Surface Modification Of Carbon Nanotubes For Adsorption Of Low-concentration CO₂from Confined Space

In order to ensure human health and safety in confined spaces, such as submarine, space shuttle and spacecraft, it is essential to stickly control the CO2concentration of confined space. In recent years, solid amine adsorbents with advantages of low renewable energy consumption, high selectivity and easy operation, etc. have aroused extensive attention worldwide. Being a new kind of carbon materials, carbon nanotubes (CNTs) also shows potential applications in CO2adsorption. In this paper, CNTs were chemically amine-grafted for low-concentration CO2capture after three different pre-oxidative treatments, including thermal treatment, HNO3oxidation and O2oxidation.The chemical composition and textural properties of CNTs were analysed through characterization of N2adsorption/desorption isotherms, SEM, XPS, TGA, elemental analysis, etc., and the corresponding amine-grafting reaction and CO2adsorption mechanisms were also illustrated. It is observed that BET area and pore volume of CNTs were increased from127.8m2/g and0.340cm3/g to151.8m2/g and0.394cm3/g after O2oxidation, respectively. As a result of that, more amino compounds could be uniformly tethered onto CNTs and favor of mass transfer of CO2. It also demonstrated that NH2-silicon polymerization of amines with carboxyl acids on the oxidized-MWCNT surfaces generated by HNO3treatment resulted in the consumption of primary amine (NH2) groups and the congestion of the channels, which may be contributed to the low efficiencies of amine-grafting for CO2adsorption. While O2oxidation can avoid the introduction of carboxyl acids and achiece a higher adsorption capacity of28.02mg/g at25℃for2%CO2。With the increasing of temperature from15to30℃, the adsorption capacity of optimized adsorbent, o-CNT-AEAPS, increased from26.8to28.2mg/g. Thus this adsorbents can have a good adsorption performance between17～25℃that comfortable for staff in confined space. The water content in the atmosphere under room temperature is about0-4%. o-CNT-AEAPS also achieved the largest CO2 adsorption capacity with31.6mg/g with water conten increasing to4%. That’s to say that this new made adsorbent possesses good water resistance in confined space.The cylic CO2adsorption/desorption experiments of o-CNT-AEAPS showed that the adsorbed CO2could be completely desorbed via thermal desorption at75℃for15min. Results also showd that the stablity of o-CNT-AEAPS was better than that of adsorbents made through impregnation methods. The vaccum desorption efficiency under5kPa for30min was about90%. In additon, if a combination of thermal and vacuum desorption was conducted at75℃and5kPa, the time for effectively desorb CO2could be shorten to5min. The adsorption capacities of o-CNT-AEAPS were stable after10cycles of adsorption and regeneration cycles. These results suggest that the O2pretreated CNTs adsorbents with amine-grafting are promising adsorbents for CO2capture.