A catalyst is a substance which promotes certain reactions but is not one of original reactants or final products. In other words, the catalyst is not consumed in the reactions it promotes. Platinum group metals (PGM) including platinum itself, palladium, and rhodium are commonly used in emission control catalysts. Modern catalytic converters utilize a monolith honeycomb substrate which is coated with the PGM metal compounds and packaged into a stainless steel container. The honeycomb is made either of ceramics or stainless steel foil. Its structure of many small parallel channels presents high catalytic contact area to the exhaust gases. As the hot gases flow through the channels and contact the catalyst, several exhaust pollutants are converted into harmless substances. The following reactions occur in the oxidation catalyst:
The hydrocarbon emissions from LPG engines will contain a mixture of propane, butane, ethane, and other compounds. Both CO and hydrocarbons are converted in the oxidation catalyst to carbon dioxide and water vapor which are non-toxic gases. The conversion of CO and HC in the catalyst requires oxygen, as shown in the reaction equations. Usually there is not enough oxygen in the exhaust gases of LPG engines to burn all of the pollutants. Oxidation catalyst systems frequently require that extra air, called secondary air, be introduced into the exhaust system in front of the catalyst.
Typical conversion efficiencies for carbon monoxide and various hydrocarbons (butane, propane, and methane) in the Nett® oxidation catalyst are shown in Figure 8. Catalyst activity increases with temperature. A minimum exhaust temperature of about 200°C is necessary for the catalyst to “light-off”. Higher temperatures are necessary for hydrocarbon conversion. LPG exhaust contains short carbon chain hydrocarbons which are more difficult to convert in the catalyst than those found in diesel or gasoline exhaust. As illustrated in the graph, the shorter the carbon chain the higher the conversion temperature.