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

摘要

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 (X_(4-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 biodegrad-ability. 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.