Effect Of Surface Modification On Physical Properties, Surface Chemistry And CO₂ Adsorption Properties Of Activated Carbon

With the influence of green gases on global climate change becoming more and more serious, reduction of carbon dioxide emissions has turned into a worldwide problem. Adsorption is considered one of the most appealing options for CO2 capture, and the key of this technology is searching for efficient adsorbents with a high CO2 selectivity and capacity. Porous materials containing basic groups are potential adsorbents in reducing CO2 emission due to their highly developed porous structure, low corrosivity and easy regeneration. In this article, basic groups were introduced into carbon structure through kinds of surface modification methods in order to enhance their CO2 adsorption capacities.The suitable materials were prepared through three steps. Firstly, three types of activated carbons were oxidized by nitric acid, including coal-based activated carbon (MAC), coconut shell activated carbon (YAC) and pitch-based spherical activated carbon (LAC) to choose the best matrix material. The effect of nitric acid oxidation on surface chemistry and pore structure of activated carbons were characterized by titration, Fourier transform infrared (FTIR) spectroscopy, surface area and porosity measurement. The oxidized activated carbons were further grafted by ethylenediamine and characterized by titration. The results indicated that the structures of MAC and LAC were well maintained after oxidation. LAC was found to have the largest content of acidic groups (2.36 mmol/g). After being grafted by ethylenediamine, LAC was also found to have the largest content of basic groups (1.39 mmol/g).Secondly, series of oxidation processes were carried out using LAC as the original material. The products were characterized by titration, FTIR spectroscopy, surface area and porosity measurement. The results indicated that LAC-3 (LAC treated by nitric acid) not only had a large content of acidic groups (2.65 mmol/g), but also maintained a good pore structure. After nitric acid treament, the surface area increased from 911 m2/g to 918 m2/g. Thirdly, further modification was carried out using LAC-3 as the original material. A series of LAC containing basic groups were produced using NaOH, guanidine hydrochloride, melamine and ethylenediamine as modifiers (marked as LAC-3-1, LAC-3-2, LAC-3-3, and LAC-3-5 respectively). Their capacities for CO2 adsorption were studied through both static and column tests. The results indicated that all modifiers could increase the adsorption capacity of LAC for CO2, and this adsorption capacity correlated well with the content of basic groups. The contents of basic groups were LAC-3-1>LAC-3-2>LAC-3-3>LAC-3-5, and adsorption capacities for CO2 were LAC-3-1≈LAC-3-2>LAC-3-3>LAC-3-5. Breakthrough curves achieved in column tests showed that the adsorption rates for CO2 were LAC-3-1>LAC-3-2>LAC-3-5>LAC-3-3. After regeneration, adsorption properties of all the four modified materials especially LAC-3-5 degraded. The other three had relatively stable adsorption properties.