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Highly efficient and stable CuZr-based metallic glassy catalysts for azo dye degradation
Journal Pre-proof Highly efficient and stable CuZr-based metallic glassy catalysts for azo dye degradation Bowen Zhao, Zhengwang Zhu, Xin Dong Qin, Zhengkun Li, Haifeng Zhang
PII:
S1005-0302(20)30118-3
DOI:
https://doi.org/10.1016/j.jmst.2019.12.012
Reference:
JMST 1967
To appear in:
Journal of Materials Science & Technology
Received Date:
3 November 2019
Revised Date:
22 December 2019
Accepted Date:
30 December 2019
Please cite this article as: Zhao B, Zhu Z, Qin XD, Li Z, Zhang H, Highly efficient and stable CuZr-based metallic glassy catalysts for azo dye degradation, Journal of Materials Science and amp; Technology (2020), doi: https://doi.org/10.1016/j.jmst.2019.12.012
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Research Article
Highly efficient and stable CuZr-based metallic glassy catalysts for azo dye degradation Bowen Zhao 1, 2, Zhengwang Zhu 1, *, XinDong Qin 3, Zhengkun Li 1, Haifeng Zhang 1 1
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
Academy of Sciences, Shenyang 110016, China 2
School of Materials Science and Engineering, University of Science and Technology of China,
Shenyang 110016, China Institute of Rare and Scattered Elements, College of Chemistry, Liaoning University, Shenyang
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3
110036, China
* Corresponding author. Prof. Zhengwang Zhu, Tel.: +86 024-23971782.
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E-mail address: [email protected] (Z.W. Zhu).
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Highlights
Current alloys as efficient catalysts can be efficiently applied for dye wastewater treatment.
2.
The Cu47.5Zr46Al6.5 alloy can completely degrade AO Ⅱ azo dye within 30 min.
3.
The Cu47.5Zr46Al6.5 alloy shows excellent degradation behaviors and cyclic stability.
4.
The inherent mechanism of catalytic performance and cycle stability can be explained in detail.
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1.
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[Received 3 November 2019; Received in revised form 22 December 2019; Accepted 30 December 2019]
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Abstract: Metallic glasses with the unique disordered atomic structure and metastable nature have been recently applied to degrade the azo dyes and other organic pollutants based on their superior catalytic performance. In this work, the functional properties of six CuZr-based metallic glassy ribbons with the different nominal components in degrading Acid Orange Ⅱ (AO Ⅱ) azo dyes were investigated. The Cu47.5Zr46Al6.5 metallic glassy ribbons could exhibit the more advanced catalytic performance for degradation process, which could completely degrade azo dye aqueous solution within
30 min. Additionally, the Cu47.5Zr46Al6.5 metallic glassy ribbons also showed the excellent cyclic stability along with approximately 97.68% degradation efficiency after 10 cycles. These excellent catalytic performance and stability are closely related to the synergistic effect of exposed copper nanoparticles and produced copper oxides in the reaction, which contributes to accelerate the generation of more hydroxyl radicals (·OH) to react with dye molecules. Our findings can be able to develop a novel potential metallic glassy material for the functional application of wastewater treatment.
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Key words: metallic glasses; catalytic performance; degradation; azo dyes Introduction
Dyeing wastewater contains a variety of biologically toxic and carcinogenic organic
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pollutants, which can seriously cause environmental damage and endanger human health[1-6]. Thus, the treatment of contamination in wastewater has attracted great attention in terms of high
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efficiency and economy[7]. The conventional treatment approaches of dyeing wastewater, including
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physical adsorption[8,9], biological method[10], zero-valent metals reduction[11-13], advanced oxidation processes[14-16], as well as electrochemical technology[17,18], and so on, have been widely utilized in recent years, but there are still certain shortcomings and limitations, which include long enough
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degradation time, low degradation efficiency and lack applicability. Consequently, to develop a new multifunctional and environment-friendly catalytic material is the most critical priority for the
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moment, because the high efficient catalysts can play an important role in the field of wastewater treatment[19,20].
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Metallic glasses (MGs) as the high efficient catalysts can contribute to the functional application for the excellent degradation of azo dyes and organic pollutants, due to many miraculous and superior properties of metallic glasses which include disorder atomic structure, broadly tunable atomic components, sufficient active sites and high corrosion resistance[21-26]. At present, the catalytic performance of various metallic glasses in degrading the azo dye aqueous solution have been studied, such as Mg-based[27-30], Al-based[31,32], Co-based[33], and Fe-based[20,34-39] metallic glasses, etc. Mg60Zn35Ca5[28] metallic glassy powders obtained by high energy ball milling technology and Mg70Zn25Ca5[29] amorphous alloy powders produced by gas atomization can all
exhibit catalytic capability and reaction efficiency in degrading azo dyes. It is also reported that the superior degradation performance by Al91-xNi9Yx (x = 0, 3, 6, 9)[31] metallic glassy ribbons can be demonstrated in both alkaline and acidic solutions. Ball-milled Co78Si8B14[33] metallic glassy powders can degrade the azo dyes of AO Ⅱ at an ultrafast rate, and which is faster than that of the relative
crystalline
organizations
and
Fe0
powders.
In
addition,
Fe78Si9B13[22,35]
and
(Fe0.99Mo0.11)78Si9B13[36] metallic glassy ribbons have much higher degradation efficiency than their crystalline counterparts. Fe80B20[20] metallic glasses with a flexible grid structure is responsible for purifying Direct Black 15 (DB 15) azo dyes with 30 min at room temperature. Although all of these metallic glassy materials can exhibit the excellent capability in degrading azo dyes and organic
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pollutants, one common tough challenge that is limited durability and stability due to the severe corrosion tendency has appeared in a cyclical reaction. In order to solve the above problems, it is surprisingly found that CuZr-based metallic glasses have outstanding catalytic behavior and cycle
synergistic effect of generated oxides and ·OH radicals.
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stability, besides the higher degradation efficiency and rate can be attributed to the perfect
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In this work, the new functional CuZr-based metallic glassy ribbons with six different nominal compositions are successfully fabricated and the corresponding amorphous structures are
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accurately characterized. Moreover, the degradation performance of AO Ⅱ aqueous solution by Cu47.5Zr46Al6.5 metallic glassy ribbons is systematically investigated under the various experimental conditions. It is obviously found that the current MG alloys exhibit both the high degradation
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efficiency and rate. The catalytic degradation reaction mechanism is also discussed briefly according to surface topographies of the used ribbons, and the reason for the superior durability and
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stability than other typical metallic glassy ribbons in the re-used reactions is further explained.
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2. Experimental
2.1. Preparation of CuZr-based metallic glassy ribbons The alloy ingots with nominal compositions of Cu45Zr48.5Al6.5, Cu46Zr47.5Al6.5, Cu46Zr46.5Al7.5,
Cu47Zr46.5Al6.5, Cu46Zr46Al8 and Cu47.5Zr46Al6.5 were prepared by arc-melting high pure elements (Cu: 99.9 wt%, Zr: 99.9 wt% and Al: 99.9 wt%) under a Ti-gettered argon atmosphere, respectively. Metallic glassy ribbons with a thickness of ~50 μm and a width of ~2 mm were prepared by single roller melt-spinning under an argon atmosphere. Amorphous structural feature of melt-spun ribbons
was verified by X-ray Diffraction (XRD, Rigaku D/max 2400, America) with Cu-Kα radiation, differential scanning calorimetry (Netzsch DSC 204 F1) and transmission electron microscopy (TEM, FEI Tecnai F20, America) 2.2. Azo dyes degradation measurement Commercial available AO Ⅱ (purchased from Shanxi Tond Chemical CO., Ltd, China) directly dissolved in distilled water to form azo dye aqueous solution with a concentration of 100 mg/L. The surface area of 1000 cm2/L of 2×8 mm metallic glassy ribbons as the catalysts and 100 mL
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simulated dye aqueous solution were both added into Erlenmeyer flask (250 mL), which was placed on a magnetic stirrer with adjustable temperature and stirring number. The solution system with the different pH was regulated by adding diluted HCl solution (0.1 mol/L) and diluted NaOH solution (0.1 mol/L). About 3 mL reaction solution was taken out for centrifugation at each 5 min interval,
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and dye supernatant was analyzed by UV-vis spectroscopy (Purkinje General TU-1900, China). The phase and structure nature of ribbons after multiple cycles of reaction were examined by XRD. The
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binding states of elements on the surfaces of the as-spun and recycled ribbons were evaluated by X-ray photoelectron (XPS, Thermo ESCALAB 250X, America) with a monochromatic Al-Kα
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X-ray source. Meanwhile, the microscopic morphology and elemental distribution of these ribbons were characterized by SEM equipped with EDS-mapping (Zeiss SUPPA 55, Germany),
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respectively. 3. Results and Discussion
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3.1. Structure of metallic glassy ribbons
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Fig. 1(a) shows the XRD patterns of six melt-spun glassy ribbons. Each of XRD patterns
exhibits a broadened diffuse peak around 2θ = 38° without any sharp Bragg peaks, demonstrating all the ribbons form a completely amorphous structure. As shown in Fig. 1(b), each of the DSC curves clearly displays the exothermic peak. The amorphous nature was also verified by DSC measurements. Additionally, Fig 1(c) and (d) present HRTEM image and the corresponding of SAED pattern of melt-spun Cu47.5Zr46Al6.5 metallic glass ribbons, further indicating the ribbons are in amorphous state.
3.2. Degradation of azo dyes by metallic glassy ribbons The AO Ⅱ aqueous solution was used to examine the catalytic degradation capacity of the current CuZr-based metallic glasses. It is found that the strong characteristic absorption peak located at λmax = 484 nm in the UV-vis spectra producing in the azo bond (-N = N-). The intensity of the bond is proportional to the concentration of the reaction solution. Fig. 2(a) presents the UV-vis spectra of AO Ⅱ aqueous solution decomposed by Cu47.5Zr46Al6.5 metallic glassy ribbons at 40 ℃ under the acidic condition of pH = 2 without mixing into any other chemical reagent. It can be obviously seen that the intensity of the UV-vis absorption peak gradually becomes weaker with the
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extension of reaction time. Eventually, AO Ⅱ aqueous solution was decolorized thoroughly within 30 minutes. The influence of the Cu content and Al content on the degradation rate was illustrated in Figs. 2(b) and (c), respectively. The six CuZr-based metallic glassy ribbons with the different nominal compositions can all exhibit the unexpected high performance for zao dye degradation
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because of the minor differences in the components of those samples. However, it is clearly shown that the degradation rate was enhanced with the increase of the Cu content in the metallic glassy
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ribbons. It is possible that the ribbon samples are more likely to expose numerous copper nanoparticles owning to strong catalytic property throughout the reaction, which can provide even
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more reaction active sites and create a larger amount of ·OH radicals to react with dye molecules to further improve degradation rate. The similar behavior occurs to the alloy of the different Al
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contents. The increase of Al content can also accelerate the rate of dye degradation, due to the fact that the Al element in the ribbons are contacted with reduction hydrogen [H], hydrogen ion H+ or even extremely strong oxidized ·OH radicals generated in the entire solution system, which will be
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further elaborated in the following part of the mechanism. The degradation process can be confirmed to the first-order reaction kinetics following Ct/C0 = (1 - Cf/C0) exp (-kt) + Cf/C0，where
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Ct and C0 are expressed as the dye concentration at reaction time and initial state individually, Cf is the final residual concentration, t is the reaction time, and k represents the reaction rate constant [40]. Further, the parameter k can be referenced as the degradation rate and the degradation efficiency is calculated by the formula η = 1 - Cf/C0. Fig. 2(d) displays the degradation efficiency and degradation rate of the AO Ⅱ aqueous solution by six different CuZr-based metallic glassy ribbons. The degradation efficiency is relatively close, and the degradation rate illustrates a rising trend, which consists with the above-mentioned data analysis. In contrast with the other five samples, the
Cu47.5Zr46Al6.5 metallic glassy ribbons accomplish the highest prominent degradation rate constant of k = 0.165 and simultaneously demonstrates a superb degradation efficiency of η = 97.68%. Therefore, the CuZr-based metallic glassy ribbons exhibit an outstanding degradation effect on the AO Ⅱazo dyes. As is well known, the different environmental conditions have a significant impact on the azo dye degradation process. Herein, the effects of various experimental parameters, including the surface area of ribbons, reaction temperature and initial pH were systematically investigated. Fig. 3(a) displays the effect of surface area of ribbons on the degradation behaviors of AO Ⅱ aqueous solution. When the surface area of ribbons increased from 700 cm2/L to 1300 cm2/L, the
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degradation reaction of AO Ⅱ aqueous solution continuously maintains a much faster rate and can be completely degraded within 40 min. Besides when the ribbons with surface area of 1000 cm2/L and 1300 cm2/L were selected, the dyes degradation time can be rapidly promoted within 30 min.
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All of the above results declare that the surface area of ribbons performs the significant influence throughout the degradation process, which reduces the degradation time.
The reaction temperature is another important factor, which seriously affects the degradation
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rate of AO Ⅱ aqueous solution. Fig. 3(b) shows the degradation rate of AO Ⅱ dyes at various
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temperatures, which is in range of 303 K-333 K. As can be seen, increasing the temperature of the solution can remarkably improve the dye degradation ability, which indicates that the reaction belongs to thermally activated process [40]. It is obvious that there is a certain difference of the
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degradation rate in the first fifteen minutes, and along with the time growing, AO Ⅱ aqueous solution is almost fully degradation within 40 minutes under the four temperature conditions. The
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proposed reason is that higher temperature can accelerate the thermal motion of dye molecules in solution. On the other hand, generating amount of ·OH radicals with strong oxidation performance,
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providing more sources to approach and react with the dye molecules, can notably enhance the dye degradation. In contrast, when the temperature is low, the solution cannot supply sufficient ·OH radicals, causing a lower reaction rate and speed. All the results mentioned above demonstrate that the CuZr-based metallic glassy ribbons can be able to create the excellent and favorable catalytic capability for applications in azo dye aqueous solution treatments in high temperature system. The effect of pH range of 2 - 10 on the degradation of AO Ⅱ aqueous solution has been studied and the results are shown in Fig. 3(c). AO Ⅱ aqueous solution was completely degraded within 30 min when the pH of solution equals to 2. Furthermore, the highest degradation efficiency of dye aqueous
solution, namely 97.68%, was obtained only at initial solution pH = 2 under other optimum reaction conditions currently. However, in an acidic solution with a pH of 3, the AO Ⅱ aqueous solution was decomposed hardly. And almost no degradation reaction occurs in neutral solution of pH = 7 or alkaline solution of pH = 10. Therefore, it can be elaborated that the factor of pH values is a decisive and effective requirement in the dye degradation. Fig. 3(d) illustrates the degradation efficiency of dyes of the first, third, fifth, eighth and tenth cycle, respectively. After each trial, the catalysts were recycled by some simple filtration followed by washing with water and ethanol. Similar degradation efficiency of dye aqueous solution was obtained in the ten repeated reactions, revealing that the catalyst exhibits outstanding cycle stability performance. The possible reason is
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that the formed oxidation products on the surface of the glassy ribbons containing potential and excellent catalytic properties can be able to provide more active sites to achieve faster degradation performance after 10 cycles.
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Next, we investigated the catalytic behavior for degradation of AO Ⅱ aqueous solution in two solution systems, which were adjusted with diluted H2SO4 solution and HCl solution, respectively.
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Surprisingly, two distinct phenomena were observed, which AO Ⅱ aqueous solution was completely degraded in a solution adjusted with diluted H2SO4 solution and not in another case. It
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may be doubted that whether Cl- in solution play a crucial role to contribute to dye degradation course. To prove that concentration of Cl- is key to the observation of the successful accomplish of the dye degradation. A control experiment by adding four different chemical substances (only 1.4
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mM NaCl, only 0.7 mM H2SO4, 1.4 mM NaCl and 0.7 mM H2SO4, 1.4 mM NaCl) into solution separately was investigated. The purpose is to ensure that the concentration of H+ or Cl- is exactly
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consistent, reaching the basic requirements of the comparative test. The experimental results are shown in Fig.4 (a). It is observed that when only one of H+ and Cl- is present in the solution alone,
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no degradation reaction occurs throughout the process. However, when H+ and Cl- coexist in the solution, the samples show the perfect degradation behaviors. It is known that the corrosion reaction can take place at specific concentrations or long enough reaction time due to attacking on materials by Cl-. In this work, the appearance of pitting is responsible for the action of relatively low concentration of Cl- on the surface of ribbons in the presence of H+ as a depolarizer, leading to the bareness of a large amount of fresh copper nanoparticles on the surface of the samples. Exposed copper nanoparticles can plenty destroy dye macromolecule to achieve decomposition due likely to so-called size effect. That is to say, generated high surface energy and unstable atoms are easily
adsorbed and bonded to other atoms or molecular groups. Moreover, many active atoms with large surface area were fabricated by reducing particle size to improving the catalytic capability. In addition, the copper nanoparticles as efficient catalysts coordinating partial ·OH radicals formed in solution fright against dye macromolecule, thereby further enhancing degradation performance. Fig. 4(b) further shows the effect of the degradation rate of dyes on the different concentration ratios of H+ and Cl-. It has clearly revealed that when the concentration of H+ and Cl- are both 1.4 mM, dye solution can be degraded positively. On this basis, the degradation rate reduced significantly when one of the concentrations of H+ or Cl- will cut down 0.4 mM, but still retained efficient when the concentration of H+ or Cl- raise to 2.4 mM, indicating that the optimal concentration of H+ and Cl-
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should be at least greater than 1.4 mM. This part can further explain previous report of the degradation effect in two acidic dye solution with pH = 2 and pH = 3 in Fig. 3(c), because the concentration of H+ and Cl- in the solution system of pH = 2 is even higher than those in the solution of pH = 3.
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The good synergic effect of H+ and Cl- in the degradation process of AO Ⅱ aqueous solution
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was demonstrated for further SEM observation. Fig. 4(c-f) display the surface morphology of Cu47.5Zr46Al6.5 metallic glassy ribbons after degradation in NaCl, H2SO4, NaCl and H2SO4 and HCl
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solution system, respectively. The smooth and glossy surface morphology of the ribbons without any precipitates is shown in Fig. 4(c) and 4(d). Rather, the irregular and rough surface morphology is clearly discovered in Fig. 4(e) and 4(f), and it is obviously found that some loose and spares
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oxidation products were distributed on the surface of the ribbons. It is correctly proved again that H+ and Cl- coexist to contribute to dye degradation behaviors. Further, stability and reusability test
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of Cu47.5Zr46Al6.5 amorphous catalysts in terms of dye degradation were conducted under identical and optimal reaction conditions.
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3.3. Structure and surface morphology Fig. 5 shows that the XRD curves of fresh and reacted Cu47.5Zr46Al6.5 ribbons after first, third,
fifth, eighth and tenth cycles, respectively. The samples after three cycles exhibit the sharp crystalline diffraction peaks at 2θ = 43° and 2θ = 50°, which mark the unique characteristic peak of elemental copper. Moreover, the relative intensity of the peaks gradually increase with the increase of the cycle number, whereas the copper characteristic peaks appear approximately near 2θ = 50°
after eighth and tenth cycles. The above analysis results show that the copper nanoparticles separate out on the surface of the ribbons, which is consistent with the previous assumption. In order to further certificate the reaction mechanism of degradation process, the variation of surface morphologies for Cu47.5Zr46Al6.5 metallic glassy ribbons before and after the reaction was discussed. As seen from Fig. 6(a), the surface of the initial ribbons is quite smooth with metallic luster. An irregular surface topography dispersed with fine and uniform oxidation products after the first use is shown in Fig. 6(b). In addition, the EDS patterns (inset of Fig. 6(a) and Fig. 6(b)) show the initial ribbons and the first reacted ribbons, separately. Compare to two curves, the peaks are both determined as Cu, Zr and O elements. It is found that the formation oxidation products can be
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amply verified on the basis of dramatically increasing of the large amount of O element. Nevertheless, the content of Cu element has sharply reduced after degradation reaction, which tends to increase the contact area between Cu element and reaction solution, thereby realizing the
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abundant generation of ·OH radicals[41,42]. Compared to the first reused ribbon surface, the content of the oxides were gradually increased after the third and eighth use, as shown in the Fig. 6(c) and
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Fig. 6(d). After the tenth use (Fig. 6(e)), an exaggerated situation, expanding the corrosion pit and increasing the oxide particles size, is clearly presented. Fig. 6(f) shows a low magnification photo of
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the surface morphology of eighth run recycled amorphous ribbons. The countless oxidation products attaching to the inside and edged of the etch pit are first discovered on the surface of the eighth use ribbons. Fig. 6(g) and (h) show a low magnification photo of the surface morphology and
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the corresponding mapping result of Cu, Zr, Al and O elemental distributions of tenth run recycled amorphous ribbons. It is confirmed that the area where Zr and Al elements have no existence
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belongs to CuxO deposits, promoting mass electron transfer[43]. The more and more substances, emerging from a serious of oxidizing reaction of Cu, CuxO and other elemental, were generated and
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collected on the surface of ribbons along with reactions. The formation of surface oxidation may be the reason for speeding up the degradation of dyes. The catalytic active sites can be uniformly distributed on the entire material surface due to the single-phase structure of the amorphous alloy. Besides, the high activation energy exists on the surface of the metallic glassy material depending on the metastable nature, which is beneficial to the catalytic degradation reaction. This phenomenon can indicate that more catalytically active sites are provided and electron transfer rates are enhanced, resulting in higher degradation efficiency.
To further obtain the chemical states of elements and electronic structure on the surface, the XPS analysis of the surface of the as-spun and reacted ribbons after first, third, fifth, eighth and tenth cycles were investigated, respectively. Fig. 7(a) shows the strong binding energies of Cu 2p, Zr 3d, Al 2p, C 1s and O 1s in XPS full range spectrum, which can confirm no changes in composition of the Cu47.5Zr46Al6.5 ribbons before and after the degradation reaction[31]. And the results are consistent with the EDS analysis in Fig. 6(a) and (b). As shown in Fig. 7(b), the XPS spectrum of Cu 2p3/2 in the fresh CuZr-based ribbons includes only one peak that locates at 932.4 eV assigned to metal state Cu0. The oxidized state Cu+ a can be observed for the ribbons after the first cyclic reaction based on the relevant Cu2O satellite peak. However, the spectrum of Cu 2p3/2
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can be divided into two peaks, locating at 932.4 eV corresponded to mixed valence Cu0/Cu+ and approximate 933.6 eV corresponded to oxidized state Cu2+ after the third cyclic reaction, respectively. It can clearly reveal that degradation reaction leads to the formation of Cu2O and CuO.
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The catalytic ability for degrading dyes is further enhanced the owning to synergistic effect of the generated Cu2O phase and bare copper nanoparticles. More importantly, forming the Cu2O phase on
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an amorphous matrix material can provide more catalytically active sites and accelerate electron transfer. Fig. 7(c) displays the comparative XPS spectrum of Zr 3d5/2 for ribbons before and after
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the degradation. Zirconium is a reactive metal and Zr is usually found as ZrO2, which promotes amorphous state formation in alloy and does not take part in the catalytic degradation reaction. The Zr 3d5/2 peaks in the fresh CuZr-based ribbons locate at 178.8 eV corresponded to metal state Zr0
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and 182.4 eV corresponded to oxidized state Zr4+ after the reaction. The position of Zr 3d5/2 peak can all shift towards the higher binding energy assigned to oxidized state Zr4+ after the reaction. Fig.
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7(d) displays the comparative XPS spectrum of Al 2p among the as-spun and reacted ribbons. The no significant changes in the position and shape of Al 2p peak can be observed visually after the
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catalytic degradation. Aluminum usually forms a passivation film without a hitch on the surface, and oxide peak position may vary with the thickness of the film, which is near 74.7 eV corresponded to oxidized state Al3+ before and after the reaction. Based on the analysis of the surface micromorphology and electronic structure, the copper nanoparticles and Cu2O phase can be evenly distributed on the surface of the ribbons, which is conducive to achieving more excellent degradation performance. 3.4. Proposed mechanism
Based on the above analysis, the schematic diagram of the degradation of AO Ⅱ dyes by using Cu47.5Zr46Al6.5 metallic glassy ribbons with higher activity and stability can be illustrated in Fig. 8. The most probable catalytic mechanism can be divided into following two pathways: corrosion reaction and catalytic degradation process. As for the pitting reaction, the interaction between H+ and Cl- can attracted to the extraordinary role to contribute to the formation of fresh copper nanoparticles[44-46]. Thus, appropriate concentrations of H+ and Cl- will invade and attack the surface of the ribbons brazenly, bringing about the copper nanoparticles to be exposed naturally in solution. Moreover, the plenty of oxidation products can be adsorbed on the inside and edged of the etch pit tightly, which
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supporting the extensive catalytic activity sites depending on the size effect. The exposed copper nanoparticles and the produced Cu2O with strong reducing ability can directly reduce the dye molecules in the solution.
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As for the degradation process, the reaction pathway is extremely similar to Fenton-like reaction, which can utilized the strong oxidizing ·OH radicals produced through activating and
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dissociating O2 in the presence of H+ in contact with Cu0/Cu+ to degrade pollutants[47-49]. Based on the above section, it can be clearly indicated the precipitated copper nanoparticles and Cu2O phase
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on the Cu ligament surface can both destroy O2 molecules to develop ·OH radicals under acidic conditions, which oxidize azo dye molecules to degrade into non-toxic and harmless carbon dioxide (CO2), water and other species.
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On the other hand, a mere handful of Al elements in the catalysts also have more and less a certain contribution in the catalytic degradation experiments, which corresponds to the results in Fig.
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2(c). In summarize, the degradation mechanism can actually be divided into two portions. The first part can be interpreted as the direct reduction of zero-valent Al. Secondly, new reduced hydrogen
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[H] can act on the reduction of azo bonds under the action of surface catalysis[31,32,50]. 4. Conclusions
An efficient catalyst of CuZr-based metallic glassy ribbons was synthesized via melt-spinning technology successfully. The catalysts clearly demonstrated the excellent and miraculous degradation ability towards azo dyes of AO Ⅱ, which could be obtained complete decomposition within 30 min using Cu47.5Zr46Al6.5 metallic glassy ribbons under the optimum conditions. In addition, the high degradation efficiency of AO Ⅱ is attributed to the generation of ·OH radicals
through the haptoreaction between H+/O2 and Cu ligaments and Cu2O nano-crystals during the reaction, further explaining the reason for superior durability and stability than other typical glassy ribbons catalysts in the re-used reactions. The research work indicates the Cu based metallic glassy ribbons as high activity catalysts are benefited in the degradation of azo dyes containing wastewater. Acknowledgements The work was supported by the National Natural Science Foundation of China (Nos. 51801209, 51790484, U1738101), DongGuan Innovative Research Team Program (2014607134), LiaoNing
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Revitalization Talents Program (XLYC1802078 and XLYC1807062), the fund of Shenyang National Laboratory for Materials Science, Shenyang Amorphous Metal Manufacturing Co., Ltd.,
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Shenyang 110000, China.
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Figure captions
Fig. 1. (a) XRD patterns and (b) DSC curves of melt-spun CuZr-based metallic glass ribbons, (c) HRTEM image and (d) SAED pattern of the melt-spun Cu47.5Zr46Al6.5 metallic glass ribbons.
Fig. 2. (a) Typical UV-vis spectra of AO Ⅱ aqueous solution degraded by Cu47.5Zr46Al6.5 metallic glassy ribbons, (b) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Cu contents as a function of reaction time, (c) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Al contents as a function of
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reaction time, (d) comparison of degradation efficiency and degradation rate for six different composition ribbons (AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃ , pH = 2).
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Fig. 3. Effect of (a) surface area of ribbons, (b) temperature and (c) pH on the normalized concentration of AO Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) the
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degradation of AO Ⅱ aqueous solution with Cu47.5Zr46Al6.5 metallic glassy ribbons at the different cycles (if not mentioned, AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃,
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pH = 2).
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Fig. 4. Effect of (a) additive types and (b) c(H+) and c(Cl-) on the normalized concentration of AO Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, surface morphology of the Cu47.5Zr46Al6.5 metallic glassy ribbons after degradation with the different additive types of (c) NaCl
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solution, (d) H2SO4 solution, (e) NaCl and H2SO4 solution, (f) HCl solution (if not mentioned, AO
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Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃, pH = 2).
Fig. 5. XRD patterns of the reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
Fig. 6. (a) SEM image of EDS analysis (inset) of melt-spun Cu47.5Zr46Al6.5 metallic glassy ribbons, (b) SEM image of EDS analysis (inset) of first reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (c) SEM image of third reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) SEM image and (f) low magnification SEM image of eighth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (e) SEM image,
(g) low magnification SEM image and (h) mapping results of tenth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
Fig. 7. XPS spectrum of (a) the full range spectra, (b) Cu 2p, (c) Zr 3d and (d) Al 1s in binding energy regions for the as-spun and reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
Fig. 8. Schematic illustration of the degradation mechanism of AO Ⅱ azo dyes using CuZr-based
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metallic glassy ribbons.
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Figure list:
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Fig. 1. (a) XRD patterns and (b) DSC curves of melt-spun CuZr-based metallic glass ribbons, (c)
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HRTEM image and (d) SAED pattern of the melt-spun Cu47.5Zr46Al6.5 metallic glass ribbons
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Fig. 2. (a) Typical UV-vis spectra of AO Ⅱ aqueous solution degraded by Cu47.5Zr46Al6.5 metallic
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glassy ribbons, (b) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Cu contents as a function of reaction time, (c) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Al contents as a function of
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reaction time, (d) the comparison of degradation efficiency and degradation rate for six different composition ribbons (AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃ ,
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pH = 2).
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Fig. 3. Effect of (a) surface area of ribbons, (b) temperature and (c) pH on the normalized concentration of AO Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) the
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degradation efficiency of AO Ⅱ aqueous solution with Cu47.5Zr46Al6.5 metallic glassy ribbons at the different cycles (if not mentioned, AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L,
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temperature: 40 ℃, pH = 2).
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Fig. 4. Effect of (a) additive types and (b) c(H+) and c(Cl-) on the normalized concentration of AO
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Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, surface morphology of the Cu47.5Zr46Al6.5 metallic glassy ribbons after degradation with the different additive types of (c) NaCl solution, (d) H2SO4 solution, (e) NaCl and H2SO4 solution, (f) HCl solution (if not mentioned, AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃, pH = 2).
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Fig. 5. XRD patterns of the reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
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Fig. 6. (a) SEM image of EDS analysis (inset) of melt-spun Cu47.5Zr46Al6.5 metallic glassy ribbons, (b) SEM image of EDS analysis (inset) of first reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (c) SEM image of third reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) SEM image and (f) low magnification SEM image of eighth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (e) SEM image, (g) low magnification SEM image and (h) mapping results of tenth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
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Fig. 7. XPS spectrum of (a) the full range spectrum, (b) Cu 2p, (c) Zr 3d and (d) Al 1s in binding
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energy regions for the as-spun and reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
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Fig. 8. Schematic illustration of the degradation mechanism of AO Ⅱ azo dyes using CuZr-based
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metallic glassy ribbons.
PII:
S1005-0302(20)30118-3
DOI:
https://doi.org/10.1016/j.jmst.2019.12.012
Reference:
JMST 1967
To appear in:
Journal of Materials Science & Technology
Received Date:
3 November 2019
Revised Date:
22 December 2019
Accepted Date:
30 December 2019
Please cite this article as: Zhao B, Zhu Z, Qin XD, Li Z, Zhang H, Highly efficient and stable CuZr-based metallic glassy catalysts for azo dye degradation, Journal of Materials Science and amp; Technology (2020), doi: https://doi.org/10.1016/j.jmst.2019.12.012
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Research Article
Highly efficient and stable CuZr-based metallic glassy catalysts for azo dye degradation Bowen Zhao 1, 2, Zhengwang Zhu 1, *, XinDong Qin 3, Zhengkun Li 1, Haifeng Zhang 1 1
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
Academy of Sciences, Shenyang 110016, China 2
School of Materials Science and Engineering, University of Science and Technology of China,
Shenyang 110016, China Institute of Rare and Scattered Elements, College of Chemistry, Liaoning University, Shenyang
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3
110036, China
* Corresponding author. Prof. Zhengwang Zhu, Tel.: +86 024-23971782.
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E-mail address: [email protected] (Z.W. Zhu).
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Highlights
Current alloys as efficient catalysts can be efficiently applied for dye wastewater treatment.
2.
The Cu47.5Zr46Al6.5 alloy can completely degrade AO Ⅱ azo dye within 30 min.
3.
The Cu47.5Zr46Al6.5 alloy shows excellent degradation behaviors and cyclic stability.
4.
The inherent mechanism of catalytic performance and cycle stability can be explained in detail.
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1.
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[Received 3 November 2019; Received in revised form 22 December 2019; Accepted 30 December 2019]
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Abstract: Metallic glasses with the unique disordered atomic structure and metastable nature have been recently applied to degrade the azo dyes and other organic pollutants based on their superior catalytic performance. In this work, the functional properties of six CuZr-based metallic glassy ribbons with the different nominal components in degrading Acid Orange Ⅱ (AO Ⅱ) azo dyes were investigated. The Cu47.5Zr46Al6.5 metallic glassy ribbons could exhibit the more advanced catalytic performance for degradation process, which could completely degrade azo dye aqueous solution within
30 min. Additionally, the Cu47.5Zr46Al6.5 metallic glassy ribbons also showed the excellent cyclic stability along with approximately 97.68% degradation efficiency after 10 cycles. These excellent catalytic performance and stability are closely related to the synergistic effect of exposed copper nanoparticles and produced copper oxides in the reaction, which contributes to accelerate the generation of more hydroxyl radicals (·OH) to react with dye molecules. Our findings can be able to develop a novel potential metallic glassy material for the functional application of wastewater treatment.
1.
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Key words: metallic glasses; catalytic performance; degradation; azo dyes Introduction
Dyeing wastewater contains a variety of biologically toxic and carcinogenic organic
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pollutants, which can seriously cause environmental damage and endanger human health[1-6]. Thus, the treatment of contamination in wastewater has attracted great attention in terms of high
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efficiency and economy[7]. The conventional treatment approaches of dyeing wastewater, including
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physical adsorption[8,9], biological method[10], zero-valent metals reduction[11-13], advanced oxidation processes[14-16], as well as electrochemical technology[17,18], and so on, have been widely utilized in recent years, but there are still certain shortcomings and limitations, which include long enough
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degradation time, low degradation efficiency and lack applicability. Consequently, to develop a new multifunctional and environment-friendly catalytic material is the most critical priority for the
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moment, because the high efficient catalysts can play an important role in the field of wastewater treatment[19,20].
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Metallic glasses (MGs) as the high efficient catalysts can contribute to the functional application for the excellent degradation of azo dyes and organic pollutants, due to many miraculous and superior properties of metallic glasses which include disorder atomic structure, broadly tunable atomic components, sufficient active sites and high corrosion resistance[21-26]. At present, the catalytic performance of various metallic glasses in degrading the azo dye aqueous solution have been studied, such as Mg-based[27-30], Al-based[31,32], Co-based[33], and Fe-based[20,34-39] metallic glasses, etc. Mg60Zn35Ca5[28] metallic glassy powders obtained by high energy ball milling technology and Mg70Zn25Ca5[29] amorphous alloy powders produced by gas atomization can all
exhibit catalytic capability and reaction efficiency in degrading azo dyes. It is also reported that the superior degradation performance by Al91-xNi9Yx (x = 0, 3, 6, 9)[31] metallic glassy ribbons can be demonstrated in both alkaline and acidic solutions. Ball-milled Co78Si8B14[33] metallic glassy powders can degrade the azo dyes of AO Ⅱ at an ultrafast rate, and which is faster than that of the relative
crystalline
organizations
and
Fe0
powders.
In
addition,
Fe78Si9B13[22,35]
and
(Fe0.99Mo0.11)78Si9B13[36] metallic glassy ribbons have much higher degradation efficiency than their crystalline counterparts. Fe80B20[20] metallic glasses with a flexible grid structure is responsible for purifying Direct Black 15 (DB 15) azo dyes with 30 min at room temperature. Although all of these metallic glassy materials can exhibit the excellent capability in degrading azo dyes and organic
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pollutants, one common tough challenge that is limited durability and stability due to the severe corrosion tendency has appeared in a cyclical reaction. In order to solve the above problems, it is surprisingly found that CuZr-based metallic glasses have outstanding catalytic behavior and cycle
synergistic effect of generated oxides and ·OH radicals.
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stability, besides the higher degradation efficiency and rate can be attributed to the perfect
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In this work, the new functional CuZr-based metallic glassy ribbons with six different nominal compositions are successfully fabricated and the corresponding amorphous structures are
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accurately characterized. Moreover, the degradation performance of AO Ⅱ aqueous solution by Cu47.5Zr46Al6.5 metallic glassy ribbons is systematically investigated under the various experimental conditions. It is obviously found that the current MG alloys exhibit both the high degradation
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efficiency and rate. The catalytic degradation reaction mechanism is also discussed briefly according to surface topographies of the used ribbons, and the reason for the superior durability and
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stability than other typical metallic glassy ribbons in the re-used reactions is further explained.
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2. Experimental
2.1. Preparation of CuZr-based metallic glassy ribbons The alloy ingots with nominal compositions of Cu45Zr48.5Al6.5, Cu46Zr47.5Al6.5, Cu46Zr46.5Al7.5,
Cu47Zr46.5Al6.5, Cu46Zr46Al8 and Cu47.5Zr46Al6.5 were prepared by arc-melting high pure elements (Cu: 99.9 wt%, Zr: 99.9 wt% and Al: 99.9 wt%) under a Ti-gettered argon atmosphere, respectively. Metallic glassy ribbons with a thickness of ~50 μm and a width of ~2 mm were prepared by single roller melt-spinning under an argon atmosphere. Amorphous structural feature of melt-spun ribbons
was verified by X-ray Diffraction (XRD, Rigaku D/max 2400, America) with Cu-Kα radiation, differential scanning calorimetry (Netzsch DSC 204 F1) and transmission electron microscopy (TEM, FEI Tecnai F20, America) 2.2. Azo dyes degradation measurement Commercial available AO Ⅱ (purchased from Shanxi Tond Chemical CO., Ltd, China) directly dissolved in distilled water to form azo dye aqueous solution with a concentration of 100 mg/L. The surface area of 1000 cm2/L of 2×8 mm metallic glassy ribbons as the catalysts and 100 mL
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simulated dye aqueous solution were both added into Erlenmeyer flask (250 mL), which was placed on a magnetic stirrer with adjustable temperature and stirring number. The solution system with the different pH was regulated by adding diluted HCl solution (0.1 mol/L) and diluted NaOH solution (0.1 mol/L). About 3 mL reaction solution was taken out for centrifugation at each 5 min interval,
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and dye supernatant was analyzed by UV-vis spectroscopy (Purkinje General TU-1900, China). The phase and structure nature of ribbons after multiple cycles of reaction were examined by XRD. The
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binding states of elements on the surfaces of the as-spun and recycled ribbons were evaluated by X-ray photoelectron (XPS, Thermo ESCALAB 250X, America) with a monochromatic Al-Kα
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X-ray source. Meanwhile, the microscopic morphology and elemental distribution of these ribbons were characterized by SEM equipped with EDS-mapping (Zeiss SUPPA 55, Germany),
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respectively. 3. Results and Discussion
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3.1. Structure of metallic glassy ribbons
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Fig. 1(a) shows the XRD patterns of six melt-spun glassy ribbons. Each of XRD patterns
exhibits a broadened diffuse peak around 2θ = 38° without any sharp Bragg peaks, demonstrating all the ribbons form a completely amorphous structure. As shown in Fig. 1(b), each of the DSC curves clearly displays the exothermic peak. The amorphous nature was also verified by DSC measurements. Additionally, Fig 1(c) and (d) present HRTEM image and the corresponding of SAED pattern of melt-spun Cu47.5Zr46Al6.5 metallic glass ribbons, further indicating the ribbons are in amorphous state.
3.2. Degradation of azo dyes by metallic glassy ribbons The AO Ⅱ aqueous solution was used to examine the catalytic degradation capacity of the current CuZr-based metallic glasses. It is found that the strong characteristic absorption peak located at λmax = 484 nm in the UV-vis spectra producing in the azo bond (-N = N-). The intensity of the bond is proportional to the concentration of the reaction solution. Fig. 2(a) presents the UV-vis spectra of AO Ⅱ aqueous solution decomposed by Cu47.5Zr46Al6.5 metallic glassy ribbons at 40 ℃ under the acidic condition of pH = 2 without mixing into any other chemical reagent. It can be obviously seen that the intensity of the UV-vis absorption peak gradually becomes weaker with the
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extension of reaction time. Eventually, AO Ⅱ aqueous solution was decolorized thoroughly within 30 minutes. The influence of the Cu content and Al content on the degradation rate was illustrated in Figs. 2(b) and (c), respectively. The six CuZr-based metallic glassy ribbons with the different nominal compositions can all exhibit the unexpected high performance for zao dye degradation
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because of the minor differences in the components of those samples. However, it is clearly shown that the degradation rate was enhanced with the increase of the Cu content in the metallic glassy
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ribbons. It is possible that the ribbon samples are more likely to expose numerous copper nanoparticles owning to strong catalytic property throughout the reaction, which can provide even
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more reaction active sites and create a larger amount of ·OH radicals to react with dye molecules to further improve degradation rate. The similar behavior occurs to the alloy of the different Al
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contents. The increase of Al content can also accelerate the rate of dye degradation, due to the fact that the Al element in the ribbons are contacted with reduction hydrogen [H], hydrogen ion H+ or even extremely strong oxidized ·OH radicals generated in the entire solution system, which will be
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further elaborated in the following part of the mechanism. The degradation process can be confirmed to the first-order reaction kinetics following Ct/C0 = (1 - Cf/C0) exp (-kt) + Cf/C0，where
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Ct and C0 are expressed as the dye concentration at reaction time and initial state individually, Cf is the final residual concentration, t is the reaction time, and k represents the reaction rate constant [40]. Further, the parameter k can be referenced as the degradation rate and the degradation efficiency is calculated by the formula η = 1 - Cf/C0. Fig. 2(d) displays the degradation efficiency and degradation rate of the AO Ⅱ aqueous solution by six different CuZr-based metallic glassy ribbons. The degradation efficiency is relatively close, and the degradation rate illustrates a rising trend, which consists with the above-mentioned data analysis. In contrast with the other five samples, the
Cu47.5Zr46Al6.5 metallic glassy ribbons accomplish the highest prominent degradation rate constant of k = 0.165 and simultaneously demonstrates a superb degradation efficiency of η = 97.68%. Therefore, the CuZr-based metallic glassy ribbons exhibit an outstanding degradation effect on the AO Ⅱazo dyes. As is well known, the different environmental conditions have a significant impact on the azo dye degradation process. Herein, the effects of various experimental parameters, including the surface area of ribbons, reaction temperature and initial pH were systematically investigated. Fig. 3(a) displays the effect of surface area of ribbons on the degradation behaviors of AO Ⅱ aqueous solution. When the surface area of ribbons increased from 700 cm2/L to 1300 cm2/L, the
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degradation reaction of AO Ⅱ aqueous solution continuously maintains a much faster rate and can be completely degraded within 40 min. Besides when the ribbons with surface area of 1000 cm2/L and 1300 cm2/L were selected, the dyes degradation time can be rapidly promoted within 30 min.
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All of the above results declare that the surface area of ribbons performs the significant influence throughout the degradation process, which reduces the degradation time.
The reaction temperature is another important factor, which seriously affects the degradation
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rate of AO Ⅱ aqueous solution. Fig. 3(b) shows the degradation rate of AO Ⅱ dyes at various
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temperatures, which is in range of 303 K-333 K. As can be seen, increasing the temperature of the solution can remarkably improve the dye degradation ability, which indicates that the reaction belongs to thermally activated process [40]. It is obvious that there is a certain difference of the
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degradation rate in the first fifteen minutes, and along with the time growing, AO Ⅱ aqueous solution is almost fully degradation within 40 minutes under the four temperature conditions. The
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proposed reason is that higher temperature can accelerate the thermal motion of dye molecules in solution. On the other hand, generating amount of ·OH radicals with strong oxidation performance,
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providing more sources to approach and react with the dye molecules, can notably enhance the dye degradation. In contrast, when the temperature is low, the solution cannot supply sufficient ·OH radicals, causing a lower reaction rate and speed. All the results mentioned above demonstrate that the CuZr-based metallic glassy ribbons can be able to create the excellent and favorable catalytic capability for applications in azo dye aqueous solution treatments in high temperature system. The effect of pH range of 2 - 10 on the degradation of AO Ⅱ aqueous solution has been studied and the results are shown in Fig. 3(c). AO Ⅱ aqueous solution was completely degraded within 30 min when the pH of solution equals to 2. Furthermore, the highest degradation efficiency of dye aqueous
solution, namely 97.68%, was obtained only at initial solution pH = 2 under other optimum reaction conditions currently. However, in an acidic solution with a pH of 3, the AO Ⅱ aqueous solution was decomposed hardly. And almost no degradation reaction occurs in neutral solution of pH = 7 or alkaline solution of pH = 10. Therefore, it can be elaborated that the factor of pH values is a decisive and effective requirement in the dye degradation. Fig. 3(d) illustrates the degradation efficiency of dyes of the first, third, fifth, eighth and tenth cycle, respectively. After each trial, the catalysts were recycled by some simple filtration followed by washing with water and ethanol. Similar degradation efficiency of dye aqueous solution was obtained in the ten repeated reactions, revealing that the catalyst exhibits outstanding cycle stability performance. The possible reason is
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that the formed oxidation products on the surface of the glassy ribbons containing potential and excellent catalytic properties can be able to provide more active sites to achieve faster degradation performance after 10 cycles.
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Next, we investigated the catalytic behavior for degradation of AO Ⅱ aqueous solution in two solution systems, which were adjusted with diluted H2SO4 solution and HCl solution, respectively.
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Surprisingly, two distinct phenomena were observed, which AO Ⅱ aqueous solution was completely degraded in a solution adjusted with diluted H2SO4 solution and not in another case. It
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may be doubted that whether Cl- in solution play a crucial role to contribute to dye degradation course. To prove that concentration of Cl- is key to the observation of the successful accomplish of the dye degradation. A control experiment by adding four different chemical substances (only 1.4
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mM NaCl, only 0.7 mM H2SO4, 1.4 mM NaCl and 0.7 mM H2SO4, 1.4 mM NaCl) into solution separately was investigated. The purpose is to ensure that the concentration of H+ or Cl- is exactly
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consistent, reaching the basic requirements of the comparative test. The experimental results are shown in Fig.4 (a). It is observed that when only one of H+ and Cl- is present in the solution alone,
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no degradation reaction occurs throughout the process. However, when H+ and Cl- coexist in the solution, the samples show the perfect degradation behaviors. It is known that the corrosion reaction can take place at specific concentrations or long enough reaction time due to attacking on materials by Cl-. In this work, the appearance of pitting is responsible for the action of relatively low concentration of Cl- on the surface of ribbons in the presence of H+ as a depolarizer, leading to the bareness of a large amount of fresh copper nanoparticles on the surface of the samples. Exposed copper nanoparticles can plenty destroy dye macromolecule to achieve decomposition due likely to so-called size effect. That is to say, generated high surface energy and unstable atoms are easily
adsorbed and bonded to other atoms or molecular groups. Moreover, many active atoms with large surface area were fabricated by reducing particle size to improving the catalytic capability. In addition, the copper nanoparticles as efficient catalysts coordinating partial ·OH radicals formed in solution fright against dye macromolecule, thereby further enhancing degradation performance. Fig. 4(b) further shows the effect of the degradation rate of dyes on the different concentration ratios of H+ and Cl-. It has clearly revealed that when the concentration of H+ and Cl- are both 1.4 mM, dye solution can be degraded positively. On this basis, the degradation rate reduced significantly when one of the concentrations of H+ or Cl- will cut down 0.4 mM, but still retained efficient when the concentration of H+ or Cl- raise to 2.4 mM, indicating that the optimal concentration of H+ and Cl-
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should be at least greater than 1.4 mM. This part can further explain previous report of the degradation effect in two acidic dye solution with pH = 2 and pH = 3 in Fig. 3(c), because the concentration of H+ and Cl- in the solution system of pH = 2 is even higher than those in the solution of pH = 3.
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The good synergic effect of H+ and Cl- in the degradation process of AO Ⅱ aqueous solution
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was demonstrated for further SEM observation. Fig. 4(c-f) display the surface morphology of Cu47.5Zr46Al6.5 metallic glassy ribbons after degradation in NaCl, H2SO4, NaCl and H2SO4 and HCl
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solution system, respectively. The smooth and glossy surface morphology of the ribbons without any precipitates is shown in Fig. 4(c) and 4(d). Rather, the irregular and rough surface morphology is clearly discovered in Fig. 4(e) and 4(f), and it is obviously found that some loose and spares
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oxidation products were distributed on the surface of the ribbons. It is correctly proved again that H+ and Cl- coexist to contribute to dye degradation behaviors. Further, stability and reusability test
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of Cu47.5Zr46Al6.5 amorphous catalysts in terms of dye degradation were conducted under identical and optimal reaction conditions.
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3.3. Structure and surface morphology Fig. 5 shows that the XRD curves of fresh and reacted Cu47.5Zr46Al6.5 ribbons after first, third,
fifth, eighth and tenth cycles, respectively. The samples after three cycles exhibit the sharp crystalline diffraction peaks at 2θ = 43° and 2θ = 50°, which mark the unique characteristic peak of elemental copper. Moreover, the relative intensity of the peaks gradually increase with the increase of the cycle number, whereas the copper characteristic peaks appear approximately near 2θ = 50°
after eighth and tenth cycles. The above analysis results show that the copper nanoparticles separate out on the surface of the ribbons, which is consistent with the previous assumption. In order to further certificate the reaction mechanism of degradation process, the variation of surface morphologies for Cu47.5Zr46Al6.5 metallic glassy ribbons before and after the reaction was discussed. As seen from Fig. 6(a), the surface of the initial ribbons is quite smooth with metallic luster. An irregular surface topography dispersed with fine and uniform oxidation products after the first use is shown in Fig. 6(b). In addition, the EDS patterns (inset of Fig. 6(a) and Fig. 6(b)) show the initial ribbons and the first reacted ribbons, separately. Compare to two curves, the peaks are both determined as Cu, Zr and O elements. It is found that the formation oxidation products can be
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amply verified on the basis of dramatically increasing of the large amount of O element. Nevertheless, the content of Cu element has sharply reduced after degradation reaction, which tends to increase the contact area between Cu element and reaction solution, thereby realizing the
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abundant generation of ·OH radicals[41,42]. Compared to the first reused ribbon surface, the content of the oxides were gradually increased after the third and eighth use, as shown in the Fig. 6(c) and
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Fig. 6(d). After the tenth use (Fig. 6(e)), an exaggerated situation, expanding the corrosion pit and increasing the oxide particles size, is clearly presented. Fig. 6(f) shows a low magnification photo of
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the surface morphology of eighth run recycled amorphous ribbons. The countless oxidation products attaching to the inside and edged of the etch pit are first discovered on the surface of the eighth use ribbons. Fig. 6(g) and (h) show a low magnification photo of the surface morphology and
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the corresponding mapping result of Cu, Zr, Al and O elemental distributions of tenth run recycled amorphous ribbons. It is confirmed that the area where Zr and Al elements have no existence
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belongs to CuxO deposits, promoting mass electron transfer[43]. The more and more substances, emerging from a serious of oxidizing reaction of Cu, CuxO and other elemental, were generated and
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collected on the surface of ribbons along with reactions. The formation of surface oxidation may be the reason for speeding up the degradation of dyes. The catalytic active sites can be uniformly distributed on the entire material surface due to the single-phase structure of the amorphous alloy. Besides, the high activation energy exists on the surface of the metallic glassy material depending on the metastable nature, which is beneficial to the catalytic degradation reaction. This phenomenon can indicate that more catalytically active sites are provided and electron transfer rates are enhanced, resulting in higher degradation efficiency.
To further obtain the chemical states of elements and electronic structure on the surface, the XPS analysis of the surface of the as-spun and reacted ribbons after first, third, fifth, eighth and tenth cycles were investigated, respectively. Fig. 7(a) shows the strong binding energies of Cu 2p, Zr 3d, Al 2p, C 1s and O 1s in XPS full range spectrum, which can confirm no changes in composition of the Cu47.5Zr46Al6.5 ribbons before and after the degradation reaction[31]. And the results are consistent with the EDS analysis in Fig. 6(a) and (b). As shown in Fig. 7(b), the XPS spectrum of Cu 2p3/2 in the fresh CuZr-based ribbons includes only one peak that locates at 932.4 eV assigned to metal state Cu0. The oxidized state Cu+ a can be observed for the ribbons after the first cyclic reaction based on the relevant Cu2O satellite peak. However, the spectrum of Cu 2p3/2
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can be divided into two peaks, locating at 932.4 eV corresponded to mixed valence Cu0/Cu+ and approximate 933.6 eV corresponded to oxidized state Cu2+ after the third cyclic reaction, respectively. It can clearly reveal that degradation reaction leads to the formation of Cu2O and CuO.
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The catalytic ability for degrading dyes is further enhanced the owning to synergistic effect of the generated Cu2O phase and bare copper nanoparticles. More importantly, forming the Cu2O phase on
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an amorphous matrix material can provide more catalytically active sites and accelerate electron transfer. Fig. 7(c) displays the comparative XPS spectrum of Zr 3d5/2 for ribbons before and after
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the degradation. Zirconium is a reactive metal and Zr is usually found as ZrO2, which promotes amorphous state formation in alloy and does not take part in the catalytic degradation reaction. The Zr 3d5/2 peaks in the fresh CuZr-based ribbons locate at 178.8 eV corresponded to metal state Zr0
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and 182.4 eV corresponded to oxidized state Zr4+ after the reaction. The position of Zr 3d5/2 peak can all shift towards the higher binding energy assigned to oxidized state Zr4+ after the reaction. Fig.
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7(d) displays the comparative XPS spectrum of Al 2p among the as-spun and reacted ribbons. The no significant changes in the position and shape of Al 2p peak can be observed visually after the
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catalytic degradation. Aluminum usually forms a passivation film without a hitch on the surface, and oxide peak position may vary with the thickness of the film, which is near 74.7 eV corresponded to oxidized state Al3+ before and after the reaction. Based on the analysis of the surface micromorphology and electronic structure, the copper nanoparticles and Cu2O phase can be evenly distributed on the surface of the ribbons, which is conducive to achieving more excellent degradation performance. 3.4. Proposed mechanism
Based on the above analysis, the schematic diagram of the degradation of AO Ⅱ dyes by using Cu47.5Zr46Al6.5 metallic glassy ribbons with higher activity and stability can be illustrated in Fig. 8. The most probable catalytic mechanism can be divided into following two pathways: corrosion reaction and catalytic degradation process. As for the pitting reaction, the interaction between H+ and Cl- can attracted to the extraordinary role to contribute to the formation of fresh copper nanoparticles[44-46]. Thus, appropriate concentrations of H+ and Cl- will invade and attack the surface of the ribbons brazenly, bringing about the copper nanoparticles to be exposed naturally in solution. Moreover, the plenty of oxidation products can be adsorbed on the inside and edged of the etch pit tightly, which
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supporting the extensive catalytic activity sites depending on the size effect. The exposed copper nanoparticles and the produced Cu2O with strong reducing ability can directly reduce the dye molecules in the solution.
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As for the degradation process, the reaction pathway is extremely similar to Fenton-like reaction, which can utilized the strong oxidizing ·OH radicals produced through activating and
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dissociating O2 in the presence of H+ in contact with Cu0/Cu+ to degrade pollutants[47-49]. Based on the above section, it can be clearly indicated the precipitated copper nanoparticles and Cu2O phase
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on the Cu ligament surface can both destroy O2 molecules to develop ·OH radicals under acidic conditions, which oxidize azo dye molecules to degrade into non-toxic and harmless carbon dioxide (CO2), water and other species.
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On the other hand, a mere handful of Al elements in the catalysts also have more and less a certain contribution in the catalytic degradation experiments, which corresponds to the results in Fig.
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2(c). In summarize, the degradation mechanism can actually be divided into two portions. The first part can be interpreted as the direct reduction of zero-valent Al. Secondly, new reduced hydrogen
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[H] can act on the reduction of azo bonds under the action of surface catalysis[31,32,50]. 4. Conclusions
An efficient catalyst of CuZr-based metallic glassy ribbons was synthesized via melt-spinning technology successfully. The catalysts clearly demonstrated the excellent and miraculous degradation ability towards azo dyes of AO Ⅱ, which could be obtained complete decomposition within 30 min using Cu47.5Zr46Al6.5 metallic glassy ribbons under the optimum conditions. In addition, the high degradation efficiency of AO Ⅱ is attributed to the generation of ·OH radicals
through the haptoreaction between H+/O2 and Cu ligaments and Cu2O nano-crystals during the reaction, further explaining the reason for superior durability and stability than other typical glassy ribbons catalysts in the re-used reactions. The research work indicates the Cu based metallic glassy ribbons as high activity catalysts are benefited in the degradation of azo dyes containing wastewater. Acknowledgements The work was supported by the National Natural Science Foundation of China (Nos. 51801209, 51790484, U1738101), DongGuan Innovative Research Team Program (2014607134), LiaoNing
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Revitalization Talents Program (XLYC1802078 and XLYC1807062), the fund of Shenyang National Laboratory for Materials Science, Shenyang Amorphous Metal Manufacturing Co., Ltd.,
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Shenyang 110000, China.
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Figure captions
Fig. 1. (a) XRD patterns and (b) DSC curves of melt-spun CuZr-based metallic glass ribbons, (c) HRTEM image and (d) SAED pattern of the melt-spun Cu47.5Zr46Al6.5 metallic glass ribbons.
Fig. 2. (a) Typical UV-vis spectra of AO Ⅱ aqueous solution degraded by Cu47.5Zr46Al6.5 metallic glassy ribbons, (b) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Cu contents as a function of reaction time, (c) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Al contents as a function of
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reaction time, (d) comparison of degradation efficiency and degradation rate for six different composition ribbons (AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃ , pH = 2).
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Fig. 3. Effect of (a) surface area of ribbons, (b) temperature and (c) pH on the normalized concentration of AO Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) the
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degradation of AO Ⅱ aqueous solution with Cu47.5Zr46Al6.5 metallic glassy ribbons at the different cycles (if not mentioned, AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃,
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pH = 2).
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Fig. 4. Effect of (a) additive types and (b) c(H+) and c(Cl-) on the normalized concentration of AO Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, surface morphology of the Cu47.5Zr46Al6.5 metallic glassy ribbons after degradation with the different additive types of (c) NaCl
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solution, (d) H2SO4 solution, (e) NaCl and H2SO4 solution, (f) HCl solution (if not mentioned, AO
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Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃, pH = 2).
Fig. 5. XRD patterns of the reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
Fig. 6. (a) SEM image of EDS analysis (inset) of melt-spun Cu47.5Zr46Al6.5 metallic glassy ribbons, (b) SEM image of EDS analysis (inset) of first reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (c) SEM image of third reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) SEM image and (f) low magnification SEM image of eighth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (e) SEM image,
(g) low magnification SEM image and (h) mapping results of tenth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
Fig. 7. XPS spectrum of (a) the full range spectra, (b) Cu 2p, (c) Zr 3d and (d) Al 1s in binding energy regions for the as-spun and reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
Fig. 8. Schematic illustration of the degradation mechanism of AO Ⅱ azo dyes using CuZr-based
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metallic glassy ribbons.
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Figure list:
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Fig. 1. (a) XRD patterns and (b) DSC curves of melt-spun CuZr-based metallic glass ribbons, (c)
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HRTEM image and (d) SAED pattern of the melt-spun Cu47.5Zr46Al6.5 metallic glass ribbons
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Fig. 2. (a) Typical UV-vis spectra of AO Ⅱ aqueous solution degraded by Cu47.5Zr46Al6.5 metallic
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glassy ribbons, (b) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Cu contents as a function of reaction time, (c) the normalized concentration of AO Ⅱ aqueous solution at 484nm treated by samples of different Al contents as a function of
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reaction time, (d) the comparison of degradation efficiency and degradation rate for six different composition ribbons (AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃ ,
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pH = 2).
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Fig. 3. Effect of (a) surface area of ribbons, (b) temperature and (c) pH on the normalized concentration of AO Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) the
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degradation efficiency of AO Ⅱ aqueous solution with Cu47.5Zr46Al6.5 metallic glassy ribbons at the different cycles (if not mentioned, AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L,
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temperature: 40 ℃, pH = 2).
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Fig. 4. Effect of (a) additive types and (b) c(H+) and c(Cl-) on the normalized concentration of AO
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Ⅱ aqueous solution using Cu47.5Zr46Al6.5 metallic glassy ribbons, surface morphology of the Cu47.5Zr46Al6.5 metallic glassy ribbons after degradation with the different additive types of (c) NaCl solution, (d) H2SO4 solution, (e) NaCl and H2SO4 solution, (f) HCl solution (if not mentioned, AO Ⅱ: 100 mg/L, surface area of ribbons: 1000 cm2/L, temperature: 40 ℃, pH = 2).
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Fig. 5. XRD patterns of the reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
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Fig. 6. (a) SEM image of EDS analysis (inset) of melt-spun Cu47.5Zr46Al6.5 metallic glassy ribbons, (b) SEM image of EDS analysis (inset) of first reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (c) SEM image of third reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (d) SEM image and (f) low magnification SEM image of eighth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons, (e) SEM image, (g) low magnification SEM image and (h) mapping results of tenth reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
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Fig. 7. XPS spectrum of (a) the full range spectrum, (b) Cu 2p, (c) Zr 3d and (d) Al 1s in binding
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energy regions for the as-spun and reacted Cu47.5Zr46Al6.5 metallic glassy ribbons.
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Fig. 8. Schematic illustration of the degradation mechanism of AO Ⅱ azo dyes using CuZr-based
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metallic glassy ribbons.