An investigation of CO2 adsorption kinetics on porous magnesium oxide

Abstract

MgO is a promising adsorbent for CO2 capture. While, its adsorption kinetics under different conditions is not sufficiently investigated. Furthermore, the rate-limiting step in CO2 adsorption of MgO cannot be found according to previous study. In this work, the CO2 adsorption kinetics of porous MgO was investigated under wide range of CO2 partial pressure and adsorption temperature. Different adsorption models were used to investigate the adsorption mechanism of porous MgO. It was found that porous MgO showed obvious two-stage adsorption process with a rapid initial CO2 uptake and a subsequent slow adsorption process under wide range of CO2 partial pressure and adsorption temperature. Pseudo-second order model accurately predicted the CO2 adsorption behaviors of porous MgO in all employed adsorption conditions, which indicated the validity of pseudo-second order model in the prediction of chemisorption behavior. The two-stage adsorption process of porous MgO was not caused by the adsorption product generated on the exterior of adsorbent particle. According to the rate-limiting models, the CO2 adsorption process of MgO was controlled by film diffusion and intraparticle diffusion. At the initial adsorption process, film diffusion resistance governed the adsorption rate. The intraparticle diffusion resistance became a primary factor in the later adsorption step. The narrow pore of porous MgO hindered the diffusion of CO2 molecule and led to a long time to adsorption equilibrium. This study suggests that porous MgO should be prepared with large pore so as to minimize the intraparticle diffusion resistance.