Effect of chlorine on microstructure and activity of Pd/CeO2catalysts

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Abstract
The evolution of microstructure and activity in benzene hydrogenation of Pd/CeO2 catalysts, prepared from Pd chloride or Pd nitrate, upon reduction at 573–973 K has been studied by X-ray diffraction (XRD), selected-area electron diffraction (SAD), high-resolution transmission electron microscopy (HRTEM) and gas chromatography. The crucial role of chloride from the metal precursor in structural transformations of the ceria support has been established and attributed to its ability to form stable cerium oxychloride at temperatures as low as 573 K. A. rapid decline to zero of the activity of Pd/CeO2 catalysts in benzene hydrogenation was observed when the temperature of reduction was increased from 573 to 773 K. This effect could not be explained by sintering, but XRD data indicated that a metal–support reaction leading to Pd–Ce alloy formation was likely to be the cause. At higher temperature (873 K) the coverage of Pd particles by a thin overlayer (probably CeOx) was observed by HRTEM.

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