Interfaces in MOF-Derived CeO2–MnOX Composites as High-Activity Catalysts for Toluene Oxidation: Monolayer Dispersion Threshold

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Abstract
A series of CeO2–MnOX catalysts with different Ce contents was prepared using Mn–BTC MOF as a sacrificial template for toluene oxidation. Interestingly, the performance of CeO2–MnOX increased rapidly only when the Ce content lower than 5%. The 1%-, 3%- and 10%-Ce-content samples exhibited the T90 value of 325 °C, 291 °C and 277 °C, respectively. XRD shows that the catalyst phase changes significantly before (Mn3O4 only) and after (Mn2O3, Mn3O4 and CeO2) 3% Ce loading. All other results indicated that the Ce–Mn interface properties of different Ce content composite oxides was quite distinguishable in terms of removal and energy efficiency. XRD and XPS results further showed that there a Ce monolayer dispersion threshold existed on the interface of MnOX (3.2 wt%, confirmed by XPS), which caused the difference in performance increment. The dispersed Ce could be divided into a monolayer dispersion state (1–3%) and a crystalline phase state (>3%), according to the existence form, which corresponded to the significant and minor enhancements of toluene conversion rate. Importantly, the Ce in monolayer dispersion state obviously improved the redox properties of catalysts interface, while the Ce in crystal state not. The interfaces with monolayer dispersion Ce result in more abundant metal ion states, oxygen vacancies, better electron transfer performance and catalytic activity.

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