Dissociative adsorption of CCl4 on the Fe3O4(111)-(2 x 2) selvedge of α-Fe2O3(0001)

The surface reactions of CCl4 with the Fe3O4(111)-(2 x 2) selvedge of naturally occurring alpha-Fe2O3(0001) singlecrystals have been investigated using synchrotron X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). CCl4 was found to dissociate on the Fe3O4 surface at 100 K producing chemisorbed Cl and adsorbed CCl2. TPD shows that the large majority of the dissociatively adsorbed CCl2 fragments extract lattice oxygen and desorb as phosgene at >275 K. However, the XPS spectra show no evidence for the formation of surface-bound phosgene, at 100 K, indicating that its formation involves two steps. The first step, dissociation, is spontaneous at 100 K, whereas the second, oxygen atom abstraction to form phosgene, requires thermal excitation. Cl chemisorption yielded two separate species, the mono- and dichloride terminations of surface iron sites. The identification of these two surface terminations is based on the coverage dependence and the surface temperature history of their Cl 2p(3/2) peak intensity. For example, heating to >450 K allows the monochloride to transform into iron dichloride, indicating Cl adatom mobility at these temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.