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.

The adsorption and photochemical properties of reduced and stoichiometric anatase TiO 2 nanoparticles, prepared by annealing in vacuum and air, respectively, at different temperatures up to 500 °C and 2 days have been investigated. Combined X-ray diffraction and Raman spectroscopy results suggest that vacuum annealing leads to a defective, oxygen vacancy rich surface region with an accompanying decrease of the crystalline core. The surface chemical properties of the reduced and calcined TiO 2 nanoparticles were studied by means of SO 2 adsorption measured by in situ diffuse reflectance Fourier transform spectroscopy. On pristine TiO 2 nanoparticles SO 2 adsorption leads to a broad absorbance band centered at 1140 cm −1 . In contrast, SO 2 does not adsorb on stoichiometric TiO 2 obtained after long-term annealing in air at 500 °C. However, after the same heat treatment in vacuum, SO 2 is shown to bind strongly on well-defined adsorption sites associated with a narrow absorbance band at 1150 cm −1 . The increased adsorption on reduced TiO 2 is attributed to formation of subsurface oxygen vacancies and reactive Ti 3+ species at the surface that promote SO 2 bonding. A surface-sulfite species (HSO 3 − ) was identified as the major adsorbate on both the as-prepared and the vacuum-annealed sample, and a formation mechanism involving reaction with hydrogen from surface hydroxyl groups is proposed. During UV illumination, SO 2 is photoadsorbed on TiO 2 during SO 2 exposure in an inert He gas atmosphere. In contrast to dark SO 2 adsorption, this reaction does not involve surface defects, since the concentration of photoadsorbed SO 2 did not significantly change on the deeply reduced TiO 2 nanoparticles. On the basis of these findings, a new mechanism for the formation of surface-bound SO 3 2− during UV illumination on the stoichiometric surface is proposed, which should be generally applicable for other similar adsorbates and semiconducting oxides.

The catalytic and photon-induced oxidation of NO 2 on anatase TiO 2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO 3 . Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface−NO 3 − species, where NO 2 bonds to the lattice oxygen, and freely adsorbed NO 3 − ions. These products can be obtained either by dissociative adsorption of HNO 3 or by catalytic/photocatalytic oxidation of NO 2 , which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface−NO 3 − species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N 2 O 5 ) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO 2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO 2 crystal. In particular, a stable surface−NO 3 − and NO 3 − ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO 2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO 2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N 2 O during continued UV light illumination.

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