Grand canonical Monte Carlo (GCMC) simulation is used to study the adsorption of pure SO using a functionalized bilayer graphene nanoribbon (GNR) at 303 K. The functional groups considered in this work are OH, COOH, NH, NO, and CH. The mole percent of functionalization considered in this work is in the range of 3.125%-6.25%. GCMC simulation is further used to study the selective adsorption of SO from binary and ternary mixtures of SO, CO, and N, of variable composition using the functionalized bilayer graphene nanoribbon at 303 K. This study shows that the adsorption and selectivity of SO increase after the functionalization of the nanoribbon compared to the hydrogen terminated nanoribbon. The order of adsorption capacity and selectivity of the functionalized nanoribbon is found to follow the order COOH > NO > NH > CH > OH > H. The selectivity of SO is found to be maximum at a pressure less than 0.2 bar. Furthermore, SO selectivity and adsorption capacity decrease with increase in the molar ratio of SO/N mixture from 1:1 to 1:9. In the case of ternary mixture of SO, CO, N, having compositions of 0.05, 0.15, 0.8, the selectivity of SO over N is higher than that of CO over N. The maximum selectivity of SO over CO is observed for the COOH functionalized GNR followed by NO and other functionalized GNRs.

The structure and dynamics of water droplets on a bilayer graphene surface are investigated using molecular dynamics simulations. The effects of solid/water and air/water interfaces on the local structure of water droplets are analyzed in terms of the hydrogen bond distribution and tetrahedral order parameter. It is found that the local structure in the core region of a water droplet is similar to that in liquid water. On the other hand, the local structure of water molecules at the solid/water and air/water interfaces, referred to as the interface and surface regions, respectively, consists mainly of three-coordinated molecules that are greatly distorted from a tetrahedral structure. This study reveals that the dynamics in different regions of the water droplets affects the intermolecular vibrational density of states: It is found that in the surface and interface regions, the intensity of vibrational density of states at ∼50 cm−1 is enhanced, whereas those at ∼200 and ∼500 cm−1 are weakened and redshifted. These changes are attributed to the increase in the number of molecules having fewer hydrogen bonds in the interface and surface regions. Both single-molecule and collective orientation relaxations are also examined. Single-molecule orientation relaxation is found to be marginally slower than that in liquid water. On the other hand, the collective orientation relaxation of water droplets is found to be significantly faster than that of liquid water because of the destructive correlation of dipole moments in the droplets. The negative correlation between distinct dipole moments also yields a blueshifted libration peak in the absorption spectrum. It is also found that the water–graphene interaction affects the structure and dynamics of the water droplets, such as the local water structure, collective orientation relaxation, and the correlation between dipole moments. This study reveals that the water/solid and water/air interfaces strongly affect the structure and intermolecular dynamics of water droplets and suggests that the intermolecular dynamics, such as energy relaxation dynamics, in other systems with interfaces are different from those in liquid water.

In this work, a comparative study on water-stable microporous adsorbents is conducted computationally in the quest of a suitable adsorbent for post-combustion CO2 capture. In this regard, three metal–organic frameworks (MOFs), two covalent organic frameworks (COFs), and a single-wall carbon nanotube (SWCNT) are investigated under the same flue gas conditions. The simulation results show that the pure component adsorption capacity for CO2 follows the order SWCNT > InOF-1 > COF-300 > UiO-66 > COF-108 > ZIF-8 at post-combustion conditions. Further, these materials are impregnated with ionic liquids to examine the effect on the CO2 separation ability of these materials. The adsorption capacity enhances by incorporating ionic liquids, especially [EMIM]­[SCN] compared to [EMIM]­[BF4] as a result of a stronger interaction and being less bulky in nature. We further tested the effect of the presence of other components of flue gas on the selectivity of CO2 over N2, and we found that the presence of SO2 and water vapor reduces the CO2 selectivity in all of the materials considered in this work. Performance in terms of CO2 selectivity of these materials is tested in the presence of all major components of flue gas, and we found that, under the same thermodynamic conditions, it follows the order InOF-1 > COF-300 > UiO-66 > SWCNT > COF-108 ≈ ZIF-8. The CO2/N2 selectivity increases significantly after impregnating materials with ionic liquids. In the presence of water, InOF-1 completely discard N2, showing infinitely large selectivity for CO2/N2. In humid conditions, the difference in selectivity between pristine and composite materials decreases significantly.

The selective adsorption behaviours of carbon dioxide, methane and nitrogen on bundles of functionalized CMK-5 are investigated at 303 K using grand-canonical Monte Carlo simulations. Functional groups (-OH, -COOH) cause a significant enhancement in CO2 uptake (up to 19.5% at a pressure of 38.13 bar for -COOH). On the other hand, the adsorption amount of methane decreases with respect to bare CMK-5 by ∼13% (at 38.13 bar) upon functionalization. Furthermore, functionalized CMK-5 with different pore sizes (4 nm, 6 nm, 8 nm) and inter-tube distances (d = 0 to 1.5 nm) are used to investigate the adsorption behaviour of flue gases. While the pore diameter is seen to reduce the isosteric heat of adsorption, the inter-tube distance of 0.25 nm shows the highest uptake of CO2 at p ≤ 18 bar, followed by 0.5 nm for the pressure range of 18 < p ≤ 30 bar, whereas for p > 30 bar, d = 1.0 nm shows the maximum uptake. For methane and nitrogen, the maximum adsorption is obtained at d = 0.25 nm in the studied pressure range. The selective adsorption of CO2 in binary mixtures is investigated using ideal adsorption solution theory. CO2-N2 selectivity is found to increase significantly by surface functionalization of CMK-5 compared to pure CMK-5. The maximum selectivity of CO2-CH4 using -COOH functionalized CMK-5 is found to be ∼10 for an equimolar CO2-CH4 mixture at a pressure of 38.13 bar.

Grand canonical Monte Carlo simulations are conducted to investigate the adsorption ability of a 3-D graphene sponge (GS) to separate acidic gases from flue gas stream. To assess the adsorption capacity of GS, first, adsorption of pure component flue gas is studied at a temperature of 303 K and varying pressure up to 2.5 bar. Subsequently, the adsorption capacity and selectivity of GS are investigated for a ternary mixture (CO 2 /SO 2 /N 2 ) of flue gas under the same conditions. This study shows that the maximum adsorption capacity of GS for pure component flue gas is observed for SO 2 followed by CO 2 and N 2 . The adsorption uptake decreases with an increase in pore size of GS. At 1 bar, the amount of adsorption of SO 2 and CO 2 are ∼13 mmol/g and ∼2.6 mmol/g, respectively. Upon increasing the average pore size to 20 Å, the excess amount decreases by 56% and 58% for SO 2 and CO 2 , respectively. The adsorption capacities of GS for CO 2 and SO 2 are better than other carbonbased adsorbents except for CNT bundles. In the case of a ternary mixture of N 2 , CO 2 , and SO 2 in the mole ratios of 0.8, 0.15, and 0.05, we found that the adsorption behavior follows the same order as in the pure component flue gas adsorption. However, the adsorption amount decreases significantly from that of pure component adsorption amount in GS. The adsorption amount of SO 2 and CO 2 at postcombustion conditions decreases to 1.3 mmol/g and 0.5 mmol/g, respectively, which further decreases upon increasing the average pore size. Selectivity analysis of adsorption shows that the adsorption selectivity of SO 2 over N 2 is the maximum followed by the selectivity of CO 2 over N 2 and SO 2 over CO 2 . Both selectivity and uptake capacity decreases with increase in average pore size of GS.