Development of porous N/O hybrid carbon from spent tires for CO2 capture in the framework of circular economy: Parallel upgrading of pore and surface chemical structure

In this work, spent tires were processed into porous hybrid carbon materials for CO2 capture. An efficient heteroatom doping and pore structure optimization process in parallel was proposed. In the optimal hydrothermal conditions, addition of ammonium oxalate and melamine could significantly increase the specific surface area of STAC from 703.76 to 905.04 m2/g and dope with abundant N (4.41 at%) and O (7.32 at%) heteroatoms. Then the mechanism of pore widening and doping of carbon materials by pre-oxidation, small molecules, and molecular fragments is also elaborated. The extensive characterization of spent tire porous hybrid carbon (STAC-NO2) demonstrates that it is rich in numerous CO, C-OH, N-6, and N-5 structures, which provided adsorption sites for CO2 via electrostatic interaction. The STAC-NO2 exhibits a foamy coral-like structure, with abundant micro-mesopores. The Hierarchical porous, rough pore surface, and abundant amorphous structure greatly enhanced the CO2 capture capacity. Thermodynamic studies show that CO2 capture by STAC-NO2 is a thermodynamically controlled exothermic, entropy-decreasing physical adsorption process. Moreover, kinetic studies have demonstrated that the adsorption process involves chemical adsorption. At temperatures under 103.66 °C (ΔG0 < 0), the adsorption reaction occurs spontaneously. The isosteric heat of adsorption and regeneration test prove its cyclic renewability. This study provides valuable guidance for the high value-added transformation and utilization of spent tires in the circular economy.