TPD characterization of surface adsorption sites of model Ziegler-Natta polymerization catalysts

The surface adsorption sites of titanium chloride films that can be used as model Ziegler-Natta polymerization catalysts have been investigated with temperature-programmed desorption (TPD) of various probe molecules. The adsorption sites can be differentiated depending on the heat of desorption of the molecule. Two titanium chloride films were produced by electron-induced chemical vapor deposition of TiCl₄ and co-deposition of Mg and TiCl₄ on gold substrates. The surfaces of both films were chlorine-terminated. 1,3,5-Trimethylbenzene (mesitylene) was found to be an effective and non-destructive molecule for TPD characterization of adsorption sites on the model catalysts. The film produced by the electron-induced chemical vapor deposition had a distribution of adsorption sites that were not thermodynamically stable and subject to high-temperature annealing, while the film prepared by the co-deposition had a stable surface structure that was composed of domains of the MgCl₂(001) basal plane structure.