Abstract
The performance of Pd/SiO2 catalysts modified with La and Nb oxides in the selective hydrogenation of acetylene was investigated. The amounts of H2 chemisorbed on the catalysts were significantly reduced when the catalysts were reduced at high temperatures, e. g., 500°C. This is because the Pd surface is covered with small patches of partially reduced La and Nb oxides after the high temperature reduction, which is the characteristic of a well-known strong metal support interaction (SMSI) phenomenon. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) results indicated that the Pd surface was also modified electronically, such that the adsorption of ethylene to the Pd surface was weakened. Such behaviors of metal oxides affected the ethylene selectivity and lifetime of the catalysts. The ethylene selectivity of Pd-La/SiO2 and Pd-Nb/SiO2 was improved due to the geometric and electronic modifications of Pd by the oxides. Pd-La/SiO2 showed more significant improvement in the ethylene selectivity than Pd-Nb/SiO2. The activity of Pd-Nb/SiO2 was comparable to that of Pd/SiO2, even though the Pd surface was partially covered with Nb oxide, because the oxide had an additional hydrogenation activity. The catalyst deactivation was also retarded when the Pd surface was modified with La and Nb oxides. Thermogravimetric analysis (TGA) and temperature-programmed oxidation (TPO) results showed that Pd-La/SiO2 and Pd-Nb/SiO2 produced less amounts of coke and the coke species were more volatile than in the case of Pd/SiO2.
