Reconstruction of solid surfaces is generally accompanied by changes in surface activities. Here, via a combined experimental and theoretical study, we successfully identified that a trace amount of potassium dopant restructures the mineral anatase TiO2(001) single-crystal surface from an added molecule (ADM) termination to an added oxygen (AOM) one without changing the (1×4) periodicity. The anatase TiO2(001)-(1×4)-ADM surface terminated with 4-fold coordinated Ti4c and 2-fold coordinated O2c sites is (photo)catalytically active, whereas the anatase TiO2(001)-(1×4)-AOM surface terminated with O2c and inaccessible 5-fold coordinated Ti5c sites is inert. These results unveiled a mechanism of dopant-induced transformation from a reactive to an inert TiO2(001)-(1×4) surface, which unifies the existing arguments about the surface structures and (photo)catalytic activity of anatase TiO2(001)-(1×4).

Revisited a spectrokinetic approach.

Supported Au catalysts are highly selective and size-sensitive in catalytic hydrogenation of alkynes under mild conditions. Using thermal-programmed desorption and density functional theory calculations, we study the hydrogenation reactions of C2 hydrocarbons with atomic H and clarify the site-specific selective hydrogenation of C2H2 on Au(997) at low temperatures. On atomic H(a) covered Au(997), hydrogenation of C2H2 goes with 100% selectivity to C2H4 at steps, yet no hydrogenation occurs at terraces; adsorbed C2H4 on neither steps nor terraces reacts with H(a). DFT calculations suggest that the increased adsorption free energies and appropriate reaction barriers of C2 species at steps lead to the step-site specific semihydrogenation of C2H2. These results elucidate the elementary surface reactions between C2 hydrocarbons and atomic H on Au surfaces at the molecular level and significantly deepen the fundamental understanding of the unique selectivity of Au catalysts.

The crystalline-to-vitreous phase transformation of a SiO2 bilayer supported on Ru(0001) was studied by time-dependent LEED, local XPS, and DFT calculations. The silica bilayer system has parallels to 3D silica glass and can be used to understand the mechanism of the disorder transition. DFT simulations show that the formation of a Stone-Wales-type of defect follows a complex mechanism, where the two layers show decoupled behavior in terms of chemical bond rearrangements. The calculated activation energy of the rate-determining step for the formation of a Stone-Wales-type of defect (4.3 eV) agrees with the experimental value. Charge transfer between SiO2 bilayer and Ru(0001) support lowers the activation energy for breaking the Si-O bond compared to the unsupported film. Pre-exponential factors obtained in UHV and in O-2 atmospheres differ significantly, suggesting that the interfacial ORu underneath the SiO2 bilayer plays a role on how the disordering propagates within the film.

Surface chemistry and photochemistry of small molecules on the rutile TiO2(001) and TiO2(011)-(2 × 1) surfaces were studied by low energy electron diffraction, thermal desorption spectroscopy, and x-ray photoelectron spectroscopy. It was found that the TiO2(001) surface mainly exhibits the defects of Ti interstitials in the near-surface region, while the TiO2(011)-(2 × 1) surface mainly exhibits the defects of double-oxygen vacancies. The defect structures of TiO2 surfaces strongly affect their adsorption and thermal/photodesorption behaviors. On the TiO2(001) surface, CH3OH and H2O dissociatively adsorb at the surface Ti sites near Ti interstitials; O2 molecularly adsorbs at the surface Ti sites adjacent to Ti interstitials, forming photoactive O2 species that undergoes a hole-mediated photodesorption process; CO adsorbs at the nearest surface Ti sites close to the Ti interstitials, but CO2 does not, and the resulting CO species is photoactive; and both CO and CO2 species adsorbed at the normal Ti4+ sites are photoinactive. On the TiO2(011)-(2 × 1) surface, O2 adsorbs only at the double-oxygen vacancy sites, and the resulting O2 species dissociates to form two oxygen atoms to refill in the oxygen vacancies upon heating; CO2 adsorbs at the double-oxygen vacancy sites, but CO does not, and the resulting CO2 species is photoactive; and both CO and CO2 species adsorbed at the surface Ti4+ sites are photoinactive. These results broaden the fundamental understandings of the chemistry and photochemistry of TiO2 surfaces, and the established structure-reactivity relation of small molecules on TiO2 surfaces is useful in probing complex structures of TiO2 powder catalysts.

Fundamental understandings of surface chemistry and catalysis of solid catalysts are of great importance for the developments of efficient catalysts and corresponding catalytic processes, but have been remaining as a challenge due to the complex nature of heterogeneous catalysis. Model catalysts approach based on catalytic materials with uniform and well-defined surface structures is an effective strategy. Single crystals-based model catalysts have been successfully used for surface chemistry studies of solid catalysts, but encounter the so-called “materials gap” and “pressure gap” when applied for catalysis studies of solid catalysts. Recently catalytic nanocrystals with uniform and well-defined surface structures have emerged as a novel type of model catalysts whose surface chemistry and catalysis can be studied under the same operational reaction condition as working powder catalysts, and they are recognized as a novel type of model catalysts that can bridge the “materials gap” and “pressure gap” between single crystals-based model catalysts and powder catalysts. Herein we review recent progress of surface chemistry and catalysis of important oxide catalysts including CeO2, TiO2 and Cu2O acquired by model catalysts from single crystals to nanocrystals with an aim at summarizing the commonalities and discussing the differences among model catalysts with complexities at different levels. Firstly, the complex nature of surface chemistry and catalysis of solid catalysts is briefly introduced. In the following sections, the model catalysts approach is described and surface chemistry and catalysis of CeO2, TiO2 and Cu2O single crystal and nanocrystal model catalysts are reviewed. Finally, concluding remarks and future prospects are given on a comprehensive approach of model catalysts from single crystals to nanocrystals for the investigations of surface chemistry and catalysis of powder catalysts approaching the working conditions as closely as possible.

The formation of HCOOCH3 initiated by photooxidation of CH3OH on TiO2 surfaces is interesting, but the mechanism remains ambiguous. Using thermal desorption spectroscopy and X-ray photoelectron spectroscopy, we herein have comparatively studied the adsorption of HCOOCH3 on the rutile TiO2(110) surface, the coadsorption and photooxidation of CH3OH and HCHO on the rutile TiO2(110) surface, and the adsorption and photooxidation of CH3OH on the rutile TiO2(011)-(2 x 1) surface. The HCOOCH3 formation from the photooxidation of CH3OH was identified as a relay reaction of photocatalytic reactions, followed by thermal reactions. CH3OH is photocatalytically oxidized to CH3O and HCHO species, and HCHO is photocatalytically oxidized to HCO species, and then the CH3O and HCO species undergo thermal coupling reactions to produce HCOOCH3 during the subsequent heating process. The TiO2 surface structure was observed to affect not only the photocatalytic oxidation process of CH3OH to produce CH3O and HCO species but also the subsequent thermal coupling reaction of CH3O and HCO species to produce HCOOCH3. CH3OH on the TiO2(110) surface is more photoreactive than on the TiO2(011)-(2 x 1) surface. A higher fraction of HCHO is produced for CH3OH/TiO2(011)-(2 x 1) than for CH3OH/TiO2(110), while a smaller faction of HCOOCH3 is produced. These results broaden our fundamental understanding of the reaction mechanism of CH3OH photocatalysis on TiO2 surfaces.

Photocatalytic water reduction to hydrogen over oxide semiconductors is one of the most extensively investigated artificial photocatalytic reactions, but the nature of the active species has not yet been elucidated. Here, we successfully prepared Pt/rutile TiO2110) surfaces with hydrated proton species via co-adsorption of hydrogen and water and observed the photocatalytic reduction of hydrated protons to H2 upon UV light illumination. These results provide experimental evidence to prove hydrated protons as the active species for photocatalytic water reduction to hydrogen and demonstrate the occurrence of photocatalytic reduction of hydrated protons to H2 within the H-bonding network on the catalyst surface instead of directly on the catalyst surface. The Pt-TiO2 interface is capable of dissociating water to form hydroxyl groups that facilitate the formation of H-bonding network on the catalyst surface to enhance the photocatalytic H2 production. Our results greatly advance fundamental understanding of artificial photocatalytic water reduction.

Aligning the intrinsic photocatalytic activities of various types of TiO2 facets is of great importance and interest in TiO2 photocatalysis. Herein, we report a comparative study of the intrinsic photocatalytic activities of anatase TiO2(001)-(1 x 4) and rutile TiO2(110)-(1 x 1) surfaces probed by photostimulated desorption of O-2 with combined thermal desorption spectroscopy, photostimulated desorption spectroscopy, and density functional theory calculations. Photostimulated desorption behavior of O-2 depends on both TiO2 surface structures and O-2 adsorption configurations. O-2 species adsorbed on the anatase TiO2(001)-(1 x 4) surface exhibit a higher intrinsic photostimulated desorption reactivity than those on the rutile TiO2(110)-(1 x 1) surface, which can be associated with their electronic structures and subsequent hole transfer processes from TiO2. These results provide, for the first time, direct and unambiguous experimental evidence for the higher intrinsic photocatalytic activity of the anatase TiO2(001) facet than those of other TiO2 facets and highlight the key role of structures of both TiO2 surfaces and adsorbed species in TiO2 photocatalysis.

The positive and negative effects of water on low-temperature CO oxidation were comprehensively investigated on a Pt(111) surface by thermal desorption spectroscopy, X-ray photoelectron spectroscopy, and polarization-modulated infrared reflection absorption spectroscopy. At 110 K, the interaction between H2O and preabsorbed O-2 forms the {O-2(a)center dot(H2O)(n)} complex on the Pt(111) surface through hydrogen bonding. Upon heating the surface to 170 K, the O-2 in the {O-2(a)center dot(H2O)(n)} complex is subject to dissociation. On the O-2-saturated Pt(111) surface (50 L O-2/Pt(111)), part of the chemisorbed O-2 on Pt(111) can be displaced by H2O and forms {O-2(a)center dot(H2O)(n)} trapped by H2O molecules at high H2O coverage (>0.44 ML H2O). At this point, high H2O coverage also weakens the molecular oxygen dissociation. Regarding the CO oxidation, a new CO2 production channel by the interaction of CO with the {O-2(a)center dot(H2O)(n)} complex at similar to 153 K has been discovered by exposing H2O to the CO/O-2/Pt(111) surface at 110 K, which dominates the low-temperature (<200 K) CO reaction processes. On the other hand, high H2O coverage can also weaken the CO2 production by replacing adsorbed O-2 on Pt(111). These results provide deep insights into the fundamental understanding for the effect of water in low-temperature CO oxidation.

Using low-energy electron microscopy and local photoelectron spectroscopy, water formation from adsorbed O and H-2 on a Ru(0001) surface covered with a vitreous SiO2 bilayer (BL) was investigated and compared to the same reaction on bare Ru(0001). In both cases the reaction is characterized by moving reaction fronts. The reason for this might be related to the requirement of site release by O adatoms for further H-2-dissociative adsorption. Apparent activation energies (E-a(app)) are found for the front motion of 0.59eV without cover and 0.27eV under cover. We suggest that the smaller activation energy but higher reaction temperature for the reaction on the SiO2 BL covered Ru(0001) surface is due to a change of the rate-determining step. Other possible effects of the cover are discussed. Our results give the first values for E-a(app) in confined space.

Iron oxide films epitaxially grown on close-packed metal single crystal substrates exhibit nearly-perfect structural order, high catalytic activity (FeO) and room-temperature magnetism (Fe3O4). However, the morphology of the films, especially in the ultrathin regime, can be significantly influenced by the crystalline structure of the used support. This work reports an ultra-high vacuum (UHV) low energy electron/synchrotron light-based X-ray photoemission electron microscopy (LEEM/XPEEM) and electron diffraction (µLEED) study of the growth of FeO and Fe3O4 on two closed-packed metal single crystal surfaces: Pt(111) and Ru(0001). The results reveal the influence of the mutual orientation of adjacent substrate terraces on the morphology of iron oxide films epitaxially grown on top of them. On fcc Pt(111), which has the same mutual orientation of adjacent monoatomic terraces, FeO(111) grows with the same in-plane orientation on all substrate terraces. For Fe3O4(111), one or two orientations are observed depending on the growth conditions. On hcp Ru(0001), the adjacent terraces of which are ‘rotated’ by 180° with respect to each other, the in-plane orientation of initial FeO(111) and Fe3O4(111) crystallites is determined by the orientation of the substrate terrace on which they nucleated. The adaptation of three-fold symmetric iron oxides to three-fold symmetric substrate terraces leads to natural structuring of iron oxide films, i.e., the formation of patch-like magnetite layers on Pt(111) and stripe-like FeO and Fe3O4 structures on Ru(0001).

Interaction of hydrogen with TiO2 plays a vital role in TiO2-based photocatalysis and thermal catalysis. In this work, we compared thermal-, photo-, and electron-induced reactivity of various types of hydrogen species on a rutile TiO2(110) surface formed by atomic H exposure at 320 and 115K by means of thermal desorption spectroscopy, X-ray photoelectron spectroscopy and low energy electron diffraction. Atomic H interaction with rutile TiO2(110) at 115K forms surface TiH hydride, surface hydroxyl group, and chemisorbed water. Upon heating, surface TiH hydride reacts to produce H2 while surface hydroxyl groups react to form both water and H2. Atomic H interaction with rutile TiO2(110) at 320K strongly reduces TiO2 due to the continuous formation and desorption of water and forms surface hydroxyl groups and likely subsurface/bulk hydrogen species. Upon heating, hydrogen forms as the only gas-phase product and its desorption activation energy decreases with the subsurface/bulk reduction extent of rutile TiO2(110). Surface TiH hydride exhibits photo-induced reactivity while both surface TiH hydride and surface hydroxyl group exhibit electro-induced reactivity. These results have important implications for understanding the hydrogen-involved thermal and photo reactions on TiO2-based catalysts.

Fundamental understanding of complex FT synthesis is of great interest. We have employed X-ray photoelectron spectroscopy and temperature-programmed desorption to comparatively investigate CH2I2 adsorption and reactions on clean, hydrogen- and CO-covered Co(0001) surfaces. Surface chemistry of CH2I2 was demonstrated to sensitively depend on available vacant surface sites on Co(0001). Upon adsorption on clean Co(0001) surface at 110 K, CH2I2 undergoes stepwise decomposition reactions to produce carbon adatoms, CH(a) and CH2(a) species at small coverages, and chemisorbs both dissociatively and molecularly at large coverages. Upon heating, CH2(a) species facilely undergoes surface reactions to produce CH, C3H6, and C2H4 in gas phase and CH(a) species on the surface at low temperatures. CH(a) species undergoes surface reactions to produce CH4 in gas phase and C2H2(a) species on the surface at higher temperatures, and both CH(a) and C2H2(a) species undergo further surface reactions to produce H-2 in gas phase and carbon species on the surface. Coadsorbed H adatoms and CO molecules were found to strongly affect surface chemistry of CH2I2 and the resulting CH species on Co(0001) via suppressing the decomposition reactions and promoting the carbon carbon bond coupling reactions. These results add novel insights in fundamental understanding of complex FT synthesis.

AbstractMit niederenergetischer Elektronenmikroskopie und lokaler Fotoelektronenspektroskopie wurde die Bildung von Wasser aus absorbierten O und H2 auf einer Ru(0001)‐Oberfläche untersucht, die mit einer glasartigen SiO2‐Bilage (BL) bedeckt war, und mit der gleichen Reaktion auf der reinen Ru(0001)‐Oberfläche verglichen. In beiden Fällen tritt eine fortschreitende Reaktionsfront auf. Der Grund hierfür kann mit der Anforderung zusammenhängen, dass die O‐Adatome Platz freigeben müssen, damit weiterer H2 dissoziativ adsorbiert. Wir finden scheinbare Aktivierungsenergien ( ) für die Frontbewegung von 0.59 eV ohne Deckschicht und 0.27 eV mit Deckschicht. Wir schlagen vor, dass bei der SiO2‐BL‐bedeckten Ru(0001)‐Oberfläche die geringere Aktivierungsenergie aber höhere Reaktionstemperatur durch eine Änderung des geschwindigkeitsbestimmenden Schritts verursacht wird. Weitere mögliche Auswirkungen der Deckschicht werden diskutiert. Unsere Resultate ergeben die ersten Werte für im eingegrenzten Raum (“confinement”).

Investigating the surface chemistry of formaldehyde on the surface of TiO2 is important in understanding the thermal catalytic and photocatalytic reactions of formaldehyde on TiO2-involved catalysts. By combining thermal desorption spectroscopy and X-ray photoelectron spectroscopy, we studied the adsorption, thermo-induced surface reactions, and photo-induced surface reactions of formaldehyde on the rutile TiO2(011)-(2 x 1) surface. The dominant thermal-catalytic reaction is the formation of ethylene by a reductive carbon carbon formation reaction of formaldehyde adsorbed at the oxygen vacancy sites, and the dominant photocatalytic reaction is the formation of formate, assisted by the bridge O-2c sites, followed by carbon monoxide formation at elevated temperatures. The surface intermediates of formaldehyde reactions to ethylene and carbon monoxide on the rutile TiO2(011)-(2 x 1) surface were identified. The effect of the surface structure of the rutile TiO2(011)-(2 x 1) surface, particularly the oxygen vacancy, on the thermal-catalytic and photocatalytic activity toward formaldehyde was revealed by studying the coadsorption of water and formaldehyde. These results broaden our fundamental comprehension on the reaction mechanism of formaldehyde on the TiO2 surfaces.

Understanding the fundamental processes taking place on Co surfaces during the Fischer-Tropsch (FT) synthesis is of great interest and importance. We herein report a self-anticoking mechanism of a cobalt surface by subsurface oxygen. The active carbidic carbon species for FT synthesis tends to transform into the inactive graphitic carbon species on clean Co(0001) and poisons the Co surface. Subsurface atomic oxygen on Co(0001) can stabilize the active carbidic carbon species and quench the transformation process. These results reveal, to the best of our knowledge, for the first time the reactivity of various surface species on Co surfaces that dynamically maintain a delicate balance to enhance the long-term stability of Co catalysts during FT synthesis.

Employing TiO2 anatase (001)-(1x4), rutile (110) and rutile (011)-(2x1) single crystal surfaces, we comprehensively studied the effects of TiO2 surface structures on the competitive adsorption of water and methanol by means of low energy electron diffraction, thermal desorption spectra and X-ray photoelectron spectroscopy. The relative adsorption strengths of chemisorbed methanol and water vary with the TiO2 surface structures and the adsorption sites. This leads to TiO2 surface structure-dependent competitive adsorption of water and methanol. The chemisorption of CH3OH on TiO2 anatase (001)-(1x4) surface is seldom affected by pre-covered water at low coverages but is affected by pre-covered water at high coverages; the chemisorption of CH3OH on TiO2 rutile (110) surface is seldom affected by pre-covered water; and the chemisorption of CH3OH on TiO2 rutile (011)-(2x1) surface is affected by pre-covered water even at low coverages. These results deepen the fundamental understandings of surface chemistry on TiO2 surfaces.

Photocatalytic reaction of methanol on an anataseTiO(2)(001)-(1 x 4) reconstructed' surface, a prototype reaction for photocatalysis, was studied by means of X-ray photoelectron spectroscopy, thermal desorption spectrum, and densityfunctional theory calculations, Photocatalytic oxidation reaction, was observed toexclusively occur at the Ti-4C sites of the (1 x 4) added row but not at the Ti-5C sites-of the (1 x 1) basal surface, The accompanying density functional theory calculation results demonstrate that the valence band maximum is localized at the-oxygen atoms of the (1 x 4) added row and the methoxy species bonded to the Ti-4C sites, respectively, for the clean and methanol-covered, anatase TiO2(001)-(1 x 4) surfaces. This leads to-a Ti-4C site-specific oxidation of the methoxy species by photogenerated holes. These results reveal a concept of surface reconstruction-induced site-specific charge separation and photocatalytic reaction on oxide ph-otocatalysts that will greatly deepen the understanding of the vital role of oxide surface structure in photocatalytic reactions.