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Scientific Reports Volume 11, Article Number: 22699 (2021) Cite this article
Azo dyes and nitrophenols are highly toxic, affecting the photosynthesis cycle of aquatic organisms, and have been widely used in various industries. Industrial disposal increases the accumulation of azo compounds in the environment. In this study, we synthesized a low-cost, PdO-doped NiO heterogeneous mixture through a simple hydrothermal combined calcination process. The morphology results prove that the spherical PdO nanoparticles are uniformly doped with NiO nanoparticles. The band gap values of metal oxide NiO, PdO and PdO-NiO composite materials are found to be 4.05 eV, 3.84 eV and 4.24 eV, respectively. The high optical band gap (Eg) value of the composite material indicates that the PdO interface and the NiO interface are tightly combined in the composite material. The catalytic activity of PdO-NiO was analyzed for the reduction of different toxic azo compounds, namely 4-nitrophenol (NP), 2,4-dinitrophenol (DNP), 2,4,6-trinitro Phenol (TNP), methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) and their mixtures in the presence of NaBH4. For the first time, a large number of toxic azo compounds were reduced to non-toxic compounds with a high reduction rate. The proposed PdO-NiO catalyst exhibits excellent rate constants of 0.1667, 0.0997, and 0.0686 min-1 for NP, DNP, and TNT, and 0.099, 0.0416, and 0.0896 min-1 for MB, RhB and MO dyes, respectively, which is better than before. The reported rate constant is higher for catalysts. PdO-NiO mainly completes the reduction of the azo compound mixture within 8 minutes. In addition, PdO-NiO showed a stable reduction rate of azo compounds in five cycles without significant loss. Therefore, the proposed low-cost and high-efficiency PdO-NiO catalyst may be a promising catalyst for the degradation of azo compounds.
The rapid industrial growth in the era of globalization has caused environmental pollution problems. Aromatic nitrophenols (4-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenol) and azo dyes (methylene blue, rhodamine B and methyl orange) are The main factors causing environmental pollution1,2,3,4. Azo dyes are widely used in leather factories, metal electroplating plants, food companies, paint industry, paper pulp, especially textile industry5. However, during the dyeing process, only 5% of the dye is absorbed, and the remaining dye is released into the environment. This process can lead to environmental pollution and human health problems such as bleeding, nausea and skin ulcers6. Another highly toxic organic pollutant is nitrophenol. This aromatic and inert compound is highly toxic and will be discharged into the environment through wastewater from the papermaking, pesticide, agrochemical, dye and pharmaceutical industries7,8. Human exposure to these toxic nitrophenols can adversely affect kidney and liver function, and cause cancer in the organism9. Even short-term exposure can cause serious health diseases to humans, such as nausea, cyanosis and lethargy10. Therefore, the US Environmental Protection Agency lists nitrophenol as the most toxic pollutant to humans and organisms11. Therefore, these toxic azo compounds need to be removed from the environment. So far, the degradation achieved through ultrafiltration, aerobic or anaerobic treatment, adsorption, advanced oxidation process (AOP) and coagulation technology has been widely used to remove these nitrophenols and azo dyes12, 13. However, the aforementioned technology is said to be more expensive and less effective. On the other hand, catalytic reduction is an emerging process for reducing nitrophenol and azo dyes, in which organic compounds are reduced to a hydrogenated form, which is a non-toxic and environmentally friendly product. In particular, the reduction product of nitrophenol, namely aminophenol (AP), is an important intermediate in the industrial production of agrochemicals, drugs and corrosive coatings.
However, the reduction of BH4− p-nitrophenol and azo dyes is catalyzed by many noble metals, such as Pt, Au, Ag and Ru14,15,16,17. However, the scarcity and high cost of catalysts hinder actual development. Therefore, the size and shape control synthesis of catalysts has recently triggered a huge research effort, which has led to the manufacture of various morphologies, such as wires, rods, bristles, plates, polyhedrons, and branched nanostructures. Therefore, the solubility of all these metals should be recorded by metals with similar high catalytic activity. In addition, a recent study by Mahmoud et al.18 provides clear evidence that the reaction continues on the surface.
In particular, bimetallic nanocrystals are a very attractive combination of precious metals and non-precious metals (inexpensive primary conversion metals), which provides opportunities to reduce the use of precious metals and the total cost of catalysts. For example, Toshima et al. [19] Bimetallic Pd-Ni NPs are more effective for hydrogenation of nitrobenzene than Pd NPs. The structure of bimetallic nanomaterials has aroused people's interest in its potential applications in organic reactions, fuel cells, and sensing devices. Therefore, several synthesis methods have been developed, such as water heating systems, electrochemical deposition, seed-mediated growth, and current conversion. In this study, we developed a bimetallic Pd-Ni oxide through a simple hydrothermal preparation method. The characteristic of the synthetic sample is to determine its basic physical and chemical properties through the specialization of basic testing techniques. The morphological results show that the spherical PdO nanoparticles are uniformly doped with NiO. NaBH4 was used to catalytically reduce azo dyes and aromatic nitrophenol to evaluate the catalytic efficiency of the obtained catalyst. For the first time, we demonstrated the reduction of azo compounds in a larger volume than the traditional reduction process. Compared with pure NiO, PdO-doped NiO shows an effective catalytic reduction of azo compounds. The PdO-NiO catalyst exhibits an excellent rate constant in reducing azo compounds, which is higher than other noble metal catalysts reported. Our findings provide certain insights for the catalytic reduction based on oxygen-containing transition bimetallic catalysts.
The crystalline properties and phase purity of the synthesized NiO, PdO and PdO-NiO nanocomposites were confirmed by XRD analysis, as shown in Figure 1a. All three XRD patterns show a highly crystalline nature, which confirms the purity of the sample. In the PdO-NiO mixed metal oxide, the strong diffraction peaks at 2θ = 37.16, 43.24, 62.81, 75.32 and 79.34° point to the cubic (101), (012), (110), (113) and (202) The plane NiO phase is consistent with the height of the standard JCPDS NO: 01-071-475120. On the other hand, the sharp peaks at 2θ = 34.54 and 55.79° point to the (101) and (112) planes, corresponding to the tetragonal phase of PdO, with an average lattice parameter of 3.043 Å, which is highly correlated with the standard JCPDS NO: 043-102421. Interestingly, no metal diffraction peaks and other impurity phases are observed in the mixed PdO-NiO nanocomposite. This confirmed that the metal source (Pd) was completely oxidized to metal oxide (PdO) and formed a hybrid PdO-NiO nanocomposite. In addition, the diffraction patterns of single metal oxides (PdO, NiO) are compared in Figure 1a.
(a) X-ray diffraction pattern and (b) FTIR spectra of NiO, PdO and PdO-NiO (c) TGA curve of PdO-NiO nanocomposite.
The sharp peaks of each metal oxide confirm the high crystallinity of the tetragonal phase of PdO and the cubic phase of NiO. Both diffraction patterns of a single metal oxide are closely related to standard JCPDS documents: 01-071-4751 and 043-1024, respectively. The average grain size and micro-strain of NiO, PdO and PdO-doped NiO were calculated using Scherrer analysis and WH analysis and plotted in Figure 1b. A small change in the crystallite size of the catalysts (PdO, NiO and PdO-NiO) is observed in Figure 1b, which is due to the difference in the crystal distribution in the catalyst. The average crystallite sizes of PdO, NiO and PdO-NiO were found to be 10.8, 7.8 and 7.36 nm, respectively. PdO doping reduces the crystallite size in PdO-NiO composites, which is consistent with previous reports. The calculated crystallite size, d-spacing, micro-strain and binding energy of PdO, NiO and PdO-NiO are shown in Table 1. In addition, the experimentally calculated d-spacing values of pure PdO and NiO correlate well with the theoretical values and are shown in Table SI. 1. The synthesized catalyst shows that the negative and positive slopes of ε correspond to compressive stress and tensile stress, respectively.
In addition, chemical bonds and functional groups were analyzed by FTIR spectrometer. Figure 1c shows the FTIR spectra of the synthesized NiO, PdO, and PdO-NiO nanocomposites. Similar spectra were observed for the three catalysts. Due to the phenomenon of surface adsorption, the absorption peak at the high frequency region of 3200-3400 cm-1 belongs to the OH stretching vibration of water molecules. In addition, three absorption peaks appear at 1398.2, 1237.8 and 1057.8 cm-1, which are attributed to CO, CH2 and C=O stretching vibrations, which are closely related to the XPS analysis data. The metal oxide bonding peak appears in the frequency range of 480.2-702.6 cm-1. Therefore, the tensile frequencies of Pd-O and Ni-O in the PdO-doped NiO sample confirmed the formation of the mixed PdO-NiO nanocomposite. After that, the thermal stability of PdO-NiO nanocomposites was studied by TGA analysis. Figure 1d shows the thermal analysis of the PdO-NiO nanocomposite. The first weight loss (7%) starts in the temperature range of 65 to 180 °C. Due to the H2O molecule, evaporation was observed in the range of 187 to 574 °C, and then continued to lose about 10% in weight. A significant weight loss of 20% was observed above 600 °C, which may be attributed to unreacted CO3 combustion23. Therefore, XRD, FTIR and TGA spectroscopy studies confirmed the formation of hybrid PdO-NiO nanocomposites.
The electronic state and chemical bond of PdO-doped NiO composite materials were analyzed by using XPS spectroscopy. Figure 2 shows the XPS spectrum of the PdO-NiO nanocomposite, the wide-scan spectrum of PdO-NiO (Figure SI.1), showing the presence of Pd(3d), Ni(2p), O(1s) and C(1s) ) Elements. The deconvoluted Pd 3d XPS spectrum in Figure 2a shows that the two main peaks with binding energies of 336.4 and 342.2 eV correspond to the spin-orbit double peaks of Pd 3d5/2 and Pd 3d3/2, respectively, which confirms the form of the Pd2 ion PdO24,25 in PdO-NiO nanocomposites. In addition, the satellite peaks of the Pd species appear at the binding energies of 339.2 eV and 345.3 eV. In the Ni 2p spectrum (Figure 2b), Ni 2p3/2 and Ni 2p3/2 spin-orbit double peaks are observed at 854.9 eV and 872.5 eV, corresponding to Ni-O and Ni-OH and their corresponding satellite peaks, respectively Located at binding energies of 867.2 eV and 879.1 eV26,27. In addition, the O 1s spectrum (Figure 2c) shows two peaks with binding energies of 529.3 eV and 534.2 eV, which are attributed to the MO and M-OH species. The obtained XPS spectrum of PdO-NiO nanocomposite is closely related to XRD and EDX analysis.
High-resolution X-ray photoelectron spectra of PdO-NiO nanocomposites (a) Pd 3d, (b) Ni 2p, (c) O 1s and (d) C 1s spectra.
The morphology and element composition of the NiO, PdO and PdO-NiO nanocomposites were carefully observed by FE-SEM. Figure 3 shows the SEM topography images of NiO, PdO and PdO-NiO nanocomposites at low and high magnifications. The pure metal oxide NiO and PdO samples in Figure 3a-d show the porous crystal morphology of each element with high purity. On the other hand, the PdO-doped NiO samples in Fig. 3e and f show uniform, monodisperse, spherical crystal morphology. This confirms the uniform distribution of PdO and NiO. Therefore, PdO doping enhances the surface area of the catalyst. In addition, the element composition and element distribution of the PdO-NiO sample are also analyzed, as shown in Figure 3g and h. EDX spectroscopy and element mapping clearly confirmed the presence of Pd, Ni and O elements in high-purity composite materials.
FESEM images (a, b) NiO (c, d) PdO, (e, f) PdO-NiO and (g, f) EDS spectra and elemental mapping of PdO-NiO nanocomposites.
The detailed morphology and particle size distribution of PdO-NiO NPs were measured by HR-TEM, and the results are shown in Figure 4. Figures 4a-c show typical HRTEM images of the synthesized PdO-NiO nanocomposite. The obtained TEM image confirmed the uniform distribution of spherical PdO-NiO NPs, which is consistent with the FESEM results. In Figure 4c (inset), the histogram shows that the formed PdO-NiO nanoparticles are uniformly distributed, with an average particle size of about 9.64 ± 2.1 nm, which is closely related to the XRD crystallite size. In addition, the SAED pattern was analyzed to understand the crystallinity and crystal quality of PdO-NiO nanoparticles, as shown in Figure 4d. The clear ring structure indicates the polycrystalline nature of PdO-NiO. The obtained diffraction ring d-spacing value corresponds to the (101), (012), (110), (113), and (202) planes of NiO nanoparticles. Figure 4e shows the lattice fringes of PdO-doped NiO nanoparticles. The stripes show the lattice planes of the two metal oxides. The interplanar spacing value of 0.1997 nm corresponds to the (012) plane of the NiO phase, and the d-spacing value 0.2145 nm corresponds to the (110) plane of PdO in the composite material. This is closely related to the XRD d spacing value. The element mapping in Figure 4f shows the presence of uniformly distributed Ni, Pd, and O elements similar to the SEM mapping. The morphology results of the synthesized catalyst are closely related to XRD, FTIR and XPS analysis.
High-resolution transmission electron microscope HRTEM image (ac), (d) SAED pattern, (e) plane spacing and (f) elemental mapping of PdO-NiO nanocomposite.
The UV-Vis absorption spectra of the synthesized catalysts NiO, PdO and PdO-NiO were analyzed, and the respective results are shown in Figure 5a. The absorption spectrum shows the strongest absorption maximum of all three catalysts at 234.8 nm. In addition, the characteristic absorption bands of NiO and PdO were observed at 338.2 nm and 422.1 nm, respectively. On the other hand, the characteristic absorption bands of PdO-NiO samples were not observed. In addition, the band gap energies of the three catalysts are calculated by using the Schuster-Kubelka-Munk function.
(a) UV-Vis absorption spectrum and (b) (αhν)2 Vs hν diagrams of NiO, PdO, and PdO-NiO nanocomposites (the inset shows the band gap energy of the catalyst).
The band gap energy (Eg) is obtained by extrapolating the photon energy, and the result is shown in Figure 5b. The calculated band gaps (Eg) of NiO, PdO, and PdO-NiO are 4.05 eV, 3.84 eV, and 4.24 eV, respectively (Figure 5b, inset). PdO doped with NiO increases its band gap value. This indicates that the PdO interface and the NiO interface are tightly combined in the composite material. The band gap value of the obtained catalyst is much higher than the reported band gap energy. The band gap energy is highly dependent on the size of the particles. The band gap energy increases with the decrease of the particle size, which confirms that the synthesized catalyst is nano-scale. The band gap energy (Eg) of the PdO-NiO catalyst is closely related to the FESEM and TEM results.
The photoluminescence (Pl) spectra of NiO, PdO, and PdO-NiO materials were measured at an excitation wavelength of 325 nm and shown in Figure 6. Figure 6a shows the Pl spectra of pure NiO, PdO and PdO-NiO nanocomposites. Since 3d8 electrons of the Ni2 ion are excited from the conduction band to the valence band, the blue/violet emission of all three samples is observed at 364 nm. It can be seen from Figure 6a that the strength of the PdO-NiO nanocomposite is lower than that of pure NiO and PdO, which indicates that the electron transfer between NiO and PdO is higher, which is closely related to the electrochemical results. The deconvoluted PL spectra of NiO, PdO and PdO-NiO materials are shown in Figure 6b-d; each sample is fitted with four peaks, as shown in Figure 6b-d. The UV emission at 364 nm (3.4 eV) corresponds to the near band edge (NBE) excitation of NiO25. The obtained PL spectra confirmed that the PdO-NiO nanocomposite has higher conductivity than pure metal oxide.
(a) are the PL spectra and (bd) deconvolution spectra of NiO, PdO and PdO-NiO nanocomposites, respectively.
The electrode kinetics of NiO, PdO and PdO-NiO modified GC electrodes were explored at different scan rates in 1 M KOH at room temperature. In addition, the resistance of the above-mentioned electrodes was monitored in the impedance analysis and shown in Figure 7. Figure 7a, NiO/GC shows a pair of clear redox peaks around 0.49 V and 0.44 V, respectively, corresponding to the reversible reaction between Ni2 and Ni3 26. In addition, the redox peak current increases linearly with the increase of the scan rate.
(ac) Cyclic voltammetry and (d) EIS curves of NiO, PdO and PdO-NiO nanocomposites in 1 M KOH electrolyte solution.
Figure 7b shows the CV mode of PdO/GC electrode in 1 M KOH solution. Due to the formation of oxyhydroxide on the electrode surface in alkaline medium, it produces poor palladium oxide peaks and peaks at 0.52 V and 0.53 V, respectively. Palladium reduction peak. The PdO-NiO NPs modified GC electrode in Figure 7c shows clear Ni2 and Ni3 kinetics, and the peak current is eight times higher. This confirms the effective electron transfer between NiO and PdO in the composite. This is closely related to the PL result. In addition, the impedance spectra of the three electrodes were obtained with a fixed overpotential (500 mV s-1) in a 1 M KOH electrolyte, as shown in Figure 7d. In Figure 7d, the Rct values of pure metal oxide NiO/GC and PdO/GC electrodes are 236 Ω and 2702 Ω, respectively. The bimetallic oxide PdO-NiO/GC shows 425.7 Ω, which is lower than pure PdO. Because the electron conversion between each metal oxide is superior.
Azo compounds are highly toxic to the environment and humans. Especially nitrophenol is listed as the most dangerous chemical in the world. Therefore, the reduction of nitrophenol has received the most attention. Generally, the nitrophenol reduction reaction is thermodynamically favorable under optimized conditions (E0 =-0.76 V), and NaBH4 is used as a reducing agent (E0 =-1.33 V) 27,28. However, due to the kinetic barrier between the reducing agent and the reactants, the reduction rate is prolonged in the absence of a catalyst. Therefore, catalytic reduction of nitrophenol is a good way to convert into non-toxic aminophenol (AP) in the presence of NaBH4 as a reducing agent. It is easy to monitor the reduction reaction with an ultraviolet-visible spectrometer. It is well known that NaBH4 alone cannot reduce nitrophenol to aminophenol. As shown in Figure SI. 2 The absorption peaks of fresh nitrophenol (NP, DNP and TNP) appear at 300-370 nm, respectively. When reducing agent is added, the peak shifts to 402-450 nm28. In addition, due to the formation of corresponding nitrophenol ions in the alkaline solution, the color of the solution changed from light yellow to dark yellow. However, no reduction was achieved within 2 hours, indicating that the nitrophenol ion is very stable to NaBH4. In addition, three nitrophenols were used to explore the catalytic activity of NiO, as shown in Figure SI. 3. The catalytic reduction ability of pure NiO to nitro compounds is very poor. In contrast, the PdO-NiO catalyst showed excellent activity for nitrophenol, as shown in Figure 8.
The catalytic activity and kinetic rate of PdO-NiO nanocomposites for the reduction of nitrophenol in NaBH4 solution (a,b) NP, (c,d) DNP and (e,f) TNP.
It can be seen that the absorbance of nitrophenol at 400 nm gradually decreases with the reaction time, which confirms that PdO-NiO promotes the electron and hydrogen transfer between the reactants. Because PdO-NiO has higher active sites. The PdO-NiO catalyst completed the reduction reaction of NP, DNP and TNP in 10, 13, and 25 minutes, respectively. In addition, the aminophenol absorption peaks of all three nitrophenols appear near 300 nm. On the other hand, the dark yellow solution became colorless, indicating the formation of aminophenol. The rate constant κapp of each nitrophenol is calculated from the ln (At/Ao) Vs curve. time. The proposed PdO-NiO catalyst exhibits excellent rate constants of 0.1667, 0.0997, and 0.0686 min-1 for NP, DNP and TNT, respectively, which are higher rate constants than previously reported catalysts (Table SI.2). Generally, the mechanism of reduction of nitrophenol to aminophenol follows many intermediate steps, from nitro to nitroso, then hydroxylamine, and finally to aminophenol. These reactions require electron transfer and active hydrogen atoms. Here, BH4− ions generate active hydrogen atoms on the surface of the catalyst, and then the PdO-NiO catalyst enhances electron transfer. Therefore, the PdO-NiO catalyst can effectively accelerate the reduction of NP. In addition, the comparison of the catalytic reduction performance of nitrophenol with different catalysts is shown in Table SI. 2. The reduction mechanism of nitrophenol and PdO-NiO catalyst and NaBH4 is shown in Figure SI. 6.
In addition, the catalytic activity of PdO-NiO composites was explored by adding NaBH4 in the presence of PdO- to reduce organic azo dye compounds, such as methylene blue (MB), rhodamine B (RhB) and methyl orange (MO). NiO catalyst 29. As shown in Figure 9, the reduction of each dye was monitored at different absorption peaks. In the presence of PdO-NiO, the intensity of each dye at its respective wavelength decreases linearly with time. In addition, the reduction rate κapp of each dye is calculated from the ln (At/Ao) Vs curve. time. The reduction rates of PdO-NiO catalyst for MB, RhB and MO are 0.099, 0.0416 and 0.0896 min-1, respectively, and their catalytic activity is better than that of previous reports. In addition, the azo dye solution became colorless, indicating complete reduction in the presence of PdO-NiO. In the presence of NaBH4, the pure NiO catalyst has poor reduction activity for azo dyes (Figure SI.4). In addition, the table shows the comparison of the catalytic reduction performance of azo dyes using different catalysts. SI. 3. In addition, PdO-NiO nanocomposites were used to test the reducibility of mixtures of nitrophenol (NP, DNP, and TNP) and azo dyes (MB, RhB, and MO). The results are shown in Figure 10. At first, the azo compound formed a dark solution, and then PdO-NiO was added in the presence of NaBH430, which quickly became colorless and became a transparent solution. A complete reduction of the azo compound was achieved in 8 minutes. Therefore, the proposed PdO-NiO is a promising catalyst for wastewater treatment. In addition, different loadings of PdO-NiO catalysts (3-10 mg) were used to study the effect of catalyst loading on the reduction of azo compound mixtures and are shown in the table. SI. 4. This shows that the reduction rate increases with the increase of the catalyst loading. In addition, the catalytic reduction performance of various catalysts on toxic azo compounds is shown in the table. 2. The PdO-NiO catalyst shows superior reduction performance than previously reported catalysts.
The catalytic activity and kinetic rate of PdO-NiO nanocomposites for the reduction of azo dyes in NaBH4 solution (a,b) MB, (c,d) RhB and (e,f) MO.
The reaction process of azo compound mixture (4-NP, 2,4-DNP, 2,4,6-TNP, MB, RhB, MO) with PdO-NiO and NaBH4. Conditions: dye: 100 ppm, 25 ml, nitrophenol: 0.12 mM, 25 ml, NaBH4: 0.1 M, 5 ml and catalyst: 3 mg.
In addition, the reduction mechanism of azo dyes on PdO-NiO catalyst is shown in Figure SI. 7.
After the complete reduction reaction, the catalyst properties were analyzed to understand the stability of PdO-NiO. In Figure 11a, the FTIR spectrum shows that there is no significant change before and after the mixture is reduced by catalytic reduction. In addition, the SEM image (Figure 11b) also shows that the morphology of PdO-NiO has not changed. In addition, the HR-TEM image (Figure 11c) was also analyzed to study the change in particle size after catalytic reduction, showing that there was no significant change in particle size. This proves that PdO-NiO is highly stable under reducing conditions.
(a) FTIR spectrum of PdO-NiO nanocomposite before and after reduction of azo compound mixture (b) FE-SEM image (c) HR-TEM image of PdO-NiO after reduction of azo compound mixture.
In summary, we synthesized a low-cost, PdO-doped NiO heterogeneous mixture through a simple and cost-effective hydrothermal combined calcination process. XRD, XPS, FE-SEM and HR-TEM techniques confirmed that spherical PdO nanoparticles were uniformly doped with NiO nanoparticles. The bimetallic oxide composite PdO-NiO shows a high band gap value of 4.24 eV. PdO-NiO has enhanced superior catalytic activity than pure NiO. The proposed PdO-NiO catalyst exhibits excellent rate constants of 0.1667, 0.0997, and 0.0686 min-1 for NP, DNP and TNT, and 0.099, 0.0416 and 0.0896 min-1 for MB, RhB and MO dyes, respectively. This is a higher rate constant than previously reported catalysts. In addition, PdO-NiO can complete the reduction of the azo compound mixture in 8 minutes. In addition, PdO-NiO showed a stable reduction rate of azo compounds in five cycles without significant loss. In addition, a reasonable mechanism for catalytic reduction of nitrophenol and azo dyes is also proposed. Therefore, the proposed low-cost and high-efficiency PdO-NiO catalyst may be a promising catalyst for the degradation of azo compounds.
Metal precursors (Ni(NO3)2·6H2O) (< 99.95%), (PdCl2) (< 99.99%), (Na2CO3) (< 99.99%) and (KOH) (ultra pure) All chemicals are from Dae- Ringier Chemical Korea. Dyes such as methylene blue, rhodamine B and methyl orange were purchased from Korea Dae-Jung Chemical Company. Aromatic nitrophenols (NP, DNP, TNP) and NaBH4 (< 99.99%) were purchased from Sigma Aldrich, South Korea.
The bimetallic oxide (PdO-NiO) is synthesized by a simple co-precipitation method. First, 0.443 g PdCl2 and 8.36 g Ni(NO3)2·6H2O are dissolved in 25 mL of deionized water until they become a homogeneous solution. Then increase the pH of the solution to 9.5 by slowly adding Na2CO3 solution. Then, a green slurry is formed and mixed quickly at 60°C for 5 hours. The precipitate was then separated by vacuum filtration and washed thoroughly with deionized water until the pH of the filtrate solution was 7. The resulting crystals were then dried in an electric oven at 100°C for 12 hours, and then they were fine-grained. Finally, the dried particle sample was calcined in an air atmosphere at 500°C for 3 hours. In a similar way, single metal oxides (NiO and PdO) are only synthesized with their respective metal sources. The possible reaction mechanisms for the formation of PdO-NiO, PdO and NiO are described.
The crystal phases and structures of PdO-NiO, NiO and PdO were obtained from an X-ray diffractometer (Bruker AXS, Germany). The functional groups and metal oxide peaks of the catalyst were analyzed by ATR-FTIR (iS5, Thermo Scientific, USA). The morphological image and elemental composition of the metal oxide are monitored by (FE-SEM) Hitachi S-4800 II (Japan). HR-X-ray & UV photoelectron spectrometer (XPS, Thermo science, USA) is used to analyze metal bonding. Analyze high-resolution images and mapping images of the catalyst from the HR-TEM (Hitachi) instrument. The PL excitation and emission spectra were obtained by using a HORIBA Fluoromax spectrophotometer. A UV-Vis spectrophotometer (Shimadzu Corporation) was used to systematically study the catalytic reduction of nitrophenol and dye molecules.
A standard three-electrode system is used for electrochemical analysis. The glassy carbon electrode (GCE) modified by the catalyst (PdO-NiO, NiO and PdO) was used as the working electrode, and the platinum wire and Ag/AgCl were fixed as the counter electrode and the reference electrode, respectively. In the range of -0.2 to 0.9 V, the electrochemical activity of the catalyst was analyzed in a 1 M KOH electrolyte. In addition, the electrochemical impedance analysis of the catalyst modified GCE was analyzed with different amplitudes in 1 M KOH solution.
Use NaBH4 as a reducing agent to carry out the nitrophenol reduction reaction in an aqueous medium. In this work, three different nitrophenol (2-NP, 2,4-DNP and 2,4,6-TNP) solutions with an initial concentration of 0.1 mM were used for nitrophenol reduction. First, add 5 ml of freshly prepared 0.1 M NaBH4 to 50 mL of nitrophenol solution, then mix 2 mg/ml PdO-NiO, and then monitor the reduction of nitrophenol every 1 minute. Similarly, analyze the reduction of azo dyes (MB, RhB, and MO) with 50 ppm 100 ml azo dye solution, and monitor the ʎ max of each dye every 1 minute.
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This work was supported by the Korea Water Cluster (2020 Water Technology Commercialization Micro-Cluster Project Laboratory). This work was also supported by Hongik University's "Project to Establish a Regional Specialized Smart City Graduate School".
Department of Materials Science and Engineering, Hongik University, 2639-Sejong-ro, Jochiwon-eup, Sejong-city, 30016, Korea
AG Ramu & Dongjin Choi
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AGR: Conceptualization, investigation, manuscript writing, revision. DC: Supervision, editing, project management, fund acquisition.
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Ramu, AG, Choi, D. Use high-activity bimetallic PdO-NiO nanocomposite materials to perform efficient simultaneous catalytic reduction of a variety of toxic dyes and nitrophenol wastewater. Scientific Report 11, 22699 (2021). https://doi.org/10.1038/s41598-021-01989-7
DOI: https://doi.org/10.1038/s41598-021-01989-7
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