Design of hydrodechlorination catalysts on the basis of chloromethanes-metallic active sites interactions

Abstract

The adsorption of chloromethanes, namely trichloromethane (TCM), dichloromethane (DCM) and monochloromethane (MCM), and hydrogen on metal clusters of Pd, Pt, Rh and Ru was studied by molecular simulation to learn on the design of more efficient catalysts for the hydrodechlorination (HDC) of those species. All the above metals showed high dechlorination ability, following in all cases the order TCM > DCM > MCM. The computational study consigns that dissociative adsorption of chloromethanes is promoted on the zero-valent metal (M0) sites while non-dissociative adsorption, associated to catalyst deactivation, is strongly favored on the electro-deficient ones.The dechlorination ability of M0 species follows the order Ru > Rh > Pt > Pd, with greater differences in the dechlorination of TCM. Conversely, hydrogenation ability remarkably decreases from Pd and Pt to Rh and Ru catalysts. This determines a more favorable balance of reactant adsorption energies in the HDC with Pd, especially for TCM HDC. Based on the theoretical findings, catalysts with high proportion of zero-valent metal species, supported on CeO2-Sm2O3 (CS), were prepared, characterized and tested in the HDC of TCM and DCM in lab-scale continuous fixed bed experiments. The conclusions obtained by simulation were supported by the experimental results. The high proportion of zero-valent species in the Pd/CS catalyst, besides the favorable interactions between active sites and both chloromethanes and H2, allow an outstanding performance of the catalyst in the HDC of those chloromethanes, with a significantly higher dechlorination activity and stability than the other metals for the HDC of TCM.