Unveiling the structure and reactivity of rare earth oxides and gold catalysts through density functional theory

Abstract

In the field of heterogeneous catalysis and electrocatalysis, rare earth oxides (REOs) are increasingly gaining attention as a relatively unexplored group of materials, while nanoscale gold catalysts have demonstrated significant potential in low-temperature applications. These two kinds of catalyst systems can not only act as the active components for the catalysis reactions separately, but can also be combined into one metal-oxide hybridized system. Considering the complexity of mixed metal-oxide systems, it is essential to first study the surface properties and reactivity of individual components, such as rare earth oxides and nanostructured gold. Density functional theory (DFT) enables the modelling of REOs and gold, and provides a deeper microscopic understanding of their catalytic properties. However, a reliable and universal computational approach is required for modelling their properties, particularly REOs due to the strong correlation of the f-orbital electrons and the lack of systematic studies. Therefore, the first step of this work was to systematically assess the performance of the currently-available methods, here applying PBE+U and HSE06 in describing the structural parameters and energetic properties of the selected hexagonal lanthanide sesquioxides and the cubic fluorite-type cerium dioxide. Several representative surfaces were constructed to evaluate the transferability of the obtained optimal parameter U for bulk REOs to simulate surface properties, such as the electronic structure and adsorption properties, comparing the PBE+U level to the hybrid HSE06 level. After determining the optimal method for describing bulk REOs and surfaces, we further selected the La2O3(0001) surface to investigate the influence of the selected s-block, d-block and f-block heteroatoms on the surface Lewis acidity/basicity and surface reducibility through replacing the surface La atom by the dopants. Finally, we focused on the gold catalysts and elucidated the possibility of self-organized oxygen species formed on the selected Au(221) surface, followed by the investigation of the reactivity for CO and O2 on the surface. The main results of our work can be summarized as follows. The HSE06 hybrid functional has been found to accurately reproduce lattice parameters and certain energetic properties compared to experimental values. The PBE+U method can only reproduce the results of HSE06 or experimental values if the U parameter is selected from an appropriate range of values. Typically, the U parameter values must be adjusted for different Ln2O3 (denoting lanthanide sesquioxides) or CeO2 to accurately describe different properties. Most bulk oxide structural parameters and reaction energies can be accurately described by PBE+U with a relatively small U parameter. However, a larger U value is needed to simulate surface electronic properties of Ln2O3 or CeO2, such as 4f electron localization. For the La2O3(0001) surface, a distinct linear relationship was found between the surface reducibility, as measured by oxygen vacancy formation energy and the band center of the unoccupied La5d states or the occupied O2p states of the surface La or O atoms. It was noted that the formation of an oxygen vacancy shifts the La5d and O2p band centers downwards with respect to the Fermi level, which can be offset to various degrees by low-valence s-block and d-block dopants (Cu or Ni), but not so much by the high-valence d-block dopants and f-block dopants chemically similar to La atoms. The d- or p-band center relative to the Fermi level is more indicative of the surface reducibility, compared to the Lewis acidity/basicity measured by adsorption energies of molecules with Lewis basic/acidic properties. The Lewis acidity of the surface La atoms does not change significantly upon doping, yet the Lewis basicity of O atoms varies slightly. In a study of the role of self-organized surface oxygen structures for gold catalysts, we constructed a double oxygen (O) chain along the step edge on the Au(221) surface, with or without oxygen vacancies that may form on gold-based catalysts under practical catalytic conditions. Accompanied by an increased adsorption strength of CO and O2 on the oxygen chain with vacancies, CO and O2 also become more activated. The dissociation of O2 has a factor of two lower activation energy compared to the regular Au(221) surface. We considered two types of mechanisms for CO oxidation: dissociative and associative. Both mechanisms may compete on the O chain with two adjacent vacancies depending on the given thermodynamic conditions when CO and O2 co-adsorb on the chain or the step edge on the Au(221) surface. A more favorable pathway can be found with the initial co-adsorption state of CO* at a terrace site and O2* adsorbed at a vacancy site.