Hydrogen-assisted dechlorination of 1,2-dichloroethane on active carbon supported palladium–copper catalysts

A series of active carbon supported palladium–copper catalysts with varying amounts of both metal components and overall metal loading 2 wt.%, prepared by incipient wetness impregnation were characterized by temperature programmed reduction (TPR), hydrogen chemisorption, temperature programmed (palladium) hydride decomposition (TPHD), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). TPHD and XRD studies indicated that the bimetallic Pd–Cu/C catalysts exhibited a considerable degree of alloying. Pd/C, Cu/C and Pd–Cu/C catalysts were investigated in the hydrodechlorination (HdCl) of 1,2-dichloroethane in the gas phase at a relatively low reaction temperature (210–230 °C). All catalysts showed fair stability with time on stream (up to ∼15 h). The Pd/C and Pd-rich Pd–Cu catalysts essentially retained their initial activity while the bimetallic samples with higher Cu content exhibited some deactivation at the initial stage of reaction. Cu-rich bimetallic samples exhibited the highest selectivities toward ethene (desired reaction product), >90%. For all bimetallic Pd–Cu/C samples, this selectivity gradually increased with time on stream. XPS studies of freshly reduced and post-reaction catalysts did not support the hypothesis that during hydrodechlorination the surface of Pd–Cu is being gradually enriched in copper, as could be expected of a higher affinity of copper to chlorine. Post-reaction deposits investigated by the temperature programmed hydrogenation (TPH) showed substantial amounts of chlorine on the copper catalyst, whereas all palladium-containing Pd–Cu/C samples exhibited only carbon-containing deposit, with C2-hydrocarbons desorbing at a relatively low temperature (300 °C). Suggested interpretation of catalytic data includes both the role of C2Hx species building up on catalyst's surface during hydrodechlorination as well as the role of Pd–Cu mixed sites in the mechanism of hydrogen-assisted dechlorination of 1,2-dichloroethane.

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► Pd–Cu/C catalysts are active and stable in hydrodechlorination of 1,2-dichloroethane (1,2-DCA).
► Ethene selectivity is very high for Cu-rich catalysts.
► Post-reaction deposit had Cl (on Cu), and C2Hx (on Pd–Cu) removable in H2 at 300 °C.
► Gradual surface blocking by C2Hx species makes Pd–Cu/C more selective for ethene.
► Ligand effect explains synergy in hydrodechlorination of 1,2-DCA for Pd–Cu catalysts.