Understanding the Influence of N-Doping on the CO2 Adsorption Characteristics in Carbon Nanomaterials

Compared to metal-organic frameworks and zeolites, carbon-based materials are particularly interesting for gas capture applications due to their better stability against moisture and corrosive flue gases. Increasing the accessible surface area and incorporation of heteroatoms are generally the two different strategies put forward to further enhance the adsorption characteristics of carbon materials. The influence of nitrogen incorporation on the gas adsorption characteristics, especially CO2 adsorption, is however a controversial research topic with conflicting conclusions reported by various studies. In the present work, using vertically aligned carbon nanotubes (VACNTs), we investigate for the first time the influence of N-doping on the CO2 adsorption characteristics of carbon materials. The present study aims to shed additional light on the parameters known so far as well as those that are currently not considered but are additionally responsible for gas adsorption in functionalized carbon materials. To this end vertically aligned carbon nanotubes (VACNTs) are ideal carbon model structures for gas adsorption studies as they have a well-defined, reproducible mesoporous pore structure with a chemically homogeneous surface. Thus, the interfering influence of micropores, which are generally present in carbon-based adsorbents, is avoided, making a straightforward interpretation of the exact influence of N-doping possible for the first time. N-incorporation in the form of pyridinic or pyrrolic/pyridonic groups is achieved by using a N-2 rf plasma treatment. The presence of these nitrogen functionalities is found to have a beneficial influence on the CO2 adsorption characteristics over a wide range of pressure (0-36 bar). The nature of interaction is determined by calculating the isosteric heat of adsorption and finally by comparing the adsorption characteristics of as-prepared and N-doped VACNTs with oxygen functionalized VACNTs; the importance of determining the oxygen functional groups in the adsorbent is presented.