Heterogeneous catalytic redox reactions of neptunium ions in the aqueous system HNO3-HCOOH

The redox behaviours of neptunium(VI,V) ions in aqueous solutions containing nitric and formic acids in the presence of Pt/SiO2 catalyst have been studied. It was shown that neptunium(VI), being quite stable in 0.1-2.0 M HNO3 and 0.1-1.5 M HCOOH, is reduced effectively to neptunium(V) in the presence of ∝ 1 wt.% Pt/SiO2. In the presence of 0.05-0.5 M urea (anti-nitrous agent) the reduction of neptunium(VI) proceeds to neptunium(IV) through the stage of the neptunium(V) formation. The effects of the solid-to-liquid mass to volume ratio, the concentration of urea, the concentration of nitric acid, the concentration of formic acid and the temperature on the rates of the Np(VI) and Np(V) reductions in the presence of ∝ 1 wt.% Pt/SiO2 were investigated. It was proposed that both reactions proceed through the stage of the HCOOH activated adsorption on Pt catalyst. Mechanisms of the Np(VI) and Np(V) catalytic reductions by formic acid were postulated. (orig.)