Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Sol-gel entrapped Au 0 - and Ag 0 -nanoparticles catalyze reductive de- halogenation of halo-organic compounds by BH 4 - Jaydeep Adhikary a, ⁎ , Dan Meyerstein a,b, ⁎ , Vered Marks a , Michael Meistelman c , Gregory Gershinsky d , Ariela Burg e , Dror Shamir f , Haya Kornweitz a , Yael Albo c, ⁎⁎ a Chemical Sciences Dept., Ariel University, Ariel, Israel b Chemistry Dept., Ben-Gurion University, Beer-Sheva, Israel c Chemical Engineering, Biotechnology and Materials Dept., Ariel University, Ariel, Israel d Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Israel e Chemical Engineering Dept., Sami Shamoon College of Engineering, Beer-Sheva, Israel f Chemistry Dept., Nuclear Research Centre Negev, Beer-Sheva, Israel ARTICLE INFO Keywords: Sol-gel Silica matrix Gold and silver nanoparticles Dehalogenation Mechanistic study 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 bor- ohydride catalyzed by sol-gel silica entrapped Au 0 and Ag 0 nanoparticles. The results indicate that the me- chanism of reduction of Br 3 CCO 2 - differs from that of Cl 3 CCO 2 - . Calculated by DFT, the source of this dif- ference lies in the larger bond strength of CeCl compared to that of CeBr and the weaker M 0 eC 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 AgeC and AueC 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. 1. Introduction Halogenated organic compounds are widely used as starting mate- rials, intermediates and solvents in the chemical industry, in agri- cultural chemistry, and in medicinal chemistry [1,2]. Considered major environmental pollutants [3], however, representatives of these che- micals comprise most of the top one hundred species in the list of ha- zardous compounds [4,5]. Chief among the halo-organic compounds are halo-acetic acids (HAAs), whose toxicity, strong bioaccumulation and persistence constitute significant threats to the environment and to human health through, among other things, their deleterious effects on water quality [6]. The formation of these halo-acetic acids (HAAs) during the treatment of water and wastewater with chlorine or bromine for disinfection purposes is the most significant source of halo-acetic acids in wastewater [7,8]. The U.S. Environmental Protection Agency proposed a maximum contaminant level of 60 μg/L for the sum of the concentrations of five HAAs, i.e., mono-chloro-acetic acid (MCAA), di- chloro-acetic acid (DCAA), tri-chloro acetic acid (TCAA), mono-bromo- acetic acid (MBAA) and di-bromo-acetic acid (DBAA) [9]. Typical ha- logenated disinfection by-products, exhibit high toxicity, geno-toxicity, and carcinogenicity [10]. Consequently, researchers the world over are working to develop a viable method for their removal from waste streams [11]. Recently it was shown that gold metal nanoparticles (Au 0 - NPs) entrapped in SiO 2 sol-gel matrices catalyze the de-halogenation of HAAs by BH 4 - [12]. The results indicated that the mechanisms of these degradation processes depend on the rate of BH 4 - addition and differ for the de-halogenation of Cl 3 CCO 2 - and Br 3 CCO 2 - [12a]. It is commonly assumed that the de-halogenation of X 3 CCO 2 - proceeds in steps, wherein X 2 CHCO 2 - and XCH 2 CO 2 - are formed as successive inter- mediates [13]. However, recent results for the Au 0 -NPs catalyzed de- bromination of Br 3 CCO 2 - suggested that this is not always the case [12a], where de-bromination of Br 3 CCO 2 - is leading to succinic acid https://doi.org/10.1016/j.apcatb.2018.08.040 Received 17 April 2018; Received in revised form 1 August 2018; Accepted 17 August 2018 ⁎ Corresponding authors at: Chemical Sciences Dept., Ariel University, Ariel, Israel. ⁎⁎ Corresponding author. E-mail addresses: adhikaryj86@gmail.com (J. Adhikary), danmeyer@bgu.ac.il (D. Meyerstein), yaelyt@ariel.ac.il (Y. Albo). Applied Catalysis B: Environmental 239 (2018) 450–462 Available online 18 August 2018 0926-3373/ © 2018 Published by Elsevier B.V. T