The advancement of new adsorption techniques, such as Direct Air Capture (DAC) for removing CO₂ from the atmosphere, is becoming increasingly important to achieve climate goals by reducing unavoidable and legacy CO₂ emissions. During DAC, a solid sorbent is used to capture CO₂ from the ambient humid air with a current level of approximately 420 parts per million.

The binding of CO₂ molecules on the solid sorbents surface by adsorption can be achieved by both physisorption and chemisorption. Desorption usually happens in a closed system after the removal of excess air, applying a combination of increased temperature, reduced pressure, or steam purge. Sorbents suitable for Direct Air Capture (DAC) should possess high CO₂ working capacities, demonstrate high selectivity towards CO₂ adsorption, be readily available in large quantities, exhibit long-term stability, and be cost-effective [1].

In this context, activated carbons (AC) seem to hold great potential, as they have been produced for decades and are readily available on an industrial scale worldwide. As a well-known sorbent and filter material, they provide large specific surface areas, variable pore width distributions and large pore volumes. It is believed that in AC particularly micropores with estimated pore widths of 0.7 nm to 0.8 nm could be beneficial for CO₂ adsorption [2].

The introduction of heteroatoms, such as N or S, into the carbon skeleton can also significantly influence the adsorption properties. [3]. As a substrate material, AC allow for further functionalization, for example by impregnation or grafting with different kinds of amines that bind CO₂ by a reversible chemisorption process [4].

In this screening study, we will present a simple method to measure CO₂ breakthrough curves under DAC conditions. The CO₂ uptake of porous carbons was determined using a temperature-swing-adsorption approach under varying relative humidity. Additionally, special attention was given to their cyclability in several adsorption–desorption cycles.

The aim of this screening study was to investigate a series of non-functionalized, heteroatom-doped, and amine-functionalized porous carbons for their potential to adsorb CO₂. The carbon adsorbents were thoroughly characterized through elemental analysis and low-temperature N₂ physisorption. Additional N₂ and CO₂ isotherms were measured at room temperature to determine the selectivity of CO₂ over N_2.