Lan Li, Eric Cockayne, Laura Espinal and Winnie Wong-Ng


The manganese oxide OMS-2 (Octahedral Molecular Sieve) material, also known as α-MnO2, is of great interest for carbon             capture and storage applications. Our experiments show that OMS-2 exhibits CO2 sorption hysteresis at pressures larger than 7 bar. The hysteretic behavior is strongly dependent on time, temperature and pressure. To better understand the atomic structures and sorption mechanism we have performed first-principles total energy calculations. OMS-2 materials have (2 × 2) + (1 × 1) tunnel structures, built from chains of edge-sharing MnO6 octahedra. The small (1 × 1) tunnel is empty while the large (2 × 2) tunnel contains additional species, such as K+ and H2O, to stabilize the structure. Our first-principles calculations indicate that the binding energy of H2O in the OMS-2 is 0.39 eV, smaller than that of K+ by 4 eV, and that therefore H2O can be easily removed from the OMS-2, but K+ remains. We find that K+ behaves as a “gate keeper”, which blocks the diffusion of CO2 in the OMS-2 due to high energy barrier (5.44 eV/CO2 molecule). Increasing the concentration of CO2 can dramatically lower the energy barrier for CO2 to bypass K+. The gate-keeping behavior accounts for the observed sorption hysteresis. Based on the experimental and computational results, a possible CO2 sorption mechanism by OMS-2 will be discussed in the poster.