Noble metal-catalyzed homogeneous and heterogeneous processes in treating simulated nuclear waste media with formic acid

摘要

Simulants for the Hanford Waste Vitrification Plant feed containing the major non-radioactive components Al, Cd, Fe, Mn, Nd, Ni, Si, Zr, Na, CO3(sup 2)-, NO3-, and NO2- were used to study reactions of formic acid at 90(degrees)C catalyzed by the noble metals Ru, Rh, and/or Pd found in significant quantities in uranium fission products. Such reactions were monitored using gas chromatography to analyze the CO2, H2, NO, and N2O in the gas phase and a microammonia electrode to analyze the NH4+/NH3 in the liquid phase as a function of time. The following reactions have been studied in these systems since they are undesirable side reactions in nuclear waste processing: (1) Decomposition of formic acid to CO2 + H2 is undesirable because of the potential fire and explosion hazard of H2. Rhodium, which was introduced as soluble RhCl3-3H2O, was found to be the most active catalyst for H2 generation from formic acid above (approximately) 80(degrees)C in the presence of nitrite ion. The H2 production rate has an approximate pseudo first-order dependence on the Rh concentration, (2) Generation of NH3 from the formic acid reduction of nitrate and/or nitrite is undesirable because of a possible explosion hazard from NH4NO3 accumulation in a waste processing plant off-gas system. The Rh-catalyzed reduction of nitrogen-oxygen compounds to ammonia by formic acid was found to exhibit the following features: (a) Nitrate rather than nitrite is the principal source of NH3. (b) Ammonia production occurs at the expense of hydrogen production. (c) Supported rhodium metal catalysts are more active than rhodium in any other form, suggesting that ammonia production involves heterogeneous rather than homogeneous catalysis.