The surface structure of small metallic particles on supported catalysts is determined using weak-beam techniques. It is found that surfaces of the particles have a rough structure corresponding to incomplete layers and steps. This structure can be reproduced with a theoretical calculation of the particle structure assuming that the gross crystal structure of the particle is retained but it has one or more incomplete shells of atoms. The calculation enables us to determine the probability of various sites as a function of particle size. Sites of coordination from B2 to B6 are calculated. The results indicate that single-atom and B2 sites, which do not oscillate in population with increasing particle size, are most likely responsible for catalytic activity in hydrogenolysis. On the other hand, sites of higher coordination (B3-B6), which have an oscillatory behavior with size, are most likely responsible for isomerisation and particle size effects.
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