Heterogeneous Catalytic Reduction of Perchlorate in Water with Re−Pd/C Catalysts Derived from an Oxorhenium(V) Molecular Precursor

Abstract

The molecular Re(V) complex, chlorobis(2-(2'-hydroxyphenyl)-2-oxazoline)-oxorhenium(V), Re(O)(hoz)(2)Cl, has been investigated as a suitable precursor, when combined with activated carbon powder containing 5 wt % Pd, to provide a heterogeneous catalyst for the reduction of aqueous perchlorate by hydrogen. Two general methods for catalyst preparation have been adopted: first, by a standard "incipient wetness" impregnation of the carbon powder with handling under largely aerobic conditions for convenience and, second, by a completely anaerobic procedure maintaining a hydrogen atmosphere during adsorption of the complex in water onto the powder. Both types of catalyst were efficient for the complete reduction of perchlorate to chloride within a few hours at room temperature over a range of initial concentrations (2-200 ppm) under 1 atm of H(2) and acidic conditions (pH 2.7-3.7). The perchlorate reduction profiles displayed pseudo-first-order kinetics, and the rates were insensitive to excess chloride. Complete reduction of perchlorate was observed even at pH 5.9 in a phosphate buffer over the course of two weeks. Under comparable conditions, chlorate reduction proceeded ca. 10 times more quickly than perchlorate reduction. The impregnated catalyst was examined by STEM/EDS, which revealed a wide distribution in Pd nanoparticle sizes and also suggested that the Re complex did not aggregate preferentially on or near the Pd particles. XPS of this material provided evidence for reduced Pd after the reaction, but only a +7 oxidation state was seen for the Re sites both pre- and postreduction. Elemental analyses of the catalyst materials taken pre- and postreduction showed variable amounts of Re loss (0-50%) but relatively unchanged amounts of nitrogen. These results show the need to maintain a reducing atmosphere during the preparation and operation of the catalyst in order to achieve optimum activity and stability.