M. A. San Miguel

Oxygen vacancies on TiO2 (110) from first principles calculations

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.

Experimental and molecular dynamics simulation analysis of LaCrO3 precipitation in chromia scales

Experimental characterization of grain and grain boundaries of chromia scales upon addition of La has been carried out with a variety of techniques. The effect of lanthanum addition on the high temperature oxidation is explained based on simulations suggested by the experimental results. The grain boundary precipitation mechanism of perovskite phases suggested by our simulations is through the extraction of Cr atoms from the oxide scale.

A theoretical study of the NaTiO2 (001) rutile interaction

A theoretical study of the reduction of the TiO2 (001) rutile surface by atomic Na is reported. The study is based on ab initio embedded cluster calculations carried out for two different sites of this surface. The first one (A) is represented by a TiO4−4 cluster and the second (B), an oxygen ending site, by a cluster of formula TiO8−6. The calculations show that adsorption of Na is possible on both sites although site A is energetically favored. The electron transfer process has been examined from state averaged complete active self consistent field (CASSCF) calculations. The analysis of the these wave functions shows that the Na 3s electron is transferred to a Ti 3d orbital of the surface only on site A. For site B such a transfer would involve an excited state of the cluster. The dissociation curves for several electronic states are reported.

An ab initio study of the CH2O adsorption on the MgO (100) surface. Effects of replacing the active Mg2+ ion by different metallic cations

Abstract A theoretical analysis of the interaction between formaldehyde, CH2O, and the MgO (100) surface is reported. Ab initio Hartree-Fock calculations are carried out using an embedded cluster approach in which the environment is described by both total ion potentials and point charges. The cluster selected to describe the CH2O-surface interaction is CH2O[MgO5]8−, in which the formaldehyde oxygen is coordinated to the Mg2+ ion. The effect of the addition of metallic cations to the catalyst is analysed by replacing the cluster Mg2+ ion by Na+, Al3+, Sc3+ and Ti4+ so as to modify the Lewis acid properties of the surface. The different contributions to the adsorbate-surface interaction are analysed, and the theoretical vibrational frequencies are discussed.