Surface characterisation of Pd–Ag/Al 2O 3 catalysts for acetylene hydrogenation using an improved XPS procedure

Abstract

Effects of pretreatment on the surface of alumina-supported Pd–Ag catalysts with oxygen or oxygen-containing compounds (NO, N 2O, CO and CO 2) have been studied using an improved X-ray photoelectron spectroscopy (XPS). Surface analysis was performed either before or after the selective hydrogenation of acetylene. Analysis of the surface after reduction shows evidence of a Pd–Ag alloy. The binding energy of the Pd 3d is not affected by pretreatment, whereas a significant shift of the Ag 3d is revealed after NO and N 2O pretreatment. The surface after reaction shows no state change of either Pd or Ag compared to those measured prior to reaction, which is in agreement with the reactivity test; therefore surface modification occurs after pretreatment and is retained even after 8 h on stream. No carbonaceous deposits are formed after 8 h on stream. Ethylene gain enhancement by NO and N 2O pretreatment is a result of strong adsorption on the surface which may block the sites responsible for ethylene hydrogenation without facilitating carbonaceous deposits for hydrogen spillover. On the other hand, pretreatment with O 2, CO or CO 2 increases the Pd active sites, which increases C 2H 2 hydrogenation activity.