A.P.J. Jansen

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.

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.

Combining density-functional calculations with kinetic models: NO/Rh(111)

We present a dynamic Monte-Carlo model involving lateral interactions and different adsorption sites (top, fcc and hcp). Using this model in combination with kinetic parameters from UHV experiments and lateral interactions derived from DFT calculations we have reproduced the ordering behavior of NO on Rh(111) during adsorption and the temperature programmed desorption (TPD) of NO from Rh(111) under UHV conditions. The formation of c(4×2)-2NO domains at 0.50 ML coverage is shown to depend strongly on the next-next-nearest-neighbor repulsion between the NO adsorbates in our model. The formation of the (2×2)-3NO structure at higher coverage follows from the avoidance of the strong next-nearest-neighbor repulsion in favor of the occupation of the top sites. A single-site model was able to reproduce the experimental TPD, but the lateral interactions were at odds with the values of the DFT calculations. A three-site model resolved this problem. It was found that all NO dissociates during TPD for initial coverag...

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.

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.

Quantifying lateral adsorbate interactions by kinetic Monte-Carlo simulations and density-functional theory: NO dissociation on Rh(100)

The dissociation of NO on Rh(100) surfaces has been modelled by kinetic Monte-Carlo (MC) simulations including nearest neighbour (NN) and next-nearest neighbour (NNN) interactions, and zero-coverage kinetic parameters obtained from experiments. Two approaches were performed to derive the lateral interactions. First, the interactions were quantified by fitting the MC model to the experimental data. All interactions between NO, N and O were found to be repulsive. Nearest-neighbour interactions involving atoms are typically on the order of 20–30 kJ mol−1; between molecules they are below 10 kJ mol−1. All next-nearest neighbour interactions were smaller than 10 kJ mol−1. The simulations show that at higher initial NO coverage, the nitrogen and oxygen atoms were created by dissociation form c(2 × 2) islands, thereby compressing the NO areas and impeding their dissociation. Second, interactions estimated using DFT calculations were significantly higher than the ones estimated from fitting the experiments. Monte-Carlo simulations based on interactions obtained from DFT provide a description that is only qualitatively useful.

The heterogeneity of the hydroxyl groups in chabazite

Two different clusters that have the topology of chabazite but different shapes have been used as a model for the Bronsted sites in chabazite. One of the clusters consists of eight tetrahedral atoms (8T) arranged in a ring and the other represents an intersection of two 8T rings. The adsorption of water and methanol on the two stable proton positions in chabazite has been studied using the B3LYP functional. The coordination of water and methanol with respect to the zeolite fragments were found to be similar, but with methanol situated closer to the acid site than water. The anharmonic zeolite OH stretch frequencies were found to be in the range of 2170–2500 cm−1 and 1457–2074 cm−1 in the presence of water and methanol, respectively. As a measure of the acidity of the bridging hydroxyl groups in chabazite the shift of the zeolite OH stretch frequency upon adsorption has been used. We have found that the proton attached to the oxygen atom O1 to be more acidic than the proton attached to the oxygen atom O3. ...

Role of surface diffusion in the ordering of adsorbed molecules: dynamic Monte Carlo simulations of NO on Rh(111)

The saturation coverage of molecules adsorbed on metal surfaces is often seen to increase with temperature of adsorption, and may be accompanied by the ordering of the molecules into periodic structures at higher temperatures. The case of NO on Rh(111) presents a specific example of this behavior. Modelling the adsorption process by means of Monte Carlo simulations in which diffusion and lateral interaction are considered indicates that both the increase of the saturation coverage and the ordering with increasing adsorption temperature are in agreement with an enhanced mobility of the adsorbed molecules.

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.

Methods for parallel simulations of surface reactions

We study the opportunities for parallelism for the simulation of surface reactions. We introduce the concept of a partition and we give new simulation methods based on cellular automaton using partitions. We elaborate on the advantages and disadvantages of the derived algorithms compared with dynamic Monte Carlo algorithms. We find that we can get very fast simulations using this approach trading accuracy for performance and we give experimental data for the simulation of the Ziff model.