Effect of TiO2 crystalline phase composition on the physicochemical and catalytic properties of Pd/TiO(2) in selective acetylene hydrogenation.

Pd/TiO(2) catalysts have been prepared using TiO(2) supports consisting of various rutile/anatase crystalline phase compositions. Increasing percentages of rutile phase in the TiO(2) resulted in a decrease in Brunauer-Emmett-Teller surface areas, fewer Ti(3+) sites, and lower Pd dispersion. While acetylene conversions were found to be merely dependent on Pd dispersion, ethylene selectivity appeared to be strongly affected by the presence of Ti(3+) in the TiO(2) samples. When TiO(2) samples with 0-44% rutile were used, high ethylene selectivities (58-93%) were obtained whereas ethylene losses occurred for those supported on TiO(2) with 85% or 100% rutile phase. X-ray photoelectron spectroscopy and electron spin resonance experiments revealed that a significant amount of Ti(3+) existed in the TiO(2) samples composed of 0-44% rutile. The presence of Ti(3+) in contact with Pd can probably lower the adsorption strength of ethylene resulting in an ethylene gain. Among the five catalysts used in this study, the results for Pd/TiO(2)-R44 suggest an optimum anatase/rutile composition of the TiO(2) used to obtain high selectivity of ethylene in selective acetylene hydrogenation.