On the mechanism of sulfur exchange

The most general observation is to be made on the basis of the tracer studies that only a part of surface sulfur is mobile or exchangeable on catalysts with sulfides and oxides. Supported Co and Ni contain only mobile sulfur, if sulfided by thiophene and contain both mobile and immobile sulfur, if they are sulfided by H2S/H2.[ 1 This is interpreted with the differences in the metal-sulfur bond strengths, as the edge S-atoms are held more weakly than the top S-atoms of the slab.I This follows from the results of some radiosulfur tracer studies of metal single crystals that different surface S-species are formed at different surface sulfur concentrations, and on different crystal faces of Pt, and 

Note that the surface monolayers of different structures PtS2 or PtS are formed in different experimental conditions, as witnessed by studies with sulfur-35. 

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