Novel nitrogen-enriched activated carbon with tunable microporosity from agricultural and plastic waste for CO2 adsorption

Abstract

Low-cost nitrogen-enriched activated carbons (PNACs) with high microporosity were prepared by a simple one-step activation doping process using KOH as a chemical activator and melamine formaldehyde (MF) as a nitrogen source. These PNACs have shown great potential as effective CO2 adsorbents. The resulting PNACs were mainly microporous structure, with surface areas ranging from 736 to 2144 m2/g and total pore volumes ranging from 0.565 to 1.366 cm3/g. The CO2 adsorption capacities of PNACs were in the range of 2.53-6.07 mmol/g and 1.59–4.13 mmol/g at 0 °C and 25 °C under 1 bar, respectively. Additionally, the CO2/N2 selectivity of the PNACs ranges from 11.15 to 26.86 at a CO2 partial pressure of 0.1 bar. The experimental isotherm data were analyzed using various equations and the Toth isotherm equation was found to be the most accurate mathematical model to describe the CO2 adsorption isotherm of the activated carbon. This result further confirms that PNACs have a non-homogeneous pore structure. The exceptional CO2 adsorption properties can be attributed to the high microporosity and nitrogen content of the PNACs. The relationship between the physicochemical properties and CO2 adsorption performance of the activated carbon was further investigated by relevance analysis. At 0 °C and 1 bar, it was observed that the pore structure played a predominant role in the CO2 adsorption process of PNACs. During this condition, PNACs mainly exhibited physical adsorption, which was primarily governed by the microporous filling mechanism. However, at 25 °C and 1 bar, both microporosity and nitrogen content contribute equally to the CO2 adsorption effect. This observation confirms that the pore structure and nitrogen functional groups work synergistically to influence the CO2 adsorption performance of PNACs. The adsorption mechanism of PNACs can be described as physical adsorption, driven by the cooperation between nitrogen functional groups and the filling of micropores. This innovative one-step activation doping process successfully converts agricultural and plastic waste into sustainable and high-performance CO2 adsorbents. It provides new insights for mitigating the greenhouse effect and addressing the issue of plastic pollution.