EFFECT OF THE METAL-SUPPORT INTERACTION ON THE CATALYTIC PROPERTIES OF PALLADIUM FOR THE CONVERSION OF DIFLUORODICHLOROMETHANE WITH HYDROGEN - COMPARISON OF OXIDES AND FLUORIDES AS SUPPORTS
B. Coq et al., EFFECT OF THE METAL-SUPPORT INTERACTION ON THE CATALYTIC PROPERTIES OF PALLADIUM FOR THE CONVERSION OF DIFLUORODICHLOROMETHANE WITH HYDROGEN - COMPARISON OF OXIDES AND FLUORIDES AS SUPPORTS, Journal of physical chemistry, 99(28), 1995, pp. 11159-11166
The reaction of difluorodichloromethane with hydrogen has been studied
between 433 and 523 K and atmospheric pressure, over Pd catalysts sup
ported on graphite and oxides or fluorides of Al, Ti, and Zr. In CF2Cl
2 hydrogenation, CH2F2 and CH4 represented more than 95% of the produc
ts. The catalytic properties of fluoride supported catalytsts did not
undergo any change as a function of time. In contrast, Pd supported on
oxides showed changes in selectivty during the first hours on stream.
This was ascribed to the reaction of the oxide support with HF releas
ed during the reaction. Alumina and titania were nearly completely con
verted to the corresponding fluorides, but not zirconia. The selectivi
ty to the desired product CH2F2 was 56% for Pd/graphite and reached 90
% for Pd/ZrF4. The kinetic study suggested that the selectivity was co
ntrolled by the bond strength between a carbene-like species CF2 and t
he surface. The strength of this interaction is supposed to vary with
electron availability at the Pd surface, and this hypothesis was then
investigated by infrared spectroscopy using the adsorption of CO on Pd
/Al2O3 and Pd/AlF3. The results show that the morphology of the Pd par
ticles was little affected by the support and that AlF3-supported Pd b
ecomes electron deficient, due to the strong Lewis acidity of the supp
ort. This effect is mainly a short-range effect which is better induce
d by supports made up of a mixture of fluorides, oxyfluorides, and hyd
roxyfluorides, rather than pure fluorides. Catalytic properties simila
r to those of Pd/AlF3 and ZrF4 can be simulated with Pd/graphite sampl
es promoted with small amounts of aluminum or zirconium.