First Principles Studies of the Surface Chemistry of NiFe2O4 and NixCo3-xO4 Spinel

Abstract

Nickel cobaltite, NiCo2O4, and nickel ferrite, NiFe2O4, are spinel oxides with interesting catalytic properties. Nickel cobaltite oxidizes carbon monoxide and methane, while nickel ferrite is an electrocatalyst for water oxidation. These materials have been recently the focus of intense research aimed at modifying their activities and improving their performances. This thesis describes our theoretical studies of the structural and electronic properties of nickel cobaltite and nickel ferrite, their surfaces, and their interactions with probe molecules.

The inverse spinel nickel cobaltite is a promising technological material with complex electronic and magnetic properties. Understanding these properties is important for the development of novel electronic devices and as a basis for the study of their surface and catalytic properties. We have investigated the bulk electronic and magnetic properties of nickel cobaltite using Density Functional Theory (DFT) calculations augmented with on-site Hubbard U repulsion on 3d electrons (DFT+U). Starting from an analysis of nickel doped cobalt oxides, we found that nickel acts as a p-type dopant in Co3O4. NiCo2O4 has a ferrimagnetic half-metallic ground state with fractional valence on Ni and Co cations at tetrahedral sites (Td), caused by the partial occupancy of Ni and Co(Td)’s eg states. We also determined the formation energies of two relevant defects, namely NiCo(Td) exchanges and oxygen vacancies, as a function of the values of the U terms. Facile NiCo(Td) exchange, as observed experimentally, was obtained using U values that are significantly smaller than those predicted by linear response theory. Our computed bulk O-vacancy formation energies suggest that NiCo2O4 is an active oxidant similar to Co3O4.

We next extend our study to NiCo2O4 (NCO) surfaces, focusing on the structure, defects and reactivity of (001) surfaces. Our results suggest that the formation of surface oxygen vacancies (VO) on the NCO (001) surface is strongly affected by the neighboring cation in the 3rd layer. In particular, Ni in the 3rd layer significantly reduces the VO formation energy. As a result, VO formation is generally much easier on NCO (001) than on Co3O4 (001) surfaces, suggesting that NCO may be a better catalyst than Co3O4 for oxidation reactions based on the Mars Van Krevelen mechanism. VOs on reduced NCO surfaces can be healed through dissociative water adsorption at room temperature. In contrast, adsorption of molecular oxygen at VOs is energetically unfavorable under ambient conditions, suggesting that O2 adsorption may represent the thermodynamic limiting step for oxidation reactions on NCO (001) surfaces.

We again use DFT+U calculations to investigate the mechanism of the low temperature CO oxidation reaction (COOR) on Co3O4(110)/(001) and NiCo2O4(001) as well as methane oxidation on NiCo2O4(001). Our results indicate that the COOR is controlled by the thermodynamics of surface re-oxidation on (001) surfaces and by the kinetic barrier for CO2 formation on the on Co3O4 (110) surface. The COOR is inhibited by water adsorption at surface oxygen vacancies. For methane oxidation, the computed barrier of the first C-H bond agrees well with experimental observations.

Nickel ferrite, NiFe2O4, is another spinel oxide with interesting properties and applications, particularly as a catalyst for water oxidation. We have used DFT+U calculations to study the structure, electronic properties, and energetics of the NiFe2O4(001) surface and its interaction with water both in the absence and in the presence of surface oxygen vacancies. In a humid environment, water adsorbs dissociatively on the surface oxygen vacancies leading to the formation of surface hydroxyls. At high temperature, water desorbs leaving a surface containing oxygen vacancies. These defects could represent useful reactive sites for various catalytic reactions. CO and methane oxidation on NiFe2O4 are slightly less favorable in comparison to NiCo2O4, even though the reaction pathways are similar.