The Enhancement of CO2 and CH4 Capture on Activated Carbon with Different Degrees of Burn-Off and Surface Chemistry.

Activated carbon derived from longan seeds in our laboratory and commercial activated carbon are used to investigate the adsorption of methane (CH4) and carbon dioxide (CO2). The adsorption capacity for activated carbon from longan seeds is greater than commercial activated carbon due to the greater BET area and micropore volume. Increasing the degree of burn-off can enhance the adsorption of CO2 at 273 K from 4 mmol/g to 4.2 and 4.8 mmol/g at 1000 mbar without burn-off, to 19 and 26% with burn-off, respectively. This is because an increase in the degree of burn-off increases the surface chemistry or concentration of functional groups. In the investigation of the effect of the hydroxyl group on the adsorption of CO2 and CH4 at 273 K, it is found that the maximum adsorption capacity of CO2 at 5000 mbar is about 6.4 and 8 mmol/g for cases without and with hydroxyl groups contained on the carbon surfaces. The opposite behavior can be observed in the case of methane, this is due to the stronger electrostatic interaction between the hydroxyl group and carbon dioxide. The simulation results obtained from a Monte Carlo simulation method can be used to support the mechanism in this investigation. Iron oxide is added on carbon surfaces with different concentrations to reveal the effects of ferric compounds on the adsorption of CO2. Iron at a concentration of about 1% on the surface can improve the adsorption capacity. However, excessive amounts of iron led to a limited adsorption capacity. The simulation result shows similar findings to the experimental data. The findings of this study will contribute to the progress of gas separation technologies, paving the way for long-term solutions to climate change and greenhouse gas emissions.

Monte Carlo Simulation and Experimental Studies of CO2, CH4 and Their Mixture Capture in Porous Carbons.

Adsorption of carbon dioxide and methane in porous activated carbon and carbon nanotube was studied experimentally and by Grand Canonical Monte Carlo (GCMC) simulation. A gravimetric analyzer was used to obtain the experimental data, while in the simulation we used graphitic slit pores of various pore size to model activated carbon and a bundle of graphitic cylinders arranged hexagonally to model carbon nanotube. Carbon dioxide was modeled as a 3-center-Lennard-Jones (LJ) molecule with three fixed partial charges, while methane was modeled as a single LJ molecule. We have shown that the behavior of adsorption for both activated carbon and carbon nanotube is sensitive to pore width and the crossing of isotherms is observed because of the molecular packing, which favors commensurate packing for some pore sizes. Using the adsorption data of pure methane or carbon dioxide on activated carbon, we derived its pore size distribution (PSD), which was found to be in good agreement with the PSD obtained from the analysis of nitrogen adsorption data at 77 K. This derived PSD was used to describe isotherms at other temperatures as well as isotherms of mixture of carbon dioxide and methane in activated carbon and carbon nanotube at 273 and 300 K. Good agreement between the computed and experimental isotherm data was observed, thus justifying the use of a simple adsorption model.

Explanation of the unusual peak of calorimetric heat in the adsorption of nitrogen, argon and methane on graphitized thermal carbon black.

Heats of adsorption and adsorption isotherms of argon, nitrogen and methane on a perfect graphitic surface and a defective graphitic surface are studied with a Grand Canonical Monte Carlo Simulation (GCMC). For the perfect surface, the isosteric heat versus loading shows a typical pattern of adsorption of simple fluids on graphite. Depending on adsorbate, degree of graphitization and temperature, a spike in the heat curve versus loading is observed when the first layer is mostly covered with adsorbate molecules. The heat spike is observed for argon and nitrogen at 77 K while for argon at 87.3 K it is no longer present. These simulation results are consistent with the experimental data of J. Rouquerol, S. Partyka and F. Rouquerol, J. Chem. Soc., Faraday Trans. 1, 1977, 73, 306. In the case of methane we observe heat spikes at low temperatures, 84.5, 92.5 and 104 K. The heat spike shifts to higher loading with temperature and it then disappears at high temperatures. These observations are in qualitative agreement with the experimental data of A. Inaba, Y. Koga and J. A. Morrison, J. Chem. Soc., Faraday Trans. 2, 1986, 82, 1635. In all cases where heat spikes are observed, the GCMC simulation results indicate that the heat spike is associated with the squeezing of molecules into the already dense first layer, and the rearrangement of molecules to form a highly structured fluid of this layer. While this squeezing into the first layer is happening, molecules continue to adsorb onto the relatively sparse second layer.

Adsorption of water in finite length carbon slit pore: comparison between computer simulation and experiment.

The effects of surface dimensions and topology on the adsorption of water on a graphite surface at 298 K were investigated using the grand canonical Monte Carlo (GCMC) simulation. Regarding the surface topology, we specifically considered the functional group and its position on the surface. The hydroxyl group (OH) is used as a model for the functional group. For describing the interaction of water, we used the potential model proposed by Muller et al., and the simulated isotherms of water in slit pores are found to depend on the position and concentration of the functional group. The onset of adsorption shifts to lower pressure when the concentration of functional group increases or when the functional group is positioned at the center of the graphene surface. The configuration of a group of functional groups also affects the adsorption isotherm. In all cases investigated, we have found that the hysteresis loop always exists, and the loop size depends on the concentration of the functional group and its position. Finally, we tested the molecular model of water adsorption on a functional graphite pore against the experimental data of a commercial activated carbon. The agreement is found to be satisfactory when the model porous solid is composed of pores having width in the range between 10 and 20 A and functional groups positioned at the center of the graphitic wall.