Bioinspired Complex-Nanoparticle Hybrid Catalyst System for Aqueous Perchlorate Reduction: Rhenium Speciation and Its Influence on Catalyst Activity

A highly active catalyst for reduction of the inert water contaminant perchlorate (ClO4-) to Cl- with 1 atm H-2 at 25 degrees C is prepared by noncovalently immobilizing the rhenium complex Re-v(O)-(hoz)(2)Cl (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline) together with Pd-0 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 ClOx- intermediates, whereas Pd-0 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 Re-v(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 Re-V and R-VII species serves as the main catalytic cycle for ClO4- reduction. Under reducing conditions, approximately half of the immobilized hoz-coordinated Re-V is further reduced to R-III, which is not directly reactive with ClO4-. A small fraction of the lioz-coordinated Re-VII species can dissociate to ReO4- and free hoz, which are then reductively reitmnobilized as a less reactive mixture of Re-v, Re-III, and Re-I 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 nanopatticles.