3.1 SDBD and DDBD treatment of benzene series
3.1.1 Discharge parameters and air tube analysis
Discharge parameters are an essential aspect for the performance of DBD reactor. Through different output voltages, different discharge powers of SDBD and DDBD were obtained (Fig. S1). The discharge power of DDBD was higher than that of SDBD under different output voltages. With the increase in voltage, the difference in the discharge power increased gradually, reaching the value of 15.5 W. Fig. 2 showed the relationship between SED, airflow and output voltage. For a certain voltage, with the increase of flow rate, the reactor's SED decreased gradually, whereas SDBD attenuated faster than the DDBD. For a constant airflow, the SED increased gradually with the increase of output voltage. Compared with the DDBD, the SED of SDBD increased slightly. It can be seen that, in the DBD, the SED was positively correlated with the output voltage and negatively correlated with the airflow. The preservation capability of SDBD for SED was weaker than that of DDBD.
3.1.2 Effect of initial concentration on the degradation rate and by-products
The degradation efficiencies of SDBD and DDBD for benzene and toluene mixture decreased gradually with the increase of initial concentration, and the degradation rate decreased from the highest value of 78% to the lowest value of 3% (Fig. 3). When the energy density was kept constant, the high-energy electrons and active particles were limited with the increase of pollutant concentration. The removal rate decreased with the increase of initial concentration. The removal rate of VOCs in DDBD was higher than that of SDBD because the SED of DDBD was higher than that of SDBD and released more electricity. The degradation rate of toluene was significantly higher than that of benzene (about 11 - 21% higher), which was due to the fact that toluene has one more methyl entity than benzene that is easier to fall off (Wu et al., 2013; Liu et al., 2017; Najafpoor et al., 2018; Dang et al., 2016).The EEs of the two reactors for the VOCs’ mixture were different from each other. The EE of DDBD was slightly higher than that of SDBD by 0.1 - 0.25 g/kWh (Fig. 3), indicating that the EEs of the two reactors for benzene and toluene treatment were not significantly different because the SED of the two reactors was constant. The SED and degradation efficiency of DDBD for toluene, and benzene were higher than those of SDBD. From the perspective of the reactor structure, the discharge breakdown of a single medium was relatively easy. The two layers of the medium increased the thickness of barrier, which led to increased energy consumption and reduced EE. With the increase in the initial concentrations of benzene and toluene, the overall EE first increased, became flat, and then, decreased. In the case of constant SED, the electric energy generated by the device was constant, and the active substances in the device were constant. With the increase in the concentration of benzene series, the increased by-products consumed more electricity, which degraded the pollutants. However, when the increase in the initial concentration reached to a certain extent, the need for electric energy was limited, which interfered with the by-products. Therefore, the degradation efficiency and energy utilization rate decreased for the process.
In the air tube ( Fig. S2) with the output voltage of 12 kV and flow rate of 275 L/h, O3 concentration produced by SDBD was about 3787.43 ppm, and that produced by DDBD was about 2144.47 ppm. For the constant SED, the transformation rate of O3 increased with the build-up of output voltage (Fig. 4a), indicating that part of O3 generated by the plasma device was involved in the degradation of VOCs, while some part of O3 got transformed and degraded (Mao et al., 2017; Chen et al., 2018). The higher the benzene series concentration, the more O3 consumption. When the concentration increased to about 100 ppm, the consumption rate of O3 was slower than in the previous stage. The concentration of NO2 in SDBD was 196.54 ppm, whereas that in DDBD was 73.21 ppm. Fig. 4b showed that, when the initial concentration was 20-80 ppm, NO2 in SDBD was slightly higher than that in DDBD. After 80 ppm, the transformation rate of NO2 in DDBD was higher than that in SDBD, which may be due to the presence of minor ·O and other substances in SDBD compared to DDBD. With the increase of concentration, limited ·O preferentially participated in the degradation of VOCs and intermediates, which reduced the transformation of NO2 in SDBD. The fluctuation in the rate of transformation of NO2 in SDBD was unstable, which may have been caused by the instability of SDBD.
The concentration of NO was 13.25 ppm in SDBD and 0.981 ppm in DDBD. As the initial concentration of VOCs increased, the transformation rate of NO in SDBD increased significantly (Fig. 4c), which was due to the reason that there were more substances such as ·O in SDBD than in DDBD that contributed to the transformation of NO into NO2. Fig. 4d shows the relationship between the CO2 selectivity and the degradation of benzene and toluene in the two reactors. The mineralization of SDBD was slightly lower than that of DDBD during the degradation of benzene. The mineralization of the two reactors gradually decreased with the increase of initial concentration. The reason why the mineralization of SDBD was lower than that of DDBD was that the discharge of DDBD was stable and more active substances were produced in stable states. Furthermore, more active substances were involved in the process of ring-opening of benzene and toluene, which effectively degraded the VOCs and converted them into CO2. The efficiency was higher, so the CO2 selectivity of DDBD was higher than that of SDBD. For the phenomenon that the degree of mineralization decreased with the increase in the concentration of VOCs. This could be due to the reason that the SED was constant, and the total amount of active substances were also constant. When the concentration of VOCs increased, the limited active substances were not sufficient to transform VOCs into CO2, resulting in an increase in the intermediate products, which led to a decrease in the selectivity of CO2. The second reason is that, with the increase in the concentration of VOCs, the intermediate products increased, which increased the consumption of electricity and led to a decrease in the degree of mineralization. In addition, the selectivity of CO2 in SDBD was relatively unstable, which was caused by the unstable discharge of electric energy and the unstable amount of intermediate products.
3.1.3 Effect of output voltage on the degradation rate and by-products
As the output voltage increased, the degradation rate of VOCs became larger (Fig. S3), which was due to the increase in the electric field’s intensity and more active substances, thus improving the efficiency of the degradation of VOCs. For the same output voltage, the degradation efficiency of VOCs of SDBD was lower than that of DDBD because the DDBD was more stable and produced more effective active substances, thereby improving the degradation efficiency of VOCs. The output voltage was positively correlated with the EE (Fig. S3). As the SED increased, the electric field intensity increased, and the active substances increased the species involved in the degradation of VOCs, which in turn increased the energy efficiency. Beyond the voltage of 13 kV, the energy efficiency could not be increased. Although the active substances produced by the reactor increased, the concentration of the degraded target pollutants became limited, and therefore, the EE became limited.
When the concentration of VOCs, flowrate and output voltage had values of 110 ppm, 275 L/h, and 10-13 kV, the transformation rate of O3 increased with the increase of voltage (Fig. S4a). Due to the increase of SED, although the generated O3 increased, the active substances increased as well. Most of the O3 transformed into active substances and participated in the degradation reactions of VOCs. When the output voltage lied within the range of 13–16 kV, with the increase of voltage, the O3 transformation rate decreased. Moreover, the O3 concentration generated at this stage increased sharply, and the concentration of VOCs was limited. The excess O3 cannot participate in the transformation, so the transformation rate decreased. Within the output voltage range, the difference of O3 transformation between the two reactors was not obvious. The transformation rate of NO2 in SDBD was higher than that in DDBD after the output voltage of 10 kV (Fig. S4b). When the concentration and flow rate of VOCs were constant, the build-up of voltage led to the increase of active substances and the consumption of more ·O, which reduced the formation of NO2 and increased the number of intermediate products. The generated NO was involved in the degradation of intermediate products, which also reduced NO2. In addition, the increase of O3 concentration also promoted the transformation of NO2. The transformation rates of NO and NO2 in SDBD were also higher than those in DDBD ( Fig. S4c).
The degree of mineralization of SDBD in benzene degradation was significantly lower than that of DDBD (Fig. S4d). DDBD generated electrical energy more stably than SDBD, which in turn generated more active substances consistently. More active substances were involved in the ring-opening process of benzene and toluene (Han et al., 2020), which improved the degradation efficiency of VOCs into CO2. Therefore, DDBD had higher CO2 selectivity than the SDBD. Similarly, the mineralization of DDBD was higher than that of SDBD. Beyond the output voltage of 12-13 kV, the selectivity of CO2 tended to stabilize. When the intermediate product increased to the maximum, it gradually degraded into CO2 and reached a certain degree of stability. Even if the active substance increased, the mineralization did not change, and the selectivity of CO2 tended to stabilize.
3.1.4 Effect of airflow on the degradation rate and by-products
The degradation efficiency of VOCs by SDBD and DDBD decreased with the increase of flow rate (Fig. S5). Due to the decrease of residence time, the contact time between the VOCs and active substances decreased. The energy efficiency of SDBD was higher than that of DDBD (Fig. S5). With the increase in the flow rate, the energy efficiency increased. When the concentration of VOCs and the output voltage were constant, SED decreased, and the degradation rate of VOCs exhibited little change, which led to an increase in energy efficiency. However, due to the limitation of residence time, the energy efficiency increased slightly.
With the change in the gas flow rate, the transformation rate of O3 fluctuated significantly (Fig. S6a) because the formation and transformation of O3 were greatly affected by the residence time. Another reason was that the flow had a significant impact on SED. The overall performance of DDBD for O3 transformation was better than SDBD because DDBD discharge was more stable. Furthermore, its SED was less affected by flow than that of SDBD, and therefore, it would produce more active substances to promote the transformation of O3. The transformation rates of NO2 and NO both decreased with the increase of airflow (Fig. S6b). There was little difference between the SDBD and DDBD for the transformation of NO2. The transformation of NO in SDBD was higher than that in DDBD (Fig. S6c). This could be due to two reasons. The first was the shorter residence time and the shorter contact reaction time for NO2 and active substances. The second was that the SED decreased with the increase in the flow rate, and the amount of active substance produced decreased.
With the increase in the flow rate, the mineralization of SDBD and DDBD gradually decreased, which was also consistent for the degradation of toluene (Fig. S6d). This could be due to the reason that both the SED and the number of active substances decreased. The limited active substances were not sufficient to degrade benzene and toluene to CO2, resulting in an increase in the intermediate products and a decrease in the selectivity of CO2. The second reason was that the residence time decreased, due to which, the contact time between the intermediate products and the active substances decreased, thus decreasing the degree of mineralization. SDBD was slightly less mineralized than DDBD during the degradation of benzene.
3.2 Preparation of rGO-based catalyst
3.2.1 Preparation of GO
The FG morphology was stacked. The layers were closely connected, while the edges were clearly defined and compact (Fig. 5a). Both the GO1 and GO2, prepared using different methods (Fig. 5b,c), were significantly different from flake graphite in morphology after the IHM and ultrasonic exfoliation. For GO1, although the layer spacing increased, the morphology and appearance were not ideal. The degree of oxidation stripping was also not high. On the other hand, by adding KMnO4 in multiple batches, GO2 was intercalated by orderly oxidation. GO2 was complete and orderly than GO1.
BET surface area (SBET), total pore volume (Vtotal), and average pore diameter (dpore) of the flake graphite were low, and the two kinds had significant differences compared with the FG (Table S1). The physical performance was significantly improved, and the preparation of GO from the flake graphite was relatively complete. Furthermore, SBET of GO2 increased by about 11% compared with GO1, however Vtotal and dpore did not increase significantly.
XRD characterization (Fig. 6a) showed that the FG had a characteristic graphite peak at 26.5°, whereas both the GO1 and GO2 had a characteristic peak of GO at 10.6°, and 42.5°. According to Bragg's law, the layer spacing of GO2 was nearly three times larger than that of FG, indicating that introduction of KMnO4 in batch mode can effectively increase the layer spacing of GO.
Figure 6b showed no peak of FG, indicating that there was no functional group. A series of characteristic peaks appeared in GO2, indicating that the polar groups increased sharply, and O-H appeared at 3375 cm-1. The absorption peak at 1621 cm-1 belonged to the unoxidized vibration peak of C=C, whereas the absorption peak at 1261 cm-1 was ascribed to the C-H stretching vibration (Kizil et al., 2002; Almasian et al., 2010). The absorption peak at 1135 cm-1 was ascribed to C-O-C. These polar functional groups all indicated that GO introduced more hydrophilic groups, which is also the reason for GO's hydrophilic solid properties.
3.2.2 Preparation of catalyst using ion-assisted hydrothermal method
In the absence of [BMIM]PF6, the nanocrystals showed irregular morphology and local dispersion, whereas most of them had severe agglomeration (Fig. 7a). The manganese catalyst was effectively dispersed on the rGO after the addition of [BMIM]PF6. The addition of [BMIM]PF6 could effectively improve the degree of dispersion and reduce the degree of agglomeration (Fig. 7b). The excessive addition of [BMIM]PF6 made the nanocrystals exhibit irregular morphology, and the agglomeration phenomenon occurred locally (Fig. 7c).
When [BMIM]PF6 was not added, the formation of crystals was incomplete, and the layer edge was crimped, which is not conducive to the presence of nucleation center of the catalyst (Fig. 7d). When the dosage of [BMIM]PF6 was 0.25 mL, the morphology and structural characteristics of the prepared FeOx/rGO catalyst were apparent. The crystal dispersion was a little high (Fig. 7e). When the dosage of [BMIM]PF6 was 0.5 mL, the crystal dispersion was not too high (Fig. 7f). The TiOx/rGO catalyst prepared with the addition of 0.25 mL of [BMIM]PF6 had good structural morphology (Fig. 7g). The crystal formation of 0 mL (Fig. 7h) and 0.5 mL (Fig. 7i) was not complete, and the lamellar edges were crimped, which was not conducive to nucleation.
Within the 0-0.25 mL range, the SBET and Vtotal of the three catalysts increased, whereas dpore decreased. Within the range of the added amount of 0.25-0.5 mL, excessive addition led to poor dispersion performance of the catalyst, agglomeration and other phenomena, which all resulted in smaller specific surface areas (Table S2). When the dosage of [BMIM]PF6 was 0.25-0.5 mL, SBET, Vtotal, and dpore of the three catalysts showed little change.
In the absence of [BMIM]PF6, the characteristic peaks of MnOx/rGO with different intensities appeared. When 0.25 mL [BMIM]PF6 was added, some characteristic peaks of Mn3O4 appeared (Fig. 8a). When there was no [BMIM]PF6, there were two characteristic peaks of FeOx/rGO at 27.3° and 42.5°, both of which belonged to Fe3O4 (JDPDF 26-1136). When 0.5 mL of [BMIM]PF6 was added, the characteristic peak at 27.3° was weaker than that when there was no [BMIM]PF6, while the characteristic peak at 42.5° became narrower and sharper. Compared with the characteristic diffraction peak when 0.5 mL of [BMIM]PF6 was added, the characteristic peak at 0.25 mL was sharper, narrower and higher in intensity, indicating that better crystals were formed due to relatively intact lattices and a higher degree of crystallization under the condition of 0.25 mL of ion-assisted solution (Fig. 8b). All diffraction peaks of TiOx/rGO catalyst belonged to TiO2 (JDPDF 21-1272). With the increase in the dosage of [BMIM]PF6, the diffraction peak of C=O gradually weakened (Fig. 8c).
Figure S7a shows the FT-IR characteristics of three catalysts prepared with different additives. The MnOx/rGO prepared without [BMIM]PF6 had an O-H vibration stretching peak at 3487 cm−1, while the addition of 0.25 mL [BMIM]PF6 significantly weakened its peak intensity and introduced some new groups (O-H group, H2O group, and C-O-H group). Moreover, the addition of 0.5 mL [BMIM]PF6 had no prominent characteristic peak compared with the first two, which may be due to the addition of too much ionic liquid that resulted in incomplete hydrothermal reduction reaction and crystallization of [BMIM]PF6 without introducing more new groups. For FeOx/rGO, when [BMIM]PF6 dosage was 0-0.25 mL, there was an O-H stretching vibration peak at 3487 cm-1, whereas the peak intensity decreased for the added amount of 0.5 mL (Fig. S7b). The more substantial characteristic peak (O-H group) at 3487 cm-1 was of the 0.5 mL [BMIM]PF6 dosage spectrum. In contrast, the characteristic peak intensity of [BMIM]PF6 for 0 mL and 0.25 mL was slightly weaker (Fig. S7c). For the three dosage conditions of the ionic liquid, the preparation results of the catalyst were ideal when the [BMIM]PF6 dosage was 0.25 mL. The crystal had enhanced physical properties, many active centers for the nucleation of catalyst, and its internal structure and functional groups were excellent.
3.3 DDBD combined with the three catalysts
3.3.1 Degradation of benzene series using MnOx/rGO, FeOx/rGO, TiOx/rGO combined with DDBD system
The degradation rate of MnOx/rGO+DDBD was the highest among the four combinations of benzene and toluene degradation (Fig. 9a), with the value for benzene reaching to around 82% and that for toluene 86%. Compared to the DDBD device without a catalyst system, the degradation rate increased by 20-43%. The degradation efficiency of FeOx/rGO+DDBD was second only to MnOx/rGO+DDBD, which was determined by the properties of Mn and Fe catalysts (Qin et al., 2021). In another words, VOCs and active substances induced a redox reaction on the surface of the catalyst, and its degradation capability was closely related to the redox capability of the catalyst itself. Mn has a stronger catalytic capacity and higher redox capacity (Durán et al., 2009). The degradation efficiency of TiOx/rGO+DDBD was lower than the first two, among which TiOx/rGO mainly relied on the ultraviolet (UV) light generated by plasma to trigger the catalytic effect (Zhao et al., 2015; Souza et al., 2006; Brückner et al., 2021). However, this system consisted of a series device. The UV light generated by DBD was relatively weak on the catalyst at the tail end, due to which, its catalytic effect was not very ideal. The EE of MnOx/rGO+DDBD was the highest (Fig. 9b), which was 31-54% higher than that of DDBD without a catalytic system. The EE was related to the SED and degradation efficiency. The degradation efficiency of the MnOx/rGO+ DDBD system was the highest, and its energy efficiency was also the highest.
The O3 transformation rate of FeOx/rGO+DDBD was the highest (Fig. 9c), reaching the value of about 80%. Compared with the DDBD without a catalyst system, the O3 transformation rate of FeOx/rGO+DDBD increased by about 43.5% because the introduced FeOx/rGO transformed the excess O3 into active substances, such as superoxide free radicals. The NO transformation of MnOx/rGO+DDBD was about 71%, and the NO2 transformation was about 73%, which was about 47% higher than a single DDBD processing unit. The MnOx/rGO+DDBD system had the highest degree of mineralization (Fig. 9d). The reaction of active substances with manganese-based catalysts resulted in more VOCs being effectively degraded and more intermediate products transformed into CO2.
3.3.2 XPS characterization of the three catalysts
The most obvious atomic content was that of C, followed by O (Table S3). Catalysts were combined with graphene to produce -COOH. The surface of the catalyst adsorption caused -OH/CO32−. Moreover, O2− was combined with the three transition metal lattice oxygen. The oxygen content of the three transition metal catalysts included adsorbed water (-COOH ), adsorbed oxygen on the surface of the catalyst (-OH/CO32−), and lattice oxygen (O2−) (Huang et al., 2012; He et al., 2019; Zeng et al., 2019; Kara et al., 2014). Overall, the proportion of each catalyst was not much different from the theoretical value (Fig. 10a). MnOx/rGO catalyst contained the highest content of lattice oxygen, whereas TiOx/rGO catalyst had the most surface adsorbed oxygen. Moreover, FeOx/rGO lied in between the two(Fig. 10b). Generally speaking, lattice oxygen and surface adsorbed oxygen often determine the excellent catalytic performance. Therefore, all the three transition metals had a high catalytic capability.
In the high-resolution spectra, manganese mainly existed in the form of Mn2p1 and Mn2p3, whereas iron mainly existed in the form of Fe2p1. Furthermore, titanium mainly existed in the form of Ti2p3. The high-resolution O1s spectrum was assigned to -COOH at 532.1 eV, to -OH/CO32− at 531.4 eV, and to O2− at 530.9 eV. For the high-resolution spectral image of Mn2p (Fig. 10c), a prominent characteristic peak near 653.8 eV belonged to Mn2p1. At the binding energy of 642.1 eV, its characteristic peak belonged to Mn2p3. According to the comparison of XPS binding energies and spacing, MnOx/rGO catalyst mainly existed as Mn3O4. For Fe2p high-resolution spectrograph (Fig. 10d), the characteristic peak was not apparent at 724.7eV, which should belong to Fe2p1. The characteristic peak of Fe2p3 (710.3 eV) was more potent than that of Fe2p3. According to the XPS Binding Energy (B.E.) results and the binding energy spacing, FeOx/rGO catalyst mainly existed in the form of Fe3O4. In the Ti2p spectrum (Fig. 10e), the characteristic peak at the binding energy of 464.9 eV belonged to Ti2p1, whereas the characteristic peak at the binding energy of 1458.2 eV belonged to Ti2p3. According to the XPS B.E. results and spacing, the TiOx/rGO catalyst mainly existed as TiO2.