Influence of pore structure and nitrogen-containing functional groups on the competitive adsorption of high-pressure CO2/H2 by biochar

With moderate N content and rich micropore, NC-AC shows the highest CO 2 uptake. • N-doping and pore structure control low/high-pressure CO 2 sorption, respectively. • Optimal pore size for CO 2 adsorption increases with adsorption pressure. • Pyrrolic N exhibits the most significant enhancement in CO 2 adsorption. In steam methane reforming (SMR) for hydrogen production, efficient CO 2 removal is essential. Although ultra-micropores and nitrogen doping enhance CO 2 adsorption on carbon adsorbents, their combined effects under high-pressure conditions remain unclear. Herein, a nitrogen-doped biochar (NC), an activated biochar (AC), and a nitrogen-doped activated carbon (NC-AC) were prepared from waste coconut shell. AC exhibited a highly developed pore structure (specific surface area of 1755 m 2 /g and micropore volume of 0.644 cm 3 /g), whereas NC was rich in nitrogen content (16.49 wt%). NC-AC combined a moderate nitrogen content (3.06 wt%) with an abundant porous network. High-pressure CO 2 adsorption isotherms revealed a pressure-dependent evolution in the relative contributions of nitrogen-containing functional groups and pore structure: the contribution of nitrogen-containing functional groups was more pronounced at low pressures (<0.5 bar), the contribution of pore structure gradually increased at intermediate pressures (0.5–2 bar), and pore structure played a more pronounced role at high pressures (>2 bar). Dynamic column breakthrough experiments with a CO 2 /H 2 mixture (20/80 vol%) demonstrated that NC-AC exhibited the highest CO 2 uptake. Molecular simulations showed that at low pressure, the 0.6 nm pore exhibited the highest CO 2 adsorption, while at 10 bar, the 0.8 nm pore surpassed due to its strong interaction forces and large packing space. In the 0.8–1.0 nm pores, CO 2 preferentially formed multilayer adsorption near the pore walls and pushed H 2 toward the pore center. Among nitrogen-functional groups, pyrrolic N (N-5) significantly enhanced the CO 2 adsorption capacity and selectivity. Download: Download high-res image (94KB) Download: Download full-size image