Adsorption energies of NH3 and NH+4 in zeolites corrected for the long-range electrostatic potential of the crystal

Abstract

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 a part 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 NH, and NH: on three different clusters embedded in the potential of a chabazite crystal, and we have compared them with the adsorption energies of NH3 and NH: 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 -20 to -2-3 kJ/mol.

Quantum Chemical Investigations of Structural Parameters of PVDF-based Organic Ferroelectric Materials

“…Within the QCC methods there are different approximations levels, i.e., the Moller-Plesset (MP2) and Configuration Interaction (CI), which are some of the most accurate and involve electronic correlation. The required time for running a calculation is about N 5 , where N is the basis set employed for the atomic orbital description (Teunissen, 1994). Other approximations such as the Hartree-Fock (HF) (N 4 ) and the Density Functional (DFT) (N 3 ) level of theory require shorter computing time (Sierra, 1993;Evleth et al, 1994) with the possibility of applying them to a larger system size, having less accuracy but similar phenomena description to those of MP2 and CI, (further details are reported in Das & Whittenburg in 1999).…”

“…Quantum mechanics (QM) methods, which we also denote as QCC (Quantum Chemical Calculations) can provide valuable information about the electronic and structural phase transitions for several polymer backbones giving an insight into structure-property relationships, which are not yet completely elucidated (Broahurst et al 1978;Jungnickel, 1999;Tashiro et al, 1988;Casalini & Roland, 2001;Gao & Scheinbeim, 2000;Casalini & Roland, 2002;Nicholas, 1997;Nicholas et al, 1991;Gómez-Zaravaglia & Fausto, 2004;Teunissen et al, 1994;Sierra, 1993;Evleth et al, 1994;Das & Whittenburg, 1999). Therefore, the first principles-based materials models are helpful for the design of materials with improved properties.…”

Computational-chemical advances in heterogeneous catalysis

“…Whereas trends can be analysed by variation of the Si-H bonds that terminate the clusters, large cluster calculation are needed to solve this issue conclusively. According to our estimates [ 11 ] and that of others [ 12 ] clusters of at least 16 are needed to resolve this issue conclusively.…”

“…Large embedded clusters are needed to satisfactorily study changes in composition. The long range electrostatic effects can be accounted for by embedding the cluster in the electrostatic potentials as computed by the use of program as the CRYSTAL [ 11 ] routine.…”

Assessment of Molecular Modeling & Simulation

“…A widely accepted approach involves termination with hydroxyl groups, whereby the hydrogens are fixed at lattice positions that anchor the cluster to the actual zeolite (van Santen and Kramer 1995;Sauer 1994;Sauer et al 1994;Nicholas 1999;Gonzales, Bell and Chakraborty 1997;Frash and van Santen 1999). The long-range Madelung interactions can be accounted for by introducing electric field effects into the Hamiltonian (Teunissen et al 1994).…”