Particle Size and PdO–Support Interactions in PdO/CeO2-γ Al2O3 Catalysts and Effect on Methane Combustion

Abstract

In this study, we investigated the effects of sequential impregnation in two PdO/CeO2/Al2O3 nanocatalysts (4Pd-20CeO2/Al2O3 and 20CeO2-4Pd/Al2O₃) on catalytic properties, particle sizes, and metal oxide–support interactions. Pulse chemisorption indicated significantly higher dispersion and smaller particle size in the 20CeO2-4Pd/Al2O₃ catalyst. STEM images of the 4Pd-20CeO2/Al2O₃ catalyst showed PdO nanoparticles on the surface of crystalline Al2O₃. In the 20CeO2-4Pd/Al2O3 catalyst, PdO nanoparticles were strongly embedded on ceria indicating PdO-ceria interactions. Both supports were on separate sites in the two catalysts suggesting weak interactions. PdO particle sizes were 6–12 nm in the 4Pd-20CeO2/Al2O₃ catalyst and 4–8 nm in the 20CeO2-4Pd/Al2O₃ catalyst. Methane conversion was 100% at 275 °C after a 20-min run with the 4Pd-20CeO2/Al2O3 catalyst compared to 25% conversion by the 20CeO2-4Pd/Al2O₃ catalyst under same conditions. The support alumina could stabilize the PdO species and facilitated oxygen migration on the surface and from the bulk in the 4Pd-20CeO2/Al2O3 catalyst. The lower activities in the 20CeO2-4Pd/Al2O₃ catalyst could be due to inaccessibility of PdO active sites at low temperature due to embedment of PdO nanoparticles on ceria. We could infer from our data that sequence of impregnation in catalyst synthesis could significantly influence catalytic properties and methane combustion due to PdO–support interactions.