(765a) Pd/CeO2 Catalyst Derived from One-Pot Generated Pd@Ce-Based Metal-Organic Framework for Efficient CO Oxidation

In heterogeneous catalysis, the development of catalytic materials with highly dispersed active sites is always a hot topic, since the exposed number of active sites without obvious sintering under reaction conditions determines the intrinsic catalytic activity. The normal route to increase the dispersion of active metals is to minimize their size, and then load onto high surface area supports. This process often uses stabilizing agent such as polyvinylpyrrolidone (PVP) to control the metal particle size, which however is difficult to be removed from the particle surface thus inhibiting the catalytic performance to a certain extent [1]. Encapsulation of metal particles inside porous materials is another effective way to generate highly dispersed catalyst, especially inside microporous or mesoporous materials with high surface area [2,3]. Comparing to conventional zeolites and inorganic porous materials, metal-organic frameworks (MOFs) are a class of newly emerged porous materials that possess tunable pore structures and diverse functionalities [4,5], which are promising as good candidates for efficient encapsulation of active metals inside their fine-tuned pore structures. The challenge in this area is to develop a facile fabrication method to in situ generate metals with well controlled size inside MOF pore structures.

Dimethylformamide (DMF) is a commonly used solvent for the preparation of MOFs, which could also serve as a reducing agent and solvent for the synthesis of precious metal particles such as palladium [6,7]. The dual function of this reagent implies the possibility to merge the fabrication of MOFs with the in situ generation of precious metal particles inside their pore structures simultaneously. Herein, we used a solvothermal method for in situ generating palladium clusters inside the pore structures of Ce-BTC (Pd@Ce-BTC). After pre-treatment of Pd@Ce-BTC in different atmospheres (Air or N₂) at varying temperatures, a series of highly dispersed Pd/CeO₂ catalysts have been successfully prepared. For reference, the Ce-BTC MOF was also pretreated with air or N₂ and then used as supports for Pd through DMF reduction method. The samples were characterized systematically using X-ray diffraction (XRD), Raman spectrum, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM/EDS), X-ray photoelectron spectroscopy (XPS), H₂-temperature programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Comparing to the samples pre-treated in air flow, the Pd/CeO₂ catalyst derived from in situ synthesized Pd@Ce-BTC pre-treated in N₂ flow exhibited much better catalytic performance in CO oxidation. The high activity of the optimal Pd/CeO₂ catalyst was closely associated with its rich Pd-CeO₂ interfaces, since the residual carbon species formed under N₂ pre-treatment condition acted as effective physical barriers for isolating the in situ generated Pd-CeO₂ active phases.

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