Catalytic Converters: An Introduction to the Science and Environmental Concerns
An Introduction to the Science and Environmental Concerns
by Carter Lavin
Introduction:
Devices used in the decontamination of air can be divided into two
general categories based on their methods of purification: physical and
chemical. Catalytic converters utilize chemical reactions between the catalyst
and the pollutant to remove the contamination. While they have the advantage of
logistical simplicity, high efficiency and scalability, which makes them ideal
for treating exhaust from internal combustion engines, these devices have their
drawbacks that make their net worth debatable. They primarily work by
converting highly dangerous pollutants into benign substances or less dangerous
pollutants. They are not an ideal technical solution to air pollution as they
cannot fully eliminate contamination, just decrease its threat, but their
expanded use may result in lowering pollution levels. However, due to the
energy intense and environmentally damaging nature of refining the necessary
materials and constructing catalytic converters, extensive environmental
life-cycle analyses must be done in order to determine if they have a net
environmental gain and should be encouraged to be spread to new applications.
While the understanding of catalytic science has been largely neglected
until the later half of the 20th century, catalytic technology has
been used extensively since 1904 when Fritz Haber first synthesized ammonium
using an iron oxide as the catalyst (Maugh). The use of catalytic processes to
purify air started getting studied more in depth as the 1950s brought about the
beginning of environmental concern in the United States and regulations on air
pollution (Piver). When the United States passed its first federal regulation
on air pollution control, the Air Pollution Control Act of 1955, industries
like refining, power generation and automotive started to receive pressure to
work to limit pollution, which lead to the increase of investment in research
and development of many technologies including catalytic converters (EPA, Maugh).
However, as further understanding of the details of what occurs in a catalytic
reaction have not yielded great technological applications, the majority of
research has focused on surface area sciences (Maugh).
How they function:
There are two types of catalytic reactions that take place in a
catalytic converter: reduction catalysts and oxidation catalysts, and they both
work on the concept of transform pollutants into less harmful substances. Reduction
catalysts are the first stage in the purification process where the converter
utilizes a catalyst, usually a precious metal like platinum or rhodium. These
precious metals break apart the chemical bonds of nitric oxides by making
stronger bonds with the nitrogen than the nitrogen was bonded to the oxygen.
Thus, the nitrogen leaves its bond with oxygen to bond with the catalyst, while
the left over oxygen atoms bond with each other and form the highly stable
oxygen molecule of O2. The diatomic oxygen molecule will not re-bond
with the nitrogen as the valence of the oxygen molecule is completely filled
with the double bond so a great deal of energy is needed to disassociate the
double bond and allow the oxygen to reform with the nitrogen. The nitrogen
atoms that are bonded to the catalyst will then bond with each other to form N2,
then release the catalyst and continue to flow to the second part of the
catalytic converter (Twigg).
The second part of the converter, the oxidation catalyst, has the
opposite type of reaction as the reduction catalyst. In the oxidation catalyst,
the remaining pollutants, generally carbon monoxide and hydrocarbons are burned
to encourage bonding between the carbon and hydrogen atoms of the pollutants
and the oxygen in the environment to produce carbon dioxide and water. This
phase has the potential of creating unintended molecules if the pollutants in
the air coming from the engine react with other substances in the air such as
sulfur or parts of the catalyst. The second phase is important to purifying the
airflow as the primary combustion process, which occurs in the engine, may
result in incomplete combustion which creates toxins like carbon monoxide and
various hydrocarbons. While the two types of reactions in catalytic converters
are complete opposites, one creating chemical bonds and the other of breaking
them, they are improved with similar construction techniques and are part of
the same whole.
Increasing the time allowed for a chemical reaction to occur increases
its efficiency. In the case of catalytic converters, this can be achieved by
slowing down air flow or increasing the converter’s surface area. While slowing
the air flow is possible in industrial uses due to venting controls, it is
difficult to achieve in regards to transit as maintaining a certain level of
air flow is vital to increasing combustion efficiency in the engine which in
turn reduces the amount of carbon monoxide and hydrocarbons. If the air flow in
the system were slowed to augment the pollutant’s time in the catalytic
converter, engine combustion efficiency would decrease thereby creating a
higher need to use the converter so it would be self-defeating. Surface area
can be increased by expanding the size of the catalytic converter, but as they
are reliant upon precious metals like platinum, this could drastically increase
their cost. Also, in its main application, transit, space is a limited variable
as the converter needs to fit on the automobile. The way most manufactures
increase surface area in a cost effective manner is by constructing the
converter to house a porous, honeycomb-like structure with many groves (Maugh).
By maximizing the amount of surface area exposed of the catalyst, the need for
more of the catalytic material is lowered which is very desirable due to the
high cost of the precious metals involved.
The catalytic processes in air decontamination can be applied to a wide
variety of pollution sources like factories and power plants. However, the high
cost of the catalysts, the precious metals, and their potential to be made into
a relatively small size makes them economically unsuitable for large scale
applications but useful for transit. Also, there is a large selection of other,
similarly efficient but less costly, methods to purify air in industrial
processes such as sedimentation chambers, filters, absorption chambers so
catalytic reactions are generally not selected for industrial processes.
Meanwhile, most other methods of air pollution are neither as scalable nor
portable as catalytic converters and so they have become the de facto exhaust
treatment technology on automobiles.
Disadvantages,
Logistical and Environmental:
The disadvantages of catalytic converters are in their physical
limitations and their byproducts from their use and manufacturing. All other
types of air purifying methods can only function within certain parameters and
catalytic converters are no exception. As the catalytic process is dependent on
the exposure of the reactants, efficiency can be reduced greatly and the
reaction can be stopped if a physical barrier is created between the reactants
due to sedimentation build up. This requires some sort of cleaning to remove
the sedimentation which adds to the cost of devices in both added materials and
loss of time they can be used efficiently. Also, the temperature within the
converters must be maintained below high extremes to prevent deterioration of
the catalysts, a task which is can be difficult as the chemical reaction that
takes place is generally exothermic and the reaction takes place using
combustion exhaust.
Converters are relatively sensitive to different types of fuels that may
contain substances that can cause sedimentation build up or “catalyst
poisoning.” The issue of different fuel types poisoning catalysts received
attention in the 1980s as world-wide supplies of “sweet” crude oil, oil with low
sulfur content, became depleted causing a shift in use to “sour” crude oil
which contains more sulfur and trace metals that can poison catalysts (Cohn).
The change in fuel type required a change in catalytic converter design and
refining techniques of the fuel to make them more compatible to converters. As
the depletion of sour crude oil continues and as alternative fuels like
biodiesel, ethanol, and oil shale become adopted, more retooling of catalytic
converters will be needed as well as new refining techniques for the variety of
fuels to properly adapt the catalysts and their design to the fuel change and
prevent catalytic poisoning.
From an environmental standpoint, there are two great disadvantages to
catalytic converters. The first and most obvious in terms of air pollution is
that they convert harmful contaminants to less harmful ones. This does not mean
that their byproducts are harmless; they are simply less harmful in regards to
certain parameters. Catalytic converters can be used to oxidize the toxin carbon
monoxide or hydrocarbons to form the less immediately dangerous compound of carbon
dioxide, which is formed with both types of pollutants, and water, which is
only in the case of hydrocarbons. They may also be used to convert the smog
inducing compounds of nitric oxides into the greenhouse gas of nitrous oxide.
Thus, catalytic converters transform a local air pollutant, a contaminant that
mainly damages the area near its point of emission, to locally benign
greenhouse gases which are global pollutants. The end product of these
greenhouse gases has always been an expected result of the use of catalytic
converters and until recently, it was not considered a problem. However, now
that the dangers of greenhouse gases in regards to climate change have started
to be understood, the aggregated benefit of catalytic converters is being
brought into question (Wald).
The second great issue with using catalytic reactions to decontaminate
air in terms of an overall environmental standpoint is that the extraction
process of the precious metals needed in these reactions is very destructive.
To reach ores containing platinum, companies used mechanized drills and
explosives to create holes in mountains. The generated waste rock can cause
mine tailings and contaminate the local ecosystem. Mine tailings contaminate
watersheds as erosion brings toxic sediments like lead or arsenic from the
waste into rivers and streams. Some companies, like the Stillwater Mining
Company, the only platinum and palladium mining company in the United States,
backfill their emptied mine shafts with waste rock and sand to limit mine
tailings (Mined). This process does not completely eliminate mine tailings,
sources of contamination of watersheds, but it does help protect the tailings
erosion which is the primary cause of the dispersion of their toxins (Arizona).
However, the vast majority of platinum mining occurs in South Africa where
environmental regulations are more lax than in the United States and so this
positive technique is likely not widely used resulting in higher levels of
environmental poisoning from these tailings.
Once the ore is extracted, it needs to be concentrated, smelted, and
refined into a useable product. Due to the relatively high melting point of
platinum, a simple technique to refine the ore is to melt all the other
impurities, a process that requires temperatures over 1000° Celsius (Metal). Sustaining these temperatures
requires high levels of energy which are likely created from burning fossil
fuels which contribute to local air pollution and climate change. The issues
with smelting is not restricted to the pollution from the energy source, this
can theoretically be overcome by adapting renewable energy sources, another issue
with the smelting process is it melts or even vaporizes toxic elements that can
contaminate air, water and soil. For example, if the smelting process results
in incomplete combustion, it has been found that dioxin-like compounds may form
(Jordan). These waste products could be recaptured and possibly separated for
use, but this would require more mechanization and higher economic costs. Another
technique used in refining platinum, which is also used in the gold mining
process, is that of applying strong acids to the ores as the precious metals
are impervious to their effect. However, this results in a slurry of liquid
toxins that have been extracted from the ore and currently there is not an
effective way of disposing this waste.
Once the ore is refined into a
useable level of purity of platinum it then needs to be shipped to a factory to
create it into the final product, usually a catalytic converter, which is then
shipped to an automotive factory and installed (Metal). Here ends the bulk of
the environmental impact of the catalytic converters, with the caveat that each
added weight to a vehicle, however small increases its fuel consumption as the
vehicle needs to transport a heavier mass. While this final stage of
manufacturing and transporting the end product is common throughout the
globalized economy, it is important to keep in mind so as to calculate the
total environmental cost of this air purifying device.
These environmental issues stem from the basic principle that in order
to manufacture any product designed to decontaminate, especially one reliant on
rare precious metals, there is an environmental cost from each stage of the
product’s life. Catalytic reactions are used to purify the air, but in their
fabrication, they cause soil, water and air contamination and in their use,
they convert local carbon-based pollutants to global pollutants. While there
are many social justice implications of this issue, that of transferring
pollution from one comparatively wealthy region to another poorer region, and of
passing the burden of a regional decision to be a burden on the world, that is
not the focus of this paper.
A severe and more directly
relevant issue is that of the overall environmental impact of the converters.
As it is difficult to make a comparison between different types of pollution
such as between air and water pollution, we should focus on the total effect on
air quality of catalytic converters. And while a full investigation of a
life-cycle environmental cost-benefit analysis of catalytic converters is
beyond this paper, it is import to investigate to see if the amount of
pollutants they take from vehicle exhaust in their life time is greater than
the amount of air pollutants required in their fabrication before we can
suggest their expanded use.
Possible
Improvements:
Reduction catalysts may be able to be improved to convert nitric oxides
into the separate compounds of nitrogen and oxygen which are non pollutants,
but it is physically impossible for oxidation catalysts to convert a
hydrocarbon or carbon monoxide to a more benign substance than the greenhouse
gas carbon dioxide. There inability to make carbon molecules into a completely
innocuous substance is not a failing within the technology, but rather a
reflection of the fundamental issue with burning carbon fuels. Catalytic
converters that are used to purify air contaminated with nitric oxides must be
improved to eliminate the formation of nitrous oxide, which is 300 times more
potent than carbon dioxide as a greenhouse gas (Wald). If this change is
effected, catalytic converting technology will be nearly perfected and as
benign as possible. If further environmental protection is desired, it must
come from reducing the need to use these catalysts or improving manufacturing
techniques and not from improving the technology itself.
Improvements can be made in the expanded use of catalytic converters and
current legal reforms in California are working to foster this diffusion (Barringer).
While catalytic converters are nearly always used in automobiles and used in
various forms of heavy industry, they are not used in smaller applications of
engines such as lawn mowers. These smaller, gasoline or diesel powered devices
are responsible for emitting far greater quantities of pollutants than cars or
trucks per unit of fuel due to their lack of regulation in terms of fuel
efficiency and emissions. Although there is currently a regulation battle of
the broader application of catalytic converters, if passed pollution levels of
carbon monoxide, hydrocarbons and smog will drop in California. However as the
use of lawn mowers and similar small-scale engines are less prevalent than
automobile use, the drop in pollution levels following the adoption of new
regulation will probably be less precipitous than the drop caused by the Clean
Air Act. This case of new lawn mower regulation is just one example of how the
use of catalytic converters can be spread to curtail end point pollution in a
wider application than is currently used.
Conclusion:
Catalytic technology use for decontaminating air is a scientifically simple concept. In its most common form, the automotive catalytic converter, catalysts are used to complete the previously uncompleted combustion in order to eliminate carbon monoxide and hydrocarbons in exhaust and they are used to break the bonds between nitrogen and oxygen in harmful nitric oxides to allow them to reform as diatomic nitrogen or oxygen molecules. As environmental concern for cleaner air has grown, governmental regulation has encouraged private industry to invest in this technology and to maximize efficiency they have expanded research in surface area sciences. This effective investment and adaptation of the technology has resulted in drops in the occurrence and gravity of smog; however, they have increased emissions of climate change inducing greenhouse gasses. Their wide use came as a result of growing environmental concerns in the developed world to solve their air pollution problems, but their adaptation has increased air, water and soil pollution problems in other, poorer, parts of the world as well as exacerbated the growing global threat of climate change. Further technological development and deployment of catalysts to purify air at more emission points could reduce local air pollution where adapted, but this would come at the expense of air quality in other parts of the world and further the ecological disaster of climate change. In terms of catalytic air decontamination, the question is not how can we better use this technology to improve air quality in the region, but rather does the continued use of this technology provide an overall improvement in global environmental quality?
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