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

摘要

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 (ClO_4~-) with H_2 at a rate ～100 times faster than the first generation ReO_x-Pd/C catalyst prepared from perrhenate (ReO_4~-). However, the immobilized Re(hoz)_2 complex was shown to partially decompose and leach into water as ReO_4~-, 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) Re~Ⅴ(hoz)_2 oxidation by ClO_4~- and its reduction intermediates ClO_X~-, (2) Re~Ⅶ(hoz)_2 reduction by Pd-activated hydrogen, and (3) hydrolytic Re~Ⅶ(hoz)_2 decomposition. When Re~Ⅴ(hoz)_2 oxidation is faster than Re~Ⅶ(hoz)_2 reduction, the Re~Ⅶ (hoz)_2 concentration builds up and leads to hydrolytic decomposition to ReO_4~- and free hoz ligand. Rapid Re~Ⅴ(hoz)_2 oxidation is mainly promoted by highly reactive ClO_X~- formed from the reduction of ClO_4~-. To mitigate Re(hoz)_2 decomposition and preserve catalytic activity, ruthenium (Ru) and rhodium (Rh) were evaluated as alternative H_2 activators to Pd. Rh showed superior activity for reducing the ClO_3~- intermediate to Cl~-, thereby preventing ClO_x~- buildup and lowering Re complex decomposition in the Re(hoz)_2-Rh/C catalyst. In contrast, Ru showed the lowest ClO_3~- 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.