- Metal-oxides are very important in industrial applications such as thermal barrier coatings, corrosion protection, and heterogeneous catalysis. The wide variety of applications for metal-oxides are derived from the array of properties of metal-oxides. It is, therefore, important to understand and predict the conditions of synthesis and operation to optimize the properties for which metal-oxides are used. Here we use density functional theory (DFT) to study and predict the effects of local conditions such as temperature and oxygen pressure on Pt/γ-Al2O3 interface and the role Cl concentration in the initial stages of the degradation of α-Cr2O3.
The chemical composition of the interface between a metal and an oxide determines the nature of the chemical bonding and has an influence on the stability and strength of the interface, which has important implications for sintering and the size of catalyst clusters. Since catalytic activity and selectivity is impacted by the size of the clusters, it is essential to understand, and predict, the structure and interactions at the Pt/γ-Al2O3 interface as a function of temperature and oxygen partial pressure. Density functional theory was used to study the interfaces between the (111) planes of Pt and three terminations of γ-Al2O3 based on HRTEM analysis of imbedded nanoparticles. The most stable interface was determined by comparing the interfacial energy for the different terminations determined from ab initio thermodynamics as a function of temperature and oxygen pressure. Of the three interfacial terminations, the O terminated interface, which has strong electrostatic interaction between the Pt atoms and the oxygen atoms at the interface, was the most stable at standard conditions (298K , 0.21 atm). In addition to determining the interfacial termination, information which are not accessible experimentally, the calculations provide information about electron structure and chemical bonding at the interface allowing us to predict changes in the interface termination/structure as a function of temperature and oxygen partial pressure. This work, therefore, provides the complement to experimental study of the atomic structure of the interface between γ-Al2O3 and Pt nanoparticles.
We also studied the effects of surface concentration (coverage) of Cl on the degradation of α-Cr2O3 as a model system for the thin, protective, self-repairing metal-oxide layer on stainless steel. The often-debated role of Cl in the initial stages of α-Cr2O3 degradation is general described by two previously proposed models, the ion exchange(IE) and point defect (PD) passive film degradation models, are studied. In order to compare the two models, we calculated and determined the thermodynamic feasibility of the critical steps of Cl enhanced α-Cr2O3 degradation as described in the two models. Both models begin with the substitution of adsorbed OH and H2O by Cl which is favorable at low Cl coverages and becomes less favorable with increasing Cl coverage. In the ion exchange model, the substitution is followed by Cl insertion which is unfavorable at low Cl coverages except in the presence of an O vacancy near the surface but becomes favorable with increasing Cl coverage and exothermic at full coverage. This agrees with experimental studies in which Cl was identified inside the passive film at high Cl concentrations. Likewise, Cr vacancy formation, as proposed in the point defect model, is favorable at higher coverage (<10/12ML) where Cr atoms on the surface are saturated by three Cl atoms. This suggests that the initial stages of the degradation of α-Cr2O3 depend on Cl surface concentration and the presence of O vacancy near the surface. DFT calculations are also performed to determine the energy barrier for the diffusion of Cl through the α-Cr2O3 bulk following the insertion of Cl according to the ion exchange degradation model. The lowest energy barrier of 1.1 eV favors Cl diffusion across the (0001) plane via an unoccupied chromium cation site to the neighboring O vacancy, but this is a relatively high energy barrier compared to what is expected for high diffusivity paths such as, cracks and grain boundaries.
These studies show the capability of DFT calculations to provide detailed atomistic and molecular insights into how variables such as temperature and oxygen pressure effect metal/metal-oxide interfaces and how the oxide degradations process changes with Cl concentration. In the case of Pt/γ-Al2O3, no changes in the chemical composition of the O-terminated interface was found at experimentally accessible temperature and oxygen pressure, albeit showing increasing stability with oxygen pressure. However, the mechanism for the initial stages for the Cl-induced degradation of α-Cr2O3 was found to be dependent on concentration suggesting the influence of local conditions on the degradation process.