Electron-Mediated CO Oxidation on the TiO2(110) Surface during Electronic Excitation

Spectroscopic and kinetic characterization of photogenerated charge carriers in photocatalysts

The catastrophic consequences of increased power consumption, such as drastically rising CO₂ levels, natural disasters, environmental pollution and dependence on fossil fuels supplied by countries with totalitarian regimes, illustrate the urge to develop sustainable technologies for energy generation. Photocatalysis presents eco-friendly means for fuels production via solar-to-chemical energy conversion. The conversion efficiency of a photocatalyst critically depends on charge carrier processes taking place in the ultrafast time regime. Transient absorption spectroscopy (TAS) serves as a perfect tool to track those processes. The spectral and kinetic characterization of charge carriers is indispensable for the elucidation of photocatalytic mechanisms and for the development of new materials. Hence, in this review, we will first present the basics of TAS and subsequently discuss the procedure required for the interpretation of the transient absorption spectra and transient kinetics. The discussion will include specific examples for charge carrier processes occurring in conventional and plasmonic semiconductors.

X-ray-Induced CO2 Formation via CO Reaction with TiO2 at Cryogenic Temperature

Cosmic rays, γ-rays, and X-ray-driven reactions on interstellar grains have been the main focus in understanding the extraterrestrial synthesis of complex organic molecules. Herein, using temperature-programmed desorption and X-ray photoelectron spectroscopy, we report for the first time X-ray-induced CO₂ production from a CO-covered rutile TiO₂(110) surface at 110 K, in addition to X-ray-induced CO desorption. The X-ray-induced CO₂ production was found to follow a Mars and van Krevelen (MvK) mechanism via the reaction between adsorbed CO and surface lattice oxygen of TiO₂. Defects of TiO₂ suppress the X-ray-induced CO₂ production but promote the X-ray-induced CO desorption. These findings suggest a novel X-ray-induced CO-to-CO₂ reaction on oxide surfaces at cryogenic temperature likely occurring in the interstellar medium and also warn us to keep in mind the possibility of X-rays inducing chemical reactions during structural characterizations of oxides with X-ray-involved techniques.

Surface chemistry of TiO2 connecting thermal catalysis and photocatalysis

Chemical reactions catalyzed under heterogeneous conditions have recently expanded rapidly from traditional thermal catalysis to photocatalysis due to the rising concerns about sustainable development of energy and the environment. Adsorption of reactants on catalyst surfaces, subsequent surface reactions, and desorption of products from catalyst surfaces occur in both thermal catalysis and photocatalysis. TiO₂ catalysts are widely used in thermal catalytic and photocatalytic reactions. Herein we review recent progress in surface chemistry, thermal catalysis and photocatalysis of TiO₂ model catalysts from single crystals to nanocrystals with the aim of examining if the surface chemistry of TiO₂ can bridge the fundamental understanding between thermal catalysis and photocatalysis. Following a brief introduction, the structures of major facets exposed on TiO₂ catalysts, including surface reconstructions and defects, as well as the electronic structure and charge properties, are firstly summarized; then the recent progress in adsorption, thermal chemistry and photochemistry of small molecules on TiO₂ single crystals and nanocrystals is comprehensively reviewed, focusing on manifesting the structure-(photo)activity relations and the commonalities/differences between thermal catalysis and photocatalysis; and finally concluding remarks and perspectives are given.

Introducing catalysis in photocatalysis: What can be understood from surface science studies of alcohol photoreforming on TiO2

Mechanisms in heterogeneous photocatalysis have traditionally been interpreted by the band-structure model and analogously to electrochemistry. This has led to the establishment of 'band-engineering' as a leading principle for the discovery of more efficient photocatalysts. In such a picture, mainly thermodynamic aspects are taken into account, while kinetics are often ignored. This holds in particular for chemical kinetics, which are, other than those for charge carrier dynamics, often not at all considered for the interpretation of the catalysts' photocatalytic performance. However, while being usually neglected in photocatalyis, they are a traditional and powerful tool in thermal catalysis and are still applied with great success in this field. While surface science studies made substantial contributes to thermal catalysis, analogous studies in heterogeneous photocatalysis still play only a minor role. In this review, the authors show that the photo-physics of defined materials in well-defined environments can be correlated with photochemical events on a surface, highlighting the importance of well-characterized semiconductors for the interpretation of mechanisms in heterogeneous photochemistry. The work focuses on contributions from surface science, which were obtained for the model system of a titania single crystal and alcohol photo-reforming. It is demonstrated that only surface science studies have so far enabled the elucidation of molecularly precise reaction mechanisms, the determination of reaction intermediates and assignment of reactive sites. As the identification of these properties remain major prerequisites for a breakthrough in photocatalysis research, the work also discusses the implications of the findings for applied systems. In general, the results from surface science demonstrate that photocatalytic systems shall also be approached by a perspective originating from heterogeneous catalysis rather than solely from an electrochemical point of view.

Modulation of the Reduction Potential of TiO2- x by Fluorination for Efficient and Selective CH4 Generation from CO2 Photoreduction

Photocatalytic reduction of CO₂ holds great promises for addressing both the environmental and energy issues that are facing the modern society. The major challenge of CO₂ photoreduction into fuels such as methane or methanol is the low yield and poor selectivity. Here, we report an effective strategy to enhance the reduction potential of photoexcited electrons by fluorination of mesoporous single crystals of reduced TiO_{2- x}. Density functional theory calculations and photoelectricity tests indicate that the Ti³⁺ impurity level is upswept by fluorination, owing to the built-in electric field constructed by the substitutional F that replaces surface oxygen vacancies, which leads to the enhanced reduction potential of photoexcited electrons. As a result, the fluorination of the reduced TiO_{2- x} dramatically increases the CH₄ production yield by 13 times from 0.125 to 1.63 μmol/g·h under solar light illumination with the CH₄ selectivity being improved from 25.7% to 85.8%. Our finding provides a metal-free strategy for the selective CH₄ generation from CO₂ photoreduction.

Adsorption and Photodesorption of CO from Charged Point Defects on TiO2(110)

The adsorption and photochemistry of CO on rutile TiO₂(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption, and angle-resolved photon-stimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (V_O). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (<0.02 eV). UV irradiation stimulates diffusion and desorption of CO at low temperature. CO photodesorbs primarily from the vacancies with a bimodal angular distribution, indicating some scattering from the surface, which also leads to photostimulated diffusion. Hydroxylation of V_O's does not significantly change the CO PSD yield or the angular distribution, which suggests that photodesorption can be initiated by recombination of photogenerated holes with excess electrons localized near the charged point defect (either V_O or bridging hydroxyl).

Scenarios of polaron-involved molecular adsorption on reduced TiO2(110) surfaces

The polaron introduced by the oxygen vacancy (Vo) dominates many surface adsorption processes and chemical reactions on reduced oxide surfaces. Based on IR spectra and DFT calculations of NO and CO adsorption, we gave two scenarios of polaron-involved molecular adsorption on reduced TiO₂(110) surfaces. For NO adsorption, the subsurface polaron electron transfers to a Ti:3d-NO:2p hybrid orbital mainly on NO, leading to the large redshifts of vibration frequencies of NO. For CO adsorption, the polaron only transfers to a Ti:3d state of the surface Ti_5c cation underneath CO, and thus only a weak shift of vibration frequency of CO was observed. These scenarios are determined by the energy-level matching between the polaron state and the LUMO of adsorbed molecules, which plays a crucial role in polaron-adsorbate interaction and related catalytic reactions on reduced oxide surfaces.

Liquid-Jet X-ray Photoelectron Spectra of TiO(2) Nanoparticles in an Aqueous Electrolyte Solution

Titania has attracted significant interest due to its broad catalytic applications, many of which involve titania nanoparticles in contact with aqueous electrolyte solutions. Understanding the titania nanoparticle/electrolyte interface is critical for the rational development of such systems. Here, we have employed liquid-jet ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the solid/electrolyte interface of 20 nm diameter TiO2 nanoparticles in 0.1 M aqueous nitric acid solution. The Ti 2p line shape and absolute binding energy reflect a fully oxidized stoichiometric titania lattice. Further, by increasing the X-ray excitation energy, the difference in O 1s binding energies between that of liquid water (O 1sliq) and the titania lattice (O 1slat) oxygen was measured as a function of probe depth into the particles. The titania lattice, O 1slat, binding energy decreases by 250 meV when probing from the particle surface into the bulk. This is interpreted as downward band bending at the interface.

Crystal-plane-dependent metal–support interaction in Au/TiO2

The metal–support interactions between Au and different TiO₂ crystal planes are investigated to identify their crystal-plane-dependent properties.

Multiple Nonthermal Reaction Steps for the Photooxidation of CO to CO2 on Reduced TiO2(110)

The photooxidation of CO on reduced, rutile TiO2(110) is studied on a millisecond time scale. For CO coadsorbed with a saturation coverage of chemisorbed O2 (θsat), the CO2 photon-stimulated desorption (PSD) signal is initially zero, increases to a maximum after several tens of milliseconds, and then decreases at longer times. The initial CO2 PSD signal increases ∼5 times more quickly for an oxygen coverage of 0.5θsat. The initial rate of increase of the CO2 PSD signal is proportional to the flux of UV photons. The results show that two or more nonthermal reaction steps are required to photooxidize CO adsorbed on TiO2(110). The intermediate species involved in the reactions is stable for at least 100 s at 30 K. Previous models had suggested that CO photooxidation required only one nonthermal reaction. The likely initial and final charge states of the system suggest that an electron-mediated reaction and a hole-mediated reaction are needed for the complete photooxidation reaction.