Ceramic Catalyst Substrates

W. Addy Majewski

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• Introduction
• Material Characterization Parameters
• Cordierite
• Commercial Substrates

Introduction

The catalyst substrate is a crucial component influencing performance, robustness, and durability of catalytic converter systems. The design targets for ceramic catalyst substrates include:

• high geometric surface area (GSA),
• large open frontal area (OFA),
• low thermal mass and heat capacity,
• high use temperature,
• low coefficient of thermal expansion,
• good coatability,
• washcoat compatibility,
• strength, and
• oxidation resistance.

Synthetic cordierite is a magnesium-alumino-silicate ceramic material which meets most of these requirements. Practically all today’s ceramic monolithic catalyst substrates are made of cordierite.

A ceramic catalyst substrate is shown in Figure 1. Ceramic monoliths are manufactured in an extrusion process. Raw materials such as alumina, talc, clay (kaolinite), and silica are reduced to powder and blended into a water-based paste. Additives which may be added in the process include lubricants (e.g., ethylene glycol), bonding agents (e.g., methylcellulose), and fluxes (alkaline hydroxides). Extrusions made from the paste are dried, cut to length, and calcined at temperatures above 1400°C.

Substrates slightly change their dimensions during calcination. Parts of a smaller size—such as substrates for light-duty engines—can be manufactured in a one-step process through predicting that change and extruding parts to dimensions which are slightly different from the specifications of finished product. Larger diameter parts, on the other hand, might require a multi-step production process. The substrates are extruded and calcined to larger dimensions, followed by processing to meet their dimensional specifications: they are cut to the final length, their contours are machined, the outer skin is applied using ceramic paste, and the parts are oven-dried.

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