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Article

Preparing Cu2O/Al2O3 Coating via an Electrochemical Method for the Degradation of Methyl Orange in the Process of Catalytic Wet Hydrogen Peroxide Oxidation

by
De-bo Liu
1,*,
Ping Zhang
2 and
Jian Wang
2
1
Faculty of Engineering, Huanghe Science and Technology College, No. 666 Zijingshan South Road, Zhengzhou 450063, China
2
Institute of Intelligent Manufacturing, Xinxiang Vocational and Technical College, Xinxiang 453006, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(11), 1308; https://doi.org/10.3390/catal12111308
Submission received: 10 July 2022 / Revised: 15 September 2022 / Accepted: 19 September 2022 / Published: 25 October 2022
(This article belongs to the Special Issue Advances on Catalysts Based on Copper)
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Abstract

To improve the catalytic efficiency and decrease the reaction temperature of wet air oxidation technology, a Cu2O/Al2O3 coating was prepared on the surface of aluminium alloys by anodizing technology, and subsequent heating treatment. Then, the Cu2O/Al2O3 coating and 3 wt.% H2O2 was used to degrade methyl orange. The influence of the coating’s microstructure, crystalline component on the degradation rate of the methyl orange was studied. The microstructure of the coating was observed by scanning electron microscope. Results proved that the coating was composed of micropores, and Cu2O was evenly dispersed on the surface and pores in the Al2O3 coating. X-ray diffraction pattern analysis demonstrated Cu2O and Al2O3 characteristic peaks were found after the coating was treated at 300 °C, showing that amorphous Cu2O and Al2O3 were transformed into crystalline oxide. A UV-vis spectrophotometer was used to measure the absorbance of methyl orange, and it was found that the maximum absorption wavelength of methyl orange is 460 nm. At that wavelength, the suitable degradation condition of methyl orange was studied, and results showed that when electrochemical deposition time was 30 min and catalyst dosage was 8 g, the degradation rate of methyl orange could reach 92% at 25 °C for 120 min. Furthermore, when the catalyst was reused 9 times, the degradation rate still reached 75%. Based on the above results, a kinetic equation between the degradation rate of methyl orange and catalyst dosage was derived. The microstructure and crystalline component of the catalyst after different reuse times were characterized, and results showed that the catalytic efficiency of the Cu2O/Al2O3 coating decreased with a decrease in the coating’s specific surface area and the ratio of Cu2O in the coating.

1. Introduction

Dyes have the characteristics of high organic content, high salinity, complex composition, high chromaticity, and poor biodegradability. They enrich color but damage the environment [1]. Most of these dyes are toxic, non-biodegradable and carcinogenic due to their large size and complex structure. In fact, most dyes are composed of thiocarboxylic acid groups, nitro groups, carboimino groups, and especially azo groups, which are difficult to degrade. Azo dyes have high toxicity and are harmful to various water microorganisms, fish species. Furthermore, they cause serious damage to the humans, including damage to kidneys, the reproductive system, liver, brain, and central nervous system they are exposed to azo dyes for a long time. Azo dyes are known to be mutagens and carcinogens due to the presence of amines, and related metabolites. Thus, the treatment of azo dyes wastewater has become an urgent problem [2]. Because their accumulation creates potential risks and threats to aquatic organisms and human being [3], studying an effective treatment for azo dye wastewater has become a great challenge to researchers all over the world [4,5,6]. Many methods, such as adsorption [7], membrane separation [8], extraction [9], chemical oxidation [10], coagulation [11], wet air oxidation [12], photocatalytic oxidation [13], and microbial degradation [14], can be adopted to degrade azo wastewater. Wet air oxidation technology (WAO) is an advanced oxidation technology developed during the last century that is suitable for treating harmful, and toxic wastewater. However, the degradation of pollutants is often carried out at high temperature and high pressure, and high operating costs and low degradation efficiency limit its application. The standard oxide electrode potential of OH is 2.80 V, as H2O2 can produce OH, which has a strong oxidative capacity to degrade pollutants. Therefore, during the WAO process, adding H2O2 can improve the degradation efficiency of pollutants. However, the addition of H2O2 cannot decrease the activation energy of degradation, and the service life of ·OH is short. As a result, the degradation efficiency cannot satisfy requirements for pollution treatment [15]. If a suitable catalyst is mixed during the WAO process, the activation energy of the reaction can be reduced, and the pollutants can be completed degraded in a shorter time and under milder conditions. Therefore, the catalytic wet hydrogen oxidation method (CWPO) was studied to improve the WAO technology [16]. Due to the strong oxidative capacity of OH and low activation energy, the CWPO method has moderate reaction conditions, short treatment periods, and low treatment cost. The catalyst’s activity and selectivity can be adjusted to degrade different organic matters. According to the status of the catalyst, the CWPO process can be divided into homogeneous and heterogeneous degradation processes [17]. Soluble Cu, Fe, Ni and Mn salts have been used as catalysts for degradation of wastewater. In this process, the CWPO process is homogeneous reaction, and the Fenton oxidation method is used most to degrade dye wastewater [18]. When solid catalysts are used to decompose H2O2, OH is generated to degrade organic matters and the CWPO process is a heterogeneous reaction [19]. In the homogeneous process, the active components of the catalyst can be dissolved in the wastewater, but this decreases the catalytic activity and causes secondary pollution. Compared with the homogeneous process, the heterogeneous process does not require subsequent treatments, and has therefore received extensive attention [20].
The reaction process of CWPO can be seen in Table 1, and it consists of three stages [21], in the process of CWPO, preparation of a suitable catalyst is key to improving the degradation efficiency of pollutants.
Compared with precious metals [22,23] and rare earths [24], Cu-based catalysts have higher activity and a lower price. Due to its uneven atomic distribution, high specific surface area, excellent physicochemical properties, and the occurrence of wide range of oxidation states of copper, copper and copper (I) oxide nanoparticles have diverse activity and promote several types of reactions including oxidations, reductions, cross coupling, A3 coupling, electrocatalysis and photocatalysis [25]. The reaction mechanism of Cu-based catalysts is similar to that of Fe3+/Fe2+ catalysts during the CWPO process, but their kinetic constants are 4–5 orders of magnitude higher than that of the Fenton reaction [26]. Therefore, a Cu-based catalyst can produce more OH faster, and the degradation efficiency of dye pollutants can be improved. The intermediate products of dye wastewater degradation are organic acids. The pH range of a Cu-based catalyst is wider than that of an iron-based catalyst, and the complexes between organic acids and a Cu-based catalyst are decomposed more easily compared to those of an Fe-based catalyst. Therefore, Cu-based catalysts are better in obtaining a complete degradation product, and have become the most promising catalysts in the process of CWPO [27]. As nano Cu (I) oxide has several active sites and oxygen vacancies, and can also adsorb and excite O2 as a highly active oxygen species, it has higher activity than micron Cu (I) oxide [28]. However, nano Cu (I) oxide is easily washed away and agglomerated, and suffers from photo-wavering and photo-corrosion, which limits its application [29]. When it is supported on the surface of porous materials, such as γ-Al2O3 [30], TiO2 coatings [31], activated carbon [32], and porous polymers [33], the process of wastewater treatment can be simplified, and its service life can be prolonged. To reduce the particle size and improve the stability of nano Cu, Cu (I) oxide [34], and composite oxides [35] can be used. Because γ-Al2O3 has a high specific surface area, and good thermal stability and mechanical stability, it is widely used as a carrier in various catalysts. Because γ-Al2O3 has both acidic and alkaline centers and an activated H-H, C-H bond, it can provide active components to a variety of catalytic reactions, and can also be used as a bifunctional catalyst [36]. γ-Al2O3 has tetrahedral structure, which can enhance catalytic activity by forming a composite oxide with Cu (I) oxide. When Cu (I) oxide is loaded with γ-Al2O3, the oxygen atom of the Cu (I) oxide interacts with a tricoordinated Al atom. When each Cu atom attaches to a tricoordinated O atom bound to hexacoordinated Al atoms, the Cu-O distance and Cu-O-Cu angle are changed, and the catalytic activity of Cu (I) oxide can be improved [37]. Therefore, using Cu2O/γ-Al2O3 to degrade dye wastewater has received extensive attention [38,39].
In the process of CWPO, H2O2 is used to promote the degradation of pollutants. Normally, the degradation efficiency of pollutants increases with increased temperature. However, the decomposition rate of H2O2 also increases with increased reaction temperature. Therefore, degradation should be conducted at room temperature. Highly dispersed Cu (I) oxide is easily agglomerated, so improving the degradation efficiency and stability of the Cu2O/γ-Al2O3 catalyst is a challenge in the CWPO process. Using aluminium alloys as the anode, in H2SO4 solution, an anodic coating with a regular porous structure can be prepared easily on the surface [40]. This sample can be employed as the cathode, and Cu used as the anode. In the same electrolyte, Cu (I) oxide can be deposited in the holes on the surface of aluminium alloys uniformly [41]. Although the Al2O3 and Cu2O in the coating are amorphous, they can be transformed into crystalline structures by a heating method [42]. The crystalline transformation temperature for amorphous alumina into γ-Al2O3 is 300 °C [43], thus, the heating temperature is set at 300 °C. Based on the above, to enhance the efficiency and stability of a Cu2O/Al2O3 catalyst, a Cu2O/Al2O3 coating was prepared by an electrochemical method. The aperture distribution of the coatings was analyzed by scanning electron microscopy (SEM) and Image J software. The crystalline composition of the catalyst was studied by energy dispersive spectrometer (EDS), an X-ray diffractometer (XRD), and X-ray photoelectron spectroscopy (XPS). The catalytic efficiency and stability of a Cu2O/Al2O3 coating on methyl orange was studied by UV-Vis spectroscopy (UV-Vis) at room temperature.

2. Results and Discussion

2.1. Microstructure of the Cu2O/Al2O3 Coating

After the samples were anodized at a current density of 50 mA/cm2, reaction temperature of 4 °C for 40 min in 10 wt.% H2SO4 solution and with electrochemical deposition for 30 min, the micro surface and cross-section of the Cu2O/Al2O3 coating without and with heating temperatures of 200, 300, and 400 °C were studied by scanning electron microscopy. Results are shown in Figure 1. After the treatment, there was a Cu2O/Al2O3 coating on the surface of aluminium alloys, and the coating was closely bonded with the substrate. Figure 1a,c,e,g shows that the heating treatment increased the number of micropores on the surface. However, with increase in heating temperature, microcracks on the surface increased. Moreover, the coating thickness became uneven, which would lead to the catalyst being easier to separate from the substrate and decrease the catalytic activity of the Cu2O/Al2O3 coating. Figure 1b,d,f,h shows that there were through-pores in the coating, and with the increased heating temperature, the pore sizes increased. As methyl orange is organic, with a benzene ring and the azo bond, it is effective only when the pore size is big enough. When the heating temperature was 200 °C, the pore size was too small. When the heating temperature was 400 °C, there were too many cracks in the coating, and when the heating temperature was 300 °C, there were few cracks and a suitable pore size, which could enhance the catalytic activity of the Cu2O/Al2O3 coating.

2.2. Elements and Crystalline Structure Analysis of the Cu2O/Al2O3 Coating

Image J software was used to study the distribution of pore sizes on the Cu2O/Al2O3 coating before and after being heated at 300 °C; the results were shown in Figure 2a. It can be seen that when the Cu2O/Al2O3 coating was prepared on the surface of aluminium alloys, there were many through-pores in the coating. Most of the pore diameters were less than 10 nm, and a few were more than 100 nm. After the coating was heated at 300 °C, the ratio of pore sizes less than 10 nm decreased, and that of pore sizes more than 100 nm increased. Although the ratio of mesopores varied slightly, the ratio of the large pore, mesopores, and small pores was more uniform, which was conducive to improving the catalytic performance of the Cu2O/Al2O3 coating in degrading organic compounds and its intermediates. The distribution of Cu, Al, and O was studied by an energy dispersive spectrometer, and the results are displayed in Figure 2b. After electrochemical deposition and heating treatment, Cu2O was dispersed on the surface or in the pore of anodic coatings uniformly. Even when magnification of the SEM photos was 20,000 times, Cu (I) oxide particles could hardly be seen, indicating that Cu (I) oxide was loaded in the Al2O3 coating in the form of nano Cu2O.
Compared with amorphous Cu (I) oxide, crystalline Cu (I) oxide has better catalytic activity; however, Cu (I) oxide nanoparticles obtained by electrochemical deposition technology are amorphous. To transform amorphous Cu (I) oxide into a crystalline structure, samples were heated at 300 °C with an anodic current density of 50 mA/cm2, a reaction temperature of 4 °C for 40 min in a 10 wt.% H2SO4 solution. Samples were prepared with and without the coating via electrochemical deposition for 30 min, and were heated at 300 °C. Their XRD spectra are shown in Figure 3. Before the Al2O3 and Cu2O/Al2O3 coatings were heated at 300 °C, their XRD spectra were smooth curves, illustrating that the coatings were composed of amorphous components. When the coatings were heated at 300 °C, obvious characteristic peaks appeared [44], which proved the amorphous Cu (I) oxide and Al2O3 were transformed into crystalline structures. At 44.3°, 45.4° and 66.6°, there were Al2O3 characteristic peaks in the XRD spectra of the Al2O3 and Cu2O/Al2O3 coatings, while at 38.2°, 77.9° and 85.1°, there were Cu2O characteristic peaks in the XRD spectra of the Cu2O/Al2O3 coating, but not in the Al2O3 coating. This shows that amorphous coatings could be transformed into crystalline Cu2O/Al2O3 coatings by anodizing, electrochemical deposition, and the heating treatment.
Figure 4 illustrates full and narrow range X-ray photoelectron spectroscopy of Cu2O/Al2O3 coatings prepared with an anodic current density of 50 mA/cm2, a reaction temperature of 4 °C for 40 min in a 10 wt.% H2SO4 solution, and after electrochemical deposition of the coatings and heating at 300 °C. The binding energy values corresponding to the optoelectronic peaks of Cu 2p are 929.66 eV and 949.52 eV [45], respectively. Since the binding energy complies well with the standard value (932.5 eV) of Cu2O, and the standard value of Cu 2p 3/2 binding energy in different copper oxides, it can be confirmed that the Cu (I) oxide in the Al2O3 coating was Cu2O.

2.3. Degradation of Methyl Orange Catalyzed by the Cu2O/Al2O3 Coating

The catalytic performances of Cu2O/Al2O3 coatings were studied by assessing degradation of a methyl orange solution at a concentration of 20 mg/L. The effect of process parameters on the degradation of methyl orange is illustrated in Figure 5. It can be seen that without catalyst, methyl orange was not degraded within 180 min. When using the Al2O3 coating as a catalyst, the degradation of methyl orange was low within 180 min, whereas the degradation rate of methyl orange increased when using the Cu2O/Al2O3 coating as the catalyst, especially when 3% or 8% H2O2 was mixed in the solution and the coating was heated at 300 °C. The results show that the crystalline Cu2O/Al2O3 coating has a good degradation effect on methyl orange in the process of CWPO. Furthermore, using H2O2 as an oxidant can increase the degradation rate of methyl orange. However, increasing the H2O2 concentration can only accelerate the degradation efficiency, but not improve the degradation rate of methyl orange [46].
Based on the above research, using H2O2 with a concentration of 3 wt.% as the oxidant, and the Cu2O/Al2O3 coating as the catalyst, methyl orange can be degraded within 120 min. However, the effect of process parameters, such as load mass of Cu (I) oxide, degradation temperature, catalyst dosages, and stability was not clear. To study the suitable degradation condition of methyl orange in the process of CWPO, the influence of the above parameters was investigated, and results are presented in Figure 6. It can be seen from Figure 6a that within 30 min, the absorbance of methyl orange decreased with the increase of electrochemical deposition time. When the electrochemical deposition time increased to 35 min, the absorbance decreased. In addition, it can be seen from Figure 6b that the absorbance of methyl orange at 25 °C was lower than that at 15 °C and 40 °C.
Figure 6c shows that the absorbance of methyl orange decreased with an increased catalyst dosage from 3 to 8 g, and when the catalyst increased to 10 g, the absorbance of methyl orange was maintained. Moreover, Figure 6d illustrates that the absorbance of methyl orange gradually increased with increasing of catalyst reuse times. According to the absorbance of methyl orange solution is linearly related to its concentration. Compared with the initial methyl orange solution, its absorbance decreased by different degrees with different deposition times, catalytic temperature, and catalyst dosage. The smaller the absorbance, the higher the degradation efficiency of the Cu2O/Al2O3 coating. To further study the influence of catalytic condition on the degradation rate of methyl orange, the degradation rate of methyl orange at different conditions was investigated. Results are shown in Figure 7.
Figure 7a shows that with prolonged treatment time, the degradaton rate of methyl orange increased. When the deposition time of Cu (I) oxide was less than 30 min, the degradation of methyl orange increased, while when the deposition time increased to 35 min, the degradation rate of methyl orange was maintained. The main reason is that the load mass of Cu (I) oxide increased with the increase of deposition time. When the deposition time was short, the catalytic activity of Cu2O/Al2O3 coatings increased with the increase in load mass of Cu (I) oxide. However, in the action of electrochemical deposition, Cu (I) oxide was absorbed not only in the pore but also on the surface of the Al2O3 coating. When the deposition was too long, most absorbance was on the surface of the coating, and removed during the subsequent cleaning process. In addition, the micro pores in the Al2O3 coating were sealed by Cu (I) oxide, which reduced the active site of the catalyst, and decreased the activity of the Cu2O/Al2O3 coating. It can be seen from Figure 7b that the degradation rate of methyl orange at 25 °C was much higher than at 15 °C and 40 °C, mainly because when the temperature is low, the catalytic activity of the Al2O3 coating is low, while when the temperature is high, the decomposition rate of H2O2 is too fast, both of which decrease the degradation rate of methyl orange. When the temperature was 25 °C, the rate of ·OH generation was high, and the decomposition rate of H2O2 was slow. Thus, the Cu2O/Al2O3 coating had a higher catalytic efficiency at 25 °C. From Figure 7c it can be seen that when the catalyst dosage was 3, 5 and 8 g, the degradation rate of methyl orange was 33.4, 61.2 and 92.0%, respectively. However, when the catalyst dosage increased to 10 g, the degradation rate of methyl orange increased little. The main reason is the catalytic reaction in the CWPO process is not only related to the catalytic activity but also to the adsorption and detachment rate. When the catalyst dosage reached 8 g, the reaction rate was no longer the rate-determining step. Therefore, the methyl orange degradation rate changed insignificantly. Moreover, in Figure 7d, it can be seen that the degradation rate of methyl orange gradually decreased with the increase of catalyst reuse times. After the catalyst was reused 9 times, the degradation rate of methyl orange still reached 75%, which proved the catalyst had good stability. Although the degradation rate of methyl orange was lower than that in the literature, during the CWPO process the degradation efficiency of Cu2O/Al2O3 coating was higher, and the reaction temperature was lower than that in the literature [47,48].
During the CWPO process, the degradation of methyl orange is first-order reaction. In our study, the concentration of H2O2 was 3 wt.%, and the initial concentration of methyl orange was20 mg/L; therefore, the kinetic model is [49]:
ln ( C t ) = A − k t ⋅ ln ( C t )
where Ct represents the concentration of methyl orange at different times, t represents reaction time, and k represents the reaction rate constant. k consists of three parts, namely, Cu2O electrochemical deposition time (k1), reaction temperature (k2), and catalyst dosage (k3).
According to Formula (1), taking ln(Ct) and t as straight lines, the slope is the reaction rate constant of methyl orange under different conditions. The degradation rates of methyl orange under different conditions were studied. The linear relation between the logarithm of degradation rate and operation parameters were fitted using Origin 9.1. Results are shown in Figure 8 and Table 2.
It can be seen that with the prolongation of deposition time, the reaction rate constant increased from 0.71 to 0.90, but when the deposition time reached 30 min, the degradation rate constant reached 0.89, showing that methyl orange could be rapidly degraded when the deposition was 30 min. The degradation rate constant of methyl orange at 25 °C was 0.84, while when reaction temperatures were 15 °C and 40 °C, the degradation rate constant were 0.59 and 0.67, respectively. This shows the degradation rate of methyl orange at 25 °C was the fastest. With increases of catalyst dosage from 3 g to 10 g, the degradation rate constant of methyl orange increased from 0.31 to 0.92, but when the catalyst dosage reached 8 g, the constant reached 0.92, indicating that further increasing the catalyst dosage could not accelerate the degradation rate of methyl orange. With increased reuse times, the degradation rate constant decreased slowly. When the reuse reached 9 times, the degradation rate constant was 0.74, and the catalyst still had high degradation efficiency, proving that the Cu2O/Al2O3 coating has excellent stability during CPWO process.
To further study why the degradation rate of methyl orange decreases with increased reuse times, scanning electron microscopy was used to investigate the micro surface structure and crystalline structure of the coating. Results ae showed in Figure 9 and Figure 10. It can be seen from Figure 9 that with the increase of reuse times, the size and quantity of the micro cracks in the Cu2O/Al2O3 coating increased, and big pores appeared in the coating. It is well known that the catalytic activity of the coating decreases with decreasing surface area; therefore, the degradation rate of methyl orange decreased with the increase of the coating’s reuse time. It also can be seen that the pore size of the coating decreased first and then increased with increasing reuse time. The probable reason is that methyl orange can be absorbed on the surface of the coating. After the catalyst was immersed in methyl orange solution, methyl orange was deposited on the surface of the coating, which decreased the pore size. With the further increase of reuse time, the methyl orange promoted the recrystallization of Cu2O in the coating. Therefore, the pore size of the Cu2O/Al2O3 coating increased.
An X-ray diffractometer was used to study the crystalline structure of the Cu2O/Al2O3 coating at different reuse times. Results are shown in Figure 10. It can be seen from Figure 11 that at about 36°, 58°, 80° and 85°, there are Cu (I) oxide characteristic peaks, at about 42°, 45°, 68° 70° and 75° there are Al2O3 characteristic peaks, and at about 47°, 52°, 62° and 82°, there are Cu (II) oxide characteristic peaks. It can be seen from Figure 9a–i that with the increase of reuse times, the peak intensity of Cu (II) oxide increases, while the peak intensity of Cu (I) oxide decreases, showing that some Cu (I) oxide in the coating is transformed into Cu (II) oxide. The main reason is the Cu (I) oxide in the coating is a metastable oxide. During the process of CPWO, with the action of H2O2 some Cu (I) oxide is transformed into Cu (II) oxide. Because Cu (I) oxide is the active species for degradation of methyl orange, the catalytic activity of the Cu2O/Al2O3 coating decreases with the decrease of Cu (I) oxide. Therefore, the degradation rate of methyl orange decreases with increasing reuse times.
With the increase of reuse times, a wide peak appeared in the range of 20–30°. The possible reason is that some alumina or Cu (I/II) oxide in the coating combined with water to form an amorphous product, which also led to decrease of catalyst activity. Figure 11 shows the infrared absorption peaks of Cu2O/Al2O3 coatings after catalyst reuse 0, 4, and 9 times. The peaks at 3406 cm−1 can be assigned to the O-H stretching vibrations, the peaks at 1630 cm−1, 648 cm−1 correspond to Cu-O stretching vibrations [50] and the peaks at 838 cm−1 can be assigned to Al-O stretching vibrations [51]. It can be seen that with increasing reuse times, the characteristic peak intensity of O-H increased, but the peak intensity of Al-O decreased, while the peak intensity of was Cu-O maintained. The FTIR spectra showed that crystalline Al2O3 or Cu (I) oxide combined with H2O formed amorphous products.
The degradation of methyl orange by other catalysts reported in the literature were compared with our current study and are shown in Table 3. It can be seen that the degradation efficiency of the Cu2O/Al2O3 coating is close to that of nanoparticle catalysts reported in the literature, while the reuse time can reach 9 times, and does not require other auxiliary measures such as light energy. Thus, the degradation process of methyl orange can be simplified.

3. Experiment

3.1. Experimental Materials

Each of the experimental reagents was provided by Tianjin Kemiou Chemical Reagent Co. Ltd. (Tianjin, China), and commercial aluminium alloy 1060 (20 × 20 × 1 mm3) was used as a specimen.

3.2. Experimental Process

3.2.1. Preparation of Cu2O/Al2O3 Coating

Using a DC supply, aluminium alloys were used as anodes and stainless steel was used as the cathode, respectively. The anodizing procedure was conducted with a stirring speed of 60 rpm and a current density of 50 mA/cm2 at 4 °C for 40 min in a 10 wt.% H2SO4 solution. Coatings were subjected to an anodizing treatment as cathodes, and a copper plate was used as the anode. Electrochemical deposition Cu (I) oxide on the surface of the coating lasted for 30 min. Finally, the Cu2O/Al2O3 coating was heated at 300 °C for 2 h.

3.2.2. Degradation of Methyl Orange

Methyl orange powder has a maximum absorbance at 460 nm (Figure 12a), which changes with its concentration linearly (seen in Figure 12b). A volume of 200 mL of methyl orange with a concentration of 20 mg/L was used to simulate dye wastewater, and the degradation of methyl orange was produced in a heated magnetic stirrer reactor at atmospheric pressure and 25 ± 1 °C for 3 h. Then, 2 mL methyl orange was removed from the original solution, and the degradation rate of methyl orange was determined by measuring its absorbance at 460 nm using a 723 N UV-Visible spectrophotometer.

3.3. Characteristics

The radiation emitted by the sample surface was detected by a Fourier transform infrared (FT-IR) spectrometer (JASCO FT/IR-6100). An X-ray diffractometer (XRD, (D/max-rB, RICOH, Tokyo, Japan) with CuKa sources was employed to examine the phase compositions for diverse coatings. The current applied and accelerating voltage were 30 mA and 40 kV, respectively. Scanning electron microscopy (SEM, S-4700, Hitachi, Tokyo, Japan) was used to investigate the cross-section and surface microstructures of the coating, while the porosity of the coatings was statistically analyzed by Image J software. An energy dispersive spectrometer was used for quantitative and qualitative analysis of element points, lines, and surfaces. The resolution of the secondary electron image of SEM was less than 3.0 nm, with an element detection range of Be(4)–Am(95). An AXIS-ULTRA X-ray photoelectron spectroscopy (XPS) was used to analyze the film. The analysis involved a Ka line of Al (150 W, 15 kV, 1486.6 eV), a vacuum degree of 10−9 Torr, and the carbon peak (284.8 eV) of the hydrocarbons on the surface of the standard sample. Before the test, Ar+ ions were used to bombard the inner cavity to remove the contamination layer on the sample surface. In addition, DR 5000 UV-vis spectroscopy was conducted to determine the absorbance of Cu2O/Al2O3 at different wavelength ranges. A 723 N UV-vis spectroscopy was used to test and compare the absorbance of methyl orange solution at 464 nm wavelength before and after photocatalysis treatment. Then, the degradation rate of the sample under room temperature and normal pressure conditions could be obtained. Scans were performed in the range of 200 to 800 nm at the scan rate of 1 nm/s−1. Formula (2) was used to determine absorbance before and after the photocatalytic treatment [55].
A = lg(It/I0)
In Formula (10), A is absorbance, It is transmitted light intensity, and I0 is the incident light intensity. According to Formula (2), the catalytic activity of Cu2O/Al2O3 can be determined by the degradation rate of methyl orange.
η = ( C 0 − C t ) C 0 × 100 % = ( A 0 − A t ) A 0 × 100 %
In Formula (3), η refers to the degradation rate of methyl orange (%), C0 and Ct represent methyl orange concentration (mg/L) before and after photocatalytic degradation, respectively, and A0 and At are the absorbances of methyl orange before and after treatment.

4. Conclusions

A Al2O3 coating was prepared on the surface of aluminium alloys by an anodizing technology with a current density of 50 mA/cm2 at 4 °C for 40 min in a 10 wt.% H2SO4 solution. Then, Cu (I) oxide particles were deposited electrochemically in the coating for 30 min.
The microstructure and crystalline structure of the coating was studied by SEM, XRD, EDS, and XPS, and results showed that the crystalline Cu (I) oxide was evenly dispersed in the pore and on the surface of the Al2O3 coating.
Suitable catalytic conditions were studied by measuring the absorbance of methyl orange, and results showed that the degradation rate of methyl orange could reach 92% when the electrochemical deposition time was 30 min, with a catalyst dosage of 8 g and a temperature of 25 °C for 120 min. The degradation rate was 75% after the catalyst was reused 9 times. The catalytic efficiency and condition of the catalyst was improved. Compared with similar catalysts reported in the literature, the Cu2O/Al2O3 coating has a similar degradation efficiency and higher stability. In addition, the degradation process of methyl orange can be simplified.

Author Contributions

Conceptualization, P.Z. and D.-b.L.; methodology, D.-b.L.; software, P.Z.; validation, P.Z. and D.-b.L.; formal analysis, P.Z.; investigation, D.-b.L.; resources, P.Z.; data curation, P.Z.; writing—original draft preparation, P.Z.; writing—review and editing, D.-b.L. and J.W.; visualization, P.Z. and J.W.; supervision, P.Z.; project administration, D.-b.L. and J.W.; funding acquisition, D.-b.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Zhengzhou Key Laboratory of Fiber Reinforced Polymer Composites (Grant No. 02032146) and Science and Technology Research Project of Henan Provincial Department of Science and Technology (Grant No. 212102210249).

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest to this work.

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Figure 1. Micro surface and cross-section of Cu2O/Al2O3 coating. (a) Micro surface and (b) cross-section of Cu2O/Al2O3 coating without heating; (c) micro surface; (d) cross-section of Cu2O/Al2O3 coating heating at 200 °C; (e) micro surface and (f) cross-section of Cu2O/Al2O3 coating heating at 300 °C; (g) micro surface and (h) cross-section of Cu2O/Al2O3 coating heating at 400 °C.
Figure 1. Micro surface and cross-section of Cu2O/Al2O3 coating. (a) Micro surface and (b) cross-section of Cu2O/Al2O3 coating without heating; (c) micro surface; (d) cross-section of Cu2O/Al2O3 coating heating at 200 °C; (e) micro surface and (f) cross-section of Cu2O/Al2O3 coating heating at 300 °C; (g) micro surface and (h) cross-section of Cu2O/Al2O3 coating heating at 400 °C.
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Catalysts 12 01308 g002
Figure 2. (a) Pore size and (b) element distribution of Cu2O/Al2O3.
Figure 2. (a) Pore size and (b) element distribution of Cu2O/Al2O3.
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Catalysts 12 01308 g003
Figure 3. X-ray diffraction (XRD) patterns of Al2O3 and Cu2O/Al2O3 coating before and after being treated at 300 °C.
Figure 3. X-ray diffraction (XRD) patterns of Al2O3 and Cu2O/Al2O3 coating before and after being treated at 300 °C.
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Catalysts 12 01308 g004
Figure 4. The (a) full range and (b) narrow range XPS spectrum of the Cu2O/Al2O3 coating.
Figure 4. The (a) full range and (b) narrow range XPS spectrum of the Cu2O/Al2O3 coating.
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Catalysts 12 01308 g005
Figure 5. Degradation rate comparison of methyl orange by different coatings.
Figure 5. Degradation rate comparison of methyl orange by different coatings.
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Catalysts 12 01308 g006
Figure 6. Influence of (a) electrochemical deposition time, (b) degradation temperature, (c) catalyst dosage, and (d) reuse times on the absorbance of methyl orange.
Figure 6. Influence of (a) electrochemical deposition time, (b) degradation temperature, (c) catalyst dosage, and (d) reuse times on the absorbance of methyl orange.
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Catalysts 12 01308 g007
Figure 7. Influence of (a) electrochemical deposition time, (b) degradation temperature, (c) catalyst dosage, and (d) reuse times on the degradation rate of methyl orange.
Figure 7. Influence of (a) electrochemical deposition time, (b) degradation temperature, (c) catalyst dosage, and (d) reuse times on the degradation rate of methyl orange.
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Catalysts 12 01308 g008
Figure 8. Fitting of methyl orange degradation kinetics model at different (a) electrochemical deposition time, (b) degradation temperature, (c) catalyst dosage, and (d) reuse times.
Figure 8. Fitting of methyl orange degradation kinetics model at different (a) electrochemical deposition time, (b) degradation temperature, (c) catalyst dosage, and (d) reuse times.
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Catalysts 12 01308 g009
Figure 9. Micro surface photograph of the Cu2O/Al2O3 coating after being reused (a) once, (b) twice, (c) 3 times, (d) 4 times, (e) 5 times, (f) 6 times, (g) 7 times, (h) 8 times and (i) 9 times.
Figure 9. Micro surface photograph of the Cu2O/Al2O3 coating after being reused (a) once, (b) twice, (c) 3 times, (d) 4 times, (e) 5 times, (f) 6 times, (g) 7 times, (h) 8 times and (i) 9 times.
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Catalysts 12 01308 g010
Figure 10. X-ray diffraction (XRD) patterns of Cu2O/Al2O3 coating after nine reuse times.
Figure 10. X-ray diffraction (XRD) patterns of Cu2O/Al2O3 coating after nine reuse times.
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Catalysts 12 01308 g011
Figure 11. FT-IR spectrum of Cu2O/Al2O3 coating after reuse 0, 4, and 9 times.
Figure 11. FT-IR spectrum of Cu2O/Al2O3 coating after reuse 0, 4, and 9 times.
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Catalysts 12 01308 g012
Figure 12. UV-Vis spectra of methyl orange at different (a) wavelengths and (b) concentrations.
Figure 12. UV-Vis spectra of methyl orange at different (a) wavelengths and (b) concentrations.
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Table 1. The reaction process of CWPO.
Table 1. The reaction process of CWPO.
Initiation of chainRH + O2 → R·+ HOO (RH represents organic matter)(1)
2RH + O2 → 2R·+ H2O2(2)
H2O2 + M → 2OH· (M represents catalyst)(3)
Development or transmission of chainRH + ·OH → R·+ H2O(4)
R·+ O2 → ROO·(5)
ROO·+ RH → ROOH + R·(6)
Termination of chainR·+ R·→ R-R(7)
ROO·+ R·→ ROOR(8)
ROO·+ ROO·→ ROH + RCOR2 + O2(9)
Table 2. Reaction rate equation, reaction rate constant, and variance of methyl orange degradation at different condition.
Table 2. Reaction rate equation, reaction rate constant, and variance of methyl orange degradation at different condition.
Reaction ParametersValueReaction Rate EquationKR2
Deposition time (min)15Ln (Ct) = −10.45 + 0.71 t0.710.98
20Ln (Ct) = −13.65 + 0.81 t0.810.98
25Ln (Ct) = −14.61 + 0.85 t0.850.98
30Ln (Ct) = −15.11 + 0.89 t0.890.98
35Ln (Ct) = −14.75 + 0.90 t0.900.98
Temperature (°C)15Ln (Ct) = −6.62 + 0.59 t0.590.99
25Ln (Ct) = −8.17 + 0.84 t0.840.95
40Ln (Ct) = −9.13 + 0.67 t0.670.97
Catalyst dosage (g)3Ln (Ct) = −2.66 + 0.31 t0.311.00
5Ln (Ct) = −4.02 + 0.57 t0.570.98
8Ln (Ct) = −12.12 + 0.92 t0.920.96
10Ln (Ct) = −11.56 + 0.92 t0.920.95
Reuse times1Ln (Ct) = −11.93 + 0.91 t0.910.96
2Ln (Ct) = −12.07 + 0.90 t0.900.96
3Ln (Ct) = −12.79 + 0.88 t0.880.97
4Ln (Ct) = −13.43 + 0.88 t0.880.96
5Ln (Ct) = −15.14 + 0.87 t0.870.95
6Ln (Ct) = −15.47 + 0.83 t0.830.96
7Ln (Ct) = −16.64 + 0.81 t0.810.94
8Ln (Ct) = −17.21 + 0.77 t0.770.95
9Ln (Ct) = −16.99 + 0.74 t0.740.94
Table 3. Comparison of degradation efficiency of Cu2O/Al2O3 coating with other catalysts reported in the literature.
Table 3. Comparison of degradation efficiency of Cu2O/Al2O3 coating with other catalysts reported in the literature.
CatalystReaction ConditionDegradation EfficiencyReference
Cu2ODye concentration: 10 mg/L94.54% in 4 h[52]
CuO/ZnODye concentration: 5 ppm
pH 6.8, Catalyst dosage: 0.6 g
Light source: UV-light		54.1% in 4 h[53]
CuO/NC NPsDye concentration: 30 ppm
Catalyst dosage: 0.015 g
Light source: UV and visible		97.18% in 4 min (UV)
95.96% in 4 min (Vis)		[54]
SiNWs–CuNPsDye concentration: 20 ppm
Dye amount: 20 mL
Light source: visible (5 LEDS)		89% in 120 min[3]
Cu2O/Al2O3Dye concentration: 20 mg/L
Catalyst dosage: 8 g
H2O2 concentration: 3 wt.%		92% in 120 minCurrent study
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Liu, D.-b.; Zhang, P.; Wang, J. Preparing Cu2O/Al2O3 Coating via an Electrochemical Method for the Degradation of Methyl Orange in the Process of Catalytic Wet Hydrogen Peroxide Oxidation. Catalysts 2022, 12, 1308. https://doi.org/10.3390/catal12111308

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Liu D-b, Zhang P, Wang J. Preparing Cu2O/Al2O3 Coating via an Electrochemical Method for the Degradation of Methyl Orange in the Process of Catalytic Wet Hydrogen Peroxide Oxidation. Catalysts. 2022; 12(11):1308. https://doi.org/10.3390/catal12111308

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Liu, De-bo, Ping Zhang, and Jian Wang. 2022. "Preparing Cu2O/Al2O3 Coating via an Electrochemical Method for the Degradation of Methyl Orange in the Process of Catalytic Wet Hydrogen Peroxide Oxidation" Catalysts 12, no. 11: 1308. https://doi.org/10.3390/catal12111308

APA Style

Liu, D.-b., Zhang, P., & Wang, J. (2022). Preparing Cu2O/Al2O3 Coating via an Electrochemical Method for the Degradation of Methyl Orange in the Process of Catalytic Wet Hydrogen Peroxide Oxidation. Catalysts, 12(11), 1308. https://doi.org/10.3390/catal12111308

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