CCl4 chemistry on the reduced selvedge of a α-Fe2O3(0 0 0 1) surface: a scanning tunneling microscopy study

Scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) were used to study the degradation of CCl4 on the reduced selvedge of a natural single crystal α-Fe2O3(0 0 0 1) surface in ultrahigh vacuum. Before exposure to CCl4, STM images indicate that approximately 85% of the reduced surface exhibits a Fe3O4(1 1 1) 2 × 2 termination, while the remaining 15% is terminated by 1 × 1 and superstructure phases. Images obtained after room temperature dosing with CCl4 and subsequent flashing to 600 K reveal that chlorine atoms are adsorbed only on surface regions with the Fe3O4(1 1 1) 2 × 2 termination, not on 1 × 1 and superstructure regions. Chlorine atoms from dissociative adsorption of CCl4 are observed to occupy two distinct positions located atop lattice protrusions and in threefold oxygen vacancy sites. However, in companion chemical labeling experiments, chlorine atoms provided by room temperature, dissociative Cl2 adsorption on this surface are found to occupy sites atop lattice protrusions exclusively. The clear dissimilarity in STM feature shape and brightness at the two distinct chlorine adsorption sites arising from CCl4 dissociation as well as the results of the Cl2 chemical labeling experiments are best explained via reactions on a Fe3O4(1 1 1) 2 × 2 selvedge terminated by a 1/4 monolayer of tetrahedrally coordinated iron atoms. On this surface, adsorption atop an iron atom occurs for both the CCl4 and Cl2 dissociative reactions. A second adsorption site, assigned as binding to second layer iron atoms left exposed following surface oxygen atom abstraction resulting in the formation of phosgene (COCl2), only appears in the case of reaction with CCl4. The reaction mechanism and active site requirements for CCl4 degradation on iron oxide surfaces are discussed in light of this evidence and in the context of our previously reported results from Auger electron spectroscopy (AES), LEED, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy studies.