Electrochemical generation of N2 O5

Abstract

A process is provided for the electrochemical generation of N 2 O 5 in HNO 3 , whereby a solution of N 2 O 4 in HNO 3 is electrolyzed. An electrolytic cell for the electrolysis is also provided, having substantially parallel electrodes in electrode compartments separated by a cell membrane. The anode is of Pt, Nb, Nb/Ta 40:60 alloy with a Pt coating. The cathode is Pt, stainless steel, Nb, Nb/Ta 40:60 alloy. The cell membrane is preferably a perfluorinated cationic exchange membrane. In use N 2 O 5 forms in the anolyte and N 2 O 4 increases in the catholyte. A suitable design of cell and its use in a single- or multi-stage electrolysis process is also described.

We claim: 
     
       1. A method for the electrochemical generation of dinitrogen pentoxide comprising: providing an electrochemical cell having an anode situated in an anode compartment and a cathode situated in a cathode compartment, the anode and cathode comprising plates configured in a substantially parallel relationship;   continuously passing anolyte comprising a first solution of dinitrogen tetroxide in nitric acid through the anode compartment;   continuously passing catholyte comprising a second solution of dinitrogen tetroxide in nitric acid through the cathode compartment; .[.and.].   .Iadd.between the anolyte and catholyte providing a membrane between the anode and cathode compartments for allowing the transfer of ions; and.Iaddend.   while the anolyte and catholyte are passing through the anode and the cathode compartments respectively, applying a potential difference between the anode and cathode so that an electrical current passes through the cell and dinitrogen pentoxide forms in the anode compartment;   repeatedly passing the anolyte through the anode compartment and maintaining at a constant level either the potential difference between the anode and the cathode or the electrical current passing through the cell. 14   
     
     
       2. A method as claimed in claim 1, including constantly replenishing the anolyte with dinitrogen tetroxide, in order to maintain the required concentration of dinitrogen tetroxide in the anolyte. 
     
     
       3. A method as claimed in claim 1, wherein the starting concentration of dinitrogen tetroxide in the anolyte is between 5 wt % and saturation. 
     
     
       4. A method as claimed in claim 3, wherein the starting concentration of dinitrogen tetroxide in the anolyte is between 10 wt % and 20 wt %. 
     
     
       5. A method as claimed in claim 1, including maintaining the concentration of dinitrogen tetroxide in the catholyte between 5 wt % and saturation. 
     
     
       6. A method as claimed in claim 5, including maintaining the concentration of dinitrogen tetroxide in the catholyte between 10 wt % and 20 wt %. 
     
     
       7. A method as claimed in claim 1, including maintaining the temperature of the catholyte and the anolyte between 5° C. and 25° C. 
     
     
       8. A method as claimed in claim 1, including maintaining the cell current density between the anode and the cathode plates between 50 Amps.m -2  and 1500 Amps.a -2 . 
     
     
       9. A method as claimed in claim 1, including maintaining the cell voltage between 1 volt and 20 volts. 
     
     
       10. A method as claimed in claim 9, including maintaining the anode potential vs SCE between +1.0 volt and +2.5 volts. 
     
     
       11. A method as claimed in claim 1, including passing the anolyte through two or more of the electrochemical cells connected in series so as to operate in a multi-stage process, such that the anolyte passes repeatedly through each cell as it progresses through said cells in turn. 
     
     
       12. A method as claimed in claim 11, including operating the last of said cells connected in series so as to reduce the dinitrogen tetroxide concentration in the anolyte to less than 3 wt %. 
     
     
       13. A method as claimed in claim 11, including operating the multi-stage process in a steady state with a constant composition at each stage. 
     
     
       14. A method as claimed in claim 11, including continuously monitoring the density of the anolyte with sensors in at least one of the said stages to control the operating conditions of the process.