Hydrodechlorination of carbon tetrachloride to chloroform in the liquid phase with metal-supported catalysts.: Effect of the catalyst components

The effect of the nature of catalyst components has been analyzed in the hydrodechlorination of carbon tetrachloride to chloroform. Active carbon, alumina, silica, and sepiolite were used as supports and Ru, Rh, Pd, Pt, and Ni as the metallic phase. The catalysts were characterized by CO chemisorption, TPR, and XPS. Reaction was carried out at 373 and 413 K in a slurry semibatch reactor. Dispersion of the catalysts containing different metals are very similar, while the values corresponding to Pd supported on different carriers follow the trend Pd/Al2O3 > Pd/carbon > Pd/SiO2 approximate to Pd/sepiolite. The XPS analysis showed that Pd, Ru, and Rh are homogeneously distributed on the carbon, Pt is preferentially deposited on the external surface, and Ni is inside the pores. On the other hand, Pd on silica or alumina is mainly located on the external surface of the support. Pd/C is highly active and selective to chloroform, but Pd supported on the other carriers is less active and leads the reaction toward hexachloroethane. Hydrodechlorination activity of carbon-supported catalysts are in the order Pd/C much greater than Pt/C > Rh/C > Ru/C > Ni/C, all of them being very selective toward chloroform. The results of the reaction are interpreted in terms of the spillover of hydrogen on carbon, the distribution of the metal on the support, and the density of states of the metals.