First-principles studies on degradation of aqueous amines for carbon dioxide capture
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Chemical absorption with aqueous amine-based solvents has been the most promising incumbent technology for post-combustion CO₂ capture from flue gas. However, its extensive operation is severely limited by the large cost attributed to the enormous energy requirement for solvent regeneration and degradation issues leading to makeup of amine solvent loss. First-principles atomistic modeling can provide key insights into elucidating chemical phenomena pertinent to degradation behavior in CO₂-loaded aqueous amine solution, which is often extremely challenging to be experimentally characterized. In this dissertation, our first-principles works on illuminating the molecular mechanisms governing solvent degradation of aqueous amine during CO₂ capture are presented. Using density functional theory based ab initio molecular dynamics with enhanced sampling techniques, we identify elementary reactions governing CO₂ capture and degradation. Molecular mechanisms of thermal and oxidative degradation of aqueous amine solvents are discussed in perspective of both thermodynamics and kinetics. We systematically investigate on the factors prevailing key reaction rates, such as amine functional groups, the steric hindrances, classes of amines (primary and secondary), concentration of amines, solvation nature, and temperature conditions. These factors may largely affect relative strengths of both inter- and intramolecular hydrogen bond interactions in CO₂-loaded aqueous amine solution. Our theoretical studies further illustrate the importance of an atomistic-level description of solvation structure and dynamics that may primarily govern CO₂ reaction with aqueous amine solvents and associated degradation mechanisms. This dissertation highlights the key role of first-principles computational modelling in accurately describing mechanistic understandings on CO₂ capture by aqueous amine solvents and associated degradation processes. The enhanced atomisticlevel descriptions will provide more complete explanations for experimental characterizations and valuable suggestions on how to optimize existing solvents and design more cost-efficient solvents for carbon capture processes.