Synthesis of ZnWO4/Cu2O Composites and E cient Catalytic Properties for 4-nitrophenol Removal


 Preparation of ZnWO4/Cu2O composite materials by environmental protection experimental method. ZnWO4 nanorods were prepared by a simple hydrothermal method, and then ZnWO4/Cu2O composites were prepared by chemical precipitation method. The experimental method is simple and clean. Due to the unique structural characteristics of Cu2O, the wide band gap of ZnWO4 was improved and the ZnWO4/Cu2O composite with better catalytic performance was generated. Various tests show that the catalytic performance and stability of ZnWO4/Cu2O is not lower than that of ZnWO4. Among them, ZnWO4/Cu2O-5 has the best catalytic degradation effect. The Cu-H generated during the degradation of 4-nitrophenol (4-NP) prevents the oxidation of Cu+, and the neutral product 4-aminophenol (4-AP) is easier to fall off, which improves its recycling efficiency. These encouraging results show that the ZnWO4/Cu2O composites have great potential to degrade 4-NP.

1 Introduction 4-nitrophenol (4-NP) has high toxicity and long half-life in the natural environment, which causes serious pollution to the water environment. It is easy to accumulate in humans and animals to cause cancer or physical deformity [1,2]. 4

-aminophenol (4-AP) as the intermediate of 4-NP catalytic
degradation is widely used in medicine, dyes, pesticides and other elds. Among many methods for reducing 4-NP to 4-AP, catalytic hydrogenation reduction method is the best, which has the advantages of high conversion e ciency and mild operating conditions [3]. Among them, noble metal nanoparticles have the best catalytic effect [4,5]. But it is expensive, limited reserves and di cult to large-scale promotion. Scientists are more focused on the development of cheap, abundant non-precious metal catalysts.
Cu 2 O is a typical P-type direct band gap semiconductor material that can be excited by visible light. Its band gap is about 2.17 eV, and has good photoelectric properties [6,7]. Cu 2 O nanoparticles, not only has high photocatalytic activity, bactericidal activity, photoelectron conversion activity, but also has the advantages of high surface energy and large speci c surface area [8]. This makes Cu 2 O have applications in photocatalysis [9], photohydrolysis of hydrogen in aquatic products [10], CO oxidation [11], CO 2 reduction [12], and gas sensing [13]. However, the band gap of Cu 2 O is narrow. And its reaction rate is slow, easy to be oxidized, low cycle utilization. In order to eliminate these shortcomings, researchers have made many efforts.
Since scientists found ZnWO 4 , people have had a strong interest in it. ZnWO 4 is a wide band gap semiconductor photocatalyst. Zn 2+ and W 6+ have electronic con gurations of d10 and d0, and are located at the centers of ZnO 6 and WO 6 . Therefore, they have high electron mobility, which have a matching valence band relationship with Cu 2 O [14]. In this experiment, ZnWO 4 /Cu 2 O composites are prepared according to the band gap characteristics of the two. Explore whether the photocatalytic properties of composite materials have improved.
Therefore, in this study, ZnWO 4 nanorods with stable structure were prepared by simple hydrothermal method, and ZnWO 4 /Cu 2 O was prepared by chemical precipitation method. Encouraging experimental results have been obtained, among which ZnWO 4 /Cu 2 O-5 has the best catalytic degradation effect. The modi ed catalyst improved the separation ability of electron-hole and enhanced the photocatalytic activity. The oxidation of Cu + was prevented by Cu-H generated in the catalytic degradation process.
The neutral degradation product 4-AP made it easier to fall off and improved its recycling e ciency. The e cient recycling of ZnWO 4 /Cu 2 O-X is in line with the concept of environmental protection, which is a more economical and feasible process. and the ZnWO 4 /Cu 2 O catalyst is characterized by XRD, SEM, UV-Vis, TEM, XPS and HRTEM to determine the nature of catalytic activity of catalyst. The pH was adjusted to 9 with an intelligent pH meter, stirred for 30 min, and transferred to a stainless steel autoclave. The temperature was adjusted to 453 K for 12 h. After the reaction, the precipitate was collected and washed several times with deionized water and anhydrous ethanol. Then the sample was dried by vacuum drying box, and the powder was grinded by bowl after drying.  Tecnai G2F30 transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) were used for transmission. UV-Vis diffuse re ectance spectra were measured by UV-3600 spectrophotometer. The scanning range was from 200 nm to 800 nm. The crystal phase of the sample was characterized by BRUCKER D8ADVANCE X-ray powder diffractometer in the range of 10-80°. X-ray diffraction (XRD) was performed with a 250Xi diffractometer. The energy spectrum was analyzed by SU8020 eld emission scanning electron microscope (SEM).

Evaluation of catalysts performance
2.4.1 Catalytic activity for hydrogenation of catalyst test 10 mg catalyst was dissolved in 10 ml deionized water, and ultrasonic dispersion was uniform. 10 ml 0.24 M NaBH 4 aqueous solution was added to 90 ml 2.2×10 −4 mol/L 4-NP. 0.02 ml catalyst solution was accurately removed by liquid gun and added to the above mixed solution. Add the catalyst and start timing. Remove 3.5 ml of solution every other time in the colorimetric dish. The ultraviolet spectrophotometer was immediately used to monitor the change of 4-NP ion absorption peak at 400 nm of purple wavelength in the scanning range of 250 nm-500 nm. The reaction rate constant is calculated by absorbance. The whole process was continuously stirred in a constant temperature water bath at 25℃.    Fig. 6. Fig. 6b shows the spectra of Zn 2p, showing two peaks of 1021.9 eV and 1044.7 eV, corresponding to Zn 2p 3/2 and Zn 2p 1/2 , respectively. Fig. 6c is the XPS spectrum of W4f, showing two peaks of 35.8 eV and 37.9 eV, corresponding to W 4f 7/2 and 4f 5/2 , which determines the existence of W 4f. Fig. 6d  and 6.4 nm, and the pore volumes are 1.3×10 −3 cm 3 /g and 2.8×10 −3 cm 3 /g, respectively. It indicates that the combination of the two is conducive to improving the speci c surface area, pore size and pore volume, but the space for improvement is very small. The relationship between C t /C 0 and t is shown in Fig. 9a. It can be seen that the conversion rate of ZnWO 4 /Cu 2 O-5 is the highest and that of Cu 2 O is the lowest at the same time. So the catalytic activity of ZnWO 4 /Cu 2 O-5 is higher than that of Cu 2 O.

Hydrogenation catalyst stability test
The relationship between ln (C t /C 0 ) and t is shown in Fig. 9b. It can be seen that the reaction rate of The results showed that the activation factors of Cu 2 O and ZnWO 4 /Cu 2 O-5 were 31×10 −3 s −1 mg −1 and 126×10 −3 s −1 mg −1 , respectively, indicating that its catalytic performance was better than that of most catalysts. As shown in Table. 2.  Finally, the stability test was carried out. Due to the small amount of catalyst, in order to avoid the loss caused by recycling, the stability of the catalyst was veri ed by adding 4-NP repeatedly. The speci c operation steps were shown in Section 2.4.2. The relationship between C t /C 0 and t in the 15 cycles is shown in Fig. 10a. It can be seen from the gure that after 15 cycles, the degradation effect changed little, still reaching more than 95%. It indicates that catalytic stability was good. The time needed to convert the same mass of 4-NP is shown in Fig. 10b. It can be seen from the gure that the time used increases with the increase of cycles. But it remains unchanged after increasing to a certain extent, which indirectly indicates that it has good cycle stability.

Effect of reaction temperature on catalytic hydrogenation of 4-NP
In order to study the effect of temperature on the activity of the catalyst, the activity tests are carried out at 10°C, 25°C, 40°C, 55°C and 70°C ( Fig. 11a ), and their k values were obtained by linear tting of kinetics (   Fig. 11b ). Which are 10.88×10 −3 s −1 , 25.20×10 −3 s −1 , 24.42×10 −3 s −1 , 31.93×10 −3 s −1 and 31.49 ×10 3 s −1 , respectively. It can be seen that the variation is large at low temperature, but not obvious at high temperature. It indicates that temperature has some in uence on activity of the catalyst. The catalytic effect remained unchanged after the temperature reached a certain level in the high temperature stage, and 50°C was the critical point. Fig. 11a The relationship between C t /C 0 and time at different temperatures and b kinetic tting graph 3.6. Valence analysis of catalyst elements before and after reaction In order to study the reaction mechanism of 4-NP conversion to 4-AP, the ZnWO 4 /Cu 2 O after reaction is characterized by XPS. The full spectrum of ZnWO 4 /Cu 2 O shown in Fig. 12a indicates that there is no additional impurity element after the reaction. Fig. 12b and c shows that peaks of Zn 2p are 1021.7 eV (Zn 2p 3/2 ) and 1044.8 eV (Zn 2p 1/2 ), and the peaks of W 4f are 35.3 eV (W 4f 7/2 ) and 37.5 eV (W 4f 5/2 ).
Compared with Fig. 6a  Cu + /Cu. Since it is di cult to distinguish the two by XPS, the valence state of copper ions can only be determined by X-ray induced Auger electron spectroscopy ( XAES ). The obtained gure shows three independent peaks. The main peak at about 570 eV is considered to represent Cu + , while the peak at about 567 eV is considered to represent CuO [17,18].     Diagram of the catalytic reaction mechanism of ZnWO4/Cu2O