Effect of structural ordering of the carbon support on the behavior of Pd catalysts in aqueous-phase hydrodechlorination

Catalysts consisting of Pd supported on virgin and heat-treated carbon blacks (homemade and commercial), graphites (natural and synthetic) and commercial carbon nanofibers were prepared and tested in the aqueous phase hydrodechlorinaton (HDC) of 4-chlorophenol (4-CP) under near ambient conditions (30 °C, 1 atm) in order to explore the effect of the support on the catalytic behavior. The homemade graphitized supports were prepared from commercial carbon black (CB) and from a carbon black-like material (CBPE) obtained from pyrolysis of low-density polyethylene. All the catalysts prepared yielded complete 4-CP conversion, although a wide range of activity was observed (10.7–173.5 mmol g_Pd⁻¹ min⁻¹). The graphitized carbon black provided the most active catalysts, showing Pd nanoparticles around 3 nm in size and a well-balanced contribution of Pd species (Pdⁿ⁺/Pd⁰ = 0.9). Substantial differences of activity were found between the graphitized and not graphitized supports, even for catalysts with similar Pd nanoparticle size. The higher activity of the catalysts with graphitized supports can be partly associated to a more balanced initial Pdⁿ⁺ to Pd⁰ ratio. Moreover, higher selectivity to hydrogenation products (cyclohexanone) was also achieved with those catalysts due to a higher contribution of the Pd⁰ species.