Effect of gold on catalytic behavior of palladium catalysts in hydrodechlorination of tetrachloromethane

• Pd-Au show better than Pd resistance to deactivation.
• Pd-Au are also more selective than Pd towards hydrocarbons (desired products).
• Performance of Pd-Au depends on alloying degree: ill-mixed Pd-Au rapidly deactivates.
• Inadequately mixed Pd-Au/C may be upgraded by leaching unalloyed Pd with HNO3.

Catalytic gas-phase hydrodechlorination (HdCl) of tetrachloromethane was investigated over Au/Sibunit and Pd/Sibunit carbon prepared by impregnation, and Pd-Au/Sibunit carbon prepared by either reductive deposition of Au onto Pd or successive impregnation of Pd/Sibunit carbon with a solution of gold salt carried out at aerobic conditions. The activated (by reduction in H2 at 400 °C) catalysts were characterized in terms of H2 and CO chemisorptions, XRD and (S)TEM-EDS measurements. Presence of smaller (<2 nm) metal particles in 2.8 wt.% Pd/Sibunit (dmean 3.1 nm) evidenced by TEM was not confirmed by XRD and gas chemisorptions. XRD and (S)(TEM)-EDS showed a reasonable quality of Pd-Au alloying in catalysts prepared by reductive deposition. Compared with the monometallic palladium, active carbon-supported bimetallic Pd-Au catalysts (∼0.24 g charges) showed very good activity (conversions up to 92%), resistance to deactivation during ∼70 h runs and high selectivity to nonchlorinated products (up to ∼80%) in the reaction of hydrodechlorination of tetrachloromethane, carried out at 90 °C. However, their behavior greatly depends on the quality of Pd-Au alloying: well mixed Pd-Au particles of Pd-Au maintain a very good catalyst performance, while the increasing presence of small unalloyed Pd particles leads to rapid deactivation. A large part of these small Pd particles can be leached out from such insufficiently homogenized catalystsby dissolution in 10% nitric acid. This treatment, followed by catalyst re-reduction results in the remarkable improvement of catalytic performance, especially the catalyst's stability in a long-term operation. During the reaction, carbon species originating from CCl4 enter palladium (but not PdAu) bulk, leading to the formation of the palladium carbide phase, PdCx, which seems to be less effective in hydrogen activation, and results in massive accumulation of nonreactive deposits containing both carbonaceous as well as chlorine species leading to rapid catalyst deactivation.

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