Spectroscopic and DFT computational study of TiO2 catalyst deactivation in propylene selective oxidation


Simon Podkolzin, Simon.Podkolzin@Stevens.edu, Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030 and T. A. Nijhuis, t.a.nijhuis@tue.nl, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, Helix, STW 1.26, Eindhoven, 5600 MB, Netherlands.
Density Functional Theory (DFT) calculations and in-situ infrared (IR) spectroscopic measurements were used to study deactivation of Au/TiO2 catalysts in selective propylene oxidation to propylene oxide. Surface models of TiO2 for DFT calculations were chosen based on an agreement between predicted and experimental IR frequencies for adsorbed propylene oxide. The calibrated DFT models were then used to evaluate energetics of alternative reaction pathways for C3 surface species and to propose a mechanism that is consistent with time evolution of IR spectra under reaction conditions. Titania-based catalysts are widely used in oxidation reactions, and it is usually assumed that their deactivation is due to hydrocarbon oligomerization and coking. Our results suggest that it is not the case, at least for the studied reaction. In our proposed mechanism, deactivation proceeds through decomposition of C3 species into stable oxidized C1 and C2 species that cannot easily desorb from the surface. In addition, our mechanism suggests that this decomposition of C3 species requires adjacent Ti surface sites and, therefore, provides an explanation for significantly slower deactivation for materials with isolated Ti sites.