Due to CO2 prominent role within the carbon cycle, and the surging needs for mitigating its emissions in atmosphere by implementing capture and storage technologies, it is essential to understand CO2 phase behaviour in a wide range of pressure and temperature conditions. Despite its modest structural complexity, carbon dioxide displays a complex phase behaviour. So far, seven different crystalline structures of CO2 have been discovered, five of which are molecular, while the remaining two are non-molecular. Here we focus on understanding polymorphic transitions between molecular phases of CO2 by applying Well Tempered Metadynamics (WTmetaD), a state-of-the-art enhanced sampling method. To accelerate phase transition events with WTmetaD, we have designed ad hoc Collective Variables, which convey structural information about CO2 polymorphs, and account for the slow process of reorientation of CO2 molecules during phase transition. We apply our computational protocol to tackle three key questions: i. What is the role of entropy and pressure-volume work in the stabilisation of CO2 polymorphs? ii. How does pressure and temperature affect polymorphic transition rates? iii. How does confinement in pores affect thermodynamics and kinetics of polymorphic transitions? Experimental findings report distinct polymorphs as thermodynamically stable in different regions of the phase diagram. However, rankings of CO2 polymorphs based on lattice-energy are rather insensitive to variations in pressure and temperature. We devise a strategy to breakdown free energy differences into internal energy, pressure-volume work and entropy contributions. Depending on temperature and pressure, the thermodynamic stability results dominated by different contributions. Complementarily we compute characteristic lifetimes of CO2 metastable phases across the phase diagram, by applying infrequent WTmetaD. Finally, CO2 capture and storage often involves the adsorption onto porous stationary phase. Therefore we apply our simulation protocols to shed light on the impact of confinement on thermodynamics and kinetics of CO2 polymorphic transitions in pores of varying size and geometry.

Polymorphic transitions of CO2 molecular crystals: Thermodynamics, kinetics and confinement effects

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.