High entropy oxides (HEO) represent an attractive family of catalysts that offer properties surpassing those of the constituent materials. These oxides are highly disordered with multiple components, based around a simple crystal structure. Their unique sluggish kinetics, high lattice distortions, variable oxygen vacancies, and tunable local electronic structures provide properties critical to catalysts’ durability and selectivity. The exploration of HEO nanoparticles as catalysts is just beginning. Recent reports demonstrate their high potential for applications as both catalysts and supports for reactions such as CO oxidation, CO<sub>2</sub> hydrogenation, and oxidative desulfurization. However, progress in this field is limited, primarily due to the lack of guiding design principles, which is substantially hindered by the unpredictable and complex nature of the local atomic and chemical configurations - details that are difficult to analyze by theoretical calculations or probe by most characterization tools. As a result, little is known even about the chemical distributions at the surface and sub-surface regions - information critical to the understanding of active sites and the optimization of catalyst structures for specific reactions. Here, we elucidate the detailed lattice occupancy of each element at the surface and subsurface of a model HEO catalyst (CeLaTiYHfZr)O<sub>x</sub>, by combining a suite of complementary scanning transmission electron microscopy (STEM) techniques with sophisticated data analysis. Not only the atomic site distribution of cations but also that of oxygen vacancies are revealed, allowing us to elucidate the responses of catalyst surfaces to redox environments. We focus on the surface elemental diffusion, the structural stability, and the regulation of oxygen vacancy concentrations at the atomic scale under oxidation, neutral, and reduction atmospheres as a function of temperature. Our results provide valuable structural information to theoretical calculations, initial insights into the working mechanism of HEO catalysts, and guidance for exploring their specific catalytic applications.

Atom-by-atom elucidation of lattice occupancy and redox dynamics of high entropy oxide (HEO) catalysts

ACS Spring 2022

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