Automotive exhaust catalysis

For the conversion of automotive exhaust gases the three-way catalyst (TWC) enables the removal of the three pollutants CO, NO and hydrocarbons ( HC ) in the following manner (Eqs. 10-1 to 10-4)  

Industrial Catalysis A Practical Approach, Second Edition. Jens Hagen Copyright 2006 WE.EY-VCH Verlag GmbH Co KGaA.Weinheim ISBN 3-527-31144-0 

The three-way catalyst compositions is 70% cordierite substrate (MgAl204) and the washcoat  

The three-way catalyst represents a remarkably successful area of catalytic technology. The main shortcoming of the three-way catalyst is only, that it is not good enough under lean (oxygen-rich) conditions. 

Transportation is a major source of air pollution, particularly in urban environments. The total number of cars, trucks, buses and motorcycles in the world in 1990 was estimated at about 650 million. All these vehicles produce exhaust, and an esti- 

Concepts of Modem Catalysis and Kinetics. I. ChorkendorfF, J. W. Niemantsverdriet Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30574-2 

Unbumt gasoline and cracked hydrocarbons such as ethylene and propylene are also substantial constituents of exhaust. Gasoline contains additives such as benzene, toluene and branched hydrocarbons to achieve the necessary octane numbers. The direct emission of these volatile compounds, e.g. at gas stations, is a significant source of air pollution. Leaded fuels, containing antiknock additions such as tetra-ethyl-lead, have been abandoned because lead poisons both human beings and the three-way exhaust catalyst, especially for the removal of NO by rhodium. 

In the USA, the Clean Air Act of 1970 established air-quality standards for six major pollutants particulate matter, sulfur oxides, carbon monoxide, nitrogen oxides, hydrocarbons, and photochemical oxidants. It also set standards for automobile emissions - the major source of carbon monoxide, hydrocarbons, and nitrogen oxides. An overview of the major standards is given in Tab. 10.2. The levels of, for example, the European Union (1996) are easily achieved with the present catalysts. The more challenging standards, up to those for the ultralow emission vehicle, are within reach, but zero-emission will probably only be attainable for a hydrogen-powered vehicle. 

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G. B. Fisher and co-workers. The Kole of Ceria in Automotive Exhaust Catalysis and OBD-II Catalyst Monitoring, SAE 931034, Society of Automotive Engineers, Warrendale, Pa., 1993. 

The reduction of NO by propene is of great importance in automotive exhaust catalysis. 

The ultimate direct utilization of electrochemical promotion in commercial reactors (in the chemical industry and in automotive exhaust catalysis) will depend on several technical and economical factors6 which are intimately related to the following technical considerations and problems ... 

Another interesting case - which immediately illustrates how opportunistic the concept of reaction orders for catalytic reactions may be - is that of CO oxidation, an important subreaction in automotive exhaust catalysis ... 

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

Marin GB, Hoebink JHBJ. Kinetic modeling of automotive exhaust catalysis. Cattech 1997 2 137-148. 

The dissociation of NO is a crucial elementary step in the mechanism of NO reduction by CO in automotive exhaust catalysis. The overall reaction here is ... 

P.L. Silveston, Automotive exhaust catalysis under periodic operation. Catalysis Today 25 175 (1995). 

An effort has been made in this work to evaluate the utility of surface parameters determined in UHV surface science experiments for understanding the high pressure kinetics of certain catalytic reactions. We have chosen two test reactions of considerable significance in automotive exhaust catalysis,... 

Despite the fact that perovskite-type oxides have been suggested as substitutes for noble metals in automotive exhaust catalysis [1], relatively few studies were devoted to the synthesis, characterization and catalytic activity of these catalysts. 

Most of the bimolecular surface reactions follow the complex LH mechanism, i.e. a scheme consisting of more than one step. A well-known example is the oxidation of CO to produce CO2, i.e. the CO + O —> CO2 on Pt(l 11). The catalytic oxidation of CO on a metal surface is very important from a technological point of view, as it plays a major role in automotive exhaust catalysis. In theory, one may consider three distinct mechanisms for this reaction to occur ... 

Since 1970, the perovskite type oxides, typically rare earth oxides with a (ABO3) formula, have been suggested as substitutes for noble metals in automotive exhaust catalysis (1). The most studied perovskites are LaM03 ( M = first row transition metal ) (2,3,4), where M is considered as the active site of the catalyst. The cobaltites show good activity as oxidation catalysts, the reactivity seems to depend on the facility of cobalt to undergo the transition Co Co m, which may be correlated to an oxygen non stoichiometry, and to the spin state of the cation (5). Furthermore, series of LaM03 oxides revealed similar profiles for CO adsorption studies as for NO adsorption, with NO adsorption maxima for M = Mn and Co (6). The reactivity of these catalysts has been shown not only to depend on the surface area, but also on the preparation process (7). 

In this section, the applications of perovskite-type materials in automotive exhaust catalysis are shortly presented. The latest advances in the field are included, with particular emphasis on structure-activity relationship. The section is devoted to two separate parts (a) application of perovkite oxides in model reactions related to three-way catalysis and (b) application of perovskite oxides under simulated or real exhaust conditions. 

Recently, great attention has been paid to noble metals (NMs)-promoted perovsldtes as potential candidates for automotive exhaust catalysis, since NMs incorporation to perovsldte structure prevents the sintering, reducing also volatilization losses at high temperatures [50]. Hence, it has been reported that BaCeOs perovsldtes with low levels of Pd substitution exhibited superior oxidation activity compared to highly dispersed Pd/Al203 catalysts [50]. 

Platinum and palladium have high activities for total oxidation. This property is exploited in automotive exhaust catalysis. Automobile exhaust contains toxic gases such as CO, NO, and hydrocarbons which contribute to formation of photochemical smog and acid rain. Since 1978, catalysts based on platinum, rhodium, and sometimes palladium, supported on a monolithic carrier, are applied to convert exhaust gases to less harmful products. The so-called threeway catalyst enables the following three overall reactions... 

The present effort makes no attempt to match in scope the previous review we shall confine ourselves to work concerning chemical poisoning and coking as the primary mechanism of deactivation but retain the classification according to scale — individual kinetics and mechanism, intraparticle problems, and chemical reactor problems. Sintering has been admirably covered in a recent review (2), and the subject of automotive exhaust catalysis (which is almost wholly an exercise in catalyst mortality) will be treated in one forthcoming (3). 

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