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 (R 2 = 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).

Role of porosity and polarity of nanoporous carbon spheres in adsorption applications

“…1c-f), the sizes of CO 2 -activated RFC spheres (RFC_C50, C120, C250, and C380) are reduced to 400 AE 20, 358 AE 19, 301 AE 18, and 256 AE 27 nm, respectively, which is attributed to carbon loss during the activation process (CO 2 (g) + C(s) / 2CO(g)). 39,[41][42][43] Such carbon loss is also identied under TEM investigation, in which the highly porous nature of the RFC_C380 sphere compared to the RFC sphere is observed ( Fig. 3a and b).…”

“…4a). 39,[41][42][43] According to PSD analysis determined by the NLDFT model, sharp increases in ultramicropores centered around 0.6-0.7 nm are identied for RFC_C60 and C120 samples, strongly suggesting that relative ne-tuning of ultramicropores is possible for carbonaceous materials with CO 2 activation (Fig. 4b).…”

Co3O4 Nanocrystals on Crab Shell‐derived Carbon Nanofibers (Co3O4@CSCNs) for High‐performance Supercapacitors

“…The activated CSCNs ( i.e. , CSCN_C60 and CSCN_C180) present generally more tortuous and thinner features of CSCN bundles along with CO 2 activation times (Figure (d) and (e)), which are probably attributed to prolonged carbon loss through CO 2 activation process (C [s] + CO 2 [g] ➔ 2CO [g]) under high temperature (900°C). According to TEM images, such trends are also identified that relatively thinner and more porous features of CSCN bundles were observed along CO 2 activation times under TEM measurements (CSCN, CSCN_C60, and C180 for Figure (f)–(h), respectively).…”

“…The CSCN showed a nitrogen content of 4.6 wt % originated from the constituent of organic nanofibers, N ‐acetylglucosamine. Upon a prolonged activation process, the N contents of activated carbons gradually decreased, probably due to the less stable nature of N species incorporated within carbon framework …”

New and Advanced Porous Carbon Materials in Fine Chemical Synthesis. Emerging Precursors of Porous Carbons

“…In the case of NH 3 treatment, there will be also the introduction of N-containing groups on carbon's surface, although this type of activation/surface modification is usually coupled with other oxidant gas such as CO 2 to improve porosity development [27][28][29][30][31].…”