Influence of nickel loading on reactivity of Ni/Fe bimetallic nanoparticles toward trichloroethene and carbon tetrachloride†

Bimetallic Ni/Fe-nanoparticles were developed to enhance the dechlorination reactivity of nano-sized zero-valent iron. The physical structures of Ni/Fe-NPs with an Ni loading ranging from 0.5 wt% to 20 wt% and their structure-dependent reactivity variations towards trichloroethene (TCE) and carbon tetrachloride (CT) were fully investigated. A Ni-accumulated surface was observed for the Ni/Fe-NPs with a high Ni loading (20 wt%), and the structure of the other Ni/Fe NPs was identified as a Ni/Fe alloy-like structure, with the 5 wt% Ni/Fe NPs owning the highest surface area and Fe⁰ content. While the best CT dechlorination rate was 2.5-fold that of B-nZVI at 5 wt% Ni loading, the best TCE reduction was 12-fold of B-nZVI at a medium Ni loading (3–5 wt%). Given that the primary TCE degradation mechanism was via atomic hydrogen (H*), and the degradation of CT proceeds via direct electron transfer, the more efficient reduction mechanism for the Ni/Fe NP system was preferably H* reduction. The variation in the reduction rate and the by-product yield between the medium loading (3–5 wt%) and low/high (0.5 wt% and 20 wt%) loading was more significant for TCE than CT. It was found that the medium Ni loading (3–5 wt%) obviously boosted the β-elimination of TCE to VC due to the good storage of H* in the Ni catalyst. The production of H* and enhanced electron migration rate were well demonstrated by the CV curve and Tafel curve, respectively. The location of direct electron transfer and H* catalyst in the bimetallic Ni/Fe system was further discussed.