Mechanism of the cyclo-oligomerisation of C2H2 on anatase TiO2 (101) and (001) surfaces and their reduction: An electron paramagnetic resonance and density functional theory study

Dehydroxylated, hydroxylated and hydrated anatase TiO2 samples have been exposed to acetylene at room temperature. The interaction leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and is accompanied by the appearance of Ti3+ ions, as shown by electron paramagnetic resonance (EPR) spectra. Fully or partly dehydroxylated samples show higher reactivity, whereas the hydrated samples are chemically inert. The experimental results point towards a crucial role of the more reactive (001) facets of anatase nanoparticles. Density functional theory calculations show that acetylene physisorbs on the anatase (101) surface without activation of the C-H bond. The reduced (101) surface (O vacancies) leads to acetylene activation but not to dissociative adsorption. In contrast, the dehydroxylated (001) anatase surface is very active and leads to the spontaneous splitting of the C-H bond with formation of Ti-C2H and OH groups. This is followed by subsequent additions of C2H2 molecules with formation of PAHs. During the dissociation of C2H2, radical species do not form and electrons are not transferred to the surface because direct Ti-C covalent bonds form on the surface. However, the ring closure in the formation of the aromatic compounds leaves behind hydrogen atoms that donate their valence electrons to the oxide. This results in the appearance of EPR-active Ti3+ centres. On the surface of things: Acetylene oligomerisation occurs on the surface of anatase TiO2 leading to the formation of polycyclic aromatic hydrocarbons and a reduced titania surface. The mechanism of the reaction has been elucidated by EPR measurements and DFT calculations (see figure).