SELECTIVE HYDROGENATION OF ACETYLENE ON Pd/TiO2 CATALYSTS PREPARED BY ELECTROLESS DEPOSITION METHOD

Abstract

In the present work, support Pd catalysts are commonly used in industry to remove trace amount of acetylene from ethylene feed in the production of polyethylene. Anatase nanocrystalline TiO2 samples were synthesized by a sol-gel method and subjected to thermal treatment under N2, H2, O2, and air atmospheres at 350°C employed as supports for preparation of 1%Pd/TiO2 catalysts. The catalysts were characterized by X-ray diffraction (XRD), BET surface areas, electron spin resonance (ESR), and X-ray photoelectron spectroscopy (XPS). The surface Ti3+ defects significantly increased when treated under H2 and N2, comparing to air and O2 with no changes in the crystallite size, specific surface area, and pore structure. Two different preparation methods including electroless deposition (ED) and incipient wetness impregnation (I) were used to prepare 1%Pd/TiO2 catalysts. The 1%Pd catalyst supported on the TiO2 calcined under H2 prepared by ED exhibited the best catalytic performances in the selective hydrogenation of acetylene corresponding to their high Pd dispersion on the TiO2 support. In addition, Ag-Pd/TiO2 having incremental surface coverages of Ag prepared by ED. An increased selectivity of ethylene and higher turnover frequencies for acetylene conversion were observed at high Ag coverages. The adsorption of acetylene on Pd sites as strongly adsorbed ethylidyne on large Pd ensembles led to ethane formation whereas adsorbed π-bonded species on small Pd ensembles favored the formation of ethylene. Kinetics study of Ag-Pd/TiO2 catalysts reveals that the apparent activation energy for the selective hydrogenation of acetylene Ag decreased from 12.08 ± 0.1 kcal/mol to 7.74±0.2 kcal/mol with increasing Ag coverage.