Molecular Dynamics Simulations for Xe Absorbed in Zeolites
- PDF / 427,737 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 60 Downloads / 184 Views
... ... ...
............ ....... ....................... ............
.
Fig. (a) One of dehydrated zeolite1. NaA. (b) a-cage Unit cell of zeolite A IP O 4-11.
We chose the commercial simulation software Cerius 2 [9], which was run on SGI Indy and Power Onyx platforms for model building, simulations and analysis. The systems considered were (I) a Xe atom and (II) eight Xe atoms distributed in the unit cell of NaA, and (III) a Xe atom in the unit cell of A1PO-11. The simulations (I) and (III) were aimed at investigating Xe distribution, adsorption sites and diffusion at low Xe-loading conditions. A lengthy enough MD simulation of type (II) should allow the cage occupancy statistics to reach the equilibrium distribution. This might require excessively long trajectories and in (II), we chose to distribute the number of Xe-atoms at different cages as 0, 0, 0, 1, 1, 1, 2 and 3 roughly according to the distribution obtained in ref. 4. In all simulations the potentials were chosen to be very close to those in ref. 4 to preserve comparability. The software limits the functional forms of the interaction potentials, and fitting to the available forms was done. For the Xe-Xe potential, the exp-6 form Vxe-xe(r) =Do [y-6eY(10 -4-6
6
(1)
was fitted to the Maitland-Smith Xe-Xe potential [4] by weighting with the Boltzmann distribution at 300 K. The fit was very good around the potential minimum, r 0 , and deviates most (1.4 x 10-22 J) at about 6 ± 1 A. The effective Xe-zeolite interaction [4] was described with Lennard-Jones (LJ) potentials
VXeNa/o(r) = Do
-2 (L)6
(2)
The potentials and corresponding parameters for Xe-Xe and Xe-cage interactions are listed in Table I. The potential functions for interactions between the atoms in zeolite lattice were found from built-in libraries of Cerius 2 , where we adopted the burchartl.01 [10] and UNIVERSALI.01 [11] force fields. The former was used among the elements 0, Al, Si and P of the cage, and the latter one was applied to interactions involving Na. The charges of the exchange cations Na were assigned according to Kiselev [12], where the number of close oxygen atoms is the key factor. As the Na(2) at the 8-rings connecting adjacent a-cages are off-center, the coordination numbers for Na(1), Na(2) and Na(3) are six, three and four, respectively. The charges +1 and +1 were given for six- and threecoordinated cations, and +2 was interpolated for the four-coordinated ones. Neutrality of the unit cell was balanced by a slight collective adjustment of the charges -0.2 of all oxygen atoms. During the simulations some cations escape from their initial sites leading to a somewhat obscure situation with the coexistence of differently charged free Na ions. Periodic boundary conditions were applied and both electrostatic and dispersion forces were calculated with the Ewald lattice summation method [13]. Neighbor lists for nonbonded interactions were used to reduce the simulation effort; the time interval between their updates was 100 fs in calculations (I) and (III), but was reduced to 2
Data Loading...
