[Q&A Question 259] 2017.09.17

Temporary poisoning and permanent poisoning

Reversible poisoning, irreversible poisoning, selective poisoning

Catalyst poisoning can be divided into reversible poisoning, irreversible poisoning, and selective poisoning. Reversible poisoning: When a poison adsorbs or combines with the active center, the bonds formed are relatively weak; it is possible to remove the poison using appropriate methods, thereby restoring the catalyst’s activity without affecting its properties. This type of poisoning is known as reversible poisoning or temporary poisoning.   Irreversible poisoning: When a poison interacts with the active components of a catalyst to form strong chemical bonds, it becomes difficult to remove the poison using conventional methods in order to restore the catalyst’s activity; this type of poisoning is known as irreversible or permanent poisoning.   Selective poisoning: When a catalyst becomes poisoned, it may lose its catalytic ability for a certain reaction, but it may still remain catalytically active for other reactions; this phenomenon is known as selective poisoning. There is a useful aspect to selective poisoning; for example, in a cascade reaction, if the toxin only poisons the active sites that trigger subsequent reactions, it is possible to halt the reaction at the intermediate products, thereby obtaining the desired intermediate products with high yields.

One type is when the toxin has a weak effect on the active component, and the activity can be restored using simple methods; this is known as reversible poisoning or temporary poisoning. Another type is irreversible poisoning, for which it is not possible to restore activity using simple methods. To reduce the activity of side reactions, it is sometimes necessary to selectively poison the catalyst. 　　Catalysts can lose their activity during use due to various factors, one of the important ones being poisoning. There are several possible causes of catalyst poisoning: minor impurities present in the raw material, strong adsorption (usually chemical adsorption) on the active centers, or chemical reactions with the active centers that result in the formation of other substances – all of these can poison the active centers. Additionally, such toxic substances may also be present in the reaction products ； During the preparation of the catalyst, impurities present in the support interact with the active components, and may also poison the active centers. Identification and effects of catalyst poisoning There are various types of poisons, and for a given catalyst, it is only by considering the reaction it catalyzes that it can be determined which substances are toxic. In other words, poisons affect not only the catalyst itself but also the reaction catalyzed by that catalyst. During the period of stable activity, catalysts often experience a significant decline in their activity due to contact with small amounts of impurities; this phenomenon is known as catalyst poisoning. Substances that cause a catalyst to lose its catalytic activity are called catalyst poisons. If activity can be restored after the toxic factor is removed, it is called temporary poisoning; otherwise, it is called permanent poisoning. 　　Some catalysts are toxic in certain reactions; some of these toxins have a temporary effect, while others are permanent. For example, in the synthesis of ammonia, iron-based catalysts are affected by water and oxygen as toxins. When such poisoning occurs, the catalyst can be reactivated through reduction or heating methods. This type of poisoning is temporary, or reversible ； Compounds of sulfur or phosphorus are also toxic to this catalyst and this reaction; when poisoning occurs as a result of them, it is very difficult to reactivate the catalyst, and this represents permanent poisoning, or irreversible poisoning. The latter type of poisoning can be prevented. 　　Poisoning not only affects the activity of catalysts, leading to a decrease in their efficiency, but it also impacts their selectivity. Poisoning is a practical problem that often arises when catalysts are used in chemical manufacturing, yet it is not yet fully understood. As for how to prevent poisoning and remove its effects, this must be addressed through practical experience

Temporary poisoning (reversible) Permanent poisoning

It is divided into reversible poisoning and irreversible poisoning.