Mechanism and Mitigation of the Decomposition of an Oxorhenium Complex-Based Heterogeneous Catalyst for Perchlorate Reduction in Water

A biomimetic heterogeneous catalyst combining palladium nanoparticles and an organic ligand-coordinated oxorhenium complex on activated carbon, Re(hoz)2-Pd/C, was previously developed and shown to reduce aqueous perchlorate (ClO4 -) with H2 at a rate ∼100 times faster than the first generation ReOx-Pd/C catalyst prepared from perrhenate (ReO4 -). However, the immobilized Re(hoz)2 complex was shown to partially decompose and leach into water as ReO4 -, leading to an irreversible loss of catalytic activity. In this work, the stability of the immobilized Re(hoz)2 complex is shown to depend on kinetic competition between three processes: (1) ReV(hoz)2 oxidation by ClO4 - and its reduction intermediates ClOx -, (2) ReVII(hoz)2 reduction by Pd-activated hydrogen, and (3) hydrolytic ReVII(hoz)2 decomposition. When ReV(hoz)2 oxidation is faster than ReVII(hoz)2 reduction, the ReVII(hoz)2 concentration builds up and leads to hydrolytic decomposition to ReO4 - and free hoz ligand. Rapid ReV(hoz)2 oxidation is mainly promoted by highly reactive ClOx - formed from the reduction of ClO4 -. To mitigate Re(hoz)2 decomposition and preserve catalytic activity, ruthenium (Ru) and rhodium (Rh) were evaluated as alternative H2 activators to Pd. Rh showed superior activity for reducing the ClO3 - intermediate to Cl-, thereby preventing ClOx - buildup and lowering Re complex decomposition in the Re(hoz)2-Rh/C catalyst. In contrast, Ru showed the lowest ClO3 - reduction activity and resulted in the most Re(hoz)2 decomposition among the Re(hoz)2-M/C catalysts. This work highlights the importance of using mechanistic insights from kinetic and spectroscopic tests to rationally design water treatment catalysts for enhanced performance and stability.