A novel core-shell structured Fe@CeO2-ZIF-8 catalyst for the reduction of NO by CO

Abstract

Zeolitic imidazolate framework-8 (ZIF-8) based catalysts are considered as promising candidates for the selective catalytic reduction of NO by CO (CO-SCR) reaction due to their large specific surface area. However, the structure of ZIF-8 will be damaged during the synthesis procedure of the catalyst, leading to a decrease in their physical properties, which in turn affects the catalytic performance of the catalyst. Herein, a CeO2-ZIF-8 core-shell structure was synthesized and doped with Fe, resulting in the formation of Fe@CeO2-ZIF-8, for CO-SCR reaction. Characterization results show that the Fe@CeO2-ZIF-8 core-shell catalyst has better morphology, thermal stability, and higher specific surface area than the FeCe co-impregnated ZIF-8 catalyst (FeCe@ZIF-8 (IM)). Moreover, the reducibility of the reactive Fe species in the Fe@CeO2-ZIF-8 core-shell catalyst is also enhanced. As a result, the Fe@CeO2-ZIF-8 core-shell catalyst exhibited higher low-temperature CO-SCR catalytic activity with its NO conversion reaching 99% at 250 °C versus the FeCe@ZIF-8 (IM) catalyst (89% at 250 °C). The experimental results were further verified by density functional theory (DFT) calculations including Electron Localization Function (ELF), Valence Charge Density Difference (VCDD), Projected Density of States (PDOS) and Bader charge analyses. The calculation results revealed that the core-shell configured Fe@CeO2-ZIF-8 catalyst has a stronger active metal-support charge transfer and better capacity for the adsorption of the reactants. The reaction mechanism over Fe@CeO2-ZIF-8 core-shell catalyst was investigated via in situ DRIFTS experiments, and results showed that the nitro compounds and formate species are critical intermediates in this reaction at a higher temperature range (300–350 °C). This work provides a new strategy for the synthesis of ZIF-8 based catalysts with better properties for CO-SCR reaction.