Sintering describes the loss of catalytic active surface of a catalyst as a result of structural modifications. This is a thermally activated process that is physical in nature and occurs both in supported (engl. supported) as well as unsupported catalysts. Sintering typically leads to agglomeration (engl. agglomeration) small, finely distributed metal particles, which significantly reduces their specific surface and thus the catalytic activity (see Chemisorption).

mechanisms of sintering

Two main mechanisms of sintering are distinguished:

• Nuclear Migration: Individual metal atoms leave their original positions and migrate across the catalyst support surface (migration) or through the gas phase. These particles aggregate when they collide with other metal particles. Since larger particles are more stable in terms of energy, their size increases at the expense of the smaller ones. The formation of a few large particles compared to many small ones leads to a reduction in the active surface.
  
  
  
• Particle migration: Similarly, entire metal particles can migrate on the carrier surface. When two particles collide, they fuse and form larger agglomerates.

The sintering rate is largely determined by the Temperature. Sintering is accelerated at higher temperatures. Typically, when heating up, there is a rapid loss of surface area, which then slows down. This indicates a transition from particle migration at lower temperatures to atomic migration at higher temperatures. An extreme form of sintering is the phase transformation of the crystal stable structure of the catalyst, as in the conversion of α-To the₂O₃ zu γ-To the₂O₃ which involves a decrease in the inner surface (see BET) In addition to the temperature, the Atmosphere play a crucial role. Oxidizing atmospheres such as oxygen (O₂) often promote sintering more than reducing or inert gases such as hydrogen (H₂) or nitrogen (N₂).

Sintering is generally irreversible, but there are a few exceptions in which redispersion of the active components can be achieved. One example of this is the catalytic reforming of heavy gasoline on Pt/Al₂O₃ catalysts, in which the active metal surface is restored by adding chlorine (Cl) and then reducing it. Nevertheless, in most cases the only option after sintering is to replace the catalyst, so the primary aim is to slow down sintering processes, for example by using promoters in catalysts such as zirconium oxide (ZrO₂) to stabilize the γ-To the₂O₃ structure versus phase transformations.