| A series of heterogeneous catalysts capable of fully reducing perchlorate to chloride in water at room temperature using hydrogen gas have been developed. These systems represent the first examples of heterogeneous catalysts capable of efficiently reducing perchlorate in conditions amenable with drinking water treatment applications. The primary catalyst systems are all comprised of a Re(VII) or Re(V) precursor supported on activated carbon decorated with Pd nanoparticles.;The Re(VII) precursors, methyltrioxorhenium (MTO) or the cheaper perrhenate, both exhibited pseudo first-order reaction kinetics. Perchlorate was reduced to below levels of detection (5 ppb). Chloride was the only product observed and good mass balance was achieved. The Re normalized rate constant, k(Re), under standard conditions was 2.9 Lh-1(g-Re)-1. Reduction of perchlorate was found to be insensitive to product inhibition up to 1000 ppm Cl-, but both rate and extent of reaction were highly dependent on pH. Lack of buildup of any intermediates, along with the observed pH dependence, suggest a proton-assisted oxygen atom transfer from perchlorate to the active form of the catalyst is the rate determining step in the reduction. The catalyst is believed to operate on a Re(V)-Re(VII) couple, with activated H2 reducing Re(VII) to a coordinatively unsaturated Re(V) active site which is capable of abstracting an oxygen atom from perchlorate, releasing chlorate, and reforming the Re(VII) species.;To prove a Re(V) precursor is capable of forming an active catalyst the Re(V) complex chlorobis(2-(2'-hydroxyphenyl)-2-oxazoline)-oxorhenium(V) (Re(O) 2C1(hoz)2), was added to the standard Pd/C support by the incipient wetness technique. Under standard conditions the normalized reduction rate constant, k(Re), was 27.6 Lh-1(g-Re)-1, clearly indicating that the ligand field plays a significant role in determining catalyst activity. An examination of the dependence of the observed reduction rate on pH revealed the involvement of one proton in the rate determining step of the catalytic cycle. The reduction of perchlorate by Re(O)2Cl(hoz) 2/Pd/C follows pseudo-first order kinetics with chloride being the only product observed. XPS analysis of this material showed binding energies consistent with Re(VII) on the surface both before and after use as a reduction catalyst.;To probe the ligand effect, a series of catalyst were prepared by supporting trans-[Re(O)2L4]+ X, (L = pyridine, 4-methylpyridine, 4-methoxypyridine, or 4dimethylaminopyridine, X = Cl - or [ReO4]-) or trans-[ReO 2(L-L)2]I (L-L = ethylenediamine) on nominally 5 wt.% Pd on activated carbon by incipient wetness. All of the supported Re(V) complexes examined by XPS yielded binding energies showing only a +7 oxidation state, while examination of an unsupported complex clearly showed both Re(V) and Re(VII) as being present. Unlike previous materials, these complexes had never been demonstrated as oxygen atom transfer catalysts. All materials completely reduced perchlorate to chloride in the presence of 2mM excess Cl- while exhibiting pseudo-first order kinetics. Supported catalyst activity increased with the e--donating ability of the para-substituent on the pyridine ligands. |
