## 1 Introduction  
Automotive industries are dealing with significant advancements in pollutant control strategies to comply with increasingly stringent emission standards [1]. Especially in heavy-duty truck and bus engines, compressed natural gas (CNG) has become more and more attractive as alternative fuel in terms of emissions and performance in comparison to traditional fuels, reducing adverse health effects and social costs of air pollution [2]. For spark ignition (SI) stoichiometric CNG engines, the most suitable pollutant abatement system is the three-way catalytic converter (TWC). Similar to SI gasoline engines, this device permits control of NO\({}_{\text{x}}\), unburned methane (CH\({}_{4}\)), and other pollutant emissions (CO, NMHC). The simultaneous conversion of these species is possible exclusively in a very tiny range of inlet stream composition around stoichiometric conditions [3]. Compared to gasoline engines, this optimum operating point is further reduced because of a sudden drop in NO\({}_{\text{x}}\) conversion efficiency, as soon as a slightly lean \(\lambda\) value is achieved, and a non-complete conversion of total hydrocarbons (THCs) both in lean and in rich conditions [4]. Previous studies have widely demonstrated the differences in conversion efficiencies between a steady-state test and dynamic conditions [5; 6]. Some species of the washcoat layer may oxidize or reduce depending on the exhaust gas composition. For TWCs, the most significant of these components is cerium, which acts a stabilizer and a medium for oxygen storage. These effects have a considerable impact on the \(\lambda\) value inside the catalyst, which must be appropriately investigated in order to better manage TWC behavior. Under real-world driving conditions, several deviations of the air-to-fuel ratio (AFR) from the stoichiometric value take place because of fuel cutoff phases, engine misfire, and response lag of the fuel control system [7]. In this scenario, the numerous tasks and the mentioned working issues typical of this aftertreatment device have required, since its introduction, a development of reliable and predictive numerical models capable of analyzing specific operating conditions. Modeling approaches are generally categorized as 0D, 1D, and 3D, with increasing accuracy and complexity levels. A zero-dimensional approach is solely used for steady-state conditions, as only the mean exhaust gas mass flow is considered, thus reducing the reacting device to an element that performs the chemical conversion according to kinetic parameters given as input. More complex is the 3D approach, used when the characterization of flow distribution inside the reactor is required. The accuracy is certainly improved because it permits to identify radial diffusion effects of the flow inside the catalyst; the real limitation is represented by high computational efforts, which do not allow an investigation in a very wide range of operating conditions, such as the variables of a dynamic cycle, in times compatible with project targets. A good compromise is represented by 1D models, which allow to simulate a thermo-fluid dynamic behavior of the whole exhaust system during both steady-state and transient conditions with a reduced computational effort [8]. Calibration of the kinetic reaction model inside the catalyst is certainly one of the most challenging topics. Several studies are available in literature, mostly concerning traditional gasoline spark ignition (SI) engines [9; 10; 11]. An interesting modeling approach, based on 104 reaction steps, of a TWC kinetic scheme with an exhaust mixture from natural gas-fueled engines was proposed by [PERSON] et al. However, in this study, only steady-state (SS) conditions were analyzed; thus, oxygen storage phenomena and perturbations in AFR were not considered [12]. Very little information is available in the literature on the specific features of TWC systems applied to natural gas vehicles. In this respect, an important contribution is provided by [PERSON] et al.