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.