Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
Nitrogen and Oxygen Codoped Porous Carbon with Superior CO2 Adsorption Performance: A Combined Experimental and DFT Calculation Study Yanxia Wang, Xiude Hu, Jian Hao, Rong Ma, Qingjie Guo,* Hongfeng Gao, and Hongcun Bai State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, China Downloaded via BUFFALO STATE on July 17, 2019 at 09:51:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Nitrogen and oxygen codoped porous carbons (NOCKs) were obtained by nitrogenization, preoxidation, and chemical activation. Considering the activation reagent amount and modification temperature, the pore structure conducive to CO2 adsorption was obtained. NOCK-400-1 exhibits maximum CO2 capacity of 6.77 mmol g−1 at 0 °C and 4.46 mmol g−1 at 25 °C, 1 bar. It also presents high dynamic CO2 adsorption capacity under 15% CO2/85% N2 at ambient temperature and excellent adsorption regenerability. The results show that the improvement of CO2 adsorption performance is mainly due to the synergistic effect of codoping nitrogen and oxygen. The codoping method effectively improves the relative contents of pyrrolic-N, pyridinic-N, and phenolic hydroxyl with promoting the synthesis of amorphous carbon. Furthermore, the codoping method enhances the porosity of NOCKs with less consumption of KOH. The density functional theory (DFT) calculations also demonstrate two kinds of van der Waals actions (namely, dispersion interaction and electrostatic attraction) for CO2 adsorption on the nitrogen and oxygen codoped carbon surface. Additionally, the physical adsorption mechanism on the heterogeneous surface of adsorbents is confirmed by adsorption isotherm and thermodynamic study. Therefore, nitrogen and oxygen codoped porous carbons are a promising sorbent for CO2 capture, which provides the effective information for carbon design. sample. Sethia et al.14 synthesized N-enriched activated carbons that showed extraordinary CO2 capacity of 5.39 mmol g−1 at 25 °C and high CO2/N2 selectivity. They concluded that N content played important roles in CO2 adsorption for carbons with the same microporous structure. Additionally, Liu et al.15 prepared O-containing porous carbons and suggested that the CO2/N2 selectivity mainly depended on oxygen content and ultramicropore structure. In other studies, the action mechanisms of heteroatom are also discussed. Sun et al.16 indicated that both dispersion interaction (for quaternary-N (N-Q)) and electrostatic interaction (for pyrrolic-N (N-5) and pyridinic-N (N-6)) had major contribution to enhancing CO2 adsorption by quantum chemical calculations. Furthermore, Liu et al.17 utilized the same approach and reported that the O-containing functional groups, especially hydroxyl and carbonyl, could attract more CO2 due to their high electron densities. The above discussion implies that N/O-doped carbon materials show enhanced CO2 adsorption performance.
1. INTRODUCTION The increasing emissions of greenhouse gases have a direct link to global warming and ecological deterioration. Carbon dioxide (CO2), mainly emitted by the combustion of fossil fuels (coal, natural gas, and oil), is the primary component present in the atmosphere. Therefore, the capture and store of CO2 is a momentous matter for improving the global climate as well as realizing the utilization of CO2. One crucial challenge now is how to capture and store CO2 efficiently. Adsorptive removal of CO2 by porous solid-based adsorbents has gained considerable attention. Currently, candidate materials for CO2 adsorption include zeolites,1,2 metal−organic frameworks,3,4 and porous carbons.5,6 Among those, porous carbon materials are considered as viable solid adsorbents due to their low cost, easily controllable chemical and textural structure, and excellent thermal/chemical stabilities.7,8 To boost CO2 adsorption performance, porous carbons need to have a high specific surface area from micropores because of the micropore filling mechanism of CO2.9 Moreover, various heteroatoms, such as nitrogen (N), oxygen (O), sulfur (S), and boron (B), have been introduced into the carbon framework to enhance CO2 adsorption and selectivity.10−12 Yue et al.13 fabricated N-doped carbons with CO2 adsorption capacity of 3.71 mmol g−1 at 25 °C, which was increased by 40.53% compared with that of the nondoped © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
March 15, 2019 June 25, 2019 June 27, 2019 June 27, 2019 DOI: 10.1021/acs.iecr.9b01454 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
when self-assembly of carbon structures was considered to take place. Subsequently, the NOC was mixed with KOH at a certain mass ratio. The mixture was placed in a horizontal quartz tubular reactor, heated to 700 °C under N2 atmosphere, and held for 60 min. The cooling samples were washed with 1 M HCl for 2 h, then washed with deionized water until the filtrate was neutral, and finally dried at 110 °C for 12 h. The obtained samples were denoted as NOCK-x-y, where x represents the modification temperature and y represents the mass ratio of KOH:NOC. For comparison, a nondoped sample CK-400-1 and two monatomic doped samples NCK-400-1 and OCK400-1 were synthesized. 2.2. Materials Characterization. The surface morphology of samples was observed using scanning electron microscopy (SEM, Quanta 400, FEI NanoPorts) and high-resolution transmission electron microscopy (TEM, Tecnai G2 20, FEI NanoPorts). The elemental contents were determined using an elemental analyzer (Vario EL cube, Elementar), taking the average of five measuring data as the result. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250Xi (Thermo Fisher Scientific) equipped with an Al Kα (1486.6 eV) source, to identify the functional groups on the surface. The carbon structures were assessed using a Raman spectrometer (DXR, Thermo Fisher Scientific) using a 532 nm laser excitation under a laser power of 6 mW in the backscattering arrangement. N2 adsorption−desorption isotherms at −196 °C were measured on an automatic physical sorption analyzer (autosorb IQ, Quantachrome). Before the adsorption measurements, all samples were degassed at 200 °C for more than 6 h under high vacuum conditions. The specific surface area (SBET) was calculated using the Brunnauer-Emmett-Teller (BET) method from the adsorption branch in the relative pressure (P/P0) range of 0.01−0.20. The total pore volume (Vtotal) was determined from the amount of N2 adsorbed at the P/P0 of 0.99, and the micropore (
