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Thermal decomposition of glycine molecules on cerium oxide films is defined by the type of cerium cations on the surface.

We present the combined Mössbauer and X-ray absorption spectroscopy investigation of the electronic structure and local site symmetry of Fe in olivine structured LiFePO4 (LFP) with crystallite size (CS).

We studied the coverage- and temperature-dependent proton transfer and self-metalation reactions of tetraphenylporphyrin molecules containing a carboxyl functional group (MCTPP) on rutile TiO2(110)...

The effect of the superstoichiometric Ti intercalation on the electronic structure of TiSe2 was studied by using X-ray photoelectron spectroscopy in nonresonant and resonant modes along with the DOS (density of states) calculations. It was shown that the presence of the Ti atoms in the interlayer space leads to the formation of the Ti 3dz2/Ti 3dz2 hybridized band between the Ti atoms in the regular lattice positions and Ti atoms in the interlayer space. The charge transfer to the conduction band was not observed in this case.

We have prepared and characterized atomically well-defined model systems for ceria-supported Pt–Co core–shell catalysts. Pt@Co and Co@Pt core–shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields Co–CeO2(111) solid solution at low Co coverage (0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt–Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt–Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.

Quantitative assessment of the charge transfer phenomena in cobalt oxides and cobalt complexes is essential for the design of advanced catalytic materials. We propose a method for the evaluation of the oxidation state of cobalt oxides with mixed valence states using resonant photoemission spectroscopy. The method is based on the calculation of the resonant enhancement ratio (RER) from the heights of the resonant features associated with the Co3+ and Co2+ states. The nature of the corresponding states was corroborated by means of density functional calculations. We employed a well-ordered Co3O4(111) film to calibrate the RER with respect to the atomic Co3+/Co2+ ratio. The method was applied to monitor the reduction of a well-ordered Co3O4(111) film to CoO(111) upon annealing under exposure to isopropanol. We demonstrate that this method yields the stoichiometry of cobalt oxides at a level of accuracy that cannot be achieved when fitting the Co 2p core level spectra.

We report on the interaction of CH3NH3PbI3 substrates with AlOx films prepared by atomic layer deposition at room temperature. We use synchrotron radiation-based photoemission spectroscopy and stud...

Tin oxide is a material that attracts attention due to variety of technological applications. The main parameters that influence its properties are morphology, crystalline structure and stoichiometry. Researchers try to develop nanostructured thin films with tunable parameters that would conform its technological applications. Herein, we report on the preparation and characterization of highly porous SnO2 and Pd-doped SnO2 thin films. These films were deposited in the form of nanorods with controllable geometry. Such morphology was achieved by utilizing glancing angle deposition (GLAD) with assisted magnetron sputtering. This arrangement allowed preparation of slanted pillars, zig-zag structure, vertically standing posts, spiral posts and “bush”-like structures. We calculated that slanted pillars feature the highest surface area among the listed. Then, sets of slanted pillars were deposited and studied in more details. Tin oxide films were thoroughly characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoelectron spectroscopy (SRPES). The influence of substrate annealing during the deposition and Pd doping on the morphology, crystalline structure and stoichiometry of the films are discussed. GLAD with assisted magnetron sputtering allowed us to deposit broad range of SnO2 nanostructures while annealing of the substrate during deposition affected the films crystallinity. Also, we found out that doping of the SnO2 films with Pd leads to alloy phase formation. These findings can be applied in variety of applications including gas sensing, catalysis, optics and electronics.

Achieving high stability of supported noble metal nanoparticles with respect to sintering is one of the major challenges in electrocatalysis. In this study, we explored the role of metal–support interaction in stabilizing the morphology of a well-defined model electrode consisting of Pt nanoparticles supported on well-ordered Co3O4(111) films on Ir(100). We employed X-ray photoelectron spectroscopy coupled with an electrochemical cell to analyze changes in the oxidation states of both the supported Pt nanoparticles and Co3O4(111) support as a function of electrode potential. We found that immersion into the aqueous electrolyte at pH 10 (phosphate buffer) has no effect on the integrity and chemical composition of the Co3O4(111) film in a potential window between 0.5 and 1.4 VRHE. At lower potentials, reduction of the Co3O4(111) to Co(OH)2 and metallic Co is accompanied by rapid dissolution of the film. In the presence of supported Pt particles, metal–support interaction gives rise to the formation of parti...

2-Propanol and its dehydrogenated counterpart acetone can be used as a rechargeable electrofuel. The concept involves selective oxidation of 2-propanol to acetone in a fuel cell coupled with reverse catalytic hydrogenation of acetone to 2-propanol in a closed cycle. We studied electrocatalytic oxidation of 2-propanol on complex model Pt/Co3O4(111) electrocatalysts prepared in ultra-high vacuum and characterized by scanning tunneling microscopy. The electrocatalytic behavior of the model electrocatalysts has been investigated in alkaline media (pH 10, phosphate buffer) by means of electrochemical infrared reflection absorption spectroscopy and ex-situ emersion synchrotron radiation photoelectron spectroscopy as a function of Pt particle size and compared with the electrocatalytic behavior of Pt(111) and pristine Co3O4(111) electrodes under similar conditions. We found that the Co3O4(111) film is inactive towards electrochemical oxidation of 2-propanol under the electrochemical conditions (0.3–1.1 VRHE). The electrochemical oxidation of 2-propanol readily occurs on Pt(111) yielding acetone at an onset potential of 0.4 VRHE. The reaction pathway does not involve CO but yields strongly adsorbed acetone species leading to a partial poisoning of the surface sites. On model Pt/Co3O4(111) electrocatalysts, we observed distinct metal support interactions and particle size effects associated with the charge transfer at the metal/oxide interface. We found that ultra-small Pt particles (around 1 nm and below) consist of partially oxidized Pt δ + species which show minor activity towards 2-propanol oxidation. In contrast, conventional Pt particles (particle size of a few nm) are mainly metallic and show high activity toward 2-propanol oxidation.

Oxide supports can modify and stabilize platinum nanoparticles (NPs) in electrocatalytic materials. We studied related phenomena on model systems consisting of Pt NPs on atomically defined Co3O4(111) thin films. Chemical states and dissolution behavior of model catalysts were investigated as a function of the particle size and the electrochemical potential by ex situ emersion synchrotron radiation photoelectron spectroscopy and by online inductively coupled plasma mass spectrometry. Electronic metal–support interaction (EMSI) yields partially oxidized Ptδ+ species at the metal/support interface of metallic nanometer-sized Pt NPs. In contrast, subnanometer particles form Ptδ+ aggregates that are exclusively accompanied by subsurface Pt4+ species. Dissolution of Cox+ ions is strongly coupled to the presence of Ptδ+ and the reduction of subsurface Pt4+ species. Our findings suggest that EMSI directly affects the integrity of oxide-based electrocatalysts and may be employed to stabilize Pt NPs against sintering and dissolution.

The decomposition mechanisms of ethanol and ethylene glycol on well-ordered stoichiometric CeO2(111) and partially reduced CeO2−x (111) films were investigated by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature programmed desorption. Both alcohols partially deprotonate upon adsorption at 150 K and subsequent annealing yielding stable ethoxy and ethylenedioxy species. The C–C bond scission in both ethoxy and ethylenedioxy species on stoichiometric CeO2(111) involves formation of acetaldehyde-like intermediates and yields CO and CO2 accompanied by desorption of acetaldehyde, H2O, and H2. This decomposition pathway leads to the formation of oxygen vacancies. In the presence of oxygen vacancies, C–O bond scission in ethoxy species yields C2H4. In contrast, C–C bond scission in ethylenedioxy species on the partially reduced CeO2−x (111) is favored with respect to C–O bond scission and yields methanol, formaldehyde, and CO accompanied by the desorption of H2O and H2. Still, scission of C–O bonds on both sides of the ethylenedioxy species yields minor amounts of accompanying C2H4 and C2H2. C–O bond scission is coupled with a partial recovery of the lattice oxygen in competition with its removal in the form of water.

Hydroxamic acids are an interesting anchor group for organic molecules on oxide surfaces, but very few studies exist on their adsorption on well-defined single crystal surfaces under well-defined conditions. In the present paper, we use X-ray photoelectron spectroscopy to follow the adsorption of benzohydroxamic acid on a rutile TiO2(110)-(1×1) single-crystal surface. We compare evaporation in ultrahigh vacuum with deposition from ethanol and acetonitrile solutions. Furthermore, we carried out density functional theory calculations to support the assignment of adsorbates. Several species appear on the surface, which are most evident in the N 1s region. The low-coverage species can be explained as originating from a hydroxamate, and decomposed species related to oxygen vacancies or defects. Identification of the high-coverage species, however, remains elusive, and several possibilities are discussed.

Electrical characterisation of perovskite solar cells consisting of room-temperature atomic-layer-deposited aluminium oxide (RT-ALD-Al2 O3 ) film on top of a methyl ammonium lead triiodide (CH3 NH3 PbI3 ) absorber showed excellent stability of the power conversion efficiency (PCE) over a long time. Under the same environmental conditions (for 355 d), the average PCE of solar cells without the ALD layer decreased from 13.6 to 9.6 %, whereas that of solar cells containing 9 ALD cycles of depositing RT-ALD-Al2 O3 on top of CH3 NH3 PbI3 increased from 9.4 to 10.8 %. Spectromicroscopic investigations of the ALD/perovskite interface revealed that the maximum PCE with the ALD layer is obtained when the so-called perovskite cleaning process induced by ALD precursors is complete. The PCE enhancement over time is probably related to a self-healing process induced by the RT-ALD-Al2 O3 film. This work may provide a new direction for further improving the long-term stability and performance of perovskite solar cells.

Graphene layers were prepared by isothermal decomposition of different unsaturated hydrocarbons on Pt(111) single crystal surface and characterized by SRPES and LEED. Four different alkenes (ethene, propene, 1-pentene, 1-hexene) were used in order to elucidate the eventual role of the carbon chain length in the graphene growth kinetics. The process of dehydrogenation was followed stepwise from sample temperatures of 160 K (molecular adsorption) to over 1000 K, at which the surface carbon remains almost exclusively in the form of graphene. Reaction intermediates were identified for each alkene and the corresponding reaction rates were determined. The attachment of carbon during the thermally activated growth of graphene islands on Pt(111) and similar metals is predicted to proceed via prior formation of small clusters or chains in order to overcome a large energetic and spatial barrier which exists between a single adsorbed C atom (monomer) and the graphene layer. In accordance with this assumption the gro...

Electronic metal–support interactions play a key role in the design of heterogeneous catalysts, as they provide a tool for tuning catalytic properties and enhancing catalyst stability. In this work...

Electrocatalysis is at the heart of our future transition to a renewable energy system. Most energy storage and conversion technologies for renewables rely on electrocatalytic processes and, with increasing availability of cheap electrical energy from renewables, chemical production will witness electrification in the near future1-3. However, our fundamental understanding of electrocatalysis lags behind the field of classical heterogeneous catalysis that has been the dominating chemical technology for a long time. Here, we describe a new strategy to advance fundamental studies on electrocatalytic materials. We propose to 'electrify' complex oxide-based model catalysts made by surface science methods to explore electrocatalytic reactions in liquid electrolytes. We demonstrate the feasibility of this concept by transferring an atomically defined platinum/cobalt oxide model catalyst into the electrochemical environment while preserving its atomic surface structure. Using this approach, we explore particle size effects and identify hitherto unknown metal-support interactions that stabilize oxidized platinum at the nanoparticle interface. The metal-support interactions open a new synergistic reaction pathway that involves both metallic and oxidized platinum. Our results illustrate the potential of the concept, which makes available a systematic approach to build atomically defined model electrodes for fundamental electrocatalytic studies.

The influence of a small amount of Si in a Ni single crystal on the interface formation between aluminum oxide and Ni has been investigated. The interface was formed by in-situ growth of the oxide by simultaneous supply of Al and oxygen onto Ni(1 1 1) in an ultrahigh vacuum chamber equipped with XPS apparatus. The oxide growth and the interface formation were compared between Si-containing Ni(1 1 1) and pure Ni(1 1 1). It was revealed that Si segregated on the surface of Ni and oxidized, forming an epitaxial thin alumino-silicate film. Valence band spectra demonstrated that the band offset between the oxide and Ni (energy level difference between the valence band top and the Fermi level) is different due to the oxidized Si segregation at the interface.

Understanding the correlation between structure and reactivity of oxide surfaces is vital for the rational design of catalytic materials. In this work, we demonstrate the exceptional degree of structure sensitivity of the water dissociation reaction for one of the most important materials in catalysis and electrocatalysis. We studied H2O on two atomically defined cobalt oxide surfaces, CoO(100) and Co3O4(111). Both surfaces are terminated by O2– and Co2+ in different coordination. By infrared reflection absorption spectroscopy and synchrotron radiation photoelectron spectroscopy we show that H2O adsorbs molecularly on CoO(100), while it dissociates and forms very strongly bound OH and partially dissociated (H2O)n(OH)m clusters on Co3O4(111). We rationalize this structure dependence by the coordination number of surface Co2+. Our results show that specific well-ordered cobalt oxide surfaces interact very strongly with H2O whereas others do not. We propose that this structure dependence plays a key role in ...

The interplay between the metal-support interaction and stability with respect to sintering has been investigated for Pt nanoparticles supported on well-ordered Co3O4(111)/Ir(100) films in UHV and under oxidizing conditions by means of synchrotron radiation photoelectron spectroscopy (SRPES) and near ambient pressure X-ray photoelectron spectroscopy (NAP XPS). The electronic metal-support interaction between Pt and Co3O4(111) associated with charge transfer results in partial reduction of Co3O4(111) yielding partially oxidized Ptδ+ species at the interface. The stability of the supported Pt particles is coupled with the oxidation state of Ptδ+ species, which can be reduced or oxidized depending on the Pt coverage and reactive environment. Annealing of Pt/Co3O4(111)/Ir(100) in UHV triggers the reduction of Ptδ+ species. At higher temperature, reverse spillover of oxygen to the Pt nanoparticles is accompanied by reduction of Co3O4(111). Under these conditions, the oxidation state of Ptδ+ species depends strongly on Pt coverage. Thus, at low Pt coverage (0.3 ML Pt), Ptδ+ is converted to Pt4+, at intermediate coverage (1.3 ML Pt), Ptδ+ remains stable, and at high Pt coverage (1.93 ML), Ptδ+ is reduced to Pt0. Sintering of Pt particles is associated with the reduction of the Ptδ+ species. This process is prevented under oxidizing conditions due to the formation of an interfacial oxide PtOx. The formation of an interfacial PtOx is observed at 300 K under exposure to 1 × 10−6 mbar O2 at Pt coverages below 1.3 ML. Using NAP XPS, we observe the formation of an interfacial PtOx at high Pt coverage (2.0 ML) in an oxygen atmosphere (1 mbar) at 300 K while the formation of surface PtOx is kinetically hindered and occurs above 550 K only.

The adsorption of adenine, one of the structural units of DNA and RNA, on nanostructured cerium oxide was studied using synchrotron radiation based techniques: photoelectron and X-ray absorption spectroscopies. Using a systematic approach, we studied this biomolecule’s bonding to the inorganic surface and examined the effects of different stoichiometry, morphology and composition of cerium oxide films, as well as two methods of molecular deposition (evaporation in vacuum and deposition from aqueous solution). The adenine molecule chemisorbs on the stoichiometric (IV) cerium oxide intact via nitrogen atoms, independent of the oxide morphology and deposition technique. Annealing of the adenine adlayer at 250 °C causes CeO2 surface partial reduction, along with the partial deprotonation of the nitrogen. The reaction of adenine with ex situ prepared CeO2 films (nanostructured compact and porous) activates Ce4+ cation reduction not only on the surface but also in the subsurface layers accompanied by water deso...

Mixed CuO(2 × 1)–CuWO4 layers on a Cu(110) surface have been prepared by the on-surface reaction of the CuO(2 × 1) surface oxide with adsorbed (WO3)3 clusters. The adsorption and decomposition of methanol on these well-defined CuO–CuWO4 surfaces has been followed by high-resolution X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD) to assess the molecular surface species and their concentration, while the state of the surface oxide phases before and after methanol decomposition has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and XPS. Surface methoxy species form the primary methanol decomposition products, which desorb partly by recombination as methanol at 200–300 K or decompose into CHx and possibly CO. The most reactive surfaces are mixed CuO–CuWO4 phase, with CuWO4 coverages 0.5–0.8 monolayer, thus pointing at the importance of oxide phase boundary sites. ...

As-deposited Pt-Ni and annealed Pt-Ni alloy catalyst films prepared by magnetron co-sputtering were investigated in order to quantify their ageing during accelerated durability test. The parameters of the durability test were chosen to simulate severe potential conditions that may occur at start-up/shut-down cycles of a fuel cell. Using in situ electrochemical atomic force microscopy complemented with ex situ energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, synchrotron radiation photoelectron spectroscopy and X-ray diffraction provided step-by-step correlation of chemical and morphology changes induced by catalyst’s aging processes. We show that the as-deposited Pt-Ni films face severe chemical stability problem which leads to almost complete destruction of the alloy during the durability test. On the other hand Pt-Ni films annealed in vacuum are more resistive to morphological and chemical changes. It was confirmed that upon annealing due to redistribution of Pt and Ni atoms Pt-skin is formed on catalyst surface, which is responsible for a better stability during the aging test.

Using a combination of LEED, high-resolution XPS, XSW, and NEXAFS, we have studied the adsorption of cobalt(II) 5,10,15,20-tetraphenylporphyrin (CoTPP) on Ag(100) at 300 K. In agreement with previous studies on Ag(111), we find a charge transfer from the silver surface to the porphyrin molecule, reducing the metal center. At high coverages we observe a square-shaped 1.41 × 1.41 nm2 adsorption structure, which becomes more open at lower coverages. Because of the superior energy resolution of the Diamond i09 beamline, we are able to resolve a low-binding-energy shoulder in the C 1s spectrum, originating from the lower carbon atoms in the rotated phenyl rings. This is confirmed by XSW measurements, which also gives the adsorption heights of the other atoms in the molecule. In addition, the XSW and complementary NEXAFS measurements yield information about the rotation of the phenyl rings and the deformation of the macrocycle.

The stability and the conversion of atomically dispersed Pt2+ species to sub-nanometer Pt particles have been investigated as a function of the Sn concentration in Pt–CeO2 films by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and angle-resolved X-ray photoelectron spectroscopy in combination with density functional calculations. The deposition of Sn onto the Pt–CeO2 films triggers the reduction of Ce4+ cations to Ce3+ yielding Sn2+ cations. Consecutively, the redox coupling between the Ce3+ and Pt2+ species triggers the reduction of Pt2+ species yielding sub-nanometer Pt particles. The onset of reduction of Pt2+ species is directly related to the concentration of Ce3+ centers which, in turn, is controlled by the concentration of Sn2+ cations in the Pt–CeO2 film. On average, the formation of 6Ce3+ centers corresponding to the adsorption of 3Sn atoms gives rise to the reduction of one Pt2+ species. The analysis of the depth distribution of Sn atoms in the Pt–CeO2 films revealed preferential adsorption of Sn2+ at the surface followed by diffusion of Sn2+ ions into the bulk at higher Sn coverages. Density functional modeling suggested that the adsorption of three Sn atoms in the vicinity of the Pt2+ species results in a rearrangement of the local coordination accompanied by substantial destabilization of the Pt2+ species followed by its conversion to Pt0 atoms. The formation of sub-nanometer Pt particles is coupled with re-oxidation of two Ce3+ centers per one Pt2+ species reduced. Annealing of the Pt–CeO2 films in the presence of metallic Sn also leads to the reduction of the Pt2+ species due to thermally triggered oxidation of metallic Sn residues followed by diffusion of Sn2+ into the bulk. Annealing of the Pt–CeO2 films to temperatures above 600 K results in a loss of Sn yielding sub-nanometer Pt particles supported on nearly stoichiometric and Sn-free CeO2 films.

Iridium-based materials are among the most active bifunctional catalysts in heterogeneous catalysis and electrocatalysis. We have investigated the properties of atomically defined Ir/CeO2(111) model systems supported on Cu(111) and Ru(0001) by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), scanning tunneling microscopy, and temperature programmed desorption. Electronic metal-support interactions in the Ir/CeO2(111) system are accompanied by charge transfer and partial reduction of CeO2(111). The magnitude of the charge transfer depends strongly on the Ir coverage. The Ir/CeO2(111) system is stable against sintering upon annealing to 600 K in ultrahigh vacuum (UHV). Annealing of Ir/CeO2(111) in UHV triggers the reverse oxygen spillover above 450 K. The interaction of hydrogen with Ir/CeO2(111) involves hydrogen spillover and reversible spillover between 100 and 400 K accompanied by the formation of water above 190 K. Formation of water coupled with the strong reduction of CeO2(111) represents the dominant reaction channel upon annealing in H2 above 450 K. The interaction of Ir/CeO2(111) with oxygen has been investigated at moderate and NAP conditions. Additionally, the formation and stability of iridium oxide prepared by deposition of Ir in oxygen atmosphere was investigated upon annealing in UHV and under exposure to H2. The oxidation of Ir nanoparticles under NAP conditions yields stable IrOx nanoparticles. The stability of Ir and IrOx nanoparticles under oxidizing conditions is hampered, however, by encapsulation by cerium oxide above 450 K and additionally by copper and ruthenium oxides under NAP conditions.

We have studied the adsorption and interfacial reactions of 2H-tetraphenylporphyrin (2HTPP) with cobalt-terminated Co3O4(111) and oxygen-terminated CoO(111) thin films using synchrotron-radiation X-ray photoelectron spectroscopy. Already at 275 K, we find evidence for the formation of a metalated species, most likely CoTPP, on both surfaces. The degree of self-metalation increases with temperature on both surfaces until 475 K, where the metalation is almost complete. At 575 K the porphyrin coverage decreases drastically on the reducible cobalt-terminated Co3O4(111) surface, while higher temperatures are needed on the non-reducible oxygen-terminated CoO(111). The low temperature self-metalation is similar to that observed on MgO(100) surfaces, but drastically different from that observed on TiO2(110), where no self-metalation is observed at room temperature. Fil: Wechsler, Daniel. Universitat Erlangen-Nuremberg; Alemania Fil: Fernández, Cynthia Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina Fil: Tariq, Quratulain. Universitat Erlangen-Nuremberg; Alemania Fil: Tsud, Nataliya. Elettra - Synchrotron Ligght Laboratory; Italia Fil: Prince, Kevin C.. Consiglio Nazionale delle Ricerche; Italia. Istituto Officina dei Materiali; Italia. Elettra - Synchrotron Ligght Laboratory; Italia Fil: Williams, Federico José. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universitat Erlangen-Nuremberg; Alemania Fil: Steinrück, Hans Peter. Universitat Erlangen-Nuremberg; Alemania Fil: Lytken, Ole. Universitat Erlangen-Nuremberg; Alemania

Polycrystalline cerium oxide thin films (15 nm) deposited on a glassy carbon substrate were used as an electrode in a mediator-free, non-enzymatic electrochemical sensor for hydrogen peroxide. The electrode surface was characterized by X-ray photoelectron spectroscopy, resonant photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy. The electrode sensitivity, detection limit and pH range of sensor stability were determined by applying electrochemical techniques: cyclic voltammetry and chronoamperometry. It was found that the sensor reactivity to H2O2 is directly related to the presence of electroactive cerium centres of 3+ character on the electrode surface. The Michaelis–Menten mechanism of catalase-like activity of ceria film is suggested as an explanation of the data and discussed. The results confirmed the sensing abilities of technologically well-accessible nanostructured cerium oxide films for hydrogen peroxide detection without using a mediator, i.e. the enzymatic properties of CeO2/GC electrode.

The stability of atomically dispersed Pt2+ species on the surface of nanostructured CeO2 films during the reaction with methanol has been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The isolated Pt2+ species were prepared at low Pt concentration in Pt–CeO2 film. Additionally, Pt2+ species coexisting with metallic Pt particles were prepared at high Pt concentration. We found that adsorption of methanol yields similar decomposition products regardless of Pt concentration in Pt–CeO2 films. A small number of oxygen vacancies formed during the methanol decomposition can be replenished in the Pt–CeO2 film with low Pt concentration by diffusion of oxygen from the bulk. In the presence of supported Pt particles, a higher number of oxygen vacancies leads to a partial reduction of the Pt2+ species. The isolated Pt2+ species are reduced under rather strongly reducing conditions only, i.e. during annealing under continuous exposure to methanol. Reduction of isolated Pt2+ species results in the formation of ultra-small Pt particles containing around 25 atoms per particle or less. Such ultra-small Pt particles demonstrate excellent stability against sintering during annealing of Pt–CeO2 film with low Pt concentration under reducing conditions.

The influence of phosphorus and water on methane catalytic combustion was studied over Pd@CeO2 model catalysts supported on graphite, designed to be suitable for X-ray Photoelectron Spectroscopy/Synchrotron Radiation Photoelectron Spectroscopy (XPS/SRPES) analysis. In the absence of P, the catalyst was active for the methane oxidation reaction, although introduction of 15% H2O to the reaction mixture did cause reversible deactivation. In the presence of P, both thermal and chemical aging treatments resulted in partial loss of activity due to morphological transformation of the catalyst, as revealed by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analysis. At 600 degrees C the combined presence of PO43- and water vapor caused a rapid, irreversible deactivation of the catalyst. XPS/SRPES analysis, combined with operando X-ray Absorption Near Edge Structure (XANES) and AFM measurements, indicated that water induces severe aggregation of CeO2 nanoparticles, exposure of CePO4 on the outer layer of the aggregates and incorporation of the catalytic-active Pd nanoparticles into the bulk. This demonstrates a temperature-activated process for P-poisoning of oxidation catalysts in which water vapor plays a crucial role. (C) 2015 Elsevier B.V. All rights reserved.

Histidine adsorption from neutral aqueous solution on cerium oxide substrates was studied by photoemission with use of synchrotron radiation, soft X-ray absorption spectroscopy and nuclear magnetic resonance. Polycrystalline oxide films and oxide nanoparticles were used as ceria substrates. Independent of the morphology of the support, histidine binds to the oxide through the carboxylic group while the imidazole ring does not participate in the interface formation. Compared to deposition of molecules by evaporation in vacuum, the presence of the solution during adsorption does not alter the histidine bonding to cerium oxide. The present results clearly demonstrate the applicability of the model ( in-situ ) studies of the histidine/CeO 2 interface to the biocompatible techniques of cerium oxide functionalization.

The stability and atomic ordering in Pt–Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt–Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt–Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt–Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt–Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.

A Pt-Co nanoalloy thin film catalyst was prepared by using simultaneous magnetron sputtering of Pt and Co. The catalyst was characterized during accelerated degradation test using in-situ electrochemical atomic force microscopy complemented with ex-situ techniques such as energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and synchrotron radiation photoelectron spectroscopy. The combined results gave the full step-by-step picture of the catalyst behavior during the aging test.

Proton exchange membrane (PEM) water electrolysis (PEMWE) is getting more attention in recent years as a promising alternative in context of energy storage from renewables. However high prices of platinum and iridium, currently considered to be the state-of-the-art electrocatalysts, prevent wider commercialization of this technology. In this paper, we present unconventional and cost-effective preparation method of anode catalyst, containing low amount of noble metal. Thin Ir film is magnetron sputtered on TiC-based support sublayer, hot-pressed on anode side of Nafion® N115 PEM. Following three parameters were systematically varied and their impact on PEMWE in-cell performance was evaluated: total TiC-based support material loading on the PEM, ionomer content within the support sublayer and Ir catalyst loading on top of the support sublayer. In addition, TiC-based sublayer underwent accelerated aging procedure, followed by photoelectron spectral analysis to prove its ability to withstand high anodic potentials. Remarkable PEMWE in-cell performances were obtained, considering amount of used Ir; 1.74 V (with ∼80 μg cm−2 of Ir), 1.72 V (with ∼160 μg cm−2 of Ir) and 1.71 V (with ∼240 μg cm−2 of Ir) at 1 A cm−2 and 80 °C.

Doping of oxides for catalytic applications represents one of the most used strategies for improving their catalytic performance. Model catalyst systems for doped oxides that would contain the dopant atoms in a well-defined geometry allowing for investigation of relationships between structure and reactivity are however rare. Here we report on preparation and structural properties of two-dimensional W-doped ceria on W(110) substrate. This model system adopts geometry of two-dimensional Ce 6 WO 12 (100)-oriented thin film, including isolated W 6+ ions that donate the charge to Ce 3+ ions and act as high-valence dopants. The system represents a model catalyst for prospective use in investigating the role of W 6+ active sites in CeO x -WO x catalysts and high valence dopants in ceria in general.

We have investigated the stability and the reactivity of atomically dispersed Pt, Pd, and Ni species on nanostructured CeO2 films by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. All three metals reveal specific similarities associated with the high adsorption energy of atomically dispersed Pt2+, Pd2+, and Ni2+ species that exceeds the corresponding cohesive energies of the bulk metals. The corresponding Pt–CeO2, Pd–CeO2, and Ni–CeO2 model catalysts have been prepared in the form of thin films on CeO2(111)/Cu(111) substrates and investigated experimentally under ultrahigh vacuum conditions. The atomically dispersed Pt2+, Pd2+, and Ni2+ species were formed exclusively at low concentrations of the corresponding metals. High concentrations resulted in the presence of additional metal oxide phases and emergence of metallic particles. We found that under the employed experimental conditions the Pd–CeO2 film...

The electronic metal-substrate interaction plays an important role in the surface reactions on supported metal nanoparticles. We study the interaction between rhodium clusters and cerium oxide substrate having various stoichiometries, CeOx (2.00 > x > 1.67), by means of photoelectron spectroscopy. Our results show that rhodium deposited on substrates with stoichiometry of 2.00 > x > 1.93 induces reduction of cerium oxide. On the other hand, cerium oxide with a higher degree of reduction (1.93 > x > 1.67) becomes partially oxidized after the Rh deposition. Density functional theory simulations of Rh adatom adsorbed on the CeO2(111) surface and on an oxygen vacancy at the CeOx(111) surface show that there is a charge transfer between rhodium and cerium oxide substrate. While Rh adsorption on the CeO2(111) surface leads to electron depletion at the Rh adatom and cerium oxide reduction, Rh adsorption on the oxygen vacancy at CeOx(111) leads to electron accumulation about the Rh adatom and partial oxidation of cerium oxide. The results clearly demonstrate that the electronic metal-substrate interaction occurs on Rh/CeOx systems and strongly depends on the stoichiometry of cerium oxide. These findings could be beneficial for designing catalysts with specific properties.

The adsorption of adenine on the CeO2(1 1 1)/Cu(1 1 1) surface in vacuum was studied by photoelectron spectroscopy, resonant photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy, and the present work describes in detail the bonding of the molecule to the ordered stoichiometric cerium dioxide film. The experimental findings were supported by density functional theory (DFT + U) analysis of different adsorption geometries of adenine on CeO2(1 1 1). The phase with adenine lying flat on the surface dominates on CeO2(1 1 1) up to 0.1 monolayer (ML) of adenine coverage. The mobility of single molecules was apparently sufficiently high to allow diffusion and formation of chain structures, which were observed to be stable in the temperature range from 25 to 250 °C. Beyond 0.1 ML, adenine molecules adsorb predominantly in an upright orientation. This phase, stable up to 120 °C, is according to theory stabilised via N3/Ce4+ and N9H/O2–. It was further complemented by experimental findings demonstrating free N10H2 groups in adsorbed molecules. Thus, the saturation coverage of adenine on CeO2(1 1 1), 0.23 ML, is most likely characterised by a combination of parallel and upright bound molecules.

Binding of functionalized organic molecules to oxide surfaces is an important step in the rational design of molecular devices. In the present investigation, we used synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy to determine the binding mode, electronic structure and adsorption geometry of phenylphosphonic acid (PPA) on TiO2(110)–(1 × 1). We found that PPA multilayers desorb below 380 K leaving a compact PPA monolayer adsorbed on the surface, which remains stable up to 780 K. In the 380–520 K temperature range, molecules are anchored to the surface via a single P–O–Ti covalent bond (monodentate configuration). Furthermore, the phenyl ring is tilted ~45° with respect to the surface plane and it either forms 45° or is randomly oriented with respect to [001] crystallographic direction. Raising the temperature above 520 K partially transforms the monodentate configuration to a mixed one- and twofold deprotonated bidentate binding mode, presumably after surface hydroxyl groups leave the surface as water molecules. This change in molecular binding does not alter the molecular electronic structure nor the adsorption geometry, which remain essentially unchanged.