Pd-Re/Al 2O 3: Characterization and catalytic activity in hydrodechlorination of CCl 2F 2

Abstract

A series of alumina-supported Pd-Re catalysts were prepared and characterized using X-ray diffraction, chemisorption, and temperature-programmed methods. A different stability of Pd(-Re)-hydride phases was exploited; i.e., the amount and decomposition temperatures of the β-hydride phases for different PdxRe1−x/Al2O3 catalysts were found strongly correlated with x, suggesting a considerable extent of interaction between Pd and Re in reduced Pd-Re/Al2O3 catalysts. The results from other characterization methods render a similar conclusion. The Pd-Re/Al2O3 catalysts were tested in the reaction of hydrodechlorination of CCl2F2. Rhenium appeared inactive in this reaction. Adding Re to Pd/Al2O3 introduces substantial changes in the catalytic behaviour. All Pd-Re bimetallic samples strongly deactivated with time-on-stream, whereas the activity of Pd/Al2O3, after an initial increase, was fairly stable. Selectivity patterns for the bimetallic samples also differed from that of palladium, showing the increase in the selectivity to methane during the stabilization period. The steady-state activity of Pd-rich (up to 25 at% Re) bimetallic samples was much lower than that of Pd; however, it slightly increased with further Re addition, reaching a mild maximum at 50 at% Re. Such an activity pattern resembles very much the behaviour of alumina-supported Pt-Re catalysts in hydrocarbon conversions. The selectivity towards CH2F2 changed only slightly with the bimetallic composition. X-ray diffraction indicated the presence of carbon dissolved in Pd or Pd-rich phases in spent Pd-Re/Al2O3 catalysts. Differences in lattice parameter of the carbonized phases depended on the nominal composition of Pd-Re, verifying a considerable extent of Pd-Re mixing in the supported catalysts. The results of catalytic screening are compatible with such a conclusion.