Surface termination dependence of the reactivity of single crystal hematite with CCl4

We describe ultrahigh vacuum Auger electron spectrometric measurements of the uptake of chlorine following the room temperature exposure of single crystal hematite, alpha-Fe2O3, to CCl4. We compare the surface chemistry of two specific surface phases formed on the basal plane of alpha-Fe2O3: the Fe3O4(111)-(2 x 2) "selvedge" and the alpha-Fe2O3/Fe1-xO "biphase." For Fe3O4 (111)-(2 x 2) an estimated saturation level of Cl of similar to75% of a monolayer is readily attained. Carbon uptake is well below that expected for simple stoichiometric dissociative chemisorption, consistent with desorption of organic products during the surface reaction. Low energy electron diffraction measurements suggest that, dependent upon preparation procedures, at least two types of alpha-Fe2O3/Fe1-xO biphase structures can be formed. Surprisingly, upon exposure to CCl4, Cl uptake does not occur on either of these biphase surfaces, despite the fact that these surfaces are thought to have the same surface concentrations of iron and oxygen as Fe3O4(111). The dramatic difference between the reactivity of the Fe3O4 and biphase surfaces suggests that the active site for the dissociative adsorption of CCl4 on Fe3O4(111)-(2 x 2) comprises both an iron cation and an oxygen anion with a surface-normal-oriented dangling bond that is uncapped by iron cations. Electron stimulated and thermal desorption of Cl from the saturated Fe3O4(111)-(2 x 2) selvedge is also reported. (C) 2002 Elsevier Science B.V. All rights reserved.