Comparison between the ability of sulphuric acid, nitric acid and urea as chemical activators of commercial activated carbons for CO2 capture

Physical adsorption process has been considered as a promising technique for CO2 capture, due to low energy consumption and high efficiency. In this way, several adsorbents including activated carbons [1], zeolites [2] and MOFs [3] have been extensively studied for CO2 adsorption in the recent years. Activated carbon has attracted much attention, because of amorphous porous form, hydrophobic character and low energy for desorption [4]. To preparation of this adsorbent, physically (thermally) and chemically methods are the two main strategies. In the first method, the materials are carbonized in the temperature range of 400-850 0C, while in the second one by using some chemical components activation takes place by heating the mixture of precursor and dehydrating agent or oxidant. Hereby, based on BLUE Map Scenario of the International Energy Agency (IEA) [5], an activated carbon (Norit ROX 0.8) has been modified by using different chemical and thermal treatments, following the procedures described elsewhere [6]. In order to obtain adsorbents with smaller particle size, commercial activated carbon was first grinded and sieved to particle sizes ranging from 0.106 to 0.250 mm, resulting in a powder activated carbon (PAC). Three of the materials were prepared directly from the PAC sample by liquid phase treatments with hydrogen peroxide, sulphuric acid and nitric acid, resulting in PACHP, PACSA and PACNA materials, respectively. The treatment with hydrogen peroxide was performed wetting 25 g of PAC with 500 mL of hydrogen peroxide solution 30% (w/v) at room temperature for 24 h. In sulphuric acid oxidation, 25 g of PAC was immersed in 500 mL of 18 M acid solution for 3 h at 423 K. Oxidation with nitric acid was carried out boiling 25 g of PAC in 500 mL of 5 M nitric acid solution for 3 h. After the liquid phase treatments, all samples were thoroughly washed with distilled water until the neutrality of the rinsing waters and further dried in oven for 18 h at 383 K, resulting in samples PACHP, PACSA and PACNA, respectively. The other two samples were obtained in successive treatments of the PACNA material. 2 g of PACNA was immersed in 50 mL of 1 M urea solution and kept in a 125 mL stainless steel high pressure batch reactor under its own atmosphere at 473 K for 2 h, the recovered solids being thoroughly washed with distilled water until the neutrality of the rinsing waters, and further dried overnight in oven at 383 K, resulting in the PACNAU material. Then, a gas phase thermal treatment was applied, in which 1 g of PACNAU was heated, under a N2 flow (100 cm3 min−1), at 393 K, 673 K and 873 K during 60 min at each temperature and then at 1073 K for 240 min, resulting in the PACNAUT material.This work was a result of the projects “VALORCOMP” (0119_VALORCOMP_2_P), funded by FEDER through Programme INTERREG V A Spain - Portugal (POCTEP) 2014–2020, under the Portugal 2020 Partnership Agreement, through FEDER; POCI-01-0145-FEDER-006984 –funded by FEDER through COMPETE2020 – POCI –by FCT and by national funds through FCT - Fundação para a Ciência e a Tecnologia with the Project POCI-01-0145-FEDER-016517 funded by FEDER through COMPETE2020 - (POCI).