A highly active catalyst for reduction of the inert water contaminant perchlorate (ClO4 –) to Cl– with 1 atm H2 at 25 °C is prepared by noncovalently immobilizing the rhenium complex ReV(O)­(hoz)2Cl (hoz = 2-(2′-hydroxyphenyl)-2-oxazoline) together with Pd0 nanoparticles on a porous carbon support. Like the Mo complex centers in biological oxyanion reductases, the immobilized Re complex serves as a single site for oxygen atom transfer from ClO4 – and ClO x – intermediates, whereas Pd0 nanoparticles provide atomic hydrogen reducing equivalents to sustain redox cycling of the immobilized Re sites, replacing the more complex chain of electron transfer steps that sustain Mo centers within oxyanion reductases. An in situ aqueous adsorption method of immobilization was used to preserve the active ReV(O)­(hoz)2 structure during bimetallic catalyst preparation and enable study of Re redox cycling and reactions with ClO4 –. Heterogeneous reaction kinetics, X-ray photoelectron spectroscopy, and experiments with homogeneous model Re complexes are combined to obtain insights into the catalytic reaction mechanisms and the influence of Re speciation on catalyst reactivity with ClO4 –. Redox cycling between hoz-coordinated ReV and ReVII species serves as the main catalytic cycle for ClO4 – reduction. Under reducing conditions, approximately half of the immobilized hoz-coordinated ReV is further reduced to ReIII, which is not directly reactive with ClO4 –. A small fraction of the hoz-coordinated ReVII species can dissociate to ReO4 – and free hoz, which are then reductively reimmobilized as a less reactive mixture of ReV, ReIII, and ReI species. This study provides an example wherein highly active metal complexes that were originally developed for homogeneous organic phase catalysis can be incorporated into heterogeneous catalysts for practical environmental applications. Findings suggest a general blueprint for developing hybrid catalysts combining single-site transition metal complexes with hydrogen-activating metal nanoparticles.

Pd-based catalyst treatment represents an emerging technology that shows promise to remove nitrate and nitrite from drinking water. In this work we use vapor-grown carbon nanofiber (CNF) supports in order to explore the effects of Pd nanoparticle size and interior versus exterior loading on nitrite reduction activity and selectivity (i.e., dinitrogen over ammonia production). Results show that nitrite reduction activity increases by 3.1-fold and selectivity decreases by 8.0-fold, with decreasing Pd nanoparticle size from 1.4 to 9.6 nm. Both activity and selectivity are not significantly influenced by Pd interior versus exterior CNF loading. Consequently, turnover frequencies (TOFs) among all CNF catalysts are similar, suggesting nitrite reduction is not sensitive to Pd location on CNFs nor Pd structure. CNF-based catalysts compare favorably to conventional Pd catalysts (i.e., Pd on activated carbon or alumina) with respect to nitrite reduction activity and selectivity, and they maintain activity over multiple reduction cycles. Hence, our results suggest new insights that an optimum Pd nanoparticle size on CNFs balances faster kinetics with lower ammonia production, that catalysts can be tailored at the nanoscale to improve catalytic performance for nitrite, and that CNFs hold promise as highly effective catalyst supports in drinking water treatment.

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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.