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

Abstract The surface reactions of CCl 4 with the Fe 3 O 4 (1 1 1)-(2×2) selvedge of naturally occurring α-Fe 2 O 3 (0 0 0 1) single-crystals have been investigated using synchrotron X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). CCl 4 was found to dissociate on the Fe 3 O 4 surface at 100 K producing chemisorbed Cl and adsorbed CCl 2 . TPD shows that the large majority of the dissociatively adsorbed CCl 2 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.