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Figure 1. Catalysts are used in all sorts of applications, from chemical plants to catalytic converters in cars. Image credit: ASU.
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Catalysis is one of the longest-established uses for nanoparticles. Aluminium, iron, titanium dioxide, clays, and silica have all been used as catalysts in nanoparticle form for many years.
Until quite recently, however, the properties of these particles which make them so useful as catalysts has not been fully understood. Nanoparticle catalysts have a very large surface area, which has a straightforward positive effect on reaction rate. However, there are structure- and shape-based properties at the nanoscale, which can also effect the catalytic activity of a material.
As scientists gain a better understanding of how the physical properties of nanoparticles affect their catalytic properties, and how fabrication parameters can in turn affect those physical properties, they can design nanocatalysts which are highly active, highly selective, and highly resilient.
This will enable industrial chemical reactions to become more resource efficient, consume less energy, and produce less waste. This will help to counter the environmental impact caused by our reliance on chemical processes in every aspect of our society, from agriculture to cosmetics.
Homogeneous Nanocatalysts
Homogeneous catalysts are used in the same medium as the reactants - for nanoparticles this typically means a solution or suspension of nanoparticles in a solvent.
The most important issue to consider when designing a nanocatalyst for use in a solution is to prevent aggregation - nanoparticles are naturally attracted towards one another in these conditions, and will clump together to form larger particles if not prevented from doing so, removing their large surface area and other benefits.
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Figure 2. Nanoparticle catalysts can be much more active and selective than the same materials in bulk or larger particle form - partly due to the increased surface area, but also due to additional size effects. Image credit: UW Madison.
The most effective way to stabilize nanoparticles in solution is to attach long chain molecules to the surface. These make it impossible for the nanoparticles to get so close that they stick together. However, they can also reduce the access to the nanoparticle surface for the reacting molecules, decreasing overall catalytic activity.
The other main concern with homogeneous nanocatalysts is recovery. Nanoparticles are notoriously difficult to remove from a solution, and the extra steps needed to do so could completely negate the process simplification due to using the catalyst in the first place.
If the nanoparticles cannot be recovered, they pose an environmental risk, as well as threatening the profitability of the process. Most nanoparticles cannot be destroyed by incineration, and the effects of nanoparticle accumulation in ecosystems are largely unknown.
Heterogeneous Nanocatalysts
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Figure 3. Heterogeneous catalysts are much easier to remove from the reaction mixture, and are also more adaptable to continuous flow processes. Image credit: UMN
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An option which is often considered to be more environmentally friendly is heterogeneous catalysis. This involves a catalyst which is in a different phase to the reactants. The catalyst is usually solid, or immobilized on a solid inert matrix. This gets around the issue of waste and recoverability, as the solid catalyst can in most cases simply be filtered out.
A great deal of research has been done to investigate the catalytic potential of various nanoparticle-support systems. Recent examples include palladium, iron, gold, nickel and platinum nanoparticles. Supports used range from silica or aluminum to carbon fibres.
Another area of heterogeneous nanocatalysts which has been explored is nanostructured solids. Nanoporous materials can be manufactured by growing the solid material around a molecular template. Nanoscale features can also be etched into the surface of a catalyst using standard lithography techniques - this can allow a degree of control over reactant flow on the catalyst surface, as well as increasing surface area.
Nanoporous Gold Catalysts
In August 2012, research published in Nature Materials examined the catalytic mechanisms in nanoporous gold. Whilst gold in its bulk form is inert, nanoporous or nanoparticulate gold is a good catalyst for oxidation of carbon monoxide, making it of interest for emissions treatment in catalytic converters.
Nanoporous gold is preferable to the nanoparticles due to its stability and recoverability, so understanding how it catalyzes reactions in greater detail is of great interest.
The research found that the catalytic activity centres around concentrations defects in the gold surface. The nanoporous gold is made by removing the silver from a gold/silver alloy - silver atoms left behind on the gold surface make the defects on the surface significantly more stable, allowing them to act as catalytic active sites.
Graphene and Carbon Nanotube Catalysts
Grafoid Inc. and CVD Equipment Corporation have announced a Joint Development Agreement to investigate the potential of combinations of graphene and carbon nanotubes as catalytic materials.
The project is promising, as both companies have experience working with these materials, and both materials have also shown interesting and unique chemical and electrical properties. The spatial restrictions caused by their unusual dimensionality causes changes in their electronic structure. Because of this, the materials have shown promise as electrocatalysts.
Conclusion
Catalysts are a crucial part of the modern chemical industry, and are used in a huge number of chemical processes all over the world. R&D departments are constantly pushing to improve the performance and lifetime of their catalysts, which the profitability of much chemical manufacturing hinges on.
Nanomaterials in catalysis are offering a whole new array of parameters which scientists can tweak to try and find the perfect catalyst. As this research goes on, we will understand more and more about how catalysts behave at the nanoscale, and we will able to make chemicals ever more efficiently.
References and Further Reading