Preparation of a model Ziegler-Natta catalyst: Electron irradiation induced titanium chloride deposition on magnesium chloride thin films grown on gold

The preparation and surface science study of a model catalyst for the Ziegler-Natra polymerization is reported. Ultra-thin films of MgCl2 and TiCl4 are prepared by gas phase deposition on gold polycrystalline foil in ultra-high vacuum conditions and are characterized by AES, XPS, ISS and TPD. TiCl2 can be deposited on monolayer and multilayer MgCl2 films at 330 K only after the irradiation of the magnesium halide with 1 keV electrons. The electron bombardment is effective both in UHV prior to TiCl4 deposition and during deposition in a background pressure of TiCl4. The MgCl2 films are Cl terminated when deposited. Electron irradiation of MgCl2 induces its reduction to metallic Mg, with the enrichment of Mg atoms at the film surface. One monolayer of TiCl4 can be chemisorbed on this surface at 330 K. The chemisorbed TiCl4 reacts with the defect rich electron irradiated MgCl2 substrate, with the formation of a mixed titanium/magnesium chloride film of high thermal stability. The TiCl4 monolayer is chemisorbed on the mixed chloride surface with a heat of adsorption of 38 kcal/mol.