Noble Metal Catalyzed Hydrogen Generation from Formic Acid in Nitrite-Containing Simulated Nuclear Waste Media

• Published in: Environmental science & technology 1996-04, Vol.30 (4), p.1292-1299
• Main Authors: King, R. B, Bhattacharyya, N. K, Wiemers, K. D
• Format: Article
• Language: English
• Subjects: Applied sciences; Catalysis; Chemistry; Decomposition; Exact sciences and technology; Hydrogen; Ions; Metals; Nuclear fuels; Pollution; Q1; Radioactive wastes; Rhodium; Wastes
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es950524c

The Hanford Waste Vitrification Plant (HWVP) is being designed by the U.S. Department of Energy to immobilize high-level nuclear waste. Simulants for the HWVP feed containing the major nonradioactive components Al, Cd, Fe, Mn, Nd, Ni, Si, Zr, Na, CO3 2-, NO3 -, and NO2 - were used as media to evaluate the stability of formic acid toward hydrogen evolution by the reaction HCO2H → H2 + CO2 catalyzed by the noble metals Ru, Rh, and/or Pd found in significant quantities in uranium fission products. Small-scale experiments using 40−50 mL of feed simulant in closed glass reactors (250−550 mL total volume) at 80−100 °C were used to study the effect of nitrite and nitrate ion on the catalytic activities of the noble metals for formic acid decomposition. Reactions were monitored using gas chromatography to analyze the CO2, H2, NO, and N2O in the gas phase as a function of time. Rhodium, which was introduced as soluble RhCl3·3H2O, was found to be the most active catalyst for hydrogen generation from formic acid above ∼80 °C in the presence of nitrite ion in accord with earlier observations. The apparent homogeneous nature of the nitrite-promoted Rh-catalyzed formic acid decomposition is consistent with the approximate pseudo-first-order dependence of the hydrogen production rate on Rh concentration. Titration of the typical feed simulants containing carbonate and nitrite with formic acid in the presence of rhodium at the reaction temperature (∼90°C) indicates that the nitrite-promoted Rh-catalyzed decomposition of formic acid occurs only after formic acid has reacted with all of the carbonate and nitrite present to form CO2 and NO/N2O, respectively. The catalytic activities of Ru and Pd toward hydrogen generation from formic acid are quite different from those of Rh in that they are inhibited rather than promoted by the presence of nitrite ion. Palladium is also an active catalyst for the reduction of nitrate to N2O by formic acid in feed simulant media.