Catalyst poisoning Poisoning) refers to the Deactivation of a catalyst by adsorbing undesirable substances, so-called poisons, on its active centers (see chemisorption). These poisons, such as H₂S, HCl or heavy metals, usually enter the system as impurities in the feed gas stream (external poisoning) and lead to a reduced Activity and in some cases also to a reduced selectivity of the catalyst. In industrial processes, controlling and preventing catalyst poisoning is therefore a key challenge.
There is a fundamental difference between Gifts and inhibitors: Poisons usually bind strongly and irreversibly to the active centers of the catalyst, while inhibitors only adsorb weakly and reversibly, which allows for easier regeneration. Poisons can also be classified according to their effect: With "selective" poisons, specific active centers, often the most effective ones, are deactivated first. This means that the relationship between the catalytic activity and the amount of adsorbed poison is not linear. In contrast, "non-selective" poisons act evenly on all active centers, which leads to a proportional loss of activity. This can also be used, among other things, to specifically poison a catalyst in order to prevent unwanted side reactions.
An example of increasing the selectivity is the treatment (engl. tempering) of Pt-Re/Al₂O₃ reforming catalysts with sulfur compounds. The support material is treated with sulfur compounds, which reduces the hydrocracking activity. The desired hydrogenation reactions, which are catalyzed by the transition metals, remain largely unaffected
Another aspect is Reversibility of poisoning. Reversible poisoning, for example by water (H2O) in ammonia synthesis can be eliminated simply by removing the poison from the reaction mixture. In contrast, irreversible poisons form stable bonds with the catalytically active metals, which results in permanent deactivation of the catalyst. A classic example is the poisoning of three-way catalysts in vehicles by lead (Pb), which was a major factor in the Europe-wide ban on leaded petrol in 2000 ("Super Unleaded").
Overall, the avoidance of catalyst poisons in the feed gas stream is a key goal to Lifespan of catalysts in industrial processes. In practice, this is often done by adsorbents like for example C&CS #1247 – a doped activated carbon for desulfurization – is used. In addition, catalytic conversion processes, such as those used by HDMax® introduced, to convert catalyst poisons into chemical compounds which can be optimally absorbed by the adsorbents.
In summary, catalyst poisoning is a complex and challenging phenomenon, but it can be effectively controlled by innovative techniques and materials to ensure the efficiency and sustainability of industrial processes.
