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Environmental and Carbon Dioxide Issues
Single step synthesis of nitrogen-doped porous carbons for CO2 capture by low-temperature sodium amide activation of petroleum coke Linli Rao, Shenfang Liu, Jiao Chen, Linlin Wang, Liying An, Pupu Yang, and Xin Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03473 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018
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Energy & Fuels
Single step synthesis of nitrogen-doped porous carbons for CO2 capture by lowtemperature sodium amide activation of petroleum coke Linli Raoa, Shenfang Liua, Jiao Chena, Linlin Wangb, Liying Ana, Pupu Yanga, Xin Hu*,a, aKey
Laboratory of the Ministry of Education for Advanced Catalysis Materials,
Zhejiang Normal University, Jinhua 321004, China bCollege
of Engineering, Zhejiang Normal University, Jinhua 321004, PR China
*Corresponding
author’s e-mail:
[email protected]; phone: 86-151-0579-0257; fax: 86-
579-8228-8269
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Abstract In this work, highly efficient nitrogen-doped porous carbonaceous CO2 sorbents were synthesized by sodium amide activation of petroleum coke at a temperature range of 400-500°C. The as-obtained sorbents exhibit excellent CO2 uptake of 3.84 mmol/g (25°C) and 5.93 mmol/g (0°C) under atmospheric pressure. It is found that in addition to the two well-accepted factors, i.e. narrow micropore volume and nitrogen content, the pore size and pore size distribution also exhibit important effects on deciding CO2 uptake under ambient condition for these adsorbents. Furthermore, these petroleumcoke-derived nitrogen-enriched carbonaceous sorbents also exhibit other merits such as high selectivity of CO2 over N2, excellent recyclability, fast adsorption kinetics, suitable heat of adsorption, and excellent dynamic CO2 uptake. This paper wishes to offer more perception and useful information in preparing highly-efficient nitrogen-doped porous carbonaceous CO2 adsorbents.
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Energy & Fuels
1. Introduction Industrial development has caused rapid increase in atmospheric CO2 concentration, which brings serious adverse consequences such as ocean acidification and climate changes.1 To mitigate CO2 emission, the fundamental solution is to reduce the production of CO2 by using new clean energy sources such as wind, solar or hydrogen energies. However, due to the relative lag in the progress of new energy techniques, the capture of CO2 is a straightforward solution to solve this problem which has achieved a common consensus.2, 3 Among many CO2 capture techniques such as amine scrubbing,4 membrane separation5 and ionic liquid absorption,6, 7 adsorption by solid adsorbents is regarded as a highly promising technique for the removal of CO2 from flue gas.8-12 The essential of this adsorption technique is to develop highly efficient CO2 solid adsorbents, which should have excellent CO2 adsorption ability and selectivity of CO2 over N2, rapid adsorption kinetics, suitable adsorption heat, and high chemical/mechanical stabilities. A large variety of porous materials have been researched for CO2 adsorption, such as zeolites,13 porous metal oxide,14 covalent organic frameworks (COFs),15 zeolitic imidazolate frameworks (ZIFs)16 and Metalorganic framework (MOFs),17,
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porous carbons,19-23 and porous polymers.24,
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Compared with other adsorbents, porous carbonaceous materials exhibit unique superiority such as easy preparation, low-cost, high surface area, tunable porosity, good chemical stabilities, and hydrophobicity. Correspondingly, extensive studies have been focused on the CO2 capture properties of various carbonaceous materials under 1 bar and 25 ° C. For instance, Lee et al. obtained porous carbonaceous sorbents via KOH 3
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activation of petroleum coke, which exhibits a maximum CO2 adsorption ability of 3.68 mmol/g.26 Coconut shell and phenolic resin derived porous carbons developed by Hu27 and Jaroniec23 et al. possess a maximum CO2 uptake of 4.26 and 4.55 mmol/g, respectively. Deng and co-workers reported porous carbons, synthesized by KOH activation of biomass precursor, exhibiting an excellent CO2 uptake of 5.0 mmol/g.28 It has been well documented that narrow micropores (600ºC), N-6 and N-5 will transform to N-Q and N-Q is the more stable N species compared with N-6 and N-5.50 However, it has been reported that the outer edge nitrogen functional groups, i.e. N-5 and N-6, have greater surface affinity to CO2 than the N-Q.51 Thus, the N-5 and N-6 species of these nitrogen-doped porous carbons are expected to be more favorable for the CO2 capture suggesting another advantage of relatively low activation temperature in this work. 3.2. SEM, TEM and XRD analysis The morphology, detailed porosity and phase structure of the selected Nincorporated carbonaceous sorbent (PS-450-2) were examined by scanning electron 7
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microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), respectively. Figure 2a shows the SEM image of P, which exhibits a dense and particulate-like morphology with a smooth surface. After NaNH2 activation, some wrinkles appear on the surface of PS-450-2 (Fig. 2b), but no pores/defects can be found on this sample. A further view at larger magnification (Fig. 2c) could not find any pores/cavities on the surface either. These phenomena imply the lack of large voids for the as-synthesized nitrogen-doped carbons in this work, which can be attributed to the low reaction temperature employed. TEM was additionally exploited to observe the detailed porosity of PS-450-2. As demonstrated in Fig. 2d, many worm-holes like microporous pores can be found, which is formed by the stacking or cross-linking of randomly oriented graphene sheets. XRD pattern of PS-450-2 was exhibited in Figure S1 (Supporting Information), in which two peaks are located at 2θ equal to 23 and 43°. These two peaks can be assigned as (002) and (100) crystal plane of graphite carbon.52 However, the broad diffraction and low intensity of both peaks suggest the low graphitization and amorphous nature of PS-450-2, which is in accordance with the TEM observation (Fig. 2d). 3.3. Pore textural characteristics Nitrogen sorption isotherms were measured at 77 K by a Beshide 3H-2000PS2 sorption analyzer for all the nitrogen-enriched carbonaceous sorbents to determine their pore textural properties. From Figure 3, all the nitrogen isotherms demonstrate a typical type I shape according to IUPAC classification.53 The steep increase of adsorbed N2 amount at P/P00.2 suggests the multilayer adsorption on the external surface. Moreover, it is found that at any fixed NaNH2/P ratio, the samples prepared under 400ºC show a very narrow knee at P/P0
