Catalytic Converters in Cars (Chemistry For Life Sciences) - Assignment Example

?Catalytic Converters Draw a labelled enthalpy profile diagram for a catalysed and un-catalysed reaction. Use your diagram to explain how catalysts speed up the rate of a reaction. Fig. 1. Enthalpy profile diagram for un-catalysed and catalysed reactions (Source: Baron M., 2010). A chemical reaction occurs due to collisions between particles in the reacting system. The minimum energy needed in the colliding particles to produce the reaction is known as the activation energy. The combined energy of the colliding particles is called the collision energy. Only when the collision energy is equal to or greater than the collision energy, reaction takes place. In reactions that are not instantaneous, one of the ways by which the rate of the reaction can be increased is through decreasing the activation energy. This can be done by adding a catalyst. As shown in the enthalpy diagram (Fig. 1) the uncatalysed reaction has a high activation energy. In the presence of a catalyst, an alternative reaction pathway with a lower activation energy is followed (Baron, 2010). The details of this pathway are: when a catalyst is added, it first combines with the reactants to form an intermediate complex (also called the transition state) having a lower activation energy. In the next step, the complex breaks down releasing the product(s), and the unchanged catalyst which is now free to interact with fresh reactant molecules to continue the reaction. The catalysed reaction proceeds faster as the activation energy is reduced. 2. List the gases emitted by a car engine. For each one specify any health or pollution problems associated with it. Explain, with balanced symbol equations, how each of these gases can be converted into less polluting gases. The major gases emitted by a car engine are (a) nitrogen, (b) oxides of nitrogen NOx), (c) carbon dioxide, (d) carbon monoxide (CO), (e) volatile organic compounds, VOCs (mainly unburnt hydrocarbons, HC), and (f) water vapour. Of the above gaseous products of combustion, nitrogen is inert and non-polluting. Besides, it is a major constituent (78%) of air. Therefore, it does not require any treatment. Similarly, carbon dioxide and water vapour are also generally harmless except that carbon dioxide contributes to global warming. Carbon monoxide, VOCs i.e., unburnt hydrocarbons and oxides of nitrogen require to be made less polluting through suitable conversion reactions (AECC, Association for Emissions Control by Catalyst, 2011). Carbon monoxide and hydrocarbons can be made less polluting through oxidation to carbon dioxide (CO2) and CO2 and water, respectively, as follows: [O] 2CO + O2 ------------------> 2 CO2 Carbon monoxide Pt-Palladium Carbon dioxide (highly hazardous) catalyst (less harmful) [O] CxH2x+2 + [(3x+1)/2]O2 -------------> xCO2 + (x+1)H2O Hydrocarbon Pt-Palladium Carbon water (hazardous) catalyst dioxide Oxides of nitrogen are converted into nitrogen and oxygen by a reduction reaction: (Reduction) 2NOx ------------> N2 + xO2 Nitrogen oxide Pt-rhodium catalyst 3. The majority of catalysts in cars are made from the precious metals platinum and palladium, which are examples of heterogeneous catalysts. Explain this term, and describe (with the help of diagrams) the adsorption and desorption mechanism by which heterogeneous catalysts work. Heterogeneous catalysts are those that catalyse a reaction in which the reactants are present in a different phase. For example, the platinum and palladium, and platinum and rhodium used in cars are catalysts in the solid phase that act upon gaseous reactants such as CO, unburnt HC, and nitrogen oxides. The basic principle of heterogeneous catalysis is that when the reactant molecules are concentrated on the catalyst surface, their collision frequency is much enhanced than when they are in the gas phase (Delpierre and Sewell, 2011) www.physchem.co.za). The mechanism of heterogeneous catalysis with gaseous reactants and a solid catalyst involves a number of steps as shown in Fig. 2 below. Step 1. Diffusion of the reactant gases on to the catalyst. Step 2. The reactants are bound to that is, adsorbed on to the surface of the catalyst at the active sites. Step 3. The adsorbed reactant molecules undergo weakening due to distortion and consequently breaking of the covalent bonds in their structure. Step 4. New bonds are formed, generating product molecules which are adsorbed on the surface of the catalyst. Step 5. The product molecules are desorbed from the catalyst and diffuse away, leaving the active sites available for a new set of reactant molecules and the continuation of the reaction. Fig. 2. Various steps involved in heterogeneous catalysis. (Source: http://www.physchem.co.za/OB12-che/catalysis.htm). 4. Explain why the structure of the catalyst and its position within the car are important factors in determining the chemical efficiency of the catalytic converter. Catalytic converters use expensive metals like platinum, palladium and rhodium as the heterogeneous catalyst. The metals are deposited as thin layers onto a ceramic honeycomb monolith substrate. The honeycomb structure increases the surface area, yet minimises the amount of metal used (Clark, 2007). The catalytic converter works most efficiently at somewhat high temperatures. Thus, when the car engine is started cold, the catalytic converter does not do the job of reducing the pollution emitted in the exhaust. This problem can be solved by locating the catalytic converter closer to the engine. Such a position enables hot exhaust gases to reach the converter and heat it up faster. However, the downside to this is that the life of the converter is reduced if it is exposed to extremely high temperatures. In most of the present day cars, the converter is positioned under the front passenger seat which is at a safe distance from the engine to keep the temperature at levels that are not harmful to the converter. 5. What developments are currently being made in the technology of catalytic converters? The expensive metals that are currently being used as catalysts could very soon be replaced by a cheap mineral called perovskite, mixed with strontium (Weise, 2010). Perovskite (Pv) is a calcium titanium oxide having the chemical formula, CaTiO3. Single nanotechnology is being applied to control the size of precious metal particles to less than 5 nm in diameter. This technology will be used to develop a catalyst that comprises of single-nanosized precious metal particles embedded in fixed positions (Venter, 2009). Pre-Ignition Catalytic Converter (PICC) technology that turns ordinary fuel into plasma. The plasma created will be injected into the engine process to yield greater mileage. References AECC, 19 April 2011. Three-way catalysts. http://www.aecc.eu/en/Technology/Catalysts.html. Accessed on 21 April 2011. Baron M., 2010.Rate of reaction. eprints.lincoln.ac.uk/2366/23/Chem_Reaction_Kinetics.ppt. Accessed on 20 April 2011. Clark J., July 2007. Types of catalysis. http://www.chemguide.co.uk/physical/catalysis/introduction.html. Accessed on 21 April 2011. Delpierre and Sewell, No date. Electronic Science Tutor. http://www.physchem.co.za/OB12-che/catalysis.htm. Accessed on 21 April 2011. Venter I., 23 January 2009. New catalytic converter technology to use 70% less PGMs, of which South Africa is the biggest producer. http://www.miningweekly.com/article/new-catalytic-converter-technology-to-use- 70-less-pgmsof-which-south-africa-is-the-biggest-producer-2009-01-23. Accessed on 21 April 2011. Weise E., 26 May 2010. New catalytic converter material could make for cleaner, cheaper cars. USA Today. http://content.usatoday.com/communities/sciencefair/post/2010/03/new-catalytic- converter-material-could-make-for-cleaner-cheaper-cars/1. Accessed on 21 April 2011. Read More