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 (Mx: 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 Mx 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 Mx 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 M0.1 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, M0.1 also displayed high hydrophobicity character, making it ideal for CO2 adsorption under humid conditions. Moreover, the Mx 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 Mx does not need any additional alkali activation before application, thus simplifying the implementation process, reducing costs, and complexity.