J. Karl Johnson

Molecular simulation of hydrogen adsorption in single-walled carbon nanotubes and idealized carbon slit pores

The adsorption of hydrogen gas into single-walled carbon nanotubes (SWNTs) and idealized carbon slit pores is studied by computer simulation. Hydrogen-hydrogen interactions are modeled with the Silvera-Goldman potential. The Crowell-Brown potential is used to model the hydrogen-carbon interactions. Calculations include adsorption inside the tubes, in the interstitial regions of tube arrays, and on the outside surface of isolated tubes. Quantum effects are included through implementation of the path integral formalism. Comparison with classical simulations gives an indication of the importance of quantum effects for hydrogen adsorption. Quantum effects are important even at 298 K for adsorption in tube interstices. We compare our simulations with experimental data for SWNTs, graphitic nanofibers, and activated carbon. Adsorption isotherms from simulations are in reasonable agreement with experimental data for activated carbon, but do not confirm the large uptake reported for SWNTs and nanofibers. Although ...

Zwitterion Functionalized Carbon Nanotube/Polyamide Nanocomposite Membranes for Water Desalination

We have shown from both simulations and experiments that zwitterion functionalized carbon nanotubes (CNTs) can be used to construct highly efficient desalination membranes. Our simulations predicted that zwitterion functional groups at the ends of CNTs allow a high flux of water, while rejecting essentially all ions. We have synthesized zwitterion functionalized CNT/polyamide nanocomposite membranes with varying loadings of CNTs and assessed these membranes for water desalination. The CNTs within the polyamide layer were partially aligned through a high-vacuum filtration step during membrane synthesis. Addition of zwitterion functionalized CNTs into a polyamide membrane increased both the flux of water and the salt rejection ratio. The flux of water was found to increase by more than a factor of 4, from 6.8 to 28.7 GFD (gallons per square foot per day), as the fraction of CNTs was increased from 0 to 20 wt %. Importantly, the ion rejection ratio increased slightly from 97.6% to 98.6%. Thus, the nanotubes imparted an additional transport mechanism to the polyamide membrane, having higher flow rate and the same or slightly better selectivity. Simulations show that when two zwitterions are attached to each end of CNTs having diameters of about 15 Å, the ion rejection ratio is essentially 100%. In contrast, the rejection ratio for nonfunctionalized CNTs is about 0%, and roughly 20% for CNTs having five carboxylic acid groups per end. The increase in ion rejection for the zwitterion functionalized CNTs is due to a combination of steric hindrance from the functional groups partially blocking the tube ends and electrostatic repulsion between functional groups and ions, with steric effects dominating. Theoretical predictions indicate that an ideal CNT/polymer membrane having a loading of 20 wt % CNTs would have a maximum flux of about 20000 GFD at the conditions of our experiments.

Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.

An accurate H2–H2 interaction potential from first principles

We have calculated the potential energy surface extrapolated to the complete basis set limit using coupled-cluster theory with singles, doubles, and perturbational triples excitations [CCSD(T)] for the rigid monomer model of (H2)2. There is significant anisotropy among the 37 unique angular configurations selected to represent the surface. A four term spherical harmonics expansion model was chosen to fit the surface. The calculated potential energy surface reproduces the quadrupole moment to within 0.58% and the experimental well depth to within 1%. The second virial coefficient has been computed from the fitted potential energy surface. The usual semiclassical treatment of quantum mechanical effects on the second virial coefficient was applied in the temperature range of 100–500 K. We have developed a new technique for computing the quantum second virial coefficient by combining Feynman’s path integral formalism and Monte Carlo integration. The calculated virial coefficient compares very well with publis...

Molecular simulation of hydrogen adsorption in charged single-walled carbon nanotubes

The adsorption of molecular hydrogen gas onto charged single-walled carbon nanotubes (SWNTs) is studied by grand canonical Monte Carlo (GCMC) computer simulation. The quadrupole moment and induced dipole interaction of hydrogen with “realistically” charged (0.1 e/C) nanotubes leads to an increase in adsorption relative to the uncharged tubes of ∼10%–20% for T=298 K and 15%–30% for 77 K. Long-range electrostatic interactions makes second layer (exohedral) adsorption significantly higher. Hydrogen orientation-ordering effects and adsorption anisotropy in the electrostatic field of the nanotube were observed. The geometry of nanotube arrays was optimized at fixed values of charge, temperature, and pressure. In general, negatively charged nanotubes lead to more adsorption because the quadrupole moment of hydrogen is positive. Calculated isotherms indicate that even charged nanotube arrays are not suitable sorbents for achieving the DOE target for hydrogen transportation and storage at normal temperatures, unl...

Diffusivities of Ar and Ne in Carbon Nanotubes

Atomically detailed simulations are used to compute the self-diffusivity and transport diffusivity of Ar and Ne through single walled carbon nanotube (SWNT) pores at room temperature. The diffusivities are computed over a range of loadings, corresponding to external equilibrium bulk pressures ranging from 0 to 100 bar. The diffusivities in carbon nanotubes are compared with diffusivities of the same gases in silicalite, a common zeolite, under the same conditions. We find that self-diffusivities are one to three orders of magnitude faster in carbon nanotubes than in silicalite, depending on loading. The transport diffusivities are about three orders of magnitude faster in nanotubes than in silicalite over all loadings studied. The equilibrium adsorption isotherms and computed diffusivities are used to predict fluxes through hypothetical membranes of nanotubes and silicalite. The fluxes for both Ar and Ne are predicted to be four orders of magnitude greater through nanotube membranes than through silicalite membranes of the same thickness.

Adsorption and separation of hydrogen isotopes in carbon nanotubes: Multicomponent grand canonical Monte Carlo simulations

Adsorption isotherms of hydrogen isotopes from molecular simulations in carbon nanotubes and interstices are presented. The adsorption of pure isotopes follows Henry’s law up to moderate coverages. A modified path integral grand canonical Monte Carlo (PI-GCMC) technique for mixture adsorption is presented and applied to adsorption of isotope mixtures in carbon nanotubes. Adsorption isotherms of H2–T2 mixtures in nanotubes and interstices are determined at 20 and 77 K. Selectivities for T2 over H2 are calculated over a range of pressures. Selectivity in the nanotubes and interstices increases with pressure until the nanotube is saturated. Comparison of simulation results with predictions based on ideal adsorbed solution theory (IAST) shows good agreement up to moderate loadings. At higher loadings, selectivities determined from multicomponent simulations remain roughly constant, whereas IAST predicts continued increase in selectivities. Isotherms for H2–D2 and the selectivities of D2 over H2 are determined...

Molecular simulation of xenon adsorption on single-walled carbon nanotubes

Adsorption of xenon on single-walled (10,10) carbon nanotubes at a temperature of 95 K has been studied by molecular simulation and the results have been compared with recent experiments [A. Kuznetsova, J. T. Yates, Jr., J. Liu, and R. E. Smalley, J. Chem. Phys. 112, 9590 (2000)]. Simulations indicate that adsorption takes place primarily on the inside of the nanotubes at the experimental conditions. Interstitial and external adsorption were found to be negligible in comparison with adsorption inside the nanotubes. The coverage computed from simulation of 0.06 Xe–C is in good agreement with the experimentally measured value of 0.042 Xe–C. The isosteric heat of adsorption from simulation ranges from about 3000 to 4500 K as a function of coverage, which is consistent with the experimental desorption activation energy of 3220 K. Adsorption on the external surfaces of the nanotubes is observed to take place at Xe pressures that are larger than those probed in the experiments. The good agreement between simula...

Assessing nanoparticle size effects on metal hydride thermodynamics using the Wulff construction

The reaction thermodynamics of metal hydrides are crucial to the use of these materials for reversible hydrogen storage. In addition to altering the kinetics of metal hydride reactions, the use of nanoparticles can also change the overall reaction thermodynamics. We use density functional theory to predict the equilibrium crystal shapes of seven metals and their hydrides via the Wulff construction. These calculations allow the impact of nanoparticle size on the thermodynamics of hydrogen release from these metal hydrides to be predicted. Specifically, we study the temperature required for the hydride to generate a H(2) pressure of 1 bar as a function of the radius of the nanoparticle. In most, but not all, cases the hydrogen release temperature increases slightly as the particle size is reduced.

Reaction Mechanism of Monoethanolamine with CO2 in Aqueous Solution from Molecular Modeling

We present a theoretical study of the reaction mechanism of monoethanolamine (MEA) with CO₂ in an aqueous solution. We have used molecular orbital reaction pathway calculations to compute reaction free energy landscapes for the reaction steps involved in the formation of carbamic acids and carbamates. We have used the conductor-like polarizable continuum model to calculate reactant, product, and transition state geometries and vibrational frequencies within density functional theory (DFT). We have also computed single point energies for all stationary structures using a coupled cluster approach with singles, doubles, and perturbational triple excitations using the DFT geometries. Our calculations indicate that a two-step reaction mechanism that proceeds via a zwitterion intermediate to form carbamate is the most favorable reaction channel. The first step, leading to formation of the zwitterion, is found to be rate-determining, and the activation free energies are 12.0 (10.2) and 11.3 (9.6) kcal/mol using Pauling (Bondi) radii within the CPCM model at the CCSD(T)/6-311++G(d,p) and CCSD(T)/6-311++G(2df,2p) levels of theory, respectively, using geometries and vibrational frequencies obtained at the B3LYP/6-311++G(d,p) level of theory. These results are in reasonable agreement with the experimental value of about 12 kcal/mol. The second step is an acid-base reaction between a zwitterion and MEA. We have developed a microkinetic model to estimate the effective reaction order at intermediate concentrations. Our model predicts an equilibrium concentration for the zwitterion on the order of 10⁻¹¹ mol/L, which explains why the existence of the zwitterion intermediate has never been detected experimentally. The effective reaction order from our model is close to unity, also in agreement with experiments. Complementary ab initio QM/MM molecular dynamics simulations with umbrella sampling have been carried out to determine the free energy profiles of zwitterion formation and proton transfer in solution; the results confirm that the formation of the zwitterion is rate-determining.