Comparative Study of the Characteristics and Activities of Pd/γ-Al2O3 Catalysts Prepared by Vortex and Incipient Wetness Methods

Catalytic Oxidation of Methane

Methane (the major component of natural gas) is one of the main energy sources for gas-powered turbines for power generation, and transport vehicles [...]

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

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.

Pd4S/SiO2: A Sulfur-Tolerant Palladium Catalyst for Catalytic Complete Oxidation of Methane

Sulfur species (e.g. H2S or SO2) are the natural enemies of most metal catalysts, especiallypalladium catalysts. The previously reported methods of improving sulfur-tolerance were toeffectively defer the deactivation of palladium catalysts, but could not prevent PdO and carrierinteraction between sulfur species. In this report, novel sulfur-tolerant SiO2 supported Pd4Scatalysts (5 wt. % Pd loading) were prepared by H2S–H2 aqueous bubble method and applied tocatalytic complete oxidation of methane. The catalysts were characterization by X-ray diffraction,Transmission electron microscopy, X-ray photoelectron Spectroscopy, temperature-programmedoxidation, and temperature-programmed desorption techniques under identical conditions. Thestructural characterization revealed that Pd4S and metallic Pd0 were found on the surface of freshlyprepared catalysts. However, Pd4S remained stable while most of metallic Pd0 was converted toPdO during the oxidation reaction. When coexisting with PdO, Pd4S not only protected PdO fromsulfur poisoning, but also determined the catalytic activity. Moreover, the content of Pd4S could beadjusted by changing H2S concentration of H2S–H2 mixture. When H2S concentration was 7 %, thePd4S/SiO2 catalyst was effective in converting 96% of methane at the 400 °C and also exhibitedlong-term stability in the presence of 200 ppm H2S. A Pd4S/SiO2 catalyst that possesses excellentsulfur-tolerance, oxidation stability, and catalytic activity has been developed for catalyticcomplete oxidation of methane.