On the performance of Pd and Rh catalysts over different supports in the hydrodechlorination of the MCPA herbicide

• In the aqueous-phase HDC of MCPA Pd catalysts were more active than Rh ones.
• Catalysts supported on mesoporous materials showed higher activity.
• Catalysts with metal particle size of 3.1–3.7 nm yielded higher reaction rates.
• Pd/Al2O3 exhibited the highest activity and the best stability.

Commercial and own-made Pd and Rh catalysts supported on alumina, zeolite Y (ZY) and two kinds of silica (Fumed Silica (FS) and SBA-15), were tested in the aqueous phase hydrodechlorination of the 4-chloro-2-methylphenoxyacetic acid (MCPA) herbicide. Pd catalysts showed higher activity than Rh and regarding the supports, the following sequence was found: Al2O3 > FS > ZY >> SBA-15. The catalysts with metal particle sizes within the range of 3.1–3.7 nm, associated with Mn+/M° ratios between 0.7 and 1.7, showed higher activities. The most active catalysts of each series, the Al2O3-supported ones, showed a distribution of the metallic phase approaching an egg-shell configuration opposite to the rest. The Pd/Al2O3 catalyst, which was the most active, suffered a significant loss of activity upon the early stages on stream that could be ascribed to the formation of palladium chloride complexes as well as to deposition of species involved in the reaction.

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