Highly surface-active Ca(OH)$_2$ monolayer as a CO$_2$ capture material

Greenhouse gas emissions originating from fossil fuel combustion contribute significantly to global warming, and therefore the design of novel materials that efficiently capture CO$_2$ can play a crucial role in solving this challenge. Here, we show that reducing the dimensionality of bulk crystalline portlandite results in a stable monolayer material, named portlandene, that is highly effective at capturing CO$_2$. Based on theoretical analysis comprised of ab-initio quantum mechanical calculations and force-field molecular dynamics simulations, we show that this single-layer phase is robust and maintains its stability even at high temperatures. The chemical activity of portlandene is seen to further increase upon defect engineering of its surface using vacancy sites. Defect-containing portlandene is capable of separating CO and CO$_2$ from a syngas (CO/CO$_2$/H$_2$) stream, yet is inert to water vapor. This selective behavior and the associated mechanisms have been elucidated by examining the electronic structure, local charge distribution and bonding orbitals of portlandene. Additionally, unlike conventional CO$_2$ capturing technologies, the regeneration process of portlandene does not require high temperature heat treatment since it can release the captured CO$_2$ by application of a mild external electric field, making portlandene an ideal CO$_2$ capturing material for both pre- and post-combustion processes.