## 3 Results and Discussion
### Effects of SO\({}_{2}\) and H\({}_{2}\)O on NO Removal Activity  
In real applicationsincoal-fired power plants, small concentrations of H\({}_{2}\)O and SO\({}_{2}\) still remain in the flue gas after the electrostatic precipitator and desulphurization devices. Therefore, the combined effect of SO\({}_{2}\) and H\({}_{2}\)O should be considered for the SCR reaction. To explore the SO\({}_{2}\) and H\({}_{2}\)O effects on the honeycomb crodierite-based Mn-Ce/Al\({}_{2}\)O\({}_{3}\) catalyst in terms of NO removal efficiency, transient response experiments were performed at 100 \({}^{\circ}\)C. Figure 2a shows the results of the catalytic activity in the presence of SO\({}_{2}\) (50 ppm) over Mn-Ce/Al\({}_{2}\)O\({}_{3}\). In the absence of SO\({}_{2}\), the steady-state NO conversion over the Mn-Ce/Al\({}_{2}\)O\({}_{3}\) was approximately 80%, indicating that the catalyst has high activity at low temperature. AfterSO\({}_{2}\) (50 ppm) was introduced, NO conversion decreased rapidly to 72% at 24 h. After SO\({}_{2}\) was shut off, NO conversion increased slightly but could not be completely recovered. During the NO conversion process in the presence of SO\({}_{2}\), irreversible deactivation of the catalyst occurred. This result is attributed to the SO\({}_{2}\) transformation into ammonium sulfates with NH\({}_{3}\), blocking the active sites on the surface of the catalyst, and competition with NO for adsorption onto the active sites [21]. Furthermore, Figure 2b shows the results of the catalytic activity in the presence of H\({}_{2}\)O (5 vol %) over Mn-Ce/Al\({}_{2}\)O\({}_{3}\); when H\({}_{2}\)O addition began, the NO conversions of Mn-Ce/Al\({}_{2}\)O\({}_{3}\) decreased to ca. 13%. Over the next 23 h, the NO conversion was nearly unchanged and almost recovered to the initial activity after H\({}_{2}\)O was removed, which resulted from the competitive adsorption between H\({}_{2}\)O and NH\({}_{3}\)/NO\({}_{x}\).  
In addition, the stability of the Mn-Ce/Al\({}_{2}\)O\({}_{3}\) catalyst was also tested at the same temperature and the results are presented in Figure 2c, indicating thatthere is no deactivation during the successive testing performed over the 168 h period at 100 \({}^{\circ}\)C in the absence of SO\({}_{2}\) and H\({}_{2}\)O. Thus, the Mn-Ce/Al\({}_{2}\)O\({}_{3}\) catalyst appears to be highly stable for NO elimination at low temperatures under the examined period of operation. Nevertheless, NO conversion by Mn-Ce/Al\({}_{2}\)O\({}_{3}\) decreased quickly to 58% when SO\({}_{2}\) and H\({}_{2}\)O were introduced into the gas flow simultaneously and the NO conversion was nearly stable over the next 124 h. The NO conversion then increased quickly to 70%, after the SO\({}_{2}\) and H\({}_{2}\)O were removed. Meanwhile, the passivation effect was noticeably enhanced in the co-presence of SO\({}_{2}\) and H\({}_{2}\)O because in the atmosphere with O\({}_{2}\) and H\({}_{2}\)O, SO\({}_{2}\) reacts with NH\({}_{3}\) to form (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) and NH\({}_{4}\)HSO\({}_{4}\), which are deposited on the catalyst surface, resulting in its deactivation [14]. According to the above results, it can be concluded that the Mn-Ce/Al\({}_{2}\)O\({}_{3}\) catalyst exhibits some degree of SO\({}_{2}\) and H\({}_{2}\)O resistance and that SO\({}_{2}\) has a more significant inhibiting effect on the NH\({}_{3}\)-SCR catalytic activity than H\({}_{2}\)O [22; 23]. Combined with the NH\({}_{3}\)-SCRefect of sulfation over this catalyst, we attempted to understand the interaction mechanism between SO\({}_{2}\) and the honeycomb crodierite-based Mn-Ce/Al\({}_{2}\)O\({}_{3}\) catalyst.  
Figure 1: NO conversion byMn–Ce/Al\({}_{2}\)O\({}_{3}\) catalyst as a function of temperature. Reaction conditions: 200 ppm NO, 200 ppm NH\({}_{3}\), air balance, gas hour space velocity (GHSV) = 1667 h\({}^{-1}\).  
Physico-Chemical Characterization of Honeycomb Cordierite-Based Mn-Ce/Al\({}_{2}\)O\({}_{3}\) Catalysts