Exhaust emission control, platinum

The principle components of modern exhaust emission control catalysts are identified. They comprise (a) a ceramic substrate, (b) a high surface area wash coat, (c) base metal promoters and/or stabilisers and (d) platinum group metals either singly or in combination. 

Platinum Catalysts for Exhaust Emission Control The Mechanism of Catalyst Poisoning by Lead and Phosphorus Compounds... 

Exhaust emission standards since the 1981 model year vehicles have required the use of three-way catalysts, either alone or in combination with an oxidation catalyst. Three-way catalysts are designed to operate in a very narrow range about the stoichiometric air/fuel ratio. In this range the HC and CO are subject to oxidation and the NO, compounds undergo reduction. The downstream oxidation catalyst in a dual bed system is generally used as a "clean-up catalyst lo further control HC and CO emissions. The most common catalytic combination in three-way uses is platinum/rhodium. Current production applications use these elements in a relatively rich proportion of 5 1 lo 10 1. whereas the respective mine ratio is about 19 1. 

The six platinum group metals, platinum, palladium, ruthenium, osmium, rhodium, and iridium, usually occur together in nature. These metals are not often found in artifacts. These metals are rare and have only been widely used in industry and for ornaments since the early twentieth century. Most platinum used today is as a catalyst in the systems used to control car exhaust emissions, in dentistry, and to make surgical tools, jewelry, and electrical equipment. 

F Propane, butane, or liquefied petroleum gas (LPG) has seen practical service in passenger automobiles for 30 years or more. Because LPG is used in the vapor phase, it pollutes less than gasoline but more than natural gas. A number of cars in the Clean Air Car Race ran on LPG. The table below lists the results and those for natural gas. It must be kept in mind that these vehicles were generally equipped with platinum catalyst reactors and with exhaust-gas recycle. Therefore the gains in emission control did not come entirely from the fuels. 

In the US and Japan automobile exhaust catalysts containing the noble metals platinum, palladium and rhodium are being used for the control of carbon monoxide, hydrocarbons, and nitrogen oxides in order to satisfy regulatory emission control requirements and such catalysts will be introduced in Europe in the near future. 

Base Metal vs. Platinum Catalysts. When the emission control performances of these two types of catalyst were compared, two points were evident. First, the base metal and platinum catalysts controlled exhaust HC and CO equally (or nearly so) at very large catalyst volumes (low space velocities). This had been reported for laboratory evaluations of these catalysts (14) thus these data for vehicles confirm the findings from steady-state bench tests. Second, initial activity of the platinum catalysts was very high. There was essentially no change in the emissions control performance of the platinum catalysts even at very small catalyst volumes, and this behavior can be utilized in two ways. 

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 25.8) where they catalyse the conversion of hydrocarbon wastes, CO and NO. (see Box 15.7) to CO2, H2O and N2. In 2008, the manufacture of catalytic converters used 81% of the rhodium, 47% of palladium and 44% of platinum consumed worldwide. The growth rate of environmental catalyst manufacture by companies such as Johnson Matthey in the UK is driven by legislative measures for the control of exhaust emissions. Regulations in force in the US and Europe have had a major impact on the levels of emissions and have improved the quality of urban air. Tighter control of vehicle emissions has now been introduced in most parts of Asia. 

Three way catalysts are generally used today for controlling exhaust emissions from automotive Internal combustion engines. Cerium oxide is widely employed as an additive in three way catalysts because of its abilities to store oxygen and to improve dispersion of platinum.[1] One drawback of the three way catalyst is that it tends to give rise to thermal deactivation caused by crystallization of cerium oxide, sintering of platinum and... 

PGM catalyst technology can also be appHed to the control of emissions from stationary internal combustion engines and gas turbines. Catalysts have been designed to treat carbon monoxide, unbumed hydrocarbons, and nitrogen oxides in the exhaust, which arise as a result of incomplete combustion. To reduce or prevent the formation of NO in the first place, catalytic combustion technology based on platinum or palladium has been developed, which is particularly suitable for appHcation in gas turbines. Environmental legislation enacted in many parts of the world has promoted, and is expected to continue to promote, the use of PGMs in these appHcations. 

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

A very important aspect of gas sensors in automotive exhaust-gas environments is aftertreatment of the electrodes to control a specific sensor behavior. For example, to measure nonequilibrium raw emissions, the sensor needs excellent catalytic ability. Various methods are known to improve electrodes in Zr02-based sensors. One well known method is to increase the active platinum surface area and the three-phase boundary area by partial reduction of zirconia close to the electrode. This occurs when the ceramic is exposed to a reducing atmosphere at high temperatures or when an electrical cathodic current is applied through the electrode and electrolyte. A similar effect can be achieved by chemical etching of the elec-... 

Supported platinum and base metal catalysts were evaluated in vehicle tests with converter volumes of 600-4300 cm3. The initial oxidation activity of the catalysts was determined as the vehicle was operated over the 1975 FTP. The ability of the base metal catalysts to control exhaust HC and CO emissions was strongly dependent on the catalyst volume. HC and CO conversion decreased quite rapidly as the converter size was decreased. 

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