Facile Synthesis of Pd/Cu2O Octahedral with Enhanced Photocatalytic Activity and Its Application of Degradation of Direct Red 278
Mahboubeh Sepahvand
1
(
Department of Chemistry, Islamic Azad University, Arak Branch, Arak, Iran
)
Reza Fazaeli
2
(
Department of Chemistry, Faculty of Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran
)
Saeed Jameh-Bozorghi
3
(
Department of Chemistry, Faculty of Science, Hamedan Branch, Islamic Azad University, Hamedan, Iran
)
Al Niazi
4
(
Department of Chemistry, Islamic Azad University, Arak Branch, Arak, Iran
)
Keywords: Degradation, Octahedral, CCD, Pd/Cu2O, DR 278,
Abstract :
In this study, copper oxide (Cu2O) particles were synthesized by octahedral shape and then were loaded with palladium (Pd). Cu2O and Pd/Cu2O, which was prepared by X-ray diffraction (XRD), scanning electron microscopy (SEM)/ Energy-dispersive X-ray spectroscopy (EDS), Brunauer-Emmett-Teller (BET) analysis and Barrett-Joyner-Halenda (BJH), Transmission electron microscopy (TEM) and Diffuse Reflectance Spectroscopy (DRS) methods, was identified. Degradation of Direct Red 278 (DR 278) was investigated by Cu2O and, Pd-Cu2O octahedral. The photocatalytic activity of Cu2O was improved by the loaded Pd. The effect of factors such as pH, time (min), mass of catalyst (g) and concentration (mg/L) was evaluated by central composite design (CCD). Among the factors, pH, mass of catalyst, concentration and time have been affected the degradation process, respectively. The highest efficiency of dye degradation was obtained at optimum conditions with pH = 2.95, mass of catalyst 0.11 (g), concentration 177.56 (mg/L) and time 11.02 (min), 99.99%. Based on the results, Pd/Cu2O octahedral is a very effective catalyst for the destruction of textile effluents. The kinetic study results indicated that pseudo first-order has a good agreement with the experimental data.
Facile Synthesis of Pd/Cu2O Octahedral with Enhanced Photocatalytic Activity and Its Application of Degradation of Direct Red 278
Abstract:
In this study, copper oxide (Cu2O) particles were synthesized by octahedral shape and then were loaded with palladium (Pd). Cu2O and Pd/Cu2O, which was prepared by X-ray diffraction (XRD), scanning electron microscopy (SEM)/ Energy-dispersive X-ray spectroscopy (EDS), Brunauer-Emmett-Teller (BET) analysis and Barrett-Joyner-Halenda (BJH), Transmission electron microscopy (TEM) and Diffuse Reflectance Spectroscopy (DRS) methods, was identified. Degradation of Direct Red 278 (DR 278) was investigated by Cu2O and, Pd-Cu2O octahedral. The photocatalytic activity of Cu2O was improved by the loaded Pd. The effect of factors such as pH, time (min), mass of catalyst (g) and concentration (mg/L) was evaluated by central composite design (CCD). Among the factors, pH, mass of catalyst, concentration and time have been affected the degradation process, respectively. The highest efficiency of dye degradation was obtained at optimum conditions with pH = 2.95, mass of catalyst 0.11 (g), concentration 177.56 (mg/L) and time 11.02 (min), 99.99%. Based on the results, Pd/Cu2O octahedral is a very effective catalyst for the destruction of textile effluents. The kinetic study results indicated that pseudo first-order has a good agreement with the experimental data.
Keywords: Pd/Cu2O, octahedral, degradation, DR 278, CCD
INTRODUCTION
Textile industry is one of the most influential industries in the economy of many countries. Effluents of these industries have always been as a government concern [1-3]. The wastewater in these chemical industries is often highly toxic and resistant to biodegradation, low BOD and high COD. The quality of the wastewater changes over time and may include many types of dyes, detergents, sulfide compounds, solvents, heavy metals and mineral salts [4-8]. Dyes are considered as one of the most dangerous species in industrial effluents, which interfere with biological activities in waters and toxic, mutagenic and carcinogenic. Dyes have a complex chemical structure and high molecular weight, which enter to the environment, due to various processes. Since the discharge of this wastewater causes significant environmental damages, it is important to use a proper wastewater treatment method. Physical and chemical processes such as adsorption [9], membrane filtration [10], coagulation [11], biological oxidation [12], chemical degradation [13] are used for wastewater treatment from textile that the disadvantages of these methods include low efficiency, high cost, non-recycling of materials and secondary pollution. Nowadays, photocatalytic methods have been considered to eliminate these disadvantages from new methods of removal toxic pollutants and chemicals from air and wastewater [14]. Among the various semiconductors, TiO2 is one of the most suitable compounds for various environmental applications [15,16]. But, due to limitations in the TiO2 synthesis method including the difficulty of the method and its high costs, synthesis of Cu2O is a good alternative for TiO2 replacement, because of its simplicity of the synthesis method, cost-effectiveness and relatively high efficiency. Cu2O is a very good choice for photocatalytic activities. This compound as p-type semiconductor with band gap (2.0-2.2 eV) has optical, electronic and catalytic unique properties; therefore, it has a high potential for use in solar cells, gas sensors, and the degradation of organic pollutants [17-22]. In recent years, various morphologies of Cu2O, including nanospheres, nanowires, nanocubes and core-shell structures have been synthesized. The basis of semiconductor photocatalysts is based on electron/hole excitation. Increasing impurity to semiconductors is widely used to adjust the structure of semiconductor strips to increase the absorption of light or to coordinate the reduction potential of the target reaction. In this study, Cu2O with octahedral morphology was synthesized in a simple and cost-effective manner. In order to enhance its photocatalytic properties, Pd was loaded on Cu2O octahedral. The photocatalytic degradation of DR 278 was investigated by Cu2O and Pd/Cu2O octahedral. The effect of parameters such as pH, time, mass of catalyst and concentration was evaluated by CCD and was finally optimized.
MATERIAL AND METHODS
Chemical
Copper (II) acetate (Cu (CH3COO)2), D-glucose (C6H12O6 97.5%), Palladium(II) nitrate dihydrate ~40% (Pd(NO3).2H2O2), hydrogen peroxide (H2O2 30%) and titanium dioxide (TiO2) degussa p25 was purchased from Sigma-Aldrich. Direct Red 278 was purchased from Colorant Limited Company.
Chemical Structure of DR 278 is shown Fig. 1.
Synthesis of Cu2O octahedral
First, 1.8 g of Cu (CH3COO)2 was dissolved in 100 mL of distilled water and was stirred for 30 min at room temperature (Solution 1). Then, 0.99 g of C6H12O6 was dissolved in 50 mL of distilled water and was placed on a magnetic stirrer for 20 min at room temperature (Solution 2). Then solution 1 and 2 were mixed together and were placed for 4 h at 90 ° C in a water bath on a magnetic stirrer. The resulting precipitate was washed several times with distilled water and then was dried for 2 h at 80 ° C for in oven.
Synthesis of Pd/Cu2O octahedral
Pd was loaded Cu2O via wet impregnation method. 1 g of Cu2O was added to 50 mL of distilled water and was placed on a magnetic stirrer (mixture 1). Then, 0.04 g of Pd(NO3).2H2O2 was added to 50 mL of distilled water and was placed on a magnetic stirrer (mixture 2). Finally, mixture 2 was added dropwise to mixture 1. The mixture was placed on a magnetic stirrer for 24 h without heating. The resulting precipitate was washed several times with distilled water and was dried for 2 h at 100 ° C in the oven. After drying in an oven, the sample was calcinated for 4 hour at 550°C.
Design of experiments
Design of experiments (DOE) is a systematic method to do a series of experiments systematically for identifying the importance of process factors, their interactions and their control in achieving optimal response. The purpose of this method is to obtain reliable and appropriate results based on a limited number of observations. Most of the tools in this field are divided into two groups, which include test design and analysis tools in the field of test design, methods such as full factorial method, Latin-square based methods. In the field of test analysis, analysis of variance and its derivatives as well as regression analysis are known as the most important tools. The response surface methodology (RSM) is used to the minimum number of tests optimize multivariate processes [23]. DOE saves time and materials by decreasing the number of experiments. CCD is one of the types of statistical schemes, which is used to model response levels and is a suitable alternative to factorial experiments [24,25]. This scheme was proposed by Box and Wilson (1951) and was completed by Box and Hunter 1957. Using the central composite scheme, maximum information can be extracted using the least run through the distribution of experimental points in the desired range. In this study, based on the CCD method, RSM was used to investigate the effect of independent variables including X1(pH), X2(Time (min)), X3(mass of catalyst (g)) and X4 (concentration (mg/L)) for degradation DR 278 at 3 levels, which is shown in Table 1.
(1)
An amount of 100 mL of DR278 with different concentration and pH using Pd/Cu2O octahedral with different of mass (g), and 0.01 mL H2O2 (30%) was poured into quartz cell and exposed to UV-C light reactor for different time. After reaction time, all experiments were centrifuged at 7000 rpm for 15 min. Subsequently, the DR 278 concentration was analyzed by UV-Vis spectrophotometer. Degradation of pollutants was calculated according to the following equation:
(2) 
=Degradation
A0 is the initial and Af is the final absorbance of the DR278 solution.
Study of reaction kinetics
In optimal conditions carried out reaction kinetics. The reaction mixture was irradiated under UV-C light for 2 to 12 min. After Pd/Cu2O octahedral reaction at different times, the samples was centrifuged for separation of the catalyst at a speed of 10,000 rpm for 30 minutes. Then, the DR 278 concentration was analyzed by UV-Vis spectrophotometer.
Pd/Cu2O Octahedral Recyclability
For this purpose, after the reaction in UV-C exposure, the Pd/Cu2O octahedral was separated by centrifugation, after which the catalyst was added to 50 mL of distilled water and placed on a magnetic stirrer for 12 h. The catalyst was then washed several times with absolute ethanol and dried at 80 °C for 2 h. The recycled Pd/Cu2O octahedral was again used in the degradation DR 278.
RESULTS AND DISSECTIONS
In Cu2O octahedral the peaks observed at 29.64, 36.52, 42.42, 61.55, 73.73 and 77.61° corresponding to (110), (111), (200), (220), (311) and (222) crystal faces the cubic structure of Cu2O, respectively (JCPDS No. 01-077-0199). Also, in Pd-Cu2O octahedral the peaks observed at 40.11, 46.50, and 68.11o which can be readily attributed to the (111), (200) and (220), related to Pd. The XRD results show that, Pd were successfully loaded on the surface of Cu2O (Fig.2). Using XRD peak broadening analysis with the Debye-Scherer (DS) and Williamson-Hall (W-H) the crystallite size of the catalysts and the strain were found.
Debye-Scherer Method
In the Debye-Scherrer (DS) method, crystallite sizes, which are calculated from each diffraction peak in the pattern, should have the same value. However, because of a systematic error, same crystallite size cannot be obtained from each diffraction peak. To eliminate the error, DS Equation (3) can be rearranged using the least square method. By this rearrangement, the DS method was evolved [26]:
D=
(3)
D is the crystallite size of synthesized particles, k is the shape factor, λ is the wavelength of CuKα radiation (λ = 0.154 nm), β is the peak broadening and θ is the Bragg angle.
Williamson–Hall analysis
The crystallite size, lattice strain, lattice stress and energy density of crystal can be estimated by the Williamson–Hall (W-H) analysis. This method relies on the principle that the approximate formula for size broadening DS and strain broadening (Bε), vary quite differently with respect to Bragg angle, θ. Size broadening can be related to DS formula as in Eq. (4). The strain induced broadening arising from crystal imperfection and distortion can be related as in Eq. (4)
Bε = Cε tan θ (4)
Where, ε is either maximum tensile strain alone or maximum compressive strain alone which can be calculated from the observed broadening and C is the constant equals to 4 that depends on the assumptions made concerning the nature of the inhomogeneous strain. One contribution varies as 1/cosθ and the other as tanθ. If both crystallite size and strain contributions present independently to each other, then their combined effects can be determined by convolution. The simplification of W–H is to assume the convolution is sum of DS and Bε [27].
Bhkl = BDS + Bε (5)
Bhkl = kλ Dcosθ + 4ε tan θ (6)
Bhkl cosθ = kλ D + 4ε sin θ (7)
In this research the crystallite sizes Cu2O and Pd/Cu2O octahedral calculated by DS and W-H. The results demonstrate in Table 2.
Fig. 3 shows SEM images of Cu2O and Pd/Cu2O octahedral. Synthesized Cu2O has octahedral morphology in nano size with smooth surfaces. Based on the SEM images of Pd/Cu2O octahedral, the Pd particles are well loaded on Cu2O octahedral. EDS was analyzed in order to determine the exact composition of Pd-Cu2O octahedral. Fig. 4 shows EDS spectrum of the Pd-Cu2O octahedral in which Cu 66.83%, O 26.98%, and Pd 6.19 % are present. As it is shown in Fig 5, Via TEM imaging was used to examine Pd/Cu2O octahedral. Dark spots indicate that the Pd particles are loaded very well on Cu2O octahedral. Determination of the optical band gap (Eg) in semiconductor is a key issue in understanding the extent of on electronic properties and it usually involves some analytical approximation in experimental data reduction and modeling of the light absorption processes. In the past decade, several studies focused on the optical properties of semiconductor. The band gap property of the synthesized Cu2O and Pd/Cu2O octahedral was evaluated using Tauc plot established on UV-vis absorption spectra [28]. The equation is as follows:
(αhν) 1 / n = A (hν-Eg) (3)
Where, h is the Planck constant, ν is the applied frequency, α is the absorption coefficient, Eg is band gap, and A is a proportional constant. The band gap values for Cu2O and Pd/Cu2O octahedral of 2.1 and 1.7 (eV) were determined by Tauc method. The diagrams are shown in Fig. 6. Based on the results, it can be concluded that by loading Pd on Cu2O Octahedral, the band gap is decreases. Specific surface measurement, diameter, volume and cavities size distribution of material have important applications. Catalysts are substances, which are very important to know about their specific surface area. Nowadays, various methods are used to measure porosity, shape of holes and specific surface area of materials. BET/BJH method is among these methods, which is much simpler, more accurate and reliable than other methods. In this study, Cu2O and Pd/Cu2O was analyzed by BET/BJH method, the results have been shown in Table 3. Based on the results, the surface area has been increased and total pore volume has been decreased by loading of Pd on Cu2O octahedral. According to the absorption/desorption curves model, it can be concluded that the hysteresis ring is for Cu2O and Pd/Cu2O octahedral of type D (Fig. 7).
Effect of Pd (wt%) on the process of photocatalyst degradation of DR 278
Increasing impurities, due to strong physical and chemical properties such as high specific surface area and decreasing size, causes to enhance the performance of the particles. In addition, impurities decrease the electron recombination by trapping the electrons created during the adsorption process, thereby increase the efficiency of the photocatalytic process. Several experiments were conducted to investigate the effect of weight percent of Pd on Cu2O octahedral. Based on the results in Table 4, it can be concluded that the highest percentage of degradation is Pd/Cu2O octahedral (1 and 2%) under UVC light. Since Pd 1% is less costly, further experiments were done to optimize the process by Pd-Cu2O 1% catalyst under UVC light.
Effect of pH
The pH, as one of the most important factors affecting photocatalytic reactions, influences the surface charge of catalyst particles. The effect of pH on the efficiency of degradation DR 278 was evaluated by changing the initial pH. HCl (1M) and NaOH (1M) were used to change the pH of the solutions. Based on the results, pH changes have a significant effect on process efficiency.
Analysis of variance (ANOVA)
Experiments were performed to investigate the effect of parameters, the results of which are shown in Table 5. Statistical assumptions are the basis of one or more variable statistical tests. Important conditions for analyzing multivariate data are the assumptions of normality, linearity and uniformity of data dispersion. If one or more of these assumptions are ignored, it occurs in the statistical results of bias or distortion. Normality is the most basic assumption of multivariate analysis. If this assumption does not exist, some specific statistical tests are invalid and not usable. Fig. 8 shows the normalization assumptions of data that the data is almost normal and the results show how the residues follow a normal distribution. This indicates a very good correlation between the results obtained by the experimental method and the values predicted by the statistical method. "Analysis of Variance" (ANOVA) technique is one of the most practical statistical methods in analyzing data. In this method, the variance of the total data is divided into two or more sections based on one or more factor variables. Normally, statistical studies are performed at 95% confidence level (alpha = 0.05) at a 95% confidence level if the P-value is smaller than 0.05. Regarding the ANOVA in Table 6, the R-Squared and Adj R-Squared values are 0.8969 and 0.8899, respectively, indicating the correctness of the model.
Among factors pH, concentration of DR 278, time and mass of catalyst were affected respectively.
Effect of pH
A solid surface in contact with a liquid or solution phase may have some electrical charge, in this case the electric charge accumulated on the solid surface with the liquid or solution in contact with it will be different. This difference in charge causes an electric potential difference on both sides of the solid phase and the soluble or liquid phase is referred to as the zeta potential. The pH parameter can affect the surface charge of the photocatalyst, the degree of ionization of the various pollutants, the cleavage of functional groups on the photocatalytic active sites as well as the structure of the dye molecule, so the pH of the solution is an important parameter during the dye degradation process. In this study, it is concluded that by decreasing the pH, the efficiency of photocatalytic removal increases because at low pH, direct reduction by electrons in the conduction band plays an important role in the decomposition of DR278 removal, which leads to a reduction gap in remove DR278. As the pH increases, the number of negatively charged sites increases. Since negative sites on the photocatalyst surface cannot be effective in adsorbing dye anions, increasing the pH will decrease the photocatalyst degradation capacity.
Fig. 9 shows zeta potential results for Pd/Cu2O octahedral at different pHs. The zeta potential is an indication of the surface potential, so it determines the magnitude of the electrical double layer repulsion. The total interaction between particles is the sum of the electrical double layer and the van der Waals interaction, which is determined by the magnitude of the Hamaker constant of the material. In this research, the point of zero charge of Pd/Cu2O octahedral was found to be at pHpzc= 6.8. The surface charge of the Pd/Cu2O octahedral photocatalyst becomes more negative with increasing pH, which in turn causes electrostatic reactions, and as a result, the negatively charged azo dye will be less adsorbed. When the pH of the solution is higher than pHpzc (pH=6.8), the negative charge on the surface causes electrostatic reactions that are effective in adsorbing cationic species. However, when the pH of the solution is lower than the pHpzc under acidic conditions, a positive charge is created on the photocatalyst surface, which is effective in adsorbing anionic species. Therefore, it can be concluded that negatively charged organic matter such as azo dye is well removed by the photocatalyst under acidic conditions.
Effect of mass of catalyst, concentration of DR 278 and time in degradation of DR 278
Based on the results by increasing mass of photocatalyst, many active sites increased and the efficiency was enhanced by increasing catalyst mass. On the other hand, increasing the photocatalyst concentration increases the turbidity and prevents the penetration of light into the solution and also decreases the rate of degradation. In this research, with increasing the photocatalyst mass from 0.02 to 0.1, the removal efficiency increased and then had a steady trend.
Reaction time is one of the most important parameters in the advanced oxidation process. In fact, the reaction time is the time required to reach the purification targets. High reaction time causes energy consumption and refining costs. Therefore, optimizing the process time saves operating costs. Also, by passing the time, the production of more hydroxyl radicals and the rate of dye removal will be increased. In the present study, more than 90% of dye removal occurs within 10 min, which suggests the catalyst has a very high activity. To evaluate the effect of concentration, solutions with concentrations of 100-300 (mg/L) were prepared. Based on the results, efficiency was decreased by increasing dye concentration. The reason is that, increasing the concentration of the dye decreases the penetration of UV light and the occupation of the active sites on the catalyst surface, reducing the production of OH, as a result, it decreases efficiency (Fig 10).
Optimal conditions
The highest efficiency of dye degradation was obtained at optimum conditions with pH = 2.95, mass of catalyst 0.11 (g), concentration 177.56 (mg/L) and time 11.02 (min), 99.99%. Based on the results, Pd/Cu2O octahedral is a very effective catalyst for the destruction of textile effluents (Fig. 11).
Comparison of Pd/Cu2O octahedral and TiO2 performance
Under optimal conditions, DR278 photocatalytic degradation was performed by TiO2 and Pd/Cu2O octahedral photocatalysts and the degradation efficiencies were 49.67 and 99.99%, respectively. Based on the results, it can be concluded that Pd/Cu2O octahedral catalyst has a very high performance in the degradation efficiency of DR 278.
The stability of Pd/Cu2O octahedral
One of the important factors in a catalyst’s performance is the number of cycles of using that. For this purpose, after the degradation of DR278, was separated, washed, and used for the same reaction. The results indicated that after 5 times of using the catalyst, the reaction efficiencies decreased 3.45% for Pd/Cu2O octahedral.
The kinetic study
The pseudo first-order (Logergren), pseudo second-order (Elovich and Blanchard) and intraparticle diffusion kinetic models were used to analyze the kinetic data [1]. The result is shown in Table 7. The kinetic results showed that the Logergren model, with the highest correlation coefficient, was the best fit with the experimental data.
ACKNOWLEDGEMENT
The authors gratefully acknowledge to Colorant Limited Company, India. Mr. Bharat J Trivedi
CONCLUSIONS
Azo dyes are widely used in the textile industry. These types of dyes are not easily eliminated from the textile wastewater and their entry into the environment can have irreparable consequences. In this study, Cu2O particles were synthesized by octahedral shape and then were loaded with Pd. The photocatalytic degradation of DR 278 was investigated by Cu2O and Pd-Cu2O octahedral. The photocatalytic activity of Cu2O was improved by the loaded Pd. Among the factors, pH, mass of catalyst, concentration and time affected the degradation process respectively. The highest efficiency of dye degradation was obtained at optimum conditions with pH = 2.95, mass of catalyst 0.11 (g), concentration 177.56 (mg/L) and time 11.02 (min), 99.99%. The results show that, Pd/Cu2O is effective photocatalyst for the degradation of DR 278; its photocatalytic ability is stronger than that of Cu2O octahedral, which is expected to be very effective for textile effluent treatment of wastewaters. The kinetic results showed that the Logergren model, with the highest correlation coefficient, was the best fit with the experimental data.
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