Download pdf - COST Redox General Meeting 2012...COST Redox General Meeting 2012, Prague 3 Workshop Programme Wednesday, September 26 14:00‐14:10 Opening 14:10‐14:35 Environmental effects and

COSTRedoxGeneralMeeting2012

InitialWorkshopoftheCMSTCOSTActionCM1104

Reducibleoxidechemistry,structureandfunctions

September 26‐28, 2012 Prague, Czech Republic

Programme and Book of Abstracts

organized by Surface Science Group Department of Surface and Plasma Science Charles University in Prague

COST Redox General Meeting 2012, Prague

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WorkshopProgramme

Wednesday,September26

14:00‐14:10 Opening

14:10‐14:35 Environmental effects and reactivity of iron oxide films on Pt(111)

Livia Giordano

14:35‐15:00 Understanding the high reducibility of gallium(III)‐doped ceria

Monica Calatayud

15:00‐15:20 Insight into the adsorption of water on the clean CeO2(111) surface with van der Waals and hybrid density functionals

Delia Fernández‐Torre

15:20‐15:40 Polarity in low dimensions: MgO nano‐islands on Au(111)

Jacek Goniakowski

15:40‐16:00 Simulated Photoemission Spectra of Hydroxylated MgO(100) at Elevated Temperatures

Lauro Oliver Paz‐Borbón

16:00‐16:30 Coffee Break

16:30‐16:55 DFT and HR‐STEM investigations into morphology and electronic structure of Co3O4 redox nanooxide

Filip Zasada

16:55‐17:15 Free energy calculation for the H abstraction from propane by V2O5/SiO2: Molecular dynamics compared to harmonic approximation

Witold Piskorz

17:15‐17:35 Design of novel WO3‐based materials with tailored optical, photo‐redox or catalytic properties

Cristiana Di Valentin

17:35‐17:55 Packing Defects into Ordered Structures

Henrik H. Kristoffersen

17:55‐18:15 Ordered Array of Single Au Adatoms on Fe3O4(001) with Remarkable Thermal Stability

Zbynek Novotny

18:15‐18:40 A novel 2‐D ternary Cu‐tungstate phase on Cu(110)2x1‐O surface oxide

Falko P. Netzer

20:00‐23:00 Dinner


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Thursday,September27

8:30‐8:55 Unified Picture of the Excess Electron Distribution at the TiO2(110) Surface

Jacques Jupille

8:55‐9:20 (Sub)surface oxygen vacancies on TiO2 anatase (101) and O2 adsorption

Martin Setvín

9:20‐9:40 Exploring the electronic structure and optical excitations of F‐doped TiO2: Relevance to Photocatalysis

Francesc Illas

9:40‐10:00 Domain walls of CO adlayers on Pd nanoparticles

Geoff Thornton

10:00‐10:20 Mechanism of Surface Reactions: Insights from First Principles Calculations

Javier Fdez. Sanz

10:20‐11:00 Coffee Break

11:00‐11:25 (VOn)m (n = 1, 2; m = 1 − 3) submonolayers supported on the CeO2(111) surface: Insights into structure and reactivity of a complex catalytic system from density functional theory

Joachim Paier

11:25‐11:50 In‐situ nanoscale redox chemistry of cerium oxide islands on Ru(0001)

Jan Ingo Flege

11:50‐12:10 Recent progress in density functional studies of ceria‐based nanostructures

Konstantin M. Neyman

12:10‐12:30 DRIFTS and XPS operando studies of an inverse CeO2/CuO water‐gas shift catalyst

Arturo Martínez‐Arias

12:30‐12:50 Thin film cerium oxide based catalysts for fuel cell application

Vladimír Matolín

12:50‐13:50 Lunch


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13:50‐14:15 Decomposition pathways of carboxylic acid on Pt/ceria model catalysts

Yaroslava Lykhach

14:15‐14:35 DFT study of Zn‐Ti oxide photocatalysts

José C. Conesa

14:35‐14:55 Atomistic structure of cobalt‐oxide/phosphate nanoparticles for catalytic water oxidation

Stefano Fabris

14:55‐15:15 Influence of carbon‐carbon bond order and silver loading on the oxidative function of silver‐alumina catalysts in absence and presence of NO

Hanna Härelind

15:15‐15:35 The application of high‐resolution IR spectroscopy and isotope labeling for detailed investigation of TiO2/gas interface reactions.

Svatopluk Civiš

15:35‐18:00 Coffee Break & Poster Session

16:30‐20:00 MC Meeting

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Friday,September28

8:30‐8:55 Imidazolium‐based ionic liquids interacting with ordered cerium dioxide surfaces: a synchrotron‐radiation photoelectron spectroscopy study

Mathias Laurin

8:55‐9:20 Tuning metal‐support interactions in reducible Pd@CeO2 core shell based catalysts

Paolo Fornasiero

9:20‐9:40 Active sites for methane dissociation over metallic and oxidized palladium

Henrik Grönbeck

9:40‐10:00 Potential method for detecting oxygen vacancies in oxides

Filip Tuomisto

10:00‐10:20 Facilitated lattice oxygen depletion in consolidated TiO2 nanocrystal powders: a spectroscopic O2 adsorption study

Oliver Diwald

10:20‐11:20 Coffee Break & Lab Tour

11:20‐11:45 First principles calculations of formation and migration of oxygen vacancies in

La1‐xSrxCo1‐yFeyO3‐ perovskites

Yuri A. Mastrikov

11:45‐12:05 Solar Cells with Overall Water Splitting Using Oligoaniline‐Crosslinked [Ru(bpy)2(bpyCONHArNH2)]

+2 Dye/Iridium Oxide Nanoparticle Arrays On Three–Dimensionally Ordered Macroporous Gold‐Nanoparticle Doped Titanium Dioxide (3‐DMGN‐TiO2) Photonic Crystals Modified Electrodes

Huseyin Bekir Yildiz

12:05‐12:25 Electronic processes at metal/oxide interfaces

Alexander L. Shluger

12:25‐12:45 Impact of oxygen scavenging on electronic properties of thin HfO2 layers

Valery V. Afanas’ev

12:45‐12:55 Closing Remarks


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ORALPRESENTATIONS


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EnvironmentaleffectsandreactivityofironoxidefilmsonPt(111)

Livia Giordano1, Gianfranco Pacchioni1, Jacek Goniakowski2, Claudine Noguera2 1Dipartimento di Scienza dei Materiali, Università di Milano‐Bicocca, Milano, Italy 2CNRS, INSP and Université Pierre et Marie Curie, Campus de Boucicaut, Paris, France

e-mail: [email protected]

Oxide ultra‐thin films of nano‐metric thickness may display novel properties related to the presence of the oxide/metal interface [1]. The possibility of exchange charge with the metallic substrate and the structural flexibility of the film allow the stabilization of charged adsorbates and unusual stoichiometries.

In the case of iron oxide ultrathin films on Pt(111) these combined effects affect the chemical reactivity of the system. In oxygen rich ambient (oxygen partial pressure in the mbar range) the FeO/Pt(111) monolayer incorporates oxygen and evolves in a new phase of FeO2 stoichiometry, stabilized by a charge transfer from the platinum support [2,3,4]. The new phase has been shown to be catalytically active in CO oxidation [2,3].

The stability of the different phases and their possible hydroxylation under realistic conditions have been addressed by considering the interaction of water with the FeO/Pt(111) film and the phase diagram of the film exposed to a mixture of H2O and O2. The results indicate that the formation of the active phases (FeO2 and FeOOH) is driven by the oxygen chemical potential also in presence of water [5].

[1] L. Giordano, G. Pacchioni, Acc. Chem. Res., 44, 1244, (2011). [2] Y.‐N. Sun, Z.‐H. Qin, M. Lewandowski, E. Carrasco, M. Sterrer, S. Shaikhutdinov, H.‐J. Freund, J. Catal., 266, 359 (2009). [3] Y.‐N. Sun, L. Giordano, J. Goniakowski, M. Lewandowski, Z.‐H. Qin, C. Noguera, S. Shaikhutdinov, G. Pacchioni, H.‐J. Freund, Angew. Chem. Inter. Ed., 49, 4418 (2010). [4] L. Giordano, M. Lewandowski, I.M.N. Groot, Y.N. Sun, J. Goniakowski, C. Noguera, S. Shaikhutdinov, G. Pacchioni, H.‐J. Freund, J. Phys. Chem. C, 114, 21504 (2010). [5] F. Ringleb, Y. Fujimori, H. Wang, H. Ariga, E. Carrasco, M. Sterrer, H.‐J. Freund, L. Giordano, G. Pacchioni, J. Goniakowski, J. Phys. Chem. C, 115, 19328 (2011).


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Temperature ( C)

% re

duct

ion

Understandingthehighreducibilityofgallium(III)‐dopedceria

M. Calatayud1, F. Tielens1, M. J. Vecchietti2, S. Collins2, S. Bernal3, A. Bonivardi2

1 Université Pierre et Marie Curie and CNRS, Paris, France 2 Universidad Nacional del Litoral and CONICET, Santa Fe, Argentina. 3 Facultad de Ciencias, Universidad de Cádiz, Puerto Real, Spain.


Doping ceria with alio‐cations is known to be an efficient and versatile way of modulating the textural, structural and chemical properties of the pure oxide. Thus, some of us have chosen the ceria‐gallia system as a support for the study of the water gas shift reaction (1‐3). This work presents the oxygen storage capacity (OSC) of the ceria and gallium‐doped cerium(IV) oxide, together with periodic DFT calculations of the oxygen vacancy formation energy.

Oxygen Storage Capacity To obtain quantitative information about the redox properties of the materials the oxygen storage capacity (OSC) of the synthesized supports was measured. Figure 1 shows the results of OSC as the percentage of reduction of Ce assuming that all the Ce (IV) is reduced to Ce (III). It can be observed that doping CeO2 with 5 and 20% of Ga, the percentage of reduction at 300 °C is 4 and 10 times higher for the latter sample compared with pure ceria. When the temperature is increased the extent of reduction also increases, reaching the equivalent of 22 and 47% of Ce (IV) for Ce95Ga05 and Ce80Ga20. It is clear that in the mixed oxides, most of the weight loss of the sample in the reducing atmosphere can be attributed to the elimination of water caused by the reduction of Ce (IV) to Ce (III). Then, it can be concluded that doping ceria with Ga (III) improved markedly the reducibility of pure cerium oxide.

Fig. 1: left, evolution with temperature of H2‐OSC expressed as % reduction of Ce(IV) to Ce(III). Right, the removal of an oxygen in the stoichiometric Ce75Ga25 model induces a strong geometrical relaxation (dist. in Å).

DFT calculations Total energy calculations were performed with the PBE functional as implemented in

the VASP code. The cost of formation of an oxygen vacancy Evac is used as reactivity index for the reducibility, Evac= Estoichiometric – [ Ereduced + ½ EO2 ]. The Evac in pure CeO2 is found to be 3.23 eV, that for pure gallia is 4.00 eV while for the mixed oxide Ce75Ga25 it is 2.54 eV. These results clearly show that the doped material is more reducible than the pure ones in agreement with the OSC measurements and previous DFT calculations [4]. The presence of oxygen vacancies in the mixed oxide is found to strongly modify the local environment of the lattice ions (Fig. 1). Besides, the formation of an oxygen vacancy leads to two reduction electrons occupying Ce 4f states.

[1] J Vecchietti, et al. Topics Catal. 54 (2011) 201 [2] G Finos, S Collins, G Blanco, E del Rio, JM Cies, S Bernal, A Bonivardi. Catal. Today 180 (2012) 9 [3] S Collins, G Finos, R Alcantara, E del Rio, S Bernal, A Bonivardi. Appl. Catal. A: General 388 (2010) 202 [4] P Quaino, O Syzgantseva, L Siffert, F Tielens, C Minot, M Calatayud. Chem. Phys. Lett. 519‐520 (2012) 69


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Figure 1. Side view of the most stable structures for molecular and ion pair adsorption. Color code: oxygen atoms in the first surface layer are red and those in the third surface layer are orange, hydrogen atoms are white, and cerium atoms are pale yellow.

InsightintotheadsorptionofwateronthecleanCeO2(111)surfacewithvanderWaalsandhybriddensityfunctionals

Delia Fernández‐Torre1, Krzysztof Kośmider1, Javier Carrasco2, M. Verónica Ganduglia‐Pirovano2, Rubén Pérez1

1Dpto. de Física Teórica de la Materia Condensada, UAM, E‐28049 Madrid, Spain 2Instituto de Catálisis y Petroleoquímica, CSIC, E‐28049 Madrid, Spain


Water interacting with ceria surfaces is an essential step in many relevant catalytic processes. In spite of this, a detailed picture of the adsorption of H2O on the most stable facet of CeO2, the clean (111) surface, is still lacking. Previous experimental and theoretical works [1] have concluded that the molecule sits on top of a Ce ion. However, further structural information, e.g. if water adsorbs molecularly or as an ion pair (see Fig. 1), is not yet clear. Currently, the only answer to this question comes from theoretical studies based on Density Functional Theory, and these have reached contradicting conclusions. In this work [2], we study this issue employing higher levels of theory (van der Waals and hybrid density functionals) and also the same levels considered previously (GGA, GGA+U). We find that all methods point to the same answer: both structures are similarly stable, and they are thus expected to coexist. When van der Waals interactions are taken into account, the binding energy of water increases by ~0.18 eV, improving the agreement with the available experimental values. Our calculations also indicate that these two species can be distinguishable by their infrared spectra. Finally, we will also discuss the energy barriers connecting the molecular structure and the ion pair, and also connecting equivalent adsorption positions.

If time allows it, we will also present recent advances in the interpretation of non‐contact atomic force microscopy images of titanium dioxide [3].

[1] Henderson et al., Surf. Sci., 526, 1, (2003); Gritschneder et al, Nanotechnology 18, 044025 (2007); Watkins et al., J. Phys. Chem. C 111, 15337 (2007); Fronzi et al., Phys. Chem. Chem. Phys. 11, 9188 (2009); Yang et al., J. Phys. Chem. C 114, 14891 (2010). [2] Fernández‐Torre et al., J. Phys. Chem. C 116, 13584 (2012). [3] Yurtsever et al., Phys. Rev. B 85, 125416 (2012).


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Polarityinlowdimensions:MgOnano‐islandsonAu(111)

Yi Pan1, Stefania Benedetti2, Philipp Myrach1, Niklas Nilius1, Livia Giordano3, Claudine Noguera4, Jacek Goniakowski4

1Fritz‐Haber‐Institut der Max‐Planck‐Gesellschaft, Berlin, Germany 2National Research Center on nanoStructures and bioSystems at Surfaces, Modena, Italy 3Dipartimento di Scienza dei Materiali, Università di Milano‐Bicocca, Milano, Italy 4Institut des Nanosciences de Paris, CNRS and Université Pierre et Marie Curie, Paris, France

e-mail: [email protected]; [email protected]

Extended polar objects present an electrostatic instability which induces substantial modifications of the surface charge density. Microscopic surface processes related to polarity compensation have been extensively studied in the last decades, also as a potential tool for tuning surface electronic and structural properties [1]. More recently it has become clear that polarity does also concern nano‐objects, but that the relevant electrostatic forces and the response they induce may differ from the known ones [2]. Indeed, below a critical size, nano‐objects may sustain finite dipole moments which drive strongly size‐ and dimensionality‐ dependent properties. At small sizes, polarity effects may also extend beyond the surface region and drive structural transformations of the entire object, resulting in novel structures, with no bulk counterparts. Since oxide nano‐objects, such as ultra‐thin films and nano‐islands, are often synthesized on metal substrates, their polarity characteristics are additionally modified by the electrostatic coupling between their structure and the interface charge transfer [3].

We will illustrate the above concepts with the recent results on monolayer MgO islands on Au(111), explored with low‐temperature scanning tunneling microscopy and density functional theory [4]. In particular, we will show that the observed electronic characteristics of the (111)‐oriented MgO islands are fully compatible with their predicted induced‐polar character, with their structure and their out‐of‐plane dipole moment resulting from a coupling between the film rumpling and the electron exchange at the MgO/Au interface. We will also identify the in‐plane polarity induced by polar island edges, and we will analyze its effect on the relative stability of MgO islands of different orientations and under different experimental conditions.

[1] J. Goniakowski, F. Finocchi, C. Noguera, Rep. Prog. Phys. 71, 016501 (2008). [2] J. Goniakowski, C. Noguera, L. Giordano, Phys. Rev. Lett 93, 215702 (2004); 98 205701 (2007); J. Goniakowski, C. Noguera, Phys. Rev. B 83, 115413 (2011). [3] J. Goniakowski, C. Noguera, Phys. Rev. B 79, 155433 (2009); J. Goniakowski, L. Giordano, C. Noguera, Phys. Rev. B 81, 205404 (2010). [4] Y. Pan et al., J. Phys. Chem. 116, 11126 (2012); N. Nilius et al., submitted Phys. Rev. B.


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SimulatedPhotoemissionSpectraofHydroxylatedMgO(100)atElevatedTemperatures

Lauro Oliver Paz‐Borbón, Anders Hellman, Henrik Grönbeck

Competence Centre for Catalysis, Chalmers University of Technology, SE‐41296 Göteborg, Sweden


The understanding of how water adsorbs and dissociates on oxide surfaces is important in several applied fields, where heterogeneous catalysis is only one example [1]. In this work, we have performed a combination of static density functional theory (DFT) calculations and ab‐initio molecular dynamics (AIMD) simulations to investigate the stability of hydroxylated MgO(100) and, in particular, to evaluate the photoemission O1s core level shifts (CLS) as a function of coverage and temperature. From our results, we observe that thermal vibrations and rapid proton‐exchange at elevated temperatures yields broad features in the simulated photoemission signal (Figure 1), in good agreement with recent experimental observations [2]. Moreover, the results provide further evidence that the stable structure of hydroxylated MgO(100) consists of a partly dissociated water monolayer; a configuration which is also found to be stable even if a second monolayer of water is added. Detailed analysis of the O1s CLS for adsorbed hydroxyl groups at different coverage reveals a pronounced effect on hydrogen bonding to neighboring H2O molecules

As the degree of electron localization, in principle could affect stability and CLS, we have compared the

results obtained with a standard gradient corrected functional (PBE) with one including exact‐exchange (PBE0). For the investigated systems, we find that PBE and PBE0 yield quantitatively similar results [3].

[1] P.A. Thiel; T.E. Madey; Surf. Sci. Rep. 7, 211 (1987) [2] J.T. Newberg, et al.; Surf. Sci. 605, 89 (2011) [3] L.O. Paz‐Borbon, A. Hellman, H. Grönbeck; J. Phys. Chem. C 116, 3545 (2012)

Figure 1. CLS and bond lengths as a function of time for an OsH group during a simulation at 300 K. Inset (a) shows OsH in the energetically preferred situation when the proton is adsorbed at the surface site. Inset (b) shows a snapshot when the proton has been transferred from the surface site to an (OH)ads group forming water. Red and blue lines indicate the corresponding bond lengths and CLS for the OsH group. The black line shows the calculated CLS of the neighbor (OH)ads group.


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DFTandHR‐STEMinvestigationsintomorphologyandelectronicstructureofCo3O4redoxnanooxide

F. Zasada, W. Piskorz, P. Stelmachowski, J. Kaczmarczyk, A. Kotarba, Z. Sojka

Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30‐060 Krakow, Poland


Many important properties of nanostructured oxides depend on their size, dimensionality and shape. Depending on the synthesis method, faceted polyhedral nanocrystals expose well defined crystallographic planes, allowing for sensible investigations into structure‐reactivity relationships at practical conditions. This assists in designing of new catalytic materials of enhanced activity and selectivity by preferential development of more active planes and enhancement of the density of active sites. One of the most attractive oxide materials with remarkable record of widespread applications is cobalt spinel. This oxide has recently received a great deal of theoretical and practical attention because of its outstanding activity in many important redox processes such as low temperature decomposition of N2O, oxidation of methane or as promising substitute for platinum in oxygen reduction reaction (ORR) for fuel cells applications. Noting the particular structure with disparate concentration and atomic arrangements of the active sites it is not surprising that distinct surface planes of Co3O4 exhibit different redox properties.

Figure: Equilibrium Wulff shape (left) together with reconstructed STEM shapes (middle) of cobalt nanocrystals viewed along the [110] direction and HRTEM image (right) of the exposed Co3O4 plane

In this study we present synthesis of the Co3O4 nanocrystals with defined shapes, their characterization

by STEM, and DFT modelling of morphology and electronic properties relevant for their redox behavior. High resolution transmission electron microscopy revealed that the synthesized samples exhibit well developed faceted nanocrystallites. The observed rhombicuboctaedral morphology of the nanospinel was then rationalized using periodic PW91 and PW91+U density functional calculations. The results enabled us to characterize the surface structure and reconstruction of the exposed low index (100), (110), and (111) planes, and to predict the equilibrium habit of the cobalt spinel crystallites, by matching 3D Wulff polyhedra to 2D STEM images along three different [100], [110] and [111] directions (Figure 1)[1]. The spin‐polarized DFT+U and B3LYP calculations of the band structure and the work function (WF) were complemented by Kelvin probe and H2‐TPR measurements, and discussed in terms of redox properties the Co3O4 nanocrystals.

[1] F. Zasada, W. Piskorz, P. Stelmachowski, A. Kotarba, J.‐F.Paul, T. Plocilnski, K. J. Kurzydlowski, Z. Sojka, J. Phys. Chem. C, 115, 6423, (2011)


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Figure 1. Constrained MD: thermodynamic integration for In1 → Ts1‐MD/Ts1‐MD‐BS → In2 for singlet (S), triplet (T), and broken‐symmetry singlet (BS) spin states

FreeenergycalculationfortheHabstractionfrompropanebyV2O5/SiO2:Moleculardynamicscomparedtoharmonicapproximation

Witold Piskorz1, Marek Sierka2, Joachim Sauer3 1Jagiellonian University, Ingardena 3, 30‐060 Kraków, Poland 2Friedrich Schiller University, Löbdergraben 32, Jena, Germany 3Humboldt University, Unter den Linden 6, 10099 Berlin, Germany


In the presentation the comparison of the thermodynamical functions obtained from the harmonic oscillator/rigid rotor (HO‐RR) approximation to those obtained from the molecular dynamics (thermodynamic integration, TI [1]) is discussed. The studied reaction is the oxidative dehydrogenation of propane. The model, vanadyl‐exchanged silsesquioxane cluster (SVO), is adapted from [2]. To facilitate the ab initio molecular dynamics (AIMD) the QMPOT [3] hybrid QM:QM formalism is used. In both HO‐RR and AIMD calculations the DFT level of theory and the semi‐empirical dispersion approximation [4] is used. The novel QM:QM AIMD was tested for the simulation the dipole IR spectra for a few selected temperatures through the application of the Wiener‐Khinchin theorem. In all thermodynamic MD calculations the NVT ensemble at 800 K, a Nosé‐Hoover chain thermostat, and the time step of 1 fs were used. The hydrogen transfer from the propane molecule to the vanadyl group was studied. First, the unconstrained simulations of the system before and after H transfer were done, and, due to the repulsive character of the SVO/propane system the TI was introduced. The resultant reaction energetic profile for the H transfer is shown in the Figure 1. In this figure the starting structure was SVO/propane (In1, singlet spin state), the resultant structure was SVOH/i‐propyl radical (In2, triplet spin state), and the transition structure (Ts1‐MD‐BS, broken symmetry singlet spin state). In the TI calculations the free enthalpy of the activation barrier (ΔGTs1‐ΔGIn1) was lowered by 32 kJ/mol, and the In2 (ΔGIn2‐ΔGIn1) was found 19 kJ/mol less stable, comparing to the HO‐RR.

[1] D. Frenkel, B. Smit, “Understanding Molecular Simulation”, Academic Press 2002. [2] C. Tuma, T. Kerber, J. Sauer, Angew. Chem. 49, (2010) [3] J. Sauer, M. Sierka, J. Comp. Chem., 21, 2000. [4] S. Grimme, J. Comput. Chem., 27, 2006. [5] X. Rozanska, R. Fortrie, J. Sauer, J. Phys. Chem. C, 111, (2007).


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DesignofnovelWO3‐basedmaterialswithtailoredoptical,photo‐redoxorcatalyticproperties

Cristiana Di Valentin, Fenggong Wang, Gianfranco Pacchioni

Dip. di Scienza dei Materiali, Università di Milano‐Bicocca, via Cozzi 53 20125 Milano, Italy


In this presentation we review some recent examples, based on density functional theory calculations, of different approaches to design novel WO3‐based materials with peculiar photo‐redox, optical or catalytic properties: doping, controlled defectivity or oxide clusters deposition on activated surfaces.

Periodic calculation have been performed with hybrid functional methods and an atomic basis set approach, as implemented in the CRYSTAL code. Hybrid functionals provide a more accurate description of the semiconductors electronic structure (e.g. band gap values, spin localization) with respect to standard LDA or GGA functionals.

The first example is doped‐WO3 [1] for photocatalytic water splitting. Doping inorganic solids with impurity atoms is a consolidated method to modify their chemical and electronic properties. Since the position of the conduction and valence band edges of WO3 fits the oxidation potential for O2 evolution but not the reduction potential for H2 evolution, a positive shift of both edges would be highly beneficial for one‐photon water splitting. Calculations indicate that such an effect can be obtained by W substitution with larger metal cations such as Hf [2].

The second examples is defective WO3‐x which is well‐known for its electrochromic properties above a certain degree of oxygen deficiency. The origin of the electrochromic effect in WO3 is, however, still an open issue. We propose [3] a rationalization based on a charge state variation of the oxygen vacancy defects and we also show that these present rather anisotropic properties.

The third example is a catalytic system based on (WO3)3 clusters deposited on the rutile (110) TiO2 surface which is found to catalyze the formaldehyde polymerization reaction at very low temperatures. We propose [4] a novel radical reaction path with very low activation barriers which is triggered by the presence of extra electrons on the (WO3)3 clusters as induced by a defective TiO2 support.

[1] F. Wang, C. Di Valentin, G. Pacchioni, J. Phys. Chem. C, 116, 8901, (2012). [2] F. Wang, C. Di Valentin, G. Pacchioni, ChemCatChem, , 4, 476, (2012). [3] F. Wang, C. Di Valentin, G. Pacchioni, Phys. Rev. B, 84, 073103, (2011). [4] C. Di Valentin, M. Rosa, G. Pacchioni, J. Am. Chem. Soc., ASAP (2012).


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PackingDefectsintoOrderedStructures

Henrik H. Kristoffersen, Lasse B. Vilhelmsen, Jess Stausholm‐Møller, Umberto Martinez, Bjørk Hammer

iNANO and Department of Physics and Astronomy, Aarhus University, Denmark


A prepared rutile TiO2 crystal is often in a reduced state. In this work we show that energy can be gained by packing the defects associated with this reduction into ordered structures. This is illustrated by the packing of oxygen vacancies at <1‐11> steps and the packing of Ti interstitials into strand structures.

Using density functional theory (DFT) and a genetic algorithm (GA) the atomic structure of <1‐11> steps on the rutile TiO2 (110) surface is resolved and found to involve a reconstruction [1]. The reconstruction places one extra TiO2 unit per unit cell at a half‐height position between the upper and lower terrace levels. This optimizes the coordination of titanium atoms. In connection with the reconstructed step it is further found that oxygen vacancies are significantly more stable at the step than on the (110) terrace.

Subsequently a combined STM and DFT study have found evidence for the existence of O vacancies at steps through the apparent depletion of terrace vacancies close to steps and the adsorption of ethanol into O vacancies at steps [2].

Secondly strand structures can form on the rutile TiO2 (110) surface. Using DFT and GA we searched through a range of stoichiometries for possible strand structures. We found that some of these structures were indeed more stable than the equivalent number of Ti interstitials [3]. Termination of a strand on the (110) surface was found to be the most unfavorable part of the strand (left figure). As a consequence strands reduce the number of terminations by growing from <1‐11> steps. These theoretical findings are in good agreement with experimental observations. In particular it was seen with STM that a stepped surface (right figure) promote strand growth.

[1] U. Martinez et al., Phys. Rev. B 84, 205434 (2011). [2] U. Martinez et al., Accepted Phys. Rev. Lett. (2012) [3] R. Bechstein et al., Phys. Rev. Lett. 108, 236103 (2012).


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OrderedArrayofSingleAuAdatomsonFe3O4(001)withRemarkableThermalStability

Zbynek Novotny, Giacomo Argentero, Zhiming Wang, Michael Schmid, Ulrike Diebold, Gareth S. Parkinson*

Institut für Angewandte Physik , Technische Universität, Wiedner Hauptstr. 8‐10/134, 1040 Wien, Austria


We present a scanning tunneling microscopy investigation of gold deposited at the magnetite

Fe3O4(001) surface at room temperature. This surface forms a reconstruction with (2 × 2)R45° symmetry[1], where pairs of Fe and neighboring O ions are slightly displaced laterally, forming undulating rows with 'narrow' and 'wide' adsorption sites. At fractional monolayer coverages, single Au adatoms adsorb exclusively at the 'narrow' sites, with no significant sintering up to annealing temperatures of 400 °C[2]. The strong preference for 'narrow' site is possibly related to charge and orbital ordering within the first subsurface octahedral layers of the reconstructed Fe3O4(001) surface[3]. Because of their high

thermal stability, the ordered Au atoms at Fe3O4(001)−(2 × 2)R45° should provide useful insights into the chemical reactivity of single atomic species.

Figure 1: STM image (30 × 30 nm2, Vsample = +1 V, Itunnel = 0.38 nA) of 0.12 ML Au (1 ML = 1.42 × 1014 atoms/cm2) deposited at room temperature. Au adatoms are located between the surface Fe(B) rows. The inset shows two Au

adatoms in high resolution, with (2 × 2)R45° periodicity indicated by cyan; the nodes are centered in the wide sites of the surface reconstruction. The Au adatoms are located in the center of the cell, i.e. at the narrow sites.

This material is based upon work supported as part of the Center for Atomic‐Level Catalyst Design, an

Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number #DE‐SC0001058.

[1] Pentcheva, R. et al. Phys. Rev. Lett 94, 126101 (2005). [2] Novotny, Z. et al., Phys. Rev. Lett. 108, 216103 (2012). [3] Lodziana, Z. Phys. Rev. Lett. 99, 206402 (2007).


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Anovel2‐DternaryCu‐tungstatephaseonCu(110)2x1‐Osurfaceoxide

M. Denk1, D. Kuhness1, M. Wagner1, S. Surnev1, F. Negreiros2, L. Sementa2, G. Barcaro2, A. Fortunelli2, F.P. Netzer1

1Surface and Interface Physics, Karl‐Franzens University Graz, A‐8010 Graz, Austria 2CNR‐IPCF Pisa, Italy


W oxides have interesting applications in the fields of catalysis, gas sensors or electrochromic devices, which are all related to their tendency towards non‐stoichiometry and their ease of changing the oxidation state. Most applications involve fundamental surface processes, which require characterization at the atomic scale for high‐level use in nanoscopic device technology. Here we explore the feasibility to fabricate metal supported W oxide nanostructures via a novel self‐assembly route by condensation of (WO3)3 clusters on Cu surfaces, deposited directly from the gas phase: we report the formation of a well‐ordered 2‐D ternary CuWOx phase by reaction of (WO3)3 with the Cu(110)(2x1)‐O surface oxide. A beam of (WO3)3 molecules, formed by vacuum sublimation of WO3 powder at 900‐1000°C in a thermal evaporator, has been directed onto a Cu(100)2x1‐O surface at low temperature and the surface has been annealed subsequently at 600K. At low temperature (<15K), the (WO3)3 clusters adsorb in intact molecular form, albeit in a somewhat distorted cluster geometry [1].

Upon heating to 600K, the (WO3)3 molecules dissociate and react with the Cu‐O surface oxide, forming a 2‐D monolayer with a well‐ordered incommensurate quasi‐square structure. The latter has been characterized by high‐resolution STM imaging, LEED and DFT simulations. The chemical nature and electronic structure of this novel 2‐D W‐O‐Cu layer has been investigated by XPS core level spectroscopy, NEXAFS and valence band spectra with use of synchrotron radiation. The unusually sharp W 4f core level spectra, the specific NEXAFS fingerprint, and the evolution of the valence band all suggest that this 2‐D surface oxide is of a ternary Cu‐tungstate CuWOx type. The geometric structure and the electronic properties of this ternary surface oxide are discussed in the light of recent DFT calculations. In view of the excellent order of this Cu‐tungstate phase, we have recorded extensive angle‐resolved photoemission data as a function of photon energy, and the electronic behaviour of this 2‐D CuWOx phase will be presented in form of respective k‐resolved band maps.

Work supported by the ERC Advanced Grant SEPON.

[1] M. Wagner, S. Surnev, M.G. Ramsey, G. Barcaro, L. Sementa, F. Negreiros, A. Fortunelli, Z. Dohnalek, F.P. Netzer, J. Phys. Chem. C115, 23480 (2011)


19

UnifiedPictureoftheExcessElectronDistributionattheTiO2(110)Surface

P. Krueger1, J. Jupille2, S. Bourgeois1, B. Domenichini1, A. Verdini3, L. Floreano3, A. Morgante3,4 1ICB, UMR 6303 CNRS‐Université de Bourgogne, BP 47870, F‐21078 Dijon, France 2Institut des NanoSciences de Paris, UPMC‐CNRS, Jussieu, F‐75252 Paris, France 3CNR‐IOM, Laboratorio TASC, Basovizza, SS14, km 163.5, I‐34149 Trieste, Italy 4Dipartimento di Fisica, Universita` di Trieste, I‐34127 Trieste, Italy


Titanium dioxide, an inert insulator in stoichiometric form, can be easily reduced into an n‐type semiconductor TiO2‐x that makes the material of huge technological relevance. The reduction of titania via

Ti4+ + e‐ →Ti3+, results in excess electrons (EE) that populate localized Ti 3d band gap states. Puzzling issues are the EE distribution and the lattice or interstitial nature (Tiint) of the Ti

3+ ions. Regarding the location of Ti3+ ions, density functional theory (DFT), DFT + U scheme and DFT‐Hartree‐Fock hybrid functionals (the two latter to better account for self interaction corrections) are far from consensus. EE are suggested to be trapped either on sixfold (Ti1) and fivefold (Ti2) coordinated surface Ti, or on subsurface beneath Ti1 or beneath Ti2 (Ti labelled as in Figure). The unique capability of resonant photoelectron diffraction (RPED) to map out the distribution of Ti 3d states was previously demonstrated in the study of TiO2(110) surfaces involving bridging oxygen vacancies [1].

However, the analysis did not discard the suggestion that Tiint atoms could play a role in EE formation [2,3]. This has prompted the analysis of Na/TiO2(110) since Na adatoms are predicted to produce similar EE as vacancies [4], while the formation of Tiint is not expected. The pivotal observation is the similarity between the Na/TiO2 RPED and the Ob‐vac RPED patterns [5]. Data were fitted by locating the Ti

3+ ions on the Ti lattice sites. Attempts to model EE on Tiint failed. A unified model of the reduced TiO2(110) surface emerges, with EE located on subsurface beneath Ti2 > second subsurface beneath Ti1 > Ti1 [4] (Figure). The qualitative agreement of the present findings with DFT approaches [5,6] shows the charge distribution of the Ti 3d states is dictated by electrostatics. It is essentially an intrinsic property of the titania surface that is independent on the way EE are created.

Figure. Location of excess charge near the TiO2(110) surface. These are derived from the resonant photoelectron diffraction (RPED) analysis of two cases: (a) Na/TiO2(110). (b) Defective TiO2(110). The small balls are Ti (light gray), O (dark gray), and Na atoms and V denotes an O vacancy. The shaded spheres symbolize the EE located on a given type of Ti site with diameters proportional to the amount of charge. Note the similarity between the two EE distributions.

[1] Krueger et al. Phys. Rev. Lett. 100, 055501 (2008). [2]Wendt et al., Science 320, 1755 (2008). [3]Papageorgiou et al., Proc. Natl. Acad. Sci. U.S.A. 107, 2391 (2010). [4] Albaret et al., Phys. Rev. B 65, 035402 (2001). [5] Krueger et al. Phys. Rev. Lett. 108, 126803 (2012). [6] Deskins et al., J. Phys. Chem. C 113, 14583 (2009).


20

(Sub)surfaceoxygenvacanciesonTiO2anatase(101)andO2adsorption

Martin Setvín, Philipp Scheiber, Martin Fidler, Michael Schmid, Ulrike Diebold

Institute of Applied Physics, TU Vienna, Wiedner Hauptstrasse 8‐10/134, A‐1040 Vienna e-mail: [email protected]

Titanium dioxide is a versatile material that finds applications in a wide range of technical fields. While the rutile phase of TiO2 is well‐investigated in surface science, it is the (metastable) anatase phase that is present in most nanomaterial and often considered the technologically more relevant polymorph.

Surface O vacancies (VO), which form easily on rutile but are not present at anatase (101) [1] are of particular interest for the surface chemistry of this material. The VO’s can be produced on anatase (101) by bombarding the surface with energetic electrons or by using electric field of the STM tip. The surface VO’s migrate to subsurface sites at temperatures as low as 200 K [2].

We use O2 as a model molecule to illustrate influence of the surface and subsurface vacancies on the adsorption process on the anatase(101) surface. Three distinct different adsorption configurations are found for oxygen. We discuss possible adsorption geometries of these configurations.

Work supported by ERC Advanced Research Grant ‘OxideSurfaces’.

[1] Y. He et al., Phys. Rev. Lett. 102 (10) (2009) 106105 [2] P. Scheiber et al., Phys. Rev. Lett., in press


21

ExploringtheelectronicstructureandopticalexcitationsofF‐dopedTiO2:RelevancetoPhotocatalysis

Sergio Tosoni,1,2 Daniel Fernandez Hevia,2,3 Francesc Illas1 1 Departament de Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, C/ Martí i Franquès 1, E‐08028 Barcelona, Spain 2 Departamento de Química, Universidad de Las Palmas de Gran Canaria, Campus Universitario de Tafira, 35017 Las Palmas de Gran Canaria, Spain 3 INAEL Electrical Systems S.A., C/ Jarama 5, 45007 Toledo, Spain


In this work we investigate the atomic and electronic structure of fluorine doped (F‐doped) stoichiometric and reduced rutile, anatase and brookite polymorphs of TiO2. First, we present a methodological analysis of the performance of several density functional schemes (GGA, GGA+U and hybrid functionals) in reproducing experimental structures and band gap of rutile and anatase; PBE+U providing the best choice and the one chosen to study suitable large supercell models of F‐doped bulk, stoichiometric and reduced rutile, anatase and brookite. We find that F‐doping is thermodynamically stable for all polymorphs. In anatase, the Ti3+ gap states arising from doping are well located in the middle of the band gap (left panel below) which is convenient for photocatalysis purposes. However, in the case of rutile and brookite F‐doping does not lead to a remarkable reduction of the band gap. We also present evidence that the presence of oxygen vacancies does not change significantly the electronic properties of F‐substituted TiO2 albeit slightly stabilize the resulting system [1]. Next, we investigate the effect of F‐doping on the optical spectra of all these systems using response theory. This approach is able to reproduce the main features of experiments and high level quasiparticle calculations for undoped titania, but at a much lower computational cost, thus allowing the study of doped titania, which requires large supercells. While the simulated spectra of F‐substituted brookite and rutile do not show any significant new feature, a relatively intense new band near the visible region is predicted for F‐substituted anatase (right panel below). This allows one to suggest assigning the spectral features near the visible region, observed on multiphase F‐doped titania samples, to the presence of anatase. The physical origin of the new absorption band in F‐doped anatase is attributed to the presence of Ti3+ centers [2].

[1] S. Tosoni, O. Lamiel‐Garcia, D. Fernandez‐Hevia, J. M. Doña and F. Illas, J. Phys. Chem. C 116, 12738 (2012). [2] S. Tosoni, D. Fernandez Hevia, O. González Díaz and F. Illas, J. Phys. Chem. Lett. 3, 2269 (2012).


22

DomainwallsofCOadlayersonPdnanoparticles

Chi Ming Yim1, Chris Muryn2, Chi Pang1, Geoff Thornton1 1London Centre for Nanotechnology, UCL 2Chemistry Department, University of Manchester,


In this work (111)‐oriented metallic Pd nanoparticles supported on rutile TiO2(110) have been studied using the complementary techniques of STM and XPEEM. X‐ray photoemission electron microscopy reveals that the nanoparticles are composed of metallic palladium, separated by the bare TiO2(110) surface. Quasi‐hexagonal particles act as mimics for supported particles in catalysis. Here there is an interest in the interaction between adsorbate domain walls that are not present on single crystal surfaces. Using CO as a model adsorbate, we find domains of /3x/3‐R30˚ at 0.33 ML, top‐bridge and bridge‐bridge c(4x2)‐2CO at 0.5 ML, as well as 2x2‐3CO at 0.75 ML. Complex structures are formed at intermediate coverages, with domain walls between some adlayers and coexisting adlayers of differing symmetry. Fluxional behavior of domains is observed following STM tip pulses.


23

MechanismofSurfaceReactions:InsightsfromFirstPrinciplesCalculations

J. Fdez. Sanz,1 J. Graciani,1 J. Plata,1 L. Feria,1 J. A. Rodriguez2 1Dep. Physical Chemistry, University of Seville, Spain 2Dep. Chemistry, Brookhaven National Laboratory, Upton, NY, USA


Although surface reactions are ubiquitous in chemical processes, their mechanisms are far from being understood. Commonly, the experimental chemist faces too many variables and possibilities to propose and assess a given mechanism, making it almost impossible to go further than a reasonable hint. The knowledge of the reaction sites as well as the involved species bring details that when mixed together with the experimental data allows a deeper view of the elementary steps that usually are related with a surface chemical transformation. In this talk we focus on the water gas shift reaction, WGS, a chemical process that allows for obtaining clean molecular hydrogen: CO+H2O → CO2 +H2. Bulk like phases or extended surfaces of coinage metals show low catalytic activity that improves when supported on a metal‐oxide. The mechanism involves several steps that can take place at different sites of the catalyst: the metal, the support or the interface. Besides the dispersion effect, the role of the support is to increase the interaction with water and facilitate its dissociation. DF calculations show that supported CeOX nanoparticles (NPs) are highly efficient in water splitting (Fig. 1, left). Furthermore The M/CeOx /TiO2 (110) surfaces display outstanding activity for the WGS, in the sequence: Pt > Cu > Au (Fig. 1, right) . Such a high catalytic activity reflects the unique properties of the mixed‐metal oxide at the nanometer level. STM and DF calculations show that Ce deposition on TiO2 (110) at low coverage gives rise to Ce2O3 dimers specifically aligned, indicating that the substrate imposes on the ceria NPs unusual coordination modes enhancing their chemical reactivity.

Fig 1. Left: water dissociation on CeOx/TiO2(110). The process is exothermic by 0.70 eV and the activation barrier is of only 0.04 eV. Right: catalytic activity of gold NPs supported on TiO2(110) and CeOx/TiO2(110).

[1] F. Yang et al. J. Am. Chem. Soc.133, 3444 (2011). [2] J. B. Park et al. J. Am. Chem. Soc. 132, 356 (2010).


24

(VOn)m(n=1,2;m=1−3)submonolayerssupportedontheCeO2(111)surface:Insightsintostructureandreactivityofacomplexcatalyticsystemfromdensityfunctionaltheory

Christopher Penschke, Joachim Paier, Joachim Sauer

Institut für Chemie, Humboldt‐Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany


In an effort to investigate ceria supported vanadia as a catalyst for oxidative dehydrogenation (ODH) reactions, we examine (VO)m and (VO2)m (m = 1, 2, 3) supported on CeO2(111) by applying density functional theory corrected for onsite Coulomb correlation (DFT+U) subject to periodic boundary conditions. Emphasis is put on structure, electron distribution, relative stability (by means of agglomeration energies as well as ab initio thermodynamics) and vibrational frequencies. Reactivity of individual systems is tested by the oxygen defect formation energy and the hydrogenation energy, which have been proposed as reactivity descriptors for ODH reactions. In agreement with experiment and previous calculations [1,2], our results suggest that vanadyl‐terminated monomers, i.e. VO2, represent the most active species. The dimer of VO2, preferring an open, chain‐like structure at the CeO2(111) surface, is the only other system out of the tested ones associated with a similarly high reactivity compared to the monomer. The more active species have several characteristic features in common: (i) the projected density of states shows a significantly narrower O(2p)−Ce(4f) gap and the valence band maximum consists almost exclusively of O(2p) states of the oxygen atoms bound to vanadium, (ii) the structures contain one specially exposed, anchoring oxygen atom, which may stabilize adsorbed hydrogen by forming a hydrogen bond, and (iii) these species benefit from a huge energy gain by virtue of relaxation as well as reconstruction effects upon oxygen removal. However, these two species are less favored thermodynamically compared to the trimer of VO2, which dominates large parts of the phase diagram. Calculated IR vibrational spectra of the supported VO2 monomer, dimer, and trimer confirm the experimentally observed blue‐shift of the V=O stretching mode upon aggregation. These findings provide substantial evidence that the observed reactive species consist of metastable monomers and dimers of VO2.

[1] M. Baron, H. Abott, O. Bondarchuk et al., Angew. Chem. Int. Ed., 48, 8006, (2009). [2] M. V. Ganduglia‐Pirovano, C. Popa, J. Sauer et al., J. Am. Chem. Soc. 132, 2345 (2010).


25

In‐situnanoscaleredoxchemistryofceriumoxideislandsonRu(0001)

J. I. Flege1, B. Kaemena1, S. D. Senanayake2, A. Meyer1, J. T. Sadowski3, E. E. Krasovskii4, J. Falta1 1 Institute of Solid State Physics, University of Bremen, Germany 2Department of Chemistry, Brookhaven National Laboratory, USA 3Center for Functional Nanomaterials, Brookhaven National Laboratory, USA 4Donostia International Physics Center, San Sebastian, Spain


Cerium oxide is an enormously versatile material and has attracted tremendous interest due to its wide range of already existing and potential applications in, e.g., catalysis, energy harvesting and storage, sensing, and microelectronics. Its inherent structural variability combined with its electronic complexity arises from the unfilled shell of highly‐localized 4f electrons in the valence band, making it a particularly attractive model system in fundamental studies of electron correlation and chemical bonding in binary metal oxides. Here, we report on the growth [1] of cerium oxide on Ru(0001) (Fig. 1), its temperature and pressure dependence as well as its oxidation state using a combination of intensity‐voltage low‐energy electron microscopy (I(V)‐LEEM), low‐energy electron diffraction (LEED), resonant photoemission spectroscopy (RPES), and ab initio scattering theory [2]. We demonstrate that, due to the distinct structure of the Ce‐5d states, major differences for fully‐oxidized and reduced ceria, as confirmed by quantitative RPES, exist in the k||=0 projected bandstructure, which determines the electron reflectivity, i.e., the I(V) energy dependence. Furthermore, we illustrate these results by performing in‐situ LEEM during methanol oxidation on nanoscale cerium oxide islands grown on Ru(0001). Using distinct I(V) fingerprints for Ce3+ and Ce4+ oxidation states, we are able to image and identify local (~10x10 nm2) reduction of the cerium oxide islands upon methanol decomposition and subsequent thermal annealing [3].

Acknowledgement: Research carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE‐AC02‐98CH10886. The authors acknowledge partial support from the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010‐19609‐C02‐02).

Figure 1. Temperature dependent island density and shape of cerium oxide islands on Ru(0001).

[1] B. Kaemena, J. I. Flege, S. D. Senanayake, A. Meyer, J. Sadowski, J. Falta, submitted. [2] J. I. Flege, A. Meyer, J. Falta, E. E. Krasovskii, Phys. Rev. B 84, 115441 (2011). [3] J. I. Flege et al., submitted.


26

Recentprogressindensityfunctionalstudiesofceria‐basednanostructures

Konstantin M. Neyman

ICREA and Dept. de Química Física & IQTCUB, Universitat de Barcelona, Barcelona, Spain


We overview our recently developed approaches to model ceria nanostructures present in heterogeneous catalysts and beyond, based on density functional calculations [1,2,3]. Applications of this modelling strategy are illustrated by results of our studies focusing on the question how the nanostructuring of such important catalytic supporting material as CeO2 affects (or may affect) its role in catalysis.

In particular, (i) we show that the oxygen vacancy formation in ceria nanoparticles, crucial for the oxygen‐storage capacity, can be dramatically facilitated [4,5] thus offering a possible key to the observed remarkable reactivity of ceria at the nanoscale; (ii) we demonstrate that ceria nanostructures, unlike bulk samples with extended surface terraces, expose oxygen atoms mobile enough to migrate to supported on them platinum species [6]. Thus, nanostructuring of ceria support opens up new reaction paths on the technologically important Pt/ceria catalysts (see Figure) [7].

[1] S. T. Bromley, I. de P. R. Moreira, K. M. Neyman, F. Illas, Chem. Soc. Rev. 38 (2009) 2657 [2] N. Nilius, S. M. Kozlov, J.‐F. Jerratsch, M. Baron, X. Shao, F. Viñes, S. Shaikhutdinov, K. M. Neyman, H.‐J. Freund, ACS Nano 6 (2012) 1126 [3] A. Migani, K. M. Neyman, S. T. Bromley, Chem. Commun. 48 (2012) 4199 [4] A. Migani, G. N. Vayssilov, S. T. Bromley, F. Illas, K. M. Neyman, Chem. Commun. 46 (2010) 5936 [5] A. Migani, G. N. Vayssilov, S. T. Bromley, F. Illas, K. M. Neyman, J. Mater. Chem. 20 (2010) 10535 [6] G. N. Vayssilov, A. Migani, K. Neyman, J. Phys. Chem. C 115 (2011) 16081 [7] G. N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G. P. Petrova, N. Tsud, T. Skála, A. Bruix, F. Illas, K. C. Prince, V. Matolin, K. M. Neyman, J. Libuda, Nature Materials 10 (2011) 310


27

DRIFTSandXPSoperandostudiesofaninverseCeO2/CuOwater‐gasshiftcatalyst

A. López‐Cámara, M. Monte‐Caballero, J. C. Conesa, A. Martínez‐Arias

Instituto de Catálisis y Petroleoquímica. CSIC. Madrid. Spain.


Copper‐cerium oxide catalysts constitute an interesting cheaper alternative to precious metals‐based systems for the water gas shift (WGS) reaction used in hydrogen production from fossil fuels or biomass feedstocks. Among possible different configurations of such type of catalysts, the inverse one in which copper oxide acts as support of ceria apparently displays most promising characteristics [1,2]. It is normally assumed that ceria‐based WGS catalysts operate, at least at relatively high reaction temperature (above ca. 573 K), through a redox mechanism by which CO is activated on Cu sites and extract oxygen from the Ce oxide which is then reoxidized by water in the presence of metallic copper [1]. However, many details of the reaction mechanism are still unknown and in particular how this type of catalysts operate at relatively low temperature around 473 K, most interesting from a practical point of view.

In this context, an inverse CeO2/CuO catalyst prepared by microemulsion [1,3] has been explored by XPS (ISISS station, BESSY synchrotron, Berlin), under either vacuum or 0.5 mbar of flowing gas containing 7% CO and/or 20% H2O (He balance) while monitoring gas composition with MS. XPS and Cu Auger spectra show that both Cu and Ce are reduced to Cu+ and (partially to) Ce3+ upon interaction with CO at 473 K. Unexpectedly, subsequent introduction of H2O to the reactant mixture at the same temperature, instead of oxidising the catalyst surface, generates some metallic copper and slightly increases the reduction degree of the ceria component; i.e. apparently H2O acts as reductant rather than as oxidant of the catalyst surface under the examined conditions. Rationalization of such result is done on the basis of the following points [3]: a) analysis of the gas phase evolution shows a mismatch in the balance between CO consumption and CO2 production; in particular, a slightly higher amount of CO2 is apparently produced suggesting that any carbonate‐like species present in the catalyst prior to H2O introduction in the reactant mixture becomes decomposed in the presence of H2O. b) This has been demonstrated by performing in parallel a similar set of experiments monitored by DRIFTS which has allowed confirming that H2O promotes the decomposition of certain carbonate species proposed to be formed at ceria‐copper interfacial sites.

The main conclusion is that interfacial carbonates formed upon interaction with CO impede the progress of redox processes leading to formation of metallic copper and partially reduced ceria (considered the most active state of this type of WGS catalysts [1,2]), thus reinforcing the idea of an important (deactivating) catalytic role of carbonate species in ceria‐based catalysts for this type of process. The presence of H2O in the reactant mixture has the beneficial role of promoting the decomposition of such type of carbonate species.

[1] L. Barrio et al. J. Phys. Chem. C 114, 3580 (2010). [2] J.A. Rodriguez et al. Angew. Chem. Int. Ed. 48, 8047 (2009). [3] A. López Cámara et al. Catal. Sci. Technol. (in press, d.o.i.: 10.1039/c2cy20399e).


28

200 nm

Thinfilmceriumoxidebasedcatalystsforfuelcellapplication

Vladimír Matolín

Charles University in Prague, V Holesovickach 2, 18000 Prague, Czech Republic


Powering of electronic devices by microfabricated power sources, including micro‐proton exchange

membrane fuel cells (‐PEMFC), are being actually investigated in laboratories world‐wide. The possibility of co‐fabrication of a power source on the same substrate as the electric circuit offers many advantages, including a reduction in size and weight, increased processing efficiency, and lower cost. The important issue of planar type fuel cells is a preparation of large specific surface area catalysts grown by thin film deposition techniques which are compatible with planar technology.

Recently we showed by fuel cell activity and electron microscopy measurements the possibility of preparation of porous large surface and high activity nanostructured thin film catalysts by depositing the catalysts in form of Pt‐Ce‐O solid solutions on different carbon substrates by magnetron sputtering [1‐3]. Figure shows example of CNTs coated by the porous Pt‐CeO2 catalyst film.

Chemical composition of the films was investigated by x‐ray synchrotron radiation photoelectron spectroscopy in soft and hard X ray region. Resonant PES has been performed by measuring Ce 4f resonant profiles of both Ce4+ and Ce3+ states. The Pt‐doped sputtered cerium oxide films contained high concentration of cationic platinum Pt2+ and Pt4+ which were highly active species for hydrogen dissociation to protonic hydrogen H+.

[1] Vaclavu, M., Matolinova, I., Myslivecek, J., Fiala, R. and Matolin, V., J. Electrochem. Soc., 156, B938, (2009). [2] Matolin, V., Cabala, M., Matolinova, I., Skoda, M., Vaclavu, M., Prince, K.C., Skala, T., Mori, T., Yoshikawa, H., Yamashita, Y., Ueda, S. and Kobayashi, K., Fuel Cells, 10, 139 (2010) [3] Matolin, V., Matolinova, I., Vaclavu, M., Khalakhan, I., Vorokhta, M., Fiala, R., Pis, I., Sofer, Z., Poltierova‐Vejpravova, J., Mori, T., Potin, V., Yoshikawa, H., Ueda, S. Kobayashi, K., Langmuir, 26, 12824, (2010).


29

DecompositionpathwaysofcarboxylicacidonPt/ceriamodelcatalysts

Yaroslava Lykhach1, Viktor Johánek2, Armin Neitzel1, Tomáš Skála2,3, Nataliya Tsud2, Kevin C. Prince3,4, Vladimír Matolín2, Jörg Libuda1,5

1Lehrstuhl für Physikalische Chemie II, FAU Erlangen‐Nürnberg, Erlangen, Germany; 2Department of Surface and Plasma Science, Charles University, Prague, Czech Republic; 3Sincrotrone Trieste SCpA, Basovizza‐Trieste, Italy; 4IOM, Basovizza‐Trieste, Italy 5Erlangen Catalysis Resource Center, FAU Erlangen‐Nürnberg, Erlangen, Germany


Future clean energy technologies may employ catalytic production of hydrogen from bio‐derived oxygenates.[1] A detailed understanding of the related reaction mechanisms on reforming catalysts requires model studies of simple compounds on well‐defined catalytic surfaces. In the current study, we use formic acid, as the simplest representative of a carboxylic acid, and acetic acid, as the simplest representative of a carboxylic acid which contains an additional C–C bond. The decomposition pathways of both molecules were investigated by means of synchrotron photoelectron spectroscopy on well‐ordered stoichiometric CeO2(111)/Cu(111), partially reduced CeO2‐x/Cu(111), and Pt/CeO2/Cu(111) model catalysts. We use the oxidation state of ceria as an indicator of the redox processes occuring on the catalyst surface. The evolution of the Ce3+ concentration, D(Ce3+), was monitored by means of Resonant Photoemission Spectroscopy (Figure 1).[2] We found that decomposition of formic and acetic acids occurs without change of the surface oxidation state on stoichiometric CeO2 (green, Figure 1). In contrast, strong reoxidation (cyan, Figure 1) and reduction (violet, Figure 1) occurs with CeO2‐x, as a result of redox transformations of the adsorbate involving the support. On Pt/CeO2, decomposition processes involve spillover of hydrogen and decomposition fragments between Pt and CeO2 support.

Figure 1. D(Ce3+) as a function of temperature on CeO2(111), CeO2‐x/Cu(111), and Pt/CeO2/Cu(111) exposed to 10 L of formic acid at 160 K.

[1] C. Rioche, S. Kulkarni, F.C. Meunier, J.P. Breen, R. Burch, Appl. Catal., B, 61 130 (2005). [2] G.N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G.P. Petrova, N. Tsud, T. Skala, A. Bruix, F. Illas, K.C. Prince, V. Matolín, K.M. Neyman, J. Libuda, Nature Materials, 10 310 (2011).


30

Fig. 1 Band structure of N‐containing Zn,Ti oxide spinel computed with hybrid DFT

Fig. 2 a) Periodic slab model used in the calculations on the ZnO‐anatase interface. b) Band alignment at the interface predicted by hybrid DFT calculations

DFTstudyofZn‐Tioxidephotocatalysts

José C. Conesa

Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie 2, 28049 Madrid, Spain


Recent work [1,2] has reported the photocatalytic properties of Zn‐Ti oxide systems, with or without N added for visible light activity. Here hybrid DFT periodic calculations (CRYSTAL code) of some such systems are presented [3], using a recently proposed relationship between the HF exchange mixing coefficient and the dielectric constant [4] (computed as well with the same DFT treatment, if needed) which yields equally reliable gaps for all phases studied.

The results show first that spinel mixed oxide phases, with composition between Zn2TiO4 and Zn2Ti3O8, have bandgaps which approach that of TiO2 or are larger, depending on octahedral cation disordering and thus on synthesis conditions; rhombohedral ZnTiO3 has a larger gap. In these systems the conduction band consists mainly of Ti 3d orbitals; photoexcited electrons should lead thus to a chemical behaviour more similar to those in TiO2 than to those in ZnO.

Secondly, substitution of O by N anions (e.g. via high temperature treatment with NH3), which rises the valence band edge due to the presence of N‐centred states (see e.g. Fig. 1), is shown to be energetically more favourable in the spinel phases than in pure TiO2, thanks to the presence of vacant cationic sites in the undoped structure, which allows charge compensation with deletion rather than formation of ion vacancies upon N incorporation.

Finally, TiO2‐ZnO junctions in composite photocatalysts are modelled with periodic alternating layers having good epitaxial match (Fig. 2a). The band alignment predicted using electrostatic potential as reference [Error! Bookmark not defined.] (Fig. 2b) implies that ZnO bands lie lower than those of anatase, aiding the electron‐hole separation and influencing the type of reactions produced.

[1] see e.g. G. Marcí et al., J. Phys. Chem. B, 105, 1033, (2001); S. Liao et al., J. Photo‐chem. Photobiol. A, 168, 7 (2004); C. Karunakaran et al. J. Hazard. Mat. 176, 799 (2010). [2] see e.g. S.A. Mayén‐Hernández et al., Solar En. Mat. Solar Cells 91, 1454, (2007); J.‐Z. Kong et al., J. Haz. Mat. 171, 918, (2009); P.H. Borse et al., J. Cer. Proc. Res. 13, 42, (2012); F. Grasset et al. Adv. Mater. 17, 294, (2005); T. Hisatomi et al., Chem. Lett. 36, 558, (2007). [3] Work reported in J.C. Conesa, J. Phys. Chem. C, accepted, doi: 10.1021/jp306160c; ibid., Catal. Today, submitted. [4] A. Alkauskas et al., Phys. Status Solidi B 248, 775, (2011).


31

Atomisticstructureofcobalt‐oxide/phosphatenanoparticlesforcatalyticwateroxidation

Xiao Liang Hu, Simone Piccinin, Alessandro Laio, Stefano Fabris

CNR‐IOM Istituto Officina dei Materiali, DEMOCRITOS Simulation Center and Scuola Internazionale Superiore di Studi Avanzati (SISSA), Via Bonomea 265, Trieste, Italy


Solar‐driven water splitting is a key photochemical reaction that underpins the feasible and sustainable production of solar fuels. An amorphous cobalt‐oxide/phosphate catalyst (Co‐Pi) based on earth‐abundant elements has been recently reported to efficiently promote water oxidation to protons and dioxygen, a main bottleneck for the overall process [1,2]. X‐ray adsorption spectroscopy suggests the presence of a network of octahedral CoO6 units, arranged into molecular‐size crystalline units, but the structure of this material remains largely unknown.

We exploit ab‐initio and classical atomistic simulations combined with metadynamics to build a realistic and statistically meaningful model of Co‐Pi [3]. The calculations demonstrate the emergence of molecular‐size crystallites in amorphous nanoparticles initially formed by a disordered Co‐O network and phosphate groups. The crystallites consist of bis‐oxo bridged Co centers assembled into cubane motifs, which form ordered structures by sharing corners or faces. These crystallites always exposes Co sites at the nanoparticle surface and incorporate at least one surface phosphate group at a cubane termination. Our amorphous nanoparticles of ~1nm can contain more than one crystallite, consisting of up to four cubane units. The nanoparticles are structurally stable up to high temperatures. Nanoparticles contaning several cubane motifs ‐ which are believed to be essential for the catalytic activity ‐ display a very good agreement with the experimental EXAFS spectra of Co‐Pi grains. We propose that the cubane rich portion of our amorphous nanoparticles are reliable structural models of the Co‐Pi catalyst surface. The structural models proposed in this work are the starting point for a more quantitative approach to reveal the water oxidation mechanism promoted by these materials.

[1] M.W. Kanan, D.G. Nocera, Science 321, 1072 (2008). [2] M. Risch, V. Khare, I. Zaharieva, L. Gerencser, P. Chernev, H. Dau, JACS 13, 6936 (2009) [3] X.L. Hu, S. Piccinin, A. Laio, S. Fabris, submitted (2012).


32

Influenceofcarbon‐carbonbondorderandsilverloadingontheoxidativefunctionofsilver‐aluminacatalystsinabsenceandpresenceofNO

Hanna Härelind, Fredrik Gunnarsson, Seyyed Majid Sharif Vaghefi, Magnus Skoglundh, Per‐Anders Carlsson

Competence Centre for Catalysis, Chalmers University of Technology, Göteborg, Sweden


The influence of carbon‐carbon bond order, here systematically represented by prototypical C2H6, C2H4 and C2H2, on the formation of oxidation products and surface species in the absence and presence of NO has been studied for silver‐alumina catalysts with different silver loadings (2 and 6 wt%) [1]. The catalysts were prepared with a sol‐gel method including freeze‐drying, which results in small silver species uniformly distributed throughout the alumina matrix. The performance of the catalysts was investigated by temperature programmed extinction‐ignition experiments using a continuous gas flow reactor. The evolution of surface species during reactant step‐response experiments was studied in situ by diffuse reflection Fourier transform infrared spectroscopy (Figure 1).

The results show that activation for oxidation generally proceeds more easily with increasing bond order of the hydrocarbon. For example, C2H2 shows the highest conversion at low temperatures. Furthermore, the use of hydrocarbons with high bond order, i.e., C2H2, as reductant for lean NOx reduction results not only in the highest peak activity but also in considerable high activity in a wide temperature range mainly thanks to high activity at low temperature. With increasing silver loading, the oxidation reactions are favored such that both the hydrocarbon and NO activation occur at lower temperatures. Several types of adsorbates, e.g., carbonate, acetate, formate, enolic and isocyanate/cyanide surface species, are present on the catalyst during reaction. Generally the nature of the surface species and likely also the surface processes are more influenced by the carbon‐carbon bond order than the silver loading. This needs to be considered when designing catalysts for emission control systems. Especially for applications using homogeneous fuels with short hydrocarbons, this may provide opportunities to tailor the catalyst functionality for the needs at hand.

Figure 1. Evolution of surface species during reactant step‐response experiments monitored by in situ DRIFTS

[1] H. Härelind, F. Gunnarsson, S.M. Sharif Vaghefi, M. Skoglundh and P.‐A. Carlsson, ASC Catal., 2, 1615, (2012).


33

Theapplicationofhigh‐resolutionIRspectroscopyandisotopelabelingfordetailedinvestigationofTiO2/gasinterfacereactions.

Svatopluk Civiš, Martin Ferus

J. Heyrovský Institute of Physical Chemistry, v.v.i., Dolejškova 3, 18223 Prague 8, CZ


Titanium dioxide is an attractive material for (photo)catalysis, solar cells, electrochromics and batteries [1], while various fundamental studies would benefit from the accessibility of a totally 18O exchanged material. The main motivation for synthesis of Ti18O2 was the investigation of surface effects during titania/gas interaction [2]. Here we report, for the first time, the preparation of pure Ti18O2 both in anatase and rutile forms. The interaction with carbon dioxide was investigated with the aim to explore oxygen isotope exchange at the Ti18O2/CO2 interface. For this purpose, high‐resolution Fourier transform infrared spectroscopy of the gas phase was adopted. In the present study, we have explored the oxygen isotope 16O/18O exchange between gaseous C16O2 and solid Ti

18O2. Although there have been several earlier works aimed at isotope exchange reactions involving carbon dioxide, the reactions have been typically studied either on the C18O2/Ti

16O2 system [3] or on a complicated gaseous mixture containing, besides C16O2, also 18O2 or H2

18O and ordinary Ti16O2 [4], [5]. To the best of our knowledge, the isotope exchange reaction at the C16O2(g)/Ti

18O2(s) interface is investigated here for the first time. Carbon dioxide offers several advantages as the target molecules for these studies: (i) The isotope exchange can be readily followed by high‐resolution FTIR; (ii) It is the final product of the photocatalytic oxidation of organic molecules; (iii) the adsorption mechanism of CO2 is known in detail and moreover, it is of prospective environmental impact for CO2 removal from the atmosphere (for review of isotope effects in atmospheric CO2 and other gases see Ref. [6]). The present measurement in dark mixtures has, as its primary goal, the determination of the time‐scale of the spontaneous isotope exchange between carbon dioxide and solid TiO2. The profiles of the individual lines of selected isotopologues (isolated lines in the spectrum) were fitted and quantified. The quantification of the spectra was carried out on the basis of calibration measurements of the absorption spectra of individual isotopologues (reference gases) of carbon dioxide at different pressures. The concentrations of individual isotopologues determined from the intensity profiles of the individual rotation‐vibration lines are characterized by the exponential decrease of the 16O‐C‐16O isotopologue and the exponential increase of the 18O‐C‐18O isotopologue. The 18O‐C‐16O acts as an intermediate in the mixture and its concentration remains almost constant.

Acknowledgements: This work is part of the research programs of the Grant Agency of the Academy of Sciences of the Czech Republic Grant No. IAA400400705 and Grant Agency of the Czech Republic Grant No. P208/10/2302.

[1] Chen, X.; Mao, S.S. Chem. Rev., 107 (2007) 2891. [2] Kavan, L. Adv. Sci. Technol., 51 (2006) 20. [3] Yanagisawa, Y. Energy Convers. Mgmt., 36 (1995) 443. [4] Liao, L.F.; Lien, C.F.; Shieh, D.L.; Chen, M.T.; Lin, J.L., J. Phys. Chem. B, 106 (2002) 11240. [5] Sato, S. J. Phys. Chem., 91 (1987) 2895. [6] Brenninkmeijer, C.A.M.; Janssen, C.; Kaiser, J.; Rockmann, T.; Rhee, T.S.; Assonov, S.S., Chem. Rev., 103 (2003) 5125.


34

Imidazolium‐basedionicliquidsinteractingwithorderedceriumdioxidesurfaces:asynchrotron‐radiationphotoelectronspectroscopystudy

Mathias Laurin1, Yaroslava Lykhach1, Stefan Schernich1, Nicola Taccardi2, Valentin Wagner2, Nataliya Tsud3, Tomáš Skála3, Kevin C. Prince4, Vladimír Matolín3, Peter Wasserscheid2, Jörg Libuda1

1FAU Erlangen‐Nürnberg, PC II, Egerlandstr. 3, 91058 Erlangen, Germany 2FAU Erlangen‐Nürnberg, CRT, Egerlandstr. 3, 91058 Erlangen, Germany 3Charles University, Mathematics and Physics, V Holešovičkách 2, 18000 Prague 8, CZ 4Sincrotrone Trieste SCpA, Strada Statale 14, km163.5, 34149 Basovizza‐Trieste, Italy


In this work we present the first surface science study of an ionic liquid (IL) thin film on a reducible oxide surface. ILs are molten salts liquid below 100°C. They exhibit unusual properties such as an extremely low vapor pressure. This allows them to be handled and studied using standard surface science techniques. Among the applications of ILs, new concepts have been proposed in catalysis, like "solid catalysts with ionic liquid layer" (SCILL), which make use of a thin IL film to tune the properties of supported heterogeneous catalysts. Cerium dioxide (CeO2) is an active support used in many catalytic processes. Thin CeO2(111) films epitaxially grown on Cu(111) provide a versatile model of this catalyst support for fundamental studies. The stoichiometric surface can be prepared in vacuo and may be reduced by different methods, including exposure to methanol.[1]

We study the interactions of [C6C1im][Tf2N] thin films with these surfaces. A monolayer of IL was deposited in situ from a home‐built evaporator.[2] Molecular orientations and thermal decomposition of the IL were followed by synchrotron‐radiation photoelectron spectroscopy (PES). And the changes in the oxidation state of CeO2 were probed in parallel by resonant PES. We find that the decomposition mechanism and the thermal stability of the IL film sensitively depend on the oxidation state of the support. Evidences of a controlled functionalization of the reduced oxide are presented. Possible interactions and decomposition mechanisms are discussed.

[1] V. Matolín, J. Libra, M. Skoda, N. Tsud, K. Prince et al., Surf. Sci., 603, 1087, (2009). [2] M. Sobota, I. Nikiforidis, W. Hieringer, N. Paape et al., Langmuir, 26, 7199 (2010).


35

Tuningmetal‐supportinteractionsinreduciblePd@CeO2coreshellbasedcatalysts

Matteo Cargnello1, J.J. Delgado Jaen3, Maurizio Prato1, Raymond J. Gorte3, Paolo Fornasiero1 1Department of Chemical and Pharmaceutical Sciences, ICCOM‐CNR, Consortium INSTM, University of Trieste, via L. Giorgieri 1, 34127 Trieste, Italy 2Departamento de Ciencia de los Materialese Ingeniería Metalúrgica y Química Inorgánica, Universidad de Cádiz, Campus Río San Pedro, 11510, Puerto Real, Cádiz, Spain. 3Department of Chemical and Biomolecular Engineering, University of Pennsylvania, 19104 Philadelphia, Pennsylvania, United States


The tailored positioning of the building blocks at the nanometer scale can dramatically improve the performance of the materials through electronic and steric interactions. Heterogeneous catalysts that are used in a wide variety of industrial and environmental applications already take advantage of the synergy between a support and the supported phases. Some oxides, such as the reducible ceria, can participate in the catalytic cycle by providing reactive oxygen through formation of vacancies. Core‐shell structures are special cases where metal‐support interactions are enhanced leading to advanced materials for catalytic and photocatalytic applications.[1]

Here, we describe a supramolecular approach in which single units composed of a Pd core and a CeO2 shell are pre‐organized in solution and then deposited onto various supports, from pristine and modified hydrophobic alumina to oxidized Multi Walled Carbon Nanotubes (MWCNT). The reactivity of the final catalysts strongly depends on the reaction atmosphere, reducing vs oxidizing conditions as a result of the redox behaviour of the ceria component. Under oxidizing conditions, the enhanced metal‐support interactions lead to exceptionally high methane oxidation activity, with complete conversion below 400 °C and outstanding thermal stability under demanding conditions unising Pd@CeO2‐modified alumina.[2] Vice versa under reducing conditions, electronic deactivation occurs on Pd@CeO2‐modified alumina, while Pd@CeO2‐MWCNT shows promising and stable catalytic performances.[3]

[1] Cargnello, M.; Fornasiero, P.; Gorte, R. J., Catal. Lett., 2012, 142, 1043 [2] Cargnello, M.; Delgado Jaén, J.J.; Hernández Garrido, J.C.; Bakhmutsky, K.; Montini, T.; Calvino Gámez, J.J.; Gorte, R.J.; Fornasiero, P., Science 2012, 337, 713, [3] Cargnello, M.; Grzelczak, M.; Rodríguez‐González, B.; Syrgiannis, Z.; Bakhmutsky, K.; La Parola, V.; Liz‐Marzán, L.; Gorte, R. J.; Prato, M.; Fornasiero, P., J. Am. Chem. Soc., 2012, 134, 11760


36

Activesitesformethanedissociationovermetallicandoxidizedpalladium

Anders Hellman, Adriana Trinchero, Henrik Grönbeck

Competence Centre for Catalysis, Chalmers University of Technology, Göteborg, Sweden


It is generally a challenge to determine the active phase of heterogeneous catalysts during operating conditions. This is valid, in particular, for structurally ill‐defined catalysts that are realized as metal particles dispersed on a porous oxide. One example is the active phase of palladium during complete methane (CH4) oxidation to water and carbon dioxide. This important reaction has recently regained interest because bio‐ and natural gas are used as fuels in automotive applications. Methane is the main component in such fuels and there is currently a pressing need for catalysts with enhanced low temperature activity. The high activity measured for catalysts based on supported palladium has previously been attributed to different Pd/PdOx phases [1], such as reduced (metallic) palladium, metal supported surface oxides and bulk metal oxide.

In this contribution [2], the active phase of Pd during methane oxidation is investigated by density functional theory calculations an compared to in situ surface x‐ray diffraction measurements. Low activation energies for methane dissociation are calculated for either under‐coordinated Pd sites in PdO or metallic surfaces. The results are in full agreement with the experiments.

The rate determining step (RDS) in CH4 oxidation is generally assumed to be the removal of the first H atom, whereupon adsorbed CH3 and H are formed. The present work support this view, and the dissociative adsorption of methane can be used as a measure of the catalytic activity. Our results show that a low barrier for methane dissociation is obtained either on under‐coordinated Pd‐sites in PdO or on metallic Pd, where the lowest barrier is calculated for an under‐coordinated Pd site on PdO(101). The atomistic rational behind the high activity of metallic sites is their ability to polarize methane during the dissociation. On the other hand, the high activity of the PdO(101) surface is an intrinsic balance between the electronic structure and the geometrical structure of the under‐coordinated Pd site. In particular, our calculations reveal a marked difference in the entrance channel for CH4 adsorption on PdO(100) and PdO(101). Whereas there is an initial repulsion over PdO(100), an attractive potential that facilitates dissociation is present over PdO(101). The difference is analyzed and found to be an interesting consequence of the detailed electronic structure.

The experiments consider Pd(100) for which an oxide film is formed during oxidizing conditions. A high CH4 conversion is observed in the presence of a thick epitaxial [PdO(101)] film supported on the metal surface or when the temperature is high enough to decompose the palladium oxide.

[1] R. Burch, P.K. Loader, and F.J. Urbano, Catalysis Today 27, 243 (1996). [2] A. Hellman, A. Resta et al., J. Phys. Chem. Lett. 3, 678 (2012).


37

Potentialmethodfordetectingoxygenvacanciesinoxides

Esa Korhonen and Filip Tuomisto

Department of Applied Physics, Aalto University, POB 11100, FI‐00076 Aalto, Finland


Positron annihilation spectroscopy is an experimental method that is particularly suitable for the identification vacancy defects in semiconductors [1]. In a semiconductor material, positrons can get trapped at negative and neutral vacancy defects, and at negatively charged non‐open volume defects given the temperature is low enough. The trapping of positrons at these defects is observed as well‐defined changes in the positron‐electron annihilation radiation. The combination of positron lifetime and Doppler broadening techniques with ab‐initio theoretical calculations provides the means to deduce both the identities and the concentrations of the vacancies. Performing measurements as a function of sample temperature and illumination gives detailed information on the charge states of the detected defects. It should be noted that isolated O vacancies in oxides tend not to be detectable by positrons due to (i) their likely positive charge and (ii) their small size preventing positron localization.

We have recently shown that nitrogen (N) vacancies and nitrogen vacancy complexes in InN can be detected and identified using positrons [2], provided they appear as complexes with indium (In) vacancies. We will here show results obtained with positron annihilation spectroscopy in thin‐film Sb‐doped SnO2, and Sn‐ and Mg‐doped In2O3. These results suggest that similarly as N vacancies in InN, O vacancies prefer to agglomerate around cation vacancies, making them detectable by positrons. Further, the concentration of these O vacancies tends to be higher near the layer/substrate interface than in the bulk of the layer. Positron annihilation methods may thus provide a means for identifying and in some cases even quantifying O vacancies in oxides.

[1] F. Tuomisto, Defect Characterization in Semiconductors with Positron Annihilation Spectroscopy, in Springer Handbook of Crystal Growth, Eds. G. Dhanaraj, K. Byrappa, V. Prasad, and M. Dudley (Spinger‐Verlag Berlin Heidelberg, 2010), pp. 1551 – 1580. [2] C. Rauch, I. Makkonen, and F. Tuomisto, Phys. Rev. B 84, 125201 (2011).


38

FacilitatedlatticeoxygendepletioninconsolidatedTiO2nanocrystalpowders:aspectroscopicO2adsorptionstudy

Michael J. Elser, Nicolas Siedl, Oliver Diwald

Institute of Particle Technology, Friedrich‐Alexander University Erlangen‐Nuremberg, Cauerstrasse 4, 91058 Erlangen (Germany)


In polycrystalline transition metal oxides, minute changes in the lattice oxygen concentration can significantly affect their chemical and catalytic activity and determine their crossover from ionic to electronic conductivity regimes. Related phenomena are critically important for a broad spectrum of applications that involve solid‐state electrolytes, sensing devices, and materials employed for printable electronics.

On TiO2 nanocrystal powders pressure‐induced consolidation and subsequent annealing generates

mesopores inside the resulting pellet as it leads to a decrease of specific surface area. This loss corresponds to the introduction of particle‐particle interfaces and grain boundaries that were found to be susceptible to facilitated lattice oxygen depletion during vacuum annealing as compared to the free particle surfaces.[1,2] We measured quantitatively the uptake of molecular oxygen on the consolidated sample. As a function of repeated cycles of vacuum annealing and adsorption the amount of paramagnetic and diamagnetic surface oxygen species was determined using mass spectroscopy in combination with FT‐IR, Electron paramagnetic resonance (EPR) and UV‐Vis‐NIR diffuse reflectance. From this data we finally were able to estimate the degree of substoichiometry in these TiO2‐x nanoparticle networks.[3] Whereas the small size of nanoparticles, their confined volume, and the associated high surface‐to‐volume ratios give rise to new desirable properties as compared to bulk materials, the interfaces between the particles provide additional functionality.

[1] M. J. Elser, T. Berger, D. Brandhuber, J. Bernardi, O. Diwald, E. Knözinger, J. Phys. Chem. B, 110, 7605 (2006) [2] S.O. Baumann, M. J. Elser, M. Auer, J. Bernardi, N. Hüsing, O. Diwald, Langmuir 27, 1946 (2011) [3] M. J. Elser, O. Diwald, J. Phys. Chem. C 116, 2896 (2012)


39

FirstprinciplescalculationsofformationandmigrationofoxygenvacanciesinLa1‐xSrxCo1‐yFeyO3‐perovskites

Yu. A. Mastrikov1 and E. A. Kotomin

Institute of Solid State Physics, University of Latvia, Kengaraga str. 8, Riga, Latvia


Complex ABO3‐‐type perovskites exhibit a perceptible ionic conductivity, leading to their promising use

as electrolytes ((La,Sr)(Ga,Mg)O3‐), oxygen permeation membranes, and SOFC cathodes

((La,Sr,Ba)(Mn,Fe,Co)O3‐). Oxygen stoichiometry strongly affects transport properties of the materials, which, in turn, defines suitability of the material for targeted applications. The oxygen migration in those perovskites occurs via the vacancy mechanism, in which the vacancy moves through a bottleneck, formed by one B‐ and two A‐site cations. The concentration and mobility of oxygen vacancies are two major factors determining bulk transport properties and the surface oxygen incorporation rate. Although oxygen vacancies are being extensively studied, our understanding of their effects on behavior of the materials is far from complete.

Based on DFT calculations, we analyze the formation and migration of oxygen vacancies in

(La,Sr)(Co,Fe)O3‐ (LSCF) perovskites. The atomic structure of transition states, charge redistribution and variation of B‐metal oxidation state, diffusion activation energies for oxygen ion migration are obtained.

We explore differences with Ba1‐xSrxCo1‐yFeyO3‐ (BSCF) [1, 2], which exhibit considerably lower migration barriers than other perovskites.

[1] E.A. Kotomin et al, SSI 188, 1, 2011 [2] R.Merkle et al, J El. Chem. Soc. 159, B 219, 2012


40

SolarCellswithOverallWaterSplittingUsingOligoaniline‐Crosslinked[Ru(bpy)2(bpyCONHArNH2)]+2Dye/IridiumOxideNanoparticleArraysOnThree–DimensionallyOrderedMacroporousGold‐NanoparticleDopedTitaniumDioxide(3‐DMGN‐TiO2)PhotonicCrystalsModifiedElectrodes

Huseyin Bekir Yildiz, Oktay Talaz

Department of Chemistry, Karamanoglu Mehmetbey University 70100 Karaman, Turkey


This project aims the construction of photoelectrochemical cell system splitting water into hydrogen and oxygen using UV‐vis light under constant applied voltage. Oligoaniline‐crosslinked 2‐(4‐aminobenzyl)malonic acid functionalized IrO2.nH2O [1, 2] nanoparticles and visible light absorbing dye, [Ru(bpy)2(bpyCONHArNH2)

+2], arrays [3] on three–dimensionally ordered macroporous gold‐nanoparticle doped titanium dioxide (3‐DMGN‐TiO2) photonic crystals [4] modified electrodes will be used as (photo)anode and nanostructures based on bonding of Pt nanoparticles by using electropolimerization on poly 4‐(2,5‐di(thiophene‐2‐il)‐1 H‐pyrrol‐1‐il) benzenamin P(SNS‐NH2) conducting polymer [5] modified gold electrode will act as cathode [6]. If the system is operated under light and constant potential; water will be oxidizingly splitted by IrO2.nH2O catalyst, and production of oxgen in anode and production of hydrogen in cathode will certainly occur. Electropolymerization of the visible region sensitive [Ru(bpy)2(bpyCONHArNH2

)+2] and 2‐(4‐aminobenzyl)malonic acid functionalized IrO2.nH2O nanoparticles on 3‐DMGN‐TiO2 films results in connection with the conductive oligoaniline bridges. The transfer of the electrons arising from the oxidation of water from nanoparticles into the valence band of [Ru(bpy)2(bpyCONHArNH2)

+2] will be more faster due to the presence of conductive oligoaniline crosslinks. Within the help of these conductive crosslinks; the electron flow from [Ru(bpy)2(bpyCONHArNH2)

+2] conductive band into the conductive band of TiO2 and then into the electrode will be very fast. The doping of the TiO2 photonic crystal film with gold nanoparticles will also enhance this fast electron transfer. Hence; this fast electron transfer will be competitive with the probable electron transfer in the reverse system or will be faster. Under these circumstances there will be an important increase in the quantum efficiency of the system.

[1] H.B. Yildiz et al., Adv. Funct. Mater. 18, 3497‐3495, (2008). [2] W.J Youngblood et al., J. Am. Chem. Soc. 131, 926‐927, (2009)..F. Author, S. Author, [3] S. Anderson et al., J. Chem. Soc. Dalton Trans. 10, 2247‐2261, (1985). [4] N. Wei et al., Biosens. Bioelectron. 26, 3602–3607, (2011). [5] E. Yildiz et al., J. Electroanal. Chem. 61, 247‐256, (2008). [6] R. Tel‐Vered et al., Small 6, 1593‐1597, (2010).


41

Electronicprocessesatmetal/oxideinterfaces

Sanliang Ling1, Keith P. McKenna2,3, Alexander L. Shluger1,2 1 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK 2 WPI‐AIMR, Tohoku University, Sendai, 980‐8577, Japan 3 Department of Physics, University of York, Heslington, York YO10 5DD, UK


Metal/oxide interfaces are ubiquitous in microelectronics, catalysis and energy materials applications. Calculating the geometric and electronic structures of interfaces and defect properties at interfaces and inside the constituent materials is still a challenge for materials modeling. I will present the results of our recent calculations of the structure and properties of MgO/Ag, HfO2/Pt and HfO2/TiN interfaces and will discuss the charge states and stability of defects at these interfaces. The results demonstrate the importance of using the techniques providing correct band gaps and band offsets as well as structural parameters of interfaces. In particular, I will demonstrate that using the Auxiliary Density Matrix Method (ADMM) within CP2k methodology allows us to treat both the metal and the oxide at the interface at the right level of accuracy and predict the oxide induced metal work function shifts in good agreement with experiments. I will also discuss the stability of defects at HfO2/Pt and HfO2/TiN interfaces in relation to forming processes in resistive RAM devices.


42

PHOTON ENERGY (eV)

2 3 4 5 6

YIE

LD (

ELE

CT

RO

N/P

HO

TO

N)

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

-1 V

+1 V

-1 V

+1 V

TiNx/Hf/HfO2/TiNx

TiNx/HfO2/TiNx

ImpactofoxygenscavengingonelectronicpropertiesofthinHfO2layers

V. V. Afanas’ev,1 A. Stesmans,1 L. Pantisano,2,3 Ch. Adelmann2

1Department of Physics, University of Leuven, Celestijnenlaan 200D, B‐3001 Belgium 2IMEC, Kapeldreef 75, B‐3001, Belgium 3Present address: Global Foundries, Malta, NY 12020, USA


With the relentless downscaling of the active components in electronic integrated circuits, oxygen scavenging is seen as a crucial technological step allowing one to achieve the desired functionality of oxide insulators. For example, to attain a sub‐0.5 equivalent oxide thickness (EOT) of the gate insulator in Si metal‐oxide semiconductor transistors, a thin SiOx interlayer between the substrate crystal and high‐

permittivity (high‐) top insulating layer must be eliminated. This is achieved by thermal treatment with a layer of reactive metal (scavenger), e.g., Ti or Hf, deposited on top of the gate insulating stack. During this treatment, oxygen from the interlayer is consumed by the scavenger allowing one to reduce the EOT. Another application of oxygen scavenging consists in enhancing the concentration of oxygen vacancies to enable a reliable switching behaviour of resistive random access memory cells. In all these processes one

ends up with a chemically reduced high‐ oxide layer which electronic properties may differ significantly from the initially deposited stoichiometric oxide.

In our work we examined the impact of oxygen scavenging on the electronic properties of the most

extensively applied high‐ oxide in devices: HfO2. Application of a 5‐nm thick metallic Hf scavenger between a 10‐nm thick HfO2 layer and top TiNx metal layer appears to transform the initial mixture of amorphous and tetragonal hafnia phases into the monoclinic HfO2 which corresponds to the lowering of the Hf coordination from 8 O atoms to 7. As a result, in the samples with the Hf scavenger included the bandgap width of HfO2 is reduced from 5.9 to 5.6 eV as illustrated in the photocurrent yield spectra shown in Fig. 1. The most dramatic changes, however, concern the electron barrier height between the Fermi level of the bottom TiNx electrode and the conduction band of HfO2: It is reduced by ≈1/3 of its initial value

suggesting formation of positively charged defects in the O‐deficient hafnia. Not only the high‐ oxide develops a high defect density upon oxygen scavenging but also the materials it contacts show an enhanced defect density: For instance, in the (100)Si/SiOx/HfO2/TiNx/a‐Si structure, the density of Si dangling bonds in the Si//SiOx/HfO2 stack is found to be approximately 3 times higher after the oxygen scavenging anneal than at the initial un‐annealed Si//SiOx/HfO2 interface. These results indicate that while the oxygen scavenging is instrumental in achieving the desired properties of the insulating oxide, it may also cause reliability problems by introducing defects.

Figure 1. Semi‐logarithmic spectral plots of the photocurrent quantum yield in TiNx/HfO2/TiNx capacitors without and with a 5‐nm Hf oxygen scavenger layer. The spectra reveal both the onset of internal electron photoemission from the metal electrodes into HfO2 (below 3 eV) as well as the threshold of the oxide intrinsic photo‐conductivity (above 5 eV).


43

POSTERPRESENTATIONS


44

SurfaceChemistryChamberforanalysisofOxides

Jiří Pavelec, Michael Schmid, Ulrike Diebold, Gareth Parkinson

Vienna University of Technology, Wiedner Hauptstrasse 8‐10/134, 1040 Wien, Austria


A new UHV chamber for the investigation of oxide surface chemistry (e.g. In2O3, SnO2, Fe3O4, perovskites, TiO2, ZrO2) is described. The combination of a specially designed sample mount design and a molecular beam dosing setup solves difficulties associated with performing Temperature Programmed Desorption experiments on bulk oxide samples. Further spectroscopic techniques available on the system include Ultra Violet Photoelectron Spectroscopy, X‐ray Photoelectron Spectroscopy using a monochromatic

X‐ray source, Low Energy Electron Diffraction, and Ion Scattering Spectroscopy. Utilization of a liquid helium flow cryostat allows us to follow reactions from 20‐1200 K, but also allows physisorption of inert gasses such as Kr, a useful probe of surface defects, and also the basis of the Buffer Layer Assisted Growth (BLAG) technique for the in‐situ preparation of monodisperse nanoparticles. An Electro spray Deposition System is used for the deposition of

non‐volatile organic molecules that otherwise cannot be studied in UHV. This surface chemistry instrument provide an ideal complement to our existing STM based studies.

P01


45

LOHCdehydrogenationonoxide‐basedmodelcatalysts

M. Amende, M. Sobota, S. Schernich, B. Sanmartín‐Zanón. I. Nikiforidis, O. Höfert, W. Zhao, Y. Lykhach, C. Papp, W. Hieringer, M. Laurin, D. Assenbaum, P. Wasserscheid, H.‐P. Steinrück, A. Görling, J. Libuda

Friedrich‐Alexander‐Universität Erlangen‐Nürnberg


Recently the concept of reversible hydrogenation of organic compounds as a convenient method for hydrogen storage, denoted as Liquid Organic Hydrogen Carriers (LOHCs), has been discussed in the literature (see e.g. [1]). Among several heterocyclic aromatic hydrocarbons, N‐ethylcarbazole (NEC) has been identified as an attractive candidate, with hydrogen storage being performed through reversible hydrogenation to dodecahydro‐N‐ethylcarbazole (H12‐NEC).

Our first studies on the dehydrogenation of H12‐NEC on Pd(111) and Al2O3‐supported Pd model catalysts under UHV conditions led to a general picture of the dehydrogenation and degradation mechanisms. By combining infrared reflection absorption spectroscopy (IRAS), high‐resolution X‐ray photoelectron spectroscopy (HR‐XPS), and density functional theory (DFT) calculations, it was shown that dehydrogenation on clean Pd nanoparticles starts at 170 K and stepwise proceeds with increasing temperature. The mechanism involves activation of the molecule at the α‐carbon position and proceeds via formation of an aromatic five‐membered ring (H8‐NEC). The C‐N bond of the ethyl side chain is cleaved at 300 ‐ 350 K, followed by further decomposition to atomic fragments, residing on the surface of the catalyst up to high temperature (figure 1).[2]

Comparative mechanistic and kinetic studies on Pt single crystals and nanoparticles confirm that C‐N bond scission is also possible on Pt‐based catalysts. However, we could clearly demonstrate that the onset of C‐N bond breakage is shifted by approximately 100 K to higher temperature as compared to Pd catalyst. This indicates a substantially higher activation energy and explains the slower degradation of the LOHC on Pt catalysts.

Moreover, the morphology of the noble metal deposits strongly affects C‐N bond breakage. Our findings indicate that the increased number of defect sites (e.g. edges and corners) on small Pt aggregates is also responsible for a substantial lowering of the activation barrier for C‐N bond scission.

[1] K. Eblagon et al., Int. J. Hydrogen Energy, 35, 11609‐21, (2010). [2] M. Sobota et al., Chem. Eur. J., 17, 11542‐11552, (2011).

Figure 1: Suggested reaction and decomposition pathway of H12‐NEC on Al2O3‐supported Pt and Pd model catalysts.

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Theoreticaldesignofabundantandstableoxygenevolutioncatalysts

Niels Theis Bendtsen and Jan Rossmeisl

CAMd, Technical University of Denmark, Building 311, 2800 Kongens Lyngby.


The oxygen evolution reaction is a complex reaction and requires a significant overpotential for most known, stable catalysts. Furthermore, these stable catalysts consist of expensive materials, such as the oxides of Pt, Ir and Ru. The reason for the needed overpotential can be attributed to a universal linear scaling between two intermediates (the binding of the adsorbates OH and OOH to a site on the catalyst surface) in the four step reaction mechanism for oxygen evolution [1].

Theoretical design can be used to find better catalysts for oxygen evolution reaction using two different approaches: Firstly, acknowledge the scaling relation and try to design cheaper and more stable catalysts from more abundant materials. As oxidized metal surfaces [2], pure oxides, perovskites [1] and doped TiO2 [3] surfaces have been investigated thoroughly, other mixed oxides are the logical next step to investigate.

The other approach is to try and break the scaling somehow. This is a much more challenging task but it could have major consequences if the concepts from theory can be applied to experimental catalyst design. A concept which successfully breaks the scaling between OH and OOH in the oxygen evolution reaction may be used for other reactions which face similar scaling relations between intermediates.

[1] Man, I.C., et al., ChemCatChem, 3, 1159, (2011). [2] Rossmeisl, J., et al., Chemical Physics, 319, 178, (2005). [3] García‐Mota, M., ChemCatChem, 3, 1607, (2011).

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In‐situDRIFTSonMoltenSaltModifiedSupportedCatalystsforMethanolSteamReforming

A. Kaftan1, M. Kusche2, B. Morain1, F. Faisal1, H. Niedermeyer2, P. Wasserscheid2, M. Laurin1, J. Libuda1 1University Erlangen‐Nürnberg, Egerlandstr. 3, 91058 Erlangen, Physikalische Chemie II 2University Erlangen‐Nürnberg, Chemische Reaktionstechnik


Towards renewable energy sources, hydrogen is considered a possible energy carrier. With a hydrogen storage capacity of 12.5 wt%, methanol is a potential compound for chemical hydrogen storage. The hydrogen stored can be released by methanol steam reforming which is typically a heterogeneously catalyzed reaction. If coupled with fuel cell applications, it is particularly important to reduce the amount of carbon monoxide formed during the reaction to avoid poisoning of fuel cell catalyst. In this work, a commercially available oxide‐supported catalyst (5 wt% platinum on alumina) is modified by alkali‐acetate salts according to the SCILL‐approach.[2] The catalytic performance has been tested in a continuous gas phase fixed bed reactor and the product gases were analyzed via gas chromatography (GC). Here we focus on the characterization of the catalyst, which was performed with CO as a probe molecule with Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).

We have studied a molten salt mixture of three alkali‐acetates (Li/K/Cs‐[OAc], 0.2/0.275/0.525) on Pt/alumina with a melting point of 120 °C. The catalytic results showed a significant decrease of CO formation for Li/K/Cs‐[OAc] coated catalysts, while the activity was doubled in comparison with the salt‐free catalyst. This enhancement, however, strongly depends on the salt loading. Our DRIFT spectra of the uncoated catalyst show two bands for on‐top and bridge‐bonded CO whereas the coated catalysts reveal an additional band which is well known for CO adsorbed on potassium‐doped platinum (alkali‐doping).[3] With increasing salt loading, the on‐top band is suppressed and the bridge‐bonded band is enhanced. This indicates that CO molecules in the vicinity of alkali metals are depleted from the on‐top sites and enhanced interaction occurs at the bridge sites. A systematic study of the single salts reveals that potassium has the most significant influence on the active platinum species.

[1] G.A. Olah, Beyond Oil and Gas: The Methanol Economy, Wiley‐VCH, 2009. [2] U. Kernchen et al., Chem. Eng. Technol. 30(8), 985‐994 (2007). [3] H.P. Bonzel, Surface Science Reports 8, 43‐125 (1987).

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Laser‐inducedsurfaceoxidationofmetallicthinfilms

S. Petrović1*, B. Gaković1, M. Trtica1, B. Salatić2, D. Pantelić2, B. Jelenković2

Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia 1Institute VINCA, University of Belgrade, POX 522, 11001 Belgrade, Serbia 2Institute of physics, University of Belgrade, Pregrevica 118, Belgrade, Serbia


Laser processing of the material may modify the composition, chemical state, and structure of the irradiated surface. Laser irradiation of metals and alloys leads to surface oxidation in air or in controlled oxygen‐rich atmosphere. The laser‐induced process of surface oxidation depends on the initial material

composition, microstructure, and ambient conditions. The advantages of laser‐enhanced material oxidation with respect to conventional methods are confining the process to a small area, short processing time, and formation of new types of oxides.

A study of the effects of laser irradiation on the composition, structure and morphology of bimetallic tungsten–titanium (WTi) thin film and (Ni/Ti) multilayer structure is reported. The

WTi thin films and multilayers composed of five (Ni/Ti) bilayers were deposited by d.c. ion sputtering on (100) Si wafers, to a total thickness of about 200 nm. Laser processing was done by a non‐focused Nd:YAG laser beam with low fluences to prevent ablation and exfoliation of materials.

Laser irradiation of a WTi thin film, in air ambient condition, led to significant changes of the chemical composition and morphology on the surface of the WTi thin film. The distribution of components on the surface and in subsurface region was indicated that an oxide layers were formed at the surface due to reduction of the W and Ti concentration and a relatively high concentration of oxygen. A very thin oxide layer was formed, a mixture of WO3 and TiO2. This mixture in form of a thin oxide layer is applied as photocatalyst and as chemical sensors for the selective catalytic reduction of gaseous pollutants and the degradation of organic compounds. The thickness of this oxide layer increased with the number of laser pulses and reached a value of 70 nm after 100 pulses. Several first atomic layers are dominated by TiO2 due to titanium surface segregation. After the laser modifications a grain‐conical nanostructure formed on the surface with higher mean surface roughness of about 44 nm. Nanometric conical structure made up of the cones about 50 nm wide in the base and height about 20‐30 nm. When the irradiation of WTi thin films was carried out in various ambient conditions (nitrogen, helium and oxygen), only different surface concentrations of oxygen were obtained, which caused the formation of a different granular nanostructure.

Through irradiation of the nanocomposite (Ni/Ti) multilayer thin films, using a 100 laser pulses, a complex structure was created, resulting in the following distribution of composition within the multilayer Ni/Ti system: (i) the initial Ni/Ti multilayer structure, with separated layers, is preserved at the interface with the Si substrate, whereby increasing the adhesion of the thin film; (ii) the formation of NiTi intermetallic compounds with excellent properties at the mid‐depth of the multilayer system; (iii) the surface segregation of Ti and the formation of a TiO2 oxide phase at the top of the irradiation surface. The morphology of this system, after 100 accumulated pulses, was found to have quite a regular parallel ripple structure, which covered a large central area of the irradiated zone. This specific nanostructure on the surface can increase the surface activity for a certain number of catalytic reactions.

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EnhancingmobilityofF‐centresatMgO(100)andCaO(100)surfacesbyadsorptionofAuclusters

Željko Šljivančanin

Vinča Institute of Nuclear Sciences (020), P.O.Box 522, RS‐11001 Belgrade, Serbia


By means of first‐principles calculations based on density functional theory we investigated mobility of neutral oxygen vacancies (F‐centres) on reduced MgO(100) and CaO(100) surfaces. In the line with previous studies we observed very high activation energies for diffusion of bare vacancies. Yet, upon adsorption of small gold clusters, the mobility of the F‐centres profoundly changes. According to the calculations performed for Au dimers and trimers chemisorbed at the F‐centres on the MgO(100) and Ca(100) surfaces, the activation energies for diffusion of the gold‐vacancy complexes are almost 1 eV lower compared to the energy barriers required for diffusion of pristine F‐centres on these surfaces.

[1] Ž. Šljivančanin, manuscript in preparation.

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GoldontitaniamodelcatalystinvestigatedbyinsituX‐rayPhotoelectronSpectroscopy:towardsidentificationofoxygenspeciesactiveinCOoxidation

K. Dumbuya1, G. Cabailh2, R. Lazzari2, J. Jupille2, L. Ringel1, M. Pistor1, O. Lytken1, H.‐P. Steinrück1, J. M. Gottfried1

1Lehrstuhl für Physikalische Chemie II, Uni. Erlangen‐Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany 2Institut des Nanosciences de Paris, UPMC and CNRS, 4 Place Jussieu, 75252 Paris, France


Numerous experiments were dedicated to the study of the active site in the CO oxidation over supported gold catalysts. In situ photoelectron spectroscopy (in situ XPS), which allows analysis in reactive conditions in the mbar range, is a powerful technique in this regard [1]. For oxygen on Au nanoparticles on TiO2, Willneff et al. [2] observed Au 4f core‐level shifts of +0.3 and +0.9 eV, which they assigned to bulk‐like metallic gold and to intermediate gold species, respectively; similar +0.3‐ 0.4 eV shifts were recorded by Herranz et al. [3]; Jiang et al.

reported a very different value of +2.3 eV [4], which was explained by photoelectron induced dissociative chemisorption of oxygen.

To revisit these conflicting results that question the applicability of in situ XPS to characterize the catalytic reaction itself, Au/TiO2(110) model catalysts were analyzed by in situ XPS in the presence of O2, CO and CO + O2 mixture in the 0.1–1 mbar range [5]. Two different samples were prepared, with particles close to the optimum size for the catalytic oxidation of CO (~ 2 nm) [6], and larger than that size (~3 nm). Under an oxygen pressure of 1 mbar, a new component shifted by +2.4 eV evolved at all particle sizes. The activation was much more efficient on the 2 nm particles than on the 3 nm particles.

Absent in 0.1 mbar O2, consistently with Wilneff et al. [2] and Herranz et al. [3], the +2.4 eV Au 4f shift appeared in 1 mbar O2 as seen by Jiang et al. [4]. It was attributed to a photoinduced activation of oxygen and a further oxidation of gold. Photons are required since XPS performed in vacuum after transfer of a Au/TiO2(110) sample exposed to 20 mbar O2 only evidenced ~ +0.5 eV Au 4f shift. But, although experiments at Lawrence Berkeley National Laboratory [3,4] involved a photon flux 5 orders of magnitude higher than that used in the present laboratory work, they show the same pressure‐ dependent activation of oxygen as herein. Therefore, the intensity of the +2.4 eV Au 4f shift only depends on the oxygen coverage (equivalent to pressure since all experiments were done at room temperature) in the photon flux range which was explored. Such an occurrence of a pressure‐ dependent activation reconciles previously conflicting XPS data [2‐5]. Finally, the rapid disappearance of the +2.4 eV Au 4f component in a 1:1 mixture of CO and O2 demonstrated the high reactivity of the activated oxygen species toward CO. Rates of evolution and consumption of this component were found to depend on gold coverage (and thus, particle size). They were higher for the smaller particles.

[1] M. Salmeron and R. Schlögl, Surf. Sci. Rep. 63 (2008) 169 [2] E. A. Willneff et al., J. Am. Chem. Soc. 128 (2006) 12052. [3] T. Herranz et al., Catalysis Today 143 (2009) 158. [4] P. Jiang et al., J. Am. Chem. Soc. 132 (2010) 2858. [5] K. Dumbuya et al, Catal. Today, 181 (2011) 20‐25. [6] I. Laoufi et al., J. Phys. Chem. C 115 (2011) 4673.

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AtomicforcemicroscopyonAl2O3thinfilmspreparedbyAtomicLayerDepositiononSi(001)substrates

B. Hoff, L. Assaud, E. Moyen, K. Pitzschel, L. Santinacci, C. Barth, M. Hanbücken

Aix Marseille University, CNRS, CINaM UMR 7325, 13288 Marseille Cedex 09, France


Nanoporous anodic aluminum oxide (AAO) membranes allow fabricating cost‐efficient self‐ordered arrays of vertical and insulating nanopores on the length scale of a wafer. They are essentially used as templates to fabricate a great variety of challenging functional nanostructures in optics, cell imaging, energy storage, photovoltaics, and microelectronics (e.g., magnetic data storage, ultra‐capacitors). Recently, we could produce a new family of AAO membranes, which exhibit sub‐10 nm pore diameters with low porosity and a high degree of ordering [1]. Such ordered templates are quite promising to grow Metal‐Insulator‐Metal (MIM) ultra‐capacitors by Atomic Layer Deposition (ALD) techniques. A major objective is to produce capacitors with a high efficiency, high loading capability and high cyclability.

A requisite for producing such MIM structures is to prepare an insulating oxide film with a high dielectric constant and with a thickness of only a few nanometers. The films need to be almost atomically flat and in particular quite homogeneous such that they exhibit a low density of defects, which would otherwise lead to undesired effects like leakage currents. A mean to study the quality of such insulating films is Atomic Force Microscopy, which strength and driving force is in versatility of different methods like nc‐AFM, conductance AFM (C‐AFM) and Kelvin Probe Force Microscopy (KPFM) [2].

In this contribution we discuss first results of the AAO membranes and MIM systems, which are composed of TiN‐Al2O3‐TiN. We then focus on Al2O3 thin films, which were grown in parallel on flat Si(001) wafers and studied by nc‐AFM and KPFM in ultra‐high vacuum. We discuss the oxide thin film quality, the role of defects in the film, the contribution of nc‐AFM, C‐AFM and KPFM in the film analysis and in particular the future prospective in view of HfO2 thin films in MIM structures.

[1] E. Moyen, L. Santinacci, L. Masson, W. Wulfhekel and M. Hanbücken, Adv. Mat., in print (2012), http://onlinelibrary.wiley.com/doi/10.1002/adma.201200648/abstract [2] C. Barth, A. S. Foster, C. R. Henry and A. L. Shluger, Adv. Mater 23 (2011) 477

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Optimizationofthesynthesisparametersofvanadiumaluminiummixedoxides�

Codruta G. Rotaru, Roxana S. Marin, Mihaela Florea

University of Bucharest, Faculty of Chemistry, Bucharest, 030016, Romania


Introduction: The rigorous control of the different preparation parameters is crucial for developing highly active and selective catalysts. Thus, understanding the key factors of the preparation method of vanadium aluminium mixed oxides is an important point towards the controlled design of these materials. This work is a part of the research program aimed at a systematic investigation of the parameters

controlling the preparation of V‐Al oxide catalysts. The effect of the precipitation pH and the molar concentration of the ammonium metavanadate solution on the physico‐chemical properties and catalytic activity of vanadium–aluminum oxide catalysts were investigated in this study.

Materials and methods: Vanadium aluminium oxide was prepared by co‐precipitation of aluminium nitrate (0.08M) and ammonium metavanadate solutions (0.02, 0.04, 0.06, 0.12 M) at different pHs (5, 7, 8 and 10) followed by hydrothermal treatment for 1 hour at 60°C. The synthesized materials were dried and calcined at different temperatures. For the characterization various techniques were used: BET, XRD, DR‐UV‐VIS, PSD, TG‐DTA. For the catalytic properties investigations the liquid phase oxidation of toluene was carried out in a round bottom flask at 60°C with acetonitrile as a solvent and aqueous H2O2 as oxidant.

Results and discussions: The results of this study show that the texture (Table 1), chemical composition, reducibility and nature of the vanadium oxide species in V‐Al‐Ox catalysts are strongly influenced by the vanadium concentration solution and co‐precipitation pH. The main effects produced are summarized as follows: (i) the specific surface area has a maximum value for the samples prepared at a vanadium concentration in solution of 4x10‐2 mol/L, and after that, decreases continuously when the vanadium concentration increases, (ii) smaller particle sizes are observed for the samples precipitated at higher vanadium concentration solutions, (iii) the reduction temperature decreases continuously with increasing vanadium concentration, and the hydrogen consumption per amount of catalyst follows a similar trend as the variation of the vanadium content in the samples and (iv) RAMAN spectroscopy reveals for all samples the formation of polymeric tetrahedral vanadate species [VOx]n

n‐ for higher concentration solutions. Table 1. Surface area and particle size for samples prepared at different vanadium solution concentrations.

Sample Conc. V (mol/l)

V/Al Surface area (m2/g)

Particle size (µm)

VAlO0.1‐0,02 0,02 0,1 342 46.9 VAlO0.25‐0,02 0.25 217 19.3 VAlO0.1‐0,04 VAlO0.25‐0,04

0,04 0.1 0,25

430 261

12.4 0.2

VAlO0.1‐0,06 VAlO0.25‐0,06

0,06 0.1 0,25

398 n.a.

9.6 0.2

VAlO0.1‐0,12 VAlO0.25‐0,12

0,12 0.1 0,25

288 200

0.2 0.2

In summary, we have synthesised high surface area vanadium‐aluminium oxide materials, by controlling different preparation parameters, and tested in selective oxidation in liquid phase of toluene to benzaldehyde under mild conditions. The highest activity was obtained for the catalyst with V/Al=0.25, [V]=0.02 and co‐precipitation pH 5.

Acknowledgements: The authors kindly acknowledge the UEFISCDI and MECTS for the financial support (grant PN‐II TE 105/2011).

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DFTstudyoftheTwo‐DimensionalElectronGasattheLaAlO3onSrTiO3Interface

Zoran S. Popović

INN‐“Vinča”, lab. (020), PO Box 522, 11001 Belgrade, Serbia


The intrinsic carrier density at the LaAlO3/SrTiO3 interface found in transport measurements (1‐21013 cm‐2) is much smaller than the 0.5 electrons per unit cell (3.51014 cm‐2) needed to suppress the "polar catastrophe". From a density‐functional study [1] we find two types of electrons occupying multiple subbands at the interface leading to a rich array of transport properties. The first type consist of: (a) Ti(3dxy) localized states from the first layer having a strong 2D character and (b) Ti(3dxz/yz) states which are also prone to localization due to their heavy masses along planar direction. The second type of electrons are from the Ti(3dxy) spreading over several Ti layers into the bulk. These electrons have small masses along the plane and are expected to contribute to transport.

[1] Zoran S. Popović, Sashi Satpathy, and Richard M. Martin, Phys. Rev. Lett. 101, 256801 (2008).

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AtomicscalecharacterizationoftheMgAl2O4(100)surface

Filippo Federici1,3, Morten K. Rasmussen, Sampo Kulju1, Thomas N. Jensen2, Flemming Besenbacher2, Jeppe V. Lauritsen2, Adam S. Foster3

1Dept. of Physics, Tampere University of Technology, Finland 2Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Denmark 3COMP, Dept. of Applied Physics, Aalto University, Finland

e-mail: [email protected]; [email protected]

Metal oxide spinels are an important class of materials in both ceramics technology and materials science. The prototypical ternary metal oxide spinel, magnesium aluminate spinel (MgAl2O4), is widely used,

for instance, as a membrane in solid oxide fuel cells and in heterogeneous catalysis, either as a support for active metal nanoclusters or as a catalyst in its own right. MgAl2O4

is a ternary metal oxide with the prototypical spinel structure. Its crystal structure defines a larger group of so‐called spinel minerals, which adopt the general formula A2+B2

3+O42‐. Recently it was

possible in interplay between NC‐AFM and surface x‐ray diffraction (SXRD) experiment together with density functional theory calculations and NC‐AFM simulations to reveal an atomistic model of the (100) surface structure and the prevalent defect types on this metal oxide [1,2], including the role of hydrogen [3]. This was a key step in developing the potential applications of this surface.

In a further step, we used the MgAl2O4 (100) surface as a substrate to grow thin films of aluminium

oxide – a particularly common material in studies of catalysis. By combining high‐ resolution non‐contact atomic force microscopy with first principles simulations we demonstrate that this film remains crystalline through several layers of growth. Simulations characterize the structure of the alumina film and its interface, and predict its electronic properties – with particular emphasis on its role as a nanocatalytic substrate.

[1] M. Rasmussen, A. S. Foster, B. Hinnemann, F. Canova, S. Helveg, K. Meinander, N. Martin, J. Knudsen, A. Vlad, E. Lundgren, A. Stierle, F. Besenbacher, and J. V. Lauritsen, Phys Rev Lett 107, (2011) 036102 [2] M. Rasmussen, A. S. Foster, F. Canova, B. Hinnemann, S. Helveg, K. Meinander, F. Besenbacher, and J. V. Lauritsen, Phys. Rev. B 84 (2011) 235419 [3] Filippo F. Canova, Adam S. Foster, Morten K. Rasmussen, Kristoffer Meinander, Flemming Besenbacher and Jeppe V. Lauritsen, Nanotechnology 23 (2012) 325703

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AdsorptioninducedrearrangementofPdZnsurfacealloys

H. H. Holzapfel1, C. Weilach1, S. M. Kozlov2, K. M. Neyman2,3, G. Rupprechter1 1 Institute of Materials Chemistry, Vienna University of Technology, Vienna, Austria 2 Universitat de Barcelona c/ Martí i Franquès, 1, 08028 Barcelona, Spain 3 Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain


Pd/Zn surface alloys have been identified as very interesting catalysts for use in chemical engineering such as methanol steam reforming, because they rather produce hydrogen and CO2 than CO from methanol, which is an unwanted catalyst poison, whereas on metallic Pd surfaces CO production is favored. New Density Functional Theory (DFT) calculations and contradictory results of experimental studies have motivated us for new investigations of the formation, surface structure and catalytic properties of PdZn surface. Combining DFT calculations with experimental data gives strong indications for structural rearrangement of Pd and Zn atoms in PdZn surface alloys on Pd(111) at elevated CO coverage.

Pd/Zn model catalysts where then prepared by physical vapor deposition of Zn onto a clean Pd(111) single crystal substrate in ultrahigh vacuum. The unalloyed Pd(111)/Zn crystal was heated up stepwise to successively higher temperatures and cooled down in in 10‐6 mbar CO. For each temperature ramp, a CO‐TPD was recorded. The surface alloys on Pd(111) were modeled by 6 layers thick p(4×3) supercell. Depending on the thickness of surface alloy under consideration, top 4 layers of the slab consisted of either stoichiometric PdZn alloy or pure Pd and were relaxed (together with adsorbed CO molecules) [1].

However, the onset of alloying starts at about 473 K and was significantly shifted to higher temperatures when the annealing was performed in a mbar CO environment. A possible surface reconstruction (s. figures below) as suggested by recent DFT calculations of the relative stability of reconstructed zigzag layers of PdZn/Pd(111) with respect to row structure depending on number of layers arranged in zigzag structure and CO coverage might be responsible for this effect [1,2].

The CO coverage, obtained from our experimental TPD measurements, which was 0.50 ±0.02 ML for a 2

ML PdZn/Pd(111) system and slightly lower, 0.47 ±0.02 ML for a >4 ML PdZn system. This is also a hint of zigzag rearrangement. According to theory, for Pd and Zn atoms in row structure, CO is stable only at coverage ≤0.33 ML, with decrease of stability at higher coverage. On rearranged zigzag surface, CO is stable even at coverage up to 0.5 ML.

To further pursue this issue, we are currently starting to perform STM studies of the reconstructed surface of PdZn alloys on Pd(111) in the presence of adsorbed CO.

[1] C. Weilach, S. M. Kozlov, H. H. Holzapfel, K. Föttinger, K. M. Neyman, G. Rupprechter, 2012, J. Phys. Chem. C (accepted) [2] He, X., Huang, Y., Chen, Z.‐X., Phys. Chem. Phys., 2011, 13, 107‐1091

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SynthesisofAnilineFunctionalizedRutheniumDyesforUsingVisibleLightWaterSpittingwithSolarCell

Oktay Talaz, Huseyin Bekir Yildiz

Department of Chemistry, Karamanoglu Mehmetbey University 70100 Karaman, Turkey


Dye‐sensitized solar cells (DSCs) provide a technically and economically credible alternative for increasing energy demands and concerns over global warming. Dye‐sensitized solar cells (DSCs) based on the ruthenium(II) dyes have high thermal and chemical stability [1]. Dye‐sensitized solar cells (DSCs) based on the ruthenium(II) dyes have been shown to be very efficient candidates for photovoltaic technology due

to their high stablity and outstanding redox properties and good response to natural visible sunlight [2]. Ruthenium(II) dyes has to interact with the semiconductor, and thus a range of attaching functionalities have been screened [3] thus we will use aniline functionalized ruthenium (5) dyes for using visible light water spitting with solar cell in our project.

Figure 1. Synthesis of Aniline Functionalized Ruthenium

[1] B. O’Regan and M. Gratzel, Nature, 353, 737‐740, (1991). [2] B. Yoo et al., J. Mater. Chem., 20, 4392‐4398, (2010). [3] V. Aranyos et al., Sol. Energ. Mat. Sol. C. 64, 97‐114, (2000).

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UnderstandingStructureandFunctionofNi/CeO2Catalysts:ATheoreticalPerspective

Javier Carrasco1, Laura Barrio1, M. Verónica Ganduglia‐Pirovano1, Ping Liu2, José A. Rodriguez2 1Institute of Catalysis and Petrochemistry‐CSIC, Madrid, Spain 2Chemistry Department, Brookhaven National Laboratory, Upton, NY, USA


Noble metals nanoparticles supported by a metal oxide are used for hydrogen production through the steam reforming of hydrocarbons and the water‐gas shift (WGS) reaction. However, the high cost of these materials limits their application. As a less expensive alternative, nickel‐ceria‐based catalyst have recently been proposed [1,2]. The challenge of characterizing the surface structure of powder catalysts and in identifying the active species have motivated the preparation of experimental model catalysts such as Ni/CeO2(111) [3]. The activity and selectivity of Ni/CeO2 catalysts are strongly dependent on the size of the dispersed particles and metal‐support interactions that can modify the chemical reactivity of Ni. Specifically, small Ni coverages display a high activity for the WGS reaction and a very low activity for CO methanation, whilst medium and large coverages produce systems that catalyze the production of methane. Here, we consider computational model Ni/CeO2(111) catalysts to investigate the structure, electronic properties and the adsorption of C, H, and CO species as a function of the nickel coverage. We focus on highly dispersed Ni1 and Ni4 clusters supported on the CeO2(111) surface. Density functional theory with the PBE functional augmented with a Hubbard‐like U term (DFT+U) and periodic boundary conditions are used.

Single Ni atoms adsorb on top of subsurface O atoms. Upon adsorption each Ni atom donates two electrons to the support creating two Ce3+ ions per adsorbed Ni atom. The Ce3+ ions are in next‐nearest neighbor position to the subsurface O. This is consistent with experimental findings (XPS, UPS) [2]. At higher Ni coverages we find that nickel is not fully oxidized and that Ni2+ and Ni0 species coexist within the Ni4 cluster, also in agreement with experimental evidence. The relevance of metal‐support and metal‐metal interactions on Ni/CeO2(111) model catalysts to the CO methanation reaction as a function of metal coverage is discussed by considering the adsorption of C, H, and CO on the supported Ni1 and Ni4 clusters as well as on the clean CeO2(111) and Ni(111) surfaces.

[1] L. Barrio et al., J. Phys. Chem. C 114, 12689 (2010). [2] G. Zhou et al., Angew. Chem. Int. Ed. 49, 9680 (2010). [3] S. D. Senanayake et al., Top. Catal. 54, 34 (2011).

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a) b) c) d) Fig.1: LEED patterns at E=80 eV of a 2 ML CeO2 film on Pt(111) (a) before reduction (cCe3+ < 10%) and (b) after reduction (cCe3+ >80%). STM images (size 125 nm x 125 nm) of a 10 ML CeO2 film on Pt(111) (c) before reduction (cCe3+ < 5%) and (d) after reduction (cCe3+ >50%).

ReversiblereductionofceriumoxideultrathinfilmsonPt(111)

F. Pagliuca1,2, P. Luches2, L. Amidani3, F. Boscherini3, S. Valeri1,2 1Dip. di Scienze Fisiche Mat. e Inf., Università di Modena e Reggio Emilia, Modena, Italy 2S3, Insituto Nanoscienze – Consiglio Nazionale delle Ricerche, Modena, Italy 3Dipartimento di Fisica, Università di Bologna, Bologna, Italy


The possibility to have an accurate and reliable control of reversible reduction/oxidation processes in cerium oxide based materials is a challenge for a number of relevant applications of these systems. The use of model systems in this context allows the application of advanced surface science techniques, which can

give a detailed picture of the process. For this purpose we used thin epitaxial films of cerium oxide, with a flat surface morphology and a high crystal quality grown on Pt(111) [1]. The concentration of Ce3+ ions in the films can be reversibly modified by thermal treatments in UHV and in O2 atmosphere. The modifications of stoichiometry, electronic properties, structure and morphology of the films have been studied by conventional (XPS,LEED,STM) and

innovative (High Energy Resolution Fluorescence Detected X‐ray Absorption Near Edge Structure HERFD‐XANES) techniques.

Several factors have been found to influence the final degree of reduction of the films, among these annealing time and temperature, and film thickness play a significant role.

The reduction process is more relevant in the surface layers than in the deeper ones. The film surface shows some changes after a significant reduction (cCe3+ >50%), with the appearance of different contrast in the STM images and of a reconstruction in the LEED patterns (Fig.1). The large scale film morphology instead remains unaltered. The original stoichiometry, surface morphology and structure of the films can be brought back to the original one by heat treatments in O2.

An HERFD‐XANES experiment on the Ce L3 adsorption edge has been carried out in order to probe the empty valence shell electron configuration within a few eV above the Fermi level with high energy resolution before, after and during the reduction/oxidation process. Furthermore, the interactions between 3d hole and 4f electrons in the final state can provide valuable information on the 4f electron configuration.

[1] P. Luches, F. Pagliuca and S. Valeri, J. Phys. Chem. C 115, 10718 (2011).

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Adsorptionanddecompositionofaceticacidonceria‐basedmodelcatalysts

Armin Neitzel1, Yaroslava Lykhach1, Viktor Johánek2, Nataliya Tsud2, Tomáš Skála2,3, Kevin C. Prince3,4, Vladimír Matolín2, Jörg Libuda1,5

1Department Chemie und Pharmazie, Friedrich‐Alexander‐Universität Erlangen‐Nürnberg, Erlangen, Germany 2Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Prague, Czech Republic 3Sincrotrone Trieste SCpA, Basovizza‐Trieste, Italy 4IOM, Basovizza‐Trieste, Italy 5Erlangen Catalysis Resource Center, FAU Erlangen‐Nürnberg, Erlangen, Germany


The adsorption and decomposition of acetic acid on stoichiometric CeO2(111)/Cu(111), partially reduced CeO2‐x/Cu(111), Pt/CeO2/Cu(111) and Pt(111) were investigated by Synchrotron Radiation Photoelectron Spectroscopy (SRPES) and Resonant Photoemission Spectroscopy (RPES). RPES is considered to be one of the most surface sensitive methods for determining the surface oxidation state of the cerium cations in ceria‐based model catalysts.[1,2] Therefore, RPES provides fundamental insight into redox processes on ceria surfaces.

On fully stoichiometric CeO2(111)/Cu(111), RPES and SRPES revealed no involvement of the ceria redox processes in decomposition of acetic acid. In contrast, on CeO2‐x/Cu(111) decomposition of acetic acid results in strong re‐oxidation of ceria between 250 K and 400 K, followed by reduction and C‐C bond cleavage at temperatures above 400 K. As a result, decomposition products like formate, alkoxy, and alkyl species, as well as atomic carbon were identified on the surface.

Additionally, the role of co‐adsorbed oxygen in decomposition of acetic acid has been investigated on Pt(111). It was found that C‐C bond cleavage occurs only in presence of oxygen ad‐atoms. Resulting methoxy species were found to decompose to hydrogen and CO during annealing of the adsorbate.

On Pt/CeO2/Cu(111) hydrogen spillover and reverse spillover accompanied by reduction and re‐oxidation of ceria, respectively, were observed during annealing. Furthermore, CO, alkoxy species, alkyl moieties, and elemental carbon were detected as decomposition products. At higher temperatures, the onset of reverse oxygen spillover from ceria to Pt particles was observed.[4] Adsorption of oxygen on Pt/CeO2/Cu(111) leads to re‐oxidation of ceria. Formation and decomposition of methoxy species has been observed similar as on Pt(111). Removal of oxygen ad‐atoms from Pt/CeO2/Cu(111) resulted in recovery of the initial reduced state of ceria.

[1] A. Trovarelli, Catalysis by Ceria and Related Metals, Imperial College Press, London, 2002. [2] V. Matolín, I. Matolínová, L. Sedláček, K. C. Prince, T. Skála, Nanotechnology 2009, 20(21), 215706. [3] Y. Lykhach, T. Staudt, M. Vorokhta, T. Skála, V. Johánek, K. C. Prince, V. Matolín, J. Libuda, J. Catal. 2012, 285 (1), 6‐9. [4] G. N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G. P. Petrova, N. Tsud, T. Skála, A. Bruix, F. Illas, K. C. Prince, V. Matolín, K. M. Neyman, J. Libuda, Nature Materials 2011, 10, 310‐315.

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Superchargedoxygenstoragecapacityofceriaatthenanoscale

Peter Broqvist, Jolla Kullgren, Kersti Hermansson*

Department of Chemistry‐Ångström, Uppsala University, Box 528, S‐75121 Uppsala, Sweden


Ceria (CeO2) is used in solid oxide fuel cells and for the purification of exhaust gases in vehicle emissions control. Behind these technically important applications of ceria lies one overriding feature, namely ceria's exceptional reduction‐oxidation properties, enabled by the duality of the cerium atom which can display two oxidation states (+IV and +III).

The surface chemistry of ceria is rich. Oxygen vacancies are rather easily created, leading to population of the 4f electron states of the cerium ions. Traditionally, much of the technical functionality of ceria has been ascribed to oxygen vacancies, such as ceria's important oxygen storage capacity (OSC). However, in this contribution we will provide an alternative view on the special red‐ox properties of nano‐sized ceria particles, and the OSC in particular.

Recent experiments have shown that small nanoparticles of ceria, with an average diameter of approximately 4 nm, possess a dramatically enhanced OSC compared to larger particles [1]. These oxygen species were found to be more reactive and therefore accessible at lower temperatures. In

Ref. 1, they were assigned to superoxo ions (O2), formed from oxygen adsorption. In the present

contribution, we have performed extensive theoretical simulations based on the density functional theory ("DFT+U"), to elucidate the underlying mechanism that governs the formation of these reactive oxygen species found experimentally on ceria at the nanoscale.

The common understanding of the OSC mechanism in ceria is based on the creation and regeneration of localized oxygen vacancies (see many references of experimental and theoretical reports in the literature). This mechanism holds very well for extended surfaces and large crystallites of ceria but fails to explain the enhanced OSC observed at the nanoscale.

Our proposed OSC mechanism for nanoceria instead involves nanoparticles that do not expose any oxygen vacancy sites, but are stable during both oxidizing and reducing conditions. The mechanism fully utilizes the ability of the nanoparticles to adsorb many oxygen molecules and to respond dynamically to the surrounding environment. Our mechanism thus predicts a significant increase in the oxygen storage capacity, and for small particles only, in agreement with experiment.

[1] J. Xu et al., Chem. Commun. 46, 1887 (2010).

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Operandoinfraredspectroscopyonionicliquidcrystalbasedcatalystssupportedonoxidepowdermaterials

Bruno Morain1, Marek Sobota1, Xinjiao Wang2, Florian Kohler3, Berthold Melcher3, Peter Wasserscheid3, Karsten Meyer2, Mathias Laurin1, Joerg Libuda1

1University Erlangen‐Nürnberg, Egerlandstr. 3, 91058 Erlangen, Physikalische Chemie II. 2University Erlangen‐Nürnberg, Anorganische und Allgemeine Chemie. 3University Erlangen‐Nürnberg, Chemische Reaktionstechnik.


Ionic Liquids (ILs) are molten salts with melting point below 100°C. When used in catalytic applications with active transition metal complexes dissolved in thin IL films on porous supports, these systems are called “Supported Ionic Liquid Phase” (SILP) catalysts [1–2]. ILs may show liquid crystalline behaviour (ILC) when containing long alkyl chains. If used as catalytic reaction media, switching from the liquid crystalline to the liquid phase may have an influence on the activity and the selectivity of the reaction. We aim at understanding the influence of the confinement in porous supports on the catalytic behaviour of ILC SILP systems.

In a first step, different loadings of the ILCs 1‐octadecyl‐ and 1‐hexadecyl‐3‐methylimidazolium trifluorosulfate [C18C1Im][OTf] and [C16C1Im][OTf] confined in supports with increasing pore size are studied using Differential Scanning Calorimetry (DSC) and Diffuse Reflectance IR Fourier Transform Spectroscopy (DRIFTS). The results reveal an influence of the confinement on the smectic‐to‐liquid phase transition and the existence of a contact layer with a thickness of about 2 nm. The latter shows a single glass transition at temperatures 30 K lower than that of the bulk [3]. In a second step, nickel complexes are dissolved in the ILC 1,3‐didodecylimidazolium tetrafluoroborate [C12C12Im][BF4]. Investigations on these systems using IR Reflection Absorption Spectroscopy (IRAS) on planar surfaces and DRIFTS on powders show the retained liquid crystalline transitions for complex concentrations from 1 to 10 wt% [4]. Finally, we apply operando DRIFTS to the hydroformylation of olefins and measure the activity and the selectivity with an online gas chromatograph. We found little influence of the phase with the systems studied and propose new ways of performing temperature‐switchable catalysis.

[1] A. Riisager et al., Ind. Eng. Chem. Res., 44, 9853, (2005). [2] M. Sobota et al., Langmuir, 26(10), 7199, (2010) [3] F.T.U. Kohler, B. Morain et al., ChemPhysChem, 12, 3539, (2011). [4] X. Wang et al., J. Mater. Chem., 22, 1893, (2012).

IL - support interactions

littleinteractions

Thin contact layerIL

IL

Support = SiO2

IL Bulk-like

325 335 345 355 365 375 385 395 405

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

Temperature [K]

Se

lect

ivit

y

370 ± 5 K

341 K

liquidlc (smectic)solid

Fig 1: Silica pore coated with ionic liquid Fig 2: Catalytic selectivity vs. temperature

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Interactionofionicliquids(ILs)withaluminaandceriathinfilms

Stefan Schernich1, M. Sobota1, Y. Lykhach1, M. Laurin1, N. Paape1, V. Wagner1, N. Taccardi1, P. Wasserscheid1, J. Libuda1, N. Tsud2, V. Matolín2, T. Skála3, K. C. Prince3

1Friedrich‐Alexander‐University Erlangen‐Nuremberg, Erlangen, Germany 2Charles University, Prague, Czech Republic 3Sincrotrone Trieste SCpA, Basovizza‐Trieste, Italy


Towards a more general understanding of IL thin‐film catalysts at the microscopic level, we have prepared single‐crystal‐based model systems under ultrahigh vacuum (UHV) conditions. Using a novel UHV‐

compatible evaporator, ultrathin films of imidazolium/Tf2N‐type ILs have been deposited onto Al2O3/NiAl(110) and CeO2‐x/Cu(111) (x = 0; 0.25). These systems are studied with Infrared Reflection Absorption Spectroscopy (IRAS) and Synchrotron‐Radiation Photoelectron Spectroscopy (SR‐PES).

At 298 K, [C4C1Im][Tf2N] adsorbs molecularly on Al2O3/NiAl(110) and wets the oxide surface. A comparison between IRAS spectra and density functional theory (DFT) calculations reveals a preferential interaction of the Tf2N‐ anion with the Al2O3 surface in cis‐configuration via the SO2‐groups within the first monolayer (fig 1).[1] Upon heating to 373 K the IL desorbs almost quantitatively.

Whereas Al2O3 appears to be rather inert towards [C4C1Im][Tf2N], the situation is drastically different for adsorption of its analogue, [C6C1Im][Tf2N], on thin films of ceria grown on a Cu(111) single crystal. Moreover, the oxidation state of the ceria film has a major influence on the surface chemistry. On stoichiometric ceria, a stable surface species is formed after cation deprotonation and dealkylation at 300‐350 K (fig 2). In contrast, on reduced ceria no cation decomposition is observed at all. Anion decomposition to sulfite/sulfates, CFx, and nitrogen is found at 425 K on the stoichiometric surface and at 350 K on the reduced surface, respectively. The temperature difference is attributed to a stronger interaction of the Tf2N‐ anion with exposed Ce

3+‐centers within the reduced film. When heating up further, oxides, fluorides, and sulfides are formed and diffuse into the reduced film, thereby acting as oxidation agents.

[1] M. Sobota, I. Nikiforidis, W. Hieringer, N. Paape, M. Happel, H.‐P. Steinrück, A. Görling, P. Wasserscheid, M. Laurin, J. Libuda, Langmuir 2010, 26(10), 7199‐7207

Fig. 1: Comparison of calculated spectra for (a) trans‐ and (b) cis‐conformers of the [Tf2N]‐ anion with (c) IRAS absorbance spectra acquired in the submonolayer and multilayer regions of [C4C1Im][Tf2N] on Al2O3/ NiAl(110).

Fig. 2: C1s SR‐PES spectra of [C6C1Im][Tf2N] on CeO2/Cu(111). A structure of the IL for peak assignment is shown as inset. Im´, alkyl´, anion´ denote peaks of decomposition products.

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