Volkswagen have been in the news for all the wrong reasons over the past week. The company admitted to installing a ‘defeat device’ in millions of cars which made it appear in emissions tests that they emitted far lower levels of particular pollutants than they actually did in normal road conditions. Plenty of articles have looked at the particulars of the scandal since the story broke last week, so as well as considering the pollutants involved in the VW story, here we’ll also take a closer look at how we use chemistry to remove pollutants from vehicle emissions.
There a number of pollutants in vehicle emissions that we aim to keep to as low a level as possible. These include hydrocarbons, which are simply remnants of the car’s fuel that remain unburnt, and particulate matter (for example, soot). Additionally, carbon monoxide, a poisonous, colourless gas, is another pollutant we aim to remove. The pollutants that have been at the forefront of the Volkswagen story, however, are the nitrogen oxides. These are generated as byproducts of the combustion reaction that occurs in vehicle engines, and can pose a number of environmental problems.
Though nitric oxide (NO) is often produced as a byproduct in greater quantities than nitrogen dioxide (NO2), it’s the latter that’s often more of a concern. Nitric oxide can quite easily react with atmospheric oxygen to form nitrogen dioxide, which can cause a range of environmental and health issues. If breathed in, it can cause inflammation, coughing, and wheezing, potentially exacerbating asthma.
Nitrogen dioxide can also react in the atmosphere to form ground-level ozone. Unlike stratospheric ozone, which acts as a protective shield from the sun’s radiation, ground-level ozone can have similar health effects to nitrogen dioxide, can damage crops, and also acts as a potent greenhouse gas. Due to all of this, nitrogen oxides are one of the main pollutants targeted when we’re looking to reduce vehicle emissions – and there are a number of ways to do that.
If you drive a petrol-fuelled car, it’ll be fitted with a three-way catalytic converter. These are so-called because they deal with the three major pollutants from vehicle emissions we’ve discussed: unburnt hydrocarbons, carbon monoxide, and nitrogen oxides. They do this via the use of small amounts of precious metals, such as platinum, palladium, and rhodium, which act as catalysts for the conversion of the pollutants into less hazardous products: carbon dioxide, water, and nitrogen.
Often more than one precious metal will be required in three-way catalytic converters, as they catalyse different reactions. Rhodium acts as a reduction catalyst, which helps reduce the nitrogen oxides to nitrogen and oxygen. Palladium acts as an oxidation catalyst, helping to oxidise carbon monoxide to carbon dioxide, and unburnt hydrocarbons to carbon dioxide and water. Platinum, meanwhile, can carry out both roles, but due to its expense and other side reactions isn’t always suitable for all purposes.
The one issue with three-way catalytic converters is that they don’t work well in diesel-powered vehicles. This is because the oxygen-rich nature of a diesel vehicle’s exhaust gases makes the conversion of nitrogen oxides inefficient. Because of this, diesel vehicles still use two-way catalysts, which only deal with carbon monoxide and unburnt hydrocarbons. Further methods are needed to remove the nitrogen oxides; one of these, not detailed in the graphic, is exhaust gas recirculation (EGR), which replaces air in the engine’s intake gases with exhaust gases, reducing the amount of nitrogen available to form nitrogen oxides.
Selective Catalytic Reduction
One of two methods that can be used to remove nitrogen oxides from diesel emissions, selective catalytic reduction (SCR) involves the injection of a fluid into the exhaust stream in order to bring about their removal. Urea is the chemical commonly sprayed into the stream, where it produces ammonia. The ammonia adsorbs onto a catalyst, and can then react with the nitrogen oxides, producing nitrogen and water.
SCR can achieve a reduction in nitrogen oxide levels of up to 90%, and is considered to be the best method for reducing diesel nitrogen oxide emissions to levels that will be required by future emission limitations. However, it does require regular topping up of the exhaust fluid. Additionally, a large limitation is the fact that SCR is not effective at removing nitrogen oxides when the exhaust gas is at a low temperature. Its optimum temperature range is 250˚C–427˚C, so when vehicles first start up, or when they are sat in traffic, the temperature can be below this and removal of nitrogen oxides can be impacted.
Nitrogen Oxide Adsorbers
These are another method that can be used in diesel engines to remove nitrogen oxides; they’re sometimes also referred to as lean nitrogen oxide traps (LNT). Any nitric oxide produced by the combustion of the diesel fuel is converted to nitrogen dioxide by reaction with oxygen over a platinum catalyst, and subsequently stored in the form of nitrates by a storage material. The conversion of nitric oxide is important, as nitrogen dioxide is more effectively stored by the storage materials used.
The chemical identity of the storage material used is an oxide of either an alkaline earth metal, alkali metal, or rare earth metal. Barium oxide is one of the most commonly used. Eventually, as the storage of the nitrogen dioxide continues, the ‘trap’ will become full. At this point, the exhaust conditions must be temporarily change to reduce the amount of oxygen. This is often accomplished by injecting diesel fuel.
This injection causes the nitrogen dioxide to desorb from the storage material, and it is then passed over a rhodium catalyst, and reduced to nitrogen in the conventional manner common to a normal catalytic converter. The overall chemistry of these nitrogen oxide adsorbers is, in reality, a little more complex than this generalised explanation, but it serves as a suitable overview. It can be effective for removing nitrogen oxides from the exhaust of smaller vehicles, but is not as effective as SCR for larger vehicles.
The Volkswagen ‘Defeat Device’
The majority of the Volkswagen vehicles that were found to have discrepancies between their on-road emissions and their emissions during emissions tests were fitted with nitrogen oxide adsorbers. However, the exact manner in which the company managed to achieve the lower emissions in the tests isn’t yet fully known. What we do know is that the cars in question contained a ‘defeat device’.
The ‘defeat device’ isn’t actually a physical device, but merely part of the car’s software. It was able to detect when the car was in test conditions – it’s speculated that it did this by monitoring whether the steering column was being turned, or whether the car’s traction control was turned off) – and when it detected this, it in some way tuned the engine’s performance. The outcome of this was lower nitrogen oxide emissions, allowing the cars to pass the test. The defeat device was found in both vehicles using SCR, and those using nitrogen oxide adsorption, so it doesn’t seem to have been specific to one type of emissions control.
As scrutiny of emissions of cars of other manufacturers now increases, it’s entirely possible that others will have been found to have fiddled their results with similar devices. It’s clear that, if we want diesel vehicles to be able to meet nitrogen oxide emissions targets, for now at least we’re going to have to be willing to accept a drop in performance; after all, this was the only way in which Volkswagen was able to get their vehicles to pass the emissions tests!
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References & Further Reading
- What is selective catalytic reduction? – Diesel Technology Forum
- Diesel catalysts – DieselNet Technology
- The science behind the Volkswagen emissions scandal – Q Schiermeier, Nature
- Critical topics in exhaust gas aftertreatment – M V Twigg