Photoinduced\nAdsorption and Oxidation of SO₂ on Anatase TiO₂(101)

Abstract

Adsorption of molecules is a fundamental\nstep in all heterogeneous\ncatalytic reactions. Nevertheless, the basic mechanism by which photon-mediated\nadsorption processes occur on solid surfaces is poorly understood,\nmainly because they involve excited catalyst states that complicate\nthe analysis. Here we demonstrate a method by which density functional\ntheory (DFT) can be used to quantify photoinduced adsorption processes\non transition metal oxides and reveal the fundamental nature of these\nreactions. Specifically, the photoadsorption of SO₂ on\nTiO₂(101) has been investigated by using a combination\nof DFT and in situ diffuse reflectance infrared Fourier transform\nspectroscopy (DRIFTS). The combined experimental and theoretical approach\ngives a detailed description of the photocatalytic desulfurization\nprocess on TiO₂, in which sulfate forms as a stable surface\nproduct that is known to poison the catalytic surface. This work identifies\nsurface-SO₄^2– as the sulfate species responsible for the surface poisoning and\nshows how this product can be obtained from a stepwise oxidation of\nSO₂ on TiO₂(101). Initially, the molecule binds\nto a lattice O₂^– ion through a photomediated adsorption process and\nforms surface sulfite, which is subsequently oxidized into surface-SO₄^2– by\ntransfer of a neutral oxygen from an adsorbed O₂ molecule.\nThe work further explains how the infrared spectra associated with\nthis oxidation product change during interactions with water and surface\nhydroxyl groups, which can be used as fingerprints for the surface\nreactions. The approach outlined here can be generalized to other\nphoto- and electrocatalytic transition metal oxide systems.