Exhaust catalytic converters

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

Several length-scales have to be considered in a number of applications. For example, in a typical monolith reactor used as automobile exhaust catalytic converter the reactor length and diameter are on the order of decimeters, the monolith channel dimension is on the order of 1 mm, the thickness of the catalytic washcoat layer is on the order of tens of micrometers, the dimension of the pores in the washcoat is on the order of 1 pm, the diameter of active noble metal catalyst particles can be on the order of nanometers, and the reacting molecules are on the order of angstroms cf. Fig. 1. The modeling of such reactors is a typical multiscale problem (Hoebink and Marin, 1998). Electron microscopy accompanied by other techniques can provide information on particle size, shape, and chemical composition. Local composition and particle size of dispersed nanoparticles in the porous structure of the catalyst affect catalytic activity and selectivity (Bell, 2003). 

The NO reduction by CO is a very important reaction taking place in the car exhaust catalytic converters. It has received considerable attention on Rh single crystals [210] and much less on Pd [211,212]. On supported model catalysts many fewer experiments have been performed than for CO oxidation, and only in recent years [212-216]. From the work from Goodman s group it appears that the reactivity of Pd single crystals decreases when the surfaces become more open, and on supported clusters it decreases with cluster size [212]. We have studied this reaction by molecular beam techniques [209]. The steady-state reaction rate as a function of... 

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. 

The three-way exhaust catalytic converter is used to complete the combustion of carbon monoxide and unburned fuel elements and to remove the NO and N02 produced during combustion. The primary components of the catalytic converter are the catalyst and its physical support. The catalyst is composed of a high-surface-area support that incorporates the primary catalytically active materials, typically mixtures of one or more of platinum, palladium, or rhodium (precious group metals, PGMs). The catalyst also contains promoters that improve the efficiency and stability of the PGM. Of these, the most important is cerium oxide, a... 

The automobile test was carried out using a 3.8 liter-V6 engine with a 2 liter exhaust catalytic converter. A non-standard 2,150 Km urban driving was carried out in order to compare fresh and used catalysts. 

Zinc Di(organo) Di(thio) Phosphates, commonly referred to as ZDDPs, are the most widely used and effective anti-wear/anti-oxidation additives in engine oil. Phosphorus contained in the ZDDP molecules has been shown to partially volatize during engine operation [2]. Unfortunately, volatile phosphorus in the exhaust stream degrades the iunction of the exhaust catalytic converter, and as a consequence, there has been pressure to reduce the amount of ZDDP in engine oil. 

The rate of oxidation of CO in an automobile exhaust catalytic converter has been approximated by the expression... 

Reduction or elimination of smog requires that the ingredients essential to its formation be removed from automobile exhaust. Catalytic converters reduce the levels of NO and hydrocarbons, two of the major ingredients of smog. (See the Chemistry Put to Work Catalytic Converters in Section 14.7.)... 

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