Tuning the surfaces of palladium nanoparticles for the catalytic conversion of Cr(VI) to Cr(III)

This work reports the feasibility of using Pd nanoparticles as innovative catalysts in the conversion of reducible contaminants from toxic to benign forms. Cr(vi) is a known carcinogen while the trivalent chromium salts are believed to be non-toxic. The ability of Pd nanoparticles to catalyze the rapid reduction of Cr(vi) to Cr(iii) using reactive sulfur intermediates produced in situ was therefore studied. Using a microchamber set at 130°C, the reduction mixture consists of palladium nanoparticles and sulfur (PdNPs/S), which generated highly reducing sulfur intermediates that effected the reduction of Cr(vi) to Cr(iii) by 1st order reaction kinetics. UV-visible spectroscopy and cyclic voltammetry were employed to monitor the reduction process. The results showed that 99.8% of 400 μM Cr(vi) was reduced to Cr(iii) by PdNPs/S in one hour compared to 2.1% by a control experiment consisting of sulfur only. The rate of Cr(vi) reduction was found to be dependent on temperature and pH and was greatly enhanced by the addition of PdNPs. Subsequent application of this approach in the reduction of Cr(vi) in soil and aqueous media was conducted. In contrast to the control experiments with and without PdNPs or sulfur, greater than 92% conversion rate was obtained in the presence of PdNPs/S within 1 hour. This represents over a 500-fold improvement in conversion rate compared to current microbial approaches. XPS analysis provided the confirmation regarding the oxidation states of Cr(vi), Cr(iii) and the nature of the reactive intermediates. This work offers PdNPs/S as a new interface for the reduction of high oxidation state heavy metal pollutants.