MATRIX EFFECTS ON COUPLED CHEMICAL-REACTIONS IN SOLID-STATE VOLTAMMETRY - DISPROPORTIONATION OF URANIUM(V) IN A SILICA XEROGEL

The reduction of UO22+ in silica prepared by an acid-catalyzed sol-gel process was studied by cyclic voltammetry in the absence of a contact ing liquid phase. The silica was prepared to have residual water on th e order of 20% by weight. The U-V that was generated electrochemically from UO22+ disproportionated rapidly in the xerogel matrix at pH valu es where it was stable on the time scale of the experiment in aqueous solution. The voltammetry was not altered when the silica was template d with Triton X-114 to increase the pore diameter; hence, electrostati c interaction between the anionic sites on the xerogel backbone and th e cationic electrolysis products was postulated as the cause of the en hanced disproportionation rate. Comparison with the voltammetry of UO2 2+ in Nafion(R) supported the hypothesis of U-IV formation by dispropo rtionation of the U-V. Doping the xerogel with La3+, which has a high affinity for anionic sites, blocked the formation of U-IV; a quasi-rev ersible one-electron reduction of UO22+ was observed, thereby providin g a positive test of the hypothesis. (C) 1998 Elsevier Science S.A. Al l rights reserved.