Heterogeneous catalysis catalytic converters

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. 

Before deriving the rate equations, we first need to think about the dimensions of the rates. As heterogeneous catalysis involves reactants and products in the three-dimensional space of gases or liquids, but with intermediates on a two-dimensional surface we cannot simply use concentrations as in the case of uncatalyzed reactions. Our choice throughout this book will be to express the macroscopic rate of a catalytic reaction in moles per unit of time. In addition, we will use the microscopic concept of turnover frequency, defined as the number of molecules converted per active site and per unit of time. The macroscopic rate can be seen as a characteristic activity per weight or per volume unit of catalyst in all its complexity with regard to shape, composition, etc., whereas the turnover frequency is a measure of the intrinsic activity of a catalytic site. 

Biocatalysis is a rather special case, somewhere between homogeneous and heterogeneous catalysis. In most cases, the biocatalyst is an enzyme - a complex protein that catalyzes the reactions in living cells. Enzymes are extremely effident catalysts. An enzyme typically completes 1000 catalytic cycles in one second. Compared to this, conventional homogeneous and heterogeneous catalysts are slow and inefficient (100-10000 cydes per hour). Speed, however, is not the only advantage enzymes specialize in converting one specific reactant into another... 

Ask the average man on the street for an example of catalysis, and you will most likely hear about the thing on the exhaust pipe that reduces engine emissions . As we shall see, the remarkable story of the catalytic converter is not only an excellent example of catalysis, but also highlights the connection between heterogeneous catalysis, green chemistry, and sustainable development. 

Heterogeneous catalysts are present in a different physical state from the reactants. A typical heterogeneous catalytic reaction involves a solid surface onto which molecules in a fluid phase temporarily attach themselves in such a way to favor a rapid reaction. Catalytic converters in cars utilize heterogeneous catalysis to break down harmful chemicals in exhaust. 

Heterogeneous catalysis is also utilized in the catalytic converters in automobile exhaust systems. The exhaust gases, containing compounds such as nitric oxide, carbon monoxide, and unburned hydrocarbons, are passed through a converter containing beads of solid catalyst (see Fig. 12.16). The catalyst promotes the conversion of carbon monoxide to carbon dioxide, hydrocarbons to carbon dioxide and water, and nitric oxide to nitrogen gas to lessen the environmental impact of the exhaust gases. However, this beneficial catalysis can, unfortunately, be accompanied by the unwanted catalysis of the oxidation of SO2 to SO3, which reacts with the moisture present to form sulfuric acid. 

Heterogeneous catalysis is also used in the catalytic converters of automobile exhaust systems. The exhaust gases, containing compounds such as nitric oxide, carbon monoxide, and unburned hydrocarbons, are passed through a converter containing beads of solid catalyst (see Fig. 15.18). The... 

For a simple example of heterogeneous catalysis, think of the catalytic converter in your car. It s made of a platinum-rhodium alloy that is finely deposited on aluminum oxide (specifically y-alumina) to decompose nitrogen oxides to less toxic nitrogen, as well as promote the combination of O2 with toxic CO and hydrocarbons to form CO2. 

Heterogeneous catalysis has a role in the atmospheric chemistry of ozone in the troposphere as well. Catalytic converters in automobiles are filled with a porous ceramic material, which provides a surface that catalyzes the removal of CO and NO (nitrogen oxides) ftom the exhaust. (Nitrogen oxides initiate the formation of ozone and other lung irritants in photochemical smog. We will examine this process in detail in Section 11.8.) The process by which catalytic converters operate is shown in Figure 11.14. The types of steps shown there are found in most examples of heterogeneous catalysis. 

One of the figures of merit for catalytic activity is the turnover number (tn) defined as the number of the molecules converted per surface site per second. For oxidative heterogeneous catalysis, the turnover number should be in the range from 10 to 100 to make a given material an effective catalyst. However, to date, the turnover number for methanol oxidation at the noble metal catalyst in the operational fuel cell is ca. 0.06, still far below the needed performance level. The following formula can be used to calculate the tn from the current density [213] ... 

Catalysis. Platinum and Pt alloys are preferably applied in heterogeneous catalysis as wire nets or powders with a high specific surface area ranging from 20 to 1000 m /g ( platinum black, palladium black ) on carbon or AI2O3 supports. The catalytic effec-tivity is structure-sensitive. Figure 3.1-300 show an example of the catalytic action of Pt for the reaction rate and the product selectivity on different crystal planes [1.218]. Pt—Pd—Rh alloys are the main active constituents of catalytic converters for automobile exhaust gas cleaning. 

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