Fundamental investigation of gas adsorption in micro- and mesoporous carbons [electronic resource]
Keywords: Energy Resources Engineering

To mitigate CO2 emissions from direct combustions of fossil fuels into the atmosphere, alternate energy sources with zero carbon emissions offer ultimate solutions. However, technologies based on thermodynamically efficient and large-scale economic electricity generation from non-carbon-based energy sources are still in development. Therefore, carbon capture combined with utilization and sequestration could serve as a bridging strategy to a time when noncarbon energy technologies are broadly deployed. As one of the attractive options, CO2 captured by carbon-based sorbents as well as CO2 sequestration in unmineable coalbeds require a thorough understanding of CO2 adsorption properties in micro- and mesoporous carbon materials. A major obstacle is insufficient understanding of the molecular-scale processes involving CO2 sorption on organic matter with structural and/or chemical heterogeneities at pressures and temperatures of interest for both carbon capture and sequestration applications. Current fundamental investigations of gas sorption onto functionalized carbon surfaces involve the characterization of carbon-based samples by experimental methods, understanding of electronic properties of functionalized carbon surfaces by density functional theory (DFT), and the thermodynamic property predictions using a Monte Carlo (MC) method within the grand canonical ensemble. Complex pore structures not only for coals, but also for other carbon-based porous materials have frequently been modeled as a collection of independent, noninterconnected slit pores with perfect graphitic walls. However, the agreement between the MC and experimental adsorption experiments often involve structurally and chemically heterogeneous systems. With the chemistry of the organic matrix unknown in the current system it is crucial that the initial models for MC are as representative of the chemistry as possible. Within the complex heterogeneous structure of the organic matrices of coal and other carbon-based porous materials, there likely exists a combination of defect sites and dangling bonds. The presence of volatile components such as water vapor, methane, and nitrogen- and sulfur-containing compounds is also expected. Indeed, these defects and functional groups are expected to play a role in the adsorption mechanisms associated with CO2 on these systems depending on the local temperature and pressure. For instance, if the temperature and pressure conditions favor surface-bound water or various forms of dissociated water (e.g., hydroxyl or carbonyl groups at the carbon surface) at the surface this may lead to complex CO2-water-surface interactions. Rather than CO2 interacting directly with a surface, it may react indirectly via a shared proton. As the first step, carbon-based materials (e.g. coal, gas shale) have been characterized by different techniques incuding FTIR, SEM, NMR cryoporometry, and Quantachrome porometry, to understand the chemical composition, surface functionality, porosity, pore-size distribution (PSD), etc. As the second step, density functional theory (DFT) calculations including a van der Waals correction have been performed to investigate the electronic properties of the graphitic surfaces and the adsorbed phase of molecular CO2. With a Bader charge analysis, DFT investigations also assisted in setting up models for the initial framework required to carry out statistical modeling. Grand canonical Monte Carlo (GCMC) was employed in the next step to simulate the macroscopic adsorption thermodynamic properties. Di_erent potential models of the CO2 molecule were compared as well. The implementation of the GCMC method yielded the adsorption isotherms of CO2 on various functionalized carbon surfaces, as well as the selectivity for multi-component systems at various temperature/pressure conditions for both carbon capture and sequestration applications. The effects of various pore sizes, potential models, temperatures, and surface heterogeneities have been investigated. Our current investigation revealed that oxygen-containing functional groups will act to enhance the CO2-surface adsorption. CO2 is preferentially adsorbed in the oxygen-containing functionalized graphitic slit pores over N2 and CH4 due to its strong permanent quadrupole moment. Furthermore, an algorithm of pore-size distribution (PSD) optimization is developed and verified by testing several carbon-based samples on comparing the simulated adsorption isotherms with available experimental adsorption data, including activated carbon (AC) and gas shale rocks.