## 3 Results and Discussion
### Catalytic Evaluation  
In NH3-SCR tests using the as-synthesized zeolite catalysts, Figures 10 and 11 depict NOx conversions as a function of the reaction temperatures ranging between 120 and 550 \({}^{\circ}\)C. As can be seen, the NOx conversion rose with temperature as the NH3-SCR process was initiated, reached a maximum, and then declined. This is particularly noticeable in the results manifested by 0.05 MCu-NH4-CHA and 0.1 MCu-NH4-CHA-RT, which perhaps is associated with their inherent structural and catalytic characteristics that are, to a certain extent, different from the 0.1 M Cu-NH4-CHA. Among the synthesized catalysts, 0.1 MCu-NH4-CHA exhibited the best catalytic performance for NOx conversion. The catalyst showed 100% conversion with steady thermal stability over a wide range of higher reaction temperatures between ca. 261 and 520 \({}^{\circ}\)C. Interestingly, this zeolite catalyst achieved 50% NOx conversion at 192 \({}^{\circ}\)C and reached complete 100% NOx conversion at temperatures from 261 \({}^{\circ}\)C. This result reveals a significant catalytic improvement compared to the conventional Cu-based chabazite (Cu-CHA) catalysts, whose NOx conversion efficiency is relatively low at temperatures below 200 \({}^{\circ}\)C. Thus, NOx must first be stored over conventional zeolite catalysts and converted at higher temperatures [49]. In addition, while the local Cu/Al ratio varied among the samples, the 0.1 M Cu-NH4-CHA is the material that has the highest Cu/Al ratio and, much more likely, will have a higher Si/Al ratio, too. It has been generally considered that samples with a higher Si/Al ratio are thermally stable [50; 51]. It is known that the thermal stability of Cu ion-exchanged zeolite-based catalysts also depends on the Cu/Al ratio [50], as confirmed by this study's results (Table 2). Therefore, it can be speculated that the enhancement of the overall thermal stability of 0.1 M Cu-NH4-CHA over a wide temperature range during NOx conversion could be attributed to the higher Cu/Al ratio. Furthermore, the abundance of stable Cu species, particularly Cu\({}^{2+}\) in the framework of Cu-based CHA zeolite catalysts, is responsible for better NOx conversion during NH3-SCR [52]. To further support this point, it was indicated that different Cu active site locations were the reason why Cu-based zeolite catalysts exhibited different catalytic activity profiles [9; 43; 53]. Therefore, as Cu\({}^{2+}\) species are well distributed and stabilized within 0.1 M Cu-NH4-CHA, as shown in the XPS profile of the ion-etched sample in Figure 9; thus, 0.1 M Cu-NH4-CHA displayed the best catalytic performance relative to 0.05 M Cu-NH4-CHA and 0.1 M Cu-NH4-CHA-RT over a wider operating temperature range. It is essential to point out that the NO\({}_{\rm x}\) conversion efficiency of 0.1 MCu-NH\({}_{4}\)-CHA zeolite catalyst presents a promising result, as its NO\({}_{\rm x}\) conversion index outperformed the earlier reported Cu-CHA zeolite catalysts, which achieved 100% NO\({}_{\rm x}\) conversion only at temperatures from 220 to 380 \({}^{\circ}\)C [52]. Similarly, Cu-based zeolite catalysts mainly showed high NH\({}_{3}\)-SCR activity in the temperature window of 350-550 \({}^{\circ}\)C, where Cu-ZSM-5, Cu-SSZ-13, and Cu-BEA showed maximum NO\({}_{\rm x}\) conversion at temperatures of 425, 450, and 550 \({}^{\circ}\)C, respectively [53]. In another similar study, ion-exchanged Cu-based SSZ-13 catalysts achieved approximately 80% NO\({}_{\rm x}\) conversion at temperatures between 175 and 400 \({}^{\circ}\)C [54]. Furthermore, the NO\({}_{\rm x}\) conversion rate of V-based catalysts with Sb remained at 80% at temperatures between 270 and 450 \({}^{\circ}\)C [55], far below the catalytic performance of 0.1 MCu-NH\({}_{4}\)-CHA reported in this study. This study, therefore, clearly shows that the NO\({}_{\rm x}\) catalytic performance of 0.1 M Cu-NH\({}_{4}\)-CHA is superior to similar and other DeNO\({}_{\rm x}\) catalysts, as indicated above.