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.