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Enhancing Direct Air Capture through Potassium Carbonate Doping of Activated Carbons

N. van Dongen, A. J. F. van Hoof, S. Calero, J. M. Vicent-Luna

TL;DR

This work addresses DAC material design by examining CO$_2$ and H$_2$O adsorption in activated carbons with explicit surface functionalities and with embedded K$_2$CO$_3$. Using GCMC simulations and two modeling approaches for K$_2$CO$_3$ incorporation, it shows that surface functional groups and potassium carbonate doping lower adsorption onset pressures and create additional adsorption sites, with water clustering enhanced through hydrogen-bond networks around dopants. CO$_2$ uptake can increase by roughly 3–5x under DAC-relevant conditions when functional groups are present, and water uptake rises by 1.5–3x depending on doping strategy, highlighting a trade-off between CO$_2$ capture and water co-adsorption. The findings provide molecular-level design guidance for DAC sorbents and underscore the importance of balancing pore chemistry and dopant distribution to optimize performance in humid ambient conditions.

Abstract

Direct air capture of carbon dioxide (CO$_2$) is one of the most promising strategies to mitigate rising atmospheric CO$_2$ levels. Among various techniques, adsorption using porous materials is a viable method for extracting CO$_2$ from air, even under humid conditions. However, identifying optimal adsorbent materials remains a significant challenge. Moreover, the performance of existing materials can be improved by doping with active species that boost gas capture, a relatively unexplored field. In this study, we perform atomistic simulations to investigate the adsorption, structural, and energetic properties of CO$_2$ and water in realistic models of activated carbons. We first analyze the impact of explicitly considering surfaces containing functional groups, which aims to imitate the chemical environment of experimental samples. Additionally, we introduce potassium carbonate within the pores of the adsorbent to evaluate its effect on CO$_2$ and water adsorption. Our results demonstrate that both functional groups and potassium carbonate enhance adsorption, primarily by shifting the adsorption onset pressures to lower values. Specifically, potassium carbonate clusters act as extra adsorption sites for CO$_2$ and water, facilitating the nucleation of water molecules and promoting the formation of a hydrogen bond network within the activated carbon pores.

Enhancing Direct Air Capture through Potassium Carbonate Doping of Activated Carbons

TL;DR

This work addresses DAC material design by examining CO and HO adsorption in activated carbons with explicit surface functionalities and with embedded KCO. Using GCMC simulations and two modeling approaches for KCO incorporation, it shows that surface functional groups and potassium carbonate doping lower adsorption onset pressures and create additional adsorption sites, with water clustering enhanced through hydrogen-bond networks around dopants. CO uptake can increase by roughly 3–5x under DAC-relevant conditions when functional groups are present, and water uptake rises by 1.5–3x depending on doping strategy, highlighting a trade-off between CO capture and water co-adsorption. The findings provide molecular-level design guidance for DAC sorbents and underscore the importance of balancing pore chemistry and dopant distribution to optimize performance in humid ambient conditions.

Abstract

Direct air capture of carbon dioxide (CO) is one of the most promising strategies to mitigate rising atmospheric CO levels. Among various techniques, adsorption using porous materials is a viable method for extracting CO from air, even under humid conditions. However, identifying optimal adsorbent materials remains a significant challenge. Moreover, the performance of existing materials can be improved by doping with active species that boost gas capture, a relatively unexplored field. In this study, we perform atomistic simulations to investigate the adsorption, structural, and energetic properties of CO and water in realistic models of activated carbons. We first analyze the impact of explicitly considering surfaces containing functional groups, which aims to imitate the chemical environment of experimental samples. Additionally, we introduce potassium carbonate within the pores of the adsorbent to evaluate its effect on CO and water adsorption. Our results demonstrate that both functional groups and potassium carbonate enhance adsorption, primarily by shifting the adsorption onset pressures to lower values. Specifically, potassium carbonate clusters act as extra adsorption sites for CO and water, facilitating the nucleation of water molecules and promoting the formation of a hydrogen bond network within the activated carbon pores.

Paper Structure

This paper contains 11 sections, 2 equations, 16 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Monte Carlo simulation
  4. Modelling CS1000a with potassium carbonate
  5. Results and Discussion
  6. Effects of surface chemistry
  7. Effects of potassium carbonate
  8. Conclusion
  9. Fit information
  10. RDF potassium carbonate verification
  11. Figures

Figures (16)

  • Figure 1: Comparisons of computed adsorption isotherms of CO$_2$ (a) and H$_2$O (b) in CS1000a$_{\text{f}}$ (solid lines) and CS1000a$_{\text{nf}}$ (dashed lines) at $300.15$ K. Lines are fits through the data points obtained using RUPTURA.Sharma2023RUPTURA The vertical line in (a) indicates the partial pressure of CO$_2$ equivalent to 420 ppm CO$_2$ (at 1 atm), its current approximate atmospheric concentration.Lan2023 The vertical line in (b) represents the saturation pressure at the simulation temperature.
  • Figure 2: Snapshots showcasing adsorption in CS1000a$_{\text{f}}$ at $300.15$ K under various partial pressure conditions. (a-c) Adsorption of CO$_2$ at $316$, $10000$, and $316228$ Pa respectively. (e-f) Adsorption of H$_2$O at $357$, $1784$, and $316228$ Pa respectively. These conditions for water correspond to $10\%$ relative humidity, $50\%$ relative humidity, and saturation.
  • Figure 3: Comparisons of computed heats of adsorption of CO$_2$ (a) and H$_2$O (b) in CS1000a$_{\text{f}}$ (solid lines) and CS1000a$_{\text{nf}}$ (dashed lines) at $300.15$ K. The horizontal dash-dotted lines represent the heats of vaporization of the respective components.
  • Figure 4: Schematic explanation of the two routes to create structures containing K$_2$CO$_3$. In the first case, the K$_2$CO$_3$ units are randomly added to an empty structure. In the second case, previously optimized small clusters of $5$ K$_2$CO$_3$ were initially added to the pores. Then, the ions of K$_2$CO$_3$ are relaxed within the structure to accommodate the clusters next to a wall correctly.
  • Figure 5: Comparisons of computed adsorption isotherms of CO$_2$ (a-c) and H$_2$O (d-f) in CS1000a$_{\text{f}}$ at $300.15$ K for varying amounts of K$_2$CO$_3$. Figures (a,d) show isotherms in which the K$_2$CO$_3$ was added randomly into the carbon, while (b,e) show isotherms in which the K$_2$CO$_3$ was added in clusters of $5$. Lines in these Figures are fits through the data points obtained using RUPTURA.Sharma2023RUPTURA In Figures (c,f), isotherms using both methods of K$_2$CO$_3$ inclusion are shown, scaled by the isotherm without K$_2$CO$_3$. The black vertical line in the CO$_2$ isotherms indicates the partial pressure equivalent to 420 ppm (at 1 atm), its current approximate atmospheric concentration.Lan2023
  • ...and 11 more figures