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

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.