Catalysts in catalytic converters

Furthermore, lead compounds poison the platinum metal-based catalysts in catalytic converters, so that efforts to abate air pollution by automobiles through the use of catalytic converters (Section 8.4.2) are dependent on the use of lead-free fuels. Fuels of sufficiently high octane... 

The catalyst in catalytic converters reduces the activation energy of the reaction. 

In another of the many interactions between problems and their solutions, catalysts in catalytic converters are poisoned by lead. For this reason, cars with catalytic converters are required to use only unleaded gasoline. One negative side effect of the use of catalytic converters is an increase in N2O emission. The converters reduce NO and NO2 to N2O, which has less immediate effects but has a greenhouse effect (described later in this chapter). 

In order to generate acceptable power levels, however, low-temperature PEM fuel cells need catalyzed electrodes. Thus far, the electrocatalysts used in PEM fuel cells have been exclusively Pt based. However, Pt is an industrial metal, mainly mined in South Africa (with about 75 % of the world production) and Russia (about 15 % of the world production). Today, 50 10 % of Pt production of the 200,000 kg/year is used as chemical catalyst in catalytic converters for internal combustion engine vehicles. A mere glance at Fig. 10.1 immediately emphasizes the link between the cost of Pt, that of cmde oU, and the health of the economy. In other words, one cannot hope for a strong economy and for Pt price to stay low at the same time. 

Oxidation. Carbon monoxide can be oxidized without a catalyst or at a controlled rate with a catalyst (eq. 4) (26). Carbon monoxide oxidation proceeds explosively if the gases are mixed stoichiometticaHy and then ignited. Surface burning will continue at temperatures above 1173 K, but the reaction is slow below 923 K without a catalyst. HopcaUte, a mixture of manganese and copper oxides, catalyzes carbon monoxide oxidation at room temperature it was used in gas masks during World War I to destroy low levels of carbon monoxide. Catalysts prepared from platinum and palladium are particularly effective for carbon monoxide oxidation at 323 K and at space velocities of 50 to 10, 000 h . Such catalysts are used in catalytic converters on automobiles (27) (see Exhaust CONTHOL, automotive). 

Selective catalytic reduction (SCR) is cmrently the most developed and widely applied FGT technology. In the SCR process, ammonia is used as a reducing agent to convert NO, to nitrogen in the presence of a catalyst in a converter upstream of the air heater. The catalyst is usually a mixture of titanium dioxide, vanadium pentoxide, and hmgsten trioxide. SCR can remove 60-90% of NO, from flue gases. Unfortunately, the process is very expensive (US 40- 80/kilowatt), and the associated ammonia injection results in an ammonia slip stream in the exhaust. In addition, there are safety and environmental concerns associated with anhydrous ammonia storage. 

Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176,177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 37-43 Copper chromite, oxidation of CO over, 86-88... 

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

By far the most important use of the platinum metals is for catalysis. The largest single use is in automobile catalytic converters. Platinum is the principal catalyst, but catalytic converters also contain rhodium and palladium. These elements also catalyze a wide variety of reactions in the chemical and petroleum industry. For example, platinum metal is the catalyst for ammonia oxidation in the production of nitric acid, as described in Pt gauze, 1200 K... 

Transition metals and their compounds are used as catalysts. Catalysts you may already know are Iron In the Haber process (Industrial production of ammonia) platinum in the Ostwald process (Industrial production of nitric acid) and platinum, rhodium and palladium In catalytic converters. 

Japanese chemists succeeded in obtaining good yields of methane by reaction of H2 with a mixture of carbon monoxide and carbon dioxide, at temperatures as low as 270 °C, by use of a special mixed catalyst containing nickel as the most important metallic constituent. Why is nickel used In the same vein, why is platinum or platinum-rhodium alloy (but not nickel) used in catalytic converters for automobile exhausts (See also Section 17.4.)... 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

The principal consumption of PGE is as a catalyst, especially the use of platinum, or the more favored palladium because of its superior high-temperature performance, in catalytic converters in motor vehicles. Among the diverse other chief uses are electrical and electronic applications, jewelry, fabrication of laboratory equipment, and dental repairs. 

Development of models that descnbe the flow in monolith channels in catalytic converters has received great attention [68,69]. See also Ref. 58 and references therein. As with channel interaction, the models applicable to monolith combustion catalysts are similar to those for catalytic converters... 

Palladium has two primary uses as a catalyst and in making jewelry and specialized alloys. A catalyst is a substance used to speed up a chemical reaction without undergoing any change itself. Palladium catalysts are used in breaking down petroleum to make high quality gasoline and other products. It is also used in the production of some essential chemicals, such as sulfuric acid (H2SO4), which is used in paper and fabric production. The catalytic converters used in automobiles today may also contain a palladium catalyst. A catalytic converter is a device added to a car s exhaust system. It helps the fuel used in the car burn more efficiently. 

These three reactions, catalyzed in catalytic converters, are all exothermic and ther-mod)mamically favored. Unfortunately, other energetically favored reactions are also accelerated by the mixed catalysts. All fossil fuels contain sulfur compounds, which are oxidized to sulfur dioxide during combustion. Sulfur dioxide, itself an air pollutant, undergoes further oxidation to form sulfur trioxide as it passes through the catalytic bed. 

Figure 61. Tailpipe emission of CO, HC and NO c from a gasoline fueled passenger car equipped with a three-way catalytic converter, in the US-FTP 75 vehicle test, as a function of the number of catalysts in the converter at fixed total catalyst volume (monolith catalyst with 62 cells cm three-way formulation with Pt 0.83gl , Rh 0.16gl fresh condition and after high temperature aging for 20 hours on an engine bench).

Because of its ability to chemisorb NO dissociatively, Rh is the key catalyst in TWC converters for the reduction of NOx. Due to its rarity in nature in comparison with the other noble metals Pt and Pd ( 1 15) and its consequent significantly higher cost, a reduction in the amount of Rh present in automotive exhaust catalytic converters, via appropriate enhancement (promotion) of the catalytic activity of the other noble metals components (Pt or Pd) would be highly desirable. 

If special conditions are required for a reaction to take place, such as the presence of a catalyst, the conditions or the catalyst can be written above or below the arrow. Platinum metal catalyzes (speeds up) the decomposition of nitric oxide to its constituent elements in catalytic converters. 

The platinum-group metals Rh, Pd and Pt play a vital role in keeping the environment devoid of pollutants originating from vehicle exhausts. They are present in catalytic converters (which we discuss in detail in Section 26.7) where they catalyse the conversion of hydrocarbon wastes, CO and NO c (see Box 14.8) to CO2, H2O and N2- The growth rate of environmental catalyst manufacture by companies... 

The demand for the rare earth metals increased over the last two decades of the 20th century, and the demand for cerium oxides in motor vehicle catalytic converters (see equations 26.39 and 26.40) has been a major contributing factor. In 2001, the major use (34% of the total consumption) for rare earth metals in the US was in glass polishing and ceramics. Petroleum catalysts and catalytic converters... 

The development of catalytic converters has recently encompassed the use of zeolites, e.g. Cu-ZSM-5 (a copper-modified ZSM-5 system), but at the present time, and despite some advantages such as low light-off temperatures, zeolite-based catalysts have not shown themselves to be sufficiently durable for their use in catalytic converters to be commercially viable. 

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