Causes of catalyst deactivation

Catalysts are subject to a variety of factors during use and can lose their activity either sharply or slowly. The causes of catalyst deactivation are complex. They can be summarized into some categories as follows.

1. Permanent deactivation

The catalyst active component is inactivated by the action of some foreign components (poisoning), often permanently. These foreign components are mostly chemical reactions or ion exchange with the active component of the catalyst resulting in changes in the active component. For example, acid catalysts are neutralized by bases, precious metal catalysts are poisoned by sulfides or nitrides, etc. 

The deactivation of catalyst poisoning is often manifested as a rapid decrease in activity. Permanent deactivation is also caused by the loss of active components due to wear and tear or sublimation during use, and such deactivation is often difficult to recover simply.

2. Non-permanent deactivation occurs when the active component is covered and gradually deactivated. For example, the reaction process generates carbon, which covers the active component or blocks the pore of the catalyst, so that the reactants cannot contact with the active component. These coverings can be removed by certain methods, such as deactivation by carbon accumulation can be revived by carbon burning and regeneration.

3. Wrong operation leads to catalyst deactivation, such as excessive reaction temperature, violent pressure fluctuations leading to confusion or crushing of the catalyst bed, etc. Such deactivation cannot be recovered.

Catalytic reactions are divided into single-phase reactions and multi-phase reactions. Single-phase reactions are carried out in the gaseous or liquid state, and the temperature, pressure and other conditions during the reaction are easier to regulate and less dangerous. In multiphase reactions, the catalytic effect occurs at the phase interface and on the surface of the catalyst, where the temperature and pressure are more difficult to control. 

It is more dangerous. Catalytic reactions should be carried out with the correct choice of catalyst, appropriate amount of catalyst, good heat dissipation, prevention of intense local reactions, and strict temperature control. If the process of catalytic reaction can be carried out continuously, using automatic temperature regulation system, it can reduce the danger.

In the catalytic reaction, when the impurities in the raw gas and the catalyst react, explosive hazards may be generated, which is very dangerous. For example, in the reaction of ethylene catalytic oxidation synthesis of acetaldehyde, due to the large amount of copper salt in the catalyst system. 

If the raw gas contains acetylene too high, the acetylene and copper reaction to generate acetylene copper, which is a red precipitate, spontaneous combustion point at 260 ~ 270 ℃, in the dry state is very easy to explode, in the air under the action of easy oxidation and combustion. Hydrocarbons and catalytic thorns in the role of metal salts to generate insoluble block, not only make the catalyst composition changes. And the block is also very easy to cause explosions.

In the catalytic reaction process, some produce hydrogen chloride, corrosion and poisoning risk; some produce hydrogen sulfide, the risk of poisoning is greater. In addition, hydrogen sulfide in the air has a wide explosive limit, the production process and the risk of explosion. In the catalytic reaction that produces hydrogen, there is a greater risk of explosion, especially under high pressure, the corrosive effect of hydrogen embrittlement of metal high-pressure containers, thus causing destructive accidents.