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Catalytic Converters

Catalytic converters are devices installed as part of the exhaust gas piping assembly associated with an internal combustion engine to reduce the concentrations of Carbon Monoxide (CO), unburned hydrocarbons and oxides of nitrogen (NOx) generated by the fuel combustion process. Originally developed from catalytic science used in the oil refining industry, catalytic converters were initially used on a wide-scale basis on automobiles, but now are common on many types of engines including mobile, stationary, off-road, recreational vehicle, marine, locomotive and outdoor power equipment.

As a general chemistry concept, a catalyst is a substance that causes or accelerates a chemical reaction without itself being consumed as part of the reaction. Inside a catalytic converter, the catalytic materials enhance the chemical reactions through which CO, unburned hydrocarbons and NOx are converted to less-toxic or inert compounds including Carbon Dioxide (CO2), water (H2O) and Nitrogen (N2). Catalytic converters are credited with significant and indispensable improvements to air quality, the environment and human health.

The catalyst portion of a catalytic converter is comprised of three main components:
  1. The substrate
  2. the washcoat
  3. The catalytic components

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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