I think the element is oxygen
2 NO₂ ↔ N₂O₄ + 61 kJ
The reaction is endothermic so by increasing the temperature the reaction moves to the left (decrease the concentration of N₂O₄)
I found this online and I hope it helps. Pls. see attachment.
The Germans fired over 150 TONS of chlorine gas at 2 colonial divisions by the French so that they could take over