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Langmuir 2006, 22, 4158-4164
Reductive Hydrodechlorination of Trichloroethylene by Palladium-on-Alumina Catalyst: 13C Solid-State NMR Study of Surface Reaction Precursors Watanee Sriwatanapongse, Martin Reinhard,* and Christopher A. Klug‡ Department of CiVil and EnVironmental Engineering, Stanford UniVersity, Stanford, California 94305 ReceiVed NoVember 15, 2005. In Final Form: February 19, 2006 Adsorption of trichloroethylene (TCE) on alumina-supported palladium catalysts (Pd/Al2O3) was studied in the presence and absence of hydrogen using 13C-solid state NMR. Carbon-13 NMR spectra indicate that at low coverage strongly adsorbed species are formed while at high coverage additional physisorbed species are present. Carbon-13 spin-echo amplitude data measured as a function of pulse separation, τ, was used to determine the 13C-13C intramolecular dipolar coupling and the carbon-carbon bond length of adsorbed species. Results indicate that a substantial fraction of the chemisorbed carbon species had undergone carbon-carbon bond scission forming single-carbon fragments, suggesting that the activation energy for carbon-carbon bond scission is comparable to the heat of adsorption. For the remaining surface species, the double bond is elongated to 1.46 ( 0.03 Å and is suspected to be chemically bonded ethynyl. At room temperature, adding an excess of hydrogen to catalyst that is covered to saturation with TCE precursors produces only in a small amount of ethane, indicating the fraction of surface species that are hydrodehalogenation precursors is small.
Introduction Catalytic reductive hydrodehalogenation is an innovative technology for the treatment of groundwater contaminated with halogenated hydrocarbons, such as chlorinated solvents and pesticides.1-3 Treatment involves adding hydrogen to the contaminated water followed by contacting with a palladium (Pd) catalyst.4 For chlorinated ethylenes, the reaction is rapid even at ambient temperature and pressure and the products formed, ethane and hydrochloric acid, are inconsequential.5 Optimized, the process is nearly quantitative and competitive to conventional technologies such as air-sparging and activated carbon, which produce secondary waste streams, and biological methods, which can yield toxic byproducts such as dichloroethylene and vinyl chloride. Unlike ethylene hydrogenation on platinum, which has been investigated in great detail,6 the surface reaction mechanism for hydrodechlorination has not been established. It has been reported that ethylene forms a π-bond at high ethylene pressures (∼1 atm) and ethylidyne (MtCCH3) bonded to Pt at low ethylene pressures (
