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From the gas kinetic theory, for a 1 atmosphere pressure and at 300K, and for a surface of 1 cm2 there are 2.9 1023 collisions of molecules every second.
(Garbor, A. Somorjai: Introduction to Surface
Chemistry and Catalysis, Wiley, New York 1994, French version:
Chimie des surfaces et catalyse Trad. Marie-Paul Delplancke, Ediscience,
Paris 1995).
Let's imagine a 28 cm size cubic volume (22.4 liters)
and somewhere onto one side, a small 10x10Å catalyst surface
of 100Å2.
This will result to a number of 2,9 109
collisions/second for this small surface.