Dear Kumar Gaurav ,

          I think you have an error in the question.  It is  Cu metal or Zn metals used for reduction not their ions.  Please  see the reference you obtained that information from it and rewrite the question in a correct form.  Thanks and good luck.

Dear Fadhil

Thanks for your rectification. Cu metal as substrate materials are being used to reduce nitrate ions.Why this is so?

Dear Michael

Will you please refer the highlighted text in the attached published paper that supports the point in my question. 

Dear Kumar Gaurav ·

       Thank you  Mr.Guarav.  Infact oxidation and reduction of a sytem depends on thr reduction potential of the reactant half cells.  In your case, if you look in tables of Standard Reduction Potentials of Analytical Chemistry texts, you can find your answer.

NO3(-)+ 4H(+) +3e = NO(g) +2H2O  E(o) = 0.955 V (which means reduction of N from

(+5) to (+2).  It can be reduced to HNO2 ( +5 to +3), (Eo = 0.940) and so on. But  reduction potential of  Cu(2+) + 2e = Cu...Eo = 0.337 V and for Zn(2+) to Zn (Eo=-0.763).  Surely you understand that the lower red.potential of both Cu(2+) and Zn(2+) than that of NO3(-), cannot reduce NO3(-).  Although Zn metal is a stronger reducing agent compared to Cu metal but the necessity to an acidic medium such as HCl, can dissolve Zn metal more easily in the medium of the reduction. This will prevent the process of reduction which makes  Cu metal more suitable. There may be other reasons too, thus you have to consult references.  Thank you and Good luck. 

Copper electrode is often used as cathode for electrochemical reduction of nitrate ions since it performs the electrocatalytic properties (low overvoltage etc.) for this process. However the main product of nitrate reduction at copper electrode in neutral media is nitrite, which is undesirable (nitrite is much more toxic to human health than nitrate). So many researchers have modified the copper electrode for the use in nitrate reduction.