What Are Pelleted Catalyst Supports?

Pelleted catalyst supports, also known as "beaded" or "particulate" supports, were used in the early years of automotive emission control technology. In the U.S. GM was the major company using pelleted catalysts, while Ford and others used monoliths.

The pellets were spherical particles made of γ-Al₂O₃, 2.5 to 5 mm in diameter. Stabilizers and active components (i.e., precious metals) were incorporated into the pellets. A volume of these "beads" was placed into a steel shell and contained between two screens to form the catalytic converter. The screens could be spring-loaded to minimize catalyst losses due to attrition. Nevertheless, the attrition resistance of the alumina pellets remained the major concern which has never been successfully resolved.

With advancements in the ceramic monolith support technology, the pelleted supports gradually became obsolete. Ceramic honeycombs with parallel, open channels exhibited several advantages over the pelleted style supports:

• No catalyst loss due to attrition
• Lower pressure loss due to high frontal open area and straight channels
• Smaller size and less weight than pelleted converters.

Today, almost all automobiles are equipped with either ceramic or metallic honeycomb supported catalysts. In the off-road emission control applications, monolithic, metallic or ceramic catalyst supports became the standard. The pelleted design is obsolete and increasingly difficult to find on the market.