologies of Zn4(CO3)(OH)6·H2O, ZnO and 1.5% AgZnO samples, respectively. From the images we can see that, Zn4(CO3) (OH)6 and ZnO are in the shapes of flowerlike microsphere, while 1.5% Ag-ZnO sample are only in the shape of microsphere losing the flower morphology. Fig. 2(d–f) are the higher magnification SEM images and we can see that Zn4(CO3)(OH)6·H2O, ZnO and 1.5% Ag-ZnO are composed with nanosheets, porous nanosheets and nanoparticles, respectively. The porous nanosheet of ZnO with 1.5% silver is broken because of the corrosion of AgNO3 solution during the photo-deposition progress. Fig. 2(g–i) are the TEM images of 1.5% Ag-ZnO sample. The images show that ZnO microspheres are composed of uniform nanoparticles with an average particle size of around 30 nm. The lattice spacings of 0.247 nm and 0.235 nm match with the ZnO (1 0 1) and Ag (1 1 1) crystallographic planes. In addition, the BET surface area and a pore diameter distribution of ZnO and 1.5% Ag-ZnO samples were characterized, as shown in Fig. 3
The phases and compositions of the serial samples were investigated by X-ray diffraction (XRD) patterns shown in Fig. 1. Fig. 1a shows the XRD patterns of the prepared Zn4(CO3)(OH)6·H2O and ZnO samples, responding to standard card of JPCDS No. 11-287 and 75-576 respectively. The XRD results prove that Zn4(CO3)(OH)6·H2O is completely decomposed into pure ZnO. Fig. 1b is the XRD results of Ag-ZnO samples, and we can see that the peak of Ag appears when the deposition amount is over 1.3%. In addition, Fig. S1 shows no change in XRD patterns of the ZnO and 1.5% Ag-ZnO samples before and after degradation of methane and ethylene under (UV–vis) light illumination, indicating the ZnO and 1.5% Ag-ZnO photocatalysts are very stable during the degradation of methane and ethylene. The surface morphologies of the as-prepared samples were characterized by SEM and TEM. Fig. 2(a–c) correspond to the lower
Fig. 3. (a) Nitrogen adsorption/desorption isotherm curves and (b) a pore diameter distribution of ZnO and 1.5% Ag-ZnO samples. 495
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Fig. 4. (a) UV–vis diffuse reflectance spectra (DRS) of ZnO and Ag-ZnO samples with depositing different proportions of Ag, (b) Photocatalytic performance of AgZnO samples with depositing different proportions of Ag. (Beijing, Perfect light 300 W Xenon lamp, λ > 320 nm).
that the photocatalytic activities are greatly improved after the amount of Ag was over 1%. The 1.5% Ag-ZnO sample shows best performance and can finish degradation in 120 min. From the results, we confirm the amount of 1.5% as an appropriate deposition ratio for further investigation. The activity of as prepared ZnO photocatalyst with porous microspheres was carried out through the photocatalytic degrading of CH4 and C2H4 experiments. Fig. 5a is the degradation curves of 250 ppm methane removing with 1.5% Ag-ZnO with porous microspheres, commercial TiO2, 1.5% Ag-ZnO (Ref) nanoparticles prepared with precipitation method [35] and Fig. 5b is the results of photodegradation of 2500 ppm ethylene. From the experimental results we can find that 1.5% Ag-ZnO photocatalyst can’t degrade both methane and ethylene under the irradiation of visible light though the sample shows good visible light response, indicating that the oxidation ability coming from surface plasmon resonance under visible irradiation is not enough to break the CeH bonds. In the degradation of CH4, ZnO with micro flowers, TiO2 and Ag-TiO2 show bad performance even under the
and Table S1. According to the Barret-Joyner-Halenda (BJH) model analysis, the porous microspheres show a wide pore size distribution of 10–50 nm calculated from the adsorption branch of the isotherm, which verifies the mesoporous structure of ZnO. The BET surface area and average pore diameter of flowerlike ZnO are 23.18 m2/g and 16.2 nm. For the 1.5% Ag-ZnO sample, the BET surface area and average pore diameter of ZnO are 23.06 m2/g and 14.2 nm. These results prove that the BET surface area is not influenced by the change of morphology and both ZnO and the 1.5% Ag-ZnO samples have porous structures with big BET surface areas. The optical properties of as-prepared ZnO and Ag-ZnO samples were studied using UV–vis diffuse reflectance spectra (DRS). As shown in Fig. 4a, with the deposition of Ag, the samples show more intense absorption in visible light region, which can be attributed to the surface plasmon resonance (SPR) of nano silver [10,30–32]. The photocatalytic performances of ZnO and Ag-ZnO samples with depositing different amount Ag were firstly evaluated by photodegradation of 250 ppm CH4 under the irradiation of UV–vis light, as shown in Fig. 4b. We can see
Fig. 5. Photodegradation of methane (a) and ethylene (b) with ZnO, 1.5% Ag-ZnO (with porous microspheres), TiO2 and 1.5% Ag-TiO2 photocatalysts; CO2 concentration curves in photodegradation of 250 ppm methane (c) and 2500 ppm ethylene (d) with 1.5% Ag-ZnO. (Beijing, Perfect light 300 W Xenon lamp, λ > 320 nm). 496
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photocatalysts still kept high efficiency in photo-oxidation experiments for four cycles’ photocatalysts tests. This indicates 1.5% Ag-ZnO is a stable photocatalyst for photodegrading CH4 and C2H4. Photoluminescence spectra were used to illuminate the separation mechanism of photogenerated carriers. Typically, the higher PL intensity means the higher recombination of electron hole pairs [10,38–41]. Fig. 6a is the PL spectra of ZnO, 1.0% Ag-ZnO, 1.5% AgZnO and 2.0% Ag-ZnO excited at 350 nm. The emission intensity of 1.5% Ag-ZnO is lower than that of ZnO, indicating that the separation of photo-generated electrons and holes in Ag-ZnO is more efficient than that in ZnO. The lifetime of charge carriers in ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO were examined by time-resolved PL spectrum, as shown in Fig. 6b. The average lifetimes of ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO are 2.98 ns, 4.18 ns, 4.44 ns and 3.78 ns, respectively. When loading 1.5 wt% Ag on ZnO, the lifetime of charge carriers gets increased compared to pristine ZnO. The charge recombination is efficiently suppressed after loading Ag nanoparticles on ZnO, resulting in higher photocatalytic activity. Thus, 1.5% Ag-ZnO photocatalyst has high photocatalytic performance. The electrochemical characterizations were further studied to understand the photocatalytic mechanism. Fig. 7a is the photocurrent responses of ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO at different potentials, and the attained photoelectrical results are consistent with the photoluminescence results in Fig. 6. The 1.5% Ag-ZnO sample has higher photocurrent response than ZnO which means higher separation efficiency of the carriers [42]. The electrochemical impedance spectra (EIS) results in Fig. 7b prove that 1.5% Ag-ZnO sample has smaller impedance which is benefited to the separation of photogenerated carriers. In the photocatalytic progress, both the photogenerated electrons and holes can be active factors for photocatalytic reaction. The electrons can reduce the dissolved O2 to generate superoxide radical and the holes can oxide H2O to generate hydroxyl radicals which both can oxide the organic molecules. Here, we investigated the electrochemical oxygen reduction reaction (ORR) of ZnO and 1.5% AgZnO. From the polarization curves in Ar and O2 saturated Na2SO4 solution (Fig. 7c), we can see that the current densities in O2 saturated solution are much higher than in Ar saturated solution both for the ZnO and 1.5% Ag-ZnO. This can prove the existing of ORR, what’s more to be noted is the ORR onset potential 1.5% Ag-ZnO is obvious higher than the pure ZnO. The bigger current and the higher onset potential prove that the depositing of Ag can greatly promote the ORR process and this means the electrons can reduce O2 more efficiently into superoxide radical which is benefited to the photocatalytic oxidation reaction [43]. From the above characterizations, we can conclude that the modification of Ag plays an important role on both enhancing the efficiency separating of carriers and promoting the ORR process. X-ray photoelectron spectroscopy (XPS) was used to investigate the
irradiation of UV–vis light. In contrast, the activity of 1.5% Ag-ZnO photocatalyst with porous microspheres is excellent and over 98% CH4 is removed in 120 min. For the degradation of C2H4, the above four photocatalysts all show degradation ability under the irradiation of UV–vis light. It should be noted that pure TiO2 has better performance than ZnO, but the photocatalytic activity of 1.5% Ag-ZnO photocatalyst has better efficiency than 1.5% Ag-TiO2 after same amount of Ag is deposited. 99% of 2500 ppm ethylene can be removed in 150 min by 1.5% Ag-ZnO photocatalyst. As shown in Fig. 5, 1.5% Ag-ZnO with porous microspheres has better photocatalytic performance than the conventional nanoparticle Ag-ZnO catalyst (Fig. S2), which indicates that the porous structure is benefit for the excellent activity of photocatalyst. It is an important index that whether the organic molecules can be completely oxidized into nontoxic CO2 in photocatalytic reactions without secondary pollution. The oxidation production during the photocatalytic process was investigated. For the photodegradation of CH4, CO2 and a small amount of CO are generated when using ZnO as photocatalyst and only CO2 is generated when using 1.5% Ag-ZnO as photocatalyst, as shown in Fig. S3a and b. The concentrations of CO2 during photocatalytic reaction are shown in Fig. 5c, and we can see that the concentrations of generated CO2 increase with decrease of CH4. The amount of produced CO2 is nearly the theoretical amount which should be generated. While in the photodegradation of C2H4 by ZnO and 1.5% Ag-ZnO as photocatalysts, a small amount of CO is generated (Fig. S3c and d). Fig. 5d is the concentration curves of C2H4 and generated CO2 in photodegradation of C2H4, and the generated CO2 is a little less than the theoretical amount which should be generated. This is due to a small amount of generated intermediate CO, as shown in Fig. S4. In order to further probe the photodegradation process, we investigated photocatalytic activity of 1.5% Ag-ZnO for CO removing, as shown in Fig. S5. Our experiment proves 1.5% Ag-ZnO photocatalyst has high activity in CO degradation and 250 ppm CO is degraded in 60 min. The degradation rates of 1.5% Ag-ZnO for CH4, C2H4 and CO in our experiments are evaluated by k = Q/t, where Q is the initial concentration of reactant and t is the photodegradation time. The degradation rates k of CH4, C2H4 and CO are calculated to be 2.08, 13.89 and 4.17 ppm/min, respectively. The different rates indicate that C2H4 is more easily degrading than CO and CH4, and CH4 is the hardest to be oxidized. This can also give an explanation that no CO was detected in low concentration CH4 photodegradation experiment when using 1.5% Ag-ZnO photocatalyst. From the results, we can conclude that 1.5% AgZnO photocatalyst has stronger mineralizing ability than pure ZnO. The stability of photocatalysts is a very important property for photocatalysts, and it concerns the potentiality of industrial application. The stability of 1.5% Ag-ZnO photocatalyst was evaluated thorough recycle photodegradation experiments, as shown in Fig. S6. The
Fig. 6. (a) Photoluminescence (PL) spectra of ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO (λexc = 350 nm), (b) Time-resolved PL spectra for ZnO, 1.0% AgZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO detected at 471 nm, the excitation source is a 377.8 nm laser. 497
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Fig. 7. (a) Photocurrent responses, (b) EIS plots and (c) Polarization curves of ZnO and 1.5% Ag-ZnO in Ar and O2 saturated Na2SO4 solution.
Fig. 8. XPS spectra of 1.5% Ag-ZnO photocatalyst before and after degrading methane.
elemental composition and surface chemical status of the 1.5% Ag-ZnO photocatalyst before and after reaction as shown in Fig. 8. In Fig. 8a, two peaks located at 368.13 eV and 374.13 eV are assigned to Ag 3d5/2 and Ag 3d3/2 of Ag0 respectively, which agrees well with previous literatures [44,45]. The O 1s region is decomposed into three peaks. The main peak located at 530.26 eV is attributed to the crystal lattice oxygen in ZnO, and the other two kinds of oxygen contributions can be ascribed to oxygen vacancy (Ovac, 531.62 eV) and chemical adsorbed oxygen on the catalyst surface (Oads, 532.57 eV), respectively (Fig. 8b) [46]. Fig. 8c shows Zn 2p XPS peaks of Ag-ZnO, which are located at 1022.47 and 1045.62 eV, resulting from Zn 2p3/2 and Zn 2p1/2, respectively [46]. In general, XPS spectra of 1.5% Ag-ZnO and ZnO photocatalysts (Fig. S8) show that the binding energies of Ag, Zn and O elements have no change after photo-oxidation reaction, and this further indicates that as-prepared photocatalysts have good stability in gas phase catalysis. Based on the above experimental results we propose a possible mechanism for photodegradation as shown in Fig. 9. Benefited to the SPR of nano silver, the Ag-ZnO catalyst has very good visible light response, but the oxidation ability of SPR is not able to degrade the
Fig. 9. Scheme of the photocatalytic reaction.
methane and ethylene gases. As we all know, both the photogenerated electrons and holes can be active species in photocatalytic reaction. It has been reported that Ag is considered as one of most promising electrocatalysts for the oxygen reduction reaction [47–49], and this means the photogenerated electrons can reduce O2 into superoxide radical more easily on the Ag particles in the photocatalytic progress [50–52]. More superoxide radical will be generated and enhance the 498
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photocatalytic activity [53–55]. So, the main contributions of Ag nanoparticles are promoting the separation of carriers, promoting the reduction of O2 into superoxide radical and realizing the absolute oxidation of the gases. Besides, the special porous sphere microstructure of Ag-ZnO makes it have better activity than normal Ag-ZnO nanoparticles catalyst. Benefited to the high efficiency of the carriers’ separation, the more generated species and the porous structure, the Ag-ZnO microsphere shows good photocatalytic activity.
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4. Conclusions
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In summary, we have prepared a porous Ag-ZnO microsphere photocatalyst and the prepared photocatalyst shows high mineralizing ability and stability on photodegradation of methane and ethylene at room temperature. The mechanism analysis reveals that the photocatalytic activities are mainly caused by the photo-exciting of ZnO. The main effects of Ag are promoting the separation of carriers, interfacial charge transfer, promoting the reduction of O2 into superoxide radical and accelerating the mineralization process. Our experiments show that the porous Ag-ZnO microsphere photocatalyst may be a potential photocatalyst for hazardous gases removing, because of its high mineralizing ability.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51602179, 21333006, 21573135 and 11374190), the National Basic Research Program of China (the 973 program, 2013CB632401). P. Wang acknowledges support from the Recruitment Program of Global Experts, China. B. Huang acknowledges support from the Taishan Scholars Program of Shandong Province.
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Full Length Article
Porous Ag-ZnO microspheres as efficient photocatalyst for methane and ethylene oxidation: Insight into the role of Ag particles ⁎
Xianglin Zhua,1, Xizhuang Lianga,1, Peng Wanga, , Ying Daib, Baibiao Huanga, a b
T
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State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China School of Physics, Shandong University, Jinan 250100, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag-ZnO Porous structure Photocatalytic Methane and ethylene oxidation
In this paper, porous microsphere Ag-ZnO photocatalyst was prepared and used in photocatalytic degrading of methane and ethylene at room temperature. The Ag-ZnO photocatalyst showed good stability and high efficiency under the simulated sunlight. Both methane and ethylene can be oxidized into CO2. In the photocatalytic reactions, the decorating of Ag nanoparticles plays key roles in promoting the separation of photo-generated carriers, promoting the oxygen reduction process and realizing the complete mineralization of gas molecules.
1. Introduction As two kinds of common hydrocarbons, methane (CH4) and ethylene (C2H4) have shown important applications in energy field and industrial production. Methane is one of the most widely used fuels and ethylene is widely used for the industrial production. However, the existences of low concentration methane and ethylene play negative influences on some certain fields. Methane is one kind of harmful greenhouse gas, which is over twenty times than that of equivalent mass carbon dioxide and has contributed about 20% global warming during the past decades [1–4]. Ethylene is harmful in the fresh-keeping industry at low concentration [5]. Reducing low concentration of CH4 and C2H4 is meaningful for the environment and economy. The best advisable way is conversing CH4 and C2H4 to CO2. Photocatalyst has been used to decompose organic pollutant into carbon dioxide and water only with solar energy, which is low cost, efficient and no secondary pollution. But it is still a challenge to break methane and ethylene because high CeH bond energies under normal reaction conditions [6–8]. Photocatalyst has attracted a lot of attention in energy conversion and pollution purification recent years [9–22]. Among the various photocatalysts, semiconductor photocatalysts is one of the best choices because of their good stabilities and adjustable band gaps and high activity. ZnO with a band gap of 3.2 eV, has been extensively studied in photocatalysis and shows high photo-oxidation ability. Most of the traditional studies about ZnO’s photocatalytic are carried out in water solution [23–29]. However, ZnO is not so stable when used in water solution because of the photochemical corrosion. It is reported that
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1
Corresponding authors. E-mail addresses: [email protected] (P. Wang), [email protected] (B. Huang). These authors contributed equally to this work.
https://doi.org/10.1016/j.apsusc.2018.06.127 Received 4 May 2018; Received in revised form 11 June 2018; Accepted 14 June 2018 Available online 21 June 2018 0169-4332/ © 2018 Published by Elsevier B.V.
some of O2− in ZnO can be oxidized into O2 gradually by photogenerated holes during the photodegradation process, which then induces the dissolving of Zn2+ in water irreversibly. People have tried to enhance the stability of ZnO by enhancing the separation efficiency of the carriers trough constructing surface heterojunctions or loading noble metal on the surface etc [28,29], while it is still unstable for ZnO used in water phase. In the gas phase photocatalytic reaction carried out in air conduction, there is much slower humidity compared to that in the water phase which can avoid the dissolving the Zn2+. Thus, ZnO is more suitable for the photocatalytic application in gas conduction. The decorating of Ag nanoparticles on semiconductor photocatalysts can enhance the photocatalytic activity and has been one of the research hotspots in photocatalytic field [30–37]. Study finds that the loading of a small amount of silver will significantly enhance the photocatalytic activity through promoting the separation efficiency of carrier and extending light absorption [25,56–59]. Herein, we synthesized ZnO porous microflowers by the thermal decomposition of the microflowers precursor. Then Ag-ZnO microspheres were prepared through a simple photodepositing of Ag particles progress. The prepared Ag-ZnO photocatalyst has excellent activity in methane and ethylene photocatalytic degradation benefited to the porous structure, higher carriers’ separation efficiency, and more active species.
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2. Experimental section
BaSO4 was used as a reference. The nitrogen adsorption/desorption isotherms of samples were measured by a Kubo-X1000 apparatus to investigate the Brunauer-Emmett-Teller (BET) surface area of the photocatalysts. A 300 W Xe lamp (320–2500 nm) was used as light source (PLS-SXE 300, Beijing Perfectlight Technology Co. Ltd.). The concentration of gases was determined with Shimadzu GC-2014C and CEAULIGHT GC-7920.
2.1. Materials and reagents All the reagents used in the experiment, including Zn(NO3)2·6H2O, CO(NH2)2, ethanol, methanol, AgNO3 and TiO2 were purchased from Sinopharm Chemical Reagent and without further purification. 2.2. Preparation of ZnO with porous microflowers
2.5. Photocatalytic experiments
Firstly, Zn4(CO3)(OH)6·H2O microflowers precursor was prepared through a simple one-step hydrothermal method. Typically, 30 mmol Zn(NO3)2·6H2O and 60 mmol urea were dissolved in 80 ml deionized water and then transferred into 100 ml Teflon-lined autoclaves, which were then sealed at 120 °C for 2 h. After cooling down to room temperature, precursor was filtered and washed with absolute ethylalcohol and deionized water three times and then dried at 60 °C for 5 h. ZnO with porous microflowers was got by heat treating the Zn4(CO3) (OH)6·H2O sample directly at 400 °C for 2 h.
The photocatalytic activities of as-prepared samples were evaluated through CH4 and C2H4 degradation under UV–visible light irradiation. Typically, 0.5 g photocatalyst was dispersed at the bottom of quartz glass reactor (Volume: 400 ml), and then CH4 or C2H4 was injected. The system was stirred and kept in dark for two hours to attain adsorption saturation. Then, photocatalytic experiment was carried out under irradiation of 300 W Xe lamp. The reaction temperature of ethylene and methane in the reactor was maintained at 5 °C and 25 °C by a circulating cooling system, respectively. 0.5 ml of gas was taken out every 15 or 30 min and injected into gas chromatography to detect the concentration of gases. Simultaneous determination of ethylene or methane concentrations were performed with an online gas chromatograph (Shimadzu GC-2014C and CEAULIGHT GC-7920) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Photoelectrochemical characterizations were evaluated in a quartz cell through three-electrode system based on CHI660C electrochemical work station. A 300 W Xe lamp with an AM 1.5G filter (100 mW/cm2) was used as the light source. The scan rate for the linear sweep voltammetry was 10 mV/s. A Pt plate and a commercially available Ag/ AgCl electrode were used as counter electrode and reference electrode. 0.05 g photocatalyst was spin coated on the FTO glass, which was served as working electrode. 0.2 M Na2SO4 solution was used as the electrolyte.
2.3. Preparation of Ag-ZnO with porous microspheres 0.6 g ZnO microflower was dispersed in 10% methanol aqueous solution and certain amount of AgNO3 (0.01 M) was added under stirring. All these processes were conducted in dark. The mixed solution was put under the irradiating of UV–vis light for half an hour. The precipitate was then filtered and dried, finally heated at 300 °C for 30 min. The as-prepared Ag-ZnO composites with different weight ratios of Ag species and ZnO were denoted as X Ag-ZnO (X = 0.1%, 0.5%, 1.0%, 1.3%, 1.5%, 1.7%, 2.0%). The theoretical mass ratio (Ag/ Zn = 1.5%) in the samples shows the best performance than others, which will be referred to as 1.5% Ag-ZnO in the following. In order to compare the performance of our synthetic photocatalysts, we have also synthesized other representative materials. 1.5% Ag-TiO2 photocatalysts were also prepared according to the above method, besides that ZnO was replaced with commercial anatase titanium dioxide. 1.5% Ag-ZnO (Ref) photocatalysts were also prepared according to a previous literature [35].
2.6. Electrochemical experiments Electrochemical oxygen reduction reaction (ORR) measurements were carried out in a three-electrode glass cell with Pt plate and Ag/ AgCl in saturated KCl solution used as the counter electrode and the reference electrode, respectively. Linear sweep voltammograms were recorded by exposing photoelectrodes in the dark at a scan rate of 10 mV/s. The electrolyte used in all measurements was an aqueous solution containing 0.2 M Na2SO4 at a pH value of 6.8. Before experiment, Argon (99.999%) was bubbled for 0.5 h to remove the dissolved O2 to obtain Ar saturated solution, and oxygen (99.999%) was bubbled for 0.5 h to obtain O2 saturated solution.
2.4. Materials characterization XRD patterns of the samples were carried out on a Bruker D8 Advance X-ray diffractometer with Cu Kα irradiation (λ = 0.154056 nm). The morphologies of as-prepared samples were characterized using scanning electron microscopy (SEM, Hitachi S4800) with an Energy Dispersive Spectrometer (EDS). UV–vis diffuse reflectance spectra (DRS) were characterized using a Shimadzu UV 2550 spectrophotometer equipped with an integrating sphere, and
Fig. 1. XRD patterns of Zn4(CO3)(OH)6·H2O, ZnO and Ag-ZnO samples with depositing different proportions of Ag. 494
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Fig. 2. SEM and TEM images of the as-prepared samples: (a), (d) Zn4(CO3)(OH)6·H2O. (b), (e) ZnO. (c), (f) and (g-i) 1.5% Ag-ZnO. Scale bars: (a-c) 2 μm, (d-g) 100 nm, (h-i) 5 nm.
3. Results and discussion
magnification morphologies of Zn4(CO3)(OH)6·H2O, ZnO and 1.5% AgZnO samples, respectively. From the images we can see that, Zn4(CO3) (OH)6 and ZnO are in the shapes of flowerlike microsphere, while 1.5% Ag-ZnO sample are only in the shape of microsphere losing the flower morphology. Fig. 2(d–f) are the higher magnification SEM images and we can see that Zn4(CO3)(OH)6·H2O, ZnO and 1.5% Ag-ZnO are composed with nanosheets, porous nanosheets and nanoparticles, respectively. The porous nanosheet of ZnO with 1.5% silver is broken because of the corrosion of AgNO3 solution during the photo-deposition progress. Fig. 2(g–i) are the TEM images of 1.5% Ag-ZnO sample. The images show that ZnO microspheres are composed of uniform nanoparticles with an average particle size of around 30 nm. The lattice spacings of 0.247 nm and 0.235 nm match with the ZnO (1 0 1) and Ag (1 1 1) crystallographic planes. In addition, the BET surface area and a pore diameter distribution of ZnO and 1.5% Ag-ZnO samples were characterized, as shown in Fig. 3
The phases and compositions of the serial samples were investigated by X-ray diffraction (XRD) patterns shown in Fig. 1. Fig. 1a shows the XRD patterns of the prepared Zn4(CO3)(OH)6·H2O and ZnO samples, responding to standard card of JPCDS No. 11-287 and 75-576 respectively. The XRD results prove that Zn4(CO3)(OH)6·H2O is completely decomposed into pure ZnO. Fig. 1b is the XRD results of Ag-ZnO samples, and we can see that the peak of Ag appears when the deposition amount is over 1.3%. In addition, Fig. S1 shows no change in XRD patterns of the ZnO and 1.5% Ag-ZnO samples before and after degradation of methane and ethylene under (UV–vis) light illumination, indicating the ZnO and 1.5% Ag-ZnO photocatalysts are very stable during the degradation of methane and ethylene. The surface morphologies of the as-prepared samples were characterized by SEM and TEM. Fig. 2(a–c) correspond to the lower
Fig. 3. (a) Nitrogen adsorption/desorption isotherm curves and (b) a pore diameter distribution of ZnO and 1.5% Ag-ZnO samples. 495
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Fig. 4. (a) UV–vis diffuse reflectance spectra (DRS) of ZnO and Ag-ZnO samples with depositing different proportions of Ag, (b) Photocatalytic performance of AgZnO samples with depositing different proportions of Ag. (Beijing, Perfect light 300 W Xenon lamp, λ > 320 nm).
that the photocatalytic activities are greatly improved after the amount of Ag was over 1%. The 1.5% Ag-ZnO sample shows best performance and can finish degradation in 120 min. From the results, we confirm the amount of 1.5% as an appropriate deposition ratio for further investigation. The activity of as prepared ZnO photocatalyst with porous microspheres was carried out through the photocatalytic degrading of CH4 and C2H4 experiments. Fig. 5a is the degradation curves of 250 ppm methane removing with 1.5% Ag-ZnO with porous microspheres, commercial TiO2, 1.5% Ag-ZnO (Ref) nanoparticles prepared with precipitation method [35] and Fig. 5b is the results of photodegradation of 2500 ppm ethylene. From the experimental results we can find that 1.5% Ag-ZnO photocatalyst can’t degrade both methane and ethylene under the irradiation of visible light though the sample shows good visible light response, indicating that the oxidation ability coming from surface plasmon resonance under visible irradiation is not enough to break the CeH bonds. In the degradation of CH4, ZnO with micro flowers, TiO2 and Ag-TiO2 show bad performance even under the
and Table S1. According to the Barret-Joyner-Halenda (BJH) model analysis, the porous microspheres show a wide pore size distribution of 10–50 nm calculated from the adsorption branch of the isotherm, which verifies the mesoporous structure of ZnO. The BET surface area and average pore diameter of flowerlike ZnO are 23.18 m2/g and 16.2 nm. For the 1.5% Ag-ZnO sample, the BET surface area and average pore diameter of ZnO are 23.06 m2/g and 14.2 nm. These results prove that the BET surface area is not influenced by the change of morphology and both ZnO and the 1.5% Ag-ZnO samples have porous structures with big BET surface areas. The optical properties of as-prepared ZnO and Ag-ZnO samples were studied using UV–vis diffuse reflectance spectra (DRS). As shown in Fig. 4a, with the deposition of Ag, the samples show more intense absorption in visible light region, which can be attributed to the surface plasmon resonance (SPR) of nano silver [10,30–32]. The photocatalytic performances of ZnO and Ag-ZnO samples with depositing different amount Ag were firstly evaluated by photodegradation of 250 ppm CH4 under the irradiation of UV–vis light, as shown in Fig. 4b. We can see
Fig. 5. Photodegradation of methane (a) and ethylene (b) with ZnO, 1.5% Ag-ZnO (with porous microspheres), TiO2 and 1.5% Ag-TiO2 photocatalysts; CO2 concentration curves in photodegradation of 250 ppm methane (c) and 2500 ppm ethylene (d) with 1.5% Ag-ZnO. (Beijing, Perfect light 300 W Xenon lamp, λ > 320 nm). 496
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photocatalysts still kept high efficiency in photo-oxidation experiments for four cycles’ photocatalysts tests. This indicates 1.5% Ag-ZnO is a stable photocatalyst for photodegrading CH4 and C2H4. Photoluminescence spectra were used to illuminate the separation mechanism of photogenerated carriers. Typically, the higher PL intensity means the higher recombination of electron hole pairs [10,38–41]. Fig. 6a is the PL spectra of ZnO, 1.0% Ag-ZnO, 1.5% AgZnO and 2.0% Ag-ZnO excited at 350 nm. The emission intensity of 1.5% Ag-ZnO is lower than that of ZnO, indicating that the separation of photo-generated electrons and holes in Ag-ZnO is more efficient than that in ZnO. The lifetime of charge carriers in ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO were examined by time-resolved PL spectrum, as shown in Fig. 6b. The average lifetimes of ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO are 2.98 ns, 4.18 ns, 4.44 ns and 3.78 ns, respectively. When loading 1.5 wt% Ag on ZnO, the lifetime of charge carriers gets increased compared to pristine ZnO. The charge recombination is efficiently suppressed after loading Ag nanoparticles on ZnO, resulting in higher photocatalytic activity. Thus, 1.5% Ag-ZnO photocatalyst has high photocatalytic performance. The electrochemical characterizations were further studied to understand the photocatalytic mechanism. Fig. 7a is the photocurrent responses of ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO at different potentials, and the attained photoelectrical results are consistent with the photoluminescence results in Fig. 6. The 1.5% Ag-ZnO sample has higher photocurrent response than ZnO which means higher separation efficiency of the carriers [42]. The electrochemical impedance spectra (EIS) results in Fig. 7b prove that 1.5% Ag-ZnO sample has smaller impedance which is benefited to the separation of photogenerated carriers. In the photocatalytic progress, both the photogenerated electrons and holes can be active factors for photocatalytic reaction. The electrons can reduce the dissolved O2 to generate superoxide radical and the holes can oxide H2O to generate hydroxyl radicals which both can oxide the organic molecules. Here, we investigated the electrochemical oxygen reduction reaction (ORR) of ZnO and 1.5% AgZnO. From the polarization curves in Ar and O2 saturated Na2SO4 solution (Fig. 7c), we can see that the current densities in O2 saturated solution are much higher than in Ar saturated solution both for the ZnO and 1.5% Ag-ZnO. This can prove the existing of ORR, what’s more to be noted is the ORR onset potential 1.5% Ag-ZnO is obvious higher than the pure ZnO. The bigger current and the higher onset potential prove that the depositing of Ag can greatly promote the ORR process and this means the electrons can reduce O2 more efficiently into superoxide radical which is benefited to the photocatalytic oxidation reaction [43]. From the above characterizations, we can conclude that the modification of Ag plays an important role on both enhancing the efficiency separating of carriers and promoting the ORR process. X-ray photoelectron spectroscopy (XPS) was used to investigate the
irradiation of UV–vis light. In contrast, the activity of 1.5% Ag-ZnO photocatalyst with porous microspheres is excellent and over 98% CH4 is removed in 120 min. For the degradation of C2H4, the above four photocatalysts all show degradation ability under the irradiation of UV–vis light. It should be noted that pure TiO2 has better performance than ZnO, but the photocatalytic activity of 1.5% Ag-ZnO photocatalyst has better efficiency than 1.5% Ag-TiO2 after same amount of Ag is deposited. 99% of 2500 ppm ethylene can be removed in 150 min by 1.5% Ag-ZnO photocatalyst. As shown in Fig. 5, 1.5% Ag-ZnO with porous microspheres has better photocatalytic performance than the conventional nanoparticle Ag-ZnO catalyst (Fig. S2), which indicates that the porous structure is benefit for the excellent activity of photocatalyst. It is an important index that whether the organic molecules can be completely oxidized into nontoxic CO2 in photocatalytic reactions without secondary pollution. The oxidation production during the photocatalytic process was investigated. For the photodegradation of CH4, CO2 and a small amount of CO are generated when using ZnO as photocatalyst and only CO2 is generated when using 1.5% Ag-ZnO as photocatalyst, as shown in Fig. S3a and b. The concentrations of CO2 during photocatalytic reaction are shown in Fig. 5c, and we can see that the concentrations of generated CO2 increase with decrease of CH4. The amount of produced CO2 is nearly the theoretical amount which should be generated. While in the photodegradation of C2H4 by ZnO and 1.5% Ag-ZnO as photocatalysts, a small amount of CO is generated (Fig. S3c and d). Fig. 5d is the concentration curves of C2H4 and generated CO2 in photodegradation of C2H4, and the generated CO2 is a little less than the theoretical amount which should be generated. This is due to a small amount of generated intermediate CO, as shown in Fig. S4. In order to further probe the photodegradation process, we investigated photocatalytic activity of 1.5% Ag-ZnO for CO removing, as shown in Fig. S5. Our experiment proves 1.5% Ag-ZnO photocatalyst has high activity in CO degradation and 250 ppm CO is degraded in 60 min. The degradation rates of 1.5% Ag-ZnO for CH4, C2H4 and CO in our experiments are evaluated by k = Q/t, where Q is the initial concentration of reactant and t is the photodegradation time. The degradation rates k of CH4, C2H4 and CO are calculated to be 2.08, 13.89 and 4.17 ppm/min, respectively. The different rates indicate that C2H4 is more easily degrading than CO and CH4, and CH4 is the hardest to be oxidized. This can also give an explanation that no CO was detected in low concentration CH4 photodegradation experiment when using 1.5% Ag-ZnO photocatalyst. From the results, we can conclude that 1.5% AgZnO photocatalyst has stronger mineralizing ability than pure ZnO. The stability of photocatalysts is a very important property for photocatalysts, and it concerns the potentiality of industrial application. The stability of 1.5% Ag-ZnO photocatalyst was evaluated thorough recycle photodegradation experiments, as shown in Fig. S6. The
Fig. 6. (a) Photoluminescence (PL) spectra of ZnO, 1.0% Ag-ZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO (λexc = 350 nm), (b) Time-resolved PL spectra for ZnO, 1.0% AgZnO, 1.5% Ag-ZnO and 2.0% Ag-ZnO detected at 471 nm, the excitation source is a 377.8 nm laser. 497
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Fig. 7. (a) Photocurrent responses, (b) EIS plots and (c) Polarization curves of ZnO and 1.5% Ag-ZnO in Ar and O2 saturated Na2SO4 solution.
Fig. 8. XPS spectra of 1.5% Ag-ZnO photocatalyst before and after degrading methane.
elemental composition and surface chemical status of the 1.5% Ag-ZnO photocatalyst before and after reaction as shown in Fig. 8. In Fig. 8a, two peaks located at 368.13 eV and 374.13 eV are assigned to Ag 3d5/2 and Ag 3d3/2 of Ag0 respectively, which agrees well with previous literatures [44,45]. The O 1s region is decomposed into three peaks. The main peak located at 530.26 eV is attributed to the crystal lattice oxygen in ZnO, and the other two kinds of oxygen contributions can be ascribed to oxygen vacancy (Ovac, 531.62 eV) and chemical adsorbed oxygen on the catalyst surface (Oads, 532.57 eV), respectively (Fig. 8b) [46]. Fig. 8c shows Zn 2p XPS peaks of Ag-ZnO, which are located at 1022.47 and 1045.62 eV, resulting from Zn 2p3/2 and Zn 2p1/2, respectively [46]. In general, XPS spectra of 1.5% Ag-ZnO and ZnO photocatalysts (Fig. S8) show that the binding energies of Ag, Zn and O elements have no change after photo-oxidation reaction, and this further indicates that as-prepared photocatalysts have good stability in gas phase catalysis. Based on the above experimental results we propose a possible mechanism for photodegradation as shown in Fig. 9. Benefited to the SPR of nano silver, the Ag-ZnO catalyst has very good visible light response, but the oxidation ability of SPR is not able to degrade the
Fig. 9. Scheme of the photocatalytic reaction.
methane and ethylene gases. As we all know, both the photogenerated electrons and holes can be active species in photocatalytic reaction. It has been reported that Ag is considered as one of most promising electrocatalysts for the oxygen reduction reaction [47–49], and this means the photogenerated electrons can reduce O2 into superoxide radical more easily on the Ag particles in the photocatalytic progress [50–52]. More superoxide radical will be generated and enhance the 498
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photocatalytic activity [53–55]. So, the main contributions of Ag nanoparticles are promoting the separation of carriers, promoting the reduction of O2 into superoxide radical and realizing the absolute oxidation of the gases. Besides, the special porous sphere microstructure of Ag-ZnO makes it have better activity than normal Ag-ZnO nanoparticles catalyst. Benefited to the high efficiency of the carriers’ separation, the more generated species and the porous structure, the Ag-ZnO microsphere shows good photocatalytic activity.
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4. Conclusions
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In summary, we have prepared a porous Ag-ZnO microsphere photocatalyst and the prepared photocatalyst shows high mineralizing ability and stability on photodegradation of methane and ethylene at room temperature. The mechanism analysis reveals that the photocatalytic activities are mainly caused by the photo-exciting of ZnO. The main effects of Ag are promoting the separation of carriers, interfacial charge transfer, promoting the reduction of O2 into superoxide radical and accelerating the mineralization process. Our experiments show that the porous Ag-ZnO microsphere photocatalyst may be a potential photocatalyst for hazardous gases removing, because of its high mineralizing ability.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51602179, 21333006, 21573135 and 11374190), the National Basic Research Program of China (the 973 program, 2013CB632401). P. Wang acknowledges support from the Recruitment Program of Global Experts, China. B. Huang acknowledges support from the Taishan Scholars Program of Shandong Province.
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