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Highly Enhanced Gas Sorption Capacities of N-Doped Porous Carbon Spheres by Hot NH3 and CO2 Treatments

Kim, Hee Soo, Kang, Min Seok, Yoo, Won Cheol
The Journal of Physical Chemistry C 2015 v.119 no.51 pp. 28512-28522
adsorption, ammonia, carbon, carbon dioxide, carbonization, catalysts, coordination polymers, hydrogen, micropores, models, nitrogen, nitrogen content, perfluorocarbons, porosity, surface area
Highly enhanced CO₂ and H₂ adsorption properties were achieved with a series of phenolic resin-based carbon spheres (resorcinol–formaldehyde carbon (RFC) and phenol–formaldehyde carbon (PFC)) by carbonization of RF and PF polymer (RFP and PFP) spheres synthesized via a sol–gel reaction and subsequent activation with hot CO₂ or NH₃ treatment. Monodisperse and size-tunable (100–600 nm) RFC and PFC spheres had intrinsic nitrogen contents (ca. 1.5 wt %), which are attributed to the synthesis conditions that utilized NH₃ as a basic catalyst as well as nitrogen precursor. A series of CO₂-activated and N-doped RFC and PFC spheres showed almost perfect correlation (R² = 0.99) between CO₂ adsorption capacities and accumulated pore volumes of fine micropores (ultramicropore <1 nm) obtained using the nonlocal density functional theory (NLDFT) model. Interestingly, NH₃ activation served not only as an effective method for heteroatom doping (i.e., nitrogen) into the carbon framework but also as an excellent activation process to fine-tune the surface area and pore size distribution (PSD). Increased nitrogen doping levels up to ca. 2.8 wt % for NH₃-activated RFC spheres showed superior CO₂ adsorption capacities of 4.54 (1 bar) and 7.14 mmol g–¹ (1 bar) at 298 and 273 K, respectively. Compared to CO₂-activated RFC spheres with similar ultramicropore volume presenting CO₂ uptakes of 4.41 (1 bar) and 6.86 mmol g–¹ (1 bar) at 298 and 273 K, respectively, NH₃-activated nitrogen-enriched RFC was found to have elevated chemisorption ability. Moreover, prolonged activation of RFC and PFC spheres provided ultrahigh surface areas, one of which reached 4079 m²g–¹ with an unprecedented superb H₂ uptake capacity of 3.26 wt % at 77 K (1 bar), representing one of the best H₂ storage media among carbonaceous materials and metal–organic frameworks (MOFs).