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The air we breathe

Sponsored by Engine exhaust is a major contributor to pollution in cities. But

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Engine exhaust is a major contributor to pollution in cities. But BASF's innovative engineering and clever catalytic chemistry is helping to produce cleaner air

16 August 2017

With their sporty curves and chrome fittings, few would disagree that classic 1960s cars have the aesthetic edge over modern cars. But beneath the stylish bodywork lies a guilty secret. The toxic emissions from one 1960s car are about equal to those from 100 modern cars.

That's thanks in no small part to the invention of the catalytic converter. In the last four decades, these devices have prevented the release of more than one billion tonnes of pollutants.

But consumers, politicians and activists want more. Modern catalytic converters can reduce more than 90 per cent of the harmful stuff in exhaust fumes. Now scientists and engineers at chemicals giant BASF are working to increase that figure even further to help vehicle makers meet ever more demanding clean air laws.

At the heart of this effort is the catalytic converter, first developed for cars in 1973 by researchers at the US mineral refining company, the Engelhard Corporation. The work was triggered by the 1970 US Clean Air Act which forced car-makers to dramatically reduce harmful emissions.

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Catalytic converters are honeycombed ceramic bodies with a huge internal surface area. This surface is coated with a formulation containing metals, such as platinum, palladium and rhodium, which catalyse various reactions. The first converters oxidised carbon monoxide into carbon dioxide and unburnt hydrocarbons into carbon dioxide and water. Later models, called "three-way" converters, also split harmful NOx (nitrogen oxides) into nitrogen and oxygen.

How four-way catalytic conversion works

Further clean air regulation has pushed this technology forward. Between 1992 and 2014, a series of European regulations reduced carbon monoxide emissions from new petrol cars by two-thirds and hydrocarbon and NOx emissions by almost an order of magnitude.

BASF has been a significant player in these reductions. In 2006, it bought Engelhard and today it is one of the world's leading suppliers of catalytic converters for the automotive industry, employing catalysis scientists worldwide.

These experts are working to cut emissions even further. One recent BASF innovation is a synthetic catalyst called copper chabazite that helps remove 95 per cent of NOx from diesel exhaust. This works in conjunction with urea fed into the exhaust stream, which decomposes into ammonia.

The copper chabazite contains regularly-spaced pores where the ammonia and nitrogen oxide molecules react to form nitrogen and water.

"Copper chabazite provides both the excellent catalytic activity required for the NOx conversion, and the structural stability needed for long-term durability," says Ahmad Moini, a member of the team that patented this technology in 2009.

The way catalytic converters are made is important too. At the BASF emissions catalysts facility at Nienburg in Germany, orange robotic arms orchestrate the coating of ceramic bodies in a creamy slurry containing chabazite. The coating takes place in a precise way to ensure the pores in the material are completely and rapidly coated.

It's here that BASF has developed the next generation of converters. Its team has designed these to meet the upcoming European legislation that reduces by an order of magnitude the permissible amount of soot, or particulate matter pollution, that petrol engines can emit.

One way to do this is to fit an additional filter that removes this soot. But BASF scientists have come up with a better solution that combines particulate filtering with conventional "three way" catalytic conversion. Their trick was the catalysation of a tailored porous filter.

The new structure is a honeycomb of long parallel tunnels that allow the exhaust to enter. The gases pass through the tunnel walls where they are broken down in catalytic reactions. The converted gases then pass into another set of parallel tunnels and out of the converter.

The pores in the tunnel walls are too small for soot to enter and this is trapped and later burnt to form carbon dioxide. The new devices are called EMPRO™ Four-Way Conversion catalysts.

BASF had to overcome significant engineering challenges to make these devices. For example, it had to ensure that the filtering process did not significantly increase engine resistance, which would reduce power or increase fuel use. "We achieved that partly by developing a new process to apply our catalyst composition into the porous walls of the filter rather than onto them," says Torsten Neubauer, BASF Vice President Environmental Catalysis Research Europe. The first cars with BASF four-way conversion components hit the market last year.

Another challenge for Neubauer and his colleagues is to cater for hybrid cars. These constantly switch from electric motors to an internal combustion engine, which ends up running at a lower temperature than conventional car engines. So BASF is investigating ways to make catalysts more effective at these lower temperatures.

"We are working with vehicle manufacturers, parts makers and catalyst substrate suppliers to further reduce emissions, including in newer vehicle types," says Tilo Horstmann, Vice President BASF Mobile Emissions Catalysts. "I’m confident that by combining our key competencies, we can provide innovative solutions to drive sustainable motoring to the next level."

Diesel cars have had a mixed press. Not long ago, politicians were promoting their fuel efficiency – diesels produce up to 20 per cent less carbon dioxide than petrol equivalents.

But more recently, they have taken against diesel cars, especially after the VW emissions testing scandal, arguing that the nitrogen oxides they produce are an unacceptable health burden in cities. As a result, the mayors of Paris, Madrid, Athens and Mexico City have announced plans to ban diesel cars by 2025.

Now automotive industry leaders say this is a step too far. "We are seeing some cities proposing aggressive and irrational diesel-bashing moves," says Tilo Horstmann, Vice President BASF Mobile Emissions Catalysts.

Diesel vehicles do emit more NOx – on average 16.5 times as much as petrol versions, according to the UK Consumers’ Association. However, this masks a huge variation within the diesel fleet between older and newer cars.

Under European regulations, light diesel vehicles sold since 2014 must emit no more than 80 milligrams/kilometre of NOx in lab tests, compared to 500 mg/km for those sold before 2000. The equivalent limit for petrol engines is 60mg/km. From September, tougher standards will cut pollution further, limiting emissions even under real world driving conditions, not just in the laboratory.

But diesel engines are also more fuel efficient. So a fall in sales of new diesel cars will reduce the European Union's chances of meeting its target of cutting average CO2 emissions of new cars from 130 g/km in 2015 to 95 g/km in 2021.

And therein lies the dilemma, says Frank Mönkeberg, Head of Application Engineering, BASF Mobile Emissions Catalysts. "Without diesel, Europe will not be able to comply with this limit."

This article appeared in print under the headline "The air we breathe"

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