I remember somethink about it being procuced form ammonia (NH3) or nitrate (NO3), both using metals as catalysts. I found a pdf I hope it helps.

Nitrifying bacteria carries enzymes (catalyst) to convert ammonia to nitrite. However because usually coupling of different bacteria or mechanism to use NO2 as alternative e- acceptor fast forwards the reaction to produce nitrate. https://en.wikipedia.org/wiki/Nitrifying_bacteria