In the Haber–Bosch process, the gases hydrogen H2 and nitrogen N2 are passed at high temperatures, in the range of 400-450 degrees Celsius, and at high pressures, close to 200 atmospheres, over a catalyst of iron prepared from a form of iron oxide called magnetite.
The reaction 2N + 3H2 --> 2NH3 takes place here. This reaction is not efficient as nitrogen is a very stable molecule. Only close to 15% of the hydrogen and nitrogen sent over the catalyst is converted to ammonia. So the gases have to be repeatedly passed over the catalyst.
After each cycle, the temperature of the gases is reduced a little. When this is done the high pressure helps convert the ammonia from gaseous to liquid form while the hydrogen and nitrogen continue to remain gases. The liquid ammonia is extracted from the mixture and the remaining hydrogen and nitrogen gases are used for another cycle of being passed over the catalyst to create more ammonia.