Periodic DFT modeling of bulk and surface properties of MgCl2

Abstract

MgCl(2) is the preferred support for the industrial Ziegler-Natta catalysts, and is believed to act as a template for the epitactic chemisorption of the active Ti species. As the first step of a thorough computational modeling of these systems, we studied the bulk and surface structure of the ordered alpha and beta phases of MgCl(2) by means of periodic DFT (B3LYP) methods using localized basis sets. The layer structure of both phases was reproduced satisfactorily with the inclusion of a (small) empirical dispersion correction ("DFT-D") as a practical method to describe the attraction between the layers. Surface models were studied on slabs with adequate thickness. It appears that various surfaces exposing 5-coordinated Mg are very similar in energy and are the lowest non-trivial surfaces. Cuts exposing 4-coordinated Mg are significantly less stable; both kinetic and equilibrium models of crystal growth indicate that they should normally not be formed to a significant extent. "Nano-ribbons" of single, flat chains of MgCl(2), sometimes proposed as components of the disordered delta phase, were also evaluated, but are predicted to be unstable to rearrangement. Implications for the role of MgCl(2) as catalyst support are discussed.