Exploitation of pomelo peel developing porous biochar by N, P co-doping and KOH activation for efficient CO2 adsorption

In order to improve the exploitation of biochar-based adsorbents for CO₂ capture, novel biochar adsorbents were synthesized with pomelo peel by effective two-step carbonization and activation, (NH₄)₂HPO₄ and KOH were taken as the dopant and activator, then a series of N, P co-doped pomelo peel-based biochar (NPBCs) was prepared, subsequently, the characteristics and adsorption capacity of NPBCs were studied. The results showed that N, P co-doping and KOH activation synergistically promoted the development of meso-, micro-, ultra-micropores and nanocapsules structure in NPBCs, and enabled NPBCs maintaining a certain N, P content. The optimum sample of NPBCs reached the highest CO₂ adsorption capacities of 3.41 and 5.74 mmol/g at 298.15 K, 273.15 K and 1 bar. The adsorption kinetics and isotherms of CO₂ on biochar were perfectly described by PFO model and Sips model respectively, indicating that the adsorption was a rapid process and dominated by physical effect. Measurements of the thermal range of CO₂ adsorption (15.59–26.01 kJ/mol) confirmed strong affinity of the NPBCs to CO₂ molecules. CO₂/N₂ selectivity up to 165 was recorded at ambient temperature and low CO₂ partial pressure. The loss of adsorption capacity of NPBCs was only 9.36% after 10 cycles showing good cyclic stability.