The size and shape of metallic nanoparticles play a crucial role in their catalytic activity in the conversion of carbon dioxide CO2 to carbon monoxide CO . This is because the size and shape of the nanoparticles directly influence their surface properties, which in turn affect the adsorption, activation, and reaction of the CO2 molecules on the catalyst surface.1. Size effect: As the size of the metallic nanoparticles decreases, the surface-to-volume ratio increases, which means that a larger fraction of the atoms is exposed on the surface. This leads to an increase in the number of active sites available for the CO2 molecules to adsorb and react. However, there is an optimal size range for the nanoparticles, where the catalytic activity is maximized. If the nanoparticles are too small, they may suffer from high surface energy and tend to agglomerate, leading to a decrease in the available active sites and a reduction in catalytic activity.2. Shape effect: The shape of the metallic nanoparticles also has a significant impact on their catalytic activity. Different shapes expose different crystal facets on the surface, and each facet may have distinct catalytic properties due to variations in atomic arrangement and electronic structure. For example, cubic nanoparticles may expose 100 facets, while octahedral nanoparticles may expose 111 facets. Some shapes may have a higher density of active sites or provide better adsorption and activation of CO2 molecules, leading to enhanced catalytic activity.In summary, the size and shape of metallic nanoparticles can significantly affect their catalytic activity in the conversion of CO2 to CO. Optimizing these parameters can lead to the development of more efficient catalysts for CO2 conversion and other important chemical reactions.