Comprehensive study of ultra-microporous nitrogen-doped activated carbon for CO2 capture

A series of strictly microporous nitrogen-doped activated carbons were used to delineate the role of nitrogen and micropores in CO2 adsorption. A wide range of activation conditions, including the KOH to carbon weight ratio (1-2), temperature (550-700 °C) and time (1-2 h) were explored to generate activated carbons with adjustable pore sizes and nitrogen contents. The materials were characterized using X-ray diffraction, elemental analysis, X-ray photoelectron spectroscopy and Ar, N2, H2 adsorption/desorption at −196 °C. CO2 uptake, heat of adsorption, and CO2/N2 selectivity were determined from the adsorption/desorption isotherms measured at different temperatures. The optimized materials exhibited unusually high nitrogen content (22.3 wt%) and high surface area (1317 m2/g), in addition to large pore volume (0.27 cm3/g) comprised of ultra-micropores less than 0.7 nm in diameter. This material showed an extraordinary CO2 uptake of 23.7 wt% (5.39 mmol/g) at 25 °C and 1 bar, one of the highest uptakes reported so far for any activated carbon. Its CO2/N2 selectivity at 25 °C was 237 and 62 at 0.01 and 1 bar, respectively. The results led to the conclusion that both nitrogen content and ultra-micropores played important roles for CO2 adsorption, the latter being predominant.