CO2 adsorption mechanisms in hydrated silica nanopores: Insights from grand canonical Monte Carlo to classical and ab initio molecular dynamics

TL;DR

This study combines multiple simulation techniques to elucidate how water content influences CO2 adsorption, mobility, and chemical reactions in silica nanopores, revealing complex hydration-dependent behaviors and reaction mechanisms.

Contribution

It provides a comprehensive multi-method analysis of CO2 adsorption in hydrated silica nanopores, including novel insights from ab initio simulations on chemical processes.

Findings

CO2 uptake decreases with increasing water content.

Hydration alters CO2 distribution from pore center to surface.

AIMD uncovers proton transfer and carbonate formation mechanisms.

Abstract

Understanding interfacial phenomena in confined systems is important for optimizing CO2 capture technologies. Here, we present a comprehensive investigation of CO2 adsorption in hydrated amorphous silica nanopores through an integrated computational approach combining grand canonical Monte Carlo (GCMC), classical molecular dynamics (MD), and ab initio molecular dynamics (AIMD) simulations. The excess adsorption isotherms reveal a marked hydration dependence, with CO2 uptake decreasing from 7.6 to 2.6 mmol/g as water content increases from 1 to 15 wt%. Analysis of adsorption kinetics demonstrates a distinctive bimodal process, characterized by rapid initial uptake followed by slower diffusion-limited adsorption, with the latter becoming increasingly dominant at higher hydration levels. Classical MD simulations reveal an inverse correlation between hydration and CO2 mobility, with self-diffusion coefficients decreasing across the studied hydration range. Density profile analysis indicates a hydration-induced transition in CO2 distribution from central pore regions to surface-proximate domains, accompanied by restructuring of interfacial water networks. Notably, AIMD simulations capture previously unrecognized chemical processes, including proton transfer mechanisms leading to surface silanol formation with characteristic O-O distances of 2.4-2.5 {\AA}, and spontaneous CO2 hydration yielding carbonate species through water-mediated reaction pathways. These findings demonstrate the dual role of confined water as both a spatial competitor and reaction medium for CO2 capture, providing molecular-level insights with quantum mechanical accuracy for design of carbon capture materials.