EMSL and Institute for Integrated Catalysis (IIC) Catalysis Workshop

Within the context of significantly accelerating scientific progress in research areas that address important societal problems, a workshop was held in November 2010 at EMSL to identify specific and topically important areas of research and capability needs in catalysis-related science

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