Synthesis of nitrogen-doped mesoporous carbon nanospheres using urea-phenol-formaldehyde resin for efficient CO2 adsorption–desorption studies

Global warming led by excessive CO₂ emission is a significant challenge. CO₂ capture is recognised as an efficient way to mitigate this issue. In this study, we successfully synthesized a series of activation-free nitrogen-doped mesoporous carbon nanospheres (M_x: where x is ratio of urea/phenol) via an aqueous synthesis route, using urea-phenol-formaldehyde resin as a precursor and triblock copolymer F127 as a soft template. These M_x exhibited nitrogen contents ranging from 0.48 % to 1.52 % and with high surface areas within the range of 486.382 to 683.891 m²g⁻¹. Furthermore, they demonstrated a uniform pore channel diameter of around 3.2 nm. The incorporated nitrogen atoms primarily in the forms of pyrrolic, pyridine, and amine groups, offers abundant adsorption sites for CO₂. The CO₂ adsorption and desorption performance of as-synthesized M_x were systematically studied under various CO₂ feed concentrations, including 10 % CO₂ by volume, compressed air (mimicking direct air capture (DAC)), and 10 % CO₂ by volume at 90 % relative humidity, all at 298 K and ∼1 atm. Interestingly, the M_0.1 sample displayed exceptional CO₂ capture performance, achieving a capacity of 2.53 mmol g⁻¹ (or 4.8 mmol m⁻²) at a 10 % CO₂ by volume feed. This outstanding CO₂ adsorption capacity can be attributed to the synergistic effects of ordered mesopore channels, abundant structural micropores, and nitrogen functionalities, facilitating efficient CO₂ adsorption and desorption. Additionally, M_0.1 also displayed high hydrophobicity character, making it ideal for CO₂ adsorption under humid conditions. Moreover, the M_x displayed remarkable stability and recyclability, positioning them as promising and environmentally friendly adsorbents for CO₂ capture and separation under practical operating conditions. Additionally, the proposed M_x does not need any additional alkali activation before application, thus simplifying the implementation process, reducing costs, and complexity.