W. Juszczyk et al., HYDRODECHLORINATION OF CCL2F2 (CFC-12) OVER GAMMA-ALUMINA SUPPORTED PALLADIUM CATALYSTS, Applied catalysis. A, General, 166(2), 1998, pp. 311-319
Alumina displays low catalytic activity at the initial stage of the re
action of CCl2F2 with hydrogen, giving mainly halogen exchange product
s, and this activity quickly decays with time-on-stream. In the case o
f Pd/Al2O3 catalysts, the contribution of the support is negligible at
180 degrees C. Catalytic activity of Pd/gamma-Al2O3 in CCl2F2 hydrode
chlorination strongly depends on metal dispersion: poorly dispersed Pd
samples exhibit the highest turnover frequencies. The same samples al
so show the highest selectivities towards the formation of CH2F2. Time
-on-stream behaviour and considerable amounts of carbon found in used
catalysts suggest that the catalytic properties of Pd/Al2O3 are regula
ted by incorporation of carbon into Pd lattice, or the formation of Pd
carbide. Poorly dispersed Pd catalysts contain a higher proportion of
plane atoms and, therefore, are subjected to a more severe carbiding.
Such a transformation generates surfaces which bind freon molecules l
ess strongly, resulting in higher activity and selectivity to partial
dehalogenation, i.e formation of CH2F2. High-temperature reduction at
600 degrees C does not much change the overall activity of Pd/Al2O3. H
owever, the selectivity to CH2F2 is somewhat increased. It is believed
that the Pd-Al2O3 interface changes upon high-temperature reduction,
leading to a Pd-Al compound. At corrosive conditions of hydrodehalogen
ation of CCl2F2, the Pd-Al would be converted to AlFx species much mor
e easily than the Al2O3 species at the Pd-Al2O3 interface of mildly re
duced Pd/Al2O3 catalysts. (C) 1998 Elsevier Science B.V.