The role of water vapour on CO2 mobility on calcite surface during carbonation process for calcium looping: A DFT study

The presence of water vapour has been proven to stimulate carbonation for the calcium looping process, while its effect mechanism during the slow reaction stage still lacks insight understanding at the atomic level, where H₂O molecules may interact with the product layer to affect the transportation of CO₂. This study tried to reveal the role of water vapour on CO₂ mobility on the calcite surface through density functional theoretical (DFT) calculation. H₂O molecule has higher adherence to calcite surface than CO₂, and it can react with CO₃ to generate HCO₃⁻ and OH⁻ ions. Then, the formed OH⁻ ion above the surface layer can adsorb CO₂. Moreover, H₂O can also react with O²⁻ defect to form OH⁻ ions, and the OH ion at the deeper position can still adsorb CO₂ through a two-step process with remarkable energy barriers. However, the formation of HCO₃⁻ can degrade the energy barriers to CO₂ release from the calcite surface, owning to the weaker CO bond. Two directions of CO₂ movement between anions were involved in this investigation, including crossing through the surface layer to the second layer and the movement inside the surface layer, where the higher mobility of CO₂ from HCO₃⁻ to O²⁻ ion occurs in both movement directions. For the case inside the surface layer, the movement from O²⁻ ion to HCO₃⁻ has a higher energy barrier, indicating that the stimulation by H₂O is a one-way effect, and the enhancement by H₂O for CO₂ mobility is caused by the reaction with CO₃²⁻ other than O²⁻ ion.