Is heteroatom doping of activated carbons always a good strategy for enhancing CO2 adsorption?

Abstract

This study shows the substantial impact of textural properties, with greater weight than N-doping, on CO2 adsorption by carbon materials over a pressure range from 1 to 25 bar and a temperature range from 273 to 323 K. It also highlights the importance of providing volumetric CO2 adsorption capacities when presenting newly developed materials. These results were obtained by exploring the influence of O- and N-doping on CO2 uptake on 14 activated carbons (ACs) with distinct specific surface areas, from 1736 to 3200 m2/g, and micropore fractions from 37 to 96 %. Based on 3 commercial ACs, N content was maximized by hydrogen peroxide oxidation and urea treatment, reaching around 11 at.%. O-N-doping mainly impacted the largest pores, reducing material textural properties and thus CO2 adsorption, although it improved CO2/N2 selectivity, especially up to 5 bar and between 273 and 323 K. The contribution of heteroatoms to CO2 uptake was particularly significant at low pressures and high temperatures, with threshold pressures increasing at higher temperatures. Breakthrough simulations were also carried out and the results demonstrate that, despite a lower specific surface area, the O-doped material exhibited longer breakthrough times than its commercial counterpart. This can be attributed to a balance between CO2 adsorption capacity, governed by the textural properties, and tap density, i.e., adsorption capacity per unit volume.