It was shown that catalysts used in the slurry phase hydrogenation of nitroaromatic compounds are post-passivated during exhibition in air for a longer period of time. This post passivation hampers a reactivation of the catalyst and is decelerated with increasing catalyst particle size. Furthermore a fast mixing during feeding in the nitroaromatic compound into the reactor and sufficient hydrogen supply to the catalyst surface have to be ensured to avoid an oxidic attack of the nitroaromatic compound towards the nickel catalyst and to obtain a slow deactivation of the this catalyst.