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 degrees 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 degrees C was significant when the reaction was conducted at low temperatures, i.e. 40 degrees C and 50 degrees C instead of 60 degrees C. Based on the characterization of catalysts by temperature-programmed reduction (TPR), H-2 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 degrees C. La2O3 interacted with I'd 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 degrees C showed the highest ethylene selectivity at a reaction temperature of 60 degrees 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. (c) 2006 Elsevier B.V. All rights reserved.