Narrow Results

Nanocrystalline cerium oxide (CeO₂) particles prepared by the novel two-stage precipitation method were used for the catalysis of CO oxidation. Firstly, two shapes, i.e. particulate (P-) and needle-like (N-), CeO₂ nanoparticles were formed via proposed temperature-arranged routes. The crystalline structure, morphology, particle size, and surface area of samples were characterized by using XRD, TEM, HRTEM, and BET techniques. Furthermore, the morphological effect of the CeO₂ samples on the catalytic activity of CO oxidation was investigated. From the experimental results, it indicated that the prepared samples were all nonporous and fcc-structured CeO₂. The CeO₂ particles, as precipitating at 90°C for 5 min and then aging at 90°C, were particulate, whereas they were needle-like by aging at 0°C. The CO oxidation reaction showed that the catalytic activity of N-CeO₂ nanoparticles was higher than that of P-CeO₂, attributing from the exposed higher-energy {100} and {110} facets for N-CeO₂ nanoparticles. Moreover, the calcined samples with higher degree of crystallinity showed further promotion in catalytic activity. It was also worthy to note, that by replacing the CeO₂ catalyst by Pd/CeO₂, a large increase in the CO conversion was found, especially catalyzed by Pd/N-CeO₂.

The effects of pretreating atmospheres (H₂, O₂ and Ar) on the activity for CO oxidation and Pd dispersion of Pd/Pr₆O₁₁ catalyst were investigated. The result shows after the Pd/Pr₆O₁₁ catalyst was pretreated by hydrogen or oxygen at 550°C, its catalytic activity for CO oxidation can obviously increase, and the activity of H₂-pretreated catalyst is superior to that of O₂-pretreated catalyst, which is attributed to an increase of Pd dispersion. The catalyst pretreated by Ar behaves the worst catalytic activity for CO oxidation. The results of CO pulse, CO chemisorption and in situ DRIFT testing indicate that the surface species (hydroxyl species or active oxygen species) generated during pretreating processes can prevent Pd particles from immigrating and sintering, resulting in an increase of the Pd dispersion on the surface of support.