Understand the three major properties of catalysts: activity, selectivity and stability

In the field of chemistry, catalyst is an extremely important substance. It is a substance that can change the rate of chemical reactions without changing its mass and chemical properties before and after the reaction.

Catalysts have three major characteristics: catalytic activity, selectivity, life or stability.

1. Catalytic activity

Catalytic activity is one of the most remarkable properties of catalysts. In chemical reactions, the catalyst reduces the activation energy required for the reaction through its own action. Activation energy is like an "energy threshold" that a chemical reaction needs to cross. The presence of a catalyst lowers this threshold, making it easier for the reacting molecules to react, thus greatly speeding up the rate of the chemical reaction.

For example, in the process of industrial ammonia synthesis, the use of iron catalysts greatly increases the reaction rate. Originally, nitrogen and hydrogen reacted to form ammonia very slowly under certain conditions, but after adding a suitable catalyst, the reaction rate was significantly improved. The rate of catalytic reaction is an important criterion for measuring catalytic activity, which is usually expressed by the conversion amount of reactants or the amount of product generated per unit time. The higher the catalytic activity, the more products are produced in the same time, and the more obvious the promotion of chemical reactions.

2. Selectivity

Selectivity is another key characteristic of catalysts. Different catalysts have specific selectivities for different chemical reactions. A catalyst often only has a significant acceleration effect on a certain type of reaction, but has little or even no acceleration effect on other reactions.

For example, in the petrochemical industry, when catalytically converting different hydrocarbon compounds, it is necessary to select catalysts with different selectivities. Taking the catalytic conversion of ethanol as an example, certain catalysts can be used to selectively convert ethanol into ethylene, while other catalysts can convert it into ether. The selectivity of the catalyst determines the directionality of the catalytic action, which is particularly important in fields such as organic synthesis. By selecting the appropriate catalyst, the direction of the chemical reaction can be precisely controlled to obtain the desired target product, reduce the occurrence of side reactions, and improve the efficiency of the reaction and the purity of the product.

3. Life span or stability

Thermal stability:

Catalysts may face various temperature conditions during chemical reactions. Thermal stability refers to the ability of a catalyst to maintain its catalytic performance and structural stability at different temperatures.

For example, in automobile exhaust purification devices, catalysts need to function stably for a long time in the high-temperature environment generated when the engine is operating. If the thermal stability of the catalyst is insufficient, structural changes, loss of active components, or sintering may occur at high temperatures, resulting in reduced catalytic performance or even failure.

Therefore, during the design and preparation of catalysts, it is necessary to consider the thermal stability of the material, select appropriate carriers and active components, and adopt appropriate preparation methods to improve the thermal stability of the catalyst.

Mechanical stability:

In practical applications, catalysts may be affected by various mechanical forces, such as air flow impact, stirring, pressure changes, etc. Mechanical stability refers to the catalyst's ability to withstand these mechanical forces without breaking, wearing out, or pulverizing.

For catalysts in fixed-bed reactors, they need to have sufficient mechanical strength to prevent breakage due to extrusion, collision, etc. during the filling and reaction processes. For the catalyst in the fluidized bed reactor, it needs to have good wear resistance to avoid excessive wear driven by the gas flow.

In order to improve the mechanical stability of the catalyst, its mechanical strength can be enhanced by optimizing the shape, size, pore structure and other physical properties of the catalyst, as well as selecting appropriate binders and reinforcing materials.

Anti-toxic stability:

In the chemical reaction system, there may be some substances that are toxic to the catalyst, called "poisons". These poisons may react chemically with the active center of the catalyst or cover the surface of the catalyst, resulting in a decrease in the activity and selectivity of the catalyst. Antitoxic stability refers to the ability of a catalyst to resist the influence of these poisons.

For example, in some industrial catalytic reactions, the raw materials may contain trace amounts of sulfur, phosphorus, arsenic and other impurities, which can poison the catalyst. In order to improve the anti-toxic stability of the catalyst, some measures can be taken, such as pretreating the raw materials to remove poisons, adding anti-toxic agents to the catalyst, or developing new catalysts with anti-toxic properties.