Comparison of activated carbon-supported Pd and Rh catalysts for aqueous-phase hydrodechlorination

Different commercial and own-made catalysts, synthesized by incipient wetness impregnation, based on Pd and Rh supported on activated carbon have been investigated for hydrodechlorination (HDC) in aqueous phase using 4-chlorophenol as target compound at 100 mg/L starting concentration. The experiments were performed in a continuous stirred basket tank reactor at 30 °C and 1 atm, fairly mild conditions for a potential application to industrial wastewater. Although all the catalysts tested showed a high dechlorination activity (X4-cp ≥ 95% except for the commercial Rh/AC catalyst) which remained practically stable during long-term experiments, the own-made catalysts yielded a higher selectivity (>40%) to the most hydrogenated reaction products, cyclohexanone and cyclohexanol. This last is by far the main reaction product with the own-made Rh catalyst, which is important given its lower toxicity and higher biodegradability. The highest catalytic activity was associated with metal particle sizes within the 3–4 nm range, indicating that the HDC reaction with these catalysts is structure sensitive. Modification of the surface composition of the active carbon support upon HNO3 oxidation entailed a more homogeneous distribution of the active phase with improved metallic dispersion. However, the catalyst with the smallest metal particle (<2 nm) was not the most active, resulting these particle sizes less effective for HDC.

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► A higher activity and stability was exhibited by Pd and Rh/AC own-made catalysts.
► Rh catalysts showed a high selectivity to the most advisable product.
► The highest activity was associated with metal particle sizes within 3–4 nm.
► The HDC reaction with these catalysts is structure sensitivity.