SCALE-UP STUDIES OF THE ELECTROSYNTHESIS OF DINITROGEN PENTOXIDE IN NITRIC-ACID

The method for the electrosynthesis of N2O5 in nitric acid hy anodic o xidation of N2O4 has been scaled up to produce quantities of 15 to 50 kg of solution containing 20 to 30 weight percent N2O5. A two- or thre e-cell, divided, plate-and- frame electrolyzer operated in the bipolar mode was employed to test various combinations of candidate electrode coatings and separators, and to study the electrochemical characteris tics of the process. Two sizes of electrolyzers were used, each having single-electrode areas of 0.096 and 0.25 m(2). The best performing an ode/substrate materials were either Pt-Lr on niobium, or IrO2 on alumi num; the best cathode materials were Pt or Pt-Ir on niobium. The prefe rred cell separator is a hydrophilic, porous polytetrafluorene-ethylen e diaphragm, but an FEP-polymer anion-exchange membrane is also satisf actory. Production of N2O5 was achieved with chemical yields of 80 to 90% and current efficiencies of 50 to 70%. Maximum current densities w ere in the range of 0.1 to 0.2 A cm(-2); Cell voltages were 3 to 5 V a nd specific energies were 1.6 to 1.8 kWh kg(-1). In the electrolyzer c atholyte, N2O4 is generated at nearly theoretical yield, and could be recovered and recycled as an anolyte feedstock. The E-0' of the N2O5/N 2O4 couple in anhydrous nitric acid was estimated to be + 1.66 +/- 0.0 2 V vs. SHE.