Edwin Flikkema

Effect of cation distribution on self-diffusion of molecular hydrogen in Na3Al3Si3O12 sodalite: a molecular dynamics study.

The diffusion of hydrogen in sodium aluminum sodalite (NaAlSi-SOD) is modeled using classical molecular dynamics, allowing for full flexibility of the host framework, in the temperature range 800-1200 K. From these simulations, the self-diffusion coefficient is determined as a function of temperature and the hydrogen uptake at low equilibrium hydrogen concentration is estimated at 573 K. The influence of the cation distribution over the framework on the hydrogen self-diffusion is investigated by comparing results employing a low energy fully ordered cation distribution with those obtained using a less ordered distribution. The cation distribution is found to have a surprisingly large influence on the diffusion, which appears to be due to the difference in framework flexibility for different cation distributions, the occurrence of correlated hopping in case of the ordered distribution, and the different nature of the diffusion processes in both systems. Compared to our previously reported calculations on all silica sodalite (all-Si-SOD), the hydrogen diffusion coefficient of sodium aluminum sodalite is higher in the case of the ordered distribution and lower in case of the disordered distribution. The hydrogen uptake rates of all-Si-SOD and NaSiAl-SOD are comparable at high temperatures (approximately 1000 K) and lower for all-Si-SOD at lower temperatures (approximately 400 K).

Hydroxylation of silica nanoclusters (SiO2)M(H2O)N, M = 4, 8, 16, 24: stability and structural trends

Employing global optimisation and ab initio calculations, we follow the step-wise molecular hydroxylation of (SiO2)M(H2O)N, M = 4, 8, 16, 24, cluster species from their anhydrous state up to a N:M ratio (R(N/M)) of ≥0.5. Increasing N from zero for low R(N/M) values, significantly and progressively, energetically stabilises all cluster sizes. In all cases, this initial steep decrease in energy levels off at a well-defined threshold R(N/M) value to a linear regime where the decrease in energy per hydroxylation by a water molecule reaches a stable minimum value. Analysis of the structures of the globally optimised clusters for each size, M, and hydroxylation, N, reveals that the initially anhydrous cluster structures have increasingly tetrahedral SiO4 centres until the transition to the linear hydroxylation regime, whereupon the average deviation from tetrahedrality starts to increase. With increasing R(N/M) the smaller clusters (M = 4, 8) tend to open up and incorporate increasing numbers of Si-OH groups, seemingly approaching the limit of separate Si(OH)4 monomers. The larger clusters considered (M = 16, 24), however, are more resistant to structural disruption and with increasing R(N/M) energetically prefer not to form more Si-OH groups but, instead to form hydrogen bonds with subsequent water molecules on their surfaces. Such behaviour is also found to be more energetically favourable than the formation of fully-hydroxylated Si16O24(OH)16 and Si24O36(OH)24 cage isomers for R(N/M) = 0.5. We further found that the threshold R(N/M) value at which the transition to the linear hydroxylation regime is encountered follows an inverse power law with respect to increasing cluster size M which may indicate the existence of a more general fundamental basis underlying our results.