Pore Characteristics for Efficient CO Storage in Hydrated Carbons.

Development of new approaches for carbon dioxide (CO) capture is important in both scientific and technological aspects. One of the emerging methods in CO capture research is based on the use of gas-hydrate crystallization in confined porous media. Pore dimensions and surface functionality of the pores play important roles in the efficiency of CO capture. In this report, we summarize work on several porous carbons (PCs) that differ in pore dimensions that range from supermicropores to mesopores, as well as surfaces ranging from hydrophilic to hydrophobic. Water was imbibed into the PCs, and the CO uptake performance, in dry and hydrated forms, was determined at pressures of up to 54 bar to reveal the influence of pore characteristics on the efficiency of CO capture and storage. The final hydrated carbon materials had HO-to-carbon weight ratios of 1.5:1. Upon CO capture, the HO/CO molar ratio was found to be as low as 1.8, which indicates a far greater CO capture capacity in hydrated PCs than ordinarily seen in CO-hydrate formations, wherein the HO/CO ratio is 5.72. Our mechanistic proposal for attainment of such a low HO/CO ratio within the PCs is based on the finding that most of the CO is captured in gaseous form within micropores of diameter <2 nm, wherein it is blocked by external CO-hydrate formations generated in the larger mesopores. Therefore, to have efficient high-pressure CO capture by this mechanism, it is necessary to have PCs with a wide pore size distribution consisting of both micropores and mesopores. Furthermore, we found that hydrated microporous or supermicroporous PCs do not show any hysteretic CO uptake behavior, which indicates that CO hydrates cannot be formed within micropores of diameter 1-2 nm. Alternatively, mesoporous and macroporous carbons can accommodate higher yields of CO hydrates, which potentially limits the CO uptake capacity in those larger pores to a HO/CO ratio of 5.72. We found that high nitrogen content prevents the formation of CO hydrates presumably due to their destabilization and associated increase in system entropy via stronger noncovalent interactions between the nitrogen functional groups and HO or CO.