## 1 Introduction  
Coal, as a porous medium, possesses a complex pore structure, and pores are the main sites where gas is stored. Coalbed gas mainly consists of methane (CH\({}_{4}\)) and its homologous alkanes, CO, H\({}_{2}\)S, H\({}_{2}\), etc. A total of 80-90% of the gas in coal seams and surrounding rocks exists in an adsorbed state. The pore size distribution of coal not only affects the storage state of gas in pores but also significantly influences the interaction between pores and gas or liquid molecules [1]. The pore structure of coal is an important indicator of coal's ability to adsorb gas and the mobility of gas [2]. A systematic study of the pore structure of coal and its adsorption law for gas is of great significance for in-depth research on the occurrence of coalbed methane and the mechanism of coal and gas outburst. At present, scholars at home and abroad have already achieved some important results in the research on the pore structure of coal. According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), the pore sizes in a coal matrix and other porous materials are divided into micropores (diameter \(\leq\) 2 nm), mesopores (diameter 2-50 nm), and macropores (\(\geq\)50 nm) [3]. [PERSON] et al. [4] proposed a new pore classification method. Based on the occurrence and migration characteristics of methane in coal, the research results show that this new pore classification method can describe the pore structure of coal more accurately, thus providing more effective theoretical guidance for coal gas drainage. [PERSON] et al. [5] conducted in-depth research on the pore structure of tectonic coal, and the results show that the pore structure characteristics of tectonic coal are important factors affecting the effect of gas drainage. [PERSON] et al. [6] used N\({}_{2}\) and CO\({}_{2}\) as probe molecules in experiments. Through isothermal adsorption experiments, they found that CO\({}_{2}\) adsorption can characterize the microstructure of coal more accurately. In 2022, [PERSON] et al. [7] Adopted the high-pressure isothermal gas adsorption method to study the adsorption performance of three fractal dimensions, characterized the differences in the microstructure of coal, reflected the influence of the microstructure of coal on the gas adsorption ability, and provided a valuable basis for the adsorption evaluation of different coal rank coal seams for gas. From a microscopic perspective, [PERSON] et al. [8] adopted the molecular dynamics method and studied the adsorption characteristics of methane in pores with a diameter of 4 nm based on the grand canonical ensemble Monte Carlo simulation (GCMC). They studied the influence and mechanism of isothermal adsorption curves, concentration distribution, and diffusion coefficients on the methane adsorption behavior under different conditions, providing a theoretical basis for revealing the interaction mechanism between methane and coal with different metamorphic degrees. In 2018, [PERSON] et al. [9] used the Materials Studio 2018 molecular simulation software to study the adsorption characteristics of methane by carbon materials with different pore sizes. Through the interpolation method, they obtained the optimal pore sizes of single-layer and double-layer carbon materials for methane adsorption. [PERSON] et al. [10] constructed coal pore structure models with pore diameters of 1, 2, and 4 nm by using graphene sheets and simulated the adsorption behavior of methane in pores of different sizes. They concluded that the adsorption amount is positively correlated with the pore size of pure gas adsorption, and the adsorption amount increases with the increase in pressure and pore size. [PERSON] et al. [11] constructed different slit pore models of coal and carried out a series of molecular simulations.