Hydrodechlorination of trichloroethylene to hydrocarbons using bimetallic nickel-iron nanoparticles

High surface-area nickel-iron nanoparticles (1:3 Ni:Fe) have been studied as a reagent for the dehalogenation of trichloroethylene (TCE). Ni-Fe (0.1 g) nanoparticles reduced TCE from a 40-mL saturated aqueous solution (24 ppm) to <6 ppb in 120 min. The dehalogenation reaction, based on the surface area normalized rate constant, was 50-80 times slower using nanoiron or iron filings, respectively. On the bimetallic particles, the reaction occurs by nickel-catalyzed hydrodechlorination. As the iron actively corrodes, the cathodically protected nickel surface chemisorbs hydrogen ions. TCE adsorbed to the Ni surface is thus hydrogenated. This reaction competes kinetically with the evolution of molecular hydrogen. Hydrogenation of the C-Cl bond results in the formation of linear as well as branched saturated and unsaturated hydrocarbons. The final TCE degradation products are predominantly even-numbered saturated hydrocarbons, such as butane, hexane, and octane. The toxic dehalogenation products vinyl chloride (VC), 1,1-dichloroethylene (1,1-DCE), cis-dichloroethylene (cis-DCE), and trans-dichloroethylene (trans-DCE) form only in trace amounts and do not persist. A kinetic solvent isotope effect of k_H2O/_D2O = 14 was observed, which indicates that C-H bond formation is rate-determining. This explains why VC and DCE, which are formed at zerovalent iron surfaces by a reaction in which electron transfer is the rate-determining step, do not accumulate. The fast reaction rate and the absence of toxic side products suggest that bimetallic nanoparticles containing good catalysts for hydrogenation (e.g., Ni, Pd, Pt) should be improved materials for the in situ or ex situ dehalogenation of chlorinated organics.