A well-tuned engine still produces many pollutants caused by the incomplete combustion of the fuel. The three regulated harmful compounds from exhaust gas mixtures are hydrocarbons (in the form of unburned gasoline), carbon monoxide (formed by the combustion of gasoline), and nitrogen oxides (created when the heat in the engine forces nitrogen in the air to combine with oxygen). Carbon monoxide is a poison for any air-breathing animal. Nitrogen oxides lead to smog and acid rain, and hydrocarbons produce smog.
A catalytic converter is a device that uses a catalyst to convert the three harmful compounds into harmless ones. The catalyst formulation comprises three key constituents: precious metals (platinum, palladium, rhodium), alumina, and rare-earth-based materials which enhance catalytic activity of the metals. Ceria is typically used in this capacity. It is white when pure, but is usually pale yellow on account of the nonstoichiometric phases between Ce2O3 and CeO2. The nonstoichiometric ceria is responsible for the following functions to remove the pollutants from the motor vehicle engine exhaust:
- a
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Promotion of the water-gas shift reaction: CO + H2O ⇒ CO2 + H2.
- b
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Enhancement of the NOx reduction capability of rhodium.
- c
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Oxygen storage. In this role, ceria provides elemental oxygen in fuel-rich but air-poor conditions, to ensure oxidation of unburnt hydrocarbons and the removal of CO. It accomplishes this by going nonstoichiometric to CeO2−x. In leaner (fuel-deficient, air-rich) conditions, it re-oxides to CeO2, i.e., it stores oxygen during the air-rich periods.
Catalytic converters are an integral part of all automobile exhaust systems manufactured since the mid-1970s (in the United States as a result of the 1970 Clean Air Act)
Diesel Exhaust Control
Pramod Thakur Ph.D., in Advanced Mine Ventilation, 2019
11.3.3 Catalytic Converter
A well-designed catalytic converter placed in the exhaust system next to the engine can drastically reduce all gaseous emissions, including carbon monoxide, unburnt hydrocarbons, and soluble organic fractions (containing polynuclear aromatics). It is a compact, durable, reliable, and a low-cost component. It should, therefore, be an integral part of all diesel engines approved for underground coal mines. Table 11.9 shows the characteristics of a well-designed catalytic converter.
Table 11.9. Characteristics of a Well-Designed Catalytic Converter
| CO | Reduced by 80%–95% |
| HC | Reduced by 85%–90% |
| DPM | Reduced by 25%–35% |
| Odor control | Very good |
| Influence on engine | Low-pressure drop; no fuel penalty |
| Reliability | Very good |
| Durability | ≥5000 h |
The core of a catalytic converter is an open-channel ceramic monolith or a metallic honeycomb substrate that provides support to the catalyst. Cordierite (2MgO-2Al2O3-5SiO2) is the most popular ceramic material used for the substrate. It has many good characteristics, such as high surface area, large open frontal area, low heat capacity, low thermal expansion coefficient, and good mechanical strength [7].
Metallic substrates are made of metal, silica, iron, chromium, and aluminum alloys. They have higher surface areas and low-pressure drop but are more expensive. Both types of substrates are coated with catalysts formulated with noble metals, such as platinum, palladium, and rhodium. The catalyst application is a two-step process. The first step is a wash-coat of aluminum, silica, titanium, cerium, and other compounds. In the second step, the noble metal catalysts are impregnated on to the wash-coat.
The catalytic converter oxidizes all gases such as CO, NO, and HC to CO2, NO2, and CO2 and water. The catalysts need to have a temperature above 300°C for efficient performance. This is the reason they are installed next to the engine. The catalytic converter usually lasts a long time. Five thousand hours of life is common when ultralow sulfur fuel is used. It is always designed to match the engine exhaust without creating too much back pressure. The current cost varies from $5000 to $10,000 depending on the size of the equipment. The surface of the catalytic converter is always coated with insulating material to keep the surface temperature below 302°F. For permissible engines, they are water-jacketed.