Abstract
Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photo-activated dehydrogenation of ethanol on reduced and oxidized rutile TiO2(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO2.
Highlights
Catalysis and photo-catalysis are fields of paramount importance both with a view on the demands on the chemical industry and the challenges in future renewable energy generation as well as to sustain our environment
A 2PPE study revealed the formation of CH3CHO and reports on a photo-induced excited state ~2.4 eV above EF for EtOH/TiO2(110) that was associated with the dissociation of EtOH on 5f-Ti sites[36]
We studied the interaction of EtOHTi and EtOTi species with a stoichiometric supercell without an OHTi group, which is denoted [TiO2(110)]0 (Fig. 6d–f). This was done to simulate the thermally activated dehydrogenation reaction as opposed to the photo-catalytic reaction. For this supercell we found that the reaction of EtOTi to CH3CHOTi is hindered by a high barrier, ~1.42 eV [cf
Summary
Catalysis and photo-catalysis are fields of paramount importance both with a view on the demands on the chemical industry and the challenges in future renewable energy generation as well as to sustain our environment. EtOH is an essential solvent and it could be used as feedstock in a possible green chemistry in the future[7,8,9] These expectations, and the fact that alcohols serve as model molecules of catalytic and photo-catalytic processes[4,10,11,12], have stimulated considerable research efforts towards the thermal and photo-catalytic oxidation of EtOH on TiO2. Following cycles of Ar+ sputtering and vacuum-annealing, the TiO2(110) crystals are reduced, leading to the creation of bulk defects and Obr vacancies on the surface[1,10,12,40,41] This leads to changes in electronic properties, where the empty Ti3d orbitals become populated, leading to a state within the ~3.1 eV wide band gap ~0.85 eV below the Fermi level (EF)[1,40,41,42]. Applying pump–probe laser ionization techniques, it has been suggested that, in the presence of oxygen, methyl radicals are produced during the photo-catalytic oxidation of EtOH37
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