Catalytic Descriptors for the Design of Ziegler–Natta Catalysts Revealed by the Investigation of the Cl–Ti(0001) Interaction by Density of States Calculations

• Published in: Journal of physical chemistry. C Vol. 121; no. 38; pp. 20871 - 20876
• Main Authors: Symianakis, Emmanouil, Karakalos, Stavros, Ladas, Spyridon
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28-09-2017
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b06980

The deposition of MgCl2 on Ti(0001) followed by annealing above 450 °C has been previously reported to result in the desorption of the Mg atoms, the formation of a Cl/Ti(0001) interface, and a new contribution at binding energy (BE) 3.5 eV. We present density of states results from DFT calculations obtained for the Cl/Ti(0001) interface and compare them with reported spectra obtained by photoelectron spectroscopy with synchrotron radiation. Our results suggest that the new contribution at BE 3.5 eV is a Cl 3p peak originating from Cl atoms relaxing at long distances from the surface, whereas the Cl 3p peak at BE 7.0 eV should be attributed only to Cl atoms strongly adsorbed at either one of the hollow sites of the Ti(0001) surface. Our calculations also support an interpretation for published experimental observations on the valence band of TiCl4 adsorbed on a Au surface. The comparisons of DFT calculations with experimental results highlight significant features of the Ti–Cl interaction at distances longer than the usual Ti–Cl bond, the high sensitivity of the BE of Cl 3s and Cl 3p peaks to the distance between Ti and Cl, as well as the influence of the interaction of the Cl atom with the substrate on the Ti–Cl bond and consequently on the Ti–C bond which determines the olefin polymerization. These observations provide an explanation for many features of the Ziegler–Natta heterogeneous catalysts, identify two catalytic descriptors, Ti–Cl bond length and the BE of the Cl 3p peak, and provide the basis for the development of future Ziegler–Natta catalytic systems engineered at the atomic level.