Exploring the dynamic surface features of high and medium entropy oxides

A set of rock-salt oxides was prepared starting from the high entropy oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O (HEO5) by progressively reducing the number of components, while preserving the native rock-salt structure. The obtained medium entropy oxides were Co0.2Ni0.4Cu0.2Zn0.2O (MEO4) and Ni0.6Cu0.2Zn0.2O (MEO3). The performance of the prepared materials towards the selected prototypical probe reaction (CO oxidation) was investigated and correlated with structural and spectroscopic features. The catalytic activity was evaluated over a wide temperature range (50-400 °C). The medium entropy oxide with three cations (MEO3) exhibited the best catalytic performance and became active at low temperature (50 °C). All the materials achieved complete conversion (99.9%) with full selectivity to the desired product (CO2 selectivity 99.9%) at medium-high temperatures (250 °C < T < 300 °C). We demonstrated that the catalyst surfaces could exhibit different local compositions, with metal enrichment and re-organization induced by the experimental conditions, leading to the formation of novel potentially active surface species. Configurational entropy is a stabilizing term, whose decrease enhances the catalytic activity of the materials, promoting a progressive destabilization of Ni(II) in favor of reduced Ni species on MEO3.