Origin of Coverage Dependence in Photoreactivity of Carboxylate on TiO₂(110): Hindering by Charged Coadsorbed Hydroxyls

The influence of reactant coverage on photochemical activity was explored using scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We observed diminished reactivity of carboxylate species (trimethyl acetate, TMA) on TiO₂(110) as a function of increasing coverage. This effect was not linked to intermolecular interactions of TMA but to the accumulation of the coadsorbed bridging hydroxyls (HO_b) deposited during (thermal) dissociative adsorption of the parent, trimethylacetic acid (TMAA). Confirmation of the hindering influence of HO_b groups was obtained by the observation that HO_b species originated from H₂O dissociation at O-vacancy sites have a similar hindering effect on TMA photochemistry. Though HO_b’s are photoinactive on TiO₂(110) under ultrahigh vacuum conditions, UPS results show that these sites trap photoexcited electrons, which in turn likely (electrostatically) attract and neutralize photoexcited holes, thus suppressing the hole-mediated photoreactivity of TMA. This negative influence of surface hydroxyls on hole-mediated photochemistry is likely a major factor in other anaerobic photochemical processes on reducible oxide surfaces

Direct Observation of Site-Specific Molecular Chemisorption of O₂ on TiO₂(110)

Molecularly chemisorbed O₂ species were directly imaged on reduced TiO₂(110) at 50 K with high-resolution scanning tunneling microscopy (STM). Two different O₂ adsorption channels, one at bridging oxygen vacancies (V_O) and another at 5-fold coordinated terminal titanium atoms (Ti_5c), have been identified. While O₂ species at the Ti_5c site appears as a single protrusion centered on the Ti_5c row, the O₂ at V_O manifests itself by a disappearance of the V_O feature. It is found that the STM tip can easily dissociate O₂ species, unless extremely low magnitude of the tunneling parameters are used. The O₂ molecules chemisorbed at low temperatures at these two distinct sites are the most likely precursors for the two O₂ dissociation channels, observed at temperatures above 150 and 230 K at the V_O and Ti_5c sites, respectively

Characterization of the Active Surface Species Responsible for UV-Induced Desorption of O₂ from the Rutile TiO₂(110) Surface

We have examined the chemical and photochemical properties of molecular oxygen on the (110) surface of rutile TiO₂ at 100 K using electron energy loss spectroscopy (EELS), photon stimulated desorption (PSD), and scanning tunneling microscopy (STM). Oxygen chemisorbs on the TiO₂(110) surface at 100 K through charge transfer from surface Ti³⁺ sites. The charge-transfer process is evident in EELS by a decrease in the intensity of the Ti³⁺ d-to-d transition at ∼0.9 eV and formation of a new loss at ∼2.8 eV. On the basis of comparisons with the available homogeneous and heterogeneous literature for complexed/adsorbed O₂, the species responsible for the 2.8 eV peak can be assigned to a surface peroxo (O₂^2–) state of O₂. This species was identified as the active form of adsorbed O₂ on TiO₂(110) for PSD. The adsorption site of this peroxo species was assigned to that of a regular five-coordinated Ti⁴⁺ (Ti_5c) site based on comparisons between the UV exposure-dependent behavior of O₂ in STM, PSD, and EELS data. Assignment of the active form of adsorbed O₂ to a peroxo species at normal Ti_5c sites necessitates reevaluation of the simple mechanism in which a single valence band hole neutralizes a singly charged O₂ species (superoxo or O₂^–), leading to desorption of O₂ from a physisorbed potential energy surface

Diffusion and Photon-Stimulated Desorption of CO on TiO₂(110)

Thermal diffusion of CO adsorbed on rutile TiO₂(110) was studied in the 20–110 K range using photon-stimulated desorption (PSD), temperature-programmed desorption (TPD), and scanning tunneling microscopy. During UV irradiation, CO desorbs from certain photoactive sites (e.g., oxygen vacancies). This phenomenon was exploited to study CO thermal diffusion in three steps: first, empty these sites during a first irradiation cycle, then replenish them with CO during annealing, and finally probe the active site occupancy in the second PSD cycle. The PSD and TPD experiments show that the CO diffusion rate correlates with the CO adsorption energystronger binding corresponds to slower diffusion. Increasing the CO coverage from 0.06 to 0.44 monolayer (ML) or hydroxylation of the surface decreases the CO binding and increases the CO diffusion rate. Relative to the reduced surface, the CO adsorption energy increases and the diffusion decreases on the oxidized surface. The CO diffusion kinetics can be modeled satisfactorily as an Arrhenius process with a “normal” prefactor (i.e., ν = 10¹² s^–1) and a Gaussian distribution of activation energies where the peak of the distribution is ∼0.26 eV and the full width at half-maximum (fwhm) is ∼0.1 eV at the lowest coverage. The observations are consistent with a significant electrostatic component of the CO binding energy on the TiO₂(110) surface which is affected by changes in the surface dipole and dipole–dipole interactions

Deprotonated Water Dimers: The Building Blocks of Segmented Water Chains on Rutile RuO₂(110)

Despite the importance of RuO₂ in photocatalytic water splitting and catalysis in general, the interactions of water with even its most stable (110) surface are not well understood. In this study we employ a combination of high-resolution scanning tunneling microscopy imaging with density functional theory based <i>ab initio</i> molecular dynamics, and we follow the formation and binding of linear water clusters on coordinatively unsaturated ruthenium rows. We find that clusters of all sizes (dimers, trimers, tetramers, extended chains) are stabilized by donating one proton per every two water molecules to the surface bridge bonded oxygen sites, in contrast with water monomers that do not show a significant propensity for dissociation. The clusters with odd number of water molecules are less stable than the clusters with even number and are generally not observed under thermal equilibrium. For all clusters with even numbers, the dissociated dimers represent the fundamental building blocks with strong intradimer hydrogen bonds and only very weak interdimer interactions resulting in segmented water chains

Light Makes a Surface Banana-Bond Split: Photodesorption of Molecular Hydrogen from RuO₂(110)

The coordination of H₂ to a metal center via polarization of its σ bond electron density, known as a Kubas complex, is the means by which H₂ chemisorbs at Ru⁴⁺ sites on the rutile RuO₂(110) surface. This distortion of electron density off an interatomic axis is often described as a ‘banana-bond.’ We show that the Ru–H₂ banana-bond can be destabilized and split using visible light. Photodesorption of H₂ (or D₂) is evident by mass spectrometry and scanning tunneling microscopy. From time-dependent density functional theory, the key optical excitation splitting the Ru–H₂ complex involves an interband transition in RuO₂ which effectively diminishes its Lewis acidity, thereby weakening the Kubas complex. Such excitations are not expected to affect adsorbates on RuO₂ given its metallic properties. Therefore, this common thermal cocatalyst employed in photocatalysis is, itself, photoactive

Dynamics, Stability, and Adsorption States of Water on Oxidized RuO₂(110)

Identifying and understanding how excess oxygen atoms affect the adsorption of water on metal oxides is crucial for their use in water splitting. Here, by means of high-resolution scanning tunneling microscopy and density-functional calculations, we show that excess oxygen atoms on the stoichiometric RuO₂(110) significantly change the clustering, conformation, and deprotonation equilibrium of adsorbed water. We considered two reactive scenarios during which the stoichiometric surface was exposed (i) first to oxygen, followed by water, and (ii) first to water, followed by oxygen. In both cases, the [OH-OH] complex on Ru rows is the dominant species, showing a significant difference from water-only adsorption on the stoichiometric surface in which the [OH-H₂O] species is found to be prevalent. Surface reactivity at almost full O coverage is also addressed; there we show that site selectivity of the surface for H adsorption and dissociation of H₂O is hindered, notwithstanding the increase of the dynamic motion of both species. We found that the work function of RuO₂ can serve as a descriptor for the reactivity of this surface to water and its constituents

Dimerization Induced Deprotonation of Water on RuO₂(110)

RuO₂ has proven to be indispensable as a co-catalyst in numerous systems designed for photocatalytic water splitting. In this study, we have carried out a detailed mechanistic study of water behavior on the most stable RuO₂ face, RuO₂(110), by employing variable-temperature scanning tunneling microscopy and density functional theory calculations. We show that water monomers adsorb molecularly on Ru sites, become mobile above 238 K, diffuse along the Ru rows, and form water dimers. The onset for dimer diffusion is observed at ∼277 K, indicating a significantly higher diffusion barrier than that for monomers. More importantly, we find that water dimers deprotonate readily to form Ru-bound H₃O₂ and bridging OH species. The observed behavior is compared and contrasted with that observed for water on isostructural rutile TiO₂(110)

Dimerization Induced Deprotonation of Water on RuO₂(110)

RuO₂ has proven to be indispensable as a co-catalyst in numerous systems designed for photocatalytic water splitting. In this study, we have carried out a detailed mechanistic study of water behavior on the most stable RuO₂ face, RuO₂(110), by employing variable-temperature scanning tunneling microscopy and density functional theory calculations. We show that water monomers adsorb molecularly on Ru sites, become mobile above 238 K, diffuse along the Ru rows, and form water dimers. The onset for dimer diffusion is observed at ∼277 K, indicating a significantly higher diffusion barrier than that for monomers. More importantly, we find that water dimers deprotonate readily to form Ru-bound H₃O₂ and bridging OH species. The observed behavior is compared and contrasted with that observed for water on isostructural rutile TiO₂(110)

Low-Temperature Reductive Coupling of Formaldehyde on Rutile TiO₂(110)

The formation and coupling of methylene upon dissociation of formaldehyde on reduced TiO₂(110) are studied using variable temperature scanning tunneling microscopy (STM). In agreement with prior studies, formaldehyde preferably adsorbs on the bridging-bonded oxygen vacancy (V_O) defect site. V_O-bound formaldehyde couples with Ti-bound formaldehyde forming a diolate species, which appears as the majority species on the surface at 300 K. Here, STM images directly visualize a low-temperature coupling reaction channel. Two V_O-bound formaldehyde molecules can couple and form Ti-bound species, which desorbs above ∼215 K. This coupling reaction heals both V_O sites indicating the formation and the desorption of ethylene. We also directly observed the diffusion of methylene groups to nearby empty V_O sites upon dissociation of the C–O bond in V_O-bound formaldehyde, which suggests that the ethylene formation occurs via coupling of the methylene groups. Statistical analysis shows that the sum of visible reaction products on the surface can only account for a half of the consumption of the initial V_O coverage, which further supports the desorption of the coupling reaction product, ethylene, after formaldehyde exposure between 215 and 300 K