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

Abstract

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).