## 3 Results
### Effect of SO\({}_{2}\) and CO on the Decomposition of N\({}_{2}\)O Catalyzed by MgO (100)  
Other flue gas components in furnaces will affect the catalytic performance of N\({}_{2}\)O decomposition on the surface of MgO (100). Thus, the adsorption of SO\({}_{2}\) on the MgO (100) surface was studied. The stable adsorption structure after geometric optimization is shown in Figure 7, and the adsorption energy, geometric structure, and Mulliken charge transfer after structure optimization are listed in Table 2.  
Figure 5: The surface reduction process of MgO (100) following the Langmuir–Hinshelwood (LH) mechanism. Figure 6: The surface reduction process of MgO (100) following the Eley–Rideal (ER) mechanism. As seen in Figure 7 and Table 2, when SO\({}_{2}\) molecules are adsorbed on the MgO (100) surface with adsorption configurations a, c, and d, the adsorption energies are \(-\)0.238 eV, \(-\)0.239 eV, and \(-\)0.239 eV, respectively, and the distances between the adsorbed SO\({}_{2}\) molecules and the surface are 1.855 A, 1.837 A, and 1.837 A, respectively. Further, the charges transferred from the MgO (100) surface to the adsorbed SO\({}_{2}\) molecules after adsorption are 0.440 \(e\), 0.448 \(e\), and 0.447 \(e\), respectively. As shown in Figure 7 and Table 2, compared with configurations a, c, and d, when other adsorption configurations b, e, f, g, and h are adsorbed on  
\(0.143\,e\), \(0.202\,e\), and \(0.130\,e\), respectively [27]. Regarding adsorption energy, geometry, and charge transfer, SO\({}_{2}\) formed strong chemical adsorption with its S-terminal at the oxygen top site, bridge site, and vacancy adsorption of MgO (100). When SO\({}_{2}\) is adsorbed at the surface Mg site at its S end or its O end on the MgO (100) surface, its interaction with the surface is also weak. In addition, by comparing the adsorption energy, structure parameters, and charge transfer of N\({}_{2}\)O on the MgO (100) surface, it was found that the interaction between SO\({}_{2}\) and the MgO (100) surface is much stronger than that between N\({}_{2}\)O and the surface. The oxygen site on the surface of MgO (100) is the common active site for SO\({}_{2}\) adsorption and N\({}_{2}\)O decomposition. However, because of the strong interaction between the SO\({}_{2}\) gas molecules and the surface, the existence of SO\({}_{2}\) will hinder the catalytic decomposition of N\({}_{2}\)O on the MgO (100) surface. When CO exists, it reduces N\({}_{2}\)O molecules in the flue gas homogeneously and affects the catalytic decomposition process of N\({}_{2}\)O on the surface of MgO (100) [28]. As shown in Figure 8, N\({}_{2}\)O molecules will form an adsorbed atomic oxygen on the surface of MgO (100) after oxygen transfer. Thus, if there is only N\({}_{2}\)O in the gas atmosphere, the activation energies of the surface reduction according to the LH and ER mechanisms are 0.840 eV and 1.498 eV, respectively. In the presence of CO, the reaction energy diagram of the surface-adsorbed atomic oxygen is shown in Figure 8, wherein the activation energy of the whole reaction process is only 0.086 eV, with 3.934 eV released. Therefore, the existence of CO can improve the rate of surface reduction, but the catalytic activity of the MgO (100) surface for N\({}_{2}\)O decomposition is determined by the transfer process of oxygen atoms. Thus, the existence of CO will not improve the catalytic activity of the MgO (100) surface. When comparing the homogeneous reduction of N\({}_{2}\)O via CO and the heterogeneous reduction of CO under the catalysis of MgO (100), we found that the heterogeneous reduction of N\({}_{2}\)O by CO is a two-step reaction. First, the N\({}_{2}\)O molecule undergoes the transfer process of oxygen atoms and must cross the energy barrier of 1.601 eV. Then, the surface reduction process occurs, which requires energy of 0.086 eV.