ADSORPTION ENERGIES OF NH3 AND NH4+ IN ZEOLITES CORRECTED FOR THE LONG-RANGE ELECTROSTATIC POTENTIAL OF THE CRYSTAL

The calculation of adsorption energies in zeolites in the cluster approximation has the advantage of a reduced computational effort compared to that of the periodic approach. However, in the cluster approximation, the long-range electrostatic effects of the infinite crystal are ignored and there are boundary effects. In order to remove apart of the disadvantages of the cluster approach, we developed a method to embed a cluster in, a zeolite crystal by imposing an electrostatic potential on it. This potential adds the long-range electrostatic effects of the crystal and subtracts the electrostatic potential of the boundary of the cluster. It is calculated from the charge distribution of the crystal obtained with an ab initio calculation using the CRYSTAL program. We calculated the adsorption energies of NH3 and NH4+ on three different clusters embedded in the potential of a chabazite crystal, and we have compared them with the adsorption energies of NH3 and NH4+ in the crystal. If a cluster is used that has dangling bonds four, or more, bonds away from the adsorbate, i.e., the adsorption site is described well covalently, embedding the cluster reduces the error in adsorption energies (relative to the crystal calculations) from approximate to 20 to approximate to 2-3 kJ/mol.