17 research outputs found
Infinitely-fast diffusion in Single-File Systems
We have used Dynamic Monte Carlo (DMC) methods and analytical techniques to
analyze Single-File Systems for which diffusion is infinitely-fast. We have
simplified the Master Equation removing the fast reactions and we have
introduced a DMC algorithm for infinitely-fast diffusion. The DMC method for
fast diffusion give similar results as the standard DMC with high diffusion
rates. We have investigated the influence of characteristic parameters, such as
pipe length, adsorption, desorption and conversion rate constants on the
steady-state properties of Single-File Systems with a reaction, looking at
cases when all the sites are reactive and when only some of them are reactive.
We find that the effect of fast diffusion on single-file properties of the
system is absent even when diffusion is infinitely-fast. Diffusion is not
important in these systems. Smaller systems are less reactive and the occupancy
profiles for infinitely-long systems show an exponential behavior.Comment: 8 pages, 5 figure
Hartree-fock-slater linear combination of atomic orbital calculations of the valence electron-distribution in neutral and charged IR clusters
Bonding of hydrogen atoms to iridium tetramer clusters: the effect of magnesium(2+) ions
Local-d.-approxn. calcns. were made of at. H adsorption on tetrahedral Ir4 clusters. The hydrogen atom prefers twofold or threefold coordination. The effect of the presence of a Mg2+ ion next to the Ir4 on the adsorption is studied. It changes the calcd. adsorption energy by less than 10%, but the Ir-H stretch frequencies may change as much as 25%. A comparison with earlier results of H2 adsorption on the same tetrahedral Ir4 clusters supports the hypothesis that polarization of the metal particle by a Mg2+ ion in zeolites promotes H2 dissoc
A computational study of the influence of the ceria surface termination on the mechanism of CO oxidation of isolated Rh atom
The reaction mechanism for CO oxidation by isolated Rh atoms stabilized on CeO2(111), CeO2(110) and CeO2(100) surfaces is investigated by a combination of Density Functional Theory and kinetic Monte Carlo calculations. On Rh/CeO2(111), one adsorbed CO molecule on Rh was found to form a stable intermediate structure with surface O. The reaction cycle cannot be closed because of the strong adsorption of the CO2 complex. The presence of a second adsorbed CO significantly decreases the desorption energy, thus opening a possible reaction path. Formation of the oxygen vacancy is accompanied by reduction of surface cerium. On Rh/CeO2(110), adsorbed CO can easily react with a ceria surface O atom due to the lower Ce–O bond energy. Since surface O atom migration is much more facile on Rh/CeO2(110) than on Rh/CeO2(111), CO2 desorption is also more easy for the former surface. Molecular oxygen will adsorb on the resulting vacancy. After desorption of the second CO2 product molecule by reaction of adsorbed CO with another surface O atom, the adsorbed oxygen molecule migrates spontaneously to the vacancy site and dissociates with negligible barrier. The role of molecular oxygen is to heal the oxygen vacancy rather than being involved in a direct reaction with adsorbed CO. The Rh/CeO2(100) model was found to be inactive for CO oxidation, mainly because of the geometric constraints for the adsorbed CO molecule to react with one of the surface O atoms, despite the low Ce–O bond energy of the CeO2(100) surface. The main reason is the large distance between the C of adsorbed CO and the ceria O surface atoms. The particularities of the CO oxidation mechanism for isolated Rh atoms on these ceria surfaces are in agreement with the experimental activity trends
Lattice-gas modeling of electrochemical Langmuir-Hinshelwood surface reactions
We consider the lattice–gas modeling of simple A+A and A+B electrochemical Langmuir–Hinshelwood surface reactions and compare exact Monte Carlo cyclic voltammograms with those obtained in the mean-field and quasi-chemical or cluster approximations. For A+A reactions, we demonstrate the importance of local correlations between adspecies when a repulsive interaction between neighboring species exists. The mean-field approximation (MFA) cannot account for such local correlations (i.e. ordered adlayers), whereas the quasi-chemical approximation gives results quite close to the Monte Carlo simulations. For the A+B reaction, poor mixing caused by low surface mobility of the adspecies, may result in local correlations (island formation) that cannot be treated satisfactorily by the MFA. A cluster approximation is able to partly bridge the gap between the mean-field and Monte Carlo results
The role of water in silicate oligomerization reaction
The silicate oligomerization reaction is key to sol–gel chemistry and zeolite synthesis. Numerous experimental and theoretical studies have been devoted to investigating the physical chemistry of silicate oligomers in the prenucleation stage of siliceous zeolite formation. Most of the previous quantum chemical computational work has used gas phase models or continuous solvent models for silica oligomerization. Here we apply Car–Parrinello molecular dynamics simulations with explicit inclusion of water molecules to investigate the reaction pathway for the anionic bond formation of siliceous oligomers. The rates of SiO–Si bond formation of linear or ring containing silicate oligomers become substantially enhanced, compared to gas phase results. The formation of 3-ring oligomer is more favorable than the formation of higher branched and ring silica oligomer
Role of Water in Silica Oligomerization
he silicate oligomerization reaction is key to sol-gel chemistry and zeolite synthesis. Numerous experimental and theoretical studies have been devoted to investigating the physical chemistry of silicate oligomers in the prenucleation stage of siliceous zeolite formation. Most of the reported computational studies of silica oligomerization employ a quantum chemical phase model supplemented with a continuum model for the solvent. We report a density-functional theory based molecular dynamics simulation of silica oligomerization in a bulk solution of explicit water molecules. Our study provides thermodynamics, kinetics, and mechanism of the reaction pathway for the anionic bond formation of siliceous oligomers. We reveal that in the water cleavage step a direct and a water mediated proton transfer pathways may occur and show that changing from a continuum to a explicit water model the rates of SiO-Si bond formation of linear and three-ring oligomers are significantly enhanced, whereas the overall thermodynamics becomes less favorable
Ab-initio kinetics of heterogeneous catalysis : NO+N+O/Rh(111)
We show that advances in two fields of computational chemistry, Dynamic Monte Carlo simulations and Density-Functional Theory calculations, are now making it possible to do ab-initio kinetics of realistic surface reactions. We present results of simulations of Temperature-Programmed Desorption experiments of NO reduction to N2 and O2 on the Rh(111) surface. Kinetic parameters were obtained from Density- Functional Theory calculations with the Generalized Gradient Approximation, making this one of the first, and up till now the most complex, example of ab-initio kinetics in heterogeneous catalysis. Top, hcp, and fcc sites are all involved and also lateral interactions are necessary to understand the kinetics of this system
First-principles predictions of potential hydrogen storage materials : nanosized Ti(core)/Mg(shell) hydrides
MgH2 is one of the most promising hydrogen storage materials. However MgH2 is thermodynamicly too stable, leading to a too high desorption temperature of 300 °C at atmospheric pressure, which is a major impediment for practical applications. In this study, aiming to tune the thermodynamic stability of the MgH2, nanosized two-dimensional Mg/Ti/Mg sandwich and three-dimensional Ti(core)/Mg(shell) hydrides have been investigated by using density functional theory calculations. For both structures, four types of hydrogen atoms can be distinguished: on the surface of the Mg (Hsurf), within the Mg (HMg), at the Mg/Ti interface (HMgTi), and within the Ti (HTi). For the dehydrogenation reaction, the hydrogen desorption from the hydride is in the order Hsurf, HMg, HMgTi, HTi. The desorption energy of Hsurf is unexpectedly high. As expected, due to the well-preserved fluorite structure of the partially hydrogenated hydride, the desorption energy of HMg is significantly lower than that of bulk rutile MgH2. The further desorption of HMgTi and HTi becomes more difficult due to the strong Ti-H bonding. We propose that partial hydrogenation without adsorption of Hsurf and partial dehydrogenation without desorption of HMgTi and HTi would keep the fluorite symmetry with its favorable thermodynamics. The reversible hydrogen capacity (HMg) of the Mg/Ti/Mg sandwich structure is low, whereas the reversible hydrogen capacity of the Ti(core)/Mg(shell) is calculated to be reasonable high. Our results predicted Ti(core)/Mg(shell) structures are potential useful materials for hydrogen storage application
Lattice-dynamical study of the structure and elasticity of dodecasil-3C at elevated temperatures
Using free-energy minimizations and three interat. potentials, the authors calcd. the structure of dodecasil-3C without guest mols. at several temps. For this structure, the interat. potentials predict substantially different free energies relative to a-quartz. Dodecasil-3C without guest mols. is triclinic for temps. between 0 and 1200 K. The positions of the O atoms in the exptl. structure of calcined dodecasil-3C may not be well resolved. Also, the authors present predictions for the elastic consts. C11, C12, and C44 that are in reasonable agreement with expt. on material with guest mols. As the symmetry of dodecasil-3C is lower than cubic which is reported at the time of the elasticity measurements, it might be interesting to perform addnl. measurements on this compd. to compare them with the authors' prediction