Reducing the size of a particle increases the ratio of surface area to mass . Because the reactive portion of the particle is at its surface, increasing the relative surface area will increase reactivity for a given amount of material To illustrate, consider a spherical particle 0 . 1 millimeter (mm) in diameter. Its surface area is 3 x 10-8 m2 . If technicians mill the same mass of material into 100-nm-sized spheres, then the total surface area increases to 3 x 10-5 m2 . Decreasing the diameter of the particle by a factor of 1000 increases the surface area by a factor of 1000. If reactive sites cover the surface of the particle, then — all else being equal — this decrease in particle size would increase reactivity substantially This effect accounts for the increased efficiency of nanoscale catalysts compared to their bulk counterparts
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