Noble metal catalyzed hydrogen generation from formic acid in nitrite-containing simulated nuclear waste media

摘要

Simulants for the Hanford Waste Vitrification Plant (HWVP) feed containing the major non-radioactive components Al, Cd, Fe, Mn, Nd, Ni, Si, Zr, Na, CO(sub 3)(sup 2(minus)), NO(sub 3)-, and NO(sub 2)- were used as media to evaluate the stability of formic acid towards hydrogen evolution by the reaction HCO(sub 2)H (yields) H(sub 2) + CO(sub 2) 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(degree)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 CO(sub 2), H(sub 2), NO, and N(sub 2)O in the gas phase as a function of time. Rhodium, which was introduced as soluble RhCl(sub 3)(center dot)3H(sub 2)O, was found to be the most active catalyst for hydrogen generation from formic acid above (approx)80(degree)C in the presence of nitrite ion in accord with earlier observations. The inherent homogeneous nature of the nitrite-promoted Rh-catalyzed formic acid decomposition is suggested by 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 ((approx)90(degree)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 CO(sub 2) and NO/N(sub 2)O, respectively. The catalytic activities of Ru and Pd towards hydrogen generation from formic acid are quite different than those of Rh in that they are inhibited rather than promoted by the presence of nitrite ion.