Catalysts are compounds that can trigger a chemical reaction without being affected themselves. For example, enzymes are natural catalysts that control many important chemical reactions in living organisms. Catalysts are also used in automobiles to reduce emissions of certain harmful compounds. For an automobile’s internal combustion engine to operate, a controlled combustion reaction needs to occur inside the vehicle’s engine. But this reaction also produces harmful burnt gases that contribute significantly to air pollution. And good air quality is very important for an individual’s overall health. In order to reduce air pollution, modern automobiles are equipped with a device called a catalytic converter that reduces emissions of three harmful compounds found in car exhaust:
- carbon monoxide (a poisonous gas)
- nitrogen oxides (a cause of smog and acid rain)
- hydrocarbons (a cause of smog)
These are converted into less harmful compounds before leaving the car’s exhaust system. This is accomplished using a catalyst, which gives the device its name. The catalyst used in a catalytic converter is a combination of platinum (Pt), palladium (Pd), and rhodium (Rh). These metals coat a ceramic honeycomb (or ceramic beads) contained within a metal casing that is attached to the exhaust pipe. The catalytic converter’s honeycomb structure provides the maximum surface area on which reactions can take place while using the least amount of catalyst.
A reduction and oxidation reaction occurs inside the device. Carbon monoxide (CO) in converted to carbon dioxide (CO2). Nitrogen oxides (NOx) are broken down into nitrogen gas (N2) and oxygen gas (O2). And hydrocarbons (HC) are converted into carbon dioxide (CO2) and water (H2O). First of all, the catalytic converter uses a reduction catalyst composed of platinum and rhodium to reduce the nitrous oxides. As the nitrous oxide molecules (NO and NO2) pass through the device, the catalyst removes the nitrogen atom, allowing the free oxygen to form oxygen gas (O2). The nitrogen atom that is attached to the catalyst reacts with other attached nitrogen atoms to form nitrogen gas (N2).
- Reduction Reaction 1: 2NO → N2 + O2
- Reduction Reaction 2: 2NO2 → N2 + 2O2
In the second stage of the reaction, an oxidative catalyst of platinum and palladium decreases emissions of carbon monoxide (CO) and unburned hydrocarbons (HC).
- Oxidation Reaction 1: 2CO + O2 → 2CO2
- Oxidation Reaction 2: H4C2 + 3O2 → 2CO2 + 2H2O