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One-step hydrothermal synthesis of Cu-doped MnO2 coated diatomite for degradation of methylene blue in Fenton-like system
Journal of Colloid and Interface Science 556 (2019) 466–475
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
One-step hydrothermal synthesis of Cu-doped MnO2 coated diatomite for degradation of methylene blue in Fenton-like system Yu Xiao a,b,1, Wangchen Huo a,1, Shaoning Yin a, Debin Jiang a, Yuxin Zhang a,⇑, Zhiqiang Zhang a,⇑, Xiaoying Liu c, Fan Dong d, Jinshu Wang e, Gang Li f, Xuebu Hu f, Xiaoya Yuan g, Hong-Chang Yao h a
State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China Chongqing Institute of Modern Construction Industry Development, Chongqing 400060, PR China Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, PR China d Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, PR China e School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, PR China f College of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, PR China g College of Materials Science and Engineering, Chongqing Jiaotong University, Chongqing 400074, PR China h College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou City, Henan Province 450001, PR China b c
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 7 May 2019 Revised 22 August 2019 Accepted 22 August 2019 Available online 24 August 2019 Keywords: Cu doped MnO2@diatomite MB degradation Fenton-like reaction Nanocomposites Density functional theory
a b s t r a c t In this work, we have synthesized Cu-doped MnO2@diatomite successfully though a one-step hydrothermal approach. Meanwhile, application for degradation of methylene blue in Fenton-like system was investigated. The compounds were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), Inductively Coupled Plasma analysis (ICP) and UV–vis spectroscopy measurements, beam scanning electron microscope (FIB/SEM), energy dispersive X-ray spectrometer (EDS). The observations revealed that copper was indeed intercalated into layered structure of MnO2 and Density functional theory (DFT) calculations predicted that Cu2+ intercalated MnO2@diatomite brought about the narrowing of band gap and the enhancing of charge mobility during catalysis. Electron Density Difference of CuMnD demonstrated excellent oxidation ability to dissociate H2O2 and generate hydroxyl radical (OH) to degrade the MB. Moreover, the proper copper doping of sample is more easily to form oxygen defect, which generate more surface hydroxyl groups as reaction sites for surface adsorption. In addition, the degradation efficiency of CuMnD was tremendously influenced by the initial pH, H2O2 dosage and copper
⇑ Corresponding authors. 1
E-mail addresses: [email protected] (Y. Zhang), [email protected] (Z. Zhang). The authors equally contributed this work.
https://doi.org/10.1016/j.jcis.2019.08.082 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Y. Xiao et al. / Journal of Colloid and Interface Science 556 (2019) 466–475
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content of catalyst. Ultimately, 0.02-25-CuMnD along with molar ration of Cu/Mn with 0.4402 showed the best degradation efficiency which was about 96.2% within 4 h with 16.5 mM of H2O2 and pH 2.06. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
2.1. Synthesis of Cu-doped MnO2 @diatomite
Waste water treatment has attracted great attention because it contains a good deal of heavy metals, organic dye and other chromophoric groups, which would pollute the environment and threat peoples’ life. Methylene blue (MB) as a familiar cationic phenothiazine dye comes from textile, printing and leather industries [1,2]. Various techniques, especially adsorption and catalytic oxidation have been wildly utilized to remove MB from wastewater on account of low energy requirement and easy operation [1–5]. However, MB may not be degraded completely by adsorption, moreover, some adsorbents will illustrate low adsorption efficiency and long adsorption time [2]. What’s more, there is still a challenge to discovery low-cost, high-efficiency and harmless materials, meanwhile to expound the actual removal mechanism of mixtures for different contaminants in practical applications [3]. Diatomite has been employed as a promising templates for the porous compounds, due to its high porosity, low density, and excellent chemical durability [6–8]. They are also utilized for adsorbing organic dye sand removing heavy metal [7,9]. However, diatomite individual used as a catalyst for wastewater was seldom reported because of its own boundedness. Hence, many researches have introduced Mn/MnO2 [9–13], Fe/ferrihydrite [14,15], TiO2 [16–18] and NiO [19] to modify the surface and properties of diatomite. Among them, MnO2 has been one of most popular candidates on account of its’ unique layers or tunnels structure, high economy efficiency, abundance and high redox potential [20]. It is known that doping nanoparticles can improve conductivity and capacitance of MnO2 effectively. Hence, many researchers have endeavored to dope metals or metal oxides into MnO2. Doping candidates include V [21], Cu/CuO [22–26], Al [26,27], ZnO [28], Co [29–31], and Fe [29,31,32]. What’s more, the catalytic performance of MnO2further promoted via doping other active components. For example, Gao et al. synthesized transition metal-doped MnO2 for CO oxidation catalysts, which showed much enhanced CO oxidation activity [33–36]. Zhang et al. incorporated Cu into MnO2 exhibiting admirable catalytic activity for benzotriazole in the Fenton-like system [37]. McKendry et al. doped cobalt into layered manganese oxide compounds which enhanced water oxidation capacity [38]. Among them, copper cation might be a prime candidate on account of its low coat and environmentally benign nature [39]. Nevertheless, for all we know, the research concerning the synthesis of copper-doped MnO2@Diatomite for a catalytic degradation for MB in a Fenton-like system seldom been reported. The objective of this work was to synthesis a catalyst copperdoped MnO2@Diatomite (CuMnD) with different morphologies by adjusting the concentration of KMnO4 and the molar ratio of Cu/ Mn to promote the property of degradation of MB in a Fentonlike process. We discuss how the Cu dopant could activate the H2O2 for catalytic oxidation elaborately. DFT calculations conform to our experimental observations and supply insight into the role of Cu in enhanced MnO2@Diatomite activity.
Method for synthesizing CuMnD and crystal structure of CuMnO2 are schematically demonstrated in Scheme 1. Before our hydrothermal process, the diatomite should first be purified through the reported chemical method [40]. In a typical synthesis, disperse 50 mg of the treated diatomite into 70 mL of KMnO4 (0.02 M) solution and magnetic stirring to obtain uniform solution. Concentration of doping copper sulfate pentahydrate was 0, 1, 3, 5, 7, 10, 15, 20, 25 at.%, representing the molar ratio of Cu to Mn in the raw materials. Secondly, certain content of copper sulfate pentahydrate mixed to mixture then stirred it for 20 min to form homogeneous suspension. After that, the mixture were poured into a 100 mL Teflon-lined stainless steel autoclave, sealed and maintained at 160 °C for 12 h under one-step hydrothermal synthesis. Finally, the as-synthesized sample was thoroughly rinsed with distilled water, then dried at 60 °C for 12 h. For simplicity, composites designated as 0.02-0-CuMnD, 0.02-1-CuMnD, 0.02-3-CuMnD, 0.025-CuMnD, 0.02-7-CuMnD, 0.02-10-CuMnD, 0.02-15-CuMnD, 0.0220-CuMnD and 0.02-25-CuMnD, respectively. For comparison, KMnO4 with other concentration (0.01 M and 0.04 M) were also synthesized and researched.
2. Materials and experimental methods The original diatomite employed in this paper bought from Tianjin Damao Chemical Reagent Company. All chemical reagents were analytical grade and employed without further treatment.
2.2. Fenton-like process of Cu-doped MnO2@diatomite The catalytic capacity of CuMnD estimated by degrading MB. We adjusted the pH value of the MB solution via 0.1 M HCl from initial value to 2.06 before the degradation reaction. In a typical process, add the proper CuMnD into a 250 mL glass beaker with 100 mL of known MB solution, after that continuous stir it at 303 K for 10 min to obtain adsorption equilibrium of MB. Afterwards, add a certain concentration of H2O2 to the system to activate the Fenton-like reaction. The concentrations of MB solutions before and after degradation were tested using a TU-1901 spectrophotometer (Model UV Win5.1.0) with wavelength at 663 nm. The catalytic efficiency for removal of MB could be evaluated via Eq. (1).
MB Remov al ð%Þ ¼
C0 Ct 100% C0
ð1Þ
where C0 and Ct (mg L1) refer to the initial concentration of MB and concentration at time t, respectively. Moreover, effects of pH (2.06– 10.50), H2O2 amount (3.3–33 mM) on the Fenton-like reaction of CuMnD and the different content of cooper doped MnO2@Diatomite were researched for optimizing the experimental condition. 2.3. Materials characterization The crystal structure and chemical composition information of synthesized CuMnD were measured by powder X-ray diffraction at a scan rate of 4° min1 from 5 to 80° (XRD, D/max 2500, Cu Ka). Kratos Axis Ultra X-ray photoelectron spectroscope (XPS, Al Ka source) was employed for investigating the surface performance. The morphological investigations were accomplished via focused ion beam scanning electron microscope (Zeiss Auriga FIB/SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The composition of the samples were established by ICP analysis. The oxygen defect the samples were measured by ESR.
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Scheme 1. Schematic illustration of the synthesis of Cu-MnO2@Diatomite (the magnification images show the structure of pores and the crystal structure of Cu-MnO2).
2.4. Density functional theory calculations Density functional theory (DFT) [41,42] were calculated rely on the Vienna ab-initio simulation package (VASP5.4) [43]. The projector augmented wave (PAW) method was employed to connect the core-valence, and generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) [44] was applied to achieve exchange-correlation. A plane-wave basis set with cutoff energy 500 eV [45] and the 7 11 5 Monkhorst Pack grid samples were utilized, and the force of atom convergence was set as 0.01 eV/Å. A 72-atom 2 3 2 super cell of bulk MnO6 was firstly relaxed, then Cu atoms were introduced and the sites of defects were shown in Fig. 1. The energy of defect formation (DE) is defined as follows
DE ¼ Ed 12Ep ECu
ð2Þ
Or DE ¼ Ed 12Ep ECu EMn
ð3Þ
where Ed , EP , ECu and EMn stand for the total energy of MnO2 with Cuintercalated defects, pure MnO2 and Cu species, respectively. 3. Results and discussion 3.1. Characterization of CuMnD 3.1.1. The crystal feature of the as-synthesized CuMnD Fig. 1a and S1 present the constituent and crystallite feature of the CuMnD seen from X-ray diffraction pattern. The strong diffraction peaks are observed at 21.9°, 28.3°, 31.2°, and 36.0◦, could be respectively assigned to (1 0 1), (1 1 1), (1 0 2), and (2 0 0) facets, planes of crystalline SiO2 with a cristobalite structure (JCPDS Card No. 76–0938), indicating an excellent crystallinity. In addition, we discovered from the XRD spectra of MnO2-covered diatomite (0CuMnD) that diffraction peaks at about 12.3°, 24.8° and 65.5° are nearly conform to the standard XRD spectrum of potassium manganese oxide hydrate crystal (JCPDS Card No. 86-0666). There are without redundant diffraction peaks originating from the copper species in the XRD spectra when the content of Cu is low, indicating the purity of as-synthesized MnO2@diatomite samples and uniform doping of Cu cations in layers of d-MnO2. However, when molar ration of Cu/Mn in deposited oxides is beyond 0.4402 (Table S1), the new XRD diffraction peaks of samples at 13.9°, 16.6°, 28.0° are assigned to (0 0 2), (0 1 2), (0 0 4), conform to the
Fig. 1. (a) XRD patterns of 0.02 M MnO2@diatomite with Cu (CuMnD). (b) The crystal structure of 0.02-0-CuMnD. (c, d) The crystal structure of 0.02-25-CuMnD with intercalation and substitution, respectively.
standard XRD spectra of Cu4(OH)6SO4 with brochantite structure (JCPDS Card No. 85-1316). Generally, the dopant atoms introduced into crystal lattice would change lattice parameter. Based on the Bragg equation, we observed that XRD peaks from 8.5 to 13.5° (Fig. S2) transfer slightly to a bigger angle after Cu ions were intercalated into MnO2, revealing that metal ions introduced into lattice and generated lattice contraction. For interlayer structure of d-MnO2 crystals, Cu2+ in solution could easily inserted into interlayer space of d-MnO2. Besides, certain Cu2+ in system solution introduced into MnO2 and substituted the Mn to generate Cu-substituted MnO2. Hence, Cu-doped dMnO2 would with two possible configurations: Cu-intercalated
Y. Xiao et al. / Journal of Colloid and Interface Science 556 (2019) 466–475 Table 1 Binding energies of Cu atom occupied in different d-MnO2 sites. Cu-intercalated d-MnO2
Cu-substituted d-MnO2
7.6799 eV
4.9608 eV
and Cu-substituted d-MnO2. To gain an atomic-scale understanding of experimental phenomena and to research impact of Cu doping, we studied electronic structures of these two possible configurations by means of the first principle calculations: Cuintercalated (Fig. 1c) and Cu-substituted d-MnO2 (Fig. 1d). Meanwhile, Fig. 1b revealed the crystal structure of 0.02-0-CuMnD. We investigated the formation energy of Cu atoms in different sites to discuss the Cu doping in d-MnO2. The calculated formation energy is following the Eq. (2) or (3), and listed in Table 1. It could be discovered Cu-intercalated d-MnO2 is more stable to seize Cu atom powerfully. And it’s difficult to come into being Cusubstituted d-MnO2. Furthermore, the lattice parameter of dMnO2 calculated is a = 4.83, b = 2.77, c = 7.31, respectively. Nevertheless, it caused lattice contraction after Cu-intercalated with lattice constant of a = 4.69, b = 2.73 and c = 5.60. It is indicated that the results of calculations accord with the XRD.
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3.1.2. The elemental analyses using ICP The existences of doping element Cu and the atomic rations of Cu, K and Mn for various Cu-doped MnO2@diatomite were detected by ICP spectrometer and illustrated in Table S1. In this research, the existence of K+ is speculated to make valence balance and therefore, insure the crystal structures of MnO2 stable [29]. Compared with 0-CuMnD, the K/Mn molar rations in all CuMnD samples have a great decrease. It is noteworthy that as the Cu/ Mn molar ration increases, the K/Mn molar ration decreases. This may illustrate that portion copper ions may be introduced into interlamination of d-MnO2 crystals to substitute potassium and obtain stable structure of d-MnO2. Other copper ions might exhibit on surface ofd-MnO2@Diatomite. In addition, the Cu/Mn molar rations of the samples 0.02-5-CuMnD, 0.02-10-CuMnD, 0.02-15CuMnD, 0.02-20-CuMnD are more than 0.4402, which may generate copper compound to alter inner structure of the d-MnO2. The results also conform to the XRD patterns. 3.1.3. X-ray photoelectron spectroscopy The surface constituents and valence states of elements of CuMnD (0.02 M) samples were detected by XPS (Fig. 2). The results from XPS illustrated in Table S2. From Fig. 2a, Si, C, K, O and Mn can be found in the survey scan of 0-CuMnD. With the increasing of
Fig. 2. XPS spectra of CuMnD (0.02 M): (a) survey scan, (b) O1s, (c) Cu2p, (d) Mn2p, (e) K2p and (f) Si2p.
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Fig. 3. Characterization of the 0.02-0-CuMnD. (a, b) Typical SEM image; (c) element mappings of O, Si, Mn, and K.
Fig. 4. Characterization of the 0.02-25-CuMnD. (a, b) Typical SEM image; (c) element mappings of O, Si, Mn, Cu and K.
Fig. 5. (a, b) Density of states of sample 0.02-0-CuMnD and 0.02-25-CuMnD with intercalation. (c) Electron density difference of 0.02-25-CuMnD with intercalation.
practical molar ratio of Cu/Mn, Cu can be observed but K decreases, finally disappear. It is an evidence that the partial copper doing may substitute potassium. Fig. 2b illustrated O 1s spectra, peaks at 530.0, 531.1 and 532.9 eV are in accord with binding energies of lattice oxygen, surface hydroxyl group and O–Si from diatomite structure, respectively [46,47]. 25-CuMnD shows a weaker lattice oxygen peak and stronger surface hydroxyl group than 0-CuMnD, illustrating copper intercalation provided more oxygen defect generating more surface hydroxyl groups which was effective active sites for surface adsorption [48] and catalytic oxidation [49]. Seen
the Cu 2p spectrum (Fig. 2c), two obvious peaks at 934.1 and 953.9 eV were in accord with Cu 2p3/2 and Cu 2p1/2. What’s more, the presence of Cu2+ could be further verified by a satellite located at 962.2 eV and a strong satellite line between 940.0 and 945.5 eV [37]. The Mn 2p spectra (Fig. 2d) illustrated two distinct peaks at 654.4 and 642.6 eV, stand for Mn 2p1/2 and Mn 2p3/2, respectively. There are two peaks at 642.2 and 643.8 eV prove the existence of Mn3+ and Mn4+ on the surface, respectively [33,50]. What’s more, the atomic percent of Mn4+/Mn3+ were 0.4550, 0.3673, 0.3174 and 0.4692 on the surface of 0-CuMnD, 3-CuMnD, 25-CuMnD,
Y. Xiao et al. / Journal of Colloid and Interface Science 556 (2019) 466–475
and 15-CuMnD, respectively (Table S2). Therefore, the copper can influence the ratio of the species of Mn. It also illustrated that the atomic percent of Cu/Mn increased from 0 to 0.4123 and as it increased, the Mn4+/Mn3+ firstly decreased and then increased. In addition, two peaks were detected according to Fig. 2e, which locked at 292.8 and 295.7 eV associating to binding energies of K 2p1/2 and K 2p3/2 of weak potassium peaks [51]. However, the K 2p fade away along with the augment of Cu. So that the atomic percent of K/Mn decreased from 0.3663 to 0. It is absolutely that the laws of XPS results are accordance with the ICP analysis. Furthermore, Fig. 2f showed the photoelectron peaks of Si 2p and only one strong peak at 103.7 eV was observed, conforming to characteristic of siloxane groups (Si–O–Si) of diatomite [46].
3.1.4. The morphology of as-prepared Cu-MnO2@Diatomite The morphology of the samples were examined via SEM. Morphology and microstructure of 0.02-0-CuMnDwere shown in Fig. 3a and b. It can be seen the unabridged diatomite is diskshaped with diameter of 30 lm. The sizes of macrospores arrays are almost 0.5 lm, which distribute in whole diatomite. The special pore structure may enable effective MB solution contact the deposited oxide. Figs. 3 and 4 and Figs. S3–S4 show the images of Cu-doped MnO2 covered on the diatomite with different concentration of KMnO4. The pictures illustrate that the Cu doping markedly affect morphology of d-MnO2 film. Fig. 3a and b illustrate the typical structure of the undoped d-MnO2 film with 0.02 M KMnO4, it can be seen considerable fine grains and few nanosheets. When 25 at. % Cu is doped into it, the film consists of numerous tiny nanosheets and grow homogenously on the skeletons of diatomite (Fig. 4a, b). Moreover, the concentration of KMnO4 greatly influence the morphology of CuMnD. These numerous greater nanosheets interknit to form network in sample 0.04-0-CuMnD when the concentration of KMnO4 increased to 0.04 M (Fig. S3a, b). Furthermore, an increasing number of nanosheets generate, some them interconnect to construct flower-like structure to spread on/in diatomite as doping concentration of Cu increasing to 25 at. % (Fig. S4a, b). Therefore, the presence of copper may make tremendous impact on self-assembly of d-MnO2 nanosheets [24]. SEM results confirm that copper introducing not only influence the crystal feature of the synthesized oxides slightly when the molar ration of Cu/Mn is under 0.4419, but also it definitely presents a significant variation of oxide surface morphology. In addition, EDS patterns and element mapping images in Figs. S5-6, 3e, 4e, S3e, and S4e displayed the presence of transition-metal copper ions in doped MnO2@diatomite and element in samples distributed uniformly. At the same time, ratio of K+ decrease with the increase content of Cu doping, which is in accord with observations of ICP analysis.
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3.1.5. The density functional theory calculations of CuMnO2@Diatomite All observations above indicated Cu-doped MnO2 nanosheets@diatomite prepared resoundingly via a simple one-step hydrothermal method. Fig. 5b illustrated density of states (DOS) of intercalation Cu doped d-MnO2. It could be discovered Fermi energy lever obtained the conduction band minimum (CBM) compared with sample without doping Cu (Fig. 5a). Furthermore, some impurity peaks were brought in near the conduction-band minimum and valance-band maximum (VBM), which bring out band gap narrow down. In which, the black line stands for the spin up and the red line means the spin down (Fig. 5b). Looking at the graph of electron density difference (Fig. 5c), the path of electron transmission has been changed on account of the intercalated Cu ion. We can see the electron firstly aggregate to Cu and then diffuse to other atoms, which may cause lattice contraction. It is in accord with observations of XRD analysis. All these indicated that capacity of electronic transmission of d-MnO2 promoted by Cu doping.
3.2. Fenton-like performance of the as-prepared 0.02-CuMnD Based on the first principal calculation, we surmised Cu ions doped into interlayer of d-MnO2 and oxidation capacity of Cu doped d-MnO2@Diatomite should have an improvement. In order to prove the assumption, we researched degradation performance of Cu doped MnO2@Diatomite as catalyst for MB degradation in a Fenton-like system.
3.2.1. Influence of pH for degradation The pH value regarded as a significant influence factor for removal of MB dye with MnO2. Plentiful researches have illustrated that acidic condition is beneficial to generate hydroxyl radicals for H2O2 in Fenton-like system [52,53]. In our research, the influence of pH on absorption of MB in the scope of pH 2.06 to 10.50 without adding H2O2 was researched. As illustrated in Fig. S7, the MB removal efficiency obtained the especially maximum value when pH is 2.06, which is 39% within 120 min. Nevertheless, there displayed a tremendous recession of the MB removal efficiency as the pH increases from 3.73 to 10.50 compared to the pH of 2.06 which is about 10%, and remains relatively stable. We can see from the inset of Fig. 6, the adsorption of MB with 0.02-25-CuMnD performed very fast within 10 min and finally achieved equilibrium no more than 60 min with pH = 2.06. Meanwhile, Fig. S8 showed the UV–vis absorbance spectra and times profiles of MB adsorption with pH range of 3.73–10.50 indicating the inferior adsorption effect. When the pH is 2.06, the adsorption efficiency is better than others.
Fig. 6. (a) Time profile of the degradation of MB dye with 0.02-25-CuMnD under different pH. (b) Effect of pH on removal efficiency of the MB dye with 0.02-25-CuMnD at 30 min after H2O2 dosing. (Conditions: MB dye concentration of 20 mg L1; dosage of composite of 0.1 g L1; the H2O2 dosage of 16.5 mM; control temperature: 303 K.)
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Fig. 7. (a) Time profile of the degradation of MB dye. (b) The kinetic curve of MB degradation. (c) Effect of H2O2 dosage on the removal efficiency of the MB dye. (d) UV–vis absorbance spectra of MB dye solution after different time intervals with 0.02-25-CuMnD with16.5 mM of H2O2. (Conditions: MB dye concentration of 20 mg L1; dosage of composite of 0.1 g L1; adsorption time of 4 h; control temperature: 303 K.)
We also conducted the degradation of MB with 25-CuMnD at different conditions of pH seen in Fig. 6. It proved that the pH has greatly influence on the adsorption and degradation of MB, and the acidic environment is more conducive to MB adsorption and decomposition. After adding H2O2 for 30 min, gradually reach the degradation equilibrium. It’s obviously that the MB removal efficiency under pH 2.06 obtain the maximum value which is 98%. On the other hand, we found that the removal efficiency of MB is greatly increased after adding H2O2 compared to absorption only. So we chose the pH of 2.06 for the next experiments. 3.2.2. Effect of H2O2 dosage for degradation We also investigated the effect of H2O2 dosage for MB degradation via adjusting the content of H2O2 from 3.3 mM to 33 mM and consequences exhibited in Fig. 7 and Fig. S9. It is illustrated that the degradation efficiency of MB grows from 50.6% to 96.2% after 4 h as the content of H2O2 varies from 3.3 mM to 16.5 mM. The improvement of H2O2 content bring about more output of hydroxide free radicals. Nevertheless, there is a nose dive of MB removal efficiency from 96.2% to 58.5% along with the H2O2 dosage grow from 16.5 mM to 23.1 mM. And then a slight reduction appears with the H2O2 dosage continue to increase to 33 mM. Therefore, H2O2 dosage of 16.5 mM may be speculated the most effective in the MB removal process. Fig. 7b reveals the kinetic curve of MB degradation with different H2O2 contents. The removal of MB consists with the first-order kinetics, accounting via the equation of ln (Ct/C0) = kt, in which value of k means rate constant of dynamics. The rate constant of dynamics was 1.329 h1 with H2O2 content of 16.5 mM, which absolutely exceeded 0.017 h1 as H2O2 content was 23.1 mM. What discovered could be illustrated as follows:
HO + H2 O2 ! HO2 + H2 O
ð4Þ
Mn4þ /Cu2þ + HO2 ! Mn3þ /Cuþ + Hþ + O2
ð5Þ
Perhydroxyl radicals c(HO2) (Eq. (4)) whose oxidation capacity was inferior to hydroxyl radical was appeared due to superfluous H2O2 and hydroxyl radicals [54]. Redox process between manganese/copper ion and perhydroxyl radicals occurred (Eq. (5)), which bring out an excessive consumption of hydrogen peroxide, leading the removal efficiency of MB declined with H2O2 content varied from 16.5 mM to 23.1 mM and 33 mM. Looking at Fig. 7d and Fig. S9, characteristic peaks at 654 nm, 628 nm, 613 nm, 601 nm in accord with intermediates Azure B, Azure A, Azure C and Thionin, respectively [55,56]. Fig. S9(f) illustrated the photos of the color variation of MB solution with various H2O2 dosage after degradation by 0.02-25-CuMnD at 60 min and 180 min, demonstrating the nearly colour fading of the MB molecules with H2O2 of 16.5 mM after degradation time of 180 min. 3.2.3. The Fenton-like reaction of Cu-MnO2@diatomite Fig. S10 and Fig. 8 illustrated the time-dependent MB degradation with MnO2@diatomite doped with different content of copper when concentration of potassium permanganate is 0.02 M in the presence of H2O2. We researched the adsorption performance of CuMnD with different Cu-doping content systematically (Fig. S10). All the results illustrated that the samples reached the degradation equilibrium practically after absorbing 10 min. And when the pH under 5.02, the promotion effect of adsorption removal of MB is remarkable. It is interesting that after absorbing 120 min, mixed H2O2 didn’t change the adsorption equilibrium and the degradation efficiency didn’t be improved. It indicated that adsorption capacity of CuMnD samples are strong enough. When molar ratio of Cu/Mn was no more than 0.4402, the degradation of CuMnD for MB increased with the content of Cu increased. It may because the formation of oxygen defect generating more surface hydroxyl groups as reaction sites for surface adsorption [57– 60]. The degradation of MB with 25-CuMnD displays the best capacity than other catalysts. According to the degradation results of 0-CuMnD (Fig. 8), the degradation of MB reaches the maximum
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OH + MB (or MBþ ) ! intermediate ! degraded products ðCO2 þ H2 O þ smallgroupsÞ
Fig. 8. Degradation of MB (20 mg L1) by using different CuMnD (0.02 M). Conditions: catalyst dosage of 0.1 g L1; the H2O2 dosage of 16.5 mM.
value on account of adsorption before H2O2 dosing, and then shows a slight desorption after H2O2 dosing 10 min, finally achieves degradation equilibrium. Actually, when the molar ratio of Cu/ Mn is beyond 0.4402, the generated Cu4(OH)6SO4 which is insoluble in water may weaken the degradation efficiency of MB slightly compared to 25-CuMnD. Comparing the results of 25CuMnD with 0-CuMnD, the adsorption capacity of 25-CuMnD in ten minutes before H2O2 dosing is obvious lower than that of 0CuMnD. Speculation that it is because the copper doping changed the surface of CuMnD and the covered denser MnO2 nanosheets finally restrain the adsorption capacity. After adding H2O2, the degradation efficiency has been improved dramatically because of the strong oxidizing ability of hydroxyl radical (OH) which can mineralize the MB into small molecules. As degradation time expansion to 20 min and 30 min after H2O2 dosing, the results shows a slight desorption and then continue oxidative degradation finally achieve equilibrium at 120 min and obtain the MB removal efficiency of 96.2% after 4 h. The detail oxidative degradation process of MB dye by H2O2 can be expressed as follows:
MB + Mn4þ /Cu2þ ! MB* + Mn3þ /Cuþ
ð6Þ
MB* + Mn4þ /Cu2þ ! MBþ + Mn3þ /Cuþ
ð7Þ
H2 O2 ! OH + OH
ð8Þ
Mn3þ /Cuþ + H2 O2 ! Mn4þ /Cu2þ + OH + OH
ð9Þ
473
ð10Þ
To provide more convincing evidence, we conducted the ESR measurement with spectra shown in Fig. 9(a). It is clear to see that the samples show a symmetrical ESR signal at g = 2.001, which is typically ascribed to the existence of oxygen vacancies. So it indicates that oxygen vacancies are already present on 0-CuMnD and 25-CuMnD. However, When 25% Cu was added the signal becomes obviously stronger, which illustrates the increased amount of oxygen vacancies. Furthermore, we conducted the ESR spectroscopy of 0-CuMnD and 25-CuMnD using DMPO as spin trapping agent. Thereafter, the ESR spectra illustrates that more. O–2 radicals are produced by Cu intercalation and provide further evidence for oxygen vacancy formation. On the other hand, the ESR spectra display that the concentration of hydroxyl radicals (OH) are also increased for the 25-CuMnD sample. This is consistent with the Fenton-like reaction results that the proper copper doping of sample is more easily to form oxygen defect, which generate more surface hydroxyl groups as reaction sites for surface adsorption and degradation. To gain a deeper understanding of above experimental investigations and to research the effect of Cu-intercalation, this paper also conducted the bond length of H2O2 and electron density difference with H2O2 of 0.02-CuMnD before and after Cu-intercalation to characterize the materials’ oxidation capacity for H2O2 (Fig. 10). As illustrated in Fig. 10a and b, the bond length of H2O2 inside 0.02CuMnD before and after Cu-intercalation is 1.376 Å and 1.431 Å, respectively, indicating the Cu-intercalation can increase the band length of H2O2. Therefore, the bond of H2O2 is more likely to dissociate and generate hydroxyl radical (OH) to oxidize the organic dye into micro-molecules in Cu-intercalation system. The electron density difference of 0.02-CuMnD with H2O2 (Fig. 10c, d) showed the change of path for electron transmission. It is expected that the electrons transfer to H2O2 from Cu atoms and rupture more easily than samples without Cu doping. Particularly worth mentioning is the results of calculations absolutely conform to the experimental conclusions. 3.2.4. Possible mechanism As shown in Scheme 2, a degradation mechanism for MB with 25-CuMnD can be speculated. Firstly, the MB is absorbed on surface of CuMnD via surface hydroxyl group and Mn4+/Cu2+. Afterwards, MB occurs surface oxidation because of Mn4+/Cu2+ and generates Mn3+/ Cu+ (Eqs. (6) and (7)). Secondly, H2O2 decomposes by means of two approaches. At first, the H2O2 self-decomposition to generate OH and OH which can degrade MB (Eq. (8)). The second approach is H2O2 decomposed by catalytic decomposition via Mn3+/Cu+ to generate Mn4+/Cu2+ and OH. In the third step, the MB and dye cations are oxidized by OH, generating degraded products
Fig. 9. (a) ESR spectra of 0-CuMnD and 25-CuMnD sample; (b,c) ESR spectra of 0-CuMnD and 25-CuMnD sample, respectively, under visible light illumination using DMPO as electron spin trapping agent.
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Fig. 10. (a, b) The bond length of H2O2 inside 0.02-CuMnD before and after Cu-intercalation. (c, d) Graph of the electron density difference of 0.02-CuMnD with H2O2 before and after Cu-intercalation.
Scheme 2. Possible oxidation reaction mechanism of CuMnD Fenton-like process.
which contain CO2, H2O and other small groups (Eq. (10)). Then because of the redox reaction between Mn4+/Cu2+ and Mn3+/Cu+, the decomposition of H2O2 restarted and then the degradation of MB continued. Therefore, in whole degradation process of MB,
the oxidation and adsorption supplemented each other and improved the general removal efficiency. It should be noted that the doped copper acted as important role to decompose the H2O2 in the overall process.
Y. Xiao et al. / Journal of Colloid and Interface Science 556 (2019) 466–475
4. Conclusion In summary, copper doping MnO2 covered disk-shaped diatomite was successfully synthesized and characterized. Moreover, the Cu-MnO2@diatomite provided an adsorption platform and 0.02-25-CuMnD was verified be an excellent catalyst for Fentonlike reaction of MB oxidation. The degradation ability for MB improved obviously via an appropriate copper doping content. That was illustrated by both theoretic arithmetic and experiment. Because of copper doping, the catalyst is more easily to form oxygen defect generating more surface hydroxyl groups as active reaction sites for surface adsorption and dissociate H2O2 so that generate hydroxyl radical (OH) to oxidize the MB. The degradation efficiency of CuMnD was tremendously influenced by the initial pH, H2O2dosage and copper content of catalyst. For 0.02-25CuMnD along with molar ration of Cu/Mn with 0.4402 showed the best degradation efficiency which was about 96.2% within 4 h with16.5 mM of H2O2 and pH 2.06. In addition, the Cu doped MnO2@diatomite with proper copper content is anticipated to be a catalyst candidate with great promise for waste water treatment on account of its high efficiency. Acknowledgements The authors gratefully appreciate the financial supports supported by the Fundamental Research Funds for the Central Universities (2018CDYJSY0055 and 2019CDQYCL042), the National Natural Science Foundation of China (Grant no. 21576034), Joint Funds of the National Natural Science Foundation of ChinaGuangdong (Grant no. U1801254), the project funded by Chongqing Special Postdoctoral Science Foundation (XmT2018043), Chongqing University Postgraduates’ Innovation Project (No. CYB16014 and CYS17003), Natural Science Foundation Project of CQ CSTC (cstc2017jcyjBX0028), Technological projects of Chongqing Municipal Education Commission (KJZDK201800801), the Innovative Research Team of Chongqing (CXTDG201602014), the State Education Ministry and Fundamental Research Funds for the Central Universities (106112016CDJZR135506 and 106112017CDJXSYY0001), the Science and Technology Innovation Talents Support Program of Chongqing (CSTCCXLJRC201706), and the Youth Innovation Promotion Association of CAS (2015316). The authors would like to especially thanks to the Electron Microscopy Center of Chongqing University for materials characterizations. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.08.082. References [1] Y. He, B. Jiang, Y. Jiang, J. Chen, Y.X. Zhang, J. Hazard. Mater. 344 (2018) 230– 240. [2] Y. He, B. Jiang, J. Chen, Y. Jiang, Y.X. Zhang, J. Colloid Interface Sci. 510 (2018) 207–220. [3] H.H. Peng, J. Chen, Y. Jiang, M. Li, L. Feng, D. Losic, F. Dong, Y.X. Zhang, J. Colloid Interface Sci. 484 (2016) 1–9. [4] M. Khaksar, M. Amini, D.M. Boghaei, K.H. Chae, S. Gautam, Catal. Commun. 72 (2015) 1–5. [5] P.L. dos Santos, I.R. Guimarães, A.M. Mesquita, M.C. Guerreiro, J. Mol. Catal. AChem. 424 (2016) 194–202. [6] Y.X. Zhang, X.D. Hao, F. Li, Z.P. Diao, Z.Y. Guo, J. Li, Ind. Eng. Chem. Res. 53 (17) (2014) 6966–6977. [7] A.F. Danil de Namor, A. El Gamouz, S. Frangie, V. Martinez, L. Valiente, O.A. Webb, J. Hazard. Mater. 241–242 (2012) 14–31. [8] N. Inchaurrondo, J. Font, C.P. Ramos, P. Haure, Appl. Catal. B-Environ. 181 (2016) 481–494. [9] Y.S. Al-Degs, M.F. Tutunju, R.A. Shawabkeh, Separ. Sci. Technol. 35 (14) (2000) 2299–2310.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
One-step hydrothermal synthesis of Cu-doped MnO2 coated diatomite for degradation of methylene blue in Fenton-like system Yu Xiao a,b,1, Wangchen Huo a,1, Shaoning Yin a, Debin Jiang a, Yuxin Zhang a,⇑, Zhiqiang Zhang a,⇑, Xiaoying Liu c, Fan Dong d, Jinshu Wang e, Gang Li f, Xuebu Hu f, Xiaoya Yuan g, Hong-Chang Yao h a
State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China Chongqing Institute of Modern Construction Industry Development, Chongqing 400060, PR China Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, PR China d Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, PR China e School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, PR China f College of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, PR China g College of Materials Science and Engineering, Chongqing Jiaotong University, Chongqing 400074, PR China h College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou City, Henan Province 450001, PR China b c
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 7 May 2019 Revised 22 August 2019 Accepted 22 August 2019 Available online 24 August 2019 Keywords: Cu doped MnO2@diatomite MB degradation Fenton-like reaction Nanocomposites Density functional theory
a b s t r a c t In this work, we have synthesized Cu-doped MnO2@diatomite successfully though a one-step hydrothermal approach. Meanwhile, application for degradation of methylene blue in Fenton-like system was investigated. The compounds were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), Inductively Coupled Plasma analysis (ICP) and UV–vis spectroscopy measurements, beam scanning electron microscope (FIB/SEM), energy dispersive X-ray spectrometer (EDS). The observations revealed that copper was indeed intercalated into layered structure of MnO2 and Density functional theory (DFT) calculations predicted that Cu2+ intercalated MnO2@diatomite brought about the narrowing of band gap and the enhancing of charge mobility during catalysis. Electron Density Difference of CuMnD demonstrated excellent oxidation ability to dissociate H2O2 and generate hydroxyl radical (OH) to degrade the MB. Moreover, the proper copper doping of sample is more easily to form oxygen defect, which generate more surface hydroxyl groups as reaction sites for surface adsorption. In addition, the degradation efficiency of CuMnD was tremendously influenced by the initial pH, H2O2 dosage and copper
⇑ Corresponding authors. 1
E-mail addresses: [email protected] (Y. Zhang), [email protected] (Z. Zhang). The authors equally contributed this work.
https://doi.org/10.1016/j.jcis.2019.08.082 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Y. Xiao et al. / Journal of Colloid and Interface Science 556 (2019) 466–475
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content of catalyst. Ultimately, 0.02-25-CuMnD along with molar ration of Cu/Mn with 0.4402 showed the best degradation efficiency which was about 96.2% within 4 h with 16.5 mM of H2O2 and pH 2.06. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
2.1. Synthesis of Cu-doped MnO2 @diatomite
Waste water treatment has attracted great attention because it contains a good deal of heavy metals, organic dye and other chromophoric groups, which would pollute the environment and threat peoples’ life. Methylene blue (MB) as a familiar cationic phenothiazine dye comes from textile, printing and leather industries [1,2]. Various techniques, especially adsorption and catalytic oxidation have been wildly utilized to remove MB from wastewater on account of low energy requirement and easy operation [1–5]. However, MB may not be degraded completely by adsorption, moreover, some adsorbents will illustrate low adsorption efficiency and long adsorption time [2]. What’s more, there is still a challenge to discovery low-cost, high-efficiency and harmless materials, meanwhile to expound the actual removal mechanism of mixtures for different contaminants in practical applications [3]. Diatomite has been employed as a promising templates for the porous compounds, due to its high porosity, low density, and excellent chemical durability [6–8]. They are also utilized for adsorbing organic dye sand removing heavy metal [7,9]. However, diatomite individual used as a catalyst for wastewater was seldom reported because of its own boundedness. Hence, many researches have introduced Mn/MnO2 [9–13], Fe/ferrihydrite [14,15], TiO2 [16–18] and NiO [19] to modify the surface and properties of diatomite. Among them, MnO2 has been one of most popular candidates on account of its’ unique layers or tunnels structure, high economy efficiency, abundance and high redox potential [20]. It is known that doping nanoparticles can improve conductivity and capacitance of MnO2 effectively. Hence, many researchers have endeavored to dope metals or metal oxides into MnO2. Doping candidates include V [21], Cu/CuO [22–26], Al [26,27], ZnO [28], Co [29–31], and Fe [29,31,32]. What’s more, the catalytic performance of MnO2further promoted via doping other active components. For example, Gao et al. synthesized transition metal-doped MnO2 for CO oxidation catalysts, which showed much enhanced CO oxidation activity [33–36]. Zhang et al. incorporated Cu into MnO2 exhibiting admirable catalytic activity for benzotriazole in the Fenton-like system [37]. McKendry et al. doped cobalt into layered manganese oxide compounds which enhanced water oxidation capacity [38]. Among them, copper cation might be a prime candidate on account of its low coat and environmentally benign nature [39]. Nevertheless, for all we know, the research concerning the synthesis of copper-doped MnO2@Diatomite for a catalytic degradation for MB in a Fenton-like system seldom been reported. The objective of this work was to synthesis a catalyst copperdoped MnO2@Diatomite (CuMnD) with different morphologies by adjusting the concentration of KMnO4 and the molar ratio of Cu/ Mn to promote the property of degradation of MB in a Fentonlike process. We discuss how the Cu dopant could activate the H2O2 for catalytic oxidation elaborately. DFT calculations conform to our experimental observations and supply insight into the role of Cu in enhanced MnO2@Diatomite activity.
Method for synthesizing CuMnD and crystal structure of CuMnO2 are schematically demonstrated in Scheme 1. Before our hydrothermal process, the diatomite should first be purified through the reported chemical method [40]. In a typical synthesis, disperse 50 mg of the treated diatomite into 70 mL of KMnO4 (0.02 M) solution and magnetic stirring to obtain uniform solution. Concentration of doping copper sulfate pentahydrate was 0, 1, 3, 5, 7, 10, 15, 20, 25 at.%, representing the molar ratio of Cu to Mn in the raw materials. Secondly, certain content of copper sulfate pentahydrate mixed to mixture then stirred it for 20 min to form homogeneous suspension. After that, the mixture were poured into a 100 mL Teflon-lined stainless steel autoclave, sealed and maintained at 160 °C for 12 h under one-step hydrothermal synthesis. Finally, the as-synthesized sample was thoroughly rinsed with distilled water, then dried at 60 °C for 12 h. For simplicity, composites designated as 0.02-0-CuMnD, 0.02-1-CuMnD, 0.02-3-CuMnD, 0.025-CuMnD, 0.02-7-CuMnD, 0.02-10-CuMnD, 0.02-15-CuMnD, 0.0220-CuMnD and 0.02-25-CuMnD, respectively. For comparison, KMnO4 with other concentration (0.01 M and 0.04 M) were also synthesized and researched.
2. Materials and experimental methods The original diatomite employed in this paper bought from Tianjin Damao Chemical Reagent Company. All chemical reagents were analytical grade and employed without further treatment.
2.2. Fenton-like process of Cu-doped MnO2@diatomite The catalytic capacity of CuMnD estimated by degrading MB. We adjusted the pH value of the MB solution via 0.1 M HCl from initial value to 2.06 before the degradation reaction. In a typical process, add the proper CuMnD into a 250 mL glass beaker with 100 mL of known MB solution, after that continuous stir it at 303 K for 10 min to obtain adsorption equilibrium of MB. Afterwards, add a certain concentration of H2O2 to the system to activate the Fenton-like reaction. The concentrations of MB solutions before and after degradation were tested using a TU-1901 spectrophotometer (Model UV Win5.1.0) with wavelength at 663 nm. The catalytic efficiency for removal of MB could be evaluated via Eq. (1).
MB Remov al ð%Þ ¼
C0 Ct 100% C0
ð1Þ
where C0 and Ct (mg L1) refer to the initial concentration of MB and concentration at time t, respectively. Moreover, effects of pH (2.06– 10.50), H2O2 amount (3.3–33 mM) on the Fenton-like reaction of CuMnD and the different content of cooper doped MnO2@Diatomite were researched for optimizing the experimental condition. 2.3. Materials characterization The crystal structure and chemical composition information of synthesized CuMnD were measured by powder X-ray diffraction at a scan rate of 4° min1 from 5 to 80° (XRD, D/max 2500, Cu Ka). Kratos Axis Ultra X-ray photoelectron spectroscope (XPS, Al Ka source) was employed for investigating the surface performance. The morphological investigations were accomplished via focused ion beam scanning electron microscope (Zeiss Auriga FIB/SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The composition of the samples were established by ICP analysis. The oxygen defect the samples were measured by ESR.
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Scheme 1. Schematic illustration of the synthesis of Cu-MnO2@Diatomite (the magnification images show the structure of pores and the crystal structure of Cu-MnO2).
2.4. Density functional theory calculations Density functional theory (DFT) [41,42] were calculated rely on the Vienna ab-initio simulation package (VASP5.4) [43]. The projector augmented wave (PAW) method was employed to connect the core-valence, and generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) [44] was applied to achieve exchange-correlation. A plane-wave basis set with cutoff energy 500 eV [45] and the 7 11 5 Monkhorst Pack grid samples were utilized, and the force of atom convergence was set as 0.01 eV/Å. A 72-atom 2 3 2 super cell of bulk MnO6 was firstly relaxed, then Cu atoms were introduced and the sites of defects were shown in Fig. 1. The energy of defect formation (DE) is defined as follows
DE ¼ Ed 12Ep ECu
ð2Þ
Or DE ¼ Ed 12Ep ECu EMn
ð3Þ
where Ed , EP , ECu and EMn stand for the total energy of MnO2 with Cuintercalated defects, pure MnO2 and Cu species, respectively. 3. Results and discussion 3.1. Characterization of CuMnD 3.1.1. The crystal feature of the as-synthesized CuMnD Fig. 1a and S1 present the constituent and crystallite feature of the CuMnD seen from X-ray diffraction pattern. The strong diffraction peaks are observed at 21.9°, 28.3°, 31.2°, and 36.0◦, could be respectively assigned to (1 0 1), (1 1 1), (1 0 2), and (2 0 0) facets, planes of crystalline SiO2 with a cristobalite structure (JCPDS Card No. 76–0938), indicating an excellent crystallinity. In addition, we discovered from the XRD spectra of MnO2-covered diatomite (0CuMnD) that diffraction peaks at about 12.3°, 24.8° and 65.5° are nearly conform to the standard XRD spectrum of potassium manganese oxide hydrate crystal (JCPDS Card No. 86-0666). There are without redundant diffraction peaks originating from the copper species in the XRD spectra when the content of Cu is low, indicating the purity of as-synthesized MnO2@diatomite samples and uniform doping of Cu cations in layers of d-MnO2. However, when molar ration of Cu/Mn in deposited oxides is beyond 0.4402 (Table S1), the new XRD diffraction peaks of samples at 13.9°, 16.6°, 28.0° are assigned to (0 0 2), (0 1 2), (0 0 4), conform to the
Fig. 1. (a) XRD patterns of 0.02 M MnO2@diatomite with Cu (CuMnD). (b) The crystal structure of 0.02-0-CuMnD. (c, d) The crystal structure of 0.02-25-CuMnD with intercalation and substitution, respectively.
standard XRD spectra of Cu4(OH)6SO4 with brochantite structure (JCPDS Card No. 85-1316). Generally, the dopant atoms introduced into crystal lattice would change lattice parameter. Based on the Bragg equation, we observed that XRD peaks from 8.5 to 13.5° (Fig. S2) transfer slightly to a bigger angle after Cu ions were intercalated into MnO2, revealing that metal ions introduced into lattice and generated lattice contraction. For interlayer structure of d-MnO2 crystals, Cu2+ in solution could easily inserted into interlayer space of d-MnO2. Besides, certain Cu2+ in system solution introduced into MnO2 and substituted the Mn to generate Cu-substituted MnO2. Hence, Cu-doped dMnO2 would with two possible configurations: Cu-intercalated
Y. Xiao et al. / Journal of Colloid and Interface Science 556 (2019) 466–475 Table 1 Binding energies of Cu atom occupied in different d-MnO2 sites. Cu-intercalated d-MnO2
Cu-substituted d-MnO2
7.6799 eV
4.9608 eV
and Cu-substituted d-MnO2. To gain an atomic-scale understanding of experimental phenomena and to research impact of Cu doping, we studied electronic structures of these two possible configurations by means of the first principle calculations: Cuintercalated (Fig. 1c) and Cu-substituted d-MnO2 (Fig. 1d). Meanwhile, Fig. 1b revealed the crystal structure of 0.02-0-CuMnD. We investigated the formation energy of Cu atoms in different sites to discuss the Cu doping in d-MnO2. The calculated formation energy is following the Eq. (2) or (3), and listed in Table 1. It could be discovered Cu-intercalated d-MnO2 is more stable to seize Cu atom powerfully. And it’s difficult to come into being Cusubstituted d-MnO2. Furthermore, the lattice parameter of dMnO2 calculated is a = 4.83, b = 2.77, c = 7.31, respectively. Nevertheless, it caused lattice contraction after Cu-intercalated with lattice constant of a = 4.69, b = 2.73 and c = 5.60. It is indicated that the results of calculations accord with the XRD.
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3.1.2. The elemental analyses using ICP The existences of doping element Cu and the atomic rations of Cu, K and Mn for various Cu-doped MnO2@diatomite were detected by ICP spectrometer and illustrated in Table S1. In this research, the existence of K+ is speculated to make valence balance and therefore, insure the crystal structures of MnO2 stable [29]. Compared with 0-CuMnD, the K/Mn molar rations in all CuMnD samples have a great decrease. It is noteworthy that as the Cu/ Mn molar ration increases, the K/Mn molar ration decreases. This may illustrate that portion copper ions may be introduced into interlamination of d-MnO2 crystals to substitute potassium and obtain stable structure of d-MnO2. Other copper ions might exhibit on surface ofd-MnO2@Diatomite. In addition, the Cu/Mn molar rations of the samples 0.02-5-CuMnD, 0.02-10-CuMnD, 0.02-15CuMnD, 0.02-20-CuMnD are more than 0.4402, which may generate copper compound to alter inner structure of the d-MnO2. The results also conform to the XRD patterns. 3.1.3. X-ray photoelectron spectroscopy The surface constituents and valence states of elements of CuMnD (0.02 M) samples were detected by XPS (Fig. 2). The results from XPS illustrated in Table S2. From Fig. 2a, Si, C, K, O and Mn can be found in the survey scan of 0-CuMnD. With the increasing of
Fig. 2. XPS spectra of CuMnD (0.02 M): (a) survey scan, (b) O1s, (c) Cu2p, (d) Mn2p, (e) K2p and (f) Si2p.
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Fig. 3. Characterization of the 0.02-0-CuMnD. (a, b) Typical SEM image; (c) element mappings of O, Si, Mn, and K.
Fig. 4. Characterization of the 0.02-25-CuMnD. (a, b) Typical SEM image; (c) element mappings of O, Si, Mn, Cu and K.
Fig. 5. (a, b) Density of states of sample 0.02-0-CuMnD and 0.02-25-CuMnD with intercalation. (c) Electron density difference of 0.02-25-CuMnD with intercalation.
practical molar ratio of Cu/Mn, Cu can be observed but K decreases, finally disappear. It is an evidence that the partial copper doing may substitute potassium. Fig. 2b illustrated O 1s spectra, peaks at 530.0, 531.1 and 532.9 eV are in accord with binding energies of lattice oxygen, surface hydroxyl group and O–Si from diatomite structure, respectively [46,47]. 25-CuMnD shows a weaker lattice oxygen peak and stronger surface hydroxyl group than 0-CuMnD, illustrating copper intercalation provided more oxygen defect generating more surface hydroxyl groups which was effective active sites for surface adsorption [48] and catalytic oxidation [49]. Seen
the Cu 2p spectrum (Fig. 2c), two obvious peaks at 934.1 and 953.9 eV were in accord with Cu 2p3/2 and Cu 2p1/2. What’s more, the presence of Cu2+ could be further verified by a satellite located at 962.2 eV and a strong satellite line between 940.0 and 945.5 eV [37]. The Mn 2p spectra (Fig. 2d) illustrated two distinct peaks at 654.4 and 642.6 eV, stand for Mn 2p1/2 and Mn 2p3/2, respectively. There are two peaks at 642.2 and 643.8 eV prove the existence of Mn3+ and Mn4+ on the surface, respectively [33,50]. What’s more, the atomic percent of Mn4+/Mn3+ were 0.4550, 0.3673, 0.3174 and 0.4692 on the surface of 0-CuMnD, 3-CuMnD, 25-CuMnD,
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and 15-CuMnD, respectively (Table S2). Therefore, the copper can influence the ratio of the species of Mn. It also illustrated that the atomic percent of Cu/Mn increased from 0 to 0.4123 and as it increased, the Mn4+/Mn3+ firstly decreased and then increased. In addition, two peaks were detected according to Fig. 2e, which locked at 292.8 and 295.7 eV associating to binding energies of K 2p1/2 and K 2p3/2 of weak potassium peaks [51]. However, the K 2p fade away along with the augment of Cu. So that the atomic percent of K/Mn decreased from 0.3663 to 0. It is absolutely that the laws of XPS results are accordance with the ICP analysis. Furthermore, Fig. 2f showed the photoelectron peaks of Si 2p and only one strong peak at 103.7 eV was observed, conforming to characteristic of siloxane groups (Si–O–Si) of diatomite [46].
3.1.4. The morphology of as-prepared Cu-MnO2@Diatomite The morphology of the samples were examined via SEM. Morphology and microstructure of 0.02-0-CuMnDwere shown in Fig. 3a and b. It can be seen the unabridged diatomite is diskshaped with diameter of 30 lm. The sizes of macrospores arrays are almost 0.5 lm, which distribute in whole diatomite. The special pore structure may enable effective MB solution contact the deposited oxide. Figs. 3 and 4 and Figs. S3–S4 show the images of Cu-doped MnO2 covered on the diatomite with different concentration of KMnO4. The pictures illustrate that the Cu doping markedly affect morphology of d-MnO2 film. Fig. 3a and b illustrate the typical structure of the undoped d-MnO2 film with 0.02 M KMnO4, it can be seen considerable fine grains and few nanosheets. When 25 at. % Cu is doped into it, the film consists of numerous tiny nanosheets and grow homogenously on the skeletons of diatomite (Fig. 4a, b). Moreover, the concentration of KMnO4 greatly influence the morphology of CuMnD. These numerous greater nanosheets interknit to form network in sample 0.04-0-CuMnD when the concentration of KMnO4 increased to 0.04 M (Fig. S3a, b). Furthermore, an increasing number of nanosheets generate, some them interconnect to construct flower-like structure to spread on/in diatomite as doping concentration of Cu increasing to 25 at. % (Fig. S4a, b). Therefore, the presence of copper may make tremendous impact on self-assembly of d-MnO2 nanosheets [24]. SEM results confirm that copper introducing not only influence the crystal feature of the synthesized oxides slightly when the molar ration of Cu/Mn is under 0.4419, but also it definitely presents a significant variation of oxide surface morphology. In addition, EDS patterns and element mapping images in Figs. S5-6, 3e, 4e, S3e, and S4e displayed the presence of transition-metal copper ions in doped MnO2@diatomite and element in samples distributed uniformly. At the same time, ratio of K+ decrease with the increase content of Cu doping, which is in accord with observations of ICP analysis.
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3.1.5. The density functional theory calculations of CuMnO2@Diatomite All observations above indicated Cu-doped MnO2 nanosheets@diatomite prepared resoundingly via a simple one-step hydrothermal method. Fig. 5b illustrated density of states (DOS) of intercalation Cu doped d-MnO2. It could be discovered Fermi energy lever obtained the conduction band minimum (CBM) compared with sample without doping Cu (Fig. 5a). Furthermore, some impurity peaks were brought in near the conduction-band minimum and valance-band maximum (VBM), which bring out band gap narrow down. In which, the black line stands for the spin up and the red line means the spin down (Fig. 5b). Looking at the graph of electron density difference (Fig. 5c), the path of electron transmission has been changed on account of the intercalated Cu ion. We can see the electron firstly aggregate to Cu and then diffuse to other atoms, which may cause lattice contraction. It is in accord with observations of XRD analysis. All these indicated that capacity of electronic transmission of d-MnO2 promoted by Cu doping.
3.2. Fenton-like performance of the as-prepared 0.02-CuMnD Based on the first principal calculation, we surmised Cu ions doped into interlayer of d-MnO2 and oxidation capacity of Cu doped d-MnO2@Diatomite should have an improvement. In order to prove the assumption, we researched degradation performance of Cu doped MnO2@Diatomite as catalyst for MB degradation in a Fenton-like system.
3.2.1. Influence of pH for degradation The pH value regarded as a significant influence factor for removal of MB dye with MnO2. Plentiful researches have illustrated that acidic condition is beneficial to generate hydroxyl radicals for H2O2 in Fenton-like system [52,53]. In our research, the influence of pH on absorption of MB in the scope of pH 2.06 to 10.50 without adding H2O2 was researched. As illustrated in Fig. S7, the MB removal efficiency obtained the especially maximum value when pH is 2.06, which is 39% within 120 min. Nevertheless, there displayed a tremendous recession of the MB removal efficiency as the pH increases from 3.73 to 10.50 compared to the pH of 2.06 which is about 10%, and remains relatively stable. We can see from the inset of Fig. 6, the adsorption of MB with 0.02-25-CuMnD performed very fast within 10 min and finally achieved equilibrium no more than 60 min with pH = 2.06. Meanwhile, Fig. S8 showed the UV–vis absorbance spectra and times profiles of MB adsorption with pH range of 3.73–10.50 indicating the inferior adsorption effect. When the pH is 2.06, the adsorption efficiency is better than others.
Fig. 6. (a) Time profile of the degradation of MB dye with 0.02-25-CuMnD under different pH. (b) Effect of pH on removal efficiency of the MB dye with 0.02-25-CuMnD at 30 min after H2O2 dosing. (Conditions: MB dye concentration of 20 mg L1; dosage of composite of 0.1 g L1; the H2O2 dosage of 16.5 mM; control temperature: 303 K.)
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Fig. 7. (a) Time profile of the degradation of MB dye. (b) The kinetic curve of MB degradation. (c) Effect of H2O2 dosage on the removal efficiency of the MB dye. (d) UV–vis absorbance spectra of MB dye solution after different time intervals with 0.02-25-CuMnD with16.5 mM of H2O2. (Conditions: MB dye concentration of 20 mg L1; dosage of composite of 0.1 g L1; adsorption time of 4 h; control temperature: 303 K.)
We also conducted the degradation of MB with 25-CuMnD at different conditions of pH seen in Fig. 6. It proved that the pH has greatly influence on the adsorption and degradation of MB, and the acidic environment is more conducive to MB adsorption and decomposition. After adding H2O2 for 30 min, gradually reach the degradation equilibrium. It’s obviously that the MB removal efficiency under pH 2.06 obtain the maximum value which is 98%. On the other hand, we found that the removal efficiency of MB is greatly increased after adding H2O2 compared to absorption only. So we chose the pH of 2.06 for the next experiments. 3.2.2. Effect of H2O2 dosage for degradation We also investigated the effect of H2O2 dosage for MB degradation via adjusting the content of H2O2 from 3.3 mM to 33 mM and consequences exhibited in Fig. 7 and Fig. S9. It is illustrated that the degradation efficiency of MB grows from 50.6% to 96.2% after 4 h as the content of H2O2 varies from 3.3 mM to 16.5 mM. The improvement of H2O2 content bring about more output of hydroxide free radicals. Nevertheless, there is a nose dive of MB removal efficiency from 96.2% to 58.5% along with the H2O2 dosage grow from 16.5 mM to 23.1 mM. And then a slight reduction appears with the H2O2 dosage continue to increase to 33 mM. Therefore, H2O2 dosage of 16.5 mM may be speculated the most effective in the MB removal process. Fig. 7b reveals the kinetic curve of MB degradation with different H2O2 contents. The removal of MB consists with the first-order kinetics, accounting via the equation of ln (Ct/C0) = kt, in which value of k means rate constant of dynamics. The rate constant of dynamics was 1.329 h1 with H2O2 content of 16.5 mM, which absolutely exceeded 0.017 h1 as H2O2 content was 23.1 mM. What discovered could be illustrated as follows:
HO + H2 O2 ! HO2 + H2 O
ð4Þ
Mn4þ /Cu2þ + HO2 ! Mn3þ /Cuþ + Hþ + O2
ð5Þ
Perhydroxyl radicals c(HO2) (Eq. (4)) whose oxidation capacity was inferior to hydroxyl radical was appeared due to superfluous H2O2 and hydroxyl radicals [54]. Redox process between manganese/copper ion and perhydroxyl radicals occurred (Eq. (5)), which bring out an excessive consumption of hydrogen peroxide, leading the removal efficiency of MB declined with H2O2 content varied from 16.5 mM to 23.1 mM and 33 mM. Looking at Fig. 7d and Fig. S9, characteristic peaks at 654 nm, 628 nm, 613 nm, 601 nm in accord with intermediates Azure B, Azure A, Azure C and Thionin, respectively [55,56]. Fig. S9(f) illustrated the photos of the color variation of MB solution with various H2O2 dosage after degradation by 0.02-25-CuMnD at 60 min and 180 min, demonstrating the nearly colour fading of the MB molecules with H2O2 of 16.5 mM after degradation time of 180 min. 3.2.3. The Fenton-like reaction of Cu-MnO2@diatomite Fig. S10 and Fig. 8 illustrated the time-dependent MB degradation with MnO2@diatomite doped with different content of copper when concentration of potassium permanganate is 0.02 M in the presence of H2O2. We researched the adsorption performance of CuMnD with different Cu-doping content systematically (Fig. S10). All the results illustrated that the samples reached the degradation equilibrium practically after absorbing 10 min. And when the pH under 5.02, the promotion effect of adsorption removal of MB is remarkable. It is interesting that after absorbing 120 min, mixed H2O2 didn’t change the adsorption equilibrium and the degradation efficiency didn’t be improved. It indicated that adsorption capacity of CuMnD samples are strong enough. When molar ratio of Cu/Mn was no more than 0.4402, the degradation of CuMnD for MB increased with the content of Cu increased. It may because the formation of oxygen defect generating more surface hydroxyl groups as reaction sites for surface adsorption [57– 60]. The degradation of MB with 25-CuMnD displays the best capacity than other catalysts. According to the degradation results of 0-CuMnD (Fig. 8), the degradation of MB reaches the maximum
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OH + MB (or MBþ ) ! intermediate ! degraded products ðCO2 þ H2 O þ smallgroupsÞ
Fig. 8. Degradation of MB (20 mg L1) by using different CuMnD (0.02 M). Conditions: catalyst dosage of 0.1 g L1; the H2O2 dosage of 16.5 mM.
value on account of adsorption before H2O2 dosing, and then shows a slight desorption after H2O2 dosing 10 min, finally achieves degradation equilibrium. Actually, when the molar ratio of Cu/ Mn is beyond 0.4402, the generated Cu4(OH)6SO4 which is insoluble in water may weaken the degradation efficiency of MB slightly compared to 25-CuMnD. Comparing the results of 25CuMnD with 0-CuMnD, the adsorption capacity of 25-CuMnD in ten minutes before H2O2 dosing is obvious lower than that of 0CuMnD. Speculation that it is because the copper doping changed the surface of CuMnD and the covered denser MnO2 nanosheets finally restrain the adsorption capacity. After adding H2O2, the degradation efficiency has been improved dramatically because of the strong oxidizing ability of hydroxyl radical (OH) which can mineralize the MB into small molecules. As degradation time expansion to 20 min and 30 min after H2O2 dosing, the results shows a slight desorption and then continue oxidative degradation finally achieve equilibrium at 120 min and obtain the MB removal efficiency of 96.2% after 4 h. The detail oxidative degradation process of MB dye by H2O2 can be expressed as follows:
MB + Mn4þ /Cu2þ ! MB* + Mn3þ /Cuþ
ð6Þ
MB* + Mn4þ /Cu2þ ! MBþ + Mn3þ /Cuþ
ð7Þ
H2 O2 ! OH + OH
ð8Þ
Mn3þ /Cuþ + H2 O2 ! Mn4þ /Cu2þ + OH + OH
ð9Þ
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ð10Þ
To provide more convincing evidence, we conducted the ESR measurement with spectra shown in Fig. 9(a). It is clear to see that the samples show a symmetrical ESR signal at g = 2.001, which is typically ascribed to the existence of oxygen vacancies. So it indicates that oxygen vacancies are already present on 0-CuMnD and 25-CuMnD. However, When 25% Cu was added the signal becomes obviously stronger, which illustrates the increased amount of oxygen vacancies. Furthermore, we conducted the ESR spectroscopy of 0-CuMnD and 25-CuMnD using DMPO as spin trapping agent. Thereafter, the ESR spectra illustrates that more. O–2 radicals are produced by Cu intercalation and provide further evidence for oxygen vacancy formation. On the other hand, the ESR spectra display that the concentration of hydroxyl radicals (OH) are also increased for the 25-CuMnD sample. This is consistent with the Fenton-like reaction results that the proper copper doping of sample is more easily to form oxygen defect, which generate more surface hydroxyl groups as reaction sites for surface adsorption and degradation. To gain a deeper understanding of above experimental investigations and to research the effect of Cu-intercalation, this paper also conducted the bond length of H2O2 and electron density difference with H2O2 of 0.02-CuMnD before and after Cu-intercalation to characterize the materials’ oxidation capacity for H2O2 (Fig. 10). As illustrated in Fig. 10a and b, the bond length of H2O2 inside 0.02CuMnD before and after Cu-intercalation is 1.376 Å and 1.431 Å, respectively, indicating the Cu-intercalation can increase the band length of H2O2. Therefore, the bond of H2O2 is more likely to dissociate and generate hydroxyl radical (OH) to oxidize the organic dye into micro-molecules in Cu-intercalation system. The electron density difference of 0.02-CuMnD with H2O2 (Fig. 10c, d) showed the change of path for electron transmission. It is expected that the electrons transfer to H2O2 from Cu atoms and rupture more easily than samples without Cu doping. Particularly worth mentioning is the results of calculations absolutely conform to the experimental conclusions. 3.2.4. Possible mechanism As shown in Scheme 2, a degradation mechanism for MB with 25-CuMnD can be speculated. Firstly, the MB is absorbed on surface of CuMnD via surface hydroxyl group and Mn4+/Cu2+. Afterwards, MB occurs surface oxidation because of Mn4+/Cu2+ and generates Mn3+/ Cu+ (Eqs. (6) and (7)). Secondly, H2O2 decomposes by means of two approaches. At first, the H2O2 self-decomposition to generate OH and OH which can degrade MB (Eq. (8)). The second approach is H2O2 decomposed by catalytic decomposition via Mn3+/Cu+ to generate Mn4+/Cu2+ and OH. In the third step, the MB and dye cations are oxidized by OH, generating degraded products
Fig. 9. (a) ESR spectra of 0-CuMnD and 25-CuMnD sample; (b,c) ESR spectra of 0-CuMnD and 25-CuMnD sample, respectively, under visible light illumination using DMPO as electron spin trapping agent.
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Fig. 10. (a, b) The bond length of H2O2 inside 0.02-CuMnD before and after Cu-intercalation. (c, d) Graph of the electron density difference of 0.02-CuMnD with H2O2 before and after Cu-intercalation.
Scheme 2. Possible oxidation reaction mechanism of CuMnD Fenton-like process.
which contain CO2, H2O and other small groups (Eq. (10)). Then because of the redox reaction between Mn4+/Cu2+ and Mn3+/Cu+, the decomposition of H2O2 restarted and then the degradation of MB continued. Therefore, in whole degradation process of MB,
the oxidation and adsorption supplemented each other and improved the general removal efficiency. It should be noted that the doped copper acted as important role to decompose the H2O2 in the overall process.
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4. Conclusion In summary, copper doping MnO2 covered disk-shaped diatomite was successfully synthesized and characterized. Moreover, the Cu-MnO2@diatomite provided an adsorption platform and 0.02-25-CuMnD was verified be an excellent catalyst for Fentonlike reaction of MB oxidation. The degradation ability for MB improved obviously via an appropriate copper doping content. That was illustrated by both theoretic arithmetic and experiment. Because of copper doping, the catalyst is more easily to form oxygen defect generating more surface hydroxyl groups as active reaction sites for surface adsorption and dissociate H2O2 so that generate hydroxyl radical (OH) to oxidize the MB. The degradation efficiency of CuMnD was tremendously influenced by the initial pH, H2O2dosage and copper content of catalyst. For 0.02-25CuMnD along with molar ration of Cu/Mn with 0.4402 showed the best degradation efficiency which was about 96.2% within 4 h with16.5 mM of H2O2 and pH 2.06. In addition, the Cu doped MnO2@diatomite with proper copper content is anticipated to be a catalyst candidate with great promise for waste water treatment on account of its high efficiency. Acknowledgements The authors gratefully appreciate the financial supports supported by the Fundamental Research Funds for the Central Universities (2018CDYJSY0055 and 2019CDQYCL042), the National Natural Science Foundation of China (Grant no. 21576034), Joint Funds of the National Natural Science Foundation of ChinaGuangdong (Grant no. U1801254), the project funded by Chongqing Special Postdoctoral Science Foundation (XmT2018043), Chongqing University Postgraduates’ Innovation Project (No. CYB16014 and CYS17003), Natural Science Foundation Project of CQ CSTC (cstc2017jcyjBX0028), Technological projects of Chongqing Municipal Education Commission (KJZDK201800801), the Innovative Research Team of Chongqing (CXTDG201602014), the State Education Ministry and Fundamental Research Funds for the Central Universities (106112016CDJZR135506 and 106112017CDJXSYY0001), the Science and Technology Innovation Talents Support Program of Chongqing (CSTCCXLJRC201706), and the Youth Innovation Promotion Association of CAS (2015316). The authors would like to especially thanks to the Electron Microscopy Center of Chongqing University for materials characterizations. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.08.082. References [1] Y. He, B. Jiang, Y. Jiang, J. Chen, Y.X. Zhang, J. Hazard. Mater. 344 (2018) 230– 240. [2] Y. He, B. Jiang, J. Chen, Y. Jiang, Y.X. Zhang, J. Colloid Interface Sci. 510 (2018) 207–220. [3] H.H. Peng, J. Chen, Y. Jiang, M. Li, L. Feng, D. Losic, F. Dong, Y.X. Zhang, J. Colloid Interface Sci. 484 (2016) 1–9. [4] M. Khaksar, M. Amini, D.M. Boghaei, K.H. Chae, S. Gautam, Catal. Commun. 72 (2015) 1–5. [5] P.L. dos Santos, I.R. Guimarães, A.M. Mesquita, M.C. Guerreiro, J. Mol. Catal. AChem. 424 (2016) 194–202. [6] Y.X. Zhang, X.D. Hao, F. Li, Z.P. Diao, Z.Y. Guo, J. Li, Ind. Eng. Chem. Res. 53 (17) (2014) 6966–6977. [7] A.F. Danil de Namor, A. El Gamouz, S. Frangie, V. Martinez, L. Valiente, O.A. Webb, J. Hazard. Mater. 241–242 (2012) 14–31. [8] N. Inchaurrondo, J. Font, C.P. Ramos, P. Haure, Appl. Catal. B-Environ. 181 (2016) 481–494. [9] Y.S. Al-Degs, M.F. Tutunju, R.A. Shawabkeh, Separ. Sci. Technol. 35 (14) (2000) 2299–2310.
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