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Суралцагчид сурагчид каталитик хувиргагчийг ашиглан агаарын бохирдолыг бууруулах технологийн чухал ач холбогдлыг ойлгох болно.

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Асуулт үндсэн

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Хүрээлэн буй орчинд хор хөнөөл ихтэй утааг хор хөнөөл багатай утаа болгох боломжтой юу?

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Багш нь каталитик хувиргагчийн механизм, түүний дотор явагдах урвал ба түүний агаарын бохирдол бууруулах боломжийг танилцуулна.

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by Chris Woodford. Last updated: February 18, 2022.

Blackened buildings and choking streets-if that's your experience when you open the front door in the morning, you probably live in a big city like Los Angeles, London, Paris, or Beijing. Cars, buses, and trucks have been a great gift to the world, because they help us move ourselves (and the things we need) quickly and efficiently. But their engine pollution spoils the places where we live and harms our health. Fortunately, most vehicles are now fitted with pollution-reducing units called catalytic converters (sometimes known as "cats" or "cat-cons"), which turn the harmful chemicals in vehicle exhausts into harmless gases such as steam. Let's take a closer look at these brilliant gadgets and how they work!

Artwork: The basic concept of a catalytic converter: sitting between your car's engine and tailpipe, it takes in dirty air and removes a significant amount of pollution from it using chemical catalysts.

Why engines make pollution

Car engines run on gasoline or diesel, which are made from petroleum. Most of our petroleum is formed when the remains of tiny sea creatures rot down, heat up, and get squeezed by layers of sea-bed rocks. Petroleum is made up of hydrocarbons (molecules built from carbon and hydrogen atoms) because living organisms are mostly made from those atoms too.

In theory, if you burn any kind of hydrocarbon fuel with oxygen from the air, you release a lot of energy and make nothing but carbon dioxide and water, which are clean and relatively harmless. In practice, though, gasoline is a mixture of about 150 different chemicals, not just hydrocarbons but additives too, and it doesn't burn as cleanly as we'd like. That means you generally get some air pollution as a byproduct. The pollutant gases made by car engines include a poisonous gas called carbon monoxide, as well as VOCs (volatile organic compounds) and nitrogen oxides that cause smog (the sort of choking, cloudy vehicle pollution we all know and hate).

Photo: The columns of the Parthenon in Athens, Greece have been blackened by vehicle pollution. Athens is one of the world's most traffic-polluted cities. Photo by Michael M. Reddy courtesy of U.S. Geological Survey.

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What is a catalytic converter?

Pollutant gases are made of harmful molecules, but those molecules are made from relatively harmless atoms. So if we could find a way of splitting up the molecules after they leave a car's engine and before they get pumped out into the air, we could crack the problem of pollution-or some of it, anyway. That's the job that a catalytic converter does.

These gadgets are much simpler than they sound. A catalyst is simply a chemical that makes a chemical reaction go faster without itself changing in the process. It's a bit like an athletics coach who stands by the side of the track and shouts at the runners to go faster. The coach doesn't run anywhere; he just stands there, waves his arms about, and makes the runners speed up. In a catalytic converter, the catalyst's job is to speed up the removal of pollution. The catalyst is made from platinum or a similar, platinum-like metal such as palladium or rhodium.

A catalytic converter is a large metal box, bolted to the underside of your car, that has two pipes coming out of it. One of them (the converter's "input") is connected to the engine and brings in hot, polluted fumes from the engine's cylinders (where the fuel burns and produces power). The second pipe (the converter's "output") is connected to the tailpipe (exhaust). As the gases from the engine fumes blow over the catalyst, chemical reactions take place on its surface, breaking apart the pollutant gases and converting them into other gases that are safe enough to blow harmlessly out into the air.

Photo: An experimental new catalytic converter designed to reduce the polluting effects of unburned fuel, nitrogen oxides and particulates. Picture courtesy of Oak Ridge National Laboratory published on Flickr under a Creative Commons (CC BY 2.0) licence.

One very important thing to note about catalytic converters is that they require you to use unleaded fuel, because the lead in conventional fuel "poisons" the catalyst and prevents it from taking up the pollutants in exhaust gases.

What happens inside the converter?

Photo: Engineers are constantly trying to improve the performance of catalytic converters, for example, by developing catalysts that work more effectively at lower temperatures. This is an example of a low-temperature oxidation catalyst made from tin oxide and platinum. Photo by CPL Bryant V courtesy of NASA Langley Research Center (NASA-LaRC).

Inside the converter, the gases flow through a dense honeycomb structure made from a ceramic and coated with the catalysts. The honeycomb structure means the gases touch a bigger area of catalyst at once, so they are converted more quickly and efficiently.

Typically, there are two different catalysts in a catalytic converter:

One of them tackles nitrogen oxide pollution using a chemical process called reduction (removing oxygen). This breaks up nitrogen oxides into nitrogen and oxygen gases (which are harmless, because they already exist in the air around us).

The other catalyst works by an opposite chemical process called oxidation (adding oxygen) and turns carbon monoxide into carbon dioxide. Another oxidation reaction turns unburned hydrocarbons in the exhaust into carbon dioxide and water.

In effect, three different chemical reactions are going on at the same time. That's why we talk about three-way catalytic converters. (Some, less-effective converters carry out only the second two (oxidation) reactions, so they're called two-way catalytic converters.) After the catalyst has done its job, what emerges from the exhaust is mostly nitrogen, oxygen, carbon dioxide, and water (in the form of steam).

How effective are catalytic converters?

Cats make a big difference to emissions, with three-way converters giving a significant extra benefit over two-way converters:

Chart: Effectiveness of catalytic converters. Figures show pollutants in grams per kilometer at 80,000 kilometers. Chart drawn by Explain that Stuff.com using data for light-duty gasoline fueled vehicles from US EPA (1990), quoted in table 3.2 (page 75) of Air Pollution from Motor Vehicles: Standards and Technologies for Controlling Emissions, Faiz et al, World Bank, 1996.

Catalytic converters are mainly designed to reduce immediate, local air pollution-dirty air where you're driving-and this chart certainly seems to suggest that they're effective. Even so, people sometimes question whether they're really as green as they seem. It's important to remember that they reduce emissions rather than eliminate them completely.

One problem is that they only really work at high temperatures (over 300°C/600°F or so), when the engine has had chance to warm up. Early types of catalytic converters typically took about 10-15 minutes to warm up, so they were completely ineffective for the first few kilometers/miles of a journey (or any part of a very short journey). Modern converters warm up in only 2-3 minutes; even so, significant emissions can still occur during this time.

Chart: Catalytic converters only become efficient at high operating temperatures. This chart shows the efficiency of a typical device at converting carbon monoxide at a range of different temperatures. Nitrogen oxides are converted with slightly higher efficiency and hydrocarbons with slightly less efficiency. At high temperatures, carbon monoxide is converted with the least efficiency of the three.

Another issue is whether they increase greenhouse gas emissions. We think of carbon dioxide as a safe gas, because it's not toxic in everyday concentrations. Nevertheless, it isn't entirely harmless, because we now know it's the major cause of global warming and climate change. Some people believe catalytic converters make climate change worse because they turn carbon monoxide into carbon dioxide. In fact, the carbon monoxide your car produces would eventually turn into carbon dioxide in the atmosphere all by itself, so a catalytic converter makes no difference on that score: it simply reduces the carbon monoxide a car pumps into the street as it drives along, improving the local air quality.

But when it comes to climate change, auto engineers and environmentalists have long pointed out another serious issue. Although cats turn most nitrogen oxides into nitrogen and oxygen, they also produce small amounts of nitrous oxide (N2O) in the process, a greenhouse gas that's over 300 times more potent than carbon dioxide. The trouble is that with so many vehicles on the road, even small amounts of nitrous oxide add up to a major problem. Back in 2000, the Intergovernmental Panel on Climate Change noted: "The introduction of catalytic converters as a pollution control measure in the majority of industrialized countries is resulting in a substantial increase in N2O emissions from gasoline vehicles." Fortunately, newer catalytic converters produce dramatically less nitrous oxide than older ones. Even so, while catalytic converters have certainly helped us to tackle short-term air pollution, there are concerns that, when it comes to long-term climate change, they could be making matters worse.

How a catalytic converter works

Before catalytic converters were developed, waste gases made by a car engine blew straight down the exhaust tailpipe and into the atmosphere. The catalytic converter sits between the engine and the tailpipe, but it doesn't work like a simple filter: it changes the chemical composition of the exhaust gases by rearranging the atoms from which they're made:

1. Molecules of polluting gases are pumped from the engine past the honeycomb catalyst, made from platinum, palladium, or rhodium.

2. The catalyst splits up the molecules into their atoms.

3. The atoms then recombine into molecules of relatively harmless substances such as carbon dioxide, nitrogen, and water, which blow out safely through the exhaust.

Do catalytic converters work for diesel engines?

Only a tiny proportion of the emissions from a diesel engine (about one percent) is pollution. That one percent consists mostly of nitrogen oxides (about 50 percent) and particulates, with relatively small amounts of carbon monoxide, hydrocarbons, and sulfur dioxide.

Chart: Dirty diesels? Drawn using figures from The pollutant emissions from diesel-engine vehicles and exhaust aftertreatment systems by İbrahim Aslan Reşitoğlu et al, Clean Technologies and Environmental Policy, January 2015, Volume 17, Issue 1, itself quoting figures from Diesel Emissions and their Control by M. Khair and W. Majewski. Society of Automotive Engineers, Inc., Warrendale, PA: 2006.

Diesel engines can and do use catalytic converters, but there are several important differences from how they work in gasoline engines.

Instead of three-way catalysts, diesels use two-way oxidation catalysts (which only tackle carbon monoxide and hydrocarbons), and ones specifically designed to work with diesel exhausts, which are significantly cooler than gasoline exhausts.

Since they don't have reduction catalysts, diesel engines produce much higher tailpipe emissions of nitrogen oxides than gasoline engines. (There are various other mechanisms that diesels can use to tackle NOx emissions, but we won't go into details here.)

Catalytic converters on diesel engines do help to reduce particulate emissions (mostly soot), though only slightly; specifically, they tackle one type of particulate known as the soluble organic fraction, SOF, made from hydrocarbons bound to soot. Diesel particulate filters (DPFs) have to be used to make a significant impact on an engine's soot emissions.

Cars aside, diesel engines tend to power much bigger vehicles than gasoline engines (huge construction machines, for example), with considerably greater exhaust output. Instead of a single catalytic converter fitted between the engine and the tailpipe, they may have a number of individual units fitted in parallel to cope with the bigger exhaust gas volume (as in the diagram below).

Artwork: Big diesel engines can produce much higher exhaust volumes, so they may need to use multiple catalytic converters "in parallel." In this 1990s design by Caterpillar, the huge converter unit (gray) is around 1m (3.3ft) in diameter. Exhaust gas enters at the left (1), is evenly separated into streams by a flow distributor unit (2, blue), passes through one of seven separate catalytic converter units (3, red), is quietened by a noise muffler system (4, green), and exits, somewhat cleaned up, through the tailpipe (5). Artwork from US Patent 5,578,277: Modular catalytic converter and muffler for internal combustion engine by Scott T. White et al, Caterpillar, courtesy of US Patent and Trademark Office.

Who invented the catalytic converter?

" I like to make things happen, and that's what engineers do-they take the basic science and make things happen.. "
John J. Mooney, catalytic converter pioneer

Whom do we thank for making streets and cities safer and cleaner? French chemical engineer Eugene Houdry (1892-1962) patented what seems to have been the very first catalytic converter in the United States, filing the invention on May 5, 1950 and receiving his ( US Patent 2,674,521: Catalytic converter for exhaust gases) four years later on April 6, 1954. Houdry had previously invented catalytic cracking, the industrial process by which the many large complex organic chemicals in petroleum are separated into dozens of useful products, including gasoline. After that, he experimented with making different kinds of vehicle fuels and making them cleaner.

Although he recognized the growing problem of air pollution, his ideas were far ahead of their time: catalytic converters were "poisoned" by the lead additives used in gasoline to improve performance. Fortunately, in the 1970s, people started to recognize the dangers of lead, a toxic heavy metal. In 1973, the US Environmental Protection Agency (EPA) released a report demonstrating how lead harmed people's health, which began the slow process for removing lead from gasoline. The first practical catalytic converters appeared shortly afterward, in the mid-1970s, and have been used in cars ever since.

Artwork: Eugene Houdry's original catalytic converter from his 1950 patent. It's essentially a set of concentric metal tubes (blue) through which the exhaust gases flow. Clean air is sucked in through ventilation holes (yellow) with the help of a venturi (orange). As in a modern cat, Houdry explains that "the deposited finely divided metal catalyst is preferably platinum," although other similar metals can be used; unlike a modern cat, the catalyst (green) isn't arranged in a honeycomb but mounted in sixteen separate rings (red) at intervals along the tube, with each one working in parallel. Artwork from US Patent 2,674,521: Catalytic converter for exhaust gases, courtesy of US Patent and Trademark Office.

Houdry invented the basic oxidation catalyst for tackling carbon monoxide. Improved, three-way catalytic converters, which could also tackle nitrogen oxides, were designed in the early 1970s by Carl Keith (1920-1988), John Mooney (1929-2020), and chemical engineers at Engelhard Corporation. Apart from removing more pollutants, they start to purify the tailpipe gases much faster than earlier converters, so they're more effective on shorter journeys.

Artwork: In Carl Keith and John Mooney's improved design, there are two separate catalytic converters. Polluted gases flow from the engine (red, 10), and the exhaust manifold (orange, 11), through the first catalyst (green, 13) and then the second (25), some distance away, before exiting through the tailpipe (gray, 26). Artwork from US Patent 3,896,616: Process and Apparatus, courtesy of US Patent and Trademark Office.

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Find out more

On this website

Books

Catalytic Air Pollution Control: Commercial Technology by Ronald M. Heck, Robert J. Farrauto, Suresh T. Gulati. John Wiley & Sons, 2016. A thoroughly comprehensive guide to the topic that starts with the basic chemistry of catalysis before moving on to gasoline and diesel engine converters, stationary sources, and topics like ozone control in airplanes and cleaning up ambient air pollution.

Air Pollution from Motor Vehicles: Standards and Technologies for Controlling Emissions by Asif Faiz, Christopher S. Weaver, and Michael P. Walsh. World Bank Publications, 1996. Interesting technical report with an emphasis on how emissions are being controlled in practice in the world's most developed and polluted cities. Includes many useful figures and tables and a comparison of the effectiveness of emissions legislation in different countries. You can also download this as a PDF from the World Bank Research site.

Automobile Catalytic Converters by Kathleen C. Taylor. Springer, 1984/2012. Somewhat dated now, but still useful for background information.

News articles

Thieves Nationwide Are Slithering Under Cars, Swiping Catalytic Converters by Hiroko Tabuchi, The New York Times, 21 February 2021. Precious metals continue to make catalytic converters an attractive target for thieves.

John J. Mooney, an Inventor of the Catalytic Converter, Dies at 90 by Sam Roberts, The New York Times, 25 June 2020. Looking back at the life of the engineer who pioneered three-way catalytic converters.

Inventor in cleaner engine claim: BBC News, 22 January 2010. A Scottish inventor claims to have developed a cool-running engine that produces virtually no particulate (soot) emissions.

As Platinum Soars, the Catalytic Converter Gets Hot by Matthew Phenix. Wired, 17 February 2008. Why thieves think the soaring price of platinum makes catalytic converters worth stealing

Autos' Converters Cut Smog But Add to Global Warming by Matthew Wald. The New York Times, 29 May 1998. The EPA issues a report highlighting problems with nitrogen oxide.

Are catalytic converters really "green"?: The Guardian, Notes and Queries. Readers give their opinions on whether cats really help the planet.

Catalytic Converter: Big 'If' of 1975 by Robert W. Irvin. The New York Times, October 13, 1974. This article from the archives shows how the auto industry had serious concerns about the effectiveness of catalytic converters when they were first introduced in the mid-1970s.

Patents

US Patent 2,674,521: Catalytic converter for exhaust gases by Eugene Houdry, April 6, 1954. In this very readable patent, Houdry explains why he developed catalytic conveters and the various technical problems he had to solve in the process (such as tackling the gases produced by widely varying driving conditions).

US Patent 3,896,616: Process and Apparatus by Carl D Keith and John J Mooney, July 29, 1975. Another very readable patent, this one describes the improved three-way catalytic converter found in most modern vehicles.

US Patent 4,672,809: Catalytic converter for a diesel engine by Richard C. Cornelison and William B. Retallick, W R Grace and Co, June 16, 1987. Describes some of the challenges of making a catalytic converter work with diesel engine emissions.

US Patent 5,578,277: Modular catalytic converter and muffler for internal combustion engine by Scott T. White et al, Caterpillar, November 26, 1996. This patent explains how multiple catalytic units work together on the tailpipe emissions from a very large diesel engine.

Practical articles

Testing and repairing catalytic converters by Mort Schultz, Popular Mechanics, December 1985. A dated-but still very interesting-article that explains different types of catalytic converters and explores reasons for their failure.

Please do NOT copy our articles onto blogs and other websites

Articles from this website are registered at the US Copyright Office. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties.

Text copyright © Chris Woodford 2007, 2020. All rights reserved. Full copyright notice and terms of use.

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Advertisement by Chris Woodford. Last updated: February 18, 2022. Blackened buildings and choking streets-if that's your experience when you open the front door in the morning, you probably live in a big city like Los Angeles, London, Paris, or Beijing.

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Оюутнууд бүлгээр хуваагдаж, Padlet дээр технологийн эдгээр талыг тодорхойлоход хэрхэн нөлөөлж байгааг хэлэлцэнэ.

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Оюутнууд агаарын бохирдлыг бууруулахад каталитик хувиргагчийн үүрэг гүйцэтгэх талаар бичигдсэн ажлыг гүйцэтгэнэ.

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The development of new technology is one of the most promising frontiers for improving air quality. From catalytic converters to low-emitting consumer products, we've developed methods for preventing air pollution both indoors and outdoors.

In honor of these technological advancements, we are going to discuss six innovations that are actively preventing air pollutants from harming our atmosphere and our health. We've previously discussed regulatory and market-based solutions to air pollution, and both of these methods had their own respective pluses and minuses. Because new innovations don't usually have the same kind of cost vs. benefit relationship as these other methods, instead of going through technology as an overarching solution, we are going to discuss these innovations individually.

Multi-Pollutant Monitoring Devices

One of the topics we always love discussing is air quality monitoring; in this case, multi-pollutant air quality monitoring.

While this piece of technology does not directly reduce emissions or remove air pollution from our skies, it nevertheless is a crucial part of the air pollution control system. Pollution sources like factories, powerplants, and others emit more than one pollutant; on the flip side, some pollution control technologies control more than one pollutant. Multi-pollutant monitoring enables regulatory bodies to ensure compliance with emission limits for many different pollutants and reduces the cost and time burden associated with ambient pollution monitoring.

Catalytic Converters

Vehicular exhaust is one of the most significant sources of air pollution globally. In response to increasingly strict environmental regulations beginning in the 1970s, gasoline- and diesel-powered vehicles were equipped with catalytic converters, a device that catalyzes a redox reaction that transforms dangerous air pollutants into less-harmful pollutants.

Gasoline-powered vehicles today generally come with a three-way converter, or an oxidation-reduction catalytic converter. These devices work by exposing exhaust to a catalyst, a substance that lowers the activation energy needed for a chemical reaction, thus enabling a reaction to convert carbon monoxide to carbon dioxide and nitrogen oxide to nitrogen and carbon dioxide. Carbon dioxide and atmospheric nitrogen are both far less harmful than carbon monoxide and nitrogen dioxide, despite CO2 being a GHG, so catalytic converters are great tools for reducing the environmental and health impacts of vehicular exhaust.

Catalytic converters do have some downsides. For one, these devices can restrict the flow of exhaust, reducing fuel efficiency and vehicle performance. Catalytic converters also have a warm-up period, during which the vehicle emits an unrestricted amount of air pollution. The creation of these converters also comes with some harms; many converters require platinum or palladium, rare metals that create pollution during the refining process.

Scrubbers

Scrubbers are a type of pollution control device that removes air pollutants like sulfur dioxide, chlorine, hydrogen sulfide, and hydrogen chloride from industrial exhaust.

There are two main types of scrubbers that we will discuss: wet scrubbers and dry scrubbers. Wet scrubbers use a liquid (usually water) to absorb particles or gases from a stream of air, and can vary in energy level. A common low energy wet scrubber is a spray tower, which works by passing the exhaust through an open vessel with sprayers to distribute the liquid. As the exhaust flows through the device, the liquid picks up floating particles or absorbs the target gas. Dry scrubbers work similarly, but instead of spraying a liquid, these devices spray dry reagents into the flue stream, neutralizing gases before they can enter the atmosphere.

Scrubbers are a hugely beneficial tool for pollution control, as they prevent damaging air pollutants from harming communities near industrial centers like powerplants and water treatment facilities. Additionally, these devices do not disrupt the production process, allowing economic and industrial activity to continue without a commensurate rise in air pollution.

Chlorofluorocarbon (CFC) and Hydrochlorofluorocarbon (HCFC) Substitutes

The Montreal Protocol, signed in 1987, gradually phased out the production and use of chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs), both of which deplete the ozone layer. CFCs and HCFCs were commonly used as solvents, propellants, and refrigerants, so we needed to create alternatives that fulfilled the same purpose but with much lower ozone-depleting potential (ODP).

Today, hydrofluorocarbons (HFCs) are one of the main substitutes for CFCs and HCFCs. The names can be a bit confusing, but, long story short, HFCs don't have chlorine , which is the substance largely responsible for harming the ozone layer. The availability of HFCs and other substitutes is credited with the overwhelming success of the Montreal Protocol, as we were able to stop the production of CFCs and HCFCs without losing their respective functions.

However, HFCs are not harmless; while these substances don't hurt the ozone layer, they do contribute to climate change. HFCs live much longer in the atmosphere than CFCs and HCFCs, which means that their impacts on global climate change are likewise long-term. The atmosphere is a very complicated system that's constantly evolving, and the emissions that we produce need to likewise adapt. There are currently plans to expand the Montreal Protocol to include HFCs, as there are alternatives to these substances as well.

Low-Emitting Stoves and Heaters

Air pollution is not just an outdoor issue; each year 3.8 million people die prematurely due to indoor air pollution from inefficient cooking practices. Most of these deaths come from burning solid fuels or kerosene in enclosed spaces, allowing particulate matter levels to build up to hazardous levels.

Innovations in stove design and alternative fuels have made cooking much more efficient and cut the amount of particulate matter produced in households. Biomass stoves can now come equipped with secondary combustions, fans, or insulated combustion chambers, which burn off unused fuel and ventilate the remaining pollutants. Ultimately we want to move away from biomass stoves, but these are good stepping stones for low- and middle-income nations where electricity or natural gas are not in steady supply.

In addition to cooking, heating produces indoor air pollution. Solar water heaters and passive solar heating and cooling systems cut the necessity of solid fuel consumption, and improved ventilation designs, like stove hoods and chimneys, can help funnel out indoor air pollutants.

VOC-Free Consumer Products and Building Materials

Lastly, we are going to discuss another innovation that reduces indoor air pollution: low-emitting consumer products and building materials.

Many of the products we use every day, including paints, cleaners, adhesives, sealants, furniture, and flooring off-gas volatile organic compounds (VOCs), which can build up without proper ventilation. Due to concerns surrounding the impacts of these emissions, more consumer products and building materials are being developed and certified as low-emitting or even VOC-free. Guidelines offered by the California Department of Public Health and ASTM can be used to determine if a product is low-emitting.

Because these innovations are crucial for controlling indoor air pollution, many building standards require or offer credit for utilizing low-emitting building materials and including low-emitting consumer products as part of an indoor air quality strategy.

Many of the technologies we discussed above are big-picture, community-wide solutions. What can we, as individuals, do to protect ourselves from air pollution? For more information, check out our air pollution guide below:

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Air Pollution Solutions: 6 Pieces of Technology That Are Clearing Our Skies

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The development of new technology is one of the most promising frontiers for improving air quality. From catalytic converters to low-emitting consumer products, we've developed methods for preventing air pollution both indoors and outdoors. In honor of these technological advancements, we are going to discuss six innovations that are actively preventing air pollutants from harming our atmosphere and our health.

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About This Quiz

Catalytic converters are amazingly simple devices that have proven to be highly effective at reducing harmful emissions produced by car engines. How much do you know about catalytic converters?

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Catalytic converters are amazingly simple devices that have proven to be highly effective at reducing harmful emissions produced by car engines. How much do you know about catalytic converters?