Palladium–Silver Sol–Gel Catalysts for Selective Hydrodechlorination of 1,2-Dichloroethane into Ethylene: III. Kinetics and Reaction Mechanism

The kinetics of selective hydrodechlorination of 1,2-dichloroethane into ethylene over a Pd–Ag/SiO2 catalyst is studied. Kinetic data at 573, 596, and 647 K have been obtained by means of experimental designs in the space of the partial pressures of the four components influencing kinetics, that is, 1,2-dichloroethane, hydrogen, ethylene, and hydrogen chloride. One model among the numerous ones examined allows us to represent correctly the experimental data. It corresponds to the following mechanism: dechlorination of 1,2-dichloroethane into ethylene occurs on silver through dissociative adsorption with successive breaking of the two C–Cl bonds and desorption of C2H4. Thanks to its activation power of hydrogen by dissociative chemisorption, palladium present at the surface of the alloy supplies hydrogen atoms for regeneration of the chlorinated silver surface into metallic silver. The presence of hydrogen adsorbed on Pd also causes the undesired ethylene hydrogenation leading to a loss of olefin selectivity.