DFT study of formation and migration of oxygen vacancies in mixed oxides: Ce1-xZrxO2 and Ni-Ce1-xZrxO2

F. PRADO; P. LUSTEMBERG; G. S. OTERO; M. V. GANDUGLIA-PIROVANO; G. ZIMICZ

Santa Fe

Conferencia; VI San Luis Conference on Surfaces, Interfaces and Catalysis; 2018

The mixed oxides of the CeO2-ZrO2 system and the NiO + (Ce, Zr, Ni) O2 composite are of interest due to their catalytic role in the production of H2, reforming and partial oxidation of hydrocarbons. Doping ceria with metals like Zr has been shown to improve redox properties and thermal stability of the oxide. A technological application of this kind of materials is that of intermediate temperature solid oxide fuel cells (IT-SOFCs), specially on the fuel oxidation electrode (anode). Previous results indicate that in the CeO2-ZrO2 solid solution, even small Zr concentrations (less than 10%) decrease the bulk reduction energy values compared to that of pure ceriai.In order to understand the reasons of this enhancement, we have performed a systematic DFT+Uii (density functional theory plus Hubbard correction parameter) analysis of both, the bulk and the (111) surface of Ce1-xZrxO2 oxide with the VASPiii,iv code. In a first step, we have optimized the position of the Zr atoms. For the Ce1-xZrxO2(111) surface, we have found that the most stable structure for a single dopant is within the second OCe1-xZrxO trilayer, and that a second dopant also prefers that trilayer. In addition, the results do not show an energetic preference for a specific distance between the Zr atoms, neither at the surface nor in the bulk, indicating a random dopant distribution. To shed some light on the processes involved in the oxide reduction, we have calculated oxygen vacancy formation energies and analyzed vacancy migration pathways. This systematic study has been repeated for the Ni- Ce1-xZrxO2 system in the limit of small Ni concentration (less than 2%) with the aim of understanding the effects of the presence of Ni in the solid solution or at the surface on the atomic and electronic properties of these mixed oxides.