Selective hydrogenation of acetylene on TiO₂-added Pd catalysts

The performance of TiO₂-modified Pd catalysts, containing TiO₂ either as an additive or as a support, in the selective hydrogenation of acetylene was investigated using a steady-state reaction test. The catalyst surface was characterized by H₂ and CO chemisorption, infrared, X-ray photoelectron spectroscopy (XPS), and the temperature-programmed desorption (TPD) of ethylene. The TiO₂-added Pd catalyst reduced at 500°C, denoted in this study Pd-Ti/SiO₂/500°C, showed a higher selectivity for ethylene production than either the Pd/TiO₂ or Pd/SiO₂ catalyst. The amounts of chemisorbed H₂ and CO were significantly reduced and, in particular, the adsorption of multiply coordinated CO species was suppressed on Pd-Ti/SiO₂/500°C, which is characteristic of the well-known strong-metal-support-interaction (SMSI) phenomenon that has been observed with the TiO₂-supported Pd catalyst reduced at 500°C, Pd/TiO₂/500°C. XPS analyses of Pd-Ti/SiO₂/500°C suggested an electronic modification of Pd by TiO₂, and the TPD of ethylene from the catalyst showed the weakening in ethylene adsorption on the Pd surface. 1,3-Butadiene was produced in smaller amounts when using Pd-Ti/SiO₂/500°C than when using Pd/SiO₂/500°C, indicating that the polymerization of C₂ species leading to catalyst deactivation proceeds at slower rates on the former catalyst than on the latter. The enhanced ethylene selectivity on Pd-Ti/SiO₂/500°C may be explained by correlating the catalyst surface properties with the mechanism of acetylene hydrogenation.