Performance of La₂O₃- or Nb₂O₅-added Pd/SiO₂ catalysts in acetylene hydrogenation

Pd catalysts promoted by La₂O₃ and Nb₂O₅, which were selected among metal oxides showing a strong metal-support interaction (SMSI), were tested for acetylene hydrogenation and their kinetic behaviors were compared with those of Pd-only and TiO₂-added catalysts. The La₂O₃-added catalyst showed lower acetylene conversions but a higher ethylene selectivity and slower deactivation rates than the Pd-only catalyst. The improvement in ethylene selectivity and catalyst lifetime was significant, particularly when the catalysts were reduced at a temperature of 500 °C. On the other hand, Nb₂O₅-added catalysts showed higher acetylene conversions as well as both improved ethylene selectivity and catalyst lifetime when compared with the Pd-only catalyst. In this case, the improved selectivity attained by reducing the catalyst at 500 °C was significant when the reaction was conducted at low temperatures, i.e. 40 °C and 50 °C instead of 60 °C. Based on the characterization of catalysts by temperature-programmed reduction (TPR), H₂ chemisorption, and the temperature-programmed desorption (TPD) of ethylene, the La₂O₃ added to the catalyst modified the Pd surface in a manner similar to TiO₂, which interacted strongly with Pd, both geometrically and electronically, after reducing the catalyst at 500 °C. La₂O₃ interacted with Pd more strongly and, consequently improved catalyst performance to a greater extent than TiO₂. Nb₂O₅ interacted with Pd similar to the other two oxides but also showed activity for hydrogenation, which contributed to the beneficial performance of Nb₂O₅-added catalysts: a higher activity for acetylene hydrogenation than that of Pd-only catalysts; remarkably low deactivation rates compared with the other catalysts. Among the three types of catalysts studied, La₂O₃-added catalysts that were reduced at 500 °C showed the highest ethylene selectivity at a reaction temperature of 60 °C. On the other hand, the Nb₂O₅-added catalysts showed the highest activity and longest catalyst lifetime due to the additional activity of Nb oxides for hydrogenation. Consequently, the former catalysts would be advantageous for use at high temperatures and the latter at low temperatures.