Evaluation of CO2 interactions with S-doped nanoporous carbon and its composites with a reduced GO: Effect of surface features on an apparent physical adsorption mechanism
Introduction
Release of such gases as CH4 or CO2 to the atmosphere is an important issue facing contemporary society [1]. The main source of these gases is the consumption of fossil energy. To decrease the environmental effects of CO2, its capture technologies are under development. Some of these methods are based on adsorption processes. Such adsorbents as zeolites [1], [2], [3], functionalized mesoporous silica [4], [5], carbon nanotubes [6], metal oxides [7], Metal Organic Frameworks (MOFs) [1], [8], [9], [10] or aminated graphite oxides and their composites with Cu-based MOF [11], activated carbon xerogels [12] and activated carbons [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27] are used. In many cases basic groups (as amines) [4], [5], [24], [25], or heteroatoms providing basicity, are introduced to the surfaces [18], [19], [28].
Even though the capacity of some MOFs for CO2 adsorption is high [8], [9], [10], [11], activated carbons are still extensively investigated as CO2 separation and storage media owing to their high porosity, inertness, and a high working adsorption capacity [13], [17], [21], [23], [29]. The latter is related to the easiness of the CO2 recovery from their pore system. The volume of pores smaller than 1 nm was found is crucial to provide a high CO2 adsorption capacity, especially at ambient conditions (kinetic diameter of the CO2 molecule is 0.33 nm) [1], [13], [14], [15], [16]. Since this volume can also be linked to adsorbents' surface area, various method including soft- [15] and hard-templating [30] or KOH activation have been applied [17], [31] to increase the porosity of carbons. The CO2 adsorption capacities of the highly nanoporous N-doped carbons synthesized by KOH activation of urea-modified petroleum coke was reported to range from 3.69 to 4.40 mmol/g and from 5.69 to 6.75 mmol/g at 298 K and 273 K, respectively, under 0.1 MPa [31]. At 0.1 MPa and 298 K 4.8 mmol/g was measured on the microporous carbons synthesized using carbon-containing metal salt [32] and 8.05 mmol/g at 0.1 MPa and 273 K on activated carbon spheres [33]. An introduction of surface basicity in the form of nitrogen functional groups [24], [25], [31] also increases CO2 adsorption on carbons owing to acid–base interactions [19], [24], [28]. On such carbons CO2 adsorption capacity of 3.13 mmol/g was measured on the porous carbon monolith synthesized using l-lysine [24] and 4.24 mmol/g on the N-enriched activated carbons obtained from a biomass waste by KOH activation under 0.1 MPa at 298 K [19].
Recently, it has been indicated that CO2 adsorption on N-doped carbon is affected by hydrogen bonding interactions [19]. An enhanced adsorption of CO2 was also reported by Xia and coworkers on sulfur doped carbon obtained within the structure of zeolite EMC-2. They reported 59 kJ/mol as a heat of adsorption at low surface coverage [20]. The high capacity on chemically activated reduced-graphene oxide/poly-thiophene composite was linked to the “oxidized S-content” and the presence of pores similar in size to CO2 molecule [22]. Following this, we have studied the adsorption of CO2 on S-doped polymer-derived nanoporous carbons and its composites with reduced graphene oxide [21]. The results indicated the complexity of the adsorption process and a significant role of polar sulfur-containing groups in the enhancement of CO2 adsorption. On these materials, the high pore space utilization by CO2 molecules was found. Specifically it was proposed that acid–base interactions of CO2 in small pores with sulfur incorporated into aromatic rings of the pore walls and polar interactions of CO2 with sulfoxides, sulfones and sulfonic aids, along with hydrogen bonding of CO2 with acidic groups on the surface contribute to this effect.
Taking into account our recent findings, the objective of this paper is to further evaluate CO2 adsorption on sulfur-containing nanoporous carbon/reduced graphene oxide composites. In the approach used we analyze the CO2 interactions with the surface using the LBET class model [34], [35]. This methodology is based on the physical adsorption phenomenon. We have chosen this analysis route because in the vast majority of published reports on CO2 sequestration on activated carbons only physical adsorption is addressed. By evaluation of adsorption thermodynamics from the isotherms measured at high pressure in the cycling mode the subtle and detailed changes in the interactions of CO2 with adsorbents' surfaces are pointed out and discussed in terms of the adsorption mechanisms and the possible contributions of chemical interactions to CO2 adsorption process. One has to remember that the results we present should not be considered as axioms; instead, they are to be approached as an expression of a certain – substantial – dose of probability that the picture of the analyzed structure is what it is, and nothing else.
Section snippets
Materials
The polymer-derived carbon and composites used for this research have been addressed as CO2 adsorbents when the adsorption was measured up to 0.9 MPa at 273 K [21]. The composite is obtained from the mixture of poly(sodium 4-styrene sulfonate) and 5% GO by its carbonization at 1073 K for 40 min under nitrogen [36]. Its is referred to as CSG. The carbon from the polymer is referred to as CS. The subsamples of both materials were oxidized at 623 K for 3 h in air [38] and the suffix “O” was added
Results and discussion
The CO2 adsorption isotherms at three temperatures up to 1.5 MPa are collected in Fig. 1. Oxidation of CS carbon does not have a visible effect on the amount of CO2 adsorbed. This is an interesting finding since the surface area and volume of pores almost doubled for the oxidized sample (SBET = 397 m2/g for the initial sample and 700 m2/g for the oxidized counterpart; V<1 nm = 0.081 cm3/g vs. 0.161 cm3/g, respectively [37], [38]; Pore size distributions (PSDs) are provided in Fig. 2A. Here we
Conclusions
The results presented in this paper, based on LBET class models, show the convolution of CO2 adsorption on carbonaceous nanoporous materials. The adsorbents used have complex surfaces from the view points of their porosity and chemistry. The addition of graphite oxide to the carbon and oxidation of the nanoporous carbon and its composite further modifies the surface, which has a visible effect on the thermodynamics of CO2 interactions with these surfaces but also on the extent of its
Acknowledgment
This was partially supported by NSF grant No. 1133112 and by the AGH University of Science and Technology in Krakow Grant No. 11.11.210.217.
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