Multilayer Adsorption Characteristics of CO 2 in Organic Nanopores with Different Pore Sizes: Molecular Simulation and Ono-Kondo Lattice Model

Shale contains numerous organic micropores with significant potential for CO storage. To precisely evaluate the CO storage potential of shale reservoirs, it is essential to accurately quantify the adsorption of CO within these pores. This study used Grand Canonical Monte Carlo (GCMC) molecular simulations to analyze the CO adsorption behavior in organic micropores of varying sizes. The study clarified the number and width of the CO adsorption layers in micropores of various sizes and proposed a method for segmenting the multilayer adsorption structure. Additionally, the classic Ono-Kondo lattice (OK) model was extended to characterize pore-filling adsorption, incorporating solid-gas and gas-gas interactions. Accurate characterization of CO multilayer adsorption and precise calculation of CO absolute adsorption in micropores were achieved. Results indicate that CO exhibits pore-filling adsorption behavior in organic micropores, forming a multilayer adsorption structure governed by the pore size. Following symmetry principles, the adsorption layer structure in organic micropores can be simplified to a maximum of three layers. When only one adsorption layer forms, its width equals the gas-accessible pore size. For two or more layers, the width of the original layer stabilizes as additional layers form. The stable adsorption layer widths, from nearest to farthest from the pore wall, are 0.33, 0.45, and 0.39 nm. The improved OK model accurately describes CO excess and absolute adsorption isotherms across different pore sizes and calculates the CO density in each adsorption layer, showing high consistency with GCMC simulation results. These findings highlight the importance of understanding the CO multilayer adsorption structure for accurately estimating CO adsorption in organic micropores.