Abstract

Pd/Ce0.6Zr0.4Ox/SiO2 and Pd/Ce0.73Tb0.27Ox/SiO2 catalysts were prepared by incipient wetness impregnation method using aqueous solutions of Ce(NO3)(3)center dot 6H(2)O, ZrO(NO3)(2)center dot nH(2)O, Tb(NO3)(3)center dot 5H(2)O and Pd(NO3)(2)center dot nH(2)O precursors. The catalysts were characterized by X-ray diffraction (XRD), Temperature Programmed Reduction (TPR-H-2), CO Chemisorption, Oxygen Storage Capacity (OSC) and Infrared spectroscopy of adsorbed CO (FTIR-CO). The catalytic activity was measured in the methane oxidation reaction. XRD patterns showed that after two activity cycles, the solids crystallinity was not modified, while TPR results revealed that reduction maxima were shifted to lower temperatures, indicating an improvement in sample reducibility. The CO/Pd ratio diminished with increasing the reduction temperature, suggesting both, Pd inward diffusion/covering by Ce3+ ions (geometrical effects) and/or increase in Pd particles size. The IR spectrum of adsorbed CO reported the presence of cationic palladium species interacting with CO regardless of reduction temperature as well as carbonate species and weak interactions with ceria and silica. Zr and Tb containing samples presented a high and stable OSC after undergoing various OSC cycles. The catalytic activity was enhanced after two reaction cycles due to the formation of large PdO particles. T-50 values indicated that reduction treatments prior to methane reaction generated more active catalytic species than those obtained by oxidation treatment; however, such active species were not stable at high temperatures.