The substrate is the 'core' of the catalytic converter. It can be made of ceramic, metal foil or wire mesh. The substrate contains numerous small flow-through channels (called "cells") through which the exhaust gases pass. The substrate is sized - both its volume and its cell density - specific to the engine and associated exhaust application.
Same size metal substrate showing variations in cell density.
The function of the substrate is to have the applicable volume of exhaust-flow pass through it while minimizing flow backpressure, and to provide a large surface area to be coated with washcoat and catalytic metals.
The washcoat is a liquid product that is applied to the substrate. The washcoat contains proprietary loadings of one or more metal oxides. The washcoat functions to form an undulated layer on the surface of the bare substrate channels, which greatly increases the substrate’s surface area. The washcoat also serves as a carrier for the catalytic metals.
The catalytic materials are commonly one or more of the Platinum Group Metals (PGM), with the most commonly used PGM’s being platinum, palladium and rhodium. The catalytic metal(s) lower the reaction’s activation energy (the energy necessary for the molecules to react), thus lowering the temperature required for the reaction to take place resulting in lower operating costs.
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