REDOX POTENTIAL MONITORING AS A METHOD TO CONTROL UNWANTED NOBLE METAL-CATALYZED HYDROGEN GENERATION FROM FORMIC-ACID TREATMENT OF SIMULATED NUCLEAR WASTE MEDIA

Simulants for the Hanford Waste Vitrification Plant feed containing th e major nonradioactive components Al, Cd, Fe, Mn, Nd, Ni, Si, Zr, Na, CO32-, NO3-, and NO2- were used to study redox potential changes in re actions of formic acid at 90 degrees C catalyzed by the noble metals R u, Ph, and/or Pd found in significant quantities in uranium fission pr oducts. Such reactions were monitored using gas chromatography to anal yze the CO2, H-2, NO, and N2O in the gas phase and a redox electrode t o follow redox potential changes as a function of time. In the initial phase of formic acid addition to nitrite-containing feed simulants, t he redox potential of the reaction mixture rises typically to +400 mV relative to the Ag/AgCl electrode because of the generation of the mod erately strongly oxidizing nitrous acid. No H-2 production occurs at t his stage of the reaction as long as free nitrous acid is present. Aft er all of the nitrous acid has been destroyed by reduction to N2O and NO and disproportionation to NO/NO3-, the redox potential decreases up on further formic acid addition. Hydrogen production typically begins to occur when the redox potential of the reaction mixture becomes more negative than the AS/ASCl electrode. The experiments outlined in this paper suggest the feasibility of controlling the production of H-2 by limiting the amount of formic acid used and monitoring the redox pote ntial during formic acid treatment.