Ab-Initio Calculation of the Charge Topology of the Oxygen Active Site of the (001) Surface of Pure and Doped MgO

Abstract:

An ab-initio calculation of the charge density g(r) of the O site in the (001) surface of pure MgO and that doped with Li, Na, and K and with a MgI+ vacancy is reported. An embedded cluster (Mg4XOs with X = Mg, Mg+vacancy, Li+, Na+, and K+) in a set of point charges that simulated the rest of the crystal was employed in the calculation. The results obtained with the embedded cluster methods for the O sites were shown to be within 2% of those calculated in slabs composed by one to three layers of pure MgO with an ab-initio method for periodic structures. The Laplacian of the charge density, -▽2ϱ(r), of the O valence shell obtained with both methods showed that, in pure and perfect MgO, the o2- ion has a local maximum in -▽ϱ(r) in a direction perpendicular to the (001) surface that does not favor the H abstraction from methane. It was also found with the embedded cluster method that doping with alkali metal ions and the Mg+ vacancy formation produced important changes in the atomic graphs of the O ion present at the surface. These changes were such that the local minima in ϱ(r) of the O valence shell were produced along the O—metal bond direction. These local minima provide the sites where the electron-rich C—H bond of methane is attracted and the H abstraction occurs. The more pronounced minimum corresponded to the Li doping, decreasing when passing to Na and K and when a Mg+ vacancy was created below the O site. This trend reproduces the known reaction barriers and supports the relationship between the reactivity of the MgO surfaces and the characteristics of the topology of ϱ(r) of the valence shell of the O- ion. © 1994, American Chemical Society. All rights reserved.

Año de publicación:

1994

Keywords:

Fuente:

scopus