Degrade Methyl Orange by Combining Cu2O/Al2O3 and H2O2 as Catalyst

To efficiently degrade printing and dyeing wastewater at low costs, Cu2O/Al2O3 catalyst was prepared by electrochemical method. Under its combined action with H2O2, photocatalytic degradation of methyl orange solution, as simulated wastewater, was carried out. With the aid of X-ray diffractometer, X-ray microanalysis system and scanning electron microscope, the structure and composition of catalyst film were designed by adjusting the electrochemical parameters, and the performance of photocatalyst was evaluated by the degradation rate of methyl orange. It was found that after anodizing aluminum alloy for 2 h in a 2 mol/L H2SO4 solution under a voltage of 15 V, alumina film with reasonable pore size distribution could be prepared on the alloy surface. By electroplating the film in a 0.01 mol/L CuSO4 solution at 5°C, and then hydrothermally treating it at 200°C, the Cu2O/Al2O3 photocatalyst could be obtained. The prepared Cu2O/Al2O3 and 10% H2O2 were used jointly as photocatalyst to degrade 10 mg/L methyl orange solution for 480 min, which achieved a methyl orange degradation rate of up to 82.1%.


Introduction
With the rapid development of printing and dyeing industry, the degree of environmental harm caused by dye wastewater has also been increasing [1].An effective means of addressing such environmental pollution is to completely degrade azo compounds in dye wastewater with various photocatalysts [2].
Copper-based catalysts, with advantages like low price, wide source and high activity for photocatalytic degradation of azo dyes, have received extensive attention from numerous researchers [3] [4].By fabricating such catalysts into films, the wastewater treatment process can be simplified, which prevents the catalyst agglomeration to extend their service life, thereby promoting their broad industrial application.H2O2, as a strong oxidizing agent, can promote the complete degradation of azo dyes to ensure the quality of degraded wastewater [5].However, its actual application in the industrial production is impossible, since large amount is required for degrading azo dyes, which leads to high economic cost of wastewater treatment.To tackle the problems of large capacity of azo dye wastewater treatment and difficulty of complete dye degradation, this study prepared Cu2O/Al2O3 catalyst by electrochemical method, which was then mixed with H2O2 to degrade methyl orange, a typical azo dye, with a view to providing necessary basic data for the treatment of azo dye wastewater.

Materials and instruments
1060 aluminum alloy (commercially available); H2O2, sulfuric acid, methyl orange, ethanol, sodium hydroxide, nitric acid and sodium silicate (all analytically pure, Kemiou Chemical Reagent, Tianjin).During the anodizing process, 4000A15V DC anodizing power supply and electrolytic cell (Rongdaxin Power Technology, Shenzhen) were used.During the hydrothermal reaction process, 250-mL hydrothermal reactor (Huotong Instrument, Beijing) was used.And in the photocatalytic degradation process of methyl orange, the Microsolar 300 xenon light source (PerfectLight Technology, Beijing) was utilized.

Experimental procedure
Anodized coating was prepared using surface-pretreated (alkali etching and bright dipping) 1060 aluminum sheet (20 × 20 × 0.5 mm 3 ) as the anode; sulfuric acid (100 g/L) and sodium silicate solution (15 g /L) as the electrolyte; symmetrically-placed stainless steel sheets (200 × 200 × 2 mm 3 ) as the cathode; and DC power supply as the external power source.Copper sulfate solution with a Cu 2+ concentration of 0.01 mol/L, which served as the electrolyte, was loaded into the micropores of alumina thin film by electrochemical technique, and then the film was hydrothermally treated to yield Cu2O/Al2O3.Under a 464-nm ultraviolet excitation, methyl orange solution was degraded using the above prepared catalyst in combination with H2O2 solution as the photocatalyst.The 464-nm absorbance of methyl orange was measured at 30-min intervals.The performance of the photocatalyst was evaluated based on the absorbance changes of methyl orange solution before and after photocatalysis, thereby investigating the effect of combined Cu2O/Al2O3 coating and H2O2 on the degradation rate of methyl orange solution.

Detection methodology
The microsurface and cross-section of the coating were observed under VEGA3 XMU field-emission scanning electron microscope (SEM), and its elemental composition was assessed with an energy-dispersive X-ray spectroscopy (EDS) analyzer.The detection conditions were: secondary electron image resolution of <3.0 nm; element detection range of Be (4)-Am (95).Utilizing the D/max-2500 X-ray diffractometer (XRD), the crystal phase composition of the coating was determined under the following conditions: tube voltage 40 kV; tube current 200 mA; measuring range 20°-80°; scanning speed 10°/min.The existence form of TiO2 in the coating was analyzed with AXIS ULTRA X-ray photoelectron spectrometer at the vacuum degree 10 -9 Torr, where the carbon peak (284.8 eV) of hydrocarbon on the standard sample surface was used as the standard binding energy value.The absorbance range of methyl orange was measured with DR 5000 UV-Vis spectrometer, and the degradation rate of methyl orange was calculated according to formula (1): where A0 denotes the absorbance of methyl orange solution before the reaction; At denotes the absorbance of methyl orange solution after reacting for a period of time.

XRD analysis of photocatalyst
Compare with amorphous Cu (І) oxide, crystalline Cu (І) oxide is featured with better catalytic activity, but Cu (І) oxide nanoparticles obtained by electrochemical deposition technology are amorphous.To transform amorphous Cu (І) oxide into a crystalline structure, the samples were heated at 300 ℃.At the anodic condition of a current density of 50 mA/cm 2 , reaction temperature of 4 ℃ for 40 min within 10 wt.% H2SO4 solution, with and without the coating were electrochemical deposition for for 30 min, before and after the caoting were heated at 300 ℃, their XRD spectrum were shown in figure 1.It can be observed that before the Al2O3 and Cu2O/Al2O3 coatings were heated at 300 ℃, their XRD spectrum was smooth curves, illustrating that the coatings were composed of amorphous components.When the coatings were heated at 300 ℃, obvious characteristic peaks appeared [6], which proved the amorphous Cu (І) oxide and Al2O3 could be transformed into the crystalline structure.Besides, at 44.3°, 45.4°and 66.6°, there were Al2O3 characteristic peaks in the XRD spectrum of the Al2O3 and Cu2O/Al2O3 coating, while at 38.2°, 77.9°and 85.1°, there were Cu2O characteristic peaks in the XRD spectrum of the Cu2O/Al2O3 coating, but it could not be seen in that of the Al2O3 coating.The result demonstrated that amorphous coatings could be transformed into crystalline Cu2O/Al2O3 coatings by anodizing, electrochemical deposition, and heating treatment.

SEM analysis of photocatalyst
The anodized aluminum alloy film and the hydrothermally treated film loaded with Cu2 ions were observed by SEM.Their microsurface morphologies are displayed in figure 2. From the SEM image (x2000) of Al2O3 film shown in figure 2(a), it is also clear that the surface of the prepared alumina film was full of micropores, which could provide the cuprous oxide with a pore-rich structure.Figures 2 (b,  c) show the SEM images (x200, x2000) of surface morphology for the Cu2-loaded Al2O3 film.As is clear, under the action of electric field force, Cu2O was uniformly distributed on the Al2O3 film surface in the form of spherical particles, which were loaded stably.According to figure 2, the structure of Cu2O/Al2O3 catalyst was as follows: Cu2O particles were loaded onto the prepared Al2O3, showing uniform distribution.The dispersed Cu2O distribution on the Al2O3 film could effectively prevent the occurrence of agglomeration during multiple cycles of azo dye wastewater degradation.

EDS analysis of photocatalyst
The elemental distribution was analyzed using an EDS analyzer on SEM, and the results are displayed in figure 3.As is clear from figure 3(a), spherically dispersed particles were present on the Al2O3 film, which were independent of each other.According to Figures 2 (b, c, d), the Al and O elements were evenly distributed, indicating that homogeneous Al2O3 film was prepared on the aluminum alloy surface.The Cu element was mostly distributed in the form of spherical particles on the film, while a small portion of it was evenly distributed on the film, suggesting that after electrochemical treatment, the Cu element loaded on the Al2O3 film was not prone to agglomeration [7].Since the catalytic activity strengthens with the increasing catalyst dispersity, the photocatalysts prepared by electrochemical loading technique should have higher photocatalytic performance.

Analysis of photocatalytic performance
For In the ultraviolet range, methyl orange has the greatest absorbance [8]. Figure 4(a) depicts the absorbance changes of methyl orange under different degradation times, where 1-cm 2 Cu2O/Al2O3 sheet was used as the photocatalyst, the concentration of methyl orange solution was 5 mg/L and the volume of H2O2 solution was 2 ml.As is clear, the absorbance of methyl orange decreased with the extension of degradation time, indicating increase in the degradation rate of methyl orange [9].According to the absorbance values of methyl orange at different catalytic times, it could be degraded into low-molecular-weight organic matter by the action of TiO2/Al2O3.To identify the intermediate products in its degradation process, the molecular weight of organic matter was analyzed by GC-MS under different degradation times, and the intermediate products of methyl orange degradation were analyzed by exploiting the spectral isolation function of the instrument under different degradation times.The results are displayed in figure 4(b).As is clear, the molecular weight of methyl orange solution decreased persistently with the extension of degradation time, indicating continuous photocatalytic degradation of high-molecular-weight methyl orange into small molecule compounds [10].During the photocatalytic degradation of methyl orange by Cu2O/Al2O3, the sulfonic and amino groups at both ends of methyl orange were removed first, and then the benzene ring broke away from the azo bond to generate phenol and azobenzene.Finally, the π-bond of benzene ring opened to become straight-chain alkanes, which were decomposed into small molecules.This suggests that the Cu2O/Al2O3 catalyst preferentially degraded the straight (branched) terminal chains of organic matter, while the azo bond and benzene ring were degraded last.

Effects of process parameters on methyl orange degradation rate
Given the great influence of preparation conditions of Cu2O/Al2O3 catalyst on its photocatalytic performance, the effects of anodizing voltage, anodizing temperature, electrolyte concentration and copper sulfate concentration on the degradation rate of methyl orange were investigated, in order to further analyze the effects of catalyst preparation parameters on the methyl orange degradation.Figure 5 displays the relevant results.It is clear from fiugure 5(a) that within a DC voltage range of 5-20 V, the methyl orange photodegradation rate of prepared catalyst was the highest when the anodizing voltage was 15 V, so the appropriate anodizing voltage was identified as 15 V.As is clear from figure 5(b), within an anodizing temperature range of -2-10℃, the methyl orange photodegradation rate of prepared catalyst was the highest when the anodizing temperature was within 5-10℃, so the appropriate anodizing temperature range was identified as 5-10℃.According to figure 5(c), the degradation rate of methyl orange increased with the rising H2O2 concentration.Nonetheless, it was also found that addition of only a small amount of H2O2 could produce a great impact on the photocatalytic degradation rate of methyl orange [11].Since H2O2 is a strong oxidizing agent, the primary purpose of its incorporation into the photocatalyst system is to promote the complete degradation of methyl orange [12].However, H2O2 is not reusable, and when its concentration is excessively high, the cost of dye wastewater degradation will increase accordingly, so photocatalytic degradation should be carried out at a low concentration.From the economic perspective, its appropriate addition amount was identified to be 6 ml per 200 ml of printing and dyeing wastewater.From figure 5(d), it can be seen that the degradation rate of methyl orange failed to increase with the rising concentration of copper sulfate solution.The probable reason was that although the Cu + loaded on the Al2O3 film increased in quantity at high concentrations of copper sulfate solution, these ions were lost from the catalyst surface during the subsequent cleaning process.Moreover, the increase in Cu + quantity would lead to agglomeration in the subsequent hydrothermal process, which would be detrimental to fully exerting photocatalytic activity as well.Hence, the appropriate concentration of copper sulfate was identified as 0.01 mol/L.Under the foregoing optimized process conditions, the degradation rate of methyl orange solution reached 82.143%.

4.Conclusions
1) Cu2O/Al2O3 catalyst is prepared by electrochemical technique.XRD analysis demonstrates that the prepared catalyst contains alumina and cuprous oxide.SEM analysis reveals loading of homogeneous Cu2O particles on the prepared Al2O3 film, while EDS analysis verifies uniform distribution of Cu, Al and O elements in the catalyst.
2) Under an anodizing voltage of 15 V, an anodizing temperature range of 5-10℃, a H2O2 addition of 6 ml/200 ml and a CuSO4 concentration of 0.01 mol/L, the Cu2O/Al2O3-H2O2 photocatalyst achieves a 82.1% degradation rate of methyl orange.

Figure 3 .
Figure 3. (a) SEM image of Cu2O/Al2O3 and distributions of (b) Cu, (c) Al and (d) O on the film

Figure 4 .
Figure 4. (a) absorbance of methyl orange after treated different degradation time (b )the molecular weight of methyl orange solution in different degradation time

6 Figure 5 .
Figure 5. Influence of (a) preparation voltage, (B) oxidation temperature, (c) concentration of H2O2 solution, (d) the effect of copper sulfate concentration on the degradation rate of methyl orange