Effect of H₂S inhibition on the hydrodechlorination of polychlorinated biphenyls over Mo/Al₂O₃ and Co-Mo/Al ₂O₃ catalysts

The effect of the cobalt addition on the H₂S inhibition and subsequently on the polychlorinated biphenyl (PCB) hydrodechlorination (HDC) activity over Mo-based catalysts was investigated. The HDC activity over a Mo catalyst containing cobalt was much higher than that over the same Mo catalyst without cobalt. On the other hand, the HDC activity of the Co-Mo catalyst was more inhibited by H₂S than that of the Mo catalyst at 300 C. Thus, kinetic parameters were calculated using a Langmuir-Hinshelwood model to determine the reaction pathway of the H₂S inhibition over Mo and Co-Mo catalysts. We found that the heats of adsorption of PCB (Aroclor ^® 1242) and H₂S on the Co-Mo catalyst was higher than on the Mo catalyst, indicating that the sulfur-containing species adsorb more strongly on the catalyst containing cobalt. The results suggested that while the Co-Mo catalyst was more inhibited by H₂S, the Mo-S bonds were more stable on this catalyst than on the solely Mo catalyst. This Mo-S bond was responsible for the stabilization of the active phase, which allowed creation of a greater amount of sulfur atoms potentially labile. Thus, that explained the better HDC activity over the Co-Mo catalyst than over the Mo catalyst, despite a greater H₂S inhibition on the former.