More than 80% selectivity in the hydrodechlorination of 1,1,2-trichlorotrifluoroethane (CFC-113) has been observed over palladium catalysts containing selected metal additives such as Ag, Bi, Cd, Cu, Hg, In, Pb, Sn, Tl to chlorotrifluoroethene (3FCl) and trifluoroethene (3FH). In particular, a bismuth modified palladium catalyst supported on SiO2 and a thallium modified catalyst on active carbon provided 3FH and 3FCl with more than 90% yield at 520-600 K. Kinetic studies suggest that the reaction route is CFC-113 --> 3FCl --> 3FH and that the role of the metal additives is to suppress the hydrodechlorination of 3FCl to 3FH or further hydrogenation of 3FH. The thermodynamic estimations imply that the modifier plays another role in the inhibition of hydrodefluorination. The IR studies of carbon monoxide chemisorption on the modified palladium catalysts show that the addition of bismuth or thallium leads to a drastic suppression of bridged carbon monoxide on palladium and to the enhancement of linear carbon monoxide. This shift indicates that the addition of the promoter results in the breaking the ensembles of palladium atoms. Further evidence for this mechanism comes from the temperature-programmed reduction spectra, where a reduction peak shifts to higher temperature. X-ray diffraction spectra, where a broad peak based on palladium shifted to low angle of 2theta by the addition of thallium, indicates the formation of Tl-Pd alloy or intermetallic compound.