Entropy-Stabilized Isolated Active Pd Species within a High-Entropy Fluorite Oxide Matrix for CO2 Hydrogenation to Formic Acid

Active, stable Pd species were incorporated within the matrix of fluorite-based high-entropy oxide (HEO) (YZrLaGdHf)xO2 using an entropy-driven strategy based on a liquid-phase reaction. In situ X-ray absorption fine structure analyses and transmission electron microscopy observations at elevated temperatures under H2 confirmed that partially aggregated Pd oxide species in the as-synthesized Pd@(YZrLaGdHf)xO2 were transformed into an entropy-stabilized isolated form at 600 °C. In contrast, Pd species on the (YZrLaGdHf)xO2 surface were readily reduced with the formation of nanoclusters/nanoparticles even in the temperature range of 100 to 200 °C. This Pd@(YZrLaGdHf)xO2 exhibited enhanced activity during the hydrogenation of CO2 to formic acid with a maximum turnover number of 5000 based on the quantity of surface-exposed Pd atoms. This performance was more than eight times higher than that of the Pd/(YZrLaGdHf)xO2 on the basis of supported Pd. An assessment of the kinetics and theoretical investigations suggested that entropy-stabilized isolated Pd species facilitated both H2 dissociation and the attack of dissociated H atoms on C atoms in HCO3– ions.