Matrix effects on coupled chemical reactions in solid-state voltammetry: disproportionation of uranium(V) in a silica xerogel

Abstract

Abstract The reduction of UO22+ in silica prepared by an acid-catalyzed sol-gel process was studied by cyclic voltammetry in the absence of a contacting liquid phase. The silica was prepared to have residual water on the order of 20% by weight. The UV that was generated electrochemically from UO22+ disproportionated rapidly in the xerogel matrix at pH values where it was stable on the time scale of the experiment in aqueous solution. The voltammetry was not altered when the silica was templated with Triton X-114 to increase the pore diameter; hence, electrostatic interaction between the anionic sites on the xerogel backbone and the cationic electrolysis products was postulated as the cause of the enhanced disproportionation rate. Comparison with the voltammetry of UO22+ in Nafion® supported the hypothesis of UIV formation by disproportionation of the UV. Doping the xerogel with La3+, which has a high affinity for anionic sites, blocked the formation of UIV; a quasi-reversible one-electron reduction of UO22+ was observed, thereby providing a positive test of the hypothesis.