Sintering describes the loss of the catalytically active surface of a catalyst as a result of structural modifications. This is a thermally activated process of a physical nature that can occur with both supported and unsupported catalysts. Sintering typically leads to the agglomeration of small, finely dispersed metal particles, which significantly reduces their specific surface area and thus their catalytic activity (see chemisorption).

Mechanisms of sintering

Two main mechanisms of sintering are distinguished:

• Atomic migration: Individual metal atoms leave their original positions and migrate across the support surface of the catalyst (migration) or through the gas phase. These particles clump together when they collide with other metal particles. Since larger particles are more energetically stable, 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 area.
  
  
  
• Particle migration: Similarly, entire metal particles can migrate on the substrate surface. When two particles collide, they merge and form larger agglomerates.

The sintering rate is largely determined by the temperature. When two particles collide, they merge and form larger agglomerates. Typically, during heating, there is initially a rapid loss of surface area, which later slows down. This suggests 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 structure of the catalyst, as in the transformation of α-Al2O3 to γ-Al2O3, which results in a decrease in the internal surface area (see BET). Besides temperature, the atmosphere also plays a crucial role. Oxidizing atmospheres such as oxygen (O₂) often promote sintering more strongly than reducing or inert gases such as hydrogen (H₂) or nitrogen (N₂).

Sintering is generally irreversible, however there are a few exceptions in which redispersion of the active components can be achieved. An example of this is the catalytic reforming of heavy naphtha on Pt/Al₂O₃ catalysts, in which the active metal surface is restored by the addition of chlorine (Cl) and subsequent reduction. Nevertheless, in most cases, after sintering, only the replacement of the catalyst remains an option; therefore, the primary goal is to slow down sintering processes, for example by using promoters in catalysts such as zirconium oxide (ZrO2) to stabilize the γ-Al2O3 structure against phase transformations.