Density Functional Theory Analysis of Dichloromethane and Hydrogen Interaction with Pd Clusters: First Step to Simulate Catalytic Hydrodechlorination

Abstract

A density functional theory (DFT) analysis has been conducted for the gas-phase hydrodechlorination (HDC) of dichloromethane (DCM) with palladium catalyst to achieve a better knowledge of the reaction mechanism involved in the HDC process, which constitutes an emerging technology for the treatment of organochlorinated contaminants. The computational study included the effect of size, oxidation state, and spin configuration of Pd cluster on the adsorption of H2 and DCM reactants on the catalyst surface. Calculations described the activation of H2 by Pd clusters through a dissociative adsorption with low enthalpy values. In addition, partially and fully dissociated DCM intermediates on Pd surface were predicted by DFT calculations. Remarkably, the dissociative adsorption of DCM on Pd active sites occurs via the scission of C–Cl bonds, promoted by the formation of C–Pd linkages, implying high adsorption enthalpy. The computational results showed that DCM can be also molecularly adsorbed on both zerovalent and electrodeficient Pd species. However, the nondissociative adsorption of DCM over electrodeficient Pd cluster is remarkably favored in energy, with adsorption enthalpies (∼−50 kcal/mol) corresponding to chemisorption. Current theoretical evidence explained the deactivation of Pd/AC catalyst as a consequence of the selective poisoning of electrodeficient Pd active centers by chlorinated hydrocarbons, in good agreement with our previous experimental findings.

Deactivation behavior of Pd/C and Pt/C catalysts in the gas-phase hydrodechlorination of chloromethanes: Structure–reactivity relationship

“…This suggests a strong interaction of DCM with Pd particles which disfavors its desorption. This is consistent with the results from molecular simulation [41] and XRD, where, as indicated before, the formation of a Pd-C phase was detected in the catalyst used in the HDC of DCM. No HCl desorption was observed in this case, due in part to the lower amount of HCl formed because of the lower chlorine content of the starting molecule and the lower conversion achieved.…”

“…The chemically inert core orbitals of palladium and platinum clusters were described with the effective core potentials of Hay and Wadt [40], which include relativistic effects on valence electrons, while the external orbitals were represented with a double-ζ basis set using Dunning/Huzinaga full double-ζ basis set (Lanl2DZ), which was also used for the rest of the atoms of the species involved in this study. Thus, all the simulations were conducted at the B3LYP/Lanl2DZ computational level, which has been proved to be reliable for systems involving Pd atoms [41,42,43,44,45,46]. Full geometry optimization was done for isolated reactants, Pd and Pt clusters and reactants-metal systems.…”

“…Full geometry optimization was done for isolated reactants, Pd and Pt clusters and reactants-metal systems. In this work, Pt6 and Pd6 clusters were used as models of reference to simulate the metal active sites of the catalysts, following the criteria developed elsewhere [41]. All structures of Pd and Pt clusters have been optimized for various accessible spin multiple (singlet and triplet) states to determine the most stable structures, associated with the lowest energies.…”

“…A wide diversity of hydrocarbons of up to nine carbon atoms were also detected from flash pyrolysis of the catalyst used in the HDC of TCM. [41] where it was found that non-dissociative adsorption of CMs on Pd is thermodynamically favored in the order DCM > MCM > TCM for electrodeficient Pd and MCM > DCM > TCM for zero-valent Pd, the non-dissociative adsorption on electro-deficient Pd being by far the more favored in any case. The TPD profiles of the catalyst used in the HDC of DCM show only a small peak of DCM at a temperature around 300 ºC.…”

“…Pd [41]. The possible presence of both zero-valent and electrodeficient metallic species in the catalysts was considered.…”