Catalytic converter reactions

The implementation of very effective devices on vehicles such as catalytic converters makes extremely low exhaust emissions possible as long as the temperatures are sufficient to initiate and carry out the catalytic reactions however, there are numerous operating conditions such as cold starting and... 

We consider next perhaps the bet understood catalyzed reaction the oxidation of CO over group VIII metal catalysts. The reaction is an important environmental one since it involves the conversion of CO to CO2 in automobile catalytic converters. The mechanism is straightforward ... 

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). 

In tills chapter we consider systems in which a reaction between two gaseous species is carried out in die adsorbed state on die surface of a solid. The products of die reaction will be gaseous, and die solid acts to increase die rate of a reaction which, in die gaseous state only, would be considerably slower, but would normally yield die same products. This effect is known as catalysis and is typified in industty by die role of adsorption in increasing die rate of syndiesis of many organic products, and in die reduction of pollution by die catalytic converter for automobile exliaust. 

Aniline 77 was converted into its diazonium salt with nitrous acid and this was followed by reduction with stannous chloride to afford the corresponding arylhydrazine 78. Condensation of 78 with 3-cyanopropanal dimethylacetal 79 gave the arylhydrazone 80. Treatment of 80 with PPE resulted in cyclization to indole 81. The nitrile group was then reduced to the primary amine by catalytic hydrogenation. Reaction of the amine with excess formalin and sodium borohydride resulted in Imitrex (82). 

After each reaction stage, sulfur is removed by condensation so that it does not collect on the catalyst. The temperature in the catalytic converter should be kept over the dew point of sulfur to prevent condensation on the catalyst surface, which reduces activity. 

Olefins could he catalytically converted into shorter and longer-chain olefins through a catalytic disproportionation reaction. For example, propylene could he disproportionated over different catalysts, yielding ethylene and butylenes. Approximate reaction conditions are 400°C and 8 atmospheres ... 

Perhaps the most familiar example of heterogeneous catalysis is the series of reactions that occur in the catalytic converter of an automobile (Figure 11.12). Typically this device contains 1 to 3 g of platinum metal mixed with rhodium. The platinum catalyzes the oxidation of carbon monoxide and unburned hydrocarbons such as benzene, C6H6 ... 

One problem with heterogeneous catalysis is that the solid catalyst is easily poisoned. Foreign materials deposited on the catalytic surface during the reaction reduce or even destroy its effectiveness. A major reason for using unleaded gasoline is that lead metal poisons the Pt-Rh mixture in the catalytic converter. 

Figure 1 illustrates the process in more detail. The inert liquid is pumped upflow through the reactor at a velocity sufficient to fluidize the catalyst and to remove the reaction heat. The low Btu feed gas is passed simultaneously up the reactor where it is catalytically converted to a high concentration methane stream. The exothermic reaction heat is taken up by the liquid mainly as sensible heat and partly by vaporization (depending on the volatility of the liquid). The overhead product gases are condensed to remove the product water and to recover any vaporized liquid for recycle. The main liquid flow is circulated through a heat... 

Steady state models of the automobile catalytic converter have been reported in the literature 138), but only a dynamic model can do justice to the demands of an urban car. The central importance of the transient thermal behavior of the reactor was pointed out by Vardi and Biller, who made a model of the pellet bed without chemical reactions as a onedimensional continuum 139). The gas and the solid are assumed to have different temperatures, with heat transfer between the phases. The equations of heat balance are ... 

Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176,177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 37-43 Copper chromite, oxidation of CO over, 86-88... 

To conclude this chapter, we shall return to homogeneously catalyzed reactions. It is useful to consider the case in which two substrates are catalytically converted to products. For the net reaction... 

The strength and interrelation of catalysis, classical promotion and electrochemical promotion is illustrated in Fig. 2.3. The reaction under consideration14 is the reduction of NO by CO in presence of 02. This is a complex reaction system but of great technological importance for the development of efficient catalytic converters able to treat the exhaust gases of lean burn and Diesel engines. 

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... 

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

Catalysts can be poisoned, or inactivated. A common cause of such poisoning is the adsorption of a molecule so tightly to the catalyst that it seals the surface of the catalyst against further reaction. Some heavy metals, especially lead, are very potent poisons for heterogeneous catalysts, which is why lead-free gasoline must be used in engines fitted with catalytic converters. The elimination of... 

One form of biological poisoning mirrors the effect of lead on a catalytic converter. The activity of an enzyme is destroyed if an alien substrate attaches too strongly to the enzyme s active site, because then the site is blocked and made unavailable to the true substrate (Fig. 13.42). As a result, the chain of biochemical reactions in the cell stops, and the cell dies. The action of nerve gases is believed to stem from their ability to block the enzyme-controlled reactions that allow impulses to travel through nerves. Arsenic, that favorite of fictional poisoners, acts in a similar way. After ingestion as As(V) in the form of arsenate ions (As043 ), it is reduced to As(III), which binds to enzymes and inhibits their action. 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

Scheme 4.16 Some reactions occurring in a catalytic converter...

In this paper, we first briefly describe both the single-channel 1-D model and the more comprehensive 3-D model, with particular emphasis on the comparison of the features included and their capabilities/limitations. We then discuss some examples of model applications to illustrate how the monolith models can be used to provide guidance in emission control system design and implementation. This will be followed by brief discussion of future research needs and directions in catalytic converter modeling, including the development of elementary reaction step-based kinetic models. 

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Heterogeneous catalysts are the active ingredients in automobile catalytic converters. When combustion occurs in an automobile engine, side reactions generate small amounts of undesired products. Some carbon atoms end up as poisonous CO rather than CO2. Another reaction that takes place at the high temperatures and pressures in automobile engines is the conversion of N2 to NO. Furthermore, the combustion process fails to bum all the hydrocarbons. Hydrocarbons, CO, and NO all are undesirable pollutants that can be removed from exhaust gases... 

By far the most important use of the platinum metals is for catalysis. The largest single use is in automobile catalytic converters. Platinum is the principal catalyst, but catalytic converters also contain rhodium and palladium. These elements also catalyze a wide variety of reactions in the chemical and petroleum industry. For example, platinum metal is the catalyst for ammonia oxidation in the production of nitric acid, as described in Pt gauze, 1200 K... 

In the feed pretreatment section oil and water are removed from the recovered or converted CCI2F2. The reactor type will be a multi-tubular fixed bed reactor because of the exothermic reaction (standard heat of reaction -150 kJ/mol). After the reactor the acids are selectively removed and collected as products of the reaction. In the light removal section the CFCs are condensed and the excess hydrogen is separated and recycled. The product CH2F2 is separated from the waste such as other CFCs produced and unconverted CCI2F2. The waste will be catalytically converted or incinerated. A preliminary process design has shown that such a CFC-destruction process would be both technically and economically feasible. 

Finally, a group from General Motors has explored the mechanistic importance of the N20 + CO reaction as an intermediate step during the reduction of NO by CO on noble metal exhaust catalysts [87,88]. Quasi-linearization of the non-linear NO + CO reaction system by identifying a critical kinetic parameter revealed that, indeed, the rate of the N20 + CO conversion as an intermediate step in the overall NO + CO conversion can be two to three orders of magnitude faster than the isolated N20 + CO reaction. This suggests that the observed suppression of N20 production at higher temperatures may be due to its fast reaction with adsorbed CO once produced, and that, contrary to the accepted wisdom, the formation of N20 and its subsequent reaction with CO can make a major contribution to the kinetics of the reduction of NO by CO in three-way catalytic converters. The validity of the theoretical results was verified by both... 

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