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A metal film catalyst is a well-defined model system, which can be used to study the quantum size effects in catalysis. In this 2D or quasi-2D catalyst system, there is one dimension falling in the nano regime such that extraordinary catalytic properties may result from reduction in the size of this dimension. Here, we studied the surface reactivity of 2D Ag catalyst surfaces and found exceptionally low surface reactivity of a monolayer Ag film to CCl4 in comparison to that of the bulk Ag.
A well-defined monolayer Ag film with the superstructure of Ag/Si(111)-(√3×√3)R30° has been deposited onto the Si(111)-7×7 surface and characterized by STM. The surface reactivity of the monolayer Ag surface was probed using CCl4 molecules. XPS, UPS, STM, and PEEM results show that the monolayer Ag surface is quite inert to this molecule. CCl4 adsorbs on the surface molecularly at 110 K, which completely desorbs at 150 K. In contrast, the bulk Ag surfaces are very reactive to CCl4. Both molecular and dissociative adsorption of CCl4 on bulk Ag surfaces at 110 K have been observed. Furthermore, CCl4 adsorption on the monolayer Ag surface and the bulk Ag surface has been in-situ imaged by PEEM, which clearly shows the significant difference in the surface reactivity. UPS and XPS data demonstrate that the electronic structure of the two surfaces is different to each other. The different surface reactivity may be attributed to the variation in the Ag surface electronic structure which is strongly tuned by the Ag film thickness. |