Reduction of carbon tetrachloride by nanoscale palladized zero-valent iron@ graphene composites: Kinetics, activation energy, effects of reaction conditions and degradation mechanism

Abstract

Herein, we reported a study on using the catalytic performance of nanoscale palladized zero-valent iron@ graphene composites (PFGC) synthesized by using coprecipitation method to significantly enhance carbon tetrachloride (CT) dechlorination in the laboratory, and then the nanocomposite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunner-Emmet-Teler (BET), and X-ray photoelectron spectroscopy (XPS). The catalytic activity was investigated in consideration of several operation parameters including the initial pH, temperature, the dosage of nanocatalyst and the optimal content of palladium metal. The results showed that 50.4% and 93.5% CT (Cinitial=3mgL−1) were removed from aqueous solution by nZVI (Nanoscale zero-valent iron, 0.5gL−1) and PFGC (Nanoscale palladized zero-valent iron@ graphene composites, using Fe/GO=2:1 and Pd/Fe=0.75%, 0.5gL−1), respectively. The degradation kinetics of CT in all experiments followed pseudo first-order reaction kinetics. Moreover, it has been found that palladium metal (Pd0) accelerated hydrogen formation and activation to generate atomic hydrogen (·H) which in turn behaved as the reducing agents responsible for the dechlorination reactions as presented in the conceptual model. And the activation energies reduced from 44.42 to 30.18kJ/mol. However, it was very easy to agglomerate and form large-sized particles due to the high specific surface energy, hence, graphene as the supporting matrix graphically depicted in the SEM images increased specific surface areas to improve the dispersibility of bimetallic nanoparticles. After five successive runs, there was no significant reduction in the removal efficiency indicating a good reusability and stability of PFGC.