In the Laboratory
Demonstration of a Catalytic Converter Using a Lawn Mower Engine Mark A. Young Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
[email protected] The heterogeneous conversion of certain pollutants, specifically unburned fuel, in the exhaust stream of a small gasolinepowered combustion engine is studied using a flow-reactor apparatus. The laboratory is a vivid demonstration of the application of surface chemistry to a significant environmental concern and also illustrates the “cold-start” problem associated with catalytic converters. We have successfully implemented the experiment in one of our undergraduate courses, and it is suitable for an environmental chemistry or an instrumental analysis laboratory. The experiment can easily be completed in one, 4 h laboratory period and is usually combined with other studies of heterogeneous catalysis that focus on the fundamental aspects of surface chemistry (1). A catalyst accelerates the rate of a given reaction and, in the process, is neither created nor destroyed. These two attributes are essential for applications in environmental remediation where it is often necessary to convert stable species to less problematic products and continuous, long-term conversion is desirable (2). In a catalytic converter, the relevant process is heterogeneous in nature, involving gas-phase reactant species and a solid catalyst. In heterogeneous conversion, gas-phase species must first adsorb to active sites on the catalyst surface before chemistry can proceed. The reaction rate can be expressed in terms of the surface reaction rate constant, k, and the surface coverage, θ, which is the fraction of available surface sites occupied by adsorbed reactant species. If θ is expressed as a Langmuir isotherm, then the reaction rate, v, for a unimolecular process is (3) kKP (1Þ ν ¼ kθ ¼ 1 þ KP where K is the Langmuir equilibrium constant and P is the pressure of the reactant. In the limit of low pressure (concentration), the observed rate follows first-order-type behavior. However, at high pressures, the available surface sites are saturated and the rate exhibits zero-order kinetics, a characteristic phenomenon of heterogeneous catalysis. Mechanisms for bimolecular processes give rise to variations of eq 1 but still predict different kinetics owing to surface saturation depending on the relative pressures of the reacting species (1, 3). The temperature dependence of the catalysis rate can be complicated as both the rate constant, k, and the Langmuir constant, K, are temperature dependent and non-Arrhenius behavior may be exhibited (3). However, there should still be an optimal temperature for catalytic conversion. At low temperatures, there will not be enough energy to overcome the activation barrier of the process. At high temperatures, the 180
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surface residence time of adsorbed species will be too short to facilitate reaction. In addition, thermal decomposition of the reactants (“cracking”) to nonvolatile fragments may occur, blocking surface sites and preventing catalysis. Gasoline combustion by automobile engines produces CO2 and H2O as the main reaction products. However, incomplete combustion of the gasoline leads to the formation of CO (4) and other hydrocarbons, often referred to as VOCs (volatile organic compounds) (5). In addition, NO and NO2, together designated as NOx, are also produced from nitrogen and oxygen in air owing to the high temperatures typical of internal-combustion engines (6). If these combustion products are not removed from the engine exhaust prior to emission into the atmosphere, pollution problems, such as acid rain and smog, can result. It is possible to significantly reduce undesirable emissions by passing the engine exhaust stream through a catalytic converter located in the exhaust system. Heterogeneous conversion of potential pollutants to less harmful species constitutes an emission-abatement strategy. In response to the Clean Air Act that limited emissions of CO and VOCs into the atmosphere, the automobile catalytic converter was introduced in 1975 (7). The function of the first-generation catalytic converters was to oxidize CO and VOCs to CO2 and H2O: CO, VOC þ O2 s CO2 þ H2 O catalyst
(2Þ
The first catalysts consisted of Pt and Pd metals supported on an inert ceramic alumina support. The catalytic converter also necessitated the use of unleaded gasoline since the catalyst became deactivated upon exposure to the lead additives. Improvements in catalytic converters were mandated by limits on NOx emissions that were imposed in the early 1980s. The second-generation catalytic converters (8), introduced in 1981, were referred to as three-way catalysts because CO, VOC, and NOx were all converted. To achieve this, rhodium was added to the catalysts because it is able to efficiently reduce NOx: NOx s N2 þ O2 catalyst
(3Þ
Cerium was also added because it improved the CO oxidation properties of the converter. The three-way catalysts used in catalytic converters today have been effective in reducing environmentally harmful emissions, particularly in urban settings. However, there are some significant remaining challenges. For instance, under conditions of high air/fuel ratios that optimize gas mileage, NOx reduction is not as efficient. Perhaps the most serious challenge is the “cold-start”
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In the Laboratory
Figure 1. Diagram of the catalytic reactor and gas manifold showing the combustion engine exhaust connected to one of the flow controllers. A mechanical pump with a needle valve on the inlet is used to achieve a small negative pressure in the manifold and pull exhaust gas through the reactor.
problem. The CO and VOCs emissions are significant during the first few minutes after a cold start because catalytic oxidation is not optimized until the converter reaches higher temperatures, which can take several minutes. Currently, heating of the catalyst is accomplished passively by exposure to the hot exhaust stream. Experimental Setup The basic experimental apparatus (Figure 1) has been described previously (1). A stainless-steel manifold with solidstate flow controllers delivers a precise flow of reactant gases and an inert Ar buffer to a temperature-controlled catalytic reactor. The reactor is a fragment of a catalytic converter from a vehicle exhaust system sealed in a glass tube and placed in a clamshell oven. The chemical composition of the flow stream is monitored with an integrated quadrupole mass spectrometer (QMS 300, Stanford Research Systems) through a 1 m long capillary. The QMS employs differential pumping to sample directly from atmospheric pressure while maintaining a fast temporal resolution, ∼100 μs. Real-time tabular and graphical display of the mass spectra is facilitated by the instrument software supplied by the manufacturer. We have added another flow channel for the current experiment that is connected to the exhaust pipe of a small internalcombustion engine. The two-stroke engine was scavenged from an old lawn mower. Newer, four-stroke engines with more advanced pollution controls are not as effective in illustrating the operation of the catalytic converter owing to their cleaner exhaust. The engine is mounted on flexible rubber pedestals and enclosed in a soundproofed wooden box. The box is vented by a fan to the laboratory exhaust system, which is also connected to the engine exhaust pipe. The engine cannot be started without first switching on the exhaust fan and the same electrical switch is also used to kill the engine. To get the exhaust stream from the engine to flow through the reactor, the manifold is operated under reduced pressure. In this configuration, the exhaust gas is pulled through the reactor and vented through a mechanical pump. Experimental Procedure The catalyst is heated to the desired temperature of ∼250-350 °C, empirically determined by comparison to the temperature dependence of model catalytic reactions, such as the hydrogenation or oxygenation of ethylene (1). A reference mass spectrum is collected by sampling the engine exhaust before it enters the reactor. The flow is then diverted through the heated
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Figure 2. (Top trace) Portion of the reference mass spectrum recorded for the engine exhaust gas flow, corresponding to engine emissions prior to activation of the catalyst (the “cold-start” emissions). The prominent features can be assigned to unburned gasoline by comparison with a mass spectrum of gasoline vapor (see Figure 3). (Bottom trace) Difference spectrum illustrating the effect of the heated catalyst. Quantitative conversion of the unburned gasoline is evident as is a small negative peak at m/e = 30, possibly due to NOx conversion. The spectra have been offset but are on the same scale.
catalytic reactor, and a second spectrum is recorded. Typical mass spectral data are shown in Figure 2. Hazards The catalyst is operated at high temperatures and should be properly insulated. The engine is noisy, and ear protection or noise abatement must be in place. The engine must be adequately vented. The exhaust system can also become very hot and should be insulated or isolated from contact. The Ar gas requires a specialized regulator, and the exit flow from the reactor should be vented. There are potential hazards associated with the use of high-pressure gas; the gas cylinder must be handled with care and be properly secured. Results The mass spectral data can be analyzed to identify the various components, most prominently the expected combustion products (CO2, H2O) and air (e.g., N2) but also minor
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catalyst operating temperature. For the conditions used in the current example, 100% conversion of the hydrocarbon pollutants is observed, illustrating the effectiveness of the converter for these species. Students are able to comment on the “cold-start” problem since the reference flow corresponds to emissions from the engine exhaust before the catalytic converter reaches operational temperature. Conclusions
Figure 3. Reference mass spectrum of gasoline vapor collected by inserting the QMS capillary into the headspace of a gasoline storage can.
species associated with pollutants from the engine exhaust. Standard mass spectra, available from the “library” menu of the instrument software or from the NIST WebBook site (9), are useful for identifying characteristic fragments. One of the most obvious features is a series of peaks associated with unburned gasoline centered at m/z = 56, 70, and 85. These peaks were assigned from comparison to a spectrum of gasoline vapor (Figure 3) collected by inserting the QMS capillary into the headspace of a gasoline storage can. The effect of the catalyst can most clearly be seen by generating a difference spectrum. Since this option is not available in the instrument software, the data are exported to a graphing program, such as Kaleidagraph or a spreadsheet. The reference spectrum can then be subtracted from the mass spectrum collected from the reactor flow. A typical difference spectrum is shown in Figure 2. The negative peaks associated with gasoline indicate quantitative conversion of the unburned fuel by the catalytic converter. A small negative peak at m/z = 30 is consistent with loss of NO2 and NO (eq 3), both of which have major fragments in this mass channel. In addition, the CO2 signal at m/z = 44, 45, 46 increases as would be expected from conversion of gasoline and other VOCs by the catalyst (eq 2). It is difficult, however, to quantify either the difference peak at m/z = 30, as the signal is relatively small, or the CO2 peaks, as these signals are variable and subject to interference. The mass spectral data can be used to quantify the extent of heterogeneous conversion of the VOCs, say as a function of the
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A QMS is used to characterize exhaust emissions from a small gasoline-powered lawn-mower engine. The effectiveness of a catalytic converter is then qualitatively assessed with a temperature-controlled flow reactor. Comparison of mass spectra collected with and without the catalyst can be used to examine the extent of heterogeneous conversion of pollutants and to illustrate the “cold-start” problem. The laboratory serves as a tangible demonstration of heterogeneous catalysis in the context of an important emission-abatement strategy. The students have enjoyed the instrumentation aspects of the experiment, particularly as it places a familiar item, a lawn-mower engine, in the laboratory setting. They are also generally engaged by the environmental implications of the subject matter. Literature Cited 1. Young, M. A. J. Chem. Educ. 2009, 86, 1082–1084. 2. Environmental Catalysis; Grassian, V. H., Ed.; CRC Press: Boca Raton, FL, 2005, 3. Atkins, P. W.; de Paula, J. Physical Chemistry, 8th ed.; W. H. Freeman: New York, 2006; Chapter 25. 4. Seasholtz, M. B.; Pence, L. E.; Moe, O. A. J. Chem. Educ. 1988, 65, 820–823. 5. Fleurat-Lessard, P.; Pointet, K.; Renou-Gonnord, M.-F. J. Chem. Educ. 1999, 76, 962–965. 6. Driscoll, J. A. J. Chem. Educ. 1997, 74, 1424–1425. 7. EPA. Milestones; Mobile Source Emissions; US EPA. Available from: http://www.epa.gov/oms/invntory/overview/solutions/ milestones.htm. 8. Catalysis and Automotive Pollution Control IV. Studies in Surface Science and Catalysis; Kruse, N., Frennet, A., Bastin, J.-M. Eds.; Elsevier: Amsterdam, 1998. 9. NIST. NIST Chemistry WebBook. http://webbook.nist.gov/chemistry/.
Supporting Information Available Student handout. This material is available via the Internet at http://pubs.acs.org.
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