3.1 Morphology and microstructure analysis
Fig. 2 SEM images of (a-b) ZnWO4; (c-d) Cu2O; (e-f) ZnWO4/Cu2O
ZnWO4 SEM images prepared by hydrothermal method are shown in Fig. 2a and b. The results show that the size of ZnWO4 nanorods is uniform and the diameter is about 50 nm. Fig. 2c and d show that the Cu2O spheres synthesized by chemical precipitation method are about 500 nm to 900 nm, and the surface is rough and not smooth, which greatly improves its activity. ZnWO4/Cu2O composites formed by chemical precipitation method are shown in Fig. 2e and f. It can be seen that ZnWO4 is closely adhered to the surface of Cu2O and does not change its morphology. Close contact ensures the effective transfer of charge during catalytic hydrogenation reduction. The mapping of ZnWO4/Cu2O is shown in Fig. 3, which shows that Zn, O, Cu and W exist in ZnWO4/Cu2O, and are uniformly distributed in the sample.
The TEM and HRTEM images of ZnWO4/Cu2O composites are shown in Fig. 4. According to Fig. 4, the lattice spacing of Cu2O is 0.24 nm, which corresponds to 111 planes.The lattice spacing of 0.37 nm corresponds to 011 plane of ZnWO4. At the same time, as shown in Fig. 4c, clear stripe spacing proves that Cu2O and ZnWO4 have good crystallinity, and they have matching crystal plane spacing and lattice plane.
Fig. 4TEM and HRTEM images of ZnWO4/Cu2O
3.2 Material composition and valence analysis (XRD and XPS)
As shown in Fig. 5, the black curve is ZnWO4 nanowires. Its characteristic diffraction peaks are about 23.8°, 24.6°, 30.7°, 36.3°, 41.2°, 53.6°. Which are matched with (110), (200), (210), (211), (220), (310), (222), (320), (321), (400) and (421) crystal planes of ZnWO4 (JCPDS NO.15-0774) [15]. The red curve is Cu2O. Its characteristic diffraction peaks are 29.6°, 36.5°, 42.4°, 61.6°, 73.7°, corresponding to the (110), (111), (200), (220), (311) crystal planes of Cu2O crystal. Which are consistent with the (JCPDS NO.77-0119) [29] card. When ZnWO4/Cu2O-X composite is synthesized, with the increase of ZnWO4, the XRD pattern of ZnWO4/Cu2O-X composite does not appear the characteristic peak of ZnWO4. There is only the narrow and sharp peak of Cu2O, but the peak position is shifted. No other changes were detected, indicating the high purity of the complex. The main reason for the absence of ZnWO4 characteristic peaks may be the content and particle size. The content of Zn and Cu in the synthesized compound were determined by inductively coupled plasma atomic emission spectrometer (ICP-OES), and the results are shown in Table. 1. The low content and high dispersion of ZnWO4 can not cause the morphological change of Cu2O, which is consistent with the results of SEM and TEM.
Table 1
The ratio of the amount of Zn/Cu in the ZnWO4/Cu2O compositee
catalyst | theoretical value | actual value |
ZnWO4/Cu2O-1 | 1% | 1.3% |
ZnWO4/Cu2O-3 | 3% | 4.0% |
ZnWO4/Cu2O-5 | 5% | 6.8% |
ZnWO4/Cu2O-7 | 7% | 9.4% |
Fig. 5 XRD patterns of ZnWO4; Cu2O; ZnWO4/Cu2O-X
The characteristic peaks of Zn, W, O, Cu can be observed are shown in Fig. 6. Fig. 6b shows the spectra of Zn 2p, showing two peaks of 1021.9 eV and 1044.7 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively. Fig. 6c is the XPS spectrum of W4f, showing two peaks of 35.8 eV and 37.9 eV, corresponding to W 4f7/2 and 4f5/2, which determines the existence of W 4f. Fig. 6d is O, showing three peaks at 530.3 eV, 531.1 eV and 532.2 eV. The peak at 530.3 eV corresponds to the lattice oxygen in Cu2O, and the peak at 531.1 eV corresponds to the lattice oxygen in ZnWO4. The spectral result of Cu 2p is shown in Fig. 6e. There are two peaks at 932.9 eV and 952.3 eV, corresponding to Cu 2p3/2 and Cu 2p1/2. XRD and XPS analysis showed that Cu2O and ZnWO4 coexist in Cu2O/ZnWO4.
Fig. 6 a XPS survey spectra; High-resolution XPS spectra of b Zn 2p; c W 4f; d O 1s and e Cu 2p
3.3 N2 adsorption-desorption characterization of catalysts
The N2 adsorption-desorption isotherms of Cu2O and ZnWO4/Cu2O and their corresponding pore size distribution curves are shown in Fig. 7. It can be seen that Cu2O and ZnWO4/Cu2O are typical type IV isotherms, indicating that Cu2O and ZnWO4/Cu2O are mesoporous materials. In 0~0.9 V, the adsorption capacity of ZnWO4/Cu2O is low. While in the 0.9~1.0 V, the absorption capacity is greatly improved. On the whole, the absorption capacity of ZnWO4/Cu2O is low, indicating that the specific surface area is not large. But ZnWO4/Cu2O has stronger absorption capacity than Cu2O, indicating that its specific surface area is larger than Cu2O. Which is consistent with the test results. The BET algorithm shows that the specific surface area of Cu2O is 1.9 m2/g, and that of ZnWO4/Cu2O is 2.7 m2/g. The average pore sizes are 4.2 nm and 6.4 nm, and the pore volumes are 1.3×10−3 cm3/g and 2.8×10−3 cm3/g, respectively. It indicates that the combination of the two is conducive to improving the specific surface area, pore size and pore volume, but the space for improvement is very small.
Fig. 7 N2 adsorption and desorption curves ofa Cu2O; b ZnWO4/Cu2O
3.4 Activity detection of catalysts
The activity of the catalyst was determined by the degradation degree of 4-NP by the catalyst. The standard schematic diagram of catalytic reduction of 4-NP are shown in Fig. 8a. Under neutral and acidic conditions, the peaks of 4-NP at about 317 nm. When NaBH4 is added, the solution is alkaline. 4-NP is dissociated into ions, and the maximum absorption peak shifts to 400 nm. When the catalyst is added, 4-AP is rapidly generated. And its characteristic absorption peak is at 300 nm. There is no by-product in the whole reaction process [16]. The absorbance spectra of ZnWO4, Cu2O, ZnWO4/Cu2O-1, ZnWO4/Cu2O-3, ZnWO4/Cu2O-5, ZnWO4/Cu2O-7 changes with time are shown in Fig. 8(b-g). It can be seen from Fig. 8b that when ZnWO4 is added, the absorbance almost don’t change, which proved that ZnWO4 is no catalytic. When other catalysts are added, the absorbance changes obviously. The degradation efficiency of ZnWO4/Cu2O composite for 4-NP is better than that of Cu2O. The catalytic degradation efficiency from high to low are ZnWO4/Cu2O-5, ZnWO4/Cu2O-3, ZnWO4/Cu2O-7, ZnWO4/Cu2O-1, Cu2O. Therefore, the combination of the two can modify Cu2O and improve the catalytic activity of Cu2O.
In order to more intuitively observe the influence of ZnWO4/Cu2O-X on the catalytic performance, we selected the absorbance at 400 nm to calculate the value of Ct/C0 for comparison (C0 and Ct are the concentration of 4-NP solution at time 0 and time t). As the reaction proceeds, the ratio of Ct/C0 to time t becomes smaller and smaller. Which indicates that the content of 4-NP in the whole reaction system becomes lower and lower. The linear fitting of Fig. 8a show the linear relationship was good and conforms to the first-order kinetics. So (A0 and At are the absorbance of 4-NP solution at 0 and t, Kapp is the apparent rate constant, t is the reaction time):
ln (At/A0) = ln (Ct/C0) = -kappt 1
The relationship between Ct/C0 and t is shown in Fig. 9a. It can be seen that the conversion rate of ZnWO4/Cu2O-5 is the highest and that of Cu2O is the lowest at the same time. So the catalytic activity of ZnWO4/Cu2O-5 is higher than that of Cu2O.
The relationship between ln (Ct/C0) and t is shown in Fig. 9b. It can be seen that the reaction rate of ZnWO4/Cu2O-5 is the fastest. Linear slopes were obtained by linear fitting. The kapp values of Cu2O, ZnWO4/Cu2O-1, ZnWO4/Cu2O-3, ZnWO4/Cu2O-5 and ZnWO4/Cu2O-7 were 8.2×10−3 s−1, 15.91×10−3 s−1, 25.2×10−3 s−1, 20.6×10−3 s−1 and 6.2×10−3 s−1, respectively. In order to show the advantages of composites more clearly, the activation factors of ZnWO4/Cu2O-5 were calculated by formula 2.
k'=kapp/m 2
Fig. 9 Different catalysts of a Ct /C0 curve graph with time; b netic fitting graph
The results showed that the activation factors of Cu2O and ZnWO4/Cu2O-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.
Table 2
Comparison of the rate constant (k) and activity factor (k') of different catalysts used to reduce 4-NP to 4-AP
sample | catalyst dosage | The concentration of NaBH4 (M) | [4-NP] usage (mM) | reaction rate constant (k)×10−3 (s−1) | active factors (k')×10−3༈s−1mg−1༉ | references |
5.1wt% Co-doped CuO NPs | 1 | 8 | 0.12 | 43.80 | 43.8 | [20] |
Cu2O NPs | 1 | 0 | 0.1 | 15.7 | 15.7 | [21] |
Cu2O/MoS2/rGO | 10 | 6.25 | 12.5 | 62 | 6.2 | [22] |
Cu2O/Cu-MOF/rGO | 1 | 100 | 0.1 | 32.7 | 32.7 | [23] |
Cu2O/ZrO2 | 12 | 30 | 0.0625 | 15.97 | 1.331 | [24] |
15-cco/bv | 1 | 50 | 0.1 | 24.8 | 24.8 | [25] |
Fe3O4@ppy-MAA/Ag | 7.5 | 100 | 40 | 2.383 | 0.3177 | [26] |
Pt-NiO/G | 1 | 100 | 0.1 | 33.9 | 33.9 | [27] |
Ag-Fe bimetallic NPs | 5 | 3.333 | 0.0667 | 1.1 | 0.22 | [28] |
Cu2O/ZnWO4-5 | 0.2 | 0.025 | 0.2 | 25.2 | 126 | This work |
Fig. 10a The cycle stability test of ZnWO4/Cu2O-5 and b the time required for ZnWO4/Cu2O-5 to convert the same quality 4-NP
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 verified by adding 4-NP repeatedly. The specific operation steps were shown in Section 2.4.2. The relationship between Ct/C0 and t in the 15 cycles is shown in Fig. 10a. It can be
seen from the figure 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 figure 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.
3.5 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 fitting 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 influence 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 Ct/C0 and time at different temperatures and
b kinetic fitting 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 ZnWO4/Cu2O after reaction is characterized by XPS. The full spectrum of ZnWO4/Cu2O 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 2p3/2) and 1044.8 eV (Zn 2p1/2), and the peaks of W 4f are 35.3 eV (W 4f7/2) and 37.5 eV (W 4f5/2). Compared with Fig. 6a and b, the spectral peaks of W4f move slightly. But there is one more peak in O 1s and Cu 2p, and the peak is 532.4 eV, indicating the formation of CuO. The peaks at 934.7 eV, 954.6 eV and 943.3 eV, 962.5 eV also indicate the formation of CuO in Fig. 12e. The peaks at 932.4 eV and 952.2 eV are Cu+/Cu. Since it is difficult 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 figure 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]. The ratio of peak area of Cu2O to Cu in the catalyst decreased from 3.57 to 2.43 before and after the reaction, and the presence of CuO indicated the formation of Cu. Sasmal et al. [19] reported that Cu2O was easily reduced to Cu by NaBH4 at pH about 9, while Cu was easily oxidized to CuO, and finally formed ternary complexes. The presence of Cu is very important for the reduction of 4-NP, which can absorb hydrogen and is the action site of BH4−. The shift of the binding energy peaks of Zn 2p and W 4f and the generation and shift of the binding energy peaks of O 1s and Cu 2p indicate that they interact.
Fig. 12a The XPS survey spectrum of the recycled ZnWO4/Cu2O; the high-resolution XPS spectra for b Zn 2p, c W 4f, d O 1s, e Cu 2p, f Cu LMM of the refreshed andg the used Cu LMM
3.7 Electrochemical impedance performance test
In order to study the charge transfer process and the separation efficiency of charge carriers, electrochemical impedance spectroscopy (EIS) was performed on Cu2O, ZnWO4 and ZnWO4/Cu2O. In general, the Nyquist radius is related to the charge transfer efficiency. And the charge transfer impedance with smaller radius is low. As shown in Fig. 13, the arc size of the prepared samples is observed to be ZnWO4/Cu2O < Cu2O <ZnWO4. The interaction between Cu2O and ZnWO4 improved the electron transfer effect of ZnWO4/Cu2O composites.
Fig. 13 Electrochemical impedance (EIS) curves of Cu2O, ZnWO4 and ZnWO4/Cu2O