High-surface area graphites as supports for hydrodechlorination catalysts: Tuning support surface chemistry for an optimal performance

The performance of parent and modified high-surface area graphites (HSAGs) as supports for Pd-based hydrodechlorination catalysts is studied in this work. Catalytic hydrodechlorination of tetrachloroethylene (TTCE) has been selected as model reaction, because of its environmental relevance. Different Pd/HSAG catalysts were prepared using both parent, oxidized (using a nitric acid treatment) and pyrolysed HSAG (in order to remove the surface functional groups of the parent HSAGs). Two different preparation procedures were also tested, using either aqueous or organic solutions of the precursor (palladium chloride). Hydrodechlorination experiments have been carried out in a fixed bed reactor at 523 K, 0.5 MPa, and a space time of 24 g s mmol−1. Fresh and used catalysts were characterized by TEM (metal dispersion), TPD (evolution of surface groups), TPO (amount of carbonaceous deposits), nitrogen physisorption (morphology), and XPS (chemical composition and speciation). Obtained results show that HSAG-supported catalysts present very good performance both in terms of conversion (TOFs up to four times higher than the encountered for other conventional supports, such as activated carbon or alumina), selectivity (only small amounts of trichloroethylene being observed, in the same order of magnitude, below 10% selectivity in all the cases, that other Pd catalysts), and resistance to deactivation (more than 30 h on stream). The surface chemistry of the HSAG (concentration and type of the surface functional groups) determines both the activity of the catalyst and the main deactivation cause. Thus, catalysts prepared over the parent HSAGs show a relatively fast deactivation by fouling, ascribed to the strongest acid sites; whereas the selective removal of the surface functional groups, or the transformation of these groups during the oxidative treatment, lead to catalysts less prone to fouling. In these cases, chlorine poisoning is the main deactivation cause. The preparation procedure of the catalysts also affects their performance, being the catalysts prepared using aqueous precursors less prone to chlorine poisoning.