Catalytic activity of noble metal nanoparticles toward hydrodechlorination: influence of catalyst electronic structure and nature of adsorption

In this study, stabilized Pd, Pt and Au nanoparticles were successfully prepared in aqueous phase using sodium carboxymethyl cellulose (CMC) as a capping agent. These metal nanoparticles were then tested for catalytic hydrodechlorination toward two classes of organochlorinated compounds (vinyl polychlorides including trichloroethylene (TCE), tetrachloroethylene (PCE), and alkyl polychlorides including 1,1,1-trichloroethane (1,1,1-TCA), and 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA)) to determine the rate-limiting steps and to explore the reaction mechanisms. The surface area normalized reaction rate constant, k_SA, showed a systematic dependence on the electronic structure (the density of states at the Fermi level) of the metals, suggesting that adsorption of organochlorinated reactants on the metal catalyst surfaces is the rate-limiting step for catalytic hydrodechlorination. Hydrodechlorination rates of 1,1,1-TCA and 1,1,1,2-TeCA agreed with the bond strength of the first (weakest) dissociated C-Cl bond, suggesting that C-Cl bond cleavage, which is the first step for dissociative adsorption of the alkyl polychlorides, controlled the catalytic hydrodechlorination rate. However, hydrodechlorination rates of TCE and PCE correlated with the adsorption energies of their molecular (non-dissociative) adsorption on the noble metals rather than with the first C-Cl bond strength, suggesting that molecular adsorption governs the reaction rate for hydrodechlorination of the vinyl polychlorides.