CCl4 chemistry on the magnetite selvedge of single-crystal hematite: competitive surface reactions

Temperature programmed reaction/desorption (TPR/D) studies were undertaken to characterize the surface chemistry which occurs between CCl4 and the Fe3O4 (1 1 1) selvedge of single crystal α-Fe2O3 (0 0 0 1). Six separate desorption events are clearly observed and four desorbing species are identified: CCl4, OCCl2, C2Cl4 and FeCl2. It is proposed that OCCl2, CCl4 and C2Cl4 are produced in reactions involving the same precursor, CCl2. Three reaction paths compete for the CCl2 precursor: oxygen atom abstraction (for OCCl2), molecular recombinative desorption (for CCl4) and associative desorption (for C2Cl4). During the TPR/D temperature ramp, the branching ratio is observed to depend upon temperature and the availability of reactive sites. The data are consistent with a rich site-dependent chemistry.