Jorge Pikunic

Influence of chemical and physical surface heterogeneity on chemical reaction equilibria in carbon micropores

Recent simulation results are presented for the equilibrium yield of the ammonia synthesis reaction in various model microporous carbons. It is found that the reaction equilibria within the micropores is affected by many factors, including pore size, pore shape, connectivity, surface roughness, and surface chemical activation. In order to probe these effects, reactive Monte Carlo simulations of the reaction were performed in several microporous carbon models: smooth slit-shaped carbon pores, a realistic carbon model generated from experimental diffraction data, single-walled carbon nanotubes, and smooth slit-shaped pores activated by carboxyl surface groups. The simulations show that the ammonia conversion is most sensitive to the carbon pore width and to the amount of surface chemical activation. Effects of surface corrugation and pore connectivity on the equilibrium reaction yield are minimal.

Simulation of chemical reaction equilibria and kinetics in heterogeneous carbon micropores

Abstract We present a simulation study which shows how the equilibrium yield and kinetics of chemical reactions can be enhanced by tailoring the structure and surface chemistry of the catalyst support material. Equilibrium results are presented for the ammonia synthesis reaction, N 2 +3H 2 ↔2NH 3 , occurring within various carbon supports, representing a range of chemical and physical surface heterogeneity. Using a simulation technique known as Reactive Monte Carlo (RxMC), we find that surface activation and pore width are primary factors in determining the conversion of the ammonia synthesis reaction while effects of surface corrugation are small. We probe the kinetic effects of physical confinement within microporous carbons by studying the bimolecular hydrogen iodide decomposition reaction, 2HI→H 2 +I 2 , in carbon slit-pores and nanotubes. The rate constant of this reaction is measured by combining the quasi-equilibrium hypothesis of transition-state theory (TST) with the RxMC simulation technique. The kinetic simulations represent a new method for probing reaction kinetics in non-ideal environments and show accurate results when applied to the hydrogen iodide decomposition reaction.

Stability of porous carbon structures obtained from reverse monte carlo using tight binding and bond order hamiltonians

The constrained Reverse Monte-Carlo (RMC) technique [1,2] was used to generate atomic configurations of disordered microporous carbons in a previous work. However, a carbon structure obtained from RMC is a result of the fitting to some structural data such as obtained from X-ray diffraction; it does not guarantee the stability of the resulting models when a realistic interatomic potential is used. In the present work, we studied the stability of these RMC structures using canonical Monte-Carlo simulations. Two different descriptions of the carbon-carbon and carbon-hydrogen interactions are used, both encompassing the bonding processes characteristic of carbon chemistry. The first approach is based on a bond-order potential while the second considers a tight binding model. We found that the structures obtained from RMC simulations undergo local structural changes upon relaxation, however the porous structure of the models remains intact.

Models of porous carbons

Except for the fullerenes, carbon nanotubes, nanohorns, and schwarzites, porous carbons are usually disordered materials, and cannot at present be completely characterized experimentally. Methods such as X-ray and neutron scatter ing and high-resolution transmission electron microscopy (HRTEM) give partial structural information, but are not yet able to provide a complete description of the atomic structure. Nevertheless, atomistic models of carbons are needed in order to interpret experimental characterization data (adsorption isotherms, heats of adsorption, etc.). They are also a necessary ingredient of any theory or molecular simulation for the prediction of the behavior of adsorbed phases within carbons — including diffusion, adsorption, heat effects, phase transitions, and chemical reactivity.