This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. pubs.acs.org/JACS Article Photoinduced Adsorption and Oxidation of SO2 on Anatase TiO2(101) ̈ David Langhammer, Jolla Kullgren, and Lars Osterlund* Cite This: J. Am. Chem. Soc. 2020, 142, 21767−21774 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information 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. fi Speci cally, the photoadsorption of SO2 on TiO2(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 TiO2,in fi 2− which sulfate forms as a stable surface product that is known to poison the catalytic surface. This work identi es surface-SO4 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 TiO2(101). Initially, the molecule binds to a lattice O2 ion through a photomediated adsorption process and forms surface fi 2− sul te, which is subsequently oxidized into surface-SO4 by transfer of a neutral oxygen from an adsorbed O2 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. ■ INTRODUCTION adsorption has gained attention recently because of the technologically important superwetting properties of UV Adsorption of molecules on surfaces is the fundamental step 13,14 Downloaded via 5.241.82.182 on January 4, 2021 at 18:05:30 (UTC). preluding all heterogeneous catalytic reactions. Such adsorp- irradiated TiO2 surfaces. tion processes are well understood during thermally activated Because photoadsorption plays an important role during 1−5 catalytic reactions, but corresponding insights into photo- photocatalytic reactions involving O2 and hydroxyls, it is activated adsorption processes are much less developed,1 albeit reasonable that it should also be important in other − experimentally well established.6 8 The dominant view of photostimulated surface reactions. During artificial photosyn- See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. photoadsorption on metal oxide photocatalysts is that thesis by CO2 reduction, adsorption of CO2 and subsequent − molecular oxygen or surface cations capture photoexcited formation of CO2 under photoillumination are considered to 15 electrons, while holes are captured by hydroxyls from be crucial reaction steps. On TiO2(001), CO2 forms surface- 2− 2− 16,17 dissociated water molecules or surface oxygen atoms. This CO3 by bonding to a lattice O ion, and it has been 2− separation of excited charges is crucial for the overall shown that an accumulation of CO3 species at the surface is ffi − fi photocatalytic e ciency, since recombination of electron associated with the deactivation of TiO2 during arti cial hole pairs otherwise occurs on a much faster time scale than photosynthesis, which typically occurs within a few hours.18 the surface chemical reactions.9 On titania (TiO ), which is by 2 Similarly, the TiO2 surface becomes deactivated during NOx far the most well-studied photocatalyst due to its large oxidation when surface-NO − (Ti−O−NO −) and/or ad- abundance and high stability, excited electrons are readily 3 2 captured by O2 molecules, while the holes are localized at terminal OH− groups that form upon dissociative adsorption Received: September 9, 2020 − • Published: December 17, 2020 of H2O. This leads to the formation of O2 and OH , which together have an oxidative potential exceeding 3 eV (290 kJ/ mol). In particular, photoadsorption of O2 has attracted considerable interest, stemming back to works by Bickley et 10,11 12 al. and Boonstra and Mutsaers, while H2O photo- © 2020 American Chemical Society https://dx.doi.org/10.1021/jacs.0c09683 21767 J. Am. Chem. Soc. 2020, 142, 21767−21774 Journal of the American Chemical Society pubs.acs.org/JACS Article − 19−21 37 sorbed NO3 (ads) is formed. During photocatalytic value used in this work has been suggested by Selloni et al. oxidation of SO2, 98% of the reacted SO2 molecules are based on a linear response analysis using a defective anatase fi converted into strongly bound sulfate species that deactivate TiO2(101) slab. To localize added electrons at speci c Ti sites, the surface completely in <30 min, such that the surface must the titanium atoms were slightly displaced prior to the ionic be regenerated either by heating above 400 °C or by dissolving relaxation, such that local potential energy minima were the sulfate ions in aqueous solutions.22 The latter approach is obtained after relaxation on Ti4+ ions, which thereby were the simplest and most effective (provided that the pH is kept reduced to Ti3+. The assignment of the Ti3+ ions was made neutral), although sulfur poisoning still remains a large issue based on their characteristic magnetization (close to unity) and 22,23 during oxidative desulfurization on TiO2. Several reaction by inspecting the spin-density charge distribution. The mechanisms have been suggested to explain the transformation titanium, oxygen, and sulfur atoms were treated by using 4, 24−27 of SO2 into sulfate on TiO2, including formation of 6, and 6 valence electrons, respectively, and pseudopotentials 2− surface-SO4 through interaction with lattice oxygen and have been generated by using the projected augmented wave 2− − formation of SO4 (ads) by reactions with surface hydroxyls. It (PAW) method to account for the valence core interactions, has also been suggested that oxygen vacancies are the main as suggested by Blöchl et al.38 A plane wave basis set with a reactive adsorption sites,28 although this assertion was shown kinetic energy cutoff of 546 eV has been used to model the to be incorrect in our previous study.29 In previous studies we electronic wave functions, and all calculations have been Γ have shown that SO2 does not adsorb in the absence of UV performed at the -point only inside the Brillouin zone. illumination on crystalline and stoichiometric TiO2 at room The anatase TiO2(101) surface was modeled by using a total temperature, employing identical particles as those used in this of 192 atoms in a slab consisting of 64 TiO2 units, building a 29,30 × work. These conclusions are qualitatively supported by the (2 3) supercell with four stoichiometric TiO2 layers. The work of Baltrusaitis et al.,24 showing much larger adsorption slabs were separated by a vacuum gap of more than 15 Å. Ionic capacity on small 4 nm TiO2 nanoparticles compared to 32 nm relaxations were performed until all forces were <0.01 eV/Å by nanocrystals. Therefore, SO2 adsorption on anatase TiO2 using a conjugant gradient algorithm. Vibrational frequencies provides a good example of how excited-state chemistries were calculated by using the finite differences method facilitate molecule adsorption. implemented in VASP, that is, by displacing a selected number In this work, we simulate the photocatalytic formation of of atoms in the direction of each Cartesian coordinate to 2− surface-SO4 on TiO2(101) by localizing negative charge determine the Hessian matrix (step size 0.015 Å). Only atoms around the SO2 adsorbate. This generally describes the that were part of the adsorbate and the surface atoms situation during interband absorption in strongly correlated coordinating with the adsorbate were allowed to move during metal oxides, where excited electrons usually localize on this displacement. The calculated frequencies have been shifted ν − ν surface metal cations. The localization of charge on Ti in TiO2 by 0.08 22.159, where is the wavenumber. This relation enables the formation of a strong covalent bond between sulfur was obtained by plotting the difference between calculated 2− 2− and lattice O , such that a surface-SO3 adsorbate is formed. frequencies of molecular SO2 and the experimental gas phase 2− This acts as an intermediate during formation surface-SO4 by frequencies as a function of the wavenumber. asubsequentoxidationinvolving an adsorbed oxygen molecule. This photoadsorption mechanism can be understood ■ EXPERIMENT as a concerted activation of lattice oxygen and neighboring In situ diffuse reflectance infrared Fourier transform spectroscopy titanium, which should be generally valid for other adsorbate (DRIFTS) measurements have been performed to monitor the photo- systems, such as H2O, CO2,NO2, and so on, as well as other oxidation of SO2 on anatase TiO2 nanoparticles and to study the ff transition d- or f-metal oxides. It is thus also relevant in studies e ect of a subsequent exposure to H2O. Measurements were − of geochemical reactions involving mineral dust particles.31 33 performed on a vacuum-pumped FTIR spectrometer (Vertex 80v, Bruker Optik GmbH, Ettlingen, Germany) equipped with a custom Importantly, our results present a simple mechanism by which fi photoadsorption processes can be simulated and quantified by modi ed Praying Mantis high-temperature reaction chamber (HVC- DRM-5, Harrick Inc., USA), which enabled gas flow through the using density functional theory (DFT). To add further support fi powder sample bed and simultaneous IR and UV illumination of the to these ndings, dedicated experiments have been performed sample during measurements.