Liquid-phase hydrodechlorination of CCI4 to CHCl3 on Pd/carbon catalysts:: Nature and role of Pd active species

This study examines the influence of the preparation variables on the physicochemical properties of Pd/C catalyst and its performance in the hydrodechlorination of carbon tetrachloride and proposes a model of a palladium active site and its catalytic modus operandi which satisfactorily describes the chloroform formation from carbon tetrachloride. Several carbon-supported Pd (0.2-1.6 wt%) catalysts were prepared by incipient wetness impregnation of the support with H2PdCl4 aqueous solutions and reduced at 523 K (CPd series). Additionally, five portions of the 1.0 wt% CPd precursor were reduced at 423, 473, 623,723, and 823 K. Another two catalysts containing 1.0 wt% Pd were prepared in similarly to Pd(NO3)(2) and reduced at 523 K (NPd series). One of these last samples was calcined at 573 K prior to the reduction. The TPR results indicate that reduction of the CPd precursors is more difficult than that of NPd, which are reduced at subambient temperature. As shown by CO chemisorption, metal dispersion in NPd is lower than in CPd catalysts, in which dispersion decreases as the reduction temperature increases. X-ray photoelectron spectroscopy shows that Pd-o and Pdn+ species are present on the catalysts surface and that their proportion depends on the reduction temperature and the nature of the palladium precursor. All the catalysts containing both Pdo and Pdn+ were active and highly selective in the hydrodechlorination of carbon tetrachloride to chloroform, whereas samples having only one of these species were inactive. The results indicate that both Pd-o and Pdn+ are required for the catalytic hydrodechlorination reaction and, consequently, it is proposed that the active Pd site is dual in nature and is constituted by the association of the two species: electron-deficient and metallic palladium [Pdn+-Pd-o]. The hydrodechlorination activity is related to the Pdn+/Pd-o ratio and the maximum activity corresponds to Pdn+/Pd-o approximate to 1. We propose that H-2 chemisorbs and dissociates on Pd-o to give the covalent adatom Pd-H, whereas CCl4 chemisorbs dissociatively on the same Pdn+ site by abstraction of the slightly nucleophilic chloride anion (Cl:(-)), leading to the formation of the highly reactive activated complex [Cl-Pd-o: --> CCl3](n+). Interaction between the two surface species leads to chloroform and hydrogen chloride and to the regeneration of the active site. (C) 2002 Elsevier Science (USA).