Controlling NO x Reduction Pathways of Rh Catalysts with Oxygen-Vacancy-Rich ZnO 1– x Nanoparticles Derived from a Metal–Organic Framework

Surface and lattice vacancies in crystal structures play a critical role in improving properties, but synthesizing vacancy-rich crystals remains a significant challenge. In this research, we proposed a novel method to synthesize ZnO_1-x nanoparticles (NPs) with rich oxygen vacancies (V_O), by thermally decomposing a H₂DAB-Zn₂(ox)₃ metal-organic framework. The high-concentration V_O in ZnO_1-x NPs were confirmed by X-ray powder diffraction, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and other measurements. The lattice expansion was observed for ZnO_1-x NPs, resulting from electrostatic repulsion between Zn²⁺ and positive V_O. As the support for NO_x reduction, the homogeneously dispersed PGM/ZnO_1-x (PGM = Ru, Rh, Pd, Ir, and Pt) catalysts were prepared and exhibited remarkable improvement in activities compared with the corresponding PGM/ZnO. The activity enhancement of Rh/ZnO_1-x may be attributed to strong CO adsorption and fast NO dissociation induced by the abundant V_O, resulting in a short reaction pathway distinct from Rh/ZnO.