Photoinduced Adsorption and Oxidation of SO₂ on Anatase TiO₂(101)

Abstract

Adsorption of molecules is a fundamental step in all heterogeneous catalytic reactions. Nevertheless, the basic mechanism by which photon-mediated adsorption processes occur on solid surfaces is poorly understood, mainly because they involve excited catalyst states that complicate the analysis. Here we demonstrate a method by which density functional theory (DFT) can be used to quantify photoinduced adsorption processes on transition metal oxides and reveal the fundamental nature of these reactions. Specifically, the photoadsorption of SO 2 on TiO 2 (101) has been investigated by using a combination of DFT and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The combined experimental and theoretical approach gives a detailed description of the photocatalytic desulfurization process on TiO 2 , in which sulfate forms as a stable surface product that is known to poison the catalytic surface. This work identifies surface-SO 4 2– as the sulfate species responsible for the surface poisoning and shows how this product can be obtained from a stepwise oxidation of SO 2 on TiO 2 (101). Initially, the molecule binds to a lattice O 2 – ion through a photomediated adsorption process and forms surface sulfite, which is subsequently oxidized into surface-SO 4 2– by transfer of a neutral oxygen from an adsorbed O 2 molecule. The work further explains how the infrared spectra associated with this oxidation product change during interactions with water and surface hydroxyl groups, which can be used as fingerprints for the surface reactions. The approach outlined here can be generalized to other photo- and electrocatalytic transition metal oxide systems.

Quantum Chemical Investigation of Photocatalytical Sulfoxidation of Hydrocarbons on TiO2

“…Besides this, adsorption forms a1, r2, and r3 can be interpreted as a SO 3 species that is formed on the surface. In the literature, it is assumed that SO 2 forms anionic species such as SO 3 – , SO 3 2– , or SO 4 2– on TiO 2 with br-O atoms. ,, By examining the crystalline orbitals at the Γ-point, an interpretation of the chemical interaction between SO 2 and the surfaces can be made. Figure illustrates the important crystalline orbital near the Fermi energy of adsorbed SO 2 on anatase[101] (a1) and on rutile[110] (r2) in comparison to the highest occupied molecular orbital (HOMO) of SO 3 2– in the gas phase.…”

“…In the literature, it is assumed that SO 2 forms anionic species such as SO 3 − , SO 3 2− , or SO 4 2− on TiO 2 with br-O atoms. 7,11,12 By examining the crystalline orbitals at the Γ-point, an interpretation of the chemical interaction between SO 2 and the surfaces can be made. Figure 5 When investigating the density of states (DOS), hybridization of SO 2 orbitals with br-O orbitals on both surfaces can be found (Figure 6b,d).…”

“…The results from the literature are therefore in excellent agreement with our calculations. 7,11,12 Additionally, the gas phase excitation of SO 2 into a triplet state is blocked by the hybridization process because the LUMO of SO 2 , where an electron is excited into, is filled with electrons of the surface. In order to understand the excitation mechanism proposed by Parrino et al, the electronic structure of the system SO 2 −TiO 2 was investigated in more detail.…”

“…The adsorption of SO 2 on TiO 2 has gained renewed interest in recent years as this process not only plays a major role in the production of sulfuric acid but also may reduce SO 2 as a source of pollutants in the environment. − In this context, the rutile modification of TiO 2 has been studied more intensively in comparison to the photocatalytically more active anatase modification . It was proposed that SO 2 adsorbs on the surface in the form of an SO 3 -like species by interacting with the oxygen of TiO 2 . , Latest results for the anatase modification also suggest a similar picture . However, the chemical properties, which may change during adsorption, and the direct comparison of both modifications were investigated in less detail.…”

“…7,11 Latest results for the anatase modification also suggest a similar picture. 12 However, the chemical properties, which may change during adsorption, and the direct comparison of both modifications were investigated in less detail. The photocatalytic excitation of SO 2 and the reaction with alkanes have also been described only sparsely to date.…”