The size and shape of gold nanoparticles play a significant role in their catalytic activity in the oxidation of carbon monoxide CO . Gold nanoparticles have been found to be highly effective catalysts for CO oxidation, even at low temperatures. The catalytic activity of gold nanoparticles is influenced by several factors, including their size, shape, and the nature of the support material they are placed on.1. Size: The size of gold nanoparticles has a direct impact on their catalytic activity. Smaller nanoparticles have a higher surface area-to-volume ratio, which means that a larger number of gold atoms are exposed on the surface. This increased surface area provides more active sites for the adsorption and reaction of CO and oxygen O2 molecules. Generally, gold nanoparticles with sizes between 2-5 nm have been found to exhibit the highest catalytic activity for CO oxidation.2. Shape: The shape of gold nanoparticles also affects their catalytic activity. Different shapes, such as spheres, rods, cubes, and octahedra, have different surface structures and crystallographic facets. These facets can have varying degrees of reactivity towards CO and O2 molecules. For example, gold nanoparticles with a higher proportion of {111} facets such as octahedra have been found to be more active for CO oxidation than those with a higher proportion of {100} facets such as cubes . This is because the {111} facets have a higher binding energy for oxygen, which promotes the formation of active oxygen species necessary for CO oxidation.3. Support material: The nature of the support material on which gold nanoparticles are placed can also influence their catalytic activity. The interaction between gold nanoparticles and the support material can alter the electronic properties of the gold particles, affecting their ability to adsorb and activate CO and O2 molecules. For example, gold nanoparticles supported on reducible metal oxides such as TiO2, CeO2, and Fe2O3 have been found to exhibit higher catalytic activity for CO oxidation compared to those supported on non-reducible metal oxides such as SiO2 and Al2O3 .In summary, the size, shape, and support material of gold nanoparticles can significantly affect their catalytic activity in the oxidation of carbon monoxide. Smaller nanoparticles with specific shapes and supported on appropriate materials can enhance the catalytic performance of gold nanoparticles in CO oxidation reactions.