Is N-Doping in Porous Carbons Beneficial for CO₂ Storage? Experimental Demonstration of the Relative Effects of Pore Size and N-Doping

The relative influence of nitrogen doping and pore size of highly microporous carbon materials, with virtually identical porosity, on CO2 uptake capacity at low pressure (≤1 bar) is presented in this article. The carbon materials are prepared via a range of synthesis methods, including activation of a variety of carbon precursors (biomass, polypyrrole, or carbon nanotube superstructures) and carbonization of an organic salt (potassium hydrogen phthalate), which generated a series of carbons with closely matched porosity but which are either N-free or N-doped. The carbons have a total surface area of 920 ± 60 m2/g and micropore surface area of 860 ± 40 m2/g, values that are within ±5% of each other and within the repeatability range (or experimental error) of the porosity measurements. The carbons have identical micropore volume of 0.39–0.40 cm3/g, and similar overall pore size and apparent pore size distribution. The similar porosity values allowed a simple and straightforward analysis of the influence of N-doping on CO2 uptake without any ambiguities associated with changes in surface area and pore volume. Contrary to many previous reports wherein both N-doping and porosity varied, we show that the presence of N has no beneficial effect on the adsorption of CO2. Rather, we show that the low-pressure adsorption of CO2 on carbons is critically sensitive to the pore size, and in particular to minute changes in micropore size distribution within the pore size range 5–10 Å. The pore size also exerts greater influence on both the isosteric heat of CO2 adsorption and the selectivity for CO2 over N2.