Video: Reason of Using Nanoparticles as Catalysts

Why can nanoparticles be used as effective catalysts in very small quantities? [A] They have a high surface-area-to-volume ratio. [B] They are inflammable. [C] They are transition metals. [D] They have low reactivity. [E] they are lightweight.

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Video Transcript

Why can nanoparticles be used as effective catalysts in very small quantities? (A) They have a high surface-area-to-volume ratio. (B) They are inflammable. (C) They are transition metals. (D) They have low reactivity. Or (E) they are lightweight.

Nanoparticles are very small particles of matter. They usually contain only a few 100 atoms, and these atoms arrange themselves into spheres, 3D geometric shapes, or into nano-tubes or fibers. The sizes of these particles vary between one and 100 nanometers in diameter, length, or breadth. In certain cases, particles that are smaller than 500 nanometers are also considered nanoparticles.

Nano- means one billionth. So, one nanometer is the same as a billionth of a meter or one times 10 to the negative nine meters. A billionth of a meter is the same as 0.000000001 meters. So, a nanoparticle of, say, 20 or 17 nanometers in diameter is a very small particle indeed. So, what is so special about nanoparticles?

When particles are smaller than 100 nanometers in diameter, they exhibit very different properties to bulk materials. In bulk materials, properties are independent of and do not rely upon the size of the particles. For example, melting point, color, and reactivity remains constant in a bulk material, regardless of the size of the particles or the amount of bulk material present. Let’s use a specific example.

The element gold, Au, regardless of the amount present, if it is in the form of a bulk material and not in the form of tiny nanoparticles, it will have a melting point of 1064 degrees Celsius. It will be gold yellow in color and will be inert or unreactive. For nanoparticles, the properties depend on and are related to the size of the particles. Let’s take gold as an example again.

For gold nanoparticles, approximately 1.4 nanometers in size, the melting point is somewhere between 23 and 25 degrees Celsius. Gold nanoparticles are often dark red to black in color in solution depending on their size. And these nanoparticles are reactive and can also be used as catalysts.

The graph shows us visually how a property such as melting point depends on the particle radius. Though there are no values given, we can see a general trend. The property, in this case, melting point, is constant or almost constant for large particle sizes. And the property, in this case, melting point, changes greatly at very small particle sizes. Let’s briefly investigate particle size in terms of surface-area-to-volume ratio.

Let’s compare a large particle and a small particle. We will use cubes just to make it easier. The numbers represent the lengths of each side, and we will leave out the units for now. If we calculate the surface area of the large particle, there are six sides to the cube. The surface area of each side is length times breadth. Putting in the values, we get 150 units squared for the total surface area of the large particle. Doing the same for the small particle, we get a surface area of six units squared.

Let’s now calculate the volume of each particle. Volume is length times breadth times height. And putting in the values, for the large particle, we get a volume of 125 units cubed and for the small particle, one unit cubed. Let’s now put the surface area and volume for each into a ratio. And we get for the large particle 150 as to 125 and for the small particle six as to one. Simplifying, for the large particle, we get 1.2 as to one. And we can now compare the surface area to volume ratio for each particle.

We can see that, for the large particle, there is a small surface area as to volume ratio. We know this because there is very little difference in size between 1.2 and one. However, for the small particle, there is a large surface area as to volume. We know this because six is much bigger than one. It is this large or high surface area as to volume which give nanoparticles their very interesting properties.

And the reason why nanoparticles can be used as effective catalysts in very small quantities is answer (A) because they have a high surface-area-to-volume ratio.

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