The effects of alkali metal ions on the physiochemical and catalytic properties of Pd/NiAlOx catalysts for lean methane oxidation

Abstract

Self-assembled nickel-aluminum mixed oxides (NiAlOx) were prepared by using alkali metal ions as modifiers in a hydrothermal system and employed as the support materials for palladium catalysts. Via tuning the preparation procedure, both the ‘bonded’ and ‘deposited’ alkali metal ions were introduced to the NiAlOx materials for elucidating their effects on the physiochemical and the catalytic properties of Pd/NiAlOx catalysts for lean methane oxidation. Benefiting from the well-dispersed palladium nanoparticles, optimized electronic state of palladium phases, and the favorable structure of host materials, the optimal Pd/NiAl-Na sample exhibited excellent catalytic performance for methane oxidation. Over this sample, complete methane conversion was realized at 401 °C in the presence of 15 vol % water vapor, which was 26 °C lower than that of the Pd/NiAl catalyst (427 °C) at same reaction conditions. Due to the relatively small size of Li+ ions, certain amounts of Li+ incorporated into the NiAlOx lattice and improved the redox properties of NiAl-Li. However, its supported palladium catalysts did not show improved performance in the oxidation of methane. Furthermore, systematic characterizations revealed that the ‘deposited’ alkali metal ions resulted in the agglomeration of palladium nanoparticles and the formation of electron-deficient palladium component, thus exerting negative effect on the activities of palladium phases. It was hoped that this work could provide some fundamental understandings on the roles of alkali metal played in catalytic reaction.