Ligand-enhanced reduction of perchlorate in water with heterogeneous Re-Pd/C catalysts.

New bimetallic heterogeneous catalysts for the reduction of perchlorate in water with dihydrogen have been prepared in two ways: (1) by impregnating 5 wt % Pd on activated carbon powder with one of the complexes trans-[ReO(2)(py-X)(4)](+) (py-X = 4-substituted pyridine; X = H, Me, OMe, NMe(2)) or (2) by adsorbing perrhenate onto the Pd/C powder in the presence of the pyridine ligand under a hydrogen atmosphere. Both sets of catalysts are highly active at pH 2.7-3.0 (HCl), with observed rates increasing with varying X in the order H < Me < OMe < NMe(2), which supports a rate-determining oxygen atom transfer reaction involving the Re(V) centers. However, the materials prepared by method (1) show evidence of facile formation of perrhenate from handling under ambient conditions. A system prepared by method (2) with 2 equiv of dimethylaminopyridine per Re is especially active and stable up to pH 4.5.