The interaction of water with metal oxide surfaces is of key importance to several research fields and applications. Because of its ability to photo-catalyze water splitting, reducible anatase TiO2 (a-TiO2) is of particular interest. Here, we combine experiments and theory to study the dissociation of water on bulk-reduced a-TiO2(101). Following large water exposures at room temperature, point-like protrusions appear on the a-TiO2(101) surface, as shown by scanning tunneling microscopy (STM). These protrusions originate from hydroxyl pairs, consisting of terminal and bridging OH groups, OHt/OHb, as revealed by infrared reflection absorption spectroscopy (IRRAS) and valence band experiments. Utilizing density functional theory (DFT) calculations, we offer a comprehensive model of the water/a-TiO2(101) interaction. This model also explains why the hydroxyl pairs are thermally stable up to ∼480 K.

Ultrathin cobalt oxide films supported on noble-metal surfaces have received much attention as interesting examples of low-temperature CO oxidation catalysts. It is expected that the activity of the cobalt oxides is closely linked with the structure and morphology of the film, but a direct operando correlation between CO oxidation activity, nanoscale structure, oxidation state, and surface composition has been missing. Here, we use a combination of operando ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy to investigate varying submono-layer coverages of CoO supported on Pt(111) under CO oxidation conditions. The goal is to compare the structural and spectroscopic features as the samples are exposed to O-rich CO/O2 gas mixtures at millibar pressure and brought to temperatures where CO oxidation occurs. Upon first exposure to millibar gas mixtures, the initial bilayer CoO film is oxidized to trilayer CoO2, characterized by a preserved film morphology and Co in a predominant 3+ oxidation state. However, upon temperature increase during the CO oxidation reaction, the cobalt oxide ultrathin film undergoes dewetting into nanoparticles. On the basis of the XPS signature, we conclude that these nanoparticles have a Co3O4-like structure. The results underline the importance of operando observations of surface structures. This new insight into the Co oxide/metal interface may aid in our understanding of reactivity of metal oxide coated noble-metal particles in general.

:by means of high-resolution scanning tunneling microscopy (STM), westudied the adsorption and reaction of submonolayer NH3on rutile TiO2(110) surfaces indifferent oxidation states. On a clean, reduced TiO2(110) surface with O vacancies, NH3adsorbs at 120 K exclusively as single molecular species on surface Ti sites. On aTiO2(110) surface with H adatoms, we observed small amounts of NH3monomers inclose proximity to each other and NH3dimers, in addition to the majority of isolated NH3monomers. On such a surface, we found that NH3and H adatoms can diffuse togetheralong the [001] direction and NH3can diffuse along the [11x305;0] direction via"hydroxylbridges". On an oxidized TiO2(110) surface with O adatoms (Oot) and O2molecules, wefound dimeric NH3species and isolated OHtgroups. Interestingly, there were alsogrouped adsorbates of NH2OH stoichiometry (singles and pairs). Following annealing at 330 K, the coverage of paired NH2OHspecies was much increased. Wefind a strong affinity of NH3species to interact with Ootadatoms and Ootadatom pairs and proposedissociation of NH3species at Ootadatoms. Finally, we discuss the faith of the observed adsorbates at elevated temperature

We combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to study the molecular and electronic structure of submonolayer tungsten oxide supported on anatase TiO2(101), -(001), and rutile TiO2(110) surfaces. We found that monomeric tungsten oxide species form on all three TiO2 surfaces upon mild annealing at 400 K, with a geometry depending on the supporting facet. At similar to 600 K, surface diffusion of the monomers sets in, but the monomers remain on the surface without diffusing into the bulk even at higher annealing temperatures. As-deposited tungsten oxide at monolayer coverage is stronger oxidized than thick layers. At elevated temperatures (400-900 K), significant reduction is observed, strongly dependent on the TiO2 facet employed and bulk defects within the substrate. Among the TiO2 surfaces studied, the weakest reduction by vacuum annealing was found for tungsten oxide supported on anatase TiO2(001).

We utilized scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations to study the diffusion of ammonia (NH3) on anatase TiO2(101). From time-lapsed STM imaging, we observed monomeric and dimeric diffusion channels, and a general tendency to higher diffusion rates with increasing NH3 coverage. In surface regions where several NH3 molecules are adsorbed within a few sites, we further observed the diffusion of NH3 molecules occurring in cascades, where the diffusion of one adsorbate triggers that of others. This eventually leads to apparent diffusion barriers that are lower than expected within a single-jump model. From the DFT calculations, we obtained mechanistic insights into the two observed NH3 diffusion channels. Within the dimeric NH3 diffusion channel, one NH3 swings around another adsorbed NH3 and experiences a reduced diffusion barrier, owing to the intermolecular bonding during the event.

We combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations to study dispersed tungsta clusters on anatase TiO$_2$(101). Following two different preparation methods, we found that monomeric WO$_3$ species are the most stable configuration rather than WO$_3$ trimers, (WO$_3$)$_3$. The WO$_3$ monomers form tetrahedral WO$_4$ structures on anatase TiO$_2$(101), with one W$-$O bond and two W$-$O$-$Ti linkages per WO$_3$ monomer. Locally, the WO$_3$ monomers form well-ordered (2$\times$1) structures. The discovered geometric structure of WO$_3$ on anatase TiO$_2$(101) opens up numerous opportunities for fundamental studies addressing tungsta and accurate structure activity studies of WO$_3$/TiO$_2$ model catalysts.

We prepared vanadia-titania model catalysts with V(+5) oxidation state by sublimating V2O5 powder onto clean anatase TiO2 (101) [a-TiO2 (101)]. The V2O5/a-TiO2 (101) model catalysts with V 2 0 5 in the sub-monolayer coverage range were studied with scanning tunneling microscopy, lab-source X-ray photoelectron spectroscopy, and synchrotron-radiation X-ray photoelectron spectroscopy. On freshly prepared V2O5/a-TiO2 (101) samples, we find well-dispersed V2O5 clusters as the smallest species, together with larger particles of V2O5 stoichiometry. Upon vacuum-annealing at similar to 500 K, small V2O5 clusters agglomerate into larger particles. Upon vacuum-annealing at similar to 700 K, vanadia reduces and, eventually, disappears from the surface via V diffusion into the bulk. The V2O5/a-TiO2 (101) model catalyst can be hydroxylated by an evidence for OH groups located at the V2O5/a-TiO2 (101) interface as well as at the a-TiO2 (101 ice-assisted preparation. We find support.

The interaction of methanol with iron oxide surfaces is of interest due to its potential in hydrogen storage and from a fundamental perspective as a chemical probe of reactivity. We present here a study examining the adsorption and reaction of methanol on magnetite Fe3O4(001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The methanol desorption profile from Fe3O4(001) is complex, exhibiting peaks at 140 K, 173 K, 230 K, and 268 K, corresponding to the desorption of intact methanol, as well as peaks at 341 K and 495 K due to the reaction of methoxy intermediates. The saturation of a monolayer of methanol corresponds to ∼5 molecules/unit cell (u.c.), which is slightly higher than the number of surface octahedral iron atoms of 4/u.c. We probe the kinetics and thermodynamics of the desorption of molecular methanol using inversion analysis. The deconvolution of the complex desorption profile into individual peaks allows for calculations of both the desorption energy and the prefactor of each feature. The initial 0.7 methanol/u.c. reacts to form methoxy and hydroxy intermediates at 180 K, which remain on the surface above room temperature after intact methanol has desorbed. The methoxy species react via one of two channels, a recombination reaction with surface hydroxyls to form additional methanol at ∼350 K and a disproportionation reaction to form methanol and formaldehyde at ∼500 K. Only 20% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway.

By means of scanning tunneling microscopy (STM) measurements, we studied in situ the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O2) and hydrogen (H2), respectively. The FeO islands respond very dynamically toward O2, with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O2 dissociation and O atom incorporation. An STM movie obtained during oxidation reveals how further O2 molecules can dissociate after the consumption of all initially existing CUF sites through the formation of new CUF sites. In contrast, we found that H2 molecules only dissociate when vibrationally excited through the ion gauge and only at the basal plane of FeO islands, implying that the CUF sites are not relevant for H2 dissociation. Our STM results reveal how excess O atoms are incorporated and released in O2 and H2 and thus shed light onto the stability of inverse catalysts during a catalyzed reaction.

The adsorption of ammonia on anatase TiO2 is of fundamental importance for several catalytic applications of TiO2 and for probing acid-base interactions. Utilizing high-resolution scanning tunneling microscopy (STM), synchrotron X-ray photoelectron spectroscopy, temperature-programmed desorption (TPD), and density functional theory (DFT), we identify the adsorption mode and quantify the adsorption strength on the anatase TiO2(101) surface. It was found that ammonia adsorbs non-dissociatively as NH3 on regular five-fold coordinated titanium surface sites (5f-Ti) with an estimated exothermic adsorption energy of 1.2 eV for an isolated ammonia molecule. For higher adsorbate coverages, the adsorption energy progressively shifts to smaller values, due to repulsive intermolecular interactions. The repulsive adsorbate-adsorbate interactions are quantified using DFT and autocorrelation analysis of STM images, which both showed a repulsive energy of ∼50 meV for nearest neighbor sites and a lowering in binding energy for an ammonia molecule in a full monolayer of 0.28 eV, which is in agreement with TPD spectra.

Using a multi-technical approach, we studied the oxidation of anatase TiO2(1 0 1)-supported vanadium (V) clusters at room temperature. We found by ex situ XPS that the highest oxidation state is +4 at sub-monolayer coverage regardless of the O-2 pressure, and STM studies revealed that the initial oxidation proceeds through oxygen-induced disintegration of V clusters into monomeric VO2 species. By contrast, for 2 monolayer V coverage, a partial oxidation to V5+ is achieved. By in situ APXPS measurements, we found that V can be maintained in the V5+ oxidation state irrespective of the coverage; however, in the sub-monolayer range, an O-2 pressure of at least similar to 1 x 10(-5) mbar is needed. Our results suggest an enhanced reducibility of V in direct contact with the TiO2 support compared to V in the 2nd layer, which is in line with the observed optimum V2O5 loading in catalytic applications just slightly below a full monolayer. (C) 2018 Elsevier Inc. All rights reserved.

Low-coordinate surface sites, such as those present on high-index step edges, often exhibit chemical reactivity that markedly differs from more close-packed facets. To understand the site-specific reactivity, insight into the three-dimensional atomic arrangement of step edges is needed. Here, we employ atomic-resolution transmission electron microscopy (TEM) of nanoparticles in combination with scanning tunneling microscopy (STM) of a single crystal surface to uncover the structure of prevalent step edges on the anatase TiO2 (001) surface.

We studied the interaction of water with the anatase TiO_{2}(001) surface by means of scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory calculations. Water adsorbs dissociatively on the ridges of a (1×4) reconstructed surface, resulting in a (3×4) periodic structure of hydroxyl pairs. We observed this process at 120 K, and the created hydroxyls desorb from the surface by recombination to water, which occurs below 300 K. Our calculations reveal the water dissociation mechanism and uncover a very pronounced dependence on the coverage. This strong coverage dependence is explained through water-induced reconstruction on anatase TiO_{2}(001)-(1×4). The high intrinsic reactivity of the anatase TiO_{2}(001) surface towards water observed here is fundamentally different from that seen on other surfaces of titania and may explain its high catalytic activity in heterogeneous catalysis and photocatalysis.

Using high-resolution scanning tunneling microscopy (STM), we have studied the oxidation of rutile TiO2(110)-(1x1) surfaces with Had species at room temperature. We followed the evolution of various stable species as function of the O-2 exposure, and the nature of the ultimately dominating species in the Ti troughs is described. When O-2 saturation was accomplished using a glass-capillary array doser, we found that on-top O (O-ot) adatoms are the predominant surface species. In contrast, when O-2 was supplied via backfilfing of the chamber the predominant surface species are tentatively assigned to terminal OH groups. We argue that unintended reactions with the chamber walls have a strong influence on the formed surface species, explaining scattered results in the literature. On the basis of our STM data we propose an alternative, easy way of preparing oxidized TiO2(110) surfaces with O-ot adatoms (o-TiO2). It is certain that o-TiO2(110) surfaces prepared according to this recipe do not have any residual surface O vacancies. This contradicts the situation when oxidizing reduced TiO2(110) surfaces with O vacancies, where some O vacancies persist. (C) 2017 Elsevier B.V. All rights reserved.

To understand the structure-reactivity relationships for mixed-metal oxide catalysts, well-defined systems are required. Mixtures of vanadia and titania (TiO2) are of particular interest for application in heterogeneous catalysis, with TiO2 often acting as the support. By utilizing high-resolution scanning tunneling microscopy, we studied the interaction of vanadium (V) with the anatase TiO2(101) surface in the sub-monolayer regime. At 80 K, metallic V nucleates into homogeneously distributed clusters onto the terraces with no preference for nucleation at the step edges. However, embedding of single V atoms into TiO2 occurs following annealing at room temperature. In conjunction with X-ray photoelectron spectroscopy data and density functional theory calculations, we propose that monomeric V atoms occupy positions of regular surface Ti sites, i.e., Ti atoms are substituted by V atoms.

Understanding the cocatalyst/semiconductor interaction is of key importance for the design and synthesis of next generation photocatalytic materials for efficient hydrogen production and environmental cleanup applications. Here we investigate preformed Pd nanoparticles (NPs) supported on a series of anatase TiO2 having well-controlled but varying degrees of crystallinity and crystallite size, and explore their photocatalytic performance for H-2 production and phenol decomposition. While tuning the anatase crystallite size significantly influences the photocatalytic performance, varying the TiO2 crystallinity shows a negligible effect. Interestingly, the optimum quantum efficiency (similar to 78%) for H-2 evolution is achieved with anatase having medium crystallite size (similar to 16 nm), whereas for phenol decomposition, a promotional effect is only observed for anatase with larger crystallite sizes (>20 nm). Surface radical species and radical densities study reveal that the photogenerated charge carriers have been trapped at different sites depending on the crystallite size of anatase. While the excited electrons are only trapped in bulk lattice sites in small anatase (<16 nm), larger anatase particles provide extra surface sites for charge trapping, which benefit charge storage and transportation to Pd surface sites, leading to a more efficient utilization of charge carriers for photocatalysis. Additionally, Pd supported on medium sized anatase nm) hinders the formation of O-2(center dot-) radicals on TiO2 surfaces, thus preventing unwanted reoxidation of photogenerated H-2.

Using high-resolution scanning tunneling microscopy (STM) we have studied the oxidation of ultrathin FeO films grown on Pt(111). At the initial stage of the FeO film oxidation by atomic oxygen exposure, we identified three distinct types of line defects, all of which form boundaries between FeO domains of opposite orientation. Two types of line defects appearing bright (type-i) and dark (type-ii) in the STM images at typical scanning parameters are “metallic”, whereas the third line defect exhibits nonmetallic behavior (type-iii). Atomic-scale structure models of these line defects are proposed, with type-i defects exhibiting 4-fold coordinated Fe atoms, type-ii exhibiting 2-fold coordinated O atoms, and type-iii exhibiting tetrahedrally-coordinated Fe atoms. In addition, FeO2 trilayer islands are formed upon oxidation, which appear at FCC-type domains of the moiré structure. At high scanning bias, distinct protrusions on the trilayer islands are observed over surface O ions, which are assigned to H adatoms. The experimental data are supported by density functional theory (DFT) calculations, in which bare and hydroxylated FeO2 trilayer islands are compared. Finally, we compare the formation of O-rich features on continuous FeO films using atomic oxygen with the oxidation of Pt(111)-supported FeO islands accomplished by O2 exposure.

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photoactivated dehydrogenation of ethanol on reduced and oxidized rutile TiO2(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO2.

Using high resolution and ambient pressure X-ray photoelectron spectroscopy we show that the catalytically active FeO2 trilayer films grown on Pt(111) are very active for water dissociation, in contrast to inert FeO(111) bilayer films. The FeO2 trilayer is so active for water dissociation that it becomes hydroxylated upon formation, regardless of the applied preparation method. FeO2 trilayers were grown by oxidation of FeO(111) bilayer films either with molecular oxygen in the mbar regime, or by NO2 and atomic oxygen exposures, respectively, in the ultrahigh vacuum regime. Because it was impossible to prepare clean FeO2 without any hydroxyls we propose that catalytically highly active FeO2 trilayer films are generally hydroxylated. In addition, we provide spectroscopic fingerprints both for Pt(111)-supported FeO(111) and FeO2 films that can serve as reference for future in situ studies.