Hydrodechlorination of tetrachloromethane on alumina- and silica-supported platinum catalysts

Two series of alumina- and silica-supported platinum catalysts, representing a wide range of metal dispersion were characterized by CO chemisorption, XRD and TEM, and investigated in the hydrodechlorination of tetrachloromethane at 343–363 K. Catalytic activity of Pt/Al2O3 in CCl4 hydrodechlorination shows very strong inverse relationship with metal dispersion; highly dispersed Pt samples exhibit very low turnover frequencies. Similar, but much milder trend, was found for silica-supported Pt catalysts. This significant support effect on the catalytic behavior was attributed to an extensive surface chloriding of small Pt particles interacting with Lewis acid sites of γ-alumina. It is suggested that such interactions lead to formation of electrodeficient Pt sites, which are quickly blocked by produced chloride species. On silica, similar in kind metal-support interactions do not occur, and, in effect, deactivation of Pt/SiO2 catalysts is much less marked. Very strong dependence of catalytic activity of alumina-supported platinum on metal dispersion implies that even very small changes in Pt particle size dispersion during reaction and catalyst regeneration should not be ignored in interpreting variations in the catalytic behavior. Indeed, changes in Pt particle size caused by different regeneration procedures are found to be an important factor in shaping the ultimate hydrodechlorination behavior. Carried out in an oxidative atmosphere for an efficient removal of coke, the regeneration of a highly dispersed Pt/Al2O3 catalyst resulted in a significant growth of metal particles which, in line with the correlation between turnover frequency and metal dispersion established for this catalytic system, exhibited much better performance than that of a freshly reduced catalyst sample. In the case of highly dispersed Pt/SiO2 catalyst the regeneration also leads to decrease of metal dispersion and corresponding improvement of catalytic activity.

Large differences in catalytic behavior of Pt/SiO2 and Pt/Al2O3 in CCl4 hydrodechlorination.Figure optionsDownload as PowerPoint slideResearch highlights▶ Catalytic activity of Pt/Al2O3 in CCl4 hydrodechlorination shows very strong inverse relation with metal dispersion. ▶ Highly dispersed Pt samples exhibit very low turnover frequencies. ▶ Similar, but much milder, trend is found for silica-supported Pt catalysts. ▶ This significant support effect on the catalytic behavior was attributed to an extensive surface chloriding of small Pt particles interacting with Lewis acid sites of γ-alumina. ▶ Changes in Pt particle size caused by different regeneration procedures is important factor in shaping the hydrodechlorination behavior.