Significantly increasing porosity of mesoporous carbon by NaNH2 activation for enhanced CO2 adsorption

Abstract

Sodium amide (NaNH2), a readily available strong base, was investigated as an efficient reagent for chemical activation of mesoporous carbon (MC) in the temperature range of 400–900 °C, aiming to enhance the CO2 adsorption performance. Total surface area and pore volume of the activated MC increase greatly with the activation temperature up to 700 °C and then tend to level off. Small micropores with a diameter <1 nm are developed mainly at low temperatures (400–550 °C) and decrease continuously in volume as the activation temperature increases. Nitrogen species are incorporated onto the carbon activated at 400 °C but completely disappear at higher activation temperatures due to poor thermal stability. CO2 adsorption experiments illustrated a substantial improvement in capacities at 0 °C for the NaNH2-activated carbons (6.31 mmol/g at 1 bar and 2.06 mmol/g at 0.15 bar) in comparison to pristine MC (2.01 mmol/g at 1 bar and 1.00 mmol/g at 0.15 bar). The low-pressure CO2 capacities are well correlated with the volume of small micropores rather than the total micropore volume and surface area. The activation ability of NaNH2 was compared with those of KOH and NaOH, verifying the superiority of NaNH2 in the MC activation under relatively moderate conditions, i.e., activation reagent/MC weight ratio of two and activation temperature of 550 °C.