# Monolith (catalyst support)

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**Monolithic catalyst supports** are [extruded](/source/Extrusion) structures that are the core of many [catalytic converters](/source/Catalytic_converter),[1] most [diesel particulate filters](/source/Diesel_particulate_filter), and some [catalytic reactors](/source/Chemical_reactor#Catalytic_reactor). Most catalytic converters are used for [vehicle emissions control](/source/Vehicle_emissions_control). Stationary catalytic converters can reduce [air pollution](/source/Air_pollution) from [fossil fuel power stations](/source/Fossil_fuel_power_station).

## Properties

Monoliths for automotive catalytic converters are made of a [ceramic](/source/Ceramic) that contains a large proportion of synthetic [cordierite](/source/Cordierite), 2MgO•2Al2O3•5SiO2, which has a low [coefficient of thermal expansion](/source/Coefficient_of_thermal_expansion).[1]

Each monolith contains thousands of parallel channels or holes, which are defined by many thin walls, in a [honeycomb structure](/source/Honeycomb_structure). The channels can be square, [hexagonal](/source/Hexagon), round, or other shapes. The hole density may be from 30 to 200 per cm2, and the separating walls can be 0.05 to 0.3 mm. The many small holes have a much larger [surface area](/source/Surface_area) than one large hole. High surface area facilitates catalytic reaction or filtration. The open spaces in the cross-sectional area are 72 to 87% of the frontal area, so resistance to the flow of gases through the holes is low, which minimizes energy consumed forcing gases through the structure.

The monolith is a [substrate](/source/Substrate_(materials_science)) that [supports](/source/Catalyst_support) a [catalyst](/source/Catalyst). After the monolith is complete, a washcoat is applied that deposits oxides and catalyst(s) (most commonly [platinum](/source/Platinum), [palladium](/source/Palladium), and/or [rhodium](/source/Rhodium)) on the walls of the holes.

Alternative structures include corrugated metal and a [packed bed](/source/Packed_bed) of coated pellets or other shapes.

## Uses

- [Diesel particulate filters](/source/Diesel_particulate_filter) (DPF)
- Catalytic incineration
- [Catalyst support](/source/Catalyst_support) for chemical processes
- [Vehicle emissions control](/source/Vehicle_emissions_control)

## References

1. Williams, J.; *Monolithic structures, materials, properties and uses*; Catalysis Today, 69 (2001) 3-9

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Adapted from the Wikipedia article [Monolith (catalyst support)](https://en.wikipedia.org/wiki/Monolith_(catalyst_support)) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Monolith_(catalyst_support)?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.