Surface oxidation modification of nitrogen doping biochar for enhancing CO2 adsorption

The purpose of this paper is to demonstrate a surface oxidation modification strategy of nitrogen doping biochar for enhancing CO2 adsorption. The porous structure and surface group distribution of nitrogen doping biochars before and after modification (remarked as TF and OTFs) were characterized. Then the thermodynamic and kinetic characteristics of CO2 adsorption such as adsorption capacity, selectivity and activation energy were systematically investigated. The results indicated that the modified biochars OTFs exhibited high adsorption capacity of CO2 between 5 and 8 mmol·g-1 at 100 kPa and 273 K. The oxidation modification affected CO2 adsorption behaviors by the ways of adjusting surface pore size and surface group distributions. The former decreased the mass transfer diffusion resistance of CO2 from gas phase to biochar surface by transforming some micropores to the mesopores. And the later improved the adsorption selectivity of CO2 to the other gases such as N2 based on the interaction difference enhancement by those oxygen-containing (OFGs) and nitrogen-containing functional groups (NFGs). When the molar ratios of CO2/N2 in gas mixture were 1:1 and 15:85, the modified biochar OTF-10 exhibited the better selectivity, and the maximum selectivities of CO2 to N2 at 273 K were 44.7 and 69.4, respectively. Furthermore, the behavior of CO2 adsorption on the biochars well conformed to the quasi-first-order dynamical model, accompanied by the higher adsorption rate of CO2 and activation energy. The obtained activation energies for TF and OTF-10 were 12.98 and 16.62 kJ·mol-1, respectively. The higher activation energy for OTF-10 meant that the adsorption temperature had a stronger effect on adsorption rate of CO2.