Hydrodechlorination of dichlorodifluoromethane (CFC-12) on Pd–Pt/Al2O3 catalysts

Abstract

A series of alumina-supported palladium–platinum catalysts of 1–2.8wt.% overall metal loading were prepared by incipient wetness impregnation and investigated in the reaction of dichlorodifluoromethane with dihydrogen at 160–180°C. All samples had high metal dispersion, what, in combination with a low metal loading, made physical characterization of catalysts difficult. Temperature-programmed hydride decomposition (TPHD) study, the technique which appeared to be a promising technique for diagnosing the quality of alloying palladium-containing systems, appeared less informative, because, due to high metal dispersion, all tested samples did not show a β-PdH phase formation.A 2–2.5-fold increase in hydrodechlorination activity and a considerable enhancement of the selectivity towards difluoromethane (from 46 to ∼60%, at 180°C) are obtained upon introducing small amounts (up to 20at.%) of platinum to palladium. Further increase of platinum content lowers both the activity as well as the selectivity. This synergistic effect must follow from a considerable effect of interaction between palladium (active component) and platinum (much less active component). CHClF2 is produced in larger amounts on monometallic 1wt.% Pt/Al2O3 catalyst (selectivity 28% at 180°C and higher at lower temperatures), whereas Pd and Pd–Pt samples form only small amounts of this product. This suggests that the catalytic behavior of palladium is largely preserved in the bimetallic samples. As after reaction the most active and selective Pd–Pt catalysts (with 10 and 20at.% Pt) contained lesser amounts of carbon than the other (less active) catalysts, it is considered that small amounts of platinum introduced (to palladium) are beneficial for maintaining the catalyst surface less blocked by carbon species, and in effect, more active.