Carbon dioxide adsorbents from flame-made diesel soot nanoparticles

Carbon dioxide (CO₂) is the top contributor to global warming. On the other, soot particles formed during fuel combustion and released into the atmosphere are harmful and also contribute to global warming. It would therefore be highly advantageous to capture soot and make use of it as a feedstock to synthesize carbon-based materials for applications such as carbon dioxide adsorption. In this work, flame-made diesel soot nanoparticles were used to produce a variety of activated carbons by combined oxidative treatment with hydrogen peroxide (H₂O₂) and potassium hydroxide (KOH), and their performance towards CO₂ adsorption was evaluated. The effect of the chemical activation of soot with H₂O₂ for different reaction times and with KOH on the physicochemical properties of the activated carbons was investigated and compared to fresh soot. Interestingly, hollow aggregates of carbonaceous nanoparticles of a high interplanar distance, reduced polycyclic aromatic hydrocarbons (PAH) size, shorter PAH stacks, mesoporous structure, and a high content of oxygen functionalities along with other structural defects in PAHs were obtained in the synthesized activated carbons. Among the various analysis techniques employed, Raman spectroscopy indicated that the I_D/I_G ratio in soot decreased after simultaneous chemical treatment, though it did not indicate any enhancement in the graphitic character since the carbonyl and carboxylic containing PAHs and monovacancies (which cause defects in PAHs) also contribute to the increase in the intensity of the graphitic band. The activated carbons possessed promising CO₂ adsorption capacities, adsorption kinetics and CO₂/N₂ selectivity. For example, one of the activated carbons, following H₂O₂ treatment for 9 h and a subsequent KOH activation, exhibited a CO₂ adsorption capacity of 1.78 mmol/g at 1 bar and 25 °C, representing an increase of 161 % in capacity as compared to fresh soot. Hollow aggregates of carbonaceous nanoparticles consisting of shorter PAHs with a larger number of defects led to enhanced CO₂ adsorption rate and CO₂/N₂ selectivity on activated carbons.