Abstract:
Effect of regeneration of spent catalyst on the properties of Pd/Ag/Al₂O₃ catalysts in selective hydrogenation of acetylene has been studied. Enhancement of the catalyst activity and ethylene selectivity during C₂H₂ hydrogenation by N₂O pretreatment found only for fresh catalysts. Bulk and surface properties of the catalysts have been characterized by various techniques i.e., XRD, CO-adsorption, TPD, TPO, and XPS. The amounts of coke deposited on catalyst surface as well as oxygen concentration used in regeneration do not affect the number of metal active sites of catalysts recovered after regeneration. However, change in geometry/ morphology of metal on surface has been found. From the XRD results, palladium particle size (ca. 6 nm) was not changed after regeneration but a decrease in particle size of silver has been found since the Tamman temperature of silver is below regeneration temperature. Results from CO-adsorption indicate increasing of metal active sites of Pd-Ag catalyst after N2O pretreatment was found only in Pd-Ag catalyst prepared by sequential-impregnation. Surface analysis by XPS has confirmed the existence of surface Pd-Ag alloy with Ag enrichment after reduction. Significant shift of the Ag 3d binding energy is revealed after N₂O pretreatment. Pretreatment with N₂O create a new site for ethylene adsorption and it is postulated to be the origin of enhancement in ethylene selectivity. Significant decrease in amount of ethylene adsorbed on regenerated catalyst surface has also observed. The results from these analytical techniques reveal that changes in the surface properties of Pd-Ag catalysts during regeneration made the catalysts not able to be activated by N₂O pretreatment.
