R. Ohnishi et al., SELECTIVE HYDRODECHLORINATION OF CFC-113 ON BI-MODIFIED AND TL-MODIFIED PALLADIUM CATALYSTS, Applied catalysis. A, General, 113(1), 1994, pp. 29-41
More than 80% selectivity in the hydrodechlorination of 1,1,2-trichlor
otrifluoroethane (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 a
nd a thallium modified catalyst on active carbon provided 3FH and 3FCl
with more than 90% yield at 520-600 K. Kinetic studies suggest that t
he 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 hydrodefluor
ination. The IR studies of carbon monoxide chemisorption on the modifi
ed palladium catalysts show that the addition of bismuth or thallium l
eads 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 ensembl
es of palladium atoms. Further evidence for this mechanism comes from
the temperature-programmed reduction spectra, where a reduction peak s
hifts 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.