Properties of Nb-modified PdSio₂ catalyst for selective hydrogenation of acetylene

Abstract

Highly dispersed Pd/SiO2 were prepared by ion exchange method and modified with Niobium oxide added to the catalysts by excess-solution impregnation. All the samples were calcined at 300℃ for 2 hours, and subsequently reduced in H2 at temperatures between 300℃ and 500℃ before being used for the reaction. The catalytic performances of Pd/SiO2 and Pd-Nb/SiO2 (Nb-modified Pd catalysts) were investigated by monitoring their activity and selectivity for selective hydrogenation of acetylene. And their surface was characterized by using H2- chemisorption, Temperature-Programmed Desorption (TPD), X-ray Photoelectron Spectroscopy (XPS) and Thermo Gravimetric Analysis (TGA). Pd-Nb/SiO2 showed improved selectivity for the conversion of acetylene to ethylene in the presence of excess amount of ethylene, especially when the catalysts were reduced at high temperatures, i.e., 400oC or 500oC. And Pd-Nb/SiO2 also exhibited advanced deactivation behavior after the reduction at 500oC. That is, the catalyst deactivation was retarded when the catalysts were modified with Nb oxide. H2-chemisorption results indicated that the Pd surface was decorated with partially reduced NbOx similarly to the case of Nb2O5-supported catalysts, which is a typical strong metal support interaction (SMSI) phenomenon. Added Nb oxide reduced the adsorption strength of ethylene on the Pd surface by charge transfer from Nb oxide to Pd surface, which was confirmed by XPS and ethylene TPD experiment. TGA and DTGA results showed that the amount of green-oil produced on Pd- Nb/SiO2 is less than that on Pd/SiO2 and the size of green-oil produced on Pd- Nb/SiO2 is smaller than that on Pd/SiO2. The Nb-modified catalysts showed improved ethylene selectivity and deactivation behavior after reduction at high temperatures because the Pd surface was modified geometrically and electronically by the migration of the partially reduced Nb oxide onto the Pd surface.