Stable Palladium Oxide Clusters Encapsulated in Silicalite-1 for Complete Methane Oxidation

Abstract

Zeolite-supported metal catalysts are widely employed in a number of chemical processes, and the stability of the catalytically active species is one of the most critical factors determining the reaction performance. A good example is the Pd/zeolite catalyst, which provides high activity for methane oxidation but deactivates rapidly under the reaction conditions due to palladium nanoparticle sintering. Although coating the metals with thin shells of porous materials is a promising strategy to address the sintering of metals, it is still challenging to fix small metal particles completely inside zeolite crystals. Here, using an aminebased ligand to stabilize palladium during the zeolite synthesis, we realize the exclusive encapsulation of highly dispersed palladium oxide clusters (1.8−2.8 nm) in the microporous channels and voids of the nanosized silicalite-1 crystals. The synthesis conditions of the zeolite-supported catalyst influence the encapsulation degree and the size distribution of metal particles. Thanks to the encapsulation effect of small palladium oxide clusters, together with the inherent properties of silicalite-1 such as low acidity, high hydrophobicity, and high hydrothermal stability, the optimized Pd@silicalite-1 catalyst outperforms the traditional Pd-based catalysts prepared by wetness impregnation, exhibiting both high activity and better stability in the lean methane oxidation reaction.

Recent Advances in Complete Methane Oxidation using Zeolite‐Supported Metal Nanoparticle Catalysts

“…Several research groups have adopted this synthesis procedure and showed that the encapsulated Pd nanoparticles are highly active for the complete oxidation of methane. [16,39,50,65] Figure 5 shows a schematic illustration of the so-called in situ synthesis using ethylenediamine. The figure also shows how Wang et al reduced their catalyst under H 2 in their original work.…”

“…High-silica zeolites with the MFI framework have recently received much attention, [15,16,39,50,65,80] which may be explained by the issues with water and increasing interest in hydrophobic properties. [18,[27][28][29] For example, silicalite-1 (S-1) is an MFI zeolite that contains no Al and, therefore, has no acidic sites but high hydrophobicity and hydrothermal stability.…”

“…[15] To the best of our knowledge, these are the only reports on this phenomenon, although several groups have studied similar Pd/ZSM-5 [9,13,63] and Pd@silicalite-1 catalysts. [39,50,64,65] Friberg et al [18] investigated the resilience of zeolite-based catalysts towards NO and showed a small advantage compared to alumina supports. Very recently, Suvanto et al [66] also investigated the interaction between SO 2 and NOx (x = 1 or 2).…”

“…Compared to conventional high-surfacearea metal oxides, such as Al 2 O 3 , microporous zeolites offer several advantages, including simple loading through ionexchange, [27] hydrophobic properties, [18,39,50] encapsulation of the active nanoparticles. [27,39,50,65,67,68] and high control of the structure and composition. For instance, Petrov et al recently showed that complete removal of the acidic sites by postexchange with Na + and confinement of the Pd nanoparticles within the zeolite resulted in a highly active and stable catalyst that resisted steam-induced sintering and zeolite degradation.…”

“…Although the combustion reaction occurs under strongly oxidizing conditions, the authors observed the presence of carbonaceous species on both Pd/ZSM‐5 (Si/Al=140) [14] and Pd/silicalite‐1 (no Al) [15] . To the best of our knowledge, these are the only reports on this phenomenon, although several groups have studied similar Pd/ZSM‐5 [9,13,63] and Pd@silicalite‐1 catalysts [39,50,64,65] …”