Environ. Sci. Technol. 2010, 44, 4716–4721
Influence of Rhenium Speciation on the Stability and Activity of Re/Pd Bimetal Catalysts used for Perchlorate Reduction J O N G K W O N C H O E , †,§ J O H N R . S H A P L E Y , ‡,§ T I M O T H Y J . S T R A T H M A N N , †,§ A N D C H A R L E S J . W E R T H * ,†,§ Department of Civil and Environmental Engineering, Department of Chemistry, and Center of Advanced Materials for the Purifications of Water with Systems, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
Received January 21, 2010. Revised manuscript received April 23, 2010. Accepted May 7, 2010.
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 H2-reducing environments and as a function of Re content. In oxic solutions, Re immobilization is dictated by sorption of the Re(VII) precursor, perrhenate (ReO4-), to activated carbon via electrostatic interactions. Under H2reducing conditions, Re immobilization is significantly improved and leaching is minimized by ReO4- reduction to more reduced species on the catalyst surface. X-ray photoelectron spectroscopy shows two different Re binding energy states under H2-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 H2-reducing conditions are key elements in stabilizing the active Re surface species that are needed for sustained catalytic perchlorate treatment.
Introduction Widespread perchlorate (ClO4-) contamination of soil and water sources results primarily from improper disposal of perchlorate-containing waste, including solid rocket propellant (1, 2). In addition, natural sources of perchlorate contamination have been detected, including Chilean-based fertilizer (3). Perchlorate impairs function of the thyroid gland, which has a major role in central nervous system develop* Corresponding author phone: (217)333-3822; fax: (217)333-6968; e-mail:
[email protected]. † Department of Civil and Environmental Engineering. § Center of Advanced Materials for the Purifications of Water with Systems. ‡ Department of Chemistry. 4716
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ment and skeletal growth (4). As a result, state regulatory standards have been implemented in Massachusetts (2 ppb) and California (6 ppb), and a federal health advisory level (15 ppb) has been set, while a regulatory standard is being considered (1). With potential drinking water regulations on the horizon, increasing efforts have been directed at the development of improved technologies for treating perchlorate. Two technologies are currently considered practical for perchlorate treatment: ion exchange and biological treatment. Ion exchange is highly effective, and perchlorate-selective resins are capable of meeting low parts per billion treatment goals with short reactor residence times (5, 6). However, ion exchange only serves to separate the contaminant from the treatment stream, and periodic resin regeneration produces brines containing high concentrations of perchlorate that require further treatment. Biological treatment is accomplished by anaerobic perchlorate-respiring bacteria (PRB) reducing perchlorate to chloride using an organic carbon source (e.g., acetate, ethanol) or hydrogen as the electron donor (7). Although effective in some applications (8, 9), biological processes are sensitive to varying water conditions (e.g., pH, presence of competing electron acceptors, such as nitrate and sulfate), and there remains significant public opposition to the use of biological processes for drinking water treatment because of concerns about pathogen introduction. Biological treatment may also not be ideal for intermittent treatment applications (e.g., periodic treatment of perchlorate-contaminated ion exchange regeneration brines (9)) because of slow startup times. In recent years, there have been efforts to treat perchlorate by chemical reduction, including treatment with nanoscale zerovalent Fe (10, 11) and monometallic metal hydrogenation catalysts (e.g., Pd, Ti, Cr) (12). However, perchlorate reduction by these processes is very slow under ambient temperature and pressure conditions because of the large activation barrier for perchlorate reduction by single-electron transfer mechanisms (10, 11). Nevertheless, facile reduction of perchlorate in homogeneous solutions by using rhenium complexes that react through two-electron, oxygen-atom transfer (OAT) mechanisms has been reported (13, 14). Perchlorate reduction is coupled with Re(V) oxidation to Re(VII), which can be subsequently reduced back to Re(V), thereby completing a catalytic cycle (14). However, practical use of homogeneous catalysts is difficult and the bulk reducing agents used in these studies (e.g., hypophosphorous acid, thioether) are not suitable for water treatment applications. Recently, Hurley and Shapley (15) achieved rapid reduction of perchlorate to chloride with no observed ClOx- intermediates in aqueous systems by immobilizing Re(VII) precursors (perrhenate and methyltrioxorhenium) on a commercial palladium-onactivated carbon catalyst (Pd-AC) and supplying hydrogen as an electron donor. Much higher rates of perchlorate reduction are achieved with the Re/Pd-AC catalyst at room temperature and pressure under mildly acidic conditions compared to aqueous chemical reduction processes previously reported (no perchlorate reduction observed with hydrogen-fed Pd-AC alone). A mechanism was proposed in which Pd converts H2 into adsorbed atomic hydrogen, a potent reductant, which then reduces adsorbed Re(VII) precursors to Re(V) species that react with perchlorate by OAT reactions at the catalyst-water interface. Such a catalyst may be useful for treatment of perchlorate-contaminated water when used alone or more likely as part of a hybrid ion exchange/catalytic reduction process, where ion exchange is used to remove perchlorate from contaminated water and 10.1021/es100227z
2010 American Chemical Society
Published on Web 05/20/2010
a catalytic process is used to treat and recycle perchloratecontaminated brines, (which are often mildly acidic to prevent scaling in pipelines) produced by periodic resin regeneration. Several heterogeneous catalysts with noble metals have been reported for their applicability in the treatment of both inorganic and organic contaminants in drinking water (16-18). A common concern regarding Pd-based catalysts is cost. While Pd is more expensive than common nonplatinum metals, Davie et al. (16) demonstrated in a 100-day pilot study that Pd-catalyzed treatment of groundwater contaminated with trichloroethene is less expensive than activated carbon treatment ($8/1000 gal vs $10-36/1000 gal). Although promising, many questions regarding the use of heterogeneous Re/Pd bimetal catalysts for perchlorate treatment remain; among these is a better understanding of Re immobilization on Pd-AC and its role in the catalytic reduction process. This contribution examines the factors controlling Re sorption and leaching by the Pd-AC host material and the influence of Re speciation on both catalyst activity and long-term stability when treating perchloratecontaminated water. Specific objectives are to (i) determine the influence of Re sorption on catalyst activity, (ii) characterize the uptake and leaching of Re under variable solution conditions, (iii) characterize the redox speciation of sorbed Re under oxic versus H2-reducing conditions, and (iv) obtain mechanistic insights into the heterogeneous catalytic reaction mechanism. Results from this work will be used to improve the longevity and sustainability of catalytic treatment systems for perchlorate and related oxyanions that have proven difficult to treat (e.g., nitrate) using conventional technologies.
Experimental Section Chemical Reagents and Catalyst Preparation. Sources of chemical reagents are provided in Supporting Information (SI). The procedure used for preparation of Re/Pd-AC catalysts was modified from that reported previously (15) because initial tests with catalysts that were isolated and dried in air showed significant leaching of Re surface deposits when used in subsequent batch reactions (to be discussed). The commercial Pd-AC powder (nominal 5 wt % Pd on Degussa-type E101 activated carbon) was wet sieved to obtain particles
