## 3 Results and Discussion
### Catalytic Evaluation
Hence, the result presented by 0.1 M Cu-NH\({}_{4}\)-CHA proffers a solution towards achieving more effective use of Cu-CHA catalysts in maximizing NO\({}_{\rm x}\) conversion strategies for abatement of NO\({}_{\rm x}\) and pollution-free urban atmospheres. Similar catalytic behavior was observed between NH\({}_{4}\)-CHA and K-CHA forms of the as-synthesized zeolite catalysts. Although, as seen in Figure 11, the NO\({}_{\rm x}\) conversion also increased with temperature, the catalytic NO\({}_{\rm x}\) conversion efficiency exhibited by the K-CHA forms of zeolite catalysts was unsatisfactory. Their conversion profiles were also not maintained over a wide range of temperatures. Instead, it suddenly declined. This indicates that the catalysts were thermally unstable, and as a result, the catalysts suffered a decrease in NO\({}_{\rm x}\) conversion even at temperatures far below 400 \({}^{\circ}\)C. At ca. 330 \({}^{\circ}\)C, the K-CHA form zeolite catalysts achieved <80%, 40%, and ca. 68% NO\({}_{\rm x}\) conversion for 0.05 MCu-K-CHA, 0.1 M Cu-K-CHA, and 0.1 M Cu-K-CHA-RT, respectively. Immediately after attaining these apex conversions, the conversions dropped drastically due to deactivation. The observed deactivation may be attributed to surface modifications caused by potassium (K) masking or poisoning, as alkali metals (Na, K) and alkali earth-metals (Mg, Ca) are documented as poisons for SCR catalysts [56]. In this case, the potassium (K) masking or poisoning effect might be responsible for reducing the surface active sites of the K-CHA catalysts or initiating the transformation of isolated Cu ions to Cu\({}_{\rm x}\)O\({}_{\rm y}\)/Cu\({}^{\rm II}\)OH clusters, respectively. The masking effect of K is usually caused by its ions anchoring to Bronsted acid sites because K has more affinity for Bronsted acid sites than isolated Cu ions or  
Figure 10: NO\({}_{\rm x}\) conversion by the NH\({}_{4}\)-CHA zeolite catalyst ion-exchanged at different concentrations and temperatures. Catalyst loading: 50 mg (40–80 mesh) mixed with 50 mg silica gel. Reaction conditions: 500 ppm NO + 500 ppm NH\({}_{3}\) + 14% O\({}_{2}\)/N\({}_{2}\) + 5% CO\({}_{2}\) + 5% H\({}_{2}\)O + N\({}_{2}\). Total gas flow rate: 200 mL/min. Space velocity 240 L/h-g. Temperature ramp: 2.5 \({}^{\circ}\)C/min. Total reaction time: 172 min. directly supplanting Cu ions due to strong difference in electropositivity [57]. On the other hand, the transformation of isolated Cu ions to Cu\({}_{\text{x}}\)O\({}_{\text{y}}\)/Cu\({}^{\text{II}}\)OH clusters might be initiated by electronic interaction through which Cu species behave as anions by accepting electrons from K via filled 4s orbitals and partially filled extended 4p orbitals [58]. This demonstrates that the incorporation of K into the catalysts affected the surface active sites or acid sites generated by the incorporation of Cu in the K-CHA zeolite catalysts. Therefore, Cu species, particularly in isolated forms, are inferred to be the active sites in the NH\({}_{3}\)-SCR process, in agreement with earlier observations [59]. This can be further supported by the apparent difference in the H\({}_{2}\)-TPR profiles, where a blend of isolated Cu\({}^{+}\) and Cu\({}^{2+}\) dominated the forms of Cu species in Cu-NH\({}_{4}\)-CHA zeolite catalysts. At the same time, Cu\({}^{\text{II}}\)OH was the characteristic principal species in the Cu-K-CHA catalysts. The manifested difference in DeNO\({}_{\text{x}}\) catalytic activity between the as-synthesized ion-exchanged Cu-NH\({}_{4}\)-CHA and Cu-K-CHA zeolite catalysts finally confirmed this.