Sinter Resistance and Activity Enhancement via a Facet-Dependent Metal–Support Interaction of Pd/ZnO Catalysts in CO Oxidation

Abstract

Metal–support interaction (MSI) has been extensively investigated regarding its structural and catalytic modification in heterogeneous catalysts. Herein, we reveal how facet-dependent MSI impacts on the structure and performance of Pd/ZnO in the CO oxidation reaction. ZnO nanosheets with orthohexagonal shape which are exposed mainly with {001} facets exhibit remarkable thermal stability under air atmosphere until 600 °C, while ZnO nanorods bounded with {100} facets transform at 400 °C and totally change after calcination at 600 °C. Thus, the Pd species supported on ZnO nanorods and nanosheets also display different sintering behaviors after thermal treatment, and the particle size of Pd grows from 1.1 to 5.7 nm on {100} facets while it keeps high-dispersion on {001} facets after 600 °C calcination. In situ transmission electron microscopy was used to visualize the sintering process, demonstrating that Pd displays the Ostwald ripening and particle migration coalescence mechanism on ZnO {100} facets, while shows extraordinary stability on ZnO {001} facets. Therefore, Pd supported on ZnO nanorod exhibits deactivation behavior after high temperature treatment in CO oxidation, during which the T50 delays from 106 to 120 and 145 °C after 200, 400, and 600 °C calcination. Interestingly, Pd on ZnO nanosheet shows obvious activity enhancement from 137 to 90 and 75 °C of T50 after the same treatment, which is ascribed to the synergy between Pd and ZnO {001} facets during catalysis. This work gives in-depth understanding of the facet-dependent MSI of Pd/ZnO catalysts, and it sheds light on the rational design of heterogeneous catalysts with high efficiency and sinter resistance.