Scanning Tunneling Microscopy and Theoretical Study of Competitive Reactions in the Dissociative Chemisorption of CCl4 on Iron Oxide Surfaces

Abstract

Variable temperature scanning tunneling microscopy (VT STM) and theoretical ab initio computer simulations were used to study dissociative chemisorption and competitive surface chemistry of CCl4 on an iron-terminated Fe3O4 (111) 2 × 2 surface in an ultrahigh vacuum. The Fe3O4 (111) surface was exposed to CCl4 molecules at 224 K, slowly annealed to 500 K (0.2 K/s), and scanned at room temperature. Two different chlorine species were observed only on the iron-terminated Fe3O4 (111) 2 × 2 surface due to chemisorption of CCl4, one on top of surface-terminating iron atoms (Cl bound to surface irons) and the other at 3-fold oxygen vacancy sites (Cl bound to subsurface irons). The ratio of the number of chlorine species on top sites to the number at 3-fold oxygen vacancy sites is approximately 9:1, which is dramatically different from the 1:10 ratio observed when CCl4 is dosed at room temperature. The difference in the ratio of these two chlorine species can be explained with a competitive surface reaction picture in which phosgene evolution/surface oxygen atom abstraction, leading to chlorine species at 3-fold oxygen vacancy sites, can only favorably compete with recombination and association reactions, leading to chlorine atoms on top sites, near room temperature. Theoretical calculations were performed that predict an activation barrier of 0.16 eV for the production of phosgene from CCl4 reacting with the iron-terminated Fe3O4 (111) 2 × 2 surface.