Elucidating the origin mechanism of a morphology-dependent layered double hydroxide catalyst toward organic contaminant oxidation via persulfate activation.

Understanding how the morphology of a layered double hydroxide (LDH)-based catalyst alters its catalytic activity provides an available strategy for the rational design and fabrication of high-efficiency catalysts at a micro-scale. Herein, three nickel-iron layered double hydroxide (NiFe-LDH) catalysts including 2D-plate-like hexagon (P-NiFe-LDH), 2D/3D-flower-like solid sphere (FS-NiFe-LDH), and 2D/3D-flower-like hollow sphere (FH-NiFe-LDH) with regulable oxygen vacancies (OVs) were fabricated via a morphological regulation method of Ostwald ripening. The experimental results demonstrated that the three types of NiFe-LDH exhibited different abilities to activate persulfate (PS) for the abatement of acid orange 7 (AO7) with a sequence of FH-NiFe-LDH > FS-NiFe-LDH > P-NiFe-LDH. Particularly, the FH-NiFe-LDH with a hollow structure exhibited the most considerable activity with the first-order rate constant up to k = 0.02639 min-1, benefiting from the highly accessible surface areas, higher intrinsic activity of the exposed crystal planes, and abundant OVs. Characterizations further confirmed that these properties could profoundly allow for more exposure of active sites and enhance the reactivity of OV-connected Ni or Fe to facilitate electron transfer and generate more reactive radicals, therefore elucidating the morphologic origin of catalytic performance. Based on the quenching experiments, sulfate radicals (SO4·-), hydroxyl radicals (·OH), and oxygen radicals (O2·-) were identified to be involved in the decomposition process. Furthermore, the continuous redox cycle of Ni(II)/Ni(III)/Ni(II) and Fe(II)/Fe(III)/Fe(II) was responsible for the generation of active radicals via activating PS.