Automotive catalyst three-way

Three-way automotive catalysts based on palladium, rather than the more expensive metals platinum and rhodium, have long been desired. However, Pd is more sensitive than R to poisoning by lead (Pb) compounds [1-4], Consequently, widespread commercial use of Pd-based automotive three-way catalysts (TWC) was delayed in the U.S. until the early 1990s, by which time residual Pb concentrations in unleaded gasoline had decreased to negligible levels. The past five years have witnessed... 

Meitzler, A. H. "Application of Exhaust-Gas-Oxygen Sensors to the Study of Storage Effects in Automotive Three-Way Catalysts" SAE Paper No. 800019, 1980. 

The supports themselves are usually made by the bulk methods. Examples of impregnated catalysts indude the Pd/C hydrogenation catalysts, the Pt/Sn/Al203 dehydrogenation catalysts and the automotive three-way catalysts. There are also catalysts made by hybrid synthesis approaches, e.g., where the active precursors are mixed with a powdered support, and the mixture is then agglomerated. 

Usually the sintering of the catalytic components causes deactivation of catalysts. For example the automotive three-way catalysts deactivate by the sintering of fine particles of precious metals and alumina supports as well as CeOj when the catalysts are subjected at high temperatures for a long time. Sintering of Ce02 facilitates the... 

Fig. 4.1. Illustration of the structure gap for an automotive three-way catalysts (TWO) depicted by a schematic drawing of a monolithic, supported catalyst. The lower right image shows a TEM picture of aged automotive catalyst 12-nm Pt on AI2O3. Note, the presence of small Pt single crystals. Well-defined model catalysts such as those made by electron-beam lithography (EBL lower middle image) are used to mimic real catalyst and bridge the gap between single crystal studies (bottom left) and real-life catalysts...

The authors have been studying the effect of alkaline addition on the catalytic activity of automotive three-way catalysts. We have found that the addition of Ba to Pd or platinum(Pt) three-way catalysts is effective for improvement of catalytic activity under reducing conditions, and that the suppression of hydrocarbon(HC) chemisorption on the catalysts by the addition of Ba allowed the catalytic reaction to proceed smoothly (16,17). 

The operation of warmed-up automotive three-way catalysts is considered. Special emphasis is given to the observation that significant fractions of CO, hydrocarbon, and NO emissions in urban driving tests occur during vehicle acceleration. The increased emissions during acceleration occur as a result of increased exhaust flow rates and rich air-fuel ratio excursions of the air-fuel ratio control system. A method is presented for displaying and analyzing... 

A chemisorption teclmique developed by Koinai et al., based on CO methanation, was successfrilly used to analyze noble metal dispersions of both fresh and vehicle-aged Pt/Rli and Pd/Rli commercial automotive three-way catalysts. The teclmique is relatively rapid (< 2 hours), extremely sensitive, and largely free from complications due to adsorption of CO on non-noble metal components of the washcoat (support, promoters, stabilizers, etc.). Particle sizes of the vehicle-aged catalysts, calculated by applying the spherical particle assumption to the dispersions measured by the CO methanation method, agreed well with particle sizes calculated from x-ray diffraction line-broadening data. These results indicate that the CO methanation teclmique can be applied routinely to obtain fast and accurate measurements of noble metal surface areas in automotive catalysts retrieved from tlie field, even tliose with metal dispersions ca. 2% or less. 

Brett, P. S., et al., An Investigation into Lubricant Related Poisoning of Automotive Three Way Catalysts and Lamda Sensors, SAE Paper 890490, 1989. 

Aluminas also play an important part in the preparation of structured catalysts. They can be present on or constitute the structured support of catalysts. Common shapes are monohthic honeycombs or open-cell foams. Aluminas can be used as washcoat, a high-surface-area layer that gives the geometrical framework a suitable morphology to support and disperse the catalytically active phase, or they can be used as primer, an intermediate layer between the geometrical support and the washcoat that acts as glue between the two layers (433). Alumina plays a central role for the preparation of efficient automotive three-way-catalysts (434). 

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