A comparative XPS study of the MgCl2/SiO2/Si interface prepared under UHV conditions and by different organic solvents

Silicon oxide in combination with magnesium chloride is used as a support for the industrial preparation of some types of Ziegler-Natta catalysts. These catalysts are the best candidates for the low pressure production of linear low-density polyethylene and ethylene co-polymers from the gas phase, with improved optical properties in the film form. By many authors silica is believed to act as an inert support, where MgCl 2 simply covers its outer surface while the active species of silica do not affect the catalysts active sites for polymerization [1]. On the other hand some consider that silica behaves as a normal solid state reactant with high surface area. In other words, the active sites into the pores of each particle are accessible for reaction with precursor solutions of the reactants. The usual industrial way of preparation of this type of support is by impregnation of precursor organo-metallic MgCl 2 compounds on high surface area (300-700 m 2 g-1) silica. The study of this system at a molecular level is a particularly difficult task due to the sensitivity of magnesium chloride to oxygen and humidity. Furthermore, although surface sensitive spectroscopies can provide significant information on the nature of surface species, the surface composition and structure, the poor conductivity of both SiO 2 and MgCl 2 in combination with the porous nature of the former make surface analysis rather complicated. In the preset work the interaction between MgCl 2 and SiO 2 is investigated by X-ray Photoelectron Spectroscopy. The porous silica substrate is replaced by a " model " flat support that is electrically conductive providing a surface science compatible " realistic " system. For this purpose a Si(100) wafer is used on which a thin SiO 2 layer is grown. The oxide layer is chosen to be thicker than 3 nm to resemble the chemical properties of the real oxide [2] and thin enough to avoid electrostatic charging during XPS measurements. For the purposes of the present study MgCl 2 is applied on this support in three different ways: i) in-situ by evaporation under UHV conditions, ii) ex-situ by drop-casting from a sparse (0.1M) tetrahydrofuran (THF) solution and iii) ex-situ by drop-casting from a dense (1 M) ethanol solution. The Si2p, O1s, C1s, Mg2p and Cl2p XPS peaks and Mg(KLL) XAES peak were recorded before and after deposition. All samples where annealed at 723 K for further investigation. As shown in Table …