Automotive catalysts catalytic converters

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. 

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). 

Some catalyst supports rely on a relatively low surface area stmctural member coated with a layer of a higher surface area support material. The automotive catalytic converter monolith support is an example of this technology. In this appHcation, a central core of multichanneled, low surface area, extmded ceramic about 10 cm in diameter is coated with high surface area partially hydrated alumina onto which are deposited small amounts of precious metals as the active catalytic species. 

Automotive Catalytic Converter Catalysts. California environmental legislation in the early 1960s stimulated the development of automobile engines with reduced emissions by the mid-1960s, led to enactment of the Federal Clean Air Act of 1970, and resulted in a new industry, the design and manufacture of the automotive catalytic converter (50). Between 1974 and 1989, exhaust hydrocarbons were reduced by 87% and nitrogen oxides by 24%. 

At the heart of an automotive catalytic converter is a catalyzed monolith which consists of a large number of parallel channels in the flow direction whose walls are coated with a thin layer of catalyzed washcoat. The monolith catalyst brick is wrapped with mat, steel shell and insulation to minimize exhaust gas bypassing and heat loss to the surroundings. 

The widespread use of platinum, palladium and other metals in automotive catalytic converters has been driven by environmental considerations and the increasing costs of the metals. This has not been matched by the development of clean reprocessing technologies for the catalysts themselves. The spent catalyst metals are oxidized to their cations via leaching into concentrated acids. 

Houdry s solution to the problem was the first catalytic converter ever designed for an automotive vehicle. The catalytic converters found on almost all cars and trucks in use today are still strikingly similar to his invention. Exhaust gases passed into the converter and over a bed of platinum catalyst, then exited with a greatly reduced concentration of carbon monoxide, nitrogen oxides, and unburned hydrocarbons. Houdry obtained a patent for his device in 1956 and founded a company, Oxy-Catalyst, to manufacture and sell the new product. 

In fact, most of us benefit from the use of catalysis. Automotive catalytic converters have represented the most massive application of environmental catalysis and one of the most challenging and successful cases in catalysis, generally. Automobile catalysts deseive a few more comments. The engine exhaust emission is a complex mixture, whose composition and flow rate change continuously depending on a variety of factors such as driving conditions, acceleration, and speed. Despite the variability of the conditions, three-way catalysts have achieved the reduction of exhaust carbon monoxide, hydrocarbons, and... 

The need for controlling the exhaust emissions from automotive vehicles has been recognized since 1975. The most effective and tested method proved to be the installation of diree-way catalysts at the exhaust emission system of cars. The development and the improvement M of such catalysts was and will be a complicated effort, since a cat-alyst placed in a vehicle should simultaneously accelerate oxida-tion and reduction reactions, under continuously changing conditions of temperature and space velocity, in contrast to industrial applications where catalysts operate under fixed and MjM controlled conditions. Generally, the catalytic converter of a vehicle has to satisfy the following requirements ... 

Monolithic catalysts have found a wide range of applications in the removal of pollutants from air, especially in the automotive industry. Specifically, the demand for large surface to small volume, high conversions achieved for low retention times, and low pressure drop led to the development of monolithic supports. More information on automotive catalytic converters has been given in Chapter 1. Usually, a thin layer of alumina is deposited onto a monolith for keeping the precious metal used for air pollutants abatement dispersed. The oxidations that take place are highly exothermic and the reaction rates achieved are in turn high. Hence, the reactants diffuse only a small distance... 

The catalyst of an automotive catalytic converter is typically a form of platinum, Pt palladium, Pd or rhodium, Rd. The production of catalytic converters is by far the largest market for these precious and semiprecious metals. 

Another important application of heterogeneous catalysts is in automobile catalytic converters. Despite much work on engine design and fuel composition, automotive exhaust emissions contain air pollutants such as unburned hydrocarbons (CxHy), carbon monoxide, and nitric oxide. Carbon monoxide results from incomplete combustion of hydrocarbon fuels, and nitric oxide is produced when atmospheric nitrogen and oxygen combine at the high temperatures present in an... 

It is advisable to consider only briefly the deactivation of automotive catalysts by lead, as observed in early attempts of the implementation of catalytic converters on automobiles. These observations are summarized adequately in the earlier reviews (5-5). In particular Table 5 of the review by Yolles and Wise (4) shows that researchers engaged in the development of automotive catalysts before the middle 1960s did not contemplate... 

ICP-MS is useful for analysis of catalysts from two perspectives The composition of the catalysts must be carefully controlled, particularly because the active elements are often expensive. The catalysts are often finely distributed in a substrate material so their concentration in the bulk material may be quite low. Second, catalysts, particularly those used in automotive catalytic converters, can be a significant source of platinum group elements in the environment. Re and Pt have been measured in catalysts by ICP-MS [193], Procedures for the analysis of used catalytic converter materials by ICP-MS have been reported [355]. Accurate measurements are essential for many of these applications so isotope dilution-based concentration calibration is commonly used. 

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. 

However, for the relatively low sulfur concentrations to which three-way catalyst are typically exposed, the precious metals are usually unaffected. Poisoning of ceria is much more serious than poisoning of the precious metal at the levels of 5 to 20 ppm SO2 currently present in the typical automotive exhaust. At these concentrations, SO2 interacis primarily with the ceria-containing component of in the catalytic converter and it is this poisoning of ceria that appears to be the primary problem associated with sulfur poisoning [6-11]. The evidence for this is strong. For... 

In general, both cordierite and metallic monoliths are unsuitable as catalytic supports. To process a monolith into an active monolithic catalyst, a layer of porous catalytic support must be deposited on the walls between channels. y-Alumina appeared to be the most effective support for automotive catalysts. The alumina layer is deposited by sol-gel technique (so called washcoating). Adherence of 7-alumina to cordierite is relatively strong. However, to form the stable 7-alumina layer on a metallic surface, we need to use an appropriate alloy that is appropriately processed before the layer is deposited. Stainless steel containing chromium, aluminum, and yttrium subjected to thermal treatment under oxidizing conditions meets requirements of automotive converters. Aluminum in the steel is oxidized to form 7-alumina needles (whiskers) protruding above the metal... 

S.H. Oh, Catalytic converter modeling for automotive emission control, in Computer-Aided Design of Catalysts (E.R. Becker and C.J. Pereira, eds.). Dekkcr, New York, 1993, p. 

Not all catalysts need the extended smface provided by a porous structure, however. Some are sufficiently active so that the effort required to create a porous catalyst would be wasted. For such situations one type of catalyst is the monolithic catalyst. Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations. Typical examples include the platinum gauze reactor used in the ammonia oxidation portion of nitric acid manufacture and catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (honeycomb) or non-porous (wire gauze). A photograph of a automotive catalytic converter is shown in Figure CD 11-2. Platinum is a primary catalytic material in the monolith. 

---