Tuning CO selectivity in CO2 hydrogenation over CeO2-based high-entropy oxides: Roles of mechanochemical synthesis, entropy-stabilized phases and structural evolution

High-Entropy Oxides (HEOs) represent a promising class of multifunctional catalysts for direct CO 2 hydrogenation into value-added products. Their capacity to integrate multiple cations into a single lattice, accommodate oxygen vacancies, and stabilize dispersed active sites creates a suitable microenvironment that enhances CO 2 and H 2 reactivity, thereby boosting conversion and selectivity. Herein, a Ce 0.5 (Zn 0.1 Co 0.1 Mg 0.1 Ni 0.1 Cu 0.1 )O x catalyst was successfully synthesized via mechanochemical ball milling. The effects of milling time and calcination temperature (500 and 750 °C) on their structural evolution and catalytic properties were investigated. Physicochemical characterization revealed the formation of a fluorite-type structure with partial incorporation of transition metals into the CeO 2 lattice at 500 °C. Conversely, calcination at 750 °C induced metal exsolution driven by the formation of entropy-stabilized (NiCoCuZnMg)O x domains. These thermally induced structural transformations governed the catalytic performance of both materials. The catalyst calcined at 500 °C (BM-C500) achieved CO 2 conversions of 28–37 % with 75–85 % CO selectivity, whereas the sample calcined at 750 °C (BM-C750) exhibited nearly 100 % CO selectivity across the entire reaction temperature range. Post-reaction characterization revealed the in-situ formation of highly dispersed metallic species (mainly Ni 0 and Cu 0 ) coexisting with a partially preserved oxide matrix, highlighting a key role of dynamic metal–oxide interfacial sites in governing activity and selectivity. Additional experiments using synthesized (NiCoCuZnMg)O x entropy-stabilized oxide (ESO) alone and physically mixed with CeO 2 demonstrated that the catalytic performance cannot be explained by a simple additive effect but seems to arise from the structural integration achieved thanks to the intimated contact allowed by the mechanochemical synthesis of the Ce 0.5 (Zn 0.1 Co 0.1 Mg 0.1 Ni 0.1 Cu 0.1 )O x (BM) catalysts. Long-term stability tests identified one of the samples (BM-4h-C500) as the most robust formulation, sustaining performance with only moderate CO yield attenuation over 100 h on stream. Collectively, the results demonstrate that mechanochemical synthesis is an efficient route for preparing active, highly selective and stable HEOs/CeO 2 catalysts for the reverse Water-Gas Shift (RWGS) reaction.