SO2 adsorption on rutile TiO2(110): An infrared reflection-absorption spectroscopy and density functional theory study

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The adsorption of SO2 on single crystalline TiO2(110) has been investigated by means of polarized infrared reflection-absorption spectroscopy (IRRAS) experiments and density functional theory (DFT) calculations. IR absorption bands were detected at 1324 cm−1 and 985 cm−1 with p-polarized light incident along both the [11¯0] and [001] crystallographic directions at 123 K. When the temperature was increased to 153 K, the peak at 1324 cm−1 disappears, while a new, weak band appears at 995 cm−1. Simultaneously, a band at 995 cm−1 also emerges with s-polarized light along the [11¯0] direction. Based on the symmetry properties of the IRRAS spectra and accompanying ab initio simulations of the spectra employing a three layer model (vacuum-adsorbate-substrate), it is shown that the low temperature absorption IRRAS bands can be attributed to an SO3-like adsorbate structure. This is also the most stable adsorption structure (Ead = −0.58 eV) on the stoichiometric surface. The combined IRRAS and DFT results show that the band appearing at 995 cm−1 is associated with a surface sulfite specie which is stabilized by residual surface water. The DFT calculations also revealed that a stable adsorption structure exists on a reduced TiO2 surface, where SO2 binds strongly to an oxygen vacancy site. It is suggested that this is an intermediate that form surface sulfate upon further reactions with water, although it was not observed on the stoichiometric surface studied in this work.