Slow diffusion of methane in ultra-micropores of silicon carbide-derived carbon

We investigate macroscopic uptake kinetics of CH4 in silicon carbide-derived carbon (SiC-DC). Ultra-microprosity in SiC-DC is found based on CO2 adsorption at 273 K, but which has poor accessibility to Ar at 87 K. The adsorption kinetics of CH4 is found to follow a bidisperse pore structure model, considering relatively rapid particle scale diffusion in large micropores, and a much slower local grain (or microparticle) scale diffusion in ultra-micropores. The grain scale activation energies are comparable with values for carbon molecular sieves, and consistent with values expected for the size range of the ultra-micropores, while the activation energies for transport in the larger particle scale micropores are comparable to those for conventional activated carbons. The particle scale diffusivities compare well with the results of equilibrium molecular dynamics simulations using a hybrid reverse Monte Carlo simulation constructed model of SiC-DC, with similar activation energy. On the other hand microscopic quasi-elastic neutron scattering measurements are found to probe only short-range barriers with lower activation energy. It is anticipated that ultra-micropores will not make a significant contribution to the transport in any membrane or adsorption-based process based on SiC-DC, due to the extremely slow transport in these ultra-micropores and their small pore volume

Automated analysis and benchmarking of GCMC simulation programs in application to gas adsorption.

<p>In this work we set out to evaluate the computational performance of several popular Monte Carlo simulation programs, namely Cassandra, DL Monte, Music, Raspa and Towhee, in modelling gas adsorption in crystalline materials. We focus on the reference case of adsorption in IRMOF-1 at 208 K. To critically assess their performance, we first establish some criteria which allow us to make this assessment on a consistent basis. Specifically, the total computational time required for a program to complete a simulation of an adsorption point, consists of the time required for equilibration plus time required to generate a specific number of uncorrelated samples of the property of interest. Our analysis shows that across different programs there is a wide difference in the statistical value of a single MC step, however their computational performance is quite comparable. We further explore the use of energy grids and energy bias techniques, as well as the efficiency of the parallel execution of the simulations. The test cases developed are made openly available as a resource for the community, and can be used for validation and as a template for further studies.</p

Computational investigation on CO2 adsorption in titanium carbide-derived carbons with residual titanium

We develop a new approach for modeling titanium carbide derived-carbon (TiC-CDC) systems with residual titanium by the generation of modified atomistic structures based on a silicon carbide derived-carbon (SiC-CDC) model and the application of weighted combinations of these structures. In our approach, the original SiC-CDC structure is modified by (i) removing carbon, (ii) adding carbon and (iii) adding titanium. The new atomic scale carbide-derived carbon (CDC) structures are investigated using classical molecular dynamics simulations, and their pure CO adsorption isotherms are calculated using grand canonical Monte Carlo simulations. The system of TiC-CDC with residual titanium is modeled as weighted combinations of pure carbon CDC structures, CDC structures with titanium and a TiC crystalline structure. Our modeling is able to produce both structural properties and adsorption isotherms in accordance with experimental data. The fraction of different models in the systems successfully reflects the structural differences in various experimental TiC-CDC samples. The modeling also suggests that in partially etched TiC-CDC systems, the titanium that may be accessible to CO gas at the transitional interface may provide significant interaction sites for CO and may lead to more efficient overall gas adsorption

Effect of structural anisotropy and pore-network accessibility on fluid transport in nanoporous Ti3SiC2 carbide-derived carbon

We develop an atomistic model of disordered TiSiC carbide-derived carbon (TiSiC-DC) through hybrid reverse Monte Carlo simulation, and validate it against experimental adsorption data of Ar and CO using grand canonical Monte Carlo (GCMC) simulation. While supporting the atomistic model, the GCMC simulations reveal inadequate accessibility of narrow micropores, leading to small deviation between experimental and simulated isotherms at low pressure. It is found that the TiSiC-DC structure is lamellar and highly anisotropic, with a percolating path in only one direction, which is parallel to the lamellae, leading to anisotropic diffusion. The energy barriers for diffusion in this anisotropic structure are found to be smaller, and the diffusion coefficient larger, than in the more disordered but isotropic SiC-derived carbon, despite the larger pore volume of the latter. These findings, based on molecular dynamics simulations are confirmed by analysis of the free energy landscape, showing larger free energy barriers for SiC-derived carbon. Our findings suggest that diffusion in isotropic carbon structures is hindered by higher energy barriers, arising from greater short-range disorder, in comparison to highly anisotropic structures, consistent with recent literature observations of larger pore wall-mediated scattering in isotropic structures

Differences in the adsorption and diffusion behaviour of water and non-polar gases in nanoporous carbon: role of cooperative effects of pore confinement and hydrogen bonding

We investigate the effect of pore confinement and molecular geometry on the adsorption and self-diffusion of H2O, CO2, Ar, CH4, C3H6, SF6 and C5H12, in a realistic model of nanoporous silicon carbide derived carbon (SiC-DC), constructed using hybrid reverse Monte Carlo simulation. Adsorption isotherms, adsorbate-adsorbate and adsorbate-adsorbent contributions to the isosteric heat of adsorption are determined to study the effect of pore confinement, microporosity and molecular geometry on adsorption of these gases. We describe the cooperative effect of pore confinement and hydrogen bonding on the formation of water clusters and anomalous adsorption behaviour of water compared with non-polar gases. We find that, in contrast to literature results based on the slit-pore model, pore-filling does not occur below the saturation pressure in hydrophobic amorphous carbon materials such as SiC-DC and activated carbon fibre. We also compare self-diffusivities and activation energy barriers of water and non-polar gases in the microporous structure of SiC-DC to identify underlying correlations with molecular properties. We demonstrate that the self-diffusivity of water deviates considerably from the correlation between diffusivity and molecular kinetic diameter observed for non-polar gases. This is attributed to the reduced diffusivity of water, and its relatively large energy barrier at high loadings despite its small kinetic diameter, which is due to the blocking effect of water clusters at pore entries