Using x-ray photoelectron spectroscopy of the oxygen 1s core level, the ratio between intact (D_{2}O) and dissociated (OD) water in the hydrated stoichiometric TiO_{2}(110) surface is determined at varying coverage and temperature. In the submonolayer regime, both the D_{2}O∶OD ratio and the core-level binding energy of D_{2}O (ΔBE) decrease with temperature. The observed variations in ΔBE are shown with density functional theory to be governed crucially and solely by the local hydrogen bonding environment, revealing a generally applicable classification and details about adsorption motifs.

The CO-induced restructuring of Co(11–20) has been investigated with temperature-programmed desorption (TPD), low-energy electron diffraction, scanning tunneling microscopy (STM), and density funct...

Preparing an anatase TiO2(101) surface with a high density of oxygen vacancies and associated reduced Ti species in the near-surface region results in drastic changes in the water adsorption chemistry compared to adsorption on a highly stoichiometric surface. Using synchrotron radiation excited photoelectron spectroscopy, we observe a change in the water growth mode, from layer-by-layer growth on the highly stoichiometric surface to bilayer growth on the reduced surface. Furthermore, we have been able to observe Ti3+ enrichment at the surface upon water adsorption. The Ti3+ enrichment occurs concomitant with effective water dissociation into hydroxyls with a very high thermal stability. The water bilayer on the reduced surface is thermally more stable than that on the stoichiometric surface, and it is more efficient in promoting further water dissociation upon heating. The results thus show how the presence of subsurface defects can alter the wetting mechanism of an oxide surface.

The restructuring was studied with low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), high-resolution photoelectron spectroscopy (HRPES, MATLINE beamline, ASTRID2, Aarhus University), temperature programmed desorption (TPD) and DFT. Three theoretical model surfaces with a (3x1) periodicity were selected; one unreconstructed surface, and two with either a missing row (MR) or an added row (AR) of Co atoms along [0001], to represent the reconstruction (Figure 1). The calculations were performed with the Vienna ab initio simulations package (VASP) [3].

The anatase TiO2(101) surface and its interaction with water is an important topic in oxide surface chemistry. Firstly, it benchmarks the properties of the majority facet of TiO2 nanoparticles and, secondly, there is a controversy as to whether the water molecule adsorbs intact or deprotonates. We have addressed the adsorption of water on anatase TiO2(101) by synchrotron radiation photoelectron spectroscopy. Three two-dimensional water structures are found during growth at different temperatures: at 100 K, a metastable structure forms with no hydrogen bonding between the water molecules. In accord with prior literature, we assign this phase to chains of disordered molecules. Growth 160 K results in a metastable structure with expressed hydrogen bonding between the water molecules. At 190 K, the water molecules become disordered as the thermal energy is too high and hence the hydrogen bonds break. The result is a structure with isolated monomers. Partial dissociation is observed for all three growths, with the molecular state only slightly favored in energy (20-40 meV) over the dissociated state. Heating of a thick film leads to more dissociation compared to a bilayer, when formed at 100 K. Thus, extending the water network facilitates proton transport and hence dissociation. The results reconcile apparent conflicting experimental results previously obtained by scanning tunneling microscopy (STM) and core level photoelectron spectroscopy.

The CO oxidation behavior under excess oxygen and near stoichiometric conditions over the surface of Pd 3 Au(100) has been studied by combining near-ambient pressure X-ray photoelectron spectroscopy and quadrupole mass spectrometry and compared to Pd(100). During heating and cooling cycles, normal hysteresis in the CO 2 production, i.e. with the light-off temperature being higher than the extinction temperature, is observed for both surfaces. On both Pd 3 Au(100) and Pd(100) the (√5 × √5)R27° surface oxide structure is present during CO 2 production under excess oxygen conditions (O 2 :CO = 10:1), while at near stoichiometric conditions (O 2 :CO = 1:1) the surfaces are covered with atomic oxygen. Au as alloying element hence induces only minor differences in the observed hysteresis and the active phase compared to pure Pd. Alloying with Au thus yields a different behavior compared to Ag, where reversed hysteresis is observed for CO 2 production over Pd 75 Ag 25 (100) at similar conditions [Fernandes et al., ACS Catal. (2016) 4154].

The location of potassium (K) on Cobalt (Co) and its effect on adsorption and adsorption-induced surface restructuring is important for understanding the deactivation of Co Fischer-Tropsch catalysts and the nature of the active surface. Co(11-20) restructures by anisotropic migration of Co atoms upon CO exposure. Deposition of sub-monolayer amounts of K on Co(11-20) and the effect on the CO-induced restructuring were therefore investigated using scanning tunneling microscopy (STM), high resolution photoemission spectroscopy (HR-PES), and density functional theory calculations (DFT). The combined STM and DFT results suggest that the preferred adsorption site for K at low coverage is in the vicinity of step edges. DFT also found that diffusion of K along the [0001] direction, in between the zigzag rows of the topmost Co layer is facile. The restructuring under CO exposure with K pre-adsorbed proceeded on the terraces rather than from the step edges, in a slower and more disordered manner. HR-PES showed that the amount of CO adsorbed at saturation significantly decreased with predeposited K. The obstructed migration of Co atoms across the surface may be important in understanding why very low amounts of K on supported Co catalysts significantly reduces the activity towards hydrogenation of CO.

We present a synchrotron radiation photoelectron spectroscopy study of the adsorption and photooxidation of trimethyl acetate (TMA) on TiO2(B)(001). The TiO2(B)(001) substrate was realized in the form of 2nm thick film on Au(111). The TMA species adopt the bidentate bonding configuration, as expected for carboxylic acids on TiO2, but cannot coordinate to all surface Ti ions due to steric hindrance. The proposed arrangement of the TMA species thus allows for the formation of an overlayer with a (2 x 1) periodicity. The thermal stability is found to be comparable to that on rutile (110) although the results indicate differences in the threshold for the TMA+H -> TMAA reaction. Photolysis using both ultraviolet (UV) light and soft x-ray synchrotron radiation (SR) was studied and compared to the reaction on the reduced ruffle (110) surface. A kinetic analysis suggests that the photoreaction rate for TMA on the TiO2(B) thin film is initially two times faster than that on the reduced rutile TiO2(110) surface. The higher activity of the TiO2(B) film is assigned to a reduced influence from surplus electrons associated with reduced Ti species, thereby decreasing the probability for hole-annihilation at high TMA coverage. (C) 2017 Elsevier B.V. All rights reserved.

Thin films of TiOx on Pd(100) and Pd(111) have been investigated with respect to their properties after oxidation and reduction cycles. High-resolution photoemission spectroscopy (HRPES) and low energy electron diffraction (LEED) have been applied to characterize the thin film oxidation states and structure before and after oxidation and reduction under ultrahigh vacuum conditions. Fully oxidized TiO2 films were formed on both surfaces. These structures display Moiré patterns in LEED, in one dimension for Pd(100) and in two dimensions for Pd(111), and they have previously not been reported for TiO2/Pd. The oxidation process causes strong reduction in the interaction between the oxide thin film and the Pd substrate, most significantly for Pd(111). Reversible oxidation/reduction cycling of TiOx thin films on Pd(111) and Pd(100) was possible.

CO oxidation over Pd(100) and Pd75Ag25(100) has been investigated by a combination of near-ambient pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations, and microkinetic modeling. For both surfaces, hysteresis is observed in the CO2 formation during the heating and cooling cycles. Whereas normal hysteresis with light-off temperature higher than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd75Ag25(100). The reversed hysteresis can be explained by dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium, which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag-rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs. During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.

The growth of ultrathin TiOx (0≤x≤2) films on Pd(100) and Pd(111) surfaces by chemical vapor deposition (CVD), using Titanium(IV)isopropoxide (TTIP) as precursor, has been investigated by high resolution photoelectron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. Three different TiOx phases and one Pd–Ti alloy phase have been identified for both surfaces. The Pd–Ti alloy phase is observed at the initial stages of film growth. Density functional theory (DFT) calculations for Pd(100) and Pd(111) suggest that Ti is alloyed into the second layer of the substrate. Increasing the TTIP dose yields a wetting layer comprising Ti2+ species (TiOx, x ∼0.75). On Pd(100), this phase exhibits a mixture of structures with (3×5) and (4×5) periodicity with respect to the Pd(100) substrate, while an incommensurate structure is formed on Pd(111). Most importantly, on both surfaces this phase consists of a zigzag pattern similar to observations on other reactive metal surfaces. Further increase in coverage results in growth of a fully oxidized (TiO2) phase on top of the partially oxidized layer. Preliminary investigations indicate that the fully oxidized phase on both Pd(100) and Pd(111) may be the TiO2(B) phase.

We present a dedicated synchrotron radiation photoelectron spectroscopy (SR-PES) study of a photochemical reaction on the surface of rutile TiO2(110). The photoreaction kinetics of carboxylate species (trimethyl acetate, TMA) upon irradiation by UV and soft X-rays were monitored, and we show that it is possible to control the reaction rates from UV light and soft X-rays independently. We directly observe Ti4+ → Ti3+ conversion upon irradiation, attributed to electron trapping at Ti sites close to surface OH groups formed by deprotonation of the parent molecule, trimethylacetic acid (TMAA). TMA photolysis on two surface preparations with different oxygen vacancy densities shows that the vacancy-related charge quenches the amount of charge that can be trapped at hydroxyls upon irradiation. During the initial stages of reaction the correlation between the amount of photodepleted TMA and the amount of charge trapped in the Ti 3d band gap state is nearly 1:1. A first-order kinetics analysis reveals that the reaction rate decreases with decreasing TMA coverage. There is also a coverage-dependent difference in the electronic structure of TMA moieties, primarily involving the carboxyl anchor group. These changes are consistent with a decreased hole affinity of the adsorbed TMA and hence a decreased reaction rate. This discovery adds to the previously presented picture of a reactivity that is inversely proportional to the number of surface hydroxyls, suggesting that the balance between the amounts of TMA, OH, and trapped charge needs to be considered.

Physics, Chemistry and Applications of Nanostructures, pp. 3-11 (2015) No AccessKINETICS OF WATER ADSORPTION ON TiO2(110)M. LEETMAA, B. WANG, D. RAGAZZON, L. E. WALLE, A. BORG, P. UVDAL, A. SANDELL, and N. V. SKORODUMOVAM. LEETMAADept. of Physics and Astronomy, Uppsala University P. O. Box 516, SE-751 20 Uppsala, Sweden, B. WANGDept. of Physics and Astronomy, Uppsala University P. O. Box 516, SE-751 20 Uppsala, Sweden, D. RAGAZZONDept. of Physics and Astronomy, Uppsala University P. O. Box 516, SE-751 20 Uppsala, Sweden, L. E. WALLEDept. of Physics, Norwegian University of Science and Technology (NTNU) NO-7491 Trondheim, Norway, A. BORGDept. of Physics, Norwegian University of Science and Technology (NTNU) NO-7491 Trondheim, Norway, P. UVDALChemical Physics, Dept. of Chemistry, P.O. Box 124, and MAX-IV Laboratory P.O. Box 118, Lund University, SE-221 00 Lund, Sweden, A. SANDELLDept. of Physics and Astronomy, Uppsala University P. O. Box 516, SE-751 20 Uppsala, Sweden, and N. V. SKORODUMOVAMultiscale Materials Modelling group, Department of Materials and Engineering Royal Institute of Technology (KTH), SE-10044 Stockholm, SwedenDept. of Physics and Astronomy, Uppsala University P. O. Box 516, SE-751 20 Uppsala, Swedenhttps://doi.org/10.1142/9789814696524_0001Cited by:1 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: We have built a kinetic model of the first water monolayer growth on TiO2(110) using the kinetic Monte Carlo (KMC) method based on the parameters describing water molecule diffusion and dissociation obtained in first principle calculations. The adsorption process has also been studied with X-ray photoelectron spectroscopy (XPS). The experiment shows that the ratio between intact and dissociated molecules monitored during the growth crucially depends on temperature with more molecules staying intact at lower temperatures. Most profound composition differences are observed at low coverage (up to 0.2-0.25 ML). Our simulations reproduce the experimental trends and rationalize these observations. FiguresReferencesRelatedDetailsCited By 1Modeling Kinetics of Water Adsorption on the Rutile TiO 2 (110) Surface: Influence of Exchange-Correlation FunctionalJohan O. Nilsson, Mikael Leetmaa, Baochang Wang, Pjotrs A. Žguns and Igor Pašti et al.8 November 2017 | physica status solidi (b), Vol. 255, No. 3 Physics, Chemistry and Applications of NanostructuresMetrics History PDF download

High resolution photoelectron spectroscopy has been used to investigate water adsorption on four different TiOx ultrathin film structures, grown on Au(111) by chemical vapor deposition. Two of the structures are reduced TiOx single layer phases, forming a honeycomb (HC) and a pinwheel (PW) structure, respectively. The other two phases have TiO2 stoichiometry, one in the form of islands and one in the form of a TiO2(B)(001) extended layer. Partial water dissociation is observed for all phases but the HC phase, and the dissociation propensity and adsorbate thermal stability structure result from interplay between the atomic structure of the particular TiOx phase and defects formed in the preparation. The dissociation on the TiO2(B) film is mainly related to different types of defect sites. The TiO2 islands, interpreted as surface reconstructed rutile TiO2(100), generate the highest amount of hydroxyls with a behavior consistent with reconstruction into a mixed (100) and (110) termination. Water dissociation on the PW layer can be assigned to particular sites of the structure and it stands out by leading to oxidation of Ti species.

This study presents how a TiO2(B) film exposing the (001) face can be grown on Au(111) by chemical vapor deposition. Identification and characterization of the TiO2(B)(001) layer are carried out with low-energy electron diffraction (LEED), synchrotron radiation photoelectron spectroscopy (PES), scanning tunneling microscopy (STM) and X-ray absorption spectroscopy (XAS). Formation of the TiO2(B) film requires a two-step preparation procedure: deposition at 280°C followed by annealing to 500°C. This suggests that the interaction between a substrate and an overlayer stabilizes the TiO2(B) film, preventing the formation of thermodynamically more stable rutile islands. The study thus gives insight into how the morphology and the atomic structure of the titania overlayer can be controlled.

This thesis presents surface science studies, investigating several aspects of titanium dioxide at the atomic scale. The greater part of this work is devoted to the preparation by chemical vapor deposition (CVD) of titanium(IV) tetraisopropoxide (TTIP) of ultrathin TiO2 or TiOx films on Au(111). Four ordered structures were growth and characterized. It was also demonstrated how the morphology of the film (wetting film vs island) can be tailored. The acquired knowledge about the CVD process was exploited to load nano porous gold with titania, enhancing its catalytic activity. The reactivity towards water adsorption of the titania structures on Au(111) was also investigated. Finally, part of this work concerned the studying of the behavior of water on the stoichiometric rutile TiO2(110) surface, combining the experiments with density-functional theory (DFT) calculations and (kinetic) Monte Carlo simulations. The main experimental techniques used in this work are low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and photoelectron spectroscopy (PES).

composition of the first water layer on the stoichiometric rutile TiO2 (1 1 0) surface was studied with the use of synchrotron radiation photoelectron spectroscopy. The same H2O-OH balance was reached using different preparation procedures and there was no difference between H2O and D2O. On this basis, we rule out the possibility that the partially dissociated layer is an artifact caused by photon irradiation during measurements. Pre-deposition of Au at room temperature, primarily decorating step edges [e.g. S. A. Tenney et al. J. Phys. Chem. C 115 (2011) 11112] decreases the water uptake but does not relative amount that dissociates. This implies that the observed H2O-OH (b)alance is not controlled by dissociation at steps and subsequent out-diffusion of hydrogens on the terraces. That is, the formation of a partially dissociated water layer is an inherent property of stoichiometric terraces. (C) 2014 Elsevier B. V. All rights reserved.

•H2 reduction of the surface oxide is significantly slower for Pd75Ag25(100) compared to Pd(100).•The reduction behavior shows complex temperature dependence, not well described by Avrami kinetics.•Oxygen spillover effects during the surface oxide reduction are observed for Pd(100).•For Pd(100) the observed reduction rate is rather independent of temperature.•For Pd75Ag25(100) the reduction rate displays a non-monotonic variation with temperature.

The hydrogen solubility and permeation in Pd77%Ag23% membranes have been determined as a function of temperature and membrane thickness. Unexpectedly, the solubility of hydrogen is found to systematically increase as the membrane thickness decreases from 11.2 to 2.2µm. Topography studies by atomic force microscopy in conjunction with previously reported characterization suggest linkage of the hydrogen solubility to the density of grain boundaries. A higher average grain boundary density for thinner membranes results from the nucleation and growth proceeding during membrane fabrication by sputtering. For the membranes and conditions (no membrane pretreatment; 300–400°C; ΔpH2≤200kPa) applied here, surface phenomena affect the hydrogen transport at thicknesses below ~5µm. Determination of the solubility constants hence allowed the extraction of the bulk diffusivity parameters from the permeability measurements over the thicker membranes (6.7–11.2µm), in good agreement with reported values obtained using other techniques.

Reduction of the (root 5 x root 5)R27 degrees surface oxide on Pd(1 0 0) and Pd75Ag25(1 0 0) surfaces by H-2 has been studied using high-resolution photoelectron spectroscopy in situ at H-2 pressures 5 x 10(-9) mbar and 5 x 10(-8) mbar and selected temperatures in the range 30 degrees C to 170 degrees C. The reduction is slower on Pd75Ag25(1 0 0) compared to Pd(1 0 0) for all temperatures and pressures investigated. For Pd(1 0 0), the surface oxide reduction rate is rather independent of temperature, while for Pd75Ag25(1 0 0) a nonmonotonic variation is observed. As indicated by kinetic analysis, the complex reduction behavior is not well described by Avrami kinetics. Oxygen spillover effects contribute to this picture for Pd(1 0 0), while surface compositional effects appear to dominate the performance for Pd75Ag25(1 0 0). These findings may have implications for understanding the oxidation, reduction and hydrogen transport properties of Pd-Ag membranes. (C) 2014 Elsevier B.V. All rights reserved.