What are the reasons for catalyst deactivation during the reaction process?

The reasons for catalyst deactivation during the reaction process are mainly classified as follows:

1. Poisoning

Impurity poisoning: A small amount of impurities present in the reaction system, such as sulfides, phosphides, and heavy metal ions, may chemically react with the active sites of the catalyst, forming stable compounds and thus causing the active sites to lose their activity. For example, in some metal catalysts, sulfur atoms easily combine with metal active centers to form sulfides, covering the catalyst surface and hindering the contact between reactants and active centers.

Selective poisoning: Some impurities may only have a poisoning effect on certain active centers or specific reaction pathways of the catalyst, leading to changes in the selectivity of the catalyst. For example, in the olefin hydrogenation reaction, a small amount of carbon monoxide may preferentially adsorb on specific active sites of the catalyst, inhibiting the hydrogenation reaction of olefins, while having a relatively small effect on other reactions.

2. Coking

Reaction intermediate deposition: In organic reactions, reactants or reaction intermediates may undergo polymerization, cracking, and other reactions on the catalyst surface, generating coke precursors, which gradually form coke. This coke will cover the active centers of the catalyst, hindering the contact between reactants and active centers, and will also block the pores of the catalyst, affecting mass diffusion. For example, in petroleum cracking reactions, hydrocarbon substances easily undergo coking on the catalyst surface.

Coke structure and properties: The structure and properties of coke have a significant impact on catalyst deactivation. Amorphous coke is relatively easy to remove and has a smaller impact on catalyst activity; however, coke with a higher degree of graphitization is difficult to remove and will severely reduce catalyst activity.

3. Sintering

High-temperature effect: Under high-temperature reaction conditions, the crystal grains of the active components or support of the catalyst may grow, leading to a decrease in the specific surface area of the catalyst, a reduction in the number of active centers, and thus catalyst deactivation. For example, in metal catalysts at high temperatures, metal particles gradually aggregate and fuse, increasing the particle size.

Atmosphere influence: Certain gases in the reaction atmosphere may promote the sintering process. For example, in an oxidizing atmosphere, oxygen species on the surface of metal oxide catalysts may accelerate the migration of lattice oxygen, promoting crystal grain growth and sintering.

4. Active component loss

Dissolution and leaching: In some reaction systems, the active components of the catalyst may be dissolved or leached by the reaction medium. For example, in acidic or alkaline reaction media, some active components of metal catalysts may dissolve, leading to a decrease in catalyst activity.

Volatilization loss: At high temperatures or under specific reaction conditions, the active components of the catalyst may volatilize. For example, some organic ligands in organometallic catalysts may decompose or volatilize at high temperatures, exposing the active centers and losing stability, thus leading to catalyst deactivation.

5. Structural changes

Phase transformation: The catalyst may undergo phase transformation during the reaction process, leading to changes in its crystal structure and surface properties. The structure and electronic state of the active centers also change accordingly, thus affecting the activity of the catalyst. For example, some metal oxide catalysts may transform from a high valence state to a low valence state under a reducing atmosphere, and the crystal phase structure changes.

Mechanical damage: During the reaction process, the catalyst may be subjected to mechanical stress, such as wear and compression, leading to the breakage and pulverization of the catalyst particles, a decrease in the specific surface area, and a reduction in the exposure of active centers, thus deactivating the catalyst. This is more common in fixed-bed reactors, especially when the reaction gas flow rate is high or the catalyst particle strength is low.