Existence of ultrafine crevices and functional groups along the edge surfaces of graphitized thermal carbon black

N- and S-Doped Carbons Derived from Polyacrylonitrile for Gases Separation

The CO2 capture using adsorption can reduce the carbon footprint, increasing the sustainability of the process without the production of wastes present in commonly used industrial operations. The present research work analyses the effect of the doping-agents incorporation in carbon materials upon adsorption and separation of gases, specifically for carbon dioxide and nitrogen. The carbons precursor was polyacrylonitrile (PAN), which enabled the incorporation of nitrogen atoms in the structure, whereas sulphur doping was reached using pure sulphur after the carbonisation step. The influence of several variables (such as temperature or pressure) and characteristics of synthesised materials (mainly corresponding to surface characteristics) on carbon dioxide separation has been evaluated. Adsorption isotherms were determined for each gas (CO2 and N2) at different temperatures and pressures. Different adsorption models were evaluated to fit the experimental data. In general, the Toth isotherm described better the adsorption for both gases. Important parameters such as CO2/N2 selectivity and heat of adsorption were determined using the IAS theory and the experimental isotherms at different temperatures, respectively. Non-activated carbons generated from PAN carbonisation without sulphur addition showed the highest values of selectivity (up to 400) and adsorption heat (up to 40 kJ mol−1), mainly at low pressures and at low carbon dioxide uptakes, respectively. Furthermore, thanks to their high adsorption capacity, these carbons can be applied for carbon dioxide separation from mixtures with nitrogen.

Characterization of non-graphitized carbon blacks: a model with surface crevices

Experimental isotherms for argon and nitrogen adsorption on two non-graphitized carbon substrates, Carbopack B and Cabot BP280, do not obey Henry's Law in the range of pressures accessible to the most sensitive MKS pressure transducers. At high pressures, close to the bulk coexistence pressure (P0), the isotherms at temperatures below the bulk triple point temperature cross the P0 axis at a finite loading, a behaviour which is interpreted as incomplete wetting. It was found that the adsorbed density at P0 for Cabot BP280 is lower than that for Carbopack B which is, in turn, only slightly lower than that for the highly graphitized Carbopack F, suggesting that there is a long-range effect of the surface structure in non-graphitized carbon blacks, in the accumulation of higher layers, especially for Cabot BP280. We have carried out extensive Monte Carlo simulations to compare experimental observations with a molecular model for substrate surfaces decorated with crevices of molecular dimensions. From the analysis of the experimental data, it was found that the typical width of crevices is of the order of 0.65-0.9 nm. In the high pressure region, the crossing of the P0 axis by isotherms at temperatures below the bulk triple point temperature can be explained by an adsorbate structure which is less dense and more disordered than the fcc structure of the bulk crystal, with a consequent raising of the coexistence pressure between the adsorbate and the gas phase above P0. Adsorbate loading at the point where the isotherm crosses the P0 axis for Cabot BP280 is lower than for Carbopack B which can be attributed to a higher concentration of crevices leading to a lower adsorbate density and an irregular arrangement of atoms at the interface separating the adsorbed phase and the gas phase. This results in weaker gas-adsorbate interactions which supresses the build-up of higher layers. We suggest that the use of the adsorbed density at the bulk coexistence pressure, at temperatures below the bulk triple point temperature, can be a useful tool for assessing the presence and concentration of surface crevices on non-graphitized carbon black.

The physisorption mechanism of SO2 on graphitized carbon

Sulfur dioxide (SO2) in flue gases emitted from fossil fuel power plants dramatically reduces the CO2 capture efficiency via adsorption, which is due to the potential reaction of SO2 with basic functional groups on the adsorbent. Physisorption rather than chemisorption is preferred, because adsorbents can be more easily regenerated by either reducing the pressure or increasing the temperature. Carbon is a suitable adsorbent for SO2 capture and widely used, and therefore it is important to study SO2 adsorption onto carbon with the Monte Carlo simulation to provide microscopic details to demarcate the roles of the basal plane of the graphene layer and the functional groups in adsorption. SO2 is a polar molecule like water, as they both carry partial charges, but they interact differently with functional groups. Instead of 3D-clusters in the case of water, SO2 is localized around the functional groups and spreads over the basal plane to form 2D-molecular layers because of the strong dispersive interactions with graphite. The results indicate that the functional group has a negligible effect on the enhancement of adsorption and its role is to localize 2D-clusters of SO2 molecules. For non-graphitized carbon, we have found that the greater loadings at low pressure compared to the highly graphitized carbon is due to the presence of defects (crevices) on the basal plane surface. Finally, to describe better the experimental data, we have found that the reduction in the interactions between adsorbed molecules in the first layer is because of the repulsion of their dipoles pointing normal to the surface, a phenomenon called surface mediation and is widely used in the description of gas adsorption on surfaces.

On the explanation of hysteresis in the adsorption of ammonia on graphitized thermal carbon black

The entropic effect of temperature dominates the adsorption behaviours of ammonia from spill-over to clustering on the basal plane of graphite.