Chlorodifluoromethane Hydrodechlorination on Carbon-Supported Pd-Pt Catalysts. Beneficial Effect of Catalyst Oxidation

Abstract

Previously tested 2 wt % palladium-platinum catalysts supported on Norit activated carbon preheated to 1600 °C have been reinvestigated in CHFCl2 hydrodechlorination. An additionally adopted catalyst oxidation at 350–400 °C produced nearly an order of magnitude increase in the turnover frequency of Pd/C, from 4.1 × 10−4 to 2.63 × 10−3 s−1. This increase is not caused by changes in metal dispersion or possible decontamination of the Pd surface from superficial carbon, but rather by unlocking the active surface, originally inaccessible in metal particles tightly packed in the pores of carbon. Burning carbon from the pore walls attached to the metal changes the pore structure, providing easier access for the reactants to the entire palladium surface. Calcination of Pt/C and Pd-Pt/C catalysts results in much smaller evolution of catalytic activity than that observed for Pd/C. This shapes the relationship between turnover frequency (TOF) and alloy composition, which now does not confirm the Pd-Pt synergy invoked in the previous work. The absence of this synergy is confirmed by gradual regular changes in product selectivity, from 70 to 80% towards CH2F2 for Pd/C to almost 60% towards CH4 for Pt/C. The use of even higher-preheated carbon (1800 °C), completely free of micropores, results in a Pd/C catalyst that does not need to be oxidized to achieve high activity and excellent selectivity to CH2F2 (>90%).