Abstract
AbstractActivated carbon (AC) spheres with a diameter of 1.0–2.0 mm are synthesized from coal tar pitch for postcombustion carbon capture. The as‐prepared AC macrospheres after potassium hydroxide (KOH) activation are found to possess extraordinarily developed microporosity of which 87% is ultra‐microporosity with pore diameters less than 0.8 nm. Despite the relatively low surface area of just 714 m2 g−1 with a pore volume of 0.285 cm3 g−1, the macrospherical carbon adsorbents achieve exceedingly high CO2 uptake capacities of 3.15 and 1.86 mmol g−1 at 0 and 25 °C, respectively, with a CO2 partial pressure of 0.15 bar. Cyclic lifetime performance testing demonstrates that the CO2 uptake is fully reversible with fast adsorption and desorption kinetics. More importantly, due to their high bulk density of ≈1.0 g cm−3, the AC macrospheres exhibit extremely high volumetric CO2 uptakes of up to 81.8 g L−1 at 25 °C at 0.15 bar CO2, which represents the highest value ever reported for ACs. The high ultra‐microporosity coupled with the potassium‐modified physiochemical surface properties is found to be responsible for the outstanding CO2 adsorption performance of the pitch‐based AC macrospheres.
Highlights
As physical adsorbents, the CO2 capture capacity of carbon-based materials is largely determined by their textural properties and surface chemistries
We report a pitch-based activated carbon (AC) macrospheres with diameters of 1.0–2.0 mm, which give the highest volumetric CO2 uptake ever reported at low CO2 partial pressure (0.15 bar at 25 °C)
The N and S contents remain consistent after KOH activation
Summary
The CO2 capture capacity of carbon-based materials is largely determined by their textural properties and surface chemistries. Two main methodologies have been agreed that can markedly increase CO2 capture capacities at low partial pressures: design of ultra-micropores below 0.8 nm to enhance the micropore filling mechanism;[9] and surface modification via nitrogen doping,[10] to improve the affinity between AC adsorption sites and CO2. Given the limited volume of a capture system, it is essential to maximize both of the adsorption capacity and the density for the adsorbents under practical flue gas stream conditions.[12] In addition, most of the AC materials reported previously were produced in the power forms with particle sizes in the range of micrometers, which greatly restricts their direct practical applications on large scales with either fluidized-bed and/or moving bed operations while the further engineering of the materials for desirable shapes and particle sizes will lead to losses in CO2 adsorption performance.[6d]. The potassium-containing surface groups cannot be removed even with excessive water washing, which can be indicated by the remaining potassium, close to 6 wt% after washing with abundant deionized water
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