PNNL

Abstract

Gibbsite (aluminum hydroxide, Al(OH)3) nanoparticles, synthesized from aluminum chloride or aluminum nitrate were doped with metal ions, Cr(III) or Fe(III), then irradiated with gamma rays to determine the effect of the dopants on radiolytic hydrogen (H2) production and radical generation. The addition of Cr(III) and Fe(III) ions at concentrations of 0.5% or 5% decreased the concentration of stable oxygen-centered radicals, with the strongest suppression in Cr(III) doped samples. A decrease in H2 yields was observed with increasing Cr(III) or Fe(III) concentration with the greatest effect observed for the Fe(III) doped samples. Reduction of the Cr(III) to Cr(II) and Fe(III) to Fe(II) was also observed, probably due to scavenging of radiolytically produced electrons. However, further processes differ for Fe(III) and Cr(III) doped systems. Both ions are reduced by the free electrons leading to a decrease in H2 production, but they react differently with the oxygen radicals. Cr(III) can be oxidized by oxygen radicals whereas Fe(III) is not. Fe(II) can interact with peroxides, possible products of intermediate oxygen oxidation, converting back to Fe(III), and leaving oxygen radicals behind. These oxidation reactions lead to a difference in the observed relative effects on H2 yields and oxygen radical production between Cr(III) and Fe(III) doped gibbsite. The connection between electron scavenging and H2 production indicates that radiolytically produced electrons are precursors to H2.