Nanoparticles are one of the most common nanomaterial forms, appearing in many different compositions and sizes. The ability to tune and create nanoparticles with specific functional properties has led to them being applied in various areas. One area where they are starting to find a lot of use is as a synthetic catalyst (i.e. a non-naturally occurring catalyst – like enzymes etc are). Here, we are going to investigate the different areas where nanoparticles can be used as synthetic catalysts.
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Nanoparticles are officially defined as a particle that is between 1 and 100 nm. For several particles to be defined as a nanoparticle system, at least 50% of the particles within the system need to fall within this size range. Nanoparticles are a useful material for catalysis due to their high relative surface area – if a nanoparticle system has the same volume as a bulk material, its relative active surface area is greater than the bulk material. Therefore, nanoparticles are added in as heterogeneous catalysts (different phase catalyst) to act as a binding/adsorption site, or as catalytic support, their high active surface area utilized for various types of reactions.
Some of the most common catalysis applications below show nanoparticles being used (and are some of the most promising from a commercial perspective), but this is by no means an exhaustive list and new applications are always emerging for synthetic nanoparticle catalysts.
Organic synthesis (and organic chemical reactions) is one of the biggest areas where catalysts are used – especially in a commercial production setting – as they enable many chemical products to be made at lower costs and in larger volumes than would otherwise be possible. While catalysts have been around for many years in organic synthesis reactions, several nanoparticles are now being used for different reactions.
While there are many individual nanoparticles, nanoparticles composed of iron, platinum, palladium, copper, gold, iridium and silver (in various compositional and orientational arrangements), as well as their oxide derivatives, have found the most use in organic synthesis reactions. Some specific reactions where nanoparticle catalysts are used include epoxidation, selective oxidation, hydrogenation, oxygen-reduction, alcohol oxidation, and reductive coupling reactions. These reactions are used on a wide range of organic molecules in several different applications and commercial settings.
Biodiesel is becoming a potential candidate to replace conventional fuels due to their lower environmental impact. The production of biodiesel is reliant on the transesterification of the fatty acids in vegetable oil, animal fats or waste cooking oils, alongside alcohols and a catalyst. As it is a growing product, more efforts have been made on finding innovative ways of producing large volumes of biodiesel. Nanoparticles consisting of potassium and calcium have shown to be efficient at producing biodiesel. This high efficiency is due to the high surface area and large pore size that promote contact between the catalyst and the reactants, leading to an improved transesterification efficiency.
Photocatalysts are another large application area, making up one of the biggest catalysis areas after organic synthesis reactions, due to the number of photocatalytic reactions that can be performed. Many of the photocatalytic reactions also involve organic molecules but use light to degrade the molecules. The most common molecules which can be degraded and removed from a system by photocatalytic reactions are dyes, and various nanoparticle systems have been shown to photodegrade malachite green, methylene blue, acid orange 7 (AO7), procion red MX-5B (MX-5B) and reactive black 5 (RB5) dyes. The photodegradation mechanisms vary depending on the dye and the catalyst but most involve the forming of oxygen-based radicals under visible light or UV light irradiation.
The most common nanoparticles used in these reactions include titanium dioxide (and doped titanium dioxide), zinc oxide, aluminum oxide, and niobium oxide, as well as composite nanoparticles made up of these metal oxides. As well as removing dyes from spent products and environmental systems, the nanoparticles use photocatalytic mechanisms to remove bacteria (such as E-coli) from surfaces, facilitate photocatalytic reactions performed in photovoltaic (solar) cells, and can be used to degrade a wide number of phenol-containing (and phenol derivative-containing) molecules from various water systems (both natural and industrial wastewater systems).
“Novel Metal Nanomaterials and Their Catalytic Applications”- Gu H. and Wang J., Molecules¸ 2015, DOI: 10.3390/molecules200917070
“Applications of nano-catalyst in a new era”- Dave P. N. et al, Journal of Saudi Chemical Society, 2012, DOI: 10.1016/j.jscs.2011.01.015