The use of Silica Supported Nickel-Copper Oxide Catalyst for Photodegradation of Methylene Blue

Introduction

Textile industry is one of the fastest growing industries in Indonesia.¹ The rapid development of the textile industry poses a problem for the environment, especially the problem caused by textile dyeing liquid waste. The liquid waste contains toxic and dangerous compounds. The liquid waste in the waters can kill water organisms and block sunlight from penetrating the aquatic environment, thus disrupting the biological processes.²

Discharged dyestuffs from textile industry are generally in the form of organic compounds that are bio-nondegradable and difficult to be decomposed.³ Dyes often used in the textile industry are methylene blue, methyl orange, and Congo red. Methylene blue is a cationic heterocyclic aromatic compound commonly used as basic leather, cloth, and fabric coloring. The threshold limit value of methylene blue is 5-10 mg/L.

The conventional method for reducing the levels of methylene blue in water includes chlorination, ozonation, and adsorption using activated carbon.^4,5 New methods which are easy to implement and relatively inexpensive need to be developed. One of these is photodegradation using a combination of semi-conductor photocatalyst and ultraviolet light.^6,7 Harraz et al. (2019) was successful in engineering the photodegradation of methylene blue under UV and visible light using TiO₂@SiO₂ photocatalyst.⁸ The photodegradation will be more costly effective if undertaken under the sunlight.⁹

The use of photocatalyst has advantages because it can mineralize the total organic pollutants including the textile waste (azo compounds), the cost of their use is cheap and the process is relatively fast, non-toxic and has long-term use capability.^9,10 The ability of the photocatalyst can be improved by applying materials such as TiO₂, ZnO, Fe₂O₃, CdS, and ZnS which have semiconductor properties.^11–13 However, the materials have a relatively expensive price. Therefore, semiconductor materials need to be supported by inexpensive compounds such as silica (SiO₂).^14,15

Nickel and copper oxides are semiconductor materials. The bandgap energy of NiO and CuO is >3 eV and 2-3 eV, respectively.^16,17 To lower the bandgap of SiO₂ which is about 9 eV the nickel and copper oxides may be supported on the surface of silica. Silica-supported nickel and copper oxides are expected to work as a photocatalyst of methylene blue under sunlight.

Materials and Methods

Materials including SiO₂ (Merck, 99.3%), Ni(NO₃)₂.3H₂O (Merck, 99.99%), Cu(NO₃)₂.3H₂O (Merck, 99-104%), and Methylene Blue, are used without any prior treatment.

The (Ni-Cu)O_x@SiO₂ catalyst was prepared by mixing a proportional amount of Ni(NO₃)₂.3H₂O, Cu(NO₃)₂.3H₂O and SiO₂, and distilled water to a volume of 30 mL. The mixture was stirred using a magnetic stirrer and evaporated. Nearly dry mixture was added with ethanol, then methanol, stirred and filtered. The residue was air dried for 24 hours, and calcined at 800°C for 4 hours.

The XRD diffractogram of (Ni-Cu)O_x@SiO₂ catalyst was obtained by using X-Ray. Diffraction Rigaku Multiflex, and Cu-Kα radiation in the range 2θ from 5°-80°. The UV-Vis spectrum of the samples were recorded as a function of wavelength using the UV-Vis 1700 Pharmaspec Spectrophotometer Specular Reflectance. The samples were put into cuvettes and the absorbances were measured in a wavelength range of 200-800 nm. The samples of (Ni-Cu)O_x@SiO₂ were qualitatively analyzed using Scanning Electron Microscopy-Electron Dispersive X-Ray Analyzer JEOL JED-2300 working at 15 kV.

Dark adsorption test was undertaken by mixing 0.1 gram of the (Ni-Cu)O_x@SiO₂ catalyst with 50 ml of 10 ppm methylene blue solution in  a 100 mL Erlenmeyer. The mixture was stirred in a dark room. The absorption of the filtrate was measured at the time interval of  0, 5, 10, 15, 20, 25, 35, 65, 95, and 125 minutes, and at a wavelength of 663 nm.

Photocatalyst activity of the (Ni-Cu)O_x@SiO₂ under sunlight was measured by mixing 0.1 gram of the catalyst with 50 ml of 10 ppm methylene blue solution in a 100 mL Erlenmeyer. The mixture was exposed under the sunlight, and the absorbance of the filtrate was observed after 0, 5, 10, 15, 20, 25, 35, 65, 95, and 125 minutes of the sunlight exposure, at a wavelength of 663 nm.

Results and Disscussion

The grayish white of (Ni-Cu)O_x@SiO₂ catalysts were successfully synthesized. The XRD spectra indicate the presence of a broad-weak peak at 2θ = 21.8º (Figure 1.), which is a diffraction of SiO₂ tridymite (JCPDS No. 39-1425). Crystal size of (Ni-Cu)O_x@SiO₂ calculated using the Scherrer equation is 10.38 nm.^18,19

Figure 1: The XRD difractogram of (Ni-Cu)Ox@SiO2, Ni:Cu = (a) 0.2:0.1 and (b) 0.15:0.15. Click here to view figure

 

Unfortunately, the combination of SEM-EDX method is failed to prove the presence of both nickel and copper species on the surface of SiO₂. The micrographs only indicate the silica species, that is SiO₂ having particle size of between 0.1 – 15 µm (Figure 2). The EDX analysis suggests that both nickel and copper species are under limitation of EDX measurement (Figure 3).

Figure 2: The SEM micrograph of (Ni-Cu)Ox@SiO2, Ni:Cu = (a) 0.2:0.1 and (b) 0.15:0.15. Click here to view figure

Figure 3: The EDX spectra of (Ni-Cu)Ox@SiO2, Ni:Cu =  (a) 0.2:0.1 and (b) 0.15:0.15. Click here to view figure

 

UV-Vis Diffuse Reflectance method was applied to measure energy absorbance of samples at wavelength between 200-800 nm. It has been postulated that the UV spectrum ranges from 200-400 nm, while the visible spectrum ranges from 400-800 nm. All samples indicate peaks at UV and visible wavelenght (Table 1). These prove that the nickel and copper specieses are supported on the surface of SiO₂, since SiO₂ absorbs the energy at low wavelength.

Table 1: Absorbances of (Ni-Cu)O_x@SiO₂ catalysts.

(Ni-Cu)Ox@SiO2, Ni:Cu =	λ (nm)
	Visibel	UV
0.3:0.0	331	264
0.2:0.1	331	252
0.15:0.15	320	–

 

Bandgap energy affects the photocatalytic ability of (Ni-Cu)O_x@SiO₂ catalyst. The reflectance data (REF) of the diffuse reflectance UV-Vis measurement is used in the calculation using the Kubelka-Munk equation to obtain the bandgap energy (Eg) of (Ni-Cu)O_x@SiO₂.²⁰ The calculation of bandgap energy by the Kubelka-Munk method include a graph of the relationship between eV and (F (R’_∞xhυ))^1/2 presented in Figure 4.

Figure 4 describes that bandgap energies of (Ni-Cu)O_x@SiO₂, Ni:Cu = 0.3:0.0,  0.2:0.1 and 0.15:0.15  are between 3.40-2.58 eV, 4.62-2.31 eV, and 3.34-2.41 eV, respectively. Steiner, et al. (1992) found that NiO bandgap energy is ~4 eV,²¹ while the CuO has an energy bandgap between 1.85-2.4 eV and are effectively operated under UV and visible lights.²²  This promises that the (Ni-Cu)O_x@SiO₂ catalyst is also effectively used under sunlight.

Figure 4: The calculation of (Ni-Cu)Ox@SiO2, Ni:Cu = (a) 0.3:0.0 (b) 0.2:0.1 and (c) 0.15:0.15, bandgap energies. Click here to view figure

 

The absorption study of (Ni-Cu)O_x@SiO₂ on methylene blue is carried out in the dark room. This experiment facilitates the determination of the ability of the catalyst to absorb methylene blue without obstruction of light. The concentration of methylene blue before and after adsorption measured at any times is listed in Table 2.

Table 2: The concentration of methylene blue after adsorption by (Ni-Cu)O_x@SiO₂.

Time (minute)	The concentration of methylene blue (ppm) after adsorption by (Ni-Cu)Ox@SiO2, for Ni:Cu =
	0.3:0.0	0.2:0.2	0.15:0.15
0	8.66727	8.66727	8.66727
5	5.49894	5.49894	5.49894
10	2.47603	2.37215	2.36176
15	1.88911	2.28905	2.13842
20	1.75926	1.87872	1.16714
25	1.74368	1.54630	1.11001
35	1.55669	1.52553	0.87628
65	1.43204	1.41126	0.55945
95	1.42684	1.37490	0.55438
125	1.01652	0.96977	0.54910

 

The (Ni-Cu)O_x@SiO₂ catalyst absorption capacity was calculated by using the Langmuir and Freundlich equations.²³ A plot of the concentration of methylene blue adsorbed in each 1 gram of catalyst (C/m) versus the final methylene blue concentration (C) is used to determine the Langmuir isotherm pattern, and a plot of the log amount of methylene blue adsorbed on every 1 gram of catalyst (log X_m/m) versus a log concentration of methylene blue after adsorption (log C_t) is used to determine the pattern of Freundlich isotherm. Line equations are obtained from the plots of Langmuir and Freundlich isotherms (Table 3). The values of R² of Langmuir adsorption isotherm pattern are higher than the value of R² of  Freundich adsorption isotherm pattern, leading the conclusion that the adsorption of methylene blue by the (Ni-Cu)O_x@SiO₂ follows the Langmuir isotherm pattern. This model assumes that there is only a surface layer (monolayer) and is homogeneous of  (Ni-Cu)O_x on the surface of SiO₂ which can block adsorption into deeper layers.

Table 3: Langmuir and Freundlich isotherm’s line equations.

(Ni-Cu)Ox@SiO2, Ni:Cu=	Langmuir isotherm	Freundlich isotherm
0.3:0.0	y = 0.0743x – 0.0736 R² = 0.9757	y = -0.5398x + 1.6498 R² = 0.898
0.2:0.1	y = 0.0734x – 0.0719 R² = 0.9695	y = -0.516x + 1.6422 R² = 0.8789
0.15:0.15	y = 0.0665x – 0.0428 R² = 0.955	y = -0.369x + 1.5679 R² = 0.8247

 

The catalyst adsorption capacity can be calculated by using the line equation of the Langmuir isotherm pattern. The adsorption capacity of (Ni-Cu)O_x@SiO₂ catalyst is listed in Table 4. The decreasing of NiO_x and the increasing of CuO_x is related linearly with the increasing of the adsorsobtion capacity of (Ni-Cu)O_x@SiO₂ catalyst to methylene blue.

Tabel 4: The adsorption capacity of (Ni-Cu)O_x@SiO₂ catalyst.

(Ni-Cu)Ox@SiO2, Ni:Cu=	Adsorption capacity (mol/gram)
0.3:0.0	13.5870
0.2:0.1	13.9082
0.15:0.15	23.3645

 

The concentration of methylene blue isolated in the dark for 24 hours is higher than for 2 hours, might be due to desorption process. The methylene blue in the solution was then exposed to sunlight at period of times. The concentration of methylene blue after exposed by sunlight at period of times is presented in Figure 5. The concentation of methylene blue is gradually decreased at the first 20 minutes. The concentration is then steady figuring that the photodegration is finished. The percentages of methylene blue photodegradated under sunlight using (Ni-Cu)O_x@SiO₂, where Ni:Cu=0.3:0, 0.2:0.1 and 1.5:1.5 are 55.65%, 49.87%, and 52.11%, respectively.

Figure 5: The methylene blue photodegradation under sunlight using (Ni-Cu)Ox@SiO2, where Ni:Cu= (a) 0.3:0, (b) 0.2:0.1 and (c) 1.5:1.5. Click here to view figure

 

The reaction order kinetics for photodegradation of methylene blue are presented in Table 5. The highest R² values indicate the second reaction order. These is meant that the photodegradation reaction rate of methylene blue using the (Ni-Cu)O_x@SiO₂ catalyst under sunlight follows the second-order reaction.

Table 5: Methylene blue photodegradation order kinetics.

(Ni-Cu)Ox@SiO2, Ni:Cu=	Order	k	R2
0.3:0.0	0	0.0036	0.5293
	1	0.0043	0.5565
	2	0.0053	0.5866
0.2:0.1	0	0.0028	0.5148
	1	0.0031	0.5812
	2	0.0036	0.6476
0.15:0.15	0	0.0036	0.6903
	1	0.005	0.7631
	2	0.0071	0.8169

 

The photocatalytic reaction takes place in a heterogeneous system and the reaction rate is influenced by the adsorbance of the reactants on the surface of the catalyst. The kinetic constant k_obs is the reaction rate constant that does not take into account the role of adsorption, so that when the adsorption process becomes a part that affects the photodegradation reaction, it is necessary to determine the actual reaction rate constant (k) which has corrected the k_obs with the adsorption constant (K) where k_obs = kKL. The kinetics parameters of the k_obs are determined by fitting the curve 1/Ct to the time due to the photodegradation reaction of methylene blue with (Ni-Cu)O_x@SiO₂ following a second-order reaction. Then the reaction rate constant (k) is determined based on the k_obs listed in Table 6. The (Ni-Cu)O_x@SiO_2, Ni:Cu=0.15:0.15 shows the highest photodegradation reaction rate constant of methylene blue under sunlight, which is 0.10677•10⁴ minute^-1. This concludes that the (Ni-Cu)O_x@SiO_2, Ni:Cu=0.15:0.15 is most effectively used as a photocatalyst under sunlight.

Table 6: Methylene blue photodegradation kinetics using Ni-Cu oxide@SiO₂ catalyst.

(Ni-Cu)Ox@SiO2, Ni:Cu=	KL	kobs (minute-1)	k (minute-1)
0.3:0.0	0.0743 • 106	0.0053	0.07133 • 104
0.2:0.1	0.0734 • 106	0.0036	0.04905 • 104
0.15:0.15	0.0665 • 106	0.0071	0.10677 • 104

 

The photodegradation reaction rates of methylene blue are listed in Table 7. The difference in Ni and Cu concentrations supported onto silica does not have a significant effect on the photodegradation reaction rate equation. This proves that the (Ni-Cu)O_x@SiO₂ catalyst is effective for photodegradation of methylene blue under sunlight.

Table 7: Methylene blue photodegradation rate.

(Ni-Cu)Ox@SiO2, Ni:Cu=	Reaction rate (ppm/minute)
0.3:0.0	0.07133 • 104×C2
0.2:0.1	0.04905 • 104×C2
0.15:0.15	0.10677 • 104×C2

 

Conclusion

The as prepared (Ni-Cu)O_x@SiO₂ is representative for photodegradation of methylene blue. The nickel and copper oxides decrese the bandgap of silica making the catalyst suitable for photodegradation under visible and ultra violet lights. The catalyst adsorption follows the Langmuir isotherm pattern with adsorption capacity of (Ni-Cu)O_x@SiO₂ increases along with the increasing of copper oxide. The photocatalytic activity of the catalyst in the degradation of methylene blue under the sunlight follows a second order reaction rate.

Acknowledgements

This research was supported and fully funded by the Yogyakarta State University of Indonesia.

Conflict of Interest

There is no conflict of interest.

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