Effect of Reduction and Oxidation Treatments on Pd/ZnO Catalysts

Abstract

The effect of hydrogen reduction and subsequent calcination on Pd/ZnO catalysts was investigated by temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The TPR studies showed that ZnO was reduced by hydrogen spilt over from palladium. This reduction started mildly at room temperature and became vigorous at ca. 700 K. The XRD results revealed that Pd&z.sbnd;Zn alloys were formed on the catalyst after reduction at 700 K. The morphology of the palladium crystallites on the catalysts was found by TEM to be changed with reduction treatment. The shape of these crystallites was globular after hydrogen reduction at ambient temperature. Some local images with a contrast between the ZnO background and the palladium crystallites were observed around the crystallites after reduction treatment. These images were attributed to thin layers of Pd&z.sbnd;Zn alloys formed during the reduction. The area of these images increased with the extent of reduction and decreased on recalcination treatment. During recalcination, the Pd&z.sbnd;Zn alloy was decomposed to palladium and ZnO again. Simultaneously, the regenerated palladium crystallites were reaggregated and assumed their original global shape.