If thiophenes and related compounds land on the basal plane of the catalyst at room temprerature or higher, they do not stick. Nevertheless, increasing the basal plane area while holding the platelet edge area fixed increases the activity of the catalyst. Why?
We proposed two new roles for mobile promoters, based on our low temperature STM measurements to explain this phenomenon.
Nickel atoms are freely mobile at 77K on the MoS2 terraces. They move so quickly that we cannot image them with our low temperature STM. At 4K, we can image the atoms but they are very easy to move. (For more information on manipulating atoms, see the Manipulation Page). These atoms are mobile because the bonding of the surface of the layered MoS2 is already saturated. When nucleophilic molecules are bound to the promoter atoms, they will become even more mobile.
This image was recorded by Jim Kushmerick.
We proposed new roles for promoter atoms in catalysis. For the nickel- and/or cobalt-promoted MoS2 hydrodesulfurization catalysts, they provide a means:
To enhance sticking -- The promoter atoms can capture the reactant molecules, whereas the terrace cannot.
To transport the reactants -- The promoter atoms mobilize the reactants and can carry them across the surface. Thus the reactants can sample a much greater fraction of the surface than if they simply landed at a sticky site and were immobilized there.
