Cycling pressure-switching process enriches micropores in activated carbon by accelerating reactive gas internal diffusion in porous channels

Physical activation of activated carbon (AC) is a pore-formation and pore-widening process via gasification, which involves gas diffusion and reaction in micro-channels. Existing efforts focus on the reaction part by optimizing reaction temperature, reactive species, type of mixing, etc. In this work, a cycling pressure-switching (CPS) process has been developed to expedite the gas transport between bulk gas phase and internal surface of particles. Specifically, intermittent vacuumization operation is introduced during activation, that is to create a negative system pressure to facilitate reactive gas (CO₂) diffusion into porous channels for activation reaction and then extract product gas (CO) out of the pores through vaccumization. CPS accelerates the gas exchange and breaks the diffusion-reaction balance in traditional atmospheric pressure process. Thus, richer porous structure can be created in CPS process. By using spent mushroom substrate as precursor, AC with specific surface area of 1175 m²/g and pore volume of 0.52 cm³/g can be obtained after optimizing the cycle number, CO₂ pressure and activation time. By investigating the effect of precursor size and bulk density on pore structure development, it is found that internal diffusion is the rate-limiting factor of AC activation that can be alleviated by the CPS process.