A New Bioinspired Perchlorate Reduction Catalyst with Significantly Enhanced Stability via Rational Tuning of Rhenium Coordination Chemistry and Heterogeneous Reaction Pathway

摘要

Rapid reduction of aqueous ClO_4~- to Cl~- by H_2 has been realized by a heterogeneous Re(hoz)_2-Pd/C catalyst integrating Re(O)(hoz)_2Cl complex (hoz = oxazolinyl-phenola-to bidentate ligand) and Pd nanopartides on carbon support, but ClO_x~- intermediates formed during reactions with concentrated ClO_4~- promote irreversible Re complex decomposition and catalyst deactivation. The original catalyst design mimics the microbial ClO_4~- reductase, which integrates Mo(MGD)_2 complex (MGD = molybdopterin guanine dinudeotide) for oxygen atom transfer (OAT). Perchlorate-reducing microorganisms employ a separate enzyme, chlorite dismutase, to prevent accumulation of the destructive ClO_2~-intermediate. The structural intricacy of MGD ligand and the two-enzyme mechanism for microbial ClO_4~- reduction inspired us to improve catalyst stability by rationally tuning Re ligand structure and adding a ClO_x~- scavenger. Two new Re complexes, Re(O)(hfe)_2Cl and Re(0)(hoz)(htz)Cl (htz = thiazolinyl-phenolato bidentate ligand), significantly mitigate Re complex decomposition by slightly lowering the OAT activity when immobilized in Pd/C. Further stability enhancement is then obtained by switching the nanoparticles from Pd to Rh, which exhibits high reactivity with ClO_x~- intermediates and thus prevents their deactivating reaction with the Re complex. Compared to Re(hoz)_2-Pd/C, the new Re(hoz)(htz)-Rh/C catalyst exhibits similar ClO_4~- reduction activity but superior stability, evidenced by a decrease of Re leaching from 37% to 0.25% and stability of surface Re speciation following the treatment of a concentrated "challenge" solution containing 1000 ppm of ClO_4~-. This work demonstrates the pivotal roles of coordination chemistry control and tuning of individual catalyst components for achieving both high activity and stability in environmental catalyst applications.