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 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 ClOx– 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.