Scientists at Victoria University of Wellington are making new shapes of nanoparticles. These will help reduce poisonous emissions from car exhausts by making catalytic converters in cars more efficient.
Catalytic converters
Dr Richard Tilley and PhD student John Watt are working to increase the efficiency of catalysts used in car catalytic converters.
Catalytic converters clean up car exhausts before letting the gases into the atmosphere. One of the processes is to convert poisonous carbon monoxide to carbon dioxide. Exhaust passes over a surface coated in platinum catalyst nanoparticles. Carbon monoxide and oxygen bind to this surface, where they join to form carbon dioxide. This carbon dioxide only has weak bonds with the platinum, so it is released. The platinum is then available to repeat the process.
Platinum also helps to reduce harmful emissions of oxides of nitrogen, by changing them to nitrogen and oxygen gases. When a nitrogen oxide or nitrogen dioxide molecule meets the catalyst, the catalyst grabs the nitrogen atom and frees the oxygen. The nitrogen atoms bond with others stuck on the catalyst and are released as nitrogen gas.
Nature of science
There is normally more than one way of tackling a science problem. While Richard and John work on the shape and activity of platinum to increase efficiency, Richard Haverkamp and others at Massey University are investigating gold and other metals to replace platinum. Japanese scientists are investigating embedding the platinum nanoparticles on the surface of nanoscale ceramic balls to reduce clumping at high temperatures.
Only about 10% of the platinum catalyst is active, which is a real waste because the metal is very expensive. This is partly due to the nanoparticles clumping together at high temperatures and so reducing surface area. New, more active shapes of platinum and other catalysts, such as palladium and rhodium, should increase the efficiency of catalytic converters. There will also be less waste of a very limited, expensive resource.
New shapes of nanoparticles
Richard and John are making new shapes of nanoparticles that increase the surface area of the particles. To be a catalyst, the surface area: volume ratio of the nanoparticles is important – the larger the surface area, the more atoms are available to join in a chemical reaction.
It’s not just about surface area, though. Some of the crystal faces of nanocrystals are more catalytically active than others. Richard and John use chemicals called surfactants to increase the growth of just the more active faces, increasing the activity of the whole catalyst.
How do surfactants control size and shape?
Some of the chemical tools that make Richard and John’s work possible are surfactants. These are soap molecules with a water-loving (hydrophilic) end and a water-hating (hydrophobic) end.
When nanoparticles are made in solution, they are in the form of tiny crystals. They have regular arrangements of atoms, just like the larger crystals you are more familiar with, such as sugar and diamond. When left uncontrolled, atoms keep joining on to the nanocrystals, causing them to grow until they are no longer at the nanoscale. This is where surfactants come in.
Surfactants bind to the surfaces of the nanocrystals, surrounding them and stopping them from growing.
Surfactants can also be used to control the shape of nanocrystals. Surfactants bind to different crystal faces with different strengths. If a surfactant binds strongly to a crystal face, that face can’t grow. If a surfactant binds weakly to a face, that face can grow. Richard and John make use of this process to get some faces of nanocrystals to grow but not others. This means they can control the shape of the nanocrystals.
