Using density-functional calculations, we investigate the possibility of ammonia synthesis at supported Fe nanoclusters along catalytic routes closely resembling those in biological nitrogen fixation. To achieve similar catalytic conditions as at the active site of the enzyme nitrogenase, the clusters are passivated with either S or N atoms. From calculated potential-energy profiles for the N-2 hydrogenation, we find that low-temperature synthesis of ammonia is viable at the clusters passivated by N atoms due to the strong binding energy of the N-2 molecule in the initial adsorption step.