Promoters are added in the catalyst to speed up the reactions. They are not catalyst themselves however they are present to shoulder the possible reaction that can help in the production of the desired quantities.
For Haber process, the general reaction would be
`N_2 + 3 H_2 -> 2 NH_3 ( Delta H = -92.4 (kJ)/(mol^-1))`
It is a method of producing ammonia derived from the nitrogen in the atmosphere and hydrogen gas. Iron plays a role as the place where the gasses will be deposited before the reaction to occur. Iron is usually promoted with certain chemicals (in this case, Mo) so as to have a maximum yield of ammonia. Molybdenum helps the efficiency of the production of ammonia in the process