Sol-gel entrapped Au0- and Ag0-nanoparticles catalyze reductive de-halogenation of halo-organic compounds by BH4−

Abstract This study investigated the reductive de-halogenations of toxic Br 3 CCO 2 − , Br 2 CHCO 2 − , BrCH 2 CO 2 − , CH 3 CHBrCO 2 − , CH 2 BrCH 2 CO 2 − , CH 2 BrCHBrCO 2 − , Cl 3 CCO 2 − , Cl 2 CHCO 2 - and ClCH 2 CO 2 − by sodium borohydride catalyzed by sol-gel silica entrapped Au 0 and Ag 0 nanoparticles. The results indicate that the mechanism of reduction of Br 3 CCO 2 − differs from that of Cl 3 CCO 2 − . Calculated by DFT, the source of this difference lies in the larger bond strength of C Cl compared to that of C Br and the weaker M 0 C bond strength in Au 0 -CBr 2 CO 2 − compared to those of Au 0 -CCl 2 CO 2 − and Au 0 -CH 2 CO 2 − . Furthermore, the de-halogenation mechanisms catalyzed by Ag 0 -NPs differ from those catalyzed by Au 0 -NPs. The latter observation is attributed to the different Ag C and Au C bond strengths and to the different over-potentials for H 2 release of these M 0 -NPs. In addition, product composition depends on the rate of BH 4 − addition. Proton labeling experiments prove that nearly all the hydrogen atoms in the products originated from the water solvent and not from the BH 4 − . The detailed mechanistic conclusions that can be drawn from these results differ considerably from those commonly accepted for de-halogenation processes.