Over the last decades, automobile manufacturers have employed several strategies to decrease VOC and NOX emissions from their vehicles and thereby meet governmental standards. One early technique for NOX control was to lower the temperature of the combustion flame and thereby decrease the rate of creation of thermal nitric oxide. The temperature lowering was achieved by recirculating a fraction of the engine emissions back through the flame.
In recent decades, more complete control of NOX emissions from gasoline-powered cars and trucks has been achieved using catalytic convert- ers placed just ahead of the muffler in the vehicle’s exhaust system. The original two-way converters controlled only carbon-containing gases, including carbon monoxide, CO, by completing their combustion to carbon dioxide. However, by use of a surface impregnated with a platinum-rhodium catalyst, the modern three-way converter changes nitrogen oxides back to elemental nitrogen and oxygen using unburned hydrocarbons and the com- bustion intermediates CO and H2 as reducing agents:
2 NO 9: N2 O2 overall via, for example,
2 NO 2 H29: N2 2 H2O PROBLEM 3-7
Write and balance reactions in which NO is converted to N2 (a) by CO, and (b) by C6H14. [Hint: The other reaction product is CO2, plus H2O in the lat-
ter case.] ●
The carbon-containing gases in the exhaust are catalytically oxidized almost completely to CO2 and water by the oxygen that is present:
2 CO O29: 2 CO2
CnHm (nm/4) O29:n CO2m/2 H2O
CH2O O29: CO2 H2O
The catalyst is dispersed as very tiny crystallites, initially less than 10 nm in size. Oxygen sensors in the vehicle’s exhaust system are monitored by a computer chip that controls the intake air/fuel ratio of the engine to the stoi- chiometric amount required by the fuel in order to ensure a high level of con- version of the pollutants. The whole process is illustrated in Figure 3-9a. If the air/fuel mix is not very close to the stoichiometric ratio, the warmed catalyst will not be effective for reduction (if there is too much air), causing nitrogen oxides will be emitted into the air, or for oxidation (if there is too little air), causing CO and hydrocarbons to be emitted, as illustrated in Figure 3-9b.
Recall that the reaction of N2
with O2 has a high activation energy, so its rate is very dependent upon temperature.
Reduction
PROBLEM 3-7
Several hundred dollars’
worth of precious metals are present in each catalytic converter.
Oxidation
CO CnHm
CH2O CO2⫹ H2O
reduction NO
oxidation
catalytic converter
processes N2
Some progress has been reported in the use of less-valuable metals, such as copper and chromium, instead of the expensive platinum-group metals as cata- lysts in catalytic converters. Although the metals are recycled from old con- verters, a portion is inevitably lost in the process. Scientists have expressed concern about the environmental problem of widely broadcasting the tiny particles of platinum, palladium, and rhodium that are lost from the converters themselves during their operation.
The catalyst that reduces nitric oxide to nitrogen also reduces sulfur dioxide, SO2, to hydrogen sulfide, H2S. The emitted gases include H2S and other reduced sulfur compounds, which collectively often give vehicle emis- sions their characteristic odor of rotten eggs. In addition, the small amounts of sulfur-containing molecules in gasoline—and diesel fuel—can partially deactivate catalytic converters if sulfate particles produced from them
Catalytic converter
Exhaust to tailpipe
Porous ceramic coating Platinum
Rhodium Exhaust from motor
Catalytic converter Muffler Tailpipe
Conversion (percent)
<1 Rich 1.0
(excess fuel) CO Hydrocarbons
NOX
>1 Air/fuel ratio
100 80 60 50 30 40 90 70
20 0 10
Lean (excess air) (b)
(a)
FIGURE 3-9 (a) Modern catalytic converter for automobiles, with its position in the exhaust system indicated. [Source: L. A. Bloomfield, “Catalytic Converter,” Scientific American (February 2000): 108.]
(b) Efficiency in conversion of catalytic converter versus air/fuel ratio. [Source: B. Harrison,
“Emission Control,” Education in Chemistry 37 (2000): 127.]
Improving Air Quality: Photochemical Smog 93
during the gasoline’s combustion become attached to and thereby cover the sites of the catalyst metal, deactivating them. The maximum annual average sulfur levels in gasoline, amounting to several hundred parts per million in the past, have been reduced to 30 ppm in both the United States and Canada, and to 10 ppm in the European Union. By contrast, the maximum sulfur in gasoline for India’s cities is 150 ppm, though there are plans to lower it to 50 ppm in the future.
The sulfur is removed from gasoline during refining, usually by hydrode- sulfurization, itself a catalytic process, which reacts organic sulfur- containing molecules in the gasoline with hydrogen gas, H2, to produce hydrogen sul- fide, which then is removed. Alternatively, the sulfur-containing molecules may be removed from the fuel by absorbing them during the refining process.
In the first few minutes after a vehicle’s engine has been started up, the catalysts are cold, so the converters cannot operate effectively and there are bursts of emissions from the tailpipe. Indeed, approximately 80% of all the emissions from converter-equipped cars are produced in the first few minutes after starting. Once an engine has warmed up and the catalysts have been heated to about 300°C by engine exhaust, three-way catalysts convert 80–90% of the hydrocarbons, CO, and NOX to innocuous substances before the exhaust gases are released into the atmosphere. However, fuel-rich mix- tures are fed to the engine in the first minute or so after the vehicle engine is started, and also when high acceleration occurs, so carbon monoxide and unburned hydrocarbons are emitted directly into the air under these oxygen- starved conditions.
Research and development is underway to develop catalytic converters that would convert start-up emissions so that these are not released into the air. Various approaches being investigated include
• devising a converter that will operate at lower temperatures or that can be preheated so it begins to operate immediately,
• storing pollutants until the engine and converter are heated, and
• recirculating engine exhaust through the engine until the reactions are more complete.
Older cars (with no converters or just two-way converters) still on the road continue to pollute the atmosphere with nitrogen oxides even during their normal operation.
The maximum amounts of emissions that can legally be released from light-duty motor vehicles such as cars have gradually been decreased in order to improve air quality. The U.S. EPA sets regulations for the maxi- mum emissions per mile of CO, NOX, total and non-methane hydrocarbons for gasoline-powered vehicles. Some governments have recently instituted mandatory inspections of exhaust systems to ensure that they continue to operate properly.
Methane is much slower to react than are other hydrocarbons, so its emissions are often excluded from regulations.
Vehicles whose catalytic converters have been damaged or tampered with produce most of the emissions: typically, 50% of the hydrocarbons and carbon monoxide are released from 10% of the cars on the road. For example, a study of soot emissions from cars in The Netherlands established that 5% of vehicles accounted for 43% of pollution. A mid-1990s survey of carbon monoxide emitted by passing traffic in Denver, Colorado produced the results shown in Figure 3-10.
The great majority of Denver’s vehicles of ages up to about 12 years were rated
“good” (light green in the figure) in terms of their control of tailpipe emissions.
Most of the carbon monoxide came from cars 6–12 years old (Figure 3-10), because they were so numerous and because of the presence in that fleet of cars with “poor” or “fair” emission levels (dark green and gray in Figure 3-10, respec- tively). In recent decades, the gradual deterioration of catalytic converters is less pronounced, so the occurrence of such high emitters is now far less common.