MOF-Derived High-Entropy Oxide: A “Catalytic Engine” That Ignites the HC-SCR Reaction

Addressing the challenge of NOx pollution control in exhaust emissions from stationary and mobile sources, this study employed the ultrasonic impregnation method to synthesize high entropy-metal organic framework (HE-MOF) precursors loaded with transition metals (Fe/Co/Ni/Cu/Mn) in a one-step process. The reason for selecting these five metals stems from their known individual performances in the SCR reaction: Mn can efficiently oxidize NO to NO2 by virtue of its abundant valence states, while Co assists in NO adsorption and the generation of nitrate species; Cu (especially Cu+) can break the C–H bonds of C3H6 to form active intermediates, and Fe reduces the deep oxidation of the reducing agent to improve its utilization rate; Ni not only stabilizes the structure of the MOF precursor, but also enhances catalytic performance through electron interaction with other metals via valence cycling (Ni3+/Ni2+). Subsequently, these precursors were calcined at 350 °C to fabricate a series of catalysts, which show great potential for application in the field of denitrification. C3H6–SCR tests in a fixed-bed microreactor revealed that the SCR reactivities of the catalysts were improved gradually with the increase in the number of supported metal species. Notably, HEO-350 catalyst, maintained over 83% NO removal within the broad temperature window of 225–300 °C, with the highest NO conversion rate of 93.2% at 250 °C, and exhibited superior N2 selectivity of higher than 95%. The HEO-350 catalyst exhibits strong resistance to both SO2 and H2O. In the stability test with simultaneous introduction of 0.02 vol % SO2 and 10 vol % H2O, the NO conversion rate of HEO-350 reached 93.6% in the middle stage of the experiment. XRD and FE-SEM characterizations confirmed the successful synthesis of the precursors; XPS analysis revealed strong synergistic effects among the five metals, while H2-TPR further indicated that the synergy effect significantly enhanced the redox capacity of the catalyst. BET results showed that HEO-350 had a specific surface area of 161 m2 · g–1, much higher than that of Ni-350; the large specific surface area also endows it with significant application potential in photocatalysis and other catalytic fields. Py-FTIR characterization proved that there were abundant acid sites on the surface of HEO-350, with the total acid amount of Lewis acid and Brønsted acid reaches 80.24 μ mol/g at the optimal activity temperature, which was attributed to the lattice distortion effect of high-entropy materials. In-situ DRIFTS tests identified carboxylates and nitrates as key reaction intermediates, and a synergistic reaction mechanism on the surface of HEO-350 was proposed by integrating the characterization results.