There are many kinds of chemical catalysts, the application is extremely wide, the same reaction and even the same device of different catalyst performance is also very different, so what kind of catalyst is a good chemical catalyst, chemical catalyst and how to evaluate it?

Generally speaking, the quality and efficiency of a chemical catalyst are mainly the three comprehensive indicators of activity, selectivity and stability. Also related to these three items are the mechanical strength, toxicity resistance, geometric physical properties, macroscopic and microscopic physical structures, and economic performance of the catalyst. A more comprehensive performance overview is shown in the table below.

Catalyst performance usually refers to activity, selectivity and stability, and stability includes chemical stability (anti-toxicity) and physical stability (heat resistance, mechanical strength). Fixed bed chemical catalysts, especially selectivity and stability are more important.

       Stability of catalyst

Catalyst stability is usually expressed in terms of lifetime.

It refers to the time that the catalyst maintains a certain level of activity under the conditions of use (one-way life), or the cumulative time after regeneration (total life). It can also be expressed in terms of the total number of reaction conversions that can be achieved per active site.

According to the definition of catalyst, an ideal catalyst should be able to be used permanently. However, in fact, due to chemical and physical reasons, the activity and selectivity of the catalyst will gradually decrease with the use of time, until it is considered to be deactivated after falling below a certain value (the activity is still there, but it does not have industrial value).

The stability of the chemical catalyst includes the thermal stability of high temperature heat, the mechanical stability of friction, impact and gravity, and the anti-toxic stability of the poison.

1) Heat stability, a good chemical catalyst should be able to have a certain level of vitality for a long time under the condition of reaction temperature. High temperature reaction is relatively common, however, most catalysts have their own limit temperature, which is mainly high temperature is easy to cause the active component of the catalyst crystallite sintering growth, lattice damage or lattice defect reduction. Metal catalysts generally sinter readily above the semi-melting temperature.

2) Mechanical stability. The catalyst with high mechanical stability can withstand friction and impact between particles, between particles and fluid, between particles and vessel wall, and can not be broken or have obvious pulverization in the process of transportation, filling, self-weight load or change of reaction conditions. Generally, the mechanical stability is expressed by compressive strength and pulverization degree.

3) anti-toxic stability, due to harmful impurities (poisons) on the catalyst poisoning, so that the catalyst activity, selectivity or life of the phenomenon is called catalyst poisoning. In general, poison refers to sulfide H₂SCOS₂、 CS₂、RSH、R₁SR₂Thiophene, R₁RRR₂、RSO₃H, etc., phosphorus, arsenic, halogen compounds, primary heavy metal compounds, metal organic compounds, etc.; oxygen-containing compounds O₂、CO₂What to do₂O and so on. For metal catalysts, the former will cause permanent poisoning and the latter will cause temporary (reversible) poisoning.

The poisoning phenomenon of the catalyst can be roughly explained by the adsorption of poisons or further transformation of the surface active centers into stable surface compounds, and the active sites are passivated or permanently occupied.

In summary, the choice of a cost-effective chemical catalyst, price is not the only factor, the catalyst has high activity and selectivity, long life is a more important indicator.