New nanoparticle shapes
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 and platinum nanoparticles
Dr Richard Tilley of Victoria University of Wellington explains what catalytic converters are and the role of catalysts. He also outlines why different shapes of nanoparticles are useful.
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
An octapod nanoparticle
Transmission electron microscope image of an octapod. You can see the regular arrangement of atoms making up the octapod.
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.
Different shaped nanocrystals
This transmission electron microscope image shows octapods (not quite square) and tripods (three arms), grown using surfactants to control the shapes.
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.
Structure affects function
Prof Richard Haverkamp, of Massey University, explains why the small size of some nanoparticles helps them become good catalysts.
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 at work
While Dr Richard Tilley of Victoria University of Wellington explains how surfactant chemicals are used to control size and shape of nanocrystals, we see John Watt making gold nanoparticles in a flask.
Surfactants bind to the surfaces of the nanocrystals, surrounding them and stopping them from growing.
How to make an octapod
On the left is a computer image of a platinum nanocrystal. Each ‘dot’ is an atom. When a surfactant is used to block the square faces, only the triangular faces can grow, and the result is an octapod (right).
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.
Making gold nanoparticles in micelles
John Watt demonstrates one way to make gold nanoparticles. They form by chemical reaction inside tiny spheres called micelles, created by surfactants (soaps).
Related content
Chemical reactions and catalysts are two of the big science ideas that underpin this research story.
To explore the nanoscience connection, students can use modelling clay to construct catalyst nanoparticle shapes and calculate surface area:volume ratios with the aim of trying to develop a more efficient shape.
Useful links
Visit the The MacDiarmid Institute for Advanced Materials and Nanotechnology website.
Dr Jerome Leveneur, a Material Scientist from GNS Science and Associate Investigator for the MacDiarmid Institute for Advanced Materials and Nanotechnology, explains how scientists can manipulate the surfaces of objects to make them interact differently with fluids like coffee in a coffee cup in this video from Science on a Napkin.
