Abstract:
Effect of titania consisting of various phase compositions on Pd and Pd-Ag catalysts for selective acetylene hydrogenation has been studied. Increasing amount of %rutile phase in the TiO[subscript 2] resulted in a decrease in BET surface areas, lower amount of Ti[superscript 3+] sites, and lower Pd dispersion. Acetylene conversion was found to be merely dependent on Pd dispersion while ethylene selectivity appeared to be strongly affected by the presence of Ti[superscript 3+] in the TiO[subscript 2] samples. When TiO[subscript 2] samples with 0-44% rutile were used, high ethylene selectivities were obtained whereas ethylene losses occurred for those supported on TiO[subscript 2] with rutile phase 85 or 100%. XPS and ESR experiments revealed that significant amount of Ti[superscript 3+] existed in the TiO[subscript 2] samples composed of 0-44% rutile. The presence of Ti[superscript 3+] in contact with Pd can probably lower adsorption strength of ethylene resulting in an ethylene gain. Among all the catalysts used in this study, the results for Pd/TiO[subscript 2]-R44 suggest an optimum anatase/rutile composition of the TiO[subscript 2] used to obtain high selectivity of ethylene in selective acetylene hydrogenation. In addition, pure anatase titania has been prepared by solvothermal and sol-gel methods and employed as supports for Pd and Pd-Ag catalysts. BET surface area of the solvothermal TiO[subscript 2] was less than that of sol-gel TiO[subscript 2], However, due probably to the different synthesis routes, the amount of Ti[superscript 3+] sites on the TiO[subscript 2] prepared by solvothermal method were higher than the one prepared by sol-gel method. It was found that acetylene conversion and ethylene selectivity of Pd catalyst supported on solvothermal TiO[subscript 2] with higher Ti[superscript 3+] sites were lower than those of sol-gel TiO[subscript 2] supported one. Acetylene conversion and ethylene selectivity of Pd-Ag catalyst were found to belower than that of single metal Pd catalyst supported on TiO[subscript 2] containing Ti[superscript 3+]. However, the presence of Ag improved significantly ethylene selectivity ofthe catalysts without Ti[superscript 3+] presented. In conclusion, this study reveal both Ti[superscript 3+] and the strong-metal support interaction (SMSI) presented in Pd and Pd-Ag supported on TiO[subscript 2] significantly affect the catalyst performance in selective hydrogenation of acetylene
