A Theoretical Investigation of the Thermal and Photochemical Mechanisms of Ethylbenzene Dehydrogenation on Rutile TiO$_{2}$(110)

TL;DR

This thesis uses advanced quantum chemical methods to explore how ethylbenzene dehydrogenates on TiO2 surfaces via thermal and photochemical pathways, revealing the influence of surface oxidation and photon energy on reaction mechanisms.

Contribution

It introduces a dual-methodological quantum approach combining DFT and multi-reference methods to elucidate complex surface reaction mechanisms and the effects of surface oxidation and photon wavelength.

Findings

Both thermal and photochemical pathways are dominated by proton-coupled electron transfer on stoichiometric surfaces.

Wavelength dependence affects the reaction pathway, with 257 nm enabling excited state reactions bypassing ground state barriers.

Surface oxidation shifts the mechanism to more efficient hydrogen atom transfer via oxygen radicals.

Abstract

This master's thesis investigates the thermal and photochemical dehydrogenation of ethylbenzene (EB) to styrene on the rutile TiO$_{2}$(110) surface. A dual-methodological quantum chemical approach is used for this investigation. While industrial styrene production is energy-intensive, photocatalysis on semiconductor materials offers a promising alternative under significantly milder conditions. To elucidate the underlying mechanisms, this study employs density functional theory (DFT-PBE-D3) for geometry optimization and high-level multi-reference methods (SA-CASSCF) to accurately describe the electronic complexity of excited states and radical intermediates. The investigation reveals that, on the stoichiometric surface, both thermal and photochemical pathways are dominated by proton-coupled electron transfer (PCET). The wavelength dependence observed in the literature is explained by how the system navigates electronic manifolds. 343 nm irradiation leads to rapid relaxation into the ground state, where high kinetic barriers persist. In contrast, 257 nm excitation enables the system to persist in higher excited states (S1/T2). This allows the reaction to bypass the rate-determining ground-state barrier. Furthermore, the study demonstrates that surface oxidation causes a fundamental mechanistic shift. On oxidized surfaces, pre-adsorbed oxygen radicals (O$_{Ti}$) enable direct hydrogen atom transfer (HAT), which is more efficient than PCET on reduced surfaces. This "hydrogen scavenger" effect explains the significant increase in styrene yield. This work underscores the necessity of multi-reference treatments for complex surface reactions and provides a fundamental understanding of how surface stoichiometry and photon energy govern photocatalytic efficiency.