## 1 Introduction  
Nitrogen oxides (NO\({}_{\rm x}\)) are pollutants introduced into the environment during fossil fuel combustion in automobiles and various non-road vehicles (such as construction equipment, boats, etc.) [1]. NO\({}_{\rm x}\) is also emitted from industrial sources such as power plants, boilers, cement kilins, and turbines. NO\({}_{\rm x}\) is a common term comprising a mixture of N\({}_{2}\)O (a potent greenhouse gas with 300 times more atmospheric warming effect than CO\({}_{2}\)), NO, NO\({}_{2}\), N\({}_{2}\)O\({}_{3}\), and NO [2]. All these are formed from molecular nitrogen in the air and nitrogen contained in fossil fuels during combustion at high temperatures [3, 4]. NO\({}_{\rm x}\) emissions constitute environmental and health hazards, facilitating the depletion of the ozone layer and the formation of acid rain, haze, and photochemical smog [3]. Hence, many countries have introduced increasingly stringent regulations and policies on air pollution to reduce NO\({}_{\rm x}\) emissions. As part of these measures and enforced actions, reducing emissions directly by creating and optimizing low-NO\({}_{\rm x}\)-formation combustion processes is one strategy. Other techniques, including combustion chambers with lower NO\({}_{\rm x}\) formation, have also been developed [5]. Different approaches to reducing NO\({}_{\rm x}\) formation include removing nitrogen from flue gas (denitrification), recycling flue gas, and altering combustion parameters [6]. However, the NO\({}_{\rm x}\) generated after applying such methods is still beyond the allowed threshold limit. Thus, further advanced technologies have been developed to reduce NO\({}_{\rm x}\) emissions effectively. Among the most widely adopted technologies are three-way catalysts [7], NO\({}_{\rm x}\) storage, selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR) using a reducing agent such as ammonia to reduce NO\({}_{\rm x}\) to N\({}_{\rm 2}\) gas [8]. SCR is characterized by using a catalyst that achieves high levels of conversion of NO\({}_{\rm x}\) into N\({}_{\rm 2}\) and H\({}_{\rm 2}\)O at lower temperatures when compared with alternative technologies [6]. In this context, different materials have been used as catalysts for SCR. For example, metal oxides have been used as catalysts, such as vanadium and titanium oxides supported on tungsten, molybdenum, and other elements [6]. For instance, V\({}_{\rm 2}\)O\({}_{\rm 5}\)-WO\({}_{\rm 3}\)(MoO\({}_{\rm 3}\))/TiO\({}_{\rm 2}\) has been used as a commercially effective catalyst. However, the major constraints in the application of this material are the hazardous nature of V\({}_{\rm 2}\)O\({}_{\rm 5}\), the temperature range (300-400 \({}^{\circ}\)C), and the side reaction activity towards the oxidation of SO\({}_{\rm 2}\). Depending on the application, catalysts that work at lower temperatures are required. Generally, metal-based catalysts work best at temperatures between 232 and 427 \({}^{\circ}\)C [6]. In applications that require even lower temperatures, between 177 and 288 \({}^{\circ}\)C, platinum and palladium are the best options. On the other hand, to apply SCR under harsh conditions, zeolite-based catalysts ion-exchanged with Cu or Fe are an appealing, practical alternative for reducing NO\({}_{\rm x}\) to molecular nitrogen. Cu-exchanged zeolites have proven effective catalysts for the SCR process [9; 10; 11; 12; 13]. The hydrothermal stability of these zeolites is related to their pore size. For instance, zeolites with medium and large pore sizes (such as Cu-ZSM-5, Cu-BEA, and Cu-Y) tend to be less stable, while zeolites with small pore sizes (such as SSZ-13 and SAPO-34) have higher thermal stability and catalytic activity [14]. This stability difference, related to pore size, is connected to the fact that, compared to small pore-sized zeolites, the porous geometry of medium and large pore-sized zeolites is more susceptible to collapse, with a consequent decrease in their surface area and the sintering of surface catalytic active sites.