Catalytic Activity

Because surface area increases as particle size decreases (Section 16.6.2), nanomaterial-based catalysts show markedly increased catalytic activity compared with the same substance in bulk form -- more exposed surface area means more available active sites for a reaction to occur on. Nanoparticle catalysts are usually heterogeneous catalysts, meaning the catalyst itself is in solid form while the reaction it speeds up takes place at its surface, in contact with reactants that may be gaseous or in solution. A practical advantage of this is that nanoparticle catalysts can be separated from the reaction mixture relatively easily (since they remain a distinct solid phase) and can then be recycled for further use, rather than being consumed or lost. The chapter's examples include palladium (Pd) and platinum (Pt) metal nanoparticles, used as catalysts in hydrogenation reactions, and titanium dioxide (TiO2) and zinc oxide (ZnO), used in photocatalysis. A particularly striking example is gold: in bulk form, gold is famously chemically unreactive (which is part of why it is prized for jewellery), but gold nanoparticles have been found to be v …