3.1.Effect of reaction temperature
The effect of temperature variation on the efficiency for desulfurization and denitration of the catalyst was investigated by adjusting the temperature in the reactor. As shown in Fig. 2, the catalytic efficiency of the catalysts( V-Ce-ZSM-5 and Cu-Ce-ZSM-5) increased first and then decreased at the test temperature (50°C-300°C), and the optimum performance was obtained at 150°C. While reaction temperature increased, the oxidation groups and free radicals on the surface and inside of the catalyst were thermally activated(Bychkov et al. 2020,Hu et al. 2020 ), which significantly increases the number of activated molecules and the chemical reaction rate of the catalyst. However, with a further increase in temperature, the recombination probability of free radicals would be expedited, which is an important reason for the decrease in removal efficiency(Liu et al. 2017). So the following catalyst activity tests were all tested at a temperature of 150 ℃.
3.2.Effect of V、Cu and Ce doping
Fig. 3 shows the changes of desulfurization and denitration efficiency of different catalysts with the doping amount of V, Cu and Ce. With the increase of V and Cu loading from 5% to 9%, the desulfurization and denitrification efficiency of the catalyst increased first and then decreased. When the loading of V was 7% and the loading of Cu was 8%, the efficiency was the best. After Ce doping, the efficiency of desulfurization and denitrification was significantly enhanced. When the Ce load was 7%. When the loading of Ce is 7%, the efficiency was the best.
With the increase of the doping amount of V, Cu and Ce in the catalyst, part of the pore structure would be occupied, and the adsorption capacity of the catalyst for pollutants would be weakened. However, the oxidation capacity gradually increased, and the removal of pollutants changed to the oxidation process. The doping of multiple elements significantly increases the oxidation capacity of the catalyst, but does not always to increase, because the pollutants will first adsorb on the adsorption site, and then the oxidation reaction occurs. With the progress of the reaction, the adsorption site gradually saturated, desulfurization and denitrification efficiency will gradually decrease The adsorption and oxidation performance of the catalyst was best when the loading of V was 7%, Cu was 8% and Ce was 7%.
3.3.Effect of GO doping
Fig. 4 shows the change in desulfurization and denitration efficiency of the catalyst with GO doping amount. With the increase of the amount of GO from 0% to 0.9%, the oxidation efficiency of the catalyst for SO2 and NO increased gradually, and tends to be stable after the amount of GO was 0.5%. When the doping amount of GO was 0.5%, the catalyst showed the best catalytic effect, and the oxidation efficiencies of the catalyst for SO2 and NO were 94.60% and 83.64%, respectively.
It was evident that the addition of GO can effectively enhance the oxidation effect of gaseous pollutants, especially NO. During the reaction, the role of GO is mainly to increase the loading of active components and increase electron mobility(Kandy et al. 2019 ).
3.4.Characterization of the catalyst
3.4.1.BET analysis
Table 1 shows the BET characterization results of different catalysts. With the addition of V, Cu, and Ce elements, the total pore volume and the specific surface area of the catalyst increased gradually, while the pore size became smaller. This indicated that the infiltration of the elements improved the pore structure of the original carriers, resulting in a corresponding change in the specific surface area, thereby improving the performance of the catalysts. When the catalyst was doped with GO, the ZSM-5 was favorable for element loading(Zheng et al. 2019 ). Compared with ZSM, the specific surface area of [email protected] increased by 42.53%, resulting in an increase in the number of active sites, thus improving the catalytic activity of the catalyst.
Table 1 BET characterization results of the catalysts
Sample
|
SBET (m2/g)
|
Pore volume (cm3/g)
|
Average particle size (nm)
|
ZSM-5
|
170.47
|
0.1014
|
3.9173
|
V-ZSM-5
|
184.89
|
0.1117
|
3.6242
|
Cu-ZSM-5
|
179.46
|
0.1169
|
3.3475
|
V-Cu-Ce-ZSM-5
|
213.72
|
0.1511
|
3.1312
|
V-Cu-Ce-ZSM-5@GO0.5
|
242.97
|
0.167
|
3.005
|
3.4.2.SEM and EDS analysis
Fig. 5 (a-c) shows the SEM images of ZSM-5, V-Cu-Ce-ZSM-5, and V-Cu-Ce-ZSM-5@GO0.5, respectively. Compared with ZSM-5, the surfaces of V-Cu-Ce-ZSM-5 and V-Cu-Ce-ZSM-5@GO0.5 were agglomerated, and several particulates were uniformly dispersed on the surface of the catalyst. The results indicated that the ZSM-5 carrier could be stably loaded with V, Cu, and Ce elements during the catalyst synthesis. Furthermore, when the V-Cu-Ce-ZSM-5 catalyst was doped with GO, the porosity increased, and the particle division on the surface of the carrier was more uniform, which is consistent with the results of Zhang( Zhang et al. 2019) et al. These porous structures can significantly increase the diffusion resistance when the polluted flue gas passes through, which is beneficial to improve the effective contact area and contact time between the active components and the reactive gas, finally improve the the catalytic efficiency of V-Cu-Ce-ZSM-5@GO0.5.
Fig. 6 shows the EDS mapping images of V-Cu-Ce-ZSM-5@GO0.5. V, Cu, and Ce elements were uniformly distributed in the whole mixed catalyst. In addition, the results of EDS showed that the atomic ratios of V, Cu, and Ce were 2.79%, 2.53%, and 2.07%, respectively, which further confirmed the uniform fixation of V, Cu, and Ce nanoparticles on the surface of ZSM-5.
3.4.3.XRD analysis
According to Fig. 7(a) displaying the XRD patterns of ZSM-5, V-Cu-Ce-ZSM-5, and V-Cu-Ce-ZSM-5@GO0.5, the diffraction peaks of ZSM-5 located at 13.24° and 15.64° disappeared with the loading of V, Cu, and Ce and produced obvious diffraction peaks of V2O5 and CuO at 32.520°and 48.320°, indicating that V, Cu, and Ce interacted chemically with the ZSM-5 carrier, these elements were immobilized on the ZSM-5 carrier. Meanwhile, the position of the characteristic spectra peaks did not change with the addition of GO. However, the intensity of the characteristic peaks was enhanced, indicating that the addition of GO increased element loading and enhanced the catalytic reaction intensity of the catalyst.
Fig. 7(b) shows the XRD magnification patterns of [email protected]. After 2θ=25°, all the diffraction peaks of V, Cu, and Ce elements appear. The diffraction peaks of 2θ at 28.549°, 33.083°, 46.486°, 56.346°, and 59.093° belong to the characteristic peaks of CeO2 ( Marzouk et al. 2018), the diffraction peaks at 26.759°, 32.520°, 36.230°, and 41.289° were assigned to V2O5 (Seo et al. 2015), and the peaks at 29.847°, 38.568°, 43.848°, 48.320°, and 55.257° were the main peak patterns of CuO (He et al. 2013).
3.4.4.XPS analysis
Fig.8 shows the high-resolution XPS spectra of V-Cu-Ce-ZSM-5@GO0.5 (a). The V, Cu and Ce elements supported on the catalyst mainly exist in the form of oxides of V2O5, CeO2, and CuO. According to the oxidation binding energy data, fine spectras of V2p (b), Cu2p (c) and Ce3d (d) were simulated . Compared with the concentration of oxygen, when V exists at a low concentration, it is often necessary to find the vanadium binding energy with a lower spin-orbit splitting peak on one side of O1s peak and collect V2p and O1s regions together (507 eV – 540 eV) to achieve accurate peak fitting. V produced an asymmetric peak shape in the V2p region, while the vanadium oxide had a symmetric peak shape and a significantly split spin-orbit. Two prominent peaks of V metal were located at 517.2 eV and 524.4 eV, respectively, as shown in Fig. 8(b). The higher peak intensity indicated that vanadium oxide was dominated by V2O5 on the catalyst surface. Fig. 8(c) shows the energy spectrum of Cu2p, which could be split into two peaks, Cu2p3/2 (peak at 934.7 eV) and Cu2p1/2 (peak at 953.7 eV), these were assigned to Cu(II)(Arul et al.2020 ). Fig. 8(d) shows that the Ce3d region had well-separated spin-orbit components, each of which was further split by multiple splitting. Ce(IV) had a peak at 917eV, which was not present in the Ce(III) spectrum ( Mhatre et al.2021).