Thermodynamics and kinetics of carbon deposits on cobalt: a combined density functional theory and kinetic Monte Carlo study.

We have built a lattice gas model for cobalt-carbon interaction to investigate the thermodynamics and kinetics of carbon deposition on Co(0001) surfaces. The formation of carbon structures on cobalt is considered to be one of the causes of deactivation of a cobalt Fischer-Tropsch (FT) catalyst. The formation of graphene - the most thermodynamically stable phase under FT conditions - is kinetically inhibited during the first 30 hours of exposure of the surface to carbon, while the build-up of surface carbide is the fastest reaction. Our simulations clearly show that the kinetics of carbon deposition is the result of two competing effects: a fast subsurface diffusion and a slower surface diffusion to form a carbon-carbon bond.

Kinetic Monte Carlo modeling of silicate oligomerization and early gelation.

We present a lattice-gas kinetic Monte Carlo model to investigate the formation of silicate oligomers, their aggregation and the subsequent gelation process. In the early oligomerization stage, the 3-rings are metastable, 5-rings and 6-rings are formed in very small quantities, 4-rings are abundant species, linear and branched species are transformed into more compact structures. Results reveal that the gelation proceeds from 4-ring containing species. A significant amount of 5-rings and 6-rings, sharing Si with 4-ring, form in the aging stage. These reveal the formation mechanism of silicate rings and clusters during zeolite synthesis.

Mechanism of the initial stage of silicate oligomerization.

The mechanism of the initial stage of silicate oligomerization from solution is still not well understood. Here we use an off-lattice kinetic Monte Carlo (kMC) approach called continuum kMC to model silicate oligomerization in water solution. The parameters required for kMC are obtained from density functional theory (DFT) calculations. The evolution of silicate oligomers and their role in the oligomerization process are investigated. Results reveal that near-neutral pH favors linear growth, while a higher pH facilitates ring closure. The silicate oligomerization rate is the fastest at pH 8. The temperature is found to increase the growth rate and alter the pathway of oligomerization. The proposed pH and temperature-dependent mechanism should lead to strategies for the synthesis of silicate-based materials.

Direct versus hydrogen-assisted CO dissociation.

The mechanism of CO dissociation is a fundamental issue in the technologically important Fischer-Tropsch (F-T) process that converts synthesis gas into liquid hydrocarbons. In the present study, we propose that on a corrugated Ru surface consisting of active sixfold (i.e., fourfold + twofold) sites, direct CO dissociation has a substantially lower barrier than the hydrogen-assisted paths (i.e., via HCO or COH intermediates). This proves that the F-T process on corrugated Ru surfaces and nanoparticles with active sixfold sites initiates through direct CO dissociation instead of hydrogenated intermediates.

Effect of counter ions on the silica oligomerization reaction.

The silicate oligomerization reaction is key to sol-gel chemistry and zeolite synthesis. Numerous experimental and theoretical studies have addressed the physical chemistry of silicate oligomers in the prenucleation stage of siliceous zeolite formation. Here we report a study of a silica condensation reaction in aqueous solution in the presence of counter ions (Li(+) and NH(4)(+)). Ab-initio molecular dynamics simulations have been used to construct reaction energy diagrams including transition state free energies. Contact with Li+ as well as NH(4)(+) increases the activation energies of the dimerization step compared to the situation in the absence of counterions. The presence of NH(4)(+) has no effect on consecutive oligomerization steps. Hence NH(4)(+) will increase the relative formation rate of larger oligomers.

Mechanism of oligomerization reactions of silica.

The mechanism of the oligomerization reaction of silica, the initial step of silica formation, has been studied by quantum chemical techniques. The solvent effect is included by using the COSMO model. The formation of various oligomers (from dimer to tetramer) was investigated. The calculations show that the anionic pathway is kinetically preferred over the neutral route. The first step in the anionic mechanism is the formation of the SiO-Si linkage between the reactants to form a five-coordinated silicon complex, which is an essential intermediate in the condensation reaction. The rate-limiting step is water removal leading to the oligomer product. The activation energies for dimer and trimer formation ( approximately 80 kJ/mol) are significantly higher than those of the subsequent oligermerization. The activation energies for the ring closure reaction ( approximately 100 kJ/mol) are even higher. The differences in activation energies can be related to the details in intra- and intermolecular hydrogen bonding of the oligomeric complexes.

A computational study of the influence of the ceria surface termination on the mechanism of CO oxidation of isolated Rh atoms.

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.

A computational study of the mechanism of CO oxidation by a ceria supported surface rhodium oxide layer.

A mechanism of CO oxidation by a thin surface oxide of Rh supported on ceria is proposed: CO is oxidized by the Rh-oxide film, which is subsequently reoxidized by a ceria surface O atom. The proposed mechanism is supported by in situ Raman spectroscopic investigations.

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 oligomers.