Roles of Surface Chemistry and Texture of Nanoporous Activated Carbons in CO2 Capture

The presence of heteroatoms, such as nitrogen or oxygen, on the surface of nanoporous activated carbons (ACs) promotes CO2 uptake due to the stronger specific interactions with these heteroatoms. However, heteroatom doping post-treatments often decrease the surface area and pore volume of ACs, and it is difficult to separate heteroatom doping from textural effects. The objective of this paper was to investigate when the control of adsorption shifts from surface chemistry to textural characteristics. For this purpose, three different types of commercial ACs were doped with nitrogen and/or oxygen by an easy and cheap method and then subjected to CO2 adsorption experiments up to 32 bar at 273 K. We evaluated the effect of the surface chemistry and texture, considering the whole range of porosity on CO2 adsorption as a function of pressure. Below atmospheric pressure, CO2 adsorption is mainly controlled by surface chemistry, increasing with O + N content, and the narrowest pores are responsible for more than 60% of CO, adsorption at pressures below 0.01 bar. Above atmospheric pressure, from 1 to about 5 bar, the textural properties become more significant to finally take control of the adsorption phenomena at pressures above 5 bar. We have undoubtedly shown that the adsorption capacity is only a function of the total pore volume and that the adsorbed density is even higher than that of liquid CO2 in ultramicropores (pore diameter less than 0.7 nm) at pressures of 15 bar and above.