Catalytic converter

Catalytic converter
source
Name	Catalytic Converter
Caption	A typical catalytic converter in an exhaust system
Classification	Emission control system
Related	Exhaust manifold, Oxygen sensor, Muffler
Manufacturer	Bosch (company), Delphi Technologies, Tenneco, Faurecia

Catalytic converter. An essential component of modern automotive emission control systems, it is a device that uses a catalyst to convert harmful pollutants in exhaust gas from an internal combustion engine into less harmful substances. It achieves this through catalyzed redox reactions, primarily targeting carbon monoxide, unburned hydrocarbons, and nitrogen oxides. The widespread adoption of catalytic converters, driven by stringent environmental regulations like the Clean Air Act (United States), has significantly reduced air pollution from motor vehicles globally.

History and development

The foundational chemistry for catalytic converters was pioneered by French chemist Michel Frenkel in the early 20th century. Significant development occurred in the 1950s by engineers at Engelhard Corporation, led by Walter G. L. Jordan. The first widespread automotive application was driven by the Clean Air Act (United States) of 1970, with General Motors introducing the first commercial units on the 1975 model year Chevrolet Nova in the United States. Key innovations included the development of the three-way catalyst by researchers at Volkswagen and BASF in the late 1970s, which allowed for simultaneous reduction of multiple pollutants. Regulations in the European Union and Japan soon followed, mandating their use globally.

Construction and components

The core structure consists of a stainless steel shell housing a ceramic or metallic substrate, most commonly a honeycomb structure made of cordierite to maximize surface area. This substrate is coated with a washcoat of porous materials like alumina and ceria, which provides a high-surface-area layer for the active catalytic materials. The precious metal catalysts are dispersed within this washcoat, typically using a combination of platinum, palladium, and rhodium. The assembly is protected by a mat material, often made of intumescent material like vermiculite, which expands to hold the substrate firmly inside the shell under varying thermal conditions.

Chemical reactions and operation

The device facilitates simultaneous oxidation and reduction reactions. The oxidation reactions convert carbon monoxide to carbon dioxide and unburned hydrocarbons to carbon dioxide and water vapor. Concurrently, reduction reactions convert nitrogen oxides into nitrogen and oxygen. These processes are optimized by maintaining the air–fuel ratio near the stoichiometric point, monitored by upstream and downstream oxygen sensors in a closed-loop system managed by the engine control unit. The catalyst requires a minimum temperature, the light-off temperature, to become effective, which is why it is mounted close to the exhaust manifold.

Types and applications

The primary type is the three-way catalyst, used in virtually all modern gasoline engine vehicles. For diesel engine applications, a diesel oxidation catalyst is used to control carbon monoxide and hydrocarbons, often paired with a selective catalytic reduction system using a reagent like AdBlue to reduce nitrogen oxides. Other variants include the two-way catalyst, historically used in some North American markets, and lean-burn nitrogen oxide traps. They are universally required in vehicles sold in the European Union, United States, Japan, China, and most other major markets.

Environmental impact and regulations

The introduction of catalytic converters led to dramatic reductions in urban smog and pollutants like ground-level ozone. Their use is mandated by legislation worldwide, including the Clean Air Act (United States) enforced by the Environmental Protection Agency and standards set by the European Commission. While reducing tailpipe emissions, the mining and refining of precious metals like platinum and palladium raise concerns about environmental degradation in regions like the Bushveld Igneous Complex in South Africa. Theft of converters for their precious metal content has also become a significant issue in cities like London and Los Angeles.

Performance and maintenance

Performance can be degraded by contaminants such as lead from leaded gasoline, phosphorus and zinc from engine oil additives, and sulfur from fuel. Misfire conditions can cause overheating and melting of the substrate. Diagnosis often involves monitoring the signals from the downstream oxygen sensor and using an exhaust gas analyzer. Failure can trigger a malfunction indicator lamp on the vehicle's dashboard. While designed to last the life of the vehicle under normal conditions, physical damage from road debris or the use of improper fuels are common causes of premature failure.

Category:Emission control technologies Category:Automotive technologies