Influence of Rhenium Speciation on the Stability and Activity of Re/Pd Bimetal Catalysts used for Perchlorate Reduction

Abstract

Recent work demonstrates reduction of aqueous perchlorate by hydrogen at ambient temperatures and pressures using a novel rhenium-palladium bimetal catalyst immobilized on activated carbon (Re/Pd-AC). This study examines the influence of Re speciation on catalyst activity and stability. Rates of perchlorate reduction are linearly dependent on Re content from 0-6 wt %, but no further increases are observed at higher Re contents. Surface-immobilized Re shows varying stability and speciation both in oxic versus H(2)-reducing environments and as a function of Re content. In oxic solutions, Re immobilization is dictated by sorption of the Re(VII) precursor, perrhenate (ReO(4)(-)), to activated carbon via electrostatic interactions. Under H(2)-reducing conditions, Re immobilization is significantly improved and leaching is minimized by ReO(4)(-) reduction to more reduced species on the catalyst surface. X-ray photoelectron spectroscopy shows two different Re binding energy states under H(2)-reducing conditions that correspond most closely to Re(V)/Re(IV) and Re(I) reference standards, respectively. The distribution of the two redox states varies with Re content, with the latter predominating at lower Re contents where catalyst activity is more strongly dependent on Re content. Results demonstrate that both lower Re contents and the maintenance of H(2)-reducing conditions are key elements in stabilizing the active Re surface species that are needed for sustained catalytic perchlorate treatment.

Application of a Re–Pd bimetallic catalyst for treatment of perchlorate in waste ion-exchange regenerant brine

“…The Pd/C material was treated and characterized as described previously (Choe et al, 2010). NH 4 ReO 4 , NaClO 4 , Na 2 SO 4 , Na 2 HPO 4 and NH 4 Cl were purchased from SigmaeAldrich.…”

“…ClO 4 À treatment in most synthetic brine solutions, the active ReePd/C catalyst was prepared in situ prior to introducing ClO 4 À , using procedures modified from those reported previously (Choe et al, 2010). To facilitate comparison of catalyst performance in brine solutions with previous work conducted in DI water (Hurley and Shapley, 2007), the same catalyst loading and formulation (2 g/L, nominally 5 wt% Re and 5 wt% Pd) were used for batch reactions conducted here.…”

“…Recently, our team reported on the development of heterogeneous bimetallic catalysts, which combine small amounts of rhenium and palladium on activated carbon supports (ReePd/C), that reduce aqueous ClO 4 À to Cl À using hydrogen (H 2 ) as the electron donor under ambient temperature and pressure (Hurley and Shapley, 2007;Hurley et al, 2009;Choe et al, 2010). Rates of ClO 4 À reduction by ReePd/C were found to be many times faster than other aqueous chemical reduction processes reported, including nanoscale zerovalent iron (Cao et al, 2005;Xiong et al, 2007) and most other metal catalysts (Wang et al, 2008), and none of the intermediate oxyanions (ClO 3 À , ClO 2 À , ClO À ) were detected during reactions.…”

Physical–Chemical Processes for Nitrogen Removal

“…The sand filter and granular activated carbon were used to remove microbial byproducts and reduce subsequent biofouling of the nitrate-selective resin. Other approaches to ion exchange brine treatment and reuse include electrochemical reduction of nitrate (Jelinek et al, 2004;Paidar et al, 2004) and the potential for catalytic reduction (Choe et al, 2010). Overall regeneration using recycled brine combined with biological denitrification is the most mature process for treatment and reuse of ion exchange brine (Lehman et al, 2008).…”

Pd-catalytic hydrodechlorination of chlorinated hydrocarbons in groundwater using H2 produced by a dual-anode system

“…Pd-catalytic reductive hydrodechlorination shows a promise for the remediation of CHCs, offering many advantages over other processes such as quick reaction, no halogenated intermediates and applicability for a broad range of reducible compounds (Agarwal et al, 2007;Knitt et al, 2007;Munakata and Reinhard, 2002;Shuai et al, 2012). Although it has been tested extensively in lab (Choe et al, 2010;Janiak and Okal, 2009;Zhu and Lim, 2007), large-scale treatments in the field are still limited because of the challenges such as H 2 supply and catalyst fouling (Chaplin et al, 2012b;Kopinke et al, 2004).…”