3.1 One step chlorine substitution process
Due to the toxicity characteristics of the PCDDs isomers, the toxic equivalents of these congeners directly affected by the chlorine substitution of 12378-PeCDD. The energy barrier of chlorine substitution at positions 4#, 6# and 9# was calculated separately and it was found that further chlorine substitution of 12378-PeCDD occurred most easily at position 6#, and the results of which are presented in Table 1. According to the difficulty of chlorine substitution, the order conforms to the principle of distal preference. We have previously resolved the association of PCDDs isomers by probability analysis, and these calculated results are not completely consistent with the calculated energy barrier. At the same time, it should be noted that the action angle between the isomer molecule and the crystal cell has changed by optimizing the interaction structure. Figure 2 shows the setting of the action angle parameter and the migration rule. Therefore, further consideration for the migration of action angle is another important factor to deeply optimize the lattice oxygen interaction and explore the direction of optimization.
3.2 Effects of CaO adsorption
Table 2 presents the energy barriers for the chlorine substitution process of PCDD when it closes to the CaO modified Fe2O3 lattice oxygen surface. Firstly, the original energy barriers have been increased under the conditions of CaO modification. It indicates that the chlorine substitution is more difficult to occur, which may be one of the original causes of the CaO modification reducing the production of PCDD/Fs. At the same time, the intervention of CaO modification resulted in that the occurrence of chlorine substitution process in position 6# is no longer the easiest pathway, but adjusted to position 9#. This result indicates that CaO modification can directly change the distribution patterns of PCDD/Fs isomers, and the reduction of PCDD/Fs toxicity can be further achieved by regulating the CaO modification. It should be noted that the optimized action angle is fundamentally altered, thus suggesting that changing the optimal action angle (which alters the intermolecular collision probability) is also potentially responsible for reducing the toxicity equivalence of the PCDD/Fs isomers, especially since the energy barrier and the values of action angle at position 9# are significantly different from the other two pathways. For details of the action, it can be seen in Fig. 3.
3.3 Effects of load components and action sites
In order to further study the load components and action sites, Na2O and CaO are respectively adopted to optimize the structure through the artificial rotation of 90 degrees. The results are shown in Fig. 4 and Fig. 5. It should be pointed out that Na2O and CaO are chosen as calculation cases on the basis of the difference between bi-metal atoms and mon-metal atoms. First of all, it can be seen from Fig. 4 that when CaO is loaded on the surface of the iron-based cell, the action angle does not change much after 90 degrees of artificial rotation, but the influence on the energy barrier is very obvious. It can be inferred that, for mono-metal oxide loads, the optimization of load form mainly depends on the influence of the energy barrier, while the influence of the action angle is not obvious. In contrast, Fig. 5 shows that after the Na2O load form is artificially rotated 90 degrees, the energy barrier has little influence, but the action angle is obviously changed. Similarly, due to the inferred bi-metal oxide load, more attention needs to be paid to the change of the action angle.
3.4 Effects of Fe-based cells structure regulation
In order to further study the directional regulation of Fe-based cells structure, the method of artificially stretching iron atoms was adopted in this study to improve the Fe-based cells structure, and the results are shown in Fig. 6. It can be found that the energy barrier is obviously changed by artificially stretching the action angle between the Fe-based surface and the PCDD isomer surface, but the effect of the action angle is little. It can be inferred that the influence from the change of the iron atomic structure on the energy barrier should be considered in the subsequent optimization of the Fe-based OC. It should be pointed out that there are some differences in the change of energy barrier between different isomers.