Search Results (2)

5 wt% Pd/γ-Al₂O₃ catalysts were prepared by a modified Vortex Method (5-Pd-VM) and Incipient Wetness Method (5-Pd-IWM), and characterized by various techniques (Inductively coupled plasma atomic emission spectroscopy (ICP-AES), N₂-physisorption, pulse CO chemisorption, temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD)) under identical conditions. Both catalysts had similar particle sizes and dispersions; the 5-Pd-VM catalyst had 0.5 wt% more Pd loading (4.6 wt%). The surfaces of both catalysts contained PdO and PdO_x with about 7% more PdO_x in 5-Pd-VM. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning electron microscope (SEM) images indicated presence of PdO/PdO_x nanocrystals (8–10 nm) on the surface of the support. Size distribution by STEM showed presence of smaller nanoparticles (2–5 nm) in 5-Pd-VM. This catalyst was more active in the lower temperature range of 275–325 °C and converted 90% methane at 325 °C. The 5-Pd-VM catalyst was also very stable after 72-hour stability test at 350 °C showing 100% methane conversion, and was relatively resistant to steam deactivation. Hydrogen TPR of 5-Pd-VM gave a reduction peak at 325 °C indicating weaker interactions of the oxidized Pd species with the support. It is hypothesized that smaller particle sizes, uniform particle distribution, and weaker PdO/PdO_x interactions with the support may contribute to the higher activity in 5-Pd-VM. Full article

The search to discover a suitable catalyst for complete combustion of methane at low temperature continues to be an active area of research. We prepared a 5 wt % PdO-PdO_x/γ-Al₂O₃ catalyst by a modified Vortex-assisted Incipient Wetness Method. X-ray Photoelectron Spectroscopy showed that the original catalyst contained PdO (38%) and PdO_x (62%) on the surface and indicated that PdO_x originated from the interaction of PdO with the support. Scanning Transmission Electron Microscopy confirmed the catalyst had an average particle size of 10 nm and was well-dispersed in the support. The catalyst exhibited exceptional low-temperature activities with 90–94% methane conversion at 300–320 °C. The catalyst was active and stable after several catalytic runs with no signs of deactivation by steam in this narrow temperature range. However, the conversion decreased in the temperature range 325–400 °C. The surface composition changed to some extent after the reaction at 325 °C. A tentative mechanism proposes PdO_x (Pd native oxide) as the active phase and migration of oxide ions from the support to PdO and then to PdO_x during the catalytic oxidation. The high methane conversion at low temperature is attributed to the vortex method providing better dispersion, and to catalyst–support interaction producing the active phase of PdO_x. Full article