WTF platinum is not the only metal capable of catalyzing ammonia production. Iron is capable as well (and can be improved by adding bauxite), so using iron tanks, high temperatures and high pressure should suffice. BUT Riichiro did it in order to make the plot more epic, so we shall let it slide this time.
(I know, the reaction is exothermic, which implies that to increase efficiency one'd better lower the temperature, however it would take too long so I'd rather apply heat until the tank reaches 850ºF, in a way that , as Arrhenius equation told us, the reaction can occurs much more faster)